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Molecules 2020, 25, 3191 9 of 24 milling was successfully used by Baláž [84]. This methodology, also called mechanochemical synthesis has recently attracted a significant attention of the research world [85–91]. Physical methods are useful because of speed and not using toxic chemicals, but on the other hand disadvantages are also there which are previously mentioned [80]. 4.1.2. Chemical Methods Chemical methods represent an easy way to prepare AgNPs in solution. The most common technique for nanoparticles preparation is the reduction by reducing agents of organic or inorganic character, for example sodium ascorbate, hydrogen, N,N-dimethyl formamide, or sodium borohydride. The principle of AgNPs preparation is the reduction of Ag+ ion to metallic form Ag0 which is followed by agglomeration into oligomeric clusters leading to the formation of metallic colloidal AgNPs [92]. To avoid the agglomeration of nanoparticles, it is necessary to use stabilizing agents, such as poly(vinyl alcohol), poly(vinyl pyrrolidine), or polyethylene glycol [93–95]. 4.1.3. Biological Methods Biological synthesis of nanoparticles in general has become very popular because of its simplicity, low cost, or environmental reasons. The reduction of metallic salt is performed by a natural material including plants, plant extracts, microorganisms, or small biomolecules (amino acids, vitamins or polysaccharides) [96–99]. According to the used methodology, biological methods can be divided into in vivo and in vitro. In vivo methods use the whole cell for AgNPs biosynthesis, so nanoparticles are synthesized intra- or extracellularly, while during the in vitro process, the reduction of Ag+ ions takes places outside of a living organism (the most common are plant extracts containing the compounds with antioxidant and reducing properties—polyphenols, flavonoids, terpenes, aldehydes, carbohydrates, etc.,) [100–102]. In addition to plant extract, the extracts of edible mushrooms [103–105], extract of microorganisms [106–109], tissue extract [110], algae extract [111] essential oils [112], and simple biomolecules, such as glucose [113], starch [114], dextrin [115], pectin [99], or cellulose [116] can be used. The term in vivo describes the biological synthesis of AgNPs inside the living organism. The very first article was published by Gardea-Torresdey et al. [117], who reported the synthesis of AgNPs by living plant alfalfa (Medicago salvia). They observed that the alfalfa root is able to absorb silver from agar medium to produce AgNPs. Microorganisms are also capable of producing nanoparticles. Some microorganisms are resistant to metal, so they can survive and grow during the production of NPs, so there is no barrier to use bacteria for AgNPs synthesis [118–120]. Fungi also represent a valuable producer of silver nanoparticles because of their capability of metals bioaccumulation [121,122]. 4.2. AgNPs Antioxidant Properties Antioxidant capacities of various biological samples, pure chemicals, or isolated compounds are well known. Beside the application of AgNPs in diverse areas, the large number of articles dealing with antioxidant properties of silver nanoparticles have been published spreading in the recent decade. In the Table 3 we have summarized the methods used for silver nanoparticles antioxidant capacity determination together with the methods of preparation during recent years. As it can be seen, the number of publications reporting on biosynthesis (especially using plant extract), has prevailed over any other method of synthesizing AgNPs, which may be justified by simplicity, availability, low cost of this approach (Table 3).PDF Image | Antioxidant Activity Determination of Silver Nanoparticles
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