Chemically bonded carbon nanotubes on modified ... - Springer Link

Microchim Acta (2011) 174:295–301 DOI 10.1007/s00604-011-0621-4

ORIGINAL PAPER

Chemically bonded carbon nanotubes on modified gold substrate as novel unbreakable solid phase microextraction fiber Habib Bagheri & Zahra Ayazi & Habiballah Sistani

Received: 12 March 2011 / Accepted: 13 May 2011 / Published online: 27 May 2011 # Springer-Verlag 2011

Abstract A new technique is introduced for preparation of an unbreakable fiber using gold wire as a substrate for solid phase microextraction (SPME). A gold wire is used as a solid support, onto which a first film is deposited that consists of a two-dimensional polymer obtained by hydrolysis of a self-assembled monolayer of 3-(trimethoxysilyl)-1-propanthiol. This first film is covered with a layer of 3-(triethoxysilyl)propylamine. Next, a stationary phase of oxidized multi-walled carbon nanotubes was chemically bound to the surface. The synthetic strategy was verified by Fourier transform infrared spectroscopy and scanning electron microscopy. Thermal stability of new fiber was examined by thermogravimetric analysis. The applicability of the novel coating was verified by its employment as a SPME fiber for isolation of diazinon and fenthion, as model compounds. Parameters influencing the extraction process were optimized to result in limits of detection as low as 0.2 ng mL−1 for diazinon, and 0.3 ng mL−1 for fenthion using the timescheduled selected ion monitoring mode. The method was successfully applied to real water, and the recoveries for spiked samples were 104% for diazinon and 97% for fenthion. Keywords Carbon nanotube . Self assembled monolayers (SAMs) . Unbreakable fiber . Solid phase microextraction . Gas chromatography-mass spectrometry

H. Bagheri (*) : Z. Ayazi : H. Sistani Environmental and Bio-Analytical Laboratories, Department of Chemistry, Sharif University of Technology, P.O. Box 11365-9516, Tehran, Iran e-mail: [email protected]

Introduction SPME is a solvent-free extraction technique which integrates sampling, extraction, concentration and sample introduction in a single process and is predominantly performed on SPME fibers [1, 2]. The general fiber coatings of SPME include polydimethysiloxane and poly(dimethylsiloxane)divinylbenzene, polyacrylate, etc. Although they are commercially available, but possess some drawbacks: a) low recommended operating temperature (generally in the range of 240–280 °C), which causes carry-over and short lifetime; b) instability and swelling in organic solvents, which restrict their use with LC and c) fragility and expensive cost. The lack of proper chemical bonding of the stationaryphase coating and the relatively high thickness of the conventional fibers seem to be responsible for some drawbacks of the commercial fibers. For these reasons, recently, chemically bonded stationary phase [3–5], porous layer activated charcoal [6], polypyrrole [7, 8], polyaniline [9, 10], glassy carbon films and polymer with hydroxyfullerene [11], calix[4]arene and crown ether based on sol–gel technology [12–16] were also developed as the fiber coatings for the determination of different analytes in various samples. Carbon nanotubes (CNTs) are a new type of carbon material first found in 1991 by Iijima [17]. CNTs have drawn great attention, for their high aspect ratios, predominant electrical and mechanical properties [18–20]. Because of the larger specific area and hydrophobic characteristic of the surface, CNTs have been regarded as a new type of sorbent and have been studied for adsorption of some inorganic [21] and organic compounds [22–32]. Also it has been used for adsorption of enzymes in order to prepare biosensors [33]. MWCNTs have been characterized as superior sorbent for removing dioxins for environmental

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protection. The amount of dioxin adsorbed on CNTs is 1034 times greater than that on activated carbon [22]. It is believed that reasons for its adsorption may be primarily due to their highly hydrophobic surface [34] and unique structure with internal tube cavity [35]. In this study a new method was developed to prepare chemically bonded CNTs coating on surface of gold substrate to overcome the discussed drawbacks of SPME fibers. For this purpose it was necessary to modify both MWCNTs materials and gold surface. Modification of CNTs surface was performed by oxidation of CNTs in an acidic medium. The gold surface was modified by production of a self assembly monolayer using 3-(trimethoxysilyl)-1-propanthiol (3-TMSPT). Then a layer of 3-(triethoxysilyl)-propylamine (3-TESPA) was introduced to the modified surface of gold in order to impart amine groups to the surface in order to react with acidic functionality of the oxidized CNTs surface. After successful preparation of fiber, it was used for SPME of two organophosphorous pesticides (OPPs) from water samples, which are among the most widely applied pesticides worldwide, causing residues of these substances to be presented in agricultural products and ground waters, surface waters, lagoons and drinking water.

Experimental Reagents and standards MWCNTs with purity higher than 95%, length of 1–10 μm and number of walls in range of 3 to 15, were obtained from Plasma Chem GmbH (Germany, http://plasmachem.com). Diazinon, fenthion, 3-(trimethoxysilyl)-1-propanthiol, 3-(triethoxysilyl)propylamine, sodium chloride, methanol, ethanol, acetone, nitric acid, sulfuric acid, hydrogen peroxide, trifluoroacetic acid (TFA) and sodium dodecylsulfate (SDS) were supplied from Merck (Darmstadt, Germany, http://www.merck-chemicals.com). Instrumentation A gas chromatograph model Agilent 6820 (http://www. agilent.com), with a split-splitless injection port and flame ionization detection (FID) system, was used to determine the optimized extraction conditions. Separation of analytes was carried out using a capillary column HP-1 MS (60 m× 0.25 mm i.d.) with 0.25 μm film thickness (HewlettPackard, Palo Alto, CA, USA, http://www.chem.agilent. com). The carrier gas was Helium (99.999%) at a flow rate of 1 mL min−1. For quantitative determination, a Hewlett-Packard (HP, Palo Alto, USA) HP 6890 plus series GC equipped with a split-splitless injector and a HP 5973 mass-selective

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detector system were used. The separation of OPPs was performed on a 25 m×0.25 μm HP-5 MS column (0.25 μm film thickness). Samples were heated in a homemade glass water bath connected to a refrigerated circulating water bath (RTE-8 NESLAB, USA, http://www.thermoscientific. com). A hot plate magnetic stirrer, Snijders 34532 (Tilburg, Holland, http://www.snijders-tilburg.nl), was used to reflux CNTs in acidic medium. An ultrasonic bath (Elmasonic, Germany, http://www.elma-ultrasonic. com) was used to dispersion of CNTs in aqueous media. For verifying the chemical bonding of CNTs onto the modified Au surface, FTIR spectra were obtained using Unicam Mattson 1000 FTIR spectrometer. In order to characterize the morphological properties of the prepared SPME fiber the SEM images were obtained by a TSCAN VEGA II XMU (Berno, Czech Republic, http://www. tescan-usa.com). Preparation of SPME fiber CNTs oxidation Predetermined quantities of raw MWCNTs (0.1 g) and 50 mL nitric acid were added into a round-bottomed glass flask, and the MWCNTs mixture was sonicated for 5 min in an ultrasonic bath. Next the mixture was refluxed at 100 °C for 6 h. After dilution of the mixture with distilled water, it was filtered and washed to neutral pH, dried at 40 °C for 12 h, and weighed. Surface functionalization of Au wires First, the Au wires with length of 2 cm and thickness of 240 μm, were prepared and were immersed in piranha solution (H2SO4:H2O2, 3:1, v/v) for 10 min at 80 °C. Afterward the wires were washed with acetone in ultrasonic bath for 10 min. The prepared Au wires were immersed in the solution of 3-TMSPT in ethanol with concentration of 10−3 mol L−1 for 12 h. Afterward to introduce amino functionality to the surface, the modified gold substrates were dipped in a solution of 1.2 mL 3-TMSPA in ethanol with concentration of 10−3 mol L−1 and 30 μL TFA and 20 μL water for 12 h. Next the substrates were placed in an oven at 120 °C for 30 min. These two operations were repeated for three cycles to form a layer of 3-TMSPA on the fiber. Chemical bonding of MWCNT layer to the fiber An amount of 1 mg oxidized MWCNTs were dispersed in a 1 mL SDS solution (1%) for 30 min under sonication to prepare the MWCNTs suspension. The pretreated fibers

Chemically bonded MWCNTs on Au as novel SPME fiber

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Fig. 1 Schematic illustration of the preparation process of the MWCNTs/Au fiber

were immersed into the MWCNTs suspension for 4 h in a hot water bath (70 °C), and then it was placed in a 120 °C oven for 30 min. This procedure was repeated three times until the coating reached the desired thickness. Finally, the coated fiber was conditioned under the flow of dry nitrogen prior to the SPME experiments.

Results and discussion Fiber preparation In order to prepare the chemically bonded CNT/Au fiber, it is necessary to functionalize surface of both CNTs and Au wire. According to the literature, one way to functionalize the Au surface is based on the use of self assembled monolayers (SAMs) using bifunctional organic or inorganic compounds [36]. Compounds such as 3-trimethoxysilylpropanthiol [37], 11-trichlorosilylundecylthioacetate and thiols with two active functional groups mostly are used for this purpose [38–40]. In this work, 3-(trimethoxysilyl)-1-propanthiol was used as Fig. 2 FT-IR spectra of the MWCNTs coating after chemical bonding with the fiber

the Au surface functionalization agent to impart the chemical bond between coating and substrate. When the conditioned Au wires were immersed in 3-TMSPT solution in ethanol, 3-TMSPT molecules were bonded chemically to Au surface by thiol head. The structure and morphology of SAM of 3-TMSPT on the Au surface has been extensively studied using scanning tunneling microscopy and secondary ion mass spectrometry [41]. After preparation of a 2-D self assembled layer of 3-TMSPT on the Au wire, it was essential to introduce a kind of functionality amenable to react with carboxylic groups on CNT molecules. This was achieved using 3-TESPA with amino functional group (Fig. 1). The MWCNTs were oxidized by refluxing in nitric acid to create –COOH groups at the sidewall of the nanotubes. This activation process was verified by FTIR spectrum in which an extra peaks at 1,538, 1,708 and 3,744 cm−1 corresponding to carbonyl groups appeared for the oxidized MWCNTs while it was absent in pristine MWCNTs. The MWCNTs/Au fibers were prepared using the reaction between –COOH and –NH2 groups upon heat treatment [31, 42]. The generation of

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amide groups and chemical bonding of MWCNTs on the Au wires were confirmed by FTIR. As shown in Fig. 2, the peaks at 1,623, 1,384 and 3,415 cm−1 are corresponding to amide C = O stretching vibration, amide C–N stretching and amide N–H stretching vibrations, respectively. The peaks at 2,923 and 1,130 cm−1 are related to CH2 and Si–O in 3-TESPA molecules, respectively. This data revealed that MWCNTs are bonded chemically to the surface of modified gold substrate due to the formation of amide groups. Figure 3a shows the SEM micrograph of the prepared MWCNTs/Au fiber. It can be observed that CNTs molecules bonded on the Au surface are in a disordered manner and compactly cover the Au surface. The SEM image of fiber at low magnification order is demonstrated in Fig. 3b, indicating that the CNTs coating uniformly covered the fiber surface. The thermal stability of the CNT/Au fiber was examined by thermogravimetric analysis (TGA). The TGA thermogram (Fig. 4) shows a rather consistent stability in the temperature range of 30–400 °C. The coating desorption occurred in the temperature range of 400–800 °C with a minor weight loss of 5.3%. Optimization

Fig. 3 a–b The SEM images of the MWCNTs-bonded SPME fiber at a magnification of a 50,000 and b 500

Fig. 4 TGA thermogram of CNT/Au fiber

After successful preparation of the novel SPME coating, the preliminary feasibility tests were examined by SPME of two important OPPs. Then it was needed to optimize the influential parameters on SPME procedure. The first parameter is the extraction mode. Among three different mode of SPME, headspace extraction has advantageous due to lower interferences of matrix and no effect of sample pH and solvents on the fiber. All these features together with the sufficient volatility of selected OPP analytes encouraged us to use the headspace mode.


299 Table 1 Some analytical data obtained for SPME of OPPs using the CNT/Au fibers and GC-MS Compound LOD ng mL−1 LDR ng mL−1 R2

Diazinon Fenthion

Fig. 5 Effect of extraction temperature on extraction efficiency. Extractions were performed from a 8 mL aqueous sample solution spiked with each OPPs at 0.5 μg mL−1 for 30 min, salt content of 30% (w/v); the thermal desorption was performed for 6 min at 200 °C

Both desorption time and temperature have great influences on the peak area of extracted analytes. The desorption condition should be selected in a way that quantitative desorption occurs with the least amount of carryover and peak broadening. The time of desorption should be as short as possible while carryover effects must be avoided. The desorption temperature was varied within a range of 200 to 270 °C with desorption time of 5 min. The obtained results show that the increase of desorption temperature has led to a decrease in the analytes peak area which might be due to the OPPs compounds instability at high temperatures. The desorption time was also studied within a range of 2 to 7 min by leaving the SPME fiber in the injection port of GC. The maximum peak area was observed at desorption time of 6 min. The influence of salt addition on the extraction efficiency was investigated, as well. Usually, the presence of salt increases the ionic strength of aqueous solution and would affect the solubility of organic solutes. This effect leads to varying the partition coefficient of analytes between headspace and solution (Khs) hence the extraction efficiency may be changed. In this experiment, sodium chloride concentration of 0.0 to 30% (w/v) was tested. An increase in extraction

Fig. 6 Effect of extraction time on extraction efficiency. The analytes were extracted from a 8 mL aqueous sample solution spiked with each OPPs at 0.5 μg mL−1 at temperature of 60 °C, salt content of 30% (w/ v); the thermal desorption was performed for 6 min at 200 °C

0.2 0.3

0.5–200 0.5–200

0.9949 0.9797

RSD% RR% (n=3) 4.6 4.4

104 97

efficiency was observed by adding sodium chloride. Therefore the salt amount of 30% (w/v) was chosen as optimum value in order to perform further extractions. Extraction temperature is one of the most important parameters for the evaluation of extraction efficiency in SPME. In this work, an extraction temperature ranging from 30 to 80 °C was evaluated. As shown in Fig. 5, the amount of extracted compounds increases with enhancing temperature up to 60 °C but after this point the extraction efficiency was decreased for both diazinon and fenthion. The sorption of analyte into the sorbent is an exothermic process. Hence, the Kfh decreases with increasing temperature while the Khs increases with increasing temperature. Accordingly, the temperature of 60 °C was chosen as the optimization extraction temperature for further experiments. The SPME equilibrium time was investigated by exposing the fiber to the headspace of the sample containing the target analytes for 10 to 60 min. As shown in Fig. 6, there is an increase in responses up to 30 min, after this time the peak area for fenthion remained approximately constant, but a decrease for diazinon was observed. Method validation Based on the method development observed above, the following conditions were selected for the determination of diazinon and fenthion in water: extraction temperature of 60 ° C, extraction time of 30 min, salt concentration of 30% w/v, maximum rate of stirrer, desorption temperature of 200 °C and desorption time of 6 min. The obtained calibration graphs for both of analytes were linear in the concentration range of 0.5 to 200 ng mL−1. The regression coefficient for the analytes was rather satisfactory (R2 >0.9797). Limits of detection, based on a signal-to-noise ratio of 3/1, was 0.2 and 0.3 ng mL−1 for diazinon and fenthion, respectively, using SIM mode (Table 1). The standard deviation of the developed method for extraction of the selected OPPs from 1 ng mL−1 spiked water samples were below 4.6% (n=3). To evaluate the applicability of the developed method, extraction and analysis was performed on Zayandeh-rood river (Isfahan-Iran) water. The water sample was spiked at a concentration level of 1 ng mL−1 and the analysis was

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carried out under the optimized conditions described above. Acceptable relative recoveries were achieved for diazinon and fenthion (Table 1).

Conclusions The CNT/Au fiber was successfully prepared by covalent bonding based on the surface modification of both multiwalled carbon nanotube and the Au wire. Apparently, the prepared MWCNT-based coating demonstrates favorable extraction capability due to the high surface area. The chemical bonding between the modified gold surface and oxidized CNTs combining with the inherent stability of MWCNTs and the unbreakable property of Au fibers led to novel SPME fibers which have high thermal, chemical and mechanical stability. The novel fibers could be used in a convenient way with a long operating life which could be easily used for more than 100 times. Apparently adsorption is the predominant mechanism for trapping the analytes while according to the SEM images (Fig. 3) the fiber has approximately 13 μm of thickness.

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