An Economic Naked-Eye Visualization Detection of Heavy Metal Ions Using a Shape Memory Hydrogel

Industrial development has led to contamination of the environment with heavy metal ions, which accumulate in soil or water and pose a serious threat to human health. Simple and rapid detection methods are effective in controlling or preventing the further increase of pollution. However, existing detection methods are relatively complex or require expensive instruments and special sample handling, often resulting in secondary contamination. Therefore, there is a real need for a simple and economical on-site detection method. Herein, we describe a new method for the detection of Fe3+ ions in industrial wastewater using shape change in a hydrogel. Metal ions coordinate with the carboxyl groups in a polyacrylic acid/poly­(vinyl alcohol) shape memory hydrogel, and this fixes temporary shapes that depend on the metal ions and their concentrations. This shape change is reversible by the dissociation of ethylenediaminetetraacetic acid. In particular, this change can be well reproduced and observed with the naked eyes. The shape memory performance experiments and density functional theory (DFT) simulation calculations showed that Fe3+ ions coordinated most strongly with the carboxyl groups in the hydrogel under experimental conditions. A quantitative relationship between the shape fixation ratio and Fe3+ ion concentrations was established. According to this, an economic on-site testing device was designed to achieve analogue effluent detection. The method has potential applications for rapid detection of Fe3+ ion concentration in highly acidic industrial wastewater, which offers the possibility of practical application of shape memory hydrogels in environmental management.