Response of Biological Gold Nanoparticles to Different pH Values: Is It Possible to Prepare Both Negatively and Positively Charged Nanoparticles?

The mycelium-free supernatant (MFS) of a five-day-old culture medium of Fusarium oxysporum was used to synthesize gold nanoparticles (AuNPs). The experimental design of the study was to answer the question: can this production process of AuNPs be controllable like classical chemical or physical approaches? The process of producing AuNPs from 1 mM tetrachloroauric (III) acid trihydrate in MFS was monitored visually by color change at different pH values and quantified spectroscopically. The produced AuNPs were analyzed by transmission electron microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The presence of capping agents was confirmed by Fourier transform infrared spectroscopy (FTIR). Two AuNP samples with acidic and alkaline pH were selected and adjusted with the pH gradient and analyzed. Finally, the size and zeta potential of all samples were determined. The results confirmed the presence of the proteins as capping agents on the surface of the AuNPs and confirmed the production of AuNPs at all pH values. All AuNP samples exhibited negative zeta potential, and this potential was higher at natural to alkaline pH values. The size distribution analysis showed that the size of AuNPs produced at alkaline pH was smaller than that at acidic pH. Since all samples had negative charge, we suspect that there were other molecules besides proteins that acted as capping agents on the surface of the AuNPs. We conclude that although the biological method of nanoparticle production is safe, green, and inexpensive, the ability to manipulate the nanoparticles to obtain both positive and negative charges is limited, curtailing their application in the medical field.