Aqueous mercuric ion detection with microsphere optical ring resonator sensors

We have developed a novel optical sensor for mercuric ion Hg(II) detection in an aqueous environment by utilizing a fused silica microsphere ring resonator. Due to the high Q-factor associated with the whispering gallery modes (WGM) (>105) at the surface of the microsphere, the light interaction with the analytes on the sphere surface is significantly increased, resulting in enhanced sensitivity. The spectral position of the WGM of a microsphere shifts in response to the binding of Hg(II) to the surface of the microsphere, which is activated by the thiol group. Our experimental results show that the Hg(II) detection limit of the microsphere sensor is approximately 50 ppb (w/w) and that mercuric ions bind to thiol groups with 1:3 stoichiometry. Control experiments with zinc ions and 2-mercaptobenzothiazole (2-MBT) were also performed to ensure that Hg(II) binds selectively to thiol groups.