Development of sustainable and recyclable Cu(II) ion sensor using bioinspired "Bai Ji" photonic crystal hydrogels

Excessive Cu(II) is a constant risk to aquatic ecosystems and human health. To date, field-deployable assays that are visual, quantitative, and recyclable are rare. Objective of the presented study is to create a bio-inspired, recyclable sensor that transduce selective Cu(II) binding to an immediate optical readout for on-site monitoring. Therefore, We engineered a photonic-crystal hydrogel from carboxymethyl Bletilla striata ("Bai Ji") polysaccharide. We embedded a colloidal photonic lattice within the polysaccharide network. This design converts Cu(II) binding, which involves coordination and ion exchange at oxygenated groups, into a concentration-dependent blue shift in structural color. The material facilitates naked-eye screening and spectroscopic quantification of Cu(II) with an limit of detection (LOD) of 0.078 µmol∙L^-1 and an adsorption capacity of 82 ± 0.1 mg∙g^-1. Mechanistic analysis (kinetics, isotherms, thermodynamics, X-ray Photoelectron Spectroscopy(XPS)) supports cooperative coordination, ion-exchange, and van der Waals interactions between Cu(II) and the polysaccharide functional groups. The hydrogel is regenerable by mild elution and yields accurate recoveries in lake and tap water. This work demonstrates a sustainable and efficient platform for rapid, on-site Cu(II) monitoring in environmental waters.