Zinc oxide/copper ferrite ferrofluids derived from natural resources for antifungal applications

One of the serious global health issues is related to toxigenic fungi, especially Aspergillus flavus. To address this issue, it is crucial to develop eco-friendly and effective antifungal agents. Therefore, we develop zinc oxide/copper ferrite–soursop leaf extract nanocomposites as novel antifungal agents using a combination of sol–gel, coprecipitation, and mixing methods. Interestingly, the nanocomposites were modified into ferrofluids with varying mass of copper ferrite–soursop leaf extract to increase their stability and antifungal performance. Furthermore, this work employed natural resources such as iron sand, soursop leaf, and coconut oil as the main eco-friendly precursors. The results of X-ray diffractometry showed that the nanocomposites formed two crystal phases, namely zinc oxide and copper ferrite with hexagonal wurtzite and inverse cubic spinel structures, respectively. Meanwhile, the presence of soursop leaf extract as a surfactant in ferrofluids was indicated by several main functional groups, such as C–H, O–H, and C=O. The morphology of the zinc oxide/copper ferrite–soursop leaf extract nanocomposites tended to be more uniform, with a size that decreased from 52.44 nm to 22.68 nm. The prepared ferrofluids were tested using the well diffusion method with 3 replicates, showing the antifungal activity against Aspergillus flavus with inhibition zone diameters ranging from 7.60 mm to 8.90 mm. This increase occurred because the particle size of nanocomposites was smaller, causing them to easily penetrate the fungal cell membrane. In addition, the increase in the inhibition zone diameter can be attributed to the presence of O–H functional groups and bioactive compounds, originating from increasing leaf extract composition. Furthermore, the data analysis confirmed that the increase in the inhibition zone diameter was not due to random variation, but rather the effectiveness of the ferrofluid system, as indicated by a statistically significant result (p < 0.05). The presence of oleic acid and dimethyl sulfoxide in the ferrofluids also contributed to increasing the stability of the nanocomposites and membrane penetration so that the ferrofluids enter the fungal cells. Furthermore, the synergistic effect between zinc oxide and copper ferrite also produced reactive oxygen species and interactions with fungal cells. These findings suggest that the prepared zinc oxide/copper ferrite–soursop leaf extract ferrofluids have potential as novel antifungal agents, especially against Aspergillus flavus.

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