The study of heterogeneous catalytic reactions remains a major challenge because it involves a complex network of reaction steps with various intermediates. If the vibrational spectra of individual molecules could be monitored in real time, one could characterize the structures of the intermediates and the time scales of reaction steps without ensemble averaging. Surface-enhanced Raman scattering (SERS) spectroscopy does provide vibrational spectra with single-molecule sensitivity, but typical single-molecule SERS signals exhibit spatial heterogeneities and temporal fluctuations, making them difficult to be used in single-molecule kinetics studies. Here we show that SERS can monitor the single-molecule catalytic reactions in real time. The surface-immobilized reactants placed at the junctions of well-defined nanoparticle-thin film structures produce time-resolved SERS spectra with discrete, step-transitions of photoproducts. We interpret that such SERS-steps correspond to the reaction events of individual molecules occurring at the SERS hotspot. The analyses of the yield, dynamics, and the magnitude of such SERS steps, along with the associated spectral characteristics, fully support our claim. In addition, a model that is based on plasmonic field enhancement and surface photochemistry reproduces the key features of experimental observation. Overall, the result demonstrates that it is possible, under well-controlled conditions, to differentiate the chemical and physical processes contributing to the single-molecule SERS signals, and thus shows the use of single-molecule SERS as a tool for studying the metal-catalyzed organic reactions.

The existence of sub-nanometer plasmonic hot-spots and their relevance in spectroscopy and microscopy applications remain elusive despite a few recent theoretical and experimental evidence supporting this possibility. In this Letter, we present new spectroscopic evidence suggesting that Angstrom-sized hot-spots exist on the surfaces of plasmon-excited nanostructures. Surface-enhanced Raman scattering (SERS) spectra of 4,4'-biphenyl dithiols placed in metallic junctions show simultaneously blinking Stokes and anti-Stokes spectra, some of which exhibit only one prominent vibrational peak. The activated vibrational modes were found to vary widely between junction sites. Such site-specific, single-peak spectra could be successfully modeled using single-molecule SERS induced by a hot-spot with a diameter no larger than 3.5 Å, located at the specific molecular sites. Furthermore, the model, which assumes the stochastic creation of hot-spots on locally flat metallic surfaces, consistently reproduces the intensity distributions and occurrence statistics of the blinking SERS peaks, further confirming that the sources of the hot-spots are located on the metallic surfaces. This result not only provides compelling evidence for the existence of Angstrom-sized hot-spots but also opens up the new possibilities for the vibrational and electronic control of single-molecule photochemistry and real-space visualization of molecular vibration modes.

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Conspectus The initial observations of surface-enhanced Raman scattering (SERS) from individual molecules (single-molecule SERS, SMSERS) have triggered ever more detailed mechanistic studies on the SERS process. The studies not only reveal the existence of extremely enhanced and confined fields at the gaps of Ag or Au nanoparticles but also reveal that the spatial, spectral, and temporal behaviors of the SMSERS signal critically depend on many factors, including plasmon resonances of nanostructures, diffusion (lateral and orientational) of molecules, molecular electronic resonances, and metal–molecule charge transfers. SMSERS spectra, with their molecular vibrational fingerprints, should in principle provide molecule-specific information on individual molecules in a way that any other existing single-molecule detection method (such as the ones based on fluorescence, mechanical forces, or electrical currents) cannot. Therefore, by following the spectro-temporal evolution of SMSERS signals of reacting molecules, one should be able to follow chemical reaction events of individual molecules without any additional labels. Despite such potential, however, real applications of SMSERS for single-molecule chemistry and analytical chemistry are scarce. In this Account, we discuss whether and how we can use SMSERS to monitor single-molecule chemical kinetics. The central problem lies in the experimental challenges of separately characterizing and controlling various sources of fluctuations and spatial variations in such a way that we can extract only the chemically relevant information from time-varying SMSERS signals. This Account is organized as follows. First, we outline the standard theory of SMSERS, providing an essential guide for identifying sources of spatial heterogeneity and temporal fluctuations in SMSERS signals. Second, we show how single-molecule reaction events of surface-immobilized reactants manifest themselves in experimental SMSERS trajectories. Comparison of the reactive SMSERS data (magnitudes and frequencies of discrete transitions) and the predictions of SMSERS models also allow us to assess how faithfully the SMSERS models represent reality. Third, we show how SMSERS spectral features can be used to discover new reaction intermediates and to interrogate metal–molecule electronic interactions. Finally, we propose possible improvements in experimental design (including nanogap structures and molecular systems) to make SMSERS applicable to a broader range of chemical reactions occurring under ambient conditions. The specific examples discussed in this Account are centered around the single-molecule photochemistry of 4-nitrobenzenethiol on metals, but the conclusions drawn from each example are generally applicable to any reaction system involving small organic molecules.

Reduction of nitrobenzene is widely used for the assessment of the catalytic activities of nanoparticles, yet its mechanism is still largely unverified. Here, using the surface-enhanced Raman scattering (SERS), we have identified an intermediate of the first step in the photocatalytic reduction of nitrobenzenethiols (NBTs) on a metallic surface. The formation of the intermediate is identified by a fast red-shift of the NO symmetric-stretching peak of the SERS spectra of reacting NBTs, prior to the slow intensity decay. On the basis of the laser power dependences of the rates of spectral changes, electrochemical SERS, and quantum chemical calculations, we conclude that the intermediate is the anion radical of nitrobenzenethiol that is formed by the metal-to-molecule single-electron transfer reaction. The subsequent intensity decay of the peak, which is the rate-determining step of the whole reduction reaction, corresponds to another single-electron reduction of the anion radical into dihydroxyaminobenzenethiol or dianion of NBT.