## 3 Results and Discussion  
### Reduction Reaction of Metal Ions  
After the addition of MQLE, the color of the HAuCl4 solution changed from light brown to purple and eventually to deep ruby red with increasing irradiation time, indicating successful synthesis of Au NPs [26]. Similarly, the color of the AgNO3 solution changed from light brown to intense brown, confirming the synthesis of Ag NPs [27]. Furthermore, the mixture containing both HAuCl4 and AgNO3 changed color from light brown to dark brown [28].  
Figure 1A presents the reduction efficiency and reduction potential of the Au ion, Ag ion, and Au-Ag solutions over prolonged irradiation times. The Au ion solution was completely reduced to NPs within 50 s, whereas the Ag ion solution and the mixed solution were completely reduced within 60 s. This rapid reduction process (50-60 s) under microwave irradiation was significantly more efficient than the prolonged time (several hours) required for biogenic production using only plant extracts, microorganisms, and biopolymers [29]. Given that NP formation follows first-order reaction kinetics, the rate constants (k) for the Au ion solution, Ag ion solution, and mixed solution were 0.1093, 0.0947, and 0.0746 s\({}^{-1}\), respectively, indicating that the rate of reduction was the highest for the Au ion solution. The reduction potential of the Au-Ag solution increased from 15.2 to 54.8 mV, suggesting that biomolecules in MQLE, such as phytochemicals, facilitated the reduction in metal ions to NPs. The pH of the Au-Ag solution decreased from 5.6 to 2.6, likely due to the release of H\({}^{+}\) ions during the oxidation process facilitated by certain reductants in MQLE [28]. Notably, before irradiation, the reduction efficiency of the metal ions was approximately 35%. Therefore, even in the absence of microwave irradiation, MQLE could reduce metal ions.  
Figure 1B presents the SPR spectra of different NPs. The SPR spectra of Au NPs, Ag NPs, and Au-Ag alloy NPs were recorded at irradiation times of 50, 60, and 60 s, respectively. The absorption peaks for Au NPs and Ag NPs were observed at 562 and 426 nm,  
Figure 1: (**A**) Reduction efficiency of different metal ions and corresponding reduction potentials in the Au-Ag solution at different irradiation times. (**B**) SPR peaks for Au NPs, Ag NPs, and Au-Ag alloy NPs were observed at 562, 426, and 528 nm, respectively; the peaks were recorded at irradiation times of 50, 60, and 60 s, respectively.  
respectively. By contrast, the SPR spectrum of Au-Ag alloy NPs exhibited a single peak at 528 nm, positioned between the absorption peaks of Au NPs and Ag NPs, suggesting the formation of alloy NPs rather than a mere mixture of distinct metal particles [30]. This observation differs slightly from the SPR peaks obtained for chemically synthesized NPs, likely attributable to the effects of organic NP conjugates, which can induce either a red shift or a blue shift in the SPR peak [5]. Similar results have been reported by studies where NPs were synthesized using mannosylerlythritol lipid and citrus peel extract [31; 32].  
Regarding stability, the absorbance and spectral bandwidth of the SPR signal of Au-Ag alloy NPs, obtained at an irradiation time of 60 s, exhibited no significant changes over an 8-month storage period. However, a reduction in absorbance was observed in month 9, suggesting the formation of aggregates [33]. Nonetheless, the spectral bandwidth remained unchanged. After 12 months, alterations in both absorbance and bandwidth, along with a red shift, were observed, indicating particle enlargement and NP instability [34]. These findings suggest that Au-Ag alloy NPs can maintain stability for at least 8 months.