Discriminating Nanoparticle Dimers from Higher Order Aggregates through Wavelength-Dependent SERS Orientational Imaging

Abstract

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.

Applying Super‐Resolution Imaging Techniques to Problems in Single‐Molecule SERS

“…If nanoparticle height information is required, AFM can also be used to correlate nanoparticle structure to optical activity. For this purpose, a combined AFM/total internal reflection optical microscope is used [36][37][38]. The optical microscopy system allows the user to locate the same area investigated in the super-resolution study, while the AFM gives sub-diffraction limited structural information on the nanoparticle aggregates.…”

“…To prevent this, we functionalize glass coverslips with a layer of (3-aminopropyl) triethoxysilane (APTES) to anchor the nanoparticles to the substrate surface. In this procedure, argon plasma-cleaned coverslips are incubated in a 0.5-2% APTES-methanol solution for approximately 5 min, and then thoroughly rinsed with methanol to remove excess APTES [37]. This allows AFM characterization without disturbing aggregates on the coverslip surface.…”

Superlocalization Surface-Enhanced Raman Scattering Microscopy: Comparing Point Spread Function Models in the Ensemble and Single-Molecule Limits

“…6,27,35 Figure 4 compares the fits and (7) 74 (8) a Values in parentheses indicate the standard deviation of the fit in the last significant figure over 20 image frames. b Δx(Δy) corresponds to (x(y) 0D À x(y) 0G ) for each type of fit.…”

“…Thus, we attribute the increased variability in the centroid position in the SM-SERS sample to an effect of molecular motion, which influences how the molecule interacts with the various plasmon modes of the nanoparticle, thereby shifting its centroid. 6,7,35 To compare the calculated centroids between MM-SERS and SM-SERS samples, we calculated twodimensional spatial intensity maps, which relate the intensity of the measured SERS to the position of the calculated centroid. 8 To do this, fitted centroid locations from individual image frames associated with SERS emission are collected into 1 nm bins, and the color of the bin is determined by the average fitted intensity of the points in each bin.…”

Molecular imaging with surface-enhanced Raman spectroscopy nanoparticle reporters

“…Interestingly, the signal in biological media can actually increase due to nanoparticle aggregation, 16 resulting in the formation of “hot spots” between nanoparticles that increase the signal even more than individual nanoparticles (see the article by Sharma et al). 21 Nevertheless, such aggregation is currently nonspecific and unpredictable and is a significant error source for experiments in which SERS intensity corresponds to a biomarker. Thus, a silica coat on the surface of gold nanoparticles has been employed 10,11,22–26 to protect and stabilize the imaging component of the hybrid nanoparticle.…”