## 3 Results and Discussion
### Bacterial Viability Assays of the Cys-AuNCs and Cys-AgNCs  
For application as a fluorescent probe, nanoclusters have to be highly biocompatible in bacteria. To investigate the bacterial viabilities, the Cys-AuNCs and Cys-AgNCs with various concentrations were respectively incubated with _E. coli_. As shown in Figure 4, without incubation with the Cys-AuNCs, Cys-AgN NCs, or silver nanoparticles (AgNPs), the OD600 value of the _E. coli_ solution was measured to be \(\approx\)1.00 (the curve of _E. coli_) after culturing for 180 min. Furthermore, as seen in Figure 4a, the bacterial growth curves revealed that the bactericidal effect was increased with the concentrations of the Cys-AuNCs. The OD600 values of the _E. coli_ solutions incubated with the Cys-AuNCs at different concentrations, namely, 0.175, 0.35, 0.7, 1.4, and 2.8 mg/mL, were respectively found to be 0.99, 0.93, 0.92, 0.82, and 0.63 after culturing for 180 min. Moreover, as seen in Figure 4b, the bacterial growth curves of _E. coli_ also demonstrated that the antibacterial effect of the Cys-AgNCs increased with the concentrations. The OD600 values of the _E. coli_ solutions incubated with the Cys-AgNCs at various concentrations, namely, 0.175, 0.35, 0.7, 1.4, and 2.8 mg/mL, were respectively detected to be 0.80, 0.65, 0.59, 0.30, and 0.11 after culturing for 180 min. The antibacterial effects of the Cys-AuNCs and Cys-AgNCs can be attributed to the ROS generation that was induced by the Cys-AuNCs and Cys-AgNCs for the inactivation of the _E. coli_[47]. In comparison with the Cys-AuNCs (0.175 mg/mL), the Cys-AgNCs (0.175 mg/mL) produced a higher bactericidal effect because the release of the silver ions from the Cys-AgNCs enhanced the ROS production and inhibited the growth of the _E. coli_[48]. To examine the release of the silver ions, AgNPs (0.175 mg/mL) with an average size of 18.5 nm were incubated with the _E. coli_. As shown in Figure 4c, with the incubation of AgNPs, there was no obvious growth of _E. coli_ after culturing for 180 min. With the same concentration (0.175 mg/mL), the Cys-AgNCs expressed a lower bactericidal effect compared to that of the AgNPs because the cysteine ligand of the Cys-AgNCs could be applied as an ROS scavenger to decrease the ROS generation. The generation of ROS was quenched using cysteine on the surface of the Cys-AuNCs, resulting in its oxidized form, cystine [49; 50]. Most importantly, compared to the _E. coli_ solution, the growth curve of the _E. coli_ incubated with the Cys-AuNCs (0.175 mg/mL) presented no significant change, as shown in Figure 4a. The results indicated that the Cys-AuNCs (0.175 mg/mL) exhibited superior biocompatibility in the _E. coli_. due to the use of the cysteine ligand as an ROS scavenger.  
Figure 3: FTIR spectra of the cysteine (black), Cys–AuNCs (red), and Cys–AgNCs (blue).  
To further investigate the bactericidal activity, the _E. coli_ incubated with the same concentration of the Cys-AgNCs, Cys-AuNCs, and AgNPs (0.175 mg/mL) were each cultured on LB agar plates. As shown in Figure 5a, the colonies of the _E. coli_ incubated with sterilized water, Cys-AgNCs, Cys-AuNCs, and AgNPs were calculated to be 2.02 \(\times\) 10\({}^{9}\), 1.95 \(\times\) 10\({}^{9}\), 1.68 \(\times\) 10\({}^{9}\), and 0 CFU/mL, respectively. Moreover, the viabilities of the _E. coli_ incubated with the Cys-AgNCs, Cys-AuNCs, and AgNPs were separately calculated to be 97%, 81%, and 1%. Based on the results of the standard colony-counting method, the Cys-AuNCs exhibited superior biocompatibility in the _E. coli_ and the AgNPs revealed high bactericidal activity in the _E. coli_. Therefore, the superior biocompatible Cys-AuNCs were demonstrated as being a potential fluorescent probe for biomedical applications in bacteria.