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One of the most important advantages of SPE compared to SPME is the availability of a wider range of relatively cheap commercial sorbents. However, the low rate of diffusion and mass transfer between the packed sorbents and the target analytes generally results in a slow extraction process. Moreover, in some cases, cartridges can be blocked when complex matrices are analyzed, leading to process failure. Besides, a previous cartridge conditioning is required which also lengthens the extraction process. In order to solve these problems, dSPE—as a miniaturized version of SPE in many cases—emerged as an alternative based on the direct addition of the sorbents (without preconditioning) to samples or extracts [67, 68], improving the contact area between sorbent and the sample/extract solution. Consequently, the analytes are extracted much faster by simple manual, vortex, or ultrasound agitation, while most interferences remain in the solution (depending on the selectivity of the sorbent). Then, the sorbent can be separated by its retention into a SPE column [25] or by centrifugation and the subsequent supernatant decantation [69]. In the first case, the analytes are later eluted as in conventional SPE, while in the second, the analytes have to be desorbed under agitation, centrifugation, and decantation. Therefore, this variant saves time, efforts, and solvents, with the consequent economic savings that this entails [70, 71]. The few dSPE methods developed for the extraction of PAEs from water samples (see Table 1.2) have been based on the application of graphene [72], GO-MIP [69], MIPs microbeads [73], a commercial metal-organic framework (MOF) [25], and a temperature-sensitive polymer [74]. In this last case, a lab-synthesized β-cyclodextrin-poly(N-isopropylacrylamide) (β-CD-PNIPAM) polymer was used to carry out the extraction of three short-chain PAEs from tap and mineral water. This kind of polymers are characterized by showing a reversible phase transition in aqueous samples at a certain temperature known as low critical solution temperature (LCST). In this case, the LCST is at 32°C, so the polymer behaves as a liquid below that value while it is a solid when the temperature exceeds it. Taking advantage of this fact, the authors introduced 20 mL of β-CD-PNIPAM into the sample and mixed. Then, the temperature was increased up to 50°C obtaining a floating solid phase, which was collected after Na₂SO₄ addition in order to favor the salting-out effect. This extraction methodology, in combination with GC-MS, allowed obtaining good extraction efficiency and sensitivity, especially for DBP, which makes think about the potential of this procedure for the determination of long-chain PAEs. It is important to mention that, from an operational point of view, the use of this kind of polymers implies that the extraction step is developed in the same way as a LLE procedure and, as consequence, it could be not considered as a sorbent-based extraction methodology for some authors. However, the second part involve a desorption of the analytes from the solid polymer, since that reversible phase transition just takes place when it is into the water sample. In order to provide a better vision of this procedure, Figure 1.5 shows a scheme of a similar procedure used by Chen et al. in a previous work for the determination of different phenolic compounds—no PAEs were included [75].

Table 1.2 Some examples of the application of SPE for the analysis of PAEs in water samples.

PAEs	Matrix (sample amount)	Sample pretreatment	Separation technique	LOQ	Recovery study	Residues found	Comments	Reference
DMP, DEP, DIBP, DBP, DMEP, BMPP, DEEP, DNPP, DHXP, BBP, DBEP, DCHP, DEHP DNOP, and DNP	River and sea waters (20 mL)	dSPE using 3 mL colloidal G and vortex for 2 min, centrifugation at 3800 rpm for 5 min, and desorption with 5 mL ethyl acetate and 2 g sodium sulfate by vortex for 30 s	GC-MS	5–20 μg/L	72–117% at 20 and 50 μg/L	Nine river and 2 sea waters samples were analyzed and contained at least 1 PAE at levels from 2 to 78 μg/L	Ethyl acetate showed higher extraction efficiency than ACN, acetone and hexane as desorption solvent	[72]
DEHP	Rain, lake and river waters (600 mL)	dSPE using 20 mg GO-MIP and agitation for 30 min, centrifugation at 12000 rpm for 10 min, and desorption (twice) with 2.5 mL acetone by vortex for 1 min and subsequent sonication for 5 min	HPLC-UV	2.82 μg/L	82-92% at 5, 50, and 500 μg/L	One sample of each water were analyzed and residues were found at 0.32 ± 0.08 and 1.56 ± 0.32 μg/L in lake and river waters, respectively.	DEHP was used as the template molecule. Acetone showed higher extraction efficiency than MeOH as desorption solvent	[69]
DMP, DEP, DBP, BBP, DEHP, and DNOP	Bottle water (200 mL)	dSPE using 60 mg DMIMs and stirring for 90 min, and desorption with 5 mL dichloromethane by sonication for 15 min	GC-MS	1.03–1.35 μg/L	92.4–99.0% at 25 μg/L	Two samples were analyzed and residues of DEHP were found at 10.06 ± 0.84 and 11.90 ± 1.70 μg/L	DEP was used as the dummy template. Dichloromethane showed higher extraction efficiency than acetone, MeOH, chloroform, ethyl acetate and hexane as desorption solvent. DMIMs-dSPE method showed higher recovery values compared with non-imprinted polymers	[73]
DMP, DEP, and DBP	Tap and mineral waters (20 mL)	dSPE using 20 mg (β-cyclodextrin-poly (N-isopropylacrylamide) and water bath at 50°C for 25 min., addition of sodium sulfate for polymer condensation purposes, and desorption with 200 μL ethyl acetate by sonication for 15 min.	GC-MS	0.021–0.350 μg/L	82.2–105.6% at 5, 100, and 600 μg/L	One sample of each water were analyzed and all PAEs were found at levels from 0.14 to 4.97 μg/L	Ethyl acetate showed higher extraction efficiency than hexane, acetone and dichloromethane as desorption solvent.	[74]
BBP, DBEP, DIPP, DNPP, DCHP, DEHP, DNOP, DINP, and DEHA	Milli-Q, pond, tap and waste waters (50 mL)	dSPE using 120 mg Basolite* F300 MOF and shaking for 5 min, vacuum-dried using a SPE column for 30 min, and elution with 15-mL ACN	HPLC-MS	0.022–0.069 μg/L	70–118% at 0.375 and 1.875 μg/L	Eight samples were analyzed and residues of DEHP were found at levels from 0.21 ± 0.26 to 4.04 ± 0.23 (_ig/L in all samples	ACN showed higher extraction efficiency than dichloromethane, acetone, cyclohexane and MeOH as elution solvent	[25]
DMP, DEP, DPP, DIBP, DBP, DNPP, DHXP, BBP, DEHP, DHP, DCHP, DPhP and DNOP	Drinking water (200 mL)	m-dSPE using 20 mg MWCNTs-m-NPs under agitation for 2 min, a magnet was used for decantation, and elution with 1 mL toluene–acetone (1:4, v/v)	GC-MS/MS	0.03–0.1 μg/L	86.6-100.2% at 5 μg/L	Three samples were analyzed and no residues were detected	Toluene showed higher extraction efficiency than acetone, MeOH, hexane and ethyl acetate as elution solvent. To reduce the toxicity of toluene, different proportions toluene-acetone (1:1, 1:4 and 1:9, v/v) were tested and the mix toluene–acetone (1:4, v/v) gave similar results	[76]
DEP, DPP, DBP, DCP, and DEHP	Bottled and river waters (300 mL)	m-dSPE using 25 mg G-Fe3O4 under 3 4 agitation for 15 min, a magnet was used for decantation, and elution (in triplicate) with 0.5 mL acetone by vortex for 10 s	HPLC-UV	0.03–0.1 μg/L	80.0–106.0% at 0.5 and 5 μg/L	One sample of each water were analyzed and residues of DBP and DEHP were found at 0.12 and 0.15 μg/L, respectively, in the river water sample	Acetone showed higher extraction efficiency than MeOH and ACN as elution solvent. Coca-Cola and green tea samples were also analyzed	[78]
DMP, DEP, DAP, DIBP, and BBP	River, reservoir and sea waters (300 mL)	m-dSPE using 36 mg layered carbon-Fe3O4 under agitation for 10 min, a magnet was used for decantation, and elution (in triplicate) with 0.5 mL acetone by vortex for 10 s	HPLC-UV	0.27–0.33 μg/L	88.0–104.7% at 5 and 10 μg/L	One sample of each water were analyzed and residues of DAP and DIBP were found at 0.52 and 0.86 μg/L, respectively, in the river water sample	Acetone showed higher extraction efficiency than MeOH and ACN as elution solvent	[26]
DMP, DEP, DIBP, DBP, DMEP, BMPP, DEEP, DNPP, DHXP, BBP, DBEP, DCHP, DEHP, DIPP, DNOP, and DNP	Mineral and tap waters (9.8 mL plus 0.2 mL MeOH)	m-dSPE using 0.1 mL suspension of MWCNTs-m-NPs in water (40 mg/ml) under vortex for 3 min, a magnet was used for decantation, and elution with 1 mL acetone	GC-MS	0.016–0.13 μg/L	79.6–125.6% at 5 μg/L	Two mineral and 1 tap water samples were analyzed and contained at least 3 PAEs at levels from 0.36 to 3.3 μg/L	Acetone showed higher extraction efficiency than MeOH, ethyl acetate and hexane as elution solvent. Juice and carbonated drinks, and one perfume sample were also analyzed	[77]
DMP, DEP, DIBP, DBP, DEHP, BBP, and DNOP	River and pond waters (10 mL)	m-dSPE using 20 mg G-Fe3O4 under vortex for 15 min, a magnet was used for decantation, and elution with 0.4 mL ethyl acetate and 0.5 g anhydrous sodium sulfate by sonication for 15 min	GC-MS	0.035–0.19 μg/L	88–110% at 10,000 μg/L	One sample of each water were analyzed and residues of all PAEs except DMP were found at levels from 22.2 to 150.8 μg/L	Ethyl acetate showed higher extraction efficiency than acetone and chloroform as elution solvent	[79]
DMP, DEP, DBP, BBP, and DNOP	River, tap and mineral waters (20 mL)	m-dSPE using 20 mg Fe3O4-ZIF-8 MOF under sonication for 8 min, a magnet was used for decantation, and elution with 1 mL MeOH by sonication for 8 min	HPLC-DAD	0.3–0.8 μg/L	85.6–103.6% at 1, 10, and 100 μg/L	One sample of each water were analyzed and at least 2 PAEs at levels from 5 to 60 μg/L were detected in the river and tap water samples	Methanol showed higher extraction efficiency than ACN, chloroform and tetrahydrofuran as elution solvent	[85]
DMP, DEP, DIBP, DBP, DEHP, BBP, DNOP, DMEP, DEEP, DNPP, BMPP, DHXP, DBEP, DCHP, DPhP, and DINP	Tap and lake waters (20 mL)	m-dSPE using 20 mg Fe3O4-polypyrrole under agitation for 40 min, a magnet was used for decantation, and elution with 2 mL ethyl acetate by sonication for 60 min	GC-MS	0.018–0.068 μg/L	80.4–108.2% at 5 and 100 μg/L	One sample of each water were analyzed and at least 5 PAEs at levels from 0.10 to 6.90 μg/L were detected	An orthogonal fraction factorial design was used for optimization purposes. Ethyl acetate showed higher extraction efficiency than acetone and isopropanol as elution solvent	[81]
DEP, DPP, DBP, DIPP, DNPP, BBP, DCHP, DEHP, DNOP, DINP, DIDP and DEHA	Mineral, tap, pond and waste waters (25 mL adjusted at pH 6)	m-dSPE using 60 mg Fe3O4-PDA under agitation for 1 min, a magnet was used for decantation, and elution with 6 mL dichloromethane by agitation for 30 s	GC-MS/MS	0.009–0.02 μg/L	71–120% at 0.5 and 5 μg/L	One sample of each water were analyzed and residues of DEP and DBP were found at levels from 0.36 ± 0.46 to 4.20 ± 0.52 μg/L in the mineral, tap and waste waters	Dichloromethane showed higher extraction efficiency than acetone, MeOH and ACN as elution solvent	[22]
DMP, DEP, BBP, and DBP	Carbonated, mineral and soda waters (25 mL)	m-dSPE using 30 mg poly(1-vinyl-3-butylimidazolium bromide)-PS m-NPs under vortex for 2.5 min, a magnet was used for decantation, and elution with 7 mL ACN by sonication	HPLC-DAD	0.017-0.047 μg/L	77.8-102.1% at 2 and 20 μg/L	One sample of each water were analyzed and residues of DEP were found at 25.8 and 15.5 μg/L in the carbonate and soda waters, respectively	ACN showed higher extraction efficiency than acetone, petroleum ether and MeOH as elution solvent	[82]
DMP, DEP, DAP, DIBP, and DBP	Tap and well waters (5 mL plus 15% w/v NaCl)	m-dSPE using 15 mg Fe3O4-MIL-101(Cr) MOF under agitation for 20 min, a magnet was used for decantation, and elution with 1 mL hexane/acetone (1:1 v/v) by vortex for 3 min	GC-MS	0.3–0.5 μg/L	90.1–106.7% at 5 and 50 μg/L	One sample of each water were analyzed and no residues were detected	The use of Fe3O4-MIL-101(Cr) MOF showed higher enrichment capacity than Fe3O4 and MIL-101(Cr) MOF separately. Hexane/acetone (1:1 v/v) showed higher extraction efficiency than ethyl acetate, hexane, acetone and hexane/ethyl acetate (1:1 v/v) as elution solvent. Human plasma was also analyzed	[86]
DMP, DEP, DBP, BBP, DEHP, and DNOP	Tap, drinking and mineral waters (10 mL)	m-dSPE using 15 mg Fe3O4-MIL-100 MOF and 15-mg Fe3O4-SiO2-polythiophene under sonication for 1 min and oscillation for 15 min, a magnet was used for decantation, and elution with 1 mL ACN by agitation for 10 min	GC-MS	1.1–2.9 μg/L	76.9–109.1% at 1, 10 and 50 μg/L	One sample of each water were analyzed and no residues were quantified	A combination of 15-mg Fe3O4-MIL-100 MOF and 15-mg Fe3O4-SiO2-polythiophene gave better extraction efficiency than using 30-mg each separately. ACN showed higher extraction efficiency than acetone, ethyl acetate and hexane as elution solvent	[88]
DMP, DBP, BBP, DCHP, and DEHP	Tap and lake waters (100 mL adjusted at pH 6)	m-dSPE using 30 mg poly(1-vinylimidazole)-carboxy-latocalix[4] arene m-NPs under sonication for 15 min, a magnet was used for decantation, and elution with 0.5 mL MeOH by sonication for 5 min	HPLC-UV	0.05–0.11 μg/L	89.9–110.0% at 0.5, 1, and 5 μg/L	One sample of each water were analyzed and contained at least 1 PAE at levels from 0.4 to 8.9 μg/L	Methanol showed higher extraction efficiency than ACN and chloroform as elution solvent. The use of poly(1 -vinylimidazole)-carboxy-latocalix[4] arene m-NPs showed higher enrichment capacity than Fe3O4 and poly(1-vinylimidazole) separately. m-dSPE using poly(1-vinylimidazole)-carboxy-latocalix[4] arene m-NPs showed better results compared with SPE with C18 and Cleanert SCX cartridges. Drinks, tonic lotions, and human serum were also analyzed	[83]
DBP, DMP, DCHP, BBP, and DEP	River water (10 mL)	m-dSPE using 30 mg 3D N-Co-C/HCF MOF under agitation for 20 min, a magnet was used for decantation, and elution with 6 mL ACN by sonication for 10 min	HPLC-UV	0.077–0.377 μg/L	92.4–104.2% at 1, 10, and 50 μg/L	One sample was analyzed and contained DMP and DEP at 0.075 and 0.081 μg/L, respectively	Green tea, sports beverage and white spirit were also analyzed. ACN showed higher extraction efficiency than acetone, MeOH, and ethanol as elution solvent	[87]
DPP, DBP, DCHP, DEHP, DNOP, DIDP, BBP, DINP, DIPP, DNPP, and DEHA	Sea water (50 mL adjusted at pH 6)	m-dSPE using 120 mg Fe3O4-PDA under agitation for 1 min, a magnet was used for decantation, and elution with 1 mL dichloromethane by agitation for 30 s	GC-MS	0.0018–0.319 μg/L	79–116% at 0.4, 1, and 1.6 μg/L	Ten samples were analyzed and no residues were quantified	Sea sand was also analyzed.	[80]

MeOHMeOHMeOHMeOHMeOHACN, acetonitrile; BBP, benzylbutyl phthalate; BMPP, bis(4-methyl-2-pentyl) phthalate; DAD, diode-array detector; DAP, diallyl phthalate; DBEP, di(2-butoxyethyl) phthalate; DBP, dibutyl phthalate; DCHP, dicyclohexyl phthalate; DEEP, di(2-ethoxyethyl) phthalate; DEHA, di(2-ethylhexyl) adipate; DEHP, di(2-ethylhexyl) phthalate; DEP, diethyl phthalate; DHP, diheptyl phthalate; DHXP, dihexyl phthalate; DIBP, diisobutyl phthalate; DIDP, diisodecyl phthalate; DINP, diisononyl phthalate; DIPP, diisopentyl phthalate; DMEP, di(2-methoxyethyl) phthalate; DMIMs, dummy molecularly imprinted microbeads; DMP, dimethyl phthalate; DNOP, di-n-octyl phthalate; DNP, dinonyl phthalate; DNPP, di-n-pentyl phthalate; DPhP, diphenyl phthalate; DPP, dipropyl phthalate; dSPE, dispersive solid-phase extraction; G, graphene; GC, gas chromatography; GO, graphene oxide; HCF, hierarchical carbon framework; HPLC, high-performance liquid chromatography; LOQ, limit of quantification; m-dSPE, magnetic solid-phase extraction; MeOH, methanol; MIL, Material of Institute Lavoisier; MIP, molecularly imprinted polymer; m-NPs, magnetic nanoparticles; MOF, metal organic framework; MS/MS, tandem mass spectrometry; MS, mass spectrometry; MWCNTs, multiwalled carbon nanotubes; PAE, phthalic acid ester; PDA, poly(dopamine); PS, polystyrene; SPE, solid-phase extraction; UV, ultraviolet; ZIF, Zeolitic Imidazolate Framework.

MeOHMeOHMeOHMeOHMeOH

Figure 1.5 Scheme of a similar extraction procedure carried out by Chen et al. [75] using the same (β-CD-PNIPAM temperature-sensitive polymer as extractant for the determination of phenolic compounds in river water samples. Reprinted from [75] with permission of Elsevier. Peak identification: phenol (BP), 2,4-dichlorophenol (2,4-DCP), (β-naphthol ((β-NP), and bisphenol A (BPA).

Despite the simplicity of the dSPE, the whole procedure can be even improved and simplified if the sorbent particles can be manipulated with a magnet. Such dSPE mode is called m-dSPE and is based on the use of magnetic NPs (m-NPs), which can be applied as synthesized (though in very few cases), although they are generally functionalized or coated with other chemical species or materials, resulting in many cases a “coreshell” structure, in order to improve their selectivity, or even embedded them in the extraction sorbent to provide it with magnetic properties [50]. Despite the extraction step is performed in a similar way as in dSPE, in this case, the magnetic sorbent containing the analytes is retained in the extraction recipient using an external magnet while the sample matrix is easily discarded without the need of an additional centrifugation step or the retention of the sorbent in an empty column. Finally, the analytes are desorbed from the magnetic sorbent using a suitable solvent and, once more, the sorbent is retained with the magnet to separate the solvent containing the analytes by decantation for their determination with a suitable technique.

Although a wide variety of metals, metal oxides and alloys can be used to provide magnetic properties to the sorbent, Fe₃O₄ NPs have been used as the main support with this purpose due to their extraordinary magnetic features and the possibility of being modified by anchoring certain chemical species or materials to obtain sorbents that provide different selectivity. Therefore, as in the rest of the sorbent-based microextraction techniques, the preparation of appropriate sorbents is one of the most important aspects to be considered. In this regard, carbon-based nanomaterials like MWCNTs [76, 77] and graphene [26, 78, 79], combined with m-NPs have once again been one of the most used for the extraction of PAEs from water samples due to their well-known properties in terms of high surface area-to-volume ratio that guarantees high extraction efficiency. As an example, MWCNTs were functionalized via chemical modification by Jiao et al. [76]. Figure 1.6 shows the scanning electron microscopy and transmission electron microscopy images of m-NPs successfully combined with MWCNTs. The authors used this sorbent to study a large set of thirteen PAEs, paying special attention to the elution step because PAEs may not be easily desorbed due to the strong interaction between the sorbent and analytes. For this purpose, different common organic elution solvents (i.e., acetone, MeOH, n-hexane, ethyl acetate, and toluene) were evaluated, finding that the use of toluene provided higher recovery percentages compared to non-aromatic solvents, particularly for BBP and diphenyl phthalate (DPhP), since they contain two and three benzene rings, respectively. However, since toluene is highly toxic, toluene-acetone mixtures were also evaluated in different proportions (1:1, 1:4, and 1:9, v/v), obtaining that toluene-acetone (1:4, v/v) kept satisfactory recovery values, while the use of toluene-acetone (1:9, v/v) decreased them. Finally, the viability of this method was demonstrated for the extraction of the selected PAEs from drinking water, showing an extraordinary extraction capacity which, in combination with the inherent advantages of m-dSPE, becomes this methodology an excellent alternative to be explored for the analysis of other pollutants. Similarly, Luo et al. [77] immobilized m-NPs onto MWCNTs by ultrasound application as a simpler alternative to chemical functionalization (see Figure 1.7). In particular, appropriate amounts of the previously synthesized Fe₃O₄ NPs and MWCNTs were dispersed in dimethylformamide and sonicated, achieving the spontaneous assembling of both materials resulting in a magnetic composite. Finally, it was washed with water and suspended in water at 40 mg/mL, taking a certain volume of it for the extraction procedure. Next, the usual m-dSPE procedure was carried out for the enrichment of sixteen PAEs from mineral and tap water, juice and carbonated drinks as well as one perfume sample. The combination of this extraction procedure with GC-MS allowed obtaining a good extraction capacity in relatively short times as well as high sensitivity, becoming this methodology an interesting alternative to be considered.

Figure 1.6 Scanning electron microscope image (A) and transmission electron microscope image (B) of the MWCNTs coated m-NPs. Reprinted from [76] with permission from Royal Society of Chemistry. m-NPs, magnetic nanoparticles.

Figure 1.7 Schematic illustration of the preparation strategy for m-NPs and the m-dSPE procedure for the determination of PAEs. Reprinted from [77] with permission from Elsevier. CNT, carbon nanotubes; GC, gas chromatography; MS, mass spectrometry; MWCNTs, multi-walled carbon nanotubes.

Graphene is also able to establish strong π stacking interactions with benzene rings which already was demonstrated when it was used as sorbent in dSPE for PAEs extraction [72]. However, graphene is difficult to remove from the sample in which it has been dispersed because it is an ultralight material. Therefore, if magnetic properties are provided, its subsequent separation will be greatly facilitated while retaining its excellent adsorption capacity. Such is the case of the work developed by Wu et al. [78] in which graphene-coated m-NPs were first used for the extraction of PAEs from bottled and river water samples as well as from a soft drink and green tea samples. In this case, graphene was previously obtained from the graphite oxide exfoliation to obtain GO, which was subsequently reduced. Then, it was suspended in the alkaline solution used to synthesize the Fe₃O₄ NPs by a chemical coprecipitation process, obtaining the desired magnetic sorbent. As expected, the developed m-dSPE procedure provided high extraction efficiency, besides high enrichment factors, which resulted in LODs in the range of few μg/L, even though a HPLC-UV system was used for analytes separation and detection.

Despite carbon-based nanomaterials have been widely combined with m-NPs in a good number of applications, polymeric coatings have been, without any doubt, the most extensively used in m-dSPE because of the versatility and advantages they provide. In consequence, and as it could not be otherwise, polymer-coated m-NPs have also been widely used for the extraction of PAEs from water samples. The polymer coating protects m-NPs from oxidization and undesirable aggregation that occurs after their synthesis improves their stability and maintains their magnetic properties. Consequently, surface functionalization will also improve m-NPs dispersion while different kinds of interactions with PAEs take place, at the same time that enhances their selectivity. As several examples, Hernández-Borges’ group first applied PDA-coated m-NPs for the isolation and enrichment of PAEs from mineral, tap, pond, and waste waters [22] as well as in sea water and sea sand samples [80], using in both cases a chemical co-precipitation method to obtain the Fe₃O₄ m-NPs and taking advantage of the self-polymerization capacity of DA in weak alkaline water solutions to create a magnetic core-shell sorbent; while Zhao et al. [81] demonstrated the applicability of synthesized PPy-coated m-NPs through a chemical oxidation method which allowed the combination of both materials for the determination of sixteen PAEs in lake and tap water samples, while Liu et al. [82] and Zhou et al. [83] employed the highly hydrophilic ILs 1-vinyl-3-butylimidazolium bromide and 1-vinylimidazole to modify PS and carboxylatocalix[4]arene coated m-NPs, respectively. In these last two works, the immobilization of polymerized ILs onto m-NPs surface improved the dispersion and extraction efficiency in drinking and environmental water samples significantly due to the additional hydrogen bonding and π-π interactions. In addition, the developed sorbents could be reused at least 12 and 30 times, respectively, without a significant decrease in their adsorption capacity or carry-over.

Finally, m-NPs have also been combined with MOFs and used as sorbents in m-dSPE. In fact, and specifically concerning the extraction of PAEs, it has been the main field of application of MOFs among all sorbent-based microextraction techniques. MOFs are considered one of the nanoporous materials with the largest surface areas, characterized by highly ordered cavities and tailorable chemistry by coordinated bounds of a large variety of metal cations and organic ligands [84]. Various types of MOFs combined with the magnetic core have arouse special interest in this topic, as it is the case of Liu et al. [85], who coated Fe₃O₄ m-NPs with Zeolitic Imidazolate Framework-8 (ZIF-8) resulting in a core-shell structure. These modified m-NPs were successfully applied to the extraction of five PAEs from river, mineral and tap water samples, showing an excellent extraction capacity (recovery in the range 84%–104%) for all the analytes independently of the matrix. On contrary, Dargahi and co-workers [86] proposed other alternative sorbent based once again on the combination of Fe₃O₄ m-NPs and a MOF, but in this case, the Material of Institute Lavoisier-101 (MIL-101) was used as support on whose surface the m-NPs were deposited. This sorbent was applied to the m-dSPE of five short-chain PAEs from well water and human plasma samples, showing promising results. However, MOFs are not always used as synthesized, but they are sometimes used as the basis to create highly porous sorbents with different features. A clear example of this fact is the work of Wang et al. [87], in which they synthesized what they called a “three-dimensional magnetic porous N-Co@carbon dodecahedron/hierarchical carbon framework” (3D N-Co@C/HCF) which was used as sorbent for the extraction of five PAEs from river water, green tea, a sports beverage, and white spirit samples. The first step consisted in the synthesis of a 3D carbon-based structure based on sodium carbonate and glucose. Then, it was combined with Co(NO₃)₂ and 2-methylimidazole under stirring, obtaining a composite of ZIF-67 and the HCF, which was finally calcined at 700°C under N₂ atmosphere to obtain the desired 3D N-Co@C/HCF. This magnetic nanoporous sorbent demonstrated to provide an excellent extraction efficiency for all the target PAEs and matrices thanks to its large surface-to-volume ratio.

Apart from the previous works, the combination of MOF and polymer-coated m-NPs has also been found beneficial for the extraction of PAEs. In this sense, Li et al. [88] prepared Fe₃O₄@MIL-100 and Fe₃O₄@ SiO₂@polythiophene as mixed sorbents for m-dSPE extraction of six PAEs (DMP, DEP, DBP, BBP, DEHP, and DNOP) from tap and mineral water samples. Although PAEs contain both benzene rings and alkyl chains, the use of the MOF-coated m-NPs alone did not show enough extraction efficiency particularly for both DMP and DEP. This was attributed to the greater water solubility of these low-molecular PAEs as well as lower hydrophobic interaction with this sorbent. When the polymer-coated m-NPs were used, it did not show good adsorption capacity for the PAEs that contain longer alkyl chains, especially DNOP. This was associated to the negative effect of these long alkyl chains on the π-π interactions with the sorbent. Instead, both sorbents in a 1:1 (w/w) ratio were combined under sonication, and the mixture was used as sorbent, giving satisfactory extraction recovery values for all the PAEs and matrices.