Enhanced antibacterial activity with increasing P doping ratio in CQDs

It is an urgent challenge to develop efficient antibacterial agents against resistant bacteria in the treatment of infectious diseases. Carbon quantum dots (CQDs) have attracted much attention owing to their good stability, low toxicity and excellent biocompatibility. In this work, CQDs doped with different contents of the element phosphorus (P) were prepared by a simple hydrothermal method using valine as a carbon source, triethylamine as a nitrogen source and different volumes of phosphoric acid as a phosphorus source. The average diameter and the surface charge could be regulated from 2.89 nm to 1.56 nm and +2.58 mV to +5.47 mV by increasing the content of the element P in these CQDs. Importantly, these CQDs showed effective bacterial inhibition against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The minimal inhibitory concentration (MIC) decreased from 0.71, to 0.51 to 0.18 mg mL^-1 on E. coli and S. aureus with the increase of P element content. Furthermore, the morphologies of E. coli cells and S. aureus were damaged and became irregular upon treatment with these CQDs. The results of singlet oxygen (¹O₂) detection demonstrated that intracellular ¹O₂ was generated during the antibacterial process. We speculated that bacterial inhibition induced by these CQDs was accompanied by disruption of permeability and structural integrity, owing to strong electrostatic interactions between negatively charged bacteria and positively charged CQDs and production of singlet oxygen of CQDs. Together, this study indicates that the CQDs can be a candidate to treat resistant bacterial infections and may improve the understanding of killing pathogens by antibacterial CQD drugs.