Multimodal smart sensing via wavelength-selective hydrochromism in zero-dimensional metal halides

Smart luminescent materials that are dynamically responsive to external stimuli are crucial for advanced sensing and encryption devices; however, integrating multimodal responsiveness into a single platform remains challenging. Herein, we present a versatile zero-dimensional metal halide, Cs ₃ GdCl ₆ , doped with Yb ³⁺ , Er ³⁺ , and Tb ³⁺ to achieve distinct sensitivities to UV, X-rays, temperature, and moisture. Characterization results and analyses allowed us to uncover a unique excitation-wavelength-dependent hydrochromic mechanism in Cs ₃ Gd _0.8 Er _0.2 Cl ₆ . The material retained its yellow emission without color variation under 980 nm excitation, whereas it underwent a rapid, green-to-red hydrochromic shift under 1550 nm excitation. Kinetic analysis confirmed that the absorption cross section is the decisive factor; sufficient absorption capability is a prerequisite for populating moisture-sensitive high-energy levels via energy transfer upconversion. By exploiting these properties, we developed two distinct application platforms: a dynamic pattern plate for qualitative multi-stimuli visualization and a power-free, sticker-type humidity dosimeter capable of quantitative analysis based on Fick’s second law.