A cysteamine-modified photonic crystal hydrogel sensor for visual detection of Hg2+ ions

Abstract

Mercury ions (Hg²⁺) are highly toxic pollutants with severe environmental and health impacts, necessitating the development of rapid and cost-effective detection methods. Herein, we report a cysteamine-modified photonic crystal hydrogel (CPCH) sensor for visual detection of Hg²⁺. The CPCH was constructed by embedding superparamagnetic Fe₃O₄ colloidal photonic crystals into a poly(acrylamide-co-acrylic acid) hydrogel matrix through UV-initiated polymerization under an external magnetic field, which aligned the colloidal arrays. The as-prepared hydrogel contained abundant carboxyl groups, which were subsequently activated by EDC/NHS chemistry and covalently coupled with cysteamine molecules via amide reactions, introducing a high density of thiol (–SH) groups. Based on the hard and soft acids and bases (HSAB) theory, these N, O, and S functional groups act as specific recognition sites capable of selectively coordinating with Hg²⁺ ions. This induces hydrogel contraction and reduces the interparticle spacing of Fe₃O₄ colloidal nanoparticles, thereby triggering a significant blueshift in diffraction and a distinct structural color transformation. Experimental results demonstrate that at pH = 5, increasing Hg²⁺ concentration from 0 to 2 mM progressively enhances the diffraction shift, with the maximum response (Δλ_max = 162 nm) achieved at 1 mM. A good linear relationship was observed within the range of 0.01 to 0.5 mM. The limit of detection (LOD) was 6.9 µM. Moreover, the sensor exhibited a rapid response, reaching equilibrium within 5 min, accompanied by a change in structural color from orange-yellow to blue-purple. Additionally, the CPCH sensor demonstrates not only excellent selectivity but also favorable regenerability, enabling reuse up to five times without compromising detection performance. This facile and low-cost sensing platform provides a visually perceptible and highly selective method for Hg²⁺ detection, showing great promise for environmental monitoring and on-site analysis.