Direct Photolithographic Deposition of Color‐Coded Anti‐Counterfeit Patterns with Titania Encapsulated Upconverting Nanoparticles

Creating security labels as anti‐counterfeit measures can require multi‐step methods, clean room processing, and high‐cost equipment. Some labels also have a limited applicability due to the ease of creating a counterfeit. Herein, a photochemical metal‐organic deposition (PMOD) based approach that enables creation of high‐resolution luminescent patterns that retain nanoparticles in transparent metal oxide films is reported. This low‐cost, photoresist‐free process creates high‐resolution patterns of metal oxides without requiring processes such as etching or lift‐off. Upconverting nanoparticles (UCNPs) with tunable red/green or blue emission are prepared by doping Yb3+/Er3+ and Yb3+/Tm3+ into β‐NaYF4 hosts, respectively. Luminescent inks are prepared by suspending UCNPs in solutions with titanium di‐n‐butoxide bis(2‐ethylhexanoate). Customizable luminescent patterns are prepared by casting inks onto substrates, followed by exposure to ultraviolet light through photomasks. Photodecomposition of the titanium precursor yields amorphous oxide films encapsulating the UCNPs. Security labels are prepared by selectively patterning luminescent inks using PMOD. Distinct patterns of red‐green‐blue (RGB) luminescence are discernible only upon excitation with a near‐infrared (NIR) laser. These customizable, anti‐counterfeit labels exhibit the merits of low‐cost, high‐throughput, and simple manufacturing techniques. Yet, the versatility of customizing their emissive properties suggests a practical application as an anti‐counterfeiting measure.