Introduction

Particles with nanometers sizes in at least one dimension are commonly known as nanomaterials (NMs) [1, 2]. Due to their small size, NMs are used in many fields, such as life sciences, aerospace, electronics, chemical production, and agriculture. Many inorganic nanoparticles are employed as NMs, for example, carbon nanotubes are usually used as carriers; Iron nanoparticles are widely used due to their magnetism; Silica nanoparticles have attracted research attention for their abundant pore structure and large specific surface area. Other studies have focused on copper, gold, or silver nanoparticles [3]. Polymers and liposomes are renewable, biodegradable, and environmentally friendly, usually used as organic carriers to encapsulate pesticides [4, 5]. These materials are also called nanostructured materials (NSMs) or engineered nanomaterials (ENMs), and most applications are concentrated on efficiency and productivity enhancement.

Roco et al. were the first to note that NMs can be used for drug delivery [6]. A National Planning Workshop of America proposed the use of NMs in agriculture, mainly for rapid detection of pesticide residues and food packaging [7]. Subsequent research on the application of NMs has involved various aspects of agrochemicals such as pesticide formulations [8, 9], pesticide residues analysis [10, 11], fertilizers [12, 13], plant growth regulators (PGRs) [14], and development of bio-pesticides [15, 16], including nucleic acid pesticides and pheromone. The application of NMs in agrochemicals is shown in Fig. 1. In 2019, IUPAC selected ten chemical technologies most likely to impact human society in the future [17], and nanopesticides were ranked first for their potential low impact on the environment and human health. Benefits such as increased penetration [18], coverage [19] and uptake of active ingredients (AIs) [20] on the target in the presence of NMs will also occur for all agrochemicals with nanotechnology applications in agriculture [21]. Nano-agrochemical delivery system via NMs have great potential for improving the efficacy of agricultural inputs, alleviating environmental pollution, and saving labor costs, which contributes to the maintenance of the sustainability of agricultural systems and the improvement of food security. NMs will have prospective future in agriculture [22].

Fig. 1
figure 1

Nanomaterials loaded with various agrochemicals. Multiple release modes of active ingredients by modulating the structure of the loading system (Particle, Fiber, Layered system, Porous based, Capsule, Emulsion). The small size effect and large specific surface area of nanoparticles can adhere to the target as much as possible to improve the bioavailability of active ingredients

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Although NMs have promising applications in agrochemical delivery [23, 24], academics have also raised some concerns [25], mainly in terms of the unknown health risks for human beings and regulatory difficulties brought by the small size effect of NMs. The complexity and cost of manufacturing technology also limit the large-scale use of NMs in agriculture [26]. In this paper, we illustrate the recent advances and challenges of NMs applied as plant protection agrochemicals.

Nanomaterials for pesticide and fertilizer applications

Pesticides and fertilizers are the most widely used agrochemicals in agricultural production [27], and NMs were the earliest to be applied to these two agrochemicals which are also the most researched and reported. Constructing pesticides and fertilizers delivery system through different strategies could improve their utilization and reduce the run-off to environment, which contributes to alleviating environmental pollution and negative impact on the non-target organism caused by excessive use of agricultural inputs.

Nanomaterials for the establishment of pesticide formulations

Pesticides play a crucial role in defending against biological disasters and promoting crop productivity. Most AIs of pesticides are lipid-soluble [28]. Preparing AIs into liquid formulations requires numerous organic solvents; emulsifiers and other additives are also essential. These requirements can trigger overuse of organic solvents, posing a threat to the environment and human health. Solid formulations need solid emulsifiers and many inert materials as fillers, and they are widely used due to their easy storage and transportation. However, dust pollution is caused in the process of production and application, because of lacking effective protection measures, which is prone to harming non-target organisms as well. Wettable powder (WP) and emulsifiable concentrate (EC) are main pesticide formulations in conventional pesticide products [29, 30], but their actual AIs utilization of biological target uptake is less than 0.1% after dust drift and rainwater leaching [31].

NMs used in pesticide production mainly participate in establishing nanopesticide formulation systems to improve the bioavailability of AIs (Fig. 1), detailed reasons as follows (Fig. 2).

Fig. 2
figure 2

Nanomaterials loaded with pesticides. NMs loaded with pesticides can reduce the degradation of AIs caused by environmental influences, achieve target-controlled release of AIs by the properties of nanomaterials, thus prolonging the shelf life of pesticides; Some NMs have their own insecticidal and fungicidal activities; When applied to soil, NMs help pesticides to be fixed in the soil for plant root absorption in order to reduce the loss of pesticides by leaching

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NMs improve the stability of AIs. AIs that easily undergo photolysis and are easily decomposed can be stabilized under the protection of NMs. Avermectin (AV) is an excellent bio-pesticide that has been widely used, but it is sensitive to ultraviolet (UV) light and has a very short half-life (6 h) [32]. Zhu et al. grafted AV onto the long chain of poly (ethylene glycol)-carboxymethyl cellulose (PEG-CMC), and the obtained nanoparticles improved the stability of AV significantly over the control, and the degradation rate of AV was less than 50% after 72 h of illumination [33]. Another approach is using NMs as a protective shell for AIs. Kaziem et al. encapsulated AV in hollow mesoporous silica, then anchored cyclodextrin in the outermost layer of the particles [70]. NMs now are specifically applied as bio-fertilizer; trace element fertilizers such as Fe and Zn; major element fertilizers, including N, P, and K; medium element fertilizers, for example, Si, Ca, Mg, and S. NMs are also used in organic fertilizer. In this section, we present an overview of the application of NMs for each of the fertilizer types as above.

Table 1 Nanomaterials applied to fertilizer and their functions
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Djaya et al. developed a composite embedding process for graphite and silica composites to encapsulate the plant growth promoting rhizobacteria (PGPR) and endophytic bacteria to obtain a novel biocontrol delivery system (BDS) [71]. The efficacy of the BDS was evaluated by measuring the diameter of inhibition zone against R. solanacearum culture. The results showed that compared to control, inhibition zone of the treatment of Lysinibacillus sp. was up to 10.24 times wider, and treatment of Bacillus sp. was 12.6 times respectively, which indicated the ability of BDS to form synergistic effect and inducing antagonistic interactions.

Benzon et al. found that compared to conventional fertilizer use only, combining nano-fertilizer with conventional fertilizer significantly improved the agronomic parameters of rice [72]. The plant height, chlorophyll content, number of reproductive tillers, panicles and spikelets were increased by 3.6%, 2.72%, 9.10%, 9.10% and 15.42%, respectively. Panicle weight, total grain weight, total shoot dry weight and harvest index were also improved by 17.4%, 20.7%, 15.3%, and 2.9%, respectively. Furthermore, the authors determined that the performance of using only nano-fertilizer was not noticeable and suggested using nano-fertilizer in conjunction with conventional fertilizer to achieve good results.

Haydar et al. evaluated the biological effects of iron oxide nanoparticles as well as EDTA-functionalized iron oxide nanoparticles as a nano-micronutrient fertilizer in place of conventional iron fertilizers [73]. The effect of these fertilizers on agronomic trait parameters of mulberry (Morus alba L.) was estimated in a pot experiment. The application was carried out by both foliar spraying and soil application, and iron oxide nanoparticles and their EDTA functionalized forms were applied at two different concentrations (10 and 50 mg/kg soil). The results showed positive effects on the agronomic trait parameters of the treated plants. Iron oxide nanoparticles in the soil at a concentration of 10 mg/kg significantly improved the morphological characteristics of the plants, such as germination rate, leaf number (52.73% higher than the control), plant biomass (37.20% and 90.24% increase in shoot and root biomass, respectively, compared to the control), root properties (34% increase in root length), and also reduced the time to germination of the first leaves. The same treatments increased by 42% and 15%, respectively, compared to the control for chlorophyll and sugar content. The activity of antioxidant enzymes such as catalase, peroxidase and NADPH oxidase was also increased compared to the control. All results showed that iron oxide nanoparticles could replace the traditional iron fertilizer in the cultivation and propagation of mulberry crops.

Raliya et al. developed nano zinc fertilizer to increase the yield of pearl millet (Pennisetum americanum) [74]. In this study, the fungus Rhizoctonia bataticola TFR-6 (NCBI GenBank Accession number JQ675307) was cultivated in potato dextrose (PD) broth medium. The filtrate of fungus-free organism cells was isolated, which was used to prepare an aqueous zinc oxide solution with a concentration of 0.1 mM. The synthesis of nano zinc was performed by exposing the former zinc oxide solution to the fungal cell-free filtrate previously prepared for 62 h. The dynamic light scattering (DLS) results showed that the particle size of the synthesized nanoparticles ranged from 15 to 25 nm and had spherical with clear particle interfaces and crystalline structures as seen by TEM. Fertilization with the nano-fertilizer applied to 6-week-old pearl millet showed the following results: stem length (15.1%), root length (4.2%), root area (24.2%), chlorophyll content (24.4%), total soluble leaf protein (38.7%), plant dry biomass (12.5%), acid phosphatase (76.9%), alkaline phosphatase (61.7%), phytase (322.2%) and dehydrogenase (21%) activities were significantly higher than the control. The application of Zn nano-fertilizer increased the yield of the crop at maturity by 37.7%.

Li et al. found via field and indoor experiments that the long-term use of pesticides in tea gardens would cause the plants to accumulate excessive amounts of ROS, leading to oxidative damage in the plants and affecting the nutritional quality of tea [75]. In contrast, selenium nanoparticles activated the plant antioxidant system, resulting in significant increases in the content and activity of antioxidant enzymes in the ascorbic acid-glutathione cycle, which helped minimize pesticide induced oxidative damage. Biofortification with selenium significantly increased the accumulation of nutrients and functional substances such as proteins, carotenoids, tea polyphenols, and catechins in tea products. Metabolomic and transcriptomic studies have shown that selenium nanoparticles can induce theanine, glutamate, proline, and arginine production by regulating the glutamine-glutamate cycle. Nanoselenium exogenous supplementation promotes secondary metabolism in tea plants, which in turn increases the biosynthesis of total phenols and flavonoids in tea leaves. Nanoselenium promotes the uptake and conversion of selenium in the soil, leading to a significant increase in the production of selenium-rich teas with many key aroma components.

Alimohammadi et al. evaluated the effect of urea and Nano-Nitrogen Chelate (NNC) fertilizers on yield of sugarcane (Saccharum Officinarum) and nitrate leaching from soil [76]. Treatments included five different N contents of urea and NNC (0, 80, 112, 137 and 161 kg N ha−1). Results showed that the mean values of soil nitrate concentrations were 10.2 and 12.8 mg/kg for urea and NNC treatments, respectively, during sugarcane growth. The nitrate leaching level was high with urea fertilizer (699.0 mg l−1) and low with NNC fertilizer (183.0 mg l−1). This study also found that the application of NNC was significantly effective in increasing sugar content in sugarcane. Considering nitrate leaching and its impact on human health and the environment, NNC is potentially valuable.

Rajonee et al. prepared phosphorus and potassium compound nano-fertilizer using zeolite as a carrier material, and characterized and evaluated fertilization efficiency [77]. Material composition measured by X-ray diffraction (XRD) showed that the modified zeolite's peak position and height changed compared with unmodified zeolite, indicating that the K, P fertilizer element was successfully loaded into the zeolite and the nano-fertilizer was prepared successfully. An in vitro incubation study with the application of nano-fertilizer showed that the release of potassium was most prominent throughout the experiment and all experimental units showed the same trend to different degrees. The potassium content in the soil was maintained at a higher level with the nano-potassium fertilizer compared to control. Even though nano-fertilizer has promising performance, its large-scale implementation is limited by high cost.

Chen et al. revealed that CaSO4 reduced phosphorus loss from farmland through soil incubation experiments [78]. CaSO4 is a typical soil conditioner with strong complexation to orthophosphate. Compared to conventional crude CaSO4, nano calcium sulfate provides a larger surface area, higher solubility, and improved integration of fertilizer and soil, which further reduces soil phosphorus loss.

Ahanger et al. presented a nano-vermicompost organic fertilizer with particle sizes ranging from 1 µm to 200 nm after air-drying and grinding [79]. The influence of this fertilizer on the growth of tomatoes under drought stress was studied, and the results showed beneficial effects on growth, oxidative stress parameters, antioxidant, nitrogen metabolism, osmotic accumulation and mineral elements under drought stress, which demonstrated its beneficial contribution in preventing drought-mediated damage to tomatoes.

Currently, NMs in fertilizer are mostly reported from validation experiments illustrating that NMs can be used as a fertilizer directly or as auxiliary materials to indirectly improve fertilizer utilization. The lack of systematic theoretical studies on interactions and material exchange among NMs, plant root systems, root physical and microbial environments, and plant systems hinders the development and implementation of nano-fertilizer formulations.

The reported mechanisms by which NMs act as fertilizer or fertilizer synergists to affect plant growth can be summarized as the following aspects: First, reducing fertilizer particle size to nanoscale is expected to improve the solubility and dispersion potential of fertilizer nutrients. Furthermore, due to the small size effect of NMs, they have unique potential to easily pass through the membrane barriers of plant cell walls, thereby offering altered uptake kinetics of the applied nano-form fertilizers [80]. Lastly, NMs protect fertilizer by encapsulating it to reduce its volatilization, fixing it in the soil through NMs and reducing its leaching losses, thus improving its utilization and reducing environmental damage. NMs and nanotechnology in fertilizers will effectively promote fertilizers' absorption and utilization, thus improving the agronomic parameters of crops. It can also reduce the environmental pollution caused by the volatilization and leaching of chemical fertilizers.

New applications of nanomaterials in agrochemicals

In recent years, because of the environmentally friendly control strategies advocated by Integrated Pest Management, the rapid rise of organic farming, and the increased awareness of environmental protection and food safety, many eco-friendly agrochemicals, such as bio-pesticides have received lots of attention, NMs are subsequently applied to these chemicals [81].

Nanomaterials for bio-pesticide applications

The ecological and health threats associated with applying chemical pesticides are increasingly concerning. According to the reports of Tang and Tackenberg et al., in the past few years, bee numbers have decreased markedly and trace pesticides have been detected in 75% of the world's honey, particularly neonicotinoids, including imidacloprid, thiacloprid, and thiamethoxam. With this background, some environmentally-friendly agricultural solutions for pest control that pose little risk to humans and beneficial microorganisms have gained more attention [82, 83].

Compared with chemical pesticides, bio-pesticides are more eco-friendly, and they have become more common for pest and disease control in plants. Currently, the market share of bio-pesticides is increasing, with the following categories [84].

  1. (1)

    Microbial pesticides, developed using living microorganisms, specifically subdivided into bacterial pesticides, such as Bacillus thuringiensis (Bt), B. subtilis, and B. cereus; and fungal pesticides, such as Beauveria bassiana (Bals.), Metarhizium anisopliae, (Paecilomyces lilacinus (Thom.) Samson), and Verticillium.

  2. (2)

    Viral pesticides, including nuclear polyhedrosis virus (NPV), granulosa virus (GV), and genetically engineered rod-shaped virus.

  3. (3)

    Plant-derived pesticides are natural products in plants, some of which have insecticidal and fungicidal effects, such as EOs, bittersweet, pyrethrins, indocyanin.

  4. (4)

    Agricultural antibiotics are microbial secondary metabolites produced by bacteria, fungi, actinomycetes, and other microorganisms in the fermentation process, which can be used to control agricultural pests. Examples include AV, bupropion, erythromycin, and liuyangmycin.

  5. (5)

    Biochemical pesticides, such as vinblastine, erythromycin, and gibberellic acid.

  6. (6)

    Plant immunity elicitor-inducing antibacterial agents, which allow plants to fight against pests and diseases by enhancing their immune function, such as chitosan and activating proteins.

Bio-pesticides also include natural enemy pesticides, such as Trichogramma dendrolimi (Matsumura) and T. chilonis (Ishii).

However, bio-pesticides have their own drawbacks. For example, microbial pesticides and antibiotic pesticides are easily affected by external environment factors such as temperature, humidity, and light, making the AIs unstable. Using NMs as carriers is one way to solve the stability and utilization issues of bio-pesticides effectively (Table 2).

Table 2 Nanomaterials applied to bio-pesticide and their functions
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When Bt forms spores accompanied by the production of insecticidal crystal protein, which is a protein of length 130–140 kDa, it needs to be solubilized in an environment with a pH > 9.5 to be activated and then become toxic. The midgut of lepidopteran larvae is a strong alkaline environment, and when the larvae intake Bt, it is activated and releases 60–70 kDa of toxin, killing the target. Therefore, the smaller the particle size of Bt, the better its performance. Usually, top-down approaches of microionization are employed to make Bt in the nanoform. These approaches, including jet milling, ball milling, and high pressure homogenization, could convert the coarse powder to ultrafine powder, and are easy to scale up and give reproducible results [85]. However, the balls used in the milling may produce contamination of the final product, and heat generation during high speed milling process could reduce the viability and efficacy of Bt. Therefore, an increase of using NMs to load Bt is observed. The preparation of NMs loaded Bt could not only obtain ultrafine formulations, but also improve the stability and shelf life of biological pesticides in the environment. Zaki et al. studied the application of NMs in Bt formulations and investigated a complex of nanotubular sodium titanate and Bt as a novel nano-pesticide against cotton leaf-worm. The preparation was carried out by mixing the NMs sodium titanate and Bt in water at a mass ratio of 1:2, sonicating for 10 min, then stirring with a magnetic stirrer, and finally drying [86]. The bacterial nanocomposites showed an improvement in insecticidal activity against cotton leaf worm, Spodoptera littoralis and a stronger effect on the different biological features of pest than bacteria alone.

In order to address the poor stability and low utilization of Bt, excipients were also added to prevent settling of suspensions; preservatives were added to extend shelf life; and for powders, additives were applied to improve their wetting and spreading properties. UV shielding agents were also used to prevent rapid photolysis after spraying. These improvements resulted in a sixfold increase in the utilization of AIs [87].

Beauveria bassiana (Bals.) is a fungal microbial pesticide, usually prepared with vegetable oil and mineral oil as dispersion media to obtain oil suspensions for use. B. bassiana is susceptible to decomposition by UV irradiation, so Hersanti et al. used silica nanoparticles (Silica Nps.) and carbon fibers as carriers for B. bassiana to control potato Spodoptera litura (S. litura) [88]. Silica Nps. and carbon fibers can also be used as fertilizers to provide nutrients to plants while protecting them from biological stress. A total of 16 treatments (control treatments were included) were designed to assess the effect of applying different concentrations of the mixture B. bassiana + Silica Nps. + carbon fiber on the mortality of S. litura. The results showed that the application of the mixture of B. bassiana + Silica Nps. 5% + carbon fiber had the highest mortality rate (53.33%) on S. litura larvae, while the single B. bassiana treatment had a mortality rate of 6.67%.

Plant-derived pesticides are also important bio-pesticides, and currently, EOs and neem-derived products are the most widely used [89]. EOs, an important natural source of pesticides, are mostly lipophilic and interfere with insects' normal metabolic, biochemical, and physiological reactions, which eventually affect their behavior. NMs applied to plant EOs show a synergistic effect on EO bioactivity. Cinteza et al. mixed chitosan-coated nanosilver with EOs in specific ratios, and the physical and chemical properties and biological activity of the system remained stable after mixing. In addition, a synergistic effect from the antibacterial activity test was observed, and the mixture was effective against a wide range of microorganisms with minimal side effects [90].

NMs are applied in antibiotic bio-pesticides to raise the stability and utilization of AIs. AV is the most widely used antibiotic-based bio-pesticide. As noted earlier, NMs improved the anti-UV ability of AV, increased the biocompatibility of the AIs, and improved its utilization.

In addition, microbial pesticides and chemical pesticides can be loaded together through NMs to form a dual-functionalized system, which has a synergistic effect on the bioactivity. Cui et al. used water–oil-water double emulsion method combined with high-pressure homogenization technology to prepare PLA nanocapsules containing validamycin and thifluzamide (VTNC) for the control of rice sheath blight [91]. The VTNC shown an average diameter of 265.3 ± 0.8 nm and polydispersion index of 0.034 ± 0.030, and these spherical particles were well dispersed, smooth in surface, and equal in size. The bioactivity evaluation results of VTNC against Rhizoctonia solani. showed that dual-functionalized system was significantly better than that of the commercial formulations and a clear synergistic effect between the two AIs was observed.

Gibberellic acid (GA) is an important biochemical pesticide, and is relatively widely used. GA stimulates plant growth, such as promoting the formation of male flowers [92]. Hafez et al. used layered double hydroxides (LDH) as a potential carrier for a GA formulation [93]. GA@LDH nano-formulation was synthesized using the memory-effect property of LDH. TEM showed a hexagonal flake-like shape with a lateral dimension ranging from 10 to 20 nm. The GA@LDH nano-formulation promoted plant growth with a significant increase in stem length over the control at the same concentration, and also showed better bioavailability and improved safety compared to the control. This study highlights a potential nanohybrid formulation form of GA using the memory effect property of the LDH clay material.

As a plant immunity elicitor, chitosan stimulates plant growth, induces disease resistance in plants, and produces immunity and inactivation against various fungi, bacteria, and viruses [94]. Asgari-Targhi et al. synthesized chitosan/zinc oxide nanocomposite (CS-ZnONP) and evaluated their biologically in vitro conditions [95]. The nanocomposite was able to improve plant growth and biomass more than bare ZnONPs use. Plants with CS-ZnONP increased chlorophyll (51%), carotenoids (70%), proline (twofold) and protein (about twofold) concentrations than with bare ZnONPs. Adding CS-ZnONP to the seed medium increased enzymatic antioxidant biomarkers (catalase and peroxidase). The activity of phenylalanine ammonia-lyase showed a similar significant upward trend. CS-ZnONP treatment greatly enhanced the accumulation of alkaloids (60.5%) and soluble phenols (40%). Micropropagation experiments showed that CS-ZnONP treatment promoted organ development. Overall, CS-ZnONP can be considered as a highly effective biocompatibility elicitor.

The integrated system of pesticides and fertilizers constructed with NMs and nanotechnology can achieve the co-delivery of pesticides and fertilizers, which is of great potential in reducing labor cost and giving full play to the synergistic effects of fertilizers and pesticides [96].

Nanomaterial-mediated nucleic acid pesticides system

Nucleic acid pesticides are based on the introduction of exogenous nucleic acid fragments into the target, which interferes with the normal transcription of the target gene, resulting in the death of the target. The more studied are RNA pesticides. RNA pesticides are used to protect crops with RNA interference (RNAi) technology. RNAi delivers double-stranded (ds)RNA /small interfering (si) RNA to insects, silencing genes and affecting normal genetic coding, ultimately killing the pests (Fig. 4.) [97]. RNA-pesticides work through the initiation, effect, and cascade amplification of RNAi by Dicer-like (DCL), Argonaute(AGO), and RNA-dependent RNA polymerase (RdRP). The RNA-induced silencing complexes (RISC) assembled by functional small RNAs and AGO enzymes are the minimal functional units of gene silencing [98]. DCL participates in cleavage and processing of small RNA precursors (dsRNA or microRNA precursors, etc.) to form mature small RNA sequences [99]. AGO is implicated in the recognition and selective binding of small RNAs to promote the formation of functional RISC complexes and mediate target sequence degradation or translational repression [100]. RdRP is mainly associated with the generation and replication of secondary small RNAs, further enhancing and amplifying the silencing effect [101]. Therefore, functional small RNAs are the core elements of biologically mediated RNAi-dependent gene silencing, and DCL, AGO, and RdRP are critical for maintaining the generation, transmission, and amplification of RNAi-mediated gene silencing signals [115]. Attraction test showed that the nanoporous carrier exhibited better elicitation performance than commercial product under the same environment. Correia et al. also developed a composite membrane for the slow release of pheromones using poly (butylene adipate‑co‑terephthalate) and activated charcoal, which could protect the AI from climatic factors and prolong the shelf life of the pheromone [116]. The results of diffusion experiment showed that the volatility of pheromone rhynchophorol was 100% within 24 days for the control sample, while the volatility of composite membrane-coated rhynchophorol was only 45% after 65 days.

Fig. 5
figure 5

Nanomaterial loaded pheromone. Traditional pheromones products are unstable and easy to volatilize, resulting in short shelf life and unsatisfactory "Attract-and-kill" effect; Nanomaterial-loaded pheromones with environment friendly properties can realize slow-release, protect from decomposition under ambient conditions, and show excellent efficacy in the open orchard

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Bhagat et al. prepared pheromone methyl eugenol (ME) nanogels using low-molecular mass gelators (LMMGs)-all-trans tri (p-phenylene ethynylene) bis-aldoxime [117]. The preparation was based on the self-assembly by weak intermolecular non-covalent interactions (e.g., hydrogen bonding, π-π stacking and van der Waals) of LMMGs, which leads to specific solvent gelation. The gel system was a three-dimensional nanoscale supramolecular network structure, providing high pheromone retention capacity. In addition, the nanogel was insoluble in water, which protected ME from decomposition caused by light, temperature and humidity in the environment, thus prolonged the shelf life. The perpetration did not require the use of any additives, such as crosslinkers and antioxidants, so it was an environmentally friendly agrochemical. The results of field trap** tests showed that in the luring test of B. dorsalis, treatment 1 (ME alone) and treatment 2 (ME in nanogel) were basically equal in the first three days, while from the 8th day, treatment 2 showed a more effective effect than treatment 1. Therefore, nanogel immobilized ME was an effective agent for the control of B. dorsalis.

Future challenges in the development of nano-agrochemicals

NMs are still being explored for applications in agrochemicals, and safety assessment, production cost, evaluation standards, registration polices and public concern issues must be addressed. A critical assessment of the impact of nanoparticles is necessary before they are determined to be safe to use [118].

Environmental safety assessment

There are concerns about the potential safety risks of NMs to non-target organisms as well as to humans. Juárez-Maldonado et al. summarized that the direct interaction of NMs with microbial cells is also quite complex and may cause specific changes in microbial cell physiology and gene expression that can lead to proliferation of any kind of cell [119]; such risks may reduce the diversity and abundance of specific microbial groups in the soil [120]. Risk assessment of NMs for agrochemicals primarily remains in the laboratory stage and the real scenarios of application do not have enough risk assessment. Various application environments may lead to variable results. For the design and predict the environmental fate of the nanoscale agrochemicals, Zhang et al. proposed to integrate existing nutrient cycling and crop productivity models with nanoinformatics approaches to optimize targeting, uptake, delivery, nutrient capture, and long-term effects on soil microbial communities to design nanoscale agrochemicals that combine the best safety and functional properties [121]. Kah et al. suggested that nano-agrochemicals should be extensively tested for effectiveness and environmental effects under field conditions [122]. Further work needs to be done to evaluate nano-agrochemicals to assess the advantages and new risks of existing products in a reasonable manner.

Production cost, evaluation standards and registration policies

Though nano-enabled agrochemicals may offer a range of benefits, they are still in the early developmental stage. Several companies have deposited patents comprising numerous protocols for production and application of nano-products, whereas, the commercial nanotechnology-based products is still relatively limited in quantity and needs further development. According to the Nanotechnology Products Database available at Statistic Bank (StatNano), there are 9420 nanotechnology-based products are commercialized, only 229 nano-products launched for agriculture application [123]. Most of these products are nano-fertilizers (43%), and nanoformulations aimed at improving plant survival against disease, pest and other stress represent together 28% of these products. The high production costs and complexity of the production process make the use of NMs in agriculture a low-margin industry that cannot generate significant economic returns on the initial production investment [124]. This is a major constraint to the application of NMs in agrochemicals on a large scale. Furthermore, there is a lack of uniform methods and standards among government authorities relating to NMs [125]. The existing methods of government regulators are not applicable or cannot correctly evaluate the safety risks of nano-pesticides, and there is an urgent need to study and establish corresponding evaluation methods and standards, for the large-scale application of NMs in agrochemicals. The pesticide registration authorities in the US and European countries have issued registration policies for nano-pesticides. The US EPA has used the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) to require the registration of nano-pesticides as “new” substances and to refine relevant registration policies. The EU member states register pesticides under the Plant Protection Products Regulation, which applies to the registration of single pesticides and mixtures, regardless of size, and this includes the registration of nano-pesticides [126].

The public's acceptance of new technologies also hinders the industrialization of NMs in agriculture to a certain extent, especially as agriculture is closely related to food safety and people are likely to be sensitive to the application NMs in agriculture.

Prospects for nanomaterials applied in agrochemicals

In this review, we discussed the application of NMs and nanotechnology in plant protection, including the use of NMs in constructing nano-pesticide formulation systems, nano-fertilizers, bio-pesticides, nucleic acid pesticides, PGRs, and pheromone. We also noted the challenges of industrial production of NMs for plant protection. Although nano-agrochemical delivery is at an early stage of development, it is expected that this application will improve the efficiency of agrochemicals and reduce environment pollution. More research is required to facilitate their large-scale application. In the future, the research focus of nano-agrochemicals are as follow; (1) competition with the conventional formulations in performance, cost and scale manufacture technology, especially in the prospect of commercialization; (2) establishment of new evaluation standards of nano-agrochemicals on product quality and safety; (3) precise controlled-release systems able to response to the changes in environmental stimuli such as pH, light, temperature, enzyme activity, etc.; (4) establishment of specific using guidelines and effective large-scale field application technology of nano-agrochemicals to facilitate their extension in agriculture.

Industrial production of nano-agrochemicals prepared by NMs and nanotechnology are influenced by many factors including product economics, environmental factors, evaluation standards, using guidelines and policy issues, but the benefits of NMs and nanotechnology applied in agricultural are likely to be monumental because it can improve the efficacy, accuracy and targeting of agrochemicals. In addition, the introduction of reasonable policies and popularization of the new technology among the public are also beneficial to promote its development.