## 3 Results and Discussions  
Figure 2 shows the TEM images of the tiopronin-capped gold nanoclusters/nanoparticles. The numbers on the upper-left indicate the molar ratio of gold to tiopronin and the mean particle size. The insets on the upper-right show the size distribution obtained using ImageJ. Figure 2i shows the lattice structure of the tiopronin-capped gold nanoparticles synthesized with a gold-to-tiopronin molar ratio of 1:0.02. The size of the tiopronin-capped gold nanoclusters/nanoparticles increased with the decrease in the amount of tiopronin. The modal size, mean size, and standard deviation of the size distribution of the tiopronin-capped gold nanoclusters/nanoparticles with different gold-to-tiopronin molar ratios are presented in Table 1. The lattice plane spacings of the tiopronin-capped gold nanoparticle were measured to be 0.24 nm and 0.15 nm, which respectively correspond to the (111) and (220) planes of gold (Figure 2i). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Au:Tiopronin** & **1:3** & **1:2** & **1:1** & **1:0.5** & **1:0.2** & **1:0.1** & **1:0.05** & **1:0.02** \\ \hline
**d\({}_{\text{mode}}\) (nm)** & 0.58 & 0.81 & 1.38 & 2.21 & 2.45 & 3.69 & 4.15 & 4.38 \\
**d\({}_{\text{mean}}\) (nm)** & 0.89 & 1.17 & 2.02 & 2.75 & 3.1 & 4.28 & 4.76 & 4.79 \\
**\(\sigma\) (nm)** & \(\pm\)0.32 & \(\pm\)0.42 & \(\pm\)0.66 & \(\pm\)0.76 & \(\pm\)0.94 & \(\pm\)1.32 & \(\pm\)1.55 & \(\pm\)1.37 \\ \hline \end{tabular}
\end{table} 
Table 1: The modal size (d\({}_{\text{mode}}\)), mean size (d\({}_{\text{mean}}\)), and standard deviation (\(\sigma\)) of the size distribution of the tiopronin-capped gold nanoclusters/nanoparticles with different gold-to-tiopronin molar ratios. Figure 3 shows the crystal structure of the tiopronin-capped gold nanocluster/nanoparticle measured using XRD. Owing to their small size, the characteristics of the tiopronin-capped gold nanoclusters observed herein were similar to those of a thin film of amorphous crystals. Therefore, only the XRD pattern of the silicon wafer used as the substrate was obtained. However, as the particle size increased, the inherent crystallinity of gold gradually began to appear. The gold nanoparticles exhibited broad XRD patterns at 38.1\({}^{\circ}\), 44.3\({}^{\circ}\), 64.5\({}^{\circ}\), and 77.7\({}^{\circ}\), corresponding to Au (111), Au (200), Au (220), and Au (311), respectively. The lattice spacings obtained from TEM agreed well with those obtained via XRD, which indicated that the synthesized particles were gold nanoclusters/nanoparticles. According to the Scherrer equation (Equation (3)--which explains the correlation between particle size and the full-width half-maximum (FWHM) of a signal peak in an XRD measurement--the smaller the particle is, the wider the peak. \[\tau=\frac{K\lambda}{\beta cos\tau\theta} \tag{3}\]  
where \(\tau\) denotes the mean crystallite size, \(K\) the shape factor, \(\lambda\) the wavelength of the X-ray, \(\beta\) the line broadening at the FWHM, and \(\theta\) the Bragg angle. Figure 2: TEM images of the tiopronin-capped gold nanoclusters/nanoparticles synthesized with different gold:tiopronin ratios: (**a**) 1:3, (**b**) 1:2, (**c**) 1:1, (**d**) 1:0.5, (**e**) 1:0.2, (**f**) 1:0.1, (**g**) 1:0.05, (**h**) 1:0.02. (**i**) HRTEM image of a tiopronin-capped gold nanocluster/nanoparticle showing the lattice structure. The insets show the size distribution of the tiopronin-capped gold nanoclusters/nanoparticles, estimated from the images using a particle size analysis tool. Figure 4 shows the FT-IR results for the tiopronin and the tiopronin-capped gold nanocluster/nanoparticle; the results confirmed the binding of tiopronin to gold. The tiopronin-capped gold nanoclusters (Au:tiopronin = 1:3) showed characteristic peaks at 3419, 2954, 2925, 2852, 1662, 1635, and 1600 cm\({}^{-1}\).