Detection of Protein Orientation on the Silica Microsphere Surface Using
Transverse Electric/Transverse Magnetic Whispering Gallery Modes

M. Noto, D. Keng, I. Teraoka, and S. Arnold
Bio. Phys. J., 92, 4466-4472 (2007)

The state of adsorbed protein molecules can be examined by comparing the shifts in a narrow line resonance
wavelength of transverse electric (TE) and transverse magnetic (TM) whispering gallery modes (WGM) when the molecules
adsorb onto a transparent microsphere that houses WGM. In adsorption of bovine serum albumin (BSA) onto an aminopropylmodified
silica microsphere, the TM/TE shift ratio indicated highly anisotropic polarizability of BSA in the direction normal to the
surface, most likely ascribed to anchoring the heart-shaped protein molecule by one of its tips. The polarization-dependent resonance
shift was confirmed when the surrounding refractive index was uniformly changed by adding salt, which would simulate
adsorption of large objects.