Application of Metal-Organic Frameworks for the Extraction

Abstract

Metal-organic frameworks are materials with a lot of potential in various analytical applications, particularly in preparing samples due to their varied structure topology, good thermostability, high surface area, permanent nanoscale porosity, and adjustable pore size. However, their chemical and thermal stabilities are currently a significant limitation in the field of extraction. These materials derived from metal-organic frameworks have demonstrated good extraction performance when it comes to environmental pollutants. In this chapter, we provide a critical overview of the applications of metal-organic frameworks for the extraction, such as stir bar solid extraction, micro-solid-phase extraction, solid-phase microextraction, magnetic solid-phase extraction, and solid-phase extraction.

1. Introduction

In 1995, Yaghi and Li introduced the concept of metal-organic frameworks (MOFs) which involves the synthesis of a new crystalline structure similar to zeolites. This is achieved by coordinating Cu ions with 4, 4′-bipyridine, and nitrate ions to create large rectangular channels [1]. Microporous inorganic-organic crystalline structures that have a three-dimensional and highly ordered structure are called MOFs [2]. These structures are formed through the self-assembly of metal ions and organic ligands via coordinative bonds. MOFs have the highest known surface areas, as well as adequate mechanical resistance and thermal stability [3]. Additionally, they have uniformly structured cavities with ultralow densities, specific pore sizes, and high adsorption affinity, among other properties [4]. It is worth noting that there are a vast number of potential MOF structures that can be created by combining metal ions or clusters with various organic ligands. In fact, there are theoretically infinite possibilities. To put this into perspective, around 20,000 MOFs have already been identified, compared to only about 300 zeolites. It’s interesting to note that MOFs created through reticular synthesis using the node and connector approach can maintain their underlying structure and crystallinity even when organic linkers or inorganic secondary building units (SBUs) are expanded or chemically modified [5].

The versatility of MOFs can be adjusted through tailoring, which allows for pore functionalization using post-modifications. In Figure 1, multiple examples of MOFs used as sorbents in dispersive-based microextraction are presented, each with its unique structure, metal and organic ligand nature, and pore size(s), along with their common abbreviations.

Figure 1.

Mechanisms govern the adsorption of MOFs in aqueous solutions [6].

Materials known as MOFs are typically named after the institution that discovered them. For example, the MIL [7], UiO [8], TMU [9], and HKUST [10] families of materials were named after the researchers at the Institute Lavoisier, Oslo, Tarbiat Modares University and Hong Kong University of Science and Technology who discovered them, respectively.

There are multiple factors that can affect the stability of MOF structures, including the metal’s nature and coordination environment, pore size, dimensionality of the final frameworks, and strength of metal-ligand bonds [11, 12]. High-valent metal-carboxylate or low-valent metal-azolate frameworks are chemically and thermally stable, according to the hard/soft acid-base (HSAB) principle. MOFs can undergo powder X-ray diffraction pattern (PXRD) testing or utilize in situ PXRD, thermogravimetric analysis (TGA), temperature-programmed mass spectrometry (TPMS), etc., to assess chemical, thermal, or mechanical stability [13].

MOFs are constructed through the spontaneous assembly of metal ions and organic binders via coordinate bonds. These frameworks possess a regular structure and feature high surface area, thermal stability, as well as chemical and water resistance [14]. MOFs also have a unique pore size with a uniform cavity structure that enables high adsorption capacity and ultra-low densities. Furthermore, the shape and pore sizes of MOFs can be adjusted to create different connectivity patterns between the inorganic part and organic ligands, ranging from micro to meso pore scale [15].

Due to its unique structure and excellent performance, MOFs have a great prospect in sensing detection [16], removal [17], catalysis [18], gas capture [19], drug delivery [20], energy storage [21], nonlinear optics [22], magnetism [23], adsorption [24], separation [25], etc.

Various MOFs materials have been used in different extraction modes such as column solid-phase extraction (SPE), solid-phase microextraction (SPME), magnetic solid-phase extraction (MSPE), stir bar solid extraction (SBSE), and micro solid-phase extraction (μ-SPE), to extract analytes from various matrices including water, gas, soil, vegetable, fruit, tea, tobacco, serum, and urine. The interactions between sorbent and analyte influence the efficiency and capacity of sample preparation. MOFs have excellent chemical stability, allowing the use of different elution solvents and modes after extraction.

The purpose of this chapter book is to explore the potential of MOFs as a type of porous solid material for extracting and separating a variety of organic and inorganic pollutants from different samples. MOFs are highly advanced porous materials with promising applications in modern analytical chemistry for separation and purification processes. This comprehensive study delves into MOFs and their potential as sorbents in SPE, SPME, MSPE, SBSE, and μ-SPE. We also discuss limitations related to the use of MOFs as sorbents in solid phase-based separations and offer solutions to overcome these issues. Lastly, we examine the future possibilities and potentials of MOFs as solid sorbents.

2. Synthesis of MOFs

Various methods for synthesizing MOFs have been reported to date, including sonochemical, electrochemical, ionothermal, solvothermal, microwave, and mechanochemical approaches.

2.1 Sonochemistry method

Sonochemical synthesis uses ultrasound to chemically modify molecules. Ionic liquids have also been used as solvents in MOF synthesis. In MOF science, sonochemical synthesis aims to create a fast, eco-friendly, efficient, user-friendly, and ambient-temperature applicable method [26].

2.2 Diffusion method

One common diffusion method involves slowly bringing different substances into contact with each other. One way to do this is through solvent-liquid diffusion. This method involves creating two layers of different densities, with one layer containing a solvent that will cause precipitation and the other layer containing a solvent that will dissolve the product. These layers are separated by another layer of solvent. Over time, the precipitant solvent will diffuse into the layer containing the product, which will cause crystal growth at the interface. Another method involves using physical barriers, such as two vials of different sizes, to gradually diffuse reactants [27].

2.3 Solvothermal synthesis

The process of solvothermal synthesis involves utilizing specialized reactors that function at or above the boiling point of the solvent. Organic solvents like dialkyl formamides, alcohols, and pyridine are commonly used in this process, resulting in nanoscale shapes, improved crystallinity, and high product yields. However, achieving the desired temperature and pressure conditions requires specialized equipment like autoclaves [28].

2.4 Microwave synthesis

Metal and oxide particles can be produced through the use of microwaves. During this process, the temperature of the solution is heightened for over an hour to form nanosized metal crystals. While this technique has not been widely used for creating crystalline MOFs, it’s an important method for fast synthesis. Additionally, this method allows for proper control of the shape and size of the resulting particles [29].

2.5 Electrochemical synthesis

Electrochemical synthesis involves the use of organic ligands and electrolytes to produce metal ions from electrochemical reactions, eliminating the need for metal salts [30].

2.6 Mechanochemical synthesis

Mechanochemical synthesis, introduced in 2006, is an eco-friendly method that uses mechanical energy to carry out chemical reactions and can be performed with a small amount of solvent or even without any solvent. Mechanical force can produce chemical reactions and various physical phenomena. Mechanochemical synthesis involves breaking intramolecular bonds mechanically, leading to a chemical transformation. Mechanochemistry has been used in synthetic chemistry for a long time and is currently being used in multicomponent reactions to create pharmaceutically active co-crystals, organic synthesis, polymer science, inorganic solid-state chemistry, and many other fields. There are several reasons why the mechanically activated synthesis of MOFs is of interest, including environmental concerns [31].

3. Water stability of MOFs

Water-stable MOFs (WMOFs) are a type of MOFs that can withstand exposure to water and maintain their functional stability without any structural damage. To assess MOF water stability, we must examine changes in chemical properties when exposed to water. Internal factors include metal-ligand coordination, bond strength, porosity, ligands, metal ions, and crystallinity. External factors include humidity, pH, temperature, and more. The strength of metal-organic coordination bonds, hydrophobicity of pore surfaces, and spatial potential resistance are key factors that influence MOF water stability. It is important to have a thorough understanding of the interaction between water molecules and MOFs to maintain their structure and prevent destruction by water molecules [32].

When MOFs are in a solution with water, the weaker metal-organic coordination bonds can be damaged by water molecules. This causes the organic ligands to be replaced by water molecules or hydroxide, which can ultimately destroy the MOF structure. To improve the water stability of MOFs, it is important to enhance the strength of the metal-organic coordination bonds. The coordination bonds formed by high-valent metal ions with high charge density are stronger in the same coordination environment. According to Pearson’s hard/soft acid/base theory, stronger coordination bonds can be formed when there is a strong interaction between hard acid and hard base or soft acid and soft base. To create a sturdy framework for MOFs, the high-valent metal units need to form a rigid structure that can withstand water molecules. WMOFs are constructed by using carboxylate-based ligands (known as hard Lewis bases) and high-valent metal ions (known as hard Lewis acids) [33].

The HSAB theory states that the strength of metal-organic coordination bonds plays a crucial role in determining thermodynamic stability. This makes the metal center and organic ligands less vulnerable to water molecule attacks. In addition, kinetic factors are also taken into account. The stability of the framework is inversely proportional to the length of the metal-organic coordination bond. Longer coordination bonds are less stable due to reduced rigidity and dynamic stability. This prevents water molecule attacks by adding more steric hindrance or hydrophobic surfaces, and extra protection against metal ions [34].

In general, the stability of MOFs in water can be assessed through various methods, including scanning electron microscopy (SEM), Brunauer-Emmett–Teller (BET) surface area analysis, and power X-ray diffraction (PXRD).

4. Adsorption mechanism

Adsorption techniques are commonly used to treat wastewater by removing pollutants that are not fully extracted during the processing of the water. Organic contaminants are particularly difficult to treat due to their lower molecular weight and hydrophobicity. During the process of adsorption, pollutant molecules are drawn toward the surface of adsorbent materials by diffusion from the bulk solution to the active pores of the adsorbent (Figure 1). This process occurs due to intermolecular forces of attraction, including physisorption (such as hydrogen bonding van der Waals, and π–π interactions) and chemisorption (such as ionic interactions). Adsorbent materials possess a crucial property, i.e., their ability to undergo the adsorption process. This property is a result of their unique characteristics, such as a high selectivity toward the targeted contaminants, water and thermal stabilities, availability, large specific BET surface area, high porosity, low cost, and easy regeneration [35].

When it comes to MOF-based nanomaterials, the adsorption of target analytes is primarily influenced by the materials’ physical structure and chemical properties. Factors such as specific surface area, surface functional groups, and pore structure can directly affect the adsorption capacity. Adsorption can occur through physical or chemical interaction, with surface charge, reactivity, solubility, molecular size, and hydrophobicity being key determinants. The interaction between contaminants and adsorbent surfaces occurs through electrostatic interaction, complexation/coordination, ion exchange, and oxidation [6].

5. The application of MOFs for extraction

MOFs are hybrid organic-inorganic materials that possess exceptional features making them highly suitable as sorbents for sample pretreatment in separation science. MOFs’ pore distribution affects adsorption, confining guest molecules in a uniform space for reactions. Both rigid and flexible structures are key in using MOFs as sorbents. Their flexibility facilitates the selective adsorption of guests. The MOFs have been used in SPE, μ-SPE, MSPE, SPME, and SBSE to extract target analyets, from matrix samples (Table 1).

MOF-composite	Sample	Analyte	Extraction method	Analytical technique	LOD (ng mL^−1)	Ref.
Copper(II) isonicotinate	Coal fly ash and water	PAHs	SPE	HPLC-UV	0.002–0.014	[36]
MIL-101@graphene	Water	NSAIDs	SPE	HPLC-UV/Vis	—	[37]
HKUST-1, MOF-5, MIL-53, UiO-64, MOF-74	Wastewater	PAHs, hormones, drugs, and disinfectant	μ-SPE	UHPLC-TOF	0.04–0.22	[38]
Fe3O4@TGA@TMU-6	Environmental water	Organophosphorus pesticides	MSPE	HPLC-UV	0.5–1	[39]
Fe3O4-MWCNTs-OH@ZIF-67@IL	Porcine pancreas crude extract	chymotrypsin	MSPE	UV–vis spectrophotometer	—	[40]
MOF-199	Air	Benzene homologs	SPME	GC-FID	0.0083–0.0233	[41]
MIL-53(Al) Fiber	Water	PAHs	SPME	GC–MS/MS	0.10–0.73 ng L-1	[42]
MIL-101(Cr)	River water	Penicillin	SPME	CEC-UV	1200–4500	[43]
ZIF-8	Water, honey	Fluoroquinolones	SPME	HPLC-FLD	0.14–1.1	[44]
MIL-68@PEEK	Cosmetics and rabbit plasma	Parabens	SBSE	HPLC-MS/MS	1 pg. mL^−1	[45]
CoFe2O4/MIL-101(Fe)	Cosmetic products	N-nitrosamines	SBSE	LC–MS/MS	3–13 mg kg^−1	[46]

Table 1.

Summary of MOF-based sorbents.

6. Solid-phase extraction

SPE uses a stationary phase column to separate the substance of interest, which is then eluted using a solvent. Different formats exist, and the type of adsorbent material greatly impacts the extraction capacity. MOFs and MOF-based composites are highly effective adsorbents for antibiotics.

In 2006, Zhou and colleagues demonstrated the use of MOF in SPE. They developed an online flow injection SPE system that utilized copper (II) isonicotinate as a sorbent to determine the trace amount of polycyclic aromatic hydrocarbons using high-performance liquid chromatography-ultra-violet (HPLC-UV) the buck structure’s pores filtered guest polycyclic aromatic hydrocarbons (PAH) molecules based on their shape and size. It extracted PAHs from environmental samples via SPE, then separated and detected them through HPLC and UV. The method detected trace PAHs in certified reference material and local water samples with enhancement factors ranging from 200 to 2337 [36].

In a study conducted by Dai et al., a MIL-101(Cr) packed SPE column was used to extract sulfonamides from water samples. This was coupled with ultra-high performance liquid chromatography-tandem mass spectrometry detection. With the optimum conditions established, the detection limits achieved were 0.03–0.08 μg/L and the quantitation limits were 0.11–0.27 μg/L. The linear ranges for the analytes were 0.2–40 or 0.5–100 μg/L (r2 > 0.996), with relative recoveries in the range of 83.5–107.3%. The relative standard deviation (RSD) was between 0.2% and 8.0% (n = 6). The molecular modeling method was also employed to demonstrate the molecular interactions and free binding energies [47].

In their study, Yang and colleagues investigated the use of ZIF-8 as a sorbent column for the enrichment of tetracycline from water and milk samples. They packed 390 mg of ZIF-8 into a stainless steel column that was 3 cm long and 4.6 mm in diameter. This column was then mounted onto the HPLC injector valve, replacing the sample loop. To achieve on-line solid-phase extraction (SPE), oxytetracycline (OTC), tetracycline (TC), and chlortetracycline (CTC), the researchers loaded the sample solution at a flow rate of 3.0 mL/min for 10 minutes using a flow-injection system. The extracted analytes were then eluted into a C18 analytical column, which was 25 cm long and 4.6 mm in diameter, for HPLC separation under isocratic conditions, using a mobile phase consisting of 10% MeOH, 20% ACN, and 70% 0.02 mol L⁻¹ oxalic acid solution at a flow rate of 1.0 mL/min. Researchers optimized their method to produce enhancement factors of 35–61, with detection limits of 1.5–8.0 μg/L, quantification limits of 5.0–26.7 μg/L, and a sample throughput of 4 samples/h [48].

Zhang and team utilized the “sol-cryo” technique to pack MIL-101 into graphene, resulting in MOF@graphene. NSAIDs were enriched from tap and river water using MOF@graphene-based SPE and HPLC analysis, with satisfactory recoveries (80.8–106.9%) and good RSD (2.5–8.7%). The incorporation of MOF and graphene showed a remarkable synergy effect in pretreatment where the dispersibility of MOF was improved by graphene and the surface area of graphene was enlarged by MOF [37].

Deng and colleagues conducted a study on using MOF (PCN-224) packed into empty cartridges to extract sulfonamide residues before quantifying them through HPLC-MS (Figure 2). Sulfonamides are antibacterial compounds that can be misused in veterinary practice and livestock, leading to their accumulation in surface water and posing a threat to living organisms. It is crucial to develop efficient and straightforward methods to ensure public safety, as illegal pharmaceutical residues are frequently found in the environment and food products. The team used liquid chromatography-tandem mass spectrometry to quantify sulfonamides, reproducibility (1.7–5.1%), good repeatability (2.8–6.7%), wide linear ranges (0.5–2000 ng·L⁻¹), and achieving low limits of detection (0.07–0.47 ng·L⁻¹). This method successfully detected sulfonamides in food and drinking water samples [49].

Figure 2.

A Shematic of PCN-224 as an adsorbent for SPE of polar sulfonamide antibiotics [49].

Several approaches have been proposed to overcome the poor hydrothermal stability of moisture-sensitive MOFs, which hindered their direct applications.

Asiabi et al. used the electrospinning method to create a CH3MOF-5/PAN composite for use as a sorbent in the analysis of levonorgestrel and megestrol acetate in aqueous samples. The resulting nanofibers were packed into mini-disc cartridges to be used as SPE devices. After optimizing the conditions, the linearity ranged from 0.05 to 100 μg L⁻¹ with R² values higher than 0.999. Both analytes had a limit of detection of 0.02 μg L⁻¹. The method’s applicability was demonstrated by analyzing the analytes in urine samples, with a recovery percentage ranging from 82.8 to 94.8%. This shows the method’s capability for determining drug levels in urine samples [50].

Yang et al. discovered that adding a hydrophobic methyl group to MOF-5 resulted in CH₃MOF-5, a water-stable sorbent for SPE in PAHs determination [51].

A group of researchers, Liu et al., used a heat treatment process in an Ar atmosphere to create MOF-5-C, a type of adsorbent, for detecting trace amounts of phthalate esters in bottled water, peach juice, and soft drink samples. The MOF-5-C demonstrated great adsorption capabilities and stability, thanks to its large pore volume and solvent resistance. The adsorbent showcased good linearity in the concentration range of 0.1–50.0 ng mL⁻¹ for bottled water samples and 0.2–50.0 ng mL⁻¹ for peach juice and soft drink samples under optimal conditions. The results suggest that MOF-5-C could be used as an effective adsorbent for preconcentrating other organic compounds, making it a promising prospect [52].

7. Micro solid-phase extraction

A method called μ-SPE has gained popularity for its quick, easy, environmentally-friendly, and cost-effective analysis of samples. This method is effective due to its ability to minimize the effect of surrounding substances, its simple process, and its low use of solvents. Unlike other methods where sorbents are placed in cartridges, DSPE involves adding solid sorbents to the sample matrix with the target analyte. This process, known as the “dispersive mode,” uses magnetic stirring, mechanical agitation, ultrasounds, or vortex to improve the interaction between the adsorbent particles and analyte quickly. MOFs, or metal-organic frameworks, have recently become a popular choice for sample preparation due to their outstanding properties.

A team of researchers led by Ojaghzadeh Khalil Abad has developed a new method for extracting two anti-cancer drugs, paclitaxel and vinorelbine, from environmental water and urine samples. They achieved this using a μ-SPE technique based on a green sorbent made from a composite of chitosan, MIL-88B, and MIL-88A. The team synthesized a sorbent using hydrothermal and chemical procedures to create a MOF-on-MOF structure, which was coated with chitosan to produce the final sorbent, MIL-88B@MIL-88A@chitosan. This sorbent proved more effective at extracting drugs than the individual components. The method had a linear range of detection between 0.11–217.4 and 0.12–234.8 ng mL⁻¹, with R² values greater than 0.9931 for paclitaxel and vinorelbine, respectively. The limit of detection for both drugs was 0.03 ng mL⁻¹. The researchers also analyzed real water samples, including river water, well water, and urine samples. The relative recoveries of paclitaxel and vinorelbine in these samples were between 88 and 97.6% and 90.0–97.8%, respectively [53].

Researchers recently tested the use of a metal-organic framework called MOF-801 as an adsorbent for pesticide analysis. They combined it with evaporation for preconcentration, achieving high extraction recoveries and low limits of detection. The process also reduced the amount of organic solvent used [54].

In a recent study conducted by Tahmasebi and team, three MOFs containing azine functional groups in their pores were tested for their ability to extract heavy metal ions from water samples. Researchers evaluated the effectiveness of different MOFs in adsorbing heavy metals. TMU-5 was found to be the most effective sorbent for preconcentrating trace amounts of heavy metals. The method was optimized using flow injection inductively coupled plasma optical emission spectrometry and achieved good linearity between 0.05 and 100 μg L⁻¹. The detection limits ranged from 0.01 to 1.0 μg L⁻¹, and the method showed enhancement factors between 42 and 225 and relative standard deviations of 2.9–6.2% [55].

In a different study, five types of MOFs were utilized UiO-64, MOF-74, MIL-53, MOF-5, and HKUST-1 to simultaneously separate various analytes from aqueous samples, including PAH, hormones, drugs, and disinfectants, using miniaturized DSPE methods. Interestingly, the sorbent’s physico-chemical features, such as the metal’s nature and the structural environment of the pores (width and size), have significant effects on the adsorption process. Among the mentioned MOFs, MIL-53(Al) proved to be the most efficient for the selected group of analytes due to its “pore breathing” ability, which enables its flexible structure with variable pore sizes to adapt to this group of analytes. The entire process was successful and validated by HPLC-DAD and LC-MS [38].

In a study conducted by Samimi and colleagues, TMU-24 was found to be an effective adsorbent for removing eosin B from water. The sorbents, which have a large surface area, stability, tunability, and porosity, showed rapid adsorption kinetics (within 9 minutes) and high adsorption capacity. The optimal conditions for maximum removal of eosin B were found to be a pH of 6, an absorbent dosage of 4 mg, and a contact time of 9 minutes. The results of the study were consistent with the Langmuir model and the pseudo-second-order kinetics, indicating that eosin B was adsorbed onto the adsorbent in monolayers due to its chemical affinity. Thermodynamic parameters indicated that the adsorption process was spontaneous and exothermic. Furthermore, TMU-24 was found to be reusable for up to 6 cycles and has the potential for use in treating wastewater polluted with dyes [3].

8. Magnetic solid-phase extraction

MSPE is a modern and miniaturized technique used for sample preparation. It involves dispersing a magnetic sorbent in a liquid sample matrix and then separating it using an external magnetic field. The sorbents used in MSPE applications are typically bare magnetite NPs or magnetic hybrid materials coated with silica, carbon nanostructures, polymers, and MOFs.

Researchers Shakourian et al. developed a composite material called Fe₃O₄@ TMU-6 for MSPE of certain organophosphorus pesticides in rice and environmental water samples (Figure 3). To extract and determine the analytes, MSPE-HPLC-UV was utilized. The MOFs in the sorbents had a large surface area and unique porous structure, which enabled high affinity for the target analytes due to π-π and hydrophobic interactions between the analytes and MOF ligands. The calibration curves for phosalone, chlorpyrifos, and profenofos in water samples were linear within particular ranges. For phosalone, the range was 7.5–75 μg L⁻¹, for chlorpyrifos, it was 10–100 μg L⁻¹, and for profenofos, it was 10–150 μg L⁻¹ under optimal conditions. The limit of detection (LOD) for phosalone and profenofos in water samples was 0.5 μg L⁻¹, while for chlorpyrifos, it was 1 μg L⁻¹ [39].

Figure 3.

A schematic of MSPE of some organophosphorus pesticides [39].

A study conducted by Bahiraee and colleagues involved the use of Fe3O4@TMU-12 as an effective sorbent for MSPE of diazinon and chlorpyrifos from environmental water samples. Researchers used a combination of a simple and affordable method with high-performance liquid chromatography to detect certain compounds. They found that the magnetic framework composite was effective for diazinon and chlorpyrifos, with optimal extraction conditions, and obtained calibration curves for both compounds. The method had a high precision, with LODs of 0.5 μg L⁻¹ for diazinon and 0.8 μg L⁻¹ for chlorpyrifos [4].

Guo et al. have presented their findings on the use of MIL-101(Cr)@Fe₃O₄ nanocomposites as an adsorbent for purifying and enriching 9 mycotoxins in agricultural products. The successful synthesis of adsorbent was confirmed through various characterization methods. Coupled with analysis by ultra-high-performance liquid chromatography-tandem mass spectrometry, the study obtained good linearity (R² ≥ 0.991), high sensitivity (LOQ in the range of 0.08–0.20 μg kg⁻¹), satisfactory recovery (83.5–108.5%), and acceptable precision (RSD, 1.6–10.4%) [56].

A study by Zhou and colleagues has shown that Fe₃O₄@DUT-5, when combined with HPLC, can be used as an effective adsorbent for the trace analysis of azole fungicide residues in fruits and vegetables. The results were promising, with satisfactory enrichment factors ranging from 69.6 to 96.4 and good linearity between 10 and 1000 μg L⁻¹. The low limits of detection, ranging from 0.054 to 0.313 μg L⁻¹, were found to be below the maximum residue limits of azole fungicides in agricultural products, which is 10.0 μg kg⁻¹. The study also showed a high recovery rate, ranging from 62.3 to 100.8%, and reproducibility lower than 6.4%, meeting the requirements for pesticide residue analysis. Furthermore, the adsorption capacity remained high and stable even after 5 cycles [57].

Zhang and colleagues developed a composite material called Fe₃O₄@MIL-100 (Fe) for extracting phenyl urea herbicides (PUHs) from food and environmental water samples using the MSPE method. PUHs are harmful and have been banned in Europe, so detecting even small amounts of them in complex samples is important. The composite has a unique pore size and high specific surface area due to its MIL-100(Fe) shell structure, and a Fe3O4 core with superparamagnetic properties allows for effective magnetic separation. PUHs enter internal sites of Fe3O4@MIL-100 (Fe), followed by π–π interactions with organic ligands and H-bonding with carboxyl groups. Fe3O4@MIL-100(Fe) can be reused at least 5 times without significant loss of adsorption efficiency [40].

In a study conducted by Liu et al., they utilized carbonization to create ZIF-67-C which was then utilized in MSPE with HPLC–UV to concentrate phenyl urea herbicides in grape and bitter gourd samples. Through this process, ZIF-67-C exhibited impressive extraction capabilities for the targeted analytes and displayed potential for use in other organic pollutants [58].

9. Solid-phase microextraction

SPME is a technique introduced in the early 1990s, which combines sampling, extraction, enrichment, and introduction into one step. It uses SPME fibers to capture analytes between the fiber coating and sample matrix. This method is solvent-free, environmentally friendly, quick, and easy to use with gas chromatography. It is useful in analyzing trace organic contaminants in various fields. MOF-based SPME fibers have shown high efficiency, sensitivity, and repeatability in previous applications.

The first reported application of MOFs to SPME was in 2009, using a fiber coated with MOF-199 [41]. The SPME fiber was fabricated by in situ hydrothermal growth of thin MOF-199 films on etched stainless steel wire. The limits of detection for the benzene homologs were 8.3–23.3 ng L⁻¹.

Researchers found that MIL-53 (Al) coated fibers were the most effective for detecting 16 PAHs in wastewater using GC–MS/MS. The fiber demonstrated good linearity and low LODs under optimal conditions. Stability was also tested in two wastewater samples [42].

A recent study compared the effectiveness of Ca-BTC MOF and a hybrid Ca-BTC-MCC MOF in extracting seven drugs with varying physicochemical properties. The study used an HPLC/DAD instrument configuration in reversed-phase mode under isocratic elution mode to evaluate the extraction efficiency of both sorbents. The results showed that Ca-BTC MOF had a better extraction efficiency for all the analytes, except nirmatrelvir and ritonavir. The findings indicate that the adsorption capacity is influenced by not only the surface area of the adsorbents but also other factors, such as the morphology of the adsorbent and the physicochemical properties of the analytes [59].

A new and effective technique has been developed for creating a nanocomposite composed of MOF-5-ionic liquid functionalized graphene. This nanocomposite is capable of preconcentrating and extracting two different antibiotics from a variety of samples. The MOF-5/ILG composite possesses a large surface area and exceptional adsorption capabilities for the targeted compounds prior to GC-FID analysis. The suggested SPME exhibits a satisfactory range of linearity (0.05–500 μg L⁻¹) with an R² value exceeding 0.9981. Additionally, a novel fiber coating has been introduced for SPME-based separation of organochlorine pesticides made up of MOFs and graphite oxide composites [60].

Lin et al. created a hybrid MOF-polymer (MIL-101(Cr)–poly(EMDA–BMA)) for the in-SPME of penicillin. This was followed by capillary electrochromatography (CEC) coupled with UV analysis. The method had a recovery of 63–96.2% with high sensitivity due to the large surface area of the sorbent. Additionally, it had high reproducibility and low solvent consumption [43].

In their study, Pang et al. created a composite called ZIF-8@monolith to be used as a sorbent in an online in-tube-SPME-HPLC-FLD method for extracting and determining fluoroquinolones (FQs) in water and honey samples (Figure 4). The addition of ZIF-8 to the monolith enhanced the extraction of FQs by increasing the surface area. The method achieved low detection limits of 0.14–0.61 ng L⁻¹ and 0.39–1.1 ng L⁻¹ for water and honey samples, respectively [44].

Figure 4.

The preparation sketch of MM-CMC [44].

10. Stir bar solid extraction

Baltussen et al. introduced stir-bar sorptive extraction, a technique with high sensitivity, reproducibility, recovery, and low organic solvent consumption [61]. A technique called stir-bar sorptive extraction is incredibly sensitive, uses minimal solvents, and recovers materials well. It involves using a stir bar with a higher coating amount than SPME, which results in better extraction capacity. Researchers have started using WMOFs in SBSE, coating them with PDMS through the sol-gel method.

Wang and colleagues described a technique for securely attaching MIL-68 to a durable PEEK jacket made of poly (ether ether ketone) through covalent modification (Figure 5). To analyze methylparaben, ethylparaben, and propylparaben in cosmetics and rabbit plasma, the researchers used the stir bar extraction method along with HPLC-MS/MS. The covalent modification process was responsible for the method’s excellent sensitivity (with a LOD of approximately 1 pg. mL⁻¹) and robustness [45].

Figure 5.

PDMS/MIL-101-Cr-NH2 coated stir bar sorptive extraction [45].

In 2019, Miralles et al. used CoFe₂O₄/MIL-101(Fe) to selectively separate eight hazardous N-nitrosamines in cosmetic samples. They obtained satisfactory results with optimized conditions, including acceptable enrichment factors up to 62 and limits of detection ranging from 3 to 13 l g kg⁻¹. Finally, they achieved quantitative relative recoveries of 96–109% using standard addition calibration [46].

Lin and team used a water-stable hybrid material, Fe₃O₄@MOF-5, to coat Nd-Fe-B magnets for SBSE. MOF-5-based SBSE with GC-MS successfully detected PCBs in fish samples with a recovery rate of 94.3–97.5%. They also used a MOF-5 modified bar to increase selectivity toward PCBs in fish samples, resulting in large enrichment factors of 50–100 [62].

Hu et al. used PDMS/IRMOF-3 for estrogen extraction and PDMS/Al-MIL-53-NH₂ for PAH extraction. The methods have LODs of 0.15–0.35 μg L⁻¹ and a linear range of 1–2500 μg L⁻¹. The optimized conditions resulted in good enrichment factors ranging from 16.1 to 88.9 and a long lifetime of over 40 extraction cycles [63].

Xiao et al. developed a new method using PDMS/MIL-101-Cr-NH₂-based SBSE with GC-FPD to detect OPPs in East Lake and pond water samples, resulting in high enrichment factors and recoveries [64].

11. Conclusion

MOFs have gained popularity as sorbents due to their high surface area and specific pore sizes. Six synthetic strategies exist that determine their structure and properties. However, their resistance to common sample preparation conditions is a concern, and stability must be ensured to prevent degradation.

MOFs absorb analytes through various mechanisms, including p-p stacking, hydrogen bonding, and hydrophobic interaction. Selective binding can improve recognition and resistance to interferences. MOFs with specific sites can enhance extraction efficiency by considering the spatial configuration of the target compound.

MOFs and MOF-based composites are promising sorbents for sample pretreatment in various techniques such as SPE, μ-SPE, MSPE, SPME, and SBSE. Further research and development can increase the usage of MOFs in sample pretreatment.