Highly sensitive colorimetric sensing for heavy metal ions by strong polyelectrolyte photonic hydrogels

Taking Cu2+, Pb2+ and Ag+ as examples of target cations, we describe a general strategy for constructing photonic metal ion sensing hydrogels that allow highly sensitive and selective detection via visual color changes. The sensors consist of heavy metal ion sensitive hydrogels with inverse opal structure. The presence of target cations causes shrinkage of the hydrogels and blue shift of the refraction peak wavelength with a detection limit lower than 1 nM. The interconnected macropores and the preorganized ligands makes these sensors sensitive and selective to the target ions without significant interference from pH, while the response mechanism based on the release of water of hydration after coordination prevents interference from ionic strength and anions. With this sensory system, direct, sensitive, rapid and selective detection of heavy metal ions with a broad dynamic range is achieved without expensive instruments, providing general methods for designing colorimetric metal ion sensing materials.