Moisture-Induced Reversible Phase Conversion of Cesium Copper Iodine Nanocrystals Enables Advanced Anti-Counterfeiting

Hydrochromic materials have attracted widespread attention in the fields of anti-counterfeiting because of their ability of the reversible light absorption and/or emission properties in response to water. Here, for the first it is demonstrated that the ternary copper halides Cs 3 Cu 2 I 5 nanocrystals (NCs) possess excellent hydrochromic properties. The prepared Cs 3 Cu 2 I 5 NCs films can dynamically extract and insert CsI by exposing/removing water to realize the reversible conversion between blue-emissive Cs 3 Cu 2 I 5 and yellow-emissive CsCu 2 I 3 . Interestingly, polymethyl methacrylate (PMMA) coated Cs 3 Cu 2 I 5 can effectively avoid the extraction of CsI and maintain long-term stability in the water. Further, the hydrochromic Cs 3 Cu 2 I 5 and water-resistant Cs 3 Cu 2 I 5 @PMMA are used as the inks to synergistically act on anti-counterfeiting information to achieve multiple encryption effects, which can clearly identify and authenticate the effective information after moisture decryption. Importantly, the pattern can be re-encrypted to the invalid pattern after water evaporation. In addition, the anti-counterfeiting pattern has excellent stability during repeated encryption/decryption conversion cycles, which can not only balance the accessibility of anti-counterfeiting information but also effectively improve the security of information. This new discovery may not only deepen the understanding of Cs 3 Cu 2 I 5 but also provide new options for the design of hydrochromic materials for anti-counterfeiting information.