Whispering gallery mode (WGM) optical resonator sensors are an extremely sensitive label-free technology for detecting the binding of biomolecules in solution. To better understand the fast transient response observed with these devices, we model mass transfer to spherical and toroidal WGM sensors of identical outer radius. Finite element simulations predict a 3-10 fold higher binding frequency for toroidal sensors. These results agree to within an order of magnitude with experimental data from the literature and suggest a design strategy to improve the transient response of a sensor by making the device small only in the dimension that governs boundary layer development.

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