  Microcavity and whispering gallery mode (WGM) biosensors derive their
sensitivity from monitoring frequency shifts induced by protein binding at
sites of highly confined field intensities, where field strengths can be
further amplified by excitation of plasmon resonances in nanoparticle layers.
Here, we propose a mechanism based on optical trapping of a protein at the site
of plasmonic field enhancements for achieving ultra sensitive detection in only
microliter-scale sample volumes, and in real-time. We demonstrate femto-Molar
sensitivity corresponding to a few 1000s of macromolecules. Simulations based
on Mie theory agree well with the optical trapping concept at plasmonic
'hotspots' locations.
