Intense Hydrochromic Photon Upconversion from Lead‐Free 0D Metal Halides For Water Detection and Information Encryption

Abstract Hydrochromic materials that change their luminescence color upon exposure to moisture have attracted considerable attention owing to their applications in sensing and information encryption. However, the existing materials lack high hydrochromic response and color tunability. This study reports the development of a new and bright 0D Cs₃GdCl₆ metal halide as the host for hydrochromic photon upconversion in the form of polycrystals (PCs) and nanocrystals. Lanthanides co‐doped cesium gadolinium chloride metal halides exhibit upconversion luminescence (UCL) in the visible–infrared region upon 980 nm laser excitation. In particular, PCs co‐doped with Yb³⁺ and Er³⁺ exhibit hydrochromic UCL color change from green to red. These hydrochromic properties are quantitatively confirmed through the sensitive detection of water in tetrahydrofuran solvent via UCL color changes. This water‐sensing probe exhibits excellent repeatability and is particularly suitable for real‐time and long‐term water monitoring. Furthermore, the hydrochromic UCL property is exploited for stimuli‐responsive information encryption via cyphertexts. These findings will pave the way for the development of new hydrochromic upconverting materials for emerging applications, such as noncontact sensors, anti‐counterfeiting, and information encryption.