We present the first super-resolution optical images of single-molecule surface-enhanced Raman scattering (SM-SERS) hot spots, using super-resolution imaging as a powerful new tool for understanding the interaction between single molecules and nanoparticle hot spots. Using point spread function fitting, we map the centroid position of SM-SERS with +/-10 nm resolution, revealing a spatial relationship between the SM-SERS centroid position and the highest SERS intensity. We are also able to measure the unique position of the SM-SERS centroid relative to the centroid associated with nanoparticle photoluminescence, which allows us to speculate on the presence of multiple hot spots within a single diffraction-limited spot. These measurements allow us to follow dynamic movement of the SM-SERS centroid position over time as it samples different locations in space and explores regions larger than the expected size of a SM-SERS hot spot. We have proposed that the movement of the SERS centroid is due to diffusion of a single molecule on the surface of the nanoparticle, which leads to changes in coupling between the scattering dipole and the optical near field of the nanoparticle.

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.

This Article introduces surface-enhanced Raman scattering (SERS) orientational imaging as a powerful far-field optical technique for determining the in-plane and out-of-plane orientations of SERS-active nanoparticle dimers. Optical images of Rhodamine 6G (R6G) SERS emission patterns are measured and correlated with atomic force microscopy (AFM) images of the associated SERS-active silver nanoparticle dimers. The AFM is used to measure individual silver nanoparticle dimer orientations and height asymmetry, defining in-plane and out-of-plane angles associated with the dimer geometry. Theoretical emission pattern images based on these angles are generated using a simple dipole emission model and show excellent agreement with the experimental emission patterns. This technique provides a rapid all-optical technique to analyze the orientation of SERS active nanoparticle dimers.

Surface-enhanced Raman scattering (SERS) orientational imaging is a recently developed all-optical technique able to determine SERS-active silver nanoparticle dimer orientations by observing lobe positions in SERS emission patterns formed by the directional polarization of SERS along the longitudinal axis of the dimer. Here we extend this technique to discriminate nanoparticle dimers from higher order aggregates by observing the wavelength dependence of SERS emission patterns, which are unchanged in nanoparticle dimers but show differences in higher order aggregates involving two or more nanoparticle junctions. The ability of SERS orientational imaging to identify stacked nanoparticles in higher order aggregates is also demonstrated. The shape of the SERS emission patterns originating from trimers labeled with low and high concentrations of dye is investigated, showing that the emission pattern lobes become less defined as the dye concentration increases. Dynamic fluctuations in the SERS emission pattern lobes are observed in aggregates labeled with low dye concentrations, as molecules diffuse into regions of higher electromagnetic enhancement in multiple nanoparticle junctions.