  We detect 39 nm x 10 nm gold nanorods using a microtoroid stabilized via the
Pound-Drever-Hall method. Real-time detection is achieved with signal-to-noise
ratios up to 12.2. These nanoparticles are a factor of three smaller in volume
than any other nanoparticle detected using WGM sensing to date. We show through
repeated experiments that the measurements are reliable, and verify the
presence of single nanorods on the microtoroid surface using electron
microscopy. At our current noise level, the plasmonic enhancement of these
nanorods could enable detection of proteins with radii as small as a = 2 nm.
