Engineering a water-compatible upconversion nanoink for NIR-readable covert printing and QR authentication

Lanthanide-doped upconversion nanoparticles (UCNPs) offer an innovative route to covert optical labels that become visible under near-infrared (NIR) excitation. Here, LiYF₄:Yb, Er@LiYF₄ core–shell (CS) nanoparticles were coated with poly(acrylic acid) (PAA) and used to create a printable nanoink for water-based printing. X-ray diffraction (XRD) and transmission electron microscopy (TEM) supported retention of the tetragonal phase and particle morphology after PAA treatment, while FTIR measurements were consistent with altered surface chemistry. Photoluminescence studies revealed the increase in the intensity of the green emission by about 2.7 times upon LiYF4 shell formation. The background-corrected green peak intensity of inks with concentrations of CS-PAA of 1–20 mg mL − 1 increased by about 28 times. Four nanoink formulations were tested in this study, and the ink with the 65:35 volume ratio of 10% aqueous ethanol to glycerol was selected, and covert text was printed with a Canon PIXMA MG5320 printer. QR-code readout used NIR camera acquisition followed by smartphone decoding of the camera image displayed on a computer screen. Decoding success rates over 20 decoding attempts were 5%, 25%, and 85% for inks with concentrations of 1, 2.5, and 5 mg mL − 1 , and 100% for inks with concentrations of 7.5, 10, and 20 mg mL − 1 . These results highlight the potential of aqueous LiYF₄-based inks for NIR-readable covert printing, with promising applications in anticounterfeiting, product authentication, and covert information encoding.