Introduction: Antibiotic resistance has become one of the most pressing global health challenges, limiting the effectiveness of conventional treatments and increasing the risk of untreatable infections. This crisis has motivated the search for alternative antimicrobial approaches that are both effective and safe. Among nanomaterials, carbon dots (CDs) have attracted significant attention due to their small size, strong fluorescence, low toxicity, ease of synthesis, and low cost. CDs also possess photosensitizing properties, which allow them to generate reactive oxygen species under light irradiation. This makes them promising candidates for antimicrobial photodynamic therapy (aPDT), a method that uses light-activated agents to destroy microbial cells. The present study aimed to synthesize and characterize carbon dots with photosensitizing ability and to evaluate their antimicrobial effects against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli under light and dark conditions.
Methods: Carbon dots were synthesized using a hydrothermal method, involving treatment at 160 °C for 4 hours. After synthesis, the samples were purified and their pH adjusted. Structural and optical characterization was carried out using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and ultraviolet-visible (UV-Vis) spectroscopy, confirming the successful synthesis and optical properties of the CDs.
To evaluate antimicrobial activity, two bacterial strains were chosen: S. aureus (Gram-positive) and E. coli (Gram-negative). Antibacterial effects were assessed in 96-well plates using minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) tests. Each test was repeated three times under two conditions: light exposure using a handmade light source, and complete darkness as a control. The influence of carbon dot concentration and light exposure on bacterial growth inhibition was systematically analyzed.
Results: The synthesized carbon dots exhibited strong fluorescence and low toxicity, consistent with expectations from previous reports on their biocompatibility. Characterization confirmed their structural stability and optical responsiveness.
In antimicrobial tests, results demonstrated that increasing concentrations of CDs enhanced inhibition of both bacterial strains. Importantly, light exposure acted as a catalyst, significantly accelerating the antibacterial effect compared to dark conditions. Under illumination, MIC and MBC values were reached at lower concentrations and in shorter times, indicating a photodynamic enhancement of antimicrobial activity.
The comparative results between Gram-positive and Gram-negative bacteria suggested that both species were susceptible, although variations in sensitivity were observed, possibly due to differences in their cell wall structures. Overall, the study highlighted a concentration-dependent and light-enhanced antimicrobial performance of the synthesized CDs.
Conclusion: This study successfully synthesized and characterized photosensitizing carbon dots with strong fluorescence, low toxicity, and promising antimicrobial effects. The findings demonstrate that CDs can effectively inhibit both S. aureus and E. coli, with light significantly enhancing their activity. These results support the potential application of CDs in antimicrobial photodynamic therapy and other biomedical uses, particularly as alternatives to conventional antibiotics in the fight against microbial resistance.
The research contributes to the growing body of evidence that nanomaterials, and specifically carbon-based nanostructures, can serve as effective antimicrobial agents. By combining affordability, safety, and efficiency, CDs represent a valuable platform for future biomedical innovations aimed at addressing antibiotic resistance.