Shift of Whispering Gallery Modes in Microspheres by Protein Adsorption
S. Arnold, M. Khoshsima, I. Teraoka, S. Holler, F. Vollmer
Opt. Lett. 28, 272-274 (2003)

Biosensors based on the shift of whispering-gallery modes in microspheres accompanying protein adsorption are described by use of a perturbation theory. For random spatial adsorption, theory predicts that the shift should be inversely proportional to microsphere radius R and proportional to protein surface density and excess polarizability. Measurements are found to be consistent with the theory, and the correspondence enables the average surface area occupied by a single protein to be estimated. These results are consistent with crystallographic data for bovine serum albumin. The theoretical shift for adsorption of a single protein is found to be extremely sensitive to the target regioin, with adsorption in the most sensitive region varying as 1/R5/2. Specific parameters for single protein or virus particle detection are predicted.