## 3 Results and Discussion
### Finding a Potential Technological Application of the ZnO/Clay Composite
Figure 2e shows the first appearance of ZnO agglomerates due to a high concentration of ZnO nanoparticles. When the concentration of ZnO increases, the quantity, and size of agglomerates also rise (Figure 2f). Consequently, a dispersion mechanism can be established depending on the deposited material concentration. The clay agglomerates serve like the base of the dispersion of the ZnO (Figure 2g). At low concentrations of ZnO, from 2 to 10 wt.% (Figure 2g), the distribution of ZnO is uniform onto the clay surface. Once the surface coating is reached, the agglomeration of ZnO nanoparticles occurs (Figure 2g). The antifungal response of ZnO/Clay composites is tested by the Bauer-Kirby disk diffusion assay. The fungus selected for the assay is _Aspergillus niger_, which is incubated at 37 \({}^{\circ}\)C for three days after ZnO/Clay addition. As shown in Figure 3a,b, the presence of nanoparticulated ZnO improves the antifungal inhibition ratio of the bentonite clay. Evaluating the antifungal evolution as a function of the ZnO concentration (Figure 3c), an increase in the sporulation inhibition diameter (SID) is observed. Modified bentonite clay with Ag cation has a very low antifungal response (called B). This fact is in accordance with the lack of antifungal activity of Ag that literature reports. By contrast, it has been reported by [PERSON] et al. [33] that the use of the modified bentonite clay with Ag cations  
Figure 2: Distribution of ZnO on bentonite modified clay by dry dispersion methodology. Panels (**a**) to (**f**) show the micrographs of ZnO/clay after the incorporation of different ZnO concentrations—2% (**a**), 6% (**b**), 10% (**c**), 20% (**d**), 40% (**e**), 60% (**f**). Panel (**g**) shows the steps of composite evolution according to ZnO concentration: (1) no ZnO addition, (2) ZnO concentration from 2% to 10%, (3) concentration upper than 20%. guarantees an effective antibacterial activity. This fact should endow the ZnO/Clay system of a combined action between Ag-Zn cations, where the Ag cations guarantee the antibacterial activity, while Zn cations provide the antifungal activity. At low ZnO concentrations, between 2 to 10 wt.% (named as C to E), the SID increases exponentially. When 20 wt.% of ZnO (F) is reached, the SID behavior changes towards a more moderate improvement. This asymptotic character is maintained for higher ZnO concentrations as 40 wt.% (G) and 60 wt.% (H). Therefore, the relationship between the addition of ZnO and antifungal activity is not linear. There are two well-differentiated linear regressions whose intersection occurs at 10 wt.% (E). At concentration below 10 wt.%, ZnO nanoparticles are well dispersed onto clay, which coincides with faster efficacy growth of the antifungal activity (see Figure 3c green zone). When agglomerates appear on clay at concentration upper 10 wt.%, the growth of the antifungal response is reduced (see Figure 3c red zone). Hence, ZnO agglomerates onto clay surface are less effective with respect to the uniformly dispersed ZnO. These results prove the importance of both the uniform distribution and an adequate concentration of the ZnO nanoparticles onto the clay surface for the antifungal activity of composite. According to the promising results against fungi, the antifungal response of ZnO/Clay composite is tested in a waterborne paint matrix. The same method and conditions as in the previous antifungal test are used. The experiments are collected in Figure 4a and Figure S5 (Supplementary Materials). The waterborne paint was modified with a concentration of 0.5 wt.% of ZnO/Clay composite with a ZnO content of 10 wt.% (i.e., 0.05 wt.% of ZnO nanoparticles). The very low concentration is selected due to the adequate compromise between ZnO dispersion and antifungal response. Two conditions of the waterborne paint, fresh and dry, are taken into account. As shown in Figure 4a, the fresh paint without the presence of ZnO/Clay composite, as control, displays a poor inhibition diameter.