Novel detection scheme for optical biosensing using whispering gallery modes in clusters of dielectric particles

Abstract

We present a novel concept for an optical biosensor based on Whispering Gallery Mode (WGM) excitations in clusters of spherical microresonators. WGM are specific optical modes that arise when light is trapped by Total Internal Reflection (TIR) inside of a sphere and circulates close to its circumference. These modes are sensitive to the adsorption of (bio-) molecules onto the resonator surface upon which the WGM spectrum is shifted towards higher wavelengths. Compared to single particles, clusters of microresonators offer the advantage of being more easily detected due to their higher radiative emission power. Further, the lineshape of the spectra obtained from clusters depends crucially on their composition and therefore may be used as a fingerprint for their identification, e.g., for sensing applications in array formats. Our results demonstrate that clusters of microresonators show the same sensitivity and performance as single spheres. The adsorption of layers of polyelectrolytes and bovine serum albumin (BSA) onto clusters of spheres with 10 μm diameter has been monitored in situ. Depending on the choice of materials, we achieved a mass sensitivity limit of 50 fg, which is about 100 times more sensitive than that of state-of-the-art WGM biosensors.