## 3 Results and Discussion  
Vitamin B\({}_{12}\) comprises a _Corrin_ ring in which a cobalt atom is bound to four nitrogen molecules (molecular structure shown in Figure 1) [28]; the redox potential of cob(II)alamin/cob(I)alamin being 0.49 V at pH 7 [29]. The formation of metallic Ag, Au, and Pd nanoparticles via this greener method was scrutinized by XRD analysis. Figure 2 shows XRD patterns of the synthesized samples; Ag, Au, and Pd samples were well crystallized during the synthesis process using vitamin B\({}_{12}\) under both 3 min and 6 min of MW irradiation, thus indicating that vitamin B\({}_{12}\) effectively reduced metal ions in the solution. However, control reactions carried out at room temperature and 100 \({}^{\circ}\)C (oil bath heating) did \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Metal Precursor (mL)** & **Amount of Vitamin B\({}_{12}\) (mL)** & **Irradiation Time (min)** \\ \hline Ag & 1 & 1 & 5 & 3 \\  & 2 & 2 & 4 & 6 \\ \hline Au & 1 & 1 & 5 & 3 \\  & 2 & 2 & 4 & 6 \\ \hline Pd & 1 & 1 & 5 & 3 \\  & 2 & 2 & 4 & 6 \\ \hline \hline \end{tabular}  
* All experiments were conducted at 100 \({}^{\circ}\)C.  
\end{table} 
Table 1: Experimental conditions for the synthesis of Ag, Au, and Pd \({}^{\mathrm{a}}\). not yield specified products. For Ag samples, the peaks corresponding to (111), (200), and (220) planes were observed at 20 = 38.2, 44.4 and 64.66\({}^{\circ}\), respectively (see Figure 2a). As shown in Figure 2b for Au, the peaks at 20 = 38.17, 44.37, 64.56 and 77.55\({}^{\circ}\) were observed, which correspond to (111), (200), (220), and (311) planes, respectively. Peaks corresponding to (111), (200), and (220) planes of Pd were observed at 20 = 40.15, 46.70 and 68.18\({}^{\circ}\), respectively, for Pd samples (see Figure 2c). Interestingly, no metallic nanoparticles were observed with conventional heating at 100 \({}^{\circ}\)C  
Figure 1: The molecular structure of vitamin B12. Figure 2: XRD patterns of synthesized samples: (**a**) Ag; (**b**) Au; (**c**) Pd; and (**d**) all samples treated at 100 \({}^{\circ}\)C without the MW irradiation. (see Figure 2d). Also, no particles formed in the samples aged for 24 h at room temperature or without MW treatment (see supplemental Figure S1). These results surmise that the MW irradiation is crucial for the synthesis of noble metal particles using vitamin B\({}_{12}\). UV-visible absorption of the synthesized samples and vitamin B\({}_{12}\) was investigated (Figure 3). It can be seen from Figure 3b that the absorbance at wavelengths ranging from 500 to 570 nm increased due to surface plasmon resonance (SPR) of silver nanoparticles [30] compared to that of vitamin B\({}_{12}\); Ag nanoparticles normally exhibit resonance peaks at 400-450 nm. However, in the present study, the shift in resonance peak is due to the formation of different shapes of particles in the presence of vitamin B\({}_{12}\). The absorbance for the sample subjected to MW irradiation for 6 min was higher than the sample that was heated for 3 min. These results are in good agreement with the XRD analysis (see Figure 2a), as well as with earlier reports exhibiting the SPR effect of Ag nanoparticles at wavelengths ranging from 500 to 570 nm [30]. For Au nanoparticles, a slight increase of the absorbance at wavelengths ranging from 520 to 550 nm was noticed compared to that of vitamin B\({}_{12}\), due to the localized surface plasmon resonance of Au (Figure 3c). This indicates that the formation of Au is in accordance with outcomes from XRD analysis (see Figure 2b) and earlier documented research [31,32]. For Pd samples, the specific light absorption patterns were not discerned due to the background light absorbance of vitamin B\({}_{12}\) (see Figure 3d), but decreased absorbance around  
Figure 3: UV-visible absorbance for (**a**) vitamin B\({}_{12}\), and synthesized (**b**) Ag NPs; (**c**) Au NPs; and (**d**) Pd NPs.