The present application relates to polymer microparticle-metal nanoparticle composites, to methods of preparing polymer microparticle-metal nanoparticle composites and to uses of such composites. The methods comprise introducing into a microfluidic device, a composition comprising: a cationic metal nanoparticle precursor; a polymer microparticle precursor that comprises a plurality of photopolymerizable groups; and a photoreducer-photoinitiator; then irradiating the composition under conditions to simultaneously reduce the cationic metal and polymerize the photopolymerizable groups to obtain the composite.