Green-Synthesized Zinc Oxide Nanoparticles Show Broad Antimicrobial Potential

Background: The Growing Need for Alternative Antimicrobial Strategies

Nanoparticles (NPs) — materials with sizes between 10 and 100 nanometers — have attracted attention across science and technology for their unique properties. In medicine, they are being explored for drug delivery, antimicrobial and antioxidant applications, and diagnostics. Traditional chemical and physical methods for NP production are often costly, time-consuming, and environmentally hazardous.

Green synthesis offers an alternative: it is faster, less expensive, and eco-friendly, often producing particles with more uniform size and morphology. This approach uses plant extracts or microorganisms as natural reducing, stabilizing, and capping agents in the formation of metal or metal oxide NPs.

Zinc oxide nanoparticles (ZnONPs) have been synthesized from many plants, but until now, no study had examined ZnONPs produced from four desert plants with known medicinal properties:

• Thymelaea hirsuta – used for anti-inflammatory, cathartic, and other traditional remedies, with evidence of activity against hepatocellular carcinoma.

• Aloe vera – known for wound healing, burn treatment, antioxidant, and anti-inflammatory benefits.

• Retama monosperma – used for various health purposes and a source of natural fibers.

• Peganum harmala – containing β-carboline alkaloids with antibacterial, anticancer, and antioxidant effects.

These plants can thrive in arid, harsh environments and, in some cases, are considered invasive — making them under-utilized yet sustainable resources.

Study Objectives

The research team aimed to:

1. Biosynthesize ZnONPs from the four selected plant extracts.

2. Characterize the physical and chemical properties of these nanoparticles.

3. Assess antimicrobial activity against bacterial, yeast, and fungal strains.

4. Evaluate phytochemical content, pharmacokinetics, and molecular interactions with microbial target proteins.

How the Nanoparticles Were Made

The aerial parts of the plants were collected in Tunisia, dried, and ground into powder. Extracts were prepared in water and mixed with zinc acetate dihydrate, then heated to produce ZnONPs.

The nanoparticles were designated:

• Thymhirs.bio-ZnONP (T. hirsuta)

• Aloever.bio-ZnONP (A. vera)

• Retam.bio-ZnONP (R. monosperma)

• Harm.bio-ZnONP (P. harmala)

Characterizing the Particles

The researchers used several techniques to analyze the ZnONPs:

• UV–Vis spectroscopy showed characteristic absorption peaks at 340–360 nm, confirming ZnO formation.

• Granulometry revealed particle sizes ranging from about 255 nm (P. harmala) to nearly 980 nm (R. monosperma).

• FTIR analysis detected functional groups (O–H, C–H, C=C) from plant compounds on the particle surfaces, which act as stabilizers.

• Electron paramagnetic resonance (EPR) found paramagnetic centers in A. vera and R. monosperma ZnONPs.

Phytochemical Profiles

Using HPLC-MS, the team identified abundant phenolic acids and flavonoids, such as quinic acid, gallic acid, rutin, caffeic acid, apigenin-7-O-glucoside, and cirsiliol.

Different plants showed distinct profiles:

• P. harmala had the highest quinic acid and naringenin levels.

• T. hirsuta was rich in quercetin.

• A. vera had the highest overall phenolic content, including chlorogenic acid and catechin.
  These compounds likely contributed to nanoparticle formation and stability.

Testing Antimicrobial Activity

The nanoparticles were tested against:

• Bacteria: Staphylococcus aureus, Micrococcus luteus, Salmonella enterica Typhimurium, Escherichia coli

• Yeasts: Candida albicans, C. krusei, C. neoformans

• Fungi: Aspergillus flavus, A. niger, A. fumigatus

Bacterial Results

• T. hirsuta ZnONPs strongly inhibited S. aureus (15 mm zone).

• A. vera ZnONPs showed the highest effect overall, with 24 mm inhibition against M. luteus.

• R. monosperma and P. harmala ZnONPs also inhibited S. aureus and M. luteus, though less strongly.

Yeast Results

• A. vera ZnONPs inhibited all three Candida species tested.

• P. harmala ZnONPs were notably active against C. neoformans (16 mm).

• T. hirsuta ZnONPs inhibited only C. albicans.

Fungal Results

• P. harmala ZnONPs had strong activity against all Aspergillus species, especially A. fumigatus (16 mm).

• A. vera ZnONPs were active against all Aspergillus strains tested.

• T. hirsuta and R. monosperma ZnONPs also showed notable antifungal effects.

Importantly, the plant extracts and zinc acetate alone displayed weak or no antimicrobial activity in most cases, confirming the enhanced effect of ZnONPs.

Computational Modeling Insights

Phytochemicals from the plant extracts were docked with:

• TyrRS from S. aureus

• Secreted aspartic proteinase from C. albicans

• Wheat germ agglutinin

Key results:

• Compound 12 bound most strongly to TyrRS (–10.5 kcal/mol).

• Compound 8 showed the highest affinity for the fungal enzyme (–10.0 kcal/mol).

• Compound 16 had the best binding to wheat germ agglutinin (–8.4 kcal/mol).

Many compounds formed up to nine hydrogen bonds with target proteins, suggesting stable interactions that could explain antimicrobial effects.

Pharmacokinetic Assessment

Most identified compounds met Lipinski’s Rule of Five, suggesting good drug-likeness. Bioavailability scores ranged from 0.11 to 0.85. Many were predicted to have good gastrointestinal absorption and were not substrates for P-glycoprotein, which can limit drug effectiveness.

Synthetic accessibility scores indicated these compounds could be feasibly produced if needed for further drug development.

Novel Findings

• First report of ZnONP biosynthesis from these four desert plant species.

• Demonstrated broad-spectrum antimicrobial activity, often surpassing plant extracts or zinc salts.

• Linked phytochemical composition to nanoparticle stability and antimicrobial potency.

• Combined in vitro and in silico methods to support observed biological effects.

Practical Implications

The study suggests plant-based ZnONPs could:

• Serve as alternative antimicrobial agents, particularly against drug-resistant pathogens.

• Be developed into pharmaceutical formulations or functional food additives with antimicrobial properties.

• Offer a sustainable route for nanoparticle production using under-utilized plant resources from arid regions.

However, the authors note that further work is needed to optimize particle size, improve long-term stability, and assess cytotoxicity before clinical or industrial applications.

Conclusion

This research demonstrates that ZnONPs synthesized from T. hirsuta, A. vera, R. monosperma, and P. harmala possess significant antimicrobial potential. The findings provide a foundation for future exploration of green-synthesized nanomaterials in medicine and industry.

The translation of the preceding English text in Arabic:

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