## 1 Introduction  
Noble metals such as gold (Au), silver (Ag), and palladium (Pd) have been widely used for many applications, including catalysis, electronics, photonics, optoelectronics, sensing, and pharmaceuticals [1; 2; 3; 4; 5]. In order to synthesize noble nanometals, various methods have been established, including a polyol process, sodium borohydride (NaBH4) reduction, and photoreduction [6; 7; 8; 9; 10; 11; 12]. Nevertheless, these conventional synthesis methods have several disadvantages, including the use of noxious reagents and volatile organic solvents as well as the production of hazardous by-products [6; 7; 8; 9; 10]. Therefore, it is of great importance to eliminate the use of toxic reducing agents for the synthesis of noble metal nanoparticles.  
Several efforts have been made recently to utilize naturally occurring reducing compounds, namely vitamins and polyphenic extracts of tea and fruit [13; 14; 15; 16; 17; 18]. For example, vitamin B1 effectively reduced Pd ions and the structures of the ensuing Pd nanoparticles could be well controlled as nanobelts, nanoplates, and nanotrees at different concentrations of palladium precursors [17]. Vitamin B2 was used as a reducing and capping agent for the synthesis of metal nanoparticles [18]. In view of its antioxidative properties, water-soluble vitamin B12 is used now in the present work to synthesize Ag, Au, and Pd metal nanoparticles [19; 20].  
In addition to the use of greener reagents, alternative or renewable energy sources show significant promise in helping to reduce the amount of by-product formation and energy used. Not only do they safeguard against these harmful by-products, but the use of alternative energy helps to preserve many of the natural resources that we currently use [21]. For example, [PERSON] _et al._[22] reported that hematite could be synthesized with a microwave (MW)-assisted hydrothermal method and it is 36 times faster compared to a conventional synthesis. Microwave heating has emerged as an alternative and powerful energy source for nanomaterial synthesis [23], which can accomplish chemical transformations in a short span of time (e.g., minutes instead of hours or days) since enough energy for the chemical reaction can be directly provided by molecular vibrations due to MW [24]. A variety of highly crystalline nanomaterials with uniform sizes such as titanium dioxide, copper, and iron oxides have been expeditiously synthesized in minutes [25; 26; 27]. The fusion of both the use of safer reagents and MW heating ideally suits the development of a sustainable technology to create noble nanometals of uniform, smaller size by reducing or eliminating the formation of undesired wastes.  
In this study, vitamin B12 was used as both a reducing and capping agent for the synthesis of Ag, Au, and Pd nanoparticles in conjunction with MW irradiation. The synthesized samples were thoroughly characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectroscopy, and high resolution-transmission electron microscopy (HR-TEM). The MW irradiation was critical to synthesizing noble metals using vitamin B12; no particle formation was discerned at room temperature and oil bath heating at 100 \({}^{\circ}\)C without the use of MW irradiation.