currently the methods of choice for very accurate and selective detection. [4] Competing and more compact methods are based on colorimetric [5] or fluorescence [6] assays, X-ray fluorescence, or protein binding assays and have made considerable progress, even though some still lack the selectivity or sensitivity required for water quality monitoring. In most countries, the concentration of lead in drinking water is not allowed to exceed 5-15 ppb, whereas levels exceeding 1 ppm have been reported in heavily contaminated tap water (1 ppb w = 4.83 × 10 −9 m). [7] In-line detection of heavy metals either at the water treatment plant or inside the distribution system is considered ideal to ensure safe drinking water. [8] Here, we report on a functionalized mesoporous silica film with a very high affinity to Pb(II) which was applied to a silicon-based microphotonic resonator as a refractive index sensing platform. This combination results in a compact and comparably simple sensor capable of detecting lead with high selectivity at ppb levels in water. Hundreds of these coated microresonators can be produced in a single fabrication run using a highly parallelized process, thereby reducing the cost considerably.Micro-/mesoporous silica materials (MPS) -in essence, very porous forms of glass -have been widely used in absorbing, scrubbing, and extracting analytes from liquids. [9][10][11][12] These processes exploit the large surface area of MPS materials. Their silica frame imparts good chemical resistance, high stiffness, and good optical properties. Mesoporous silica can be coated on waveguide surfaces using sol-gel methods and is compatible with silica-based optical waveguide sensors, such as long-period fiber gratings (LPG) [13] and silicon-on-insulator waveguide sensors. [14] Different functional groups can be added to the silica networks to bind selectively to a variety of analytes, including metal ions. [13,[15][16][17][18][19][20][21][22][23][24] Another large class of porous scrubbing materials is based on naturally derived low-cost materials such as cocoa pod husks, tea waste, dried sunflower leaves, etc., but these materials are unsuitable as refractive index sensor materials, since they are not optically transparent and cannot be readily applied to refractive index sensor platforms. [25] Previous research has shown that mesoporous silica functionalized with bis[3-(triethoxysilyl)propyl]tetrasulfide (BTESPT) Silicon-on-insulator (SOI) microring resonators coated with tetrasulfidefunctionalized 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...
