# Study of Green Synthesis of Ultrasmall Gold Nanoparticles Using _Citrus Sinnensis_ [PERSON]
## 1 Introduction  
Nanotechnology is one of the most important technologies in all areas of science due to the significant difference in nature properties of nanoparticles [1; 2; 3; 4], and these properties (e.g., biological, catalytic activity, mechanical properties, melting point optical absorption, thermal and electrical conductivity) are dominated by their size, shape and distribution [5; 6]. Traditionally, nanoparticles could be produced and stabilized by two different ways. The physical methods are based mostly on the energy transfer that occurs in an irradiated material by ionizing or non-ionizing radiation, leading to the generation of reducing agents responsible for the nucleation of metallic particles. These methods include radiation [6] and laser ablation [7], among others. Most of the physical methods are green approaches, however, because of the radiation dose control and the lack of stabilizers, the production of stable gold nanoparticles (AuNPs, \(\leq\)2 nm) with narrow distribution by physical methods are rarely reported [6]. The chemical methods are a redox reaction between material and reducing agents such as sodium borohydride [8], hydrazine [9], and triethyl amine [10]. These are currently the most explored pathways for the synthesis of gold nanoparticles due to their ease of performance, high production yield and stability, despite that they are known to be biologically and environmentally toxic. Metallic nanoparticles of noble metals such as gold, silver, platinum, and palladium have beenwidely used in products ranging from cosmetic to medical and pharmaceuticals. AuNPs, noble metal nanoparticles that are characterized by stability, less toxicity, and biocompatibility, are widely used in biological systems for various novel applications [11; 12]. AuNPs could be synthesized by physical and chemical approaches, however in order to produce AuNPs with size less than 5 nm, chemical reductants and protecting agents commonly are used for the particles size tuning and stabilizing, which are often toxic and hard to remove from the AuNPs suspension [13; 14; 15]. These AuNPs synthesized by chemical agents often lead to inconsistent toxicity risks in biological tests [16; 17; 18; 19; 20; 21]. Therefore, developing an environmental friendly method that is toxic chemicals free is crucial, especially in medical and pharmaceutical areas. In our previous research, uniform small to ultra-small AuNPs with controllable size and high stability were synthesized successfully via green synthesis (using fruit juices/extracts) [22]. However, the major functional groups that are responsible for the reduction of AuNPs, the possible role in the synthesis and stabilization of ultra-small AuNPs, are still unknown. As a foodstuff source, fruit juices/extracts are not the best choice for the green synthesis at industrial-scale production, based on economic reasons. Since green synthesis is a method that uses natural products (such as plants, enzymes, fungi and algae) as both reductant and stabilizer, it is possible to find an alternative natural product source such as peel, stem and leaf to instead of fruits [23; 24; 25; 26]. Citrus peels is one of the most underutilized and geographically diverse biowaste residues. At present, the worldwide industrial citrus wastes account for 50 wt% of the original whole fruit mass, and almost 1/2 of the citrus wastes is peel [27; 28; 29]. The management of these wastes that produce odor and soil pollution still remains a major problem in the food industry [30]. At present, there are many researches related to the green synthesis of AuNPs by orange peel were conducted, however, the particles produced by those researches are usually bigger than 5 nm [2; 31; 32]. Thus, in this study, extract from biowaste _Citrus sinensis_ (_C. sinensis_) peels was delicately used as a reducing agent to prepare uniform-sized AuNPs by controlling pH and sodium hydroxide adding sequence. As a result, stable AuNPs with ultra-small size (<2 nm) were successfully obtained from freshly prepared _C.