## 3 Results and Discussions
These peaks correspond to the O-H stretching band of the carboxyl group (3419), the C-H stretching of the alkane compound (2954, 2925, 2852), C=C stretching (1662, 1600), and C=O (1635) and indicate the presence of alkane compound containing carboxylic groups. The tiopronin-capped gold nanoparticles (Au:tiopronin = 1:0.1) show characteristic peaks at 3350, 2954, 2925, 2854, 1662, 1635, and 1595 cm\({}^{-1}\). These peaks correspond to N-H stretching of the secondary amines (3350), C-H stretching of the alkanes (2954, 2925, 2854), C=C stretching (1662, 1595), and C=O stretching (1635). In addition, the broad band corresponding to the O-H stretching of the carboxyl group existed near the 3350 cm\({}^{-1}\) peak. Therefore, the FT-IR results confirmed the presence of an alkane compound containing a carboxyl group in both samples, indicating that tiopronin had bound to the gold nanocluster/nanoparticle as a ligand. Figure 5a shows the absorption characteristics of the tiopronin-capped gold nanoclusters/nanoparticles. Unlike tiopronin-capped gold nanoclusters with sizes smaller than 2.21 nm, tiopronin-capped gold nanoparticles with sizes larger than 2.45 nm have distinct surface plasmon resonance peaks at 501 (2.45 nm), 507 (3.69 nm), 510 (4.15 nm), and 511 (4.38 nm) nm due to the strong plasmonic interaction of gold. As the size of the gold nanoparticles increases, the wavelength of the light absorbed by localized surface plasmon resonance increases. Therefore, the size of the gold nanoparticles was proportional to the wavelength of the peak surface plasmon resonance (SPR). This agreed well with the results obtained via TEM. Figure 5b shows the photoluminescence for the tiopronin-capped gold nanoclusters of sizes 0.58 and 0.81 nm with the respective peaks at 827.4 and 830 nm. The fluorescence property of the gold nanoclusters observed herein was attributed to the molecular-like behaviors of the gold nanoclusters with sizes smaller than 2 nm [37; 38]. Figure 3: XRD results for the tiopronin-capped gold nanocluster/nanoparticle (black line: reference gold XRD pattern; red line: tiopronin-capped gold nanoparticle). Figure 6a shows the sizes of the tiopronin-capped gold nanoclusters/nanoparticles measured using DLS. The hydrodynamic size of the tiopronin-capped gold nanoclusters/nanoparticles increased as the amount of tiopronin decreased. This trend was consistent with the results obtained via TEM. The modal hydrodynamic sizes of the tiopronin-capped nanoclusters/nanoparticles with different gold-to-tiopronin ratios were measured to be 1.74, 2.33, 3.62, 4.85, 5.61, 6.5, and 6.5 nm. Figure 6b shows a comparison of the sizes measured using TEM and DLS. The sizes measured by DLS were slightly larger than those obtained using TEM. The average size difference with different gold-to-tiopronin ratios was calculated to be 1.83 \(\pm\) 0.4 nm. The disparity in the sizes obtained from the different measurements was attributed to the molecular length of tiopronin, which is 0.77 nm as calculated by density functional theory. Figure 4: FT-IR measurement results for the tiopronin-capped gold nanocluster (blue), tiopronin-capped gold nanoparticle (red), and tiopronin (black). An offset of 100 is given between each plot for readability. Figure 7 presents the zeta potential distribution of the tiopronin-capped gold nanoclusters/nanoparticles (Au:tiopronin = 1:3 and 1:0.1). In general, the colloidal stability of the particles can be considered excellent when the absolute amplitude of the zeta potential is greater than 30 mV. The zeta potentials of the synthesized tiopronin-capped gold nanoclusters/nanoparticles were measured to be \(-\)59.5 mV (Au:tiopronin = 1:3) and \(-\)30 mV (Au:tiopronin = 1:0.1), indicating excellent colloidal stability. Figure 5: (**a**) Absorbance curves of the tiopronin-capped gold nanoclusters/nanoparticles obtained using a UV–Vis spectrophotometer. The numbers in the legend indicate the molar ratio of gold to tiopronin.