Microcavity-Enhanced Fluorescence Energy Transfer from Quantum Dot Excited Whispering Gallery Modes to Acceptor Dye Nanoparticles

Whispering gallery mode (WGM) microcavities are emerging as potential candidates in the field of biosensing applications, as their resonance wavelengths shift with changes in the refractive index in the region of their evanescent field. Their high-quality resonance modes and accessible surface functionalities make them promising for molecular assays, but their high sensitivity makes them inherently unstable. Here, we demonstrate that WGM resonances also strongly enhance fluorescence energy transfer between donors placed inside the microcavity and acceptors placed outside. We load colloidal quantum dots (QDs) into polymeric microspheres to provide WGMs that benefit from the QD optical features when used as energy-transfer donors. Spectroscopic analysis of the emission from the microcavities shows that the high quality of WGMs enables a very efficient energy transfer to dye-loaded polymer nanoparticle acceptors placed in their vicinity. Compared to Förster resonance energy transfer, WGM-enabled energy transfer (WGET) occurs over a much more extended volume, thanks to the delocalization of the mode over a typically 105 times larger surface and to the extension of the WGM electromagnetic field to larger distances (>100 nm vs a few nm) from the surface of the microcavity. The resulting sensing scheme combines the sensitivity of WGM spectroscopy with the specificity and simple detection schemes of fluorescence energy transfer, thus providing a potentially powerful class of biosensors.