Intensity Fluctuations in Single-Molecule Surface-Enhanced Raman Scattering

Abstract

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 ...

Interplay Between Near-Field Properties and Au Nanorod Cluster Structure: Extending Hot Spots for Surface-Enhanced Raman Scattering

“…The above results show that, for SERS applications, the ee aggregation is a better choice aiming at maximizing the measured intensities. However, the nature of such coupling produces extremely localized hots pots 14 at the nanorod tips (Figure 1c), which may lead to strong spatial SERS intensity fluctuations 23 and reduced probabilities for a single-molecule event to be detected once only a molecule that reaches such small volume locations generates a measurable signal. In this sense, it is reasonable to search for ways to increase the spatial distribution of hots spots over a greater area on the AuNR surface, which necessarily must be accompanied by a field amplification in the ss interaction.…”

Correlating Single-Molecule Sers Statistics to Plasmonic Nanocluster Structure and Near-Field Properties

“…It is worth pointing out that a similar distribution profile as in Figure 6C would be obtained for AuNE dimers if the surface coverage is increased. This is a counterintuitive interpretation, 37,60,61 since the HS strength is much larger for the system with a greater surface curvature. The result is a direct consequence of a great contribution from the high degree of HS confinement observed for the ellipsoid.…”

“…[28][29][30][31][32][33][34] The magnitude of enhancements in the HSs makes possible the observation of SERS spectra that can be correlated to singlemolecule events. [35][36][37][38] Despite the obvious analytical chemistry implications of such small detection limit, the SERS spectra at such conditions may open possibilities to characterize HS properties regarding field amplifications and resonance conditions, which in turn are related to the HS structure. In the last few years we have given attention to the SERS spectra at single-molecule SERS (sm-SERS) conditions as a tool to extract physical information related to the HSs.…”

“…In the last few years we have given attention to the SERS spectra at single-molecule SERS (sm-SERS) conditions as a tool to extract physical information related to the HSs. 37,39 For instance, the ratio of anti-Stokes to Stokes intensities from a molecule, related to the local temperature in Raman measurements, in SERS spectra it also accounts for the HS resonance profile for the near-field enhancement: if the plasmon resonance is redshifted with respect to the incident laser wavelength, a larger enhancement must be expected for the Stokes if compared to anti-Stokes signals; on the other hand, if the plasmon resonance wavelength is smaller than that of the incident laser, an anti-Stokes preferential enhancement must be expected. 40,41 This information can be used to map the resonances from each HS probed by a single-molecule.…”

Cancer Diagnosis through SERS and Other Related Techniques

“…This versatile effect is one of the reasons why metals have so much potential for nano-optics. Besides, electromagnetic fields can have even more specific shapes such as tips [27] or the so-called hot spots [28]. Attending to the material, noble metals, gold and silver have attracted huge interest among the plasmonic nanoparticles.…”