Green synthesis of silver nanoparticles using the plant extract of Salvia spinosa grown in vitro and their antibacterial activity assessment

Nanotechnology is the synthesis of particles with at least one dimension in the range of 1–100 nm, resulting in high surface to volume ratios. As the particle size decreases, not only does the ratio of surface area to volume increase but also the physical, chemical and biological properties of the particles differ compared to their bulk counterparts [ 1 , 2 ]. Noble-metal nanoparticles exhibit incredible physicochemical, optoelectronic and biochemical characteristics. They are being used for various purposes in industrial and pharmaceutical applications [ 3 , 4 ]. Despite the existence of numerous metals in nature, only a few of them such as gold, silver, palladium and platinum are synthesized extensively in nanostructured form [ 5 , 6 ]. Among the above-mentioned metals, silver nanoparticles have attracted much attention due to their unique characteristics for utilizing in various applications including pharmaceutics, agriculture, water detoxification, air filtration, textile industries and as a catalyst in oxidization reactions [ 7 , 8 , 9 ]. Furthermore, their predominant property is their high antibacterial activity against a broad range of bacteria without any toxicity to animal cells [ 10 , 11 , 12 ]. The establishment of resistance to antibiotics in bacteria especially multidrug resistance compelled scientists to explore novel compounds to halt multidrug-resistant microorganisms. Bactericidal activity of Ag NPs without toxicity to human cells can make them a proper substitution for antibiotics [ 13 , 14 ]. Ag NPs are being employed for eliminating microorganisms in medical devices, implants and hospital masks [ 15 , 16 ]. They are also extensively being used in hospitals either supplemented with antibiotics or alone for preventing infection [ 17 ]. Nanoparticles are traditionally synthesized broadly by physical and chemical procedures. Chemically prepared nanoparticles are not appropriate for medical usages due to hazardous chemicals binding on their surface. Furthermore, by-products produced in chemical routes are toxic for the environment. Physical routes for synthesis of NPs have some drawbacks, too. These methods require high energy and space, and are expensive [ 18 , 19 , 20 ]. Using biological systems such as microorganisms, plants, viruses and animal cell cultures is an alternative procedure for preparation of NPs [ 21 , 22 , 23 ]. In the most recent studies, researchers utilized Gelidium amansii [ 24 ], Enteromorpha compressa [ 25 ], Phanerochaete chrysosporium [ 26 ], Bacillus brevis [ 27 ] and Daucus carota [ 28 ] for the biosynthesis of Ag NPs and investigated their antimicrobial properties. Biosynthesis of NPs is eco-friendly, time affordable, cost effective. More importantly, the biosynthesized NPs are free of hazardous material on their surface. Also, they may be coated with bioorganic compounds that make them proper for medical applications. These give biosynthesis of NPs distinct advantages over conventional methods [ 29 , 30 ]. For example, NPs are used as a new tool for targeting in cancer therapy and the most important difficulty is toxicity of NPs synthesized by previous methods. Scientists overcome this issue using biologically synthesized NPs coated with biomolecules that are more biocompatible [ 31 , 32 ].