Response of Biological Gold Nanoparticles to Different pH Values: Is It Possible to Prepare Both Negatively and Positively Charged Nanoparticles?  
[PERSON]  
1 Institute of Microbiology, Czech Academy of Sciences, 142 20 Prague, Czech Republic;  
[PERSON](P.P.); [EMAIL_ADDRESS] (O.B.); [EMAIL_ADDRESS] (M.P.);  
[EMAIL_ADDRESS] (E.N.)  
[PERSON]  
1 Institute of Microbiology, Czech Academy of Sciences, 142 20 Prague, Czech Republic;  
[PERSON](P.P.); [EMAIL_ADDRESS] (O.B.); [EMAIL_ADDRESS] (M.P.);  
[EMAIL_ADDRESS] (E.N.)  
[PERSON]  
1 Department of Medical Sciences, Shahrood Branch, Islamic Azad University, Shahrood 4318936199, Iran 1  
chemical methods, there has recently been an increased focus on using biological processes or their products in NP production. This is due to the improved properties of NPs in terms of biocompatibility and an environmentally friendly means of production [13; 15].  
Filamentous fungi are eukaryotic microorganisms that colonize almost every ecological niche on Earth. Being indispensable players in the carbon cycle, they are essential members of the ecosystem [16; 17]. They are also important pathogens of plants [18], animals, and humans [19; 20]. Nevertheless, fungi have great potential in industrial utilization in many biotechnological applications [21]. The biotechnological synthesis of NPs is one of these applications. Many filamentous fungal strains have been investigated for their use in the biological synthesis of AuNPs including the mesophilic genera _Apergillus, Penicillin, Fusarium, Trichoderma, Pacolimyes, Rhizpus, Chaetomium_, and _Stemphylium_[22; 23]. A set of 27 thermophilic genera was also tested [14]. In the biological synthesis method, the nanoparticles are usually reduced by intracellular microbial products, such as enzymes or extracellular substances, that include proteins, polysaccharides, and other secreted biomaterials [24]. Thus, the NP reduction process can take place inside or outside the microbial cells, and the reducing agents and the reduction process can be enzymatic or non-enzymatic [24]. An inherent feature of all these different types of bioreduction is that the microbial products also serve as capping agents that cover the surface of the produced NPs. Some of these agents are proteins, and their presence on the surface of the NPs affects the stability of the nanoparticles and prevents their aggregation [25]. These capping agents are responsible for the surface zeta potential of unmodified AuNPs. Several techniques are available to detect the presence of capping agents on NP surfaces such as Fourier transform infrared spectroscopy (FTIR) and sodium dodecyl sulfate-polyacrylamideide gel electrophoresis (SDS-PAGE) [25]. In this report, we analyzed the response of the capping agents (secreted by _Fusarium oxysporum_ into the culture medium) to pH variations before and after AuNP production. The experiments aimed to evaluate the relationship between the pH variations and the zeta potential, size, and agglomeration of the final AuNPs. The obtained characteristics of AuNPs after biological production could enable us to understand their activity under physiological conditions and help to answer the fundamental question: can this fungal production of AuNPs be a controllable process like the other non-biological methods?