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Trends in Analytical Chemistry 119 (2019) 115611

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Trends in Analytical Chemistry

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Recent applications of magnetic composites as extraction adsorbentsfor determination of environmental pollutants*

Meng Yu 1, Limei Wang 1, Liqin Hu, Yaping Li, Dan Luo, Surong Mei*

State Key Laboratory of Environment Health (Incubation), Key Laboratory of Environment and Health, Ministry of Education, Key Laboratory of Environmentand Health (Wuhan), Ministry of Environmental Protection, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology,#13 Hangkong Road, Wuhan, Hubei, 430030, China

a r t i c l e i n f o

Article history:Available online 25 July 2019

Keywords:Magnetic solid-phase extractionMagnetic compositesEnvironmental pollutantsSample pretreatmentExtraction adsorbents

* Dedicated to the 70th anniversary of Dalian InChinese Academy of Sciences.* Corresponding author.

E-mail address: [email protected] (S. Mei).1 These authors equally contributed to this work.

https://doi.org/10.1016/j.trac.2019.07.0220165-9936/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t

Magnetic solid-phase extraction (MSPE) is considered to be an advancing sample preparation techniquefor the separation and preconcentration of environmental pollutants at trace-levels. Magnetic compos-ites, as MSPE adsorbents, incorporate the distinct advantages of versatile nanomaterials and magneticnanoparticles, compared to traditional solid-phase extraction packing materials. The former can affordfast dispersion and efficient recycling when applied in complex sample matrices. In this review, weelaborate the applications of magnetic composites as MSPE adsorbents for the enrichment of environ-mental pollutants, reported in the last five years.

© 2019 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, increasing environmental pollution andecological imbalance have made the separation and determinationof pollutants a research hotspot. Typical pollutants, including heavymetals, polycyclic aromatic hydrocarbons (PAHs), polychlorinatedbiphenyls (PCBs), pesticides, phthalate esters (PAEs), bisphenol A(BPA), perfluorinated compounds (PFCs), organic phosphate flameretardants (OFRs), and so on [1] are widely distributed in envi-ronmental matrices and can cause harm to the human body, even atlow levels of concentrations. Against this backdrop, it is veryimportant to identify and quantify harmful pollutants in differentenvironmental matrices. However, the complexity and diversity ofthe sample matrices lead to the need for enhanced efficiency andselectivity in sample pretreatment procedures.

Solid-phase extraction (SPE) is a relatively mature technique. Itis based on the reversible interactions between the target and anadsorbent in a column through physical or chemical adsorption,followed by an elution process [2]. Selecting the proper adsorbentsfor SPE is a key concern. C18 bonded silicas and HLB-basedmaterialsare used frequently. However, the traditional SPE column may

stitute of Chemical Physics,

suffer from clogging of the adsorbent, high column pressure, andthe need for excess toxic solvent during the extraction process.Magnetic solid-phase extraction (MSPE) is a modified class of SPE,and employsmagnetic nanomaterials as adsorbents to separate andenrich target compounds in a sample matrix. Briefly, MSPE is car-ried out by dispersing a magnetic adsorbent in the sample solutionto enable adsorption of the target analyte through specific in-teractions. After the adsorption procedure is accomplished, themagnetic adsorbent containing the analyte is separated from thesample matrix by an external magnet. After the elution procedure,the target analyte is desorbed from the magnetic adsorbent anddissolved in a desorption solvent. The desorption solution enrichedwith the target analyte can then be collected for further determi-nation. Thereafter, the magnetic adsorbent is recycled. Due to theease of magnetic separation, the time-consuming and tediouscentrifuging and filtration procedures required in traditional SPEcan be avoided in MSPE, and the latter method shows other po-tential advantages such as fast separation speed, good sorbentreusability, high extraction efficiency, and convenient operation.

For MSPE, the selection of the magnetic adsorbent has animportant influence on the extraction efficiency, enrichment factor,selectivity, and anti-interference ability, and is thus a key factor forachieving good extraction performance. Magnetic materials canexhibit different types of magnetism, mainly diamagnetism, para-magnetism, ferromagnetism, antiferromagnetism, and ferrimag-netism. Magnetic nanoparticles (MNPs) composed of Fe, Co, Ni, andtheir corresponding metal oxides usually possess paramagnetism

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Abbreviation

APB 3-aminophenylboronicBDPE 4-bromodiphenyl etherBDBA 1,4-benzene diboronic acidBFRs brominated flame retardantsBMZIF bimetallic ZIFBPA bisphenol Ab-CD beta-cyclodextrinCMPs conjugated microporous polymersCNFs carbon nanofibersCNTs carbon nanotubesCOFs covalent organic frameworksCOPs covalent organic polymersCPs chlorophenolsCQDs carbon quantum dotsCTAB cetyltrimethylammonium bromideCTFs covalent triazine-based frameworksDAD diode array detectorDBDPE 4,4ʹ-dibromodiphenyl etherERY erythromycinFAAS flame atomic absorption spectrometryFID flame ionization detectorFLD fluorescence detectorG grapheneGC gas chromatographyGO graphene oxideGQDs graphene quantum dotsH3BTC benzene tricarboxylic acidsHCPs hyper-crosslinked polymersHCSs hollow carbon nanospheresHKUST-1 Hong Kong University of Science and Technology-1HPLC high-performance liquid chromatographyILs ionic liquidsICP inductively coupled plasmaIUPAC International Union of Pure and Applied ChemistryLDHs layered double hydroxidesMAA mercaptoacetic acidMACs macrolide antibioticsMagG magnetic grapheneMCNTs magnetic CNTsmSiO2 mesoporous SiO2

mTiO2 mesoporous TiO2

MDMIPs magnetic dummy MIPsmECD micro-electron capture detectorMILs Material of the Institute LavoisierMIPs molecularly imprinted polymersMISPE molecularly imprinted SPEMMIPs magnetic-MIPs

MNPs magnetic nanoparticlesMNIPs magnetic non-imprinted polymersMOFs Metal-organic frameworksMPC magnetic porous carbonMPS 3-methacryloxypropyltrimethoxysilaneMS mass spectrometryMS/MS tandem mass spectrometryMSPE Magnetic solid-phase extractionMTMOS methyltrimethoxysilaneMWCNTs multi-walled carbon nanotubesOFRs organic phosphate flame retardantOPPs organophosphorus pesticidesPA phytic acidPa-1 p-phenylenediaminePAEs pesticides, phthalate estersPAHs polycyclic aromatic hydrocarbonsPCBs polychlorinated biphenylsPCOMs porous covalenteorganic materialsPDA poly-dopaminePEI polyethyleneiminePFCs perfluorinated compoundsPILs poly (ionic liquid)sPP-CMP polyphenylene conjugated microporous polymerPPy polypyrrolePSS polystyrenesulfonate sodiumPTMS phenyltrimethoxysilaneP[VHim]Br poly (1-vinyl-3-hexylimidazolium) bromideRF resorcinol-formaldehydeRGO reduced graphene oxideSELEX systematic evolution of ligands by exponential

enrichmentSPE solid phase extractionSWCNTs single-walled carbon nanotubesTBB 1,2,4,5-tetrabromobenzeneTBBPA tetrabromobisphenol ATBP 2,4,6-tribromophenolTEOS tetraethoxysilaneTh thiopheneTp 1,3,5-triformylphoroglucinol2-D two-dimensionalUPLC ultra high-performance liquid chromatographyUV ultraviolet detector4-VP 4-vinylpyridineZIFs zeolite imidazolate frameworksZr-MOF Zirconium-based MOF-COOH carboxyl group-OH phenolic hydroxyl group-SH thiol-C-O-C- epoxy group

M. Yu et al. / Trends in Analytical Chemistry 119 (2019) 1156112

or ferromagnetism, e.g., magnetite (Fe3O4), maghemite (g-Fe2O3),and CoFe2O4 [3,4], which are widely used as magnetic cores inMSPE adsorbents. The methods of preparing MNPs include co-precipitation synthesis [5,6], hydrothermal synthesis [7], sol-vothermal synthesis [8], and sol-gel synthesis [9]. However, thepuremagnetic cores obtained by the aforementionedmethods tendto agglomerate, resulting in weakened magnetism when the MNPsare used as absorbents. In order to overcome this limitation, asuitable coating method with functionalized materials is required.Carbonaceous nanomaterials and porous materials are the mostpopular coating materials due to their structure and properties,which not only improve the stability of MNPs and prevent their

oxidation but also offer abundant reaction sites and large specificsurface area. Moreover, MNPs combined with other functionalmaterials such as silicon nanomaterials, metallic nanomaterials,ionic liquids (ILs), chitosan, and surfactants are also used as MSPEadsorbents. The interactions between the target analytes andabove-mentioned adsorbents involve electrostatic attraction,hydrogen bonding, p-p stacking, van der Waals force, hydrophobicforce, and metal ionic coordination [10]. However, a complicatedmatrix may interfere with the adsorbent through these non-selective interactions. In such cases, coating MNPs with carefullydesigned materials such as molecularly imprinted polymers (MIPs)and aptamers is a useful method. Several reviews of MSPE have

M. Yu et al. / Trends in Analytical Chemistry 119 (2019) 115611 3

been published in the past few years [11e14]. These reviewsmainlydiscussed the synthesis techniques or the application of one or twoof the above-mentioned magnetic composites. There are a limitednumber of comprehensive publications systematically reviewingthe use of various magnetic composites as MSPE adsorbents andtheir application in the enrichment of environmental pollutants.Hence, in this review, we summarize the recent advances in mag-netic composites as MSPE adsorbents for the enrichment of envi-ronmental pollutants from the year 2014 to the present (Table 1).

2. Magnetic carbonaceous nanomaterials as MSPE adsorbents

Diverse carbon-based nanomaterials are commonly used,including graphene (G), graphene oxide (GO), reduced grapheneoxide (RGO), carbon nanotubes (CNTs), carbon nanofibers (CNFs),carbon-based quantum dots, and graphitic carbon nitride (g-C3N4)[14,15], etc. Most possess superior characteristics such as goodmechanical, thermal, and chemical stability, large specific surfacearea, and numerous active sites. For the use of these nanostructuredcarbon-based materials as MSPE adsorbents to enrich environ-mental contaminants in complex sample matrices, the emphasis ison functional modification to achieve improved adsorption selec-tivity and specificity so as to minimize interference from the me-dium. To date, two main kinds of carbonaceous nanomaterials havebeen widely applied in MSPE, and these are described in turnbelow.

2.1. G-based composites

G is a typical carbon-based nanomaterial comprising single-atom thick, two-dimensional (2-D) sheets of sp2 hybridized car-bon atomswith structural arrangement in a hexagonal lattice. G hasa large surface area and numerous delocalized p-electrons, and ishighly effective for the extraction of organic compounds containinga benzene ring structure on the basis of p-p stacking [16e18].Limited by its hydrophobicity and low weight, uniform dispersionand good recycling of pure G from sample solutions are difficult. Gis also ineffective for the adsorption of a number of polar pollutantscontaining hydrophilic chemical groups. Therefore, appropriatemodification is necessary to meet the specific requirements ofpractical applications. GO is a reaction product obtained by chem-ical oxidation of G and possesses more functional groups such ascarboxyl (-COOH), phenolic hydroxyl (-OH) and epoxy groups (-C-O-C-), which can improve the selective affinity for various analytesand facilitate further modification depending on the active inter-action sites relative to pristine G [17,19]. RGO, which possessesfewer residual oxygenated groups and vacancy defects than GO, canbe synthesized by a suitable chemical reductionmethod that is nowwidely applied to obtain highly reduced GO nanosheets. The activesurfaces of GO and RGO offer the possibilities for compositeincorporation [20,21]. GO and RGO can be fixed to MNPs to form astable nanocomposite that can be more easily separated andrecovered from the sample solution and prevent the loss of GO andits derivatives during the extraction process [22].

The methods of fabricating magnetic G-based compositesmainly include the hydrothermal approach, solvothermal methodand in-situ chemical coprecipitation method [22,23]. Yang et al.[24] fabricated G-doped magnetic nanocomposites (Fe3O4/G)through a solvothermal procedure for adsorbing brominated flameretardants (BFRs), including 2,4,6-tribromophenol (TBP), tetra-bromobisphenol A (TBBPA), 4-bromodiphenyl ether (BDPE), and4,40-dibromodiphenyl ether (DBDPE). The G component offers alarge p-electron system, while the introduction of Fe3O4 nano-particles can overcome the restrictions of single G, allowing rapidand simple separation from aqueous samples, and leading to good

affinity for the target aromatic compounds through p-p stackingand hydrophobic interactions. The maximum saturation magneti-zation (Ms) of Fe3O4/G was 59.9 emu g�1, which suggested goodsuperparamagnetism. Coupled with a high-performance liquidchromatography-ultraviolet detector (HPLC-UV), low limits ofdetection (LODs) (0.2e0.5 mg L�1) and good recoveries(85.0e105.0%) of the test BFRs in environmental water sampleswere achieved. Because pure magnetic G-based materials havenarrow applications with limited adsorption properties, theygenerally fail to provide satisfactory extraction performance forcomplicated and diverse environmental pollutants without (orwith only simple) modification. Therefore, further functionalizationof G-based magnetic materials can greatly improve the selectivityfor target analytes and reduce the interference from environmentalmatrices.

Nodeh et al. [25] fabricated a nanostructured magnetic G-basedtetraethoxysilane-methyltrimethoxysilane (TEOS-MTMOS) hybridcomposite (Fe3O4@G-TEOS-MTMOS) as a MSPE adsorbent for therapid analysis of four organophosphorus pesticides (OPPs) in watersamples. The synthetic procedure involvedmodifying G nanosheetswith Fe3O4 nanoparticles and subsequently coating them withporous silica-based nanospheres via a sol-gel method. Thesefunctional components of the adsorbents provided selectiveadsorption sites for the target OPPs, including non-polar OPPs(diazinon, chlorpyrifos) via p-p stacking, and polar OPPs (phos-phamidon, dimethoate) via hydrogen-bonding and electrostaticinteractions (Fig. 1). The as-synthesized adsorbent offered higheradsorption capacities (37.18e76.34 mg g�1) than either the singleFe3O4@G or TEOS-MTMOS composite. Furthermore, detection bygas chromatography coupled with a micro electron capture detec-tor (GC-mECD) afforded low LODs (1.4e23.7 pg mL�1) and satis-factory recoveries (83e105%).

A polyethyleneimine-functionalized magnetic RGO-basednanocomposite (Fe3O4@PEI-RGO) was successfully prepared by Liet al. as a MSPE adsorbent in order to preconcentrate trace polaracidic herbicides in a food matrix [26]. PEI is a positively chargedlong-chain polymer, and the modified RGO component can providea large surface area for anion exchange events. Under optimizedextraction conditions, the as-synthesized Fe3O4@PEI-RGO nano-composite exhibited higher adsorption capacity for the five studiedherbicides (mainly through electrostatic attraction and p-p inter-action) than Fe3O4@PEI-GO, Fe3O4@PEI, and Fe3O4/RGO. As deter-mined by high-performance liquid chromatography-diode arraydetection (HPLC-DAD), the LODs of the MSPE-HPLC-DAD methodwere in the range of 0.67e2 ng g�1, with recoveries of87.41e102.52% in rice samples.

Ji et al. [27] proposed a core-shell magnetic GO composite sta-bilized by phytic acid (GOPA@Fe3O4) as a MSPE adsorbent for theextraction of PAHs from vegetable oil samples. The GOPA@Fe3O4composite was obtained via a microwave-enhanced hydrothermalmethod. Due to its super-amphiphilic property and unique affinityfrom p-p stacking, this excellent adsorbent could be disperseduniformly in hydrophobic samples and was specifically utilized forthe enrichment of aromatic compounds. The whole pretreatmentprocedure was accomplished within 5 min. Combined with HPLC-DAD, low LODs and satisfactory recoveries were achieved in therange of 0.06e0.15 ng g�1 and 85.6e102.3%, respectively.

Molaei et al. [28] functionalized magnetic GO (MGO) withpyrrole-thiophene copolymer (MGO/SiO2@coPPy-Th) as a MSPEadsorbent for extracting and quantifying heavy metal ionsincluding copper (Cu(II)), lead (Pb(II)), chromium (Cr(III)), zinc(Zn(II)) and cadmium (Cd(II)), with subsequent detection via flameatomic absorption spectrometry (FAAS). The MGO was synthesizedby a chemical co-precipitation method, then coated with a silicalayer, and finally functionalized with a conducting pyrrole-

Table 1The application of magnetic composites as extraction adsorbents for the determination of environmental pollutants.

Magnetic adsorbents Analytes Samples Detectiontechniques

Adsorptioncapacity(mg g�1)

Enrichmentfactor (EF)

LOD (ng L�1) Precision(RSD, %)

Recoveries (%) Ref.

1.1 magnetic carbonaceous nanomaterialsG-MNPs carbamate tomato samples HPLC-DAD e 364e434 0.58e2.06b 0.69e6.71 90.34e101.98 [95]

pesticidesFe3O4/G BFRs water HPLC-UV e e 200e500 1.1e7.1 85.0e105.0 [24]G/Fe3O4@PT PAHs water GC-FID e 650e1592 9e20 4.3e6.3 83e107 [96]Fe3O4@G-TEOS-MTMOS OPPs water GC-mECD 37.18

e76.34109e1247 1.4e23.7 1.3e8.7 83e105 [25]

Fe3O4@PEI-RGO polar acidicherbicides

rice HPLC-DAD e e 0.67e2b 1.07e8.82 87.41e102.52 [26]

GOPA@Fe3O4 PAHs vegetable oil HPLC-DAD e e 0.06e0.15b 3.44e8.41 85.6e102.3 [27]b-CD/MRGO OCPs honey GC-ECD e 373e506 0.52e3.21c 2.9e7.8 78.8e116.2 [97]PPy-RGOx-Fe3O4 PAEs bottled water,

beveragesGC-MS/MS e e 5e10 0.6e7.7 87.5e99.1 [98]

Fe3O4/GO Co(II),Ni(II),Cu(II),Cd(II), Pb(II)

human urine,plasma

ICP-MS 1.28e9.71 e 16e395 1.8e5.5 81e113 [99]

GOePAR@Fe3O4 Pb(II) food, water ETAAS 133 600 0.18 2.4 94.3e107 [100]Fe3O4@GO/2-PTSC Hg(II) food, water ICP-OES e 193 7.9 1.63 92.75e100 [101]GO-Fe3O4@PS PAHs water GC-FID e 695e887 3e10 4.9e7.4 95.8e99.5 [102]MGO/SiO2@coPPy-Th Cu(II), Pb(II), Zn(II),

Cr(III), Cd(II)water FAAS 80e230 36e44 150e650 ＜6 90e106 [28]

Fe3O4@MWCNTs parabens water, humanurine

GC-MS e 25e180 30e2000 4.7e9.2 81e119 [103]

Mag-MMWCNTs b-blockers environmentalwater

Chiral UPLC-MS/MS

e e 0.50e1.45 0.4e10.4 82.9e95.6 [104]

mag-MWCNTs sulfonylureaherbicides

water HPLC-DAD e 178e210 10e40 2.0e12.9 76.7e106.9 [105]

c-MWCNT-MNPs phenoliccompounds

sesame oil HPLC-MS/MS e e 0.01e13.60a 0.5e13.2 83.8e125.9 [106]

HCSs@Fe3O4-MWCNTs-COOH

herbicides wheat flour HPLC-DAD e e 0.24e0.68b ＜3.5 88.8e96.6 [31]

CNFs-Fe3O4 PAHs water GC-FID e e 8e30 3.2e11.2 90.1e100.9 [35]m-G/CNF PAHs environmental

waterGC-FID e 630e845 4e30 3.4e5.7 95.5e99.9 [107]

Fe3O4/HAP/GQDs Cu(II) food ICP-AES e 39.2 15 0.87e4.47 83.5e104.8 [108]CDs/C11eFe3O4 benzo[a]pyrene water HPLC-FLD 76.23 e 0.15 3.1e5.3 88.4e96.4 [109]g-C3N4/Fe3O4 PAEs water HPLC-UV 4.14e18.02 e 50e100 1.1e2.6 79.4e99.4 [40]g-C3N4/Fe3O4 PAHs water HPLC-UV e e 50e100 1.8e5.3 80.0e99.8 [110]Fe3O4@rGO-g-C3N4 CPs daily toner HPLC-UV e e 0.20e0.30a 5.6e9.6 80.5e104 [111]1.2 porous materialsFe3O4eNH2@MIL-101(Cr) pyrethroids environmental

waterGC-ECD e e 5e9 0.3e4.6 72.1e106.8 [45]

Fe3O4eNH2@MIL-101(Cr) sudan dyes tomato sauce HPLC-DAD e 50 0.5e2.5a 1.4e9.2 69.6e92.9 [112]Fe@MIL101(Cr) PAHs river water HPLC-VWD e 429e482 44e64 1.3e4.4 85.7e97.3 [113]Fe3O4@SiO2-MIL-101(Cr) pesticides water HPLC-DAD e e 300e1500 1.1e7.8 80.2e107.5 [114]MIL-101(Fe)eNH2@Fe3O4

eCOOHfungicides river, lake

waterHPLC-UV e e 40e400 6.5e10.2 71.1e99.1 [115]

Fe3O4@MIL-101(Fe) OPPs human hair,urine

GC-FPD e e 210e2280 1.8e9.4 74.9e94.5 [116]

Fe3O4@En@MIL-101(Fe) Cd(II), Pb(II), Zn(II),Cr(III)

agriculturalproducts

FAAS 155e198 e 150e800 4.9e7.6 87.3e110 [117]

magnetic MIL-100(Fe) PAHs environmentalwater

GC-FID e 452e907 4.6e8.9 1.7e9.8 88.5e106.6 [118]

MAA@Fe3O4-ZIF-8 PAEs water HPLC-DAD e e 80e240 2.1e5.5 85.6e103.6 [46]Fe3O4@SiO2@MOF/TiO2 triazole fungicides environmental

waterHPLC-MS/MS e e 0.19e1.20 5.16e7.57 90.2e104.2 [47]

Fe3O4@PDA@ZIF-7 PAHs rainwater,PM2.5

GC-MS e e 0.71e5.79 3.1e12.7 82.1e99.4 [119]

PSA@Zr-MOF@Fe3O4 herbicides environmentalwater

UPLC-HRMS e e 10e30 0.6e8.6 86.2e104.6 [120]

MagG@PDA@Zr-MOF pesticides tobacco GC-MS e e 10.78e45.45b 1.1e12.7 57.9e126.3 [48]Fe3O4@HKUST-1 PAHs water, fruit-tea UPLC-FLD e e 0.8e12 1.2e11 75e94 [121]Magnetic MOF-5 heterocyclic

pesticidesriver water HPLC-FLD 81e181 e 40e110 2.98e7.12 80.20e108.33 [49]

Fe3O4@SiO2eMOF-177 phenols environmentalwater

GC-MS e e 16.8e208.3 4.2e9.2 83.3e108.7 [50]

Fe3O4@DMcT@HKUST-1 Cd(II), Zn(II), Pb(II) baby food FAAS 155e190 186 100e750 5.4e8.3 90.0e106 [52]Fe3O4@BITC@HKUST-1 Cd(II), Pb(II), Zn(II),

Cr(III)vegetable FAAS 168e210 250 120e700 <7.2 80.0e114 [122]

Fe3O4@IRMOF-3 Cu(II) water ETAAS 2.4 50 73 0.4 98.0e102.0 [123]Fe3O4@TMU-8 Co(II), Cu(II), Pb(II),

Cd(II), Ni(II), Cr(III),Mn(II)

water ICP-AES e 66e232 300e1000 2.9e6.4 88e104 [124]


Table 1 (continued )





Recoveries (%) Ref.

Fe3O4@COFs 4-n-nonylphenol,4-n-octylphenol,bisphenols

plastic-packaged tea

HPLC-FLD e e 80e210 0.39e5.21 81.3e118.0 [125]

Fe3O4@PDA@COF PAEs human plasma GC-MS e e 2.5e10 2.3e6.8 92.3e98.9 [126]Fe3O4@mTiO2@COFs PCBs soils GC-MS e e 0.003e0.006b 2.6e4.9 93.1e98.1 [58]COF-LZU1@PEI@Fe3O4 PAHs water, soil HPLC-FLD e e 0.2e20 1.8e4.4 85.1e107.8 [127]bouquet-like magnetic COF PAHs environmental

waterHPLC-FLD e e 0.24e1.01 2e8 73e110 [57]

CTF/Fe2O3 PFCs environmentalwater

HPLC-MS/MS e e 0.62e1.39 1.12e9.71 81.8e114.0 [128]

Fe3O4@SiO2@CTF parabens personal careproducts

HPLC-UV e e 20 2.3e5.0 86e102 [8]

CTFs/Ni PAEs plasticpackagingmaterials

GC-FID e 59e88 150e530 0.35e1.01 70.6e119 [60]

Fe3O4-mPMF BPA, 4-tertbutylphenol, 4-tertoctylphenol,nonylphenol

river water,bottled juice

HPLC-VWD e e 20e100 3.2e6.4 85.4e109 [129]

magnetic PP-CMP hydroxylatedmetabolites ofPAHs

human urine HPLC-FLD e e 10e80 3.5e10.5 76.0e107.8 [61]

M-PPOP phenylureaherbicides

grape juice,tomato

HPLC-DAD e 51e106 100e2000.5e0.8a

3.3e6.4 80.8e117 [130]

HCP/Fe3O4 SAs natural water,milk

HPLC-AD e e 210e2500 3e10 84e105 [131]

Fe3O4@mSiO2-Me-PTSA PCBs water GC-ECD 46.3 119e147 0.16e0.91 0.26e8.54 85.25e118.60 [65]Fe3O4@mSiO2-Ph-PTSA PAHs soil GC-MS e e 0.07e0.41b 2.21e13.68 86.85e110.01 [132]Fe3O4@MCM-48 fluoroquinolones drinking water,

foodHPLC-MS/MS e 86e98 0.7e6.0 2.9e8.2 75.0e112.5 [133]

Fe3O4@cyclam-SBA-15 Pb(II) environmentalwater, food

FAAS 625 200 2020 5.002 95.05e100.35 [134]

Fe3O4@mSiO2eNH2eCMCD

highly chlorinatedPCBs

seafood GC-MS e e 0.021e0.071b 1e11 88.4e103.2 [135]

Fe3O4@fTiO2-CMCD CBs soil GC-MS 25.6 e 0.009e0.031a 2.6e5.4 87.3e104.3 [136]1.3 magnetic molecularly recognition nanomaterialsWC-TMMIPs BPA seawater HPLC-UV 1.92e8.30 e 0.02f 2.12e4.33 86.3e104.3 [137]Fe3O4@MIPs BPA milk HPLC-UV 17.98 e 3700 2.9e3.8 97.23e99.21 [74]MMIP TBBPA water, fish HPLC-UV e e 1000,15.2b 0.53e4.52 89.68e112.116 [138]MMIPs TBBPS environmental

waterHPLC-UV 1626.8e e 200 3.2e5.8 77.8e88.9 [139]

Fe3O4@void@C-MIPs PAEs environmentalwater,beverage

GC-MS 569.2 822e1423 1.6e5.2 4.1e6.7 86.1e103.1 [140]

MDMIPs PAEs food GC-MS/MS 162.0 e 0.15e1.64b 1.7e10.2 73.7e98.1 [75]MMIPs chloroacetamide

herbicidesenvironmentalwater

HPLC-UV 4.5 e 30e60 3.27e6.54 82.1e102.9 [141]

MMIP MACs foodstuff HPLC-UV 94.1 e 0.015e0.2e 0.4e6.7 82.5e113.1 [73]MMIP auramine O water UVeVis 4.14 e 1g 2.6e3.7 99.66e108.75 [142]DMIP acrylamide food HPLC-UV 24.1 e 1.3a 1.2e4.1 86.0e98.3 [143]PM-MIMs 17b-estradiol milk, water HPLC-UV 0.84 e 0.18f ＜4.4 97.5e113.0 [144]Co-MNPC@MIP AFs corn UPLC-MS/MS 35.90 e 50e70 1.7e5.1 75.1e99.4 [145]Fe3O4@IIP Ni(II) food FAAS 50 263 250 7.5 87.0e106 [146]Fe3O4@Cr(VI) IIPs Cr(VI) water FAAS 2.50 98 290 2.1 98.0e99.2 [147]Fe3O4@SiO2@IIP Pb(II) agricultural

productsFAAS 105 300 480 3.2 97.0e99.0 [148]

Fe3O4@TiO2@SiO2eIIP Co(II) water, humanurine, milk

FAAS 35.21 e 150 1.22 98.0e106.0 [149]

Apt-MNPs BPA human serum,urine

HPLC-FLD 78.4d e 1000e2000 1.5e7.3 90.8e92.3 [83]

Apt-Fe3O4/GO Pb(II) blood, urine ICP-MS e 50 50 ＜ 4.7 97.7e101.3 [150]Apt-MAMs AFs maize HPLC-FLD 342e392b e 10e25 1.64e7.99 77.5e97.8 [151]Fe3O4@PDA@UiO-66-NH2

-AptPCBs soil GC-MS 195e218d e 0.018e0.025b 1.6e4.3 89.2e95.2 [152]

Fe3O4@COFs-Apt-15 2-OH-CB 124 serum HPLC-MS 37.17e e 2.1 2.9e3.6 87.7e101.5 [84]1.4 other magnetic nanomaterialsFe3O4@SiO2 paraquat human plasma,

urineUVeVis 2.4 35 12200 0.65 92.9e105.2 [153]

Fe3O4@SiO2eC16 PCBs environmentalwater

GC-MS/MS e e 0.14e0.27 ＜9.06 75.17e101.20 [154]

Fe3O4@SiO2eNH2&F13 PFCs environmentalwater

UPLC-MS/MS e 1000 0.029e0.099 3.1e12.6 90.05e106.67 [86]

(continued on next page)


Table 1 (continued )





Recoveries (%) Ref.

PEMs/Fe3O4@SiO2 Cu(II) water, rice FAAS 14.7 95.7 230 2.1 94.4e114.1 [155]Fe3O4@SiO2-TCPP PAHs environmental

waterGC-MS e e 2e10 ＜13.0 71.1e107.1 [156]

Fe3O4@SiO2@g-MPTS Hg(II), MeHg(I) water, humanhair

ICP-MS e 50 1.6e1.9 2.6e3.4 75.6e99.6 [157]

Fe3O4@SiO2@GMA-S-SH Hg(II), MeHg(I),PhHg(I)

farmlandwater, soil, rice

HPLC-ICP-MS 32.1e142 329e380 0.4e1.4 5.0e8.3 84.3e116 [158]

Fe3O4@Au@DDTFe3O4@Au@2-ME

diphenols, PAHs water HPLC-UV e e 340e16670260e520

1.3e3.2 63.8e110.7 [89]

Fe@Ag@DMB Cd(II),Pb(II), Hg(II) water HPLC-VWD e 433e464 11e31 1.85e2.37 97.5e103.2 [159]Fe3O4@MnO2 As(III), As(V) water CHG-AFS e e 2.9e3.2 4.3e4.8 85.6e111.7 [160]MPIL@CC[4]A PAEs water, drinks,

human serumHPLC-UV 52.90

e63.77e158.7e191.3 20e310 4.6e7.7 84.3e110.8 [161]

3D-IL@mGO PAHs vegetable oil GC-MS e e 0.05e0.30a 1.8e9.0 80.2e115 [162]Fe3O4@SiO2@(PSS-PIL)n pesticides water HPLC-UV e e 500 2.0e4.6 82.5e109.3 [90]Fe3O4@SiO2@GO@ILs CPs water HPLC-MS/MS e e 0.2e2.6 0.6e8.4 85.3e99.3 [163]PIL-MNPs OPPs tea HPLC-UV e 84e161 10 4.5e11.3 81.4e112.6 [164]Fe3O4@SiO2@TiO2@CPC BPA water HPLC-UV e e 500 3.2e7.8 92e105 [92]SDS@ Fe3O4 cationic dyes food HPLC-DAD 47.4e270.3 e 0.2e0.9a 0.5e14.3 70.1e104.5 [165]Fe3O4@SiO2@MgeAl LDH PAEs water HPLC-UV e e 12.3e36.5 2.1e6.9 63e102 [166]Fe3O4@DC193C parabens water HPLC-UV e e 2400e6300 0.7e10.1 86.0e118.0 [167]GO-Chm herbicides water, rice HPLC-UV 29.41

e35.71e 2�8, 0.008

e0.03b2.46e4.01 94.33e102.67 [168]

MCM silver nanoparticles water ICP-MS e e 16e23 3.4e4.2 84.9e98.5 [94]polythiophene@CS@MNPs triazines water GC-FID e e 10e30 7e12 96e102 [169]

PT: polythiophene, OCPs: organochlorine pesticides, PAR: 4-(2-pyridylazo)resorcinol, ETAAS: electrothermal atomic absorption spectrometry, 2-PTSC: 2-pyridinecarbox-aldehyde thiosemicarbazone, ICP-OES: inductively coupled plasma-optical emission spectrometry, PS: porous polystyrene, HAP：hydroxyapatite, ICP-AES：inductivelycoupled plasma-atomic emission spectroscopy, VWD: variable wavelength detection, GC-FPD: gas chromatography-flame photometric detector, En: ethylenediamine, HRMS:high-resolution mass spectrometry, DMcT: 2,5-dimercapto-1,3,4-thiadiazole, BITC: benzoyl isothiocyanate, IRMOF: iso-reticular MOFs, TMU-8: Tarbiat Modares University-8,LZU1: Lan Zhou University-1, mPMF: mesoporous polymelamine-formaldehyde, M-PPOP: magnetic porphyrin-based porous organic polymer, SAs: sulfonamide residues, AD:amperometric detector, Ph: phenyl, fTiO2: flower like TiO2, CMCD: carboxymethyl-b-cyclodextrin, CBs: chlorobenzenes,WC-TMMIPs: water-compatible temperature andmagnetic dual-responsiveMIPs, TBBPS: tetrabromobisphenol S, C-MIPs: carbon-MIPs, UVeVis: ultravioletevisible spectrophotometry, Pu: 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole(Purpald), IIP: ion imprinted polymer, MIIP: magnetic ion-imprinted polymer, DMIP: Dummy molecularly imprinted polymers, PM-MIM: photonic-magneticresponsive molecularly imprinted microspheres, MNPC: magnetic nanoporous carbon, GFAAS: graphite furnace atomic absorption spectrometry, AFs: Aflatoxins, PEMs:polyelectrolyte multilayers, g-MPTS: g-mercaptopropyltrimethoxysilane, MeHg(I): methylmercury, PhHg(I): phenylmercury, GMA: glycidyl methacrylate, DDT: 1-dodeca-nethiol, 2-ME: 2-mercaptoethanol, DMB: dimercaptobenzene, CHG-AFS: chemical hydride generation-atomic fluorescence spectrometry, MPIL@CC[4]A: magnetic polyionicliquid with carboxylatocalix[4]arene, CPC: cetyl pyridinium chloride SDS: sodium dodecyl sulfate, LDH: allayered double hydroxide, DC193C: silicone-ethyleneoxidecopolymer, Chm: chitosan graphene oxide, MCM: magnetic chitosan microsphere, CS: chitosan.

a mg kg�1.b ng g�1b.c ng kg�1.d ng mg�1.e mg g�1.f mM.g mg L�1.


thiophene copolymer (PPy-Th), resulting in a composite rich inactive reaction sites. This MGO/SiO2@coPPy-Th nanocompositeoffered high hydrophilic performance, interaction activity, andmagnetic susceptibility as a potential MSPE adsorbent, which allowit to overcome the limitations of simple magnetized GO. This noveladsorbent presented higher preconcentration factors (36e44) andadsorption capacities (80e230 mg g�1) than other common SPEadsorbents. The LODs and recoveries obtained with this analyticalmethod ranged from 0.15 to 0.65 ng mL�1 and 90% to 106%,respectively.

2.2. CNT-based composites

CNTs are allotropes of carbon, composed of G nanosheets. Ingeneral, CNTs can be formed by monolayer or multilayer graphenesheets, which can be rolled up to form a one-dimensional hollowtubular nanostructure. Based on the number of G layers, CNTs areclassified as single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs). The adsorption mechanismof CNTs may be related to their hollow nanostructure or externalsurface containing adsorption sites. Specific chemical functionalgroups can be anchored on the sidewall or bottom of CNTs,

including oxygen-containing groups, which changes the surfacefeatures of the CNTs while making them more hydrophilic [29].Moreover, the introduction of magnetic components to preparemagnetic CNTs (MCNTs) is a common method of enhancing theseparation efficiency during analytical applications. The obtainedMCNT composites can provide large surface areas with fast masstransfer for the enrichment of target analytes in diverse environ-mental media. For example, Ricardo et al. adopted an in-situ high-temperature decomposition method to fabricate MCNT compositesas a MSPE adsorbent for extracting seven PAHs from aqueoussamples [30].

Li et al. [31] synthesized magnetic carboxylic MWCNT compos-ites enhanced by hollow carbon nanospheres (HCSs) (HCSs@Fe3O4-MWCNTs-COOH) as MSPE adsorbents for the simultaneousextraction of three polar herbicides from wheat flour samples,coupled with HPLC-DAD for further determination (Fig. 2). Thiscarboxylic MWCNT component was immobilized in the pores ofHCSs via an in-situ physical entrapment strategy, and Fe3O4nanoparticles were then introduced onto the surface of the HCS-enhanced MWCNT composites. The enhanced adsorptionbehavior resulted in better selectivity for the hydrophobic and ar-omatic herbicides based on p-p electron-donor-acceptor

Fig. 1. The illustration of basic structure and suggested adsorption sites of the pre-pared Fe3O4@G-TEOS-MTMOS adsorbent. Reprinted with permission from Ref. [25],Copyright (2017) Elsevier.


interactions, hydrogen bonding, hydrophobic interaction, andelectrostatic attraction. The proposed MSPE-HPLC-DAD methodexhibited superior analytical performance, with LODs and re-coveries in the range of 0.24e0.68 ng g�1 and 88.8e96.6%,respectively.

Fig. 2. The synthesis protocol and the proposed MSPE-HPLC-DAD procedur

Besides the excellent extraction of various organic pollutants,MCNT nanocomposites are also suitable for the adsorption ofinorganic elements from environmental matrices. Fan et al. [32]developed magnetic MWCNT composites modified with thiol-groups (CNTs-SH@Fe3O4) as an ideal adsorbent for the removal ofHg(II) from complex water samples. The thiol groups were firstdecorated onto MWCNTs, and the functionalized MWCNTs-Fe3O4composites were subsequently prepared through a chemical co-precipitation method. The resulting CNTs-SH@Fe3O4 adsorbentprovided a large surface area of 200.21 m2 g�1, average pore size of9.23 nm, and pore volume of 0.427 cm3 g�1. The maximumadsorption capacity of 172.4 mg g�1 for Hg(II) suggested enhancedaffinities based on surface physical adsorption, Lewis acid-baseinteraction, and reduction adsorption between the adsorbent(due to synergistic effects from the components of CNTs, thiolgroups, and Fe(II) in Fe3O4 nanoparticles) and Hg(II). In competitiveadsorption systems containing other ions (Cu(II)/Mg(II)/Zn(II)), theCNTs-SH@Fe3O4 adsorbent maintained good selectivity towardsHg(II). Additionally, good reusability and stability were exhibitedafter five repeated adsorption-desorption cycles, and high removalefficiency of Hg(II) was achieved in a wide pH range from 3 to 11.

In addition to the aforementioned widely studied magneticcarbonaceous nanomaterials, other novel carbon-based magneticnanomaterials for MSPE have also been reported. CNFs are theproducts of various stacking arrangements of G sheets with non-hollow cylindrical nanostructures, and possess larger size (lengthand diameter) and more abundant edge sites than CNTs [33,34].Yazdi et al. [35] produced magnetic CNFs as a MSPE adsorbent via achemical co-precipitation synthesis procedure coupled with gaschromatography-flame ionization detection (GC-FID) for adsorbingand determining PAHs. This established method achieved desirableresults with LODs and recoveries in the range of0.008e0.03 ng mL�1 and 90.1e100.9%, respectively. Carbon-basedquantum dots are 0-D spherical nanoparticles with sizes below10 nm, mainly composed of graphene quantum dots (GQDs) andcarbon quantum dots (CQDs), which can be applied to extract

e. Reprinted with permission from Ref. [31], Copyright (2018) Elsevier.

Fig. 3. The synthesis procedure for the magnetic core-shell Fe3O4@SiO2@MOF/TiO2.

Reprinted with permission from Ref. [47], Copyright (2016) Elsevier.


environmental pollutants owing to their large contact area andspecific photoluminescence properties [36,37]. Rezaei et al. [38]prepared magnetic GQDs (Fe3O4/GQDs) by a simple hydrothermalmethod for the selective extraction of BPA in water samples. TheFe3O4/GQD adsorbent showed higher adsorption efficiency for BPAthan Fe3O4/G, probably owing to its large surface-to-volume ratioand high polarity derived from the hydroxyl groups of GQDs.Regarding g-C3N4, it has a layered 2-D framework as a G analoguewith superior characteristics of chemical and thermal stability,unique photocatalytic activities, good biocompatibility, and easysurface modification, and can be obtained from nitrogen-rich pre-cursors via a thermal decomposition procedure [39]. In Wang'sstudy [40], a magnetic g-C3N4 (gC3N4/Fe3O4) adsorbent synthesizedthrough an in-situ chemical co-precipitation method was utilizedfor the enrichment and detection of PAEs in water samples prior toHPLC-UV. The analytical performance (low LODs of0.05e0.10 mg L�1 and good recoveries of 79.4e99.4%) indicated thatthe as-established MSPE-HPLC-UV method was suitable for PAEsanalysis. Furthermore, Table 1.1 lists the recent applications of thesecarbonaceous nanomaterials as MSPE adsorbents for the extractionof target pollutants from various samples prior to quantitativeanalysis.

3. Magnetic porous materials as MSPE adsorbents

Metal-organic frameworks (MOFs), porous covalenteorganicmaterials (PCOMs), and mesoporous materials are fascinatingclasses of porous materials with larger surface area and highporosity, which can provide MNPs with abundant pore structuresand large interior surfaces for modification. The combination ofMNPs and porous materials helps to accelerate the adsorption/desorption kinetics of target analytes and improve the adsorptioncapacity towards target analytes. Table 1.2 summarizes somerepresentative applications of magnetic porous materials as MSPEadsorbents developed in the last five years for environmentalremediation.

3.1. MOF-based composites

MOFs are composed of inorganic sub-units (metal ions/clusters/chains) and organic linkers (nitrogen-containing heterocyclecompounds, carboxylate ligands) joined by coordination bonds,and possess a multi-dimensional periodic network structure [41].The hydrophobicity and rich p-p bonds in MOFs make thempotentially useful as SPE adsorbents [42]. However, MOFswith non-spherical morphologies are not convenient for packing in SPE car-tridges [12]. In addition, it is difficult to separate MOFs with theadhered target compound from aqueous solutions. In this case, thehybridization of MNPs and different MOFs can overcome theaforementioned shortcomings and enable the direct separation ofMOFs with an external magnet, thus avoiding the need for tediouscentrifugation and filtration procedures.

The techniques for the synthesis of MOFs have been describedelsewhere [43]. Here, we focus on the methods of magnetizingMOFs, including embedding, encapsulating, layer-by-layer method,direct mixing, and in-situ growth of MNPs [12,44]. The widely usedembedding method (Fig. 4) is performed by adding MNPs to MOFprecursors. However, MNPs may occupy the pores of the MOF,resulting in a loss of porosity of the MOFs in this method. Theencapsulation method employs a polymer or carbonaceous layer asa buffering interface between the MOFs and MNPs to promote thegrowth of MOFs around the interface and improve the compati-bility between the MNPs and MOFs [44]. In the layer-by-layermethod, functional groups (such as carboxyl groups) on the sur-face of the MNPs are necessary to control the growth of the MOF

crystals and obtain a core-shell structure. The advantage of thedirect mixing method is that the magnetization of the MOFs andthe MSPE procedure are accomplished simultaneously in thesample solution, but it is hard to maintain structural stability incomplicated sample matrices. The in-situ growth of MNPs requiresthe pre-prepared MOFs to be structurally robust under the condi-tions used for the synthesis of the MNPs, which limits the appli-cation of this technique.

As the most commonly used MOFs in MSPE, the Material of theInstitute Lavoisier (MILs) family, the zeolite imidazolate frame-works (ZIFs) family, and zirconium-based MOFs (Zr-MOFs) showhigh resistance to water and common solvents, which facilitatestheir application in the extraction of hydrophobic target com-pounds [12]. MILs comprise trivalent metal ions (Cr, Fe) and car-boxylic acid ligands. Among them, MIL-101(Cr), prepared by thecoordination of terephthalic acid and chromium nitrate non-ahydrate, has been utilized for dye removal and extracting herbi-cides, pesticides, PAEs, and PAHs from environmental water,biological samples, and food matrices (Table 1.2). He et al. [45] re-ported the synthesis of Fe3O4eNH2@MIL-101(Cr) via an embeddingmethod. The surface area and pore volume of the synthesizedFe3O4eNH2@MIL-101(Cr) were 428.5 m2 g�1 and 0.27 cm3 g�1,respectively. Under the optimum conditions, only 10 mg of adsor-bent was required to extract pyrethroids from 50 mL of environ-mental water, and the recovery remained stable for eight cycles in areusability test. Compared to un-modified Fe3O4@MIL-101(Cr), theintroduction of eNH2 enhanced the water solubility of the MNPsand improved the selectivity for adsorption of the acid targets.Combined with a gas chromatography-election capture detector(GC-ECD), the developed method showed low detection limitsranging from 5 to 9 pg mL�1. Furthermore, the conjugation andhydrophobic effects were proved to be the main adsorptionmechanisms, along with the innovative utilization of moleculardocking. ZIF-8 composed of an imidazole-based organic linker (2-methylimidazole) and Zn(II) is the most studied ZIF, and has beenused in the adsorption of herbicides, PAHs, and fungicides inaqueous samples due to its excellent water stability. Liu and co-workers [46] successfully fabricated Fe3O4@ZIF-8 coreeshell mi-crospheres via an encapsulation method by using mercaptoaceticacid (MAA) as a buffering interface. The Ms value of Fe3O4@ZIF-8was about 60.0 emu g�1. With the help of an external magnetic

Fig. 4. Schematic illustration of the embedding process for the magnetic MOF-5 and the MSPE procedure. Reprinted with permission from Ref. [49], Copyright (2018) Elsevier.


field, Fe3O4@ZIF-8 dispersed in water can be collected within 10 s.The extraction of PAEs from water samples was accomplished in16 min with 20 mg of sorbent per 20 mL sample. Coupled withHPLC-UV, LODs in the range of 0.08e0.24 mg L�1 and an accuracy of85.6e103.6% were obtained. In another study [47], Fe3O4@SiO2@-ZIF-8 microspheres were fabricated (Fig. 3) via a layer-by-layermethod and post-modified with TiO2 to capture fungicides in wa-ter samples. The introduction of TiO2 not only strengthened theweak interactions between the target and ZIF-8, but also improvedthe mass transfer rate of the fungicides. Zr-MOFs are based on thecoordination of terephthalic acid and Zr(Ⅳ). The high oxidationstate of Zr endows the frameworks with higher chemical stability[12]. Jin et al. [48] synthesized magnetic graphene (MagG)@poly-dopamine (PDA)@Zr-MOF via an encapsulationmethod, where PDAacted as a buffering interface for the growth of Zr-MOFs andincreased the hydrophilicity of the composite. The resultantMagG@PDA@Zr-MOF sorbents had good magnetic response, highthermal stability, and much larger surface area (178 m2 g�1) thanMagG (21.13 m2 g�1) andMagG@PDA (21.25 m2 g�1). When utilizedto enrich multi-pesticide residues, the entire MSPE procedure wasquickly accomplished within 10 min and only 0.50 g of tobaccosample was needed. The recoveries for MagG, MagG@PDA, andMagG@PDA@Zr-MOF increased gradually in the range of 14e76%,29e95%, and 58e126%, respectively, indicating that both PDA andZr-MOF contributed to the adsorption process.

Compared with the above-mentioned MOFs, MOF-5, MOF-177,and Hong Kong University of Science and Technology-1 (HKUST-1)are less moisture-stable but are still extensively used after theirgrowth on MNPs and modification with functional groups(Table 1.2). Ma et al. [49] synthesizedmagnetic MOF-5 (Fig. 4) via anembedding method, and only 7 mg of the resultant sorbent was

required per 60-mL water sample, with a maximum adsorptioncapacity in the range of 81e181 mg g�1 for four heterocyclic pes-ticides. During the formation of MOFs, an interesting phenomenonwas observed, wherein larger organic ligands gave rise to a largerpore volume of the MOFs. For example, magnetic MOF-177 withpore edges larger than 10.8 Å and a higher Langmuir surface equalto 4170 m2 g�1 was obtained [50] by replacing terephthalic acid inMOF-5 with 1,3,5-tris(4-carboxyphenyl)benzene, and this MOFshowed good recoveries of 83.3e108.7% towards phenols in envi-ronmental water when combined with gas chromatography/massspectrometry (GC-MS).Wu et al. [51] constructedmagnetic copper-based HKUST-1 through a simple method, which was performed bysubsequently adding benzene tricarboxylic acid (H3BTC) ligands,Fe3O4 particles, and Cu(OAc)2$H2O to mixed solutions (dime-thylformamide:ethanol, v/v, 1:1) followed by refluxing at 70 �Cwith stirring for 7 h. The obtained Fe3O4/HKUST-1 compositepossessed a satisfactory saturation magnetization value of 44 emug�1 and higher adsorption capacities towards ciprofloxacin andnorfloxacin of 538 mg g�1 and 513 mg g�1, respectively. In addition,the adsorption capacity of the adsorbent only decreased slightlyafter ten reuse cycles. In another study [52], magnetic HKUST-1(Fe3O4@2,5-dimercapto-1,3,4-thiadiazole@HKUST-1) was fabri-cated by a procedure similar to the one described above and appliedto preconcentrate trace amounts of Cd(II), Pb(II), and Zn(II) in babyfood samples, and the sorption capacity reached 155 mg g�1,173 mg g�1, and 190 mg g�1 respectively. LODs in the range of0.10e0.75 ng mL�1 were obtained when coupled with FAAS. Theabovementioned synthesis method of HKUST-1 is easy to carry out,however, it inevitably involves the use of toxic solvents (such asdimethylformamide and dimethyl sulfoxide) and the generation ofby-product anions from the rawmaterials. Against such a backdrop,


Hu and coworkers [53] innovatively utilized Cu(OH)2 as coppersource to synthesize magnetic HKUST-1 by a facile self-templateapproach. First, Fe3O4@SiO2@Cu(OH)2 was prepared by a co-precipitation method. Next, the transformation of Cu(OH)2 toHKUST-1 was achieved by dispersing Fe3O4@SiO2@Cu(OH)2 in ul-trapure water followed by adding H3BTC. Then, the unsaturatedcopper sites in the resultant core-shell Fe3O4@SiO2@HKUST-1madeit possible to introduce sulfhydryl groups via modification withbismuthiol (BieI). Finally, the composites were successfully appliedto Hg2þ removal from water and showed high adsorption capacity(264 mg g�1).

Based on a three-dimensional skeleton, MOFs containing para-magnetic metal centers (such as Fe, Co) are potential precursorsand templates for the synthesis of magnetic porous carbon (MPC)by direct carbonization under nitrogen atmosphere and can pre-vent pore blockage during the loading of the magnetic metal oxidesvia co-precipitation. Two analogous series of MPC (A-series and B-series) were fabricated [54] by simple pyrolysis of two similar Fe-containing MOFs (Fe-MIL-88A and Fe-MIL-88B, containingaliphatic fumaric acid and aromatic 1,4-benzenedicarboxylic acid asthe organic ligand, respectively) at different carbonization tem-peratures. The highest surface area, total pore volume, andadsorption capacities towards methylene blue (equal to 490m2 g�1,0.52 cm3 g�1, and 83.9 mg g�1, respectively) were obtained for theB-series MPC when the pyrolysis temperature reached 800 �C,which indicates that both the organic ligand of the starting MOFsand the pyrolysis temperature influenced the properties of theMPC. Furthermore, the mode of transformation from Fe ions to thefinal agminated MNPs differed slightly depending on the pyrolysistemperature and organic ligand. MOFs containing non-paramagnetic metal centers can also potentially act as carboniza-tion templates for forming MPCs in the presence of paramagneticmetals (Fe, Co). Due to the lack of magnetism of ZIF-8-derived MPC,Liu et al. [55] dissolved Zn(II), Co(II), and 2-methylimidazole inmethanol to fabricate bimetallic ZIF (BMZIF), which integrated thehigh porosity and high N content of ZIF-8 with the magneticproperties and satisfactory graphitized carbon of ZIF-67. N-dopedMPC was thereafter obtained by heating BMZIF powder at 700 �Cunder nitrogen, and was successfully applied as an adsorbent forextracting organochlorine pesticides from an aqueous medium,followed by gas chromatography coupled with tandem massspectrometry (GC-MS/MS) detection. The proposed method ach-ieved an excellent LOD in the range of 0.39e0.70 ng L�1 andsatisfactory recoveries of 79.4e98.3%.

Fig. 5. Synthesis and application of the bouquet-like magnetic Tppa-1 sorbent. Reprin

3.2. PCOM-based composites

PCOMs composed entirely of light elements (C, H, O, N, B, etc.)exhibit variable topological structures, high specific surface area,and nanoscale porosity [56]. The reported PCOMs can be classifiedinto two categories according to the pore size distribution andstructural ordering: crystalline PCOMs (such as covalent organicframeworks (COFs)) and amorphous PCOMs (such as covalentorganic polymers (COPs), hyper-crosslinked polymers (HCPs), andconjugated microporous polymers (CMPs)). The introduction ofMNPs overcomes the shortcomings of PCOMs, such as hydropho-bicity and low density, and endows PCOMs with more potential forMSPE. The preparation techniques of magnetic PCOMs can also becategorized into four classes, including direct mixing of PCOMswith MNPs, one-step methods of simultaneous preparation ofPCOMs andMNPs, post-synthesis deposition of PCOMs on preparedMNPs, and post-synthesis deposition of MNPs on prepared PCOMs;the last two methods are more widely used.

Among crystalline PCOMs, COFs based on the Schiff-base reac-tion are the most widely used sorbents for PAHs, phthalate esters,and endocrine-disrupting phenols in MSPE (Table 1.2). He et al. [57]synthesized novel bouquet-shaped magnetic COFs, composed ofMNP “flowers” and COF “stems” (Fig. 5). Briefly, the surface ofamino-functionalized Fe3O4 nanoparticles was treated with 1,3,5-triformylphoroglucinol (Tp) monomers, which was necessary forthe ensuing formation of the COFs and directed growth. Tp and p-phenylenediamine (Pa-1) were then added to the surface-modifiedMNPs to grow COF (Tppa-1) nanofibers through the Schiff-basereaction by a room temperature solution-phase approach.Compared to pure Tppa-1, bouquet-shaped magnetic Tppa-1 had ahigher surface area to total pore volume ratio, but a lowerBrunauer-Emmett-Teller (BET) surface area of 247.8 m2 g�1 owingto the addition of the MNPs. Meanwhile, cross-linking with Tppa-1deceased the saturation magnetization of magnetic Tppa-1 from69.4 emu g�1 to 40.1 emu g�1, but the composite still possessed asuperparamagnetic nature. The sorbent was successfully used toextract PAHs mainly due to hydrophobic interactions, with theassistance of hydrogen bonding and p�p stacking interactions. Inaddition, the anti-interference capacity of magnetic Tppa-1 wasstable when the concentration of humic acid was less than40 mg L�1. Satisfactory recovery and low LODs in the range of73e110% and 0.24e1.01 ng L�1 were respectively obtained whenthe system was coupled with high-performance liquid chroma-tography with fluorescence detection (HPLC-FLD). Similarly, Wang

ted with permission from Ref. [57]. Copyright (2017) American Chemical Society.

Fig. 6. Schematic representation of the synthesis of covalent triazine-based frameworkgrafted magnetic particles and the extended CTF. Reprinted with permission fromRef. [8], Copyright (2018) Elsevier.


and coworkers [58] used 5-tris(4-aminophenyl)benzene and ter-ephthalaldehyde as monomers to construct COFs and loaded theframeworks onto the surface of mesoporous TiO2 (mTiO2)-modifiedMNPs. The flower-like mTiO2 not only provided a large surface areafor COF coverage, but also showed strong affinity for chlorine-containing pollutants. The surface area of Fe3O4@mTiO2@COFswas more than twice that of Fe3O4@mTiO2, and the former wasthermally stable at 400 �C. This sorbent was utilized to capture PCBsin soil. Notably, desorption of the PCBs was thermally driven andprecluded the consumption of organic solvents. According to Ge'sreport [59], magnetic melamine-based COFs were synthesizedthrough a microwave-assisted method. Briefly, melamine and ter-ephthalaldehydewere first dissolved in dimethylsulfoxide followedby adding a certain amount of amino-functionalized Fe3O4 (Fe3O4/melamine-COF ratio of 2:100, 5:100,10:100, and 15:100). Thepolymerization of melamine and terephthalaldehyde occurred in amicrowave-assisted process for 4 h. The highest BET specific surfacearea of 495 m2 g�1 was observed when the ratio of Fe3O4/mel-amine-COF was 5:100. As expected, the magnetic melamine-basedCOF with the highest Fe3O4 content possessed the highest satura-tion magnetization value of 3.59 emu g�1. To balance the surfacearea and magnetic properties, the adsorbent with a ratio of Fe3O4/melamine-COF of 10:100 was chosen for the selective removal ofHg(II). The selective adsorption for Hg(I) was tested in the presenceof other metal ions (Na(I), Cd(II), Zn(II), Ni(II), Pb(II), Mg(II), andCr(III)). The results indicated that the adsorption capacity for Hg(I)(97.65 mg g�1) was over 100 times higher than that of other metalions, which should be attributed to the high density of N groups inmelamine-based COFs.

Covalent triazine-based frameworks (CTFs) generated fromnitrile cyclation are another class of COFs widely used in MSPE(Table 1.2). In a study by Yan [60], to avoid damaging the structureof MNPs under the harsh synthesis conditions required for CTFs(400 �C for 40 h), the in-situ reduction of Ni ions on pre-preparedCTFs was used to fabricate magnetic CTF/nickel (Ni) composites.Due to the interactions between the Ni ions and nitrogen atoms ofthe CTFs, Ni(II) could be immobilized on the surface of the CTFs andreduced to Ni particles. As the mass ratio of Ni to CTFs was variedfrom 0.5 to 1.5 (m/m), the saturation magnetization increased from19.7 to 36.3 emu g�1. The ratio was finally set to unity to balance themagnetic properties of the composite and the extraction efficiencytowards PAEs. The enrichment factor was in the range of 59e88. Bycoupling with GC-FID, a satisfactory recovery of 70.6e119% wasobtained from samples of plastic packaging materials. In compari-son with the aforementioned case, Shahvar and coworkers [8]utilized the milder Friedel-Crafts alkylation reaction to fabricateCTFs on the surface of pre-prepared phenyl-functionalized MNPs(Fig. 6). In this study, Fe3O4@SiO2 was surface-modifiedwith phenylgroups by treatment with phenyltrimethoxysilane (PTMS). Friedel-Crafts alkylation was then performed by using cyanuric chloride asa node and biphenyl as a linker in the presence of AlCl3. As shown inFig. 6, having a large amount of aromatic rings in the CTFs wasbeneficial for the extraction of aromatic ring-containing contami-nants due to p-p stacking and hydrogen bond interactions. Thisexcellent sorbent with good reusability for 17 cycles was success-fully applied to the extraction of parabens from biological andenvironmental samples.

Magnetic amorphous HCP and CMP were also used as sorbentsin MSPE (Table 1.2). Zhou and co-workers [61] fabricated MNPswith 3-aminophenylboronic acid monohydrate (APB) and addedboronic-acid-functional MNPs to a mixture of monomers (1,2,4,5-tetrabromobenzene (TBB) and 1,4-benzene diboronic acid(BDBA)) to synthesize a magnetic polyphenylene conjugatedmicroporous polymer (PP-CMP) by the Suzuki coupling reaction.The resultant magnetic PP-CMP not only possessed excellent

stability due to the firm covalent linkages and rich conjugatedstructure, but also showed satisfactory adsorption of trace PAHs inhuman urine prior to HPLC-FLD.

3.3. Mesoporous materials-based composites

Mesoporous nanomaterials usually possess unique surfacestructures with pore sizes ranging from 2 to 50 nm according to thedefinition given by International Union of Pure and AppliedChemistry (IUPAC) [62]. Mesoporous materials have exhibited greatapplication value owing to their excellent structural features,including highly ordered mesoporous structures, large specificsurface area, high pore volume, controllable pore size, easy surfacefunctionalization, and good biocompatibility [63]. Among theseprominent properties, the mesoporous structures provide activecenters for versatile physicochemical interactions or modifications.Therefore, mesoporous materials are widely used in drug delivery,catalysis, sensors and adsorption applications. In this section,mesoporous silica-based materials, mesoporous titanium dioxide(mTiO2), and mesoporous carbon materials are mainly introduced,and the incorporation of magnetic nanoparticles to form magneticmesoporous composites is discussed, and these composites havealready taken on a distinctive role in the extraction of environ-mental pollutants [64].

Qin et al. synthesized methyltrimethoxy modified-mesoporoussilica layer-coated magnetic microspheres with p-toluenesulfonicacid (PTSA) as a catalyst (Fe3O4/mSiO2-Me-PTSA) for the extractionof 26 PCBs in environmental water samples [65]. Magnetic meso-porous silica (Fe3O4/mSiO2) was obtained by using cetyl-trimethylammonium bromide (CTAB) as a surfactant template withthe silica precursor TEOS via a sol-gel process. In this work,methyltrimethoxy was grafted onto the surfaces of Fe3O4/mSiO2microspheres with PTSA as a catalyst to enhance the reaction effi-ciency. The Fe3O4/mSiO2-Me-PTSA adsorbent not only provided a


large surface area (197.1 m2 g�1) and fast magnetic separationability (an Ms of 33 emu g�1), but also allowed effective adsorption(within 10 min) of the target PCBs. Besides, the high enrichmentfactors of the PCBs were in the range of 119e147, while themaximum adsorption capacity was 46.3 mg g�1 for 2,4,40-Tri-chlorobiphenyl (PCB 28). Coupled with GC-ECD, the establishedmethod showed satisfactory results including the low LODs of0.16e0.91 ng L�1 and good recoveries of 85.25e118.60%.

Zhao's group [66] fabricated Fe3O4@resorcinol-form-aldehyde@mesoporous TiO2 microspheres to remove As(V) inacidic environments. Briefly, the prepared Fe3O4 nanoparticleswere previously coated by resorcinol-formaldehyde using a modi-fied solegel coating method. Then, a concentrated ammonia solu-tion was added into an ethanolic solution of the aboveFe3O4@resorcinol-formaldehyde followed by the addition of tetra-butyl titanate. After reacting for 12 h with continuous mechanicalstirring at 45 �C， Fe3O4@resorcinol-formaldehyde@mesoporousTiO2 microspheres (Fe3O4@RF@mTiO2) were obtained. The BETsurface area and pore size of the resultant adsorbents were337m2 g�1 and 3.0 nm, respectively.When applied to remove As(V)in an acidic environment (pH ¼ 3), the inner cores of the adsorbentwere almost unchanged after five adsorption cycles. However, theinner Fe3O4 cores of Fe3O4@mTiO2 microspheres without the RFlayer gradually disappeared and became a cavity, which demon-strated the protective effect of the RF layer. In addition, it was foundthat electrostatic forces and surface coordination interactions be-tween As(V) and partially crystallized TiO2 were responsible for theexcellent As(V) adsorption performance, which exhibited a fastadsorption rate of 1.16 gmg�1 h�1 and an unprecedented saturationcapacity of 138.6 mg g�1.

In addition to the MOFs-derived porous carbon described inSection 3.1, mesoporous silica templates can also be used to prepareporous carbon with a mesoporous structure. In Yi's study [67], themesoporous template SBA-15 was added to a sucrose solution andheated at 100 �C for 6 h. Next, the mixture was boiled in a sodiumhydroxide solution to remove the silica templates. Then, theresultant mesoporous carbon was oxidized through a nitric acidtreatment to enhance the adsorption capacity for heavy metal ions.Finally, magnetization of the oxidized mesoporous carbon wascarried out employing FeCl3$6H2O and FeSO4 as magnetic species.The proposed magnetic oxidized mesoporous carbon had a BETspecific surface area of 179 m2 g�1, a total pore volume of0.18 cm3 g�1, and mean pore size of 2.9 nm. The equilibrium timeand maximum adsorption capacity for Cu(II) were 3 h and 51.4 mg/g, respectively. At last, the adsorbent was successfully applied toremove metal ions in industrial wastewater.

4. Magnetic molecular recognition composites

With respect to the aforementioned magnetic functional ma-terials, including carbonaceous and porous materials, the adsorp-tion mechanism mainly depends on their structural characteristicsto form various interactions with environmental pollutants, such aselectrostatic attraction, hydrogen bonding, p-p stacking, van derWaals force，hydrophobic interaction, and metal ionic coordina-tion, etc. However, those materials are unable to offer strongadsorption specificity and discriminability. In particular, structuralanalogues co-existing in complicated environmental matrices mayinterfere with the adsorption behavior. Herein, functional materialsfeaturing a retention mechanism based on specific molecularrecognition have been lately emphasized for a broad range of ap-plications in sample pretreatment. This review focuses on twokinds of traditional molecular recognition materials, MIPs andaptamers, as promising candidates for use in MSPE. Derived fromthe well-designed imprinted cavities complementary to their

template molecules in size, form and functional groups, or oligo-nucleotide sequences that can specifically identify target analyteswith high selectivity and affinity, MIPs and aptamer-based mag-netic molecular recognition composites have the prominent ad-vantages of higher adsorption capacities and extraction efficienciesthan the aforementioned magnetic carbonaceous and porous ma-terials. Table 1.3 summarizes the advances in the application ofMIPs and aptamer-based magnetic molecular recognition com-posites as MSPE adsorbents for environmental pollutants in the lastfive years.

4.1. MIP-based composites

MIPs are artificial, tailor-made polymer materials with highlycross-linked 3-D network structures designed on the basis of bio-mimetics. Because MIPs have the advantages of strong specificity,good stability, reusability, and large adsorption capacity, they arepromising adsorbents with high affinity and selectivity for theseparation and enrichment of target compounds and structurallyrelated analogues [68,69]. At present, a variety of improved syn-thetic strategies have been developed for MIPs (e.g., surfaceimprinting, multi-monomer imprinting, dummy imprinting,stimuli-responsive imprinting, etc.), overcoming the deficiencies oftraditional imprinting strategies such as incomplete templateremoval, heterogeneous morphology and poor dispersity in solu-tions [70]. From the perspective of the interactionmechanism, non-covalent molecular imprinting is extensively used to synthesizeMIPs currently due to its simplicity and ability to form numerousfunctional monomer-template complexes [71]. When applied inMSPE, MIPs can be synthesized in the presence of MNPs to incor-porate the advantages of high selectivity towards target com-pounds and rapid magnetic separation from the sample solution[72].

Our group adopted magnetic-MIPs (MMIPs) as a MSPE adsor-bent coupled with HPLC-UV for the selective extraction anddetection of macrolide antibiotics (MACs) in different food samples[73]. MIPs were synthesized on the surface of a 3-methacryloxypropyltrimethoxysilane (MPS)-modified Fe3O4@SiO2substrate with a surface imprinting technique by using erythro-mycin (ERY) as a template molecule. The resultant MMIPs offeredhigher adsorption selectivity than magnetic non-imprinted poly-mers (MNIPs) due to their rich bonding sites. Compared with thecurrent reported molecular imprinting SPE (MISPE)-HPLC-UVmethods, the developed analytical method afforded the simulta-neous and efficient detection of six MACs with fewer matrix in-terferences in the MSPE process, lower LODs of 0.015e0.2 mg g�1,and better recoveries of 82.5e113.1%.

Yuan et al. [74] fabricated core-shell magnetic silica-coated MIPbeads by using binary functional monomers of 4-vinylpyridine (4-VP) and beta-cyclodextrin (b-CD) through reversible addition-fragmentation chain transfer polymerization, which could pro-vide hydrogen-bonding sites and hollow truncated cavities for theenrichment of BPA from milk samples. The introduction of binaryfunctional monomers vastly promoted the adsorption selectivity ofMMIPs. The as-synthesized MMIPs were applied to the selectiveextraction of BPA and displayed superparamagnetic behavior (anMs of 35.18 emu g�1), a saturation capacity of 17.98 mg g�1, highadsorption efficiency (96.11%), and good reusability (six cycles),indicating better extraction ability for BPA than MNIPs. Combinedwith HPLC-UV analysis, the developed method achieved desirablerecoveries (97.23e99.21%) and a low LOD (3.7 mg L�1).

Lu et al. [75] designed magnetic dummy MIPs (MDMIPs) asMSPE adsorbents via surface imprinting with bis(10-methoxy-oxodecyl) ester as a dummy template and methacrylic acid as afunctional monomer, for the enrichment of ten PAEs in food


samples. This MDMIPs adsorbent provided a high Ms of 53.14 emug�1 for rapid magnetic separation, fast adsorption kinetics, and alarger saturated adsorption capacity (162.0 mg g�1) than the cor-responding MNIPs (54.8 mg g�1) towards the target PAEs. Theproposed MSPE-GC-MS/MS method was quite efficient for theseparation and determination of trace-level PAEs, with the LODsand the recoveries in the range of 0.15e1.64 ng g�1 and 73.7e98.1%,respectively.

4.2. Aptamer-based composites

Aptamers are artificial, specific, single-stranded nucleotide se-quences of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA),selected by the systematic evolution of ligands by exponentialenrichment (SELEX) in vitro, with a high affinity for binding withthe given target molecules [76]. In comparison with antibodies,aptamers can be regarded as artificial antibodies that provide manyadvantages such as high chemical stability (including at hightemperature with a certain range of pH resistance), low suscepti-bility to degradation, low cross reactivity, and prolonged shelf life.No animal experiments need to be used for their production andthey are amenable to further modification, especially for theidentification of small target molecules [77,78]. Hence, aptamerscan be regarded as ideal oligo-sorbents with high affinity andspecificity; the equilibrium dissociation constants (Kd) values are inthe picomolar range, which overcomes the deficiency of antibodiesfor various analytical purposes [79,80]. Aptamers are small andshort oligonucleotides with a complex 3-D folding structure. Due totheir specific structural properties, the recognition interactionsbetween a target molecule and its conjugated aptamer essentiallyoriginate from complementarity of the molecular structures, aro-matic ring stacking, hydrogen bonding, electrostatic interactions,and van derWaals forces [80e82]. The resulting unique recognitionabilities of aptamers for their targets tend to be stable and specific,making aptamers potentially applicable in various fields of complexsample analysis. Aptamers have aroused interest for the develop-ment of aptamer-based assays, which means that aptamers can beimmobilized on a solid support to form a high-affinity extractant,termed an oligo-sorbent, according to the results of current studies.

Su et al. [83] designed a novel magnetic oligo-sorbent byimmobilizing a BPA-specific single-stranded DNA (ssDNA) aptamer(50-biotin-CCG CCG TTG GTG TGG TGG GCC TAG GGC CGG CGG CGCACA GCT GTT ATA GAC GTC TCC AGC-30) on the surface ofFe3O4@SiO2 nanoparticles through biotin-avidin interactions forthe determination of BPA from biological samples. The resultantApt-MNPs offered excellent magnetic susceptibility, high adsorp-tion capacity (78.4 ng mg�1), and the ability to selectively identifyBPA, showing promising potential as an adsorbent in MSPE. Thedeveloped Apt-MNP adsorbent coupled with HPLC-FLD providedlow LODs as well as good recoveries (2.0 ng mL�1 and 1.0 ng mL�1,90.8% and 92.3% in serum and urine samples, respectively) forquantifying low levels of BPA in complicated bodily fluid matrices.

Jiang and co-workers prepared aptamer-modified magneticCOFs for trace-enriching one type of the main metabolites of PCBs(hydroxylated PCBs) in human serum before high-performanceliquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis [84]. The obtained core-shell structural material in-tegrated the benefits of superparamagnetism (the Ms of 39.5 emug�1) from the MNPs, large surface area and porous structure fromthe COFs, and high specific adsorption from the aptamer in thecapture of hydroxylated PCB (2-OH-CB 124). By means of covalentinteractions, the synthesized aptamer modified with amino groupswas immobilized on the surface of magnetic COFs containing car-boxylic groups. Compared to Fe3O4 and Fe3O4@COFs-COOH, the as-prepared Fe3O4@COFs-Apt presented superior selective extraction

ability for 2-OH-CB 124 from human serum samples with satis-factory recoveries (87.7e101.5%) and low LODs (2.1 pg mL �1).

In brief, due to the high affinity and selectivity of aptamers,further research efforts are warranted to develop novel aptamer-based magnetic multifunctional materials with suitable detectiontechniques, making it possible to selectively extract various envi-ronmental pollutants from complex sample matrices with goodspecificity and sensitivity, as well as eliminating interferingsubstances.

5. Other magnetic composites as MSPE adsorbents

In addition to these above-mentioned magnetic composites,MNPs can also be combined with other functional materials such assilicon nanomaterials, metallic nanomaterials, ILs, chitosan, andsurfactants to fabricate MSPE adsorbents. Table 1.4 summarizesother magnetic composites applied in MSPE for the separation andpreconcentration of environmental pollutants over the last fiveyears.

Silicon nanomaterials are now extensively used in MSPE due totheir unique characteristics such as low cost, biocompatibility,mechanical stability, and easy surface modification, and act as aprotective outer layer for a magnetic core. On this basis, function-alized magnetic silicon nanomaterials can offer more efficientadsorption properties in diverse sample matrices. Our group syn-thesized a novel magnetic silica composite that was surface-modified with amino groups and an octyl-perfluorinated chain(Fe3O4@SiO2eNH2&F13) in a one-step reaction through a sol-gelprocedure (Fig. 7A) [85], and further utilized the composite forthe selective extraction of PFCs from surface water samples, fol-lowed by ultra-high performance liquid chromatography-tandemmass spectrometry (UPLC-MS/MS) analysis (Fig. 7B) [86]. In theMSPE process, 50 mg of the adsorbent was dispersed in 500 mL ofaqueous sample, and the adsorbent-analyte composites couldreach adsorption equilibrium within 30 min. The electrostatic at-tractions and fluorine-fluorine (FeF) interactions with the addi-tional size exclusion effect contributed to high adsorptionefficiencies and fast adsorption kinetics for PFCs. Low LODs of0.029e0.099 ng L�1 and satisfactory recoveries of 90.65e106.67%were obtained. Therefore, the Fe3O4@SiO2eNH2&F13 nano-composite can be widely applied as an effective adsorbent, espe-cially for aqueous solutions with large volume, to evaluate theconcentration levels of typical PFCs in environmental watersystems.

Metallic nanomaterials include a range of inorganic or inorganichybrid nanoparticles; the commonly reported nanometer-sizedmetallic particles include Au, Ag, Cu, Al2O3, ZnO, MnO2, TiO2, andZrO2 [87], etc. On account of their specific characteristics such aslarge surface area, good chemical stability and low-temperaturemodification, metallic nanomaterials have gained interest forsample pretreatment in MSPE [15,88]. Li et al. prepared core-shellFe3O4@Au nanoparticles functionalized by thiol-containing li-gands via self-assembly as a mode switching MSPE adsorbent(normal-phase or reverse-phase SPE mode) for the extraction ofPAHs and diphenols in environmental water samples [89]. In thisstudy, a nanomagnetic Fe3O4 core was used as a carrier due to itsexcellent properties for magnetic separation and short diffusionroute, while the modified gold shells with exchangeable self-assembled monolayers wrapped around the core acted as a uni-versal interaction platform for bonding with polar and non-polartarget compounds via a dynamic thiol exchange process. Thisadsorbent could switch fromnormal-phase SPEmode for diphenolsto reverse-phase SPE mode for PAHs. Combined with the HPLC-UVmethod, the low LODs of diphenols and PAHs ranged from 0.34 to16.67 mg L�1 and 0.26e0.52 mg L�1, respectively, and the proposed

Fig. 7. A) The schematic procedure of Fe3O4@SiO2eNH2&F13 synthesis and adsorption mechanism for PFCs. Reprinted with permission from Ref. [85], Copyright (2016) Elsevier. B)The developed MSPE-UPLC-MS/MS method. Reprinted with permission from Ref. [86], Copyright (2017) Springer Nature.


method also showed wide linear ranges and high precision; thus, asensitive and effective mode-switching MSPE-HPLC-UV methodwas successfully established.

ILs and their corresponding polymers, poly(ionic liquid)s (PILs),were used as functional monomers in MSPE sorbents to capturePAHs, OPPs, triazine herbicides, PAEs, and chlorophenols (CPs) fromvarious samples. He et al. [90] successfully prepared PIL-functionalized magnetic materials with controllable degrees ofpolymerization through a facile layer-by-layer assembly technique

based on the alternate deposition of oppositely charged poly(1-vinyl-3-hexylimidazolium) bromide (P[VHim]Br) and poly-styrenesulfonate sodium (PSS) on silica-coated MNPs. Increasingthe number of bilayers led to more abundant adsorption sites butdecreased the magnetism of the adsorbents. The highest extractionefficiency for four pesticides, higher saturated magnetization, and amore homogeneous shape were achieved when the number ofassembled bilayers was optimized at ten in the resultant Fe3O4@-SiO2@(PSS-PIL)10.


Surfactants are amphiphilic substances that are composed ofhydrophobic/water-hating and lipophilic/oil-liking functionalgroups [91]. The coating of surfactants onto MNP surfaces gives riseto the formation of hemimicelles and admicelles, depending on thesurfactant concentration [92]. Huda M. and co-workers [93]extracted lead ions from aqueous solutions using an emulsionliquid membrane, which contained bis-2-ethylhexyl phosphoricacid as a carrier, H2SO4 as an internal phase, kerosene as organicphase, and Span-80 and Fe2O3 particles as surfactant and co-stabilizer, respectively. Under optimal conditions, the extractionefficiency of Pb(II) increased to 97.2% after 8.0 min of mixing timewith 0.3% membrane breakage, which indicates that the co-stabilization strategy between the surfactant and co-stabilizerwas a useful method to increase the membrane stability. Further-more, the mechanism of this co-stabilization strategy wasexplained as being the result of the electrostatic interactions be-tween Span-80 and Fe2O3 particles modifying the surface of thelatter, which resulted in various contact angles at the oil/waterinterface for stabilization.

Chitosan is a natural polymer material with a large number ofhydroxyl and free amino groups on its surface, which impart chi-tosan unique merits, such as biocompatibility and biodegradability,and allow for easy modification. The introduction of chitosan cannot only support the growth of MNPs but also inhibit the aggre-gation of MNPs. In Tolessa's study [94], magnetic chitosan micro-spheres with an average size of 2 mm were fabricated through asuspension cross-linking technique for use as an adsorbent toextract Ag nanoparticles in the presence of Ag(I). Briefly, MNPswere suspended in a 1% chitosan solution followed by vigorousshaking and ultrasonication for 15 min. Then, the mixture wasadded into a toluene solution containing Span-80, which was usedas an emulsifier. After stirring for 30 min at 500 rpm, glutaralde-hyde and an NaOH solution were added subsequently. Finally, themagnetic chitosan microspheres were separated using an externalmagnetic field. The positively charged chitosan in the resultantadsorbent improved the extraction efficiency of Ag nanoparticles(84.9e98.8%), which were negatively charged due to the coating ofnatural organic matter onto their surface. During three reuse cycles,the extraction efficiency remained approximately constant(77.2 ± 2.2%) with only a slight change. When combined withinductively coupled plasma-mass spectrometry (ICP-MS) analysis,low LODs for three Ag nanoparticles with different coatings andsizes were obtained in the range of 0.016e0.023 mg/L.

6. Conclusions and future perspectives

This review summarizes the recent developments of multi-functional magnetic composites as MSPE adsorbents for theextraction of environmental pollutants from the year 2014 to thepresent date. The main environmental pollutants mentioned in thiswork include PAHs, PCBs, PAEs, PFCs, BPA, pesticides, drug residues,and heavy metal ions found in environmental, biological, and foodmatrices. Based on the diverse physicochemical characteristics,structural morphologies, and sub-classifications of target analytes,nanostructured materials can be accurately designed for theiridentification with satisfactory extraction performance. Comparedwith conventional SPE adsorbents, the integration of multifunc-tional nanomaterials and MNPs can afford superior advantagessuch as large surface area, more adsorption sites, highly stablestructural forms, and rapid separation from the sample matrix. Onthis basis, the combination of these promising MSPEmaterials withmatching detection techniques has been extensively utilized forqualitative and quantitative analysis of trace environmental pol-lutants, despite the interference from various analogues and com-plex sample matrices.

In the future, in order to expand the application of these ver-satile nanomaterials, efforts should be directed towards over-coming the limitations of these known and widely used materialsas MSPE adsorbents for the entire procedure from synthesis topractical application, making it possible to solve identified prob-lems such as overcoming heterogeneous shapes and poor physi-cochemical stability and achieving reusability and dispersibilityunder rigorous external environmental conditions. The synthesisstrategies can be further simplified and optimized to use fewerharmful chemical reagents based on the theories of traditionalmethods. Moreover, researchers can explore new andenvironment-friendly nanomaterials to avoid the potential harmand contamination from certain nanomaterials and conform to thestandards of green chemistry. Furthermore, studies focusing onnew application modes of MSPE coupled with analytical detectiontechniques should be performed with the objective of achievingautomation, miniaturization, and high-throughput sample analysisso as to obtain portable, fast, and satisfactory application.Addressing these issues will lead to remarkable progress in multi-functional magnetic composites for sample pretreatment.

Competing financial interests

The authors declare no competing financial interests.

Acknowledgements

This research was funded by the National Key R&D Program ofChina (2017YFC0212003) and the National Natural Science Foun-dation of China (No. 21577043).

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