Whispering gallery mode (WGM) biosensors derive their unprecedented sensitivity for label-free detection of biomolecules from the high-quality (Q) factor of an optical resonance phenomenon. Plasmonic excitations in metal nanostructures which are able to localize the WGM field at the target binding site can further enhance sensitivity by local electric field amplification. Random nanoparticle layers and nanopost antennas are investigated for this purpose, and an efficiency parameter is introduced to quantitate the local field overlap with analyte molecules, a pre-requisite for single molecule detection.