  We, for the first time, report the detection and the size measurement of
single nanoparticles (i.e. polystyrene) in aquatic environment using mode
splitting in a whispering gallery mode (WGM) optical resonator, namely a
microtoroid resonator. Using this method we achieved detecting and measuring
individual synthetic hemozoin nanocrystals, which are a hemoglobin degradation
by-product of malarial parasites, dispersed in a solution or in air. The
results of size measurement in solution and in air agree with each other and
with those obtained using scanning electron microscope and dynamic light
scattering. Moreover, we compare the sensing capabilities of the degenerate
(single resonance) and non-degenerate (split mode, doublet) operation regimes
of the WGM resonator.
