Optoplasmonic microcavities based on whispering-gallery modes are sensors developed in the past decade that combine the high-quality factors of dielectric microcavities and the small nanometer-scale localization of electric fields by metal nanoparticles to achieve exceptional sensitivity for detecting single molecules in solution. These sensors have enabled the detection of single ions, enzyme activity and various ligand reactions/interactions. I will describe the photonic working principle of these sensors with a focus on the experimental aspects of biomolecular sensing. Most recent advances include the use of these sensors as probes for the bioenergetics of enzymes. Furthermore, a novel optothermal signal transduction mechanism allows for sensing the absorption of light by single molecules with altered selection rules on plasmonic nanoparticles.