Single polystyrene nanoparticles are detected from resonance wavelength fluctuations in toroidal
and spherical microcavities. The magnitude of the wavelength-shift signal follows a reactive
mechanism with inverse dependence on mode volume. By reducing the size of a microsphere
cavity we demonstrate sensitivity to single Influenza A virions. Furthermore, we introduce a
novel mechanism for trapping and accumulation of nanoparticles at the microcavity-sensorregion
by utilizing light-force exerted in evanescent field gradients.