Detection of Aqueous Lead (II) And Mercury (II) Using Micro-Ring Resonators Coated with Tetrasulfide Functionalized Mesoporous Organosilica Thin Films

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Authors
Chen, Hao
Keyword
Heavy Metal Sensor , Ring Resonator , Silicon-on-Insolator Sensor , Mesoporous Organosilica Film , Lead , Mercury , Pb , Hg
Abstract
Silicon-on-insulator (SOI) micro-ring resonators coated with bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT) and tetraethoxysilane (TEOS) co-condensation mesoporous silica coatings are reported as chemical sensors to detect Pb(II) and Hg(II) ions in aqueous solutions. The sensors were based on the interaction of the evanescent field of the guided light with the functionalized mesoporous silica films. Upon adsorption of heavy metal ions (Pb(II) and Hg(II)), the refractive index of the film increased, leading to observable shifts in the resonant wavelengths of the micro-resonators. The tetrasulfide groups were placed into some of the silica films to increase the films’ affinity to Pb(II) and Hg(II) ions. The sensors were exposed to aqueous solutions of Pb(II) and Hg(II) ions at different concentrations and time-resolved adsorption-desorption curves were recorded and analyzed. It was found that two distinct binding sites with different affinities for metal ions were present in the S4-TEOS silica films, corresponding to the tetrasulfide groups and the silanol groups. The rate constants for the absorption and desorption processes as well as the equilibrium constants were determined for each of the binding sites and ions. The initial adsorption rates of the Pb(II) and Hg(II) uptake processes changed linearly with Pb (II) and Hg(II) concentrations from 10 ppb to 1 ppm and could be used to quantify metal ions at the 30 ppb level. The adsorption and desorption processes of other ions on the same tetra-sulfide film were reported to test for possible chemical interferences – specifically of Na(I), K(I), Mg(II), Ca(II), Fe(II), Zn(II), Cd(II), and Fe(III). In addition, Pb(II), Na(I), K(I), Ca(II), Fe(II), and Fe(III) solutions were tested on mesoporous silica films without the tetrasulfide groups to test their adsorption and desorption to bare silica. These test results fitted well with a single-adsorption-site model for TEOS silica film, which confirmed the double-adsorption-site model for the S4-TEOS silica films, where the deprotonated silanol groups and the tetrasulfide groups acted as the two adsorption sites. A single-adsorption-site model was developed for the TEOS silica film, with the deprotonated silanol groups as the only adsorption sites. Tap water and seawater spiked with Pb(II) were also tested on S4-TEOS silica films.
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