**5. Carbon-based nanoparticles/nanomaterials**

Recently, carbon-based nanomaterials/particles (CNPs), which include nanotubes (i.e., double- or single-walled carbon nanotubes (DWCNT/SWCNTs)), fullerenes, and graphene oxide (GO) (**Figure 3**), have gained attention due to their potent biocidal activities. According to literature, the biocidal potency of these novel NPs is influenced by their physical/chemical properties, high adsorptive potentials, size, large surface area, and colloidal stability under wide range of pH. Increasing the NPs' surface area led to a decrease in size, with concomitant increase in adsorption and absorption (into fungi cell), which improved interaction [190–196] with subsequent inhibition of fungal growth.

The mycelia biomass and aflatoxin biosynthesis in *A*. *flavus* NRRL 3251 was negatively influenced at 10 μg mL<sup>−</sup><sup>1</sup> of fullerene C60 (fullerols C60(OH)24). The effects (growth arrest) was concentration-dependent. However, the antioxidative activity of the furrerols declined over time [197]. Hao and colleagues [198] investigated the fungicidal potentials of metal (copper oxide (CuO), ferric oxide (Fe2O3), and TiO2NPs) and carbon-based NPs (multiwalled carbon nanotubes, fullerene, and reduced graphene oxide) against *Botrytis cinerea*. The results showed that all the six NPs exhibited biocidal activity with 50 mg/L of fullerene showing the strongest antifungal effects.

Reduced graphene oxide (rGO) nanosheets inhibited the mycelial growth of *A*. *niger*, *A*. *oryzae,* and *F*. *oxysporum* with half maximal inhibitory concentrations (IC50) of 500, 500, and 250 μg/mL, respectively. The fungicidal activity as ascribed to the sharp edge of the rGO [199] which inflict injury on the cells, resulting in leaking of the cell components. Another hypothesis is that the organic functional groups on the fungi cell wall chemically interact with the ROS in rGO [200], which halts the uptake of nutrient and excretion of waste metabolites eventually killing the fungi.

Among the six carbon nanomaterials (SWCNTs, MWCNTs, GO, rGO, C60, and activated carbon (AC)) assessed for their fungicidal activity against pathogenic fungi (i.e., *F*. *graminearum* and *F*. *poae*), SWCNTs (500 μg/mL) exhibited the most potent activity, followed by MWCNTs, GO, and rGO respectively. However, the other two CNPs (C60 and AC) showed minimal activity, probably due to insufficient contact with fungal spores [201]. Conclusively, increasing the concentration of CNPs (62.5 < 125 < 250 < 500 μg/mL) increased the fungicidal potency. In a similar study, Wang et al. [202] reported that modifying the surface of MWCNTs with ▬OH,

*Mycotoxins and Food Safety*

cells are composed of cations such as H+

which vital intracellular structures are leaked out.

of HS could potentially influence the biocidal activity.

**4.1 Mechanistic action of UDHSS-NPs antimicrobial activity**

Several studies have described the biological activity of sapropel on enzymes, which confirms its antimicrobial activity. Details of these studies are discussed below in a quest to put forward a proposed mechanism by which UDHSS-NPs kill microorganisms. Environmental factors such as temperature, pH, oxygen, and moisture play a vital role in the mechanistic action of UDHSS-NPs. According to Perdue [166], HSs is complex mixture containing aliphatic, aromatic carboxyl and hydroxyl functional groups, which binds with microbial cells either on grains or in the environment (i.e., water, soil, etc.), thus alter the membrane structural intergrity and its functions. According to literature, the fungi cell walls share similarities with plant and bacterial and indeed with the extracellular matrix material of mammalian cells. The anionic surface, β1,4- and β1,3 linked polystarch forms a ribbon-like or helical (β1,3-glucan) structures which interacts with opposite charges. The cross linking of glycans of in eubacterial walls with peptides as well as phenolics and polysaccharides in plats promotes hydrogen bonding [167, 168]. Furthermore, the fungal cell wall is uniquely composed of mannoproteins, chitins, αand β-linked glucans which serves many functions including; metabolism, ion exchange as well as providing cell rigidity and shape [169]. With the latter interacting with the HS. The interactions between HS and microbial cells depend on the lipophilicity and electric potential of the HS and cell [170], coupled with the size of the UDHSS-NPs. Microbial

> , Na+ , K+

interact with UDHSS-NPs thus penetrate the cell. As documented by Lofts et al. [171], cation-HS interactions exert control on the reactivity of cation, including its bioavailability for further reaction. Studies have shown the effects of binding metals with HS on water and soil ecosystems [172–174]. Natural and artificial HS got attracted to rice cells [175], macrophyte of *Ceratophyllum demersum*, crustaceans—*Gammarus pulex*, and vertebrates—tadpoles of *Rana arvalis* [176], which support the hypothesis that HS is charged and naturally interacts with microorganisms. When HS penetrates or is taken up by a cell, the electric potential of the cell is disrupted, denying the cell the ability to provide support in terms of rigidity, shape and metabolism, thus creating pores through

In an in vivo experiment, Vigneault et al. [177] discovered that Suwannee River HA and FA enhanced the release/leakage of the fluorescent probe sulforhodamine-B (SRB) encapsulated within 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphatidylcholine (POPC) vesicles. With regards to HA, a pH from 7.6 to 5.7 enhanced its surfactant-like effect. In conclusion, HS can alter the permeability of microbial cell, to create pores via which intracellular components are leaked out, killing the microorganism. However, the concentration, functionalisation (acylation), and pH

According to Almatov and Akhmerov [178], 0.2–0.8 mg/mL mumie activated mitochondrial respiration and inhibited cellular succinate-oxidase and NADHoxidase activity (mitochondrion). Similarly, mumie triggered the outflow of

Previous studies [179–181] reported that mumie induced a dose-dependent elevation of superoxide dismutase, catalase, and glutathione peroxidase in rats. These enzymes are involved in the generation of ROS in an HA-induced antimicrobial or biological effects, which killed microorganisms and other grain storage pest. A small-molecular size humic (LMSH) extracted from the feces of *Nicodrilus* and *Allolobophora rosea* enhanced the uptake of nitrate by plant roots and the accumulation of anions in the leaves. Further molecular analysis showed that LMSH influenced gene transcription in roots and long-distance effects in shoots as observed for *Mha2* and the *ZmNrt2.1* gene, respectively [182], which indicate HS can interfere with protein synthesis in microbes. FA and HA extracted from a podzol stimulated respiration in

, Li2+, Al3+, Ca2+, Cu2+, or Pb2+ which

**146**

Ca2+ [160].

**Figure 3.** *Various carbon-based NPs [208].*

▬COOH, and ▬NH2 improved the fungicidal activity (inhibition in spore elongation and germination) than the unmodified CNTs. It is hypothesized that modified CNTs formed a stable dispersions, which favoured interaction with spores, as a results enhanced antifungal activity. The authors observed a reduction in *F*. *graminearum* spore from 68.5, 54.5, 28.3, 27.4, and 29.5 μm, when 500 μg/mL MWCNTs (control), MWCNTs-COOH, MWCNTs-OH, and MWCNTs-NH2 were applied, respectively. Moreover, previous works [203, 204] documented that biological activity of nanotubes improved upon addition of functionalized aliphatic amide (covalent) and polyethylene glycol (PEG) and/or polyoxyethylene(40)nonylphenyl ether (IGPAL) (non-covalent) chemical groups [205]. Zare-Zardini et al. [206] conducted a covalent functionalization of MWCNTs with lysine and arginine under radiation. The modified MWCNTs exhibited potent biocidal activity against all test fungi (A*. niger*, *A. fumigatus, C. albicans, P. chrysogenum, S. cerevisiae, F. culmorum, Microsporum canis, Trichophyton mentagrophytes, Trichophyton rubrum, and P. lilacinum*) compared unmodified MWCNTs. Surprisingly, the fungicidal activity of MWCNTs-arginine against all the test fungi was slightly higher than MWCNTs-lysine. The authors hypothesized that the positive charge on arginine might have enhanced the binding of NPs on the fungal membrane and altered the genetic makeup (DNA). Thus, lysine and arginine could be utilized to improve the fungicidal activity of CTNs. Recently, Katerine et al. [207] reported the fungicidal activity of cotton fabric silica-silver carbon-based hybrid NPs against *A*. sp., *Cladosporium* sp. and *Chaetomium globosum*. The fabrics with high number carbon exerted the most increased biocidal activity on *C. globosum* and *Aspergillus* sp.
