Surface modification of carbon dots with tetraalkylammonium moieties for fine tuning their antibacterial activity

In this contribution, we report a facile functionalization method of carbon dots (CDs) by tetraalkylammonium moieties using diazonium chemistry to improve their antibacterial activity against Gram-positive and Gram-negative bacteria. CDs were modified by novel diazonium salts bearing tetraalkylammonium moieties (TAA) with different alkyl chains (C2, C4, C9, C12) for the optimization of antibacterial activity. The functionalized CDs were characterized by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared (FTIR) spectroscopy, ultraviolet–visible (UV-vis) absorption spectrophotometry and fluorescence spectroscopy to confirm the covalent binding of TAA onto the CDs’ surface. Variation of the alkyl chain allows to reach the strongest antibacterial effect for CDs-C9 towards Gram-positive S. aureus (MIC 3.09±1.10 µg mL-1) and Gram-negative E. coli (MIC 7.93±0.17 µg mL-1) bacteria. In addition, the antibiofilm properties of the CDs-C9 were assessed; full biofilm inhibition was achieved after 6 h (64 µg mL-1) and 2 h (128 µg mL-1) treatment for S. aureus biofilm and >60% suppression of biofilm mass after 6 h (128 µg mL-1) for E. coli biofilm. CDs-C9 demonstrated good biocompatibility on mouse fibroblast (NIH/3T3), HeLa and U-87 MG cell lines for concentrations up to 256 µg mL-1. Collectively, our work highlights the correlation between the surface chemistry of CDs and their antimicrobial performance.