Near-infrared-light-mediated high-throughput information encryption based on the inkjet printing of upconversion nanoparticles

Information security has attracted broad attention in today's information age, and information encryption on paper has been widely studied since paper is still the most important information carrier. Fluorescent inks are commonly used in information encryption on paper, but they suffer from background fluorescence interference. Herein, we develop a background-free and easy-to-perform method for information encryption based on the inkjet printing of upconversion nanoparticles (UCNPs). The UCNPs can efficiently eliminate background fluorescence interference since phosphors in paper cannot be activated by near-infrared (NIR) light. Moreover, owing to their small size, excellent dispersibility and good stability, UCNP inks can be directly applied to commercial inkjet printers for convenient and high-throughput information encryption on paper. Information was easily printed on different kinds of paper substrates and the information can only be visualized under NIR light excitation. Furthermore, a novel information encryption strategy was designed by utilizing UCNPs with different excitation wavelengths. Only excitation at the defined wavelength can obtain the correct information. This proposed information encryption strategy can completely avoid background fluorescence interference, and it also features easy operation, high throughput as well as low costs, indicating its good promise to serve as a household encryption method in our daily life.