Metal–Organic Frameworks in Agriculture

Agrochemicals, which are crucial to meet the world food qualitative and quantitative demand, are compounds used to kill pests (insects, fungi, rodents, or unwanted plants). Regrettably, there are some important issues associated with their widespread and extensive use (e.g., contamination, bioaccumulation, and development of pest resistance); thus, a reduced and more controlled use of agrochemicals and thorough detection in food, water, soil, and fields are necessary. In this regard, the development of new functional materials for the efficient application, detection, and removal of agrochemicals is a priority. Metal–organic frameworks (MOFs) with exceptional sorptive, recognition capabilities, and catalytical properties have very recently shown their potential in agriculture. This Review emphasizes the recent advances in the use of MOFs in agriculture through three main views: environmental remediation, controlled agrochemical release, and detection of agrochemicals.


CURRENT CHALLENGES OF AGRICULTURE
Agrochemicals or agrichemicals (primarily fertilizers and pesticides) have become a fundamental part of today's agricultural systems in order to fulfill the huge requirement of food. Agrochemicals can be classified on the basis of various principles, such as toxicity, target, chemical composition and formula, mode of entry or action, and source. Here, we will classify them according to their mode of action, although their toxicity, target, and origin will also be discussed. Thus, agrochemicals can be divided into pesticides (insecticides, herbicides, fungicides, rodenticides, algaecides, molluscicides, and nematicides), fertilizers (mainly supplying macronutrients: N, P, and K), soil conditioners (improving the soil's physical and mechanical qualities), liming (Ca and Mg) and acidifying agents, and plant growth regulators (also known as phytohormones).
With a dramatic increase after the second World War, 1 the intensive use of agrochemicals has deteriorated the quality of ecosystems (living beings, groundwaters, soils) by impacting human health and, in recent years, leading to the development of pesticide-resistant strains. 2−4 Over the period of 2011− 2018, pesticide sales were around 360,000 tons per year only in the European Union (EU) with the major groups sold being fungicides, herbicides, and bactericides. This is particularly crucial since, according to the Food and Agriculture Organization (FAO) of the United Nations, agriculture occupies about 38% of Earth's terrestrial surface. 5 High and repeated doses of hazardous agrochemicals are routinely used to protect crops against pests (insects, fungi, unwanted plants, and others) and boost food productivity (e.g., increasing the number of times per year a crop can be grown on the same territory). With a global population projected to rise above 9.7 billion by 2050, food security is of increasing importance. Herbicides are the most widely used pesticides, comprising >40% of total use, while insecticides and fungicides constitute approximately 30% and 20%, respectively. Pesticide/fertilizer pollution patterns are well-established with a major pollution peak taking place a few days or weeks after agrochemical application. 6 Ideally, their toxic effect should be limited to both the target area and organisms. However, the lack of specificity of agrochemicals and their widespread use (i.e., in 2018 almost 400,000 tons of pesticides were sold in Europe) 7 allow them to leach out of the soil and enter surface water and groundwater; therefore, they are even present in drinking water. 8 On the basis of their application methods, between 10% and 75% of the pesticides do not reach their targets, 9,10 resulting in frequent contamination of terrestrial and aquatic environments. 9−11 The EU Drinking Water Directive sets the general drinking water quality standard for added concentrations of pesticides and their metabolites to be less than 0.5 μg·L −1 . Remarkably, most of the studies in this field report that ∼80% of the studied pesticides are found in concentrations much higher than the EU water quality standard (e.g., 3-fold higher concentrations of tebufenpyrad and pendimethalin in the Louros River in Greece, 12 21-and 26-fold higher concentrations of glyphosate and aminomethylphosphonic acid (AMPA) in the area of Zurich, Switzerland, 13 40-, 25-, and 20-fold higher concentrations of amitrole, diuron, and terbuthylazine in the Arc River in France, respectively, 14 and 8-, 12-, 16-, and 25-fold higher concentrations of oxadiazon, pretilachlor, bentazone, and 2-methyl-4-chlorophenoxyacetic acid (MCPA) in the Rhone River in France, respectively, 15 among others).
Despite the strict EU regulation, pesticides continue enter the food chain through water and food. Regarding other regions, the problem is magnified. For example, it is predicted that in 2050 the major part of global chemical sales will take place in Asia. During the last two decades, South-East Asian countries have shown a strong industrial growth in agriculture. 16 However, the vast majority of these countries lack the capacity to handle chemical management issues and, furthermore, they still need to develop legislation, institutions, and general awareness. Therefore, this should be considered a global environmental problem. In terms of acute toxicity to humans, many agrochemicals manifest their toxicity through biochemical and functional actions in the central and peripheral nervous system. Also, although not always easy to identify, there is evidence that links long-term exposure to some pesticides with chronic illnesses, including dermal, respiratory, liver, and kidney disorders, 17 fertility difficulties, 18,19 postponed neuropathy, 20 and cancer (e.g., sarcoma, lung, brain, gonads, liver, digestive system, and urinary tract). 20,21 In this sense, it is likely that the scale and outcome of pesticide-associated chronic effects are underestimated as the symptoms of such poisonings may be incorrectly attributed to other affects. Aside from toxicity to humans, in terms of environmental costs, the unsystematic use of agrochemicals increases pest and disease resistance, diminishes nitrogen fixation and soil biodiversity, and increases the bioaccumulation of pesticides. 22 Finally, the loss of livestock to resistant bacterial diseases also represents a considerable waste of water and energy investment as well as capital.
Apart from pesticides, fertilizers are among the major contributors to raise crop yield, and therefore, their use has been exponentially enhanced over the past decades (annually >3 million tons have been imported into the EU since 2015). 23 However, as for pesticides, the use of chemical fertilizers is limited by their poor specificity, increasing both the environmental and production costs (between 50% and 70% of total applied nitrogen is lost by volatilization 24,25 and 5−10% is lost by leaching). 26 Further, inefficiencies in the production of food are further intensified by food waste (i.e., ∼33−50% of global manufactured food spoils as consequence of microbial contamination). 27 The actual scenario of the inefficient use of fertilizers and intensive irrigation, biocides, and processed food is stressing ecosystems and leading to significant environmental collateral injuries (e.g., increasing soil erosion and degradation, loss of biodiversity, rising water withdrawals, reducing water quality, eutrophication, disruption of global nutrient cycles, and increasing the energy consumption and greenhouse gas emissions). 5 All the above suggests that water, food, nature, and animal and human health are inextricably linked to the agri-food systems.

NANOTECHNOLOGY AS A NOVEL APPROACH IN
AGROCHEMICAL DEVELOPMENT The increase of society's concern regarding the potential damage of agrochemical application in agricultural production has challenged industry and researchers to search for new efficient and safer methods against insect pests, infections, and unwanted plants or weeds. In this sense, nanotechnology research has recently received an increasing attention in agriculture. With the general aim of developing delivery nanosystems for agrochemicals, 28,29 nanopesticides and nanofertilizers have been proposed as a novel class of plant protection and growth products that promise a number of benefits to agriculture, the environment and, finally, human health. One of their key drivers is the important reduction in the quantity of agrochemicals necessary to guarantee crop protection and growth, which may be achieved by different ways, such as (i) improved apparent solubility and stability of photo-and thermolabile agrochemicals or active ingredients (AIs), (ii) controlled release and targeted delivery of AIs, and (iii) enhanced bioavailability and adhesion ( Figure 1).
Nanocarriers of agrochemicals of different natures have been described, including the known "soft" nanoparticles (NPs) (e.g., polymers, lipid, and nanoemulsions) as well as "hard" nanomaterials, such as silica NPs, 30−34 nanoclays, 35 TiO 2 , 36 carbon nanotubes, 37 or graphene oxides. 38 Nanocarriers are mainly applied in plant nutrition with the final objective of an increased efficiency of the actually used fertilizers either by enhancing the administration of elements that are poorly bioavailable (P, Zn) or by reducing losses of mobile nutrients to other natural environments (nitrate). However, long-term instability, subsequent burst agrochemical release, and associated toxicity are some of the major drawbacks that need to be addressed.

METAL−ORGANIC FRAMEWORKS AS PROMISING
MATERIALS IN AGRICULTURE Among the novel technologies proposed in agriculture, metal− organic frameworks (MOFs) have gained a significant role in the fields of the elimination of agrochemicals (adsorption and/ or photodegradation) and sensing. MOFs are considered to be a remarkable class of highly porous coordination polymers, containing inorganic nodes (e.g., atoms, clusters, or chains) and organic linkers (e.g., carboxylates, nitrogenated, or phosphonates), that assemble into multidimensional periodic lattices. 40 MOFs have been proposed for many societal and industrially relevant applications, such as adsorption, 41 separation, 42 magnetism, 43 luminescence, 44 conductivity, 45 sensing, 43 catalysis, 46 energy, 47 drug delivery, 48 etc. In particular, MOFs are promising materials in agriculture due to their interesting properties: (i) versatile hybrid compositions, which allow a huge variety of combinations, (ii) large specific surface areas and pore volumes, related to exceptional sorption capacities, (iii) simply functionalizable cavities, where specific host−guest interactions may occur, (iv) synthesis at large scale (some of them are already commercialized), and (v) an adequate stability profile, so they are stable enough to accomplish their function and, after being degraded, prevent associated toxicity in animals/plants due to their accumulation.
Different strategies have been reported in the use of MOFtype materials in agriculture. In particular, related to agrochemicals, MOFs have been proposed (i) in water remediation through the elimination (adsorption/degradation) of agrochemicals or derived products, (ii) as carriers for the controlled release of agrochemicals, and (iii) as sensors for the determination of these molecules in water or food ( Figure  2). While not many reviews have detailed the use of MOFs in the elimination of agrochemicals as contaminants in water 49−53 or their potential in the detection and quantification of these potentially toxic molecules, 51,54,55 the use of MOFs as agrochemical delivery systems is a very recent research field, initiated in 2015. 56 Grouped by their function, this Review will discuss the MOFs and MOF-based composites that have been investigated to date in the agricultural domain. In order to give a broad spectrum of benefits and drawbacks of the use of each material, particular features of each structure and its properties are also included. In the text, the most original, interesting, and promising MOFs in agriculture will be highlighted, although all the reports currently found in the literature are summarized in Tables 1 to 3.
4.1. Adsorption Processes. Regarding the adsorption of herbicides, the first study was published by Jung et al. in 2013 for the removal of 2,4-D using the MIL-53(Cr) material ([Cr(OH)(BDC)]; H 2 BDC: benzene-1,4-dicarboxylic acid, pore size of ∼8 Å). 57 MIL-53(Cr) exhibited an efficient and fast adsorption (556 mg·g −1 in 1 h) with an adsorption capacity much higher than that of activated carbon (286 mg· g −1 ) or zeolite (256 mg·g −1 ). Importantly, the adsorption of 2,4-D at a very low concentration is 5-fold greater that of activated carbon at a plateau concentration, demonstrating the utility of MIL-53(Cr) in commercial uses for consumed water with low 2,4-D levels. Finally, the recyclability of MIL-53(Cr), after washing the MOF with a mixture of water/ethanol, was also noticeable after 3 cycles, suggesting the potential application of this MOF on the herbicide's removal. In a more recent work, a series of furan-thiophene derived from Cr-MOF MIL-101(Cr) ([Cr 3 (O)X-(BDC) 3 (H 2 O) 2 ], X = OH or F; Brunauer−Emmett−Teller surface area (S BET ) ∼ 4100 m 2 g −1 ; pore volume (V p ) = 2.02 cm 3 g −1 ; pore size of 11.7 and 16 Å) was achieved via visible-light-mediated C−C bond-forming catalysis within photosensitizing porous materials. 58 The process of trapping the guest molecule was accomplished under metal-free and very mild conditions, leading to novel functionalized MOFs with more π−π stacking, H bonding properties, and outstanding adsorption capacity to eliminate herbicides from the aqueous solutions. The synthesized MOFs removed up to 96.9% of the tested herbicides from the aqueous solutions even at initially very low herbicide concentrations (30 ppm pore size of ∼11 and 8 Å; H 2 BDC-NH 2 : 2-aminoterephthalic acid) was described for the removal of herbicides in water. 59 UiO-66-NH 2 was loaded on the carbon nanotube aerogels (MPCAs) by the in situ nucleation and growth of the UiO-66-NH 2 NPs onto the carbon nanotubes (UiO-66-NH 2 @MPCA). The study on the adsorption of chipton and alachlor demonstrated that the adsorption capacity of UiO-66-NH 2 @    achieving a total GLY loading of 1516.02 mg·g −1 (or 8.97 mg· g −1 ) in only 20 min. In the second work, the same authors thoroughly investigated the mechanism governing ATZ adsorption on Zr 6 -based MOFs (UiO-66-X, where X = H, OH, NH 2 ; DUT-52; UiO-67; NU-901; NU-1000; and NU-1008) by investigating the impact of MOF used linkers and topology on ATZ uptake capacity and kinetics. 61 Among all the tested Zr-MOFs, it was found that the mesopores of NU-1000 facilitate the rapid ATZ uptake, saturating in less than 5 min. Excluding the pyrene-based linker, NU-1008 ([Zr 6 (μ-O) 4 (μ-OH) 4 (HCOO)(H 2 O) 3 (OH) 3 (TCPB) 2 ]; TCPB: 1,2,4,5-tetrakis(4-carboxyphenyl)benzene; S BET ∼ 1400 m 2 · g −1 ; pore size of ∼14 and 30 Å) removed <20% of the exposed ATZ. The pyrene-based linker seems to offer enough sites for π−π interactions with ATZ as revealed by the near 100% uptake ( Figure 4). These results indicate that the ATZ uptake in NU-1000 stems from the existence of a pyrene core in the linker of the MOF, which confirms that the π−π stacking is the main force of the ATZ adsorption. Finally, the cyclability of the MOF was demonstrated through 3 adsorption−desorption cycles.

Catalytic Processes.
The number of studies related to the catalytic degradation of agrochemicals using MOFs or MOF composites are very limited. The literature mainly focuses on the use of MOF composites with only one work based on a simple MOF. This study describes the bifunctional nanoscale porphyrinic MOF PCN-224 (or [Zr 6 (TCPP) 1.5 ]; H 2 TCPP: tetrakis(4-carboxyphenyl)porphyric; S BET = 2600 m 2 ·g −1 ; V p = 0.95 cm 3 ·g −1 ; pore size = 1.9 nm) as both the sensor for the recognition of trace NIT and the photocatalyst to enable the pesticide degradation. 62 The intense fluorescence of the probe was quenched by NIT, leading to a sensing range from 0.05 to 10.0 μg·mL −1 . The potentiality of PCN-224 in the degradation of NIT was further identified. The photodegradative effectiveness was up to 95% after only 20 min of laser irradiation, whereas no significant NIT degradation was detected under darkness, regardless of the PCN-224 presence. Therefore, this material could be established as an all-in-one nanoplatform for pesticide sensing, detection, and posterior photodegradation in agricultural farmland and other environments. Among other MOF-based composites, metallic NP composites are the most employed. The MIL-53(Fe)@AgIO 3 composite was successfully applied in the decomposition of two organophosphate pesticides (OPs; malathion methyl and chlorpyrifos) under sunlight irradiation. 63 After 1 h of solar light irradiation, 78−90% of both pesticides was individually degraded in tap and distilled water. In binary mixtures (both composites), 70% of mineralization is achieved after 3 h. Another example of metallic NPs@MOF composites is magnetic α-Fe 2 O 3 @MIL-101(Cr)@TiO 2 for the degradation of paraquat herbicide from aqueous solution. 64 A maximum photocatalytic degradation was achieved at optimal conditions (see Table 1; 87.5% after 45 min), demonstrating the utility of   these systems in the photocatalytic degradation of agrochemicals in water.
MOFs have also been used as support for the stabilization of enzymes able to degrade agrochemicals in wastewater and soil. Gao et al. described the preparation of a hierarchically porous MOF (H-MOF(Zr)) as support of the chloroperoxidase (CPO) and horseradish peroxidase (HRP) enzymes, leading to the CPO/HRP@H-MOF(Zr) composite. 63 CPO@H-MOF(Zr) and HRP@H-MOF(Zr) composites were applied in the treatment of wastewater containing IPU and 2,4-D, achieving a complete and very fast (15 min) degradation. Finally, we want to highlight the fabrication of purified esterase embedded in zeolitic imidazolate frameworks (ZIFs) for the degradation of pesticides. 66 Particularly, in this work, aryloxyphenoxypropionate herbicide-hydrolyzing enzyme, QpeH, was embedded into ZIF-10 ([Zn(Im) 2 ]; Im: imidazonate) and ZIF-8 ([Zn(Hmim) 2 ]; Hmim: 2-methylimidazole; S BET ∼ 1260 m 2 g −1 , V p ∼ 0.6 cm 3 g −1 , pore size of ∼3.4 and 11.4 Å) and tested in the degradation of quizalofop-P-ethyl in a watermelon field. Remarkably, the QpeH@ZIF-10 composite showed a slightly improved degradation efficiency compared to QpeH@ZIF-8 (88% vs 84%). Unfortunately, no ZIF degradation studies were performed in this research in an attempt to rationalize the different behaviors of both ZIF composites and their potential application in real water treatments. It should be noted that the QpeH@ZIF composites were demonstrated to affect the recovery of the bacterial community in soil.

MOFs AS AGROCHEMICAL DELIVERY AGENTS
The recent enthusiasm around the use of MOFs as agrochemical delivery agents is highlighted in Figure 3 with a significant increase in the number of papers published on the topic in the last two years. All the studies reported so far on the controlled release of agrochemicals from MOFs are summarized in Table 2, again sorted by the different types of released agrochemical: (i) herbicides, cis-1,3-dichloropropene (1,3-DCPP) and ortho-disulfides; (ii) fungicides, diniconazole, prochloraz, tebuconazole, and zoxystrobin; (iii) insecticides, chlorantraniliprole, λ-cyhalothrin, dinotefuran, imidacloprid, and thiamethoxam; (iv) fertilizers: urea; (v) plant grow regulators: gibberellin. The first MOF described as a delivery agent of agrochemicals was OPA-MOF (OPA: oxalate− phosphate−amine). In 2015, Anstoetz et al. described the use of an OPA-MOF as a microbially induced slow-release N and P fertilizer. 56 In this research, the capacity of the ureatemplated OPA-MOF as a new fertilizer with a slow release was investigated and compared with a standard P (triple superphosphate) and N (urea) fertilizer (ferralsol). The authors hypothesize that the OPA-MOF is a gradual-release fertilizer for crops grown on acidic soils, where microbial consumption of the oxalate linker gives rise to the degradation of the framework structure, thereby releasing Fe phosphate. While in the OPA-MOF treatment the hydrolysis of urea was fast, the conversion of the ammonium to nitrate was significantly diminished in comparison with the urea treatment (ferralsol). However, P uptake and yield in OPA-MOF was considerably lower than in conventionally fertilized plants. OPA-MOF was proven to have potential as an enhanced efficiency N fertilizer but not in P bioavailability. A year later, two novel OPA-MOFs were hydrothermally synthesized and fully characterized to be used again as slow-release fertilizers. 104 The framework backbone is robust and based on FeO 6 units with bidentate oxalate bridges joining adjacent Fe centers. PO 4 units have corner-sharing for all of their oxygens with the FeO 6 units. The authors studied the release of oxalate, setting it high enough to permit oxalate concentrations in the soil solution to achieve 1 mg·L −1 but also low enough to avoid fast and purely chemically driven compound degradation. The results show that, from the two synthesized materials, OPA-MOF-I has a slow solubility with an oxalate concentration of ca. 5 mg·L −1 at high loading and seems to be compatible with trials as a fertilizer in future works.
In 2017, Yaghi and co-workers described a naturally degradable MOF as a carrier of the important fumigant cis-1,3-dichlorophropene (1,3-DCPP). 105 The MOF [Ca 14 (Llactate) 20 (acetate) 8  Porphyrinic MOFs have also been described as promising carriers of fungicides. 106 Particularly, PCN-224 was loaded (30 wt %) with tebuconazole and constructed layer by layer with chitosan and pectin to get tebuconazole microcapsules. The synthesized microcapsules (Tebuc@PCN@P@C) had a dualmicrobial effect on plant bacterial and fungal diseases ( Figure  5). First, the tebuconazole previously loaded in the microcapsules was gradually released (87% in 7.25 days) after the pectin layer was decomposed by the pectinase released by the invading pathogen. Second, the singlet oxygen ( 1 O 2 ) was released from the organic linker porphyrin when the MOF NPs were exposed to light after the formation of pectin to inhibit the pathogens. The synthesized compound displayed excellent double activities of having photodynamic therapy and being microbicidal against the bacteria X. campestris pv campestris (82.4% and 18.4% under light and dark, respectively) and P. syringae pv tomato (56.3% and 9.5% under light and dark, respectively) and the fungi A. alternate (68.0%). Finally, the authors studied the safety of this compound against Chinese cabbage (Brassica rapa pekinensis) in a greenhouse environment. The results demonstrated that the synthesized microcapsules do not have a major effect on both the fresh weight and the soil plant analysis development (SPAD) value of the tested plant leaf, so the Tebuc@PCN@P@C microcapsules can be considered safe.
A very recent and complete study described a further example of a tebuconazole loaded MOF, the MIL-101(Fe) gated with Fe III -tannic acid (TA) networks. 107 The Fe III -TA complexes are able to absorb UV−vis near-infrared (NIR) lights. The design of MIL-101(Fe)-TA NPs enables the release of the tebuconazole cargo (24.1 wt %) in response to 7 stimuli (i.e., acidic pH, alkaline pH, H 2 O 2 , glutathione (GSH), phosphate, ethylenediaminetetraacetate (EDTA), and sunlight) to meet the diverse controlled release of the encapsulated cargo ( Figure 6). Tebuconazole is gradually released from the gated MIL-101(Fe) when the pH decreases to 5.0 as a result of the partial disassembly of Fe III -TA networks, and a significant delivery of the pesticide occurred when the pH increases to 9.0 owing to both the disassembly of the Fe III -TA networks and the degradation of the MIL-101(Fe). This is important since, in various parts of the plants themselves or caused by pest and pathogens, there are different pH values. Further, when crop plants suffer from biotic or abiotic stress, H 2 O 2 is rapidly produced in cells. On the basis of the Fenton reaction between H 2 O 2 and Fe II /Fe III , the release of the cargo will be induced by the degradation of MIL-101(Fe). GSH, normally found in plants and animals, is able to reduce Fe III to Fe II , causing the degradation of MIL-101(Fe) and, then, promoting the release of the encapsulated pesticide. Additionally, phosphates can induce MIL-101(Fe) degradation by competitive coordination with Fe III , and finally, the Fe III -TA networks on MIL-101(Fe) will stimulate the controlled release of the pesticide via the photothermal effect of the NIR light of sunlight. Lastly, this system demonstrated high fungicidal activities against R. solani (rice sheath blight; concentration for 50% of the maximal effect, ED 50 : 0.4960 mg·L −1 after 48 h) and F. gaminearum (wheat head blight; ED 50 : 0.5658 mg·L −1 after 48 h); good safety in seed germination, seedling emergence, and plant height of wheat by seed dressing; satisfactory control efficacies on wheat powdery mildew caused by B. graminis.
Thus, considering all the mentioned examples, the controlled release of agrochemicals from MOFs and MOF composites is an emerging research field that has demonstrated a great potential as an alternative and efficient new strategy to release plant nutrients but also control pests in agricultural applications.
In 2010, Wen et al. reported one of the first works about the application of MOFs in the efficient detection of agrochemicals. In this study, a new MOF, named [Cd(2,2′,4,4′-bptcH 2 )] n (2,2′,4,4′-bptcH 4 : 2,2′,4,4′-biphenyltetracarboxylic acid), that was thermally stable and luminescent was prepared via a hydrothermal reaction. 122 This material was tested as a solid-phase extraction (SPE) material for the detection of trace levels of organophosphate pesticide (OP) via stripping voltametric analysis. The determination of parathion methyl as a model included two main steps: parathion methyl adsorption and electrochemical stripping detection of adsorbed pesticide. The MOF modified glass carbon electrode was immersed into a sample solution containing the desired parathion methyl concentration, and the peak currents increased rapidly with the immersion time, up to 12 min, which indicated the saturation. The calculated limit of detection (LOD: 0.0006 μg·mL −1 ) is comparable with that of 0.0048 μg·mL −1 at a hanging mercury drop electrode, suggesting that the reported MOF is reliable for the determination of OPs in water. The same year, Barreto 3 ] (H 2 DPA: pyridine-2,6-dicarboxylic acid) for the determination of pesticides from four chemical classes, namely, organochlorine (endosulfan), organophosphate (malathion and parathion methyl), dicarboximide (procymidone), and carbamate (pirimicarb) in fresh lettuce (Lactuca sativa) by matrix solid-phase dispersion (MSPD) and gas chromatography−mass spectrometry (GC/MS). 123 The recoveries obtained ranged from 78% to 107% with relative standard deviation (RSD) values between 1.6% and 8.0%. The LOD and limit of quantification (LOQ) ranged from 0.02 to 0.05 mg·kg −1 and from 0.05 to 0.1 mg·kg −1 , respectively, for the different pesticides studied. Importantly, the comparison with a conventional sorbent (silica gel) showed better performance of the MOF sorbent for all tested pesticides. However, the reasons of this improvement are not investigated or discussed by authors. Later on, in 2017, Tao et al. 124 originally synthesized a tetraphenylethene-based ligand (BPyTPE: (E)-1,2-diphenyl-1,2-bis(4-(pyridin-4-yl)phenyl)ethene) with a trans conformation and prominent AIE properties. On the basis of BPyTPE, a novel 2D pillaredlayered LMOF [Zn 2 (bpdc) 2 (BPyTPE)] (H 2 bpdc: biphenyl-4,40-dicarboxylic acid) was developed showing a 3-fold interpenetration structure. The activated material (without solve) exhibits a strong blue-green emission at 498 nm with an important φ F of 99%. The emission of the MOF without solvent can be quenched selectively and effectively by 2,6-DN. Thus, the authors established a method to quantitatively and sensitivity detect trace 2,6-DN with a linear range of 0.94− 16.92 ppm and a low detection limit of 0.13 ppm.
When one considers these outstanding original works, MOFs have opened a new opportunity for the development of efficient techniques to detect agrochemicals. However, most of these materials are more or less sensitive to moisture or water and can be degraded through hydrolysis. Only few MOFs can maintain their stability in water or a moist environment. One example is the previously reported luminescent Zr-MOF CAU-24, based on the C-centered orthorhombic arrangement cluster [Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 12+ ] bridged by TCPB 4− linkers in a scu topology (H 4 TCPB: 1,2,4,5-tetrakis(4-carboxyphenyl)benzene; S BET : 1450 m 2 ·g −1 ; rhombic channels of ∼10 × 5.3 and ∼3.5 × 2.4 Å 2 , Figure 7). This material demonstrated a rapid, sensitive, and in situ      detection of OP pesticides. 125 Along the 22 pesticides tested, the synthesized CAU-24 quickly absorbs trace amounts of OP parathion methyl and indicates its presence. It has a low LOD of 0.115 μg·kg −1 (0.438 nM) with a wide linear range from 70 μg·kg −1 to 5.0 mg·kg −1 . The water stability of this Zr-MOF was investigated by suspending it in water for 24 h and monitoring by powder X-ray diffraction (PXRD), adsorption/desorption isotherms, and pore distribution. The crystalline structure and porosity of the Zr-MOF was kept in water after 24 h. Finally, the Zr-MOF was used to mimic rapid in situ imaging detection of pesticide residues on surface vegetables (lettuce and cowpea); visual signals appeared under UV light within 5 min. Therefore, this MOF has the possibility for low-cost, rapid, and in situ imaging detection of OP contamination via easy-to-read visual signals.
MOF composites have also been used in the determination of agrochemicals. In 2014, a MOF with an iron oxide enclosure was reported for the determination of OPs in biological samples. 126   The table is organized according to the agrochemical studied, followed by the MOF-based material name (or chemical formula), recovery (%), applicability, and detection limit.  Table 3). 127 ATP, an ecofriendly nature and low-cost clay, is added here to improve the hydrolytic stability of ZIF-8 as the −OH groups of ATP can selectively coordinate with the metal ions in ZIF-8. The ATP@ Fe 3 O 4 @ZIF-8 nanocomposite was applied as a sorbent for the magnetic solid-phase extraction (MSPE) of benzoylureas prior to high-performance liquid chromatography (HPLC) determination ( Figure 8). The established method was validated in terms of linearity (2.5−500 μg L −1 with satisfactory recovery of 88.29−95.99%) and precision (relative standard deviation, RSD, of <8%). Moreover, after 5 cycles, there was hardly any noticeable loss of the extraction efficiency. Finally, this method was effectively used in the determination of 6 benzoylureas in different tea infusions; the determined relative recoveries ranged from 78.8% to 114.3%.
Another example of magnetic solid-phase extraction using Fe 3 O 4 @ZIF-8-based composites is the work reported by Senosy et al. on the basis of the synthesis of Fe 3 O 4 @ APTES-GO/ZIF-8 (APTES: (3-aminopropyl)triethoxysilane; GO: graphene oxide) and its evaluation as an adsorbent for the determination of triazole fungicides in water, honey, and fruit juices. 128 Here, GO sheets were used to improve the dispersion of the adsorbent in aqueous solutions and, again, ZIF-8 to ensure enough surface area and active sites. Under the optimum conditions (extraction time, pH value of the sample, etc.), the obtained linearity of this method ranged from 1 to 1000 μg·L −1 for all analytes. The LODs and LOQs of four triazole fungicides ranged from 0.014 to 0.109 μg L −1 and from 0.047 to 0.365 μg L −1 , respectively. Moreover, this adsorbent could be reused without significant loss of its extraction recoveries. When compared with the outcomes from other studies, Fe 3 O 4 @APTES-GO/ZIF-8-MSPE could provide a higher performance and achieve satisfactory results for the analysis of trace triazole fungicides in complex matrices. Another composite, based on a nanoarchitecture of Mxene/ carbon nanohorns/β-cyclodextrin-MOF (MXene/CNHs/β-CD-MOFs), was utilized as an electrochemical sensing platform for the determination of carbendazim pesticide. 129 β-CD-MOFs combined the properties of the host−guest recognition of β-CD and porous structure, high porosity, and pore volume of MOFs, which are fundamental in achieving a high adsorption capacity of carbendazim. MXene/CNHs possess a large specific surface area, accessible active sites, and high conductivity, which allowed more mass transport channels and enhanced the mass transfer capacity and catalysis of carbendazim. 123,126,128 With the collaborative effect of both (β-CD-MOFs and MXene/CNHs), the electrode extended a wide linear range from 3.0 nM to 10.0 μM and a low LOD of 1.0 nM. Additionally, this sensor also showed high selectivity, reproducibility, and long-term stability as well as satisfactory application in tomato samples.

PERSPECTIVES IN USING MOFs IN AGRICULTURE
As a novel class of materials, MOFs exhibit a great potential in agroindustry, either to detect or eliminate agrochemicals or to achieve their sustained and controlled release. In all these scenarios, the aim is to reach the rational and environmentally friendly use of agrochemicals. Despite the novelty of MOFs in agriculture, the experience acquired in other areas (particularly biomedical and environmental ones) allow us to identify precise challenges related to their use in agriculture.
First, MOF stability under the working conditions is of crucial relevance. However, from the wide number of MOFs and MOF-based composites reported in environmental remediation (water and soil), only few discuss this critical point and mostly under conditions far from real water streams or fields. In this sense, many of these materials are built up from toxic metals (e.g., Cr, Ag) and/or harmful organic moieties (e.g., porphyrins), which can be released upon the MOF degradation. The selection of safe and stable MOFs is therefore mandatory for their use in agroindustry (mainly for environmental remediation and agrochemical controlled release). Further, it is essential to investigate the performance of MOFs under real conditions using complex water and soil compositions and/or vegetables or plants (e.g., river water, real fields or greenhouses, vegetables, products, etc.), considering concentration ranges found in nature, different temperatures, humidity, sunlight hours, soil composition, or pH in different parts of plants, among others.
Second, the cost of MOFs is of particular importance for agroindustry applications. When one takes into account that vegetables and fruits are normally popular and affordable, it is necessary to use a low-cost and long-lifetime material. Nontoxic and abundant safe precursors together with simple synthetic routes with a high space time yield (STY; kilogram of MOF produced per cubic meter of reaction per day) (toxic solvents, expensive ligands, etc.) need to be put in place for the most promising candidates. Note here that few MOFs have been produced so far at the ton scale by different companies, and thus, they are not currently commercially available. 218 To further progress through the application, specific manufacturing and devices should be considered (pellets, columns, membranes, etc.), and one needs to take into account the potential decrease in the MOF performance.
Finally, understanding the interaction of the agrochemicals and MOFs might help one further improve the resulting performances at the detection, removal, or progressive release stages. Also, research could be focused on multifunctional MOFs and MOF composites that combine, for instance, the extraction with the detection of pesticides in food matrices or the simultaneous elimination of different agrochemicals.