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

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 x 10(7) CFU/mi. of gram-positive bacteria Staphylococcus aureus and Bacillus subtilis with 30 mu g/mL of TA-CuNCs for 10 min, the bacteria are completely inhibited, while under same conditions the viabilities of gram-negative bacteria Escherichia coli 0157: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 (MIT) assay with HepG2 and 293 Tcells, giving rise to cell viability of 94.2% for HepG2 and 96.7% for 293 T by incubating 10(6) cell/mL with 200 mu g/mL of TA-CuNCs for 24 h. These results make TA-CuNCs a potential alternative as bactericide for infection treatment caused by gram-positive bacteria. (C) 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.