Balancing Amino and Thiol Groups in Ethylene-Bridged Materials: Probing Surface Interactions with Cu(II) and a Copper Phthalocyanine Dye

Abstract Combining amino and thiol groups in organosilicas can alter both metal coordination and the accessibility of the surface binding sites. Here, ethylene-bridged organosilicas were prepared by sol–gel co-condensation to compare monofunctional reference materials with a bifunctional series having a fixed amino/thiol precursor ratio and varied total functional-silane content relative to the bridging precursor. Hydrated Cu(II) ions and the bulky anionic copper phthalocyanine dye Reactive Blue 231 (RB231) were used as complementary probes of surface interactions. The amino-functionalized material exhibited the highest Cu(II) uptake, whereas the bifunctional material prepared at a molar feed ratio of ethylene-bridged, amino, and thiol silane precursors of 4:1:1 (denoted as EBHS-AT2) combined comparable metal uptake with the highest dye uptake. FTIR revealed shifts in amino group deformation bands after adsorption, while XPS showed an increased higher-binding-energy N 1s contribution after Cu(II) uptake. In EBHS-AT2, these nitrogen changes contrasted with the retention of the principal thiol-like S 2p component. Together with the Cu(II) diffuse reflectance spectra, these observations support predominantly amino-mediated metal binding. Nitrogen physisorption and ζ-potential measurements linked the enhanced RB231 uptake to the combination of developed mesoporosity and positively charged surfaces at the adsorption pH. Quantum chemical calculations identified an indirect contribution of neighboring thiol functionality: it stabilized a Cu(II) complex coordinated by two amino groups without itself entering the coordination sphere. Hydrolysis of the modeled framework fragments made the binding less exothermic. The combined evidence shows that bifunctionality influences adsorption through both donor site chemistry and the environment in which those sites interact with the adsorbate.