Bio/chemical sensors based on microsphere ring resonators.

摘要

Effective bio/chemical sensors are of interest to clinical diagnosis, homeland security, environmental monitoring and nanofabrication. Silica optical microsphere resonators have drawn increasing attention in bio/chemical sensor development. Light propagates in microsphere in the form of whispering gallery modes (WGMs). WGM has an evanescent field outside the sphere and therefore the spectral position of the WGM is sensitive to refractive index change induced by the binding/removal of bio/chemical molecules to/from the sphere surface. Various applications of microsphere resonators in bio/chemical sensing are demonstrated, viz., small-molecule detection, proteolytic activity and metal-ion detection. For small-molecule detection, we observe reduction in sphere diameter when hydrofluoric acid etches away silica molecules (60Daltons) at the sphere surface. Calculations show a detection limit of 0.4femtomole silica molecules. To detect proteolytic activity, we employ BSA/Trypsin pair. BSA is immobilized onto the sphere surface and as Trypsin cleaves away BSA molecules, we show a trypsin detection limit of 10-4 Units/ml. Detection of mercuric ions is achieved when the spectral position of WGM shifts in response to the binding of Hg(II) to the sphere surface, resulting in a Hg(II) detection limit of 50 ppb. In addition, a Refractometric sensor based on microspheres is also developed. Ethanol is added to water surrounding the microsphere to induce refractive index change and the WGM shifted in response to the change with a sensitivity of 30nm/RIU.