## 3 Results and Discussion  
### Structural and Optical Characterizations of the Cys-AuNCs and Cys-AgNCs  
To demonstrate the successful preparations of Cys-AuNCs and Cys-AgNCs, the sizes and shapes were characterized using transmission electron microscopy (TEM) (Hitachi HT7700, Tokyo, Japan). As shown in Figure 1a, the Cys-AuNCs exhibited an approximately spherical shape. In the TEM image of Figure 1b, the Cys-AgN NCs also revealed a roughly spherical shape. Furthermore, the sizes of the Cys-AuNCs and Cys-AgNCs were each measured based on 100 nanoclusters in the TEM images of Figure 1a,b. The size distributions of the Cys -AuNCs and Cys-AgN NCs are respectively shown in the histograms of Figure 1c,d, and their Gaussian fitting curves were also simulated, as shown in Figure 1c,d. According to the Gaussian fitting curves, the average sizes of the Cys-AuNCs and Cys-AgNCs were respectively calculated to be 3.9 \(\pm\) 0.021 and 4.7 \(\pm\) 0.038 nm. To further characterize the optical properties, UV-Vis spectra and fluorescence spectra of the Cys-AuNCs and Cys-AgNCs were separately examined using UV-Vis spectroscopy (JASCO V-770 Spectrophotometer, Easton, MD, USA) and fluorescence spectroscopy (JASCO spectrofluorometer FP-8500, Easton, MD, USA). As shown in Figure 2a, the UV-Vis absorption spectrum of the Cys-AuNCs revealed the lack of surface plasmon absorption of the gold nanoparticles at a wavelength of \(\approx\)520 nm. Similarly, the absorption spectrum of the Cys-AgNCs showed the disappearance of the surface plasmon absorption of the silver nanoparticles at a wavelength of \(\approx\)450 nm. The disappearances of the surface plasmon absorptions of metallic nanoparticles can be attributed to the metallic nanoclusters  
Figure 1: TEM images of the (**a**) Cys–AuNCs and (**b**) Cys–AgNCs. Histograms of the nanocluster size distributions and Gaussian fitting curves of the (**c**) Cys–AuNCs and (**d**) Cys–AgNCs. containing high oxidation states of metallic ions, which resulted in the shortage of free electrons for the generation of coherent oscillations [39; 40]. Moreover, the fluorescence spectra were examined to demonstrate the successful syntheses of the Cys-AuNCs and Cys-AgN NCs. As shown in Figure 2b, the fluorescence spectrum of the Cys-AuNCs exhibited a maximum intensity at 575 nm when using an excitation wavelength of 360 nm. The large Stokes shift (215 nm) of the Cys-AuNCs revealed their promising application as a fluorescent probe in bioimaging. The left inset in Figure 2b shows the orange-red fluorescence of the Cys-AuNCs under irradiation using a hand-held UV lamp. For the Cys-AgN NCs, the fluorescence spectrum exhibited a maximum intensity at 438 nm using an excitation wavelength of 364 nm. The right inset in Figure 2b shows the blue fluorescence of Cys-AgNCs under irradiation from a hand-held UV lamp. Furthermore, the fluorescence quantum yields of the Cys-AuNCs and Cys-AgNCs were respectively measured to be 7.2 and 0.98% using an integrating sphere. According to previous studies, the fluorescences of the Cys-AuNCs and Cys-AgNCs were validated because of the ligand-metal charge transfer between the thiolate ligand and the core of the AuNCs [41; 42; 43]. To further confirm that the cysteine conjugated with the AuNCs, the cysteine, Cys-AuNCs, and Cys-AgNCs were characterized using Fourier transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet iS10, Waltham, MA, USA). s shown in Figure 3, the characteristic IR bands of cysteine were obtained, including a broad stretching band of NH\({}^{3+}\) (\(\approx\)2974 cm\({}^{-1}\)), a stretching band of SH (\(\approx\)2551 cm\({}^{-1}\)), an asymmetric stretching band of COO\({}^{-}\) (\(\approx\)1585 cm\({}^{-1}\)), and a bending vibration of NH (\(\approx\)1536 cm\({}^{-1}\)).