2009. "Nanomaterial-Based Biosensors for Detection of Pesticides and Explosives." Chapter 26 in Nanotechnology Applications for Clean Water, ed. N. Savage, et al, pp. 377-390. William Andrew, Nowich, NY. Abstract In this chapter, we describe nanomaterial-based biosensors for detecting OP pesticides and explosives. CNTs and functionalized silica nanoparticles have been chosen for this study. The biosensors were combined with the flow-injection system, providing great advantages for onsite, real-time, and continuous detection of environmental pollutants such as OPs and TNT. The sensors take advantage of the electrocatalytic properties of CNTs, which makes it feasible to achieve a sensitive electrochemical detection of the products from enzymatic reactions at low potential. This approach uses a large aspect ratio of silica nanoparticles, which can be used as a carrier for loading a large amount of electroactive species, such as poly(guanine), for amplified detection of explosives. These methods offer a new environmental monitoring tool for rapid, inexpensive, and highly sensitive detection of OPs or TNT compounds.
2009. "Biosensors Based on Functionalized Carbon Nanotubes, Nanoparticles, and Nanowires." Chapter 4 in Handbook of Electrochemical Nanotechnology, vol. II, ed. Yuehe Lin and H. S. Nalwa, pp. 95-111. American Scientific Publishers, Stevenson Ranch, CA. Abstract In this book chapter, we will review recent progress in functionalization of nanotubes, nanoparticles, and nanowires for sensing applications.
2009. "Nanomaterials-Enhanced Electrically Switched Ion Exchange Process for Water Treatment." Chapter 14 in Nanotechnology Applications for Clean Water, ed. M. Savage, et al, pp. 179-189. William Andrew, Norwich, NY. Abstract The objective of our work is to develop an electrically switched ion exchange (ESIX) system based on conducting polymer/carbon nanotube (CNT) nanocomposites as a new and cost-effective approach for removal of radioactive cesium, chromate, and perchlorate from contaminated groundwater. The ESIX technology combines ion exchange and electrochemistry to provide a selective, reversible method for the removal of target species from wastewater. In this technique, an electroactive ion exchange layer is deposited on a conducting substrate, and ion uptake and elution are controlled directly by modulation of the potential of the layer. ESIX offers the advantages of highly-efficient use of electrical energy combined with no secondary waste generation. Recently, we have improved upon the ESIX process by modifying the conducting substrate with carbon nanotubes prior to the deposition of the electroactive ion exchanger. The nanomaterial-based electroactive ion exchange technology will remove cesium-137, chromate, and perchlorate rapidly from wastewater. The high porosity and high surface area of the electroactive ion exchange nanocomposites results in high loading capacity and minimize interferences for non-target species. Since the ion adsorption/desorption is controlled electrically without generating a secondary waste, this electrically active ion exchange process is a green process technology that will greatly reduce operating costs.
2009. "Nanocarbon-Based Nanocatalysts: Synthesis and Applications in Fuel Cells." Chapter 7 in Handbook of Electrochemical Nanotechnology, vol. I, ed. Yuehe Lin and H.S. Nalwa, pp. 145-164. American Scientific Publishers, Stevenson Ranch, CA. Abstract In this book chapter, we review the recent progress in synthesis and fuel cell applications of nanocatalysts based on carbon nanotubes, mesoporous carbon and other nanostructured carbon materials.
2009. "" Abstract This 2-volume handbook provides an overview of recent advances in the field of electrochemical nanotechnology. It will be of great interst to graduate students, scientists, and engineering professionals whose research is at the interface of electrochemistry and nanotechnology.
2009. "Supercritical Fluid Assisted Synthesis and Processing of Carbon Nanotubes ." Journal of Nanoscience and Nanotechnology 9(5):2781-2794. Abstract Carbon nanotubes (CNTs) constitute one of the most fascinating nanomaterials with specific properties and enormous applications. Taking advantages of the unique properties of supercritical fluids (SCFs), various techniques have been developed to produce and process CNTs and related nanostructured materials when conventional techniques become unviable. Herein we propose a critical review of these SCF based techniques. The most relevant characteristics of each technique and the enabled novel structures and functions which are difficult to accomplish by traditional techniques are highlighted.
2009. "Glucose Biosensor Based on Immobilization of Glucose Oxidase in Platinum Nanoparticles/Graphene/Chitosan Nanocomposite Film." Talanta 80(1):403-406. doi:10.1016/j.talanta.2009.06.054 Abstract The bionanocomposite film consisting of glucose oxidase/Pt/functional graphene sheets/chitosan (GOD/Pt/FGS/chitosan) for glucose sensing was described. With the electrocatalytic synergy of FGS and Pt nanoparticles to hydrogen peroxide, a sensitive biosensor with detection limit of 0.6 µM glucose was achieved. The biosensor also had good reproducibility, long term stability and negligible interfering signals from ascorbic acid and uric acid comparing to the response to glucose. The large surface area and good conductivity of graphene suggests that graphene is a potential candidate for sensor material. The hybrid nanocomposite glucose sensor provides new opportunity for clinical diagnosis and point-of-care applications.
2009. "EQCM Immunoassay for Phosphorylated Acetylcholinesterase as a Biomarker for Organophosphate Exposures Based on Selective Zirconia Adsorption and Enzyme-Catalytic Precipitation ." Biosensors and Bioelectronics 24(8):2377-2383. Abstract A zirconia (ZrO2) adsorption-based immunoassay by electrochemical quartz crystal microbalance (EQCM) has been initially developed, aiming at the detection of phosphorylated acetylcholinesterase (AChE) as a potential biomarker for bio-monitoring exposures to organophosphate (OP) pesticides and chemical warfare agents. Hydroxyl-derivatized monolayer was preferably chosen to modify the crystal serving as the template for directing the electro-deposition of ZrO2 film with uniform nanostructures. The resulting ZrO2 film was utilized to selectively capture phosphorylated AChE from the sample media. Horseradish peroxidase (HRP)-labeled anti-AChE antibodies were further employed to recognize the captured phosphorylated protein. Enzyme-catalytic oxidation of the benzidine substrate resulted in the accumulation of insoluble product on the functionalized crystal. Ultrasensitive EQCM quantification by mass-amplified frequency responses as well as rapid qualification by visual color changes of product could be thus achieved. Moreover, 4-chloro-1-naphthol (CN) was comparably studied as an ideal chromogenic substrate for the enzyme-catalytic precipitation. Experimental results show that the developed EQCM technique can allow for the detection of phosphorylated AChE in human plasma. Such an EQCM immunosensing format opens a new door towards the development of simple, sensitive, and field-applicable biosensor for biologically monitoring low-level OP exposures.
2009. "The durability dependence of Pt/CNT electrocatalysts on the nanostructures of carbon nanotubes: hollow- and bamboo-CNTs." Journal of Nanoscience and Nanotechnology 9(10):5811-5815. Abstract The electrochemical durability of Pt/CNT with hollow- and bamboo-structured carbon nanotubes as the support for PEM fuel cells was investigated using cyclic voltammetry (CV, 0.6-1.1V) accelerated degradation test method. Pt/CNT catalysts were characterized with cyclic voltammograms, rotating disk electrodes, and TEM images. The changes in the electrochemical surface area of Pt and the activity toward oxygen reduction reaction (ORR) before and after the degradation indicate that bamboo-structured carbon nanotubes supported Pt (Pt/B-CNT) catalyst exhibited much higher durability. TEM images indicate that the sintering of Pt nanoparticles was much less for Pt/B-CNT. These are attributed to the specific bamboo-like nanostructures which provide more “bamboo-knot” defects and edge plane-like defects. Pt-support interaction was therefore enhanced and the durability was improved.
2009. "The Corrosion of PEM Fuel Cell Catalyst Supports and Its Implications for Developing Durable Catalysts." Electrochimica Acta 54:3109-3114. Abstract Studying the corrosion behavior of catalyst support materials is of great significance for understanding the degradation of PEM fuel cell performance and developing durable catalysts. The oxidation of Vulcan carbon black (the most widely-used catalyst support for PEM fuel cells) was investigated using various electrochemical stressing methods (fixed-potential holding vs. potential step cycling), among which the potential step cycling was considered to mimic more closely the real drive cycle operation of vehicle PEM fuel cells. The oxidation of carbon was accelerated under potential step conditions as compared with the fixed-potential holding condition. Increasing potential step frequency or decreasing the lower potential limit in the potential step can further accelerate the corrosion of carbon. The accelerated corrosion of carbon black was attributed to the cycle of consumption/regeneration of some easily oxidized species. These findings are being employed to develop a test protocol for fast screening durable catalyst support.
2009. "Layer-By-Layer Assembled Hybrid Film of Carbon Nanotubes/Iron Oxide Nanocrystals for Reagentless Electrochemical Detection of H2O2." Sensors and Actuators. B, Chemical 138(1):182-188. Abstract A new approach to construct a reagentless H2O2 electrochemical sensor is described. Iron oxide magnetic nanocystals (IOMNs), as peroxidase mimetics, were employed to assemble a multilayer structure layer by layer. Polythionin was electrodeposited onto the glassy carbon electrode surface to introduce amino groups. Carboxyl functionalized multi-walled carbon nanotubes, amino functionalized IOMNs, and thionin monomers were anchored onto a polythionin-functionalized GC surface in order by carbodiimide or glutaraldehyde chemistry. The resulting multilayer construction with three layers of IOMNs and thionin mediator exhibits excellent electrochemical response to the reduction of H2O2, whereas such a modified electrode with one layer construction only yields a slight response to H2O2 of the same concentration. The tethered MWCNs enlarge the amount of immobilized IOMNs and effectively shuttle electrons between the electrode and the thionin.
2009. "Protein-Based Nanomedicine Platforms for Drug Delivery." Small 5(15):1706-1721. doi:10.1002/smll.200801602 Abstract Drug delivery systems have been developed for many years, however some limitations still hurdle the pace of going to clinical phase, for example, poor biodistribution, drug molecule cytotoxicity, tissue damage, quick clearance from the circulation system, solubility and stability of drug molecules. To overcome the limitations of drug delivery, biomaterials have to be developed and applied to drug delivery to protect the drug molecules and to enhance the drug’s efficacy. Protein-based nanomedicine platforms for drug delivery are platforms comprised of naturally self-assembled protein subunits of the same protein or a combination of proteins making up a complete system. They are ideal for drug delivery platforms due to their biocompatibility and biodegradability coupled with low toxicity. A variety of proteins have been used and characterized for drug delivery systems including the ferritin/apoferritin protein cage, plant derived viral capsids, the small Heat shock protein (sHsp) cage, albumin, soy and whey protein, collagen, and gelatin. There are many different types and shapes that have been prepared to deliver drug molecules using protein-based platforms including the various protein cages, microspheres, nanoparticles, hydrogels, films, minirods and minipellets. There are over 30 therapeutic compounds that have been investigated with protein-based drug delivery platforms for the potential treatment of various cancers, infectious diseases, chronic diseases, autoimmune diseases. In protein-based drug delivery platforms, protein cage is the most newly developed biomaterials for drug delivery and therapeutic applications. Their uniform sizes, multifunctions, and biodegradability push them to the frontier for drug delivery. In this review, the recent strategic development of drug delivery has been discussed with a special emphasis upon the polymer based, especially protein-based nanomedicine platforms for drug delivery. The advantages and disadvantages are also discussed for each type of protein based drug delivery system.
2009. "Nanomaterials for Sensing and Electrocatalysis." Journal of Nanoscience and Nanotechnology 9(4):2173-2174. Abstract This special issue provides an overview of recent advances in nanomaterials for sensing and electrocatalysis. The emergence of nanoscience and nanotechnology has led to great advances in electrochemical science and technology, and these advances may lead to a new branch of electrochemistry research-electrochemical nanotechnology-that combines electrochemical techniques with nanotechnologies to address important issues in energy, electronics, environment, and heath care.
2009. "Electrochemically synthesized ordered TiO2 and platinum nanocomposite electrode: preparation, characterization, and application to photoelectrocatalytic methanol oxidation." Journal of Nanoscience and Nanotechnology 9(4):2297-2302. Abstract In this work, the nanocomposite electrodes consisting of Pt and TiO2 nanotubular arrays have been synthesized, and the morphologies, structural, and photo-electrochemical properties of the electrodes are characterized by SEM, XRD, and electrochemical methods. Highly ordered TiO2 nanotubular arrays can be obtained through anodization of titanium. The platinum nanoparticles are electrodeposited into TiO2 nanotubes by a chronopotentiometry method. Cyclic voltammetry and XRD measurements can confirm the presence of platinum in this nanocomposite electrode. The nanostructural electrode greatly improved performances for methanol oxidation under UV-Vis illumination compared to that without illumination. An enhancement of 58% in the current density has been observed upon illumination with UV-Vis light irradiance at an intensity of 50 mW/cm2. The improved performance of the TiO2/Pt nanocomposite electrode results from a enhanced methanol oxidation by photo-generated holes in the TiO2 nanoarrays under illumination and a synergistic effectiveness between TiO2 and Pt nanoparticles.
2009. " Pt/Carbon Nanofiber Nanocomposites as Electrocatalysts for Direct Methanol Fuel Cells: Prominent Effects of Carbon Nanofiber Nanostructures ." Journal of Nanoscience and Nanotechnology 9(4):2316-2323. Abstract Carbon nanofibers (CNFs) with different microstructures, including platelet-CNFs (PCNFs), fish-bone-CNFs, and tube-CNFs, were synthesized, characterized and evaluated toward methanol oxidation reaction (MOR). The CNFs studied here showed several structures in which various stacked morphologies as well as the ordering of their size and graphite layers can be well controlled. Platinum nanoparticles have been electrodeposited on CNFs surfaces, and their electrocatalytic activities toward MOR have been studied by using cyclic voltammetry, chronoamperometry, and linear sweep voltammograms. Morphologies, textural properties, and the crystalline structure of the CNFs supports and catalysts have been characterized with transmission electron microscopy and scanning electron microscopy. The comparative tests conclude that Pt/PCNFs have the best electrocatalytic performance and good stability at room temperature. The high electrocatalytic activity and stability can be attributed to the specific microstructure of PCNFs, which have large numbers of edge-active carbon atoms on the surface of the CNFs as well as synergistic effects between CNFs and the platinum nanoparticles. The results suggest that PCNFs are excellent potential candidates as catalyst supports in direct methanol fuel cells.
2009. "Enhanced Activity and Stability of Pt catalysts on Functionalized Graphene Sheets for Electrocatalytic Oxygen Reduction ." Electrochemistry Communications 11(5):954-957. Abstract Electrocatalysis of oxygen reduction using Pt nanoparticles supported on functionalized graphene sheets (FGSs) was studied. FGSs were prepared by thermal expansion of graphite oxide. Pt nanoparticles with average diameter of 2 nm were uniformly loaded on FGSs by impregnation methods. Pt-FGS showed a higher electrochemical surface area and oxygen reduction activity with improved stability as compared with commercial catalyst. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical characterization suggest that the improved performance of Pt-FGS can be attributed to smaller particle size and less aggregation of Pt nanoparticles on the functionalized graphene sheets.
2009. "Direct electrochemistry and electrocatalysis of horseradish peroxidase immobilized in hybrid organic-inorganic film of chitosan/sol-gel/carbon nanotubes." Talanta 78(1):120-125. Abstract A hybrid organic-inorganic nanocomposite film of chitosan/sol-gel/multi-walled carbon nanotubes was constructed for the immobilization of horseradish peroxidase (HRP). This film was characterized by scanning electron microscopy. Direct electron transfer (DET) and bioelectrocatalysis of HRP incorporated into the composite film were investigated. The results indicate that the film can provide a favorable microenvironment for HRP to perform DET on the surface of glassy carbon electrodes with a pair of quasi-reversible redox waves and to retain its bioelectrocatalytic activity toward hydrogen peroxide.
2009. "Nanotechnology-Based Electrochemical Sensors for Biomonitoring Chemical Exposures ." Journal of Exposure Science and Environmental Epidemiology 19:1-18. doi:10.1038/jes.2008.71 Abstract This manuscript highlights research focused on the development of field-deployable analytical instruments based on EC detection. Background information and a general overview of EC detection methods and integrated use of nanomaterials in the development of these sensors are provided. New developments in EC sensors using various types of screen-printed electrodes, integrated nanomaterials, and immunoassays are discussed. Recent applications of EC sensors for assessing exposure to pesticides or detecting biomarkers of disease are highlighted to demonstrate the ability to monitor chemical metabolites, enzyme activity, or protein biomarkers of disease. In addition, future considerations and opportunities for advancing the use of EC platforms for dosimetric studies are covered.
2008. "Layer-by-Layer Assembly of Enzymes on Carbon Nanotubes." Chapter 6 in Biomolecular Catalysis: Nanoscale Science and Technology, vol. ACS Symposium Series 986, ed. J Kim, SH Kim, P Wang, pp. 117-128. American Chemical Society, Washington DC. Abstract The use of Layer-by-layer techniques for immobilizing several types of enzymes, e.g. glucose oxidase (GOx), horse radish oxidases(HRP), and choline oxidase(CHO) on carbon nanotubes and their applications for biosenseing are presented. The enzyme is immobilized on the negatively charged CNT surface by alternatively assembling a cationic polydiallyldimethyl-ammonium chloride (PDDA) layer and a enzyme layer. The sandwich-like layer structure (PDDA/enzyme/PDDA/CNT) formed by electrostatic assembling provides a favorable microenvironment to keep the bioactivity of enzyme and to prevent enzyme molecule leakage. The morphologies and electrocatalytic acitivity of the resulted enzyme film were characterized using TEM and electrochemical techniques, respectively. It was found that these enzyme-based biosensors are very sensitive, selective for detection of biomolecules, e.g. glucose, choline.
2008. "Detection of Cd, Pb, and Cu in non-pretreated natural waters and urine with thiol functionalized mesoporous silica and Nafion composite electrodes." Analytica Chimica Acta (620):55-63. doi:10.1016/j.aca.2008.05.029 Abstract Electrochemical sensors have great potential for environmental monitoring of toxic metal ions in waters due to their portability, field-deployability and excellent detection limits. However, electrochemical sensors employing mercury-free approaches are normally suffered from metal binding competition and fouling by organic substances and surfactants in natural waters, thus tedious sample pretreatments such as wet ashing are needed. In this work, we have developed mercury-free sensors by coating a composite of thiol self-assembled monolayers on mesoporous supports (SH-SAMMS) and Nafion on glassy carbon electrodes. With a combined benefit of SH-SAMMS as outstanding metal preconcentrator and Nafion as antifouling binder, the sensors could detect 2.5 ppb of Cd and 0.5 ppb of Pb in river water, Hanford groundwater, and seawater after 3 and 6 minutes of preconcentration and without sample pretreatment. They could also detect 2.5 ppb of Cd, Pb, and Cu simultaneously after 5 minutes of preconcentration. The electrodes have long life time and excellent single and inter-electrode reproducibility (%RSD of 5 after 8 consecutive measurements). Unlike SAMMS-carbon paste electrodes, the SAMMS-Nafion electrodes were not fouled in samples containing albumin. Successful detection of Cd in human urine was also demonstrated. Other factors including pH effect, diffusion resistance, and Tl interference on the metal detection at SAMMS-Nafion electrodes were studied. With the ability to reliably detect low metal concentration ranges without sample pretreatment and fouling, the SAMMS-Nafion composite sensors have the potential to become the next generation metal analyzers for environmental and bio- monitoring of toxic metals.
2008. "Synthesis of Lutetium Phosphate/Apoferritin Core-Shell Nanoparticles for Potential Applications in Radioimmunoimaging and Radioimmunotherapy of Cancers." Journal of Materials Chemistry 18(15):1779-1783. Abstract We report a novel approach for synthesizing LuPO4/apoferritin core-shell nanoparticles based on an apoferritin template, conjugated to the protein biotin. To prepare the nanoparticle conjugates, we used non-radioactive lutetium as a model target or surrogate for radiolutetium (177Lu). The central cavity, multi-channel structure, and chemical properties of apoferritin are well-suited for sequentially diffusing lutetium and phosphate ions into the cavity--resulting in a stable core-shell composite. We characterized the synthesized LuPO4/apoferritin nanoparticle using transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). We tested the pre-targeting capability of biotin-modified lutetium/apoferritin nanoparticle using streptavidin-modified magnetic beads and streptavidin-modified fluorescein isothiocyanate (FITC) tracer. This paper presents a simple, fast, and efficient method for synthesizing LuPO4/apoferritin nanoparticle conjugates with biotin for potential applications in radioimmunotherapy and radioimmunoimaging of cancer.
2008. "Dye-Doped Silica Nanoparticle Labels/Protein Microarray for Detection of Protein Biomarkers." The Analyst 133:1550 - 1555. doi:10.1039/b719810h Abstract Biomarkers serve as indicators of biological and pathological processes, or physiological and pharmacological responses to a drug treatment. Interleukin-6 (IL-6), a biomarker with its important biological and pathological functions, has been studied for decades. Conventional fluorescence immunoassay has been widely used for analysis of biomakers like IL-6. However, single fluorophore labeling shows its limitations of low intensity and poor stability. We report a dye-encapsulated silica nanoparticle as a label, with the advantages of high fluorescence intensity, photostability, and biocompatibility, in conjunction with microarray technology for sensitive immunoassay of IL-6 on a microarray format. The tris (2,2’-bipyridyl)ruthenium (II)chloride hexahydrate (Rubpy) dye incorporated into silica nanoparticles using a simple one-step microemulsion synthesis step. The nanoparticles are uniform in size with a diameter of 50 nm. The microarray fluorescent immunoassay approach based on dye-doped silica nanoparticle labels has high sensitivity for practical applications with a limit of detection for IL-6 down to 0.1 ng mL-1. The calibration curve is linear over the range from 0.1 ng mL-1 to 10 ng mL-1. Furthermore, results illustrated that the assay is highly specific for IL-6 in the presence of range of cytokines or proteins. The RuDS dye-labeled nanoparticles in connection with protein microarrays show the promise for clinical diagnosis of biomarkers.
2008. "Apoferritin-Templated Yttrium Phosphate Nanoparticle Conjugates for Radioimmunotherapy of Cancers." Journal of Nanoscience and Nanotechnology 8(5):2316-2322. doi:10.1166/jnn.2008.177 Abstract We report a templated-synthetic approach based on apoferritin to prepare radionuclide nanoparticle (NP) conjugates. Non-radioactive yttrium (89Y) was used as model target and surrogate for radioyttrium (90Y) to prepare the nanoparticle conjugate. The center cavity and multiple channel structure of apoferritin offer a fast and facile method to precipitate yttrium phosphate by diffusing yttrium and phosphate ions into the cavity of apofrritin, resulting a core-shell nanocomposite. The yttrium phosphate/apoferritin nanoparticle was functionalized with biotin for further application. The synthesized nanoparticle was characterized by transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS). We found that the resulting nanoparticles were uniform in size, with a diameter of around 8 nm. We tested the pre-targeting capability of the biotin-modified yttrium phosphate/apoferritin nanoparticle (yttrium phosphate/apoferritin nanoparticle) conjugate with streptavidin-modified magnetic beads and with aid of biotin-modified fluorecein isothiocyanate (FITC) tracer. This work shows that an yttrium phosphate NP conjugate provides a fast, simple and efficient method to prepare radioactive yttrium conjugate for applications in radioimmunotherapy of cancer.
2008. "Sensitive electrochemical immunoassay for 2,4,6-trinitrotoluene based on functionalized silica nanoparticle labels." Analytica Chimica Acta 610(1):112-118. doi:10.1016/j.aca.2008.01.024 Abstract We present a poly(guanine)-functionalized silica nanoparticle (NP) label-based electrochemical immunoassay for sensitively detecting 2,4,6-trinitrotoluene (TNT). This immunoassay takes advantage of magnetic bead–based platform for competitive displacement immunoreactions and separation, and use electroactive nanoparticles as labels for signal amplification. For this assay, anti-TNT-coated magnetic beads interacted with TNT analog-conjugated poly(guanine)-silica NPs and formed analog-anti-TNT immunocomplexes on magnetic beads. The immunocomplexes coated magnetic beads were exposed to TNT samples, which resulted in displacing the analog conjugated poly(guanine) silica NPs into solution by TNT. In contrast, there are no guanine residues releasing into the solution in the absence of TNT. The reaction solution was then separated from the magnetic beads and transferred to the electrode surface for electrochemical measurements of guanine oxidation with Ru(bpy)32+ as mediator. The sensitivity of this TNT assay was greatly enhanced through dual signal amplifications: 1) a large amount of guanine residues on silica nanoparticles is introduced into the test solution by displacement immunoreactions and 2) a Ru(bpy)32+-induced guanine catalytic oxidation further enhances the electrochemical signal. Some experimental parameters for the nanoparticle label-based electrochemical immunoassay were studied and the performance of this assay was evaluated. The method is found to be very sensitive and the detection limit of this assay is ~ 0.1 ng mL-1 TNT. The electrochemical immunoassay based on the poly[guanine]-functionalized silica NP label offers a new approach for sensitive detection of explosives.
2008. "Quantum-Dot-Based Electrochemical Immunoassay for High-Throughput Screening of the Prostate-Specific Antigen." Small 4(1):82-86. doi:10.1002/smll.200700459 Abstract In this paper, we demonstrate an electrochemical high-throughput sensing platform for simple, sensitive detection of PSA based on QD labels. This sensing platform uses a microplate for immunoreactions and disposable screen-printed electrodes (SPE) for electrochemical stripping analysis of metal ions released from QD labels. With the 96-well microplate, capturing antibodies are conveniently immobilized to the well surface, and the process of immunoreaction is easily controlled. The formed sandwich complexes on the well surface are also easily isolated from reaction solutions. In particular, a microplate-based electrochemical assay can make it feasible to conduct a parallel analysis of several samples or multiple protein markers. This assay offers a number of advantages including (1) simplicity, cost-effectiveness, (2) high sensitivity, (3) capability to sense multiple samples or targets in parallel, and (4) a potentially portable device with an SPE array implanted in the microplate. This PSA assay is sensitive because it uses two amplification processes: (1) QDs as a label for enhancing electrical signal since secondary antibodies are linked to QDs that contain a large number of metal atoms and (2) there is inherent signal amplification for electrochemical stripping analysis—preconcentration of metal ion onto the electrode surface for amplifying electrical signals. Therefore, the high sensitivity of this method, stemming from dual signal amplification via QD labels and pre-concentration, allows low concentration levels to be detected while using small sample volumes. Thus, this QD-based electrochemical detection approach offers a simple, rapid, cost-effective, and high throughput assay of PSA.
2008. "Magnetic Electrochemical Immunoassays with Quantum Dot Labels for Detection of Phosphorylated Acetylcholinesterase in Plasma." Analytical Chemistry 80(22):8477-8484. doi:10.1021/ac801211s Abstract A new magnetic electrochemical immunoassay has been developed as a tool for biomonitoring exposures to organophosphate (OP) compounds, e.g., insecticides and chemical nerve agents, by directly detecting organophosphorylated acetylcholinesterase (OP−AChE). This immunoassay uniquely incorporates highly efficient magnetic separation with ultrasensitive square wave voltammetry (SWV) analysis with quantum dots (QDs) as labels. A pair of antibodies was used to achieve the specific recognition of OP−AChE that was prepared with paraoxon as an OP model agent. Antiphosphoserine polyclonal antibodies were anchored on amorphous magnetic particles preferably chosen to capture OP−AChE from the sample matrixes by binding their phosphoserine moieties that were exposed through unfolding the protein adducts. This was validated by electrochemical examinations and enzyme-linked immunosorbent assays. Furthermore, antihuman AChE monoclonal antibodies were labeled with cadmium-source QDs to selectively recognize the captured OP−AChE, as characterized by transmission electron microscopy. The subsequent electrochemical SWV analysis of the cadmium component released by acid from the coupled QDs was conducted on disposable screen-printed electrodes. Experimental results indicated that the SWV-based immunoassays could yield a linear response over a broad concentration range of 0.3−300 ng/mL OP−AChE in human plasma with a detection limit of 0.15 ng/mL. Such a novel electrochemical immunoassay holds great promise as a simple, selective, sensitive, and field-deployable tool for the effective biomonitoring and diagnosis of potential exposures to nerve agents and pesticides.
2008. "Functionalized carbon nanotubes and nanofibers for biosensing applications." Trends in Analytical Chemistry. TrAC 27(7):619-626. doi:10.1016/j.trac.2008.05.009 Abstract This review summarizes the recent advances of carbon nanotube (CNT) and carbon nanofiber (CNF)-based electrochemical biosensors with an emphasis on the applications of CNTs. Carbon nanotubes and carbon nanofibers have unique electric, electrocatalytic, and mechanical properties which make them efficient materials for the use in electrochemical biosensor development. In this article, the functionalization of CNTs for biosensors is simply discussed. The electrochemical biosensors based on CNT and their various applications, e.g., measurement of small biological molecules and environmental pollutants, detection of DNA, and immunosensing of disease biomarkers, are reviewed. Moreover, the development of carbon nanofiber-based electrochemical biosensors and their applications are outlined. Finally, some challenges are discussed in the conclusion.
2008. "Electrochemical Immunoassay of Carcinoembryonic Antigen Based on A Lead Sulfide Nanoparticle Label." Nanotechnology 19(43):Art. No. 435501. doi:10.1088/0957-4484/19/43/435501 Abstract We describe a Lead sulfide nanoparticle (PbS NP) based electrochemical immunoassay to detect a tumor biomarker, carcinoembryonic antigen (CEA). Cubic PbS NPs were prepared and functionalized with thioglycolic acid (TGA), which stabilized the formed NPs and offered carboxyl groups to conjugate with CEA antibodies. PbS NP conjugated with monoclonal CEA antibody was used as a label in an immnorecognition event. After a complete sandwich immunoreaction among the primary CEA antibody (immobilized on the carboxyl-modified magnetic beads), CEA, and the PbS-labeled secondary antibody (PbS-anti-CEA), PbS labels were captured to the magnetic-bead (MB) surface through the antibody-antigen immunocomplex. Electrochemical stripping analysis of the captured PbS was used to quantify the concentration of CEA after an acid-dissolution step. The MBs and the magnetic separation platform were used to integrate a facile antibody immobilization with immunoreactions and the isolation of immunocomplexes from reaction solutions in the immunoassay. The performance of this nanoparticle based electrochemical immunoassay was successfully evaluated with human serum spiked with CEA, indicating that this convenient and sensitive technique offers great promise for rapid, simple, and cost-effective analysis of tumor biomarkers in biological fluids.
2008. "Carbon Nanotube-Based Electrochemical Sensor for Assay of Salivary Cholinesterase Enzyme Activity: An Exposure Biomarker of Organophosphate Pesticides and Nerve Agents." Environmental Science & Technology 42(7):2688-2693. doi:10.1021/es702335y Abstract Certain saliva enzymes may be useful biomarkers for detecting exposures to organophosphate pesticides and chemical nerve agents. In this regard, saliva biomonitoring offers a simple and noninvasive approach for rapidly evaluating those exposures in real time. An electrochemical sensor coupled with a micro-flow injection system was developed for a simple, rapid, and sensitive characterization of cholinesterase (ChE) enzyme activities in rat saliva. The electrochemical sensor is based on a carbon nanotube (CNT)-modified screen-printed carbon electrode (SPE), which is integrated into a flow cell. Because of the excellent electrocatalytic activity of the CNTs, the sensor can detect electroactive species that are produced from enzymatic reactions with extremely high sensitivity and at low potentials. The electrochemical properties of acetylcholinesterase (AChE) enzymatic products were studied using a CNT-modified SPE, and the operation parameters such as the applied potential and substrate concentration were optimized to achieve the best performance. The AChE enzyme activity was further investigated using the CNT-based electrochemical sensor with commercially available purified AChE and saliva obtained from naïve rats. It was found that the calibration curve is linear over a wide range of AChE concentrations from 5 pM to 0.5 nM, and the sensor is very sensitive with the detection limit down to 2 pM. The dynamics of the enzyme activity in saliva with organophosphate pesticides was further studied using this sensor. The results show that the senor can be used to characterize salivary enzyme activity and to detect the exposure to organophosphate compounds. This new CNT-based electrochemical sensor thus provides a sensitive and quantitative tool for noninvasive biomonitoring of the exposure to organophosphate pesticides and nerve agents.
2008. "The influence of the electrochemical stressing (potential step and potential-static holding) on the degradation of polymer electrolyte membrane fuel cell electrocatalysts." Journal of Power Sources 185(1):280-286. doi:10.1016/j.jpowsour.2008.07.008 Abstract The understanding of the degradation mechanisms of electrocatalysts is very important for developing durable electrocatalysts for polymer electrolyte membrane (PEM) fuel cell. The degradation of Pt/C electrocatalysts under potential static holding conditions (at 1.2V and 1.4V vs. RHE) and potential step conditions with the upper potential of 1.4V for 150s and changing to lower potential limits (0.85V and 0.60V) for 30s in each period [denoted as Pstep(1.4V_150s-0.85V_30s) and Pstep(1.4V_150s-0.60V_30s), respectively] were investigated. The electrocatalysts and support were characterized with electrochemical voltammetry, transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Pt/C degrades much faster under Pstep conditions than that under potential static holding conditions. Pt/C degrades under the Pstep(1.4V_150s-0.85V_30s) condition mainly through the coalescence process of Pt nanopaticles due to the corrosion of carbon support, which is similar to that under the conditions of 1.2V and 1.4V potential static holding; however, Pt/C degrades mainly through the dissolution/loss and dissolution/redepostion process if stressed under Pstep(1.4V_150s-0.60V_30s). The difference in the degradation mechanisms is attributed to the chemical states of Pt nanoparticles: Pt dissolution can be alleviated by the protective oxide layer under the Pstep(1.4V_150s-0.85V_30s) condition and the potential static holding conditions. These findings are very important for understanding PEM fuel cell material degradation and are also useful for developing fast test method for screening durable catalyst support materials.
2008. "Novel Catalyst Support Materials for PEM Fuel Cells: Current Status and Future Prospects." Journal of Materials Chemistry 19(1):46-59. doi:10.1039/b808370c Abstract The catalyst supports exhibit great influence on the cost, performance, and durability of polymer electrolyte membrane (PEM) fuel cells. This review paper is to summarize several important kinds of novel support materials for PEM fuel cells (including direct methanol fuel cell, DMFC): nanostructured carbon materials (carbon nanotubes/carbon nanofibers, mesoporous carbon), conductive doped diamonds and nanodiamonds, conductive oxides (tin oxide/indium tin oxide, titanium oxide, tungsten oxide) and carbides (tungsten carbides). The advantages and disadvantages, the acting mechanism to promote electrocatalysis, and the strategies to improve present catalyst support materials and to search for new ones are discussed. This is expected to throw light on future development of catalyst support for PEM fuel cells.
2008. "Ultrasensitive Electrochemical Detection of mRNA Using Branched DNA Amplifiers." Electrochemistry Communications 10(12):1847-1850. doi:10.1016/j.elecom.2008.09.028 Abstract We describe here an ultrasensitive electrochemical detection of m RNA protocol without RNA purification and PCR amplification. The new m RNA electrical detection capability is coupled to the amplification feature of branched DNA (bDNA) technology and with the nagnetic beads based electrochemical bioassay.
2008. " Multiplex Electrochemical Immunoassay Using Gold Nanoparticle Probes and Immunochromatographic Strips ." Electrochemistry Communications 10(10):1636-1640. doi:10.1016/j.elecom.2008.08.032 Abstract We describe a multiplex electrochemical immunoassay based on the use of gold nanoparticle (Au-NP) probes and immunochromatographic strips (ISs). The approach takes advantage of the speed and low cost of the conventional IS tests and the high sensitivities of the nanoparticle-based electrochemical immunoassays. Rabbit IgG(R-IgG) and human IgM (H-IgM) were used as model targets for the demonstration of the proof of concept. The Au-NPs based sandwich immunoreactions were performed on the IS, and the captured gold nanoparticle labels on the test zones were determined by highly-sensitive stripping voltammetric measurement of the dissolved gold ions (III) with a carbon paste electrode. The detection limits are 1.0 and 1.5 ng/mL with the linear ranges of 2.5-250 ng/mL for quantitative detection of R-IgG and H-IgM, respectively. The total assay time is around 25 minutes. Such multiplex electrochemical immunoassay could be readily highly multiplexed to allow simultaneous parallel detection of numerous proteins and is expected to open new opportunities for protein diagnostics and biosecurity.
2008. "Nanoparticle-Based Electrochemical Immunosensor for the Detection of Phosphorylated Acetylcholinesterase: An Exposure Biomarker of Organophosphate Pesticides and Nerve AgentsOrganophosphate Pesticides and Nerve Agents ." Chemistry - a European Journal 14(32):9951-9959. doi:10.1002/chem.200800412 Abstract A nanoparticle-based electrochemical immunosensor has been developed for the detection of phosphorylated acetylcholinesterase (AChE) adducts, which is a potential exposure biomarker for organophosphate pesticides (OP) and chemical warfare nerve agent exposures. Zirconia nanoparticles (ZrO2 NPs) were used as selective sorbents to capture the phosphorylated AChE adduct, and quantum dots (ZnS@CdS, QDs) were used as tags to label monoclonal anti-AChE antibody to track the immunorecognition events. The sandwich-like immunoreactions were performed among the ZrO2 NPs, which were pre-coated on a screen printed electrode (SPE) by electrodeposition, phosphorylated AChE and QD-anti-AChE. The captured QD tags were determined on the SPE by electrochemical stripping analysis of its metallic component (cadmium) after an acid-dissolution step. Paraoxon was used as a model OP insecticide to prepare the phosphorylated AChE adduct to demonstrate the proof of principle for this sensor technology. The paraoxon-AChE adduct was characterized by Fourier Transform Infrared Spectrum, and the binding affinity of anti-AChE to the paraoxon-AChE was validated with an enzyme-linked immunosorbent assay. The parameters (e.g., amount of ZrO2 NP, QD-anti-AChE concentration,) that govern the electrochemical response of immunosensors were optimized. The voltammetric response of the immunosensor is highly linear over the range of 10 pM to 4 nM paraoxon-AChE, and the limit of detection is estimated to be 8 pM. This new nanoparticle-based electrochemical immunosensor thus provides a sensitive and quantitative tool for biomonitoring exposure to OP pesticides and nerve agents.
2008. "Bioelectrochemical Immunoassay of Polychlorinated Biphenyl." Analytica Chimica Acta 612:23-28. doi:10.1016/j.aca.2008.01.080 Abstract A simple, rapid, and highly sensitive bioelectrochemical immunoassay method based on magnetic beads (MBs) and disposable screen-printed electrodes (SPE) has been developed to detect polychlorinated biphenyls (PCBs). The principle of this bioassay is based on a direct competitive enzyme-linked immunosorbent assay using PCB-antibody-coated MBs and horseradish peroxidase (HRP)-labeled PCB (HRP-PCB). A magnetic process platform was used to mix and shake the samples during the immunoreactions and to separate free and unbound reagents after the liquid-phase competitive immunoreactions among PCB-antibody-coated MBs, PCB analyte, and HRP-PCB. After a complete immunoassay, the HRP tracers attached to MBs were transferred to a substrate solution containing o-aminophenol and hydrogen peroxide for electrochemical detection. The different parameters, including the amount of HRP-PCB conjugates, immunoreaction time, and the concentration of substrate that governs the analytical performance of the immunoassay have been studied in detail and optimized. The detection limit of 5 pg mL-1 was obtained under optimum experimental conditions. The performance of this bioelectrochemical immunoassay was successfully evaluated with untreated river water spiked with PCBs, and the results were validated by commercial PCB enzyme-linked immunosorbent assay kit, indicating that this convenient and sensitive technique offers great promise for decentralized environmental application and trace PCBs monitoring.
2008. "A nanoparticle label/immunochromatographic electrochemical biosensor for rapid and sensitive detection of prostate-specific antigen ." Biosensors and Bioelectronics 23(11):1659-1665. Abstract We present a nanoparticle (NP) label/immunochromatographic electrochemical biosensor (IEB) for rapid and sensitive detection of prostate-specific antigen (PSA) in human serum. This IEB integrates the immunochromatographic strip with the electrochemical detector for transducing quantitative signals. The NP label, made of CdSe@ZnS, serves as a signal-amplifier vehicle. A sandwich immunoreaction was performed on the immunochromatographic strip. The captured NP labels in the test zone were determined by highly sensitive stripping voltammetric measurement of the dissolved metallic component (cadmium) with a disposable-screen-printed electrode, which is embedded underneath the membrane of the test zone. Experimental parameters (e.g., immunoreaction time, the amount of anti-PSA-NP conjugations applied) and electrochemical detection conditions (e.g., preconcentration potential and time) were optimized using this biosensor for PSA detection. The analytical performance of this biosensor was evaluated with serum PSA samples according to the “figure-of-merits” (e.g., dynamic range, reproducibility, and detection limit). The results were validated with enzyme-linked immunosorbent assay (ELISA) and show high consistency. It is found that this biosensor is very sensitive with the detection limit of 0.02 ng/mL PSA and is quite reproducible. This method is rapid, clinically accurate, and less expensive than other diagnosis tools for PSA; therefore, this IEB coupled with a portable electrochemical analyzer shows great promise for simple, sensitive, quantitative point-of-care testing of disease-related protein biomarkers.
2008. "Sensitive electrochemical detection of horseradish peroxidase at disposable screen-printed carbon electrode." Electroanalysis 20(18):2040-2046. doi:DOI: 10.1002/elan.200804287 Abstract A rapid, simple and sensitive electrochemical assay of horseradish peroxidase (HRP) performed on disposable screen-printed carbon electrode was developed. HRP activities were monitored by square-wave voltammetric (SWV) measuring the electroactive enzymatic product in the presence of o-aminophenol and hydrogen peroxide substrate solution. SWV analysis demonstrated a greater sensitivity and shorter analysis time than the widely used amperometric and differential-pulsed voltammetric methods. The voltammetric characteristics of substrate and enzymatic product as well as the parameters of SWV analysis were optimized. Under optimized conditions, a linear response for HRP from 0.003 - 0.1 U/mL and a detection limit of 0.002 U/mL (1.25×10-15 mol in 25 µL) were obtained with a good precision (RSD = 8%; n = 6). This rapid and sensitive HRP assay with microliters-assay volume could be readily integrated to portable devices and point-of-care (POC) diagnosis applications.
2008. "Electrochemical Branched-DNA Assay for Polymerase Chain Reaction-Free Detection and Quantification of Oncogenes in Messenger RNA." Analytical Chemistry 80(24):9402-9410. doi:10.1021/ac801263r Abstract We describe a novel electrochemical branched-DNA (bDNA) assay for polymerase chain reaction (PCR)-free detection and quantification of p185 BCR-ABL leukemia fusion transcript in the population of messenger RNA (mRNA) extracted from cell lines. The bDNA amplifier carrying high loading of alkaline phosphatase (ALP) tracers was used to amplify targets signal. The targets were captured on microplate well surfaces through cooperative sandwich hybridization prior to the labeling of bDNA. The activity of captured ALP was monitored by square-wave voltammetric (SWV) analysis of the electroactive enzymatic product in the presence of 1-napthyl-phosphate. The specificity and sensitivity of assay enabled direct detection of target transcript in as little as 4.6 ng mRNA without PCR amplification. In combination with the use of a well-quantified standard, the electrochemical bDNA assay was capable of direct use for a PCR-free quantitative analysis of target transcript in total mRNA population. The approach thus provides a simple, sensitive, accurate and quantitative tool alternate to the RQ-PCR for early disease diagnosis.
2008. "Effect of Nickel Microstructure on Methane Steam-Reforming Activity of Ni-YSZ Cermet Anode Catalyst ." Journal of Catalysis 258(2):356-365. doi:10.1016/j.jcat.2008.06.031 Abstract The activity of nickel-yttria stabilized zirconia (Ni-YSZ) solid oxide fuel cell (SOFC) cermet anodes for the steam reforming of methane has been investigated in the absence of electrochemical effects. The cermet was prepared by co-milling and sintering NiO and 5YSZ powders at 1375oC in air. During the high temperature sintering step, NiO dissolved into the YSZ particles to form a solid NiO-YSZ solution. During the subsequent catalyst reduction step, Ni exolved from the YSZ. As a result, many small Ni particles on the order of 10-20 nm formed at the surface of the YSZ. These small particles contribute significantly to the overall reforming activity, along with the large bulk Ni particles within the Ni-YSZ cermet. We have observed high initial activity that decreases by as much as an order of magnitude with time on stream, until the anode catalyst reaches a stable steady state activity. The time to reach this stable activity is a function of the reaction conditions and feed gas composition. Higher temperature, hydrogen partial pressure, and space velocity all accelerated the deactivation rate at a constant steam-to-carbon ratio of 3. Initial and lined out activities and average turnover frequencies were obtained for both Ni-YSZ and bulk Ni, based on a rate expression that is first order in methane and zero order in steam. Comparative tests at 750oC show high initial activity on a per-Ni site basis with both materials, but these turnover rates decline over a period of a few hours. Following lineout, there appears to be a negligible effect of Ni particle size on turnover rate. These results indicate the presence of structure sensitivity for methane reforming, but only with freshly calcined and reduced catalysts that may contain highly coordinatively unsaturated sites. There is an apparent structure insensitivity with aged catalysts where Ni particle sizes are generally 50 nm and greater. Under reaction conditions that employ high space velocities and low methane conversions, the water-gas-shift reaction does not establish thermodynamic equilibrium.
2007. "Electrochemical sensors based on nanomaterials for environmental monitoring ." Chapter 15 in Environmental Applications of Nanomaterials: Synthesis, Sorbents And Sensors, ed. Glen E. Fryxell and Guozhong Cao, pp. 401-438. Imperial College Press, London, United Kingdom. Abstract This article review work relevant to the two fastest growing nanomaterials in electrochemical sensing of metal ions: organically modified ordered mesoporous silicas (OMSs) and carbon nanotubes (CNTs). Nanostructured self-assembled monolayers on mesoporous silicas (SAMMS) materials are highly effective as electrode modifiers; they can be either mixed with conductive materials or spin-cast as a thin-film on electrode surface. The interfacial chemistry of SAMMS can be fine-tuned to selectively preconcentrate the specific metal ions of interest. The functional groups on SAMMS materials enable the preconcentration to be done without mercury, supporting electrolytes, applied potential, and solution degassing, all of which are often required in conventional adsorptive stripping voltammetric sensors. Since it was first introduced in 1991, CNTs have been widely investigated for electrochemical sensors of many important biomolecules because of their electrocatalytic and antifouling properties, biocompatibility, high surface, and mechanical strength. For trace metal analysis, CNT thin-film created by drop-coating of CNT-solvent suspensions on electrode surfaces has been explored in order to develop mercury-free sensors by exploiting the bulk properties of the CNTs. Array of low-site-density aligned carbon nanotubes has been grown on metal substrates by a non-lithographic method. Each CNT serves as a nanoelectrode which normally has greater mass transfer rate and higher mass sensitivity than conventional macroelectrodes. The array of millions of CNT nanoelectrodes provides magnified voltammetric signals for trace metal ions without the need for a signal amplifier.
2007. "Nanotubes, Nanowires, and Nanocantilevers in Biosensor Development." Chapter 3 in Nanomaterials for Biosensors, Nanotechnologies for Life Sciences, vol. 8, ed. Challa S.S.R. Kumar, pp. 56-100. Wiley-VCH, Weinheim, Germany. Abstract In this chapter, the reviews on biosensor development based on 1-D nanomaterials, CNTs, semiconducting nanowires, and some cantilevers will be introduced. The emphasis of this review will be placed on CNTs and electrochemical/electronic biosensor developments. Section 2 of this chapter gives a detailed description of carbon nanotubes-based biosensor development, from fabrication of carbon nanotubes, the strategies for construction of carbon nanotube based bisosensros to their bioappplications. In the section of the applications of CNTs based biosensors, various detection principles, e. g. electrochemical, electronic, and optical method, and their applications are reviewed in detail. Section 3 introduces the method for synthesis of semiconducting nanowires, e.g. silicon naowires, conducting polymer nanowires and metal oxide nanowires and their applications in DNA and proteins senseing. Section 4 simply describes the development for nanocantilevers based biosensors and their application in DNA and protein dignosis. Each section starts from a brief introduction and then goes into details. Finally in the Conclusion section, the development of 1-D nanomaterials based biosensor development is summarized.
2007. "Microanalyzer for Biomonitoring of Lead (Pb) in Blood and Urine ." Analytical and Bioanalytical Chemistry 387(1):335-341. doi:10.1007/s00216-006-0940-1 Abstract Biomonitoring of lead (Pb) in blood and urine enables quantitative evaluation of human occupational and environmental exposures to Pb. The state-of-the-art ICP-MS instruments analyze metals in laboratories, resulting in lengthy turn around time, and are expensive. In response to the growing need for metal analyzer for on-site, real-time monitoring of trace metals in individuals, we developed a portable microanalyzer based on flow-injection / adsorptive stripping voltammetry and used it to analyze Pb in rat blood and urine. Fouling of electrodes by proteins often prevents the effective use of electrochemical sensors in biological matrices. Minimization of such fouling was accomplished with the suitable sample pretreatment and the turbulent flowing of Pb contained blood and urine onto the glassy electrode inside the microanalyzer, which resulted in no apparent electrode fouling even when the samples contained 50% urine or 10% blood by volume. There was no matrix effect on the voltammetric Pb signals even when the samples contained 10% blood or 10% urine. The microanalyzer offered linear concentration range relevant to Pb exposure levels in human (0-20 ppb in 10%-blood samples, 0-50 ppb in 50%-urine samples). The device had excellent sensitivity and reproducibility; Pb detection limits were 0.54 ppb and 0.42 ppb, and % RSDs were 4.9 and 2.4 in 50%-urine and 10%-blood samples, respectively. It offered a high throughput (3 min per sample) and had economical use of samples (60 µL per measurement), making the collection of blood being less invasive especially to children, and had low reagent consumption (1 µg of Hg per measurement), thus minimizing the health concerns of mercury use. Being miniaturized in size, the microanalyzer is portable and field-deployable. Thus, it has a great potential to be the next-generation analyzer for biomonitoring of toxic metals.
2007. "Electrochemical Sensors for the Detection of Lead and Other Toxic Heavy Metals: The Next Generation of Personal Exposure Biomonitors." Environmental Health Perspectives 115(12):1683-1690. Abstract To support the development and implementation of biological monitoring programs, accurate and quantitative technologies for measuring xenobiotic exposure are needed. Micro-analytical based sensors that work with complex biomatrices such as blood, urine or saliva are being developed and validated. These sensor platforms will improve our ability to make definitive associations between chemical exposures and disease. Among toxic heavy metals, lead (Pb) continues to be one of the most problematic. Despite a considerable effort to identify and eliminate Pb exposure sources, this metal still remains a significant health concern, particularly for young children. Ongoing research is focused on the development and validation of portable metal analyzers that have many advantages over current available technologies, thus having the potential to become the next-generation of toxic metal analyzers. This review will highlight the development and validation of two classes of metal analyzers for the voltammetric detection of Pb, this includes: (1) a metal analyzer based on flow injection analysis and anodic stripping voltammetry (ASV) at a Hg-film electrode, and (2) mercury-free metal analyzers employing adsorptive stripping voltammetry (AdSV) and novel nanostructure materials which include the self-assembled monolayers on mesoporous supports (SAMMS) and carbon nanotubes (CNTs). These sensors have been optimized to detect Pb in urine, blood, and saliva as accurately as the state-of-the-art-ICP-MS with high reproducibility, and sensitivity, while being much more portable, field-deployable and less expensive than conventional analytical methods. It is anticipated that these improved and portable analytical sensor platforms will facilitate our ability to conduct a meaningful biological monitoring program that will enable us to have a greater understanding of the relationship between chemical exposure assessment and disease outcomes. Keywords: biomonitoring, lead (Pb), sensors, dosimetry technology, exposure assessment
2007. "Quantum-Dots Based Electrochemical Immunoassay of Interleukin-1α." Electrochemistry Communications 9(7):1573-1577. doi:10.1016/j.elecom.2007.02.024 Abstract We describe a quantum-dot (QD, CdSe@ZnS)-based electrochemical immunoassay to detect a protein biomarker, interleukin-1α (IL-1α). QD conjugated with anti-IL-1α antibody was used as a label in an immunorecognition event. After a complete sandwich immunoreaction among the primary IL-1α antibody (immobilized on the avidin-modified magnetic beads), IL-1α, and the QD-labeled secondary antibody, QD labels were attached to the magnetic-bead surface through the antibody-antigen immunocomplex. Electrochemical stripping analysis of the captured QDs was used to quantify the concentration of IL-1α after an acid-dissolution step. The streptavidin-modified magnetic beads and the magnetic separation platform were used to integrate a facile antibody immobilization (through a biotin/streptavidin interaction) with immunoreactions and the isolation of immunocomplexes from reaction solutions in the assay. The voltammetric response is highly linear over the range of 0.5 to 50 ng mL-1 IL 1α, and the limit of detection is estimated to be 0.3 ng mL-1 (18 pM). This QD-based electrochemical immunoassay shows great promise for rapid, simple, and cost-effective analysis of protein biomarkers.
2007. "Development of a Non-Invasive Biomonitoring Approach to Determine Exposure to the Organophosphorus Insecticide Chlorpyrifos in Rat Saliva." Toxicology and Applied Pharmacology 219(2-3):217-225. doi:10.1016/j.taap.2006.10.002 Abstract Abstract Non-invasive biomonitoring approaches are being developed using reliable portable analytical systems to quantify dosimetry utilizing readily obtainable body fluids, such as saliva. In the current study, rats were given single oral gavage doses (1, 10 or 50 mg/kg) of the insecticide chlorpyrifos (CPF), saliva and blood were collected from groups of animals (4/time-point) at 3, 6, and 12 hr post-dosing, and the samples were analyzed for the CPF metabolite trichlorpyridinol (TCP). Trichlorpyridinol was detected in both blood and saliva at all doses and the TCP concentration in blood exceeded saliva, although the kinetics in blood and saliva were comparable. A physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model for CPF incorporated a compartment model to describe the time-course of TCP in blood and saliva. The model adequately simulated the experimental results over the dose ranges evaluated. A rapid and sensitive sequential injection (SI) electrochemical immunoassay was developed to monitor TCP, and the reported detection limit for TCP in water was 6 ng/L. Computer model simulation in the range of the Allowable Daily Intake (ADI) or Reference Dose (RfD) for CPF (0.01-0.003 mg/kg/day) suggest that the electrochemical immunoassay had adequate sensitivity to detect and quantify TCP in saliva at these low exposure levels. To validate this approach further studies are needed to more fully understand the pharmacokinetics of CPF and TCP excretion in saliva. The utilization of saliva as a biomonitoring matrix, coupled to real-time quantitation and PBPK/PD modeling represents a novel approach with broad application for evaluating both occupational and environmental exposures to insecticides.
2007. "Nanovehicles based Bioassay Labels." Electroanalysis 19(7-8):777-785. doi:10.1002/elan.200603787 Abstract In this article, we review recent advances of our group in nanoparticle labels based bioassay. Apoferritin and silica nanoparticles have been used as nanovehicles to load large amount of markers for highly sensitive bioassay. Markers loaded apoferritin, apoferritin-templated metallic phosphate nanoparticles, and poly [guanine] coated silica nanoparticles have been prepared, characterized and used as labels for highly sensitive bioassay of protein and DNA. Dissociation and reconstitution characteristics at different pH as well as the special cavity structure of apoferritin nanovehicle provides a simple and convenient route to prepare versatile nanoparticle labels and avoid the complicated and tedious synthesis process of conventional nanoparticle labels. The optical and electrochemical characteristics of the prepared nanoparticle labels are easily controlled by loading different optical or electrochemical markers. Additionally, the use of apoferritin nanovehicle as template for synthesis of metallic phosphate nanoparticle labels offers fast route to prepare uniform-size metallic nanoparticle labels for electrochemical bioassay and avoids the traditional harsh dissolution conditions to dissolve metallic nanoparticle tags (that is, the strong-acid dissolution of quantum dots and gold nanoparticles) during the stripping analysis step. Silica nanoparticle has also been used as nanovehicle to carry thousands of poly [guanine] tracers, which was used to enhance the oxidation current of Ru(bpy)32+, resulting in enhanced sensitivity of electrochemical immunoassay. The new nanovehicle-based labels have been used for highly sensitive electrochemical detection of DNA and protein biomarkers, such as tumor necrosis factor-alpha (TNF-a). The high sensitivity and selectivity make these labels a useful addition to the armory of nanoparticle-based bioassay. The new nanovehicles based labels hold great promise for multiplex protein and DNA detection and for enhancing the sensitivity of other bioassays.
2007. "Nanomaterial Labels in Electrochemical Immunosensors and Immunoassays." Talanta 74(3):308-317. doi:10.1016/j.talanta.2007.10.014 Abstract This article reviews recent advances in nanomaterial labels in electrochemical immunosensors and immunoassays. Various nanomaterial labels are discussed, including colloidal gold/silver, semiconductor nanoparticles, and markers loaded nanocarriers (carbon nanotubes, apoferritin, silica nanoparticles, and liposome beads). The enormous signal enhancement associated with the use of nanomaterial labels and with the formation of nanomaterial–antibody-antigen assemblies provides the basis for ultrasensitive electrochemical detection of disease-related protein biomarkers, biothreat agents, or infectious agents. In general, all endeavors cited here are geared to achieve one or more of the following goals: signal amplification by several orders of magnitude, lower detection limits, and detecting multiple targets.
2007. "Electrochemical Quantification of Single Nucleotide Polymorphisms Using Nanoparticle Probes." Journal of the American Chemical Society 129(34):10394-10401. doi:10.1021/ja070429r Abstract We report a new approach for electrochemical quantification of single-nucleotide polymorphisms (SNPs) using nanoparticle probes. The principle is based on DNA polymerase I (klenow fragment)-induced coupling of the nucleotide-modified nanoparticle probe to the mutant sites of duplex DNA under the Watson-Crick base pairing rule. After liquid hybridization events occurred among biotinylated DNA probes, mutant DNA, and complementary DNA, the resulting duplex DNA helixes were captured to the surface of magnetic beads through a biotin-avidin affinity reaction and magnetic separation. A cadmium phosphate-loaded apoferritin nanoparticle probe, which is modified with nucleotides and is complementary to the mutant site, is coupled to the mutant sites of the formed duplex DNA in the presence of DNA polymerase. Subsequent electrochemical stripping analysis of the cadmium component of coupled nanoparticle probes provides a means to quantify the concentration of mutant DNA. The method is sensitive enough to detect 21.5 attomol mutant DNA, which will enable the quantitative analysis of nucleic acid without polymerase chain reaction pre-amplification. The approach was challenged with constructed samples containing mutant and complementary DNA. The results indicated that it was possible to accurately determine SNPs with frequencies as low 0.01. The proposed approach has a great potential for realizing an accurate, sensitive, rapid, and low-cost method of SNP detection.
2007. "Disposable Electrochemical Immunosensor Diagnosis Device Based on Nanoparticle Probe and Immunochromatographic Strip." Analytical Chemistry 79(20):7644-7653. doi:10.1021/ac070691i Abstract We describe a disposable electrochemical immunosensor diagnosis device that is based on the immunochromatographic strip technique and an electrochemical immunoassay based on quantum dot (QD, CdS@ZnS) labels. The device takes advantage of the speed and low-cost of the conventional immunochromatographic strip test and the high-sensitivity of the nanoparticle-based electrochemical immunoassay. A sandwich immunoreaction was performed on the immunochromatographic strip, and the captured QD labels in the test zone were determined by highly sensitive stripping voltammetric measurement of the dissolved metallic component (cadmium) with a disposable-screen-printed electrode, which is embedded underneath the membrane on the test zone. The new device coupled with a portable electrochemical analyzer shows great promise for in-field and point-of-care quantitative testing of disease-related protein biomarkers. The parameters (e.g., voltammetric measurement of QD labels, antibody immobilization, the loading amount of QD-antibody, and the immunoreaction time) that govern the sensitivity and reproducibility of the device were optimized with IgG model analyte. The voltammetric response of the optimized device is highly linear over the range of 0.1 to 10 ng mL-1 IgG, and the limit of detection is estimated to be 30 pg mL-1 in association with a 7-min immunoreaction time. The detection limit was improved to 10 pg mL-1 using a 20-min immunoreaction time. The new disposable electrochemical diagnosis device thus provides a more user-friendly, rapid, clinically accurate, less expensive, and quantitative tool for protein detection.
2007. "Apoferritin-Templated Synthesis of Encoded Metallic Phosphate Nanoparticle Tags." Analytical Chemistry 79(15):5614-5619. doi:10.1021/ac070086f Abstract Encoded metallic-phosphate nanoparticle tags, with distinct encoding patterns, have been prepared using an apoferritin template. A center-cavity structure as well as the disassociation and reconstructive characteristics of apoferritin at different pH environments provide a facile route for preparing such encoded nanoparticle tags. Encapsulation and diffusion approaches have been investigated during the preparation. The encapsulation approach, which is based on the dissociation and reconstruction of apoferritin at different pHs, exhibits an effective route to prepare such encoded metallic-phosphate nanoparticle tags. The compositionally encoded nanoparticle tag leads to a high coding capacity with a large number of distinguishable voltammetric signals, reflecting the predetermined composition of the metal mixture solution (and hence the nanoparticle composition). Releasing the metal components from the nanoparticle tags at pH 4.6 acetate buffer avoids harsh dissolution conditions, such as strong acids. Such a synthesis of encoded nanoparticle tags, including single-component and compositionally encoded nanoparticle tags, is substantially simple, fast, and convenient compared to that of encoded metal nanowires and semiconductor nanoparticle (CdS, PbS, and ZnS) incorporated polystyrene beads. The encoded metallic-phosphate nanoparticle tags thus show great promise for bioanalytical or product-tracking/identification/protection applications.
2007. "Magnetic Beads-based Bioelectrochemical Immunoassay of Polycyclic Aromatic Hydrocarbons." Electrochemistry Communications 9(7):1547-1552. doi:10.1016/j.elecom.2007.02.007 Abstract A simple, rapid, and sensitive bioelectrochemical immunoassay method based on magnetic beads (MBs) has been developed to detect polycyclic aromatic hydrocarbons (PAHs). The principle of this bioassay is based on a direct competitive enzyme-linked immunosorbent assay using PAH-antibody-coated MBs and horseradish peroxidase (HRP)-labeled PAH (HRP-PAH). A magnetic process platform was used to mix and shake the samples during the immunoreactions and to separate free and unbound reagents after the liquid-phase competitive immunoreaction among PAH-antibody-coated MBs, PAH analyte, and HRP-PAH. After a complete immunoassay, the HRP tracers attached to MBs were transferred to a substrate solution containing 3, 3´, 5, 5´- tetramethylbenzidine (TMB) and hydrogen peroxide (H2O2) for electrochemical detection. The voltammetric characteristics of the substrate were investigated, and the reduction peak current of TMB was used to quantify the concentration of PAH. The different parameters, including the amount of HRP-PAH conjugates, the enzyme catalytic reaction time, and the pH of the supporting electrolyte that governs the analytical performance of the immunoassay have been studied in detail and optimized. The detection limit of 50 pg mL-1 was obtained under optimum experimental conditions. The performance of this bioelectrochemical magnetic immunoassay was successfully evaluated with tap water spiked with PAHs, indicating that this convenient and sensitive technique offers great promise for decentralized environmental applications.
2007. "Effects of Microstructure of Carbon Nanofibers for Amperometric Detection of Hydrogen Peroxide." Analytica Chimica Acta 597(2):238-244. doi:10.1016/j.aca.2007.06.046 Abstract Carbon nanofibers (CNFs) with different microstructures, including platelet-carbon nanofibers (PCNFs), fish-bone-carbon nanofibers (FCNFs), and tube-carbon nanofibers (TCNFs), were synthesized, characterized, and evaluated for electrochemical sensing of hydrogen peroxide. The CNFs studied here can show several microstructures in which various stacked morphologies and their sizes and graphite-layer ordering can be well controlled. Glassy carbon (GC) electrodes modified by CNFs were fabricated and compared for amperometric detection of hydrogen peroxide. Sensors of PCNFs/GC, FCNFs/GC, and TCNFs/GC were used in the amperometric detection of H2O2 in a solution of 0.05 M phosphate buffered saline solution (pH 7.4).
2007. "Design and Synthesis of Self-Assembled Monolayers on Mesoporous Supports (SAMMS): The Importance of Ligand Posture in Functional Nanomaterials." Journal of Materials Chemistry 17:2863-2874. doi:10.1039/b702422c Abstract Water, and water quality, are issues of critical importance to the future of humankind. The Earth’s water supplies have been contaminated by a wide variety of industrial, military and natural sources. The need exists for an efficient separation technology to remove heavy metal and radionuclide contamination from water. Surfactant templated synthesis of mesoporous ceramics provides a versatile foundation upon which to build high efficiency environmental sorbents. These nanoporous ceramics condense a huge amount of surface area into a very small volume. These mesoporous architectures can be subsequently functionalized through molecular self-assembly. These functional mesoporous materials offer significant capabilities in terms of removal of heavy metals and radionuclides from a variety of liquid media, including groundwater, contaminated oils and contaminated chemical weapons. They are highly efficient sorbents, whose rigid, open pore structure allows for rapid, efficient sorption kinetics. Their interfacial chemistry can be fine-tuned to selectively sequester a specific target species, such as heavy metals, tetrahedral oxometallate anions and radionuclides. This manuscript provides a review of the design, synthesis and performance of the sorbent materials. The role that ligand posture plays in the chemistry of these interfacial ligand fields is discussed.
2007. "Fabrication of Poly(methyl Methacrylate) microfluidic chips by redox-initiated polymerization." Electrophoresis 28(16):2897-2903. Abstract In this report, a method based on the redox-initiated polymerization of methyl methacrylate (MMA) has been developed for the rapid fabrication of PMMA microfluidic chips.The new fabrication approach obviates the need for special equipment and significantly simplifies the process of fabricating microdevices. The attractive performance of the novel PMMA microchips has been demonstrated in connection with contactless conductivity detection for the separation and detection of ionic species.
2007. "A Sol-Gel-Modified Poly(methyl methacrylate) Electrophoresis Microchip with a Hydrophilic Channel Wall." Chemistry - a European Journal 13(22):6461-6467. doi:10.1002/chem.200700242 Abstract A sol-gel method was employed to fabricate a poly(methyl methacrylate) (PMMA) electrophoresis microchip that contains a hydrophilic channel wall. To fabricate such a device, tetraethoxysilane (TEOS) was injected into the PMMA channel and was allowed to diffuse into the surface layer for 24 h. After removing the excess TEOS, the channel was filled with an acidic solution for 3 h. Subsequently, the channel was flushed with water and was pretreated in an oven to obtain a sol-gel-modified PMMA microchip. The water contact angle for the sol-gel-modified PMMA was 27.4° compared with 66.3° for the pure PMMA. In addition, the electro-osmotic flow increased from 2.13×10-4 cm2 V-1 s-1 for the native-PMMA channel to 4.86×10-4 cm2 V-1 s-1 for the modified one. The analytical performance of the sol-gel-modified PMMA microchip was demonstrated for the electrophoretic separation of several purines, coupled with amperometric detection. The separation efficiency of uric acid increased to 74 882.3 m-1 compared with 14 730.5 m-1 for native-PMMA microchips. The result of this simple modification is a significant improvement in the performance of PMMA for microchip electrophoresis and microfluidic applications.
2006. "Carbon Nanotube Based Sensors." In Encyclopedia of Sensors, vol. 2, ed. C. A. Grimes, E. C. Dickey, and M. V. Pishko, pp. 25-51. American Scientific Publishers, Stevenson Ranch, CA. Abstract This review article provides a comprehensive review on sensors and biosensors based on functionalized carbon nanotubes.
2006. "Microfabricated Devices for Sample Extraction, Concentrations, and Related Sample Processing Technologies." Chapter 8 in Bio-MEMS: Technologies and Applications, ed. W Wang and SA Soper, pp. 213-236. CRC Press, Boca Raton, FL. Abstract This is an invited book chapter. As with other analytical techniques, sample pretreatments, sample extraction, sample introduction, and related techniques are of extreme importance for micro-electro-mechanical systems (MEMS). Bio-MEMS devices and systems start with a sampling step. The biological sample then usually undergoes some kinds of sample preparation steps before the actual analysis. These steps may involve extracting the target sample from its matrix, removing interferences from the sample, derivatizing the sample to detectable species, or performing a sample preconcentration step. The integration of the components for sample pretreatment into microfluidic devices represents one of the remaining the bottle-neck towards achieving true miniaturized total analysis systems (µTAS). This chapter provides a thorough state-of-art of the developments in this field to date.
2006. "Voltammetric Analysis of Europium at Screen-Printed Electrodes Modified with Salicylamide Self-Assembled on Mesoporous Silica." Analyst 131(12):1342-1346. doi:10.1039/b609211j Abstract Mercury-free sensors for europium (Eu3+) assay, based on chemical modification of screen-printed carbon electrodes (SPCEs), with self-assembled salicylamide ligands on mesoporous silica (Sal-SAMMS) have been developed. The preconcentration of Eu3+ at SAMMS-based sensors utilize the binding affinity between the salicylamide ligand and Eu3+, which can be accomplished at open circuit potential without electrolyte and solution degassing. This was followed by the anodic stripping voltammetric Eu detection in a new medium. Due to the strong covalent bonding of the functional groups on mesoporous silica, the SAMMS based sensors with a built-in 3-electrode system can be reused for tens of measurements with minimal degradation, enabling the establishment of the calibration curve and lowering the costs. Linear response was found in the range of 75 to at least 500 ppb Eu3+ after a 5 minute preconcentration period. The detection limit was 10 ppb after a 10 minute preconcentration, and this limit can be improved with increased preconcentration time. Reproducibility (%R.S.D) of 100 ppb Eu2+ was 10% for a single sensor (5 measurements) and 10% for 5 sensors. The reproducibility can be improved through the precision of manufacturing of the sensors, in which SAMMS modification can be done in situ, increasing the user-friendliness of the sensors.
2006. "Sensitive Immunoassay of a Biomarker Tumor Necrosis Factor-[alpha] Based on Poly(guanine)-Functionalized Silica Nanoparticle Label." Analytical Chemistry 78(19):6974-6979. doi:10.1021/ac060809f Abstract A novel electrochemical immunosensor for the detection of tumor necrosis factor-alpha (TNF-a) based on poly(guanine)-functionalized silica nanoparticles (NPs) label is presented. The detection of mouse TNF-a via immunological reaction is based on a dual amplification: 1) a large amount of guanine residues is introduced on the electrode surface through the silica nanoparticle and immunoreaction, 2) mediator-induced catalytic oxidation of guanine, which results in great enhancement of anodic current. The synthesized silica NP conjugates were characterized with atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. These experiments confirmed that poly[G] and avidin were immobilized on the surface of silica NPs. The performance of the electrochemical immunosensor was evaluated and some experiment parameters (e.g., concentration of Ru(bpy)32+, incubation time of TNF-a, etc.) were optimized. The detection of limit for TNF-a is found to be 5.0x10-11 g mL-1 (2.0 pM), which corresponds to 60 attomoles TNF-a in 30 uL. This immunosensor based on the poly[G] functionalized silica NP label offers great promise for rapid, simple, cost-effective analysis of biological samples.
2006. "Electroactive Silica Nanoparticles for Biological Labeling." Small 2(10):1134-1138. Abstract A novel electrochemical immuno-biosensor based on poly(guanine)-functionalized silica nanoparticle labels and mediator-generated catalytic reaction was described. The functionalized silica NPs conjugates were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, and electrochemistry. This immunobiosensor is very sensitive and the limit of detection was found to be down to 0.2 ng/ml (4 pM), which was attributed to signal amplification by poly[G] functionalized silica NPs and guanine catalytic oxidation. Attractive feature of this approach is feasible to develop a cheap, sensitive and portable device for multiplexed diagnoses of different proteins. This method is simple, selective and reproducible for trace protein analysis and can be extended to study protein/protein, peptide/protein, and DNA/ protein interactions.
2006. "Catalytic Adsorptive Stripping Voltammetric Measurements of Trace Vanadium at Bismuth Film Electrodes ." Talanta 69(4):914-917. Abstract Bismuth-coated glassy-carbon electrodes have been successfully applied for catalytic adsorptive-stripping voltammetric measurements of low levels of vanadium(V) in the presence of chloranilic acid (CAA) and bromate ion. The new protocol is based on the accumulation of the vanadium-chloranilic acid complex from an acetate-buffer (pH 5.5) solution at a preplated bismuth film electrode held at -0.35 V (vs. Ag/AgCl), followed by a square-wave voltammetric scan. Factors influencing the adsorptive stripping performance, including the CAA and bromate concentrations, solution pH, and accumulation potential or time have been optimized. The response compares favorably with that observed at mercury film electrodes. A linear response is observed over the 5 - 25 ug/L concentration range (two min accumulation), along with a detection limit of 0.20 ug/L vanadium (10 min accumulation). High stability is indicated from the reproducible response of a 50 ug/L vanadium solution (n=25; RSD = 3.1%). Applicability to a groundwater sample is illustrated.
2006. "Amperometric Choline Biosensor Fabricated through Electrostatic Assembly of Bienzyme/Polyelectrolyte Hybrid Layers on Carbon Nanotubes." Analyst 131(4):477-483. doi: 10.1039/b516038c Abstract We report a flow injection amperometric choline biosensors based on the electrostatic assembly of an enzyme of choline oxidase (ChO) and a bi-enzyme of ChO and horseradish peroxidase (HRP) onto multi-wall carbon nanotubes (MWCNT) modified glassy carbon (GC) electrodes. These choline biosensors were fabricated by immobilization of enzymes on the negatively charged MWCNT surface through alternatively assembling a cationic polydiallydiimethylammonium choride (PDDA) layer and an enzyme layer. Using this layer-by-layer assembling approach, bioactive nanocomposite film of a PDDA/ChO/PDDA/HRP/PDDA/CNT (ChO/HRP/CNT) and a PDDA/ChO/PDDA/ CNT (ChO/ CNT) were fabricated on GC surface, respectively. Owning to the electrocatalytic effect of carbon nanotubes, the measurement of faradic responses resulting from enzymatic reactions has been realized at low potential with acceptable sensitivity. It is found the ChO/ HRP /CNT biosensor is more sensitive than the ChO/CNT one. Experimental parameters affecting the sensitivity of biosensors, e.g. applied potential, flow rate, etc. were optimized and potential interference was examined. The response time for this choline biosensor is fast (less than a few seconds). The linear range of detection for the choiline biosensor is from 5 x10-5 to 5x10-3 M and the detection limit is determined to be about 1.0 x 10-5 M.
2006. "Disposition of Lead (Pb) in Saliva and Blood of Sprague-Dawley Rats Following a Single or Repeated Oral Exposure to Pb-Acetate." Toxicology 222(1-2):86-94. doi:10.1016/j.tox.2006.01.030 Abstract Abstract Biological monitoring for lead (Pb) is usually based upon a determination of blood Pb concentration; however, saliva has been suggested as a non-invasive biological matrix for assessing exposure. To further evaluate the potential utility of saliva for biomonitoring, the disposition of Pb was evaluated in whole blood (WB), red blood cells (RBC), plasma, parotid gland, bone, and saliva following either a single oral dose of 100 mg Pb-acetate/kg body weight in rats or ~1-week after 5 sequential daily oral gavage doses of 1, 10, or 100 mg Pb-acetate/kg/day. Saliva volume, pH, total saliva protein, and α-amylase activity were also determined. At specified times post-dosing groups of animals were anethetized and administered pilocarpine to induce salivation. Saliva was collected, the animals were humanely sacrificed, and tissue samples were likewise collected, weighed, and processed for Pb analysis. Following a single dose exposure to PB-acetate, Pb was detectable in all samples by 30 min post-dosing. For both the single and repeated dose treatments the concentration of Pb was highest in WB and RBC relative to plasma and saliva. However, the Pb rapidly redistributed (within 5-days post-treatment) from the blood into the bone compartment based on the substantial decrease in WB and RBC Pb concentration, and the concurrent increase in bone Pb following repeated exposure at all dose levels. Although there is clear variability in the observed Pb concentrations in plasma and saliva, there was a reasonable correlation (r2=0.922) between the average Pb concentrations in these biological matrices which was consistent with previous observations. The single oral dose of Pb-acetate resulted in a decrease in salivary pH which recovered by 24 hr post-dosing and a decrease in α-amylase enzyme activity which did recover within 5-days of ceasing exposure. It is currently unclear what impact these slight functional changes may or may not have on Pb salivary clearance rates. These results demonstrate a feasibility to rapidly detect Pb in saliva and suggest that saliva may correlate best with plasma Pb concentration.
2006. "Versatile Apoferritin Nanoparticle Labels for Assay of Protein ." Analytical Chemistry 78(21):7417-7423. doi:10.1021/ac060653j Abstract A versatile bioassay label based on marker-loaded-apoferritin nanoparticles (MLAN) have been developed for sensitive protein detection. Dissociation and reconstitution characteristics at different pH as well as the special cavity structure of apoferritin provide a facile route to prepare nanoparticle labels, and avoid complicated and tedious synthesis process of conventional nanoparticle labels. The optical and electrochemical characteristics of the prepared nanoparticle labels are easily controlled by loading different optical or electrochemical markers. A fluorescence marker (fluorescein anion) and a redox marker [hexacyanoferrate (III)] were used as model markers to load into the cavity of apoferritin nanoparticle and developed for microscopic fluorescence immunoassay and electrochemical immunoassay, respectively. Detection limits of 0.06 ng mL-1 (0.39 pM) and 0.08 ng mL-1 (0.51 pM) of IgG were obtained with fluorescein MLAP and hexacyanoferrate MLAN, respectively. The new nanoparticle labels hold great promise for multiplex protein detection (in connection to nanoparticles loaded with different markers) and for enhancing the sensitivity of other bioassay.
2006. "Electrochemical Proteolytic Beacon for Detection of Matrix Metalloproteinase Activities ." Journal of the American Chemical Society 128(38):12382-12383. doi:10.1021/ja0626638 Abstract This communication describes a novel method for detecting of matrix metalloproteinase-7 activity using a peptide substrate labeled with a ferrocene reporter. The substrate serves as a selective ‘electrochemical proteolytic beacon’ (EPB) for this metalloproteinase. The EPB is immobilized on a gold electrode surface to enable ‘on-off’ electrochemical signaling capability for uncleaved and cleaved events. The EPB is efficiently and selectively cleaved by MMP-7 as measured by the rate of decrease in redox current of ferrocene. Direct transduction of a signal corresponding to peptide cleavage events into an electronic signal thus provides a simple, sensitive route for detecting the MMP activity. The new method allows for identification of the activity of MMP-7 in concentrations as low as 3.4 pM. The concept can be extended to design multiple peptide substrate labeled with different electroactive reporters for assaying multiple MMPs activities.
2006. "Carbon Nanotube Templated Asembly of Protein." Journal of Nanoscience and Nanotechnology 6(4):948-953. Abstract This paper describes a novel general strategy for fabricating protein-polyion multilayers by electrostatic layer-by-layer (LBL) self-assembly on a carbon nanotube (CNT) template. Such a noncovalent functionalization method is important for preserving the activity of biomolecules and the mechanical and electrical properties of CNTs. Glucose oxidase and poly (diallydimethylammonium) chloride polymer were used as a model to investigate the LBL process on a CNT template. High-resolution transmission electron microscopy and electrochemical characterization confirm the formation of LBL nanostructures on carboxyl functionalized CNTs. We have also demonstrated the applications of these nanoshell bioreactors to the direct electrochemistry of proteins and biosensing. This strategy can be applied to assemble other biological molecules, such as antibodies, antigens, and DNA, for wide bioassay applications.
2006. "Biosensor Based on Self-Assembling Acetylcholinesterase on Carbon Nanotubes for Flow injection/Amperometric Detection of Organophosphate Pesticides and Nerve Agents." Analytical Chemistry 78(3):835-843. doi:10.1021/ac051559q Abstract A highly sensitive flow-injection amperometric biosensor for organophosphate pesticides and nerve agents based on self-assembly of acetylcholinesterase (AChE) on carbon nanotube (CNT)-modified glassy carbon (GC) electrode is described. AChE is immobilized on the negatively-charged CNT surface by alternatively assembling a cationic polydiallyldimethylammonium chloride (PDDA) layer and an AChE layer. Transmission electron microscopy images confirm the formation of layer-by-layer nanostructures on carboxyl functionalized CNTs. The unique sandwich-like structure (PDDA/AChE/PDDA) on the CNT surface formed by self-assembly provides a favorable microenvironment to keep the bioactivity of AChE and to prevent enzyme molecule leakage. The electrocatalytic activity of CNT leads to a greatly improved electrochemical detection of the enzymatically generated thiocholine product, including a low oxidation overvoltage (+150 mV), higher sensitivity, and stability. The developed PDDA/AChE/PDDA/CNT/GC biosensor integrated into a flow injection system was used to monitor organophosphate pesticides and nerve agents, such as paraoxon. The sensor performance, including inhibition time and regeneration conditions, was optimized with respect to operating conditions. Under the optimal conditions, the biosensor was used to measure as low as 0.4 pM paraoxon with a 6-min inhibition time. The biosensor had excellent operational lifetime stability with no decrease in the activity of enzymes for more than 20 repeated measurements over a 1-week period. The developed biosensor system is an ideal tool for online monitoring of organophosphate pesticides and nerve agents.
2006. "Bioelectrochemical Magnetic Immunosensing of Trichloropyridinol: A Potential Insecticide Biomarker." Electroanalysis 18(16):1605-1613. Abstract A magnetic beads-based bioelectrochemical magnetic immunosensor was developed for the fast and sensitive determination of the trichloropyridinol (TCP) biomarker in environmental samples. After liquid phrase competitive immunoreaction among a limited amount of TCP antibody coated-magnetic beads (Ab-MBs), TCP analyte, and horseradish peroxidase (HRP) labeled TCP (HRP-TCP), a magnet/glassy carbon (MGC) electrode was used to collect a TCP-Abs-MBs and a HRP-TCP-Ab-MBs immunocomplex assembly. The activity of HRP tracers bound to the beads was monitored with highly sensitive square wave voltammetry (SWV) by accumulating an electroactive enzymatic product to the MGC electrode surface under constant potential (0.5 V) during enzymatic reaction in the presence of 3’,3’,5’,5’-tetramethylbenzidine (TMB)-H2O2 substrate solution. The electrochemical characteristics of substrate and product were investigated, and the parameters of the immunoassay were optimized.
2006. "Bioassay Labels Based on Apoferritin Nanovehicles." Chembiochem 7(9):1315-1319. doi:10.1002/cbic.200600225 Abstract Here we report a nanoparticle label based on apoferritin nanovehicle loaded internally with markers for sensitive electrochemical DNA detection. The central cavity structure, the dissociation and reconstitute properties at different pHs of apoferritin provide a facile method to load and release markers. Hexacynoferrate(III) was used as model marker to load into the cavity of apoferritin protein cage. The loaded nanoparticle surface was functionalized with amino-modified DNA probe. Electrochemical DNA hybridization assay based on the hexacynoferrate loaded apoferritin nanovehicle could detect 23 atmol DNA targets in 50 ul sample solution. The concept could be readily extended to load other redox and fluorescence markers for bioassay applications. The new nanoparticle labels hold great promise for multi-target detection (in connection to nanoparticles loaded with different markers) and for enhancing the sensitivity of other bioassays.
2006. "Apoferritin Templated Synthesis of Metal Phosphate Nanoparticle Labels for Electrochemical Immunoassay." Small 2(10):1139-1143. Abstract W have introduced template-synthesized metal phosphate nanoparticle labels for electrochemical immunoassay. Such use of an apoferritin template offers a simple and convenient route to prepare metallic nanoparticle labels for electrochemical immunoassays and avoid the complicated and time-consuming nanoparticle synthesis process (QD synthesis). Releasing metal ions from metal phosphate in an acetate buffer (pH 4.6) eliminates the harsh condition in the traditional metallic nanoparticle dissolution (e.g., strong acid dissolution of QDs and gold nanoparticles). This method is ultrasensitive and its DL is low to 77fM. The simultaneous detection of multiple protein targets is easily performed by using different metal phosphate nanoparticle labels (cadmium phosphate and lead phosphate). This approach can be extended to prepare multiple metal (such as zinc, lead, cadmium, copper, indium, gold, silver) phosphate nanoparticle labels or hybrid metal (bimetallic or trimetallic with predetermined ratios) phosphate nanoparticle labels for a multiplex electrochemical immunoassay. The new nanoparticle labels could be applicable to other electrochemical bioassays, such as DNA, and is thus expected to lead to wide applications for protein diagnostics and for bioanalysis in general.
2006. "Amperometric Glucose Biosensor Based on Self-Assembling Glucose Oxidase on Carbon Nanotubes." Electrochemistry Communications 8:251-256. Abstract A flow injection amperometric glucose biosensor based on electrostatic self-assembling glucose oxidase (GOx) on a carbon nanotube (CNT)-modified glassy carbon transducer is described. GOx is immobilized on the negatively charged CNT surface by alternatively assembling a cationic polydiallyldimethylammonium chloride (PDDA) layer and a GOx layer. The unique sandwich-like layer structure (PDDA/GOx/PDDA/CNT) formed by self-assembling provides a favorable microenvironment to keep the bioactivity of GOx and to prevent enzyme molecule leakage. The direct electrochemistry behavior of GOx and electrocatalysis of H2O2 on the fabricated PDDA/GOx/PDDA/CNT electrode demonstrated that such a biosensor fabrication method preserves the activity of enzyme molecules and the mechanical and electrocatalytic properties of carbon nanotubes, enabling sensitive determination of glucose. Flow injection amperometric detection of glucose is carried out at -100 mV (vs Ag/AgCl) in 0.05 M phosphate buffer solution (pH 7.4) with wide linear response range of 15 uM- 6 mM and a detection limit of 7 uM. The PDDA/GOx/PDDA/CNT/GC biosensor showed excellent properties for the sensitive determination of glucose with good reproducibility, remarkable stability, and free of interference from other co-existing electroactive species. The present methods can be applied to assemble other enzyme molecules and biological molecules, such as antibody, antigen, and DNA, to the CNT surface for wide biosensor and bioassay applications.
2006. "Electrosynthesis, Characterization, and Application of Novel Hybrid Materials Based on Carbon Nanotube-Polyaniline-Nickel Hexacyanoferrate Nanocomposites." Journal of Materials Chemistry 16(6):585-592. Abstract Incorporating nanoclusters of nickel hexacyanoferrates (NiHCF) onto a porous polyaniline (PANI)–carbon nanotube (CNT) matrix provides a novel class of hybrid materials with a good ion exchange capacity, high stability, and a selectivity for caesium ions. The CNT-PANI-NiHCF nanocomposite films have been synthesized by electrodeposition step-by-step on glassy carbon electrodes and characterized with cyclic voltammetry (CV), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) techniques. CV and XPS investigations confirmed the formation of PANI and NiHCF on the surface of CNTs. The microscopy of NiHCF hybrid materials was characterized by SEM and TEM; the size of NiHCF particles is approximately 20 to 50 nm. The porous high surface area CNT matrix provides the high loading capacity for the deposition of NiHCF nanoparticles, while the PANI thin-film further stabilizes the nanoparticles. The selectivity for caesium ion adsorption of the hybrid materials was investigated. The high selectivity for caesium provides the base to develop a novel electrochemical ion exchange process for the treatment of nuclear wastes and radioactive-caesium contaminated waters.
2006. "Electrically Controlled Anion Exchange Based on Polypyrrole and Carbon Nanotubes Nanocomposite for Perchlorate Removal ." Environmental Science and Technology 40(12):4004-4009. Abstract A novel and stable carbon nanotube/polypyrrole nanocomposite film has been electrosynthesized and the feasibility for removing perchlorate ion through an electrically controlled anion exchange has been evaluated.
2006. "Preparation, Characterization and Anion Exchange Properties of Polypyrrole/Carbon Nanotube Nanocomposite." Journal of Nanoscience and Nanotechnology 6(2):547-553. Abstract In this study, polypyrrole (PPy) thin film was electrodeposited on carbon nanotube (CNT) backbones by applying a constant deposition potential in solution with 0.1 M pyrrole with different electrolytes such as NaCl, NaNO3, or NaClO4. The hybrid films were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and cyclic voltammetry. SEM images revealed the nanostructrure of PPy film generated on CNTs surface. The electrochemical and anion exchange properties of PPy-CNT composite film have been investigated. Nanostructured composite thin films of polypyrrole/CNTs were studied by cyclic votammetry between 0.4 and -0.8 V in aqueous solution to evaluate their cycling stability and capacity for electrically switched anion exchange. It is found that the PPy/CNTs nanocomposites can improve the anion exchange capacity and stability of the PPy-CNTs composite film, which may be attributed to the nanostructure of the polyprrole film, which offer the high aspect ratio of the film and ease of diffusion of anions in the nanostructured film, and the interaction between CNTs and PPy.
2006. "Monitoring environmental pollutants by microchip capillary electrophoresis with electrochemical detection ." Talanta 68(3):497-503. Abstract This is a review article. During the past decade, significant progress in the development of miniaturized rnicrofluidic systems has Occurred due to the numerous advantages of microchip analysis. This review focuses on recent advances and the key strategies in microchip capillary electrophoresis (CE) with electrochemical detection (ECD) for separating and detecting a variety of environmental pollutants. The subjects covered include the fabrication of ruicrofluidic chips, ECD, typical applications of microchip CE with ECD in environmental analysis, and future prospects. It is expected that microchip CE-ECD will become a powerful tool in the environmental field and will lead to the creation of truly portable devices.
2006. "Microchip Capillary Electrophoresis with Electrochemical Detection for Monitoring Environmental Pollutants." Current Analytical Chemistry 2(1):43-50. Abstract This invited paper reviews recent advances and the key strategies in microchip capillary electrophoresis (CE) with electrochemical detection (ECD) for separating and detecting a variety of environmental pollutants. The subjects covered include the fabrication of microfluidic chips, sample pretreatments, ECD, typical applications of microchip CE with ECD in environmental analysis, and future prospects. It is expected that microchip CE-ECD will become a powerful tool in the environmental field and will lead to the creation of truly portable devices.
2005. "Application of Novel Nanoporous Sorbents for the Removal of Heavy Metals, Metalloids, and Radionuclides." Chapter 19 in Trace Elements in the Environment, ed. MNV Prasad, KS Sajwan, R Naidu, pp. 369-380. Taylor and Francis, Boca Raton, FL. Abstract A new class of hybrid nanoporous materials for removing toxic heavy metals, oxyanions, and radionuclides from aqueous waste streams has been developed at the Pacific Northwest National Laboratory. These novel materials consist of functional molecules such as thiols, ethylenediamine complexed copper, and carbamoylphosphonates that are self-assembled as monolayers within the nanopores of a synthetic silica-based material. Tests indicated that these sorbents (self-assembled monolayers on mesoporous silica — SAMMS) can achieve very high sorbate loadings (~6 meq/g) very rapidly with relatively high specificity (Kd: 1×108 ml/g). Because of the specifically tunable nature of the functionalities, these nanoporous sorbents can be targeted to remove a selected category of contaminants such as heavy metals (Ag, Cd, Cu, Hg, and Pb), oxyanions (As and Cr), and radionuclides (137Cs, 129I, 237Np, and isotopes of Pu, Th, and U) from waste streams.
2005. "Deposition of Platinum Nanoparticles on Carbon Nanotubes by Supercritical Fluid Method." Journal of Nanoscience and Nanotechnology 5(11):1852-1857. doi:10.1166/jnn.2005.421 Abstract Carbon nanotube-supported platinum nanoparticles with a 5-15 nm diameter size range can be synthesized by hydrogen reduction of platinum(II) acetylacetonate in methanol modified supercritical carbon dioxide. XPS and XRD spectra indicate that the carbon nanotubes contain zero-valent platinum metal and high-resolution TEM images show that the visible lattice fringes of the Pt particles are crystallites. Carbon nanotubes synthesized with 25% by weight of Pt nanoparticles exhibit a higher activity for hydrogenation of benzene compared with a commercial carbon black platinum catalyst. The carbon nanotube-supported Pt nanocatalyst can be reused at least six times for the hydrogenation reaction without losing activity. The carbon nanotube-supported Pt nanoparticles are also highly active for electrochemical oxidation of methanol and for reduction of oxygen suggesting their potential use as a new electrocatalyst for polymer electrode fuel cell applications.
2005. "Supercritical Fluid Immersion Deposition: A New Process for Selective Deposition of Metal Films on Silicon Substrates." Surface & Coatings Technology 190(1):25-31. Abstract Supercritical CO2 is used as a new solvent for immersion deposition, a galvanic displacement process traditionally carried out in aqueous HF solutions containing metal ions, to selectively develop metal films on featured or non-featured silicon substrates. Components of supercritical fluid immersion deposition (SFID) solutions for fabricating Cu and Pd films on silicon substrates are described along with the corresponding experimental setup and procedure. Only silicon substrates exposed and reactive to SFID solutions can be coated. The highly pressurized and gas-like supercritical CO2, combined with the galvanic displacement property of immersion deposition, enables the SFID technique to selectively deposit metal films in small features. SFID may also provide a new method to fabricate palladium silicide in small features or to metallize porous silicon.
2005. "Supercritical Fluid Attachment of Palladium Nanoparticles on Aligned Carbon Nanotubes." Journal of Nanoscience and Nanotechnology 5(6):964-969. doi:10.1166/jnn.2005.133 Abstract Nanocomposite materials consisting of Pd nanoparticles deposited on aligned multi-walled carbon nanotubes have been fabricated through hydrogen reduction of palladium precursor in supercritical carbon dioxide. The supercritical fluid processing allowed deposition of high-density Pd nanoparticles (~5-10 nm) on both as-grown (unfunctionalized) and functionalized (using HNO3 oxidation) nanotubes. However, the wet processing for functionalization results in pre-agglomerated, bundle-shaped nanotubes thus significantly reducing the effective surface area for Pd particle deposition, although the bundling provides more secure, lock-in-place positioning of nanotubes and Pd catalyst particles. The nanotube bundling is substantially mitigated by Pd nanoparticle deposition on the unfunctionalized and geometrically separated nanotubes, which provides much higher catalyst surface area. Such nanocomposite materials utilizing geometrically secured and aligned nanotubes can be useful for providing much enhanced catalytic activities to chemical and electrochemical reactions (e.g. fuel cell reactions), and eliminate the need for tedious catalyst recovery process after the reaction is completed.
2005. "Screen-Printed Electrodes Modified with Functionalized Mesoporous Silica for Voltammetric Analysis of Toxic Metal Ions." Electrochemistry Communications 7(11):1170-1176. doi:10.1016/j.elecom.2005.08.018 Abstract Mercury-free sensors for lead (Pb2+) assay based on chemical modification of screen-printed carbon electrodes (SPCEs) with acetamide phosphonic acid self-assembled monoloyer on mesoporous support (Ac-Phos SAMMS) have been developed. The preconcentration of Pb2+ at SAMMS-based sensors utilize the binding affinity of the acetamide phosphonic acid and Pb2+, which can be accomplished at open circuit potential without electrolyte and solution degassing. Due to the strong covalent bonding of the functional groups on SAMMS, the SAMMS based sensors with a built-in 3 electrode system can be reused for tens of measurements with minimal degradation, enabling the establishment of the calibration curve and lowering the costs. Linear calibration curve was found in the range of 0 to at least 100 ppb Pb2+ after 5 minutes of preconcentration. The detection limit was calculated from 3S/N to be 1 ppb of Pb2+. Reproducibilty (%RSD) was found to be 5% for a single sensor (6 measurements) and 10% for 5 sensors. The reproducibility can be improved through the precision of manufacturing of the sensors, in which SAMMS modification can be done in-situ, increasing the user-friendliness of the sensors. Cadmium, lead, and copper can also be detected simultaneously at the SAMMS screen-printed sensors.
2005. "Optimization of A Portable Microanalytical System to Reduce Electrode Fouling from Proteins Associated with Biomonitoring of Lead (Pb) in Saliva." Talanta 67(3):617-624. Abstract There is a need to develop reliable portable analytical systems for on-site and real-time biomonitoring of lead (Pb) from both occupational and environmental exposures. Saliva is an appealing matrix since it is easily obtainable, and therefore a potential substitute for blood since there is a reasonably good correlation between Pb levels in both blood and saliva. The microanalytical system is based on stripping voltammetry of Pb at the microelectrochemical cell having a flow injection/flow-onto design. Samples that contain as little as 1% saliva can cause electrode fouling, resulting in significantly reduced responsiveness, irreproducible quantitations, and the need for frequent electrode regeneration. In addition, incomplete Pb release from salivary protein can also yield a lower Pb response than expected. This paper evaluates the extent of in vitro Pb-protein binding and the optimal pre-treatment for releasing Pb from the saliva samples. Even in 50% by volume of rat saliva, the electrode fouling was not observed, due to the appropriate sample pretreatment (with 1.0 M acid, followed by centrifugation at the RCF of 15200×g) and the constant flow of the sample and acidic carrier that prevented passivation by the protein. The system offered a linear response over a low Pb range (1-10 ppb), low detection limit (1 ppb), excellent reproducibility (5% RSD), and reliability. It also yielded the same Pb concentrations in unknown samples as did the ICP-MS. These encouraging results suggest that the microanalytical system represents an important analytical advancement for real-time non-invasive (i.e., saliva) biomonitoring of Pb.
2005. "Nanostructured Electrochemical Sensors Based on Functionalized Nanoporous Silica for Voltammetric Analysis of Lead, Mercury and Copper." Journal of Nanoscience and Nanotechnology 5(9):1537-1540. Abstract We have successfully developed electrochemical sensors based on functionalized nanostructured materials for voltammetric analysis of toxic metal ions. Glycinyl-urea self-assembled monolayers on mesoporous silica (Gly-UR SAMMS) was incorporated in carbon paste electrodes for the detection of toxic metal ions such as lead, copper, and mercury based on adsorptive stripping voltammetry (AdSV). The electrochemical sensor yields a linear response at low ppb level of Pb2+ (i.e., 2.5 to 50 ppb) after a 2 minute preconcentration period, with reproducible measurements (%RSD = 3.5, N = 6), and excellent detection limits (at few ppb). By exploiting the interfacial functionality of Gly-UR SAMMS, the sensor is selective for the target species, does not require the use of a mercury film, and can be easily regenerated in dilute acid solution. The rigid, open, parallel pore structure, combined with suitable interfacial chemistry of SAMMS, also results in fast analysis times (2-3 minutes). The nanostructured SAMMS materials enable the development of miniature sensing devices that are compact and low-cost, have low-energy-consumption, and are easily integrated into field-deployable units. Keywords: electrochemical sensor, glycinyl-urea, self-assembled monolayer, mesoporous silica, lead, mercury, copper, adsorptive stripping voltammetry.
2005. "Hydroxypyridinone (HOPO) Functionalized Self-Assembled Monolayers on Nanoporous Silica for Sequestering Lanthanide Cations." Journal of Nanoscience and Nanotechnology 5(4):527-535(9). doi:10.1166/jnn.2005.096 Abstract 1,2-Hydroxypyridinone (1,2-HOPO) ligands were installed as self-assembled monolayers on nanoporous silica (MCM-41) to create a superior class of sorbent materials for lanthanide cations. Lanthanides were used as a model system for the radioactive, expensive and highly hazardous actinides in the preliminary screening studies. The ligand properties of the 1,2-HOPO ligand field and the extremely large surface area of the MCM-41, coupled with the dense monolayer coating contribute to extremely high lanthanide binding capacity of the 1,2-HOPO nanoporous sorbent. At pH 4-5.9, the mass-weighted partition coefficients (Kd) for La, Nd, Eu, Lu were 354,000, 344,000, 210,800, 419,800, respectively. The rigid, open pore structure of the silica also allows for very rapid sorption. Being silica-based, the sorbent is compatible with vitrification processing into a final glasseous waste form, for subsequent disposition in a deep geological repository.
2005. "Automated portable analyzer for lead(II) based on sequential flow injection and nanostructured electrochemical sensors ." Talanta 68(2):256-261. Abstract A fully-automated portable analyzer for toxic metal ion detection based on a combination of a nanostructured electrochemical sensor and a sequential flow injection system has been developed in this work. The sensor was fabricated from a carbon paste electrode modified with acetamide phosphonic acid self-assembled monolayer on mesoporous support (Ac-Phos SAMMS) which was embedded in a very small wall-jet (flow-onto) electrochemical cell. The electrode was solid-state and mercury-free. Samples and reagents were injected into the system and flowed through the electrochemical cell by a programmatic sequential flow technique which required minimal volume of samples and reagents and allowed the automation of the analyzer operation. The portable analyzer was evaluated for lead (Pb) detection due to the excellent binding affinity between lead and the functional groups of Ac-Phos SAMMS as well as the great concern for lead toxicity. Linear calibration curve was obtained in a low concentration range (1 to 25 ppb of Pb(II)). The reproducibility was excellent; the percent relative standard deviation was 2.5 for seven consecutive measurements of 10 ppb of Pb(II) solution. Excess concentrations of Ca, Ni, Co, Zn, and Mn ions in the solutions did not interfere with detection of lead, due to the specificity and the large number of the functional groups on the electrode surface. The electrode was reliable for at least 90 measurements over 5 days. This work is an important milestone in the development of the next-generation metal ion analyzers that are portable, fully-automated, and remotely-controllable.
2005. "Monodispersed core-shell Fe3O4@Au nanoparticles ." Journal of Physical Chemistry B 109(46):21593-21601. Abstract The ability to synthesize and assemble monodispersed core-shell nanoparticles is important for exploring the unique properties of nanoscale core, shell, or their combinations in technological applications. This paper describes findings of an investigation of the synthesis and assembly of core (Fe3O4)-shell (An) nanoparticles with high monodispersity. Fe3O4 nanoparticles of selected sizes were used as seeding materials for the reduction of gold precursors to produce gold-coated Fe3O4 nanoparticles (Fe3O4@Au). Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, core-shell composition, surface reactivity, and magnetic properties have confirmed the formation of the core-shell nanostructure. The interfacial reactivity of a combination of ligand-exchanging and interparticle cross-linking was exploited for molecularly mediated thin film assembly of the core-shell nanoparticles. The SQUID data reveal a decrease in magnetization and blocking temperature and an increase in coercivity for Fe3O4@Au, reflecting the decreased coupling of the magnetic moments as a result of the increased interparticle spacing by both gold and capping shells. Implications of the findings to the design of interfacial reactivities via core-shell nanocomposites for magnetic, catalytic, and biological applications are also briefly discussed.
2005. "Iron Oxide-Gold Core-Shell Nanoparticles and Thin-Film Assembly." Journal of Materials Chemistry 15(18):1821-1832. Abstract This paper reports findings of an investigation of the synthesis of monolayer-capped iron oxide and core (iron oxide)–shell (gold) nanocomposite and their assembly towards thin film materials. Pre-synthesized and size-defined iron oxide nanoparticles were used as seeding materials for the reduction of gold precursors, which was shown to be effective for coating the iron oxide cores with gold shells (Fe oxide@Au). The unique aspect of our synthesis is the formation of Fe oxide@Au core–shell nanoparticles with controllable surface properties. The novelty of our assembly strategy is the exploitation of the ligand-exchange reactivity at the gold shells for the thin film assembly of the core–shell nanoparticles. The core–shell nanocomposites and assemblies have been characterized using TEM, XRD, XPS, FTIR, TGA, and DCP-AES techniques. In addition to evidence from TEM detection of the change in particle size, UV-Vis observation of the change in the surface plasmon resonance band, and XRD detection of disappearance of the magnetite diffraction peaks after coating the gold shell, the formation of the core–shell morphology was further confirmed by DCP-AES composition analysis of Au and Fe in the molecularly-mediated thin film assembly of Fe oxide@Au particles. The interparticle ligand exchange–precipitation chemistry at the gold shell is to our knowledge the first example demonstrating the inter-shell reactivity for constructing thin films of Fe oxide@Au particles. The results have provided important insights into the design of interfacial reactivities via core–shell nanocomposites for magnetic, catalytic and biosensing applications.
2005. "Carbon Nanotubes Based Nanoelectrode Arrays: Fabrication, Evaluation and Application in Voltammetric Analysis." Electroanalysis 17(1):79-84. Abstract Nanoelectrode arrays were fabricated based on controlled density aligned carbon nanotubes. Nanoelectrodes were demonstrated for voltammatric analysis of drug and metal ions.
2005. "Rapid, quantitative and sensitive immunochromatographic assay based on stripping voltammetric detection of a metal ion label." Analyst 130(11):1513-1517. Abstract A novel, sensitive immunochromatographic electrochemical biosensor (IEB) which combines an immunochromatographic strip technique with an electrochemical detection technique is demonstrated. The IEB takes advantages of the speed and low-cost of the conventional immunochromatographic test kits and high-sensitivity of stripping voltammetry. Bismuth ions (Bi3+) have been coupled with the antibody through the bifunctional chelating agent diethylenetriamine pentaacetic acid (DTPA). After immunoreactions, Bi3+ was released and quantified by anodic stripping voltammetry at a built-in single-use screen-printed electrode. As an example for the applications of such novel device, the detection of human chorionic gonadotronphin (HCG) in a specimen was performed. This biosensor provides a more user-friendly, rapid, clinically accurate, and less expensive immunoassay for such analysis in specimens than currently available test kits.
2005. "Ultrasensitive Voltammetric Detection of Trace Heavy Metal Ions Using Carbon Nanotube Nanoelectrode Array." Analyst 130(7):1098-1101. doi:10.1039/b419447k Abstract We describe an ultrasensitive voltammetric detection of trace heavy metal ions using nanoelectrode arrays (NEAs) that are based on low-site density carbon nanotubes (CNTs). The NEAs were prepared by sealing the side-walls of CNTs with an epoxy passive layer that reduces the current leakage and eliminates the electrode capacitance, leading to a low background current. This provides a high signal-to-noise ratio. The CNTs-NEAs coated with a bismuth film were used successfully for voltammetric detection of trace cadmium and lead (II) at the sub-ppb level. The detection limit of 0.04 g/L was obtained under optimum experimental conditions. The attractive behavior of the new carbon NEA sensing platform holds great promise for onsite environmental monitoring and biomonitoring of toxic metals.
2005. "Sequential Injection/Electrochemical Immunoassay for Quantifying the Pesticide Metabolite 3, 5, 6-Trichloro-2-Pyridinol." Electrochemistry Communications 7(12):1463-1470. Abstract An automated and sensitive sequential injection electrochemical immunoassay was developed to monitor a potential insecticide biomarker, 3, 5, 6-trichloro-2-pyridinol. The current method involved a sequential injection analysis (SIA) system equipped with a thin-layer electrochemical flow cell and permanent magnet, which was used to fix 3,5,6-trichloro-2-pyridinol (TCP) antibody coated magnetic beads (TCP-Ab-MBs) in the reaction zone. After competitive immunoreactions among TCP-Ab-MBs, TCP analyte, and horseradish peroxidase (HRP) labeled TCP, a 3, 3’, 5, 5’-tetramethylbenzidine dihydrochloride and hydrogen peroxide (TMB-H2O2) substrate solution was injected to produce an electroactive enzymatic product. The activity of HRP tracers was monitored by a square wave voltammetric scanning electroactive enzymatic product in the thin-layer flow cell. The voltammetric characteristics of the substrate and the enzymatic product were investigated under batch conditions, and the parameters of the immunoassay were optimized in the SIA system. Under the optimal conditions, the system was used to measure as low as 6 ng L-1 (ppt) TCP, which is around 50-fold lower than the value indicated by the manufacturer of the TCP RaPID Assay® kit (0.25 ug/L, colorimetric detection). The performance of the developed immunoassay system was successfully evaluated on tap water and river water samples spiked with TCP. This technique could be readily used for detecting other environmental contaminants by developing specific antibodies against contaminants and is expected to open new opportunities for environmental and biological monitoring.
2005. "Sensitive Electrochemical Detection of Enzymatically-generated Thiocholine at Carbon Nanotube Modified Glassy Carbon Electrode." Electrochemistry Communications 7(11):1163-1169. Abstract A carbon nanotube modified glassy-carbon (CNT/GC) electrode was used for enhancing the sensitivity of electrochemical measurements of enzymatically-generated thiocholine. Cyclic voltammetric and amperometric characteristics of thiocholine at CNT/GC, glassy carbon, carbon paste, and gold electrodes were compared. The CNT layer leads to a greatly improved anodic detection of enzymatically generated thiocholine product including lower oxidation overpotential (0.15 V) and higher sensitivity because of its electrocatalytic activity, fast electron transfer and large surface area. The sensor performance was optimized with respect to the operating conditions. Under the optimal batch conditions, a detection limit of 5 ×10 -6 mol/L was obtained with good precision (RSD = 5.2%, n=10). Furthermore, the attractive response of thiocholine on a CNT/GC electrode has allowed it to be used for constant-potential flow injection analysis. The detection limit was greatly improved to 0.3 ×10-6 mol/L. The high sensitivity electrochemical detection of enzymatically generated thiocholine with a CNT sensing platform holds great promise to prepare an acetylcholinesterase biosensor for monitoring organophosphate pesticides and nerve agents.
2005. "Enzyme Nanoparticles-Based Electronic Biosensor." Chemical Communications 2005(27):3481-3483. doi:10.1039/b504943a Abstract A novel method for fabricating electronic biosensors based on coupling enzyme nanoparticles and self assembly technology is illustrated. Redox horseradish peroxidase nanoparticles were prepared by desolvation with ethanol and subsequent crosslinking with glutaraldehyde. The cross-linked enzyme nanoparticles were functionalized by cysteine to introduce thiol groups on the nanoparticle surface. Immobilized enzyme nanoparticle on the gold electrode by self-assembly kept redox and electrocatalytic activities, and was used to develop reagentless biosensors for H2O2 detection without promoters and mediators. The new approach is simple, low cost and circumvents complications associated with solution systems. It is a universal immobilization method for biosensor, biomedical devices, biofuel cells and enzymatic bioreactors fabrication and expected to open new opportunities for biosensor, clinical diagnostics, and for bioanalysis, in general.
2005. "Electrochemical Stripping Analysis of Organophosphate Pesticides and Nerve Agents." Electrochemistry Communications 7(4):339-343. Abstract A sensitive electrochemical stripping voltammetric method for analyzing organophosphate (OP) compounds was developed using a carbon paste electrochemical (CPE) transducer. OPs strongly adsorb on a CPE surface and provide facile electrochemical quantitative methods for electroactive OP compounds. Operational parameters have been optimized, and the stripping voltammetric performance has been studied using square wave voltammetry. The adsorptive stripping voltammetric response is highly linear over the 1-60 mol/L methyl parathion range examined (2-min adsorption), with a detection limit of 0.05 u mol/L (10 min accumulation) and good precision (RSD=3.2%, n =10). These findings can lead to a widespread use of electrochemical sensors to detect OP contaminates.
2005. "Electrochemical Sensor for Organophosphate Pesticides and Nerve Agents Using Zirconia Nanoparticles as Selective Sorbents ." Analytical Chemistry 77(18):5894-5901. Abstract Electrochemical sensor for detection of organophosphate (OP) pesticides and nerve agents using zirconia (ZrO2) nanoparticles as selective sorbents is presented. Zirconia nanoparticles were electrodynamically deposited onto the polycrystalline gold electrode by cyclic voltammetry. Because of a strong affinity of zirconia to the phosphoric group, nitroaromatic OPs strongly bind to the ZrO2 nanoparticle surface. The electrochemical characterization and anodic stripping voltammetric performance of bound OPs were evaluated using cyclic voltammetric and square-wave voltammetric (SWV) analysis. SWV was used to monitor the amount of bound OPs and provide simple, fast, and facile quantitative methods for nitroaromatic OP compounds. The sensor surface can be regenerated by successively running SWV scanning. Operational parameters, including the amount of nanoparticles, adsorption time, and the pH of the reaction medium have been optimized. The stripping voltammetric response is highly linear over the 5–200ng/mL (ppb) methyl parathion range examined (2-min adsorption), with a detection limit of 1 ng/mL (10 min accumulation), and good precision (RSD=5.3 %, n = 10). The promising stripping voltammetric performances open new opportunities for fast, simple, and sensitive analyzing of OPs in environmental and biological samples. These findings can lead to a widespread use of electrochemical sensors to detect OP contaminates.
2005. "A Renewable Electrochemical Magnetic Immunosensor Based on Gold Nanoparticle Labels." Journal of Nanoscience and Nanotechnology 5(7):1060-1065. Abstract A particle-based renewable electrochemical magnetic immunosensor was developed by using magnetic beads and a gold nanoparticle label. Anti-IgG antibody-modified magnetic beads were attached to a renewable carbon paste transducer surface by magnets that were fixed inside the sensor. A gold nanoparticle label was capsulated to the surface of magnetic beads by sandwich immunoassay. Highly sensitive electrochemical stripping analysis offers a simple and fast method to quantify the capatured gold nanoparticle tracer and avoid the use of an enzyme label and substrate. The stripping signal of gold nanoparticle is related to the concentration of target IgG in the sample solution. A transmission electron microscopy image shows that the gold nanoparticles were successfully capsulated to the surface of magnetic beads through sandwich immunoreaction events. The parameters of immunoassay, including the loading of magnetic beads, the amount of gold nanoparticle conjugate, and the immunoreaction time, were optimized. The detection limit of 0.02 μg ml-1of IgG was obtained under optimum experimental conditions. Such particle-based electrochemical magnetic immunosensors could be readily used for simultaneous parallel detection of multiple proteins by using multiple inorganic metal nanoparticle tracers and are expected to open new opportunities for disease diagnostics and biosecurity.
2005. "PtRu/Carbon Nanotube Nanocomposite Synthesized in Supercritical Fluid: A Novel Electrocatalyst for Direct Methanol Fuel Cell." Langmuir 21(24):11474-11479. Abstract This manuscript describes a novel approach for synthesis of Pt-Ru/CNTs nanocomposites in supercritical fluid and demonstrated that Pt-Ru/CNTs nanocomposites synthesized in supercritical fluid are effective electrocatalysts for direct methanol fuel cells.
2005. "Platinum/Carbon Nanotube Nanocomposite Synthesized in Supercritical Fluid as Electrocatalysts for Low-Temperature Fuel Cells." Journal of Physical Chemistry B 109(30):14410-14415. Abstract Carbon nanotube (CNT)-supported Pt nanoparticles catalysts have been synthesized in supercritical carbon dioxide (scCO2) using platinum (II) acetylacetonate as metal precursor. The structure of the catalysts has been characterized with transmission electron micrograph (TEM) and X-ray photoelectron spectroscopy (XPS). TEM images show that platinum particles size is in the range of 5-10nm. XPS analysis indicates the presence of zero-valence platinum. The Pt-CNT exhibited high catalytic activity both for methanol oxidation and oxygen reduction reaction. The higher catalytic activity has been attributed to the large surface area of carbon nanotubes and the decrease in the overpotential for methanol oxidation and oxygen reduction reaction. Cyclic voltammetric measurements at different scan rates showed that the oxygen reduction reaction at the Pt-CNT electrode is a diffusion-controlled process. Analysis of the electrode kinetics using Tafel plot suggests that Pt-CNT from scCO2 provides a strong electrocatalytic activity for oxygen reduction reaction. For the methanol oxidation reaction, a high ratio of forward anodic peak current to reverse anodic peak current was observed at room temperature, which implies good oxidation of methanol to carbon dioxide on the Pt-CNT electrode. This work demonstrates that Pt-CNT nanocomposites synthesized in supercritical carbon dioxide are effective electrocatalysts for low-temperature fuel cells.
2005. "Novel Hybrid Materials with High Stability for Electrically Switched Ion Exchange: Carbon Nanotubes/Polyaniline/Nickel Hexacyanoferrate Nanocomposites." Chemical Communications (17):2226-2228. doi:10.1039/b500417a Abstract A novel and stable carbon nanotubes /polyaniline /nickel hexacyanoferrates composite film has been synthesized with electrodeposition method, and the possibility for removing cesium through an electrically switched ion exchange has been evaluated in a mixture containing NaNO3 and CsNO3.
2005. "Low–potential Amperometric Determination of Hydrogen Peroxide with a Carbon Paste Electrode Modified with Nanostructured Cryptomelane-type Manganese Oxides." Electrochemistry Communications 7(2):166-172. Abstract Nanostructured cryptomelane-type manganese oxides were synthesized, characterized, and evaluated for chemical sensing. Cryptomelane -type manganese oxides are nanofibrous crystals with sub-nanometer open tunnels that provide a unique property for sensing applications. Carbon paste electrodes (CPEs), modified with the nanostructured cryptomelane-type manganese oxides, were investigated for amperometric detection of hydrogen peroxide. With an operating potential of +0.3 V versus Ag/AgCl, H2O2 produces catalytic oxidation currents at the modified CPE, which can be exploited for quantitative determinations. The amperometric signals are linearly proportional to H2O2 concentration in the range 1.0×10-4 ~ 6.9×10-4 M with a correlation coefficient of 0.995 (n = 7). At a signal-to-noise ratio of 3, a detection limit of 2 M can be observed for the carbon paste electrode modified with 5.5 wt% cryptomelane-type manganese oxides. In addition, the sensor has a good stability and reproducibility. The construction and renewal are simple and inexpensive. A possible response mechanism was proposed and discussed. The significant electrocatalytic activity of the modified CPE may result from the nanostructure of cryptomelane-type manganese oxides.
2005. "Incorporation of Hydroxypyridinone (HOPO) Ligands into Self-Assembled Monolayers on Mesoporous Supports for Selective Actinide Sequestration." Environmental Science and Technology 39(5):1332-1337. doi:10.1021/es049169t Abstract In this study, three isomers of hydroxypyridinones, 1,2-HOPO, 3,2-HOPO, and 3,4-HOPO, were attached to self-assembled monolayers on mesoporous silica. (SAMMS). The HOPO-SAMMS materials have superior solid adsorbents properties: they do not suffer from solvent swelling, their rigid, open pore structure allow rapid sorption kinetics, their extremely high surface area enables the installation of high functional density, and being silica-based they are compatible with vitrification into a final glasseous waste form. Kinetics, equilibrium, and selectivity of the adsorptions of actinide on the HOPO-SAMMS at various pHs, and in the presences of other metal cations, anions and competing ligands are reported. Rapid sequestration of U (VI), Np (V) and Pu (IV) was observed. Very little competition from transition metal cations and common species was observed.
2005. "Electrocatalytic Reactivity for Oxygen Reduction of Palladium-Modified Carbon Nanotubes Synthesized in Supercritical Fluid." Electrochemistry Communications 7(3):267-274. Abstract The electrocatalytic reactivity of palladium-modified carbon nanotubes (Pd-CNTs) for the oxygen reduction reaction (ORR) was investigated at the glassy carbon electrode surface in 1 M H2SO4 saturated by oxygen. Carbon nanotubes modified by palladium nanoparticles were synthesized in supercritical carbon dioxide and characterized by transmission electron micrograph. The electrocatalytic activity of the CNTs film and Pd–CNTs film toward oxygen reduction was studied using cyclic voltammetry and linear sweep voltammetry methods. The molecular oxygen reduction at the Pd-CNTs electrode started at a more positive potential than that at the CNTs electrode. A possible reaction mechanism was proposed in which the ORR may proceed through two-step two-electron processes for the Pd-CNTs modified electrode. Experimental results revealed that Pd-CNTs possess a remarkable activity and high stability for oxygen reduction in acid medium, which implies the potential applications of the Pd–CNTs for constructing electrodes of fuel cells.
2005. "Catalytic Adsorptive Stripping Determination of Trace Chromium (VI) at Bismuth Film Electrodes." Talanta 65(1):144-148. Abstract A sensitive adsorptive stripping voltammetric protocol at a bismuth film electrode for trace measurements of chromium (VI) in the presence of diethylenetriammine pentaacetic acid (DTPA) is described. The new protocol is based on accumulation of the Cr-DTPA complex at a preplated bismuth film electrode held at –0.80V, followed a negatively-going square-wave voltammetric waveform. Factors influencing the stripping performance including the film preparation, solution pH, DTPA concentration, deposition potential and deposition time, have been optimized. The resulting performance compares well with that observed for analogous measurements at mercury film electrodes. A preconcentration time of 2 min results in a detection limit of 0.3 nM Cr(VI) and a relative standard deviation at 20 nM of 5.1% (n=25). Applicability to river water samples is demonstrated. The attractive behavior of the new “mercury-free” chromium sensor holds great promise for on-site environmental and industrial monitoring of chromium (VI).
2005. "Carbon Nanotubes (CNTs) for the Development of Electrochemical Biosensors ." Frontiers in Bioscience 10(1):492-505. Abstract Carbon nanotube (CNT) is a very attractive material for the development of biosensors because of its capability to provide strong electrocatalytic activity and minimize surface fouling of the sensors. This article reviews our recent developments of oxidase- and dehydrogenase-amperometric biosensors based on the immobilization of CNTs, the co-immobilization of enzymes on the CNTs/Nafion or the CNT/Teflon composite materials, or the attachment of enzymes on the controlled-density aligned CNT-nanoelectrode arrays. The excellent electrocatalytic activities of the CNTs on the redox reactions of hydrogen peroxide, nicotinamide adenine dinucleotide (NADH), and homocysteine have been demonstrated. Successful applications of the CNT-based biosensors reviewed herein include the low-potential detections of glucose, organophosphorus compounds, and alcohol.
2005. "Adsorptive Stripping Voltammetric Measurements of Trace Uranium at the Bismuth Film Electrode." Analytica Chimica Acta 535(1-2):9-13. Abstract Bismuth-coated carbon-fiber electrodes have been successfully applied for adsorptive-stripping voltammetric measurements of trace uranium in the presence of cupferron. The new protocol is based on the accumulation of the uranium-cupferron complex at a preplated bismuth film electrode held at –0.30 V (vs. Ag/AgCl), followed by a negatively-sweeping square-wave voltammetric waveform. Factors influencing the stripping performance, including the film preparation, solution pH, cupferron concentration, adsorption potential and time have been optimized. The resulting performance compares well with that observed for analogous measurements at mercury film electrodes. A detection limit of 0.3 g/L is obtained in connection to a 10 min adsorption time. The response is linear up to 50 g/L and the relative standard deviation at 50 g/L uranium is 3.8% (n=10; 2 min adsorption). Potential interferences are examined. Applicability to sea water samples is demonstrated. The attractive behavior of the new “mercury-free” uranium sensor holds great promise for on-site environmental and industrial monitoring of uranium.
2005. "Actinide Sequestration Using Self-Assembled Monolayers on Mesoporous Supports." Environmental Science and Technology 39(5):1324-1331. Abstract Surfactant templated synthesis of mesoporous ceramics provides a versatile foundation upon which to create high efficiency environmental sorbents. These nanoporous ceramic oxides condense a huge amount of surface area into a very small volume. The ceramic oxide interface is receptive to surface functionalization through molecular self-assembly. The marriage of mesoporous ceramics with self-assembled monolayer chemistry creates a powerful new class of environmental sorbent materials called self-assembled monolayers on mesoporous supports (SAMMS). These SAMMS materials are highly efficient sorbents, whose interfacial chemistry can be fine-tuned to selectively sequester a specific target species, such as heavy metals, tetrahedral oxometallate anions and radionuclides. Details addressing the design, synthesis and characterization of SAMMS materials specifically designed to sequester actinides, of central importance to the environmental clean-up necessary after 40 years of weapons grade plutonium production, as well as evaluation of their binding affinities and kinetics are presented.
2005. "Determination of Organophosphate Pesticides at a Carbon Nanotube/Organophosphorus Hydrolase Electrochemical Biosensor." Analytica Chimica Acta 530(2):185-189. Abstract An amperometric biosensor for oganophosphorus (OP) pesticides based on a carbon-nanotube (CNT) modified transducer and an organophosphorus hydrolase (OPH) biocatalyst is described. A bilayer approach with the OPH layer atop of the CNT film was used for preparing the CNT/OPH biosensor. The CNT layer leads to a greatly improved anodic detection of the enzymatically-generated p-nitrophenol product, including higher sensitivity and stability. The sensor performance was optimized with respect to the surface modification and operating conditions. Under the optimal conditions the biosensor was used to measure as low as 0.15 M paraoxon and 0.8 M methyl parathion with sensitivities of 25 and 6 nA/µM, respectively.
2005. "Electrochemical Sensor Based on Carbon Paste Electrode Modified with Nanostructured Crypotomelane-Type Manganese Oxides for Detection of Heavy Metals." Sensor Letters 3(1):16-21. doi:10.1166/sl.2005.004 Abstract A carbon paste electrode modified with nanostructured crypotomelane type manganese oxides was evaluated as new electrochemical sensor for the detection of heavy metal ions in aqueous media. The crypotomelane type manganese oxides are nanofibrous crystals with sub-nanometer tunnels which provide excellent sites for ion-exchanges. The adsorptive stripping voltammetry (ASV) technique involves preconcentration of the metal ions into nanostructured crypotomelane type manganese oxides under an open circuit, then electrolysis of the preconcentrated species, followed by a square-wave potential sweep towards positive values. Factors affecting the preconcentration process were investigated using lead ion as the model analyte. The voltammetric responses increased with the preconcentration time from 2 to 30 min, and also linearly with lead ion concentrations ranging from 50 to 1200 ppb. The detection limits of target metal ion were 10 ppb after 4 min preconcentration and improved to 1 ppb after 20 min preconcentration. The potential for simultaneous detection of copper, silver and lead is also discussed.
2005. "Biosensors Based on Carbon Nanotubes/Nickel Hexacyanoferrate/Glucose Oxidase Nanocomposites." Journal of Biomedical Nanotechnology 1(3):320-327. Abstract Novel hybrid films based on carbon nanotubes (CNTs)/nickel hexacyanoferrate (NiHCF) nanocomposites were synthesized, characterized, and evaluated for chemical and bio-sensing properties. Nickel hexacyanoferrate particles were electrodeposited on the porous CNT thin-film to fabricate electrochemical sensors with improved sensitivity toward hydrogen peroxide. Transmission electron microscopy illustrated the deposition of nickel hexacyanoferrate nanoparticles on the surface of carbon nanotubes. The experimental results show the electrode modified with the hybrid nanocomposite film has higher electrocatalytic activity and stability for detection of hydrogen peroxide than the electrodes modified with carbon nanotube or nickel hexacyanoferrate alone. With glucose oxidase (GOx) as an enzyme model, we constructed a biosensor based on the CNTs/NiHCF/GOx nanocomposite. Excellent linear relationship up to 1.2 mM has been attained with a slope of 5.3 μA/mM for the glucose biosensor. The response time and detection limit (S/N = 3) of the biosensor was determined to be 10 s and 1 μM, respectively. The high sensitivity to glucose of the biosensor resulted from the high surface area of carbon nanotubes and excellent electrocatalytic activity of the modifiers. The biosensor also performed with excellent reproducibility and good stability.
2005. "Amperometric Biosensors Based on Carbon Paste Electrodes Modified with Nanostructured Mixed-valence Manganese Oxides and Glucose Oxidase." Nanomedicine 1(2):130-135. Abstract Nanostructured multivalent manganese oxides octahedral molecular sieve (OMS), including cryptomelane-type manganese oxides and todorokite-type manganese oxides, were synthesized and evaluated for chemical sensing and biosensing at low operating potential. Both cryptomelane-type manganese oxides and todorokite-type manganese oxides are nanofibrous crystals with sub-nanometer open tunnels that provide a unique property for sensing applications. The electrochemical and electrocatalytic performance of OMS for the oxidation of H2O2 have been compared. Both cryptomelane-type manganese oxides and todorokite-type manganese oxides can be used to fabricate sensitive H2O2 sensors. Amperometric glucose biosensors are constructed by bulk modification of carbon paste electrodes (CPEs) with glucose oxidase as a biocomponent and nanostructured OMS as a mediator. A Nafion thin film was applied as an immobilization/encapsulation and protective layer. The biosensors were evaluated as an amperometric glucose detector at phosphate buffer solution with a pH 7.4 at an operating potential of 0.3 V (vs. Ag/AgCl). The biosensor is characterized by a well-reproducible amperometric response, linear signal-to-glucose concentration range up to 3.5 mM and 1.75 mM, and detection limits (S/N = 3) of 0.1 mM and 0.05 mM for todorokite-type manganese oxide and cryptomelane-type manganese oxide modified electrodes, respectively. The biosensors based on OMS exhibit considerable good reproducibility and stability, and the construction and renewal are simple and inexpensive.
2004. "Nanostructured Materials Synthesized In Supercritical Fluid." In Dekker Encyclopedia of Nanoscience and Nanotechnology, ed. J. A. Schwarz, C. Contescu, K. Putyera, pp. 2595-2607. Marcel Dekker, New York. Abstract This review article summarized recent work on supercritical fluid synthesis of nanomaterials from PNNL and other research groups.
2004. "Electrochemical Sensors Based on Functionalized Nanoporous Silica." In Dekker Encyclopedia of Nanoscience and Nanotechnology, ed. J. A. Schwarz, C. Contescu, K. Putyera, pp. 1051-1061. Marcel Dekker, New York. Abstract This invited review article summarizes recent work on the development of electrochemical sensors based on nanoporous silica.
2004. "Biosensors Based on Carbon Nanotubes." In Dekker Encyclopedia of Nanoscience and Nanotechnology, ed. J. A. Schwarz, C. Contescu, K. Putyera, pp. 361-374. Marcel Dekker, New York. Abstract This invited review article summarizes recent work on biosensor development based on carbon nanotubes
2004. "Environmental and Sensing Applications of Molecular Self-Assembly ." In Dekker Encyclopedia of Nanoscience and Nanotechnology, ed. J. A. Schwarz, C. Contescu, K. Putyera, pp. 1135-1145. Marcel Dekker, New York. Abstract In the last decade we have witnessed many exciting new discoveries in the ability to manipulate and measure matter at the nanometer scale. Honeycombed pores structures, spheres, icosahedra, nanotubes and nanorods, self-assembled structural hierarchies; the esthetics of the nanometer regime offers Nature’s elegance in its purest form. Understanding the driving forces behind these shapes and the self-assembly processes provides key understanding for this chemistry to be exploited for positive impact on our daily lives. For this to take place, we must not only understand how the nanoscopic structures impact the structural and chemical properties of these novel new materials, but we must also understand the critical problems that we face today and how these nanoscopic properties can be tailored to address these specific needs and critical problems.
2004. "Supercritical Fluid Synthesis and Characterization of Catalytic Metal Nanoparticles on Carbon Nanotubes." Journal of Materials Chemistry 14(5):908-913. Abstract A rapid, convenient and environmentally benign method has been developed for the fabrication of metal nanoparticlemultiwall carbon nanotube (MWCNT) composites. Nanoparticles of palladium, rhodium and ruthenium are deposited onto functionalized MWCNTs through a simple hydrogen reduction of metal--diketone precursors in supercritical carbon dioxide, and are characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses. These highly dispersed nanoparticles, with a narrow range of size distribution and good adhesion on MWCNT surfaces, are expected to exhibit promising catalytic properties for a variety of chemical reactions. Preliminary experiments demonstrate that Pd nanoparticles supported on MWCNTs are effective catalysts for hydrogenation of olefins in carbon dioxide. The Pd nanoparticleMWCNT composite also shows a high electrocatalytic activity in oxygen reduction for potential fuel cell application.
2004. "Modification of SiO2 Nanowires with Metallic Nanocrystals from Supercritical CO2." Journal of Nanoscience and Nanotechnology 4(1/2):82-85. Abstract Through hydrogen reduction of metal precursors in supercritical CO2, Cu and Pd nanocrystals were deposited onto SiO2 nanowires (NWs) to form different types of nanostructured materials, including nanocrystal-NW, spherical aggregation-NW, shell-NW composites and “mesoporous” metals supported by the framework of NWs. This supercritical fluid deposition technique represents an attractive approach for modifying NWs because of its generality and simplicity and the modified NWs could be potentially useful as catalysts and for further fabrication of multifunctional composites.
2004. "Simultaneous Detection of Cadmium, Copper, and Lead using A Carbon Paste Electrode Modified with Carbamoylphosphonic Acid Self-Assembled Monolayer on Mesoporous Silica (SAMMS)." Analytica Chimica Acta 502(2):207-212. Abstract A new sensor was developed for simultaneous detection of cadmium (Cd2+), copper (Cu2+), and lead (Pb2+), based on the voltammetric response at a carbon paste electrode modified with carbamoylphosphonic acid (acetamide phosphonic acid) self-assembled monolayer on mesoporous silica (Ac-Phos SAMMS). The adsorptive stripping voltammetry technique involves preconcentration of the metal ions onto Ac-Phos SAMMS under an open circuit, then electrolysis of the preconcentrated species, followed by a square wave potential sweep towards positive values. Factors affecting the preconcentration process were investigated. The voltammetric responses increased linearly with the preconcentration time from 1 to 30 minutes or with metal ion concentrations ranging from 10 to 200 ppb. The responses also evolved in the same fashion as adsorption isotherm in the pH range of 2-6. The metal detection limits were 10 ppb after 2 minutes preconcentration and improved to 0.5 ppb after 20 minutes preconcentration.
2004. "Selective Removal of Copper(II) from Aqueous Solutions Using Fine-Grained Activated Carbon Functionalized with Amine ." Industrial and Engineering Chemistry Research 43(11):2759-2764. Abstract This paper describes the synthesis and chracterization of functionalized activated carbon and its application for removal copper from aqueous solution.
2004. "Electrophilic Aromatic Substitutions of Amine and Sulfonate onto Fine-Grained Activated Carbon for Aqueous-Phase Metal Ion Removal ." Separation Science and Technology 39(14):3263-3279. Abstract This paper describes the synthesis and characterization of functionalized activated carbon. Evaluation of the activated carbon materials for metal removal from aqueous solution is also discussed.
2004. "Carbon Paste Electrode Modified with Carbamoylphosphonic Acid Functionalized Mesoporous Silica: A New Mercury-Free Sensor for Uranium Detection." Electroanalysis 16(10):870-873. Abstract This study reports a new approach for developing a uranium (U(VI)) electrochemical sensor that is mercury-free, solid-state, and has less chance for ligand depletion than existing sensors. A carbon-paste electrode modified with carbamoylphosphonic acid self-assembled monolayer on mesoporous silica was developed for uranium detection based on an adsorptive square-wave stripping votammetry technique. Voltammetric responses for U(VI) detection are reported as a function of pH, preconcentration time, and aqueous phase U(VI) concentration. The uranium detection limit is 25 ppb after 5 minutes preconcentration and improved to 1 ppb after 20 minutes preconcentration. The relative standard deviations are normally less than 5%.
2004. "Noninvasive Biomonitoring Approaches to Determine Dosimetry and Risk Following Acute Chemical Exposure: Analysis of Lead or Organophosphate Insecticide in Saliva ." Journal of Toxicology and Environmental Health Part A 67:635-650. Abstract There is a need to develop approaches for assessing risk associated with acute exposures to a broad-range of chemical agents and to rapidly determine the potential implications to human health. Non-invasive biomonitoring approaches are being developed using reliable portable analytical systems to quantitate dosimetry utilizing readily obtainable body fluids, such as saliva. Saliva has been used to evaluate a broad range of biomarkers, drugs, and environmental contaminants including heavy metals and pesticides. To advance the application of non-invasive biomonitoring a microfluidic/ electrochemical device has also been developed for the analysis of lead (Pb), using square wave anodic stripping voltametry. The system demonstrates a linear response over a broad concentration range (1 – 2000 ppb) and is capable of quantitating saliva Pb in rats orally administered acute doses of Pb-acetate. Appropriate pharmacokinetic analyses have been used to quantitate systemic dosimetry based on determination of saliva Pb concentrations. In addition, saliva has recently been used to quantitate dosimetry following exposure to the organophosphate insecticide chlorpyrifos in a rodent model system by measuring the major metabolite, trichloropyridinol, and saliva cholinesterase inhibition following acute exposures. These results suggest that technology developed for non-invasive biomonitoring can provide a sensitive, and portable analytical tool capable of assessing exposure and risk in real-time. By coupling these non-invasive technologies with pharmacokinetic modeling it is feasible to rapidly quantitate acute exposure to a broad range of chemical agents. In summary, it is envisioned that once fully developed, these monitoring and modeling approaches will be useful for accessing acute exposure and health risk.
2004. "Spectroscopic Characterizations of Molecularly-Linked Gold Nanoparticle Assemblies Upon Thermal Treatment." Langmuir 20(10):4254-4260. Abstract Nanostructured gold catalysts have attracted increasing interest since the pioneer work of Haruta which demonstrated unusually high catalytic activities for CO oxidation when the nanoparticles were made less than ~10 nm diameter in size and supported on oxides. It is possible to use core-shell gold nanoparticles to construct the catalyst and activate the catalytic activity by applying a thermal-treatment process. The understanding of how the core-shell surface components reconstitute in such a catalytic activation process will have important implications in expanding this approach to the design and fabrication of nanostructured catalysts. We report herein the results of a study of decanethiolate-capped gold nanoparticles of 2-nm and 5-nm core sizes assembled on planar substrates using 1,9-nonanedithiol (NDT) and 11-mercaptoundecanoic acid (MUA) as molecular linkers were studied as a model system. XPS and IR are employed to detect the identity of surface species and to analyze the elemental composition or oxidation states of the nanomaterials, from which we derive structural information about the surface reconstitution of the core-shell nanostructured catalysts. Electrochemical methods are used to characterize the electrocatalytic activities.
2004. "Glucose Biosensors Based on Carbon Nanotube Nanoelectrode Ensembles." Nano Letters 4(2):191-195. Abstract This paper describes the development of glucose biosensors based on carbon nanotube (CNT) nanoelectrode ensembles (NEEs) for the selective detection of glucose. Glucose oxidase was covalently immobilized on CNT NEEs via carbodiimide chemistry by forming amide linkages between their amine residues and carboxylic acid groups on the CNT tips. The catalytic reduction of hydrogen peroxide liberated from the enzymatic reaction of glucose oxidase upon the glucose and oxygen on CNT NEEs leads to the selective detection of glucose. The biosensor effectively performs selective electrochemical analysis of glucose in the presence of common interferents (e.g. acetaminophen, uric and ascorbic acids), avoiding the generation of an overlapping signal from such interferents. Such an operation eliminates the need for permselective membrane barriers or artificial electron mediators, thus greatly simplifying the sensor design and fabrication.
2004. "Disposable Carbon Nanotube Modified Screen-Printed Biosensor for Amperometric Detection of Organophosphorus Pesticides and Nerve Agents." Electroanalysis 16(1-2):145-149. Abstract A disposable biosensor based on carbon nanotube was develped for detection of pesticides and nerve agents.
2004. "Nanoparticle-Structured Ligand Framework as Electrode Interfaces." Electroanalysis 16(1-2):120-126. Abstract Nanostructured electrode based on gold nanoparticle network was developed for selective detection of metal ions.
2004. "Composition-Controlled Synthesis of Bimetallic Gold-Silver Nanoparticles." Langmuir 20(25):11240-11246. Abstract This paper reports finding of an investigation of the synthesis of monolayer-capped binary gold-silver bimetallic nanoparticles that is aimed at understanding the control factors governing the formation of the bimetallic compositions. The findings have important implications to the exploration of gold-based bimetallic nanoparticles for biosensing and fuell cell catalytic nanomaterials.
2004. "A Direct Route towards Assembly of Nanoparticle-Carbon Nanotube Composite Materials ." Langmuir 20(14):6019-6025. Abstract The exploration of nanoparticle-structured thin films as sensing materials desires maximum accessibility of analytes and effective mass transport within the nanostructure. This paper explores the viability of creating nanoparticle-carbon nanotube (CNTs) as composite interfacial materials to enhance such properties. We report findings of an investigation of the assembly of monolayer-protected gold nanoparticles on multi-walled CNTs. A simple and effective route has been demonstrated for assembling nanoparticles of 2-5 nm core sizes onto CNTs with controllable coverage and interparticle spatial properties. The composite nanomaterials can be dispersed in organic solvent and cast on interdigitated microelectrode surface. The skeleton-like nanocomposite materials have been examined for chemiresistor sensing of volatile organic compounds. The response profiles and sensitivities of the nanocomposites determined for the sorption of a series of vapors have been shown to exhibit different or enhanced sensing properties in comparison with similar but nanotube-free nanoparticle assemblies. The observation of these results can be attributed to a combination of three factors, the increased accessibility of analytes to the nanostructure, the enhanced mass transport characteristics, and the unique electronic properties of the nanocomposite materials. Implications of the findings to the design of nanostructured sensing materials are also discussed.
2004. "Lanthanide Selective Sorbents: Self-Assembled Monolayers on Mesoporous Supports (SAMMS)." Journal of Materials Chemistry 14(22):3356-3363. Abstract Through the marriage of mesoporous ceramics with self-assembled monolayer chemistry, the genesis of a powerful new class of environmental sorbent materials has been realized. By coating the mesoporous ceramic backbone with a monolayer terminated with a lanthanide-specific ligand, it is possible to couple high lanthanide binding affinity with the high loading capacity (resulting from the extremely high surface area of the support). This lanthanide-specific ligand field is created by pairing a “hard” anionic Lewis base with a suitable synergistic ligand, in a favorable chelating geometry. Details of the synthesis, characterization, lanthanide binding studies, binding kinetics, competition experiments and sorbent regeneration studies are summarized
2004. "Lanthanide Selective Sorbents: Self-Assembled Monolayers on Mesoporous Supports (SAMMS) ." Journal of Materials Chemistry 14(22):3356-3363. Abstract Through the marriage of mesoporous ceramics with self-assembled monolayer chemistry, the genesis of a powerful new class of environmental sorbent materials has been realized. By coating the mesoporous ceramic backbone with a monolayer terminated with a lanthanide-specific ligand, it is possible to couple high lanthanide binding affinity with the high loading capacity (resulting from the extremely high surface area of the support). This lanthanide-specific ligand field is created by pairing a “hard” anionic Lewis base with a suitable synergistic ligand, in a favorable chelating geometry. Details of the synthesis, characterization, lanthanide binding studies, binding kinetics, competition experiments and sorbent regeneration studies are summarized.
2003. "Synthesis of Nanostructured Sorbent Materials Using Supercritical Fluids." Chapter 24 in Supercritical Carbon Dioxide-Separations and Processes, ACS Symposium Series, vol. 860, ed. A. S. Gopalan, C. M. Wai, and H. K. Jacobs, pp. 370-386. Oxford University Press, Washington DC. Abstract There exists a significant need today for high capacity, high efficiency sorbent materials to selectively sequester toxic metal species from groundwater or wastestreams. We have an on-going effort at PNNL to design, synthesize, characterize and evaluate functionalized nanoporous materials for environmental remediation, sensing/detection and device applications.
2003. "Supercritical Fluid Extraction of Actinides and Heavy Metals for Environmental Cleanup: A Process Development Perspective." Chapter 3 in Supercritical Carbon Dioxide: Separations and Processes, ACS Symposium Series, vol. 860, ed. A. S. Gopalan, C. M. Wai, and H. K. Jacobs, pp. 23-35. American Chemical Society, Washington DC. Abstract The extraction of heavy metal ions and actinide ions is demonstrated using supercritical carbon dioxide (CO2) containing dissolved protonated ligands, such as -diketones and organophosphinic acids. High efficiency extraction is observed. The mechanism of the extraction reaction is discussed and, in particular, the effect of addition of water to the sample matrix is highlighted. In-process dissociation of metal-ligand complexes for ligand regeneration and recycle is also discussed. A general concept for a process using this technology is outlined.
2003. "Nanoelectrode Arrays Based on Low Site Density Aligned Carbon Nanotubes." Nano Letters 3(1):107-109. Abstract Nanoelectrode arrays (NEAs) were fabricated from the low site density aligned carbon nanotubes (CNTs). The CNTs were grown by plasma enhanced chemical vapor deposition (PECVD) on Ni nanoparticles made by the electrochemical deposition.
2003. "Supercritical Fluid Fabrication of Metal Nanowires and Nanorods Templated by Multi-walled Carbon Nanotubes." Advanced Materials 15(4):316-319. Abstract A rapid, direct and clean approach has been developed to synthesize metal (Pd, Ni and Cu) nanowires and nanorods using multi-walled carbon nanotubes (MWCNTs) as templates and supercritical CO2 as the reaction medium. Filling of metals into MWCNTs to form nanowires or nanorods was easily achieved by the simple hydrogen reduction of metal-b-diketone complexes in supercritical CO2.
2003. "Immersion Deposition of Metal Films on Silicon and Germanium Substrates in Supercritical Carbon Dioxide." Chemistry of Materials 15(1):83-91. Abstract A low temperature carbon dioxide based on immersion deposition technology (SFID) has been developed for producing palladium, copper, silver, and other metal films on silicon-based substrates in supercritical CO2. The reaction is initiated by oxidation of elemental silicon to SiF4 or H2SiF6 by HF with the release of electrons that cause the reduction of metal ions in an organometallic precursor to the metallic form on silicon surface in CO2. Only the substrate surfaces are coated with metals using this method. Based on surface analysis of the films and spectroscopic analysis of the reaction products, the mechanism of metal film deposition is discussed. The metal films (Pd, Cu, and Ag) formed on silicon surfaces by the SFID method exhibit good coverage, smooth and dense texture, high purity and a metallic behavior. Similarly, metal films can also be deposited onto geranium substrates using SFID. The gas-like properties and the high pressure of the supercritical fluids, combined with the low reaction temperature, make this SFID method potentially useful for depositing thin metal films in small features, which are difficult to accomplish by conventional CVD methods.
2003. "Decorating catalytic palladium nanoparticles on carbon nanotubes in supercritical carbon dioxide." Chemical Communications (5):642-643. Abstract Hydrogen reduction of a Pd(II)-b-diketone precursor in supercritical carbon dioxide produces palladium nanoparticles on multi-walled carbon nanotubes that exhibit promising catalytic properties for hydrogenation of olefins in carbon dioxide as well as electro-reduction of oxygen in fuel cell applications.
2003. "Voltammetric detection of lead (II) and mercury (II) using a carbon paste electrode modified with thiol self-assembled monolayer on mesoporous silica (SAMMS)." Analyst 128(5):467-472. Abstract The anodic stripping voltammetry at a carbon paste electrode modified with thiol terminated self-assembled monolayer on mesoporous silica (SH-SAMMS) provides a new sensor for simultaneous detection of lead (Pb2+) and mercury (Hg2+) in aqueous solutions. The overall analysis involved a two-step procedure: an accumulation step at open circuit, followed by medium exchange to a pure electrolyte solution for the stripping analysis. Factors affecting the performance of the SH-SAMMS modified electrodes were investigated, including electrode activation and regeneration, electrode composition, preconcentration time, electrolysis time, and composition of electrolysis and stripping media. The most sensitive and reliable electrode contained 20% SH-SAMMS and 80% carbon paste. The optimal operating conditions were a sequence with a 2-5 minute preconcentration period, then a 60-second electrolysis period of the preconcentrated species in 0.2 M nitric acid, followed by square wave anodic stripping voltammetry from –1.0 V to 0.6 V in 0.2 M nitric acid. The areas of the peak responses were linear with respect to metal ion concentrations in the ranges of 10-1500 ppb Pb2+ and 20-1600 ppb Hg2+. The detection limits for Pb2+ and Hg2+ were 0.5 ppb Pb2+ and 3 ppb Hg2+ after a 20-minute preconcentration period.
2003. "Removal of Heavy Metals from Aqueous Solution Using Novel Nanoengineered Sorbents: Self-Assembled Carbamoylphosphonic Acids on Mesoporous Silica ." Separation Science and Technology 38(15):3809-3825. Abstract Self-assembled monolayers of carbamoylphosphonic acids (acetamide phosphonic acid and propionamide phosphonic acid) on mesoporous silica supports were studied as potential absorbents for heavy and transition metal ions in aqueous wastes. The adsorption capacity, selectivity, and kinetics of the materials in sequestering metal ions, including Cd2+, Co2+, Cu2+, Cr3+, Pb2+, Ni2+, Zn2+, and Mn2+, were measured in batch experiments with excess sodium ion. The solution pH ranged from 2.2 to 5.5. The kinetics study shows that the adsorption reached equilibrium in seconds, indicating that there is little resistance to mass transfer, intraparticle diffusion, and surface chemical reaction. The competitive adsorption study found the phosphonic acid-SAMMS to have an affinity for divalent metal ions in decreasing order of Pb2+ > Cu2+ > Mn2+ > Cd2+ > Zn2+ > Co2+ > Ni2+. The measured Cd2+ adsorption isotherm was of the Langmuirian type and had a saturation binding capacity of 0.32 mmol/g.
2003. "Nanoengineered Electrochemical Sensor Based on Mesoporous Silica Thin-film Functionalized with Thiol-Terminated Monolayer." Analyst 128(7):899-904. Abstract Abstract A mesoporous silica thin film was synthesized via a surfactant-templating process, where a silica sol gel is spin-coated onto the surface of a gold electrode. Upon removal of the surfactant, a film with 3D cubic pore structure and having a primary pore size of 77 Å was obtained. Thiol (-SH) functional groups were subsequently immobilized onto the mesoporous silica thin film through a self-assembly process. The thiol-immobilized mesoporous silica (SH-SAMMS) thin film was used as the electrode sensing layer for detection of lead(II) in aqueous solutions by employing a square wave adsorptive stripping voltammetry technique. The electrode modified with thin-film SH-SAMMS utilized the binding affinity of lead(II) to the thiol ligands to perform the preconcentration step under an open circuit, with no additional electrolytes, and at the pH in which the silica thin film was stable (pH 5-6). The voltammetric responses for lead(II) increased linearly with the preconcentration time from 1 to 30 minutes. The responses also increased linearly with lead(II) concentrations ranging from 250 to 5000 ppb after 5 minutes preconcentration and from 25-100 ppb after 30 minutes preconcentration.
2003. "SUPERCRITICAL FLUID EXTRACTION OF TOXIC HEAVY METALS FROM SOLID AND AQUEOUS MATRICES." Separation Science and Technology 38(10):2279-2289. Abstract The feasibility of using dithiocarbamate chelating agents or sulfur-containing organophosphorus reagents for the supercritical fluid extraction (SFE) of toxic heavy metals from solid and aqueous matrices is investigated. Effective extraction of heavy metal ions from both sand matrix and water samples was demonstrated by using supercritical CO2 containing dithiocarbamatechelating agents. A commercially available sulfur-containing organophosphorus reagent, Cyanex 302, was used for the extraction of toxic heavy metals from wood samples. The extraction profiles were initially rapid followed by a very low level of metal extraction, indicating that the process is limited to extraction of leachable toxic metals.
2003. "Solubilization of Carbon Nanotubes by Nafion Toward the Preparation of Amperometric Biosensors." Journal of the American Chemical Society 125(9):2408-2409. Abstract We report on the ability of the perfluorosulfonated polymer Nafion to solubilize single-wall and multi-wall CNT and on the dramatically enhanced redox activity of hydrogen peroxide at CNT/Nafion-coated electrodes in connection to the preparation of oxidase-based amperometric biosensors.
2003. "X-Ray Photoelectron Spectroscopic Study of the Activation of Molecularly-Linked Gold Nanoparticle Catalysts." Langmuir 19(1):125-131. Abstract This paper reports the results of a study of the activation of core-shell assembled gold nanoparticle catalysts using X-ray photoelectron spectroscopy (XPS). The goal is to determine the surface reconstitution of the nanostructured catalysts upon electrochemical activation for the electrocatalytic oxidation of methanol. The decanethiolate-capped gold nanoparticles of 2~5 nm core sizes were assembled as catalyst thin films on electrode surfaces using 1,9-nonanedithiol and 11-mercaptoundecanoic acid as molecular linkers. The XPS results have provided two important insights into the surface reconstitution of the activated nanostructure. First, the capping/linking thiolates or dithiolates are partially removed to produce the catalytic access, with the degree of removal being dependent on the nature of the molecular linker. Second, oxygenated species are detected on the activated gold nanocrystals, demonstrating the formation of surface gold oxide and its participation in the electrocatalytic oxidation of methanol. The findings are also correlated with results from studies of surface microscopic morphology and interfacial mass flux, and provide further insights into issues related to the design and preparation of highly active nanostructured gold catalysts.
2003. "Templateless Assembly of Molecularly Aligned Conductive Polymers Nanowires: A New Approach for Oriented Nanostructures." Chemistry - a European Journal 9(3):604-611. Abstract Although oriented carbon nanotubes, oriented nanowires of metals, semidcondcutors and oxides have attracted wide attention, there have been few reports on oriented polymer nanostructures such as nanowires. In this paper we report the assembly of large arrays of oriented nanowires containing molecularly aligned conducting polymers (polyaniline) without using of a porous membrane template to support the polymer. The uniform oriented nanowires were prepared through controlled nucleation and growth during a stepwise electrochemical deposition process in which a large number of nuclei were first deposited on the substrate using a large current density. After the initial nucleation, the current density was reduced step by step to grow the oriented nanowires from the nucleation sites created in the first step. The usefulness of these new polymer structures is demonstrated with a chemical sensor device for H2O2, the detection of which is widely investigated for glucose sensors. Finally, we point out that controlled nucleation and growth is a universal phenomenon and has potential for growing oriented nanostructures of a wide range of materials.
2003. "Supercritical Fluid Extraction of Toxic Heavy Metals and Uranium from Acidic Solutions with Sulfur-Containing Organophosphorus Reagents." Industrial and Engineering Chemistry Research 42(7):1400-1405. Abstract The feasibility of using sulfur-containing organophosphorus reagents for the chelation-supercritical fluid extraction (SFE) of toxic heavy metals and uranium from acidic media was investigated. The SFE experiments were conducted in a specially-designed flow-through liquid extractor. Effective extraction of the metal ions from various acidic media was demonstrated. The effect of ligand concentration in supercritical CO2 on the kinetics of metal extraction was studied. A simplified model is used to describe the extraction kinetics and the good agreement of experimental data with the equilibrium-based model is achieved.
2002. "Laser-Machined Microdevices for Mass Spectrometry." Chapter 6 in Mass Spectrometry and Hyphenated Techniques in Neuropeptide Research, ed. J. Silberring and R. Ekman, pp. 171-192. John Wiley & Sons, Wiley Interscience, NY. Abstract In this book chapter we summarize the most recent work conducted in our lab on the development of laser-machined plastic microfluidic devices for rapid and automatic sample cleanup.
2002. "Single-Channel Microchip for Fast Screening and Detailed Identification of Nitroaromatic Explosives and Organophosphate Nerve Agents." Analytical Chemistry 74:1187-1191. Abstract A single-channel chip-based analytical microsystem which allows rapid flow-injection measurements of the total content of organic-explosive or nerve-agent compounds, as well as detailed micellar chromatographic identification of the individual ones is described. The protocol involves repetitive rapid flow-injection (screening) assays - for providing a timely warning and alarm - and switching to the separation (fingerprint identificaiton) mode only when harmful compounds are detected. While micellar electrokinetic chromatography (MEKC), in the presence of sodium dodecyl sulfate (SDS), is used for separating the neutral nitroaromatic-explosive and nerve-agent compounds, an operation without SDS leads to high-speed measurements of the 'total' explosives or nerve-agent content. Switching between the 'flow-injection' and 'separation' modes is accomplished by rapidly exchanging the SDS-free and SDS-containing buffers in the separation channel. Amperometric detection was used for monitoring the separation. Key factors influencing the sample throughput, resolution, adn sensitivity have been assessed and optimized. Assays rates of ca. 360 and 30 per hour can thus be realized for the 'total' screening and 'individual' measurements, respectively. Ultimately, such development will lead to the creation of a field-deployable microanalyzer, and will enable transporting the forensic laboratory to the sample source.
2002. "Low-Potential Stable NADH Detection at Carbon-Nanotube-Modified Glassy Carbon Electrodes." Electrochemistry Communications 4:743-746. Abstract Carbon-nanotube (CNT) modified glassy-carbon electrodes exhibiting strong and stable electrocatalytic response toward NADH are described. A substantial (490 mV) decrease in the overvoltage of the NADH oxidation reaction (compared to ordinary carbon electrodes) is observed using single-wall and multi-wall carbon-nanotube coatings, with oxidation starting at ca. ?0.05V (vs. Ag/AgCl; pH 7.4). Furthermore, the NADH amperometric response of the coated electrodes is extremely stable, with 96 and 90% of the initial activity remaining after 60min stirring of 2x10-4M and 5x10-3M NADH solutions, respectively (compared to 20 and 14% at the bare surface). The CNT-coated electrodes thus allow highly-sensitive, low-potential, stable amperometric sensing. Such ability of carbon-nanotubes to promote the NADH electron-transfer reaction suggests great promise for dehydrogenase-based amperometric biosensors.
2002. "Direct Assembly of Large Arrays of Oriented Conducting Polymer Nanowires." Angewandte Chemie International Edition 41(19):3665-3668. Abstract Although oriented carbon nanotubes, oriented nanowires of metals, semiconductors and oxides have attracted wide attention, there have been few reports on oriented polymer nanostructures such as nanowires. In this paper we report the assembly of large arrays of oriented nanowires through controlled nucleation and growth during a stepwise electrochemical deposition process in which a large number of nuclei were first deposited on the substrate using a large current density. After the initial nucleation, the current density was reduced step by step to grow the oriented nanowires from the nucleation sites created in the first step. A very different morphology was also demonstrated by first depositing a monolayer of close-packed colloidal spheres using a similar step-wise deposition process. As a result, the polymer nanofibers grew from the spheres in a radial fashion and formed the continuous three-dimensional network of nanofibers in the film. The principles of control nucleation and growth in electrochemical deposition investigated in this paper should be applicable to other electrical conducting and electrochemical active materials, including metals and conducting oxides. We also hope the oriented electroactive polymer nanostructure will open the door for new applications, such as miniaturized biosensors.
2002. "Synthesis of Carbamoylphosphonate Silanes for the Selective Sequestration of Actinides." Chemical Communications 13:1374-1375. Abstract The synthesis of carbamoylphosphonate silanes (CMPO analogs) designed for sequestering actinide cations in self-assembled monolayers on mesoporous supports (SAMMS) is described.
2001. "Development of an Integrated Micro-Analytical System for Lead in Saliva and Linkage to a Physiologically Based Pharmacokinetic Model Describing Lead Saliva Secretion ." American Industrial Hygiene Association Journal 62(3):295-302. Abstract There is a need to develop reliable portable analytical instruments for real-time monitoring of trace metals, such as lead (Pb) utilizing readily available non-invasive fluids like saliva. To interpret saliva results, an understanding of the pharmacokinetics of Pb secretion into the saliva is needed. A portable microfluidics/electrochemical device was developed for the rapid analysis of Pb based on square wave anodic stripping voltammetry, where a saliva sample flows over an electrode surface, Pb2+ is chemically reduced, accumulated, and the electric potential of the electrode scanned. To evaluate the relationship between saliva and blood Pb, rats were treated with single oral doses ranging from 20 to 500 mg Pb/kg of body weight, and 24 hours later salivation was induced by administering pilocarpine, a muscarinic agonist. Blood and saliva were collected and analyzed for Pb by inductively coupled plasma-mass spectrometry (ICP-MS) and by the micro-analytical system. The micro-analytical system was slightly less responsive (~75-85%) than ICP-MS, however the response was linear over a concentration range of 1-2000 ppb suggesting that it can be utilized for the quantitation of salivary Pb. To relate saliva levels to internal dose of Pb (e.g. blood) and to total body burden, a physiologically based pharmacokinetic (PBPK) model for Pb was modified to incorporate a salivary gland compartment. The model was capable of predicting blood and saliva Pb concentration based on a limited data set. These preliminary results are encouraging and suggest that a fully developed, micro-analytical system can be utilized as an important tool for real-time biomonitoring of Pb for both occupational and environmental exposures.
2001. "Generation of Multiple Electrosprays Using Microfabricated Emitter Arrays for Improved Mass Spectrometric Sensitivity." Analytical Chemistry 73(8):1658-1663. Abstract Arrays of micro-electrospray emitters were fabricated on polycarbonate substrates using a laser etching technique. Stable multi-electrosprays were successfully generated in the liquid flow rate range relevant to mass spectrometric applications. Comparison of electrosprays generated from the micro-fabricated emitter array and conventional fused silica capillaries showed similar spray characteristics and reliability. Higher total electrospray ion currents were observed as the number of electrosprays increased at a given total liquid flow rate. Consistent with the theoretical prediction, the total spray current at a constant total liquid flow rate was shown experimentally to be approximately proportional to the square root of the number of electrosprays. It is further projected that when total flow rate is optimized the maximum achievable total current will be proportioned to the number of emitters. Evaluation of the multi-electrospray device using a triple quadrople mass spectrometer showed a factor of 2 to 3 sensitivity enhancement for the spray numbers ranging from two to nine compared to a conventional single electrospray ionization source under the same operating conditions.
2001. "Studies on In-situ Chelation/Supercritical Fluid Extraction of Lanthanides and Actinides Using a Radiotracer Technique." Separation Science and Technology 36(5 & 6):1149-1162. Abstract Radioisotope tracer techniques were used to study the process of in-situ chelation/supercritical fluid extraction(SFE) of La3+ and Lu3+ from solid matrix using mixed ligand hexafluoroacetylacetone (HFA) and tributylphosphate (TBP) as chelating agents. A lab-built SFE extactor was used in this study and the extractor design was optimized based on the experimental results. Quantitative recovery of La and Lu was achieved when the extrator design was optimized. Extraction of uranium from real world samples was also investigated to demonstrate the capability of this chelation/SFE technology for environmental remediation applications. A novel on-line back extraction technique for the recovery of metal ions and regeneration of ligands is also reported.
2001. "Selective Sorption of Cesium Using Self-Assembled Monolayers on Mesoporous Supports ." Environmental Science and Technology 35(19):3962-3966. Abstract The synthesis of novel nano-composite sorbent material, copper ferrocyanide immobilized within a mesoporous ceramic matrix, and its use as a novel cesium sorbent material is reported in this paper. Complete removal of cesium was achieved in the presence of competing metal ions for solutions containing 2 ppm cesium under a variety of conditions.
2001. "Integrated Microfluidics/Electrochemical Sensor System for Monitoring of Environmental Exposures to Lead and Chlorophenols." Biomedical Microdevices 3(4):331-338. Abstract A microanalytical system based on a microfluidics/electrochemical detection scheme was developed. The microfluidic platform was fabricated based on a multi-layer lamination method. Fluidic microchannels were produced by sandwiching laser-machined adhesive-backed polyimide gaskets between layers of the device. Individual components, such as microfabricated piezoelectrically actuated pumps and a microelectrochemical cell were designed and fabricated into plug-in modules which can be readily plugged into (or plugged from) the microfluidic platform. This allowed rapid change-out and repair of individual components by incorporating "plug and play" concepts now standard in PC's. The detection of lead and chlorophenols were performed with the microanalytical system to demonstrate the capabilities of this new technology for on-site environmental characterization and for real-time no-invasive biomonitoring of toxic chemical mixtures.
2001. "Application of Supercritical Fluids to the Reactive Extraction and Analysis of Toxic Heavy Metals from Environmental Matrices - System Optimisation." Separation Science and Technology 36(5-6):1197-1210. Abstract The extraction of Cu2+, Pb2+, Cd2+ adn Zn2+ utilising supercritical fluid carbon dioxide containing dissolved organophosphorus reagents in shown to be feasible. Using the solubility parameter concept and investigation of a variety of physical parameters favourable extraction conditions of 60 degrees Celcius and 400 atm pressure were determined. Cyanex 302 was found to be the most favourable ligand in terms of stability and ability to complex a range of metal ions. A soil containing substantial amounts of Pb2+ and Cd2+ was studied using SFE and the techniques as found to reduce the levels of leachable metal ions to near US EPA regulatory levels.
2000. "Electrochemical Sensors for the Detection and Quantification of Lead and Other Toxic Heavy Metals: The Next Generation of Personal Exposure Biomonitors." Environmental Health Perspectives 115(12):1683-1690. Abstract To support the development and implementation of biological monitoring programs, accurate and quantitative technologies for measuring xenobiotic exposure are needed. Micro-analytical based sensors that work with complex biomatrices such as blood, urine or saliva are being developed and validated. These sensor platforms will improve our ability to make definitive associations between chemical exposures and disease. Among toxic heavy metals, lead (Pb) continues to be one of the most problematic. Despite a considerable effort to identify and eliminate Pb exposure sources, this metal still remains a significant health concern, particularly for young children. Ongoing research is focused on the development and validation of portable metal analyzers that have many advantages over current available technologies, thus having the potential to become the next-generation of toxic metal analyzers. This review will highlight the development and validation of two classes of metal analyzers for the voltammetric detection of Pb, this includes: (1) a metal analyzer based on flow injection analysis and anodic stripping voltammetry (ASV) at a Hg-film electrode, and (2) mercury-free metal analyzers employing adsorptive stripping voltammetry (AdSV) and novel nanostructure materials which include the self-assembled monolayers on mesoporous supports (SAMMS) and carbon nanotubes (CNTs). These sensors have been optimized to detect Pb in urine, blood, and saliva as accurately as the state-of-the-art-ICP-MS with high reproducibility, and sensitivity, while being much more portable, field-deployable and less expensive than conventional analytical methods. It is anticipated that these improved and portable analytical sensor platforms will facilitate our ability to conduct a meaningful biological monitoring program that will enable us to have a greater understanding of the relationship between chemical exposure assessment and disease outcomes.
2000. "Microfabricated Isoelectric Focusing Devices for Direct Electrospray Ionization-Mass Spectrometry." Electrophoresis 21(1):191-197. Abstract A novel microfabricated device for isoelectric focusing (IEF) incorporating an optimized electrospray ionization (ESI) tip was contractued on polycarbonate plates using laser micromachining. Direct microchannel IEF-ESI-MS was demonstrated using the microfabricated chip with an ion-trap mass spectrometer for characterization of protein mixtures.
2000. "Separation of Divalent Transition Metal Beta-Diketonates and Their Adducts By Supercritical Fluid Chromatography ." Talanta 52(4):695-701. Abstract A method for sparation and detection of divalent transition metal beta-diketonates by adduct formation/supercritical fluid chromatography (SFC) with an open-tubular capillary column and a FID detector is described. The crystal structures of Cu-hexaflouroacetylacetone (HFA) and Cu bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (THD) complexes have been determined by X-ray crystallography. The SFC behavior of Cu beta-diketonates shows a strong correlation with the structure of the complexes. The hydrated copper beta-diketonate complexes usually exhibit strong intermolecular interactions or decomposition in SFC. Formation of adducts with a neutral donor, such as tributyl phosphine oxide (TBPO), can greatly improve the SFC behavior and detection sensibility of Cu(II) and Mn(II) beta-diketonates. The stoichiometry and thermal stability of the adducts Cu(II) and Mn(II) beta-diketonates with TBPO in supercritical CO2 have also been investigated. The implications of utilizing adduct formation for supercritical fluid extraction(SFE) of divalent transition metals and for on-line coupled SFE/SFC analysis of divalent transition metals are discussed.
1999. "An Integrated Microfabricated Device for Dual Microdialysis and On-line ESI Ion Trap Mass Spectrometry for the Analysis of Complex Biological Samples." Analytical Chemistry 71(8):1485-1490. Abstract A microfabricated dual-microdialysis device in a single intergrated micorfabricated platform was contsructed using laser micromachining techniques for the rapid fractionation and cleanup of complex biological samples. Results suggest the potential for integration of such microfabricated devices with other sample manipulations for the rapid ESI-MS analysis of complex biological samples.
1999. "Laminated Plastic Microfluidic Components for Biological and Chemical Systems." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 17(4):2264-2269. Abstract Describes fabrication processes developed for laminated plastic microfluidic components and the fabrication of a chromium metals sensor and a microdialysis device.