Evaluation of Surface-Enhanced Raman Spectroscopy Substrates from Single-Molecule Statistics

Abstract

Accurate quantification of substrate characteristics is a central pursuit within the field of surface-enhanced Raman spectroscopy (SERS). A theory based on single-molecule SERS (SM-SERS) statistics was developed for comprehensive substrate evaluation. This approach is applicable to general substrates possessing many hotspots and is capable of quantifying hotspot strength variation using a minimal set of fitting parameters. The model was validated for simulated substrates and then applied to the SM-SERS statistics of a roughened silver electrode, for which the degree of hotspot uniformity was quantified. The fitted model parameters provide important information concerning the structure–activity relationship of hotspots and can be used to directly compare SERS substrates. Overall, our results present an experimentally determinable parameter set that potentially improves upon the widely used “average enhancement factor” metric currently used for SERS substrate evaluation.

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.…”

“…This strong field localization may lead to large SERS fluctuations at low concentrations, reducing the probability density of observing large SERS enhancement events. 13,14 In closely spaced nanoparticles (such as in AuNR clusters), the individual surface plasmon modes of adjacent nanoparticles interact, producing hybridized bonding and antibonding plasmon modes. 11 Lee et al 15 showed that by controlling the morphology of AuNRs clusters, the observed plasmonic response is dependent on the AuNRs relative orientation.…”

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

“…The slope of the TPD (k, slope of the linear fit, red, in Figure 4) shows how fast the probability density decreases with F, i.e., it is related to the HS localization. 61 Since the experimental measured intensity (I) is proportional to F, the PDF for the measured intensities is expected to follow a TPDlike distribution as in Figure 4. This means that the analysis of the intensity distribution in sm-SERS experiment may reveal the local field distribution in SERS HSs.…”

“…This means that the analysis of the intensity distribution in sm-SERS experiment may reveal the local field distribution in SERS HSs. 61 The PDF in Figure 4 is characteristic of the investigated system, i.e., 20 nm AuNS dimer in water at 633 nm light incident wavelength. This field enhancement distribution may be strongly changed with variations on metal nanoparticle shape, composition and aggregation state.…”

“…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.…”