Extinction-to-Absorption Ratio for Sensitive Determination of the Size and Dielectric Function of Gold Nanoparticles

Gold nanoparticles (AuNPs) have become an essential tool for a variety of fields across the biological, physical, and chemical sciences. The characterization of AuNPs by UV-vis spectroscopy is simple and commonly used but remains prone to error because of size and shape polydispersity and uncertainties in the dielectric function. We here propose and demonstrate a method to significantly improve this routine characterization technique by measuring not only the extinction but also the absorption spectrum. Specifically, we show that by considering the ratio of the extinction to absorption spectra, denoted eta, we are able to determine the volume of AuNPs with a significant increase in accuracy compared to the UV-vis extinction method. We also prove an important property of eta: it is independent of particle shape within the quasi-static/dipolar approximation, typically for particle sizes up to 100 nm. This shape independence results in very strong constraints for the theoretical predictions to agree with the experiments. We show that the spectral shape of eta can therefore be used to discriminate between different proposed data sets for the dielectric function of gold, a long-standing challenge in plasmonics research.