the determination of PAEs."/>
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.
1.5 Others Minor Sorbent-Based Microextraction Techniques
Although dispersive versions of SPE have been widely used due to the well-known advantages they offer, as it has been already mentioned, trends in sorbent-based extraction techniques are also focused on the miniaturization of the extraction devices, which has given place to the appearance of new modifications of conventional SPE but with reduced amounts of sorbent or slight changes on extraction devices. Some of these alternative methodologies have also been successfully applied to the analysis of PAEs in water samples (see Table 1.3).
In this sense, microextraction by packed sorbent (MEPS) is considered as a miniaturized technique derived directly from SPE which can be coupled directly to the chromatographic systems without any additional modification. MEPS only use 1–2 mg of sorbent to adsorb the analytes successfully. Concretely, it is packed between frits inside a microsyringe and extraction is performed within it by subsequent suction. After the analytes are trapped, the packed sorbent is washed with water to remove the interferences. Finally, the target analytes are eluted by appropriate solvent aspiration into the microsyringe [89]. Amiri et al. [90] used only 2 mg of synthesized hydroxyapatite NPs packed inside a 0.5-ml microsyringe for the rapid extraction of five PAEs from river, mineral and tap water samples. To evaluate the extraction efficiency, the effect of multiples drawing-ejecting cycles in the range 10–60 cycles were performed in the same vial containing 8 ml of spiked sample at 50 μg/L. The results showed that the maximum peak areas were achieved using 40 cycles for all the target analytes and then kept constant. When the same process was repeated using 8 mL of spiked sample at 100 μg/L discarding each 0.5 mL load to another vial (16 cycles), the results did not improve. Therefore, 40 cycles of draw-eject in a same vial was the best approach in terms of simplicity. With it, the developed methodology showed good sensitivity, repeatability, and relative
