Abstract:
Nanoparticles (NPs) have been steadily interesting in Physics, Chemistry, Biology, Biomedicine, etc. In
this thesis work, green synthesis of silver nanoparticles by the fungal and bacterial method was
investigated. Two different concentrations of AgNO3 viz. 10mM and 1mM were used as precursors,
whereas cell- free supernatants from A. niger and E. coli were prepared and used as source of enzyme
for the reduction and stabilization of nanoparticles. The crude enzyme was produced by A. niger, using
Solid State Fermentation (SSF). The medium used for the fermentation contained wheat bran as the
growth substrate and was incubated at 29 ± 1ºC for five days. Whereas for enzyme production by E. coli,
Luria Bertani (LB) medium was used and it was incubated at 37 ± 1 ºC overnight. The culture for the
growth of A. niger was filtered by muslin cloth and centrifuged at 12000 rpm for 20 min and the filtrate
was used as source of crude enzyme for the synthesis of Ag NPs as well as for characterization. Similarly,
the E. coli culture was first filtered by Millipore filter paper followed by centrifugation of the supernatant
at 12000 rpm for 20 min. Visual indication of the synthesis of silver nanoparticles was obtained when the
color of supernatant plus the precursor changed from yellow to orange for E. coli and from light brown
to dark brown for A. niger. The characterization of the synthesized Ag NPs as well as the supernatants
obtained from E. coli and A. niger were done using UV-Vis absorption spectroscopy to confirm the
formation of silver nanoparticles. The UV- Vis absorption peak for Ag NPs synthesized by E. coli was
seen around 416nm; whereas, the UV-Vis peak for Ag NPs synthesized by A. niger appeared around
425nm. The next characterization of the synthesized silver nanoparticle was done by Fourier Transform
Infrared spectroscopy (FTIR), which was used to reveal the functional groups accompanying the synthesized NPs. Finally the antimicrobial activity of the synthesized silver nanoparticles was tested
against three gram negative and two gram positive bacteria by disc diffusion method. Extracellular crude
extracts obtained from both microbes did not show any antibacterial effect on the tested pathogens.
However, all the four kinds of Ag NPs synthesized by both species exhibited antibacterial effect. The Ag
NPs showed stronger antibacterial activities on gram positive bacteria than on gram negative bacteria.
The results also indicated that a larger value of ZOI was produced for the Ag NPs synthesized at the
lowest concentration (1mM) of the precursor AgNO3 used for the synthesis of Ag NPs.
