Zinc Oxide Nanoparticles: Synthesis, Characterization and Applications in Agriculture

Abstract

Zinc (Zn) is a mineral that plays a vital role in the growth and development processes of different plants. Although it is required in small quantities, its presence is essential in a crop. In recent years, zinc oxide nanoparticles (ZnO NPs) have garnered significant interest in agriculture due to their unique physical and chemical properties. As a result, they can be used as alternative fertilizers to help crops experiencing mineral deficiency, stress, or fungal problems. These nanomaterials can be obtained through various synthesis methods, including sol–gel, chemical precipitation, microemulsion, and green synthesis, among others. This enables managing their size, shape, and internal arrangement, establishing their ultimate characteristics and feasible uses. In this review, we will present some of the most commonly used synthesis methods for obtaining ZnO NPs, the frequently used characterization techniques, as well as some of the positive and toxic effects caused by their application in crops.

1. Introduction

In recent decades, nanotechnology has emerged as a key tool for creating various types of nanomaterials. These nanomaterials possess unique physical, chemical, and biological properties, making them applicable in diverse fields such as electronics, optics, biomedicine, materials science, and agriculture. The properties of nanoparticles and nanomaterials lie in their minuscule size (1–100 nm), which grants them a large surface area and distinctive physicochemical characteristics [1,2]. These nanoparticles can be synthesized through various physical and chemical methods and can be composed of materials like ceramics, polymers, and metals. Among the different metal oxide nanoparticles, zinc oxide nanoparticles (ZnO NPs) stand out due to their versatility and excellent chemical stability, leading to widespread use in commercial products. Zn, an essential micronutrient for plant growth and development, has been explored in agriculture because it plays a crucial role in enzymatic functions linked to photosynthesis and energy processes in plants [1,3]. The agricultural sector is encountering significant challenges due to the impacts of climate change. Issues such as rising temperatures, increased drought frequency, soil salinity, and flooding present considerable obstacles that can adversely affect plant productivity and crop yields. It is essential to address these challenges to ensure food security and promote sustainable agricultural practices [4]. This demand has led to the uncontrolled use of inorganic agrochemicals for crop fertilization, resulting in adverse environmental effects, including soil degradation, nutrient leaching, greenhouse gas emissions, and various forms of pollution (such as water, air, and soil pollution). These issues contribute to declining soil fertility, ultimately leading to food shortages [5].

The urgent demand for sustainable agricultural practices has prompted the investigation of technologies aimed at enhancing food production while reducing the use of agrochemicals. One promising approach involves the integration of nutrients into plants via nanoparticles, which can improve the efficiency of agricultural inputs and mitigate their environmental impact. Various types of nanoparticles have been developed for functions such as growth promotion, pest control, fertilization, and herbicide application, leading to significant agricultural benefits, including increased yields, effective disease management, and improved pest and weed control, all at reduced costs and with less waste [6]. This review specifically focuses on the use of ZnO NPs as a potential nanofertilizer for crop treatment, discussing the commonly used synthesis methods for their production, their important properties, and the advantages and disadvantages associated with their use in agriculture.

2. Nanofertilizers

Nanofertilizers are materials synthesized or modified from traditional fertilizers, bulk fertilizers, or extracted from vegetative or reproductive parts of a plant, and can be synthesized using physical, chemical, and biological methods with the help of nanotechnology [7]. Physical methods require physical effort to fragment a material and mechanically grind it to achieve the desired sizes. It is a method commonly scalable to industrial levels and frequently produces irregular nanoparticles. Chemical methods involve obtaining nanomaterials from chemical reactions, starting at the atomic level, and are often high-energy syntheses that require toxic reagents [7]. However, one way to reduce the use of toxic reagents has been through green synthesis methods that allow for the production of nanoparticles at a lower price, which are less toxic and environmentally friendly, using plants, plant extracts, microorganisms, among others, as reaction precursors. In the following sections, some of the most common synthesis methods for obtaining ZnO NPs will be described in greater detail.

3. Synthesis of ZnO NPs

The synthesis methods involve the use of various techniques to produce nanoparticles, each allowing for the creation of nanomaterials with distinct morphologies, particle sizes, structures, surface characteristics, and other properties. These techniques meet specific requirements depending on the intended application of the nanoparticles. For instance, methods such as sol–gel, chemical precipitation, and green synthesis can be utilized to adapt the physical and chemical properties of ZnO NPs for their use in agriculture. By carefully selecting the synthesis method, researchers can optimize the characteristics of the nanoparticles to enhance their effectiveness as nanofertilizers while minimizing any potential negative impacts on the environment. These will be briefly described in subsequent sections.

3.1. Sol–Gel

The sol–gel process begins with a colloidal solution (sol), which later converts into a gel and then a solid. This synthesis involves three important processes: hydrolysis, condensation, and polymerization. Once the gel is obtained, it is heated to evaporate the solvents and create the finished product (Figure 1). This synthesis can produce ZnO nanoparticles with a desired composition and particle size [8]. Some researchers have synthesized ZnO NPs using the sol–gel technique with different chemical precursors.

Hasnidawani et al. [9] synthesized zinc oxide by the sol–gel method, using zinc acetate dihydrate and sodium hydroxide as precursors for the formation of the colloidal solution, which were dissolved in 10 mL and 15 mL of distilled water, respectively. These solutions were stirred individually, and after 5 min, a NaOH solution was poured into the zinc acetate solution and kept under constant stirring. Finally, 100 mL of ethanol was added dropwise to the resulting solution, yielding a white precipitate. The results show the production of ZnO nanorods with a size of approximately 85 nm.

Vishwakarma and Pal [10] used zinc acetate dihydrate, sodium hydroxide, methanol, and distilled water to obtain a colloidal solution. Initially, zinc acetate dihydrate was mixed with methanol at 25 °C for 120 min. Subsequently, NaOH was added to the previous solution and stirred for 60 min until a white material precipitated at the bottom of the vessel. The resulting material was washed, dried, and sintered at 400 °C for 30 min. The results showed the production of nanoparticles with a size between 15 and 25 nm.

Jurablu et al. [11] also synthesized ZnO nanoparticles using sol–gel. They prepared a solution with 10 mL of ethanol and 300 mL of distilled water and then added ZnSO₄·7H₂O and diethylene glycol. The resulting solution was stirred for 2 h at 85 °C to obtain the gel. The gel was then dried at 220 °C for 1 h and then ground into a fine powder. The nanoparticles were synthesized at 500 °C for 3 h, resulting in an average particle size of 28 nm.

3.2. Microemulsion

An emulsion is a continuous liquid phase in which a second, discontinuous, immiscible liquid phase is dispersed. Emulsions are categorized as either oil-in-water (O/W) or water-in-oil (W/O), depending on their composition. Liquids are classified as “water” (hydrophilic and highly polar) or “oil” (nonpolar and hydrophobic) (Figure 2). Microemulsions are isotropic, stable, and transparent liquids formed by an aqueous layer, an oil layer, and a surfactant. Furthermore, a microemulsion forms spontaneously and has a smaller droplet size (0.0015 to 0.15 µm) compared to emulsions [12].

Pineda et al. [13] synthesized ZnO NPs using the microemulsion method. Initially, the organic phase was prepared by mixing different proportions of emu oil and surfactant (1:1, Span 80: Tween 80). A zinc acetate solution was then added dropwise to the previous solution. The mixture was then stirred for 5 min at 1200 rpm and then left to stand for 24 h at room temperature. NaOH solution was added dropwise to the previous solution to precipitate the ZnO. The ZnO was washed, dried, and then sintered at 800 °C for 2 h, resulting in hemispherical nanoparticles that were 41.2 nm.

Yildirim et al. [14] synthesized ZnO nanoparticles by inverse microemulsion, using zinc acetate dihydrate as zinc precursor, sodium bis (2-ethylhexyl) sulfosuccinate as surfactant, glycerol as polar phase, and n-heptane as apolar phase. First, a solution with a surfactant dissolved in n-heptane was prepared at room temperature. Then, it was divided into two containers in equal parts [14]. In the first solution, a glycerol solution with zinc acetate was slowly added; similarly, to the second solution, a glycerol solution with sodium hydroxide was added. Both solutions were stirred at room temperature, and then the second solution was added to the first. The resulting solution was kept under constant stirring at 60–70 °C for 24 h. Finally, the powders obtained were collected, washed, dried, and calcined at 300 °C, 400 °C, and 500 °C for 3 h, yielding particle sizes ranging from 15 to 24 nm [14].

Li et al. [15] mixed heptane and hexanol in a 3:1 ratio as the oil phase and added Triton X-100 as the surfactant to create a clear microemulsion (ME) with Triton X-100. They prepared two microemulsion solutions: ME-1, which combined Zn(NO₃)₂ solution with varying PEG400 amounts, and ME-2, which added NaOH solution to ME. To synthesize ZnO nanoparticles, ME-1 was slowly combined with ME-2 while stirring. The mixture was then heated at 140 °C for 15 h. After centrifugation, the solid particles were washed and dried at 60 °C, resulting in nanoparticles with various shapes—needle-shaped, columnar, and spherical—and sizes of approximately 55, 70, and 46 nm, depending on the PEG400 amount [15].

3.3. Chemical Precipitation

This is a common method for producing nanoparticles, utilizing organometallic precursors. These precursors usually form metal oxides when they are precipitated in alkaline solutions like ammonium hydroxide, potassium hydroxide, or sodium hydroxide. This method involves nucleation and growth to control morphology and particle size [16]. Atoms combine to form a small, unstable embryo that can evolve into a stable nucleus, leading to the creation of a new phase, provided its radius exceeds a critical threshold [17]. Subsequently, particle growth occurs from the attachment of more atoms to the solid surface, resulting in the formation of nanoparticles. Figure 3 presents an overview of how ZnO NPs are chemically precipitated.

Thambidurai et al. [18] synthesized ZnO nanoparticles using a chemical coprecipitation method. Initially, two solutions were prepared: one of zinc nitrate and the other of sodium hydroxide. Then, sodium hydroxide was introduced gradually to the nitrate solution while maintaining vigorous stirring to ensure thorough mixing. The resulting precipitate was washed several times to remove impurities, then dried at 70 °C for 6 h and calcined at 400 °C for 2 h [18].

In another study, ZnO nanoparticles were synthesized by preparing an aqueous zinc nitrate solution and a potassium hydroxide solution [19]. The latter was slowly added to the zinc nitrate solution under vigorous stirring. After centrifugation for 20 min, the product was washed twice, dried, and calcined at 500 °C for 3 h, producing 20 nm nanoparticles.

Arundhathi and Maheswari [20] synthesized ZnO nanoparticles using two solutions: the first contained zinc nitrate, and the second contained ammonium carbonate. The second solution was added slowly to the first while stirring, producing a white solid, which was filtered and washed. Finally, the material was dried and calcined at 300 °C for 3 h, obtaining particles with an average size of 30 nm.

3.4. Green Synthesis

Green synthesis methods use microorganisms, enzymes, and plant extracts during manufacturing to reduce the use of toxic substances, minimize environmental pollution, and lower production costs. Phytochemicals, which act as reducing and stabilizing agents, are secreted by these natural strains and plant extracts. In addition, this synthesis method enables large-scale production of ZnO nanoparticles without extra impurities [21,22]. Basically, in green synthesis methods, ZnO NPs are synthesized from a reaction between a plant extract and a zinc salt solution. Some authors report various syntheses for obtaining nanoparticles, so a general scheme for performing this type of green synthesis is presented in Figure 4.

Ashwini et al. [23] synthesized ZnO NPs from an extract of Cayratia pedata. Initially, the extract from the plant’s leaves was obtained as follows: once the leaves were washed, they were dried at room temperature, then cut and macerated with sufficient water using a mortar and pestle. The resulting solution was boiled for 15 min, filtered, and centrifuged to remove residues. The solution was stirred at 65 °C for 20 min after the zinc nitrate was added. The sample was kept heated until a thick yellow paste was obtained. The paste was dried and calcined at 400 °C for 2 h to produce agglomerated ZnO NPs, which had an average particle size of 52.24 nm.

Azeez et al. [24] extracted Eucalyptus globulus Labill leaves by washing, dehydrating, and macerating them into a paste. This paste was mixed with distilled water, and the pH of the resulting solution was adjusted to 8 using NaOH, considering that the formation of smaller particles occurs at alkaline pH. Subsequently, the extract was heated to 60 °C, and zinc nitrate was added. The mixture was heated, forming a yellow paste, which was then calcined for 2 h at 400 °C to obtain spherical ZnO nanoparticles sized 27–35 nm.

Thiam et al. [25] extracted soluble bioactive compounds by washing, drying, and grinding Licania tomentosa leaves into a powder, mixing the powder with water, and stirring for 3 h. A filtration process was then carried out to remove residual solids from the extract, and zinc nitrate was added. ZnO nanoparticles (average size 35.68 nm) were obtained by drying the solution after 3 h of stirring and calcining the resultant brown paste at 500 °C.

Table 1 presents the synthesis methods and conditions, and the sizes of the nanoparticles obtained.

3.5. Advantages and Limitations of Synthesis Methods

The aforementioned synthetic methods enable the production of various nanoparticles, including ZnO nanoparticles (Table 2). However, each method has certain advantages and limitations, which are listed in a table for better understanding.

Table 1. Concentrated information for obtaining ZnO NPs.

Synthesis Method	Synthesis Parameters	Synthesis Conditions	Particle Size (nm)	Ref.
SOL–GEL	Precursors	Zinc acetate dehydrate 2 and sodium hydroxide 8 g	85	Hasnidawani et al. (2016) [9]
	Solvents	Distiller water
	Stirred	5 min
	Precursor	0.2 M zinc acetate dehydrate and 0.02 M NaOH	15–25	Vishwakarma and Pal (2020) [10]
	Solvent	Methanol
	Reaction time	180 min
	Heat treatment	400 °C for 20 min
	Precursors	0.015 M ZnSO4·7H2O and 1.2 g of diethylene glycol	28	Jurablu et al. (2025) [11]
	Solvent	Ethanol/distilled water
	Stirred	2 h at 85 °C
	Drying	1 h at 220 °C
	Heat treatment	500 °C for 3 h
MICROEMULSION	Precursors	1:1, Span 80: Tween 80, 0.5 M zinc acetate solution, 1.0 M NaOH solution	41.2	Pineda et al. (2018) [13]
	Stirred	5 min at 1200 rpm
	Reaction time	24 h at room temperature
	Heat treatment	800 °C × 2 h
	Precursors	0.5 M zinc acetate 0.5 M sodium hydroxide	15–24	Yildirim et al. (2010) [14]
	Solvents	n-heptano and glicerol
	Reaction time	24 h at 60–70 °C
	Heat treatment	300, 400, 500 °C for 3 h
	Precursors	Zn(NO3)2 at 0.25 mol/L NaOH at 0.5 mol/L	55, 70, 46	Li et al. (2009) [15]
	Solvents	Heptane, hexanol, and Triton X-100
	Additive	PEG400
	Reaction temperature	140 °C for 15 h
CHEMICAL PRECIPITATION	Precursors	1 M sodium hydroxide, 0.5 M zinc nitrate	25–200	Thambidurai et al. (2020) [18]
	Drying	70 °C for 6 h
	Heat treatment	400 °C for 2 h
	Precursors	0.2 M zinc nitrate solution, 0.4 M potassium hydroxide solution	20	Kumar (2012) [19]
	Stirred	Vigorous
	Spin time	20 min
	Heat treatment	500 °C for 3 h
	Precursors	1 M zinc nitrate 3.2 M ammonium carbonate	30	Arundhathi and Maheswari (2019) [20]
	Heat treatment	300 °C for 3 h
GREEN SYNTHESIS	Precursors	Extract of Cayratia pedata and zinc nitrate	52.24	Ashwini et al. (2021) [23]
	Solvent	Water
	Reaction temperature	65 °C for 20 min
	Heat treatment	400 °C for 2 h
	Precursors	Extract from Eucalyptus globulus Labill leaves, zinc nitrate, NaOH	27–35	Azeez et al. (2020) [24]
	pH	8
	Reaction temperature	60 °C
	Heat treatment	400 °C for 2 h
	Precursors	Extract from Licania tomentosa leaves, zinc nitrate	35.68	Thiam et al. (2025) [25]
	Solvent	Water
	Time of stirred	3 h
	Heat treatment	500 °C

Table 1. Concentrated information for obtaining ZnO NPs.

Synthesis Method	Synthesis Parameters	Synthesis Conditions	Particle Size (nm)	Ref.
SOL–GEL	Precursors	Zinc acetate dehydrate 2 and sodium hydroxide 8 g	85	Hasnidawani et al. (2016) [9]
	Solvents	Distiller water
	Stirred	5 min
	Precursor	0.2 M zinc acetate dehydrate and 0.02 M NaOH	15–25	Vishwakarma and Pal (2020) [10]
	Solvent	Methanol
	Reaction time	180 min
	Heat treatment	400 °C for 20 min
	Precursors	0.015 M ZnSO4·7H2O and 1.2 g of diethylene glycol	28	Jurablu et al. (2025) [11]
	Solvent	Ethanol/distilled water
	Stirred	2 h at 85 °C
	Drying	1 h at 220 °C
	Heat treatment	500 °C for 3 h
MICROEMULSION	Precursors	1:1, Span 80: Tween 80, 0.5 M zinc acetate solution, 1.0 M NaOH solution	41.2	Pineda et al. (2018) [13]
	Stirred	5 min at 1200 rpm
	Reaction time	24 h at room temperature
	Heat treatment	800 °C × 2 h
	Precursors	0.5 M zinc acetate 0.5 M sodium hydroxide	15–24	Yildirim et al. (2010) [14]
	Solvents	n-heptano and glicerol
	Reaction time	24 h at 60–70 °C
	Heat treatment	300, 400, 500 °C for 3 h
	Precursors	Zn(NO3)2 at 0.25 mol/L NaOH at 0.5 mol/L	55, 70, 46	Li et al. (2009) [15]
	Solvents	Heptane, hexanol, and Triton X-100
	Additive	PEG400
	Reaction temperature	140 °C for 15 h
CHEMICAL PRECIPITATION	Precursors	1 M sodium hydroxide, 0.5 M zinc nitrate	25–200	Thambidurai et al. (2020) [18]
	Drying	70 °C for 6 h
	Heat treatment	400 °C for 2 h
	Precursors	0.2 M zinc nitrate solution, 0.4 M potassium hydroxide solution	20	Kumar (2012) [19]
	Stirred	Vigorous
	Spin time	20 min
	Heat treatment	500 °C for 3 h
	Precursors	1 M zinc nitrate 3.2 M ammonium carbonate	30	Arundhathi and Maheswari (2019) [20]
	Heat treatment	300 °C for 3 h
GREEN SYNTHESIS	Precursors	Extract of Cayratia pedata and zinc nitrate	52.24	Ashwini et al. (2021) [23]
	Solvent	Water
	Reaction temperature	65 °C for 20 min
	Heat treatment	400 °C for 2 h
	Precursors	Extract from Eucalyptus globulus Labill leaves, zinc nitrate, NaOH	27–35	Azeez et al. (2020) [24]
	pH	8
	Reaction temperature	60 °C
	Heat treatment	400 °C for 2 h
	Precursors	Extract from Licania tomentosa leaves, zinc nitrate	35.68	Thiam et al. (2025) [25]
	Solvent	Water
	Time of stirred	3 h
	Heat treatment	500 °C

Table 2. Comparison of different synthesis methods for obtaining ZnO NPs, including advantages and limitations.

Synthesis Method	Advantages	Limitations	Main Characteristics	Ref.
SOL–GEL	Nanomaterials with a precisely defined composition can be synthesized. Most chemical reactions are achieved at low temperatures (60–80 °C). A versatile technique that allows for the obtaining of a wide range of materials. High-purity homogeneous materials can be obtained. It allows complete control of morphologies and particle size. Offers the desired rate of thermal stability and good flexibility in crystal formation, which is reproducible.	The curing process increases production times. Its implementation on an industrial scale may present problems of repeatability and consistency. The synthesis of large and complex structures can be limited by gel formation. It is a technique that is very sensitive to humidity. Use of organic solutions can be toxic.	Nanoparticles with different morphologies (spherical, lamellar, nanorod, etc.) Nanoparticles with (-OH) and (-O) groups on the surface. Smaller nanoparticles can be obtained than in coprecipitation. Porous nanoparticles. Particle sizes between 1 and 85 nm approximately	[11,26,27,28,29,30,31,32,33,34,35,36,37,38]
MICROEMULSION	Microemulsion formation is reversible. Microemulsions are simple to prepare and require no energy input. Low formation energy requirements and thermodynamic stability. Biocompatibility and versatility. Their amphiphilic character allows them to carry different drugs effectively. It creates uniformly sized and distributed nanoparticles. Low reaction temperatures (25–70 °C).	Having a limited ability to dissolve substances with high melting points. Environmental factors like pH and temperature have an impact on microemulsion stability. High levels of surfactants and co-surfactants can be toxic. Difficult on an industrial scale.	Nanoparticles with uniform shapes and sizes (from 10 nm to 10 µm). Nanoparticles with a very narrow size distribution. Nanoparticles with uniform composition. Metallic nanoparticles, oxides, semiconductors, polymers. Spherical nanoparticles are usually obtained.	[13,14,39,40,41,42,43,44,45]
CHEMICAL PRECIPITATION	Relatively low reaction temperatures (25–70 °C). Obtaining fine-sized particles. Low-cost synthesis of nanomaterials. Simple and quick preparation. Various types of nanomaterials can be obtained directly.	The nucleation and growth processes are very susceptible to the reaction conditions. Wide particle size distribution. Difficult to control the morphology and agglomeration of particles. Precipitation of some impurities during the reaction.	Create quasi-spherical nanoparticles. Nanoparticles of various sizes (8–72 nm approximately). Particle agglomerates. Nanoparticles with functional groups on the surface.	[46,47,48,49,50,51,52,53]
GREEN SYNTHESIS	It uses fewer chemicals to obtain various nanomaterials. It is an eco-friendly technique. It is typically a more economical synthesis because it utilizes extracts from plants, algae, fungi, or microorganisms. Low production cost. Most nanoparticles obtained by this method are biocompatible.	Low reproducibility due to variability of extracts. Difficult to control the size and morphology of nanoparticles. Complex reactions, particularly those involving specific reagents or conditions, are typically incompatible with this method. Inconsistent composition of plant extracts. Equipment for careful plant material sterilization.	Nanoparticles with a capping layer (biological compounds). Nanoparticles with very dispersed sizes (20–70 nm approximately). They have spherical, cubic, and hexagonal shapes.	[54,55,56,57,58,59,60]

Table 2. Comparison of different synthesis methods for obtaining ZnO NPs, including advantages and limitations.

Synthesis Method	Advantages	Limitations	Main Characteristics	Ref.
SOL–GEL	Nanomaterials with a precisely defined composition can be synthesized. Most chemical reactions are achieved at low temperatures (60–80 °C). A versatile technique that allows for the obtaining of a wide range of materials. High-purity homogeneous materials can be obtained. It allows complete control of morphologies and particle size. Offers the desired rate of thermal stability and good flexibility in crystal formation, which is reproducible.	The curing process increases production times. Its implementation on an industrial scale may present problems of repeatability and consistency. The synthesis of large and complex structures can be limited by gel formation. It is a technique that is very sensitive to humidity. Use of organic solutions can be toxic.	Nanoparticles with different morphologies (spherical, lamellar, nanorod, etc.) Nanoparticles with (-OH) and (-O) groups on the surface. Smaller nanoparticles can be obtained than in coprecipitation. Porous nanoparticles. Particle sizes between 1 and 85 nm approximately	[11,26,27,28,29,30,31,32,33,34,35,36,37,38]
MICROEMULSION	Microemulsion formation is reversible. Microemulsions are simple to prepare and require no energy input. Low formation energy requirements and thermodynamic stability. Biocompatibility and versatility. Their amphiphilic character allows them to carry different drugs effectively. It creates uniformly sized and distributed nanoparticles. Low reaction temperatures (25–70 °C).	Having a limited ability to dissolve substances with high melting points. Environmental factors like pH and temperature have an impact on microemulsion stability. High levels of surfactants and co-surfactants can be toxic. Difficult on an industrial scale.	Nanoparticles with uniform shapes and sizes (from 10 nm to 10 µm). Nanoparticles with a very narrow size distribution. Nanoparticles with uniform composition. Metallic nanoparticles, oxides, semiconductors, polymers. Spherical nanoparticles are usually obtained.	[13,14,39,40,41,42,43,44,45]
CHEMICAL PRECIPITATION	Relatively low reaction temperatures (25–70 °C). Obtaining fine-sized particles. Low-cost synthesis of nanomaterials. Simple and quick preparation. Various types of nanomaterials can be obtained directly.	The nucleation and growth processes are very susceptible to the reaction conditions. Wide particle size distribution. Difficult to control the morphology and agglomeration of particles. Precipitation of some impurities during the reaction.	Create quasi-spherical nanoparticles. Nanoparticles of various sizes (8–72 nm approximately). Particle agglomerates. Nanoparticles with functional groups on the surface.	[46,47,48,49,50,51,52,53]
GREEN SYNTHESIS	It uses fewer chemicals to obtain various nanomaterials. It is an eco-friendly technique. It is typically a more economical synthesis because it utilizes extracts from plants, algae, fungi, or microorganisms. Low production cost. Most nanoparticles obtained by this method are biocompatible.	Low reproducibility due to variability of extracts. Difficult to control the size and morphology of nanoparticles. Complex reactions, particularly those involving specific reagents or conditions, are typically incompatible with this method. Inconsistent composition of plant extracts. Equipment for careful plant material sterilization.	Nanoparticles with a capping layer (biological compounds). Nanoparticles with very dispersed sizes (20–70 nm approximately). They have spherical, cubic, and hexagonal shapes.	[54,55,56,57,58,59,60]

3.6. ZnO Nanoparticles Synthesized for Agricultural Purpose

Some of the research on ZnO NPs synthesized for use in agricultural applications reports the following:

Mazhar et al. [61] synthesized ZnO NPs to increase the concentration of Zn in wheat and rice grains grown in saline soils. These ZnO NPs were obtained using the co-precipitation method. Initially, a NaOH solution was added drop by drop to a ZnSO₄·7H₂O solution in a 2:1 ratio. The mixture obtained was stirred for 12 h, after which the precipitate was filtered and washed with deionized water three times. Subsequently, the precipitate was dried in an oven at 105 °C and finally calcined at 550 °C for 2 h. According to their characterization by scanning electron microscopy, these nanoparticles had a spherical morphology and a particle size of approximately 51 nm. They were suspended in deionized water and applied to wheat and rice crops. In addition, they were compared with other sources of Zn, such as ZnSO₄·7H₂O and bulk ZnO. According to the results obtained, ZnO NPs showed better results compared to other sources of Zn in both normal and stressed soil conditions. The authors suggest that this could be due to their greater absorption and translocation in plants, improving Zn fortification in both grains.

Itroutwar et al. [62] prepared ZnO NPs using brown seaweed Turbinaria ornata (T. ornata) extract to improve the quality and yield of rice seeds. Initially, 400 mL of a 0.1 M zinc acetate solution was prepared and mixed with 80 mL of seaweed extract under magnetic stirring. To the mixture formed, 400 mL of 0.2 M NaOH was added and stirred vigorously at 60 °C for 3 h. After this time, the formation of white precipitates corresponding to the ZnO NPs was observed, which were collected, washed, and dried. Finally, the powders obtained were calcined at 400 °C for 4 h. According to the results obtained by transmission electron microscopy, dispersed nanoparticles with rod-like, spherical, and hexagonal morphologies with sizes between 15 and 52 nm were obtained. Different concentrations of ZnO NPs were suspended in deionized water, and part of it was used as nanopriming in physically healthy rice seeds, while another part was applied via foliar application in a microplot experiment. The results reported by the authors indicate that seeds treated with nanoparticles showed 100% seed germination and better agronomic characteristics. In addition, microplot experiments revealed that a dose of 10 mg/L improved grain weight. Overall, it was observed that grain and foliar applications increased the zinc content in rice crops.

Ahmed et al. [63] synthesized ZnO NPs as a nutrient in a tomato crop and evaluated their impact. The nanoparticles were prepared by adding 2 mL of 0.01% polyvinyl alcohol (PVA) to a 1 M zinc sulfate heptahydrate solution. 2 M NaOH was then added dropwise to the resulting solution, which was stirred for 18 h. After this time, a white precipitate formed, which was washed, dried, and calcined at 450 °C for 3 h. The resulting material was applied foliarly three times: during the seedling, vegetative, and fruiting stages. The results were compared with a conventional zinc fertilizer. Of the different concentrations studied, the treatment with 100 ppm of ZnO NPs improved yield (200%), physiological traits, and tomato quality.

Azim et al. [64] synthesized ZnO nanoparticles from an extract of Vernonia cinerea L. (family Asteraceae). First, a 0.02 M aqueous solution of dehydrated zinc acetate (CH₃COO)₂·H₂O was mixed with 80 mL of double-distilled water under continuous stirring. Then, 20 mL of the extract was gradually added, followed by a 2 M NaOH solution. The resulting solution was kept under stirring at a controlled temperature until it changed from white to milky white and then to pale yellow. Finally, the sample was washed and dried. These nanoparticles were applied to a tomato crop, and the results were compared with a conventional fertilizer (zinc sulfate). According to the authors of this manuscript, the nanoparticles synthesized from the extract significantly increased plant growth and development compared to conventional fertilizer, which, while offering good results, did not produce such significant improvements. Furthermore, it is emphasized that a concentration of 50 mg L⁻¹ yielded the best results and that higher doses negatively impacted photosynthesis and the antioxidant defense system.

Adil et al. [65] synthesized ZnO NPs using the precipitation method, where a 0.5 M NaOH solution was added dropwise to a 0.25 M ZnSO₄ solution, maintaining a reaction temperature of 65 °C and a pH of 5.5 until ZnO precipitates were obtained. These precipitates were collected, washed, and dried, yielding nanoparticles with an average size of 53.79 nm. The effect of these nanoparticles on the physiological parameters of wheat (Trilicum aestivum) under saline conditions was then determined, and their effect was compared with that of conventional ZnSO₄ fertilizer. According to the authors’ reports, the nanoparticles increased chlorophyll a and b content, plant height in the vegetative and maturity stages, shoot and spike length, fresh and dry root weight, and consequently, grain yield. Although conventional fertilizers improve the physical parameters of wheat under saline conditions, ZnO NPs have more significant effects, making them a better option for increasing crop productivity.

4. Characterization of ZnO NPs

Characterizing ZnO NPs is crucial for understanding their crystalline structure, properties, and potential applications in agriculture. The characteristics of nanoparticles, including particle size, morphology, crystallinity, and chemical composition, significantly influence their properties. Therefore, it is essential to gather comprehensive information using various techniques, which are briefly outlined below:

4.1. X-Ray Diffraction (XRD)

X-ray diffraction is a non-destructive characterization technique that enables the analysis of the microstructure of a variety of materials, including minerals, polymers, plastics, metals, and ceramics [66]. XRD analyses are widely used to obtain technical information on crystalline structures, lattice parameters, imperfections, crystallographic orientations, and the degree of crystallinity in a sample. Additionally, they serve to confirm the formation and purity of crystalline phases [67]. The fundamental principle of X-ray diffraction relies on the constructive interference of monochromatic X-rays with crystalline samples. These X-rays are generated by a cathode ray tube, then filtered to produce monochromatic radiation, and subsequently collimated to focus and direct them toward the sample. When incident X-rays interact with the sample, constructive interference occurs provided the conditions satisfy Bragg’s law [68].

nλ = 2d sin(θ)

where n is the diffraction order, λ is the wavelength of the incident beam in nm, d is the interplanar distance in nm, and θ is the angle of the diffracted beam in degrees (the angle between the lattice planes and the incident beam) [69].

From the diffraction patterns obtained, it is possible to determine the unit cell size and symmetry, the crystalline phase present, and the location of atoms and defects in the crystal [70].

4.2. Scanning Electron Microscopy (SEM)

Electron microscopy represents a significant advancement in imaging technology, particularly with the use of Scanning Electron Microscopy (SEM). This technique allows for detailed surface and microstructural characterization of a variety of organic and inorganic materials at scales ranging from micrometers to nanometers. Modern SEM equipment can achieve impressive magnifications of 300,000× and even up to 1,000,000×, with resolutions between 2 and 4 nm [71,72]. SEM is commonly utilized to analyze properties such as surface quality, grain size, morphology, shape, and chemical composition [73,74]. Some SEM setups are equipped with Energy Dispersive X-ray Spectroscopy (EDS), which, when combined with SEM, offers qualitative and semi-quantitative analysis of the samples.

Both SEM and EDS can provide essential information on the chemical composition of the materials being studied [72]. Essentially, SEM operates by magnifying the surface structure of a sample using a beam of electrons that is generated in a vacuum and directed by electromagnetic lenses to create high-resolution images. These images represent the sample’s surface topography and are formed from electrons reflected by the interaction of the electron beam with the material [74]. This enables a detailed examination of the micro- and nanostructures present in the sample.

4.3. Transmission Electron Microscopy (TEM)

TEM analyses are a significant alternative for exploring structural, morphological, and chemical properties in materials that are smaller than 100 nm, and in some cases, these analyses can be conducted at the atomic scale. This form of characterization has seen extensive application in fields such as biology, materials science, and engineering [75]. It is particularly useful for observing crystalline defects and second-phase precipitates, and it uniquely enables the differentiation between linear and planar defects within a crystal [76].

The technique employs a collimated electron beam directed at a sample, which must not exceed a thickness of 500 nm to allow for adequate electron transmission. The interaction between the sample’s atoms and the incident beam produces an image, with its resolution being dependent on the wavelength of the electrons used [77].

Modern TEMs come equipped with X-ray detectors and electron spectrometers, which facilitate qualitative analysis of nearly all elements in the periodic table, except hydrogen and helium, achieving nanometric resolution and atomic sensitivity. Through energy-filtered electron imaging and X-ray mapping, researchers can gather detailed information regarding the composition of the sample [76]. TEM serves as an exceptionally comprehensive analytical tool for materials researchers, allowing for the analysis of elemental chemistry, local atomic bonding, dielectric constant, sample thickness, and band gap [76]. When it comes to nanoparticles, TEM is frequently utilized to ascertain their chemical composition, size, morphology, and crystallinity.

4.4. Fourier Transform Infrared Spectroscopy (FT-IR)

In recent decades, FT-IR spectroscopy has become a useful technique for studying biological molecules and complex systems like tissues and cells [78]. FT-IR spectroscopy obtains its results by analyzing the absorption, reflection, emission, or photoacoustic spectrum using the Fourier transform [79]. This technique allows for the simultaneous analysis of multiple frequency components quickly, making it faster than traditional spectrophotometers [80]. FT-IR is a type of vibrational spectroscopy. It provides information about the structure and molecular environment of a sample. The technique works by shining infrared light on a sample. This light is absorbed, creating vibrations that correspond to the sample’s molecular structure. The result is absorption bands with specific frequency and amplitude for each sample [78]. For nanoparticles, FT-IR spectroscopy helps identify functional groups and molecular structures on their surfaces. It also examines how they interact with biological entities [81].

4.5. Ultraviolet-Visible Spectroscopy (UV-Vis)

UV-Vis spectroscopy is an economical, easy, adaptable, and non-destructive technique used for the analysis of organic compounds and some inorganic compounds. This method is predicated on the interaction between light and matter, resulting in the absorption, emission, or scattering of incident electromagnetic radiation. These interactions can be detected at wavelengths ranging from 200 to 780 nm [82]. UV-Vis is extensively utilized for monitoring chemical reactions in which reactants and products display distinct absorption characteristics. Additionally, it serves to confirm the presence of specific analytes, their forms, and their concentrations within a substance [83]. In essence, this technique involves a light source passing through a sample, with a detector positioned on the opposite side to record the transmitted light. From this transmittance, the absorption of the sample at each wavelength can be ascertained [84]. In the context of nanoparticles, UV-Vis characterization is often employed to study concentrations in chemical loading and release systems [85]. For nanoparticles with plasmonic surfaces, this method can also provide insights into their optical properties, which are influenced by particle shape and size [86].

5. Mechanism of Action in Plants

Zinc is an essential micronutrient for plant growth and development, acting as a cofactor for more than 300 proteins. Unfortunately, approximately 50% of soils used for cereal production currently exhibit deficiencies in this element, causing serious complications for agriculture and living organisms [87]. One strategy implemented to counteract zinc deficiency in soils and increase nutrient content in crops involves the use of nanoscale materials. These can be used in smaller doses, leading to better results.

5.1. Properties of ZnO NPs

After iron, zinc oxide (ZnO) is the second most common metal oxide. It is a safe, inexpensive, biocompatible, and easily obtained material, usually in the form of a white powder [48]. ZnO exists in three forms: (1) rock salt, (2) wurtzite, and (3) sphalerite. The first is very rare and occurs under high-pressure conditions, the second occurs at room pressure and temperature, and the third requires the presence of sulfur, low oxygen availability, and moderate temperatures (100–200 °C). From the above, it can be concluded that the most common form obtained from ZnO synthesis is wurtzite, which has a hexagonal crystal structure with lattice parameters a = 0.325 nm and c = 0.521 nm [1,88]. In the hexagonal unit cell, O² anions occupy the cell vertices, while Zn²⁺ cations are in alternating tetrahedral cavities, that is, each anion is surrounded by four cations at the tetrahedron vertices [1,89].

ZnO nanoparticles can be synthesized using various methods, resulting in particles with distinct sizes, shapes, and properties. Some of the shapes that can be achieved include nanospheres, nanorods, nanoplates, nanocages, nanowires, nanotubes, nanorings, and nanoflowers [90]. Among their important properties are their ability to reflect ultraviolet rays, have anti-inflammatory properties, mild anti-itch properties, high thermal and mechanical stability, and antimicrobial activity, among others [8,89,90]. Their properties have led to diverse applications in various areas, including materials science, the cosmetics industry, drug delivery, antimicrobial agents, solar cells, optoelectronic devices, product additives, batteries, pigments, food, agriculture, and more [1,8,90]. Crops are now being fertilized with these nanoparticles.

5.2. Interaction of ZnO NPs and Possible Mechanisms of Action and Transport

ZnO NPs can serve as nanofertilizers in crops. They are typically applied either foliarly or directly into the soil, and their accumulation within plants is influenced directly by the concentration at which they are exposed. However, the precise significance of root and leaf structures in this process has not yet been clearly established [91]. In the case of foliar applications, nanoparticle absorption can occur through various pathways, including stomata, transport proteins, endocytosis, and trichomes. At the root level, they can penetrate through root tissues, lateral root development zones, and areas of root damage [92]. The entry and transport of nanoparticles are influenced by several factors, such as size, shape, surface characteristics, exposure duration, and plant species. Once inside the plant, these nanoparticles can move through mechanisms like diffusion, mass flow, and phloem loading [93].

It is important to mention that although all factors are significant, particle size is the primary determinant for penetration into plant tissues. The cell wall acts as a barrier to the entry of nanoparticles into plant cells, presenting a pore diameter that varies between 5 and 20 nm depending on the plant species. NPs that fall within this size range can cross the cell wall, reaching the plasma membrane and subsequently being transported to various tissues through symplastic transport. This ability to penetrate and move within the plant is crucial for the effective delivery of nutrients and other beneficial compounds provided by ZnO NPs [94].

According to Sharif et al. [95], the band gap of ZnO NPs can vary based on factors such as particle size, crystallinity, and surface defects. This band gap is closely linked to the release of divalent Zn²⁺ cations, particularly under acidic or neutral pH conditions. When the band gap is narrow (less than 3.37 eV), it is typically associated with a higher density of defects, lower lattice stability, and an increased release of Zn²⁺ ions. Conversely, a wider band gap (ranging from 3.6 to 4.0 eV) correlates with greater crystallinity and a slower, more controlled dissolution of the nanoparticles. Thus, band gap measurements can provide valuable insights into the potential behavior of ZnO NPs in agricultural soils, helping to predict how they might interact within the environment.

Most plants obtain their nutrients through their roots, and Zn is absorbed more efficiently when it exists in the form of the Zn²⁺ ion. Typically, once Zn²⁺ ions are present in the soil, they move to the roots via mass flow and diffusion and are subsequently transported through the xylem. The absorption of Zn²⁺ from the soil solution is actively enhanced by specific membrane transporters located in the root, which facilitate its passage across the plasma membrane and its distribution into the cytoplasm [96].

6. Effects of ZnO NPS on Plants

Zinc is a critical micronutrient for plants, involved in numerous cellular functions including enzyme activation, metabolic processes, physiological functions, and maintaining ionic balance. Despite its importance, more than half of the world’s soils are deficient in zinc, which adversely impacts plant growth [97]. Traditionally, these zinc deficiencies have been addressed through the application of conventional fertilizers, such as urea, triple superphosphate, diammonium phosphate, single superphosphate, mono-ammonium phosphate, and nitrogen-phosphorus-potassium fertilizers. While these fertilizers provide essential nutrients, their efficiency remains below 30% [98]. Furthermore, the high quantities typically applied can lead to significant environmental issues, including leaching, volatilization, and eutrophication, along with broader concerns related to pollution, climate change, and economic ramifications [98].

In light of these challenges, recent advancements have introduced ZnO NPs as a viable alternative for targeted crop fertilization. Due to their nanometric size and unique surface characteristics, these nanoparticles can be more readily absorbed and utilized by plants. The use of ZnO NPs as nanofertilizers has been shown to enhance nutrient concentration within plants, thereby improving growth outcomes and resulting in higher yields and improved fruit quality [99].

Recent studies have demonstrated the positive effects of nanofertilizers, particularly zinc oxide nanoparticles (ZnO NPs), on various crops. For instance, Wang et al. [93] reported that using ZnO NPs as a foliar nanofertilizer on rice plants resulted in increased yields by enhancing grain weight and the number of spikelets. Additionally, the zinc content in the grains improved, indicating better fruit quality [97].

Subba et al. [100] evaluated the antifungal properties of ZnO NPs against Fusarium equiseti, a fungus affecting tomato plant leaves. Their results showed that after seven days of application, fungal growth was suppressed by 77.6% to 85.1% when using ZnO NPs at concentrations of 750–1200 ppm. This suggests that ZnO NPs could be an effective solution for controlling fungal diseases in crops.

Sharma et al. [101] utilized ZnO NPs derived from Cassia occidentalis leaf extract to enhance the germination process of rice seeds (Oryza sativa). Their findings indicated over a 50% increase in plant dry weight, water uptake, and root length compared to treatments with zinc sulfate and hydropriming. They also observed improvements in plumule length and soluble sugars.

Moreover, Umar et al. [102] applied ZnO NPs to corn crops, resulting in increased zinc levels in both shoots and grains. This application not only enhanced corn growth and yield but also significantly boosted the grain’s zinc content by 82%. While the benefits of ZnO NPs are evident across diverse crops, it is essential to note that their use can also lead to negative effects in certain instances. Careful consideration and further research are necessary to fully understand the implications of nanofertilizer applications in agriculture.

7. Toxicity and Environmental Impact

7.1. ZnO NPs on Soil

Soil functions as a solid ecological matrix comprising natural colloidal materials and nanoparticles. It serves as the primary sink for nanoparticles released into the environment, especially when compared to water and the atmosphere. The interactions between soil and metallic nanoparticles constitute a complex phenomenon that can significantly impact the ecosystem. Once these nanoparticles come into contact with the soil, they can be absorbed and transported to various locations through runoff and leaching. Furthermore, the degradation products of these nanoparticles can affect a wide array of biotic and abiotic processes [103].

In addition, ZnO NPs have a strong affinity for soil colloids and demonstrate higher sorption capacity compared to Zn²⁺ cations. Nonetheless, various soil properties, such as pH, texture, organic matter content, structure, degree of compaction, and the microbial community, significantly influence the bioavailability of these nanoparticles [103,104].

Although Zn is an essential element for plants, repeated and prolonged addition to soil as a fertilizer or through soil remediation technologies can lead to its accumulation over time [105]. This accumulation, particularly in soils, has detrimental effects on food safety, phytotoxicity, soil organisms, and agricultural productivity. Contaminated soils with elevated Zn levels can pose risks to human health and the ecosystem, primarily through direct ingestion or contact with the contaminated soil, thereby impacting the soil–plant–human or soil–plant–animal–human food chain [106].

In plants, excess Zn can accumulate in the leaves, causing necrosis on the leaf surface, which negatively affects growth and leads to potential health issues [92]. Thus, while Zn plays a vital role in plant development, careful management is necessary to avoid the adverse effects associated with its accumulation in agricultural settings.

It is essential to note that while nanoparticles can have an environmental impact, crops are inevitably exposed to various nanoparticles. This exposure arises not only from the direct use of fertilizers but also from their pervasive presence in our daily lives, found in vehicle exhaust, industrial emissions, incineration processes, accidental leaks, and more [92]. Therefore, synthesizing these nanoparticles efficiently and using them thoughtfully could help mitigate their environmental impact.

7.2. Toxicity in Plants

Ecosystem studies often treat nanoparticle impacts as if they were exclusively human-made. However, according to Handy et al., natural nanoparticles have existed since ancient times, with evidence found in glacial ice cores that date back about 10,000 years. This indicates that natural nanoparticles are far more numerous than their manufactured counterparts [107]. Zinc, an essential microelement, plays a crucial role in plant growth and development. Typically, most crops contain zinc values ranging from 30 to 200 µg Zn g⁻¹ dry weight [108].

Zinc is present in rocks and soils primarily as the divalent ion Zn²⁺, which can be readily absorbed by plants or can form complexes with organic matter. Due to its relatively low concentration in soils, zinc often serves as a limiting nutrient for a variety of crops [91]. Although zinc toxicity is uncommon in plants, research primarily focuses on alternatives to increase zinc levels in crops. However, high levels of this trace element can be toxic to soil and lead to structural and functional abnormalities in plants [89]. Typically, when a plant is exposed to excessive zinc, it exhibits red pigmentation on the petiole and veins, growth suppression, weakened structural integrity, and chlorosis, as the surplus zinc interferes with iron uptake [109,110].

Boonyanitipong et al. [110] studied the effects of ZnO NPs on rice (Oryza sativa L.) seed germination and found that high concentrations of nanoparticles (100 mg/L) adversely impacted the development of rice roots, leading to reductions in both root length and root number. Rajput et al. [104] further emphasized that the toxicity of zinc oxide nanoparticles is affected by their morphological characteristics, specifically their shape and size. Notably, significant concentrations of these nanomaterials are present in marine and estuarine sediments, with elevated levels of zinc detected in sediment water compared to other elements. This finding raises concerns regarding potential toxicity to aquatic species. Nevertheless, it is crucial to evaluate contamination levels in soils and sediments to accurately assess the associated risks [104].

Akanbi-Gada et al. [111] investigated the impact of ZnO NPs on tomato plants (Solanum lycopersicum L.) using different concentrations: 300, 600, and 1000 mg of ZnO NPs per kilogram of soil. The findings indicated that root absorption of zinc increased with higher concentrations of the nanoparticles. Moreover, it was noted that ZnO NPs induced oxidative stress and reduced the activity of enzymes responsible for mitigating stress in the roots. Additionally, the levels of total phenols, flavonoids, β-carotene, and lycopene in the fruit were significantly diminished by at least 4.8%. ZnO NPs are increasingly found in the environment from various sources. While their use in agriculture typically yields positive effects for crop treatment, it is essential to consider that the unregulated application of these nanomaterials could adversely affect plants and ecosystems [112].

The presence of excess nanoparticles can disrupt various physiological processes in plants, including nutrient absorption, oxidative processes, and genetic regulation. High concentrations of these nanoparticles can interfere with the transport of electrons in mitochondria and chloroplasts, leading to an increase in reactive oxygen species (ROS) [94]. It is important to note that plant cells are equipped with a natural antioxidant defense system comprising enzymes such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferase (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR). These enzymes work to minimize, buffer, and eliminate excess ROS. When ROS levels are elevated, they can alter protein structures, induce lipid peroxidation, damage nucleic acids, and potentially lead to apoptosis or necrosis in plant cells [113,114]. Furthermore, while low concentrations of ROS can activate tolerance responses in plants treated with nanoparticles, high concentrations are detrimental and can harm plant health.

8. Future Trends and Conclusions

There are various methods for synthesizing ZnO NPs, each capable of influencing properties such as size and shape. The synthesis method chosen is closely tied to the characteristics of the ZnO NPs, meaning that their properties are dependent on the method employed and vice versa. Selecting the appropriate synthesis technique is crucial based on the desired outcomes. ZnO NPs have shown promise as alternative fertilizers in crop treatment, yielding positive effects such as fungal inhibition, enhanced seed germination rates, increased mineral content in the edible portions of plants, and improved yields, among other benefits. However, it is important to note that high concentrations of these nanoparticles can lead to toxicity issues that may adversely affect plant growth and development. Therefore, further research is necessary to fully understand the mechanisms behind these nanomaterials and their true effects.