Nanomaterials in Extraction Techniques

Chapter 11

Nanomaterials in Extraction Techniques Krystyna Pyrzynskaa a

University of Warsaw, Department of Chemistry, Pasteura 1, 02-093 Warsaw, Poland *E-mail: [email protected]

11.1 Introduction Sample pretreatment has been the subject of intense research over the past 20 years. The low concentration of target analytes, the large amount of potential interferents and the incompatibility of the sample matrix with the instrumental techniques are the main reasons for these research efforts. Because sample preparation is still one of the most time- and labor-consuming parts of the analytical procedure, having a clear influence on the quality of the final analytical results, attempts are being made to improve this step in terms of speed, reliability and sensitivity. Miniaturization and automation as well as economical and safety aspects are also taken into consideration. The observed trends can be attributed not only to the development of new modalities, but also to the improvement of existing ones by the use of innovative approaches and materials. Nanoparticles (NPs) have been extensively employed to design novel extraction techniques focused on isolation and/or preconcentration of target analytes from different types of samples. Nanoparticles can be defined as particles with one or more dimensions in the nanometer range, taking 100 nm as an arbitrary limit.1 Nanoparticles can RSC Detection Science Series No. 9 Advanced Environmental Analysis: Applications of Nanomaterials, Volume 1 Edited by Chaudhery Mustansar Hussain and Boris Kharisov © The Royal Society of Chemistry 2017 Published by the Royal Society of Chemistry, www.rsc.org

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have amorphous or crystalline form, and their surfaces can act as carriers for liquid droplets or gases. Their high ratio of surface to volume, the possibility for surface functionalization and favorable thermal features provide the flexibility needed in a broad range of analytical application, especially in sample treatment.2–7 The physicochemical properties of nanomaterials can be controlled through structural design, incorporation of suitable components or modification of their surfaces. Extensive reviews covering the chemical and structural characterization of various nanostructured materials have been published.8,9 Hybrid nanoparticles in particular possess exceptional properties enabling further improvement of analytical methods. In the preparation of these multifunctional nanomaterials, various strategies are used to attain a combination of targeting specificity, magnetic properties and analytical capability.10–12 The surface modification of nanomaterials by functional molecules, particles or polymers can improve the separation and preconcentration efficiency, analytical selectivity and method reliability. The aim of this chapter is to provide an updated and critical revision of the most important features and applications of nanomaterials (metallic, carbon-based and hybrid) in sorptive extraction techniques. Emphasis is placed on the description of the different works provide interesting results regarding their application in this analytical field. Solid-phase extraction and liquid-phase microextraction techniques reported during the period 2012– 2014 are presented. Some developments published in 2015 are also included. Interested readers can find more details regarding earlier contributions in review papers.13–16

11.2 Nanoparticles Used in Environmental Analysis Buzea et al.17 proposed the classification of NPs according to different criteria, namely dimensionality, morphology, composition, uniformity and agglomeration state (Figure 11.1). An alternative complementary classification divides nanoparticles into two main groups, namely organic (carbonaceous and polymeric) and inorganic (metallic and metal-oxide), according to their chemical composition. Whereas monofunctional nanomaterials provide a single function, hybrid nanomaterials combine the properties of their nanoconstituents, which can be highly useful towards simplifying analytical methods and exploring new challenges and applications relying on their synergistic effects.10 Preparation of core/shell nanoparticles allows different properties to be combined in one material by adjusting the composition of the core and the shell. In the preparation of these multifunctional nanomaterials, various strategies are used to attain a combination of targeting specificity and analysis capability.18–21 Another new trend is to couple different NPs with magnetic materials.11,12,16,22 Among these materials, iron oxides, such as magnetite (Fe3O4) and maghemite (γ-Fe2O3), play a major role. Magnetic nanoparticles (see also Chapter 12, Section 12.3.2) have received considerable attention owing to their small size and high surface area, providing better kinetics and greater

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Figure 11.1 Classification of nanostructured materials. Reproduced from ref. 17 with permission from Springer. Copyright © 2007, American Vacuum Society.

extraction capacity for analytes. The coating of nanomaterials by inorganic substances (e.g. silica, alumina, MnO2, carbon nanotubes, graphene) and organic substances (molecularly imprinted polymers, chitosan, polypyrrole, surfactants) stabilizes the magnetic shells, prevents their oxidation and can also be used for further functionalization to obtain multifunctional magnetic nanomaterials for different applications. Particularly, silanol groups on the silica coating provide many possibilities for further surface functionalization. Magnetic NPs can be attached to the desired molecules, conferring magnetic properties to the targets, and then allowing their manipulation and transportation to a specific location through the control of MNPs by an external magnetic field. Thus, they are very useful for sample clean-up and analyte preconcentration. More specific information about the composition, properties and applications of magnetic solids in analytical chemistry can be extracted from the cited reviews.12,23 Surfactants can be physically adsorbed on the surface on active NPs forming monolayers or bilayers on these surfaces; these structures are known as hemimicelles and admicelles, respectively. Hemimicelles comprise monolayers of surfactants adsorbing with their head-groups protruded into the solution and the obtained sorbents have a high affinity towards non-polar analytes. After saturation of the oxide surface, the hydrophobic interactions between the tails of the surfactant hydrocarbon chains result in the formation of admicelles. The outer surface of hemimicelles is hydrophobic, whereas that of admicelles is ionic, which provides two different mechanisms


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(hydrophobic and electrostatic interactions) for retention of target compounds. Thus, simultaneous extraction of analytes with a range of polarities is possible.24 The number of commercially available surfactants is enormous; hence, the degree of hydrophobicity and the charge of the sorbent can be easily modified according to the analyte’s nature.

11.2.1 Metallic Nanoparticles Among metallic nanomaterials, gold and silver nanoparticles are the most popular in environmental analysis25–27 (see also Chapter 12, Section 12.4.5). One of the reasons is the ease of preparation using citrate or hydroxylamine as the reducing agents. The strong affinity that exists between Au- or Ag-NPs and polycyclic aromatic hydrocarbons (PAHs) was used for extraction and preconcentration of these compounds from drinking water.28,29 Nano-structured surfaces of gold and silver also exhibit high affinity for Hg(ii).30–33 Lo et al.33 reported a new adsorbent for removal of different mercury species (Hg2+. methylmercury, ethylmercury and phenylmercury) in natural waters prepared by mixing Au-NPs (13 nm in diameter) with Al2O3 particles (50–200 µm). That adsorbent provides a synergic effect to the components as it has higher affinity for mercury species and other metal ions (Cd2+, Co2+, Ni2+, Pb2+ and Cr3+) than Al2O3 and Au-NPs alone. It was demonstrated that sequential use of the Au-NPs–Al2O3 and Al2O3 adsorbents allowed selective separation of inorganic and organic Hg species prior to their determination by inductively coupled mass-spectrometry (ICP MS) with a very low detection limit (0.03–0.14 pM). Rational design of coatings or surface modification of metallic nanoparticles so that they can specifically bind an analyte of interest is usually the key point for a desired application. 1-(2-Pyridylazo)-2-naphthol (PAN) was used as the complexing agent for preconcentration of manganese in biological samples using AuNPs,34 while morin was employed for the adsorption of lead.35 On the basis of hard–soft interaction theory, Ag-nanoparticle-loaded activated carbon modified with 2-(4-isopropylbenzylideneamino)thiophenol was prepared and used as an efficient sorbent for the separation and preconcentration of a series of metal ions at low concentrations.36 Metal oxide nanoparticles, such as Al2O3, TiO2, ZrO2 and CeO2, offer high thermal, mechanical and chemical stabilities as well as large specific surface area and high adsorption capacity. Several researchers have used nano-sized iron oxide to enrich metal ions and organic pollutants from complex environmental matrices.37–41 Homogeneous distribution of dispersed superparamagnetic nanoparticles, such as Fe3O4, in solution causes favorable mass transport to surfaces and can permit magnetic capture of depleted material. To enhance the selectivity of these sorbents, they can also be functionalized with polymers,42–45 selective ligands,46,47 ionic liquids48 or organic frameworks.49 In particular, the molecular imprinting technique has attracted great interest because of its high selectivity (in terms of size, shape and functionality) for target molecules.50 This technique consists of the formation of ligand-selective recognition sites in synthetic polymers, in which a template is employed in

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order to facilitate the recognition site formation during the covalent assembly of the bulk phase by the polymerization or polycondensation process. The subsequent removal of the entire template is necessary to permit recognition taking place in the vacated spaces by the template species. When molecular imprinting polymer nanoparticles also contain magnetic components, the adsorption can be achieved by dispersing them in solution, and they are then easily separated from the matrix by applying an external magnet. They were applied in binding studies of tributyltin,43 copper44 and gold45 ions. Titanium dioxide nanotubes also present several exceptional properties that have been exploited in different extraction techniques.51 Several research groups have already reported different production procedures for nanoparticles containing hybrid metal oxide/metal oxide for the selective preconcentration of inorganic and organic analytes.52–60 Selected examples of the recent developments and applications are presented in Table 11.1. Table 11.1 Selected examples of hybrid oxide metal nanomaterials used in sample treatments.

Nanomaterial (mass in mg) Analyte SiO2@Al2O3@ Cu(ii) TiO2 (190)

pH

Eluent

EFa

Sorption capacity mg g−1

9.1

HNO3

49

1.4

n.a.

915

200

82.0

SiO2@Al2O3 + Phos5.0 ferrocene phates (50) SnO2@Sb2O3 Pb(ii) 5.0 (25) TiO2@ZrO4 Bisphe- 2.0 (100) nol A SiO2@Al2O3@ Cr(iii) TiO2 (200)

SiO2@TiO2@ Fe3O4 (40) ZrO2@B2O3 (200) Aminated CoFe2O4@ SiO2 (20) a

5.0

HNO3

Dichloro10 methane 17.6

Cd(ii), 8.0 Cr(iii), Mn(ii), Cu(ii) As(v) 3.0

HNO3

100

HCl

20

98.04

Cd(ii)

HCl

50

5.0

Enrichment factor; n.a.: not available.

Ref.

Tap and min- 53 eral waters, ethanol fuels Coastal 54 waters

Tap, lake, seawater n.a. Tap, ground and river waters 0.44 Tap, lake, mineral water, artificial saliva, parenteral solutions 59.3, 27.8, Tap and lake 15.4, waters 33.2

HCl

8.0

Sample

55 56 57

58

Tap and under- 59 ground waters Tap and lake 60 water


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11.2.2 Carbon Nanotubes Since the first report by Cai et al.61 of the application of carbon nanotubes (CNTs) as an adsorbent for the determination of bisphenol A and alkylphenols in water, their application in extraction techniques for the enrichment/ removal of various analytes has been studied extensively and is discussed in several reviews.4,5,13,62,63 The characteristic structures and electronic properties of carbon CNTs allow them to interact strongly with organic molecules via non-covalent forces, such as hydrophobic interactions, hydrogen bonding, π–π stacking, and electrostatic and van der Waals forces. These interactions as well as their hollow and layered nanosized structures make them good candidates for use as sorbents. Functional groups in organic molecules, which can form hydrogen bonds (e.g. –OH, NH2) as well as the presence of aromatic rings, promote adsorption of these molecules adsorption by these. Multi-walled carbon nanotubes (MWCNTs) are preferred over single-walled carbon nanotubes (SWCNTs) as the presence of concentric graphene sheets results in enhanced interaction with the analytes. Long MWCNTs (5–15 µm) show higher sorption capacities than those of shorter ones (1–2 µm).64 See also Chapter 12, Section 12.4.2.3. Different oxidizing reagents, such as HNO3, H2O2 and KMnO4, have been used for introduction of oxygen-containing functional groups (–OH, –C=O and –COOH) on the surface of CNTs.65 Oxidized CNTs show exceptionally high adsorption capacity and efficiency for the removal of heavy metal ions. Two different methods, ultrasonication and irradiation under UV-light, with concentrated HNO3 were tested and then the resulting materials were evaluated for Pb(ii) sorption.66 The UV-light method increased the CNTs’ surface acidity and presented a sorption capacity value of 511.99 mg g−1, while for ultrasonication method only 342.36 mg g−1 was obtained. Zawisza and Sitko67 proposed electrochemically assisted sorption on oxidized MWCNTs for preconcentration of Cr(iii), Mn(ii), Co(ii), Ni(ii), Cu(ii) and Zn(ii) from water samples. The method was based on the application of an electric field to support the sorption process at pH 4. After the preconcentration process, the analytes were directly determined by energy dispersive X-ray fluorescence method with the elimination of the use of organic solvents for elution. The proposed method can be combined with many techniques allowing the measurement of solid samples. In many cases, oxidation is used for the introduction of different molecules by covalent and non-covalent methods to increase the selectivity. There are numerous possibilities for functionalization of the surface of CNTs, and some common functionalized forms (see also Chapter 12, Section 12.3.2) are shown in Figure 11.2. A particular case of non-covalent functionalization is the endohedral filling of CNTs with atoms or small molecules. Functionalization also enhances interaction with polymers and other materials, thus facilitating the formation of composites. Recently, it has been reported that nanocomposites containing CNTs and metal oxides have more efficient sorption properties than CNTs, caused by interactions between them.69–73 Soft

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Figure 11.2 Examples of different possibilities of functionalization of CNTs. (a)

Noncovalent exohedral functionalization with polymers. (b) Defectgroup functionalization. (c) Non-covalent exohedral functionalization with molecules through π-stacking. (d) Sidewall functionalization. (e) Endohedral functionalization of, in this case, SWCNT. Reproduced from ref. 68 with permission from Springer. Copyright © 2010, Springer-Verlag.

materials resulting from the combination of CNTs and ionic liquids present a synergistic effect for some properties, probably owing to the special configuration of carbon nanotubes.74,75 For separation and preconcentration of trace amounts of metal ions, different ligands have been used for the functionalization of carbon nanotubes, such as ethylenediamine,76 8-aminoquinoline,77 poly(N-phenylethanolamine),78 phenyl-iminodiacetic acid,79 4-(2-thiazolylazo)resorcinol,80 2-(5-bromo-2-pyridylazo)-5-diethyl aminophenol81 or 1-(2-pyridylazo-2-naphthol).82 The stable isotope 56Fe was employed to simulate the radioactive 55Fe, 59 Fe and 52Fe ions existing in wastewaters generated by nuclear medicine and then the removal efficiency of 56Fe in these wastes was evaluated using chitosan-functionalized CNTs.83 At pH 5, the sorption capacity of that sorbent was 51.0 mg g−1. Table 11.2 presents some recent examples of applications of CNTs with different modifiers in the determination of metal ions in various samples. Magnetic molecularly imprinted polymers based on carbon nanotubes were used for extraction of carbamates.84 MWCNTs functionalized with amino-terminated alkyl chains were employed to investigate the preconcentration of perfluorooctanoic acid and perfluorooctane sulfonate from surface water samples.85


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11.2.3 Graphene Graphene, a novel two-dimensional structured carbon nanomaterial, is composed of a single layer of sp2 networks of carbon atoms. It has attracted tremendous attention and research interest owing to its high electronic conductivity, good thermal stability, excellent mechanical strength and large surface area. The hexagonal arrays of carbon atoms in graphene sheets are ideal for strong interactions with other molecules. In addition, graphene can be easily chemically functionalized through graphene oxide (GO). It is common to treat graphene as the parent form of graphite, fullerenes and carbon nanotubes owing to the similarity in the chemical structure. A significant advantage of graphene over fullerenes and CNTs is that it can be synthesized from graphite, a very common and cheap material, without using metal catalysts, thus, a cleaner material can be obtained.86 The most efficient method for low-cost, large-scale production of graphene involves oxidation of graphite, subsequent exfoliation into individual GO sheets by sonication and final reduction, usually by hydroxylamine or hydrazine. Unlike graphene, graphene oxide is highly soluble in water and possesses many reactive groups, but still maintains the basic framework of graphene. This feature is very useful in preparing functional graphene materials or graphene composites87 (see also Chapter 12, Section 12.4.2.2). Liu et al.88 reported the first application of graphene for extraction of chlorophenols from environmental water samples. The comparison with other tested materials (C18 silica, graphitic carbon, CNTs) showed that graphene gave the best results. CNTs also had a good absorption capacity for the analytes but gave poor recovery owing to incomplete elution. Recently, graphene nanomaterials have been used for the adsorption of halogenated aliphatic compounds,89 phenanthrene and biphenyl,90 methylene blue (MB) dye,91 nucleosides92 and carbamate pesticides.93 Graphene nanosheets and graphene oxide exhibited comparable or better adsorption capacities than those of carbon nanotubes and granular activated carbon (AC) in the presence of synthetic organic contaminants.90 The adsorption capacities of methylene blue onto AC, GO and CNTs were 270.27, 243.90 and 188.68 mg g−1, respectively.91 However, these values normalized by the BET surface area followed the order of AC < CNTs < GO, indicating that adsorption of MB onto carbonaceous materials was owing not only to the large surface area but also to π–π electron donor–acceptor interactions and electrostatic attraction between positively charged dye ions and negatively charged adsorbents. Graphene oxide is particularly interesting for removal of metal ions owing to its extremely hydrophilic properties and the presence of functional groups containing oxygen atoms.87,94–96 The maximum adsorption capacities at pH 5 for Cu(ii), Zn(ii), Cd(ii) and Pb(ii) were 294, 345, 530 and 1119 mg g−1, respectively.94 The adsorption capacity of GO for lead ions is much higher than that of any of the currently reported sorbents including nanomaterials (Tables 11.1 and 11.2). The difference in the metal ions’ affinity for GO was observed

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Table 11.2 Recent applications of CNTs functionalized with a modifier for enrichment/separation of metal ions.

Metal

Modifier

pH

Pb(ii)

Ethylene 5.0 diamine Cd(ii), Pb(ii), 8-Amino6.4 Ni(ii) quinoline

Pd(ii)

Poly(N4.0 phenyl ethanolamine) Fe(iii), Cu(ii), Phenyl5.5 Pb(ii) iminodi acetic acid Cd(ii) Cd(ii), Pb(ii), 4-(2-thi7.0 Zn(ii), azolylazo) Ni(ii) resorcinol As(v) 5-Br-PADAP 9.5 Cd(ii) PAN 8.0

Eluent

Sorption capacity, EFa mg g−1

Sample

Ref.

HNO3

60

River water

76

HCl

181 201, 150, 172

157.19

Tap, river, min- 77 eral, seawater, fish, sediment samples Thiourea + 44 101.5 Tap, spring, 78 HCl lake, seawater, soil samples HCl 100 64.5, 30,5, River and tap 79 17.0 water CH3COOH 25 3.9, 1.0, Macaroni, rice, 80 6.2, 4.6 lentil, spinach, lichen HNO3 50 — 81 HNO3 25 — Tap water, 82 lobster

a

EF: enrichment factor; 5-Br-PADAP: 2-(5-bromo-2-pyridylazo)-5-diethyl aminophenol; PAN: 1-(2-pyridylazo-2-naphthol).

in binary mixtures (Figure 11.3). Adsorption of Cd(ii) and Zn(ii) on GO was sharply decreased in the presence of Cu(ii) and Pb(ii), which were adsorbed preferentially. The affinity order of GO for these metals (i.e. Pb(ii) > Cu(ii) ≫ Cd(ii) > Zn(ii)) agrees well with metal electronegativity and the first stability constant of the associated metal hydroxide.94 The formation of Pb(ii) complexes in aqueous solution with dithizone97 as well as Cd(ii) with 1-(2-pyridylazo)-2-naphthol98 has been used for preconcentration of these metal ions using graphene nanoparticles from environmental waters. Graphene nanosheets modified with amino groups were also used in the analysis of food samples.99,100 Researchers have also tried to combine graphene with other nanomaterials to avoid the problems with its agglomeration and loss through the pores of the grit in SPE columns. Covalent binding of GO nanosheets to support spherical silica,101–103 polymers104 and chitosan105 was proposed. Su et al.106 prepared magnetic material Fe3O4 and SiO2 polyaniline–GO nanocomposite and used it for the preconcentration of rare earth elements in tea leaves and environmental water samples. Fe3O4 particles on the surface of graphitic carbon nitride nanosheets were employed for the isolation of phenolic acids in plant extracts.107


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Figure 11.3 The single and competitive adsorption of Cu(ii), Zn(ii), Cd(ii) and Pb(ii) on graphite oxide. (a) The amount of metal ions (mmol g−1) adsorbed on GO. (b) The relative amount of adsorbed metal ions (mole percentage). The initial amount of each metal: 5 mmol g−1 of GO, pH = 5. Reproduced from ref. 95 with permission from The Royal Society of Chemistry.

11.3 A pplications of Nanoparticles in Sorptive Extraction Techniques Sorptive extraction techniques play a unique role in analytical chemistry, particularly in the sample preparation step. One of their attractive features is the selectivity that is available from a variety of phases. They are used for the removal of potential interferences, analyte preconcentration and conversion (if needed) of the analyte into a more suitable form for detection or

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separation. In recent decades, solid-phase extraction (SPE) (see also Chapter 12, Section 12.3.1) has played a crucial role in sample preparation and a large number of new sorbents, such as molecularly imprinted polymers, magnetic nanomaterials, carbon nanoparticles and their hybrids, have been proposed and applied. Newer sorptive extraction formats, such as solid-phase microextraction (SPME), are the most recent efforts in sorptive extraction; thus, they will be addressed separately.

11.3.1 Solid-Phase Extraction SPE involves partitioning of analytes between a liquid and a solid sorbent phase. There are many variations of this technique owing to the shape and the size of the sorbent bed as well as the type of adsorbent. Over the past few years, several modifications have been discovered, most being based on miniaturization and automation of various stages of SPE, such as microextraction by packed sorbent (MEPS),108 stir-bar sorptive extraction (SBSE),109 dispersive micro solid-phase extraction (DMSPE)110,111 or matrix solid-phase dispersion (MSPD),112 where nano-sized solid materials have also been used. In most proposed applications of nanomaterials for separation and preconcentration purposes, an extraction is developed with a packed column or cartridge. Carbon nanotubes can be also used in a disc format, which possesses a larger surface area than a cartridge, resulting in good mass transfer and fast flow rate. The triple-layered CNTs disk system showed good extraction efficiency when the sample volume was up to 3000 mL.113 Oxidized CNTs in the format of sheets were applied for adsorption of divalent heavy metals from environmental water samples.114 The pipette tip solid-phase extraction method is a miniaturized form of SPE, where the conical cartridge with small inner diameter requires a tiny amount of sorbent and low solution consumption. This technique was applied for extraction of sulfonamides in environmental waters.115 Graphene (1.0 mg) was packed into a 100 µL pipette tip using degreased cotton at both ends to avoid sorbent loss and 10 mL of sample solution was loaded into the cartridge. The analytes retained on the cartridge were then eluted with 1.0 mL of 5% ammonia–methanol solution. A similar procedure was used for the extraction of lead in water and hair samples with 50.0 mg of MWCNTs packed into a 100 µL pipette tip.116 Elution was done with HNO3, obtaining an enrichment factor of 100. One of the variations of the solid-phase technique that considerably reduces the time and simplifies the extraction is dispersive SPE, in which extraction is not carried out in the column, cartridge or disk but is dispersed in the liquid sample. Compared with classic SPE methods, pre-conditioning of the sorbent is not necessary, simplifying its performance and reducing the extraction time. Particularly, nanoparticles containing magnetic components allow convenient and highly efficient enrichment.22,97 In this procedure, NPs are dispersed into the sample solution, and after adsorption process they are easily separated from the matrix by applying an external magnet while the solution is


295

Figure 11.4 Process of magnetic solid-phase extraction, where Magnetic Particle is (“MP”).

discarded. Then, the target analytes are desorbed with a suitable solution and NPs are regenerated for reuse, as shown schematically in Figure 11.4. Integration of SPE using nanomaterials with flow injection methodology has been reported.117–122 Solid-phase extraction is the most attractive procedure for automation, owing to its easy implementation, ability to be combined with different detection techniques and its high capability in preconcentration. Automation of SPE reduces reagent and sample consumption as well as analysis time. Moreover, all steps in online SPE are performed in a closed system, thus, the risk of sample contamination is minimized. An example is the speciation analysis of selenium in selenium-enriched yeast cells by HPLC-ICP-MS after (on-chip) magnetic solid-phase extraction, as proposed by Chen et al.122 Sulfonated polystyrene-coated Fe3O4 magnetic nanoparticles were employed to adsorbed selenoamino acids and selenopeptides based on cation exchange interactions between the sulfonic groups on the surface of the hybrid NPs with cationic selenoamino acids and selenopeptides. A fully automated flow-based dynamic extraction setup furnished with a CNTs-packed column for determination of readily bioaccessible (watersoluble) Cr(vi) species in soils is presented in Figure 11.5.119 The manifold was designed to accommodate bidirectional column extraction followed by processing of extracts via either inline column clean-up/preconcentration or automated dilution at will, along with Cr(vi) derivatization and flow-through spectrophotometric detection. Four different commercially available carbon nanomaterials—carbon nanofilters, bare CNTs, oxidized CNTs and CNT-NH2—were evaluated as SPE materials. Regardless of the functionalization of the carbon nanoparticles, the retention efficiency was virtually the same. However, bare CNTs were selected as the sorbent material owing to the better sensitivity of the analytical method.

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Figure 11.5 Schematic diagram of the compact flow analyzer integrating automated dynamic extraction, isolation and speciation/determination of readily bioaccessible Cr(vi) in soil samples. C: carrier (deionized water); R: reagent (1,5-diphenylcarbazide); E: eluent (99 : 1 (v/v) MeOH/0.65 mol L−1 H2SO4). Reproduced from ref. 119 with permission from Springer. Copyright © 2011, Springer-Verlag.

11.3.2 Solid-Phase Microextraction Solid-phase microextraction (SPME) is a simple and solvent-free extraction technique that is applied in preconcentration and separation of analytes from complex matrix samples.123 In SPME, a fiber or wire coated with a solid sorbent is exposed to the liquid sample (determination of both volatile and non-volatile analytes) or headspace above the sample (volatile analytes). After adsorption equilibrium is reached, the wire is withdrawn from the sample and the analytes are thermally desorbed in the case of gas chromatography detection or eluted with a suitable solvent for liquid chromatography. A high extraction efficiency, good sensitivity and successful matrix-analyte separation can be obtained only if a suitable coating is selected. Thus, the most important factor in the SPME technique is the affinity of the solid phase (fiber coating) for the analyte. The polarity and thickness of the fiber coating should be chosen to match the analyte. There are several methods for coating deposition onto the fibers, including dipping, coating adhesion, chemical bonding, sol–gel technology and electrochemical methods.124–127 Physical deposition is a fast method for fibre preparation; however, the obtained products have low thermal stability as well as low resistance to organic solvents and strong acid/basic solutions.


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The coatings obtained using the sol–gel method owing to chemical bonding have sufficient stability, but the main problem is the fragility of the fusedsilica fiber as the most commonly used support. In electrochemical deposition, the polymer film can be coated directly on metal wires, which have better mechanical strength than that of silica fibers. Carbon nanostructures (see also Chapter 12, Section 12.4.2.3.4), silica and titanium nanoparticles are most commonly reported materials in the literature for the development of new coatings for SPMESometimes, they can be combined with other nanomaterials or modified by introducing functional groups on their surface, like in SPE applications, to modify the extraction efficiency. Zhang et al.128 prepared a graphene-coated SPME fiber following deoxidization of GO-NPs. The whole fabrication included a few processes (Figure 11.6). The silica fiber was cut into small pieces and then treated to expose the maximum number of silanol groups. The fiber was then immersed into 3-aminopropyltriethoxysilane (APTES) solution as a cross-linking agent for 12 h at room temperature. Subsequently the produced fiber was inserted into an aqueous graphene oxide dispersion for 2 h in a 60 °C water bath and after conditioning was deoxidized by hydrazine to generate the graphene-coated SPME fiber. The fiber was exposed to the headspace above water solutions for polycyclic aromatic hydrocarbons extraction. Under the optimized extraction and determination conditions, the LODs of eight PAHs were in the range of 1.52–2.72 ng L−1 and recoveries were 72.7–101.7% for river water, pond water and soil samples. Table 11.3 summarizes some new applications of nanomaterials in SPME. A critical discussion of the state of the art of SPME in the field of environmental analysis is presented in the recent review by Pawliszyn et al.140 Among SPME techniques, in-tube SPME, using the open-tubular fused-silica capillary column as the SPMS device instead of the SPME fibre, shows additional advantages for easy automation and online coupling with separation or detection methods.141 In this mode, the analytes are extracted and preconcentrated from a solution directly into the inner surface of a capillary tube coated with a thin film of an appropriate extractive phase. Desorption can be performed using a static or dynamic approach. In the former, a solvent is introduced into the capillary and the desorbed analytes are sent to the injection valve of the chromatographic system. In the dynamic approach, the mobile phase is passed through the capillary column for desorption. Generally, static desorption is used when the analytes strongly interact with the capillary coating. Filtering of the sample solution before extraction is necessary to prevent clogging of the capillary and blocking the flow. Nanostructured coatings appear to be a useful option to improve the performance of these SPME modes, especially to increase the extraction efficiency. Graphene was immobilized onto the inner surface of PTFE microtubes previously modified with polydopamine and the resulting coating was used for the determination of PAHs in environmental samples.142 The use of a nanostructured polypyrrole–polyaniline composite as the extraction phase

ref. 128. Copyright (2011) American Chemical Society.

Figure 11.6 The processes for fabricating graphene-coated SPME fibers. a)–e) Sequential steps. Reproduced with permission from

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Table 11.3 Recent application of carbon nanomaterials in solid phase micro extraction (SPME).a

Carbon nanomaterial

Analyte

Sample

PEG-grafted CNTs

BTEX

PEG-grafted CNTs PEG-grafted CNTs CNTs onto steel wire CNTs onto steel wire with PIL

PAHs

Tap, mineral, waste and well water Saffron

Furan

Analytical technique LOD µg L−1

Ref.

GC-FID

0.0006–0.003

129

GC-FID

0.001–0.05

130

Fruits, juices, GC-FID milk, wheat River water GC-FID

Phenolic compounds Halogenated Groundwater aromatic of industrial hydrocarbons park CNTs onto steel PAHs Tap and river wire with PILs water CNTs/polypyrrole Fluoroquinolo- Urine and soil on Pt wire nes samples Graphene Triazine Tape, lake and herbicides sea water Graphene oxide– PAHs River and silica fiber pond water Graphene–poly- Tricyclic antide- Plasma, urine, aniline fiber pressants milk and hair CNTs-polypyrVolatile aroRiver, minrole–TiO2 matic eral and onto steel hydrocarbons wastewater wire

0.00025–0.001 131 0.01–0.02

132

GC-FID

0.05–2

133

GC-FID

0.001–0.0025

134

HPLC-UV 0.5–1.9

135

HPLC-UV 0.05–0.2

136

GC-FID

0.005–0.08

137

GC-FID

0.10–0.35

138

GC-FID

0.03–0.09

139

a

PEG: polyethylene glycol; PIL: polymeric ionic liquid.

on the inner surface of a stainless steel tube was reported for online preconcentration and determination of parabens.143 It is not just the chemical properties of NPs that are useful for developing new in-tube SPME approaches. Their physical properties, such as magnetism, can be another possibility to increase extraction efficiency.144,145 Molinar-Martinez et al.144 proposed silica-supported Fe3O4 magnetic nanoparticles immobilized on the inner surface of a bared fused-silica capillary. In the presence of a magnetic field, the NPs creating regions with different magnetic field gradients. Analytes with diamagnetic properties are trapped in the regions with minimum gradient field, increasing the adsorption of the analytes inside the interconnecting network created by the SiO2 adsorbent phase. The analytes are desorbed from the capillary column by combining the mobile phase and the change of magnetic field polarity. The utility of this methodology was demonstrated for organophosphorus pesticides.145

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11.4 Conclusions The use of nanoparticles in the preparation procedures for sample clean-up and the preconcentration of analytes is a topic of growing interest in analytical chemistry. Owing to the nanoscale effect, NPs have unique physical and chemical properties that make them superior candidates as adsorbents. Thus, they have been extensively exploited in sorptive extraction techniques, such as solid-phase extraction and microextraction. Nanomaterials modification with different functional groups can dramatically promote the extraction efficiency, which addresses the requirements of some specific applications. The involvement of nanoparticles in the extraction techniques increases as commercial firms introduce them into the market for applications with accurate physicochemical characterization. It is worth mentioning that only the applications of nanoparticles in the sorptive extraction techniques are presented and discussed in this chapter. Investigations and application of these nanomaterials in different fields of analytical chemistry, such as chemical sensors/biosensors, stationary phases or buffer additives, other extraction techniques or optical devices has become a very hot research area. It is expected that the applications of nanomaterials in analytical chemistry will greatly promote the multidisciplinary research, especially at the interface of material science and analytical chemistry.

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