# Study of Green Synthesis of Ultrasmall Gold Nanoparticles Using _Citrus Sinnensis_ [PERSON]
## 3 Results and Discussion
### Tuning AuNPs Size and Size Distribution  
Similar with foodstuff sources (juice/extracts) [22], the morphology and size of the synthesized AuNPs by biowaste (_C. sinensis_ peel extract) were influenced by NaOH in solutions. In the present study, the synthesized particles in the high pH solution showed a reduction of particle sizes. Meanwhile, these particles possessed a narrower size distribution in a spherical shape. However, limited quantity of Au nanoparticles was synthesized using high pH solution. Very few free Au\({}^{3+}\) ions could participate in the AuNPs generation reaction, since the reaction of Au\({}^{3+}\) and OH\({}^{-}\) (Au\({}^{3+}+\) OH\({}^{-}=\) Au(OH)\({}_{3}\)) occurred first, and the reduced absorbance in UV-vis spectra was observed as a result. In order to restrict the particle size growth and increase the quantity of AuNPs, most Au\({}^{3+}\) ions are ought to receive electrons from the functional groups of _C. sinensis_ peel extract instead of hydroxyl ions in solution. To avoid Au(OH)\({}_{3}\) formation, the order of adding hydroxyl ions solution was changed according to our former result, specifically [22]. First, the _C. sinensis_ peel extract was added into and mixed with excess Au\({}^{3+}\) ions solution. Then, the hydroxyl ions were added into the prepared reaction mixture. Hence, Au\({}^{3+}\) ions are possible to react with the functional groups in the _C. sinensis_ peel extract to produce AuNPs during the early stage of reactions instead of Au(OH)\({}_{3}\) generation. Then, pH of the solution was increased to impose restriction on the further growth of nanoparticles by starting reaction of Au\({}^{3+}\) and OH\({}^{-}\). When compared to the former protocol (adding hydroxyl ions before reaction), the amount of hydroxyl ions, gold ions and peels extract were constant. However, the intervention time of hydroxyl ions on the reaction was different. As shown in Figure 4, all maximum absorbance for each curves are higher than those of the method that pH was adjusted before the reaction (see  
Figure 3: TEM images of AuNPs formed in _C. sinensis_ peel extract at different pHs adjusted before the reaction: (**a**) pH = 3 (control); (**b**) pH = 6; (**c**) pH = 8; (**d**) pH = 10; (**e**) pH = 11. Table 1), which indicates that more Au\({}^{3+}\) ions are reduced to AuNPs instead of Au(OH)\({}_{3}\) by reacting with hydroxyl ions. Moreover, the present of lower \(\lambda_{\text{max}}\) = 522 (compare to \(\lambda_{\text{max}}\) = 529 when pH = 11 adjust before reaction) demonstrates that even smaller (compare to fruit extract source) AuNPs (\(<\)2 nm) are formed during this reaction sequence. Figure 5 illustrates the TEM images of nanoparticles synthesized using the _C. sinensis_ peel extract at different pHs which are adjusted after the reaction. It could be observed from the images that the size distribution of particles is much narrower than those in Figure 3 which were prepared by the method of pH adjustment before reaction, and most of the particles are spherical. Furthermore, the nanoparticles sizes are decreased to smaller than 5 nm (4.91 \(\pm\) 3.03 nm, Figure 5a) from 9.33 \(\pm\) 2.37 (Figure 3b) with the new pH adjusting sequence. The number of large particles is limited (Figure 5c,d) compare to Figure 3d,e. For _C. sinensis_ peel extract with the new pH adjusting method (adjust after reaction happened), all the AuNPs sizes are restricted to smaller than 2 nm (see Table 2) with good dispersity when the pH was 11 (Figure 5d). Furthermore, no aggregation of AuNPs was found at such experimental condition. The results of the study indicate that hydroxyl ions could be used to tune the sizes and size distributions of AuNPs by choosing different NaOH adding opportunity windows. Optimal dosage of hydroxyl ions can restrict further growth of small nanoparticles and optimize size distribution.