Nanoparticle-integrated hydrogels as versatile colorimetric sensors

Nanoparticles have been extensively employed as probes in colorimetric sensors due to their ease of surface functionalization and combined optical and catalytic properties. However, their intrinsic tendency to aggregate impairs the unique plasmonic properties and diminishes catalytic activity associated with their nanoscale size, potentially compromising long-term performance. To address these limitations, recent studies have focused on integrating nanoparticles into hydrogels, thereby limiting nanoparticle diffusion to overcome challenges in colorimetric sensing applications. In particular, nanoparticle-integrated hydrogels can preserve the colloidal stability of nanoparticles, ensuring practical applicability that is unattainable with free nanoparticles for on-site scenarios. This article comprehensively reviews recent advances in nanoparticle-integrated hydrogels for colorimetric sensing applications, with particular emphasis on their abilities to detect biological and environmental targets. The unique physicochemical properties of these hydrogels provide ideal platforms for deploying various polymeric and metallic nanoparticles that are highly sensitive to environmental changes and can visually indicate the presence of specific targets. Hydrogels also enhance sensor performance owing to their high-water contents, concentration effect, and distinctive swelling properties, which amplify sensor responses to target molecules. In the end, this review article outlines promising avenues for future research in this dynamic field.