In Situ Synthesis of Luminescent CdSe Quantum Dots Embedded in Cd(II) Metallohydrogels: A Hybrid Material for Selective and Naked-Eye Sensing of Hg₂²⁺ Ions

In situ synthesis and stabilization of quantum dots (QDs) with unique features toward environmental remediation is an area of current interest. Our present study has addressed the spontaneous formation of a metallohydrogel of Cd(II) and a mixed ligand system (sodium azide and 4,5-dicyanoimidazole). The gelation was highly selective to all three components and was only achieved when the components were present together. Impact of different pure and mixed solvent systems on metallogelation was also studied. Spontaneous and high-yield growth of fibrous bundled nanostructures, majorly resulting from the metal-ligand coordination in association with solvent-anticipated hydrogen-bonding interactions, are observed. The overall gel integrity and the corresponding nanostructure exhibit stimulus-responsive behavior toward various external stimuli. The pristine Cd(II) metallogel was then explored as a scaffold for in situ fabrication of CdSe QD) which remain fastened with the metallogel fibers. The QD-tethered metallogel was luminescent in nature where the emission behavior could be easily tuned. Finally, this hybrid luminescent gel [i.e., CdSe-tethered Cd(II) metallohydrogel] was successfully employed as a material for selective and naked-eye sensing of toxic heavy metal ion Hg₂²⁺ down to the micromolar level. This study thus highlights the potential of such metallo-supramolecular hydrogel systems in photophysics and environmental remediation.