Nanoconfined Ionic Liquids
2.5 Analytical Applications of Ionic Liquids Confined in a Solid Matrix
2.5.2 Solid‐Phase Microextraction
1‐Hexylpyridinium hexafluorophosphate [C6Py][PF6] ionic liquid was for the first time confined in silica material by the acid‐catalyzed sol–gel processing. The obtained pyridinium IL‐modified silica was applied as a SPE sorbent for removal of trace levels of Fe(III) ions from aqueous samples. Under optimal experimental conditions, the limit of detection and limit of quantification were 0.7 and 2.5 μg l−1, respectively [193]. The developed method was validated by the analysis of certified reference material and applied successfully to the isolation and determination of iron in several water samples.
In other research, a novel dummy molecularly imprinted polymer (DMIP) was prepared through a sol–gel process using bisphenol A as the dummy template and synthesized 1‐(triethoxysilyl)propyl‐3‐aminopropylimidazole bromide [{(C2O)3SiC3} C3NH2IM][Br] as a functional monomer. This novel DMIP was applied as a SPE mate- rial to develop a method for the simultaneous determination of nine organochlorine pesticides (OCPs) in environmental and food samples. The developed method was applied to the analysis of water, rice, and tea leaf samples spiked with nine OCPs. The OCPs were simultaneously determined with satisfactory recoveries from 65.65% to 129.61% and a good relative standard deviation (n = 3) of 0.91–11.47%, indicating acceptable accuracy and precision of the developed method [194].
In another study, a new solid‐phase extractant (SG‐[N‐phenacylPyr][NTf2]) was pro- posed by Marwani et al. [195]. The material was based on a hybrid combination of the hydrophobic character of newly synthesized ionic liquid with silica gel (SG) properties, without the need for partial treatment by chelating compounds. The selectivity of SG‐[N‐phenacylPyr][NTf2] towards different metal ions, including Co(II), Fe(II), Fe(III), La(III), and Ni(II), was investigated. The adsorption isotherm fit well with the Langmuir adsorption model. A kinetic study also demonstrated that the adsorption of La(III) on the SG‐[N‐phenacylPyr][NTf2] phase was in accordance with the pseudo second‐order kinetic model. The proposed method was applied to real water samples with satisfac- tory results.
In some studies [196, 197], the performance of the IL‐supported sorbents turned out to be favorable in terms of preconcentration factors and limits of detection achieved for other ionic materials and processes of metals extraction. For example, a [C4C1IM][PF6]‐
modified silica sorbent was used to preconcentrate Cd2+ after the addition of the chelat- ing agent dithizone. Recoveries for 150 ml of lake water and tap water were greater than 95% and at least 20 extractions could be made with the same sorbent without a decrease in the recovery of Cd2+.
Nanoconfined Ionic Liquids 51
this technique [208]. Extraction is performed either by direct immersion (DI) of the fiber in the investigated gaseous or liquid medium (DI‐SPME) or by extraction of ana- lytes from the headspace (HS) of the sample (HS‐SPME) [209].
Over two decades of studies on new sorption materials for SPME, numerous original and review papers have been published [210–212]. Ionic liquids, which display many favorable properties, are also used as extraction media in SPME [213]. Their high vis- cosity improves the quality of fiber coating, while using the proper ionic liquid (cation–
anion pair) can result in high selectivity of the extraction process. First attempts at coating the SPME fiber with ionic liquids were undertaken in 2005 [214]. However, the obtained fibers could be used only once and had to be re‐coated after each extraction.
In addition, a low efficiency of analyte enrichment due to thin extraction layer is another disadvantage of the proposed solution. To partially eliminate this problem, the fiber was first coated with a thin layer of the polymer Nafion and then with the ionic liquid film.
The polymer increased the surface wettability and therefore enabled the formation of a thicker layer of ionic liquid. Unfortunately, it was necessary to wash off the sorbent layer after the analysis and re‐coat the fiber, which seriously limited the practical use of this technique [215]. The problems related to low durability of fibers coated with ionic liq- uids have been eliminated by introducing polymeric ionic liquids [216]. These com- pounds are characterized by high thermal stability and therefore allow for a long‐term usage of the fibers (even up to 150 extractions) without requiring the stationary phase re‐coating. Polymeric ionic liquids are deposited on the fiber surface via cross‐linking with silica particles [217]. Silicon elastomers deposited on the fiber surface can also be impregnated with an ionic liquid [218] by the sol–gel process [175, 184, 219, 220] and in situ cross‐linking onto stainless steel fibers coated with the microstructured-silver layer [221, 222].
Liu et al. [175] published an article presenting a new SPME fiber coating pre- pared from ionic liquid by sol–gel technology. Two functionalized ionic liquids, 1‐allyl‐3‐methylimidazolium hexafluorophosphate and 1‐allyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide, were used as selective coating materials to prepare chemically bonded IL‐based organic–inorganic hybrid sorption material for SPME.
These fibers were prepared with the addition of γ‐methacryloxypropyltrimethoxysilane as a bridge using sol–gel method and free radical cross‐linking technology. The under- lying mechanisms of the sol–gel reaction were proposed, and the successful binding of these functional ILs to the sol–gel substrate was confirmed by Fourier‐transform infra- red spectroscopy. These IL‐based sol–gel coatings had a porous surface structure, high thermal stability, a wide range of pH stability, strong solvent resistance, and good coat- ing preparation reproducibility. They also had high selectivity and sensitivity towards strongly polar compounds. The applicability of these IL‐based sol–gel coatings was evaluated through the analysis of phenolic environmental estrogens (PEEs) in two real water samples. The detection limits were quite low, varying from 0.0030 to 0.1248 μg l−1. The relative recoveries were between 83.1% and 104.1% for lake water and between 89.1% and 97.1% for sewage drainage outlet water.
A similar solution presented by Zhou et al. [176] was the preparation of new selective task‐specific IL‐based‐SPME fibers by the sol–gel method and free radical cross‐linking technology. A novel crown ether functionalized ionic liquid, 1‐allyl‐3‐(6‐oxo‐benzo‐
15‐crown‐5‐hexyl)imidazolium hexafluorophosphate, was used in the studies. The obtained coating was characterized by a highly porous surface structure, stable
performance at high temperature (to 340 °C) and in different solutions (water, organic solvent, acid, and alkali), and good coating preparation reproducibility. A newly devel- oped sol–gel crown ether functionalized IL‐based coating was superior for alcohols, phthalate esters, phenolic environmental estrogens, fatty acids, and aromatic amines due to the introduction of a benzo‐15‐crown‐5 functional group in the IL structure.
Moreover, the material provided higher or comparable extraction efficiencies for most analytes studied than the commercial PDMS, PDMS/DVB, and PA fibers did.
Ionic liquids and sol–gel technology have been also investigated by Shu et al. [177].
This work demonstrates that the performance of the ionic‐liquid‐based coatings can be simply tuned by changing the counter‐anions incorporated into the ionic liquid struc- tures. Three alkoxyl‐functionalized ionic liquids, 1‐(3‐triethoxysilylpropyl)‐3‐methyl imidazolium hexafluorophosphate ([TESPMIM][PF6]), 1‐(3‐triethoxysilyl propyl)‐3‐
methyl imidazolium tetrafluoroborate ([TESPMIM][BF4]), and 1‐(3‐triethoxysilyl propyl)‐3‐methyl imidazolium bis(trifluoromethanesulfonyl)imide ([TESPMIM]
[NTf2]), were used as selective coating materials to prepare ionic‐liquid‐based organic–
inorganic hybrid fibers for SPME. These ionic liquid‐based sol–gel coatings showed a porous surface structure, strong solvent resistance, wide pH application range, good coating preparation reproducibility, specific selectivity, and extractability of both polar and nonpolar compounds, such as phenolic environmental estrogens, fatty acids, aromatic amines, alcohols, phthalate esters, and polycyclic aromatic hydrocarbons.
All fibers are highly thermally stable: [TESPMIM][PF6]‐, [TESPMIM][BF4]‐, and [TESPMIM][NTf2]‐coated fibers are stable up to 300, 285, and 454 °C, respectively. The different counter‐anions in ionic liquid structures are characterized by different steric hindrance, nucleophilicity, hydrophobicity, and ability to form hydrogen bonds, result- ing in significant differences in the characteristics of the ionic‐liquid‐based SPME fibers, which afforded higher selectivity of [TESPMIM][PF6]‐ and [TESPMIM][BF4]‐
coated fibers towards strong polar analytes, with lower selectivity towards medium polar or nonpolar analytes in comparison to [TESPMIM][NTf2]‐based fiber.
Tian et al. [178] proposed a new ionic liquid–calixarene coated SPME fiber. The coated fiber has been applied for the isolation of triazines from fruit and vegetable sam- ples. Under optimal conditions, the limits of detection of atrazine, simazine, ametryn, and cyanazine were 3.3, 4.4, 8.8, and 13.0 μg kg−1, respectively. Notably, the intra‐day and inter‐day relative standard deviations were less than 10%. The proposed method has been successfully applied to the determination of four triazines in fruit and vegeta- ble samples and the accuracy was assessed through recovery experiments. Based on the results, the method developed in this work has been proved to be simple, fast, selective, and sensitive for monitoring the triazines in fruit and vegetable samples and may be an alternative for other similar solutions [223].
Another novel approach was developed by Pang et al. [179]. They developed a new method for the isolation of organophosphate esters (a class of emerging pollutants), widely used as flame retardants and plasticizers, from aqueous samples. The 1‐hexade- cyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide, [C16C1IM][NTf2], ionic liquid confined within a hybrid network by sol–gel technology in the form of a SPME fiber was applied for extraction of six organophosphate esters from water samples. The coating thickness of the obtained SPME fiber was ∼35 μm with good thermal stability and long lifetime. Under optimized conditions, the limits of detection were in the range 0.04–0.95 μg l−1, and the repeatability and reproducibility of the method (RSD%) was
Nanoconfined Ionic Liquids 53
less than 13% and 29%, respectively. The developed method was successfully applied to determine six organophosphate esters in three real water samples, with recoveries in the range 64.8–125.4%.
An interesting solution has been suggested by Sarafraz‐Yazdi et al. [180]. An HS‐
SPME method with ionic liquid mediated sol–gel sorbents was applied for extraction and preconcentration of BTEX (benzene, toluene, ethylbenzene, and o‐xylene) from water samples at ultra‐trace levels. Three different coating fibers were prepared: (i) poly(dimethylsiloxane) (PDMS), (ii) coating prepared from poly(dimethylsiloxane) in the presence of ionic liquid as co‐solvent (PDMS‐IL‐HT), and (iii) coating prepared from poly(dimethylsiloxane) in the presence of ionic liquid as co‐solvent and condi- tioned at a lower temperature than decomposition temperature of ionic liquid (PDMS‐
IL‐LT). The obtained SPME fibers had many advantages such as high thermal and chemical stability due to the chemical bonding of the coating with the silanol groups on the fused‐silica surface. These fibers have shown a long shelf‐life of up to 180 extrac- tions. Under optimal conditions, the dynamic linear range of the methods utilizing PDMS‐IL‐HT, PDMS, and PDMS‐IL‐LT fibers were 0.3–200 000, 50–200 000, and 170–150 000 pg ml−1, respectively, whereas the detection limits were 0.1–2, 15–200, and 50–500 pg ml−1 respectively. The relative recoveries obtained for the spiked water sam- ples at 20 pg ml−1 were in the range 91.2–103.3%. The developed method was success- fully applied in the analysis of real water samples.
Pena‐Pereira et al. [181] developed a new hybrid silica‐based material with immobi- lized ionic liquid, 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4MIM][NTf2]), and evaluated the material as solid‐phase microextraction fiber coat- ing. High loadings of the IL were confined within the hybrid network with sol–gel tech- nology. For the obtained fibers energy dispersive X‐ray spectrometry (EDX) analysis confirmed the successful immobilization of [C4MIM][NTf2] within the three‐dimen- sional solid‐like network (Figure 2.10). The obtained ionogel SPME fibers exhibited
15000 10000
5000 0 C
OF Si
S
0 2 4 6
keV
20 μm Si 20 μm S
C 20 μm
20 μm 20 μm O
F
8 10
Figure 2.10 EDX spectrum of [C4MIM][NTf2]‐ionogel fiber and the corresponding elemental mapping of C, O, F, Si, and S. Source: Reprinted from Reference [181]. Copyright 2014 American Chemical Society. Reproduced with permission of the American Chemical Society.
high extractability for aromatic volatile compounds, good sensitivity, and precision when combined with a gas chromatograph with barrier ionization discharge (GC‐BID) detection. The authors applied a central composite design to assess the effect of experi- mental parameters on the extraction process. The obtained ionogel SPME fiber coat- ings enabled the achievement of excellent enrichment factors (up to 7400). The limits of detection (LOD) were found in the range 0.03–1.27 μg l−1, whereas the repeatability and fiber‐to‐fiber reproducibility were 5.6% and 12.0% on average, respectively. The results obtained in this work allow the suggestion that ionogels can be promising coating materials for future applications of SPME and related sample preparation techniques.
The same authors [182] in further research prepared new sorbent coatings based on other ionic liquids confined in a silica network for the SPME technique. Ionogels derived from three different ILs based on the anion bis(trifluoromethanesulfonyl)imide (NTf2), namely 1‐butyl‐3‐methylpyridinium bis(trifluoromethanesulfonyl)imide ([C4C1Py]
[NTf2]), 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([C4C1Pyrr]
[NTf2]), and 1‐butyl‐1‐methylpiperidinium bis(trifluoromethanesulfonyl) imide ([C4C1Pip][NTf2]), were obtained on the outer surface of optical fibers by sol–gel tech- nology. Structures of the obtained ionogels were characterized by scanning electron microscopy (SEM) and energy dispersive X‐ray spectrometry (EDX). Then, the obtained ionogel‐based fibers were applied to the headspace solid‐phase microextraction (HS‐SPME) of volatile chlorinated organic compounds in combination with gas chroma- tography with barrier ionization discharge detection. The highest extractability was observed for the fiber covered with ionogel based on [C4C1Pyrr][NTf2]. Limits of detec- tion for the developed method were between 11 and 151 ng l−1 for the target compounds.
The inter‐day repeatability, intra‐day reproducibility, and fiber‐to‐fiber reproducibility were less than 8.5%, 9.6%, and 16.9%, respectively.
Another simple, low‐cost and sensitive method utilizing ILs in order to determine organophosphate esters (OPEs) in water samples has been developed with the utiliza- tion of HS‐SPME followed by gas chromatography‐flame photometric detection [183].
The ionic liquid (1‐allyl‐3‐methylimidazolium tetrafluoroborate, [allyl‐C1IM][BF4])‐
based coating was developed by sol–gel technology and implemented for analyte extraction. The prepared coating was stable at high temperatures (up to 335 °C) and in contact with a range of solvents. It could be used at least 200 times without an obvious decrease in extraction efficiency. The developed method was successfully applied to the determination of OPEs in lake water, wastewater, sewage treatment plant effluent, and tap water with recoveries varying from 75.2% to 101.8%. The results demonstrate that the method is highly effective for the analysis of OPEs in water samples.
Another ionogel‐based SPME fiber has been obtained from [allyl‐C1IM][NTf2] ionic liquid [184]. The obtained fiber was successfully applied to the determination of phtha- late esters (PAEs) in agricultural plastic films by ultrasonic extraction combined with solid‐phase microextraction–gas chromatography. The [allyl‐C1IM][NTf2]‐OH‐TSO fiber showed comparable, or in some cases even higher, response to most of the inves- tigated PAEs as commercial PDMS, PDMS–DVB, and PA fibers. The carryover prob- lem, often encountered when using commercial fibers, had been eliminated when desorption was performed at 360 °C for 8 min. Some of the PAEs studied were detected at very high concentrations in investigated agricultural plastic film samples, which may pose a potential risk of crop damage, environmental contamination, and human exposure.
Nanoconfined Ionic Liquids 55
A promising solution was proposed by Ebrahimi et al. [185] where ionic liquid mediated sol–gel sorbents were applied to hollow fiber solid‐phase microextraction.
The developed method was applied with the aim of extracting the pesticides diazinon, fenitrothion, malathion, fenvalerate, phosalone, and tridemorph from human hair and water samples. The analytes were subsequently analyzed by high‐performance liquid chromatography and diode array detection (HPLC–DAD). The sol–gel nanocompos- ites were reinforced with nanoparticles, such as carboxylic functionalized multi‐walled carbon nanotubes (COOH‐MWCNTs), amino functionalized multi‐walled carbon nanotubes, nano SiO2, nano TiO2, and nano MgO comparatively to promote extraction efficiency. In these devices, the innovative solid sorbents were developed by the sol–gel method via the reaction of tetraethyl orthosilicate (TEOS) with 2‐amino‐2‐hydroxym- ethyl‐propane‐1,3‐diol (TRIS). Under basic conditions (pH 10–11), the gel growth pro- cess in the presence of ionic liquid and nanoparticles was initiated. Then, the sol was injected into a polypropylene hollow fiber segment for the in situ gelation process.
Parameters affecting the efficiency of HF‐SPME were thoroughly investigated. Linearity was observed over the range 0.01–25 000 ng ml−1 with detection limits between 0.004 and 0.095 ng ml−1 for the pesticides in the aqueous matrices and 0.003–0.080 ng ml−1 in hair matrices. The relative recoveries in the real samples ranged from 82.0% to 94.0% for the pesticides store seller’s hair and the work researchers’ hair. Results revealed the great possibilities of HF‐SPME–HPLC‐PDA for analysis of pesticides in biological and environmental samples.
Another material reported in the literature, for effective use in hollow fiber solid phase microextraction (HF‐SPME), was heteropolyacid‐based supported ionic liquid mediated sol–gel hybrid organic–inorganic material. A Keggin‐based IL in conjunction with sol–gel was evaluated. This study showed that the Keggin‐based IL sol–gel gener- ated porous morphology provides an effective extraction media. The method was developed for the extraction of the organophosphorus pesticides (OPs) – diazinon, fenitrothion and malathion from human hair samples. The OPs were subsequently ana- lyzed with high‐performance liquid chromatography and photodiode array detection (HPLC–PDA). Under basic conditions (pH 10–11), the gel growth process in the pres- ence of IL was initiated. Afterward, this sol was injected into a polypropylene hollow fiber segment for an in‐situ‐gelation process. Parameters affecting the efficiency of HF‐SPME were thoroughly investigated. Linearity was observed over a range of 0.02–
50 000 μg g−1 and 0.0001–25 000 ng ml−1 with detection limits between 0.0074 and 1.3000 μg g−1 and 0.00034 and 0.84 ng ml−1 for the OPs in hair and aqueous matrices, respectively. The relative recoveries in the real samples, for OPs in the storekeeper’s hair, ranged from 86 to 95.2% [186].