Tetrasulfide-Functionalized Mesoporous Silica on Nanowire Ring Resonators for Detection of Aqueous Lead, Pb(II)

Silicon-on-insulator (SOI) microring resonators coated with tetrasulfide-functionalized mesoporous silica (MPS) (S-4-MPS) are reported as sensors to detect Pb(II) ions in aqueous solutions from 10 ppb to 1 ppm. The sensors are based on the interaction of the evanescent field of the guided light with the functionalized mesoporous silica films. Upon absorption of metal ions, the resonant wavelengths of the microresonators shift due to the increased refractive index of the film. The sensors are exposed to aqueous solutions of Pb(II) ions at different concentrations and time-resolved absorption-desorption curves are obtained. The concentrations of Pb(II) ions are determined using the Lorentz-Lorenz model. It is found that two distinct binding sites with different affinities for Pb(II) are present in the S-4-MPS silica films, corresponding to tetrasulfide groups and, likely, silanol groups. The number of available binding sites corresponds to 1/25 of the total number of S-4-groups. Equilibrium constants for absorption of Pb(II) to both binding sites are obtained from the rate constants for the absorption and desorption processes. The initial absorption rates depend linearly on the concentration of Pb(II) and are used to determine the 3 sigma limit of detection as 58 ppb(m) in deionized water and 350 ppb(m) in tap water.