2009. "Self-assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-ion Insertion ." ACS Nano 3(4):907-914. Abstract We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in-situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in presence of a percolated graphene network embedded into the metal oxide electrodes.
2009. "Clay Nanoparticle-Supported Single-Molecule Fluorescence Spectroelectrochemistry." Nano Letters 9(2):655-658. Abstract We report single-molecule fluorescence spectroelectrochemistry on a clay-modified ITO electrode using cresyl violet as a redox fluorescent probe. Ensemble averaged experiments show that cresyl violet displays well-defined cyclic voltammograms when adsorbed on the clay-modified electrode. By probing the fluorescence intensity of a single cresyl violet molecule absorbed on clay surface, we can trace the redox reaction of individual molecules induced by the cyclic voltammetric potential scanning. Inhomogeneous interfacial electron transfer dynamics of the immobilized single cresyl violet molecules on the clay-modified surface were observed.
2008. "High Throughput Operando Studies using Fourier Transform Infrared Imaging and Raman Spectroscopy." Review of Scientific Instruments 79:074101. doi:10.1063/1.2949389 Abstract A prototype operando high throughput (OHT) reactor designed and built for catalyst screening and characterization combines Fourier transform infrared (FT-IR) imaging and Raman spectroscopy in operando conditions. Using a focal plane array (FPA) detector (HgCdTe FPA, 128×128 pixels, and 1,610 Hz frame rate) for the FT-IR imaging system, the catalyst activity and selectivity of all parallel reaction channels can be simultaneously followed. Each image dataset possesses16,384 IR spectra with a spectral range of 800 to 4000 cm-1 with an 8 cm-1 resolution. Depending on the signal-to-noise ratio, 2 to 20 seconds are needed to generate a full image of all reaction channels from a dataset. Results on reactant conversion and product selectivity are obtained from FT-IR spectral analysis. Six novel Raman probes, one for each reaction channel, were specially designed and house built at Pacific Northwest National Laboratory (PNNL), to simultaneously collect Raman spectra of the catalysts and possible reaction intermediates on the catalyst surface under operando conditions. As a model system, methanol partial oxidation reaction on silica-supported molybdenum oxide (MoO3/SiO2) catalysts has been studied under different reaction conditions to demonstrate the performance of the OHT reactor.
2008. "Single-molecule Fluorescence Spectroelectrochemistry of Cresyl Violet." Chemical Communications (43):5490-5492. doi:10.1039/b812161c Abstract We coupled scanning fluorescence microscopy with a potentiostat via a three-electrode electrochemical cell to enable single-molecule fluorescence spectroelectrochemistry of cresyl violet in aqueous solution, where the single-molecule fluorescence intensity of cresyl violet is modulated synchronously with the cyclic voltammetric potential scanning.
2008. "Photoswitchable Nanoparticles Enable High-Resolution Cell Imaging: PULSAR Microscopy." Journal of the American Chemical Society 130(46):15279-15281. doi:10.1021/ja805948u Abstract Fluorescence imaging has transformed biological sciences and opened a window to reveal biological mechanisms in real time despite Abbe’s diffraction limit restricts current microscope resolution to 300 nm?.HDH2 Recently, two high-resolution fluorescence microscopic techniques emerged: one uses a special photoactivatable green fluorescent proteinHDH3 and the other employs a pair of cy3/cy5 dyes.HDH4 Both avoid Abbe’s diffraction limit by photoswitching nearby fluorophores off. Thus, photoswitching fluorescence between a bright and a dark state promises to deliver a wealth of information regarding biological phenomena at the nanoscale. The ideal probe is a key-enabling single molecule that can be photoswitched on and off. Such wonderful properties, albeit implausible to imagine at first, were realized in spiropyran derivatives. While being photoswitched, one molecule alternates red-fluorescence on-and-off. Using such photo-actuated unimolecular logical switching attained reconstruction (PULSAR) microscopy, we achieved high-resolution fluorescence imaging down to 80 nm? in nanostructures and cellular organelles.
2007. "Fluctuating Two-State Light Harvesting in a Photosynthetic Membrane." Journal of Physical Chemistry C 111(25):8948-8956. doi:10.1021/jp071493y Abstract How light is converted into chemical energy in a natural photosynthetic system is of great interest in energy sciences. Using single-molecule and single-vesicle fluorescence spectroscopy and imaging, we have observed fluctuating inter-molecular protein energy transfers in the photosynthetic membranes of R. sphaeroides. Our results suggest that there are dynamic coupled and non-coupled states in the light-harvesting protein assembly.
2007. "Submicron and Nanoscale Inorganic Particles Exploit the Actin Machinery to be Propelled Along Microvillilike Structures into Alveolar Cells." ACS Nano 1(5):463-475. doi:10.1021/nn700149r Abstract The growing commerce in micro- and nanotechnology is expected to increase our exposure to submicron and nanoscale particles. One of the main targets of this exposure are the cells that line the respiratory tract, among them are the alveolar type II epithelial cells that have microvilli at their exposed apical surface. Here we show a pathway by which positively charged inorganic submicron and nanoscale particles take advantage of the actin turnover machinery within filopodia and microvilli-like structures to guide and advance their way into these cells. Our observations bring a new view of how submicron and nanoscale inorganic matter can be assimilated into the cellular environment and take advantage of its machinery. While the pathway that we describe can be exploited for a targeted drug delivery, it also points to properties of submicron or nanoscale particles that should be avoided in order to reduce particle internalization and possible toxicity.
2006. "Tip-Enhanced Near-Field Raman Spectroscopy Probing Single Dye-Sensitized TiO2 Nanoparticles." Applied Physics Letters 88(9):Art. No. 093121. Abstract The correlated metallic tip-enhanced Raman spectroscopy and atomic force microscopy (AFM) technique was used to characterize an interfacial electron transfer system of dye-sensitized titanium oxide (TiO2) nanoparticles. We have obtained the near-field Raman spectra that are associated with the photo-induced charge transfer reaction in Ru(4, 4’-dicarboxy-2,2’-bipyridine)2(NCS)2- sensitized TiO2 single nanoparticles. This method demonstrates that tip-enhanced near-field Raman spectroscopy is an effective approach for probing in homogeneous interfacial electron transfers with nanoscale spatial resolution.
2006. "Revealing Two-State Protein-Protein Interaction of Calmodulin by Single-Molecule Spectroscopy." Journal of the American Chemical Society 128(31):10034-10042. Abstract We report a single-molecule fluorescence resonance energy transfer (FRET) and polarization study of conformational dynamics of calmodulin (CaM) interacting with a target peptide, C28W of 28 amino-acid oligomer. The C28W peptide represents the essential binding sequence domain of the Ca-ATPase protein interacting with CaM, which is important in cellular signaling for the regulation of energy in metabolism. However, the mechanism of the CaM-C28W recognition complex formation is still unclear. The amino-terminal (N-terminal) domain of the CaM was labeled with a fluorescein-based arsenical hairpin binder (F1AsH) that enables our unambiguously probing the CaM N-terminal target-binding domain motions at a millisecond timescale without convolution of the probe-dye random motions. By analyzing the distribution of FRET efficiency between F1AsH labeled CaM and Texas Red labeled C28W and the polarization fluctuation dynamics and distributions of the CaM N-terminal domain, we reveal slow (at sub-second time scale) binding-unbinding motions of the N-terminal domain of the CaM in CaM-C28W complexes, which is a strong evidence of a two-state binding interaction of CaM-mediated cell signaling.
2005. "Probing Inhomogeneous Vibrational Reorganization Energy Barriers of Interfacial Electron Transfer." Journal of Physical Chemistry B 109(34):16390-16395. Abstract We report an atomic force microscopy (AFM) and confocal Raman microscopy study on the interfacial electron transfer of a dye-sensitization system, alizarin adsorbed upon TiO2 nanoparticles. Resonance Raman and absorption spectral analyses revealed the distribution of the mode-specific vibrational reorganization energies encompassing different local sites (~250 nm spatial resolution), suggesting spatially inhomogeneous vibrational reorganization energy and different Franck-Condon coupling factors of the interfacial electron transfer. We found that the total vibrational reorganization energy was inhomogeneous from site to site, and specifically, the mode-specific analyses indicated that the energy distributions were inhomogeneous for bridging normal modes and homogeneous for nonbridging normal modes, especially for modes far away from the alizarin- TiO2 coupling hydroxyl modes. Our results demonstrate a significant step forward in characterizing site-specific inhomogeneous interfacial charge transfer dynamics.
2005. "Cholesterol Dictates the Freedom of EGF Receptors and HER2 in the Plane of the Membrane." Biophysical Journal 89(2):1362-1373. doi:10.1529/biophysj.104.056192 Abstract The flow of information through the EGF receptor (EGFR) is shaped by molecular interactions in the plasma membrane. The EGFR is associated with lipid rafts, but their role in modulating receptor mobility and subsequent interactions is unclear. To investigate the role of nanoscale rafts in EGFR dynamics, we used single-molecule fluorescence imaging to track individual receptors and their dimerization partner, HER2, in the membrane of human mammary epithelial cells. We found that the motion of both receptors was interrupted by dwellings within nanodomains. EGFR was significantly less mobile than HER2. This difference was likely due to F-actin because its deploymerization led to similar diffusion patterns between the EGFR and HER2. Manipulations of membrane cholesterol content dramatically altered the diffusion pattern of both receptors. Cholesterol depletion led to almost complete confinement of the receptors, whereas cholesterol enrichment extended the boundaries of the restricted areas. Interestingly, F-actin deploymerization partially restored receptor mobility in cholesterol depleted membranes. Our observations suggest that membrane cholesterol provides a dynamic environment that facilitates the free motion of EGFR and HER2, possible by modulating the dynamic state of F-actin. The association of the receptors with lipid rafts could therefore promote their rapid interactions only upon ligand stimulation.
2005. "Nernatic Solvation of Segmented Polymer Chains." Nano Letters 5(9):1757-1760. doi:10.1021/nl051108l Abstract We examine the effect of polymer chain segmentation on the recently discovered ability of nematic solvents to elongate and align polymer chain solutes. Coordinated single molecule spectroscopy and beads-on-a-chain simulations are used to study the orientational and conformational order of a series of segmented conjugated polymers, dissolved in the nematic liquid crystal, 5CB. The order parameters for alignment and elongation are both observed to decrease with increasing segmentation, reflecting an interplay among conformational entropy, solvation anisotropy, and bending energy of the polymer chain.
2005. "Single-Molecule Triplet-State Photon Antibunching at Room Temperature." Journal of Physical Chemistry B 109(20):9861-9864. Abstract We have demonstrated probing single-molecule metal-to-ligand charge transfer (MLCT) dynamics at room temperature. Using photon antibunching measurements under CW laser excitation, non-classical photon statistics, and excitation power dependent measurements, we were able to selectively measure the single-molecule MLTC state lifetime. This work demonstrated the first single-molecule photon antibunching of triplet excited state and presents a step forward in studying single-molecule electron transfer in proteins using MLTC complex as an electron transfer donor or acceptor.
2004. "Single-Molecule Study of Protein-Protein Interaction Dynamics in a Cell Signaling System." Journal of Physical Chemistry B 108(2):737-744. Abstract We report a combined single-molecule fluorescence and molecular dynamics (MD) simulation study of protein-protein interactions in a GTP-binding intracellular signaling protein Cdc42 in complex with a downstream effector protein WASP. A 13- kDa WASP fragment which binds only the activated GTP-loaded Cdc42 was labeled with a novel solvatochromic dye and used to probe hydrophobic interactions significant to Cdc42/WASP recognition. Our single-molecule fluorescence measurements have shown conformational fluctuations of the protein complex and suggested multiple conformational states at a wide range of time scales might be involved in protein interaction dynamics. Single-molecule experiments have revealed the dynamic disorder or protein-protein interactions within the Cdc42/WASP complex, which may be important for regulating downstream signaling events.
2004. "Probing Nanosecond Protein Motions of Calmodulin by Single-Molecule Fluorescence Anisotropy." Applied Physics Letters 85(12):2420-2422. Abstract Proteins operate as part of molecular networks that perform specific cellular functions. To understand the complex moleucular network, it is important to study individual processes within the network such as protein motions and protein-protein interactions.
2004. "Probing Nanosecond Protein Motions of Calmodulin by Single-Molecule Fluorescence Anisotropy." Applied Physics Letters 85(12):2420-2422. Abstract Proteins operate as part of molecular networks that perform specific cellular functions. To understand the complex molecular network, it is important to study individual processes within the network such as protein motions and protein-protein interactions.
2004. "Correlated Atomic Force Microscopy and Flourescence Lifetime Imaging of Live Bacterial Cells." Colloids and Surfaces. B, Biointerfaces 34:205-212. Abstract We report on the imaging of living bacterial cells by using a new correlated tapping-mode atomic force microscopy (AFM) and confocal al fluorescence lifetime imaging microscopy (FLIM). Different methods of preparing the bacterial sample were explored for optimal imaging of Gram-negative Shewanella oneidensis MR-1 cells on poly-1-lysine coated surfaces and agarose gel coated surfaces. We have found that the agarose gel containing 99% buffer can provide a local aqueous environment for single bacterial cells. Furthermore, the cell surface topography can be characterized by tapping-mode in-air AFM imaging for the single bacterial cells that are partially embedded. Using in-air rather than under-water AFM imaging of the living cells significantly enhanced the contrast and single-to-noise ration of the AFM images. Near-field AFM-tip enhanced fluorescence lifetime imaging (AFM-FLIM) holds great promise for obtaining fluorescence images beyond the optical diffraction limited spatial resolution. We have previously demonstrated near-field AFM-FLIM imaging of polymer beads beyond the diffraction limited spatial resolution. Here, as the first step of applying AFM-FLIM on imaging living bacterial cells, we demonstrate a correlated and consecutive AFM topographic imaging, fluorescence intensity imaging, and FLIM imaging to characterize cell polarity.
2004. "Placing Single-Molecule T4 Lysozyme Enzymes on a Bacterial Cell Surface: Toward Probing Single-Molecule Enzymatic Reaction in Living Cells." Biophysical Journal 87:656-661. Abstract TheT4 lysozyme enzymatic hydrolyzation reaction of bacterial cell walls is an important biological process, and single-molecule enzymatic reaction dynamics had been studied under physiological condition using purified E. Coli cell walls as substrates. Here, we report progress toward characterizing the T4 lysozyme enzymatic reaction on a living bacterial cell wall using a combined single-molecule placement and spectroscopy. Placing a dye-labeled single T4 lysozyme molecule on a targeted cell wall by using a hydrodynamic micro-injection approach, we monitored single-molecule rotational motions during binding, attachment to, and dissociation from the cell wall by tracing single-molecule fluorescence intensity time trajectories and polarization. The single-molecule attachment duration of the T4 lysozyme to the cell wall during enzymatic reactions was typically shorter than photobleaching time under physiological conditions.
2004. "Correlated Topographic and Spectroscopic Imaging by Combined Atomic Force Microscopy and Optical Microscopy." Journal of Luminescence 107(1-4):4-12. Abstract Near-field scanning microscopy is a powerful approach to obtain topographic and spectroscopic characterization simultaneously for imaging biological and nanoscale systems. To achieve optical imaging at high spatial resolution beyond the diffraction limit, aperture-less metallic scanning tips have been utilized to enhance the laser illumination local electromagnetic field at the apex of the scanning tips.
2004. "Intermittent Single-Molecule Interfacial Electron Transfer Dynamics." Journal of the American Chemical Society 126(30):9374-9381. Abstract We report on single molecule studies of photosensitized interfacial electron transfer (ET) processes in Coumarin 343 (C343)-TiO2 nanoparticle (NP) and Cresyl Violet (CV+)-TiO2 NP systems, using time-correlated single photon counting coupled with scanning confocal fluorescence microscopy. Fluorescence intensity trajectories of individual dye molecules adsorbed on a semiconductor NP surface showed fluorescence fluctuations and blinking, with time constrants distributed from sub-milliseconds to several seconds.
2003. "Single-Molecule Nanosecond Anisotropy Dynamics of Tethered Protein Motions." Journal of Physical Chemistry B 107(2):618-626. Abstract Confined and hindered protein motions are generally found in living cells, with tethered rotational motions of proteins or protein domains particularly associated with and relevant to the early events of molecular interactions in cell signaling at extra- and intracellular membrane surfaces. Ensemble-averaged time-resolved fluorescence anisotropy has been extensively applied to study the protein rotational and conformational motion dynamics under physiologically relevant conditions. However, the spatial and temporal inhomogeneities of the non-synchronizable stochastic protein rotational and conformational motions are extremely difficult for such ensemble-averaged measurements to characterize. Here, we report on a demonstration of the single-molecule nanosecond anisotropy and its application to studying the tethered protein motion of a T4 lysozyme on a biologically comparable surface under water. The rotational motions of the tethered proteins are confined in a half-sphere volume primarily defined by the linker and the surface. We have observed dynamic inhomogeneity of the rotational diffusion dynamics, i.e., diffusion rate fluctuation, due to interactions between the proteins and the surface. However, we also found that the long-time averages of the dynamically inhomogeneous diffusion rates of single molecules are essentially homogeneous amongst the single molecules examined.
2003. "Correlated Topographic and Spectroscopic Imaging Beyond Diffraction Limit by Atomic Force Microscopy Metallic Tip-Enhanced Near-Field Flourescence Lifetime Microscopy." Review of Scientific Instruments 74(7):3347-3355. Abstract A new approach is demonstrated for simultaneous topographic and spectroscopic imaging applying near-field optics (NSOM) with spatial resolution beyond the optical diffraction-limit. The method combines atomic force microscopy (AFM) in the metallic-tip tapping mode and near-field scanning confocal fluorescence lifetime imaging microscopy (FILM). The AFM metallic tip was formed by sputter-coating a Si tapping mode tip with Au to form a spherulitic shape at the tip apex, allowing a high local field enhancement under laser illumination, which was necessary for a strong optical signal. A simulation used to finite element method (FEM) to further evaluate the near-field enhancement originating from the metallic Au-coated AFM tapping mode tip. We have demonstrated that spatially mapping the change in fluorescence lifetime and intensity is a promising approach to semi-quantitative and chemically specific imaging at AFM spatial resolution. The spherulitic Au-coated AFM tip not only gives adequate spatial AMF tapping-mode imaging spatial resolution but is apparently “environmentally friendly” to soft samples, such as polymeric dye-labeled nano-spheres and even biological specimens like POPO-3 labeled DNA.
2003. "Probing Single-Molecule T4 Lysozyme Conformational Dynamics by Intramolecular Fluorescence Energy Transfer." Journal of Physical Chemistry B 107:7947-7956. Abstract We demonstrate the use of single-molecule spectroscopy to study enzyme conformational motions of T4 lysozyme under hydrolysis reaction of the polysaccharide walls of E. Coli B cells.By attaching a donoracceptor pair of dye molecules site-specifically to noninterfering sites on the enzyme, the hinge-bending motions of the enzyme are measured by monitoring the donor-acceptor emission intensity as a function of time. The overall enzymatic reaction rate constants are found to vary widely from molecule to molecule. The dominant contribution to this static inhomogeneity is attributed to enzyme searching for reactive sites on the substrate.