A method of preparing a metal nanoparticle can include depositing an ink on a substrate using nanolithography to form a polymer reactor, wherein the ink comprises at least one metalloporphyrin functionalized polymer comprising a metal ion, porphyrin, and a polyethylene oxide polymer; and thermally annealing the polymer nanoreactor under conditions sufficient to form the nanoparticle. The thermal annealing can include a first stage that includes annealing the polymer nanoreactor at a temperature of about 100° C. to about 350° C. for a time sufficient to aggregate the metal ions in the polymer reactor, and a second stage that includes annealing the aggregated polymer nanoreactor at a temperature of about 500° C. to about 600° C. for a time sufficient to reduce the metal ion and decompose a polymer component of the polymer nanoreactor to thereby form the metal nanoparticle.