*3.1. Characterization of the AuNPs*

The synthesized AuNPs had an SPR peak at 519 nm (Figure 1a), indicating that the AuNPs were approximately 14 nm in size. The narrow peak suggested that the majority of the AuNPs were uniform and monodispersed [53]. The AuNPs had an absorbance value of 2.422 OD, and a concentration of 9.83 nM. DLS results revealed that the AuNPs had an average hydrodynamic diameter of 14.69 nm and a PDI of 0.175, the latter confirmed that the AuNPs were mostly monodispersed. The AuNPs showed a ζ- potential of −27.7 mV, indicating that the AuNPs were highly stable. TEM images revealed that the AuNPs were mostly spherical in shape and were relatively monodispersed (Figure 1b). This is due to electrostatic stabilization with a negatively charged layer of citrate ions [44]. The core diameters of AuNPs were found to be 13.98 ± 1.19 nm (Figure 1c), further corroborating the results obtained from the UV–vis spectrophotometry. The SPR results obtained in this study were similar to the results obtained by Stiolica et al., whereby 14 nm AuNPs also had an SPR of 519 nm. In contrast, the reported AuNPs had a ζ- potential of –51 mV [41], which was lower than the a ζ- potential obtained in this study.
