Reversible photo-induced formation of iron alginate hydrogels

Sodium alginate is well-known to be crosslinked by various polyvalent metal ions. While calcium ions (Ca²⁺) have been the most used, the crosslinking of alginate with other metal ions has received much less attention in the literature. For instance, Fe²⁺ and Fe³⁺ ions can also crosslink sodium alginate, though with varying strengths. A change in the charge of the iron ion can significantly affect the hydrogel's crosslinking density, potentially leading to full dissolution. This study demonstrates a novel approach to reversibly control alginate hydrogel formation and dissolution using visible light as an external stimulus. Visible light irradiation (450 nm) leads to the decomposition of the iron-containing sandwich complex (ISC). Liberated Fe²⁺ ions undergo quick oxidation by potassium peroxydisulfate and the resulting Fe³⁺ ions crosslink alginate chains to form a hydrogel. Conversely, treatment with 405 nm visible light induces a redox reaction between lactic acid and Fe³⁺ ions. The recovery of Fe³⁺ into Fe²⁺ leads to the hydrogel's full de-crosslinking and reversion to a solution. Notably, this process can be performed in a single step via visible light irradiation. The photochemical processes rapidly lead to gelation and re-gelation occurring within minutes. Envisaged applications of reversible photo-induced gelation are under discussion.