Mode-directed photopatterning of whispering gallery mode optical resonators

Multimodal optical resonators can integrate multiple sensing functions on a single device by assigning specific tasks to different modes. To facilitate such expanded functionality, this study demonstrates a photopatterning approach in which resonantly-amplified light within whispering gallery mode (WGM) sensors is used to direct chemical modification of the corresponding surface region addressed by the mode. A Ru(II) metallo–organic complex containing a caged aminopropyltriethoxysilane (APTES) moiety, [Ru(tpy)(biq)(APTES)](PF₆)₂, was synthesized and applied as a covalently immobilized layer to solid supports to be patterned, including spheroidal silica WGM resonators. Exciting a mode caused the area exposed to the light to be deprotected, leaving behind a pattern of reactive amine groups available for further derivatization. A two-photon deprotection process enabled the use of near-IR sources for patterning. The photopatterning technique was applied to self-referenced measurements, in which signals from two modes, a sensing and a reference mode, were used to detect specific binding of avidin against a much larger background of nonspecific adsorption. This was accomplished by patterning the sensing mode with biotin to specifically bind avidin while the reference mode tracked nonspecific adsorption.