Spiropyran-Functionalized Polydimethylsiloxane Sponges for Reversible Adsorption of Metal Ions

Toxic metal ion pollution poses significant risks to ecosystems and human health, necessitating effective remediating strategies. Various methods have been developed for the removal of metal ions from the environment. However, the existing methods for metal ion removal are often complex and do not ensure retrieval of the removed ions or reusability of the retrieving material. In this work, we show the preparation of a methacrylate spiropyran-functionalized polydimethylsiloxane sponge (PDMS-SP) to capture metal ions from the environment. The photoresponsive SP content of the samples is about 0.23%, endowing these materials with the unique feature of reversible complexation that can be remotely modulated by light. Spiropyran can reversibly isomerize to merocyanine in these sponges; the latter isomer can make complexes with some toxic cations: Al³⁺, Zn²⁺, Fe³⁺, Co²⁺, Sn²⁺, and Hg²⁺. The sponges can also complex with Ca²⁺ and Mg²⁺. UV–vis spectroscopy was used to calculate the complexation yields and adsorption capacity. As an example, the Fe³⁺ adsorption capacity of the PDMS-SP sponge was calculated as 2.60 ± 0.15 mg g^–1. We also demonstrate that the ions can easily be retrieved from the ion-adsorbed sponge by rinsing the sponge with a solvent under visible light. The sponges can be reused for at least 5 cycles without fatigue, mechanical deformation, or significant loss of adsorbent activity.