Ground state depletion microscopy for imaging interactions between gold nanowires and fluorophore-labeled ligands.

Ground state depletion with individual molecule return (GSDIM) is used to interrogate the location of individual fluorescence bursts from fluorophore-labelled DNA molecules on gold nanowire surfaces. Carboxytetramethyl rhodamine (TAMRA)-labelled double-stranded DNA molecules were bound to the surface of gold nanowires via gold-thiol linkages. Individual fluorescence bursts were spatially localized using point spread function fitting and used to reconstruct the image of the underlying nanowire. While the reconstructed images reproduce the size and shape of the nanowire structures, plasmonic coupling between the nanowire and fluorophore is observed, indicating that the location of the observed fluorescence may not precisely correlate with the location of the emitting fluorophore. Thus, plasmonic coupling is an important factor when using super-resolution imaging techniques to study plasmonic nanostructures.

Super-resolution SERS imaging beyond the single-molecule limit: an isotope-edited approach.

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 reveals a difference between SERS and luminescence centroids.

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.

Shedding Light on Surface-Enhanced Raman Scattering Hot Spots through Single-Molecule Super-Resolution Imaging.

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.