A Concept of Haze Matching for Cloaking of Bulk Anticounterfeiting Patterns Made of Silk Fibroin-Based Upconversion Nanocomposites

Since 2010, anticounterfeiting technology utilizing NaYF₄-based upconversion nanoparticles (UCNPs), which can emit visible light under invisible near-infrared (NIR) excitation, has been extensively studied. Typically, upconversion patterns and codes are realized by printing viscous nanocomposites composed of a polymer matrix and UCNPs. There are two technical issues: the first is to select a sustainable and environmentally friendly polymer matrix, and the second is to increase the concentration of UCNPs doped in the polymer matrix, considering the extremely low quantum yield of UCNPs. However, when a large amount of UCNPs are concentrated in a unit volume, scattering by nanoparticles increases, creating an opaque pattern. Because bulk patterns are intuitively identifiable, codes can be generated based solely on differences in the colors emitted by NIR excitation. One could consider making the pattern transparent to a level similar to the background by extremely reducing the concentration of UCNPs, but this would exponentially increase the output of the 980 nm light required for decoding. As alternatives, invisible UCNP-doped microbarcodes or microparticles have been proposed, but their processing involves expensive lithography and still requires inconvenient microscopy systems for decoding. Therefore, the practicality of UCNP-based anticounterfeiting platforms in actual fields outside the laboratory is very limited.<br/><br/>In this study, we propose for the first time the concept of haze matching, a convenient way to conceal bulk anticounterfeiting patterns by making the background opaque to a similar level as the pattern. This approach has the advantage of being able to easily control the optical characteristics of the background regardless of the haze level of the pattern, allowing the use of high-concentration UCNP patterns with bright upconversion emission. Regenerated silk fibroin extracted from silkworm cocoons was used as a sustainable and eco-friendly medium to disperse UCNPs. Dopant-free NaYF₄ nanoparticles synthesized under the same hydrothermal conditions were prepared as a medium for haze matching. Based on optical analysis of the transmittance and haze at various nanoparticle concentrations, we established the optimal conditions for the background to completely conceal the bulk pattern with extremely high concentrations of UCNPs. A variety of bulk codes and tags that appear selectively only under NIR excitation have been successfully demonstrated through printing/painting techniques. In particular, opaque anticounterfeiting films containing haze-matched codes and tags were more difficult to identify on white surfaces such as paper or ceramics.<br/><br/>The strategy for upgrading the sustainable nanocomposite-based anticounterfeiting platform proposed in this study will provide new options for the practical use of UCNPs.