In our latest cover aarticle in Adv. Sus. Sys Hao Chen, John Saunders, Sogol Borjian and Xiaowei Wu were coating microphotonic tandem ring resonators with tetrasulfide‐functionalized mesoporous silica films to create remarkably sensitive and specific sensors for lead, Pb(II) in water. Using simply additivity rules for material polarizability, the refractive index changes of the films were quantified. Rate constants, equilibrium constants, and binding energies for Pb(II) adsorbing to tetrasulfide sites and to silanol sites can be obtained.