Sol-gel monoliths for optical copper (II) determination

Heavy metal ions are used in a variety of industrial applications and many are considered pollutants. Currently, metal ion concentration measurements are determined off-site, using techniques that are costly, time-consuming, and inconvenient. This work describes the development of optical Cu²⁺ sensors prepared through the sol-gel process. Initial work was carried out exploring the basic properties of sol-gel glasses with amine ligands incorporated directly into the sol-gel matrix. The sol-gel process was modified to produce physically robust, optically transparent, crack-free sol-gel monoliths incorporating either diamine NH₂(CH₂)₂NH(CH₂)₃Si(OMe₃)₃ or monoamine H₂N(CH₂)₃Si(OMe)₃ directly into the sol-gel matrix using Si(OMe)₄ as a cross-linking agent. The kinetics of cupric ion uptake by the gels in Cu²⁺ solutions of various concentrations was studied by measuring the change in the absorbance of the blue metal:ligand complexes within the gels over time using UV-VIS spectrophotometry. A simple correlation between the rate of complex formation and [Cu²⁺] in solution was observed. The gels containing diamine ligand could not be regenerated, and could therefore only be used as one-time Cu²⁺ probes, possibly in environmental applications. The gels containing monoamine ligand could be regenerated with EDTA and used in subsequent Cu²⁺ uptake-removal cycles, and thus showed potential as industrial Cu²⁺ ion sensors.