A water-based ink for use in an ink jet printer for printing machine-readable indicia on a substrate is disclosed. The ink jet ink comprises fluorescent nanoparticles having a diameter between about 30 nm and about 100 nm; and an aqueous liquid vehicle. The aqueous liquid vehicle comprises water and a water soluble vehicle in sufficient amounts to achieve an ink viscosity and surface tension effective for application of the ink jet ink to the substrate in a predetermined pattern by ink jet printing.