Isolation and characterization of a native strain of Aspergillus niger ZRS14 with capability of high resistance to zinc and its supernatant application towards extracellular synthesis of zinc oxide nanoparticles

Zinc oxide nanoparticles have quite a few applications in the fields of biology، optics، mechanics، magnetism، energy، hygiene and medicine. Due to serious problems associated with physiochemical synthesis of ZnO nanoparticles، including environmental pollution، complicated and costly processes، there is a growing need to develop a simple biological procedure for synthesis of nanoparticles to achieve the monodisperse-sized particles with a higher purity، low energy consumption and a cleaner environment. We conducted this investigation to screen and isolate native fungi strains capable of high zinc metal tolerance ability and a potential for extracellular synthesis of ZnO nanoparticles using fungal secretions as biological catalysts. 15 different strains of fungi were isolated from soil samples collected from lead and zinc mines of Angoran-Zanjan using conventional enrichment process and characterized initially based on macroscopic and microscopic characteristics and colony morphology. The intrinsic tolerance of the isolated strains to zinc toxic metal was measured in the synthetic and complex media using the agar dilution method. The supernatants of isolated fungi were incubated with zinc acetate solution in a shaker incubator for 72h; then، the strain that was able to synthesis ZnO nanoparticle was identified. The ZnO nanoparticles formation was investigated by using spectroscopic techniques and microscopic observations. Among the 15 isolated strains، the strain ZRS14 had highest zinc metal tolerance ability and was selected and identified as Aspergillus niger strain ZRS14 (GenBank accession number KF414527) based on morphological and molecular phylogenetic analysis. For synthesis of ZnO nanoparticles by isolated A. niger ZRS14، fungal cell-free filtrate of the strain was collected and incubated in the presence of zinc acetate solution at a final concentration of 250 mg/l zinc metal ion at 28º C for 72 h on a rotary shaker (150 rpm) under dark conditions. Results obtained by visual observations، spectral data achieved from UV–vis، XRD spectrum and SEM micrographs revealed the extracellular formation of ZnO nanoparticles with narrow size distribution and average particle size of 32 nm with the collected cell-free filtrate of A. niger isolate ZRS14. Discussion and Owing to the extracellular synthesis of ZnO nanoparticles، the obtained results in the current study suggest that the A. niger strain ZRS14 has a considerable potentiality that can be efficiently used as an eco-friendly biocatalyst for the preparative synthesis of ZnO nanoparticles. The present investigation is the first report on the biological synthesis of ZnO nanoparticles using newly isolated strain of Aspergillus niger ZRS14.