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

摘要

Silicon-on-insulator (SOI) microring resonators coated with tetrasulfide-functionalized mesoporous silica (MPS) (S4-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 interactionof the evanescent ield of the guided light with the functionalized mesoporous silica ilms. 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 toaqueous solutions ofPb(II) ions at different concentrations and time-resolved absorp-tion-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 S4-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 S4-groups. Equilibrium constants forabsorption 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 ofPb(II) and are used to determine the 3a limit of detection as58 ppbm in deionized water and 350 ppbm in tap water.