In whispering-gallery-mode (WGM) microresonator based optical biosensors, the mode shift or splitting induced by
biomolecules are typically regarded as the sensing signals. Here we propose several schemes to improve the performance
of WGM based biosensing. In the mode shift sensing scheme, we propose to use a novel exterior plasmonic WGM
supported by a metal-coated toroidal microresonator, and the detection sensitivity reaches 500 nm/RIU (refraction index
unit). In the mode splitting scheme, we study and discuss how to extend the detection range, which is of importance in
detecting small-sized biomolecules. Finally, the single nanoparticle detection is realized experimentally by using mode
splitting sensing scheme in the aqueous environment.