In order to advance the field of single-molecule surface-enhanced Raman scattering (SM-SERS), a better understanding of colloid morphology and hot spot properties in noble metal nanoparticle aggregates is crucial. We present super-resolution optical studies of surface-enhanced Raman scattering (SERS) from rhodamine 6G (R6G) molecules adsorbed onto silver colloid aggregates correlated with scanning electron microscope (SEM) images of those same aggregates. The scattering intensity maps of the SERS signal, obtained from the super-resolution fits, are overlaid with SEM topographical images of the colloids to map the shape of the SERS hot spot and the spatial origin of SERS intensity fluctuations with sub-5 nm resolution. These results have vital implications for developing reproducible and robust substrates capable of SM-SERS.

Super-resolution optical imaging of Rhodamine 6G surface-enhanced Raman scattering (SERS) and silver luminescence from colloidal silver aggregates are measured with sub-5 nm resolution and found to originate from distinct spatial locations on the nanoparticle surface. Using correlated scanning electron microscopy, the spatial origins of the two signals are mapped onto the nanoparticle structure, revealing that, while both types of emission are plasmon-mediated, SERS is a highly local effect, probing only a single junction in a nanoparticle aggregate, whereas luminescence probes all collective plasmon modes within the nanostructure. Calculations using the discrete-dipole approximation to calculate the weighted centroid position of both the |E|(2)/|E(inc)|(2) and |E|(4)/|E(inc)|(4) electromagnetic fields were compared to the super-resolution centroid positions of the SERS and luminescence data and found to agree with the proposed plasmon dependence of the two emission signals. These results are significant to the field of SERS because they allow us to assign the exact nanoparticle junction responsible for single-molecule SERS emission in higher order aggregates and also provide insight into how SERS is coupled into the plasmon modes of the underlying nanostructure, which is important for developing new theoretical models to describe SERS emission.

Super-resolution imaging of single-molecule surface-enhanced Raman scattering (SM-SERS) reveals a spatial relationship between the SERS emission centroid and the corresponding intensity. Here, an isotope-edited bianalyte approach is used to confirm that shifts in the SERS emission centroid are directly linked to the changing position of the molecule on the nanoparticle surface. By working above the single-molecule limit and exploiting SERS intensity fluctuations, the SERS centroid positions of individual molecules are found to be spatially distinct.

Super-resolution imaging has recently been utilized to develop a better understanding of the properties of surface-enhanced Raman scattering (SERS) hot spots. SERS hot spots are much smaller than the diffraction limit of light, and therefore, obtaining a clear picture of the enhanced electromagnetic (EM) fields comprising these hot spots is a challenging task. In this Perspective, we discuss recent work applying super-resolution imaging to single-molecule SERS (SM-SERS) of rhodamine 6G (R6G) adsorbed to randomly assembled silver colloidal aggregates, allowing the shape, size, and local enhancement of the hot spots to be imaged with <5 nm resolution. The results are compared with studies applying super-resolution imaging to surface-enhanced fluorescence (SEF) of analytes diffusing into silver nanoparticle hot spots. Both studies show a strong correlation between emission intensity and position, allowing the EM field enhancements of SERS hot spots to be mapped with sub-5 nm resolution.

Super-resolution surface-enhanced Raman scattering (SERS) is used to investigate local surface potentials on plasmonic gold and silver colloidal aggregates using the redoxactive reporter molecule, Nile Blue A. This molecule is electrochemically modulated between an oxidized emissive state and a reduced dark state. The diffraction-limited SERS emission from Nile Blue on the surface of a single plasmonic nanoparticle aggregate is fit to a two-dimensional Gaussian to track the position of the emission centroid as a function of applied potential. Potential-dependent centroid positions are observed, consistent with molecules experiencing site-specific oxidation and reduction potentials on the nanoparticle electrode surface. Correlated structural analysis performed with scanning electron microscopy reveals that molecules residing in nanoparticle junction regions, or SERS hot spots, appear to be reduced and oxidized at the most negative applied potentials as the potential is cycled. F rom light harvesting 1,2 to trace chemical sensing 3−10 and electrocatalysis, 11−13 plasmonic nanoparticles represent a promising material with a diverse array of applications that are highly dependent upon nanoparticle geometry. Although ensemble studies can elucidate information about a population of nanoparticles, these studies often overlook how subtle differences in nanoparticle structure can influence their properties. To develop a more complete nanoscale understanding of the effect of nanoparticle geometry upon optical or other behaviors, our group and others have used correlated structural and super-resolution optical studies to study molecular interactions with plasmonic substrates. 14−28 In recent work, we used this approach to study the redox reaction of Nile Blue A (NB) at the surface of aggregated silver nanoparticles using surface-enhanced Raman scattering (SERS) as a reporter of the redox state of the molecule. 19 Although the oxidized form of NB is resonant with 642 nm laser excitation and produces strong SERS signal, the reduced form produces little to no SERS. 11,29−31 Thus, NB acts as a simple on/off probe, and potential cycling allows us to move from the ensemble to single-molecule regime with ease. Our earlier studies using super-resolution imaging to localize the SERS emission as the applied potential was modulated suggested that NB molecules in different locations on an aggregated nanoparticle structure experienced different local potentials, indicating a relationship between local redox chemistry and nanoparticle structure. 19