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Nanomaterials 2020, 10, 1146 3 of 24 color is an indicator of synthesis of silver NPs (AgNPs) [8,20]. For intracellular synthesis, the biomass is washed thoroughly with sterile water after culturing microorganisms under optimum growth conditions and incubated with metal ion solution. As mentioned above, the color change serves as an indicator of NPs synthesis. Then NPs are collected by ultra sonication, centrifugation and washing [21]. Here, we review various metallic NPs synthesis through the utilization of microorganisms. A diverse group of bacteria are living in soil, water, plants and animals. They can live in various soil pH, salinity, temperature and nutrient conditions. In aquatic environment, bacteria can be found in normal to highly saline water in deep-sea and even in the ice with a freezing temperature. Some of them can be occurred in heavily contaminated or hyper accumulated soils and plants. Pseudomonas stutzeri and Pseudomonas aeruginosa can survive even in high concentrated metal ion conditions [22,23]. Thiobacillus ferrooxidans, T. thiooxidans and Sulfolobus acidocaldarius can reduce ferric to the ferrous ion while living on elemental sulfur as an energy source [24]. Therefore, bacteria possess their own mechanisms by which they can survive and uptake nutrients for their growth and multiplication. They can reduce the metallic substances and utilize energy for themselves. Bacteria are evolving many defense mechanisms like sequestration intracellularly, pumping efflux, changing concentration of metal ions and precipitation extracellularly to overcome various stresses [25]. These types of mechanisms of bacteria can be applied in the green synthesis of NPs. Recently, bacterial strains belonging to Acinetobacter calcoaceticus, Bacillus amyloliquefaciens, Bacillus megaterium, Bacillus licheniformis, Escherichia coli, Lactobacillus sp. and Pseudomonas stutzeri have been used in for the biosynthesis of AgNPs [26]. Silver NPs can be produced by both intracellular and extracellular biosynthesis and these NPs have shown antimicrobial activity against many pathological organisms [27]. The culture supernatant of Pseudomonas rhodesiae was incubated with AgNO3 solution and AgNPs were synthesized. A clear surface plasmon resonance peak at 420–430 nm in the range of 350–450 nm was confirmed as a featured peak of AgNPs by UV–Visible spectroscopy. The reduction of Ag+ and stabilization of the AgNPs was identified by Fourier-transform infrared spectroscopy. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed to measure the size of the AgNPs synthesized with supernatant of P. rhodesiae. The AgNPs were generally spherical and uniform with a range of 20–100 nm in diameter. X-ray diffraction analysis was used to observe the crystalline nature of the P. rhodesiae mediated AgNPs [8]. An extracellular biosynthesis of AgNPs was carried out using B. cereus SZT1, isolated from wastewater-contaminated soil. The AgNPs were spherical shapes and their particle size ranged from 18 to 39 nm [18]. The culture filtrate of endophytic Pseudomonas poae strain CO was used to synthesize AgNPs with the size of 19.8–44.9 nm [20]. The AgNPs were biosynthesized using the culture supernatant of Stenotrophomonas sp. BHU-S7, which was isolated from agricultural farm soil [28]. The synthesis of gold NPs (AuNPs) using Bacillus subtilis isolated from Hatti Gold Mine was reported in a study. The microorganisms isolated from gold mine might be highly resistant to gold ions toxicity and could be used to synthesize AuNPs efficiently. The synthesis of ultra-small palladium and platinum NPs were done by using Shewanella loihica PV-4 within the size range of 2–7 nm [29]. Ochrobactrum sp. was used to synthesize tellurium NPs and this strain might serve as an effective nanofactory to convert the toxic tellurite oxyanions into useful NPs [30]. In addition to the isolation of bacteria from terrestrial environments some marine bacterial cultures have been utilized as nanofactories for synthesis of NPs. A novel bacterium Stenotrophomonas was used for green synthesis of AgNPs and AuNPs. Here, the secretory proteins with low molecular weight present in the supernatant play a key role for biosynthesis of AgNPs and AuNPs [31]. Another marine strain, Kocuria flava, was able to synthesize copper NPs with a size of 5 to 30 nm [32]. In a previous study, AgNPs were synthesized from Pseudomonas stutzeri AG259 through the process involving NADH-dependent reductase enzyme which provides electrons to oxidize NADH to NAD+. The donation of electron from NADH causes the bioreduction of Ag ions to AgNPs [33]. Pseudomonas aeruginosa SM1 can synthesize various NPs intracellularly, such as Ag, Fe, Co, Ni, Li, Pd, Pt and Rh NPs [34]. Moreover, some researchers have shown the synthesis of NPs without involvingPDF Image | Plant and Microbe-Based Synthesis of Metallic Nanoparticles
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