Molecular weight dependence of a whispering gallery mode biosensor

We report on molecular weight dependence measurements for an optical resonance biosensor. A dielectric microparticle is evanescently coupled with an optical fiber for the resonance stimulation, and a shift of the resonance wavelength is measured to monitor protein monolayer formation on the microparticle surface. Wavelength shifts for proteins over two orders of magnitude in molecular weight are measured. We show that the shift is proportional to molecular weight to the one-third power. Our result demonstrates that the optical resonance biosensor provides protein size information upon detection. This molecular weight dependency differentiates optical resonance sensing from electrical detection using field-effect transistors.