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Around 20 years ago, the first reports of singlemolecule surface-enhanced Raman scattering (SM-SERS) caused a revolution in nanotechnology. Several researchers were quick to recognize the importance of a technique that can provide molecular vibrational fingerprinting at the SM level. Since then, a large amount of work has been devoted to the development of nanostructures capable of SM-SERS detection. A great effort has also been geared toward elucidating the different mechanisms that contribute to the effect. The understanding of the concept of plasmonic SERS hotspots, the role of chemical effects, and the dynamics of atomic and cluster rearrangements in nanometric domains has significantly advanced, driven by new computational and experimental methods used to study SM-SERS. In particular, SERS intensity fluctuations (SIFs) are now recognized as a hallmark of SM-SERS. Interpretation of SM-SERS data must take into consideration temporal and spatial variations as a natural consequence of the extreme localization inherent to surface plasmon resonances. Further analysis of variations in spectral signature, due to either molecular reorientation or photo (or thermal) processes, pointed to a new area that combines the power of SERS fingerprinting at the SM level to modern concepts of catalysis, such as hot-electrons-driven chemistry. This large body of work on the fundamental characteristics of the SM-SERS effect paved the way to the interpretation of other related phenomena, such as tip-enhanced Raman scattering (TERS). Despite all the fundamental progress, there are still very few examples of real applications of SM-SERS. In recent years, our research group has been studying SIFs, focused on different ways to use SM-SERS. The obvious application of SM-SERS is in analytical chemistry, particularly for quantification at ultralow concentrations (below 1 nM). However, quantification using SM-SERS faces a fundamental sampling problem: the analytes (adsorbed in very small amounts, i.e., low surface coverage) must find rare SERS hotspots (areas with intense electric field localization that yields SERS). This limitation leads to strong temporal and spatial variations in SERS intensities, which translates into very large error bars in an experimental calibration curve. We tackled this problem by introducing the concept of "digital SERS". This approach provided a roadmap for SERS quantification at ultralow concentrations and a potential pathway for a better understanding of the "reproducibility problem" associated with SERS.In this Account, we discuss not only the analytical applications but also other implementations of SM-SERS demonstrated by our group. These include the use of SM-SERS as a tool to probe colloidal aggregation, to evaluate the efficiency of SERS substrates, and to characterize the energy of localized resonances. SERS involves a series of random processes: hotspots are rare; surfaces/clusters constantly reconstruct; and molecules diffuse, adsorb, and desorb. All these pathways contribute to strong fluctuations ...

Time-dependent fluctuations in surface-enhanced Raman scattering (SERS) intensities were recorded from a roughened silver electrode immersed in diluted solutions of rhodamine 6G (R6G) and congo red (CR). These fluctuations were attributed to a small number of SERS-active molecules probing regions of extremely high electromagnetic field (hot spots) at the nanostructured surface. The time-dependent distribution of SERS intensities followed a tailed statistics at certain applied potentials, which has been linked to single-molecule dynamics. The shape of the distribution was reversibly tuned by the applied voltage. Mixtures of both dyes, R6G and CR, at low concentrations were also investigated. Since R6G is a cationic dye and CR is an anionic dye, the statistics of the SERS intensity distribution of either dye in a mixture were independently controlled by adjusting the applied potential. The potential-controlled distribution of SERS intensities was interpreted by considering the modulation of the surface coverage of the adsorbed dye by the interfacial electric field. This interpretation was supported by a two-dimensional Monte Carlo simulation that took into account the time evolution of the surface configuration of the adsorbed species and their probability to populate a hypothetical hot spot. The potential-controlled SERS dynamics reported here is a first step toward the spectroelectrochemical investigation of redox processes at the single-molecule level by SERS.

It is generally expected that aggregates of metal nanoparticles are more efficient surface-enhanced Raman scattering (SERS) probes than individual nanoparticles, due to the enhancement of the electric field in the interparticle gaps. We show that, for asymmetric nanoparticles, such as gold nanorods (NRs), this is not always the case: the plasmonic behavior of NRs depends on the mutual orientation of the NRs in the ensemble. We report the results of experimental studies and theoretical analysis of the optical properties of clusters of side-by-side assembled gold NRs. Ensembleaveraged SERS spectroscopy showed a reduction in SERS intensity. Comprehensive finite-difference time-domain simulations showed a reduction of electric field intensity as the number of NRs per cluster increased. This is due to destructive interference as the radial component of the surface plasmon modes of the NRs in the cluster interact with each other. The present work expands our understanding of the configuration-specific optical behavior of asymmetric gold nanoparticles. Furthermore, it offers guidance toward the "design rules" for the development of colloidal NR systems for sensing applications.

The anomalies in the anti-Stokes to Stokes intensity ratios in single-molecule surface-enhanced resonance Raman scattering were investigated. Brilliant green and crystal violet dyes were the molecular probes, and the experiments were carried out on an electrochemically activated Ag surface. The results allowed new insights into the origin of these anomalies and led to a new method to confirm the single-molecule regime in surface-enhanced Raman scattering. Moreover, a methodology to estimate the distribution of resonance energies that contributed to the imbalance in the anti-Stokes to Stokes intensity ratios at the electromagnetic hot spots was proposed. This method allowed the local plasmonic resonance energies on the metallic surface to be spatially mapped.

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The SERS enhancement factor (SERS-EF) is one of the most important parameters that characterizes the ability of a given substrate to enhance the Raman signal for SERS applications. The comparison of SERS intensities and SERS-EF values across different substrates is a common practice to unravel the performance of a given substrate. In this study, it is shown that such a comparison may lack significance if we compare substrates of very distinct nature and optical properties. It is specifically shown that the SERS-EF values for static substrates (e.g. immobilized metallic nanostructures) cannot be compared to those of dynamic ones (e.g. colloidal metal nanoparticle solutions), and that the optical properties for the latter show strong dependence on the metal-molecule interaction dynamics. The most representative experimental results concerning the dynamic substrates have been supported by generalized Mie theory simulations, which are tools used to describe the substrate complexity and the microscopic information not usually taken into account.