Zinc Complexation Driven Small Spherical Copper Nanocluster Assemblies with Size-Dependent High Antibacterial and Anticancer Activities

A stable, narrow-sized (7.95 nm), spherical fluorescent assembly of Cu nanoclusters (NCs) was synthesized in aqueous media by zinc-ion mediated complexation reaction using mercapto propanoic acid (MPA) and l-cysteine (CYS) as stabilizing ligands. The extent and size of the assemblies could be tuned by varying the concentration of Zn²⁺ ions, offering a convenient strategy to control the morphology and emission properties of the Cu NCs assemblies. Notably, the synthesized Zn Cu NCs assemblies could be stored in solid form for over 2 months without structural or photophysical degradation and could be redispersed on demand, retaining their fluorescence and size. The assemblies exhibited bright red fluorescence with a quantum yield of 7.37%, a fluorescence lifetime of 361.32 ns, and a delayed luminescence lifetime of 163.46 μs, making them promising candidates for cell imaging. The biological activity of these assemblies-both antibacterial and anticancer-was size dependent, with smaller sized assemblies showing enhanced efficacy. In antibacterial studies, the smaller assemblies maintained significant activity against Staphylococcus aureus even 2 weeks after synthesis. In cancer studies, the Zn Cu NCs assembly demonstrated strong anticancer activity against HeLa cervical cancer cells with an IC₅₀ value of 21.1 μg/mL, likely due to increased ROS generation. The red emissive assemblies also enabled effective fluorescence imaging of HeLa cells, demonstrating their dual function as therapeutic and diagnostic (theragnostic) agents. These findings underscore the role of zinc ion-induced controlled assembly formation in producing small, robust, reusable fluorescent Cu NC assemblies, and point toward their multifunctional potential in biomedical applications.