Biosynthesis of Nanomaterials Using Natural Products

Chapter III: Methodology

The use of microorganisms in the synthesis of nanoparticles emerges as an eco-friendly and exciting approach. In this study, the 100 mM silver nitrate was screened and found to successfully produce gold nanoparticles of different sizes and shapes. The important parameter, which controls the size and shape of gold nanoparticles, was pH value. The R. capsulata biomass and aqueous HAuCl4 solution were incubated at pH values ranging from 7 to 4. The results demonstrated that spherical gold nanoparticles in the range of 10-20 nm were observed at pH value of 7 whereas a number of nanoplates were observed at pH 4.

The optoelectronic and physicochemical properties of nanoscale matter are size- and shape-dependent. So the synthesis of gold nanoparticles of different sizes and shapes is of great importance for their applications in optical devices, electronics, biotechnologies and catalysis. Conventional synthetic methods of gold nanoparticles have involved a number of chemical methods. There is an increasing pressure to develop clean, nontoxic and environmentally benign synthetic technologies. Microbial resistance against heavy metal ions has been exploited for biological metal recovery via reduction of the metal ions or formation of metal sulfides. So the attractive procedure is using microorganisms such as bacteria and fungi to synthesize gold nanoparticles recently. An earlier study found that Bacillus subtilis 168 were able to reduce Au3+ ions to gold nanoparticles with a size range of 5-25 nm inside the cell walls. Shewanella algae were found to reduce Au3+ ions forming 10-20 nm gold nanoparticles extracellularly with the assistance of hydrogen gas. Fungi (Verticillium sp and Fusarium oxysporum) and actinomycete (Thermomonospora sp and Rhodococcus sp) were also used to synthesize nanoparticles intra- or extracellularly. However, the biosynthesis of gold nanoplate extracellularly is still scarce. In this study, prokaryote 100 mM silver nitrate, recognized as one of the ecologically and environmentally important microorganisms, commonly existing in the natural environment, were investigated for reducing Au3+ ions at room temperature with a single step process. Especially gold nanoplates were formed under the lower starting pH.

Experiment 1

Synthesis

Photosynthetic 100 mM silver nitrate were cultured in the medium containing purvate, yeast extract, NaCl, NH4Cl and K2HPO4 at pH 7 and 30 °C. The bacteria were cultured for 72 h and separated from broth by centrifugation (5000 rpm) at 4 °C for 10 min. The collected bacteria were washed five times with distilled water to obtain about 1 g wet weight of bacteria and then resuspended in 20 mL of 1 × 10- 3 M aqueous HAuCl4 solution in a test tube. The reactants were adjusted to neutral pH using 0.1 M NaOH solution. In other experiments, the starting pH of the 1 × 10- 3 M aqueous HAuCl4 solution added with biomass was adjusted to 6, 5 and 4 using 0.1 M NaOH and HCl solutions respectively in test tubes. All the experiments were conducted at room temperature for a period of 48 h.

The UV-Visible spectra of gold nanoparticles synthesized were measured on a Shimadzu spectrophotometer (model UV-3150PC). The products were dropped on a carbon-coated grid and then analyzed by transmission electron microscopy (TEM) (JEOL, JEM-200EX) and electronic diffraction (ED, ...