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Trends Trends in Analytical Chemistry, Vol. 34, 2012

Towards greater mechanical,thermal and chemical stabilityin solid-phase microextractionHabib Bagheri, Hamed Piri-Moghadam, Mehrnoush Naderi

Solid-phase microextraction (SPME) is a fast, solvent-free technique, which, since its introduction in the 1990s, has been

increasingly applied to sample preparation in analytical chemistry. Conventional SPME fibers are fabricated by making a physical

bond between the usual silica substrate and the polymeric coatings. However, some applications are limited, as the lifetime and

the stability of conventional SPME fibers cannot meet the demands of analyzing relatively non-volatile compounds with more

polar moieties. There have been attempts to analyze less volatile compounds by increasing the thermal, physical and chemical

stability of the fibers. In this review, we present some new developments in the use of sol-gel technology, molecularly-imprinted

polymers (MIPs) and electrochemical deposition to prepare thermally-stable, chemically-bonded, unbreakable SPME fibers.

ª 2012 Elsevier Ltd. All rights reserved.

Keywords: Coating polymer; Electrochemical deposition; Modifier; Molecularly-imprinted polymer (MIP); Polarity; Polymerization; Sample

preparation; Sol-gel technology; Solid-phase microextraction (SPME); SPME fiber

Habib Bagheri*,

Hamed Piri-Moghadam,

Mehrnoush Naderi

Environmental and Bio-

Analytical Laboratories,

Department of Chemistry,

Sharif University of

Technology, P.O. Box 11365-

9516, Tehran, Iran

*Corresponding author.

Tel.: +98 21 66165316;

Fax: +98 21 66012983;

E-mail: [email protected]

126

1. Introduction

Solid-phase microextraction (SPME) is aninnovative technique that was introducedin early of 1990s [1]. In SPME, sampling,extraction and preconcentration are per-formed in single step. SPME is simple, fast,solvent free and cost effective. In SPME, asmall amount of extracting phase, dis-persed on a solid support, is exposed to thesample for a well-defined period of time.Afterwards, a partitioning equilibriumbetween the sample matrix and extractionphase is reached. If this option is selectedfor analysis, convection conditions do notaffect the sample amount extracted. An-other option is to utilize short time pre-equilibrium extraction, and, if convectionor agitation is constant, the amount ofanalyte extracted is related to time. Then,desorption of analytes should be per-formed as fast as possible to avoid anypossible peak broadening. In GC, the rapiddesorption happens inside the high tem-perature injection port, while, in HPLC,the fast desorption is achievable using amobile phase with strong eluting power.

SPME is an alternative to liquid-liquidextraction (LLE) and solid-phase extrac-tion (SPE), and, in contrast to them, is

0165-9936/$ - see front matter ª 2012 Elsev

regarded as a non-exhaustive extractiontechnique. Although the use of SPME fi-bers is increasingly popular, their rela-tively low recommended operatingtemperature (generally in the range 240–280�C), their instability and swelling inorganic solvents (greatly restricting theiruse with HPLC), fiber breakage, and loss ofcoatings are obstacles to the analysis ofsome chemicals. Except for fiber break-ages, the others are due to the physicalbonds between the coatings, the extract-ing phase, and the solid substrates. Lowoperating temperature and loss of coatingsaffects the performance and the life-timesof the fibers. Preparation of chemically-bonded SPME fibers has been the center ofattention for many researchers to over-come some of these problems.

2. Preparation of fibers using sol-geltechnology

2.1. Fundamentals of sol-gel technologyInorganic polymerization, better known asthe sol-gel process, is a general method forpreparing oxides by the wet route at roomtemperature. Sol-gel technology providesa resourceful approach to synthesize

ier Ltd. All rights reserved. doi:10.1016/j.trac.2011.11.004

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Trends in Analytical Chemistry, Vol. 34, 2012 Trends

inorganic polymer and organic–inorganic hybrid mate-rials under extraordinarily mild conditions, so it can beused to obtain products of various sizes, shapes andformats (e.g., fibers, films, monoliths and monosizedparticles). This technology has found increasing appli-cations in many areas, and, in analytical chemistry, it isused to prepare SPME fibers.

Sol-gel technology, developed by Malik and co-work-ers [2,3], can be an excellent alternative to conventionalcoating-preparation methods, due to its inherentadvantages and performance. These include a single-stepmanufacturing process, material homogeneity at themolecular level, chemical bonding between the sorbentand the fused-silica surface, high thermal and solventstability, and the porous structure of the hybrid material.Moreover, sol-gel organic–inorganic hybrid materialsprovide desirable sorptive properties that are difficult toachieve using purely organic or inorganic materials.Fig. 1 shows the chemical transformations occurringduring the main steps of the sol-gel process.

Precursor, coating polymer, catalyst and deactivatingagent (non-polar fibers) are required for preparation ofSPME sol-gel-based fibers. Moreover, the functionality ofthe solid support is one of the main issues in preparingSPME fibers using sol-gel technology.

2.2. Sol-gel technology for preparing SPME fibersResearch concerning the preparation of new SPME fibersincludes three sol-gel-based features in selecting (i) pre-cursors, (ii) coating polymers, and (iii) modifiers toachieve fibers with specific physical capabilities and thedesired polarity and selectivity.

2.2.1. Precursors. Sol-gel-based non-polar SPME fiberswere introduced by Malik et al. [2,3] to improve theperformance of traditional fibers. The protective poly-imide coating was removed from a 1-cm end segment ofa 200-lm o.d. fused-silica fiber, and the exposed outersurface was coated with a bonded sol-gel layer ofpoly(dimethylsiloxane) (PDMS). They used methyltrime-thoxysilane (MTMOS) as a precursor for preparing thechemically-bonded PDMS fiber. Because of chemicalbonding between the sol-gel PDMS coating and thefused-silica SPME fibers, sol-gel-coated PDMS fibers ex-hibit thermal stability greater than conventionally-coated PDMS fibers. The sol-gel PDMS fibers can beroutinely used up to 320�C (and higher) without anysigns of bleeding, whereas conventionally-coated PDMSfibers begin to bleed around 200�C. Enhanced thermalstability of sol-gel-coated fibers can overcome the sam-ple-carryover problem often encountered in SPME ofpolar solutes with conventional PDMS fibers.

Sol-gel coatings possess a porous structure and re-duced coating thickness that enhances extraction andmass-transfer rates in SPME. High-temperature condi-tioning of sol-gel-coated PDMS fibers led to consistent

improvement in peak-area repeatability for SPME-GCanalysis. Enhanced thermal stability allowed the use ofhigher injection-port temperatures for efficient desorp-tion of less-volatile analytes and should extend the rangeof analytes that can be handled by SPME-GC techniques.Mackenzie explained that the sol-gel network originatingfrom an alkyl derivative of tetraalkoxysilane (TAOS)precursor possesses a more open structure and caneffectively minimize the stress during drying and crack-ing [4]. This is the reason for the choice of MTMOS as aprecursor instead of tetraethoxysilane (TEOS) and tet-ramethoxysilane (TMOS).

One of the most important advantages of the sol-gelroute is the possibility of designing the material structureand properties through proper selection of the sol-gelprecursor and other building blocks. Due to the greatimportance of the precursor to selectivity, allyloxy bis-benzo 16-crown-5 trimethoxysilane was first used asprecursor to prepare the sol-gel-derived bisbenzo crownether/hydroxyl-terminated silicone oil (OH-TSO) coatingfor SPME [5]. The extraction efficiencies of the newcoating for organophosphorous pesticides (OPs) werestudied. The new crown-ether fiber presented a largerresponse to OPs than the commercial fibers when theywere used to extract OPs from an aqueous solution at thesame concentration and the same extraction conditions.

An ultrathin phenyl-functionalized SPME fiber coatingwas developed by sol-gel deposition to control thethickness of the fiber coating [6]. PTMOS and MTMOSwere the sol-gel precursors used at different proportions,together with different water contents, catalysts andreaction times. The film thickness was in the range 0.2–1 lm and could not be increased by multi-coating pro-cesses, and, apparently, a dense, non-porous micro-structure was obtained. This coating exhibited a stronghydrophobic character, as shown by the capability ofextraction of long-chain and polar aromatic compounds,comparable to that of 100 lm PDMS and 65lm carbo-wax–divinylbenzene (CW–DVB).

Although the sol-gel technique has overcame somesignificant shortcomings of SPME and capillary-mic-roextraction (CME) techniques by providing an effectivemeans of chemical immobilization for sorbent coatings, anarrow window of pH stability in silica-based materialsshould have been resolved. In 2004, Malik et al. [7]prepared sol-gel TiO2–PDMS-coated capillaries andshowed the possibility of using them in on-line CME-HPLC operation. This led to a significant improvement inpH stability and extraction sensitivity. Sol-gel TiO2–PDMS-coated microextraction capillaries possess excel-lent pH stability and retain their extraction characteris-tics intact, even after prolonged treatment with highlyalkaline (pH 13) NaOH solution. Direct chemical bond-ing of the coating to capillary inner walls provides thesecoatings with excellent solvent resistance, and makessol-gel TiO2–PDMS-coated capillaries very suitable for

http://www.elsevier.com/locate/trac 127

Figure 1. Chemical transformations occurring during the main steps of the sol-gel process.


on-line sample preconcentration in CME-HPLC analysis,as they were used effectively in the extraction of differentclasses of analytes with good sensitivity and run-to-runrepeatability. Low-ppb and sub-ppb level limits of detec-tion (LODs) were achieved for PAHs, ketones, andalkylbenzenes in CME-HPLC analysis using the sol-gelTiO2–PDMS-coated microextraction capillaries in con-junction with UV detection (Fig. 2).

A novel zirconia-based hybrid organic–inorganic sol-gel coating was also developed for CME (in-tube SPME)in 2005 [8]. Zirconia possesses much better alkaliresistance than other metal oxides (e.g., alumina, silica,and titanium). It is practically insoluble within a widepH range (1–14) [9–11]. Zirconia also shows outstand-ing resistance to dissolution at high temperatures[12,13]. Besides the extraordinary pH stability, excellentchemical inertness and high mechanical strength aretwo other attractive features. Alhooshani et al. [8] usedzirconium (IV) butoxide as the precursor and silanol-terminated poly(dimethyldiphenylsiloxane) (PDMDPS)as the coating polymer.

Alumina, due to its high surface area, mechanicalstrength, and thermal and chemical stability, has beenwidely used in sample purification and analytical sepa-rations. The surface chemistry of alumina is quite dif-

Figure 2. (A) Hydrolysis of titanium (IV) isopropoxide, and (B) po

128 http://www.elsevier.com/locate/trac

ferent from that of silica. Based on references reported,silica has a weak Brønsted acidity due to the silanolgroups, while alumina provides Lewis acidity and basi-city as well as low concentration of Brønsted-acid sites.Both Brønsted and Lewis acid–base characteristics areresponsible for the chromatographic performance ofalumina. Compared to silica, alumina is capable of bothion and ligand exchange. The ligand-exchange ability ofalumina originates from the presence of Lewis-acid siteson the surface (i.e. coordinatively unsaturated Al3+, andwater molecules or other easily displaced ligandscoordinatively bonded to the sites). Lewis basic analytescontaining polar functional groups (e.g., carboxylic,phenolic-OH or amino groups) can substitute for thesurface hydroxyl group or coordinated water moleculesand form complexes with the metal ions of the oxidesurface, so ligand-exchange interaction can play animportant role in selective extraction of these com-pounds by alumina-based hybrid materials. Preparationof alumina-based hybrid organic–inorganic sol-gelsorbents from a highly reactive precursor, aluminumsec-butoxide, and a sol-gel-active organic polymer (hy-droxyl-terminated polydimethylsiloxane) was reported in2006 by Liu [14]. The selectivity and the efficiency ofthis novel fiber were investigated by analyzing polar

lycondensation of hydrolysis product, titanium hydroxide.


functional compounds (e.g., fatty acids, phenols, alco-hols, aldehydes and amines).

Although SPME sol-gel fibers are becoming increas-ingly popular and can overcome the chemical andthermal instability of traditional fibers, the easy breakageof SPME sol-gel fibers is a major drawback. Although useof fused silica as substrate has many advantageous, it isalso responsible for fiber breakage.

In recent years, preparation of unbreakable fibers wasthe focus of many researchers. In 2006, the firstunbreakable SPME fiber fabricated by sol-gel depositionwas reported by Azenha et al. [15]. They used titaniumwire as a new, unbreakable substrate and showed that,under suitable conditions, alkoxysilanes and hydrosil-anes can form Ti-O-Si bonds. Before the deposition,titanium wire was immersed in 1 M NaOH for 1 h toincrease the number of surface titanol groups and finallywashed successively with 0.1 M HCl, water, and meth-anol. Then, PDMS was chemically bonded to the Ti wireby sol-gel technology.

A fiber with MTOMS precursor on the NiTi alloy assubstrate was also prepared by Carasek et al. in 2008 asan unbreakable fiber [16,17]. They described the use ofzirconium oxide electrolytically deposited onto an NiTialloy as a new substrate for sol-gel reactions. Electro-lytically-coated fiber with ZrO2 (NiTi–ZrO2) was pro-posed as an efficient, unbreakable substrate forpoly(ethylene glycol) SPME coating using sol-gel reac-tions. Wires of NiTi were used as a support for electro-deposition of ZrO2. NiTi wires previously electrodepositedwith ZrO2 were dipped into NaOH 1.0 mol L�1 solutionfor 1 h for their surface activation, followed by neutral-ization by rinsing with hydrochloric acid 0.1 mol L�1

and deionized water. This activation leads to a greater

Figure 3. NiTi–ZrO2 surface-bon

exposure of the silanol groups to the silica rod, favoringchemical bonding between the support and the poly-meric coating, which, in turn, leads to greater thermalstability of the fiber. The applicability of the proposedNiTi–ZrO2–PDMS fiber was evaluated through extractionof benzene, toluene, ethylbenzene and o-xylene (BTEX)from the headspace of aqueous samples (Fig. 3).

Recently, Bagheri et al. [18,19] developed a new wayto prepare unbreakable CME and SPME fibers. They usedself-assembled monolayers (SAMs) to functionalize thesubstrate. SAMs are ordered monomolecular films,which are spontaneously formed from immersing a solidsubstrate in a solution containing amphifunctionalmolecules. The amphifunctional molecule has a headgroup, which usually has a high affinity for the solidsurface, a tail (typically an alkyl chain), and a terminalgroup that can be used to control the surface propertiesof the resultant monolayer. The molecular forces be-tween the tails are primarily responsible for the order ofthe monolayer. The most extensively studied SAMs aresilanes, which are used to modify hydroxyl-terminatedsurfaces, and organosulfur compounds. The affinity ofsulfur for gold, platinum, copper and silver could there-fore be used.

There are several processes for preparation ofunbreakable fibers. First, Au wires with length 2 cm andthickness 240 lm, were prepared and immersed inpiranha solution (H2SO4:H2O2, 3:1, v/v) for 10 min at80�C. Afterwards, the wires were washed with acetonein an ultrasonic bath for 10 min. The prepared Au wireswere immersed in the solution of 3-(mercaptopropyl)trimethoxysilane (3MPTMOS) in ethanol with concen-tration of 10�3 mol L�1 for 12 h. Afterwards, to intro-duce amino functionality to the surface, the modified

ded sol-gel PDMS coating.



gold substrates were dipped in a solution of 1.2 mL3-TESPA in ethanol with concentration of 10�3 mol L�1

and 30 lL trifluoroacetic acid (TFA) and 20 lL water for12 h. Next, the substrates were placed in an oven at120�C for 30 min. These two operations were repeatedfor three cycles to form a layer of 3-TMSPA on the fiber.Then, 1 mg oxidized multi-wall carbon nanotubes(MWCNTs) were dispersed in a 1 mL sodium dodecyl-sulfate (SDS) solution (1%) for 30 min under sonicationto prepare the MWCNT suspension. The pretreated fiberswere immersed in the MWCNT suspension for 4 h in ahot water bath (70�C), then placed in a 120�C oven for30 min. This procedure was repeated three times untilthe coating reached the desired thickness. Finally, thecoated fiber was conditioned under the flow of drynitrogen prior to the SPME experiments. Bagheri et al.also used the SAM-sol-gel technique to prepareunbreakable CME for on-line extraction of compounds inconjunction with high-performance liquid chromatog-raphy (HPLC) [19]. The inner surface of a copper tube,intended to be used as an HPLC loop, was electrode-posited by metallic Cu followed by the SAMs of 3MPT-MOS. Then, polyethylene glycol (PEG) was chemicallybonded to the –OH sites of the SAM already covering the

Table 1. Different precursor and their advantages

Precursor Advantages

Methyltrimethoxysilane(MTMOS)

Overcome problems associated withcracking and shrinkage ofTEOS

Allyloxy bisbenzo 16-crown-5trimethoxysilane

Overcome lack of enough selectivelyof precursors such as MTMOS and TEOfor extraction of organophosphorous pe

Ultrathin phenyl-functionalized

Capability of extraction of long chainand apolar aromatic compounds

Titanium (IV)isopropoxide

pH-resistant

Zirconium(IV)butoxide

It is practically insoluble within a widerange (1–14), outstanding resistance toat high temperatures, excellent chemicainertness and high mechanical strength

Aluminum sec-butoxide High surface area, mechanical strengththermal and chemical stability

Phenyl-terminateddendrimer with atriethoxysilyl root

Both polar and nonpolar analytes wereextracted from aqueous samples on thedendrimer capillary and provided exceldetection sensitivity

3-(Trimethoxysilyl)propylmethacrylate (TMSPMA)

It is a bifunctional reagent, contains bomethacrylate and alkoxysilane groups

3-Cyanopropyltriethoxysilane The cyanopropyl moiety in sol-gel CN-coatings provided effective extraction oanalytes such as free fatty acids, alcohowithout requiring derivatization, pH adsalting out procedures

Anilinemethyltriethoxysilane(AMTEOS)

Selective for analysis of aromatic comp

3-(trimethoxysilyl) propylamine (TMSPA)

Impart polar moiety into the coatingnetwork for the extraction of polar com


inner surface of the copper loop using sol-gel technology.The homogeneity and the porous surface structure of theSAM and sol-gel coatings were examined using thescanning electron microscopy (SEM).

Different precursors, which have been used in sol-gelSPME along with their advantages and disadvantages,are summarized in Table 1.

2.2.2. Coating polymers and modifiers. Wang et al. [26]used PEG as the main sol ingredient to prepare a novelhigh-performance SPME fiber in 2000. It showed a highsample capacity for both polar and non-polar com-pounds.

After that, several attempts were made to obtain fiberswith different characteristics using different coatingpolymers and modifiers. Crown ethers have been widelyused as chromatographic stationary phases by otherresearchers, due to their good selectivity originatingfrom their cavity structure and the strongelectronegative effect of heteroatoms on the crown-etherring. In 2001, Zeng et al. [27] used three crown etherscontaining hydroxyl groups as a modifier to coat thefused-silica rods using sol-gel technology, and the fiberswere investigated by extraction of aromatic amines from

Disadvantages Ref.

Lack of selectivity [2,3]

Ssticides

– [5]

Having a dense,non-porous microstructure

[6]

– [7]

pHdissolutionl

– [8]

, – [14]

efficientlysame sol-gellent

Lack of selectivity, tree-likebranched architecture makes itdifficult for coating modifierand other stationary phases

[20]

th – [21]

PDMSf highly polarls, and phenolsjustment or

– [22]

ounds, High porosity – [23]

pounds– [24,25]


aqueous samples with headspace SPME. These fibersexhibited high thermal and solvent stability. The sol-gel-derived hydroxydibenzo-14-crown-4-coated fiber dem-onstrated the best affinity for several aromatic aminederivatives. The extraction ability showed no significantdecline after it was used 150 times. Open crown ether isalso a type of non-cyclic polyether compound. Thestructural units of open crown ether are similar to that ofcyclic polyether crown ethers. Open crown not onlypossesses the selectivity of crown ether, but also is easilysynthesized and is non-toxic [28].

Calixarenes are regarded as the third generation ofsupramolecules next to crown ethers and cyclodextrins.They are a class of cyclic oligomers prepared fromformaldehyde and para-substituted phenols via cycliccondensation under alkaline conditions. Considering theoutstanding capacity of the calixarene as receptors,mainly based on their variable chemical modificationpotential and their conformational pliability allowing akind of induced fit to the shape of suitable guest mole-cules, a sol-gel-coated calix[4]arene fiber was firstdeveloped by Zeng et al. [29–32].

Carbon-based materials are regarded as good sorbentsand have been widely used in trapping and separatingorganic compounds. There are also some reports aboutusing carbon materials as SPME fibers {e.g., polycrys-talline graphites [33], modified pencil lead [34], acti-vated carbon [35]}.

Hydroxyfullerene as a novel coating was also reported[36]. Fullerene has a unique spherical shape, a conju-gated three-dimensional p-electronic system, is used asstationary phase for LC and GC, and has shown a widerange of operational temperatures, good thermal stabil-ity and good selectivity for aromatic compounds, espe-cially exhibiting strong affinity for planarpolychlorinated biphenyl (PCB) molecules.

In recent years, ionic liquids (ILs) have gained popu-larity in a number of fields, due to their perceivedadvantages over traditional solvents. They are consid-ered ‘‘green’’ because they are remarkably less hazard-ous than their conventional counterparts, due tonegligible vapor pressures, low flammability, good ther-mal stability, tunable viscosities, low corrosion tenden-cies, and varying degrees of solubility with water andorganic solvents.

IL-mediated sol-gel sorbents was first reported byMalik et al. [37,38] for CME. ILs were used as co-solventsand as porogens in the sol-gel system. The advantages ofusing ILs as solvents for reactions include their ability tobe recycled, high thermal stability, and the improvedstability of reactants in ILs. The advantages of ILs asporogens, instead of organic molecules in sol-gelsystems, include the effect that the cation and the anionportions of the IL have on pore structure and distribu-tion, and the ability of ILs to decompose from sol-gelsystems without leaving residues behind. Their investi-

gation revealed that, for PDMS sol-gels, the addition of aphosphonium-based IL, trihexyltetradecylphosphoniumtetrafluoroborate (TTPT), slowed down gelation by about1.5 h in comparison with the sol-gel that did not containthe IL. The slower gelation in the IL-mediated sol-gelscan be attributed to the increased viscosity of the solsolution due to the addition of the IL. It is reasonable toassume that the IL did not play a role in extractions,since the thermal decomposition temperature of TTPT is190�C, and the IL-mediated sol-gel PDMS microextrac-tion capillaries were heated in an inert environment to300�C. It can therefore be safely assumed that, duringthermal conditioning, the IL had decomposed and thedecomposition products were at least partially removedfrom the capillary with the purging helium flow.

The preconcentration abilities of the two types of sol-gel PDMS capillaries (PDMS-IL and PDMS-no IL) werealso compared to determine the effect of the IL on theresulting sol-gel coating. Results showed that the sol-gelPDMS-IL coating provided significantly greater extrac-tion capability than the PDMS-no IL. This, in turn,translated into lower LODs for the sol-gel PDMS-IL mic-roextraction capillary. Malik et al. [37,38] also preparedIL-mediated polar sol-gel microextraction capillaries:PEG, polyTHF, and BMPO in conjunction with BMPT (N-Butyl-4-methylpyridinium tetrafluoroborate) and TTPT.The PEG-IL sol-gel coating appeared to be more porousthan its counterpart prepared without IL. Sol-gel coat-ings with greater porous morphology obtained with thehelp of ILs can be expected to provide better performancein extraction.

Bonded IL polymeric material for SPME GC analysiswas reported by Shirey et al. [39]. In this work, attentionwas devoted to the extraction of hydrophilic and polarcompounds (e.g., short-chain alcohols and amines). Forthis purpose, completely new and different ILs wereprepared and tested as coatings for SPME. To reducecontamination and/or loss of adsorbent, the different ILderivatives were covalently bonded to silica microparti-cles that were subsequently used to prepare a porous,mechanically strong fiber coating. Both headspace andimmersion techniques were tested to evaluate thecapabilities of the newly developed bonded IL-basedsorbents with a large variety of polar solutes. Four newIL were prepared and bonded onto 5lm silica particlesfor use as coatings in SPME. Two ILs contained styreneunits that allowed for polymerization and greater carboncontent of the bonded silica particles. Two polymeric ILsdiffering by their anion were used to prepare two SPMEfibers that were used in both headspace and immersionextractions and compared to commercial fibers. In bothsets of experiments, ethyl acetate was used as an internalstandard to take into account coating volume differencesbetween the fibers. The polymeric IL fibers are veryefficient in headspace SPME for short-chain alcohols.Immersion SPME could also be used with the IL fibers for



short-chain alcohols as well as for polar and basicamines that can be extracted at pH 11 without damageto the IL-bonded silica particles. The sensitivities of thetwo IL-based fibers differing by the anion were similar.The mechanical strength and the durability of thepolymeric IL fibers were reported to be excellent.

Nowadays, reaching lower LODs is one the mainchallenges, and much research has been focused on it.CNTs are a new type of carbon material first found in1991 by Iijima [40]. It can be described as a graphitesheet rolled up into a nanoscale tube of single-wall car-bon nanotubes (SWCNTs) or with additional graphitetubes of MWCNTs. Because of their unique geometricalstructure, CNTs exhibit excellent mechanical and ther-mal properties. For example, the highly developedhydrophobic surface of CNTs showed strong sorptionproperty towards various compounds compared with aplanar carbon surface [41]. MWCNTs have been char-acterized as superior sorbent for removing dioxins forenvironmental protection. CNTs have a curved surface,so they are expected to show a stronger binding affinityfor hydrophobic molecules compared with a planarcarbon surface. Furthermore, the internal pores of theCNTs are large enough to allow molecules to penetrate.A large sorption surface is also available on the outsideand in the interstitial spaces within the nanotubebundles. All these indicate that CNTs have a great abilityto physically adsorb hydrophobic organic pollutants.

MWCNT-coated fibers for SPME were first introducedby Yan et al. [42] to extract polybrominated diphenylethers from water and milk samples before gas chro-matography (GC) with electron-capture detection.

A novel MWCNT-bonded fused-silica fiber for SPME–GC analysis of phenols in water samples was alsoreported by Jiang [43]. Fig. 4 shows the preparationprocess of MWCNT-SPME fibers. Table 2 shows different

Figure 4. Preparation process of the multi-wa


coating polymers and modifiers that have been used insol-gel-based SPME.

3. Preparation of fibers with molecularly-imprinted polymers (MIPs)

3.1. Fundamentals of the MIP methodThe principle of molecular imprinting was inspired byFischer�s lock-and-key metaphor. In the first step, theselected key molecule is mixed with lock building blocks.The building blocks and the key are allowed to associatewith each other, firmly or loosely. The complexes soformed between the key and the building blocks aresubsequently glued together in order to fix the building-block positions around the key. Removing the molecularkey then leaves a construction which, if everythingworks properly, is selective for the original key. Inmolecular imprinting, molecules are used to create themarks or imprints, normally within a network polymer.A number of expressions have been used in the past todescribe the present technology (e.g., enzyme-analoguebuilt polymers, host-guest polymerization, templatesynthesis or template polymerization, creation of foot-prints, and preparation of specific adsorbents).

In molecular imprinting, the key molecules describedabove can be denoted by a variety of expressions [e.g.,templates (T), template molecules, target molecules,analytes, imprint molecules, imprint antigens or printmolecules], any of which is frequently encountered. Thelock building blocks are normally called functionalmonomers (M), although polymers have also been usedas imprinting building blocks. The molecular glue usedto fix the key-building block complexes is almost alwaysperceived as a crosslinker (X) or a crosslinking monomer.The entire imprinting procedure is performed in a

lled carbon nanotube (MWCNT)/fiber.

Table 2. Different coating polymer and modifier and their advantages

Coating polymer or modifier Advantages Ref.

Polyethylene glycol (PEG) High sample capacity for bothpolar and non-polar compounds

[26]

Crown ethers Good selectivity resulting fromits cavity structure and the strongelectronegative effect of heteroatomson the crown ether ring

[27,28]

Calixarenes Outstanding capacity [29–32]Polycrystalline graphites(pencil lead and glassy carbon)

Chemical resistance and low cost [33,34]

Activated carbon High partition coefficients (381 forbenzene and 1340 for o-xylene)

[35]

Fullerene Wide operational temperature, goodthermal stability and good selectivityfor aromatic compounds, especiallyexhibited strong affinity for planarPCB molecules

[36]

Ionic liquid-mediated sol-gel Low LOD [37,38]Bonded ionic liquid SPME High extraction efficiency, excellent

mechanical strength and durability(extraction at pH 11 without damage)

[39]

Permethylated-b-cyclodextrin Has high hydrophobic doughnut-shaped cavity [44]Amphiphilic and hydrophilicoligomers

Excellent SPME characteristics For theextraction of both non-polar and polarorganic compounds

[45]

Single and multi-wall carbonnanotubes (CNTs)

High surface area, higher enrichment factor [42,43,46–49]

Nanocomposites High surface area, high extractionefficiency, thermal stability

[50–54]


solvent that can be denoted porogen, since one of itsfunctions is to fill the space between the aggregatednetwork polymers so as to induce a porous construction[55].

3.2. MIP-based SPMEThe combination of MIP and SPME technique provides apowerful sample-preparation tool in terms of selectivity,simplicity, and flexibility.

The first attempt to use MIP in CME (in-tube SPME)was reported by Mullett et al. [56]. An automated, on-line MIP-SPME method was developed for the determi-nation of propranolol in biological fluids and showedimproved selectivity compared to in-tube stationary-phase materials, so overcoming the limitations of thenexisting SPME coating materials. Preconcentration of thesample by the MIP adsorbent increased the sensitivity,yielding low LODs.

Koster et al. [57] reported the first work dealing withthe use of MIP coatings on SPME fibers. A silica SPMEfiber was silanized, followed by in-situ synthesis of theMIP on the external surface of the fiber. They preparedthe MIP-based SPME fiber using clenbuterol as a tem-plate and demonstrated the possibility of selectiveextraction of brombuterol. This fiber was brittle and theMIP coating stripped during withdrawal of the fiber inthe needle.

Djozan et al. [58,59] used a monolithic MIP withSPME for selective extraction of diacetylmorphine andanalogous compounds. This was the first publication ofMIP-SPME followed by GC. The fiber prepared wasmonolith and flexible enough to be placed in a home-made syringe and inserted into a GC or a GC-MS injec-tion port. The MIP was prepared through thermal radi-cal copolymerization of MAA and EDMA in the presenceof DAMO as template, and acetonitrile (ACN) andazobis(isobutyronitrile) (AIBN) were also used as solventand initiator, respectively. For preparation of fibers, thepre-polymer solution was poured into a test tube andcapillary home-made glass as mold was inserted intotube and the mixture was cured. Polymeric monolithwas then withdrawn from the mold. Pulling out MIP andnon-imprinted polymer (NIP) fibers from capillary home-made glass used two methods:(1) the capillaries were immersed in 40% hydrofluoric

acid (HF) for 2 h, and, after dissolving the glass cap-illary, the polymeric fibers were washed with dis-tilled water;

(2) the fibers were mechanically withdrawn from mold.Djozan et al. also compared the extraction efficiency of

MIP with that of NIP.A novel MIP-coated SPME fiber that could be coupled

directly to HPLC was prepared by Hu et al. [60,61]. Priorto the coating procedure, the fibers were silylated by



immersing them into a 10% (v/v) 3-(methacryloxy)propyltrimethoxysilane solution in acetone at roomtemperature for 1 h. Fig. 5 shows the pretreatment offused silica and the MIP-coating process.

A simple approach for preparation of bisphenol A(BPA) MIP-coated SPME fibers was developed by Tanet al. [62]. A capillary was inserted into a larger borecapillary to form a sleeve as mold. The prepolymersolution, which comprised BPA, acrylamide (AM), 3-(trimethoxysilyl)propyl methacrylate (TRIM), AIBN andACN, was introduced into the interspace between thetwo capillaries, followed by polymerization under UVphotoirradiation. The larger bore capillary was etchedaway with hydrofluoric acid after the polymerization(Fig. 6).

A carbon monolith was also prepared by Feng et al.[63] and applied as the SPME fiber. The carbon monolithwas synthesized via a polymerization–carbonizationmethod, styrene and divinylbenzene being adopted asprecursors and dodecanol as a porogen during poly-

Figure 5. Preparation of molecularly-imprinted polyme


merization. The resultant monolith had a bimodal por-ous substructure, narrowly distributed nano-skeletonpores and uniform textural pores or through pores. Thecarbon monolith was directly used as an extraction fiberfor the extraction of phenols followed by GC-MS. Theresulting carbon monolith was demonstrated to be afavorable SPME fiber for extracting phenols as modelanalytes, while high extraction capacity, shorterextraction time and long lifespan could be achieved.

4. Electrochemically-prepared SPME fibers

4.1. Oxidized metal-based SPME fibersIn 2001, Djozan et al. [64] reported an anodized alu-minum wire as an SPME fiber. Aluminum wires wereanodized by direct current in a solution of sulfuric acid atroom temperature and were conditioned at 300�C for30 min. These fibers were used for the extraction of somealiphatic alcohols, BTEX and petroleum products from

r, solid-phase microextraction (MIP-SPME) fiber.

Figure 6. Preparing molecularly-imprinted polymer (MIP)-coated solid-phase microextraction (SPME) fibers.


gaseous samples. The results obtained proved the abilityof anodized aluminum wire to be a fiber for samplingorganic compounds from gaseous samples. This behaviorcould be due to the porous layer of aluminum oxide,which is formed on metal surfaces. This fiber was firm,inexpensive, durable and simple to prepare.

Djozan et al. also reported anodized zinc wire as anSPME fiber [65]. Zinc oxide is an interesting metal oxideand can be produced easily. It has considerable adsorp-tive and catalytic behavior. For preparation of fiber, zincmetal with high purity was washed with dilute detergentsolution and distilled water. The wire was used in threeforms:(1) polished wire without ZnO wire;(2) wire oxidized in air; and,(3) anodized zinc wire.

Type (1) fiber was prepared with mechanical polishingbefore use. Type (2) fiber was prepared with exposure toair for a long time. A thin layer of ZnO was formed in thisway. For preparation of type (3) fiber, the zinc wire wasanodized in a stainless-steel cell containing 0.2 M NaOHsolution as electrolyte at a controlled potential of 20 Vfor 20 min. The zinc wire and cell wall were used as theanode and cathode, respectively.

Comparison of these fibers showed that the ZnO layerformed on the surface of zinc wire has a major role inadsorption of the analytes with these fibers. However,when the zinc wire was anodized, the extraction effi-ciency was increased dramatically. The thickness andthe porosity of the oxide layer were increased substan-tially during the anodizing process. In-loop SPME cou-pled with HPLC using this method was also reported bythe same authors [66,67].

4.2. Conductive polymer-based SPME fibersConductive polymers are organic materials that gener-ally possess an extended conjugated p-electron systemalong a polymer backbone. They are versatile materials

in which molecular/analyte recognition can be achievedin different ways, including:(1) incorporation of counter ions that introduce selec-

tive interaction of counter ions;(2) use of the inherent and unusual multifunctionality

(e.g., hydrophobic, acid–base and p–p interactions,polar functional groups, ion-exchange, hydrogenbonding, electroactivity) of the polymers;

(3) introduction of functional groups to the monomers.These materials can be easily synthesized in both

aqueous and non-aqueous medium, chemically andelectrochemically; there are various types of dopant andadditives for use in synthesis. All these conditions andvarieties affect the chemical, mechanical, morphologicaland electronic properties of the polymers. These poly-mers can be synthesized from a wide range of commer-cial monomers, which could lead to many polymershaving different properties.

Application of SPME for extraction of ionic species hasbeen limited since the neutral charge of commerciallyavailable SPME coatings result in a low coating/samplepartition coefficient and poor analyte recoveries. Toovercome this difficulty, chemical modification of thesample or fiber surface by derivatization or addition ofcomplexing agents (e.g., crown ethers) has been devel-oped to increase the extraction efficiency [68]. Thesereactions can require expensive, toxic reagents andproduce unstable products, and the procedures are oftentime consuming and labor intensive.

Electrochemically-aided control of SPME (EASPME)using a conducting polymer for ionic species has beendeveloped by Ceylan et al. [69] as the extraction phase toaddress the above issues. This work described the use ofan SPME method with poly(3-methylthiophene) coatedplatinum micro-fiber electrodes to extract arsenate ionsfrom aqueous solutions without derivatization. The fi-bers were fabricated by cycling the working electrodebetween �0.20 and +1.7 V (versus Ag/AgCl) in an ACN



solution containing 50 mM 3-methylthiophene mono-mer and 75 mM tetrabutylammonium tetrafluoroborate(TBATFB) electrolyte. All electrochemical procedures(extraction and expulsion) were conducted in a three-electrode system. After fabrication, the conductingpolymer film was immersed in the sample solution andconverted to its oxidized, positively charged form byapplying a constant potential of +1.2 V with respect to

Figure 7. Mechanism of polymerization of m


the Ag/AgCl reference electrode. Arsenate ions migratedinto the film to maintain electroneutrality. Upon sub-sequent reversal of the potential to �0.60 V versus Ag/AgCl, the polymer film was converted to its reduced,neutral form, and the arsenate ions were expelled into asmaller volume (200 lL) of de-ionized water for analysisusing flow injection with inductively coupled plasmamass spectrometric (ICP-MS) detection. Fig. 7 shows the

onomers (e.g., thiophene and pyrrole).


mechanism of polymerization of monomers, such asthiophene and pyrrole.

Pawliszyn et al. [70] also coated polypyrrole (PPy) andpoly-N-phenylpyrrole (PPPy) on the surface of metalwires by an electrochemical method. These coated wireshave been used for SPME of volatile organic compounds.They also reported polypyrrole-coated capillary in-tubeSPME coupled with LC-electrospray ionization massspectrometry for the determination of b-blockers in urine

Table 3. Summary of some applications of fibers

Fiber type Analytes

Sol gel-based Polycyclic aromatichydrocarbons (PAHs)Organophosphorus pesticides (OPs)Phenols (Halophenols,Dimethylphenols, cresol, . . .)AldehydesKetonesAlkylbenzenesAminesEstrogensPolychlorinatedbiphenyls(PCBs)EstersBisphenol A (BPA)Polybrominated diphenylethers (PBDEs)AlcoholsFatty acidsAntiestrogensHaloanisolesMethyl tert-butyl ether (MTBE),ethyl tert-butyl ether (ETBE) andtert-amyl methyl ether (TAME)

MIP PropranololClenbuterolDiacetylmorphineTriazine

Tetracyclines

Bisphenol A (BPA)8-Hydroxy-2 0-deoxyguanosine17b-Estradiol

Electrochemicallyassisted

Oxidizedmetals-based

Alkylbenzenes, alcoholsThiophenolsPhenol and biphenylL-Dopa and L-Dopamine

Conductivepolymers-based

Arsenate ionsVOCsb-blockersInorganic anionsCationic analytesPhenolsOrganochlorine pesticidesPCBsPAHsFluoxetine and norfluoxetineenantiomers

and serum samples [71]. Due to the inherent multi-functional properties of PPy, primarily p–p interactions,base–acid interactions, and interactions from polargroups and hydrogen bonding, PPy coating can be usedfor SPME of a large range of analytes.

The first example of coupling SPME to ion chroma-tography (IC) was reported by Pawliszyn et al. [72]. Thetwo new coatings showed different selectivities tovarious organic compounds (e.g., the PPPy coating had

Matrix Ref.

Water samples [2,7,8,19,20,22,30,45,54,76]

Water samples [5,18,25]Water samples [2,6,14,16,20,22,24,

31,38,43,50,55,78]Water samples [6,8,14,20,22,37,38,51]Water samples [15,20,22,37,38,51]Water samples [6,7,15,17,23,26,28,30,51,52,78]Water samples [14,22,27,29,30,36,38,50]Water samples [45]Water samples [36]

Water and beer [16,21,28,32,79]Water samples [45]Water and milk [42,44]

Water and beer [14,20,21,37,38]Water and beer [14,21,22,38]Biological samples [77]Cork stopper [80]Urine [49,53]

Urine and plasma [56,82]Urine [57]Water samples [58]Water, rice, onion, soy bean,corn, lettuce and soil

[59,60]

Chicken feed, chickenmuscle and milk

[61]

Water, urine and milk [62]Urine [81]Fish [83]

Gaseous samples [64]Gaseous samples [65]n-Hexane [66]Aqueous solutions [67]Water samples [69]Water samples [70]Urine and serum [71]Water samples [72]Water samples [73]Water samples [74,47]Water samples [75]Water samples [84]Water samples [85]Human plasma [86]



better selectivity to aromatic compounds than the PPycoating, due to the phenyl group incorporated into thepolymer). This demonstrated that the selectivity of thecoating can be modified by introducing a new functionalgroup into the polymer.

A metal wire (platinum, gold or stainless steel) wasinserted into the needle of a Hamilton syringe and at-tached to the plunger. Polypyrrole (PPy) was directlyprepared onto the surface of the working electrode froma 0.1 M tetrabutylammonium perchlorate–ACN solutioncontaining 0.1 M corresponding monomer. A constantdeposition potential (1.0 V) was applied using a poten-tiostat.

In 2002, Ceylan et al. [73] used EASPME conducting-polymer-film material for extraction of cationic analytes.They used a PPy film with sulfated b-cyclodextrin (SbCD)as the dopant anion. The resulting poly(pyrrole-sulfatedb-cyclodextrin) (PPy-SbCD) film contained a very largeexcess of the sulfate anion sites, far above that necessaryas the counter ions compensating the positive charge ofthe cation radicals of oxidized PPy. The SbCD moleculesare permanently trapped in the PPy matrix and made ita high-capacity cation-exchanger film.

In 2005, an aniline-based polymer was electrochem-ically prepared and applied as a new fiber coating forSPME by Bagheri et al. [74]. A platinum wire was in-serted into the needle of a homemade syringe and at-tached to the plunger. The polymer film was directlyelectrodeposited on the platinum-wire surface in sulfu-ric-acid solution using a CV technique. Electrochemicaldeposition was performed using a three-electrode CVsystem by oxidation of 0.1 M aniline solution in 1.0 MH2SO4 under the nitrogen atmosphere. Prior to thepolymerization procedure, the platinum-wire surfacewas chemically cleaned in acetone using an ultrasonicbath for 15 min and was subsequently washed withdistilled water. The platinum wire (1 cm · 240 mm i.d.)was used as the working electrode; a platinum rod andan Ag/AgCl electrode were employed as the counterelectrode and the reference electrode, respectively. TheCV system was operated using a scan rate of 25 mV/s ata potential range of 0.65–0.9 V, and the number ofscans was set at 10. The fiber was then washed by dis-tilled water so that unwanted chemicals (e.g., monomersand the supporting electrolyte) were removed. The fiberwas dried under a gentle stream of nitrogen, preheatedat 80�C for 30 min and then inserted into a GC injectorat 200�C for 1 h while the helium gas was flowing. Theprepared fiber was successfully applied for extraction ofsome priority phenols from water samples.

In 2009, Zhao et al. [47] reported a novel MWCNT–polyaniline composite film-coated platinum wire forheadspace SPME-GC determination of phenolic com-pounds. The electrodeposited coating had a porousstructure, with high specific surface area and adsorptioncapacity, so it had high extraction efficiency for phenolic


compounds. Due to chemical binding between the Ptsubstrate and the coating, and the interaction of poly-aniline with the MWCNTs, the probe also showed highthermal stability (up to 320�C) and excellent re-usabil-ity. In addition, the probe had the advantages of easypreparation and low cost.

Polyphosphate-doped polypyrrole coated on steel fiberwas also reported by Noroozian et al. [47]. The process ofelectrodepositing polyphosphate-doped PPy on steel fiberwas very simple and inexpensive. The coating producedshowed much greater thermal stability than PPy coat-ings doped with other counter ions. The thermal stabilityof PPy/polyphosphate was even greater than that ofPDMS, PA, and Carbowax/PDMS commercial coatingscommonly used. These characteristics made possible theSPME/thermal desorption of highly involatile com-pounds. The coating also showed long lifetime withexcellent adhesion onto the steel surface.

Some typical applications of the prepared fibers by thementioned methods are shown in Table 3.

5. Conclusion and future trends

Nowadays, preparing unbreakable, chemically andthermally stable SPME fibers, overcoming the mainproblems of conventional fibers, has been attained usingnew techniques (e.g., sol gel, MIP and electrochemicallyassisted). These techniques can create chemical bondsbetween coating polymers and substrates. Unbreakablefibers can also be prepared by metal substrates that arealready functionalized by different techniques (e.g.,SAMs).

SPME fibers prepared by sol-gel methods are notselective to target analytes, and fibers obtained by theMIP technique do not have such rigid, strong structuresas fibers prepared by sol-gel methods, which is easier,more controllable and cheaper than the MIP technique,so coupling these methods together seems to be a newtrend in SPME fibers. Preparing sorbents by the sol-gel–MIP technique will probably be the focus in future forsynthesizing selective, stable fibers under the mild con-ditions that might be used for relatively complex bio-logical samples.

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