Luminescence Enhancement of a Gold Nanocluster Hydrogel Facilitated by Water for Erasable Water Writing and Visual Solvent Differentiation

Gold nanoclusters (AuNCs) with an ultrasmall size have attracted enormous interest due to their unique luminescence properties. However, manipulating/improving the luminescence of AuNCs through a convenient method is still challenging. Herein, effective enhancement of emission by simply using water is reported for the AuNCs stabilized by 4,6-diamino-2-pyrimidinethiol (DPT). The DPT-AuNCs could aggregate into a hydrogel in aqueous systems, accompanied with luminescence enhancement (the quantum yield increased from 0.36 to 3.07%). However, the xerogel obtained by freeze-drying was non-luminescent. Addition of water to the xerogel could trigger the emission again. Differential scanning calorimetry, X-ray diffraction, and theoretical calculations reveal that water-bridged hydrogen bonds play crucial roles in the emission of DPT-AuNCs. Furthermore, the water-sensitive luminescence of DPT-AuNCs was applied for developing erasable water writing materials and for visual differentiation of solvents (e.g., protonic organic solvents, non-protonic organic solvents, and water). This study offers a green, cost-effective, and convenient strategy for enhancing the luminescence properties of AuNCs, reveals the crucial role of water molecules in the emission of AuNCs, and presents an application in optoelectronic functional materials.