Robust SERS Enhancement Factor Statistics Using Rotational Correlation Spectroscopy

We characterize the distribution of surface-enhanced Raman spectroscopy (SERS) enhancement factors observed in individual hot spots of single Ag “nanocapsules”, encapsulated Ag nanoparticle dimers formed via controlled nanoparticle linking, polymer encapsulation, and small molecule infusion. The enhancement factors are calculated for over 1000 individual nanocapsules by comparing Raman scattering intensities of 4-mercaptobenzoic acid (MBA) measured from single SERS hot spots to intensities measured from high-concentration solutions of MBA. Correlation spectroscopy measurements of the rotational diffusion identify nanocapsules with signals dominated by single hot spots via their strong polarization response. Averaging over the entire surface of the nanocapsules, the distribution of enhancement factors is found to range from 106 to 108, with a mean of 6 × 106. Averaging only over nanoparticle junctions (where most SERS signals are expected) increases this average value to 108, with a range from 2 × 107 to 2 × 109. This significant statistical sampling shows that very high SERS enhancement factors can be obtained on a consistent basis using nanoparticle linking.