Zinc Oxide Meets Carbon Nitride in the Push for Safer Food

Foodborne hazards remain one of the defining public health challenges of the century. The most recent World Health Organization synthesis, compiled by the Foodborne Disease Burden Epidemiology Reference Group, estimated that in 2021 the 42 major foodborne infectious and chemical hazards under surveillance caused roughly 866 million illnesses, 1.52 million deaths and 57.1 million disability-adjusted life years worldwide, with the heaviest burden falling on children and on populations in low- and middle-income countries. Diarrhoeal pathogens, mycotoxin-contaminated staples and chemical residues account for a large share of that toll, and the picture is darkened by the accelerating spread of antimicrobial-resistant foodborne bacteria, which erodes the value of conventional sanitizers and preservatives and has been projected to cause millions of additional deaths annually by mid-century if left unaddressed.

Against that backdrop, a review published in Food Science & Nutrition takes stock of an unusual candidate technology: zinc oxide nanoparticles and their composites with graphitic carbon nitride, a metal-free polymeric semiconductor. Zinc oxide occupies a prominent position among metal oxide nanomaterials because zinc is classified as Generally Recognized As Safe by regulatory agencies for specific bulk applications, the compound is comparatively inexpensive to produce, and its wide direct band gap of about 3.37 electron volts confers both photocatalytic and antimicrobial functionality. Engineered nanomaterials of this kind kill microbes through mechanisms mechanistically distinct from conventional antibiotics and biocides, which may reduce the risk of cross-resistance while enabling multifunctional roles in active packaging, contaminant degradation and rapid pathogen sensing.

Pristine ZnO, however, carries real handicaps. Rapid electron–hole recombination limits its photocatalytic quantum efficiency, and its wide band gap restricts photoactivation almost entirely to ultraviolet light, which makes up only about 4 to 5 percent of ambient solar radiation and is largely absent from indoor food storage environments. There are also concerns about zinc ion leaching and associated toxicity at elevated concentrations, and about structural instability and photocorrosion under prolonged irradiation or in the acidic, halide-rich conditions found in some food matrices. These weaknesses have pushed researchers toward heterojunction architectures that extend functionality into the visible spectrum while suppressing charge recombination.

The most intensively studied pairing couples ZnO with graphitic carbon nitride, a two-dimensional polymer built from tri-s-triazine units that is cheap, thermally stable and responsive to visible light. Recent studies describe the composite as forming a step-scheme, or S-scheme, heterojunction: when the two semiconductors touch, electrons flow from the higher Fermi level to the lower one until equilibrium is reached, creating an interfacial electric field. Under illumination, the weaker photogenerated electrons in ZnO’s conduction band recombine selectively with the weaker holes in g-C3N4’s valence band, leaving strongly reducing electrons in g-C3N4 and strongly oxidizing holes in ZnO free to drive redox chemistry. The review is careful to stress that this designation should not be applied universally; the actual charge-transfer mechanism is material- and interface-dependent, and performance tests alone cannot definitively distinguish an S-scheme from a conventional type-II mechanism. Supporting evidence requires tools such as in situ X-ray photoelectron spectroscopy under irradiation, Kelvin probe measurements, Mott–Schottky analysis, electron paramagnetic resonance and density functional theory calculations. Where such data exist, they back the design: a solar-matched ZnO/g-C3N4 composite with an optimized 10 percent g-C3N4 loading more than doubled the reaction rate constant for degrading the pharmaceutical contaminant paracetamol under visible light compared with lower-loading composites, and combined theoretical and radical-scavenging experiments have identified superoxide and hydroxyl radicals as the dominant reactive species in accelerated dye degradation.

How the particles are made matters as much as how they work. Conventional routes—sol–gel processing, hydrothermal and solvothermal synthesis, mechanochemical processing, vapor synthesis, precipitation and, more recently, microfluidic and laser-ablation methods—offer tight control over particle size and morphology and dominate commercial production, but many rely on organic solvents, surfactants or elevated temperatures and pressures, raising cost, energy and residual-toxicity concerns for food-contact use. Green synthesis replaces hazardous reductants with biological metabolites from plant extracts, fungi, algae or bacteria. Reviews of that literature note a striking gap: despite substantial work on plant-mediated ZnO for antimicrobial and biomedical purposes, direct application of green-synthesized particles in food packaging or food-contact materials has been comparatively underexplored. Microbially mediated synthesis using actinobacteria, particularly Streptomyces species, has emerged as a scalable variant, because extracellular biosynthesis from cell-free supernatant simplifies downstream processing and eases scale-up.

The catalogue of biogenic successes is growing. ZnO nanoparticles from an estuarine Streptomyces isolate showed the strongest inhibition against Escherichia coli, with a 22-millimeter zone and particles of 51 to 65 nanometers. Particles from Streptomyces plicatus MK-104, about 22 nanometers across, displayed activity against plant-pathogenic fungi and a nematicidal effect at minimum inhibitory concentrations of 15.6 to 500 micrograms per milliliter. A Streptomyces baarnensis MH-133 formulation combined with an actinobacterially derived metabolite in nanoemulsion form showed enhanced activity against multidrug-resistant ESKAPE pathogens with favorable cytotoxicity profiles. Most recently, particles biosynthesized from the cell-free supernatant of an endophytic Streptomyces werraensis strain isolated from Passiflora caerulea produced predominantly spherical, wurtzite-phase nanoparticles of roughly 136 nanometers with concentration-dependent antibacterial activity against Salmonella paratyphi A and Proteus vulgaris comparable to ampicillin—19.0- and 16.4-millimeter inhibition zones respectively—alongside notable radical-scavenging activity. The review cautions, though, that residual biomolecules on biogenic particle surfaces raise unresolved questions about allergenicity, immunogenicity and long-term stability, and that biological variability can produce batch-to-batch differences in size distribution that tightly controlled chemical routes avoid.

The antimicrobial action itself rests on three interacting mechanisms: reactive oxygen species that oxidatively damage membrane lipids, proteins and nucleic acids; direct physical contact that disrupts the cell envelope; and the dissolution and release of zinc ions, which interfere with membrane transport, respiratory enzymes and metabolism. The review draws a distinction that matters for real storage rooms: reactive oxygen generation requires light, whereas zinc release and contact-mediated disruption operate in the dark. Because the mechanism does not depend on a single molecular target, these materials are considered less susceptible to the single-point-mutation resistance that plagues conventional antibiotics, though direct evidence from foodborne microorganisms is limited and the potential for cross-adaptation at sub-lethal doses warrants monitoring. Susceptibility patterns are not uniform either; Gram-negative organisms often show greater sensitivity, attributed to their thinner peptidoglycan and lipopolysaccharide-rich outer membrane, but some Gram-positive pathogens such as Listeria monocytogenes resist substantially, and food components like cysteine, methionine and polyphenols can quench the very radicals the particles generate.

On the application side, the best-developed use is antimicrobial packaging. Chitosan/gelatin hybrid films reinforced with ZnO nanoparticles derived from Cassia fistula fruit extract showed improved thermal stability, elongation-at-break and activity against E. coli and Staphylococcus aureus, and nanocomposite films combining babassu coconut mesocarp with green-synthesized ZnO reduced S. aureus contamination on cooked turkey ham in direct food-contact trials—one of the few demonstrations in a real food matrix. Photocatalytic decontamination is the second stream: flexible electrospun membranes of polycaprolactone, g-C3N4 and carbon quantum dots degraded more than 95 percent of aflatoxin B1 in solution within 30 minutes of visible-light exposure, ZnO/graphene hybrids achieved near-complete degradation of deoxynivalenol under ultraviolet light, and g-C3N4 degraded zearalenone to 96 percent in reference solution but only 50 percent in a real food-powder matrix—a recurring discrepancy the review treats as a central lesson. The third stream is sensing: silica-coated ZnO nanorod immunoassays have detected E. coli down to 50 colony-forming units per milliliter, and ZnO-nanograss platforms decorated with gold nanospeckles have enabled DNA-hybridization detection, though the demonstrated target, Staphylococcus epidermidis, is not a primary foodborne pathogen.

The safety and regulatory picture is where enthusiasm meets restraint. Bulk zinc oxide’s GRAS status does not automatically extend to nanoscale ZnO, whose altered physicochemical properties and increased cellular uptake demand separate assessment. In the European Union, food-contact nanomaterials fall under Regulation (EC) No. 1935/2004 and the plastics-specific Regulation (EU) No. 10/2011, both requiring case-by-case risk assessment, with EFSA’s 2021 guidance on nanomaterials in the food and feed chain anchoring migration-testing methodology and a 2024 review emphasizing genotoxicity assessment. A 2024 study commissioned by the European Union Observatory for Nanomaterials reported evidence of what it characterized as systematic nanoparticle migration from food-contact materials—silver being the most studied case—while noting that no general toxicity conclusions could yet be drawn. In the United States, the FDA requires safety data through the Food Contact Notification process. For g-C3N4 specifically, toxicological data remain markedly sparser than for ZnO, concentrated in wastewater contexts rather than food-contact exposure. The review’s conclusion is blunt: efficacy in model systems is well demonstrated, but translation to real food and food-contact applications remains limited, and these technologies are not ready for standardized food-chain adoption without sufficient real-food, toxicological, migration and regulatory validation.

Subject of Research: ZnO and g-C3N4/ZnO nanocomposites for food safety and preservation

Article Title: Zinc Oxide and Graphitic Carbon Nitride/ZnO Nanocomposites in Food Safety and Preservation: Synthesis, Antimicrobial Mechanisms, and Application Prospects

Article References: Zakari, D. A., Khalil, I., Olateju, K. S., ugli, S. T. M., Rakhimberdievich, N. A., Abrorbek, S., Amoka, A. G., & Moyosore, A. A. (2026). Zinc Oxide and Graphitic Carbon Nitride/ ZnO Nanocomposites in Food Safety and Preservation: Synthesis, Antimicrobial Mechanisms, and Application Prospects. Food Science & Nutrition, 14(11), Article e72509. https://doi.org/10.1002/fsn3.72509

Image Credits: AI Generated

DOI: 10.1002/fsn3.72509

Keywords: zinc oxide nanoparticles, graphitic carbon nitride, S-scheme heterojunction, food safety, antimicrobial packaging, photocatalysis, mycotoxins, green synthesis, Streptomyces, nanobiosensors, EFSA, toxicology