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Polyaniline-nylon-6 electrospun nanofibers for headspace adsorptive microextraction
Analytica Chimica Acta 713 (2012) 63–69
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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Polyaniline-nylon-6 electrospun nanofibers for headspace adsorptive microextraction Habib Bagheri ∗ , Ali Aghakhani Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran
a r t i c l e
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Article history: Received 3 July 2011 Received in revised form 9 November 2011 Accepted 10 November 2011 Available online 19 November 2011 Keywords: Polyaniline-nylon-6 nanofibers Electrospinning Headspace adsorptive microextraction Gas chromatography–mass spectrometry Chlorobenzenes
a b s t r a c t A headspace adsorptive microextraction technique was developed using a novel polyaniline-nylon-6 (PANI-N6) nanofiber sheet, fabricated by electrospinning. The homogeneity and the porosity of the prepared PANI-N6 sheet were studied using the scanning electron microscopy (SEM) and nanofibers diameters were found to be around 200 nm. The novel nanofiber sheet was examined as an extracting medium to isolate some selected chlorobenzenes (CBs), as model compounds, from aquatic media. The extracted analytes were desorbed using L-amounts of solvent and eventually an aliquot of extractant was injected into gas chromatography–mass spectrometry (GC–MS). Various parameters affecting the extraction and desorption processes were optimized. The developed method proved to be convenient and offers sufficient sensitivity and a good reproducibility. Limits of detection achieved for CBs with the developed analytical procedure ranged from 19 to 33 ng L−1 , while limits of quantification were from 50 to 60 ng L−1 . The relative standard deviations (RSD) at a concentration level of 0.1 ng mL−1 and 1 ng mL−1 were in the range of 8–14% and 5–11% (n = 3), respectively. The calibration curves of analytes were investigated in the range of 50–1000 ng L−1 and R2 between 0.9739 and 0.9932 were obtained. The developed method was successfully applied to the extraction of selected CBs from tap and river water samples. The relative recovery (RR) percentage obtained for the spiked real water samples at 0.1 ng mL−1 and 1 ng mL−1 level were 93–103% and 95–104%, respectively. The whole procedure showed to be conveniently applicable and quite easy to handle. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Because of the low concentrations of environmental pollutants in water samples, it is necessary to apply a preconcentration step prior to the instrumental analysis. The traditional sample preparation methods such as liquid–liquid extraction (LLE) and solid phase extraction (SPE), consume large amounts of samples and organic solvents, while they are time consuming and tedious. Recent approaches based on the sorptive and adsorptive microextraction, provide advantages of being solvent-less or little solvent usage, lower sample consumption, higher sample preparation speed and ease of operation [1]. The common aspects in the microextraction strategies are mostly focused on miniaturized sorbent amount and the extraction device geometry which often lead to the extraction of small fraction of the analytes from the sample into the extracting phase [2]. In order to increase the sensitivity of solid phase microextraction (SPME), which is due to its lower sample capacity, other approaches including stir bar sorptive extraction (SBSE) were developed to provide higher mass of sorbent in the sorptive
∗ Corresponding author. Tel.: +98 21 66165316; fax: +98 21 66012983. E-mail address: [email protected] (H. Bagheri). 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.11.027
extraction manner [3,4]. Although these approaches were used to improve different aspects of the whole extraction procedure, the fundamental bases are rather similar [5]. Another strategy to improve the sensitivity of the microextraction methods relies on the usage of nanostructured materials in which the interactions between the desired analytes and the sorbent are increased. This could be due to the enhancement of surface to volume ratio as the size of the sorbent particles is reduced extensively [6–8]. An emerging method for preparing nano-dimensional sorbents is electrospinning [9]. This methodology is a convenient technique for fabricating fibrous sorbent with controllable diameter, functionality and high surface to volume ratio [10–12]. In this technique by applying high voltages to a viscous polymeric solution, when the electrostatic repulsive force overcomes the surface tension of the polymeric solution, a charged polymeric jet ejects from the solution and afterwards flies toward the collector and forms the fibrous material with diameters in the scale of nano-tomicrometer. The electrospun sorbents were used in SPE [13–16], micro-SPE [10], SPME [17–19] and membrane extraction [20] of some organic and inorganic compounds. In this work, polyaniline (PANI) was synthesized inside the nylon-6 (N6) solution as a carrier polymer, and a composite of polyaniline-nylon-6 (PANI-N6) was electrospun into the fibrous
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sheet with nano-scale dimensions. Then, the prepared composite was applied for the headspace adsorptive microextraction of some selected chlorobenzenes (CBs), as model compounds, to evaluate its extraction applicability. 2. Experimental 2.1. Reagents The CB compounds 1,4-dichlorobenzene (14DCB), 1,2dichlorobenzene (12DCB), 1,2,4-trichlorobenzene (124TCB), 1,2,3-trichlorobenzene (123TCB) and 1,2,3,4-tetrachlorobenezne (1234TeCB) were purchased from Merck (Darmstadt, Germany). Standard solution (2000 mg L−1 ) of CBs mixture was prepared in HPLC-grade methanol (Merck) and stored in the refrigerator. The working standard solutions were prepared weekly by diluting the standard solution with methanol, and more diluted working solutions were prepared daily by diluting this solution with double distilled water (DDW). Ammonium peroxodisulphate and sodium chloride were purchased from Merck. Aniline was obtained from Merck and distilled before use. Nylon-6 (N6) was purchased from Kolon Industries Inc. (Seoul, Korea) and formic acid was obtained from Riedel-de Haën (Seelze-Hannover, Germany). All solvents used in this study were of analytical reagent grade or HPLC grade. 2.2. Apparatus An Agilent (Palo Alto, CA, USA) 6890 plus series GC equipped with a split/splitless injector and an Agilent 5973 mass selective detector system were used. The MS was operated in the EI mode (70 eV). Helium (99.999%) was employed as carrier gas and its flow rate was adjusted to 1 mL min−1 . The separation of CBs was performed on a 30 m × 0.25 mm HP-5 MS column (0.25 m film thickness). The GC column temperature was first started at 50 ◦ C and held for 5 min, then increased by 30 ◦ C min−1 to 80 ◦ C and held for 1 min, then ramped at 20 ◦ C min−1 to 200 ◦ C for 5 min. The injector temperature was set at 250 ◦ C in the splitless mode for 1 min. The GC–MS interface, ion source and quadrupole temperatures were set to 280, 230 and 150 ◦ C, respectively. The selected ion monitoring (SIM) mode, considering two characteristic ions for each compound (Table 1), was used for quantitative analysis. The surface morphology of the fabricated nanofiber sheet was investigated by a TSCAN VEGA II XMU SEM instrument (Brno, Czech Republic). All samples were heated in a homemade glass water
Table 1 The selected ions, scan start times and retention times of CB mixtures studied by GC–MS. Compounds
Selected ions (m/z)
Scan start time (min)
Retention time (min)
14DCB 12DCB 124TCB 123TCB 1234TeCB
146, 148 146, 148 180, 182 180, 182 214, 216
3 3 8 8 12
5.89 6.30 8.99 9.76 12.98
bath connected to a refrigerated circulating water bath (RTE-8, Neslab Instruments Inc., Portsmouth, NH, USA) and stirred using a magnetic stirrer (Baird & TatLook, London, England). A Brandenburg (West Midlands, England) regulated power supply was used for electrospinning. A KDS100 syringe pump (KdScientific Co., Holliston, MA, US) was used for the polymer solution delivery in the electrospinning process.
2.3. Electrospinning of PANI-N6 Firstly, an amount of 0.25 g N6 was dissolved in 1 mL of formic acid. Then, an amount of 0.1 g ammonium peroxodisulphate was added to this polymeric solution. After dissolving the salt, the aniline monomer (0.5 g) was added to the solution and the mixture was stirred to obtain a homogenous solution. After that, 0.5 mL of this solution was withdrawn by a 2.5 mL syringe which was eventually transferred to a syringe pump. A piece of aluminum foil (10 × 10 cm) was employed as a collector electrode. The collector and the polymer containing syringe needle were connected to the high voltage power supply terminals. The distance between the needle and the collector was set at 10 cm. Fig. 1 shows the schematic diagram of the apparatus used in the electrospinning process. A voltage of 16 kV was applied for the nanofibers production while a flow rate of 1.5 L min−1 was set for the syringe pump to deliver the polymer solution. All fibers were electrospun for 12 h to achieve the desired coating thickness. The electrospinning experiments were performed under the ventilation. After the electrospinning experiment, the collector foil was separated from the power supply and afterward a sheet with a typical dimension of 1 × 1 cm, was cut from the central part of the Al foil using a predesigned template and employed for extraction.
Fig. 1. Scheme of the electrospinning set up.
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3. Results and discussion 3.1. Preparation of PANI-N6 nanofiber
Fig. 2. Images of the PANI-N6 nanofiber sheet used for headspace adsorptive microextraction (a) before conditioning (b) after conditioning (c) scheme of sample extraction. a: nanofiber sheet, b: gauze metallic cylinder.
2.4. Headspace adsorptive microextraction procedure Prior to the extraction, the nanofiber sheet was conditioned in methanol for 10 min. In all experiments, 3 g of NaCl was added to 10 mL of aqueous sample, and then the solution was spiked with the mixture of CBs. The extraction was performed by exposing the nanofiber sheet, located inside a gauze metallic cylinder, to the headspace of solution. Then the sample vial was sealed with a PTFEfaced septum and an aluminum cap. Real images of the nanofiber sheet together with the scheme of the adsorptive microextraction device are shown in Fig. 2. The sample vial was heated in a circulating water bath and continuously stirred at a constant rate using a magnetic stirrer. After the extraction, the sorbent sheet was removed from the metallic cylinder. The nanofiber sheet was then folded and inserted inside a microvial for solvent desorption using 50 L of acetone and eventually an aliquot of the 1 L was injected into the GC–MS. To avoid any direct contact with the sheets, they were handled with two tweezers.
Conductive polymers are organic materials that generally possess an extended conjugated -electron system along the polymer backbone. Conductive polymers, due to their unique physical and chemical properties, have been extensively used as efficient sorbents for extracting various compounds [21–28]. Various forms of aniline-based conductive polymers have been shown to be quite efficient materials for SPE and SPME of different classes of compounds [19,22–24]. Since PANI contains a conjugated structure, there is an expectation that it could efficiently extract aromatic compounds easily through – and hydrophobic interactions. PANI has three main stable oxidation states: the fully reduced “leucoemeraldine” form, the 50% oxidized “emeraldine” form and the fully oxidized “pernigraniline” form (Fig. 3) [29]. Each of these can exist in the form of its base or in the form of its protonated (doped) salt. Oxidative doping of the “leucoemeraldine” or protonic acid doping of the “emeraldine” produces the conducting form. The emeraldine salt and its basic form show green and blue colors, respectively [28]. As Fig. 2a shows the produced nanofiber composite has a greenish color, indicating that the polymer has an emeraldine salt structure, but after conditioning the nanofiber sheet in solvent(s) its color was changed into a dark bluish (Fig. 2b), confirming that the polymer was converted into the emeraldine basic form. Features such as high surface area and -functional groups of the conductive polymer structure together with the functional groups of N6 (NH and C O) are surely important characteristics of the fabricated PANI-N6 nanofiber coating, making it a suitable candidate for the extraction purposes. The surface characteristics of the PANI-N6 coated nanofiber were investigated by the SEM technique. As the SEM images of the nanofiber sheet (Fig. 4) show, the composite nanofibers diameters are in the range of 100–300 nm. Among them, most of the nanofibers show a narrow distribution around 200 nm diameters. Also it is rather interesting that a layer of much narrower fibers, around 10 nm, could be observed. These thinner webs were distributed all around the nanofiber structure. Certainly these features make the nanofiber sheet very porous with high surface area, which should result in a significantly increased surface area availability and higher mass transfer during the extraction as well as the analyte desorption process. Also, the nanofibers are bead free and they have smooth morphology. One major issue regarding this type of sorbents is that they should have a rather high stability in organic solvents. The sorbent stability in various solvents was tested and it was found that the nanofiber sheet was stable in most solvents including methanol, ethanol, acetone, chloroform, dichloromethane, tetrachloromethane, diethyl ether, ethyl acetate, hexane and toluene. The sorbent sheet could be dissolved in formic acid and N,Ndimethylformamide (DMF) since both of them easily dissolve N6. The nanofiber sheet was quite stable and reusable for more than 100 times of usage. 3.2. Optimization To assess the performance of the fabricated nanofibers sorbent, CBs were used as model analytes. The extraction of these analytes from the spiked water samples was performed using the adsorptive microextraction method in the headspace mode. To optimize the method, a mixture of CBs was spiked at a concentration level of 100 ng mL−1 . Effects of different parameters such as desorption solvent and time, extraction time and temperature, ionic strength and sorbent mass on the extraction efficiency
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Fig. 3. Relationship between oxidation forms of polyaniline.
were investigated. In order to evaluate the sorbent sheet, the following initial conditions were performed; a volume of 10 mL DDW sample, saturated by NaCl (30% (w/v)), was spiked with 100 ng mL−1 of selected CBs and extractions were performed at 30 ◦ C for 30 min, while analytes were desorbed using chloroform for 15 min.
3.2.1. Desorption conditions Selecting the most appropriate solvent and time is quite essential for optimization of the desorption process. As mentioned before the polymeric nanofiber exhibits a very good stability in different solvents which could be used for the analyte desorption. Among all possible solvents, methanol, acetone, chloroform,
Fig. 4. SEM images of the PANI-N6 nanofiber sheet magnification: (A) 1000, (B) 10,000.
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Fig. 5. Influence of various solvents on the desorption efficiency. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C for 30 min, then desorbed for 15 min.
tetrachloromethane and hexane were selected for this survey. As shown in Fig. 5, acetone was acting as the best choice for the desorption process. Next, the effect of desorption time was studied in the range of 2–15 min. The results revealed that after 10 min complete desorption could be achieved without any memory effect. 3.2.2. Ionic strength The ionic strength of the solution increases by addition of the salt, and its effect on the extraction efficiency depends on the analyte properties and mode of the extraction [30]. Generally for many organic compounds, aqueous solubilities are decreased by the salt addition and this leads to the higher sorbent/sample distribution constant and increased sensitivity. For the headspace extraction techniques, the salt addition causes easy movement of the analyte to the headspace and eventually the higher sensitivity can be achieved [2]. The influence of ionic strength was studied by adding different amounts of NaCl, ranging from 0 to 30% (w/v) and the results are shown in Fig. 6. An increase in extraction efficiency was observed by adding NaCl until 30% (w/v) and this value, corresponding to 3 g of NaCl in the vial, was used as the optimum quantity for the next studies. 3.2.3. Sorbent mass For achieving the maximum sensitivity it is important to have sufficient sorbent in order to extract higher amounts of analytes. Since the exact sheet size could not be cut in a convenient way, the sorbent mass rather than their sizes were considered for
Fig. 6. Effect of salt addition on the extraction efficiency. Conditions: DDW sample was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C for 30 min, then desorbed by acetone for 10 min.
optimization. Various amounts of nanofiber sheets weighing from 0.4 to 5.7 mg were therefore used. As Fig. 7 reveals, an amount of 2 mg was sufficient for achieving the maximum extraction efficiency. 3.2.4. Extraction temperature The extraction temperature is another important parameter in the headspace analysis because it can affect the rate of the extraction and its equilibrium status. The extraction temperature has different effects on the extraction efficiency. At elevated temperatures, solutes can effectively move from the matrix into the headspace for rapid extraction by the sorbent. However, the coating/headspace distribution coefficient is also decreased with an increase of temperature, resulting in a diminution in the equilibrium amount of extracted analytes [28]. A temperature range of 25–60 ◦ C was used to study the effect of temperature on the extraction efficiency. As shown in Fig. 8, the extracted quantity of CBs mixture were increased up to 30 ◦ C and decreased at higher temperature. These results indicate that for high volatile CBs, a lower temperature of 30 ◦ C is more suitable for their isolation from the solution and subsequent headspace extraction. According to these results, an extraction temperature of 30 ◦ C was considered as the optimum value for the rest of experiments. 3.2.5. Extraction time The headspace adsorptive microextraction method is an equilibrium-based technique and there is a direct relationship
Fig. 7. Nanofiebr sheet mass optimization. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C for 30 min, then desorbed by acetone for 10 min.
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H. Bagheri, A. Aghakhani / Analytica Chimica Acta 713 (2012) 63–69 Table 2 Analytical data obtained after headspace adsorptive microextraction of CBs mixture using PANI-N6 nanofiber. Compounds
LODa (ng L−1 )
LDRb (ng L−1 )
R2
14DCB 12DCB 124TCB 123TCB 1234TeCB
21 19 26 27 33
50–1000 50–1000 50–1000 50–1000 60–1000
0.9932 0.9896 0.9927 0.9739 0.9895
a b
Limit of detection (S/N = 3). Linear dynamic range.
4. Analytical results The optimized headspace adsorptive microextraction method using the fabricated PANI-N6 nanofiber sheet was evaluated by quantitative analysis of the spiked DDW samples. The major quantitative data obtained for the selected CBs are tabulated in Table 2. The limits of detection (S/N = 3) were 19–33 ng L−1 and the limits of quantification (LOQ) were 50–60 ng L−1 (the lowest points of the calibration curves). Concentrations of CBs in surface waters are generally in the ng L−1 to g L−1 range, with maximum concentrations up to 0.2 mg L−1 in areas close to industrial sources [31]. As the data in Table 2 show, the values of LODs were below those reported by the Environmental Protection Agency (EPA) Method 8121 (10–890 ng L−1 ) [32], except for 1234TeCB (10 ng L−1 ). Therefore, the developed method could easily meet these demands for the analysis of this group of pollutants in environmental aqueous samples. The calibration curves of analytes were investigated in the range of 50–1000 ng L−1 and R2 values were in range of 0.9739–0.9932. The precision of the method was determined by performing three consecutive extractions from the aqueous solution and results are listed in Table 3. The relative standard deviations (RSD) were in the range of 8–14% and 5–11% at the concentration level of 0.1 ng mL−1 and 1 ng mL−1 , respectively. The sheet-to-sheet reproducibility was measured for three fabricated sorbents at the concentration level of 1 ng mL−1 and RSD% values less than 12% were achieved. To evaluate the applicability of the developed method, spiked (0.1 and 1 ng mL−1 ) and non-spiked water samples from Zayande-rood river (Isfahan-Iran) and Tehran drinking water were extracted and analyzed. None of the selected CBs could be detected in these samples. Relative recovery (RR) was measured as the peak area ratio of real sample and double distilled water sample spiked with analyte at the same level [33]. Relative recoveries obtained for the spiked real water samples were in the range of 93–103% and 95–104% at the concentration level of 0.1 ng mL−1 and 1 ng mL−1 , respectively (Table 3), indicating the absence of major matrix effects on the extraction performance of the nanofiber sheet. Absolute recoveries and enrichment factors of
Fig. 8. Extraction temperature optimization. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted for 30 min, using 2 mg nanofiber, then desorbed by acetone for 10 min.
Fig. 9. Extraction time profile. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C, using 2 mg nanofiber, then desorbed by acetone for 10 min.
between the extracted amounts of analyte and the extraction time [2]. The extraction time profiles were studied by varying the exposure time of the sorbent to the headspace of aqueous sample in the range of 5–75 min. As illustrated in Fig. 9 the extraction efficiencies for CBs were increased up to 30 min and the complete equilibrium was achieved after 45 min. Therefore, an extraction time of 30 min was finally chosen as the optimal extraction time for the subsequent evaluations.
Table 3 Relative standard deviations, relative recoveries, absolute recoveries and enrichment factors obtained after headspace adsorptive microextraction of different water samples spiked with CBs using the PANI-N6 nanofiber. Compounds
DDW RSDa (%) (n = 3) 0.1 ng mL
14DCB 12DCB 124TCB 123TCB 1234TeCB a b c d f
−1
8 11 13 14 12
Relative standard deviation. Relative recovery. Absolute recovery. Enrichment factor. Sheet to sheet RSD.
1 ng mL 5 8 10 11 9
Zayande-rood RRb (%) (n = 3) −1
0.1 ng mL 103 95 100 97 93
−1
1 ng mL 101 96 98 104 95
−1
Tap water RR (%) (n = 3) 0.1 ng mL 96 98 95 96 94
−1
1 ng mL 100 103 99 98 99
−1
DDW ARc
DDW EFd
−1
−1
1 ng mL 8 9 13 16 23
DDW RSD (%)f (3 sheet)
1 ng mL
1 ng mL−1
17 18 25 32 45
10 11 9 12 8
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the method were calculated [34] and their amounts were in the range of 8–23% and 17–45, respectively (Table 3). 5. Conclusions The PANI-N6 nanofiber sheet, fabricated by the electrospinning technology, has been shown to be a suitable sorbent for extracting trace amounts of CBs, as model analytes, from aqueous samples. Analytical data, tabulated in Tables 2 and 3, confirm that the electrospun nanofiber-based sorbent is interesting for microextraction of organic compounds. The fibrous and web structure provides higher specific surface area, loading capacity and sufficient mechanical stability. This nanofiber sheet can be used for more than 100 runs and it is also stable in many solvents, except formic acid and DMF, which is sufficient for many applications. Influential parameters in headspace adsorptive microextraction were investigated and optimized. The proposed method provides a rather easy, simple, rapid and inexpensive adsorptive microextraction method for the determination of organic compounds with good sensitivity and reproducibility. Acknowledgements The Research Council and Graduates School of Sharif University of Technology (SUT) are thanked for supporting the project. Also, we would like to acknowledge the Iran National Elite Foundation for their support for Ali Aghakhani. References [1] E. Baltussen, C.A. Cramers, P.J.F. Sandra, Anal. Bioanal. Chem. 373 (2002) 3–22. [2] J. Pawliszyn, Handbook of Solid Phase Microextraction, Chemical Industry Press, China, 2009. [3] M.E.C. Queiroz, P. Grossi, I.R.B. Olivares, J. Sep. Sci. 32 (2009) 813–824.
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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca
Polyaniline-nylon-6 electrospun nanofibers for headspace adsorptive microextraction Habib Bagheri ∗ , Ali Aghakhani Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 3 July 2011 Received in revised form 9 November 2011 Accepted 10 November 2011 Available online 19 November 2011 Keywords: Polyaniline-nylon-6 nanofibers Electrospinning Headspace adsorptive microextraction Gas chromatography–mass spectrometry Chlorobenzenes
a b s t r a c t A headspace adsorptive microextraction technique was developed using a novel polyaniline-nylon-6 (PANI-N6) nanofiber sheet, fabricated by electrospinning. The homogeneity and the porosity of the prepared PANI-N6 sheet were studied using the scanning electron microscopy (SEM) and nanofibers diameters were found to be around 200 nm. The novel nanofiber sheet was examined as an extracting medium to isolate some selected chlorobenzenes (CBs), as model compounds, from aquatic media. The extracted analytes were desorbed using L-amounts of solvent and eventually an aliquot of extractant was injected into gas chromatography–mass spectrometry (GC–MS). Various parameters affecting the extraction and desorption processes were optimized. The developed method proved to be convenient and offers sufficient sensitivity and a good reproducibility. Limits of detection achieved for CBs with the developed analytical procedure ranged from 19 to 33 ng L−1 , while limits of quantification were from 50 to 60 ng L−1 . The relative standard deviations (RSD) at a concentration level of 0.1 ng mL−1 and 1 ng mL−1 were in the range of 8–14% and 5–11% (n = 3), respectively. The calibration curves of analytes were investigated in the range of 50–1000 ng L−1 and R2 between 0.9739 and 0.9932 were obtained. The developed method was successfully applied to the extraction of selected CBs from tap and river water samples. The relative recovery (RR) percentage obtained for the spiked real water samples at 0.1 ng mL−1 and 1 ng mL−1 level were 93–103% and 95–104%, respectively. The whole procedure showed to be conveniently applicable and quite easy to handle. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Because of the low concentrations of environmental pollutants in water samples, it is necessary to apply a preconcentration step prior to the instrumental analysis. The traditional sample preparation methods such as liquid–liquid extraction (LLE) and solid phase extraction (SPE), consume large amounts of samples and organic solvents, while they are time consuming and tedious. Recent approaches based on the sorptive and adsorptive microextraction, provide advantages of being solvent-less or little solvent usage, lower sample consumption, higher sample preparation speed and ease of operation [1]. The common aspects in the microextraction strategies are mostly focused on miniaturized sorbent amount and the extraction device geometry which often lead to the extraction of small fraction of the analytes from the sample into the extracting phase [2]. In order to increase the sensitivity of solid phase microextraction (SPME), which is due to its lower sample capacity, other approaches including stir bar sorptive extraction (SBSE) were developed to provide higher mass of sorbent in the sorptive
∗ Corresponding author. Tel.: +98 21 66165316; fax: +98 21 66012983. E-mail address: [email protected] (H. Bagheri). 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.11.027
extraction manner [3,4]. Although these approaches were used to improve different aspects of the whole extraction procedure, the fundamental bases are rather similar [5]. Another strategy to improve the sensitivity of the microextraction methods relies on the usage of nanostructured materials in which the interactions between the desired analytes and the sorbent are increased. This could be due to the enhancement of surface to volume ratio as the size of the sorbent particles is reduced extensively [6–8]. An emerging method for preparing nano-dimensional sorbents is electrospinning [9]. This methodology is a convenient technique for fabricating fibrous sorbent with controllable diameter, functionality and high surface to volume ratio [10–12]. In this technique by applying high voltages to a viscous polymeric solution, when the electrostatic repulsive force overcomes the surface tension of the polymeric solution, a charged polymeric jet ejects from the solution and afterwards flies toward the collector and forms the fibrous material with diameters in the scale of nano-tomicrometer. The electrospun sorbents were used in SPE [13–16], micro-SPE [10], SPME [17–19] and membrane extraction [20] of some organic and inorganic compounds. In this work, polyaniline (PANI) was synthesized inside the nylon-6 (N6) solution as a carrier polymer, and a composite of polyaniline-nylon-6 (PANI-N6) was electrospun into the fibrous
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sheet with nano-scale dimensions. Then, the prepared composite was applied for the headspace adsorptive microextraction of some selected chlorobenzenes (CBs), as model compounds, to evaluate its extraction applicability. 2. Experimental 2.1. Reagents The CB compounds 1,4-dichlorobenzene (14DCB), 1,2dichlorobenzene (12DCB), 1,2,4-trichlorobenzene (124TCB), 1,2,3-trichlorobenzene (123TCB) and 1,2,3,4-tetrachlorobenezne (1234TeCB) were purchased from Merck (Darmstadt, Germany). Standard solution (2000 mg L−1 ) of CBs mixture was prepared in HPLC-grade methanol (Merck) and stored in the refrigerator. The working standard solutions were prepared weekly by diluting the standard solution with methanol, and more diluted working solutions were prepared daily by diluting this solution with double distilled water (DDW). Ammonium peroxodisulphate and sodium chloride were purchased from Merck. Aniline was obtained from Merck and distilled before use. Nylon-6 (N6) was purchased from Kolon Industries Inc. (Seoul, Korea) and formic acid was obtained from Riedel-de Haën (Seelze-Hannover, Germany). All solvents used in this study were of analytical reagent grade or HPLC grade. 2.2. Apparatus An Agilent (Palo Alto, CA, USA) 6890 plus series GC equipped with a split/splitless injector and an Agilent 5973 mass selective detector system were used. The MS was operated in the EI mode (70 eV). Helium (99.999%) was employed as carrier gas and its flow rate was adjusted to 1 mL min−1 . The separation of CBs was performed on a 30 m × 0.25 mm HP-5 MS column (0.25 m film thickness). The GC column temperature was first started at 50 ◦ C and held for 5 min, then increased by 30 ◦ C min−1 to 80 ◦ C and held for 1 min, then ramped at 20 ◦ C min−1 to 200 ◦ C for 5 min. The injector temperature was set at 250 ◦ C in the splitless mode for 1 min. The GC–MS interface, ion source and quadrupole temperatures were set to 280, 230 and 150 ◦ C, respectively. The selected ion monitoring (SIM) mode, considering two characteristic ions for each compound (Table 1), was used for quantitative analysis. The surface morphology of the fabricated nanofiber sheet was investigated by a TSCAN VEGA II XMU SEM instrument (Brno, Czech Republic). All samples were heated in a homemade glass water
Table 1 The selected ions, scan start times and retention times of CB mixtures studied by GC–MS. Compounds
Selected ions (m/z)
Scan start time (min)
Retention time (min)
14DCB 12DCB 124TCB 123TCB 1234TeCB
146, 148 146, 148 180, 182 180, 182 214, 216
3 3 8 8 12
5.89 6.30 8.99 9.76 12.98
bath connected to a refrigerated circulating water bath (RTE-8, Neslab Instruments Inc., Portsmouth, NH, USA) and stirred using a magnetic stirrer (Baird & TatLook, London, England). A Brandenburg (West Midlands, England) regulated power supply was used for electrospinning. A KDS100 syringe pump (KdScientific Co., Holliston, MA, US) was used for the polymer solution delivery in the electrospinning process.
2.3. Electrospinning of PANI-N6 Firstly, an amount of 0.25 g N6 was dissolved in 1 mL of formic acid. Then, an amount of 0.1 g ammonium peroxodisulphate was added to this polymeric solution. After dissolving the salt, the aniline monomer (0.5 g) was added to the solution and the mixture was stirred to obtain a homogenous solution. After that, 0.5 mL of this solution was withdrawn by a 2.5 mL syringe which was eventually transferred to a syringe pump. A piece of aluminum foil (10 × 10 cm) was employed as a collector electrode. The collector and the polymer containing syringe needle were connected to the high voltage power supply terminals. The distance between the needle and the collector was set at 10 cm. Fig. 1 shows the schematic diagram of the apparatus used in the electrospinning process. A voltage of 16 kV was applied for the nanofibers production while a flow rate of 1.5 L min−1 was set for the syringe pump to deliver the polymer solution. All fibers were electrospun for 12 h to achieve the desired coating thickness. The electrospinning experiments were performed under the ventilation. After the electrospinning experiment, the collector foil was separated from the power supply and afterward a sheet with a typical dimension of 1 × 1 cm, was cut from the central part of the Al foil using a predesigned template and employed for extraction.
Fig. 1. Scheme of the electrospinning set up.
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3. Results and discussion 3.1. Preparation of PANI-N6 nanofiber
Fig. 2. Images of the PANI-N6 nanofiber sheet used for headspace adsorptive microextraction (a) before conditioning (b) after conditioning (c) scheme of sample extraction. a: nanofiber sheet, b: gauze metallic cylinder.
2.4. Headspace adsorptive microextraction procedure Prior to the extraction, the nanofiber sheet was conditioned in methanol for 10 min. In all experiments, 3 g of NaCl was added to 10 mL of aqueous sample, and then the solution was spiked with the mixture of CBs. The extraction was performed by exposing the nanofiber sheet, located inside a gauze metallic cylinder, to the headspace of solution. Then the sample vial was sealed with a PTFEfaced septum and an aluminum cap. Real images of the nanofiber sheet together with the scheme of the adsorptive microextraction device are shown in Fig. 2. The sample vial was heated in a circulating water bath and continuously stirred at a constant rate using a magnetic stirrer. After the extraction, the sorbent sheet was removed from the metallic cylinder. The nanofiber sheet was then folded and inserted inside a microvial for solvent desorption using 50 L of acetone and eventually an aliquot of the 1 L was injected into the GC–MS. To avoid any direct contact with the sheets, they were handled with two tweezers.
Conductive polymers are organic materials that generally possess an extended conjugated -electron system along the polymer backbone. Conductive polymers, due to their unique physical and chemical properties, have been extensively used as efficient sorbents for extracting various compounds [21–28]. Various forms of aniline-based conductive polymers have been shown to be quite efficient materials for SPE and SPME of different classes of compounds [19,22–24]. Since PANI contains a conjugated structure, there is an expectation that it could efficiently extract aromatic compounds easily through – and hydrophobic interactions. PANI has three main stable oxidation states: the fully reduced “leucoemeraldine” form, the 50% oxidized “emeraldine” form and the fully oxidized “pernigraniline” form (Fig. 3) [29]. Each of these can exist in the form of its base or in the form of its protonated (doped) salt. Oxidative doping of the “leucoemeraldine” or protonic acid doping of the “emeraldine” produces the conducting form. The emeraldine salt and its basic form show green and blue colors, respectively [28]. As Fig. 2a shows the produced nanofiber composite has a greenish color, indicating that the polymer has an emeraldine salt structure, but after conditioning the nanofiber sheet in solvent(s) its color was changed into a dark bluish (Fig. 2b), confirming that the polymer was converted into the emeraldine basic form. Features such as high surface area and -functional groups of the conductive polymer structure together with the functional groups of N6 (NH and C O) are surely important characteristics of the fabricated PANI-N6 nanofiber coating, making it a suitable candidate for the extraction purposes. The surface characteristics of the PANI-N6 coated nanofiber were investigated by the SEM technique. As the SEM images of the nanofiber sheet (Fig. 4) show, the composite nanofibers diameters are in the range of 100–300 nm. Among them, most of the nanofibers show a narrow distribution around 200 nm diameters. Also it is rather interesting that a layer of much narrower fibers, around 10 nm, could be observed. These thinner webs were distributed all around the nanofiber structure. Certainly these features make the nanofiber sheet very porous with high surface area, which should result in a significantly increased surface area availability and higher mass transfer during the extraction as well as the analyte desorption process. Also, the nanofibers are bead free and they have smooth morphology. One major issue regarding this type of sorbents is that they should have a rather high stability in organic solvents. The sorbent stability in various solvents was tested and it was found that the nanofiber sheet was stable in most solvents including methanol, ethanol, acetone, chloroform, dichloromethane, tetrachloromethane, diethyl ether, ethyl acetate, hexane and toluene. The sorbent sheet could be dissolved in formic acid and N,Ndimethylformamide (DMF) since both of them easily dissolve N6. The nanofiber sheet was quite stable and reusable for more than 100 times of usage. 3.2. Optimization To assess the performance of the fabricated nanofibers sorbent, CBs were used as model analytes. The extraction of these analytes from the spiked water samples was performed using the adsorptive microextraction method in the headspace mode. To optimize the method, a mixture of CBs was spiked at a concentration level of 100 ng mL−1 . Effects of different parameters such as desorption solvent and time, extraction time and temperature, ionic strength and sorbent mass on the extraction efficiency
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Fig. 3. Relationship between oxidation forms of polyaniline.
were investigated. In order to evaluate the sorbent sheet, the following initial conditions were performed; a volume of 10 mL DDW sample, saturated by NaCl (30% (w/v)), was spiked with 100 ng mL−1 of selected CBs and extractions were performed at 30 ◦ C for 30 min, while analytes were desorbed using chloroform for 15 min.
3.2.1. Desorption conditions Selecting the most appropriate solvent and time is quite essential for optimization of the desorption process. As mentioned before the polymeric nanofiber exhibits a very good stability in different solvents which could be used for the analyte desorption. Among all possible solvents, methanol, acetone, chloroform,
Fig. 4. SEM images of the PANI-N6 nanofiber sheet magnification: (A) 1000, (B) 10,000.
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Fig. 5. Influence of various solvents on the desorption efficiency. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C for 30 min, then desorbed for 15 min.
tetrachloromethane and hexane were selected for this survey. As shown in Fig. 5, acetone was acting as the best choice for the desorption process. Next, the effect of desorption time was studied in the range of 2–15 min. The results revealed that after 10 min complete desorption could be achieved without any memory effect. 3.2.2. Ionic strength The ionic strength of the solution increases by addition of the salt, and its effect on the extraction efficiency depends on the analyte properties and mode of the extraction [30]. Generally for many organic compounds, aqueous solubilities are decreased by the salt addition and this leads to the higher sorbent/sample distribution constant and increased sensitivity. For the headspace extraction techniques, the salt addition causes easy movement of the analyte to the headspace and eventually the higher sensitivity can be achieved [2]. The influence of ionic strength was studied by adding different amounts of NaCl, ranging from 0 to 30% (w/v) and the results are shown in Fig. 6. An increase in extraction efficiency was observed by adding NaCl until 30% (w/v) and this value, corresponding to 3 g of NaCl in the vial, was used as the optimum quantity for the next studies. 3.2.3. Sorbent mass For achieving the maximum sensitivity it is important to have sufficient sorbent in order to extract higher amounts of analytes. Since the exact sheet size could not be cut in a convenient way, the sorbent mass rather than their sizes were considered for
Fig. 6. Effect of salt addition on the extraction efficiency. Conditions: DDW sample was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C for 30 min, then desorbed by acetone for 10 min.
optimization. Various amounts of nanofiber sheets weighing from 0.4 to 5.7 mg were therefore used. As Fig. 7 reveals, an amount of 2 mg was sufficient for achieving the maximum extraction efficiency. 3.2.4. Extraction temperature The extraction temperature is another important parameter in the headspace analysis because it can affect the rate of the extraction and its equilibrium status. The extraction temperature has different effects on the extraction efficiency. At elevated temperatures, solutes can effectively move from the matrix into the headspace for rapid extraction by the sorbent. However, the coating/headspace distribution coefficient is also decreased with an increase of temperature, resulting in a diminution in the equilibrium amount of extracted analytes [28]. A temperature range of 25–60 ◦ C was used to study the effect of temperature on the extraction efficiency. As shown in Fig. 8, the extracted quantity of CBs mixture were increased up to 30 ◦ C and decreased at higher temperature. These results indicate that for high volatile CBs, a lower temperature of 30 ◦ C is more suitable for their isolation from the solution and subsequent headspace extraction. According to these results, an extraction temperature of 30 ◦ C was considered as the optimum value for the rest of experiments. 3.2.5. Extraction time The headspace adsorptive microextraction method is an equilibrium-based technique and there is a direct relationship
Fig. 7. Nanofiebr sheet mass optimization. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C for 30 min, then desorbed by acetone for 10 min.
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H. Bagheri, A. Aghakhani / Analytica Chimica Acta 713 (2012) 63–69 Table 2 Analytical data obtained after headspace adsorptive microextraction of CBs mixture using PANI-N6 nanofiber. Compounds
LODa (ng L−1 )
LDRb (ng L−1 )
R2
14DCB 12DCB 124TCB 123TCB 1234TeCB
21 19 26 27 33
50–1000 50–1000 50–1000 50–1000 60–1000
0.9932 0.9896 0.9927 0.9739 0.9895
a b
Limit of detection (S/N = 3). Linear dynamic range.
4. Analytical results The optimized headspace adsorptive microextraction method using the fabricated PANI-N6 nanofiber sheet was evaluated by quantitative analysis of the spiked DDW samples. The major quantitative data obtained for the selected CBs are tabulated in Table 2. The limits of detection (S/N = 3) were 19–33 ng L−1 and the limits of quantification (LOQ) were 50–60 ng L−1 (the lowest points of the calibration curves). Concentrations of CBs in surface waters are generally in the ng L−1 to g L−1 range, with maximum concentrations up to 0.2 mg L−1 in areas close to industrial sources [31]. As the data in Table 2 show, the values of LODs were below those reported by the Environmental Protection Agency (EPA) Method 8121 (10–890 ng L−1 ) [32], except for 1234TeCB (10 ng L−1 ). Therefore, the developed method could easily meet these demands for the analysis of this group of pollutants in environmental aqueous samples. The calibration curves of analytes were investigated in the range of 50–1000 ng L−1 and R2 values were in range of 0.9739–0.9932. The precision of the method was determined by performing three consecutive extractions from the aqueous solution and results are listed in Table 3. The relative standard deviations (RSD) were in the range of 8–14% and 5–11% at the concentration level of 0.1 ng mL−1 and 1 ng mL−1 , respectively. The sheet-to-sheet reproducibility was measured for three fabricated sorbents at the concentration level of 1 ng mL−1 and RSD% values less than 12% were achieved. To evaluate the applicability of the developed method, spiked (0.1 and 1 ng mL−1 ) and non-spiked water samples from Zayande-rood river (Isfahan-Iran) and Tehran drinking water were extracted and analyzed. None of the selected CBs could be detected in these samples. Relative recovery (RR) was measured as the peak area ratio of real sample and double distilled water sample spiked with analyte at the same level [33]. Relative recoveries obtained for the spiked real water samples were in the range of 93–103% and 95–104% at the concentration level of 0.1 ng mL−1 and 1 ng mL−1 , respectively (Table 3), indicating the absence of major matrix effects on the extraction performance of the nanofiber sheet. Absolute recoveries and enrichment factors of
Fig. 8. Extraction temperature optimization. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted for 30 min, using 2 mg nanofiber, then desorbed by acetone for 10 min.
Fig. 9. Extraction time profile. Conditions: DDW sample containing 30% (w/v) NaCl was spiked with CBs at concentration level of 100 ng mL−1 and extracted at 30 ◦ C, using 2 mg nanofiber, then desorbed by acetone for 10 min.
between the extracted amounts of analyte and the extraction time [2]. The extraction time profiles were studied by varying the exposure time of the sorbent to the headspace of aqueous sample in the range of 5–75 min. As illustrated in Fig. 9 the extraction efficiencies for CBs were increased up to 30 min and the complete equilibrium was achieved after 45 min. Therefore, an extraction time of 30 min was finally chosen as the optimal extraction time for the subsequent evaluations.
Table 3 Relative standard deviations, relative recoveries, absolute recoveries and enrichment factors obtained after headspace adsorptive microextraction of different water samples spiked with CBs using the PANI-N6 nanofiber. Compounds
DDW RSDa (%) (n = 3) 0.1 ng mL
14DCB 12DCB 124TCB 123TCB 1234TeCB a b c d f
−1
8 11 13 14 12
Relative standard deviation. Relative recovery. Absolute recovery. Enrichment factor. Sheet to sheet RSD.
1 ng mL 5 8 10 11 9
Zayande-rood RRb (%) (n = 3) −1
0.1 ng mL 103 95 100 97 93
−1
1 ng mL 101 96 98 104 95
−1
Tap water RR (%) (n = 3) 0.1 ng mL 96 98 95 96 94
−1
1 ng mL 100 103 99 98 99
−1
DDW ARc
DDW EFd
−1
−1
1 ng mL 8 9 13 16 23
DDW RSD (%)f (3 sheet)
1 ng mL
1 ng mL−1
17 18 25 32 45
10 11 9 12 8
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the method were calculated [34] and their amounts were in the range of 8–23% and 17–45, respectively (Table 3). 5. Conclusions The PANI-N6 nanofiber sheet, fabricated by the electrospinning technology, has been shown to be a suitable sorbent for extracting trace amounts of CBs, as model analytes, from aqueous samples. Analytical data, tabulated in Tables 2 and 3, confirm that the electrospun nanofiber-based sorbent is interesting for microextraction of organic compounds. The fibrous and web structure provides higher specific surface area, loading capacity and sufficient mechanical stability. This nanofiber sheet can be used for more than 100 runs and it is also stable in many solvents, except formic acid and DMF, which is sufficient for many applications. Influential parameters in headspace adsorptive microextraction were investigated and optimized. The proposed method provides a rather easy, simple, rapid and inexpensive adsorptive microextraction method for the determination of organic compounds with good sensitivity and reproducibility. Acknowledgements The Research Council and Graduates School of Sharif University of Technology (SUT) are thanked for supporting the project. Also, we would like to acknowledge the Iran National Elite Foundation for their support for Ali Aghakhani. References [1] E. Baltussen, C.A. Cramers, P.J.F. Sandra, Anal. Bioanal. Chem. 373 (2002) 3–22. [2] J. Pawliszyn, Handbook of Solid Phase Microextraction, Chemical Industry Press, China, 2009. [3] M.E.C. Queiroz, P. Grossi, I.R.B. Olivares, J. Sep. Sci. 32 (2009) 813–824.
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