Synergistic Effects of Copper and Tin Doping in ZnS Quantum Dots: A Comprehensive Study of Optical, Electrochemical, and Antibacterial Properties

Semiconductor quantum dots (QDs) offer tunable bandgaps, high surface areas, and defect-related properties that support energy-efficient devices and antimicrobial technologies. Zinc sulfide-based nanostructures offer wide band gaps and chemical stability, but their limited conductivity and ultraviolet absorption limit their electrochemical and antibacterial performance. A systematic understanding of the influence of Cu2+ incorporation into Sn-modified ZnS quantum dots on their structure, bandgap, charge-storage behavior, and bactericidal activity remains lacking. This work investigates Cu2+-doped Zn0.9Sn0.1S quantum dots as a multifunctional material for energy efficiency and antimicrobial engineering applications. Cu-free and Cu-doped Zn0.9Sn0.1S quantum dots were synthesized by coprecipitation at room temperature and were characterized by structural, optical, and electrochemical techniques, while agar well diffusion assays were performed against Streptococcus oralis and Klebsiella pneumoniae. The nanocrystals retain a cubic ZnS lattice with increasing microstrain, reduced crystallite size, and lattice contraction upon Cu2+ doping, and exhibit band gap narrowing from 4.05 to 3.75 eV with enhanced ultraviolet absorption and defect-related visible emission. The 4% Cu-doped sample delivers a specific capacitance of 137.9 F/g, a charge-transfer resistance of 8.88 kΩ, and strong inhibition zones against the tested bacterial strains at 500 µg/mL. These findings confirmed the presence of Cu2+-doped Zn0.9Sn0.1S quantum dots as promising candidates for supercapacitor electrodes in energy efficiency systems and as antibacterial coatings for biomedical and dental engineering surfaces. Future work should investigate the cycling stability, ion-release behavior, and integration of these quantum dots into renewable energy and antimicrobial device architectures.