Whispering gallery mode biosensors using semiconductor quantum dots

We are constructing optically coupled biosensors from microspheres and semiconductor quantum dots (QDs). Detection will be accomplished through the resonance shift of whispering gallery modes (WGMs) in the microspheres as an analyte is adsorbed to the surface of the microsphere. The use of QDs in such a system allows us to remotely couple to the biosensors with an external optical system. Because the QDs have a high quantum yield and a broad absorption band, the biosensors may be excited with a low light flux well outside the spectral detection region. The resonant effect will permit sensitive detection due both to the WGM's narrow spectral linewidths as well as to multiple sampling as light circulates in the resonant cavity. Detection is local to the microsphere surface as changes in index are sampled via the WGM's evanescent field outside the microcavity. Since the microspheres are smaller than many cells, we anticipate that the system will be useful in both in vitro and in vivo environments.