# Study of Green Synthesis of Ultrasmall Gold Nanoparticles Using _Citrus Sinnensis_ [PERSON]
## 3 Results and Discussion
### FT-IR Analysis of AuNPs  
FTIR was carried out to determine the major functional groups that are responsible for the reduction of Au\({}^{3+}\) in solution. Besides, the possible mechanism in the synthesis and stabilization of the ultra-small sized AuNPs that were obtained by adding OH\({}^{-}\) into peel extract in different sequences is discussed. Figure 7 illustrates the FI-TR spectra obtained from control group (curve a: Au\({}^{3+}\) and _C. sinensis_ peel extract mixed solution, nothing reacting with Au\({}^{3+}\)), the pH adjusted to 11 before synthesis reaction (curve b) and pH adjusted to 11 after AuNPs synthesis reaction (curve c). The curve a shows several peaks indicating the complex nature of the biological material. The bands appearing at 3363, 2929, 1724, 1608, 1367, 1203 and 1070 cm\({}^{-1}\) are assigned to stretching vibration of O-H of alcohol or N-H of amines, C-H of alkanes, C=O of carboxylic acid or ester, N-C=O amide I bond of proteins, CH\({}_{2}\) of alkanes, C-O of carboxylic acid, ester, or ether, and C-N of aliphatic amines or alcohol/phenol, respectively [34]. After reaction with HAuCl\({}_{4}\), the shift of the peaks for both two different pH adjusting sequences (curve b and c) indicates that the carboxyl, hydroxyl and amide groups in _C. sinensis_ peel extract may participate in the nanoparticles synthesis (see Table 2). It also shows that the major stretching appearing at 3000-3500 cm\({}^{-1}\) indicates the presence of O-H stretch  
Figure 6: Hydrodynamic diameter measurement of AuNPs formed from _C. sinensis_ peel at pH = 11 adjusted after the reaction. Figure 7: The FT-IR spectra of AuNPs solution obtained from _C. sinensis_ peel extract at different experiment conditions: (**a**) control; (**b**) pH adjust to 11 before AuPNs synthesize reaction happen; (**c**) pH adjust to 11 after AuPNs synthesize reaction happen. due to the presence of phenols, flavonoids, benzophenones and anthocyanins [35]. A different shifting occurs here, indicating that different amount of carbonyl groups in the _C. sinensis_ peel extract were taken part into the redox reaction (see Section 3.4). A bigger shifting could be observed on curve c (3436 cm\({}^{-1}\)) than curve b (3434 cm\({}^{-1}\)) suggesting that more AuNPs were occurred and more carbonyl groups were participated in the reaction when pH adjusted after reaction starting, than the reaction when pH adjusted before reaction starting. Thus, the results (the reaction Au\({}^{3+}+\mathrm{OH}^{-}=\mathrm{Au}(\mathrm{OH})_{3}\) would decrease the generation of AuNPs and stop the growth of AuNPs size) obtained in Section 3.4 was verified. Aside the O-H stretching, the region of 2929 cm\({}^{-1}\) presents the C-H in xanthone [36]. Bands of C-H bond from _C. sinensis_ peels extract were split into two; 2944 cm\({}^{-1}\) and 2969 cm\({}^{-1}\) (curve b). No peak split is observed after the pH adjusting (curve c). These results suggest that, after the generation of AuNPs, the transmittance changed. Meanwhile, with the generation of AuNPs, the C=O stretching characteristic peak at region of 1700 cm\({}^{-1}\) is observed, and the peak in curve b shifts to red (1737 cm\({}^{-1}\)), compared to the peaks in curve a and c (1724 cm\({}^{-1}\)), which means that the consumption of carbonyl substance might be related to pH increase. The reasons for the speculation above are as follows: (1) for solution of curve a and c, they have the same properties before the reaction happened; (2) for solution of curve b, _C. sinensis_ peels extract were added into a high pH solution; (3) for solution of curve c, the extracts were added into the solution before pH adjusted, and part of -OH were reacted with Au\({}^{3+}\) to product Au(OH)\({}_{3}\) instead of effecting carbonyl substance existence in solution. The combination of this peak shift phenomenon with TEM images (Figures 3e and 5d) indicates that the existence of carbonyl substances help the size of the AuNPs stabilization [37]. Besides, C-O stretch can be found in the range of 1300-1000 cm\({}^{-1}\).