Plant-made magnesium oxide nanoparticles degrade dyes and curb bacteria

Researchers in India have synthesized magnesium oxide nanoparticles using a whole-plant aqueous extract of Ecbolium ligustrinum, a medicinal herb of the Acanthaceae family, and report that the resulting material can break down two common cationic dyes and inhibit the growth of two bacterial strains in laboratory tests. The work, published in Results in Chemistry, combines materials characterization with photocatalytic and antibacterial assays to evaluate a single eco-friendly nanomaterial for two potential applications: dye removal from contaminated water and antimicrobial action.

The motivation stems from a well-documented environmental problem. Textile, paper, paint, cosmetics, and food industries discharge organic dyes into waterways, and these non-biodegradable chemicals, along with hazardous byproducts formed by hydrolysis and oxidation during sewage treatment, threaten aquatic ecosystems, biodiversity, and public health. Among candidate remediation materials, magnesium oxide nanoparticles have attracted attention because of their high surface reactivity, wide bandgap, and chemical and thermal stability, with reported uses ranging from sensors and antimicrobial agents to wastewater treatment, catalysis, adsorbents, and fuel additives.

Green synthesis routes, which use organisms or plant extracts to produce nanomaterials, have become a focus of research because they avoid many of the chemical additives of conventional methods. In this study, the team collected E. ligustrinum plants from Thanthonimalai in the Karur district of Tamil Nadu, India, and identified the species with the help of a taxonomist. They extracted 90 grams of cleaned whole plants sequentially with solvents from non-polar to polar, including n-hexane, chloroform, ethyl acetate, acetonitrile, and triple-distilled water, then screened each concentrated residue for phytochemicals using standard colorimetric methods. Only the aqueous fraction, which tested positive for tannins, coumarins, saponins, and flavonoids, was carried forward for nanoparticle synthesis and biological testing.

The synthesis itself was straightforward. Ten milliliters of the aqueous plant extract were added to 100 milliliters of a 1 millimolar magnesium nitrate solution under constant stirring, turning the mixture yellow as phytochemicals interacted with magnesium ions. A 1 molar sodium hydroxide solution was then introduced dropwise to hold the pH at 10, and the mixture was stirred for two hours until the color deepened to dark yellow, which the authors interpret as the formation of a magnesium hydroxide precursor. After the precipitate was allowed to settle for a day, it was collected by centrifugation at 5000 rpm, washed four times with distilled water, dried, and calcined at 400 degrees Celsius for three hours. The procedure yielded approximately 35.4 milligrams of MgO powder, an estimated 87.84 percent yield based on the theoretical value.

Characterization confirmed the expected structure and composition. Ultraviolet-visible spectroscopy showed a characteristic absorption peak at 297 nanometers, and a Tauc plot analysis yielded an optical band gap of 5.1 electronvolts, a value the authors attribute to nanoscale effects and surface or defect states. Fourier-transform infrared spectroscopy revealed bands assigned to hydroxyl, alkene, amine, carboxyl, and phenolic groups, consistent with plant-derived phytochemicals capping the particle surfaces, along with a signal at 589 per centimeter indicating effective MgO formation. Powder X-ray diffraction produced peaks at 2-theta values of 37.02, 43.01, 62.32, 74.57, and 78.61 degrees, matching the reference pattern for cubic MgO and corresponding to the (111), (200), (220), (311), and (222) planes. Applying the Debye-Scherrer equation to these peaks gave an average crystallite size of 12.9 nanometers.

Further measurements filled in the texture and morphology. Nitrogen adsorption-desorption analysis gave a BET specific surface area of 36.611 square meters per gram and a total pore volume of 0.1455 cubic centimeters per gram, with a pore-size distribution spanning microporous domains centered near 1.19 nanometers and mesoporous networks averaging 15.895 nanometers, which the authors describe as a hierarchical porous architecture. High-resolution transmission electron microscopy showed predominantly spherical particles with well-defined lattice fringes and an inter-planar spacing of 0.24 nanometers, matching the (111) plane of cubic MgO calculated from X-ray diffraction. Selected area electron diffraction rings with bright spots indicated polycrystalline material, and a particle size histogram fitted with a Gaussian curve gave an average particle size of 37 nanometers, larger than the X-ray crystallite estimate, a discrepancy the authors note against the earlier XRD result. Energy-dispersive X-ray spectroscopy detected magnesium and oxygen at weight percentages of 38.19 and 61.81, deviating from the ideal stoichiometric ratio in ways the authors attribute to measurement conditions and the difficulty of quantifying light elements; they caution that the absence of extra peaks alone does not establish absolute purity.

The photocatalytic tests used methylene blue and basic fuchsin, both cationic dyes, at 25 milligrams per liter in 25-milliliter reaction volumes with a nominal catalyst loading of 96.15 milligrams per liter. Under complete darkness, methylene blue degraded fully in 105 minutes, while direct natural sunlight, delivered between 11 a.m. and 1 p.m., shortened the time to 50 minutes. The degradation followed pseudo-first-order kinetics, with the rate constant rising from 0.00621 per minute in the dark to 0.03691 per minute in sunlight. Basic fuchsin responded even faster: complete degradation took 40 minutes in the dark but only 16 minutes in sunlight, with the rate constant climbing from 0.05839 to 0.15725 per minute. In efficiency terms, methylene blue removal rose from 50.64 percent in the dark to 84.71 percent under sunlight, while basic fuchsin reached 90.37 percent in the dark and 93.47 percent in sunlight. The authors attribute the light-driven acceleration to photo-induced generation of reactive oxygen species and note that basic fuchsin molecules may bind more strongly to the catalyst surface and be oxidized more quickly. Notably, solar irradiance, reaction temperature, incident light spectrum, and catalyst-to-light distance were not measured or recorded, and no scavenger studies were performed, so the individual contributions of hydroxyl radicals, superoxide, and holes remain experimentally unconfirmed.

Antibacterial activity was assessed by the agar well diffusion method against Gram-positive Bacillus subtilis and Gram-negative Escherichia coli at doses of 5, 10, 25, 50, and 100 microliters, with ampicillin and erythromycin as standards. Inhibition zones showed a broadly concentration-dependent pattern, though not monotonically: against B. subtilis the zones measured 14, 11, 12, 12, and 15 millimeters across the dose series, while against E. coli they measured 13, 13, 11, 12, and 19 millimeters. At the highest dose, E. coli showed a larger zone than B. subtilis, suggesting somewhat greater effectiveness against the Gram-negative organism under these conditions. The plant extract itself, tested at 25 microliters, produced zones of 14 and 15 millimeters against the two species. The authors propose that the nanoparticles’ antibacterial effect may arise from reactive oxygen species generation, possible release of magnesium ions disturbing cellular homeostasis, and direct nanoparticle contact damaging the cell membrane, while emphasizing that the relative contribution of each mechanism was not experimentally distinguished and requires further study.

The authors position the work against a backdrop of prior green-synthesized nanomaterials, including chrysanthemum-mediated ZnFe2O4@ZnO composites, Ag2O/Fe2O3/TiO2 ternary heterojunctions, and ZnO:NiO nanocomposites reported for dye degradation and antibacterial activity, as well as earlier MgO syntheses using Hyphaene thebaica and Cymbopogon citratus extracts. They argue that most photocatalytic studies have focused on single-dye systems, chiefly methylene blue, and that the degradation kinetics of complex cationic dyes like basic fuchsin via surface electrostatic attraction and radical attack remain underexplored for plant-assisted MgO nanostructures. Within that framing, the present study compares two dyes under identical dark and natural sunlight conditions using the same catalyst.

The findings should be read with appropriate scope in mind. All photocatalytic and antibacterial results come from small-volume laboratory assays under uncontrolled natural sunlight, without recorded irradiance or temperature, and the mechanistic proposals for both dye degradation and bacterial inactivation remain hypothetical pending scavenger and mechanistic experiments. The EDS composition deviates from stoichiometry, and the particle size estimates from XRD and TEM differ. Nevertheless, the study documents a complete, plant-mediated route to crystalline MgO nanoparticles and reports measurable dye degradation and bacterial inhibition under the tested conditions, offering a starting point for further evaluation of E. ligustrinum-derived nanomaterials in environmental and antimicrobial applications.

Subject of Research: Green synthesis of magnesium oxide nanoparticles for dye degradation and antibacterial testing

Article Title: Superior dual applications – dye photocatalytic efficiency and antibacterial performance of eco-friendly synthesized magnesium oxide nanoparticles

Article References: Sivakami, S., Jothika, M., Sundaram, R., Paul, J. J., Thangapushbam, V., Rama, P., Monikadevi, P., Muthu, K., Priyadarshini, M., Arumugam, N., Jayakumar, J., Wadaan, M. A., & Devanesan, S. (2026). Superior dual applications – dye photocatalytic efficiency and antibacterial performance of eco-friendly synthesized magnesium oxide nanoparticles. Results in Chemistry, 32, Article 104018. https://doi.org/10.1016/j.rechem.2026.104018

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.104018

Keywords: magnesium oxide nanoparticles, green synthesis, Ecbolium ligustrinum, photocatalysis, methylene blue, basic fuchsin, antibacterial, wastewater treatment, reactive oxygen species, nanotechnology, XRD, sunlight degradation