The present study demonstrates an eco-friendly and low cost protocol for synthesis of silver nanoparticles using the cell-free filtrate of Aspergillus flavus NJP08 when supplied with aqueous silver (Ag+) ions. Identification of the fungal isolate was based on nuclear ribosomal DNA internal transcribed spacer (ITS) identities. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed the formation of spherical metallic silver nanoparticles. The average particle size calculated using Dynamic Light Scattering measurements (DLS) was found to be 17±5.9 nm. UV-Visible and Fourier transform infrared (FTIR) spectroscopy confirmed the presence of extracellular proteins. SDS-PAGE profiles of the extracellular proteins showed the presence of two intense bands of 32 and 35 kDa, responsible for the synthesis and stability of silver nanoparticles, respectively. A probable mechanism behind the biosynthesis is discussed, which leads to the possibility of using the present protocol in future "nano-factories".

Outburst of world population in the past decade has forced the agricultural sector to increase crop productivity to satisfy the needs of billions of people especially in developing countries. Widespread existence of nutrient deficiency in soils has resulted in great economic loss for farmers and significant decreases in nutritional quality and overall quantity of grains for human beings and livestock. Use of large-scale application of chemical fertilizers to increase the crop productivity is not a suitable option for long run because the chemical fertilizers are considered as double-edged swords, which on the one hand increase the crop production but on the other hand disturb the soil mineral balance and decrease soil fertility. Large-scale application of chemical fertilizers results in an irreparable damage to the soil structure, mineral cycles, soil microbial flora, plants, and even more on the food chains across ecosystems leading to heritable mutations in future generations of consumers.In recent years, nanotechnology has extended its relevance in plant science and agriculture. Advancement in nanotechnology has improved ways for large-scale production of nanoparticles of physiologically important metals, which are now used to improve fertilizer formulations for increased uptake in plant cells and by minimizing nutrient loss. Nanoparticles have high surface area, sorption capacity, and controlled-release kinetics to targeted sites making them "smart delivery system." Nanostructured fertilizers can increase the nutrient use efficiency through mechanisms such as targeted delivery, slow or controlled release. They could precisely release their active ingredients in responding to environmental triggers and biological demands. In recent lab scale investigations, it has been reported that nano-fertilizers can improve crop productivity by enhancing the rate of seed germination, seedling growth, photosynthetic activity, nitrogen metabolism, and carbohydrate and protein synthesis. However, as being an infant technology, the ethical and safety issues surrounding the use of nanoparticles in plant productivity are limitless and must be very carefully evaluated before adapting the use of the so-called nano-fertilizers in agricultural fields.

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Using natural processes as inspiration, the present study demonstrates a positive correlation between zinc metal tolerance ability of a soil fungus and its potential for the synthesis of zinc oxide (ZnO) nanoparticles. A total of 19 fungal cultures were isolated from the rhizospheric soils of plants naturally growing at a zinc mine area in India and identified on the genus, respectively the species level. Aspergillus aeneus isolate NJP12 has been shown to have a high zinc metal tolerance ability and a potential for extracellular synthesis of ZnO nanoparticles under ambient conditions. UV-visible spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction analysis, transmission electron microscopy, and energy dispersive spectroscopy studies further confirmed the crystallinity, morphology, and composition of synthesized ZnO nanoparticles. The results revealed the synthesis of spherical nanoparticles coated with protein molecules which served as stabilizing agents. Investigations on the role of fungal extracellular proteins in the synthesis of nanoparticles indicated that the process is nonenzymatic but involves amino acids present in the protein chains.

The present study demonstrates an economical and environmental affable approach for the synthesis of “protein-capped” silver nanoparticles in aqueous solvent system. A variety of standard techniques viz. UV-visible spectroscopy, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) measurements were employed to characterize the shape, size and composition of nanoparticles. The synthesized nanoparticles were found to be homogenous, spherical, mono-dispersed and covered with multi-layered protein shell. In order to prepare bare silver nanoparticles, the protein shell was removed from biogenic nanoparticles as confirmed by UV-visible spectroscopy, FTIR and photoluminescence analysis. Subsequently, the antibacterial efficacy of protein-capped and bare silver nanoparticles was compared by bacterial growth rate and minimum inhibitory concentration assay. The results revealed that bare nanoparticles were more effective as compared to the protein-capped silver nanoparticles with varying antibacterial potential against the tested Gram positive and negative bacterial species. Mechanistic studies based on ROS generation and membrane damage suggested that protein-capped and bare silver nanoparticles demonstrate distinct mode of action. These findings were strengthened by the TEM imaging along with silver ion release measurements using inductively coupled plasma atomic emission spectroscopy (ICP-AES). In conclusion, our results illustrate that presence of protein shell on silver nanoparticles can decrease their bactericidal effects. These findings open new avenues for surface modifications of nanoparticles to modulate and enhance their functional properties.