Polymer-based and stimulus-responsive carriers for controlled release of agrochemicals PDF

European Polymer Journal 177 (2022) 111432 Contents lists available at ScienceDirect European Polymer Journal...

European Polymer Journal 177 (2022) 111432 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj Polymer-based and stimulus-responsive carriers for controlled release of agrochemicals Ling Zheng a, Farzad Seidi a, *, Yuqian Liu a, Weibing Wu a, Huining Xiao b, * a Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources and International Innovation Center for Forest Chemicals and Materials, Nanjing Forestry University, Nanjing 210037, China b Department of Chemical Engineering, University of New Brunswick, Fredericton, NB E3B 5 A3 Canada A R T I C L E I N F O A B S T R A C T Keywords: Food demand has been increasing substantially world-wide with the growth of population and the depletion of Polymer arable land. To increase food production, it is necessary to improve the efficiency of agrochemicals usage while Stimulus-responsive reducing their negative impact on the ecosystem. The controlled release of agrochemicals has been regarded as Controlled release an emerging technology and critically important strategy to improve the efficiency and performance of agro­ Agrochemicals Nanomaterials chemicals. Among various carriers, the stimulus-responsive polymer-based carriers provide the controlled release of agrochemicals based on changes in environmental conditions (such as pH, redox, light, enzymes, etc.) along with preserving the environmental friendliness and affinity particularly for those originating from bioresources. Here, we review the development and application of polymers in the sustained release of agrochemicals. Additionally, the preparation methods and responsive behaviors and mechanisms of stimulus-responsive poly­ mer-based carriers, and the characteristics and advantages of those smart carriers are discussed. However, given the fact that the relevant research is still in its infancy, the large-scale application in farmlands still requires tremendous and continuous efforts to be paid by researchers. 1. Introduction efficiency of agrochemical utilization. Aiming at the better controllability over slow or sustainable release processes, controlled The global population has grown from 1 billion in 1800 to 7.9 billion release technology refers to the combination of drugs and chemically in 2020, an annual increase of about 83 million or 1.1 % per year. The active carriers through physicochemical methods such as embedding, United Nations predicts that the population will continue to grow, and cross-linking, and adsorption to release the drugs at a specific location or the total global population is expected to reach 8.6 billion by 2030, and target with a specific rate , which is also an ideal and efficient 9.8 billion by 2050 [1,2]. However, as the world’s population increases, approach for the controlled release of agrochemicals. Controlled release food demand continues to rise. At the same time, the area of arable land technology can effectively improve the utilization efficiency of agro­ continues to shrink. Therefore, we need to create agricultural technol­ chemicals and reduce the frequency of application, thereby reducing the ogies that increase food production per unit area in order to secure the loss of agrochemicals and the pollution of soil and groundwater. It food supply in the future. The use of agrochemicals can not only help represents an emerging technology for agricultural production nowa­ promote crop growth and increase crop yields, but also prevent pests days and in the future. Among various carriers for controlled release, the and diseases to improve crop quality, and play an extremely important one in gel form appears to be particularly appealing. The encapsulation role in improving agricultural production efficiency. of agrochemicals in a gel carrier usually has following advantages: (1) In recent years, in agricultural production, the use of agrochemicals extending the holding period of the same dose of agrochemicals; (2) has greatly increased food production on one hand, but on the other improving the stability of agrochemicals; (3) reducing the amount of the hand, it has also brought significant environmental and ecological im­ utilized agrochemicals; (4) mitigating environmental risks; and (5) pacts. Therefore, it is necessary to improve the use efficiency of agro­ providing a flexible response to specific stimuli from external environ­ chemicals to increase crop yields. At present, controlled release ments. technology is considered to be a promising solution for enhancing the Stimulus-responsive carrier refers to the environmentally responsive * Corresponding authors. E-mail addresses: [email protected] (F. Seidi), [email protected] (H. Xiao). https://doi.org/10.1016/j.eurpolymj.2022.111432 Received 18 April 2022; Received in revised form 14 July 2022; Accepted 17 July 2022 Available online 19 July 2022 0014-3057/© 2022 Elsevier Ltd. All rights reserved. L. Zheng et al. European Polymer Journal 177 (2022) 111432 materials that can undergo reversible structural and property changes generate a physical barrier in the soil and protect the fertilizer from under the stimulation of the changes in the external environment (such degradation by microorganisms such as algae. Furthermore, immobili­ as pH, temperature, magnetic field, light, redox potential, enzymes, etc.) zation of the fertilizers prevents the rapid solubilization of fertilizers, to release the loaded species. This technology has been widely used therefore, preventing from the groundwater pollution. Moreover, it of­ in the fields of targeted drug delivery, tissue engineering, cell culture, fers the potential in tuning nutrient application to the growth re­ sensors, and so on. Among various stimulus-responsive carriers, quirements of different crops according to growth timeline and nutrient hydrogels can absorb and retain a large amount of water while main­ requirements, which is beneficial to enhancing plant yield while taining structural integrity, which is beneficial to soil water retention requiring less fertilizer application. while loading agrochemicals. Therefore, stimulus-responsive natural Hydrogel has excellent water retention and can be used for the slow polymer-based hydrogel materials are very promising for the controlled release of agrochemicals. Hydrogels based on oxidized starch from or targeted release of agrochemicals in the agricultural field. However, various botanical sources prepared by oxidation using KMnO4/NaHSO3 systematic reviews on natural-based hydrogels as smart cargos for ag­ showed capability to absorb and release fertilizers such as urea, KNO3 rochemicals are rather limited, which has motivated us to present a and (NH4)2SO4. The technology of combining water-retaining comprehensive review on this topic. The main focuses of this review agent and fertilizer can reduce the unnecessary use of fertilizer, consist of polymer carriers for the slow release of a variety of agro­ improve the utilization of nutrients, and enable plants to continuously chemicals and various types of responsive or smart nanomaterials and and effectively supplement after fertilization. Liu et al. prepared a hydrogels enabling to trigger the release on demand as well as the hydrogel from black liquor, a by-product of pulp, by a redox radical- environmental impacts of natural polymer-based carriers. induced polymerization process with a water swelling rate of 359 g/g and high water retention (about 45 %). Additionally, it was used for 2. Polymeric carriers for sustained release of agrochemicals loading and releasing of ammonium dihydrogen phosphate. Pushpamalar et al. successfully prepared nine superabsorbent In the current agricultural development, agrochemicals are useful for carboxymethyl sago pulp (CMSP) hydrogels based on sago pulp con­ improving agricultural yields and quality, and for preventing and con­ taining cellulose, lignin and hemicellulose by electron beam irradiation. trolling of the diseases. The loss of agrochemical through dissolution, The cross-linked CMSP hydrogel was useful for soil moisture preserva­ diffusion, volatilization, precipitation, and soil fixation reduces the tion, slow release of inorganic fertilizer and also showed the average practical efficiency of agrochemicals, leading to the continuous increase biodegradation rate of around 3 wt% per week. The sustained-release in the use of agrochemicals. Excessive toxic agrochemicals enter the system was evaluated by loading potassium nitrate (KNO3) and ammo­ food chain through the water cycle, which is harmful and has a devas­ nium nitrate (NH4NO3) in CMSP hydrogels. The results showed that the tating impact on ecological safety and human health [10,11]. The loading content of KNO3 and NH4NO3 was enhanced with increasing immobilization and controlled release of agrochemicals are expected to KNO3 and NH4NO3 concentrations. Immersing these hydrogels in 20 % reduce the amount of utilized agrochemicals, which not only saves w/v KNO3 and 40 % w/v NH4NO3 aqueous solutions, afford the loading agricultural production costs, but also prevents excessive agrochemicals of 2100 % and 5200 % w/w %, respectively. In around 2 days, 50 % of from being leached through runoff and groundwater due to dissolution, the fertilizers were released and by prolonging the period to 90 days the thereby polluting the environment. release of around 90 % was achieved. In addition to hydrogels, other At present, the research on the sustained release of agrochemicals is nano/microparticles also have been applied for loading and control still in the initial stage of exploration. The applied carriers mainly release of agrochemicals. For example, spherical chitosan (CS)/urea include silica materials, metal/metal oxide nanoparticles, carbon nanoparticles crosslinked by tripolyphosphate (TPP), exhibited slow nanoparticles, hybrid silica/PAA nanocomposite hydrogel , and release of urea. This system could enhance the potato productivity polymer-based gels [14,15]. Among them, the polymer-based carriers besides mitigating nitrogen-losses and perhaps fertilizer application have great potential for the applications in the controlled release of rates, leading to 25 % saving of the nitrogen-fertilizer besides 7.23 % agrochemicals due to their unique characteristics, including good increase in potato yield over the conventional urea fertilizer treatment. biocompatibility and eco-friendliness. Typical polymer gels for such purposes include polyacrylamide-methylcellulose hydrogel, cur­ 2.2. Slow release of herbicide dlan, carboxymethyl cellulose, lignin, calcium alginate, polylactic acid, etc. Hydrogel, a very important polymer-based carrier, has excellent The use of herbicides in the agricultural production process can water retention due to its three-dimensional network and hydrophilic reduce the labor intensity and improve the effect of farmland weeding structure. The use of pesticide-loaded hydrogel provides a sustainable and the level of mechanized operations. In recent years, herbicides have supply of agrochemicals and increases soil moisture and sub-surface soil become increasingly popular among farmers. Moreover, herbicides hide moisture, reducing fertilizer run-off and irrigation frequency, thus great harm while bringing benefits to agricultural production. Indeed, changing plant quality and growth rates. The review presented high stability of the herbicides in soil can cause permanent damage to herein focuses on polymer or hydrogel carriers, those originating from the soil. Accordingly, it will not only affect the current season crops, but natural bioresources in particular. also cause serious damage to the following crops. The phytotoxicity, in A variety of agrochemicals have been encapsulated by polymer turn, has a serious impact on the yield and quality of crops. Using the carriers, including fertilizers, herbicides, plant growth regulators, in­ same herbicide for a long period will alter the paddy field environment, secticides and fungicides. Following the release behaviors of each type trigger the succession of the weed community, and then produce a of those agrochemicals controlled by tailor-modified polymers or certain degree of immunity to the herbicide used. To address this issue, it hydrogels. is recommended to load the herbicides into the carriers to reduce its usage and residue. Common herbicides are shown in Table 1. 2.1. Slow release of fertilizer Nanoprecipitation is a facile and fast way to encapsulate natural compounds by means of simple instrumentation and non-toxic chem­ Fertilizer is the most common agrochemical, divided into organic icals. Loading herbicides in nanocarriers can be used to stabilize the and inorganic fertilizers. Inorganic fertilizers mainly include elemental herbicides to protect the environment. Lignin is rich in content, low in fertilizers and compound fertilizers containing nitrogen, phosphate and price, and environmentally friendly, which can be used to prepare car­ potash, etc. Organic fertilizer, also called “farm fertilizer”, that contain riers for fungicides. Lignin is a waste product obtained from the paper organic matter has the characteristics of multiple types, wide sources, pulp manufacturing process. Lignin was applied as the substrate to and prolonged fertilizer efficiency. Loading a fertilizer in the carrier can produce diuron nanoformulation (DNF) by nanoprecipitation method 2 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Table 1 nanoparticles were prepared for loading a mixture of two herbicides Common herbicides, their applications and potential harmful effects on crops. including imazapyr (IMC) and imazapyr (IMR) with the encapsulation Name Applications and notes efficiency over 60 % for both herbicides. After 300 min in 2 mM calcium chloride solution, the release of herbicides from CS/ALG nanoparticles Imazethapyr (IMZ) An imidazolinone herbicide for the weed control in soybean and other legume crops. Imazethapyr reached to 30 % for IMC and 20 % for IMR. However, release from the applied preemergence (PRE) and/or CS/TPP nanoparticles was slower with around 9 % release for IMR in postemergence (POST) are effective in 300 min under the same conditions. controlling annual and perennial grass and Herbicide trifluralin which is unstable under photo irradiation can be broadleaf weeds [22,23]. 2-methyl-4- A systemic post-emergence phenoxy herbicide stabilized upon encapsulation in polymer carries due to the preventing chlorophenoxyacetic acid for control of broadleaf annual and perennial of the agrochemicals from direct exposure to the light. Encapsulation of (MCPA) weeds in arable and horticultural crops. trifluralin into the biodegradable microcapsules of poly(3-hydrox­ Atrazine A triazine herbicide for control of the pre- ybutyrate-co-4-hydroxybutyrate) (PHB) improved the photo-stability of emergence and post-emergence broadleaf weeds this herbicide so that the t1/2 for photodegradation for free and encap­ and some grass weeds to inhibit some perennial weeds. sulated trifluralin was around 20 and 53 h, respectively. Under the Metazachlor (MTZ) A frequently used herbicide commonly used in optimum conditions these microcapsules could load around 17 wt% winter rape, turnip, cabbages, and leek crops to trifluralin with ability to release < 60 % under ambient conditions in 10 control winter and annual grasses and broad- % aqueous methanol solution. The encapsulated trifluralin showed su­ leaved weeds. It has a selective pre-emergency effect and can inhibit the growth of plant roots. perior herbicidal activity in comparison with the free one (Fig. 1c). Metazachlor affects cell division and tissue Micro- and submicro particles of high molecular weight poly(lactic acid) differentiation, and provides good weed control were used to encapsulate metazachlor (MTZ). Particles denoted as small without harming crops [26,27]. (S) were prepared with the use of ultrasonication as a part of the Trifluralin A synthetic fluorinated dinitroaniline herbicide emulsification process with ability of MTZ loading up to 30 wt%. Par­ for selective weed control in cotton, soybean and sunflower production. By interfering with ticles denoted as Large (L) were prepared without the use of ultrasound mitosis and inhibiting microtubule assembly via showed higher loading capability of around 40 wt%. The release of the restricting the polymerization of tubulin (the active ingredient from both types of particles exhibited a typical structural protein of plant cells), it prevent bimodal profile. The S particles could release 25 % of MTZ at 200 h growth and causes plant death. Trifluralin is under dark condition in 0.02 mol L− 1 TRIS-buffer (pH = 7) whereas used to control annual gramineous weeds and broad-leaved weeds in field crops. under the same condition the L particles could releases 15 % of MTZ at Diuron A systemic phenyl urea herbicide which inhibits 300 h. plant growth by inhibiting the photosynthesis of plant PS II. It is a broad-spectrum residual 2.3. Slow release of plant growth regulators herbicide registered for pre-emergence and post- emergence to control of both broadleaf and annual grassy weed. Plant growth regulators (PGRs) are a class of compounds with extremely important roles. These plant hormones, or molecules that alter plant hormonal homeostasis and signaling, can be applied to crops. Under the optimized condition, around 5 wt% diuron loading with to enhance plant development, increase production, improve the visual 74 % encapsulation efficiency was reported. Diuron can be slowly and nutritional aspects of food, and increase the storage time or shelf life released from the carrier so that after 120 days, around 53, 62 and 67 %. Common plant growth regulators are shown in Table 2. of the diuron was released from the nancontainers, respectively, at pH Cyclodextrins (β- or γ-CD) have widely studied for loading and ranging from 5 to 9. In another work, fiber clay and magne­ releasing of hydrophobic organic cargos [42,43]. Cyclodextrin and its sium–aluminum layered double hydroxide (MgA1-LDH) and herbicide derivatives (HP-β-CD) can form inclusion complexes with the plant MCPA formed a hybrid nanostructure via a single coprecipitation step growth regulator gibberellic acid (GA3), which improve the water sol­ (Fig. 1a), in which the encapsulation efficiency of MCPA was 51 %. The ubility and pH stability of GA3, making GA3 more beneficial for agri­ single-step coprecipitation method could incorporate more herbicides, cultural applications. The slow-release of GA3 loaded in the and it was more efficient than the incorporation of MCPA by ion ex­ complexes was verified by bioactivity assay. The cyclodextrin inclusion change after the formation of the LDH/sepiolite nanoarchitecture. In complex could significantly promote the growth of mung bean and cu­ addition, such nanostructures enabled the sustained release of herbi­ cumber roots up to 254 % and 208 %, respectively, and the early sowing cides, so that the loaded MCPA was completely released within 2 weeks rates could be increased to 98 % and 279 %, respectively. in water at pH 5.5. Abscisic acid (ABA), a plant growth regulator, is extremely sensitive Herbicide molecules can be chemically or physically loaded in a to ultraviolet radiation and is easily inactivated by sunlight. Encapsu­ polymeric capsule device. Alginate (ALG) beads were prepared as a lation and controlled release of ABA can greatly improve the applica­ carrier for imazethapyr (IMZ). Addition of a small amount of cellulose bility. Alkali lignin (AL) was modified with cetyltrimethylammonium (CEL, 5 w/w-%) to the ALG network to form alginate/cellulose (ALG bromide (CTAB) to form AL-CTAB nanoparticles through self-assembly, /CEL) beads increased the loading efficiency of IMZ by 2 %. In addition, followed by loading ABA onto the nanoparticles to obtain ABA@AL- the release time of IMZ was prolonged due to CEL microfibers forming CTAB (Fig. 2a). ABA@AL-CTAB enhanced the thermal stability and tortuous pathways inside the ALG network (Fig. 1b), so that the release photolysis resistance of ABA, so that over 75 % of ABA could retain when of 50 % of IMZ in alginate-cellulose beads required 4 times longer than the particles were irradiated with 365 nm UV light for 60 h, but more that in ALG beads, which was about 44 days. In addition, atrazine- than 80 % of free ABA could degrade within 5 h irradiation. In addition, loaded lignin microparticles were prepared using solvent extraction/ the nanoparticles can also release ABA slowly, with a cumulative release evaporation and microfluidic techniques, respectively, and then the two of 35 % in deionized water for 72 h. Since ABA has the effect of inhib­ lignin microparticles were encapsulated in sodium alginate beads to iting plant germination, the ABA@AL-CTAB showed a hysteresis phe­ further slowdown the release of atrazine. Both beads lowered the release nomenon of germination initiation (Fig. 2b), which further verified the rate of atrazine, while the microfluidic-prepared microparticles exhibi­ sustained-release effect and photolysis resistance of ABA induced by the ted a lower burst release (~20 % of the encapsulated herbicide) and nanoparticles. required approximately 96 h to achieve 100 % release of atrazine. An alginate hydrogel bead was prepared in which paclobutrazol Alginate/chitosan (CS/ALG) and chitosan/tripolyphosphate (CS/TPP) (PBZ)-loaded polycaprolactone (PCL) microspheres were embedded, 3 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 1. (a) Schematic representations of (A) sepiolite and (B) layered double hydroxide structures, (C) molecular structure of 2-methyl-4-chlorophenoxyacetic acid (MCPA), and schemes of the synthesis of the hybrid MCPA-LDH/sepiolite nanoarchitectures by (D) one-step coprecipitation (Copyright 2019 from Elsevier). (b) Schematic of the mechanism of prolonging the release of IMA by alginate-cellulose beads (Copyright 2019 from Elsevier). (c) The growth of plant barnyardgrass (Echinochloa crus-galli) after (A) treating with encapsulated trifluralin, (B) treating with non-capsulated trifluralin, and (C) control without trifluralin(Copyright 2019 from Elsevier). environmental impact, insecticides are among the agricultural tools Table 2 most significantly associated with environmental harm. Their expressed Common plant growth regulators, their applications and potential harmful ef­ purpose is to kill pests; consequently, they may have lethal or sublethal fects on the crops. impacts on non-target organisms (such as organisms that recycle soil Name Application nutrients, pollinate crops, and prey on pest species) and reduce and/or gibberellic acid An important plant regulator that regulates the process of contaminate food supplies for organisms at higher trophic levels. In (GA3) plant development and growth, including seed development addition, the abuse of insecticides often causes serious ecological and and germination, stem and root growth, cell division and economic problems and induces insecticide resistance. Therefore, the flowering time. abscisic acid (ABA) It can regulate plant growth and development, not only for sustained release of insecticides is very critical. Common insecticides are promoting dormancy and regulating stomata opening, but also shown in Table 3. for resisting salt, heat and cold stress. In the classification of insecticides, essential oils (EOs), a complex of Paclobutrazol A triazole-containing plant growth retardant that is known to chemically pure compounds, are currently a very important category of (PBZ) inhibit the biosynthesis of gibberellins. It also has antifungal activities. It can also suppress the synthesis of abscisic acid and green insecticides. EOs are volatile in nature and can enhance the ac­ induce chilling tolerance in plants. tivity due to the synergistic effect between the components. Therefore, Salicylic acid (SA) A class of small-molecule phenolic substances widely present EO-based insecticides are very important for the control of stored in­ in plants, participating in various physiological and sects, because they are active against a variety of insects, have a fast biochemical processes such as transpiration, seed penetration rate, and do not adversely affect the ecosystem and human germination, flowering, etc.; inducing plants to produce various physiological traits such as disease resistance, salt health; therefor, biological insecticides are called “green insecticide”. resistance, and cold tolerance. They can reduce the number of pests and increase the crops production yield. Common essential oil agrochemicals include linalool, thymol, menthol, eugenol and carvacrol. Besides, compared with simple EO and the size of the PCL-loaded microspheres could be altered by varying emulsion, encapsulation EOs in cross-linked biopolymers improves the the homogenization speed and emulsifier concentration. The increase in stability and activity of EO and slow down the release rate of the in­ PCL microsphere size had a detrimental effect on the release of PBZ from secticides. the alginate device, the release percentage from 28 µm beads was about Alginate/chitosan/sodium lignosulfonate (ALG/CS/SL) poly­ 10 % lower than that of from 21 µm beads after 36 days in DI water electrolyte was prepared via layer-by-layer self-assembly for loading and (Fig. 2c). Therefore, the size of PCL microspheres can be adjusted to sustained release of abamectin benzoate (EB) with high encapsulation achieve higher fineness of the controlled release as needed. efficiency (~93 %) and outstanding performance of anti-photolysis (Fig. 3a). EB loaded in ALG/CS/SL required 204 h for complete 2.4. Slow release of insecticides release in DI water. Exposing this system to UV light for 10 h showed around 6 times less degradation of EB in comparison to free EB. Although other aspects of modern agriculture often have a great Hydrophobic modification of lignin can improve the encapsulation 4 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 2. (a) Schematic diagram of the formation of ABA@AL-CTAB nanoparticles (Copyright 2020 from ACS). (b) Effects of DMSO, AL-CTAB, free ABA and ABA@AL-CTAB on the germination of Oryza sativa L. and Arabidopsis thaliana seeds (AL = alkali lignin; CTAB = cetyltrimethylammonium bromide). (c) Percent release of PCL from alginate beads with different sizes of PLC microbeads (Copyright 2021 from NATURE RESEARCH). prevention and control of plant diseases. The use of fungicides has been Table 3 increasing over the past decades; whereas utilizing fungicides is a major Common insecticide, their applications and potential harmful effects on the issue related to the safety of crops. A large number of and multiple use of crops. agents with a single mechanism of action and strong selectivity for the Name Application control objects will cause the bacteria to mutate and produce drug avermectin (AVM) It exert their therapeutic effects through binding to resistance. For example, in the process of preventing and curing cu­ glutamate-gated chloride channels, resulting in consequent cumber downy mildew using the a large-scale, frequent, and high- paralysis and death of the parasites. concentration use of phenylamide metalaxyl has resulting the negative emamectin benzoate It is the product obtained by chemical transformation of (EB) abamectin, and its efficacy on lepidopteran pests is impact over a short period of time, causing the bacteria to develop significantly higher than that of abamectin. resistance within one or two years. Therefore, macro-controlling the imidacloprid (IMI) It is an ultra-efficient nicotinic insecticide and acts on the total amount of medications and the frequency of medications, and receptor of nicotinic acid acetylcholinesterase. It interferes rotating the use of drugs with multiple mechanisms of action are the with the motor nervous system of pests and causes chemical signal transmission to fail. It is widely used to control pests fundamental strategies to prevent or delay the emergence of drug with piercing–sucking mouthparts on crops. resistance. Common fungicides are listed in Table 4. High amylose corn starch (HACS) can form nanostructures with Rosemary essential oil (REO) with loading efficiency up to 10 % through efficiency for hydrophobic insecticides. Alkali lignin was modified with non-covalent complexation, which enhances the solubility and anti­ acetyl chloride and benzoyl chloride to improve its hydrophobicity, and fungal activity of REO. The cumulative release of REO loaded in nano­ the obtained acetylated lignin (ACAL) and benzoylated lignin (BZAL) structures after 25 h in 10 % ethanol and distilled water was 35 % and were used to prepare nanospheres to load avermectin (AVM) exhibited 25 %, respectively. In another study, mesoporous silica nano­ good sustained-release and anti-photolysis properties. The encapsula­ particles (MSNPs) were used in controlled release formulations of the tion efficiency of AMV-loaded nanospheres ACAL (AVM@ACAL) and EOs (lemongrass and cloves) to enable the release of EOs in water for BZAL (AVM@BZAL) for AMV was over 90 %. Almost complete release of more than 5 weeks to improve the stability of EOs in soil. The AVM and BZAL were achieved in 40 h and 50 h in acetone/water (1/1, loading efficiencies of lemongrass and cloves in MSNPs were 36 % and V/V) mixture. When the nanospheres were exposed to UV light for 50 h, 29 %, respectively. the highest AVM retention rates of AVM@ACAL and AVM@BZAL were Fabrication of lignin-modified epoxy resin (ER) nanocarriers, 67 % and 77.0 %, respectively. In another study, Peng et al. used through an inversed-phase interfacial polymerization, and encapsu­ sodium lignosulfonate (SL) hydrophobically modified with CTAB to load lating with abamectin (Fig. 4) enables the nanocarriers to reduce the 0.8 wt% active abamectin (AVM) material (SL/CTAB). The release rate damage caused by root-knot nematodes in the soil. With encapsulation of AMV was accelerated with the decrease of SL/CTAB dosage, and the efficiency of around 93 %, the nanocarriers could release around 48 % of release amount of AVM in acetone/water (1/1, V/V) species was only abamectin in 1:1 water:ethyl acetate mixture after 18 h. 56–87 % within 62 h. In addition, after 50 h of UV irradiation, the degradation rate of AVM was only 37–53 %. Indeed, the anti-photolysis 3. Types of responsive nanomaterials efficiency of encapsulated AVM was around 2–3 times higher than that of free AVM. The most energetic and ascending materials that can respond to Flash nanoprecipitation (FNP), a continuous and scalable technique external stimuli are called stimulus-responsive materials. At present, (Fig. 3c), was used for nanoparticle formation from hypromellose ace­ according to the physical and chemical properties and structure of ma­ tate succinate (HPMCAS) and lecithin encapsulated with abamectin, a terials, they can be divided into three categories: (1) exogenous stimulus biopesticide. This rapid technique afford high encapsulation efficiency responsive materials, such as temperature, light, ultrasound and mag­ (greater than90 %) along with loading content higher than 50 wt%. The netic field responsive materials; (2) endogenous stimulus responsive encapsulation enhanced the abamectin photostability more than six materials, including pH, enzymes and redox responsive materials; (3) times and reduced the release rate around 3 times in comparison to the multi-stimulus responsive materials. Multi-stimulus responsive free abamectin. Practically, complete release was achieved in 110 h from materials are not only important in life sciences, but also important to NPs. the new development of nanomaterials. In the field of agrochemicals for slow release, pH, redox, light and enzyme responsiveness as well as multi-stimulus responsiveness are the main focuses of current and future 2.5. Slow release of fungicide development. Summarized herein are the different types of responsive nanomaterials or hydrogels originating from natural polymers, poly­ Crop diseases are an important part of agricultural production saccharides in particular, as carriers for the controlled release of practices. The application of fungicides plays an important role in the 5 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 3. (a) Schematic description of EB-loaded ALG/CS/SL nanocarriers with sustained release and anti-photolysis; (b) Chemical formula of EB (Copyright 2020 from Elsevier). (c) Illustration for the fabrication of abamectin-loaded nanoparticles through the Flash NanoPrecipitation (FNP) process (Copyright 2021 from ACS). Temperature-responsive materials are characterized by having both Table 4 hydrophilic and hydrophobic functional groups in the molecular struc­ Common fungicides, their applications and potential harmful effects on the ture, among which poly(N-isopropylacrylamide) (PNIPAm) and poly (N, crops. N-dimethylaminoethyl methacrylate) (PDMAEMA) are representatives Name Application of this class of materials and are currently the most studied temperature- Rosemary essential oil A natural, volatile, phenolic compound with high responsive polymers. The responsive mechanism is associated with (REO) antimicrobial activity, antifungal, antiviral, antioxidant the lower critical solution temperature (LCST) of the polymers. When and medicinal properties. the temperature is lower than LCST, the hydrophilic amide functional azoxystrobin (AZOX) A broad-spectrum β-methoxyacrylate fungicide that inhibits mitochondrial respiration by binding to the Qo site groups form hydrogen bonds with water molecules, which increase the of the cytochrome bc1 complex and inhibiting electron solubility of the polymer; when the temperature is higher than the LCST, transfer, inducing reactive oxygen species production and the hydrogen bonds are broken, and the hydrophobic functional groups inducing apoptosis. begin to dominate, allowing the polymer to come out of solution Pyraclostrobin (PYR) It inhibits mitochondrial complex III in fungal and induces intracellular triglyceride accumulation in 3T3-L1 thereby (Fig. 5a). The LCST of temperature-responsive materials can be tuned by inhibiting spore germination, mycelial growth, and changing the ratio of hydrophilic and hydrophobic functional groups. sporulation of the target pathogenic fungi. The process of crop germination and growth is highly dependent on temperature. Too high or too low temperature will affect the yield and quality of crops. For example, the germination rate of tomato, corn agrochemicals. and other crop seeds will be reduced at low temperature. Temperature- responsive materials can release loaded agrochemicals under specific 3.1. Temperature-responsive carriers temperature stimuli, making crops resistant to adverse effects of temperature. Temperature-responsive carriers undergo reversible swelling- A thermosensitive hydrogel made of N-isopropylacrylamide shrinking changes with temperature changes. According to the (NIPAm) and n-butyl methacrylate (BMA) could encapsulate around 47 changes in response to temperature, they can be divided into two types. wt% salicylic acid (SA), a growth regulator or anti-refrigerant. With the Sensitive carrier (positive temperature response type) and heat shrink­ phase transition temperature of around 12 ◦ C (with a NIPAM:BMA able thermosensitive carrier (negative temperature response type) in weight ratio of 1: 0.2), the release of SA at T < 12 ◦ C was faster than which the swelling degree decreases with the increase of temperature. higher temperatures. The system showed the “on–off” release behaviour 6 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 4. Process for the preparation of lignin-modified epoxy resin (ER) nanocarriers loaded with abamectin (Copyright 2020 from ACS). Fig. 5. (a)The responsive mechanism of temperature-responsive polymers; (b) The “On-Off” release of salicyclic acid (SA) from thermosensitive poly (N-iso­ propylacrylamide-co-n-butyl methacrylate) hydrogel (Copyright 2015 from PUBLIC LIBRARY SCIENCE) ; (c) Synthesis procedure of biodegradable amphiphilic polyesters (Copyright 2019 from Elsevier). (Fig. 5b) so that at 1 h at 9 ◦ C induced around 30 % release of SA whereas the increase of temperature. With the phase transition temperature of by increasing temperature to 15 ◦ C the release was stopped and resumed 18 ◦ C for the nanomicelles, the release of pyrethrins reached to 32 % in again by reducing temperature to 9 ◦ C. Biodegradable amphiphilic 18 h at 18 ◦ C, whereas at 26 ◦ C, 50 % release was achieved over the same polyesters were prepared by condensation polymerization of PEG with period. 2-methylsuccinate (MSA) and 1,4-butanediol (BDO) which were able to Chlorpyrifos-loaded temperature-responsive microcapsules self-assemble into micelles in aqueous medium (Fig. 5c). These (CPF@CM) were fabricated at the O/W interface using n-hexadecane micelles could load around 20 wt% of avermectin (AVM). By having the emulsion as raw material, isophorone diisocyanate (IPDI) as cross­ LCST of 40 ◦ C, at T > 40 ◦ C, dehydration and shrinkage of the PEG linking agent, dibutyltin dilaurate (DBTDL) as catalyst, and cellulose as segment make the micelles unstable inducing the increase effect on the surfactant and shell wall material via polyaddition reaction (Fig. 6b). release rate. Complete release was archived at 45 ◦ C in 14 h in the The prepared CPF@CM had a loading efficiency of 33 wt% for CPF, water/methanol mixture. Wang et al. also used PNIPAm to modify which could effectively protect CPF from photodegradation and improve graphene oxide (GO) to prepare cyhalothrin (LC)-loaded nanocarriers its insecticidal activity. In addition, the microcapsules were (LC@PNIPAm-GO) with a LC loading efficiency of 15 %. The LCST of temperature-responsive, and the release of 36 %, 62 %, and 85 % was PNIPAm-GO was 34 ◦ C, and the cumulative release of LC reached 31 %, achieved after incubating the CPF@CM with ethanol/water mixture at 45 %, and 55 % after LC@PNIPAm-GO incubated with the ethanol–­ 15, 25, and 35 ◦ C for 24 h, respectively. Biological experiments showed water mixture (3:7, v/v) solution for 7 days at 27, 35, and 30 ◦ C, that larval mortality increased with the increase of temperature with the respectively. larval mortality of 53 % at 35 ◦ C indicating the temperature responsive In another study, three amphiphilic copolymers (including poly[2- characteristics (Fig. 6c). (2-Methoxyethoxy) ethyl methacrylate-co-octadecyl methacrylate] (P (MEO2MA–co–ODA)), monomethoxy (polyethylene glycol)-poly(D, L- Lactide-co-glycolide) (mPEG–b–PLGA) and monomethoxy (poly­ 3.2. pH-responsive carriers ethylene glycol)-poly(D, L-Lactide) (mPEG–b–PLA)) were synthesized and self-assembled with the insecticide pyrethroid to obtain The pH responsiveness of carriers is triggered by the change of the temperature-responsive nanoscale mixed micelle (MMs-Pys-7) as illus­ hydrogen ion concentration in the solution, and its responsive mecha­ trated in Fig. 6a, with the encapsulation efficiency (~78 %) and loading nism can be divided into two categories. In the first type of pH- efficiency (~10 %). MMs-Pys-7 enhanced the photostability and insec­ responsive carriers, a large number of acidic or basic functional ticidal activity of pyrethrins, and the release of pyrethrins increased with groups, especially carboxyl groups, usually exist, which are the key el­ ements to the pH responsiveness. The addition of these groups to 7 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 6. (a) Preparation of temperature-responsive nanoscale mixed micelle (MMs-Pys-7) loaded with insecticide pyrethrin and its temperature-responsive mechanism (Copyright 2018 from Elsevier). (b) Schematic diagram of CPF@CM fabrication (Copyright 2021 from Elsevier). (c)Schematic diagram of release and insecticidal activity of CPF@CM at various temperatures. natural polymers is essential for the preparation of pH responsive car­ methanol solution decreased from 99.7 % to 13.5 % in 800 min. In riers. When the pH of the environment changes, the functional groups addition, mesoporous silica nanoparticles (MSNs) are also good carriers can quickly accept or release protons to cause the swelling and shrinking for agrochemicals. Chelating copper ions (Cu2+) into MSNs nano­ of the material. The second type of pH responsive materials is particles in the presence of dopamine (MSNs-PDA-Cu, Fig. 7a) provide a accompanied by the breaking and reorganization of chemical bonds. For carrier for the controlled release of azoxystrobin (AZOX). With a loading example, in carriers containing hydrolytically cleavable bonds (imine, content of about 17 % and pH at 5.8, 7.2, and 8.6, respectively, the hydrazone, acetal, esters, etc.) the hydrolysis rate of the linkages de­ release of 60 %, 45 %, and 56 % of AZOX within 24 h was achieved. pends on the pH of the environment so that the release of loaded ag­ In another study, poly(glycidyl methacrylate-co-acrylic acid) (P(GMA- rochemicals is achieved upon the cleavage of the linkages [73,74]. AA)) was grafted on hollow mesoporous silica (HMS@P(GMA-AA)) Under field conditions, due to plant growth and soil microbial repro­ which could load around 33 wt% abamectin (Aba). HMS@P(GMA-AA) duction, the content of organic acids, phenolic acids and other sub­ was sensitive to pH, and in alkaline environment, Aba@HMS@P stances in the soil will change, resulting in pH changes in the (GMA-AA) swelled and resulted in a significant increase in the release. surrounding environment of crops. This can be used to release agro­ At pH 10, the cumulative release of Aba increased to 87 % on the 15th chemicals to promote crop growth. day under methanol–water mixture which was about 6.5 times higher Silica nanoparticles have the advantages of easy synthesis and than the release at pH 5–7 under the same condition (Fig. 7b). modification, excellent stability, and biocompatibility, and modified Porous polyethyleneimine-grafted lignin microspheres (PLM) were silica nanoparticles can effectively respond to pH, ionic strength, en­ prepared by grafting polyethyleneimine onto lignin using PEG as a pore- zymes, and redox reactions. It has received extensive attention in forming agent through a reverse suspension copolymerization method the field of drug sustained release in recent years. The grafting of algi­ (Fig. 7c). PLM has a porous structure and abundant functional groups, so nate (Alg) onto the silica surface via Ugi reaction makes the modified it has excellent adsorption performance for loading agrochemicals with silica more sensitive to pH. By increasing the pH from 3 to 8, the release the adsorption capacity of 909.09 mg/g for 2,4-dichlorophenoxyacetic of loaded pesticide (λ-cyhalothrin) in the alginate@silica carrier in 25 % acid (2,4-D). PLM was used for the pH-responsive controlled release of 8 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 7. (a) Procedure for the fabrication of MSNs-PDA-Cu nanoparticles by copper ion (Cu2+) chemical chelation of MSNs by dopamine (Copyright 2020 from Elsevier). (b) Illustration for the preparation of Aba@HMS@P(GMA-AA) for loading and release of Aba insecticide (Copyright 2019 from Elsevier). (c) Schematic diagram of the process of preparing porous polyethyleneimine-grafted lignin microspheres (PLM) by inverse suspension copolymerization (Copyright 2021 from Elsevier). (d) Plausible mechanism for the swelling behavior of CMCS-Pro hydrogels at different pH and image of mycelial growth inhibition test of G. graminis with different concentrations of Pro TC and CMCS-Pro(Copyright 2021 from Elsevier). (e) Schematic diagram of the synthesis of AMV-loaded nanocarriers AMV@P-Zein/CMC-g-PDMDAAC (Copyright 2020 from Elsevier). herbicides. The release of 2,4-D was 57 % after 72 h at pH 2 and 88 % chiral structure, and so on. Introducing metal ions with variable after 24 h at pH 11, respectively; while at pH = 7 only 10 % was released valence (such as Fe, Se, etc.) or chemical bonds (such as disulfide bonds) even after 96 h. Moreover, pH-responsive hydrogels of ionically that respond to redox environments in the gel system can produce redox Mn2+-crosslinked carboxymethyl chitosan (CMCS) were utilized for responsive gels. Although redox-responsive carriers have achieved loading of 22 wt% of prothioconazole (Pro). Lower swelling of the great development in the field of medicine, redox hydrogels loaded with hydrogel at lower pH reduced the release rate, so that the release content drugs can achieve high-efficiency and controlled release at cancerous of around 34 and 94 % were achieved, respectively, at pH 7.5 and 8.5 sites , but their research in agriculture is still in its infancy stage. after 4 h. However, the release was reduced to 15 % at pH 5.6 even after Glutathione (GSH) is an antioxidant commonly found in plants and in 32 h. The antifungal activity of the hydrogel was verified by the myce­ the soil of plant roots. It can undergo a reversible exchange reaction with lium growth inhibition test of G. graminis, and the inhibition rate disulfide bonds, break the disulfide bonds, and achieve drug release increased with the increase of Pro concentration (Fig. 7d). Under the , thus realizing the slow release of agrochemicals from redox car­ concentration of ingredients, the fungicidal ability of CMCS-Pro against riers. The redox responsive carriers have a high selective response to the G. graminis was better than that of Prothioconazole original drug(Pro release behavior of oxidants and reducing agents in the natural envi­ TC). Incorporation of CMC-g-PDMDAAC (PDMDAAC - poly dia­ ronment, and are not easy to be interfered by other stimulus signals and llyldimethylammonium chloride) into the phosphorylated zein afforded complex chemical environments in the environment. Therefore, a redox nanocarriers for loading of abamectin (AVM) (Fig. 7e) with encapsula­ responsive hydrogel is considered as a better agrochemicals’ controlled tion efficiency 82 %. This encapsulation improved the UV resistance of release carrier than a pH-responsive one. AMV by 10 %, and the release rate of AVM from the carrier was CMC was crosslinked with cystamine to afford redox-responsive decreased to 10 % from pH = 3 to pH = 9 within 300 h. hydrogels as a proper carrier for loading of agrochemicals. Reducing agents (such as DTT) induce the disulfide cleavage and 3.3. Redox-responsive carriers decomposition of the hydrogel whereas in the presence of oxidizing agents (such as H2O2) re-formation of the disulfide bond endows re- Redox responsive hydrogels can response to external redox reactions construction of the hydrogel. The sulfhydryl groups created after the or electric field stimulation by changing the self-assembly state of the gel dissociation of the hydrogel under reducing conditions could complex agent, and are accompanied by changes in color, fluorescence reactivity, heavy metal ions in the environment, so that the gel was endowed with 9 L. Zheng et al. European Polymer Journal 177 (2022) 111432 the dual functions of slow release of agrochemicals and soil remediation 3.4. Light-responsive carriers. This work represents the first attempt in utilizing unique proper­ ties of the redox-responsive systems for agricultural applications. Simi­ The light-responsive carriers can cause physical or chemical changes larly, Li et al. used carboxymethyl-β-cyclodextrin-modified nano-silica within or between molecules, such as discoloration, photolysis, poly­ to synthesize redox-responsive nanoparticles through surface cross­ merization, and isomerization, after absorbing light energy. The light- linking with cystamine that also responded to reducing agents (GSH) in responsiveness of the carriers is mainly realized by introducing photo­ the environment via disulfide cleavage, achieving the functions of slow isomerization functional groups, such as azobenzene, spiropyran (SP) release of salicylic acid and adsorption of heavy metal ions (Fig. 8a). Taking SP as an example, under irradiation of different wave­. lengths, the reversible conversion of hydrophobic SP molecules to Thiolation of the surface of MSNs by reaction with (3-mercapto­ zwitterionic MC moieties can be easily achieved (Fig. 9a). Therefore, the propyl)trimethoxysilane (MPTMS) and then conjugation with dec­ corresponding carriers have light-regulating membrane permeability, anethiol through the formation of disulfide bonds afford an redox- which endow controllable drug release under the regulation of UV–vis responsive carrier (Fig. 8b) for loading and release of SA. The cleav­ light. As agrochemical carrier, light-responsive materials have the age of disulfide bonds occurred upon exposure to GSH, which removed characteristics of convenience, efficiency and intelligence. Light can hydrophobic decanethiol groups from the surface of the carrier, leading precisely control the time and position of stimulation. Therefore, light- to the almost complete release of SA after 800 and 180 min in the responsive materials have special advantages over other materials. presence of 5 and 10 mM GSH. In addition, surface functionaliza­ Near-infrared (NIR) light-responsive materials have attracted wide­ tion of boron nitride nanosheets (BNNSs) with thiol-terminated poly­ spread attention due to their ability to adapt to their characteristics ethyleneimine (PEI-SH) and ethylene glycol diacrylate (PEG) remotely and spatially using a wealth of light sources. The near-infrared simultaneously introduced disulfide bonds via thiol oxidation to provide (NIR) photoresponsive nanocarriers of polydopamine-modified titanium a redox-responsive carrier (Fig. 8c). This carrier could load around 338 carbide (PDA@Ti3C2Tx) could physically encapsulate the pesticide EB. and 389 mg/g of tea tree essential oil (TTO) and triadimefons (Tri), This carrier could release 31 % of EB after 240 h in 30 % ethanol solution respectively. Around 14 % of TTO could be released in 600 min at 40 ◦ C. in the absence of light. NIR irradiation induced 20 % increase the release Additionally, the sustained release of TTO could provide a long-term to finally reach the release level of around 50 % during the same time antibacterial activity of the TTO-loaded system against E. coli. period. Beyond this, polyethylene glycol (PEG) and α-cyclodextrin Furthermore, encapsulation of TTO improved the stability against (α-CD) modified hollow carbon microspheres (HCMs) were used to UV–vis degradation up to 55 %. prepare photoresponsive hydrogels loaded with pesticide imidacloprid Fig. 8. (a) Schematic of the process of redox-responsive nanocarriers formation, hormone release and heavy metal ion adsorption in the presence of GSH (Copyright 2020 from ACS). (b) Schematic of functionalization of MSNs with -SH groups and assembly of decanethiol on to SA-loaded MSNs (Copyright 2015 from ACS). (c) Strategy for the fabrication of BNNS-PEI-PEG as a redox-responsive carrier for tea tree essential oil(OTT) and triadimefons(Tri) (Copyright 2021 from Elsevier). 10 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 9. (a) Reversible conversion of spiropyran (SP) under the action of light to prepare reversible bilayer permeable photochromic polymers (Copyright 2015 from ACS). (b) Schematic diagram of the preparation of HCM photoresponsive hydrogel and its controlled release of agrochemicals under the action of infrared light due to photothermal effect (Copyright 2021 from ACS). (c) Image of the root growth inhibitory effect of 2,4-D loaded CMCS-DEACMS micelles on the target plant cucumber; (i) control with deionized water, (ii) free 2,4-D and (iii) 2,4-D-loaded CMCS-DEACMS micelles (Copyright 2020 from MDPI). (CMSS = car­ boxymethyl chitosan; DEACMS = (7-diethylaminocoumarin-4-Yl) methyl succinate; 2,4-D = 2,4-dichlorophenoxyacetic acid). (IMI) (Fig. 9b). Due to the excellent photo-thermal effect of HCM, under The capsule material is broken under the action of cutting enzymes and the irradiation of infrared light, the generated heat could destroy the gel cell degrading enzymes to release the agrochemicals. Despite the network to release IMI, making the release of IMI about 2.6 times faster current research on enzyme-responsive carrier materials has made some than that under sunlight. Liu et al. prepared near-infrared progress, there are only very few studies on enzyme-responsive hydro­ responsive poly(N-isopropylacrylamide) (PNIPAm) hydrogels using gels, and researchers need to further explore. different contents of cellulose nanocrystals (CNCs) coated with poly­ Avermectin (AVM) could be loaded with 8 wt% loading efficiency in dopamine (PDA) as carrier for 5-fluorouracil (5-FU) with the loading esterase-responsive PEG-CMC nanoparticles, which were formed by efficiency around 10 %. The responsive release behavior can be remotely esterification crosslinking of CMC with PEG using the carbodiimide controlled via the “on–off” switch of NIR irradiation. When near- coupling agent. Nanoparticle encapsulation makes AVM resistant to infrared radiation was turned on, the release of 5-FU from photodegradation, leading to a sustained release effect. The release rate PDA@CNC/PNIPAm hydrogel was significantly increased. Upon turning of AMV in nanoparticles within 103 h was less than 70 %, but it was off the infrared irradiation, the release rate was reduced. The cumulative accelerated above 80 % in the presence of esterase over the same period release of 5-Fu from PNIPAm hydrogels containing 15 wt% PDA@CNC. An α-amylase responsive carrier with excellent UV and heat could reach up to 51 % after three switching “on–off” cycles in 35 min, shielding ability was prepared by anchoring α-cyclodextrin (α-CD) on much higher than that from PNIPAm hydrogel (18 %). the surface of hollow mesoporous silica (HMS). The produced CD-HMS Incorporation of (7-diethylaminocoumarin-4-Yl) methyl succinate could encapsulate about 38 wt% AMV. After 17 days the release (DEACMS) onto the backbone of carboxymethyl chitosan (CMCS) could reached to around 6 % and 42 % in the aqueous medium in the absence yield photoresponsive micelles for controlled release of herbicide 2,4- and presence of α-amylase. Indeed, the amylase hydrolyzes the alpha dichlorophenoxyacetic acid (2,4-D). With 10 wt% encapsulation, bonds in CD and degrade it into glucose monomers, resulting in the simulated sunlight exposure promoted the release of 2,4-D from the acceleration of the release. micelles such that approximately 90 % of the 2,4-D was released within Zein nanoparticles can be used as carriers to load agrochemicals with 500 min whereas in the absence of light release was around 65 %. The high encapsulation efficiency; and the nanoparticles are responsive to herbicidal activity of 2,4-D-containing micelles was verified by inhibit­ proteolytic enzymes present in insect guts. Monteiro et al. used zein ing the root growth of cucumber (Fig. 9c). The root growth inhibition for nanoparticles loaded with the plant compounds limonene (LIM) and 2,4-D-containing micelles was similar with that for free 2,4-D, which carvacrol (CVC) to perform an enzymatic degradation assay of LIM + could reduce cucumber root length and fresh weight by about 23 % and CVC-loaded nanoparticles with trypsin. Coincidentally, Camara 50 %, respectively. et al. also used zein nanoparticles to load agrochemicals, and under the action of trypsin, the nanoparticles were degraded by about 89 %. 3.5. Enzyme-responsive carriers The agrochemicals loaded in zein nanoparticles often show better bio­ logical activity and controlled release effect, and are promising in terms When pests interact with plants, a series of changes will occur, of application prospect. including changes in plant enzyme. Enzyme-responsive carriers can produce physical and chemical reactions to this stimulus. Enzyme- 3.6. Multi-stimuli-responsive carriers responsive carriers can change their hydrophilicity under the direct action of enzymes; and the charges can be introduced or removed, Multi-responsive carriers refer to the materials with two or more causing the formation or destruction of hydrogels to achieve the purpose responsive groups that enable the material to have two or more of adsorption and release of agrochemicals. Enzyme-catalyzed re­ responsive properties. Multi-response carriers have broader develop­ actions are highly selective and the reaction conditions are relatively ment prospects. mild, so they are widely used in biomedicine, environmental engineer­ pH and enzyme dual-responsive carriers are among the most studied ing, catalysis and other fields. The principle of controlled release of ones. Electrostatic self-assembly of sodium lignosulfonate (SL) and enzymatic pesticide formulations can be divided in two aspects: the dodecyldimethylbenzylammonium chloride (DDBAC), along with in situ transformation and degradation of agrochemicals to form active pesti­ mineralization with iron ions, was used to prepare pH- and laccase- cide molecules; the degradation of carrier materials containing pesticide responsive nanopesticide carriers (Fig. 10a) for loading lambda- molecules to achieve targeted applications. When lepidopteran insects cyhalothrin (LC). pH conditions in the lepidopteran gut and the con­ harm crops, they will come into contact with pesticide microcapsules. tained laccase (a lignin-degrading enzyme) could be manipulated to 11 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Fig. 10. (a)Schematic illustration of sodium lignosulfonate (SL) and dodecyldimethylbenzylammonium chloride (DDBAC) forming pH- and laccase-responsive nanopesticide carriers loaded with λ-cyhalothrin (LC) (Copyright 2021 from Wiley). (b) Procedure for the synthesis of dual pH and redox-responsive CMC- based nanogels (Copyright 2019 from ACS). (c) Preparation and response mechanism of dual-responsive DACMC-CYS-Fe hydrogels (Copyright 2019 from Elsevier). trigger the sustained release of the pesticide by the carrier. It was found resulting in a controlled release of 6-BA (6-benzyladenine). that alkaline conditions improved the release of LC by about 9 % within Enzymes and redox dual-responsive carriers are also of importance 24 h. The presence of laccase also accelerated the release of LC, with a as multi-responsive carriers in agriculture. A starch-encapsulated, AMV- 24 % increase in 50 U/g laccase solution and a 43 % increase in 100 U/g loaded, disulfide-bonded MSNs nanoparticle with dual redox and laccase solution. Grafting of hydroxypropyl cellulose (HPC) onto α-amylase responses was prepared with AMV loading efficiency about hollow mesoporous silica nanoparticles (HMS) via ester bonds renders 9.3 %. The nanoparticles are able to prevent the photo-degradation and the carriers pH/cellulase dual stimulus responsive. With loading ca­ premature release of AMV. When the nanoparticles in plant leaves are pacity of 12 wt% for pyraclostrobin (PYR), release rates of around 80, 73 eaten by insects that secrete glutathione and alpha-amylase, the struc­ and 18 % at pH values 3, 5, and 7 were obtained respectively, after 144 ture of the nanoparticles can be broken down, thereby releasing AMV for h. Furthermore, when crops are attacked by phytopathogens, the insecticidal effects. Glutathione can break the disulfide bond in the generated cellulases can be used to trigger the fast release of agro­ nanoparticles to accelerate the release of AMV. At the concentration of chemicals. Practically, at pH 7, the presence of cellulase increased the glutathione of 2 mM and 8 mM, the cumulative release of AMV reached release of around 5.4 times. In addition to its excellent dual-responsive 21.7 % and 51.3 % after 7 days, respectively. However, in the absence of behavior, this carrier could effectively prevent the photolysis of PYR added glutathione, only 8.2 % of AMV was released from the nano­. particles. Adding α-amylase to nanoparticles allowed to degrade starch The group led by Xiao reported a dual pH- and redox-responsive on the surface to accelerate drug release, so that the cumulative release CMC-based hydrogel fabricated through a three-step process (Fig. 10b) of AMV reached 72.2 % after 7 days. In addition, Chen et al. : (i) partial hydrophobization of CMC by acylating with palmitoyl introduced disulfide bonds between functionalized mesoporous silica chloride; (ii) inserting of the aldehyde groups onto the palmitoyl-CMC, microcapsules and pectin to prepare an enzyme and a redox dual- and (iii) crosslinking by reaction of the aldehyde groups with 3,3′ - responsive carrier, which could load 20 wt% of the fungicide kasuga­ dithiobis-(propionohydrazide). The presence of acylhydrazone and di­ mycin. The encapsulation improved the thermal stability and photo­ sulfide bonds rendered the gel dual-responsive to pH and redox condi­ stability of Kasugamycin; whereas the carrier possessed dual responsive tions. This system could load around 40 wt% SA and the release of SA properties toward both pectinase and glutathione. At 25 ◦ C and was obviously a function of the pH and redox condition of the medium. pH = 7.0, the cumulative release rate of kasugamycin in microcapsules Under acidic condition (pH 3.5), hydrazone linkage was cleaved, which was 51 % after 60 days. When adding pectinase, the release of kasuga­ led to the dissolution of the nanogel and complete release of SA within 8 mycin was 19 % after 1 h, more than 60 % after 13 h and reached 69 % h. Similarly, GSH solution can also significantly accelerate the release of after 30 h. Adding pectinase showed a similar trend, and the cumulative SA through cleavage of the disulfide bonds. By increasing the GSH release reached 78 % at 30 h. When pectinase and GSH acted simulta­ concentration from 10 to 20 mmol/L, the required time for complete neously, 29 % of Kasugamycin was released after 1 h, more than 80 % release of SA was reduced from 12 h to 6 h at pH = 7. More interestingly, released after 13 h, and reached 88 % after 30 h. after the release of SA from the gel, the carboxyl and thiol groups in the In addition to the multi-responsive carriers mentioned above, there gel were able to complex with heavy metal ions, resulting in an 89 % are also other types of dual-response carriers such as temperature and removal of copper(II) ions in the simulated soil leachate, thus creating a pH, temperature and light, light and pH, etc. In practical applications, dual-functional carrier for both responsive release of agrochemicals and the multi-responsive carriers can be designed as needed. Li et al. pre­ soil remediation. The same group also attempted to crosslink pared a spinosyn/chitosan formulation (SCF) with an encapsulation ef­ dopamine-modified carboxymethyl cellulose (DA-CMC) with cystamine ficiency of 60 % and dual pH and temperature responsiveness. The (CYS) to construct another type of pH- and redox-responsive hydrogel release rate of spinosyn showed excellent sustained-release properties (DACMC-CYS-Fe) in the presence of Fe3+ as a secondary crosslinking under neutral and alkaline conditions, i.e., about 80 % after 18 days at agent through ionic interactions (Fig. 10c). The hydrogel exhibited a both pH 6.8 or 7.4. Spinosyn release from SCF increased slightly with reversible sol–gel transition when exposed to pH and/or redox changes, increasing temperature, and the cumulative release of the formulation 12 L. Zheng et al. European Polymer Journal 177 (2022) 111432 after 18 days at 50 ◦ C (pH = 6.8) was about 89 %. The PDA@P­ biodegradable when exposed to natural ultraviolet rays. Specifically, the NIPAm nanosystem loaded with IMI exhibited good dual responsiveness carriers can easily degraded into water and carbon dioxide under the to temperature and infrared light. About 20 % of IMI was released within action of soil microorganisms. Nonetheless, the impact of the 5 h at 25 ◦ C, while at 40 ◦ C, the sustained release was as high as 64 % long-term use of polymer-based carriers also needs to be considered, i.e., within 5 h. And, after 30 min of NIR irradiation, the cumulative release whether there are unpredictable long-term consequences on the struc­ of IMI was 16 %, almost 3 times that of no light irradiation (5.2 %) ture of soil microbial populations and the effects on plants and their. An l- infrared light and pH dual-responsive nanocomposite was microenvironment should be taken into account. In fact, the potential prepared using graphene oxide (GO) and polydopamine (PDA) as carrier for scaling up the application of carriers in farm lands still needs to be for loading of oxamidil. Approximately 38, 48 and 65 % of oxamidil was further elaborated. released from the nanocomposite within 120 h at pH 5, 7 and 9, respectively. The release of oxamidil in the NIR-irradiation was 10 % 5. Conclusions and outlook higher than that in the non-irradiated group. The use of polymer-based carriers can considerably enhance the use 4. Environmental impact of polymer-based carriers efficiency of agrochemicals and improve crop yield and quality. Among various carriers, stimuli-responsive polymer-based carriers enable the According to the introduction in the second and third parts, encap­ controlled release of agrochemicals in a precise and smart manner, and sulating the agrochemicals in the polymer-based carrier can have a slow- have great potential for the applications related to crop insecticide and release effect to reduce the use of agrochemicals and prevent their sterilization. Moreover, polymer-based stimuli-responsive carriers can diffusion in the soil, thus creating tremendous ecological and economic have a variety of responsiveness, and create synergetic effects on the benefits. Stimuli-responsive carriers can release agrochemicals on actual controlled releases of agrochemicals, the adsorption or stabilization of need, which not only promote crop growth, but also has great applica­ heavy metal ion in soil and biosensing applications, thus maximizing tion prospects in insecticide and weed control (Fig. 11a). On one hand, their roles. Such application prospects represent an important direction the application of polymer-based carriers in agrochemicals has the ad­ for future agricultural development. On the other hand, the release of vantages of reducing the amount of agrochemicals and improving the agrochemicals like pesticide is susceptible to changes in soil type, stability of agrochemicals. On the other hand, the actual application moisture content and other influencing factors, which may cause agro­ process need to be considered from other aspects also such as whether chemical release to be out of sync with plant demand. Those concerns the polymer-based carrier has ecotoxicity, or might cause secondary need to be considered during the application of polymer-based carriers, pollution, as well as how to recycle or degrade the polymer-based and meanwhile, much attention should also be paid to the potential carriers. ecotoxicity of polymer-based carriers themselves. It is absolutely Prior to exploring their promising applications in agriculture, necessary to further evaluate the environmental impact of polymer- various natural and synthetic biodegradable polymer-based carriers based carriers in practical use, and the properties and stability of the have already been studied extensively for other applications like vaccine materials need to be further explored. The future research focuses of development , drug delivery and food processing [115–128]. polymer-based carriers are expected as follows: to explore greener or Therefore, there is also growing concern about the safety associated with bioresource-based and smart polymeric carriers; to further enhance the polymer-based carriers. Hazardous substances in polymer-based car­ loading efficiency of agrochemicals on carriers; to tailor-control the riers, when used in agriculture, interact with inorganic, organic, or release rate of agrochemicals from carriers along with the optimized, biological components in the environment, pass through the ecosystem, biodegradability of carriers. The extensive application of polymer-based and ultimately enter animals or humans. The behavior of nano­ carriers in agriculture requires continuous exploration by researchers. carriers in the soil system is shown in the Fig. 11b. After entering the soil, in addition to the processes of degradation and deposition, the Funding carriers can be also aggregated by interacting with various soil compo­ nents (organic and inorganic) according to their properties, whereas the This study was supported by Provincial Key Lab of Pulp and Paper hetero-aggregation appears to be more common. Of course, some Science and Technology and Joint International Research Lab of polymer-based carriers, those bioresource-based ones in particular, have Lignocellulosic Functional Materials, Nanjing Forestry University; the superior biodegradability and are harmless substances. For example, National Nature Science Foundation of China (Grant No.: 31730106, such hydrogels can be readily absorbed by plant tissues and are highly 31770623) and NSERC Canada. Fig. 11. (a)The nanocarrier releases the agrochemicals in the carrier under the stimulation of light, pH, enzymes, etc., and acts on the microenvironment of the plant foliar or soil, so as to achieve the functions of promoting crop growth, killing insects, weeding, and soil remediation;(b)Various reactions of of nanomaterials in soil system (Copyright 2020 from Elsevier). 13 L. Zheng et al. European Polymer Journal 177 (2022) 111432 Declaration of Competing Interest P.A. Morton, C. Fennell, R. Cassidy, D. Doody, O. Fenton, P.E. Mellander, P. Jordan, A review of the pesticide MCPA in the land-water environment and emerging research needs, Wiley Interdiscip. Rev.: Water 7 (2020), e1402. The authors declare that they have no known competing financial H.J. He, Y.P. Liu, S.H. You, J. Liu, H. Xiao, Z.H. 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