Colorimetric Recognition of Chloride Ion Concentration by Copper Complexation in Polyacrylamide-Based Hydrogels

Chloride ions (Cl – ) are commonly found in geological formations, oceans, and in industrial waste, where its concentration can span from millimolar to molar levels. However, most optical Cl – sensors are optimized for micromolar to millimolar detection and rely on organic chemo-sensor molecules that may leach or degrade under various conditions. Here, we report a mechanically ruggedized, dye-free, Cu 2+ -loaded alginate/poly(acrylamide)/Laponite double-network hydrogel as a simple colorimetric platform for the determination of chloride ion concentrations in aqueous media. The optimal compositions of acrylamide, alginate, and Laponite additives, which balances optical response detectability, overall transparency, and ruggedized mechanical property, are identified as 30.0, 1.0, and 1.0 wt %, respectively, with respect to the mass of deionized water. Here, the presence of chloride leads to a water/Cl – ligand exchange at the Cu 2+ sites in the hydrogel, leading to a bathochromic shift in a metal-centered d–d optical transition. The shift causes a visual color change from cyan to light green, as quantified by a change in Hue from 175° to 153° from digital image analysis in the Hue-saturation-value color space. Potential applications of these colorimetric hydrogels include process and wastewater monitoring in the smelting, rare-earth extraction, and sewage treatment industries, where monitoring of high concentrations of chloride is required in a rugged environment.