Improvement of antibacterial activity of copper nanoclusters for selective inhibition on the growth of gram-positive bacteria

A novel copper nanoclusters, i.e., tannic acid (TA) capped CuNCs (TA-CuNCs), exists high antibacterial activity for selectively inhibiting/against the growth of gram-positive bacteria by damaging the cell membrane. [Display omitted] In general, copper nanoclusters (CuNCs) possess very low or even virtually no bactericidal effect. Herein, we report a novel CuNCs possessing significantly high antibacterial activity, that is tannic acid (TA) capped CuNCs (TA-CuNCs). TA-CuNCs exhibit strong absorption and excitation-dependent fluorescence within pH 2–12, resulting from the functional groups of TA-CuNCs due to two prototropic equilibria, phenol ↔ phenolate and carboxylic ↔ carboxylate. There exists synergistic effect of TA and copper nanoclusters which endows TA-CuNCs remarkable antibacterial capability as a microbicide, as characterized by the effective inhibition on the growth of gram-positive bacteria by damaging the cell membrane. By incubating 1×107 CFU/mL of gram-positive bacteria Staphylococcus aureus and Bacillus subtilis with 30μg/mL of TA-CuNCs for 10min, the bacteria are completely inhibited, while under same conditions the viabilities of gram-negative bacteria Escherichia coli O157:H7 and Pseudomonas aeruginosa remain 85.0%, 72.0%, respectively. In addition, TA-CuNCs exhibit low cytotoxicity and favorable biocompatibility demonstrated by standard methyl thiazolyl tetrazolium (MTT) assay with HepG2 and 293T cells, giving rise to cell viability of 94.2% for HepG2 and 96.7% for 293T by incubating 106 cell/mL with 200μg/mL of TA-CuNCs for 24h. These results make TA-CuNCs a potential alternative as bactericide for infection treatment caused by gram-positive bacteria.