 Nanoparticles may be formed on a substrate by mixing precursor solutions deposited by an ink-jet printer, as described in patent US2013316482 (A1). A first solution is deposited on a substrate from a first ink-jet print cartridge. Then, a second solution is deposited on the substrate from a second ink-jet print cartridge. The solutions may be printed in an array of droplets on the substrate. Nanoparticles form when droplets of the first solution overlap with droplets of the second solution. In one example, the nanoparticles may be gold nanoparticles (Au NPs) formed from mixing a first solution of 1,2-dichlorobenze (DCB) and oleylamine and a second solution of gold chloride trihydrite and dimethyl sulfoxide (DMSO). The nanoparticles may be incorporated into optoelectronic devices.