Performance of label-free optical biosensors: What is figure of merit (not) telling us?

Here we study the analytical performance of label-free optical biosensors with respect to analyte-induced refractive index changes that can be measured by a biosensor (refractive index resolution). We present an analytical model that interrelates the refractive index resolution and the parameters of the optical platform of a biosensor. We demonstrate that the figure of merit (FOM), which has been widely used to design optical platforms of label-free optical biosensors, is not an appropriate metric to guide the design or predict the performance of label-free optical biosensors. Therefore, we propose an extended definition of FOM that addresses its limitations. We confirm the validity of the proposed approach by both numerical simulations and experiments. Finally, we show that the analytical model of the refractive index resolution not only makes it possible to predict the performance of a biosensor but also provides strategies for achieving optimal performance.