Island of Newfoundland, Nova Scotia, Gulf of St. Lawrence / May 13, 2017 https://go.nasa.gov/2rdnjZb (above)
Fabrication of Metal Nanoparticles from Fungi and Metal Salts: Scope and Application
Khwaja Salahuddin Siddiqi and Azamal Husen
Feb.24, 2016 [excerpts]
Nanoscale Res Lett. 2016; 11: 98.
Published online 2016 Feb 24. doi: 10.1186/s11671-016-1311-2
Fungi secrete enzymes and proteins as reducing agents which can be used for the synthesis of metal nanoparticles from metal salts. Large-scale production of nanoparticles from diverse fungal strains has great potential since they can be grown even in vitro. In recent years, various approaches have been made to maximize the yield of nanoparticles of varying shape, size, and stability. They have been characterized by thermogravimetric analysis, X-ray diffractometry, SEM/TEM, zeta potential measurements, UV-vis, and Fourier transform infrared (FTIR) spectroscopy. In this review, we focus on the biogenic synthesis of metal nanoparticles by fungi to explore the chemistry of their formation extracellularly and intracellularly. Emphasis has been given to the potential of metal nanoparticles as an antimicrobial agent to inhibit the growth of pathogenic fungi, and on other potential applications.
Keywords: Green synthesis, Metal nanoparticles, Antimicrobial, Fungi, Plant
Of all the processes developed so far, the fabrication of metal nanoparticles by the biogenic methods employing plant extract are more popular, innocuous, inexpensive, and environmentally friendly as they do not leave hazardous residues to pollute the atmosphere [1–6]. Chemical methods for the synthesis of nanoparticles are common, but their use is limited. The biogenic synthesis is, therefore, the best choice where inherently benign organic molecules do not pose a threat to human health and atmosphere. Microbes have a promising role in the fabrication of nanoparticles due to their natural mechanism for detoxification of metal ions through reduction that can be achieved extra- or intracellularly by bioaccumulation, precipitation, biomineralization, and biosorption [4, 7–12].
Use of microgranisms in the green synthesis of metal nanoparticles with special reference to the precious metals using fungi has been done [10, 13–17]. Since fungi contain enzymes and proteins as reducing agents, they can be invariably used for the synthesis of metal nanoparticles from their salts. Since some fungi are pathogenic, one has to be cautious while working with them during experiment. Fungus biomass normally grows faster than those of bacteria  under the same conditions. Although synthesis of metal nanoparticles by bacteria is prevalent, their synthesis by fungi is more advantageous  because their mycelia offer a large surface area for interaction. Also, the fungi secrete fairly large amount of protein than bacteria; therefore, the conversion of metal salts to metal nanoparticles is very fast.
Engineered metal nanoparticles of varying size and shape from the diverse fungal species and yeast are listed in Table 1. Extracellular synthesis of nanoparticles involves the trapping of the metal ions on the surface of the cells and reducing them in the presence of enzymes, while intracellular synthesis occurs into the fungal cell in the presence of enzymes. Fungi secrete extracellular proteins which have been used to remove metal ions as nanoparticles. In a broad sense, the metal nanoparticles can be extensively used in different areas of agriculture and technology [2, 5, 6, 20, 21]. Many metal nanoparticles are antibacterial and find extensive uses in medicine [6, 22–24]. The antibacterial efficiency is enhanced manifold when a nanoparticle of one metal is coupled with another such as those of copper and silver. Although in recent times several organisms have been investigated for the fabrication of nanoparticles, its mechanism is still not well understood. This review, therefore, focuses on the biogenic synthesis of metal nanoparticles by fungi and attempts to explore the chemistry of their formation extracellularly and intracellularly.
Synthesis, Mechanism, and Characterization of Metal Nanoparticles
Biogenic synthesis of metal nanoparticles involves bio-reduction of metal salts to elemental metal which may be stabilized by organic molecules present in the microbes such as fungi and bacteria. The other way of producing metal nanoparticles is biosorption where metal ions in the aqueous medium are bonded to the surface of the cell wall of the organisms. For large-scale production of nanoparticles, fungi and yeasts are preferred over other organisms (Figs. 1 and and2).2). When fungus is exposed to metal salts such as AgNO3 or AuCl4−, it produces enzymes and metabolites to protect itself from unwanted foreign matters, and in doing so, the metal ions are reduced to metal nanoparticles . The fungi also produce napthoquinones and anthraquinones [92–95] which act as reducing agents. Thus, a specific enzyme can act on a specific metal. For instance, nitrate reductase is essential for ferric ion reduction to iron nanoparticles. It was reported that for metal ion reduction, not only the enzyme was necessary but also an electron shuttle . It is well understood that nanomaterials may be beneficial or harmful to living systems [1–6]. For example, Cd, Hg, Pb, and Tl nanoparticles are toxic and produce adverse effect in mammals and plants. The toxicity also depends on their shape, size, and the nature of the specific metal ion.
Biosynthesis of gold nanoparticles from fungi has been reviewed very recently . They are resistant to oxidation and dispersed  nicely. The color corresponds to the particle size in general. For instance, yellow, red, and mauve refer to large, small, and fine nanoparticles, respectively, of varying size and morphology . It is claimed that gold nanoparticles can be stabilized by substances like ascorbic acid and citrate . Stabilization can also be achieved by polyvinyl alcohol . Enzymes are said to be responsible for the biosynthesis of gold nanoparticles. The intra- or extracellular synthesis of nanoparticles by fungi is done in a simpler manner. The gold ions are trapped by the proteins and enzymes on the surface of the fungi and get reduced. They further form aggregates of large dimensions . The gold nanoparticles synthesized from various sources have different properties. They have been checked for their cytotoxic effects against cancer . Both the intracellular and extracellular reduction of AuCl or AuCl3 follow the same pathway . Since AuCl requires one electron to give gold nanoparticles, it follows one-step reduction whereas AuCl3 requires three electrons and reduction occurs in three steps. As an example, when AuCl3 is dissolved in water, the following reactions occur at the mycelia of fungi which contain proteins, etc. and the metal nanoparticles are produced.
Application of Metal Nanoparticles
There are a myriad of applications of metal nanoparticles such as cosmetics, catalysts, lubricants, fuel additives, paints, agro-chemicals, food packaging, textile engineering, electronics, optics, environmental sensing, nanomedicine, drug and gene delivery agents, biodetection of pathogens, tumor destruction via heating (hyperthermia), magnetic resonance imaging, and phagokinetic studies [2–9, 23, 24, 133–136, 139]. Fungus-mediated synthesis of metal nanoparticles is getting much attention due to their extensive application in various sectors (Table 4).
full article here:
Baja CA & Isla de Cedros / May 13, 2017
Isla de Cedros
Sea of Okhotsk & Hokkaido Japan / May 13, 2017 https://go.nasa.gov/2qeHOYq
Sakhalin Russia & Sea of Okhotsk / May 13, 2017
Sakhalin & Tatarskiy proliv Russia (very difficult to find anything about Tatarskiy proliv)
Island of Newfoundland, Nova Scotia, Gulf of St. Lawrence / May 13, 2017 https://go.nasa.gov/2qeLaui (above)
Island of Newfoundland, Nova Scotia, Gulf of St. Lawrence / May 13, 2017 https://go.nasa.gov/2qeLrgO
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Island of Newfoundland, Nova Scotia, Gulf of St. Lawrence / May 13, 2017 https://go.nasa.gov/2rdpETN
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