## 3 Results
### Antibacterial Properties  
Antibacterial efficacy is a crucial parameter for assessing the potential of positively charged CQDs as antibacterial agents in biomedical applications. In this study, _E. coli_ was utilized as a model to evaluate the antimicrobial properties against Gram-negative bacteria. The images in Figure 5a depict the antibacterial results of _E. coli_ with the MICs of CQDs. MICs were visually assessed through tube turbidity before and after incubation with bacteria. The MICs of QCQD exhibited observable bacterial growth at various concentrations of CQDs (0.0001, 0.001, 0.01, 0.1, and 1 ppm). The results indicate that concentrations below 0.1 ppm maintained a clear and transparent solution, while concentrations of 0.1 ppm and  
Figure 4: UV–vis absorption and emission spectrum of CQDs (peak fitting of PL spectrum with two peaks (I and II)) (inset: optical images under daylight and UV light at 254 nm). After determining the minimum inhibitory concentrations (MICs), a pivotal phase in our analysis involved streaking and culturing samples on nutrient agar using the smoothing method. Subsequently, these cultures underwent a meticulous 24 h incubation at 37 \({}^{\circ}\)C, providing a comprehensive exploration of the minimal bacterial concentration (MBC). The detailed findings illustrated in Figure 5b offer profound insights into the efficacy of CQDs against _E. coli_. The observed MBC of CQDs for _E. coli_, established at 1 ppm in our experimental context, serves as a crucial indicator of their inhibitory potential. This result underscores that, even at a low concentration of 1 ppm, CQDs effectively halt the growth of _E. coli_ bacteria. A systematic approach involving nutrient agar and the incubation process ensures a robust assessment of bacterial concentration thresholds, reinforcing the reliability and significance of our findings. In Figure 6, the photograph illustrates the quantity of viable _E. coli_ bacterial cells after treatment, as evaluated through a disk-diffusion test employing CQDs, as shown in Figure 6A,B. The antibacterial rate of the CQDs against _E. coli_ is 57%, corresponding to statistical histograms after treatment with control and CQDs samples at a concentration of \(10^{-8}\), as assessed by determining the number of surviving bacteria in the culture dish, as seen in Figure 6C,D. Further details regarding the evaluation of antibacterial efficacy were elucidated using the standard plate count method. The remarkable antibacterial effectiveness of CQDs is intricately linked to their inherent physicochemical and functional properties. This efficacy is mainly attributed to the induction of oxidative stress initiated by the generation of reactive oxygen species (ROS) during photoactivation [40; 41]. These ROS, produced through this photoactivation process, exhibit versatile capabilities, including deactivating proteins, inducing nucleic acid damage, and initiating lipid peroxidation. This antibacterial mechanism acts as a robust disinfection process, specifically targeting bacteria. The consequences of this oxidative stress cascade extend to the disruption of bacterial cell walls and the deterioration of infective biofilm matrices. The insights from Figure 6  
Figure 5: (**a**) Minimum inhibitory concentration (MIC) and (**b**) minimum bactericidal concentration (MBC) of CQDs against _E. coli_. highlight the compelling antibacterial activity of CQDs, emphasizing their potential as effective agents in disinfection strategies against _E. coli._  
The observed increase in the measured inhibition diameter of bacteria after exposure to CQDs emphasizes their efficacy in hindering bacterial growth and biofilm formation. Our study was specifically designed to directly investigate the antibacterial properties inherent in the CQD sample when challenged with the _E. coli_ bacterial strain. It is noteworthy that CQDs consistently demonstrated the ability to inhibit _E. coli_ growth across various exposure times, aligning with established findings in the scientific literature [42,43].