  Scatterer induced modal coupling and the consequent mode splitting in a
whispering gallery mode resonator is demonstrated in aqueous environment. The
rate of change in splitting as particles enter the resonator mode volume
strongly depends on the concentration of particle solution: The higher is the
concentration, the higher is the rate of change. Polystyrene nanoparticles of
radius 50nm with concentration as low as 5x10^(-6)wt% have been detected using
the mode splitting spectra. Observation of mode splitting in water paves the
way for constructing advanced resonator based sensors for measuring
nanoparticles and biomolecules in various environments.
