Extraction of environmental pollutants using magnetic nanomaterials

Abstract

Magnetic nanomaterials are particularly useful for extracting and enriching a large volume of target analytes because they provide high surface-to-volume ratio, easy surface modification, and strong magnetism. This review summarizes the recent advance in synthesis, modification, and application of magnetic nanomaterials for the preconcentration of environmental pollutants. The extraction of environmental pollutants with magnetic nanomaterials is generally based on hydrophobic interaction, electrostatic attraction, and/or covalent bonding formation. Environmental pollutants adsorbed on the surface of nanomaterials are released by exchanging ligands, adjusting pH, or adding organic solvent. The liberated environmental pollutants are commonly detected by gas chromatography, high-performance liquid chromatography, and inductively coupled plasma-optical emission spectrometry. The basic principle of each nanomaterial and its advantages for the extraction of environmental pollutants are discussed in this review.

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1. Introduction

Nanomaterials have novel optical, electrical, and magnetic properties that make them attractive in diverse fields, such as biosensors,¹ separation science,² nanomedicine,³ and cell imaging.⁴ Moreover, nanomaterials act as adsorbents for enriching target analytes from a complex matrix. Compared to liquid–liquid extraction⁵ and solid phase microextraction,⁶ nanomaterials not only exhibit high surface area-to-volume ratios, but also are easily chemically functionalized. Thus, recently, numerous nanomaterials modified different types of surface ligands, such as multi-walled carbon nanotubes,^7–11 Au nanoparticles (NPs),^12–14 TiO₂ NPs,^15,16 and Fe₃O₄ nanoparticles (NPs)^17–42 have been developed for the extraction and enrichment of environmental pollutants. The binding types of nanomaterials with environmental pollutants include electrostatic interaction, covalent bonding formation, and hydrophobic adsorption. In contrast to other nanomaterials, Fe₃O₄ NPs possess strong magnetism and thereby are readily isolated from sample solutions by an external magnetic field without additional centrifugation or filtration.

This paper reviews the development of magnetic nanomaterials utilized for the extraction of environmental pollutants, including heavy metal ions, polycyclic aromatic hydrocarbons (PAH), phenolic compounds, pesticides, phthalates, drugs, and others. We also discuss the effects of surface ligand, nanomaterial concentration, eluent concentration, and eluent type on the extraction efficiency. Table 1 summarizes the extraction of environmental pollutants using different types of magnetic nanomaterials.

Table 1 Comparison of different types of magnetic nanomaterials

Material^a	Additive^b	Eluent^c	Analyte/sample volume^d	Detection^e	LOD	Reference
a MPTMS, 3-mercaptopropyltrimethoxysilane; CTAB, cetyltrimethylammonium bromide; CPC, cetylpyridinium chloride; BA, barium alginate; β-CD, β-cyclodextrin, ; C16minBr, 1-hexadecyl-3-methylimidazolium bromide; OTMABr, octadecyltrimethylammonium bromide; SDS, sodium dodecyl sulfate. b PAN, 1-(2-pyridilazo)-2-naphthol. c THF, tetrahydrofuran; ACN, acetonitrile. d PFC, Perfluorinated compounds. e FL, fluorescence; CE-ECL, capillary electrophoresis–electrochemiluminescence.
MPTMS-capped Fe3O4@SiO2 NPs	No	HCl, thiourea	Metal ions/50 mL	ICP-MS	24–107 pg/L	17
MPTMS-capped Fe3O4 NPs	No	HCl, K2Cr2O7	Tl^4+/160 mL	ICP-MS	0.079 ng L^−1	18
Bismuthiol-capped Fe3O4@SiO2 NPs	No	HNO3	Metal ions/100 mL	ICP-OES	43–85 pg/mL	19
Decanoic acid-capped Fe3O4 NPs	PAN	HCl	Metal ions/50 mL	ICP-OES	0.2–0.8 μg L^−1	20
CTAB-coated Fe3O4 NPs	CTAB	ACN	Phenolic compounds/800 mL	HPLC-FL	12–34 ng L^−1	21
CTAB-coated Fe3O4 NPs	CTAB	CH3OH	Phenolic compounds/700 mL	HPLC-UV	0.11–0.15 μg L^−1	22
CPC-coated Fe3O4 NPs	CPC	ACN	Phenolic compounds/800 mL	HPLC-FL	7–20 ng L^−1	23
β-CD-capped Fe3O4@SiO2 NPs	No	CH3OH	Bisphenol A/250 mL	HPLC-UV	20 ng L^−1	25
Magnetic MIP	No	CH3OH	Bisphenol A/200 mL	HPLC-UV	14 ng L^−1	26
C18-capped Fe3O4 microsphere	No	n-hexane	PAH/20 mL	GC-MS	0.8–36 μg L^−1	27
Tetradecanoate-coated Fe3O4 NPs	No	THF, ACN	PAH/350 mL	HPLC-FL	0.1–0.25 ng L^−1	28
Hydrophobic Fe3O4 NPs	1-octanol	ACN	PAH/20 mL	GC-MS	11.7–61.4 ng L^−1	29
BA caged Fe3O4@C18 NPs	No	ACN	PAH/500 mL	HPLC-FL	3–5 ng L^−1	30
C16minBr-coated Fe3O4 NPs	C16minBr	ACN	PAH/300 mL	HPLC-FL	0.33–8.33 ng L^−1	31
Decanoic acid-capped Fe3O4 NPs	No	HCl	Pesticides/400 mL	HPLC-MS	0.01–0.03 μg L^−1	32
Al2O3-coated Fe3O4 NPs	No	Na4P2O7	Pesticides/5 mL	CE-ECL	0.3, 30 ng mL^−1	34
Chitosan-coated Fe3O4@C18 NPs	No	ACN	Phthalate esters/500 mL	HPLC-UV	12.3–36.4 ng L^−1	35
Alginate–polymer-caged Fe3O4-titanate@C18 nanotubes	No	ACN	Phthalate esters/500 mL	HPLC-FL	11–46 ng L^−1	36
OTMABr-coated Fe3O4 NPs	OTMABr	CH3OH	Sulfonamides/500 mL	HPLC-UV	24–33 ng L^−1	37
SDS-coated Fe3O4@ Al2O3 NPs	SDS	CH3OH	Timethoprim/500 mL	HPLC-UV	0.09 μg L^−1	38
Fe3O4@ Al2O3 NPs	No	CH3COOH	Sulfonamides/2 g	LC-MS/MS	0.37–6.74 ng g^−1	39
Magnetic MIP	No	CH3OH	Tetracycline antibiotics/2 g	LC-MS/MS	0.2 ng g^−1	40
Chitosan-coated Fe3O4@C18 NPs	No	CH3OH, NH3	PFC/500 mL	LC-MS/MS	0.075–0.24 ng L^−1	41
Bare Fe3O4 NPs	NH4SCN	NaOH	Fluoride/250 mL	Absorption	0.015 μg mL^−1	42

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2. Extraction of environmental pollutants

2.1 Heavy metal ions

Monitoring the level of heavy metal ions in an aquatic ecosystem has received significant attention over many years because they are severe environmental pollutants and have adverse effects on human health. For example, Pb²⁺ is a potential neurotoxin that can cause chronic inflammation of the kidney and heart, inhibit brain development, and decrease nerve conduction velocity. Moreover, highly toxic Hg²⁺ can damage the brain, heart, kidney, stomach, and intestines. The maximum levels of Hg, Cd, As, Pb, Cr, and Cu ions in drinking water permitted by the United States Environmental Protection Agency (U.S. EPA) are 2, 5, 10, 15, 100, and 1300 μg L⁻¹, respectively. The current methods for the analysis of heavy metal ions are based on the use of sensitive element-specific detectors, such as flame atomic absorption spectrometry (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), and ICP-optical emission spectrometry (ICP-OES). Huang and Hu synthesized silica-coated Fe₃O₄ NPs and modified them with 3-mercaptopropyltrimethoxysilane (MPTMS).¹⁷ These kind of NPs contain thiol groups so that selective extraction of Cd²⁺, Cu²⁺, Hg²⁺, and Pb²⁺ is achieved through the formation of the metal ion–thiol complex. The adsorbed metal ions on the NP surface were liberated upon the addition of a solution containing 1 M HCl and 2% thiourea. ICP-MS was used to analyze the extracted metal ions. Under the optimal extraction conditions (pH, sample volume, and eluent), the limits of detection (LODs) for Cd²⁺, Cu²⁺, Hg²⁺, and Pb²⁺ were as low as 24, 92, 107, and 56 pg/L. The same group reported that MPTMS-modified Fe₃O₄ NPs were utilized as effective sorbent materials for extracting Te⁴⁺ in a wide pH range, but not for Te⁶⁺.¹⁸ In other words, this kind of NP is capable of separating a mixture of Te⁴⁺ and Te⁶⁺. The adsorbed Te⁴⁺ on the NP surface is released by adding a mixture of 2 M HCl and 0.03 M K₂Cr₂O₇. The LOD of Te⁴⁺ was determined at down to 0.079 ng L⁻¹ by combining NP-based extraction and ICP-MS. Along a similar line, Suleiman et al. modified silica-coated Fe₃O₄ NPs with bismuthiol for extracting Cr³⁺, Cu²⁺, and Pb²⁺ from environmental samples and determined the extracted metal ions by ICP-OES.¹⁹ Bismuthiol containing thiol and amine groups possesses strong affinity with Cr³⁺, Cu²⁺, and Pb²⁺ in the pH range of 3–8. The extraction of 100 mL sample volume with bismuthiol-coated magnetic NPs resulted in approximately 100-fold improvement in sensitivity. Without the silica coating, decanoic acid-capped Fe₃O₄ NPs can be directly employed for solid phase extraction of Cd²⁺, Co²⁺, Cr³⁺, Ni²⁺, Pb²⁺, and Zn²⁺ from water samples after these metal ions formed stable complexes with amine and 1-(2-pyridilazo)-2-naphthol.²⁰ The adsorbed metal ions were detached from the NP surface using a solution containing HCl and propanol. The released metal ions were determined using flow injection coupled to ICP-OES.

2.2 Phenolic compounds

Phenolic compounds are commonly used in several industrial processes to manufacture chemicals such as pesticides, explosives, drugs, and dyes. Chlorinated phenols originate from chlorine treatment of drinking water. Photochemical reaction induces the formation of nitrophenols in the presence of vehicle exhausts. Because phenolic compounds are potentially carcinogenic, their maximum permitted level in drinking water by the U.S. EPA is 0.5 μg L⁻¹. HPLC combined with UV, fluorescence, electrochemical, or MS detection has been developed for the determination of phenolic compounds. A preconcentration step is prerequisite for the analysis of phenolic compounds because their concentration is quite low in environmental water samples. Zhao et al. prepared cetyltrimethylammonium bromide (CTAB)-coated Fe₃O₄ NPs for the preconcentration of bisphenol A, 4-tert-octylphenol, and 4-n-nonylphenol in the presence of CTAB.²¹ CTAB molecules were self-assembled on the surface of Fe₃O₄ NPs through electrostatic interaction between the head group of CTAB and the oppositely charged group on the Fe₃O₄ NPs. When more CTAB molecules were present in the solution, the hydrophobic interaction between the hydrophobic tails of CTAB led to the formation of admicelles. Under this condition, the mixed hemimicelles consisting of hydrophobic hemimicelles and ionic admicelles were formed on the surface of Fe₃O₄ NPs. Thus, the adsorption of analytes could be driven by both hydrophobic interaction and electrostatic attraction. In the mixed hemimicelle region, the extraction efficiency remarkably increased with an increase in CTAB concentration. Fig. 1 illustrates the whole procedure for the extraction of phenolic compounds with CTAB-coated Fe₃O₄ NPs. A 800-fold improvement in sensitivity was achieved by extracting 800 mL of water samples with CATB-coated Fe₃O₄ NPs. These kind of NPs were also applied to the extraction of 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichorophenol, and pentachlorophenol in the presence of CTAB.²² The extraction of 700 mL of water sample with CTAB-coated Fe₃O₄ NPs resulted in 700-fold improvement in sensitivity. The LODs of four chlorophenols ranged from 0.11 and 0.15 μg L⁻¹. Zhao et al. found that the replacement of CTAB by cetylpyridium chloride in Fe₃O₄ NPs resulted in lower elution volume and more effective adsorption, thereby providing better sensitivity improvement.²³ Zhao et al. also reported the synthesis of 11-mercaptoundecanoic acid (MUA) and 11-dodecanethiol (DDT)-functionalized core/shell Fe₃O₄/Au NPs for effective adsorption of bisphenol A, 4-tert-octylphenol, and 4-n-nonylphenol.²⁴ DDT-modified Fe₃O₄/Au NPs exhibited strong hydrophobic interaction with phenolic compounds and their extraction efficiency was insensitive to pH change. By contrast, the adsorption capacity of 11-MUA-modified Fe₃O₄/Au NPs remarkably decreased above pH 10.0 because of repulsive electrostatic interaction between the negatively charged 11-MUA (pK_a = 4.8) and phenolic compounds (the pK_a of three phenolic compounds were all below 10.0). Ji et al. synthesized β-cyclodextrin-modified core/shell Fe₃O₄/SiO₂ NPs for selective and rapid enrichment of bisphenol A.²⁵ The extracted bisphenol A was eluted with methanol and then determined by HPLC with UV detection. The LOD was down to 20 ng L⁻¹ for bisphenol A after the extraction of 250 mL sample volume. The extraction procedure was completed within 25 min. Ji et al. prepared magnetic molecularly imprinted polymer (MIP) using bisphenol A as a template, vinylpyridine as a functional monomer, ethylene glycol dimethacrylate as a crosslinker, and methacryloxypropyltrimethoxysilane-modified Fe₃O₄ NP as a support.²⁶ Based on molecular recognition, selective extraction of bisphenol in environmental water and milk samples was accomplished using magnetic MIP prior to the analysis by HPLC with UV detection. The LODs of bisphenol A were found to be 14 ng L⁻¹ and 160 ng L⁻¹ in water and milk, respectively.

Fig. 1 An illustration of the steps involved in extracting phenolic compounds from environmental water by using CTAB/nanomagnets Fe₃O₄ sorbents. (From ref. 21, with kind permission of American Chemical Society).

2.3 PAHs

PAH is a complex class of condensed multiring benzenoid compounds originating from countless natural process and human activities. The U.S. EPA lists the following 16 as “Consent Decree” priority pollutants: benz[a]anthracene, pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, naphthalene, fluorene, benzo[a]pyrene, dibenzo[a,h]anthracene, indeno[1,2,3-cd]pyrene, acenaphthylene, acenaphthene, phenanthrene, anthracene, fluoranthene, chrysene, and benzo[g,h,i]perylene. The methods currently available for the separation of PAH include gas chromatography (GC) and high performance liquid chromatography (HPLC). The extraction of PAH is an important step to reduce the matrix effect and enahnce their sensitivity. Liu et al. described the use of C₁₈-functionalized Fe₃O₄ microspheres for extracting 16 PAH from tap water through hydrophobic interaction.²⁷ The microspheres possess a high magnetic saturation and thus are easily manipulated by an external magnetic field. By the combination of Fe₃O₄ microsphere-based extraction and GC-MS, the LODs of 16 PAH were reached in the range from 0.8 to 36 μg L⁻¹. Ballesteros–Gómez and Rubio compared different types of alkyl carboxylate-coated magnetic NPs to enrich PAH from surface water. These extracted PAHs were analyzed by HPLC with fluorescence detection.²⁸ Tetradecanoate-coated magnetic NPs were selected because of their better extraction recovery and stability. The limits of quantification (LOQs) for 7 PAH ranged from 0.2 to 0.5 ng L⁻¹, which are below the maximum permissible limits (3.8 ng L⁻¹) of PAH in surface and ground water permitted by the U.S. EPA. Shi and Lee recently reported a two-step extraction approach based on the integration of dispersive liquid–liquid microextraction and microsolid-phase extraction (Fig. 2).²⁹ 1-Octanol, which is immiscible with water, was employed for the extraction and enrichment of PAH from an aqueous solution. After liquid–liquid extraction, magnetic NPs retrieved 1-octanol from an aqueous solution via hydrophobic interaction. A small amount of acetonitrile was introduced to release 1-octanol and PAH from the NP surface. The obtained supernatant was directly detected by the fast GC-MS. This method provided the LOQs ranging from 0.04 to 0.21 ng L⁻¹. Zhang et al. modified octadecyl-functionalized Fe₃O₄ NPs with hydrophilic alginate polymers to improve their hydrophilicity and dispersibility in an aqueous solution.³⁰ The formed NPs were applied to the extraction and enrichment of PAH from a large volume of environmental water samples prior to the analysis by HPLC with fluorescence detection. Selective extraction of PAH was successfully achieved using this kind of NPs because the alginate polymers can block the access of humic acid and colloid through electrostatic repulsion and size exclusion. Zhang et al. recently used ionic liquid-coated Fe₃O₄ NPs for the extraction and preconcentration of PAH in water and soil samples, followed by HPLC-fluorescence analysis.³¹ Compared to 1-decyl-3-methylimidazolium bromide and CTAB-coated Fe₃O₄ NPs, 1-hexadecyl-3-methylimidazolium bromide (C₁₆minBr)-coated Fe₃O₄ NPs provided better extraction efficiency in the mixed hemimicelles region. When large volume (300 mL) of water sample was extracted with a small amount of C₁₆minBr (50 mg) and Fe₃O₄ NPs (80 mg) at pH 10.0, the LODs for pyrene, benzo[a]anthracene, and benzo[a]pyrene (BaP) were found to be 8.33, 1.67, and 0.33 ng L⁻¹, respectively.

Fig. 2 The combination of dispersive liquid–liquid microextraction (DLLME) and microsolid-phase extraction (D-μ-SPE) extraction for the enrichment of PAH. (From ref. 29, with kind permission of American Chemical Society).

2.4 Pesticides

Modern agricultural production in many countries relies heavily on the use of pesticides, and thus they are present in soil, ground and surface water, and food. Some pesticides can cause adverse effects to human health. According to the European Union Directive on Water Quality, the maximum permissible concentration of each and total pesticides in drinking water are 0.1 and 0.5 μg L⁻¹, respectively. Song et al. prepared superparamagnetic Fe₃O₄ NPs for extracting triazine pesticides from lake water.³² When Fe₃O₄ NPs were added to 400 mL sample, the capture of triazine pesticides was completed within 10 min through electrostatic attraction. After extraction and collection, 10 M HCl was added to dissolve Fe₃O₄ NPs. The obtained solution was neutralized with NaOH and then detected by HPLC-MS. The combination of Fe₃O₄ NPs and HPLC-MS provided low LOD (0.01–0.03 ng mL⁻¹) and large linear range (0.03–50 ng mL⁻¹). Shen et al. described the use of C₁₈-functionalized Fe₃O₄ NPs for cleanup and enrichment of 14 kinds of organophosphorous pesticides.³³ The extracted organophosphorous pesticides were determined by GC with a nitrogen/phosphorus detector. Hsu and Wang have recently reported the synthesis alumina-coated Fe₃O₄ NPs for selectively capturing glyphosate and its major metabolite aminomethylphosphonic acid (AMPA).³⁴ Because Al₂O₃-coated Fe₃O₄ NPs are capable of selectively binding phosphate-containing compounds, the extraction of 5 mL of glyphosate and AMPA was efficient (within 5 min) and selective. The adsorbed glyphosate and AMPA on the NP surface were liberated by ligand exchange between their phosphate groups and Na₄P₂O₇. These released analytes were then determined by capillary electrophoresis with electrochemiluminescence detection. Improvements in sensitivity of 460- and 64-fold were reported for glyphosate and AMPA. The LODs of glyphosate and AMPA were 0.3 and 30 ng mL⁻¹, respectively.

2.5 Phthalates

Phthalates are extensively used as polymer additives in many consumer products, such as plastics and cosmetics, resulting in the ubiquitous presence of these chemicals in the environment. They can migrate into food, water, soil, marine ecosystem, human urine, and human milk. Some phthalates can cause endocrine system disorders, have carcinogenic effects, and exhibit adverse reproductive effects. Current approaches to detecting phthalates include HPLC coupled with UV and fluorescence detection. Zhang et al. synthesized C₁₈-functionalized Fe₃O₄ NPs, followed by coating a layer of chitosan–tripolyphosphate polymer on the surface.³⁵ Hydrophobic C₁₈ ligands enabled Fe₃O₄ NPs to interact with phthalate ester, while the porosity and basket-like structure of chitosan–tripolyphosphate polymer excluded macromolecules, such as humic acid. When 500 mL of water samples was concentrated to 0.5 mL, the LODs were found to be 12.3, 18.7, 36.4, and 15.6 ng L⁻¹ for di-n-propyl phthalate, di-n-butyl phthalate, dicyclohexyl phthalate, and di-n-octylphthalate, respectively. By extracting the analytes from 500 mL of water samples, the LODs and recoveries for seven perfluorinated compounds ranged from 0.075 to 0.24 ng L⁻¹ and from 63% to 109%, respectively. Niu et al. prepared the composites of C₁₈-functionalized Fe₃O₄ NPs and titanate nanotubes, and coated them with alginate polymer.³⁶ Because the function of the as-prepared composites was similar to that of chitosan–tripolyphosphate/C₁₈ coated-capped Fe₃O₄ NPs, they are capable of preconcentrating phthalates from river water. When the incubation time, adsorbent amount, eluent volume, and sample volume were optimized to be 40 min, 80 mg, 6 mL, and 500 mL, the LODs of phthalates ranging from 11–46 ng L⁻¹ was achieved.

2.6 Drugs

Because antibiotics, such as sulfonamides, tetracyclines, quinolones, macrolides, and trimethoprim, are frequently used as feed additives to promote animal growth and as therapeutic agents to cure human/animal diseases, they are considered ubiquitous environmental contaminants. HPLC is one of the methods most used for the analysis of antibiotics when it is coupled with UV, fluorescence, and MS. Sun et al. showed that cationic octadecyltrimethylammonium bromide (OTMABr) formed mixed hemimicelles on the NP surface enabled Fe₃O₄ NPs to extract four sulfonamides.³⁷ Through the hydrophobic interaction between mixed hemimicelles and analytes, the adsorption of sulfonamides onto OTMABr-coated Fe₃O₄ NPs gradually increased with increasing OTMABr concentration in the mixed hemimicelle region. When 10 mL of 10 mg mL⁻¹ Fe₃O₄ NPs and 100 mg OTMABr were added to 500 mL of water samples at pH 9.0, the LODs of sulfonamides detected were at levels down to 0.024–0.033 μg L⁻¹. Based on this concept, Sun et al. used sodium dodecyl sulfate (SDS)-coated core/shell Fe₃O₄/Al₂O₃ NPs for extracting trimethioprim through hydrophobic and electrostatic interaction prior to HPLC-UV detection.³⁸ The coating of alumina was to avoid Fe₃O₄ NPs dissolving in acidic solution. The extraction efficiency was highly dependent on SDS concentration, solution pH, extraction volume, and desorption solvent. After 500 mL of trimethioprim was extracted with 0.1 g Fe₃O₄/Al₂O₃ NPs and 120 mg SDS (pH 2.0), the LOD and linear range for trimethioprim was found to be 0.09 μg L⁻¹ and 0.24–30 μg mL⁻¹. Additionally, in the absence of surfactant, core/shell Fe₃O₄/Al₂O₃ NPs provided higher adsorption capacity for sulfonamides when ultrasonic-assisted extraction was applied instead of magnetic stirring extraction.³⁹ The adsorption of sulfonamides on the NP surface was driven by hydrophobic interaction. Sulfonamides desorbed from the NP surface were determined by HPLC-MS. When extraction time, adsorbent amount, and eluent volume were optimized to be 8 min, 100 mg, and 2 mL (0.5% acetic acid), the LODs of sulfonamides were as low as 0.37–6.74 ng g⁻¹. The analysis of sulfonamides in different soil was accomplished using this kind of NP in combination with HPLC-MS. Chen et al. used MIP for selective preconcentration of oxytetracycline and its analogues from egg and tissue samples, followed by HPLC-MS/MS analysis.⁴⁰ Magnetic MIP was synthesized using hydrophobic Fe₃O₄ NP as a magnetically susceptible component, oxytetracycline as a template molecule, methacrylic acid as a functional monomer, as well as styrene and divinylbenzene as polymer matrix components. Compared to other MIP-based methods for the extraction of tetracycline and oxytetracycline, magnetic MIP provides distinct advantages, including short extraction time, less solvent consumption, good reusability, and good extraction efficiency. The LODs of oxytetracycline and its analogues in egg and tissue were in the range of 0.06–0.16 ng g⁻¹.

2.7 Other environmental pollutants

In addition to the above-mentioned environmental pollutants, perfluorinated compounds and fluoride ions are also hazardous to human health and environment. Zhang et al. prepared C₁₈-functionalized Fe₃O₄ NPs and coated them with a layer of hydrophilic chitosan–tripolyphosphate polymer through ionotropic gelation. The formed chitosan-coated C₁₈-functionalized Fe₃O₄ NPs were employed to concentrate anionic perfluorinated compounds through hydrophobic interaction with C₁₈ ligands and electrostatic attraction with cationic chitosan.⁴¹ The coating of chitosan–tripolyphosphate polymer improved the dispersion of C₁₈-functionalized Fe₃O₄ NPs and suppressed the adsorption of macromolecules on the NP surface by size exclusion or electrostatic interaction. Perfluorinated compounds were extracted from 500 mL of several environmental water samples using Fe₃O₄ NPs at pH 3.0. After being eluted from the adsorbent with 12 mL of methanol containing 0.28% ammonia, the analytes were further concentrated to 0.5 mL with a nitrogen flow. Finally, the concentrated analytes were analyzed by HPLC-ESI-MS/MS analysis. Under this extraction condition, the LODs of perfluorinated compounds were markedly improved, ranging between 0.075 and 0.24 ng L⁻¹. Without chromatographic separation, Parham and Rahbar combined Fe₃O₄ NP-based extraction with visible spectrometry for the determination of fluoride ions in various water samples.⁴² Extraction of fluoride ions is based on the formation of FeF₆³⁻ complex on the NP surface. The fluoride ions were released upon the addition of NaOH. The liberated fluoride ions were detected by the replacement reaction between colored Fe–SCN complex and fluoride ions.

Conclusions

The studies mentioned above demonstrate that Fe₃O₄ NPs are an appealing alternative for the extraction and enrichment of environmental pollutants because of their high surface-to-volume ratio, ease of modification, and strong magnetism. However, there are still numerous nanomaterials that are not utilized for the extraction of environmental pollutants. For example, analytes containing thiol (SH) or amino (NH₂) groups are adsorbed spontaneously onto the surface of Au NPs.⁴³ Thus, in the opinion of the authors, the extraction of thiol-containing degradation products of V-type nerve agents could be achieved using a composite of Au NPs and Fe₃O₄ NPs.⁴⁴ Surface-to-volume ratio of nanomaterials increases with reducing particle size, therefore the extraction efficiency of an environmental pollutant may be remarkably improved using magnetic NPs of small size and narrow size distribution.

Acknowledgements

We would like to thank National Science Council (NSC 98-2113-M-110-009-MY3) and National Sun Yat-sen University-Kaohsiung Medical University Joint Research Center for the financial support of this study.

References

1. R. Wilson, Chem. Soc. Rev., 2008, 37, 2028–2045.
2. C. Nilsson, S. Birnbaum and S. Nilsson, J. Chromatogr., A, 2007, 1168, 212–224.
3. E. Boisselier and D. Astruc, Chem. Soc. Rev., 2009, 38, 1759–1782.
4. A. Louie, Chem. Rev., 2010, 110, 3146–3195.
5. C. Mahugo Santana, Z. Sosa Ferrera, M. Esther Torres Padron and J. Juan Santana Rodriguez, Molecules, 2009, 14, 298–320.
6. H. Kataoka, Anal. Bioanal. Chem., 2010, 396, 339–364.
7. D. Du, M. Wang, J. Zhang, J. Cai, H. Tu and A. Zhang, Electrochem. Commun., 2008, 10, 85–89.
8. Y. Song, S. Zhao, P. Tchounwou and Y. M. Liu, J. Chromatogr., A, 2007, 1166, 79–84.
9. J. Muñoz, M. Gallego and M. Valcárcel, Anal. Chem., 2005, 77, 5389–5395.
10. C. Basheer, A. A. Alnedhary, B. S. Rao, S. Valliyaveettil and H. K. Lee, Anal. Chem., 2006, 78, 2853–2858.
11. S. Wang, P. Zhao, G. Min and G. Fang, J. Chromatogr., A, 2007, 1165, 166–171.
12. H. Wang and A. D. Campiglia, Anal. Chem., 2008, 80, 8202–8209.
13. H. Wang, S. Yu and A. D. Campiglia, Anal. Biochem., 2009, 385, 249–256.
14. K. Leopold, M. Foulkes and P. J. Worsfold, Anal. Chem., 2009, 81, 3421–3428.
15. E. Vssileva, K. Hadjiivanov, T. Stoychev and C. Daiev, Analyst, 2000, 125, 693–698.
16. P. Liang, T. Shi, H. Lu, Z. Jiang and B. Hu, Spectrochim. Acta, Part B, 2003, 58, 1709–1714.
17. C. Huang and B. Hu, Spectrochim. Acta, Part B, 2008, 63, 437–444.
18. C. Huang and B. Hu, J. Sep. Sci., 2008, 31, 760–767.
19. J. S. Suleiman, B. Hu, H. Peng and C. Huang, Talanta, 2009, 77, 1579–1583.
20. M. Faraji, Y. Yamini, A. Saleh, M. Rezaee, M. Ghambarian and R. Hassani, Anal. Chim. Acta, 2010, 659, 172–177.
21. X. Zhao, Y. Shi, Y. Cai and S. Mou, Environ. Sci. Technol., 2008, 42, 1201–1206.
22. J. Li, X. Zhao, Y. Shi, Y. Cai, S. Mou and G. Jiang, J. Chromatogr., A, 2008, 1180, 24–31.
23. X. Zhao, Y. Shi, Y. Cai and G. Jiang, J. Chromatogr., A, 2008, 1188, 140–147.
24. X. Zhao, Y. Cai, T. Wang, Y. Shi and G. Jiang, Anal. Chem., 2008, 80, 9091–9096.
25. Y. Ji, X. Liu, M. Guan, C. Zhao, H. Huang, H. Zhang and C. Wang, J. Sep. Sci., 2009, 32, 2139–2145.
26. Y. Ji, J. Yin, Z. Xu, C. Zhao, H. Huang, H. Zhang and C. Wang, Anal. Bioanal. Chem., 2009, 395, 1125–1133.
27. Y. Liu, H. Li and J. M. Lin, Talanta, 2009, 77, 1037–1042.
28. A. Ballesteros-Gomez and S. Rubio, Anal. Chem., 2009, 81, 9012–9020.
29. Z. G. Shi and H. K. Lee, Anal. Chem., 2010, 82, 1540–1545.
30. S. Zhang, H. Niu, Y. Cai and Y. Shi, Anal. Chim. Acta, 2010, 665, 167–175.
31. Q. Zhang, F. Yang, F. Tang, K. Zeng, K. Wu, Q. Cai and S. Yao, Analyst, 2010, 135, 2426–2433.
32. Y. Song, S. Zhao, P. Tchounwou and Y. M. Liu, J. Chromatogr., A, 2007, 1166, 79–84.
33. H.-Y. Shen, Y. Zhu, X.-E. Wen and Y.-M. Zhuang, Anal. Bioanal. Chem., 2007, 387, 2227–2237.
34. C. C. Hsu and C. W. Whang, J. Chromatogr., A, 2009, 1216, 8575–8580.
35. X. L. Zhang, H. Y. Niu, S. X. Zhang and Y. Q. Cai, Anal. Bioanal. Chem., 2010, 397, 791–798.
36. H. Niu, S. Zhang, X. Zhang and Y. Cai, ACS Appl. Mater. Interfaces, 2010, 2, 1157–1163.
37. L. Sun, L. Chen, X. Sun, X. Du, Y. Yue, D. He, H. Xu, Q. Zeng, H. Wang and L. Ding, Chemosphere, 2009, 77, 1306–1312.
38. L. Sun, C. Zhang, L. Chen, J. Liu, H. Jin, H. Xu and L. Ding, Anal. Chim. Acta, 2009, 638, 162–168.
39. L. Sun, X. Sun, X. Du, Y. Yue, L. Chen, H. Xu, Q. Zeng, H. Wang and L. Ding, Anal. Chim. Acta, 2010, 665, 185–192.
40. L. Chen, J. Liu, Q. Zeng, H. Wang, A. Yu, H. Zhang and L. Ding, J. Chromatogr., A, 2009, 1216, 3710–3719.
41. X. Zhang, H. Niu, Y. Pan, Y. Shi and Y. Cai, Anal. Chem., 2010, 82, 2363–2371.
42. H. Parham and N. Rahbar, Talanta, 2009, 80, 664–669.
43. C.-W. Chang and W.-L. Tseng, Anal. Chem., 2010, 82, 2696–2702.
44. K. A. Joshi, M. Prouza, M. Kum, J. Wang, J. Tang, R. Haddon, W. Chen and A. Mulchandani, Anal. Chem., 2006, 78, 331–336.

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