Nanoparticles suspended in the vicinity of a whispering gallery mode (WGM) biosensor are detected from fluctuations in the driving light-guide transmission. These fluctuations are described by Brownian particles perturbing the resonance wavelength in reaction to being polarized by the WGM’s evanescent field. Comparison between the autocorrelation of the measured fluctuations and theory provides a first order approximation for the nanoparticle size and lays the basis for future studies of interfacial dynamics. With this advance, the WGM biosensor goes beyond low-frequency measurements of adsorption and desorption and into a world which has been dominated by fluorescence correlation spectroscopy, but without labels.

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