Construction of bowl-shaped copper clusters and nitrate-selective sensing

Constructing metal cluster materials that combine well-defined recognition sites with highly sensitive fluorescent sensing capabilities has remained a significant challenge. Here, we employed ligand engineering to modify reported Cu₄ structures, successfully synthesizing a cluster-based bowl-shaped molecule. The ingenious introduction of urea groups endows the inner walls of bowl-shaped molecules with abundant binding sites. Additionally, the flexibility of both the ligands and the cluster allows the pocket to adaptively adjust, enabling interactions with a variety of anions, including BF₄^-, PF₆^-, HCOO^-, ClO₄^- and NO₃^-. Integrating experimental observations with theoretical calculations reveals that nitrate ions demonstrate the strongest binding affinity towards the bowl-shaped cluster molecules, and their binding significantly modulates the electronic transition gap, leading to luminescence quenching of the cluster. Based on this, we investigated the fluorescence sensing properties of the Cu₄ cluster targeting NO₃^-, and further developed a "shut-off"-type sensor for detecting NO₃^- anions, with a detection limit as low as 0.76 µM.