Amino ethyl-functionalized nanoporous silica as a novel fiber coating for solid-phase microextraction

Analytica Chimica Acta 646 (2009) 1–5

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Amino ethyl-functionalized nanoporous silica as a novel fiber coating for solid-phase microextraction Payman Hashemi a,∗ , Mohammad Shamizadeh a , Alireza Badiei b , Pezhman Zarabadi Poor b , Ali Reza Ghiasvand a , Ali Yarahmadi a a b

Department of Chemistry, Faculty of Science, Lorestan University, Khoramabad, Iran School of Chemistry, University College of Science, University of Tehran, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 25 November 2008 Received in revised form 3 April 2009 Accepted 4 April 2009 Available online 23 April 2009 Keywords: Nanoporous silica Amino ethyl-functional groups New solid-phase microextraction fiber Phenolic compounds

a b s t r a c t Nanoporous silica (SBA-15) was prepared and functionalized with 3-[Bis(2-hydroxyethyl)amino] propyl-triethoxysilane (HPTES) to be used as a highly porous fiber coating material for solid-phase microextraction (SPME). The prepared HPTES–SBA-15 particles had a lengthy morphology and a specific surface area of 790 m2 g−1 . They were characterized by N2 sorption analyses, scanning electron microscopy and thermogravimetric analysis. The prepared nanomaterial was immobilized onto a copper wire for fabrication of the SPME fiber. The fiber was evaluated for the extraction of BTEX and some phenolic compounds in combination with GC–MS. For optimization of factors affecting the extraction efficiency of the phenolic compounds, a simplex optimization method was used. The proposed fiber showed some selectivity towards the polar phenolic compounds with extraction efficiencies better than a PDMS commercial fiber. The repeatability for one fiber (n = 5), expressed as relative standard deviation (RSD), was between 6.5% and 9.8% and the reproducibility for five prepared fibers was between 8.2% and 11.3% for the test compounds. No significant change was observed in the extraction efficiency of the new SPME fiber over 50 extractions. The fiber was successfully applied to the determination of phenolic compounds in spiked river water and sewage samples. Thus, HPTES–SBA-15 fiber is a promising alternative to the commercial fibers as it is robust, selective, highly porous and easily and inexpensively prepared. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Integrating sampling, extraction, concentration and sample introduction in a single process has become accomplished by solidphase microextraction (SPME) that is predominantly performed on SPME fibers [1,2]. The use of metal wires as SPME supports with high mechanical stability makes this technique more robust for routine analysis [3]. Several studies have been made using different material coatings over the traditional fused-silica support, such as platinum [4–6], anodized aluminum [7], gold [8,9], stainless steel [3,10,11] and copper [12] wires. Ordered nanoporous silica such as MCM-41 [13], LUS-1 [14,15], and SBA-15 [16] with very high surface area, uniform open form structure and extremely narrow pore size distribution has great potential for application in many fields such as catalysts [17], preconcentration of metals [18–20], drug delivery [21], and modified carbon paste electrodes [22]. Hou et al. [11] first reported the use of MCM-41 particles as fiber coating in SPME for the extraction of aro-

∗ Corresponding author. Tel.: +98 6612202782; fax: +98 6612200185. E-mail addresses: payman [email protected], [email protected] (P. Hashemi). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.04.023

matic hydrocarbons in combination with HPLC and found that the adsorption capacity of the fiber coating can be greatly improved in this way. The use of phenyl functionalized MCM-41 particles were reported from the same laboratory [23,24] and showed to have higher stability and better selectivity. In this paper, amino ethyl-functionalized SBA-15 is synthesized and used, for the first time, as fiber coating of SPME onto copper wires and in combination with GC–MS. SBA-15 is diffused free due to thicker pore walls and larger pore sizes in comparison with MCM41 [25]. The extraction efficiency of this kind of nanoporous silica fibers in combination with GC–MS is carefully studied for some aromatic hydrocarbons.

2. Experimental 2.1. Reagents and materials Tetraethyl orthosilicate (TEOS, Merck) as silica source, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123, Aldrich) as surfactant, 3-[Bis(2-hydroxyethyl)amino] propyl-triethoxysilane solution (∼65% in ethanol, HPTES, Fluka) as amine compound, hydrochloric acid (Merck),

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ethanol (Merck), and aromatic hydrocarbons were used as received from suppliers. BTEX (benzene, toluene, ethylbenzene and pxylene) and phenolic compounds (phenol, Ph; 4-nitrophenol, Np; 2,4-dinitrophenol, Dn and 4-chlorophenol, Cp) with maximum available purities were purchased from Merck and used as received.

The coated wire was heated at 50 ◦ C for 24 h in an oven, gently scrubbed to remove non-bonded particles and assembled to the SPME device. It was then inserted into the GC injection port to be cleaned and conditioned at 200 ◦ C for 20 min in a helium environment.

2.2. Synthesis of pure SBA-15

2.6. The headspace SPME procedure

The preparation of SBA-15 was similar to the method reported by Zhao and co-workers [11]. About 2 g of P123 was stirred with 15 mL of deionized water at 35 ◦ C until fully dissolved, followed by adding 30 g of 2 mol L−1 HCl solution and dropwise addition of 4 g of TEOS. The mixture was allowed to stir at 35 ◦ C for 24 h before transferring into a Teflon bottle sealed in an autoclave, which was then heated to 100 ◦ C for 2 days in an oven. The solid was filtered off, washed three times with deionized water, and calcined at 600 ◦ C for 6 h.

A special SPME device (Azar Electrod Co., Oroumie, Iran) was used for holding and injection of the proposed fiber into the GC–MS injection port. A 10 mL glass bottle sealed with rubber septum was used as sample container. The bottle was held in the ultrasonic bath during the extraction. After a preset time, the SPME device was fixed on top of the capped vial and the fabricated fiber was exposed to the sample headspace while it was sonicating. After sample extraction, the fiber was withdrawn from the bottle and inserted into the GC–MS injection port for analysis.

2.3. Synthesis of HPTES–SBA-15 To prepare functionalized SBA-15 by post synthesis method, 1 g calcined SBA-15 was reacted with 0.065 g HPTES in 150 mL dry toluene under room temperature for 24 h. The resultant white solid was filtered off, washed with dry toluene, and dried at 80 ◦ C in an oven overnight. The schematic of reaction is shown in Fig. 1. 2.4. Characterization The N2 adsorption/desorption analyses were performed on BELSORP-miniII at −196 ◦ C. SBA-15 was degassed at 300 ◦ C for 2 h but HPTES–SBA-15 was degassed at 100 ◦ C for 4 h. Specific surface area, total pore volume, and pore diameter of samples was obtained by Brunauer–Emmett–Teller (BET) method using BELSORP analysis software. Scanning electron microscopy (SEM) images were taken by LEO 1455VP and morphology of pure SBA-15 and HPTES–SBA-15 were investigated by these images. Thermogravimetric analysis (TGA) measurements of pure SBA15 and HPTES–SBA-15 were performed on TA TGA Q50 in the temperature range from ambient to 800 ◦ C. The ramp rate used was 20 ◦ C min−1 . 2.5. Preparation of the SPME fiber A piece of copper wire with 170 ␮m diameter was twice cleaned with ethanol in an ultrasonic bath (22 KHz, model 5RS, Sonica, Italy) for 15 min and dried at 60 ◦ C. One centimeter of the wire was limed with epoxy glue and the extra-fine powdered nanoporous functionalized SBA-15 material was immobilized onto the wire.

2.7. GC–MS analysis GC–MS analysis was conducted with a Shimadzu (Japan) model GC-17A gas chromatograph coupled to a Shimadzu model GCMS-QP5050 mass spectrometer. Compounds were separated on a 30 m × 0.22 mm i.d. fused-silica capillary column coated with 0.25 ␮m film of BP-5 (Shimadzu). Ultra-pure helium at 0.6 mL min−1 was used as carrier gas. For direct sample injections the detector and injection port temperatures were 280 and 260 ◦ C, respectively. The column temperature was increased from 60 to 150 ◦ C at 4 ◦ C min−1 and then to 220 ◦ C at 10 ◦ C min−1 using a temperature program. Analyte desorption from fiber was performed in the split/splitless injection port at a temperature of 260 ◦ C for 3 min. In the optimized conditions the column temperature was initially set at 50 ◦ C, increased to 100 ◦ C at a rate of 10 ◦ C min−1 , and ramped at 2 ◦ C min−1 to 130 ◦ C and at 70 ◦ C min−1 to 260 ◦ C. Finally, it was held at 260 ◦ C for 2 min. 2.8. Field samples Two river water and two sewage water samples were collected for the study. The Khoramrood (Khoramabad, Iran) river water sample was collected from a polluted part of the river in the city center, in October 2008. The Ghomrood (Aligoodarz, Iran) river water sample was collected in a part polluted with wastewater from some industries, in September 2008. The sewage water samples were collected from two positions (Piroozi Bridge and Moallem Park) of Khoramabad, Iran, in March 2009. The samples were stored at 4 ◦ C before the analysis.

Fig. 1. Schematic representation of the reaction of SBA-15 and HPTES.

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is thermally stable, (3) major weight loss (∼3.5%) between 260 and 600 ◦ C because of removal of organic content of HPTES, and (4) slight weight loss between 600 and 800 ◦ C, this is due to dehydroxylation of silicate networks [26] and/or elimination of residual ethoxy groups. The SEM image of functionalized material is shown in Fig. 4a. SEM image shows lengthy fiber-like morphology for HPTES–SBA15 that arranged in a bundle of diameter of about 10 ␮m and length of 30–60 ␮m. The same morphology was observed for SBA-15. It can be concluded that morphology of solid was saved without change. Fig. 4b shows an optical microscopic image of a fabricated SPME fiber with a copper wire core. Uniform coating of HPTES–SBA-15 on the fiber is evident. The thickness of the coating layer was calculated from the difference between the coated and uncoated copper wire to be about 20 ␮m. With the method used for the fiber coating, no significant inhomogenity of the coating was detected. Fig. 2. Adsorption and desorption isotherms of (a) SBA-15 and (b) HPTES–SBA-15 measured at −196 ◦ C. Table 1 Specific surface area (SBET ), total pore volume (Vp ), and pore diameter (D) for SBA-15 and HPTES–SBA-15 obtained by BET method. Materials

SBET (m2 g−1 )

Vp (cm3 g−1 )

D (nm)

Pure SBA-15 HPTES–SBA-15

790 520

1.2862 0.9104

7.06 7.02

3. Results and discussion 3.1. Synthesis and characterization of the fiber In order to increase the selectivity and improve the sorption properties of SBA-15 as a coating material for SPME fibers, it was functionalized by HPTES groups. Fig. 2 shows the sorption isotherms before and after functionalization of SBA-15. As it is seen in the figure, the adsorption and desorption branches of pure and functionalized materials are parallel. This indicates that the mesopores are still fully accessible and no pore blocking takes place after the functionalization. It provides facile access for the chemical reagents or guest species. The specific surface areas, total pore volumes, and mean pore diameters are shown in Table 1. In this research no additional water was used. Therefore, functionalization was done on silanol capping approach and slight decrease of pore diameter of HPTES–SBA-15 probably shows this approach of functionalization. The TGA curve of HPTES–SBA-15 is shown in Fig. 3. There are four zones in the TGA curve: (1) weight loss up to 130 ◦ C that refers to removal of physically adsorbed water, (2) slight weight loss between 130 and 260 ◦ C that shows organic content of HPTES

Fig. 3. TGA curve of HPTES–SBA-15.

3.2. The SPME–GC–MS analysis The efficiency of the prepared HPTES–SBA-15 fiber was tested for the headspace SPME of the BTEX and the phenolic compounds. Preliminary experiments indicated that the prepared fiber can successfully adsorb all the test compounds. Before optimization of the extraction parameters, complete desorption of the collected analytes in the GC–MS injection port, and their proper separation over the column were optimized. For this purpose, different injector temperatures and desorption times were tested. The upper temperature that can be used for the desorption of the analytes from a fiber is limited by the thermal stability of its coating. A temperature of 260 ◦ C was found to be appropriate for the efficient desorption of analytes from the HPTES–SBA-15 fiber without damaging its coating. Desorption times from 1 to 3 min were investigated at this temperature and 3 min was selected for a complete desorption with no memory effect. Different temperature programs were tested for an appropriate separation of the BTEX and the phenolic compounds and the program mentioned in Section 2 was selected as optimum. 3.3. Optimization of the extraction parameters Different parameters influence the extraction efficiency in an SPME experiment. Effects of extraction temperature, sonication time, collection time, sample volume and salt concentration on the extraction of the BTEX compounds by the proposed method were optimized using a one-at-a-time process. The extraction efficiency increased with time up to 10 min and leveled off in longer times. Maximum extraction was obtained at a temperature of 40 ◦ C, sonication time of 10 min, sample volume of 10 mL and salt concentration of 10% (NaCl). Nevertheless, the extraction efficiency was not satisfactory even at the optimized conditions. Optimization of five different parameters on the extraction of the phenolic compounds was performed using a simplex optimization method [26]. A simplex method can significantly decrease the number of required experiments for the achievement of a target maximum. In this case, six (n + 1) initial experiments were designed. The conditions corresponding to the worst response was then reflected and the reflection process was repeated until no further improvement in the response was observed. Table 2 shows the conditions selected for this optimization. Fig. 5 shows the extraction efficiencies obtained for the designed experiments, expressed as response or total areas of the target peaks. The maximum efficiency was obtained for experiment no. 7. Therefore, the conditions of this experiment were selected as optimum. Because of acid–base properties of the phenolic compounds and the importance of the effect of pH on their extraction, this effect was studied individually. Fig. 6 depicts a large variation of the extraction efficiency with pH.

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Fig. 4. (a) SEM image of HPTES–SBA-15. (b) Optical microscopic image of a fabricated SPME fiber.

Table 2 Experimental conditions used for the simplex optimization. The experiments were performed at pH 5.5 using the HPTES–SBA-15 fiber. Exp. no.

Extrn. time (min)

Sonication time (min)

Extrn. temp. (◦ C)

Sample volume (mL)a

Salt concn. (%w/v)

Responseb

1 2 3 4 5 6 7 8 9

10 10 10 10 15 10 12 12.8 12.8

5 5 5 2 5 5 3.8 3.3 3.36

40 40 50 40 40 40 44 45.6 45.8

10 5 10 10 10 10 15 12 12.2

5 5 5 5 5 10 7 0.8 4.04

3344965510 3151238491 4263284477 4335582430 3461073638 333893604 5344215514 4322224435 4109415263

a b

The sample was a 20 mg L−1 solution of the phenolic compounds mixture in water. Total area of the analytes’ peaks was considered as response to be optimized.

The maximum efficiency for the phenolic compounds was achieved at pH 5.5. 3.4. Comparison of the HPTES–SBA-15 and PDMS fibers The extraction efficiency of the proposed fiber was compared with that of a commercial PDMS fiber for the BTEX and phenolic compounds. The HPTES–SBA-15 fiber contains polar amino groups and, as expected, its efficiency for non-polar BTEX compounds was lower than PDMS. On the other hand, a high tendency towards the adsorption of polar phenolic compounds was observed for the proposed fiber. Fig. 7 shows that the peak heights of all the studied phenolic compounds are higher for the SBA-15 fiber than those of PDMS. This indicates an average of about three times higher efficiency for the proposed fiber.

Fig. 5. The extraction efficiency (response) in the simplex optimization experiments. See Table 2 for the experimental conditions.

3.5. Analytical performances The repeatability of the extraction by the proposed fiber was examined by five replicated analysis of the phenolic compounds at 200 ␮g mL−1 . The relative standard deviations (RSDs) were between 6.5% and 9.8% for the analytes. The fiber-to-fiber reproducibility was also tested by fabrication of five independent HPTES–SBA-15 fibers and extraction of the phenolic compounds with them. The RSDs for the extraction of the phenolic compounds were between 8.2% and 11.3% in this case. No significant change was observed in the extraction efficiency of the proposed SPME fiber over at least 50 extractions. Thus, the fiber is stable in the high desorption temperatures used and has a reasonably long life. Calibration curves were plotted for the phenolic compounds and linear relationships between the analytes concentrations in the sample solutions and their SPME–GC–MS signals were obtained. As shown in Table 3, the linear ranges were up to 500 ␮g mL−1 for Ph and Np and up to 200 ␮g mL−1 for Dn and Cp with R2 values larger

Fig. 6. Effect of pH on the collection of the phenolic compounds by SBA-15 fiber. Other experimental conditions are as in experiment number 7 in Table 2.

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Table 3 Calibration curve data and calculated detection limits of the phenolic compounds analysed by the HPTES–SBA-15–GC–MS method. Sample

Slope

Ph Np Dn Cp

2 × 10 3 × 106 2 × 107 2 × 106 6

Linear range (␮g mL−1 )

Intercept

R2

−3 × 10 −2 × 107 1 × 108 1 × 107

0.9935 0.9996 0.9924 0.9930

7

10–500 10–500 0.1–200 0.1–200

LOD (␮g mL−1 ) 0.013 0.009 0.001 0.011

Table 4 The results obtained for the analysis of the spiked river water samples by the proposed method, under the optimized conditions. Sample

Added (␮g mL−1 )

Khoramrood river Ghomrood river

10 10

a

Founda (␮g mL−1 ) Ph

Np

Dn

Cp

10.4 (0.4) 11.1 (0.4)

11.3 (1.5) 11.2 (0.7)

9.0 (1.9) 11.0 (0.6)

9.3 (0.9) 9.2 (0.7)

The figures within parentheses are standard deviations for three replicates.

Table 5 The results obtained for the analysis of the sewage water samples by the proposed method, before and after addition of 10 ␮g mL−1 Ph and Cp, under the optimized conditions. Sample

Piroozi Bridge sewage Moallem Park sewage a b

Before the additiona (␮g mL−1 )

After the additiona (␮g mL−1 )

Ph

Cp

Ph

Cp

0.66 (0.06) N.D.b

0.47 (0.01) 0.42 (0.08)

10.74 (0.82) 10.3 (0.4)

10.47 (0.14) 10.51 (14)

The figures within parentheses are standard deviations for three replicates. N.D., stands for not detected.

robust, selective, highly porous and easily and inexpensively prepared. References

Fig. 7. Comparison of SBA-15 and PDMS fibers for the collection of phenol (Ph), 4-nitrophenol (Np), 2,4-dinitrophenol (Dn) and 4-chlorophenol (Cp). Experimental conditions are as in experiment number 7 in Table 2.

than 0.993. Limit of detections (LOD), calculated for 3, have been shown in the most left column of Table 3. 3.6. Application to field samples The HPTES–SBA-15 fibers were applied to the determination of chlorophenol compounds in two polluted river water and two untreated sewage water samples. Since the concentrations of most of the phenolic compounds in the field samples were lower than the detection limit of the method, the samples were spiked with the phenolic compounds. Tables 4 and 5 show the satisfied results obtained for the field samples. It can be concluded that the new HPTES–SBA-15 SPME fiber is a promising alternative to the commercial fibers as it is

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