Whispering gallery mode biosensors in the low-Q limit

Whispering gallery modes (WGM) of dye-doped polystyrene beads with diameters from 20 down to 1.5 μm are studied with respect to their appearance and linewidth by excitation of the entire mode spectrum within the emission range of the dye. The lowest order (q=1) modes, which travel most closely to the inner particle surface, are assigned to their individual quantum numbers by means of a least-square-fit, resulting in a precise determination of particle radius and eccentricity. On this basis, the suitability of these microscopic cavities for applications in optical (bio-)sensing is explored. Due to the low quality (Q) factors of these small cavities, particles with diameters below 6 μm exhibit only q=1 modes, thereby causing a drastic simplification of the WGM spectrum. In such spectra, the shift in the WGM positions upon molecular adsorption can be easily monitored, as we demonstrate for the adsorption of bovine serum albumin as well as multiple layers of polyelectrolytes onto the surface of particles with 2 μm diameter. Mie simulations are used to confirm our findings. With a mass sensitivity limit of 3 fg, these microscopic sensors are highly competitive in the field of label-free detection techniques. Moreover, their small size and the simplified, dye-mediated excitation and detection scheme may pave the way to remote in-vitro biosensing in the future.