Metal Ion-Triggered Collapsed Hydrogels as Materials for Bacterial Population Control.

Hydrogels are versatile soft materials extensively employed in applications ranging from biomedicine to environmental technologies owing to their high water content and tunable physicochemical properties. In this work, we describe a metal ion-induced collapse phenomenon in poly(vinyl alcohol) (PVA)-borate hydrogels triggered by Cu2+ cations, leading to a class of materials termed metal ion-triggered collapsed hydrogels (MitCH). Upon increasing the concentration of copper sulfate, conventional PVA-borate hydrogels undergo an abrupt contraction accompanied by water expulsion and formation of a rubbery, mechanically robust solid enriched in copper and boron species. Elemental and ICP analyses reveal copper loadings of up to ∼14 % wt, whereas sulfate and sodium ions are largely released during collapse, indicating extensive reorganization of the coordination environment. The collapse process occurs at neutral pH and depends critically on copper concentration, counterion identity, and the molecular weight and degree of hydrolysis of PVA. Structural characterization suggests non-crystalline structures; however, electron paramagnetic resonance clearly evidences antiferromagnetically coupled copper centers. MitCH materials remain stable in water but dissolve in nutrient-rich biological media, enabling environmentally responsive release of copper ions. This behavior was successfully exploited to control the growth of Escherichia coli, showing complete inhibition at sufficient material loadings. These findings demonstrate that MitCH materials constitute a promising platform for controlled antimicrobial applications combining simple water-based preparation methodology, relatively high metal loading, aqueous stability, and environment-responsive release.