Large-molecular-weight PVP-functionalized stacked upconversion nanoplates for high-performance individual marking

The enhancement of individual marking performance of taggants remains a challenge for special character tracking even after decades of development. Here, a method based on the polyvinyl pyrrolidone (PVP) functionalization of upconversion nanoplates (UCNPs) to improve the interaction between the taggant and skin is reported. It is shown that the molecular weight of the PVP used to decorate the surface of stacked Fe3+-doped NaBiF4:Yb/Er UCNPs (Fe-UCNPs) plays a critical role in enhancing taggant marking performance. This can be attributed to the strong noncovalent interactions between the functionalized UCNPs and skin which facilitated surface adhesion as well as the effective chemical penetration concentrated in the limited depth of the stratum corneum due to the long polymer chain and the mm-size-induced blocking of the UCNPs. We believe that these results may shine light on the exploration of individual marking taggants and may help promote the practical application of fluorescent taggants for special character tracking.