Preparation, characterization and applications, particularly for signal-stimulated molecule release, of Fe ³⁺ -cross-linked alginate hydrogels are critically overviewed.

Illustration of metal ions (Mⁿ⁺) detection using ascorbic acid-capped gold nanoparticles, with a visual color change observed by the naked eye at ∼20 ppb for Pb²⁺ and ∼30–40 ppb for Cr^3+/6+ cations.