Radish Leaves and Lemon Peels Yield a Dual-Action Nanomaterial for Water Treatment

Researchers in Iraq have developed a completely plant-based route for building a zinc oxide–selenium nanocomposite that can both destroy dye pollutants under ultraviolet light and suppress clinically relevant pathogens, and their results point to a surprisingly precise recipe: just 2 percent selenium by weight delivers the best of both worlds. The study, published in Frontiers in Ecology and Evolution, used radish leaf extract to biosynthesize zinc oxide nanoparticles and lemon peel extract to stabilize selenium nanoparticles, avoiding the toxic by-products and high costs associated with conventional chemical synthesis.

The choice of plant material was not arbitrary. Radish leaves (Raphanus sativus var. longipinnatus) are rich in glucosinolates and phenolic compounds with documented antioxidant and antibacterial effects, and these phytochemicals act as natural reducing and stabilizing agents during nanoparticle formation. Lemon peel (Citrus limon) contributes flavonoids, phenols, and essential oils that serve the same dual function for selenium, a semiconductor known for its antibacterial, anticancer, and antioxidant properties but notorious for colloidal instability when synthesized by conventional means. By using these extracts, the team produced stable selenium nanoparticles without any synthetic additives.

The synthesis process itself was carefully controlled. Radish leaves were dried at 60 degrees Celsius, ground into a fine powder, and extracted in deionized water at 80 degrees Celsius for 15 minutes before centrifugation to remove colloidal debris. Zinc acetate was then dissolved and combined with the radish leaf extract in ethanol, with sodium hydroxide adjusting the pH to 12 before heating at 70 degrees Celsius. The resulting white precipitate was washed, dried, and calcined at 450 degrees Celsius for two hours to crystallize the zinc oxide and eliminate residual organic material. Selenium nanoparticles were prepared separately by combining an 8 millimolar sodium selenite solution with lemon peel powder, incubating first at 70 degrees Celsius to trigger rapid nucleation and then at 37 degrees Celsius for 72 hours to allow controlled crystal growth, with the solution turning from light yellow to brick red as the nanoparticles formed.

X-ray diffraction confirmed that the zinc oxide adopted the hexagonal wurtzite structure, with characteristic peaks matching the standard reference pattern and no secondary phases detected. Using the Scherrer equation, the average crystallite size of pure zinc oxide was determined to be 14.15 nanometers. Selenium nanoparticles showed sharp peaks corresponding to the hexagonal elemental selenium structure, with an average crystallite size of 19.1 nanometers. When the two were combined into nanocomposites at 2, 6, and 10 weight percent selenium, the wurtzite peaks remained unchanged in position, indicating that selenium did not substitute into the zinc oxide lattice but instead formed a separate phase in intimate contact with the host matrix.

Perhaps the most striking structural finding was that the 2 weight percent selenium composite produced the smallest crystallites of any sample, at 12.51 nanometers, compared to 14.15 nanometers for pure zinc oxide and 15.22 nanometers at 10 weight percent selenium. The researchers attribute this to a Zener pinning effect, in which selenium domains dispersed at grain boundaries physically restrict crystal growth during thermal treatment. As selenium concentration increased beyond the optimum, lattice strain decreased systematically from 7.84 × 10⁻³ to 5.41 × 10⁻³, suggesting internal stress relaxation rather than major lattice distortion.

Ultraviolet-visible spectroscopy reinforced the two-phase picture. Pure zinc oxide showed a sharp absorption peak at approximately 370 nanometers, consistent with its wide bandgap of roughly 3.37 electronvolts. Selenium nanoparticles exhibited a primary peak at 275 nanometers, a significant blue shift from bulk selenium that confirms quantum confinement effects in particles smaller than the Bohr exciton radius. Tauc analysis of the composites revealed that the 2 weight percent sample displayed the highest absorption intensity at 370 nanometers, while higher selenium loadings showed progressively lower absorption, likely due to surface coverage and aggregation effects that reduce the effective optical path.

Field emission scanning electron microscopy revealed that pure zinc oxide forms dense aggregates of 4 to 6 primary crystals, with secondary particle diameters averaging 68.4 nanometers. Pure selenium nanoparticles adopted elongated one-dimensional structures with aspect ratios between 5.2 and 8.7, attributed to the pectin and flavonoid matrix of the lemon peel extract directing asymmetric growth. However, when incorporated into the zinc oxide matrix, these one-dimensional selenium structures were mechanically fragmented during magnetic stirring, resulting in more isotropic domains. Energy dispersive X-ray analysis and elemental mapping confirmed that selenium was uniformly distributed throughout the composite without agglomeration, even at the highest loading of 10 weight percent.

The photocatalytic performance was evaluated by degrading methyl orange, a common azo dye pollutant, under ultraviolet-A irradiation at 365 nanometers. Pure zinc oxide achieved 62.4 percent degradation in 120 minutes, but the 2 weight percent selenium composite reached 83.4 ± 0.7 percent under identical conditions, a statistically significant improvement validated across three independent biological replicates. The pseudo-first-order rate constant for the optimal composite was 0.0150 per minute with a half-life of 46.20 minutes, compared to 0.0081 per minute and 85.56 minutes for pristine zinc oxide. The enhancement is attributed to strengthened interfacial contact between the two phases, which promotes efficient charge separation and reduces electron-hole recombination, extending the lifetime of reactive oxygen species responsible for breaking down the dye molecules.

Antimicrobial testing against Staphylococcus aureus, Klebsiella pneumoniae, and Candida albicans revealed a nuanced picture. The 2 weight percent composite produced the largest inhibition zones, measuring 19.0 ± 0.5 millimeters against Staphylococcus aureus and 17.0 ± 0.4 millimeters against Klebsiella pneumoniae. All formulations showed strong activity against Candida albicans with minimum inhibitory concentrations at or below 32 micrograms per milliliter, likely driven by zinc ion release and reactive oxygen species generation that compromise the fungal cell wall. However, antibacterial efficacy against the bacterial strains declined as selenium concentration increased beyond 2 weight percent, a phenomenon the researchers link to a shadowing effect in which excess selenium covers active zinc oxide surface sites, combined with colloidal aggregation and competitive surface binding that reduce the effective surface area available for ion release and membrane disruption.

The authors describe the optimal 2 weight percent formulation as a

Subject of Research: Green synthesis of ZnO/Se nanocomposites: influence of selenium concentration on structural, optical properties with enhanced antimicrobial and photocatalytic performance

Article Title: Green synthesis of ZnO/Se nanocomposites: influence of selenium concentration on structural, optical properties with enhanced antimicrobial and photocatalytic performance

Article References: Al-Kilabi, R., Al-Hello, H., Mohammed, E. F., Al-Hello, M., Hassoon, O. A., Yaseen, M. A., & Al-Saidi, M. H. (2026). Green synthesis of ZnO/Se nanocomposites: influence of selenium concentration on structural, optical properties with enhanced antimicrobial and photocatalytic performance. Frontiers in Ecology and Evolution, 14, Article 1939540. https://doi.org/10.3389/fevo.2026.1939540

Image Credits: AI Generated

DOI: 10.3389/fevo.2026.1939540

Keywords: Green, synthesis, nanocomposites, influence, selenium, concentration, structural, optical, properties, enhanced, antimicrobial, photocatalytic