Production and Characterization of Silver Nanoparticles using some Soil Microorganisms

Abstract

Fifteen fungal and twenty one bacterial isolates were screened for extracellular biosynthesis of silver nanoparticles (AgNPs). Culture supernatant (CS), cell-free filtrate (CFF) and the washed biomass of the experimented microorganisms were investigated as possible sources for the biogenic reduction of silver ions (Ag+) to AgNPs (Ag0). CS of only ten of these microorganisms (3 fungi and 7 bacteria) were found to have certain capabilities toward reduction of Ag+ at the time that CFF of the three fungi had this potentiality with varying degrees. Biomasses of all investigated organisms were found to lack this activity. The three potential fungi were Aspergillus terreus, Cunninghamella phaeospora and Emericella quadrilineata. On the other hand, the bacterial isolates are belonged to the genera Aeromonas, Bacillus (5 isolates) and Exiguobacterium. Formation of AgNPs was preliminary detected by the colour change of the reaction mixtures to brown or at least brownish by visual observations and confirmed by UV-Vis spectroscopic analysis. A comparatively reasonable yield of AgNPs was obtained using the CS from the fungus C. phaeospora and CFF from E. quadrilineata so they were selected for further study in this respect. Factors affecting the biosynthetic reactions leading to the formation of AgNPs using CS from C. phaeospora and CFF from E. quadrilineata were investigated. These nanoparticles (NPs) were optimally synthesized at 100°C and pH 9 (CS) or pH10 (CFF). Series of experiments were also conducted to investigate some environmental conditions upon production of balanced yields of AgNPs. The most favorable environmental conditions for production of CS from C. phaeospora was after 96 h of fungal growth at 30oC in shake cultures at 150 rpm and adjusting the pH of the fermentation medium to 9. In case of CFF from E. quadrilineata the highest yield was obtained after 72 h of shaking using 150 rpm at 30oC and pH 9. Studies of certain mechanistic aspects of the bioreduction process were conducted. Nitrate reductase (NR) was confirmed to play a vital role. Other biomolecules especially proteins were detected by FTIR measurements as well as fluorescence emission spectrum that may play a role in both reduction and capping of the AgNPs.