The as-prepared GO and rGO materials were dispersed in acetone to a nominal concentration of 0.5 mg ml⁻¹ (Fig. 1a). Exfoliated GO produced through the modified Hummers method had sufficient functional groups (e.g., epoxides, hydroxyls, carboxylic acids) to enable suspension in acetone. After reducing GO with hydrazine hydrate, rGO was harvested and re-dispersed in acetone using sonication. The rGO suspension shows black color produced by the removal of oxygen by reduction with hydrazine hydrate. The microstructures of rGO nanosheets were observed by optical microscopy (Fig. 1b), SEM (Fig. 1c) and TEM (Fig. 1d). The optical micrograph shows the transparent characteristic of rGO nanosheets on an oxidized Si substrate. The SEM image shows rGO nanosheets with average particle (lateral) size of less than 5 μm coated on the carbon substrate. The TEM image shows a few layers of rGO nanosheets with different lateral sizes. The nanosheets randomly overlap forming different edge graphitic layers.

Fig. 1 (a) Photograph showing the dispersion of rGO (left) and GO (right) in acetone, (b) optical micrograph, (c) SEM image and (d) TEM image of rGO nanosheets.

Fig. 2 Mycelial growth of A. niger on the PDA media containing different concentrations of rGO (0–500 μg ml⁻¹).

Fig. 3 Mycelial growth of A. oryzae on the PDA media containing different concentrations of rGO (0–500 μg ml⁻¹).

Fig. 4 Mycelial growth of A. oxysporum on the PDA media containing different concentrations of rGO (0–500 μg ml⁻¹).

The concentrations of rGO nanosheets, which can totally inhibit the mycelia growths of A. niger ( Fig. 2), A. oryzae ( Fig. 3), and F. oxysporum ( Fig. 4), are at 500, 500, and 250 μg ml⁻¹, respectively. The reason, why the rGO nanosheets were effective to inhibit fungi, is probably due to the direct contact with the cell walls of fungi. After the contact, the reactive oxygen-containing functionalities of several small rGO nanosheets could chemically react with the organic functional groups of chitin and other polysaccharides on the cell walls of fungi. Notably, the antibacterial mechanism of graphene-related materials based on the direct contact with the bacterial cells was previously reported. It was also reported that the antifungal mechanism of essential oils containing oxygen-species functional groups (e.g., phenol) was due to the direct contact with the cell walls of fungi. Interestingly, it was recently reported that the cytotoxicity of GO nanosheets on A549 human cells occurred as a result of direct interactions between the cell membrane and GO nanosheets.