Whispering Gallery Mode Biosensors Consisting of Quantum Dot-Embedded Microspheres

New methods of biological analyte sensing are needed for development of miniature biosensors that are highly sensitive and require minimal sample preparation. One technique employs optical resonances, known as whispering gallery modes (WGMs), in spherical or cylindrical microstructures. The spectral positions of these resonant modes are very sensitive to the local refractive index and spectral shifts may be used to sense changes in the index. To excite these WGMs and enable remote excitation, quantum dots are embedded in polystyrene microspheres to serve as local light sources. Using a simple continuous wave excitation optical system, these sensors are demonstrated by monitoring the wavelength shift of multiple resonant modes as bulk index of refraction is changed in ethanol-water mixtures. The potential for targeted biosensing is explored through addition of a protein that adsorbs to the microsphere surface, thrombin, and one that does not, bovine serum albumin (BSA). The thrombin produced a spectral shift that was much larger than that due to the bulk index change. The BSA produced a significantly smaller shift that was slightly larger than the expected shift due to bulk index change. Most likely due to the thin, high index layer of quantum dots, microsensor response in all cases demonstrated increased sensitivity over theoretical predictions.