The appeal of the approach lies in its chemistry of substitution. Standard routes to nickel oxide nanoparticles, including sol-gel processing, chemical precipitation, hydrothermal synthesis, combustion methods, and thermal decomposition, typically demand high temperatures, expensive instrumentation, and reagents that generate environmentally harmful by-products. Plant-mediated synthesis replaces all of that with phytochemicals. Leaves of Zanthoxylum rhetsa, a Rutaceae family tree distributed across South and Southeast Asia and long used in Ayurvedic medicine, are rich in flavonoids, phenolics, alkaloids, and essential oils. In the synthesis, these biomolecules perform double duty: they chemically reduce nickel ions from a nickel chloride precursor into nanoscale nickel oxide, and they cap the growing particles, preventing them from clumping and controlling their size and shape.

The practical procedure was strikingly simple. Fresh leaves collected from Satarda village in the Sawantwadi region of Sindhudurg were shade-dried, ground, and boiled in distilled water for an hour to extract the phytochemicals. The filtered extract was then mixed one-to-one with a 0.1 molar nickel chloride solution and stirred magnetically for eight hours, during which a deep brown precipitate appeared, signaling nanoparticle formation. After centrifugation, repeated washing, drying at 110 degrees Celsius, and calcination at 550 degrees Celsius, the team obtained phase-pure brown ZR-NiONPs ready for characterization and biological testing.

X-ray diffraction confirmed that the material was genuine face-centered cubic nickel oxide, with diffraction peaks matching the standard reference pattern and indexing to the (111), (200), (220), (311), and (222) crystal planes. Applying the Scherrer equation to the peak widths gave an average crystallite size of just 12.88 nanometers, and the sharpness of the peaks indicated high crystallinity with minimal lattice strain. Ultraviolet-visible spectroscopy added a second confirmation: while the raw leaf extract absorbed at 289 nanometers, a signature of the aromatic polyphenols and flavonoids within it, the finished nanoparticles showed a distinct new absorption band at 476 nanometers, reflecting defect-induced electronic transitions characteristic of nickel oxide nanoscale particles.

Infrared spectroscopy told the mechanistic story in detail. The extract spectrum displayed a rich catalog of functional groups, including broad hydroxyl stretching from phenols, aliphatic carbon-hydrogen stretches from terpenoids, carbonyl and amide bands from proteins and phenolic compounds, and aromatic ring vibrations typical of flavonoids. After synthesis, several of these bands shifted or weakened, evidence that those biomolecules had been consumed in reducing and capping the particles, and a set of entirely new peaks between roughly 550 and 430 inverse centimeters appeared, corresponding to nickel-oxygen stretching vibrations. Electron microscopy completed the physical portrait: field emission scanning electron microscopy revealed spherical to irregular, agglomerated, rough-surfaced particles, while transmission electron microscopy resolved individual particles ranging from 10 to 42 nanometers, with a selected area electron diffraction pattern of bright concentric rings confirming their polycrystalline nature.

Two further measurements rounded out the physicochemical profile. Zeta potential analysis gave a value of minus 10.2 millivolts, indicating a moderately negative surface charge and only moderate colloidal stability, a common feature of plant-capped metal oxide nanoparticles whose stability derives from both electrostatic repulsion and steric shielding by adsorbed biomolecules. More dramatically, vibrating sample magnetometry revealed a saturation magnetization of approximately plus or minus 55 emu per gram with near-zero coercivity and remanence, a soft ferromagnetic signature. That magnetic character matters beyond the laboratory bench: soft ferromagnetic nanoparticles are candidates for spintronic devices, magnetic data storage, heterogeneous catalysis, and, in a biomedical context, magnetically guided delivery.

The biological results were the study’s most eye-catching element. In agar well diffusion tests, the nanoparticles produced inhibition zones of 18 to 23 millimeters against four clinically relevant bacterial strains, with the strongest effect against Staphylococcus aureus, followed by Bacillus cereus, Escherichia coli, and Proteus vulgaris. The authors attribute this activity primarily to reactive oxygen species. Nickel oxide nanoparticles generate superoxide radicals, hydroxyl radicals, and hydrogen peroxide at the cell surface, and these species attack membrane lipids, proteins, and DNA, ultimately rupturing the bacterial cell. Released nickel ions may add a second mechanism by binding thiol groups in cellular enzymes. Notably, the particles performed better against Gram-positive bacteria, whose thick peptidoglycan wall lacks the outer lipopolysaccharide membrane that shields Gram-negative species from nanoparticle penetration.

The antifungal and metabolic assays painted a picture of moderate but genuine multifunctionality. The nanoparticles inhibited Candida albicans with an 11 millimeter zone and Aspergillus niger with 9 millimeters, below the performance of standard drugs but consistent with other plant-derived nickel oxide systems. In a DPPH free radical scavenging assay they achieved an IC50 of 92.56 micrograms per milliliter, roughly double the potency of ascorbic acid in the same test, and in a heat-induced protein denaturation model of inflammation they reached 48.83 percent inhibition at 100 micrograms per milliliter. They also inhibited the alpha-amylase enzyme central to starch digestion with an IC50 of 97.71 micrograms per milliliter, suggesting potential as a complementary agent in managing postprandial blood glucose, and they suppressed Mycobacterium tuberculosis growth by 46.24 percent at the highest tested concentration of 1000 micrograms per milliliter in an Alamar Blue viability assay.

Perhaps the most clinically significant finding came from the cell culture work. Against MCF-7 human breast cancer cells, the nanoparticles showed dose-dependent cytotoxicity with an IC50 of 88.52 micrograms per milliliter, an effect the authors link to the same reactive oxygen species machinery that kills bacteria: excessive oxidative stress in cancer cells damages mitochondria, fragments DNA, and triggers apoptosis, with surface-bound Zanthoxylum phytochemicals potentially enhancing uptake across cancer cell membranes. Crucially, when the same particles were tested against L929 mouse fibroblast cells, a standard model for normal tissue biocompatibility, inhibition never exceeded 29.40 percent even at 1000 micrograms per milliliter, and no IC50 could be determined. The phytochemical capping layer, the researchers suggest, moderates surface reactivity and limits oxidative damage in healthy cells, which possess their own antioxidant defenses, while cancer cells, already living near their redox limits, succumb.

The authors are candid about the caveats. Plant-mediated synthesis remains difficult to standardize, since phytochemical composition varies with plant origin, season, and extraction method, and scaling the process to industrial levels is unproven. No in vivo toxicology or long-term environmental assessment has yet been performed, and the anticancer and anti-tubercular effects, while real, trail standard drugs by a considerable margin. Even so, the study stands out for integrating a full physicochemical characterization with a systematic panel of biomedical assays in a single investigation, something the authors note is rare in the field. If subsequent animal studies confirm the safety margin seen in fibroblasts, these leaf-forged ferromagnetic particles could find roles in antimicrobial coatings, drug delivery platforms, biosensors, and wound healing formulations, all built from a tree that traditional medicine has valued for centuries.

Subject of Research: Green synthesis of nickel oxide nanoparticles using Zanthoxylum rhetsa leaf extract and their multifunctional biomedical activities

Article Title: Green synthesis of nickel oxide nanoparticles using Zanthoxylum rhetsa (Roxb.) DC. leaf extract, comprehensive physicochemical characterisation and multifunctional biomedical applications

Article References: Kale, S. S., Nikum, A. P., Pawar, Y. A., Gurav, V. L., & Sathe, G. B. (2026). Green synthesis of nickel oxide nanoparticles using Zanthoxylum rhetsa (Roxb.) DC. leaf extract, comprehensive physicochemical characterisation and multifunctional biomedical applications. Discover Green Chemistry, 1(1), Article 15. https://doi.org/10.1007/s44509-026-00018-y

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00018-y

Keywords: green synthesis, nickel oxide nanoparticles, Zanthoxylum rhetsa, phytochemicals, antibacterial activity, anticancer activity, ferromagnetism, nanomedicine, reactive oxygen species, MCF-7 breast cancer cells, biocompatibility, Ayurvedic medicinal plants

The synthesis itself is deliberately simple and environmentally benign. Powdered green tea was extracted in hot distilled water, and the resulting polyphenol-rich filtrate was mixed with an aqueous nickel nitrate solution at 60 degrees Celsius. Raising the pH with dilute sodium hydroxide produced a pale-green nickel hydroxide precipitate, which was aged, washed, dried, and then calcined in air at 500 degrees Celsius to yield what the researchers call NiO-500. X-ray diffraction confirmed a single face-centered cubic nickel oxide phase, with reflections matching the (111), (200), (220), (311), and (222) planes and no extraneous peaks. The Debye–Scherrer analysis gave coherent crystallite sizes in the range of 22 to 31 nanometers, while field-emission scanning electron microscopy revealed a striking morphology: sharply faceted octahedra roughly 0.42 to 0.44 micrometers across, bounded mainly by (111) facets, each apparently an aggregate of many coherently scattering nanocrystallites.

Structural evidence shows that the nanoparticles and the polymer do more than coexist. In the diffraction patterns of the composite films, the broad chitosan halo centered near 20 degrees progressively flattens and weakens as nickel oxide content rises, signaling disruption of the polymer’s semicrystalline chain packing by interfacial interactions. Meanwhile the nickel oxide reflections reappear and intensify at their original positions, confirming that the oxide remains phase-pure inside the films with no crystalline nickel–organic adducts. A Williamson–Hall analysis separated size and strain contributions and revealed a non-monotonic strain evolution: modest tensile microstrain at low and high loadings, but an apparent compressive strain of roughly minus 5.6 times ten to the minus three at 8 weight percent, the loading at which the apparent crystallite size shrank to about 8 to 12 nanometers. The authors interpret this as a regime of maximum polymer-mediated confinement and interfacial stress, with partial strain relaxation once particles begin to touch at 12 weight percent.

Microscopy and spectroscopy corroborate this picture. Electron images of the pristine chitosan film show a smooth, featureless surface, while the composites display progressively denser bright inclusions: isolated, well-separated particles at 4 weight percent, closer spacing with small clusters at 8 weight percent, and occasional coalesced nodes and short chains at 12 weight percent, marking the onset of a percolating inorganic micro-network. Energy-dispersive X-ray spectroscopy detected nickel’s characteristic K and L emission lines in the doped films, and Fourier-transform infrared spectroscopy traced the chemistry of the interface. The O–H and N–H stretching band of chitosan red-shifted and broadened from 3251 to 3150 reciprocal centimeters with increasing nickel oxide content, and bands associated with the amine, hydroxyl, and C–O groups shifted in ways consistent with coordination to nickel–oxygen surface sites. Low-frequency bands near 424 to 447 reciprocal centimeters confirmed the nickel–oxide framework itself.

The optical consequences are dramatic. Pure chitosan absorbs weakly, with a strong ultraviolet peak at 208 nanometers, but adding nickel oxide reshapes the entire absorption spectrum. At 12 weight percent, a large, broad absorption feature centered near 310 nanometers emerges, and visible-range transmittance falls from about 96 percent for the neat film to 49 percent, while reflectivity climbs to 19 percent. Crucially, near-infrared transparency survives: even the most heavily loaded film still transmits roughly 65 percent at 1100 nanometers. The absorption edge, extrapolated from the linear region of the absorption coefficient versus photon energy, drops steadily from 4.97 electronvolts for pure chitosan to 2.13 electronvolts at the highest loading. The authors are careful to note that this apparent edge reflects defect-mediated, disorder-dominated absorption rather than a true narrowing of the intrinsic bandgap, but the practical effect is the same: the films absorb visible light far more efficiently, a property valuable for photodetectors, solar energy harvesting, and ultraviolet shielding.

Urbach energy analysis quantifies the growing disorder. The Urbach energy, a measure of band-tail states within the forbidden gap, rises from 0.515 electronvolts for pure chitosan to 1.463 electronvolts at 12 weight percent, while the steepness parameter falls from 0.0501 to 0.0177 and the electron–phonon interaction energy climbs from 13.29 to 37.74 electronvolts. Tauc analysis across multiple transition exponents shows that direct-allowed bandgap values decline from 5.3 to about 3.0 electronvolts, whereas indirect and forbidden-transition values fall much further, down to 1.16 electronvolts, consistent with localized tail states dominating the absorption onset. Compared with literature values for chitosan doped with copper, titanium dioxide, or zinc oxide, the nickel oxide system achieves substantially stronger bandgap narrowing, which the authors attribute to stronger interfacial bonding and the higher density of mid-gap defect states it creates.

The dielectric and dispersion results are equally striking. The refractive index reaches about 2.65 at 400 nanometers for the 12 weight percent film, and the static refractive index rises from 1.10 to 2.54. Fitting the dispersion data with the Wemple–DiDomenico single-oscillator model shows the dispersion energy increasing from 1.42 to 15.23 electronvolts while the oscillator energy falls from 6.31 to 2.79 electronvolts, and the oscillator strength jumps from 9.00 to 42.55. Both real and imaginary parts of the optical dielectric constant rise markedly at low photon energies, exceeding values reported for comparable polymer–metal oxide systems such as polystyrene–manganese chloride or iron oxide and titanium dioxide doped polyvinyl alcohol–chitosan blends. The lattice dielectric constant climbs from 2.0 to 6.36, and the free carrier density ratio N/m* increases by roughly a factor of three, with the plasma frequency rising from 2.59 times ten to the fourteenth to 8.38 times ten to the fourteenth hertz.

Nonlinear optical properties follow the same upward trajectory. The third-order nonlinear susceptibility grows from 1.78 times ten to the minus seventeenth to 6.04 times ten to the minus twelfth electrostatic units, the linear susceptibility rises from 0.0179 to 0.434, and the nonlinear refractive index increases from 3.05 times ten to the minus twelfth to 6.96 times ten to the minus tenth, values that outperform previously reported zinc oxide and lead-complex doped chitosan systems. Optical transport parameters improve in parallel: carrier relaxation time, optical mobility, and optical resistivity all shift by orders of magnitude in the direction of enhanced conductivity, which the authors link to nickel-oxide-induced charge delocalization and increased free carrier concentration.

Taken together, the results sketch a scalable, eco-friendly route to flexible photonic films whose properties can be tuned simply by adjusting filler loading. At low loadings the films remain transparent and well dispersed; at intermediate loading, interfacial constraint maximizes defect-mediated optical activity; and at 12 weight percent, incipient percolation boosts dielectric response and nonlinear behavior while the polymer matrix stays continuous and bendable. The researchers point to applications ranging from ultraviolet-shielding coatings and smart windows to photodetectors, optical limiters, and solar energy harvesting layers, all built from a biodegradable polymer and nanoparticles grown with nothing more exotic than tea, water, and mild heat. As demand grows for non-toxic, energy-efficient materials, the study suggests that kitchen-scale green chemistry and precise nanoscale engineering can meet in the same film.

Subject of Research: Green synthesis of NiO-doped chitosan nanocomposite films with tunable optical and dielectric properties for sustainable energy and optoelectronic applications

Article Title: Green-synthesized NiO-doped chitosan nanocomposite films with tunable optical and dielectric properties for sustainable energy and optoelectronic applications

Article References: Aziz, D. M., Mamand, D. M., Hassan, S. A., & Aziz, S. B. (2026). Green-synthesized NiO-doped chitosan nanocomposite films with tunable optical and dielectric properties for sustainable energy and optoelectronic applications. Journal of Materials Science: Polymers, 1(1), Article 14. https://doi.org/10.1007/s44493-026-00014-5

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00014-5

Keywords: green synthesis, nickel oxide nanoparticles, chitosan, nanocomposite films, optical bandgap, dielectric properties, Urbach energy, refractive index, nonlinear optics, UV shielding, optoelectronics, sustainable materials