(A) Illustration of a Whispering Gallery Mode (WGM) optical resonance in a glass microsphere. The binding of a protein to the microsphere surface increases the WGM path length by Δl, which is detected as a resonance frequency shift Δω. (B) The reactive sensing principle. A molecule binding to the microsphere surface is polarized within the evanescent field (yellow ring) of a WGM. The energy that is needed to polarize the molecule causes the resonance frequency to shift.