Green-Synthesized Zinc Oxide Nanoparticles with Enhanced Release Behavior for Sustainable Agricultural Applications

Abstract

This study presents a green and sustainable approach for synthesizing zinc oxide nanoparticles (ZnO-NPs) using Melia azedarach leaf extract as a reducing and stabilizing agent, with zinc acetate as the precursor. The synthesized nanoparticles were thoroughly characterized to assess their structural, morphological, and physicochemical properties, revealing nanoscale dimensions, enhanced crystallinity, and improved stability compared to commercial ZnO. Controlled release experiments under plant-relevant pH conditions demonstrated a gradual and sustained release of Zn²⁺ ions, accompanied by buffering effects and re-precipitation of Zn(OH)₂, highlighting their potential for long-term nutrient availability in soil systems. Unlike conventional studies that focus mainly on synthesis or characterization, this work emphasizes the functional performance of ZnO-NPs as nanofertilizers, combining eco-friendly production with practical agricultural applications. The plant-mediated synthesis yielded nanoparticles with uniform size distribution, enhanced dispersion, and stability, which are critical for efficient nutrient delivery and persistence in soil. Overall, this study provides a cost-effective, scalable, and environmentally benign strategy for producing ZnO nanoparticles and offers valuable insights into the development of sustainable nanofertilizers aimed at improving crop nutrition, soil fertility, and agricultural productivity.

1. Introduction

The overuse of chemical fertilizers has led to environmental pollution and economic challenges, prompting the exploration of nanotechnology-based solutions to improve nutrient use efficiency and crop yields [1,2]. There is a growing global demand for biocompatible, eco-friendly, and cost-effective materials to enhance nutrient availability and stimulate plant growth [3]. These materials should ideally be inert, less toxic, and more stable than widely used synthetic chemicals, ensuring efficient delivery to target sites [4].

The synthesis of nanoscale metal oxides such as ZnO typically requires external stabilizing agents [5]. However, Melia azedarach L. (commonly known as Chinaberry), a deciduous tree with a rich phytochemical profile, presents a sustainable alternative. Its extracts contain a variety of bioactive compounds—including terpenoids, flavonoids, polyphenols, saponins, tannins, amides, and carboxylic acids—which can act as both reducing and capping agents during nanoparticle formation [6]. This dual functionality enhances nanoparticle stability and eliminates the need for synthetic stabilizers [7,8]. The applicability of M. azedarach in green synthesis has been demonstrated in previous studies; Haq et al. [9] reported the successful biosynthesis of silver and magnesium oxide nanoparticles using its extracts, indicating the plant’s significant potential in environmentally friendly nanomaterial production.

Zinc oxide nanoparticles (ZnO-NPs) are a prominent class of metal oxide nanomaterials known for their unique physicochemical properties. These include high chemical stability, a broad absorption spectrum, strong electrochemical coupling, and excellent photostability. ZnO-NPs, with the molecular formula ZnO, are widely produced and used across commercial and industrial sectors [10]. One notable application is their use in the food industry as a zinc nutrient source [11,12].

Several studies have reported that the band gap of ZnO nanoparticles influences their Zn²⁺ release in soil, as it reflects particle size, crystallinity, and surface defects. Nanoparticles with a narrower band gap, indicating higher defect density, exhibit weaker Zn–O lattice stability and release Zn²⁺ ions more readily, particularly under acidic or neutral pH. In contrast, wider band gap, highly crystalline ZnO NPs dissolve more slowly, providing controlled, sustained ion release. Thus, band gap measurements can serve as a useful predictor of ZnO NP dissolution behavior in agricultural soils [13,14,15]. ZnO typically possesses a direct band gap of about 3.37 eV, and its physicochemical properties can be tuned through different synthesis approaches, including sol–gel, hydrothermal, and green synthesis using plant extracts. Such variations can modify the band gap, thereby influencing the solubility and controlled release of Zn²⁺ ions, which are vital micronutrients for plant growth [16,17,18]. Research indicates that ZnO nanoparticles can be absorbed by plants through roots or foliar pathways, affecting physiological attributes such as chlorophyll accumulation, antioxidant activity, and biomass production. The release rate of Zn²⁺ ions is strongly dependent on the band gap, with narrower band gaps often promoting faster dissolution and enhanced bioavailability for plant uptake. Hence, elucidating the correlation between the band gap energy of ZnO nanoparticles and their release kinetics is essential for advancing their agricultural applications, ensuring effective nutrient delivery while mitigating potential phytotoxic risks. Importantly, band gaps narrower than ~3.37 eV are frequently associated with accelerated photocorrosion and dissolution due to the upward shift in the valence band edge, which renders the material more susceptible to self-oxidation under illumination. This highlights the inherent trade-off between achieving visible-light activity through band gap narrowing and maintaining the structural stability of ZnO nanoparticles, thereby underscoring the need for careful tuning of their electronic properties to balance nutrient release efficiency with long-term stability [18,19].

Recent studies have highlighted the multifaceted roles of zinc oxide nanoparticles (ZnO-NPs) in enhancing plant physiology and stress resilience. In tea plants (Camellia sinensis), ZnO-NPs significantly improved photosynthetic efficiency, including increased levels of RubisCO (key CO₂-fixing enzyme), enhanced chlorophyll fluorescence, and elevated CO₂ utilization efficiency [20]. Similarly, in common bean (Phaseolus vulgaris), soil or foliar application of ZnO-NPs restored electron transport functions under stress, enhancing linear electron flow (LEF) by up to 72% compared to untreated plants, thereby supporting higher adenosine triphosphate/nicotinamide adenine dinucleotide phosphate generation suited for CO₂ fixation [21]. These findings underscore the potential of ZnO-NPs in improving photosynthesis and carbon fixation processes in plants.

Despite the extensive research on ZnO nanoparticles for agricultural applications, sustainable and cost-effective synthesis methods that also ensure controlled nutrient release remain limited. This study addresses this gap by introducing a green synthesis approach using Melia azedarach L. extract to produce ZnO nanoparticles and systematically evaluating the controlled release behavior of Zn²⁺ ions under plant-relevant pH conditions. Unlike many previous studies that focus primarily on synthesis or characterization, our work emphasizes the functional performance of ZnO-NPs as potential nanofertilizers, specifically targeting long-term nutrient availability and persistence in soil. By integrating an eco-friendly synthesis route with practical performance assessment, this investigation provides new insights for the development of sustainable nanofertilizers that enhance crop nutrition and productivity.

2. Materials and Methods

Zinc acetate dihydrate (C₄H₆O₄Zn·2H₂O) and commercial zinc oxide nanoparticles (designated as ZnOcom) were obtained from Thermo Fisher Scientific Inc., Waltham, MA, USA. and Ishihara Sangyo Kaisha, Ltd., Osaka, Japan respectively. Analytical grade of sodium hydroxide (NaOH) with purity of 98% was supplied by Sigma Aldrich, St. Louis, MO, USA. Deionized water was used as the solvent and aqueous medium to disperse the nanoparticles. Absolute ethanol (Barcelona, Spain, 99.9%) was used as a solvent in the final washing stage. Fresh Melia azedarach (Chinaberry) leaves were collected locally, washed thoroughly with distilled water, and air-dried prior to use.

2.1. Synthesis of ZnO Nanoparticles

Fresh leaves of Melia azedarach (20 g) were cut into small pieces and boiled in 200 mL of distilled water at 100 °C for 30 min. The mixture was filtered through Whatman No. 1 filter paper to remove solid residues, and the filtrate was cooled to room temperature. The freshly prepared extract was used immediately as a reducing and stabilizing agent. An equal volume (1:1, v/v) of the extract was added dropwise to 0.1 M zinc acetate solution under continuous stirring. This ratio was chosen based on preliminary optimization experiments, in which different extract-to-precursor ratios (1:0.5, 1:1, and 1:2) were tested. The 1:1 ratio produced ZnO nanoparticles with the most uniform particle size distribution, as confirmed by SEM analysis. The pH of the reaction mixture was adjusted to 10 using 1 M NaOH with a calibrated pH meter (WTW inoLab 7310, Weilheim, Germany). The suspension was left to stand for 24 h at ambient temperature to facilitate complete precipitation and stabilization of zinc hydroxide nanoparticles (designated as Zn_OH₂). The resulting precipitate was collected by centrifugation (10,000 rpm, 10 min), washed three times with distilled water to remove unreacted species, and dried at 80 °C overnight. The dried product was subsequently calcined at 500 °C for 2 h in a muffle furnace to yield crystalline ZnO nanoparticles (designated as ZnO_calc).

2.2. Physicochemical Characterizations of ZnO Nanoparticles

The particle size of ZnO_calc samples was estimated using the Malvern panalyticalzitasier (model Mastersizer 3000+ Lab, Malvern, UK). ZnO-NPs were carefully dispersed in distilled water using an ultrasonicator for 1 h before measurement. FTIR spectrometer (Lambda scientific, Guangzhou, China) and an attenuated total reflectance diamond crystal unit were used to obtain the infrared spectra. The spectra were recorded by scanning from 4000 to 650 cm⁻¹ with a resolution of 4 cm⁻¹. The morphology and chemical composition of synthesized ZnO were investigated using a Phenom XL G2 scanning electron microscope combined with an AXS EDS system (Thermo Fisher Scientific, Waltham, MA, USA). The kaolinitic clay and the synthesized product were coated with a ~300 Å thick platinum film under an argon atmosphere using an AGAR sputter coater machine (model AGB7340, Stansted, UK) in a high-vacuum evaporator. The SEM images were produced at 6 × 10⁻⁴ Pa and 15 kV accelerating voltage. The structural phases of the ZnO nanoparticles samples were investigated using an Anton Paar XRD diffractometer (XRDynamic 500, Graz, Austria) with a Cu Ka-radiation k = 1.54 Å, 40 kV, 40 mA at a 2 h range 2–70 with a scan rate of 2 deg/min, step scan size 0.02, and receiving slit of 0.3 mm. The data obtained from the XRD pattern was used to calculate the crystallite size using the Scherrer equation [22].

The surface area and pore size distribution were measured using N₂ adsorption–desorption isotherms on a Quantachrome Instruments (Anton Paar, Autosorb-iQ, Graz, Austria) analyzer. Samples were degassed under vacuum at 250 °C for 3 h to remove adsorbed moisture and volatiles. Specific surface area was calculated using the multi-point BET method, while pore size distribution was determined with AsiQwin software (v5.21) using NLDFT and the N₂ at 77 K on oxide surface kernel. This slit-pore NLDFT model, tailored for oxides, accounts for solid–fluid and fluid–fluid interactions, offering a more accurate characterization of meso- and microporous structures than classical BJH methods, which assume cylindrical macropores.

The optical band gap of ZnO_calc sample was determined using a JASCO UV–Visible spectrophotometer (Model V-730, Tokyo, Japan). The nanoparticles were dispersed in deionized water and sonicated for 15 min prior to measurement. The UV–Vis spectrum was recorded at room temperature in the wavelength range of 200–800 nm. The absorption coefficient was calculated according to the procedure described by Ayesh and Abdel-Rahem [23], taking into account the nanoparticle concentration and the 1 cm optical path length of the quartz cuvette. The band gap energy was subsequently estimated using the Tauc relation (Equation (1), [24]).

$${\left(\mathsf{\alpha }\mathrm{h}\mathrm{v}\right)}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}=\mathrm{A}\left(\mathrm{h}\mathrm{v}-\mathrm{E}\mathrm{g}\right)$$

(1)

here

α: absorption coefficient;

hv: photon energy (Planck’s constant × frequency);

A: proportionality constant;

Eg = optical band gap energy;

n = 1/2 → direct transition allowed (as in ZnO).

2.3. Release Behavior Study of Synthesized Green ZnO Nanoparticles

A quantity of 0.5 g of the synthesized green zinc oxide (ZnO) nanoparticles was dispersed in 1.0 L of distilled water. The pH of the prepared solution was adjusted to the desired value using acetic acid to simulate internal plant conditions that promote Zn²⁺ ion release. The release behavior of Zn²⁺ was evaluated over a period of two weeks. Electrical conductivity (EC) was measured periodically using a calibrated WTW Cond 3210 (Weilheim, Germany) conductivity meter to estimate the overall ionic concentration in the solution. Simultaneously, pH was monitored with a WTW inoLab pH 7310 m to assess changes in acidity that could influence the dissolution rate. This experimental setup was designed to provide insights into the sustained release of zinc ions under near-physiological plant conditions and to evaluate the potential of the ZnO_calc sample as a slow-release micronutrient fertilizer.

3. Results

3.1. Physiochemical Characterization of the Green-Synthesized ZnO NPs

The size distribution of the ZnO_calc sample (Figure 1) displays a dominant peak at approximately 122 nm, representing the majority of the nanoparticle population. This relatively uniform distribution indicates that nucleation and growth were effectively regulated by plant-derived compounds, which coated the nanoparticles and imparted electrostatic stabilization through negatively charged carboxyl groups. In addition, a minor peak observed near 5560 nm corresponds to a small fraction of larger aggregates, likely originating from insufficiently capped nanoparticles or residual organic constituents from the plant extract.

The chemical structure evolution from Zn(OH)₂ to ZnO nanoparticles was investigated using FTIR spectroscopy, with spectra of Zn_OH₂ and ZnO_calc samples presented in Figure 2a,b, respectively. The Zn_OH₂ spectrum (Figure 2a) shows a broad O–H stretching band at 3427 cm⁻¹, indicating surface hydroxyl groups and adsorbed water, and peaks at 2922 and 2853 cm⁻¹ corresponding to aliphatic C–H stretching from plant-derived phytochemicals, which likely contribute to nanoparticle stabilization. Additional bands at 1602 and 1416 cm⁻¹ are attributed to H–O–H bending and C=O/COO⁻ vibrations, while the 1105 cm⁻¹ band reflects C–O or C–O–C stretching, confirming the presence of organic capping agents [25]. Notably, peaks at 615 and 471 cm⁻¹ correspond to Zn–O stretching and Zn–O–H bending, verifying the formation of Zn(OH)₂ nanoparticles [26].

Upon calcination, the ZnO_calc spectrum (Figure 2b) exhibits marked changes. The O–H stretching band shifts to 3438 cm⁻¹ with reduced intensity, reflecting dehydration, while weak C–H and C–O bands indicate partial removal of residual organics. Absorptions at 1629 and 1424 cm⁻¹ correspond to H–O–H bending and C=O/COO⁻; stretching, suggesting minor surface-bound organics or carbonates. Sharp bands at 677 cm⁻¹ confirm Zn–O lattice vibrations, demonstrating the successful transformation of Zn(OH)₂ into ZnO nanoparticles. These results are consistent with the literature, which reports diminishing organic features and retention of characteristic lattice vibrations upon calcination of green-synthesized nanoparticles [27]. Overall, the comparison highlights the effectiveness of calcination in removing organic residues while preserving the ZnO crystalline framework.

The SEM–EDS analyses of ZnO_calc and ZnO_com nanoparticles are shown in Figure 3 and Figure 4, respectively. Both samples exhibited nanosized particles with a tendency to aggregate, a common feature of high surface energy nanomaterials. The ZnO_calc sample (Figure 3) displayed nearly spherical particles with irregular clustering, likely due to interactions with phytochemicals from the plant extract, which acted as natural capping and stabilizing agents. EDS confirmed Zn (45.70 wt%) and O (34.40 wt%) as the primary elements, along with minor amounts of Ca (15.80 wt%), Mg (2.50 wt%), Si (0.90 wt%), and P (0.70 wt%), attributed to plant-derived metabolites or associated minerals, suggesting surface adsorption of bioactive compounds that aid stabilization and functionalization. In contrast, the ZnO_com sample (Figure 4) showed stronger agglomeration, irregular morphology, and a porous texture that increases the surface-to-volume ratio. Its EDS spectrum identified Zn (38.86 wt%) and O (34.83 wt%) as the dominant constituents, with Si (4.10 wt%) confirming the presence of a stabilizing silicon coating. Additional signals included C (21.62 wt%), originating from carbon tape or surface contamination, and Cl (0.60 wt%), likely from residual precursor salts. These findings confirm the successful formation of ZnO nanoparticles in both cases, with plant-mediated ZnO_calc functionalized by phytochemicals, while the commercial ZnO_com was effectively modified with a silicon coating to enhance stability and potential performance in optical and catalytic applications.

The XRD pattern of the ZnO_calc sample (Figure 5) confirms the successful formation of the hexagonal wurtzite structure, as evidenced by the characteristic diffraction peaks listed in

Table 1. XRD Peak Parameters of ZnO_calc sample.

Miller Index	Peak Position (2θ Deg)	FWHM	Crystallite Size D (nm)
(100)	31.7	0.4456	18.5
(002)	34.4	0.8405	9.9
(101)	36.2	0.5012	16.7
(102)	47.5	0.8909	9.7
(110)	56.6	0.9489	9.5

, consistent with JCPDS card no. 36–1451. Crystallite size estimation using the Scherrer equation (Table 1) indicated an average size of ~18–20 nm, verifying the nanocrystalline nature of the sample. The noticeable broadening of the diffraction peaks can be attributed to the combined effects of nanoscale crystallite dimensions and lattice strain. Such strain is likely introduced during green synthesis, where phytochemicals present in the plant extract act as reducing and capping agents, thereby restricting crystal growth and inducing lattice distortions and surface defects that contribute to peak broadening.

Figure 6 shows the N₂ adsorption–desorption isotherm of the ZnO_calc sample. According to the IUPAC classification [28], the isotherm corresponds to type II with an H4 hysteresis loop, characteristic of mesoporous ZnO nanoparticles. This indicates a pore structure capable of accommodating substantial amounts of adsorbed gas, which is beneficial for applications requiring high surface area, such as desorption, catalysis, and controlled-release systems. The corresponding pore size distribution is presented in Figure 7, showing nanopores ranging from 4 to 24.0 nm with an average diameter of 8.5 nm. The BET surface area was measured as 12.9 m²/g, confirming the high porosity of the synthesized nanoparticles. Such mesoporosity and surface area are expected to facilitate controlled Zn²⁺ release, as the nanopores can act as reservoirs, supporting both immediate nutrient availability and sustained delivery over time in agricultural applications.

3.2. Optical Properties Measurement Using UV–Visible Absorption Spectroscopy and Energy Gap Calculations

The UV–Vis spectra of the three samples (200–800 nm), shown in Figure 8, display pronounced UV absorption with clear absorption edges, enabling band-gap estimation through Tauc analysis for a direct-allowed transition (n = 1/2). The ZnO_calc sample exhibits a sharp absorption edge in the 315–335 nm region, yielding an estimated band gap of Eg = 3.6 ± 0.1 eV, which is slightly blue-shifted relative to bulk ZnO, consistent with quantum confinement effects. Previous studies have reported band gaps of approximately 3.39 eV for ZnO nanoparticles and 3.2 eV for ZnO nano-rods, highlighting the influence of nanoscale confinement on the optical properties of ZnO [29].

In contrast, the ZnO_com sample shows a broader absorption profile extending to 390–400 nm, corresponding to Eg = 3.23 ± 0.07 eV; this red shift and band-tail formation can be linked to Si-induced surface states and increased scattering effects. Meanwhile, the Zn_OH₂-derived sample demonstrates a diffuse absorption edge around 330–355 nm, giving an apparent Eg = 3.45 ± 0.10 eV, reflecting its limited crystallinity and incomplete transformation to ZnO. These findings collectively indicate that phytochemical-assisted crystallization promotes the formation of well-defined ZnO with a wider band gap, whereas Si coating reduces and broadens the optical transition through surface modification, and Zn(OH)₂ remains optically less distinct until fully converted. Similar trends have been reported in other green synthesis studies, where ZnO nanoparticles are produced from plant extracts such as pomegranate, beetroot, and other botanicals, often exhibiting lower band gap values than chemically synthesized counterparts, enhancing both light absorption and surface reactivity [30].

3.3. Controlled Release of Zn²⁺ Ions

The combined pH and electrical conductivity (EC) profile (Figure 9) reflects the ionic release kinetics of the ZnO_calc sample. An initial sharp rise in pH (above 9 within hours) and concurrent EC increase indicate rapid surface hydrolysis and partial ZnO dissolution, followed by stabilization of pH (≈7.5–8.5) and EC (~100 µS/cm), reflecting a sustained, slower-release regime. This dual-phase release provides immediate nutrient availability while ensuring long-term Zn²⁺ supply, reducing toxicity risk. Compared to chemically synthesized ZnO nanoparticles, which often show rapid or poorly controlled release [31,32,33], our green-synthesized ZnO exhibits a more balanced and sustained release profile. These results, supported by recent field studies demonstrating improved salt tolerance and crop yield [34], indicate that the ZnO_calc nanoparticles are effective slow-release zinc sources, delivering bioavailable zinc (≤200 ppm) and potentially modulating rhizosphere pH to favor nutrient uptake. Overall, plant-mediated synthesis offers a practical and eco-friendly approach, combining controlled nutrient delivery with enhanced functional performance and environmental safety.

As summarized in

Table 2. Comparative summary of Zn²⁺ ion release behavior from ZnO synthesized by different methods.

Synthesis Method	pH Behavior over Time	EC Behavior over Time	Zn2+ Release Characteristics	Ref.
Sol–gel	Gradually increases with time as dissolution proceeds	EC steadily increases over longer period	Moderate-to-high release rate is attributed to the porous structure, which increases surface reactivity and facilitates ion exchange.	[35]
Hydrothermal	Stable pH profile; minimal variation	Low and stable EC values	Exhibits the lowest Zn2+ release rate owing to its dense crystal structure and minimal surface defects that restrict dissolution.	[36]
Precipitation	Decreases slightly with time as Zn2+ hydrolyzes	Rapid EC rise initially, stabilizing after a few hours	Moderate release rate resulting from particle aggregation and the presence of residual ions that promote initial Zn2+ release.	[37]
Present Green synthesis study	Rapid early increase (peak ≈ 10 at ~few h) then stabilizes to mildly alkaline ~8–8.5 over long term	EC: sharp early rise, then gradual increase and plateau (~170 µS/cm by 200 h). EC correlates with ionic release.	Controlled and sustained Zn2+ release is attributed to phytochemical capping agents that decrease solubility and enable gradual dissolution.

, Zn²⁺ release from ZnO nanoparticles is strongly governed by the synthesis route. Green-synthesized ZnO exhibited a biphasic release; an initial sharp rise in pH and EC, followed by gradual stabilization, indicative of controlled dissolution. Compared with sol–gel, precipitation or hydrothermal methods, the green-synthesized ZnO nanoparticles show slow ion diffusion and moderating Zn²⁺ solubility under alkaline conditions dominated by Zn(OH)₂. The observed EC–pH correlation supports previous findings (Table 2), highlighting EC as a practical proxy for ionic mobility in soils. These results demonstrate that green synthesis imparts favorable slow-release characteristics, making ZnO nanoparticles suitable for sustainable zinc fertilization.

4. Conclusions

This study demonstrates a sustainable and effective green synthesis route for ZnO nanoparticles using Melia azedarach leaf extract, yielding highly crystalline, uniformly dispersed, and bioavailable nanostructures with distinct advantages over commercial silicon-coated ZnO. Comprehensive characterization confirmed the presence of organic surface capping, enhanced crystallinity, and a nanoscale band gap consistent with improved stability and reactivity. The observed sustained Zn²⁺ release under mildly acidic conditions further supports their potential as controlled-release nanomaterials. Taken together, these results underline the suitability of green-synthesized ZnO-NPs as low-cost, eco-friendly alternatives for agricultural and environmental applications, offering functional efficiency.