Portable and writable photoluminescent chalk for on-site information protection on arbitrary substrates

Information protection is particularly important for secure communication between parties, avoiding the leakage of secret information to unknown parties. Among various strategies used for information protection in the previous reports in the literature, security inks made from photoluminescent materials are the most widely investigated. The security inks suffer from the drawbacks such as lack of universality on various substrates. In this work, we provide a promising new complement to the security ink-based protection techniques. We report a kind of portable photoluminescent chalk made from lanthanide-doped ultralong hydroxyapatite nanowires, which can directly write the covert information at anytime and anyplace on arbitrary surfaces and substrates including paper sheets, metals, fabrics, plastics, woods, walls, foams, leaves, and even human body. The as-written original information cannot be recognized under the ambient light, but it is readable upon exposure to the ultraviolet light. In this work, the dual and triple encryption strategies are also provided for the high-level information protection. It is expected that the versatile, convenient, and user-friendly strategy demonstrated herein is promising for the on-site secret information protection.