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

Microparticle Photophysics Laboratory, Polytechnic University, Brooklyn, New York 11201

Correspondence: Address reprint requests to Iwao Teraoka, E-mail: teraoka{at}poly.edu.

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 aminopropyl-modified 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.