A highly specific biochemical-to-optical signal transduction scheme combines three distinct phenomena: a biocatalytic process, a localized surface plasmon resonance effects in noble metal nanoparticles, and the swelling-shrinking transition in a stimuli-responsive hydrogel. An ultrathin polymer-nanoparticle plasmonic sensor (see figure) is designed for two important applications: analysis of biomolecules and probing (monitoring) of local properties of biomaterials.