Introduction

Solid phase extraction (SPE) is one of the widely used sample preparation technique for the extraction of both organic and bioinorganic analytes from various samples. The scope of the present work only allows for the critical review of recent work reported in literature for the selective SPE of organic compounds from various sample matrices. SPE plays a key role in the trace analysis of dilute samples where it extracts and pre-concentrates the compounds of interest. To date, different SPE sorbents have been reported in literature, and a number of them including carbon nanotubes [1]; Waters Oasis sorbents including Oasis hydrophilic lipophilic balance (HLB) [2], anionic-exchange (Oasis MAX) [3], cationic-exchange (Oasis MCX) [4]; reversed-phase C18 [5] and styrene divinylbenzene (DVB) [6] are commercially available. A variety of these sorbents have been described in a review article presented by Wen et al. [7] and some most recent applications (selected randomly) are summarized in Table 1. Some of the reported sorbents lack selectivity which often causes prolonged sample analysis times while the presence of matrix effects in the extracts results in suppression of the analyte signal during chromatographic quantitation. Therefore, to improve these drawbacks, novel sorbents for selective extraction of organic analytes from various samples including environmental, food and biological samples have been investigated. The general milestones in SPE aimed at enhancing its applicability in environmental and bio-analysis are summarized in Fig. 1. The major advances in this regard include miniaturization, selectivity, online modes and reusability. All these advancements are discussed in detail in this chapter.

Table 1 Common solid phase extraction sorbents recently reported in literature for extraction of organic analytes from environmental samples
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Fig. 1
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General milestones in solid phase extraction

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Selectivity in SPE is usually demonstrated by two distinct procedures. In one approach, the sample is directly analyzed using a chromatographic instrument without analyte isolation. The same sample is also analyzed after SPE using a selective sorbent. In this case, a selective sorbent results in a cleaner chromatogram with a characteristic analyte peak. This approach has been demonstrated by Ncube et al. [14] in which direct injection of hexane sludge extracts was compared with extracts injected after selective extraction using a smart adsorbent [14]. Their results presented as Fig. 2 show that the smart sorbent was effective in reducing the impact of matrix effects. In most cases, this approach is used to compare the performance of different adsorbents. In one example demonstrated in Fig. 3, a novel sorbent (divinyl benzene polymer monoliths) was compared for selectivity with the Waters Oasis HLB sorbent in the analysis of saccharin in a urine sample [15]. As shown in Fig. 3, their results indicated that a novel sorbent was able to remove significant amount of matrix interferences, thereby showing a better selectivity than Oasis HLB sorbent. Another procedure currently in use to demonstrate selectivity is one where the sample is spiked with the compound of interest and several other compounds that are structurally similar to the analyte, followed by sample extraction and analysis. In this instance, higher recovery of the target compound compared to others is taken as a proof of selectivity [16].

Fig. 2
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Copied from Ref. [1] with permission from Wiley-VCH

a Total ion chromatogram after direct injection of hexane sludge extract; b total ion chromatogram after passing through a molecularly imprinted polymer as a selective adsorbent; c chromatogram showing quantitation ion peaks of standards; d quantitation ion peaks of sludge extract after selective adsorption.

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Fig. 3
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Copied from ref. [2], with permission from Elsevier

The sample clean-up of saccharin in urine. (top) saccharin in neat urine, (middle) sample clean-up with Oasis HLB and (bottom) sample clean-up with divinyl benzene polymer (PDVB) monoliths.

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Over two decades ago, the improvements of SPE protocol where the main goal was to achieve a better selectivity involved the application of immunosorbents [17, 18]. In recent years, more researchers have been developing novel sorbents that are suitable for selective SPE of organic analytes. In this case, a review article was published in 2014 on selective materials for SPE which focused on molecularly imprinted polymers (MIPs) and mixed-mode ion-exchanged sorbents [19]. Thus far up to 2018 (in the last 5 years), plenty of developments and interesting applications for MIPs and mixed-mode ion exchanged sorbents have been reported. Furthermore, there is a great amount of work reported in literature on the new materials including nano sorbents and MIP composite materials that are capable of extracting selectively various organic compounds from complex samples. Some SPE sorbents recently reviewed by Płotka-Wasylka [20] are based on the use of nanostructured materials, including carbon nanomaterials, electrospun nanofibers, dendrimes and magnetic nanoparticles, molecular recognition sorbents such as aptamers, immunosorbents, MIPs, ion imprinting polymers as well as metal–organic frameworks and restricted access materials. And, therefore, the current review is only based on selective sorbents for SPE with its aim being to critically review recent materials used for selective SPE of organic compounds from various sample matrices. In this work, we focused primarily on the work published from 2014, with occasionally referring to the earlier studies.

Selective Solid Phase Extraction Sorbents

Molecularly Imprinted Polymers

The application of MIPs as selective sorbents for SPE is well exploited in the field of analytical chemistry. MIPs are synthetic polymers with highly specific recognition sites for target molecules, made from monomers that have the ability to form a complex with a template, usually the target compound in analysis, through covalent or non-covalent interactions (polar, hydrogen-bond, and van der Waals forces) that are then joined together by a cross-linking agent [21]. The removal of the template molecule by solvent extraction from the MIPs results in the formation of the binding sites that are complimentary to the template in size, shape and functional groups (Fig. 4). This subsequently influences the selective extraction of target compounds. A particular example is demonstrated in Fig. 4, where a MIP that is selective to ibuprofen (IBU) was synthesized using methacrylic acid (MAA) as a functional monomer for the purpose of hydrogen bonding with the target compound and trimethylolpropane trimethacrylate (TRIM) as a cross-linker [22]. In their case, the polymerization was initiated with 4,4-azobis(4-cyanovaleric acid) (AIVN). As shown in the Fig. 4, the special binding site for ibuprofen was achieved by removing the template from the MIP using a mixture of water and methanol. Since the template molecule plays a crucial role in imposing the selectivity of the MIP, different considerations including the use of single templates [23], multi-templates [24] and dummy templates [25] have been explored during the synthesis of MIPs.

Fig. 4
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Schematic representation of molecular imprinting copied from reference [26]

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Templates Used in the Synthesis of Molecularly Imprinted Polymers

In the context of this work, single template MIP refers to the case where a target compound is used as a template molecule. The application of single template MIPs as selective SPE sorbents has been well explored in the analysis of environmental [27], biological [23] and food samples [28]. As evidenced from the work presented by Caro et al. [23], since over a decade ago the selectivity of MIPs is usually tested in the presence of structurally related compounds. In their work, Caro et al. [23] evaluated the selectivity of a naproxen MIP for its extraction from urine samples in the presence of benzoic acid, fenoprofen, diclofenac and ibuprofen all of which have size, shape, functional groups as well as pKa values similar to that of their target compound [23]. Recently, our research group demonstrated that a MIP synthesized for SPE of ketoprofen from wastewater has the ability to selectively extract the target compound from water in the presence of triclosan, gemfibrozil and fenoprofen [27]. In certain instances, MIPs adsorb the compounds from aqueous samples through the recognition of functional groups from the template. In such cases, the washing step in SPE is optimized well for the selective desorption of untargeted compounds which act as interferences and impurities [29]. In situations of multi-analyte analysis, multi-templates MIP are developed.

Multi-template MIPs are synthesized using more than one target compound as template molecules. This innovation has received great interest in recent years [24, 30, 31]. In one case, the widely detected non-steroidal anti-inflammatory drugs (NSAIDs) in environmental samples which are naproxen, ibuprofen and diclofenac were selectively extracted by a multi-template MIP used as an SPE sorbent prior to their chromatographic separation and quantification in river water and wastewater [24]. In another study, six acidic pharmaceuticals, namely; ibuprofen, naproxen, ketoprofen, diclofenac, and clofibric acid were simultaneously imprinted and their MIP selectively extracted them from lake water, wastewater and sediments samples after ultrasonic solvent extraction [30]. Similarly, six phenolic compounds, namely; phenol, 4-chlorophenol, 2,4,6-trichlorophenol, 2,4-dichlorophenol, 2-chlorophenol and 2,6-dichlorophenol were used as multi-templates in the synthesis of MIP that resulted in their selective SPE from water samples [31]. Selectivity tests for multi-template MIPs have shown the undesirable ability of these MIPs to extract interferences through non-specific binding, however, most of interfering species are removed in the washing step of SPE. In a multi-template MIP for acidic pharmaceuticals, the recoveries of interferences were ranging from 96.3 to 97.5% in the washing solvent of SPE, which implied their selective removal from the SPE cartridge prior to the elution of target compounds [30]. Although single and multi-templates MIPs have a great potential as selective sorbents in SPE, there is a risk of template(s) leakage into real samples during extractions in cases where there is incomplete removal of the template(s) from the cavities during washing. This can have negative impact on the analytical data, and in such cases dummy template MIPs are synthesized to overcome the drawbacks.

A dummy template is a template compound whose physicochemical properties especially its functional groups and chemical structure are similar to the target analyte but do not interfere in its analytical determination [32]. Dummy MIPs are less selective than other forms of MIPs, but they have the advantage of separating the analyte signal from that of the dummy molecule in a chromatographic column. Dummy MIPs have the ability to extract selectively the group of compounds whose chemical structure closely resemble the template molecule [25, 33]. This is demonstrated in Table 2, where several compounds used as dummy templates in the synthesis of MIPs are given alongside the target compounds in the analysis. In one study, where bisphenol S was used as a dummy template, the synthesized MIP had the ability to extract bisphenols; F, E, A, B and AF with imprinting factors of 14.5, 13.8, 8.7, 5.7 and 4.2, respectively [33]. In a different perspective, due to high cost of target compounds, cheaper compounds that are structural related to them are used as dummy templates in the synthesis of MIPs. For example, citric acid was used as a dummy template in the synthesis of a MIP which was used as a selective sorbent in SPE of domoic acid from seawater [34]. The selectivity for domoic acid was observed due to its better recognition in water solutions resulting in the adsorption amount of 92.75 µg g−1 in the presence of kainic acid which was poorly adsorbed (adsorption amount of 62.14 µg g−1) [34]. Furthermore, the use of the dummy templates in molecular imprinting is beneficial in cases where safety considerations are involved [35]. For example, the synthesis of a MIP that was intended for selective extraction of acrylamide (a neurotoxic compound) from heat processed food was synthesized using propionamide as a template [35].

Table 2 Dummy templates used in the synthesis of molecularly imprinted polymers selective to various compounds
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Another advancement in the usage of templates in the synthesis of MIPs involves using a single template MIP to target several analytes with related physicochemical properties. In this regard, a molecularly imprinted microbead synthesized using diethyl phthalate was able to selectively extract diethyl phthalate and five other phthalate esters from bottled water and artificial saliva with recoveries ranging between 92 and 99%. The other phthalate esters were dimethyl phthalate, di-n-butyl phthalate, benzyl butyl phthalate, di-(2-ethylhexyl) phthalate and di-n-octyl phthalate [25]. In their study, the performance of the MIP was investigated for two non-phthalate esters (styrene and vanillic acid). Their distribution coefficients were found to be low. This was attributed to these two compounds having physicochemical properties different from the imprinted compound. As such, the molecularly imprinted microbead cavities had no specificity for styrene and vanillic acid. Other examples of single-template MIPs that have been used in the analysis of various related compounds include MIP synthesized with erythromycin template for multiplex macrolide antibiotics extraction from food samples [39] and 2,2′,4,4′-tetrehydroxybenzophenone MIP which had the ability to extract four benzophenones in tap and river water [40].

Approaches to Increase Selectivity of Molecularly Imprinted Polymers

When MIPs are applied as SPE sorbents, their selectivity can be attained by performing a selective washing or elution step after the analytes have been retained by the sorbent. It is important to perform the selective washing and elution steps due to the ability of MIPs to retain non-target compounds that have similar functional groups as the analyte. In this case, non-target compounds retained due to non-specific binding are easily removed in the washing step of SPE due to their weak interactions with the MIP. In addition, differences in the size and shape of the non-targeted compounds in comparison with the analyte lead to their desorption with the washing solvent. The removal of polar interferences is removed with polar aprotic solvents, which have the ability to dissolve the weakly polar compounds that are weakly bound by MIP. Therefore, the selectivity in molecularly imprinted solid-phase extraction is attributed to the formation of a 3D shape selective cavity that is complementary to the molecular skeleton of the template molecule [26]. In addition, the molecular size and shape might serve as the main factors for specific recognition, together with ionic interaction, hydrophobic effect and hydrogen bonding [26]. This has been evidenced using a fluconazole-imprinted MIP where selectivity was tested using fungicides (secnidazole, miconazole and tioconazole) that have similar structures and functional groups to the target molecule [41]. In their study, they found percentage recoveries of 93, 32, 51 and 35% for fluconazole, secnidazole, miconazole and tioconazole, respectively; which implied great selectivity towards their target compound, fluconazole [41]. In this case, an aqueous aprotic solvent [dimethyl sulfoxide solution (2%; v/v)] was used as a washing solvent after sample loading, which resulted in decrease in analyte recovery from a non-imprinted polymer without disturbing the MIP results. This was translated to the ability of certain organic solvents to cause disruptions of non-specific interactions between the target compound and the sorbent. This further meant that the clean-up step (washing) prior to the elution step is crucial in molecularly imprinted solid-phase extraction and the selected washing solvent should suppress the nonspecific interactions without disrupting the selective specific interactions between the imprinted cavities and the target molecule [42]. In the work done by our research group, the nonspecific interactions for competitors (triclosan, fenoprofen and gemfibrozil) were disrupted by washing the ketoprofen MIP sorbent with 5% (v/v) trimethylamine in water, thus yielding < 20% recovery for all competitors while the retained ketoprofen was eluted later-on with methanol achieving a recovery of 104% [27]. Even though both ketoprofen and competitors had the ability to undergo hydrogen bonding with the MIP, structurally the competitors were not able to fit properly into the MIP cavities thereby exposing them to the washing solvent. Thus, the nitrogen atom of trimethylamine (washing solvent) was capable of binding with hydrogen atoms of the hydroxyl group in the competitors, thereby washing them from the surface of the MIP. This proved difficulty to occur for ketoprofen, as this drug was properly fitted within the cavities of the MIP. Other solvents that have been reported as polar aprotic washing solvents to eliminate interferences due to compounds with physicochemical properties similar to the target compound include acetonitrile [43] and dichloromethane [30, 44].

Molecularly Imprinted Polymers Enclosed in Membranes or Hollow Fibers

A novel approach which is based on the combination of supported liquid membrane and MIPs has been reported in the extraction of organic analytes from aqueous media. This combination of MIPs and membrane techniques is promising to be a powerful tool as the sorbent is protected from having a physical contact with the sample matrix, while the analytes are allowed to diffuse across the membrane based on size and polarity thus resulting in better selectivity and sensitivity. The most common membrane systems used are the hollow fiber liquid phase micro-extraction (HF-LPME) and the membrane assisted solvent extraction (MASE) techniques. The MIP-membrane combination involves packing a small amount of MIP particles (≤ 100 mg) into the lumen of the membrane. A water immiscible organic solvent (the acceptor phase) is then added into the lumen. The analytes transfer from the sample solution (the donor phase) across the membrane wall into the acceptor phase and eventually adsorb onto the MIP cavities. SPE is then used to elute the analytes from the MIP cavities without the need for the washing step followed by chromatographic determination. The HF-LPME-MIP combination technique has been applied for selective extraction of sulfonamides [45] and thiabendazole [46] in environmental waters and citrus samples, respectively. Most of the applications of the MASE-MIP technique have been reported within our research group for analysis of triazines [47, 48], polycyclic aromatic hydrocarbons [14, 49] and 17β-estradiol [50].

In a different but related technique, the combination of ultrasound-assisted emulsification and micro-solid phase extraction used 4 mg of MIP sorbent placed inside the membrane for selective extraction of bisphenol A from aqueous matrices [51]. In this development, the analyte was extracted from a sample volume of 20 mL and desorbed using 200 µL of ethanol, and thus achieving a detection limit of 0.07 µg L−1 with high performance liquid chromatography-diode array detector [51]. In a different study, a cone shape polypropylene membrane containing the MIP beads was used for isolation of cocaine and its metabolites; benzoylecgonine, ecgonine methyl ester, and cocaethylene from human urine [52]. In this case, 50 mg MIP was used as sorbent and resulted in selective extraction of target analytes yielding recoveries ranging from 89 to 100%. In an innovation presented by Li et al. [53], the SPE was performed using a syringe connected with a nylon membrane filter employed as the adsorbent carrier. In their case, 80 mg of atrazine MIP was employed as a selective adsorbent for extraction of herbicides from Radix Paeoniae Alba which is a type of the traditional Chinese herbal medicines. In their method, recoveries ranging from 92 to 107% were achieved for targeted triazine herbicides.

Molecularly imprinted membranes for extraction of thymopentin from crude samples have been synthesized using 1-vinyl-3-ethyl acetate imidazolium chloride as functional monomer, N,N′-methylene bis acrylamide as cross-linking agent, N,N,N′,N′,N″-pentamethyldiethylene-triamine as ligand, and copper chloride as catalyst through surface-initiated ATRP in aqueous solution [54]. The resulting sorbent was able to isolate thymopentin from the crude samples. In a different study, nanofibrous molecularly imprinted membranes with multi-analyte selectivity were prepared by encapsulating molecularly imprinted polymer nanoparticles into electrospun polyvinyl alcohol nanofibers and resulted in binding selectivity for bisphenol A and tebuconazole [55]. Upon application in the analysis of vegetable and juice samples, their method resulted in the recoveries that were higher than 70% for both analytes from different samples [55].

Selective Mixed-Mode Ion-Exchange Polymeric Sorbents

In a review article presented by Fontanals et al. [56], mixed-mode ion-exchange polymeric sorbents have been described as dual-phase materials that have the ability to enhance selectivity and capacity of the extraction process in a single step. The same authors have also provided a list of commercially available mixed-mode ion-exchange polymeric sorbents and their applications. In their review [56], these sorbents have been pronounced as materials that generally provide cleaner chromatograms which is very important in trace analysis for the prevention of ion suppression or ion enhancement in subsequent LC-MS analysis. In addition, mixed-mode ion-exchange polymeric sorbents are known to be useful for simultaneous extraction of a wide range of organic compounds and hence categorized as class selective adsorbents. The mixed-mode sorbents extract analytes based on their interactions with ionic species while they also effectively retain non-charged species through hydrophobic and hydrophilic interactions [57]. These extracted compounds with physical and chemical properties that vary significantly are sequentially eluted from the SPE cartridge prior to chromatographic determination. Following these sentiments, Zhang et al. [57] have extracted a large number of herbicides with different polarities and acidic–neutral–basic characteristics using Oasis MAX and Bond Elut Pax SPE cartridges. In their case, the elution of retained neutral and basic herbicides was achieved with methanol, which was followed by eluting the acidic herbicides with acidified methanol.

While selective elution can be achieved when using mixed-mode ion exchange polymeric sorbents, another option is to perform selective washing for the removal of retained interferences or non-targeted compounds [58]. Gilart et al. [59] achieved selective removal of the interferences including acidic and neutral compounds binding with their strong cation-exchange sorbent through reversed-phase interactions while retaining basic analytes by cation-exchange interactions. Thereafter, the targeted basic analytes were eluted with 5% ammonium hydroxide and methanol. A similar approach was reported by Li et al. [60] where a selective washing step was optimized for efficient elimination of matrix interferences during the extraction of non-steroidal anti-inflammatory drugs from complex human urine samples using dendrimer-functionalized mesoporous silica as a reversed-phase/anion-exchange mixed-mode SPE sorbent. Several other examples reported in the period 2014–2018 where mixed-mode ion-exchange polymeric sorbents have been used in SPE are illustrated in Table 3.

Table 3 Recent applications of mixed-mode ion-exchange polymeric sorbents in SPE
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A different approach that uses a combination of mixed-mode anion-exchange cartridge (Oasis MAX) and mixed-mode cation-exchange cartridge (Oasis MCX) both supplied by Waters Corporation has been reported for SPE of fluoroquinolones from chicken plasma [69]. In this approach, the sample was first percolated in a pre-conditioned Oasis MAX cartridge with its eluate (elution with 2% phosphoric acid) being transferred to the Oasis MCX cartridge. The MCX cartridge was rinsed with methanol and target compounds were thereafter eluted with 10% ammonia in methanol, yielding recoveries of target analytes in the range of 62–85%. Both the washing step carried out with 1% ammonia in methanol and the elution step with acidified methanol were ascribed to be the most significant in matrix removal.

Selective Composite Materials

Nowadays, the composite materials which are prepared from two different polymers are used for the improvement of selective extraction taking the advantage of molecular imprinting in most cases and to improve the binding capacity while accelerating the binding kinetics [70]. Yang et al. [70] achieved selective recognition of emodin from kiwi fruit roots and adsorption equilibrium in 60 min using MIPs grafted on multi-walled carbon nanotubes as SPE sorbent. In their case, MIPs grafted on multi-walled carbon nanotubes displayed superior recognition of analyte in the presence of structurally related compounds which were rhein, chrysophanol and physcion due to the presence of emodin imprinted cavities with fixed size, shape and binding sites. While MIPs were chosen for selective recognition in their study, the selection of multi-walled carbon nanotubes was motivated by their tensile strength, chemical stability, large surface area and ultra-small size. Elsewhere, magnetic molecularly imprinted composite which possessed the properties that included large specific surface area, high surface energy, as well as magnetization was synthesized and applied for selective solid-phase extraction of p-aminosalicylic acid from aqueous and urine samples [71]. The selectivity of the magnetic molecularly imprinted composite towards p-aminosalicylic acid in the presence of structural analogues was attributed to the analyte specific binding sites on the imprinted shell. In a different situation, a hollow porous ionic liquid composite polymer was fabricated for selective SPE of hydrophilic hydroxyl benzoic acids (protocatechuic acid, 4-hydroxybenzoic acid and vanillic acid) from Pollen Typha angustifolia (a Chinese clinical medical herb used for the treatment of type 2 diabetes) [72]. In their study, the fabrication of a hollow porous ionic liquid composite polymer was performed using mesoporous Mobil Composition of Matter No. 48 nanospheres as sacrificial support, 1-vinyl-3-methylimidazolium chloride as monomer, and ethylene glycol dimethacrylate as cross-linker; and the resulting composite exhibited high adsorption capacity, fast adsorption kinetics, and excellent specific adsorption [72]. The selectivity and specificity of the hollow porous ionic liquid composite polymer was linked to strong electrostatic and π–π stacking interactions occurring between ionic liquid and hydroxybenzoic acids.

There are other various composites that have been reported in literature to be able to selectively extract organic analytes from sample matrices [73, 74]. Properties of such composites are summarized in Table 4. Most recently, Liu et al. [73] reported the application of magnetic graphene/mesoporous silica composites with carbon-functionalized pore-walls for the selective extraction of miglitol and voglibose from rat plasma. In their approach, the targeted small molecules enter the pores and are selectively retained by the amorphous carbon on the pore-wall through hydrophilic interactions while the macromolecules such as protein are excluded due to the pore size exclusion effect [73]. In addition, a composite of metal organic frameworks and magnetic carbon nanotubes was reported for selective determination of aromatic amino acids in Lanzhou lily samples where the extraction mechanism was ascribed to multiple hydrogen bonding and π–π electron-donor-acceptor interactions [74].

Table 4 The application and properties of some composite materials used in the solid-phase extraction of organic analytes
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Selective Nano Sorbents

As given in Table 5, the nano sorbents investigated recently for selective SPE of organic analytes are nylon nanofibers [76], magnetic imprinted nanoparticles [77], core–shell magnetic nano-sorbent with surface MIP coating [78], PEGylated magnetic multi-walled carbon nanotubes [79], mixed hemimicelle magnetic nanoparticles [80] and gold-polypyrrole nanocomposite sorbent [81]. The application of nanofibers as SPE sorbents was inspired by their large surface area due to the small fiber diameter which could often result in enhanced extraction capacity and sorption kinetics [76]. In comparison with the widely used Oasis HLB sorbent, the nylon 6 nano fibrous sorbent showed higher selectivity in the retention of phenolic compounds from aqueous samples. The retention of the analytes was influenced by the chemical structure of the nanofibers with nonpolar hydrophobic interactions expected to occur between the hydrophobic compounds and the methylene functionality of nylon 6 [76]. In a different perspective, the SPE sorbent made from the combination of a magnetic nano-sorbent and MIP was investigated for selective extraction of dimetridazole from food samples [78]. In their approach, the MIP sorbent was synthesized using the core–shell nanoparticles, Fe3O4@SiO2, as the supporting material due to its reliable chemical stability, compatibility, reactivity with various coupling agents, and the inherent properties of the magnetic element. The resulting MIP was easily separated by an external magnetic field after adsorption and recognition using batch adsorption experiments. Their sorbent was more selective to dimetridaloze, showing more than three times adsorbing capacity than metronidazole used as the competitor [78]. The special feature of selectivity was explained to be due to the shape of the imprinted cavity on the surface of the adsorbent that infatuated improved Lewis acid-base interaction with the template molecule [78]. Other several examples of nano sorbents are given in Table 4, demonstrating the wide applications of these sorbents to a variety of organic compounds.

Table 5 The application of Nano sorbents in selective extraction of organic analytes from complex and water samples
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Other Novel Solid Phase Extraction Sorbents

To date, several novel strategies for the improvement in selectivity of the SPE technique have been initiated (Table 6). In one approach, a sorbent used in SPE which was reported to possess the merits of antioxidation, superparamagnetism, high surface area, high supramolecular recognition and environment friendly, consisted of silica-coated Fe3O4 grafted graphene oxide and β-cyclodextrin (Fe3O4@SiO2/GO/β-CD) [82]. Upon the application of Fe3O4@SiO2/GO/β-CD to the analysis of vegetable sample, the sorbent was found to be selective towards nine plant growth regulators (4-fluorophenoxyacetic acid, 4-chlorophenoxyacetic acid, simazine, 1-naphthaleneacetic acid, 2,4-dichlorophenoxyaceticacid, atrazine, paclobutrazol, uniconazole, and tebuconazole), where their uptake was based on interaction with aromatic structures through π–π interaction, hydrophobic interaction and host–guest inclusion complex forming. In a separate study, crown ether-modified resin composite fiber showed great selectivity in the SPE of catecholamines from human urine [83]. In addition, an amino-functionalized-Fe3O4 was crosslinked with 8-hydroxy-2′-deoxyguanosine aptamer by glutaraldehyde and fixed into a steel stainless tube as the sorbent of magnetic SPE in the analysis of human urine [84]. In their study, the adsorbed 8-hydroxy-2′-deoxyguanosine was desorbed dynamically and analyzed on LC–MS. Upon application of their sorbent, they observed specific selectivity towards target compound in the presence of structurally similar and different compounds, namely; deoxyguanosine, guanosine, guanine, deoxyuridine and uric acid as well as high enrichment capacity, stability and biocompatibility. Elsewhere, Huang et al. [85] prepared a mixed-mode anion exchange sorbent based on porous poly[2-(diethylamino) ethyl methacrylate-divinylbenzene] spherical particles quaternized with 1,4-butanediol diglycidyl ether followed by triethyl amine via epoxy-amine reaction to offer strong anion exchange properties. Their sorbent showed selective isolation and purification of acidic pharmaceuticals (ketoprofen, naproxen, and ibuprofen) from neutral and basic pharmaceuticals as well as other interferences in water samples [85]. In a different scenario, a composite of metal organic frameworks and magnetic carbon nanotubes was used as the SPE sorbent and resulted in selective extraction of tryptophan, tyrosine and phenylalanine from Lanzhou lily where the adsorption was described to be based on multiple hydrogen bonding and π–π electron-donor-acceptor interactions [74]. In addition to the examples cited in this section, other several selective SPE sorbents that have been reported in literature are magnetic graphene/mesoporous silica composites with carbon-functionalized pore-walls [73], hybrid-type carbon microcoil-chitosan composite [75], polymer monoliths fabricated with high percentage of divinyl benzene [15], imprinted organic–inorganic hybrid mesoporous sorbent from nanomagnetic polyhedral oligomeric silsesquioxanes [86], phenyltrichlorosilane-functionalized magnesium oxide microspheres [87] and aminooxy- functionalized magnetic nanoparticles [88].

Table 6 Other novel solid phase extraction sorbents that are recently reported in literature
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The sorbents discussed in this section are fairly new, with majority of them tested only for selectivity in one sample matrix. It will be interesting to see their ability to properly function and show great selectivity in a broad spectrum of sample matrices. Furthermore, several tests that have not been reviewed or conducted could be revisited to address their strength and limitations. These tests may include their maximum adsorption capacities and surface area among others.

Miniaturization and New Innovations in Solid-Phase Extraction Applications Using Selective Sorbents

Miniaturization in SPE is used when the sorbent amount and sample volumes are minimal. In this SPE approach in a technique known as pipette tip SPE, a small amount (approximately 5–30 mg) of the sorbent is packed in a micro pipette tip with a capacity of 1000 µL which serves the role of the cartridge [89,90,91,92]. The sorbent loss is usually prevented by the use of cotton wool on both ends of the pipette tip. Therefore, due to small sorbent amount applied and low organic solvent required for SPE operation, this approach is deemed necessary for the realization of the green chemistry principles. In addition, the operation time of the SPE technique is significantly reduced in this approach.

Many researchers have applied MIPs as selective sorbents in pipette tip SPE for various analytes [89,90,91,92,93,94]. For example, pipette tip based SPE using MIP sorbent was applied for enantioselective determination of (−)-(2S,4R) and (+)-(2R,4S) ketoconazole in human urine samples yielding recoveries around 100% when samples were spiked with 5 µg mL−1 of the analyte [89]. In a different case, a similar technique was applied in selective determination of avermectins and milbemycins in fruit juice and water samples [90]. Their analytical method had several advantages over traditional methods which included selectivity, simplicity, reduced operational costs and low organic solvent usage. In another study [91], only 5 mg of the molecularly imprinted resin was used as a sorbent in the extraction of organochlorine pesticides in spinach. In this case, their imprinted resin had higher affinity to target compounds than the corresponding non-imprinted resin. Elsewhere, 20 mg of MIP sorbent was able to extract carvedilol enantiomers from human urine yielding the recoveries of 102% and 105%, for (+)® carvedilol and (−)-(S)- carvedilol, respectively [92]. In another study by de Oliveira et al. [93], the MIP applied in a pipette tip SPE was observed to be more selective for extraction of their analytes (ciprofloxacin, enrofloxacino, marbofloxacino and norfloxacin) from human urine than the corresponding non-imprinted polymer and C18 sorbents. The selectivity of this approach has been reported where MIPs gave high adsorption capacity and selective extraction of (Z)-3-(chloromethylene)-6-flourothiochroman-4-one from mixed solutions containing (Z)-6-fluoro-3-(hydroxymethylene)-thiochroman-4-one [94].

Further to the pipette tip technology, a miniaturized method that involved the combination of ultrasound assisted emulsification and micro-SPE where 4 mg of MIP sorbent was housed inside a membrane for selective extraction of bisphenol A from aqueous matrices has been reported by Rozaini et al. [51]. This method was applied for the analysis of bisphenol A in drinking water, mineral water, river water, lake water, as well as beverages and canned foods using high performance liquid chromatography-diode array detector yielding a detection limit of 0.07 µg L−1 [51].

Online Modes

The widely used offline mode during SPE methods has some drawbacks such as prolonged contact between the analyst and the sample leading to misinterpretation of recoveries. In addition, the offline mode is tedious and requires relatively large amounts of organic solvents. Online modes have since been introduced to eliminate these drawbacks. Effectiveness of the online mode requires that the matrix effects be at minimal to avoid noise effects. Thus, the online mode has been applied using new selective sorbents especially MIPs. In this regard, the online mode has been reported with MIPs as sorbents for analysis of ochratoxin A in wheat extracts [95], 4-nitrophenol in river water [96], cephalexin in serum [97], estrogens in various biological samples [98]. Some studies have reported an in-line mode that involves direct coupling to mass spectrometry [99, 100]. For example, Gilart et al. [100] analysed NSAIDs in environmental samples using an in-line mode in which the eluent from the MIPs was injected directly to a tandem mass spectrometer. The reduction in sample pre-treatment and contact time ensures that less solvents are used, and the entire approach is reproducible and conforms to green analytical chemistry.

Reusability

As in online SPE modes, reusability of selective sorbents has been reported mainly for laboratory-synthesized MIPs. Commercial sorbents have strict guidelines that they should be used once and discarded. It is interesting to note that researchers have not confirmed this fit. As such, reusability studies have been reported only for laboratory-synthesized MIPs. These studies have shown that MIPs can be recycled and used again and again without them losing their efficiency and selectivity towards the target molecule. The MIPs that have shown efficiency even after repeated extraction cycles include those for analysis of carbamazepine [101], polycyclic aromatic hydrocarbons [102, 103], non-steroidal anti-inflammatory drugs [104, 105] and some toxins [106, 107]. This evidence of reusability of synthesized MIPs has several advantages over commercial MIPs. It is noted that the reusability of laboratory-synthesized MIPs has hindered the commercialization of MIPs, a move which would have led to limited accessibility to the common user. In addition, the use-once approach of commercial sorbents deviates from principles of green chemistry as more solvents are used for a routine analysis with large amounts of waste generated.

Future of Selective Solid Phase Extraction Sorbents

In a recent critical review article presented by Andrade-Eiroa et al. [108], the deficiency of selectivity of most sorbents and SPE procedures was revealed. Due to the complexity of different sample matrices, the future of SPE relies largely on selective sorbents such as MIPs that have been described extensively in the present review. In addition, MIPs have been applied as selective sorbents in miniaturization techniques that are preferred over traditional extraction methods as they adhere to green chemistry principles [109]. In this regard, miniaturization is still a new research area which is likely to be well exploited in future.

Moreover, MIPs have a great future in selective SPE due to their easy preparation process, reproducibility and the ability to be re-used many times without compromising the quality of extraction. However, in a specific example, immunoaffinity chromatography (IAC) was reported to be better than the MIP sorbent which yielded better quantitative recoveries of chloramphenicol residues by ion mobility spectrometry, thus achieving improved selectivity and sensitivity [110]. Owing to high cost of monoclonal antibodies involved in the production of IAC, the method was regarded as expensive which prompts future work to be based on MIPs as sorbents of choice. Furthermore, selected MIP sorbents are now available commercially, marketed by Supelco [111] and MIP Technologies [112], and this implies that there is a probability of applying such sorbents for routine analysis. In the near future, MIP sorbents are expected to be synthesized for all the common organic compounds and used in SPE protocols. This has also been highlighted in our previous review paper [113] where MIPs for various non-steroidal anti-inflammatory drugs were discussed, and we noted the unavailability of fenoprofen MIP in the open literature.

Many researchers have proposed various materials as selective SPE sorbents. In some cases, it was prominent that the selectivity of some sorbents was investigated using real samples that are not complex. Therefore, this implies that many of the selective SPE sorbents proposed in literature could still be validated in future for their potential in the extraction of various analytes from complex samples such as wastewater, as opposed to tap or lake waters. Furthermore, due to enhanced need for greener practices in analytical chemistry, novel selective materials developed for SPE in future should adhere to green chemistry principles. This can assist in developing analytical procedures that are greener. For starters, new analytical procedures involving SPE can be assessed for greenness using tools such as Analytical Eco-Scale and Green Analytical Procedure Index proposed elsewhere [114, 115]. In this way, the assessment for greenness of new methods can be compared to the existing SPE based methods.

Conclusions

In the present work, we have reviewed the selective sorbents used in the last 5 years (2014–2018) for SPE of various analytes from complex samples prior to their chromatographic analysis. Among other sorbents, MIPs, nano sorbents and some composite materials are widely explored nowadays as selective sorbents in SPE. The application of sorbents prepared from MIPs and other materials such as multi-walled carbon nanotubes in SPE has resulted in high selectivity of the analytical methods and improvement in binding capacity towards the target compounds. The application of some composite materials as SPE sorbents resulted in improved selectivity of the analytical methods as well as in the magnetization and improved sorption kinetic properties which are greatly required during the batch adsorption process. One of the important aspects we noted with great concern is the application of novel sorbents which are categorized as selective in SPE in sample matrices that are not complex. It is our opinion that any SPE sorbent that has been recognized as selective material should be evaluated for selectivity in a complex sample matrix such as wastewater when taking the case of environmental analysis as an example. Overall, we have provided a comprehensive overview of the recent work (2014–2018) reported in literature that focused on the selective SPE of various organic compounds from several samples including food, biological and environmental.