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We have probed single surface states and the involved interfacial charge transfer coupling on the TiO(2) surface using confocal as well as tip-enhanced near-field topographic-spectroscopic imaging analysis on a niobium-doped rutile TiO(2)(110) surface. The confocal images excited with a radially polarized donut mode render ring-shaped excitation patterns typical for quantum systems with two perpendicular transition dipole moments. The tip-enhanced near-field optical images of single surface states are visualized by the strong exciton plasmon-polariton coupling localized at the subdomain boundaries with a spatial resolution of ∼15 nm (far beyond the optical diffraction limit). We suggest that the abundant surface states in the doped TiO(2) generate excitons under laser excitation which are strongly coupled to the surface plasmon-polaritons of the Au tip. Moreover, the interfacial electronic molecule-substrate coupling has been characterized by probing the molecule-perturbed surface states distribution and the associated specific Raman vibrational modes. The imaging and characterization of the surface states and their distributions on TiO(2) surfaces at nanoscale are critically relevant to a deep understanding of interfacial electron transfer dynamics and energetics involving in solar energy conversion, photocatalysis, and mechanistic understanding of surface-enhanced Raman scattering spectroscopy.

We have probed single-molecule redox reaction dynamics of hemin (chloride) adsorbed on Ag nanoparticle surfaces by single-molecule surface-enhanced Raman spectroscopy (SMSERS) combined with spectroelectrochemistry. Redox reaction at the molecule/Ag interface is identified and probed by the prominent fluctuations of the Raman frequency of a specific vibrational mode, ν(4), which is a typical marker of the redox state of the iron center in a hemin molecule. On the basis of the autocorrelation and cross-correlation analysis of the single-molecule Raman spectral trajectories and the control measurements of single-molecule spectroelectochemistry and electrochemical STM, we suggest that the single-molecule redox reaction dynamics at the hemin-Ag interface is primarily driven by thermal fluctuations. The spontaneous fluctuation dynamics of the single-molecule redox reaction is measured under no external electric potential across the molecule-metal interfaces, which provides a novel and unique approach to characterize the interfacial electron transfer at the molecule-metal interfaces. Our demonstrated approaches are powerful for obtaining molecular coupling and dynamics involved in interfacial electron transfer processes. The new information obtained is critical for a further understanding, design, and manipulation of the charge transfer processes at the molecule-metal interface or metal-molecule-metal junctions, which are fundamental elements in single-molecule electronics, catalysis, and solar energy conversion.

Fundamental understanding of the energetic coupling properties of a molecule-semiconductor interface is of great importance. The changes in molecular conformations and vibrational modes can have significant impact on the interfacial charge transfer reactions. Here, we have probed the change in the interface properties of alizarin-TiO2 system as a result of the externally applied electric field using single-hot spot microscopic surface-enhanced Raman spectroscopy (SMSERS) and provided a theoretical understanding of our experimental results by density functional theory (DFT) calculations. The perturbation, caused by the external potential, has been observed as a shift and splitting of the 648 cm(-1) peak, typical indicator of the strong coupling between alizarin and TiO2, at SMSERS. On the basis of our experimental results and DFT calculations, we suggest that electric field has significant effects on vibrational coupling at the molecule-TiO2 interface. The presence of perturbed alizarin-TiO2 coupling under interfacial electric potential may lead to changes in the interfacial electron transfer dynamics. Additionally, heterogeneously distributed dye molecules at the interface on nanometer length scale and different chromophore-semiconductor binding interactions under charge accumulation associated interfacial electric field changes create intrinsically inhomogeneous interfacial ET dynamics associated with both static and dynamic disorders.

We investigate single-cell reduction of toxic Cr(VI) by the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1 (MR-1), an important bioremediation process, using Raman spectroscopy and scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX). Our experiments indicate that the toxic and highly soluble Cr(VI) can be efficiently reduced to the less toxic and non-soluble Cr2O3 nanoparticles by MR-1. Cr2O3 is observed to emerge as nanoparticles adsorbed on the cell surface and its chemical nature is identified by EDX imaging and Raman spectroscopy. Co-localization of Cr2O3 and cytochromes by EDX imaging and Raman spectroscopy suggests a terminal reductase role for MR-1 surface-exposed cytochromes MtrC and OmcA. Our experiments revealed that the cooperation of surface proteins OmcA and MtrC makes the reduction reaction most efficient, and the sequence of the reducing reactivity of the MR-1 is: wild type > single mutant ΔmtrC or mutant ΔomcA > double mutant (ΔomcA-ΔmtrC). Moreover, our results also suggest that the direct microbial Cr(VI) reduction and Fe(II) (hematite)-mediated Cr(VI) reduction mechanisms may co-exist in the reduction processes.