Biocompatible upconversion ink for hidden anticounterfeit labeling

Ink for rapid the application of anticounterfeit labels by means of standard printing devices has been developed based on upconversion nanoparticles (nanophosphors). Printing ink is made of an aqueous dispersion of nanoparticles with a NaYF4:YbTm/NaYF4 core–shell structure at a concentration of 0.5 mg/mL. The surface of nanoparticles is modified with amphiphilic polymers. The biosafety of ink is demonstrated in primary cultures of human fibroblasts. The hidden labeling, which is invisible in ambient lighting, is performed by the method of inkjet printing on paper. A printed image is visualized by IR laser irradiation at a wavelength of 975 nm. It is demonstrated that additional modalities of protection can be obtained by encoding the spectra and intensities of the anti-Stokes luminescence lines when combining the dopant lanthanides in nanoparticles.