We propose a novel, facile and low-cost method to prepare rewritable coatings with nanoparticles buried in gelatin-alginate hydrogel layer using water as the ink. Firstly, polystyrene-co-acrylic acid (PS-co-AA) nanoparticles mixed with gelatin were coated on various substrates such as microscope slides, plastics or black paper, and appeared white after water sprayed. The transmittance from transparency to opaqueness was irreversible in the inked regions even after the complete water evaporation. It was believed that PS nanoparticles immigrated from the bulk of gelatin network to its surfaces due to the Brownian motion.
When the gelatin was swollen by water, the nanoparticles stuck to the surfaces after water evaporation. Secondly, to testify the principle, we further coated additional gelatin-alginate hydrogel layer on the top to prevent the immigration of nanoparticles to outer surfaces. It was found that ink marks can be erased after water evaporation, and such write-and-erase process can be repeated multiple times with excellent performances. These results showed the potentials of the rewritable hydrogel coatings in anti-counterfeiting and biosensing applications.