New Advances in Nano-Enabled Weed Management Using Poly(Epsilon-Caprolactone)-Based Nanoherbicides: A Review

Abstract

The number of effective herbicides available to farmers is steadily decreasing due to increasing herbicide resistance. It seems very important to address and effectively deal with the main weed management challenges (low crop yield and environmental pollution) by investigating the potential of newly introduced materials, such as biocompatible polymer-based nanoparticles. The current review aims to encourage agricultural or environmental researchers to conduct new research on the synthesis and application of modified herbicides, such as nanoherbicides, for application in weed management and to provide a comprehensive foundation on the topic. Such nanosystems could help with the promotion of the controlled release of active ingredients and extend their action time, resulting in a reduction in dose and application number; improve the physical and chemical characteristics of the herbicide to increase foliar adhesion; prevent degradation that results from environmental factors (such as sunlight, temperature, microorganisms, or pH); and decrease herbicide leaching and contamination of the environment. Furthermore, it has been indicated that some polymeric nanocarriers can penetrate biological barriers, including membranes and plant cell walls, and translocate across vascular tissues, resulting in a more efficient delivery of active ingredients. Poly(epsilon-caprolactone) is a biocompatible material that is easily decomposable by enzymes and fungi. PCL nanoparticles could be applied as nanocarriers of herbicides in agriculture due to their low toxicity, their potential for large-scale synthesis from inexpensive materials, their ability to dissolve herbicides, their high loading capacity, and their ability to help minimize the chemical decomposition of herbicides.

1. Introduction

Weeds are one of the main factors threatening agriculture, and in vulnerable agro-ecosystems, they may endanger the entire harvest. Today, most herbicides on the market are designed to eradicate or control the weed plants’ above-ground portions. Rhizomes and tubers, which serve as propagation sources for new weeds during the evolving season, are active below-ground plant portions that are unaffected by any of these herbicides. In this regard, Field bindweed (Convolvulus arvensis L.), Canada thistle (Cirsium arvense L.), purple nutsedge (Cyperus rotundus L.), and Johnsongrass (Sorghum halepense L.) could be mentioned as some of the main problematic perennial weeds [1,2]. Compared to soils where weeds are controlled, soils that are infested with weeds and weed seeds are likely to produce lower crop yields. Hence, increased crop yield might be the result of enhanced herbicide efficacy brought about by nanotechnology [3]. The use of new technology in several aspects of agriculture, including the development of effective monitoring systems, smart chemicals, and gene delivery systems for crops, nanoherbicides, and nanoformulations, among many other applications, will transform the agricultural system. By reducing the effects of environmental contamination, it will boost productivity and decrease agricultural waste that arises indirectly. Therefore, it seems necessary to introduce nanotechnology into all agricultural systems, along with further research and field application [4,5,6]. Nanotechnology involves the manipulation or self-assembly of single atoms, molecules, or molecular clusters into structures to produce materials and products with novel or different features. One of the most exciting areas of science and technology in recent years is the study of the unique features of materials that arise at the nanoscale.

Nanoherbicides are created with the aid of nanosized preparations or nanomaterial-based herbicide formulations. Nanoherbicides are characterized as herbicide formulations based on nanomaterials that make use of the potential for effective chemical delivery to a target site. The widespread concerns about food and environmental contamination have been gradually brought on by the excessive and improper use of chemical herbicides. In comparison to conventional herbicides, formulations based on nanomaterials could increase the herbicide’s efficacy, increase its solubility, and decrease its toxicity. To increase bioavailability and improve weed eradication, herbicides are coated with nanomaterials (

Table 1. Some examples of frequently used nanomaterials in crop production systems.

Nanomaterials	Effect on Crop Production System	Ref.
Polymeric nanomaterials	Efficient release of agrochemicals; Outstanding biocompatibility; Reduce the effect on nontargeted organisms.	[8]
Silver nanomaterials	Boost plant growth; Act as anti-microbial property; Decrease in pesticide concentration while increasing its efficiency.	[9]
Nano-alumino-silicates	Improve the effectiveness of pesticides.	[10]
Titanium dioxide	Enhance chlorophyll content, nutrient uptake, activity of Rubisco, and antioxidant enzymes.	[9]
Carbon nanomaterials (graphene, graphene oxide, and carbon dots)	Enhance plant seed germination and vigor index, lower transport and leaching potential of pesticides in soil.	[9,11]
Metal-based nanoparticles	Improve percent germination, regulate plant uptake of phosphorus and nitrogen.	[9]

). Nanoherbicides are made up of tiny particles containing the herbicide’s active components and have a high affinity for their target due to their huge specific surface area. The wettability and dispersion of agricultural formulations are also improved by nanoherbicides. Some of the formulations used in nanoherbicides include nanoemulsions, nanocapsules, nanocontainers, and nanocages [7].

There is a necessity to create and develop nanoherbicides that can safeguard the environment and function as both subsurface and above-ground pretenders of weeds and that truly imitate the agricultural system. The development of a target-specific herbicide chemical enclosed in a nanoparticle is directed toward a specific receptor in the roots of the target weeds, where it enters the root system and translocates to areas that block the hydrolysis of food reserves. This can cause certain weed plants to starve and eventually result in their death [3,12]. As an example, Bombo et al. [13] developed a nanoencapsulated atrazine formulation that specifically targeted the chloroplast of Brassica juncea and, via the degradation of chloroplasts, showed herbicidal activity [13]. For the first time, we review the new advances in nano-enabled weed management in agricultural systems using poly(epsilon-caprolactone)-based nanoherbicides to explain the current state of knowledge and highlight the main methodologies and research techniques.

2. Weed Control System

Weeds are plants that establish themselves in addition to crops. Any plant, regardless of its economic value, can become a weed if it grows on cropland against the will of the landowner. Currently, there are about 2000 agricultural weeds in the world that reduce crop yields; release harmful substances into the soil; create intense competition for light, moisture, nutrients, and water; create shade; and serve as a breeding ground for most crop diseases and pests. Globally, weeds cause about a 31.5% reduction in crop production and result in economic losses of USD 32 billion per year [14]. Herbicides are most widely used as pesticides, which account for more than 40% of their total use, whilst insecticides and fungicides account for about 30% and 20%, respectively [15].

The toxicity of the herbicides used in agriculture depends on their chemical stability, photodegradability, solubility, bioavailability, and sorption in soil. There are several possible solutions to minimize toxicity, including the development of carrier systems capable of modifying the properties of a compound and providing controlled release. Controlled release provides a safer environment for pesticide use and minimizes the potential threat to the environment. At the same time, by reducing the amount of pesticide used, its effectiveness is increased [16]. The major interest of these nano-objects lies in the inherent properties (biodegradability, biocompatibility, and better stability) of their morphology and size. Indeed, a reduction in the size of various structures that are below the micrometer has allowed for new properties that are not observed at more conventional sizes to be highlighted. Aliphatic polyesters, such as poly (lactic acid) (PLA), poly (lactic acid-co-glycolic acid) (PLGA), or poly(epsilon-caprolactone) (PCL), are the most commonly used to develop active substance carriers. Due to their easily scalable chemical composition, the control of their molar mass distribution can be achieved, as well as the control of their architectures, which can also be obtained using appropriate macromolecular synthesis methods [17]. Nanoencapsulation has the advantages of safer handling and the more efficient use of pesticides with less environmental exposure.

3. Challenges in Weed Management

3.1. Herbicide Resistance

With the intensive use of herbicides, it has become clear that the effectiveness of these preparations has diminished over time due to the appearance of weeds, which are resistant to their effects. The number of efficient herbicides available to farmers is steadily declining as a result of herbicide resistance. According to Heap’s [18] recent surveys, there are currently 523 unique cases (species × site of action) of herbicide-resistant weeds globally, comprising 269 species (154 dicots and 115 monocots). Weeds have evolved resistance to 21 of the 31 known herbicide sites of action and to 167 different herbicides (reported in 99 crops in 72 countries) [18]. Herbicide resistance has a significant impact on weed control, especially following the development of weeds that are resistant to numerous herbicides with various modes of action (MOAs). Following the idea of the “pesticide treadmill”, growers then adopt other classes of herbicides to supplement or replace those that have decreased in efficacy due to the evolution of herbicide resistance. This has led to the process of the continuous use of new herbicides, or combining new herbicides, and the herbicidal resistance also posed by newer herbicides is a driver of the research on herbicidal systems [19,20,21]. Weeds can adapt to any anthropogenic activity, including herbicides [22], but if they are subjected to repeated and constant disturbances, for example, via the utilization of nanoherbicides, selection pressure can be induced. Weed management practices could prevent or at least delay resistance evolution, thus preserving valuable herbicide and transgenic technology.

3.2. Herbicide Residues

The violation of hygienic standards in the storage, transport, and massive application of herbicides in their translational forms leads to their accumulation in the environment, food raw materials, and food products, which presents a threat to humans and animals, specifically pollinators. Based on their application modalities, between 10% and 75% of pesticides do not reach their targets, resulting in the frequent contamination of terrestrial and aquatic environments [15]. However, herbicides that act selectively on weeds can cause symptoms in crops. In addition, chemical components inhibit the biological components of the soil, for example, the bacteria, fungi, actinomycetes, algae, roots, and flagellates that live in it. They all participate in the formation of humus, and their disappearance inevitably leads to a deterioration in the nutritional quality of the soil [15].

The evolution of pesticides in the environment depends on their physicochemical properties, as well as on the pedoclimatic and topographic characteristics of the land. After application, they reach the soil, where they can be subjected to retention processes, transfer to groundwater, and transfer to the atmosphere via volatilization or erosion. Some pesticides such as organochlorines are persistent in soil, remaining from a few hours to several years. 2-Methyl-4-chlorophenoxyacetic acid was detected at 93% of 68 water sites surveyed along the Danube River by Loos et al. [23]. From a dust sampling campaign conducted in 2012 in 239 homes in the Rhône-Alpes Auvergne region, Béranger et al. [24] detected 125 distinct pesticides, at least once, among the 276 searched for. Several researches have indicated that glyphosate can adsorb to sediments, constituting a pathway for its dispersion in aquatic environments [25,26]. In Argentina, Ronco et al. [25] reported an average concentration of 0.6 µg/L of glyphosate in the water of the Paraná River, while the average concentration in sediment was around 742 µg/kg and 521 µg/kg for glyphosate and AMPA, respectively [25].

The resulting toxicological risks to humans are very high (death, endocrine disruption, birth defects, cancer, neurological disorders, immune disorders, etc.). The ecotoxicological risks can be alarming (destruction of earthworms, butterflies, frogs, etc.), leading to a decrease in soil fertility and agricultural yields (food insecurity). Moreover, African countries import less than 10% of the pesticides used in the world, but they accounted for half of accidental poisonings and more than 75% of fatal cases in a WHO survey [27]. Tillitt et al. [28] showed that a 30-day exposure of fathead minnow to 0.5 µg/L atrazine reduced total egg production at spawning by 25%. According to the authors, this decrease is due to an effect of atrazine on the oocyte maturation process in female specimens.

4. Current Weed Management Approaches

Agriculture has been around for over 10,000 years. Weed control has been around for about the same period of time, and the problem has not yet been radically solved [29]. Weeds are difficult to control; they are extremely simple, grow quickly, and reproduce easily and need only moisture from rainfall to grow successfully. Unlike crops, they put all their energy into surviving. Modern man’s arsenal includes several methods: chemical, biological, and agro-technical methods. Crop rotation is an important way to restore and maintain soil fertility and control weeds, pathogens (fungal and bacterial), and pests [30]. Weeds have peculiarities, including the ability to remain in the soil for years and a great potential to spread, which makes many methods less effective, including crop rotation [29,31].

5. Nanotechnology in Weed Management

According to previous research, only a small portion of herbicides is absorbed by plants. The rest is lost through one or more of the following ways: volatilization, adsorption, leaching, photodecomposition, chemical degradation, and microbial breakdown. The continuous use of herbicides results in the development of resistance in weeds toward that herbicide [18]. The science of nanoherbicide technology can be used as a tool to fabricate a slow-release nanoencapsulated pre-emergence herbicide for achieving season-long weed-free conditions without hampering the environment. Nanoherbicides are being developed to address the problems in perennial weed management and exhausting the weed seed bank. Encapsulation by nanomaterials can protect active ingredients from premature degradation and unnecessary losses.

Studies have shown that the nanoencapsulation of herbicides can produce more targeted and less toxic formulations for agricultural applications. Due to enhanced herbicidal activity in comparison with commercial formulations, the use of nanoencapsulated herbicides would allow for the application of lower dosages of the herbicide [32]. The use of lower doses of herbicides is desirable, as it reduces the long-term effects of the residues of herbicides in agricultural areas and their toxicity to the environment. Nanoherbicides can aid in the easy delivery of herbicides to weed plants, reducing the residual accumulation in soil [32].

Herbicides are coated with a semi-permeable membrane made of an organic or inorganic polymer during nanoencapsulation. The membrane system controls diffusion, ion exchange, and other mechanisms that allow the release of the toxicant [33]. Also, with the aid of nanotechnology, water chemicals can travel through the cracks created in weed seeds by the use of nano-carbon tubes, causing them to germinate quickly. As a result, this decreases the weed seed bank and interferes with the weed seed dormancy feature, which allows weeds to develop in wash out. Eliminating naturally occurring germination inhibitors in some weeds also improves germination. For instance, due to the breakdown of phenols and other germination-inhibiting biochemical components, the germination of purple nutsedge (Cyperus rotundus L.) tubers is improved when treated with nanoparticles of iron oxide [34] and zinc oxide [35]. When the glycolysis process is inhibited by herbicide-encapsulated nanoparticles, which includes silver nanoparticles in purple nutsedge (Cyperus rotundus L.), they target the receptors of weed roots and starve them by lowering their food supply.

Target-specific release is also helpful in killing weeds, without even interacting with crops, and ultimately results in a high crop yield. Nanoherbicides have the advantage that they can be further developed for the target site inhibition of the biochemical reactions of weeds [32]. Nanoencapsulations decrease herbicide buildup in soil and prevent the emergence of weeds that are resistant to them. This is because employing herbicides that are encapsulated in nanoparticles or using nanoparticles as herbicide carriers allows the active ingredient of the herbicide to be delivered directly to the target location of weeds. This lessens the likelihood of soil herbicide buildup. The extremely small dimensions of nanoherbicides allow them to blend with soil particles and prevent the growth of weed species that have grown resistant to traditional herbicides. Smart delivery systems reduce the amount of active ingredient needed. Moreover, the highly effective penetration of nanoherbicides into plant cells and direct transport into the metabolic system of the plant results in abnormalities in the targeted biochemical pathways [33,36].

In summary, using nanoformulations could potentially increase the stability, shelf life, bioavailability, environmental sustainability, and safety of the active ingredients for sustained release. The modification of surface or bulk properties at the nanoscale has a great potential for effective improvement in agricultural productivity. Some nanomaterials enhance the mechanisms of tolerance in plants under stress conditions. In this regard, these nanomaterials could be considered as effective promising tools for overcoming some of the main problems in sustainable agricultural production. Due to their unique qualities and applications, nanoformulated agricultural products are enforced in different areas [37]. Although a vast number of nanofabricated products are developed in the agricultural sector, most of them are still under investigation and have not been introduced into the market. Nowadays, there is a lack of specific regulations and legislations about risk assessments. Nevertheless, there are a few organizations actively generating new bodies for regulating and monitoring the safe application of nanoformulations. Many nanoformulated agrochemical products have been introduced to the market that could be beneficial for meeting the increasing demands of food. Therefore, the regulated use of nanotechnology following all safety and precautionary measures could revolutionize agriculture in the future [38].

6. Types of Nanomaterials for Assembling Nanoherbicides

Numerous studies have been conducted for the development of nanoherbicides due to their superior effectiveness and environmentally friendly advantages [39,40,41,42]. Hence, nanomaterials for assembling nanoherbicides can be categorized based on their purpose: they can be put together in a way to make water-insoluble active components more soluble, to gradually slow down how quickly they release their active ingredients, to complete targeted distribution, and to promote chemical permanence [43].

Based on the chemical nature of the materials, nanoherbicides have been grouped into three fundamental categories: (a) inorganic materials developed from metals, metal oxides, clay minerals, etc.; (b) organic materials developed from active ingredients that have been encapsulated in an organic nanocarrier derived from protein, polymers, lipids, etc.; and (c) hybrid nanomaterials encompassing both organic and inorganic materials [44,45,46]. Thus, nanomaterials for assembling nanoherbicides can be grouped into three types as discussed below.

6.1. Nanoherbicides Based on Inorganic Nanomaterials

Silica, metal–organic frameworks, mesoporous silica nanoparticles, clay minerals, and other inorganic materials can be used for the preparation of nanoherbicides. Some of these nanoherbicides can release ions, though some can enclose organic molecules and release them gradually [32,47,48,49,50]. For example, inorganic nanomaterials for the synthesis of nanoherbicides based on double-layer aluminum hydroxides or zinc [51], or sepiolite clay linked with magnesium–aluminum, have been extensively used for herbicide alliance since they may reduce herbicide leaching through soil and, hence, increase the transportation of the functional materials to the plant [52,53]. They can also aid the system in encapsulating hydrophobic herbicides among their layers and are used for combating Chlamydomonas reinhardtii Dang algae. Clay elements can possibly form nano-assembling herbicides since they can be biocompatible and inexpensive and have a pertinent potential for reduction [54]. Water-bearing silicates or aluminosilicates, such as kaolinite, montmorillonite, attapulgite, diatomite, and hydrotalcite, are the principal types of clay minerals. Clay minerals are often crystalline and capable of being altered by inorganic or organic cations and have a high specific surface area. These modified derivatives work well as adsorbents for a variety of organic chemicals, and their release into the environment can be regulated by their adsorption of active ingredients [55]. According to Bayat et al. [56], some metallic nanoparticles could have positive or negative effects on the seed germination or seedling growth of seeds based on different parameters, like the type of species and metal and the concentration of the metal [56]. In cases with a negative effect, it seems interesting to study the inhibiting effect of metallic nanoparticles on the germination of weed seeds.

Recently, metal–organic frameworks (MOFs) have also been employed to regulate the release of pesticides, which has become a research hotspot. Metal–organic structures are continuously repeating periodic systems made of porous crystalline substances with metal ions or clusters as the center atom and one or more organic ligands. Since MOFs have distinct benefits over other materials, including huge surface areas, customizable apertures, and a variety of structural options, they are widely used in a variety of industries [57,58]. The types of organic molecules and metal ions utilized and the techniques for connecting them are very specialized, but the composition of MOF materials defines the flexible diversity of their metal ions and ligands. As a result, the structures and properties of MOF materials are quite varied. Organic ligands that are highly stable are carboxylic acids, which are often employed in the manufacture of MOF materials. Almost all metal elements, including transitional elements and lanthanide metals, are included in the range of metal centers that is employed. Copper, iron, and zinc are often and extensively used [59,60]. Likewise, mesoporous silica nanoparticles (MSNs) have strong adsorption performance because of their large areas of contact, tunable pores, and modified interfaces, as well as being biocompatible and environmentally friendly. Research on MSN preparation has made considerable strides since Kresge published his original findings [61]. Pesticide-loaded MSNs have been created, including those with avermectin, pyromamine, hepazole, amino acids, and prochloraz. The physical adsorption approach can be used to create pesticide-loaded MSNs, which are best absorbed and transported by plants. To provide MSNs with new performance characteristics, activated groups can be added to the hydroxyl and unsaturated keys on the surface of the MSN through branch aggregation, silicide coupling, etc. The continuous release of modified MSNs into the release medium prevents light-induced degradation of the active components [62,63]. Furthermore, MSNs are used as herbicide transporters due to their reaction to pH and strong electrostatic connections [49,64].

6.2. Nanoherbicides Based on Organic Nanomaterials

Organic nanomaterials are exceptional materials for the synthesis of nanoherbicides, and they can be built on polymers, synthetic organic materials [65,66], lipids, lignocellulosic materials, proteins, and complex macromolecules such as dendrimers [67,68,69,70,71]. Generally, various techniques have been reported to create nanoherbicides, but the nanoemulsion method is the most often used method [72,73,74,75]. Polymers are broadly used in nanoherbicides for preparation because of their biodegradability, affordability, and biocompatibility [76,77].

A type of natural polymer is chitosan, a natural deacetylation of chitin that displays good characteristics like biodegradability, biocompatibility, and environmental friendliness. Chitosan can be utilized as a bacteriostatic material to control plant diseases and pests, and they can be formulated with pesticides to prepare nanopesticides [78,79,80]. Liang et al. [81] prepared chitosan nanomaterials that were loaded with avermectin using an ion-crosslinking method; it was found that these microcapsules showed excellent slow release, which efficiently enhanced the time of retention of avermectin and, hence, enhanced its photostability [81]. Also, sodium alginate is a conventional anionic polymeric polysaccharide that can be cross-linked with polyvalent metal cations. Sodium alginate was used to create calcium alginate hydrogels that had Lentinus edodes. Good environmental sensitivity was proven in the release from the hydrogels, and L. edodes released more often at higher pH values, temperatures, and Na⁺ concentrations. This effect can increase plant resistance to most viruses and encourage plant development [41]. Synthetic organic materials as compared with natural polymers have a noticeable advantage as herbicide-assembling materials, because they have great chemical and physical stability, alkali resistance, erosion resistance, and acid resistance. Allowing for the type and quantity of the surface functional groups to be changed enables highly focused and adaptable applications [82]. The use of a star cationic polymer (SPc) to form nanoscale pesticides enhanced virulence against aphids disease. The use of an SPc nanoassembly system to release cyanobenamide was evaluated, and it showed selective toxicity against the pest western flower thrips and predators [83]. It can be concluded that the use of nanomaterials for assembling nanoherbicides can be of great importance in the agricultural production system.

6.3. Nanoherbicides Based on Organic/Inorganic (Hybrid) Nanomaterials

Herbicides with hybrid nanomaterials, used for assembling nanoherbicides, have the ability to integrate the benefits of two or more materials, such as organic and inorganic materials, into a single structure. These versatile nanomaterials can have a wide range of characteristics, dimensions, morphologies, and chemical makeups. Additionally, hybrid nanoherbicides can support strong traceability, targetability, and stimulus responsiveness qualities [32]. Due to their biocompatibility, biodegradability, natural abundance, and ease of functionalization, biomass-based hybrids made of lignin, xylan, starch, and cellulose have been investigated for their potential to encapsulate active molecules and can be used in the targeted assembling of herbicides [84]. Jiang et al. [85] found that the fabrication of hybrid xylan–lignin nanoparticles gave nanomaterial amphiphilic characteristics and formed a core–shell structure. Owing to the anti-microbial properties of copper salts or copper nanoparticles, lignin-based derivatives can also be teamed with copper to create antibacterial and antifungal compounds [86,87]. Furthermore, nanohybrids containing copper oxide nanoparticles have strong antibacterial activity, besides potential as weed-controlling substances [88]. To date, a large number of biogenic metal-based nanoparticles have been synthesized [89], which are recommended to be applied in this regard.

7. PCL Polymer as an Ecofriendly Nanocarrier for Herbicides

Suitable nanocarriers could be chosen from polysaccharides like cellulose and decomposable artificial polymers (e.g., poly ε-caprolactone) with good biocompatibility and low toxicity. A large number of bacteria and fungi are able to decompose these kinds of eco-friendly materials. PCL has attracted considerable interest in biological applications as the matrix of nanocarriers due to its good biocompatibility and biodegradability. For instance, a PCL-containing pretilachlor pre-emergent herbicide showed that the PCL nanoencapsulation decreased the toxicity of the herbicide due to a reduced effect on chromosome aberration in Allium cepa [90]. Furthermore, a considerable reduction in the phytotoxic accumulation of atrazine in soil was observed, as herbicide activity improved via the reduced mobility of atrazine [91].

Poly ε-caprolactone (PCL) (Figure 1) is a synthetic aliphatic polyester and a semicrystalline polymer that is not soluble in water and is harmless to the environment. It is obtained from the polymerization of ε-caprolactone cyclic monomer, is an affordable polymer, is nearly stable in the absence of catalytic species, is a very versatile compound, and could be synthesized in different forms of micro- and nanostructures [13,92].

Since the polymer is biodegradable and biocompatible, it seems to be used in agricultural applications as a sustained-release system. It takes about 2 to 3 years to complete the hydrolytic degradation of PCL polymers. The degradation process of PCL could be considered as a bulk process in two phases: a loss of molecular weight of up to 5000 Da as a result of chain scission and the onset of polymer weight loss. The PCL degradation kinetic patterns are consistent with autocatalyzed patterns, in which free carboxylic acid (-COOH) end groups catalyze the hydrolysis process—the scission of additional ester groups [93].

As an example, Takeshita et al. [66] reported the potential of PCL nanoparticles for the encapsulation of atrazine using the nanoprecipitation method. Compared to non-nano-atrazine, this nanocarrier system, with sustained atrazine release, showed decreased toxic effects on the algae Pseudokirchneriella subcapitata, the Allium cepa model, the fish Prochilodus lineatus, and human lymphocyte cell cultures [66]. Due to this fact, PCL polymers do not show phytotoxic activity and do not affect plant structures, and they have a great potential for application in delivering active ingredients to the mesophyll of leaves [13].

8. Classical Methods for Preparation of PCL-Based Nanocapsules

Nanoprecipitation (interfacial deposition or solvent displacement) is a method developed by Fessi et al. [94] and is one of the most widely used techniques for the fabrication of nanoparticles (Figure 2). Compared to other methods, this method is less expensive, easy to perform, reproducible, and does not require a precursor emulsion like other methods, and both nanocapsules and nanospheres can be produced by using this method [95,96]. Indeed, in the literature, more than 50% of the nanoparticles used for drug delivery are prepared via nanoprecipitation [97]. This process occurs in two phases and requires the use of three basic materials: one polymer and two miscible solvents (a polymer solvent and a polymer non-solvent). The first step, or the organic phase, consists of solubilizing the polymer (polycaprolactone) in an organic solvent (notably acetone), possibly containing the active substance to be encapsulated. The second phase is the aqueous phase, or continuous phase, which consists of adding the polymer solution drop by drop and under moderate stirring to a second solvent that is miscible with the first one and the non-solvent of the polymer (most often water). The latter usually contains at least one surfactant, allowing for the stabilization of the formed particles. The spontaneous diffusion of the solvent in the aqueous phase leads to the precipitation of the polymer and the formation of solid nanocapsules [94,96].

The commonly used polymers in this approach are biodegradable polyesters, and PCL is highlighted among them because of its biocompatibility. Another aspect of biodegradable polymers is their capability to control the release of active ingredients, which arises from their high permeability. The nature and concentration of the compartments affect the physicochemical properties of polymeric nanocapsules, and changes in the properties of nanocapsules can influence their application [96].

The emulsion–solvent diffusion technique was developed by Leroux et al. [98] (Figure 3). It differs from the previous processes by the extraction step of the organic solvent, where evaporation is replaced by a diffusion step in a large volume of water. This process therefore involves an organic solvent that must have a non-zero miscibility with water [95]. The subsequent addition of water after the emulsification step causes the diffusion of the organic solvent into the aqueous phase (because of its partial miscibility with water) and, finally, the formation of particles via the precipitation of the polymer. The residual organic solvent can eventually be removed from the aqueous phase via evaporation at a reduced pressure. Nevertheless, the evaporation step has no influence on the size of the particles because they are already formed as a result of the diffusion of the entire organic phase constituting the emulsion in the large volume of water [99]. Thus, this technique is particularly interesting, especially with regard to its extrapolation to an industrial scale.

9. PCL-Based Nanoherbicides

Considering the failure of translational use and alternative herbicides in weed control, PCL nanoparticles are set up for the sustainable use of bioactive compounds, including the triazine class (ametryn, atrazine, and simazine), polycaprolactone, and metribuzin (

Table 2. Evaluating herbicidal activity of herbicide-loaded PCL nanoformulations.

Active Ingredient	Studied Subjects	Main Results	Reference
Triazine class (ametryn, atrazine, and simazine)	Preparation and characterization of three nanoherbicides; stability assessment; in vitro release kinetics studies; evaluation of genotoxicity on Allium cepa.	Encapsulation efficiency greater than 80%; stable solutions over 270 days. Reduced genotoxicity of ametryn, atrazine, and simazine on Allium cepa; reduction in the dose of application; increases in their absorption by plant; controlled-release mechanism is relaxation of polymer chains.	[101]
Atrazine	To modify surface of PCL-atrazine nanocapsules with chitosan and investigate the effect of coating on physico-chemical properties of the nanocapsules.	With addition of chitosan, the zeta potential of nanocapsules shifted from negative to positive amounts, improving their adhesion to the target substrate.	[102]
	Preparation and characterization of nano-PCL-atrazine and evaluation of its herbicidal activity on Brassica sp. and its genotoxicity on Allium cepa.	Increased herbicidal activity of atrazine; reduced mobility of atrazine in the soil; important root development; reduced genotoxicity (chromosomal aberration) in Allium cepa.	[91]
	Preparation and characterization of nano-PCL-atrazine and evaluation of herbicidal activity on Brassica juncea.	Average particle size of 240.7 nm; 10-fold increase in efficacy of commercial atrazine in controlling mustard plants.	[42]
	Analyzing the effect of PCL-atrazine on growth, physiological, and oxidative stress parameters of maize plants (Zea mays L.) grown in soil.	PCL-atrazine with concentration of 0.1 mg/mL was effective for weed control; no effects were detected, even shortly after application. Pre- and post-emergence treatment with PCL-atrazine and PCL resulted in no development of any macroscopic symptoms in maize leaves and no impacts on shoot growth.	[103]
	Preparation and application of nano-PCL-atrazine and evaluation of its herbicidal activity in field against Amaranthus viridis and Bidens Pilosa.	Increase in the efficiencies of atrazine (more than 50% for both species compared to control with 40% herbicidal efficiency); 10-times-diluted concentration (200 g/ha) of nanoherbicide also showed the same results as commercial one.	[40]
	PCL-atrazine nanocapsules in post-emergent control of an atrazine-tolerant weed, sourgrass (D. insularis), in greenhouse.	Faster and greater inhibition of sourgrass photosystem II activity and greater enhancement in dry weight for nanoformulation-treated plants (compared with commercial herbicide).	[104]
	Pre-emergence activity of atrazine via nanoencapsulation with PCL against Bidens pilosa; residual effects of nano-PCL-atrazine and conventional atrazine on soybean plants after different periods of soil treatment.	Higher seedling mortality of B. pilosa in soil treatment with nano-PCL-atrazine than in soil treatment with atrazine, even after a 10-fold dilution; greater short-term toxicity effects of nano-atrazine than atrazine, but similar intense toxicity of nano- and non-nano-atrazine in a long-term treatment of soil on soybean.	[105]
	Morphoanatomical changes in mustard (B. juncea) leaves based on the foliar uptake of nano-PCL-atrazine in a post-emergent treatment; phytotoxicity and nanoparticle uptake.	Nano-PCL-atrazine stuck to the leaf surface, penetrated mesophyll, and transported through the vascular tissue into the cells and degraded the chloroplasts causing herbicidal activity.	[13]
	Comparing the effects of nano-PCL-atrazine and pure atrazine at different concentrations on defense mechanisms, physiological responses, and nutrient displacement in lettuce (Lactuca sativa) as a non-target plant.	In short-term exposure, the growth inhibition of nano-PCL-atrazine was similar to that of atrazine; in long-term exposure, high concentrations of nano-PCL-atrazine had greater negative effects on the end points of ROS production, protein content, and alteration in enzyme activities; nano-PCL-atrazine and atrazine differently affected displacement of nutrients, such as, Cu, K, and Fe, for plant growth.	[106]
	Preparation and characterization of nano-PCL-atrazine; to study nanoherbicide–leaf relationship and effects of this system in field and greenhouse on mustard and understand nanoherbicide’s mode of action using radiometric techniques.	Nanocapsule size about 200–300 nm; increased efficiency of atrazine uptake by mustard leaves (40% increase); a 50% reduction in atrazine rate for post-emergence control of R. raphanistrum plants under greenhouse and field conditions; increased inhibition of photosystem II (PSII) activity; improvement in the distribution of the herbicide in the plant; two-fold higher weed control in field compared to conventional formulation.	[66]
	Preparation of nano-PCL-atrazine and evaluation of its effect on alveolar epithelial human lung cells.	Nano-PCL-atrazine was more toxic to human lung cells than atrazine or PCL nanocapsules.	[107]
	To study post-emergence herbicidal efficiency of PCL-atrazine (at 200 g a. i. ha−1) against Alternanthera tenella Colla in comparison to other weed species.	Nanoformulation showed higher inhibition of maximum quantum efficiency of photosystem II (up to 39%) than commercial atrazine with same concentration.	[108]
Metribuzin	Synthesis of nano-PCL-metribuzin and using it in control of Portulaca oleraceae; evaluation of its toxicity on Allium cepa for pre-emergence applications in soybean.	Particle size of 150–250 nm; encapsulation efficiency of 83.2%; low vertical movement of nanoherbicide in soil (leaching); increased stability of metribuzin; increased herbicidal activity on purslane; nanoherbicide caused less plant chromosome aberration than non-encapsulated metribuzin.	[90]
	Preparation of nano-PCL-metribuzin and application on control of Ipomoea grandifolia; evaluation of behavior of nanoherbicide in 3 types of soil; comparison of the environmental fate of nanoherbicide with that of commercial metribuzin.	Nanoparticle size of 195 ± 35 nm; encapsulation efficiency of 74.8 ± 0.5%; higher efficiency of nanoherbicide, even at the lowest dose of 48 g a.i. per ha; nosuppressive effects on soil enzymatic activities; lower retention in soil than its commercial analogue; no difference was found in the half-life of metribuzin.	[109]
	Trace nano-PCL-metribuzin in different soils; to investigate mobility and retention dynamics of PCL-metribuzin in comparison with conventional formulation.	In deep soils containing fresh organic materials, nano-PCL-metribuzin was sorbed more than commercial formulation (14.61 ± 1.41% and 9.72 ± 1.81%, respectively).	[110]
Polycaprolactone	Preparation of nanoformulation and using it on barnyard grass; to study its effects on rice as anon-target plant); evaluation of its genotoxicity effect.	Particle size 70–200 nm; encapsulation efficiency of 99.5 ± 1.3%; upon genotoxicity experiments, nanoherbicide was less toxic than commercial herbicide; nanoherbicide had no negative effect on rice plant, but a significant effect on barnyard grass.	[111]
Ametryn	Ecotoxicological evaluation of PCL-ametryn and triazin class of herbicides; evaluation of their toxicityto aquatic organisms and in cytogenetic tests employing human lymphocyte cultures.	The nanoformulations showed lower toxicity than the commercial ones.Nanoherbicides resulted in lower toxicity to the algae and higher toxicity to the microcrustacean than the herbicides alone. The cytogenetic tests showed that the nanoformulations were less toxic than conventional herbicides.	[112]

). In fact, PCL is very valuable for the encapsulation of herbicides, especially atrazine herbicides, due to their chemical composition, which is easily modulable; their biodegradable properties; their biocompatibility; their distribution control; the high colloidal stability of their molar masses; and the control of their architectures [100]. Currently, nanoprecipitation and emulsion–solvent diffusion methods are used for the production of PCL nanoherbicides, such as nano-PCL-atrazine and nano-PCL-metribuzin, which are discussed below.

9.1. Metribuzin-PCL Nanoherbicide

Metribuzin is a pre-emergence and post-emergence herbicide used in potatoes, carrots, asparagus, lavender, and lavandin and in the seed stocks of carrots, alfalfa, wheat, etc., to control broadleaf weeds and grasses. Belonging to the same class as atrazine, metribuzin is also an inhibitor of the photosynthetic pathway, photosystem II (PSII), by binding to the QB binding site of the D1 protein. Metribuzin is released to the environment through surface runoff after spraying crops (especially within two weeks of soil application), drainpipe effluent, accidental discharge, or spray drift. It is likely to reach groundwater via leaching or to be carried into surface waters.

However, there is a lack of information about nanoherbicide behavior in environmental matrices. In a study conducted by Takeshita et al. [109], a PCL-metribuzin nanoherbicide with an encapsulation efficiency of 74.8 ± 0.5% was prepared, and its stability, its loss in different soils, and its effects on soil enzymatic activity was evaluated over time. The control effects and physiological parameters of the nanoherbicide were investigated on Ipomoea grandifolia plants. There were no differences in the half-life of the nanoherbicide compared to a commercial herbicide formulation. With an encapsulation efficiency of 74.8 ± 0.5%, no suppressive effects on the enzymatic activities of soil were observed. The nanoherbicide showed good pre-emergence in weed control, even at the lowest dose of 48 g a.i. ha⁻¹, indicating that the nanoherbicide had a higher efficiency than the commercial formulation in inhibiting PSII activity and reducing pigment levels. Moreover, the mobility of the nanoherbicide was not significantly increased, indicating a low risk of contaminating groundwater [109].

Takeshita et al. [110] followed this study by tracing a PCL-metribuzin nanoformulation in different soils to investigate its mobility and retention dynamics in comparison with those of a conventional formulation. Various soil systems and soils containing fresh organic materials were used in the experiments. For the mobility analysis, soil thin-layer chromatography was applied, and for the analysis of sorption–desorption patterns, the batch method was applied, combined with radiometric methods. The retention was reversible in all soil systems, according to the sorption parameters for both the nano- and non-nanoherbicide formulations (H~1.0). In deep soils containing fresh organic materials, nano-PCL-metribuzin was sorbed more than the commercial formulation (14.61 ± 1.41% and 9.72 ± 1.81%, respectively) (p < 0.05). The authors suggested that environmental safety relies on the maintenance of the organic material in a soil system [110].

In another relevant study, Diyanat and Saeidian [90] reported the synthesis of a PCL- metribuzin nanoherbicide with a particle size of 150–250 nm and an encapsulation efficiency of 83.2%. They applied this nanoformulation for the control of purslane (Portulaca oleracea L.) and evaluated its safety by evaluating the chromosome aberration in onion, Allium cepa, and cells for pre-emergence applications in soybean crops. The nanoherbicide was more efficient in weed control than its commercial formulation, even at doses lower than those recommended for commercial formulations. The authors also reported that the encapsulation of metribuzin resulted in less vertical movement of the herbicide in the soil. Finally, they concluded that the nanoencapsulation of metribuzin in PCL reduces the negative effects on the environment [90] (Table 2).

9.2. Atrazine-PCL Nanoherbicides

Atrazine is the most widely used herbicide for encapsulation purposes in studies on PCL-based nanoherbicides (Table 2). Atrazine nanoherbicides have weed control efficacy, even with lower doses of the active substance. Atrazine is an organochlorine herbicide belonging to the triazine group. It is an organic compound actively used mainly as a weedkiller for corn, as well as for other crops such as sorghum and grapes [18]. It is still in demand in many countries because of its high killing power and low cost [113]. However, its use has been banned in the European Union and Switzerland because it is a chronic water pollutant, and it has low biodegradability [113,114]. Since the ban of atrazine in several areas, weed management in corn has become complex, as the cost of alternative methods of this pillar of weed control has increased. In addition, alternative chemical herbicides, such as terbuthylazine, metolachlor, and alachlor, used to replace atrazine are not very effective, are found in high concentrations in water, and do not have better environmental profiles than atrazine [115,116]. Over 70 species, including Lolium rigidum, Raphanus raphanistrum, Abutilon theophrastis, and Amaranthus palmeri, are estimated to be resistant to triazine [18,117]. The nanoencapsulation of the herbicide may introduce alternative herbicides with reduced risks to the environment and improved efficiency.

Grillo et al. [101] showed that PCL polymer nanocapsules containing triazine herbicides (amethrin, atrazine, and simazine) can serve as useful modified herbicide delivery systems (Table 2). The encapsulation efficiency of the herbicidal compounds was greater than 80%. The controlled release of the herbicides from the nanocapsules was mainly regulated by the relaxation of polymer chains. The nanocapsule formulations containing herbicides were less toxic to onions than free herbicides [101]. From the research results, it can be concluded that the use of these nanocapsules can improve the use of herbicides in ecological systems. In summary, encapsulation by PCL allows for a reduction in the doses of the ametryn, atrazine, and simazine applied and increases their absorption by the plant, which increases the biological efficiency of these herbicides and reduces their accumulation and mobility in the environment, the emergence of resistant strains, and their genotoxicity on non-target organisms by limiting the leaching, volatilization, and degradation of these active compounds [101].

Grillo et al. [102] continued their research by coating PCL-atrazine nanocapsules with chitosan in order to modify their surface and investigated the effect of the coating on the physico-chemical properties of the nanocapsules. According to the herbicide release kinetic profile, the mechanism of release could be explained by a combination of both the relaxation of the polymeric chain and non-Fickian diffusion. Alterations in the dynamics of electrostatic interactions between the polymeric chains of the PCL-atrazine and chitosan could describe the results. Interestingly, a greater size and polydispersion were obtained for the nanocapsules coated with a low concentration of chitosan. For the nanocapsules coated with higher concentrations, the size and polydispersion were smaller. Furthermore, with the addition of chitosan, the zeta potential of the nanocapsules shifted from negative to positive amounts, improving their adhesion to the target substrate. The coated nanocapsules had a lower association affinity, but they may have better interactions with the long-chain hydrocarbons, acids, alcohols, and triterpenes of waxy plant cuticles. This will result in an increased efficiency of absorption by the leaf [102].

By using the nanoprecipitation technique, PCL nanoparticles containing the herbicide atrazine were prepared by Pereira et al. [91] and characterized and evaluated for their herbicidal activity and genotoxicity. The PCL/atrazine nanoparticles had a diameter of 408.5 ± 2.5 nm and showed a high herbicide encapsulation efficiency (92.7 ± 1.2%), as well as good colloidal stability that was maintained for 90 days. In vitro monitoring showed that the encapsulated herbicide had no effect on non-target organisms (corn) in the medium but was highly effective on Brassica spp. compared to commercial atrazine. This confirms that encapsulation improves the bioavailability of atrazine while reducing its vertical mobility. Reducing the concentrations and amounts of atrazine applied to the treatment medium helped to reduce the toxic effects exerted on Allium cepa [91].

Oliveira et al. [42] prepared PCL nanocapsules loaded with atrazine at the average size of 240.7 nm. They applied a concentration of 1 mg/mL of the nanocapsules in the control of mustard (Brassica juncea) as the target plant, and, after 72 h, shoot growth inhibition and severe symptoms were observed due to a reduction in net photosynthesis and PSII maximum quantum yield, as well as the enhancement of leaf lipid peroxidation. The results demonstrated that the PCL-atrazine nanocapsules were 10 times more effective than a commercial formulation of atrazine; therefore, as an important result, the application of PCL-atrazine nanocapsules enables the use of lower herbicide dosages with no reduction in efficiency, providing environmental advantages [42].

As atrazine herbicide is commonly used on maize, Oliveira et al. [103] continued their research by evaluating the side effects of PCL-atrazine nanocapsules on maize crops (as non-target plants). They studied the effect of nanocapsules of PCL-atrazine on some parameters of soil-grown maize (Zea mays L.), such as growth, physiological stress, and oxidative stress parameters. They observed that, 24 h after the post-emergence treatment of the maize with 1mg/mL PCL-atrazine, there was a 1.8-fold enhancement in the peroxidation of leaf lipid as compared to water-treated control plants. Moreover, the maize plants showed a 15% decrease in photosystem II (PSII) maximum quantum yield and a 21% reduction in the assimilation rate of net CO₂ in comparison to the control. Interestingly, the analyzed parameters were not affected after four or eight days. The authors suggested that the temporary side effects of atrazine may be the result of the plant’s ability to detoxify the herbicide. Moreover, a 10-times-diluted PCL-atrazine (0.1 mg/mL) dosage was effective in controlling weeds, with no negative impact on the studied parameters, even after a short time of usage. Also, the authors did not observe any negative effect on shoot growth or in macroscopic images of the leaves [103]. Such a study gives us very useful information about the introduction of novel nanoformulations in weed management approaches.

Sousa et al. [40] studied the post-emergence herbicidal activity of PCL-atrazine nanocapsules (2000 g/ha⁻¹) against slender amaranth (Amaranthus viridis) and hairy beggarticks (Bidens pilosa), comparing the results with those of a commercial atrazine formulation. The results showed that the PCL-atrazine nanocapsules were more effective than the commercial atrazine formulation and the control (50% and 40% inhibition in photosystem II activity, respectively). A 10-times-diluted concentration of the nanoherbicide also showed the same results as the commercial one [40].

Sousa et al. [104] followed their studies by applying PCL-atrazine nanocapsules in the post-emergent control of an atrazine-tolerant weed, sourgrass (Digitaria insularis), in a greenhouse. The efficiency of atrazine was limited against D. insularis, but nano-PCL-atrazine at both developmental stages stimulated a faster and greater inhibition of sourgrass photosystem II activity than a commercial herbicide. Some of the physiological, growth, and control parameters were measured for plants that had two or four expanded leaves, and they were treated with nano-PCL-atrazine or a commercial formulation. Nano-PCL-atrazine was able to control the sourgrass, especially in the two-expanded-leaves stage. Moreover, in the nano-PCL-atrazine-treated four-leaved plants, a greater enhancement in dry weight was observed than in those treated with the commercial atrazine. In addition, the use of nano-PCL-atrazine at half-dosage showed equal or better results in weed control than the commercial formulation at full dosage [104].

Preisler et al. [105] reported an improvement in the pre-emergence activity of atrazine via nanoencapsulation with PCL against Bidens pilosa. Soil treatment with nano-PCL-atrazine resulted in a higher seedling mortality than soil treatment with atrazine, even after a 10-fold dilution. The residual effects of nano-PCL-atrazine and conventional atrazine on soybean plants were evaluated after different periods of soil treatment. In a short-term treatment of soil (17 days) with atrazine formulations, an intense toxicity to soybean plants was observed. Considering the growth inhibition parameters, nano-PCL-atrazine at 200 g/ha⁻¹ and 2000 g/ha⁻¹ had similar inhibitory effects on the soybean, suggesting that nano-PCL-atrazine enhanced the short-term residual impact of the herbicide. In a long-term treatment of soil (60 days), the soybean plant’s growth and its physiological parameters were similarly affected by nano- and non-nanoformulations, suggesting that using nano-PCL-atrazine did not increase the residual impact of the herbicide. Therefore, if a safe interval is considered from treatment with the herbicide to sowing, this nanoformulation rather than non-nanoformulations of atrazine can be used for the efficient control of weeds with no increased phytotoxicity to non-target crops [105].

Bombo et al. [13] studied the interaction between a nano-PCL-atrazine formulation and Brassica juncea plants in structural detail. They analyzed the structural changes in the leaves based on the foliar uptake of nano-PCL-atrazine in a post-emergence treatment. The nano-PCL-atrazine stuck to the leaf surface, penetrated mesophyll, and transported through the vascular tissue into the cells, degrading the chloroplasts and causing herbicidal activity. According to the obtained results, the controlled-release system could enhance the accumulation of the active ingredients at the intracellular level of target organelles [13].

Wu et al. [106] compared the effects of nano-PCL-atrazine and pure atrazine at different concentrations on defense mechanisms, physiological responses, and nutrient displacement in lettuce (Lactuca sativa) as a non-target plant. The chlorophyll pigment content, ROS production, activities of ROS scavenger enzymes (SOD, APX, CAT, POD, GST, and PPO), and macro- and micronutrient concentrations were determined in experiments with short-, medium-, and long-term exposure durations. In the short-term exposure, the growth inhibition of nano-PCL-atrazine was similar to that of atrazine, but in the long-term exposure, high concentrations of nano-PCL-atrazine had greater negative effects on the end points of ROS production, protein content, and alteration in enzyme activities than the pure atrazine. Nano-PCL-atrazine and atrazine differently affected the displacement of nutrients, such as Cu, K, and Fe, for plant growth and disrupted mineral nutrient uptake in the plants; the differences were based on the plant organ, nutrient element, and exposure time. With nanoparticle modifications, there will be a potential to reduce the amount of herbicides required by enhancing the effect time [106].

Takeshita et al. [66] prepared 200–300 nm sized nano-PCL-atrazine and studied the nanoherbicide–leaf relationship in field and greenhouse conditions in mustard plant using radiometric techniques. They explained changes in the nanoherbicide’s mode of action for understanding the improvement in post-emergence weed control efficiency upon the nanoencapsulation of atrazine. Nano-atrazine had a lower wettability and higher absorption and translocation. The higher inhibition of photosystem II activity was described upon 40% enhancement of herbicide absorption in the first 24 h. The authors suggested that the increase in nanoherbicide herbicidal activity may be the result of its stomatal uptake. Interestingly, nano-atrazine was more efficient in the field rather than in the greenhouse, and, in the field experiments, the nanoformulation of atrazine showed better herbicidal activity, two times more than a conventional atrazine. The higher absorption and mobility in plants and the higher herbicidal activity of the nanoformulation in the field provide environmental benefits, recommending more investigations into the application of such nanoencapsulation techniques in the future [66].

Considering the fact that atrazine is harmful to humans and animals, Moore et al. [107] prepared nano-PCL-atrazine and evaluated its effect on human lung cells, as atrazine may reach respiratory units via inhalation. In previous research, it was reported that nano-PCL-atrazine was safer to human cells than atrazine, but, according to the results of this investigation, 48 h after the exposure of cells to nano-PCL-atrazine with concentrations of more than 1 ppm, the dehydrogenase release of lactate notably increased, with a maximum at 5 ppm, which was three times more than the control cells. Such effects were not observed in cells treated with atrazine or PCL nanocapsules. Therefore, nano-PCL-atrazine caused more damage to human lung alveolar cells than atrazine or PCL nanocapsules; however, nanoformulations provide higher herbicidal activity and can be applied at lower dosages, thus causing less ecological contamination [107].

9.3. Pretilachlor-PCL Nanoherbicide

PCL nanocapsules containing pretilachlor herbicide, with a high encapsulation efficiency of 99.5 ± 1.3%, were prepared and investigated by Diyanat et al. [111]. The nanocapsules had an irregular shape, with a particle size in the range of 70–200 nm. Studies also showed that the nanocapsules were stable in a suspension without any aggregation for 60 days. Barnyard grass was used as a target plant and rice as a non-target plant for an evaluation of herbicide activity in a pre-emergence manner. The authors reported that the nanoherbicide had no negative effect on the rice but had a significant effect on the barnyard grass. Moreover, in genotoxicity experiments (an estimation of the mitotic index using onion cells), the nanoherbicide was less toxic than a commercial formulation. Based on the obtained results, the authors recommended that nanocapsules of pretilachlor with PCL could be used effectively in agriculture as an environmentally friendly PCL–herbicide system [111] (Table 2).

10. Behavior of PCL-Based Nano-Enabled Herbicides in Plant Systems

Regarding the design of nanoherbicide formulations, one of the main aspects is the delivery of the active ingredient across the surface of the leaf to facilitate its reaching the action sites and improve its effectiveness. According to Takeshita et al. [66], for atrazine-loaded PCL nanoparticles, kinetic assays indicated that the release of atrazine from the nanocapsule is caused by the diffusion of the active ingredient accompanied by the relaxation of the polymer matrix, with a slower rate than conventional atrazine. Therefore, the observed rapid absorption of atrazine by mustard leaf may be due to the carriage of herbicide by PCL nanocapsules through the tissue of the plant [66]. The authors proposed that nanoherbicides stay adsorbed on the surface of the leaf, waiting for accessible pathways to become available. The size, shape, composition, surface charge (zeta potential), and loading of the nanoparticles affect their translocation. The penetration of the nanoherbicide into the leaf’s barriers is a key point in designing a nanosystem. For smaller nanoparticles (<5 nm) and for the diffusion of hydrophobic compounds, cuticular pathways are the main access routes, but, for larger nanoparticles (>40 nm), trichomes, stomata, and aqueous pores are the access routes [66]. The size of the hydathode water gates always varies from a few micrometers to several micrometers, and they facilitate nanocarrier entry. Therefore, a nanoencapsulated herbicide could pass directly through the vascular system and then rapidly spread throughout the plant, enhancing the activity of the nanoherbicide [118].

The surface of plant cell walls is negatively charged, and PCL nanoparticles also have a negatively charged surface. Negatively charged nanoparticles have faster foliar penetration than positively charged ones because positively charged nanoparticles, due to electrostatic attraction, accumulate and aggregate on the surface of tissue. However, nanomaterials with a negative zeta potential, as a result of their poor interaction with cell walls, have a higher distribution in plant tissue [118]. According to Nguyen et al. [118], the penetration of nanocarriers is fast in pepper leaves and nanoherbicides, and just after 60 min of application, they are able to reach the deepest parts of the leaf. Stomata are found on both sides of the leaf in peppers like mustard, which helps fast penetration in these species; ideal pathways for the leaf penetration of nanocarriers are stomata and hydathode regions [118].

According to Bombo et al. [13], although herbicide nanocapsules enter the leaf through its natural pores, stomata, and water pores, it is probable that their translocation beyond the leaf is also mediated by symplastic and apoplastic pathways. They reported that, 36 h after application, nanoparticles were observed inside cell protoplasts, and, after 48 h, they were observed inside the chloroplasts. It is proposed that the mechanism of penetration may involve endocytosis, wherein nanocapsules cross the cell wall and reach the cell membrane, resulting in the internalization of the nanoherbicide in a vesicle in the cytoplasm. An important result of the study was that the PCL-based nanocarriers did not show phytotoxic effects, and the treatment did not cause any structural changes, as the nanoparticles were inside the cells. It could be suggested that they could be applied for the delivery of various active ingredients into the leaf mesophyll and especially for the targeting of chloroplasts [13].

Takeshita et al. [66] reported that PCL nanocarriers loaded with atrazine had a greater mobility in leaves than atrazine’s commercial formulation, as, due to the surface characteristics of the nanocarriers, they follow the water pathway in the vessels and apoplastic path more effectively than atrazine. This results in a higher distribution through the mesophyll and greater herbicide effects, which is established by measuring the inhibition of PSII activity. Due to the nanoencapsulation, atrazine showed low translocation via the phloem, but a point enhancement in the percentage of ¹⁴C-atrazine was observed in other parts of the plant in addition to the treated leaf. Moreover, using the atrazine nanoherbicide in the center of the leaf resulted in PSII inhibition in other parts of the treated leaf and in an untreated leaf, suggesting that nanoencapsulation may improve translocation via the phloem, resulting in a greater distribution of the herbicide all over the plant. The site of greater accumulation of atrazine in mustard leaves was in portions of the leaf with more cell membranes and less cell wall volumes, like at the ends of vascular systems. This intermediary permeability of atrazine between the phloem and xylem needs more study to confirm whether the process has any relevance to atrazine’s herbicidal activity [66].

11. Conclusions and Future Outlook

With the day-by-day increasing use of herbicides, applying a controlled-release formulation seems helpful considering economic and ecological aspects. A controlled-release system is developed to release the herbicide in a controlled amount within a definite time. Such controlled-release formulations reduce the negative impacts of herbicides on the environment by reducing vaporization, leaching, and degradation. Furthermore, as a result of nanoencapsulation, the leaching potential and the loss of active ingredients before reaching the target plant are reduced. The nanoencapsulation of herbicides seems to be a useful technique for promoting some of the main features, such as improving efficiency and minimizing the herbicide’s effects on the environment and human health. For a better fight against biotic and abiotic stresses (such as drought, heat, and salinity), these benefits seem to be essential in such a changing climate. Another important aspect for the development of novel nanoherbicides is that nanoparticles may enhance the adsorption of the herbicide via better adhesion to the waxy leaf surface and then by enhancing its biological activity.

Among all of the materials that have been used for this purpose, the most suitable carriers are the biodegradable ones, as they have good biocompatibility and low levels of toxicity. PCL has attracted researchers’ attention in the field of pesticide nanoencapsulation, as it is not soluble in water, can be easily degraded by enzymes and fungi, and is a relatively cheap polymer. Moreover, PCL-nanoencapsulated herbicides can easily be prepared from inexpensive materials at large scales. The triazine class (ametryn, atrazine, and simazine), metribuzin, and polycaprolactone are the herbicides that have been encapsulated with PCL. According to recent studies, PCL-nanoencapsulated herbicides that have been used in weed management could produce less toxic formulations, providing more environmentally friendly products for sustainable agricultural systems.

Regarding future perspectives, with the development of innovative controlled-release systems for herbicides, delivery systems that specifically target subcellular compartments using biorecognition motifs need to be improved to overcome the problem of the inability of current nanoformulations to target species precisely. However, for the development of smart nanoherbicides, it is necessary to precisely understand the action mechanisms of nanoherbicides on target and non-target organisms. Moreover, there are many biological factors affecting the efficacy of nanocarriers carrying herbicides during the crossing of the plant’s cell walls. Therefore, it is recommended to improve our knowledge of the details of these nanoformulations such as nanoherbicides’ modes of action in future studies. Furthermore, many more studies are needed to determine herbicides’ impacts on soil microorganisms, confirming the clear mechanism of action considering different environmental parameters. Also, further investigations are required to better understand the uncertainties related to the adverse effects of the studied nanoherbicides.

Altogether, the reports indicate that, in comparing nanoherbicides with their commercial analogues, nanoherbicides are expected to be more sustainable, efficient, and resilient, and they have lower adverse impacts on the environment. These advantages may improve crop yields, contributing toward food security and sustainable agriculture.