*3.2. Transformation of ZnO Nanoparticles by Fungal Extracellular Metabolites*

To experimentally confirm the transformation of ZnO NPs into zinc oxalate, extracellular metabolites of *A. niger* were applied to ZnO NPs. A partial transformation of ZnO NPs was observed, and a new mineral phase—zinc oxalate dihydrate—was identified (Figure 4).

**Figure 4.** Diffractogram of the partially transformed ZnO NP to Zn oxalate dihydrate.

∙ − ∙ − At the end of the experiment, the oxalic concentration decreased from the initial 11.6 mmol·L −1 to 1.2 mmol·L −1 . Thus, a 90% decrease in the initial oxalic acid concentration was observed, further confirming the ZnO NPs transformation into the zinc oxalate dihydrate by *A. niger*. Quantitatively, approximately 43 mg or 8.6% of ZnO NPs was transformed into the zinc oxalate dihydrate in the process. Only a partial transformation was observed. The recrystallization did not result in a significant change in the size of the most particles, and it ranged between 40 and 100 nm (Figure 5). ∙ − ∙ −

(**a**) (**b**) (**a**) (**b**)

**Figure 5.** Transmission electron microscopy image of (**a**) Pristine ZnO nanoparticles (**b**) Partially transformed ZnO nanoparticles to Zn oxalate dihydrate.
