Journal of Chromatography A, 1364 (2014) 53–58
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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Facile preparation and applications of graphitic carbon nitride coating in solid-phase microextraction Na Xu a , Yiru Wang a,∗ , Mingcong Rong a , Zhifeng Ye a , Zhuo Deng a , Xi Chen a,b,∗∗ a Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China b State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361005, China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Received in revised form 23 August 2014 Accepted 25 August 2014 Available online 30 August 2014 Keywords: Graphitic carbon nitride Solid-phase microextraction Gas chromatograph Acrylamide
a b s t r a c t In this study, graphitic carbon nitride (g-C3 N4 ) was used as a coating material for solid-phase microextraction (SPME) applications. Coupled to gas chromatography (GC), the extraction ability of the SPME fiber was investigated and compared with the commercial fibers of 100 m PDMS and 85 m CAR/PDMS using six target analytes including deltamethrin, nerolidol, amphetamine, dodecane, ametryn and acrylamide. The g-C3 N4 coating revealed excellent extraction ability and durability comparing with those of the commercial fibers due to its loose structure and unique physicochemical properties. The repeatability for each single fiber was found to be 3.46% and reproducibility for fiber to fiber was 8.53%. The g-C3 N4 SPME fiber was applied to the determination of acrylamide in potato chips, the linearity and detection limit was 0.5-250 g g−1 and 0.018 g g−1 , respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Solid-phase microextraction (SPME), one kind of modern sample preparation technique, has gained great attention due to its advantage of simplicity, sensitivity, rapidity, solvent free and easily coupling to GC and HPLC. It has been widely applied in fields of food [1], environment [2] and medicine [3]. As known to all, the coating of SPME fiber has obvious influence on the sensitivity and selectivity of the analytical method. Up to now, there have been some materials used as coatings for commercial SPME fibers, such as polydimethylsiloxane (PDMS), polyacrylate (PA) divinylbenzene (DVB)/PDMS, carboxen (CAR)/PDMS and carbowax (CW)/DVB [4–6]. Furthermore, in order to prepare low-cost, mechanically robust, selective, sensitive and easily fabricated SPME fiber coatings, many self-made materials have been developed including molecularly imprinted polymer [7], anodized aluminum wire [8], ionic liquid [9], metal-organic framework [10–13], zinc oxide nanorods [14] and carbon materials [15–18]. Generally, carbon materials are widely used as sorbents to trap and separate inorganic or organic compounds. Among them, for example, activated carbon [19], fullerene [20], carbon nanotube [21,22] and graphene [23,24] have already been applied as SPME fiber coatings.
∗ Corresponding author. Tel.: +86 135 99510908. ∗∗ Corresponding author. Tel./fax: +86 592 2184530. E-mail addresses: [email protected] (Y. Wang), [email protected] (X. Chen). http://dx.doi.org/10.1016/j.chroma.2014.08.081 0021-9673/© 2014 Elsevier B.V. All rights reserved.
As an analogue of graphene, g-C3 N4 has drawn much attention in recent years. It is considered as the most promising candidate to complement carbon materials. As the most stable allotrope of carbon nitride, g-C3 N4 is mainly composed of carbon and nitrogen [25,26]. Although the accurate structure of g-C3 N4 is still unclear, many reports tend to infer the structure of g-C3 N4 as a “poly (tri-s-triazine)”, which is defected-rich and N-bridged. The conjugated aromatic tri-s-triazine polymer prefers to form p-conjugated planar layers, which is like graphene [27–29]. Thus, g-C3 N4 has excellent thermal [30,31], optical [32] and photoelectrochemical [33] properties, which make it popular in many fields, such as water splitting [34,35], NO decomposition [36] and hydrogenation reactions [37]. g-C3 N4 should be a good material as a SPME coating for the extraction of organic compounds because of its unique physicochemical properties. Furthermore, owning to the extraordinary thermal and chemical stability, the presence of g-C3 N4 may strengthen the mechanical robustness of the SPME fiber coating. In this paper, g-C3 N4 was synthesized through the pyrolysis of melamine. The synthetic product was successfully characterized with X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA). Nerolidol, amphetamine, ametryn, dodecane, deltamethrin and acrylamide were chosen as model analytes to evaluate the extraction ability of the g-C3 N4 SPME fiber. Experimental results proved the excellent adsorptive properties of g-C3 N4 coating. Additionally, the prepared fiber was applied to the determination of acrylamide in potato chip samples to evaluate the tolerance of g-C3 N4 SPME fiber in complex sample matrixes.
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2. Chemicals and materials Stainless steel wires (od. 0.15 mm) were obtained from the AnTing Micro-Injector Factory (Shanghai, China). Silicone SE30 with chromatographic purity was purchased from Shanghai Chemical Reagent Corporation. All analytical grade reagents, potassium bromide (KBr), potassium bromate (KBrO3 ) and melamine, were from the National Medicines Corporation (Shanghai, China). Acrylamide (99.9%) was purchased from Alfa Aesar (MA, USA). Nerolidol from Adrich (USA) was used. Deltamethrin was from the Agro-environmental Protection Institute, Ministry of Agriculture (Tianjin, China). Amphetamine was purchased from the Institute of Forensic Science of the Ministry of Public Security P.R.C. (Beijing, China). Ametryn was obtained from Fluka (USA). Dodecane was purchased from J&K Scientific Ltd. (shanghai, China). 1000 mg L−1 stock solutions of acrylamide, nerolidol, dodecane, amphetamine and ametryn were prepared by dissolving the above compounds in methanol. 10 mg L−1 stock solution of deltamethrin was prepared by diluting 100 mg L−1 deltamethrin n-hexane solution with acetone. All the standard solutions used for SPME extraction during the whole experiment were prepared by diluting the stock solutions to the required concentration with pure water. Pure water (18.2 M) from a Millipore Autopure WR600A system (Millipore Ltd., USA) was used throughout the experiments. Potato chip samples were collected from local supermarkets in Xiamen. All the solutions mentioned above were sealed and stored at 4 ◦ C.
2.1. Instruments A commercial manual sampling SPME device and two commercial SPME fibers, 100 m PDMS and 85 m CAR/PDMS, were purchased from Supelco (Bellefonte, PA, USA). The determination of nerolidol, amphetamine, ametryn and dodecane were carried on a Shimadazu GC-2010 gas chromatograph (GC) system coupled with a flame ionization detector (FID). In the determination of deltamethrin, an electron capture detector (ECD) was selected and used. Nerolidol, amphetamine, ametryn, dodecane and deltamethrin were separated by a 30 m × 0.25 mm I.D. DB-5 capillary column. Derivated acrylamide was separated by a RtxWax capillary column (30 m × 0.32 mm I.D., 0.25 m) and detected by an ECD. The column temperature program used for the separation of nerolidol, amphetamine, ametryn and dodecane was set as follows: an initial temperature of 80 ◦ C, and stayed 3 min, then increased to 120 ◦ C at 20 ◦ C min−1 , and held 2 min, then raised to 240 ◦ C at 10 ◦ C min−1 and maintained 2 min, finally ramped to 280 ◦ C at 30 ◦ C min−1 and maintained 2 min. The detector temperature was held at 300 ◦ C and the temperature of the injector was set at 240 ◦ C. In the separation of deltamethrin, the column temperature program was set as: an initial temperature of 60 ◦ C, stayed 5 min, then increased to 170 ◦ C at 30 ◦ C min−1 , and held 2 min, then increased to 260 ◦ C at 30 ◦ C min−1 , and held 1 min, and finally ramped to 280 ◦ C at 5 ◦ C min−1 and maintained 4.5 min. The detector temperature was at 300 ◦ C and the temperature of the injector was set at 240 ◦ C. As for the separation of derivated acrylamide, the column temperature program was: an initial temperature of 80 ◦ C, stayed 2 min, then increased to 240 ◦ C at 10 ◦ C min−1 , held 2 min. The detector temperature was at 280 ◦ C and the temperature of the injector was set at 230 ◦ C. The phase identification of g-C3 N4 power was obtained by X-ray diffraction (XRD) on a Rigaku Ultima ˚ Fourier IV XRD with Cu K␣ radiation (35 kV, 15 mA, = 1.54051 A). transform infrared spectroscopy (FT-IR) spectra were recorded on a Nicolet 380 spectrophotometer. Thermogravimetric analysis (TGA) of g-C3 N4 was performed on a SDT Q600 TG/DTA thermogravimetric analyzer, the morphology of the fiber coating was observed
by an S4800 scanning electron microscope (SEM, Hitachi, Tokyo, Japan). 2.2. Synthesis of g-C3 N4 The bulk g-C3 N4 was synthesized by pyrolysis of melamine molecules under high temperature. In the synthesis, 20 g melamine was heated at 550 ◦ C for 4 h in air. The obtained yellow product was the g-C3 N4 , then the bulk g-C3 N4 was grinded into powder, which was used for the g-C3 N4 fiber coating [34]. 2.3. Preparation of g-C3 N4 SPME Fiber Before the coating fabrication, the stainless steel (17.5 cm) was sequentially washed with acetone and pure water for 10 min with ultrasonication, and then dried in an oven at 60 ◦ C. A length of 1.5 cm from the end part of stainless steel wire was immersed into a ca 0.3 g mL−1 silicone SE-30 hexane solution for 30 s, and then g-C3 N4 was stuck to the stainless steel. Finally, the g-C3 N4 coated fiber was heated in an oven at 90 ◦ C overnight. 2.4. SPME procedure The g-C3 N4 coated fiber was installed into a 5 L microsyringe. Before use, the fiber was aged at 230 ◦ C for 30 min under nitrogen to avoid any contamination or carryover of analytes. The extraction was conducted in a 20 mL amber glass vial with a PTFE-lined septum cap. During the extraction, the coated fiber was exposed and directly dipped into middle of the sample solution, and the working solution was agitated by a magnetic stirring bar with a constant stirring rate (800 rpm) at room temperature. After the extraction, the fiber was withdrawn back and immediately inserted into the GC injector for the thermal desorption and analysis. 2.5. Sample preparation Potato chip samples were grinded into powder, and 1.000 g of sample was added into a 50 mL centrifuge tube. After addition of 10 mL n-hexane, the sample was homogenized with ultrasonication for 10 min, then the supernatant n-hexane was removed. The degreasing process was repeated as the above mentioned procedure. 10 mL methanol/H2 O (1/1, v/v) solution was added into the residue to exact acrylamide from the sample through ultrasonication for 10 min. In order to get clarified aqueous layer, the tube was centrifuged at 10,000 rpm for 10 min. The residue was extracted once more. The liquor was merged and transferred into a 50 mL quantitative flask for further use. In the derivation of acrylamide, 1.0 mL H2 SO4 (10%, v/v) was added into the flask. The mixture was then placed in a refrigerator at 4 ◦ C for 10 min. 1 mL 0.1 mol L−1 KBrO3 and 1.0 g KBr power were sequentially added into the precooled solution. The mixture was mixed homogeneously and taken into a refrigerator at 4 ◦ C for 1 h. 0.1 mol L−1 Na2 S2 O3 was added into the mixture until the solution color turned to colorless. Lastly, the mixture was diluted with water to a constant volume (50 mL) for SPME experiments. 3. Results and discussion 3.1. Characterization of g-C3 N4 As shown in Fig. 1a, the phase identification of g-C3 N4 revealed that two characteristic peaks in the sample could be identified clearly. The strong diffraction peak at 27.5◦ (0 0 2) was an interlayer stacking peak of aromatic systems (d002 = 0.324 nm) of g-C3 N4 . The small peak at 13.0◦ (1 0 0) was an in-planar peak (d100 = 0.676 nm) [38]. The FT-IR spectrum of g-C3 N4 as shown in Fig. 1b displayed
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Fig. 1. (a) XRD pattern of g-C3 N4 ; (b) FT-IR spectra of g-C3 N4 ; (c) TGA curve of g-C3 N4 in air.
that a range of absorption peaks could be observed in the range of 1200–1600 cm−1 , corresponding to the stretching mode of C N heterocycles. The broad peaks in the range of 3000–3700 cm−1 were attributed to the adsorbed H2 O molecules and NH stretching vibration. Additionally, the absorption peak at 810 cm−1 was assigned to the characteristic breathing mode of the triazine units [39]. The results indicate that the aromatic g-C3 N4 owns specific structure leading to its unique absorption ability. TGA curve of powder was shown in Fig. 1c. It is clear to find that the sample began to decompose when the temperature was over 600 ◦ C, and the total decomposition occurred at the temperature of 750 ◦ C [31]. Generally, the applied temperature for a GC injector is lower than 500 ◦ C, indicating that g-C3 N4 is applicable as SPME coating.
3.2. Characterization of the g-C3 N4 coated fiber for SPME The SEM images as shown in Fig. 2 exhibited the morphology of a stainless steel wire modificated g-C3 N4 . The g-C3 N4 coating possessed a rough and hard surface from the low-magnification image of Fig. 2a. In the high magnification SEM image (Fig. 2b), typical slate-like, stacked lamellar texture of milled g-C3 N4 could be observed. The loose structure provided g-C3 N4 fiber with excellent extraction surfaces or points, and the extraction efficiency of a thick coating was better than that of a thin one (Fig. S1). The coating
thickness was approximately 140 m (Fig. 2c), which insured the enough extraction amounts for analytes.
3.3. Extraction ability of the g-C3 N4 coated fiber In order to investigate the extraction ability of the g-C3 N4 coated fiber, deltamethrin, nerolidol, amphetamine, dodecane, ametryn and acrylamide were selected as model analytes. In order to determine acrylamide rapidly and sensitively, acrylamide was brominated. The polarity of brominated acrylamide was similar to acrylamide. Since these compounds have different polarity and electron polarizabilities, it is helpful to understand the extraction ability of the fiber. In the comparison experiments, commercial 100 m PDMS and 85 m CAR/PDMS fibers were chosen. As shown in Fig. 3, the g-C3 N4 fiber exhibited practically outstanding extraction ability towards the first five analytes comparing with those of commercial fibers, and slightly higher extraction ability for acrylamide than that of CAR/PDMS. For the first five compounds, using the g-C3 N4 fiber, higher GC signal responses (3 to 19 folds) could be obtained than those using CAR/PDMS, and 2 to 4 fold higher than those obtained from PDMS. It is clear to see that the first five compounds are low-polar, while acrylamide is a kind of small molecule with high polarity. Generally, a SPME coating is hardly to be applied to extract both low- and high- polarity analytes. The high
Fig. 2. Scanning electron micrographs of a g-C3 N4 coated fiber with magnifications of (a) 110×; (b) 2500×; (c) the cross-section image with magnification of 150×.
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N. Xu et al. / J. Chromatogr. A 1364 (2014) 53–58 Table 1 Recoveries (%) of acrylamide spiked at different levels in potato chip samples by SPME-GC-ECD method. Spiking level (g g−1 )
Recoveries (%)
Intra day (n = 3)
10 50
Fig. 3. Comparison of extraction performance of six analytes by g-C3 N4 fiber with two commercial fibers. Error bars show the standard deviation of the mean (n = 3).
extraction ability of the g-C3 N4 coated towards the first five lowpolar analytes was due to the hydrophobicity of g-C3 N4 [23]. Moreover, the aromatic structure of g-C3 N4 would be helpful to increase the extraction ability of the g-C3 N4 fiber towards deltamethrin, nerolidol, amphetamine and ametryn. Hence, the synergistic effects including hydrophobic interaction, strong – stacking interaction and hydrogen bonding between g-C3 N4 and the four analytes obviously increased the extraction ability of the g-C3 N4 coating. In the extraction of acrylamide, the g-C3 N4 fiber revealed slightly higher extraction ability than that using commercial CAR/PDM fiber owing to its high affinity for acrylamide [40]. This interesting phenomenon was mainly occurred by the unique structure of g-C3 N4 since the hydrogen bonding and dipole-dipole interaction with acrylamide contributed by the C N heterocycles of the triazine units in g-C3 N4 . Furthermore, the loose structure of the fiber enhanced the available adsorptive sites of the fiber and increased the mass transfer rate between the analytes and the coating. Additionally, in this study, silicone SE-30 had few influence on the extraction ability of the new fiber coating. It mostly played a role as glue to stick g-C3 N4 to the stainless steel.
3.4. Optimization of SPME parameters for acrylamide In order to evaluate the application ability of the g-C3 N4 fiber in matrix samples, the fiber was applied to determinate acrylamide in potato chip samples. Before the applications, several parameters for SPME including extraction time, extraction temperature, desorption temperature and desorption time were investigated and optimized. 20 mL aqueous solution spiked with 0.5 g mL−1 acrylamide was used in the study. As SPME is an equilibrium-based procedure, the extraction time is one of the key factors influencing the extraction amount, which was investigated by varying the time from 10 to 50 min in the experiment. The profile of the extraction time vs. signal response of acrylamide was exhibited in Fig. 4a. The result showed that the maximum signal could be achieved when the extraction time was increased to 30 min. Therefore, in the following study, the extraction time of 30 min was chosen. In SPME applications, a fiber generally needs much time to reach its extraction equilibrium, and non-equilibrium mode is commonly applied in a SPME process, which limits the extraction efficiency. Although the g-C3 N4 coating thickness was about 140 m, total extraction equilibrium could be achieved in a short extraction time (30 min), indicating the high diffusion speed between the g-C3 N4 coating and analytes.
Inter day (n = 3)
Sample 1
Sample 2
Sample 1
Sample 2
100.9 ± 4.2 108.4 ± 1.6
83.3 ± 4.4 95.3 ± 8.5
100.7 ± 3.2 109.9 ± 8.6
80.2 ± 7.7 98.2 ± 3.4
The mass transfer rate of acrylamide from water to the g-C3 N4 fiber was affected by the extraction temperature. The effect of the extraction temperature from 15 ◦ C to 60 ◦ C was investigated. As shown in Fig. 4b, a suitable extraction temperature was found to be in the range of 15 ◦ C to 30 ◦ C. The higher extraction temperature caused the signal decrease. Thus, room temperature (about 25 ◦ C) was preferred as the extraction temperature. The desorption temperature and time affected the desorption of the target analytes from the fiber coating. The target analytes cannot be desorbed entirely at a low desorption temperature, while they may be decomposed at a high desorption temperature. In the experiment, the effect of the desorption temperature in the range of 210 ◦ C to 250 ◦ C was investigated. The profile as shown in Fig. 4c indicates that the peak area reaches the maximal value at 230 ◦ C, and decreases slightly with the increase of desorption temperature. In addition, the effect of the desorption time was investigated at 0.1, 0.5, 1, 2, 3 min at 230 ◦ C in this work. As shown in Fig. 4d, the peak area of acrylamide reached its maximal value at the desorption time of 2 min. In the following experiments, the desorption temperature was set at 230 ◦ C, and the desorption time of 2 min was selected. 3.5. Application of the g-C3 N4 fiber Before the determination of acrylamide in potato chip samples using the g-C3 N4 fiber, the method validation including linear range, limit of detection (LOD), precision and accuracy were studied under the optimized conditions. A good linear correlation of the GC peak area to the acrylamide concentration in the range of 0.5–250 g g−1 with a correlation coefficient of 0.999 could be obtained. The detection limit for acrylamide was found to be 0.018 g g−1 (S/N = 3), which was comparable with those of other similar methods (Table S1). The repeatability for each single fiber was investigated at triplicate analyses (n = 3), and the reproducibility for fiber to fiber was evaluated by using three different fibers fabricated in the same way (n = 3). RSD of single fiber repeatability was 3.46% and fiber to fiber reproducibility was 8.53%. Moreover, the g-C3 N4 fiber could be used more than 100 times without any obvious damage of the coating (Fig. S2). To assess the method accuracy, as shown in Table 1, the detection recoveries of acrylamide were detected and calculated by extracting acrylamide from the spiked potato chip samples. The spiked amount level was selected 10 or 50 g g−1 . The experimental results revealed that the intra day recoveries was in the range from 83.3% to 108.4% at the different spiked amount levels. Meanwhile, the inter day recoveries was in the range from 80.2% to 109.9% during three days. Under the optimized extraction and GC conditions, the g-C3 N4 fiber was applied to the determination of acrylamide in potato chip samples. The typical GC chromatograms for sample 1 (a), sample 1 spiked at 50 g g−1 level (b) and 50 g g−1 standard solution (c) were exhibited in Fig. 5. The amount of acrylamide in two samples was found to be 3.87 and 3.07 g g−1 , respectively, indicating the applicability of the g-C3 N4 fiber for the determination of acrylamide. The extraction performance of home-made g-C3 N4 fiber was a little better than that of commercial CAR/PDMS fiber on the
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Fig. 4. (a) The effect of extraction time on extraction efficiency. Conditions: extraction temperature, room temperature; desorption time, 3 min. (b) The effect of extraction temperature on extraction efficiency. Conditions: extraction time, 30 min; desorption time, 3 min. (c) The effect of desorption temperature on peak area. Conditions: extraction time, 30 min; extraction temperature, room temperature; desorption time, 3 min. (d) The effect of desorption temperature on peak areas. Conditions: extraction temperature, room temperature; extraction time, 30 min. Concentrations of acrylamide was 0.5 g mL−1 . Error bars show the standard deviation of the mean (n = 3).
matrix. Coupled with GC-ECD analysis, the g-C3 N4 fiber was successfully applied to analyze acrylamide in potato chip samples. Under the optimized experimental conditions, the SPME-GC-ECD method showed low detection limit, wide linear range and good recoveries in the determination of acrylamide. Moreover, g-C3 N4 presents great potential in the extraction of different polar analytes from complicated samples.
Acknowledgements The research was financially supported by the National Nature Scientific Foundation of China (No. 21105084), the Fujian Key Projects of Science and Technology (No. 2011YZ0001-1) and NFFTBS (No. J1310024), which are gratefully acknowledged.
Fig. 5. Chromatograms of extraction acrylamide from (a) potato chips (sample 1); (b) spiked potato chips (sample 1) at 50 g g−1 level; (c) 50 g g−1 standard solution.
acrylamide determination in potato chips at optimal conditions (Fig. S3). 4. Conclusions In this study, g-C3 N4 was used as a coating material for SPME fiber. The extraction efficiency of the fiber towards deltamethrin, nerolidol, amphetamine, dodecane, ametryn and acrylamide was compared with 100 m PDMS and 85 m CAR/PDMS fibers. Extraction efficiency for the first five compounds was better than those of two commercial fibers and slightly higher extraction efficiency for acrylamide compared with CAR/PDMS. The g-C3 N4 fiber was of ease to fabricated, low cost, high extraction efficiency, good selectivity, long lifespan and good tolerance to complex sample
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma. 2014.08.081.
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Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Facile preparation and applications of graphitic carbon nitride coating in solid-phase microextraction Na Xu a , Yiru Wang a,∗ , Mingcong Rong a , Zhifeng Ye a , Zhuo Deng a , Xi Chen a,b,∗∗ a Department of Chemistry and the MOE Key Laboratory of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China b State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361005, China
a r t i c l e
i n f o
Article history: Received 14 July 2014 Received in revised form 23 August 2014 Accepted 25 August 2014 Available online 30 August 2014 Keywords: Graphitic carbon nitride Solid-phase microextraction Gas chromatograph Acrylamide
a b s t r a c t In this study, graphitic carbon nitride (g-C3 N4 ) was used as a coating material for solid-phase microextraction (SPME) applications. Coupled to gas chromatography (GC), the extraction ability of the SPME fiber was investigated and compared with the commercial fibers of 100 m PDMS and 85 m CAR/PDMS using six target analytes including deltamethrin, nerolidol, amphetamine, dodecane, ametryn and acrylamide. The g-C3 N4 coating revealed excellent extraction ability and durability comparing with those of the commercial fibers due to its loose structure and unique physicochemical properties. The repeatability for each single fiber was found to be 3.46% and reproducibility for fiber to fiber was 8.53%. The g-C3 N4 SPME fiber was applied to the determination of acrylamide in potato chips, the linearity and detection limit was 0.5-250 g g−1 and 0.018 g g−1 , respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Solid-phase microextraction (SPME), one kind of modern sample preparation technique, has gained great attention due to its advantage of simplicity, sensitivity, rapidity, solvent free and easily coupling to GC and HPLC. It has been widely applied in fields of food [1], environment [2] and medicine [3]. As known to all, the coating of SPME fiber has obvious influence on the sensitivity and selectivity of the analytical method. Up to now, there have been some materials used as coatings for commercial SPME fibers, such as polydimethylsiloxane (PDMS), polyacrylate (PA) divinylbenzene (DVB)/PDMS, carboxen (CAR)/PDMS and carbowax (CW)/DVB [4–6]. Furthermore, in order to prepare low-cost, mechanically robust, selective, sensitive and easily fabricated SPME fiber coatings, many self-made materials have been developed including molecularly imprinted polymer [7], anodized aluminum wire [8], ionic liquid [9], metal-organic framework [10–13], zinc oxide nanorods [14] and carbon materials [15–18]. Generally, carbon materials are widely used as sorbents to trap and separate inorganic or organic compounds. Among them, for example, activated carbon [19], fullerene [20], carbon nanotube [21,22] and graphene [23,24] have already been applied as SPME fiber coatings.
∗ Corresponding author. Tel.: +86 135 99510908. ∗∗ Corresponding author. Tel./fax: +86 592 2184530. E-mail addresses: [email protected] (Y. Wang), [email protected] (X. Chen). http://dx.doi.org/10.1016/j.chroma.2014.08.081 0021-9673/© 2014 Elsevier B.V. All rights reserved.
As an analogue of graphene, g-C3 N4 has drawn much attention in recent years. It is considered as the most promising candidate to complement carbon materials. As the most stable allotrope of carbon nitride, g-C3 N4 is mainly composed of carbon and nitrogen [25,26]. Although the accurate structure of g-C3 N4 is still unclear, many reports tend to infer the structure of g-C3 N4 as a “poly (tri-s-triazine)”, which is defected-rich and N-bridged. The conjugated aromatic tri-s-triazine polymer prefers to form p-conjugated planar layers, which is like graphene [27–29]. Thus, g-C3 N4 has excellent thermal [30,31], optical [32] and photoelectrochemical [33] properties, which make it popular in many fields, such as water splitting [34,35], NO decomposition [36] and hydrogenation reactions [37]. g-C3 N4 should be a good material as a SPME coating for the extraction of organic compounds because of its unique physicochemical properties. Furthermore, owning to the extraordinary thermal and chemical stability, the presence of g-C3 N4 may strengthen the mechanical robustness of the SPME fiber coating. In this paper, g-C3 N4 was synthesized through the pyrolysis of melamine. The synthetic product was successfully characterized with X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA). Nerolidol, amphetamine, ametryn, dodecane, deltamethrin and acrylamide were chosen as model analytes to evaluate the extraction ability of the g-C3 N4 SPME fiber. Experimental results proved the excellent adsorptive properties of g-C3 N4 coating. Additionally, the prepared fiber was applied to the determination of acrylamide in potato chip samples to evaluate the tolerance of g-C3 N4 SPME fiber in complex sample matrixes.
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2. Chemicals and materials Stainless steel wires (od. 0.15 mm) were obtained from the AnTing Micro-Injector Factory (Shanghai, China). Silicone SE30 with chromatographic purity was purchased from Shanghai Chemical Reagent Corporation. All analytical grade reagents, potassium bromide (KBr), potassium bromate (KBrO3 ) and melamine, were from the National Medicines Corporation (Shanghai, China). Acrylamide (99.9%) was purchased from Alfa Aesar (MA, USA). Nerolidol from Adrich (USA) was used. Deltamethrin was from the Agro-environmental Protection Institute, Ministry of Agriculture (Tianjin, China). Amphetamine was purchased from the Institute of Forensic Science of the Ministry of Public Security P.R.C. (Beijing, China). Ametryn was obtained from Fluka (USA). Dodecane was purchased from J&K Scientific Ltd. (shanghai, China). 1000 mg L−1 stock solutions of acrylamide, nerolidol, dodecane, amphetamine and ametryn were prepared by dissolving the above compounds in methanol. 10 mg L−1 stock solution of deltamethrin was prepared by diluting 100 mg L−1 deltamethrin n-hexane solution with acetone. All the standard solutions used for SPME extraction during the whole experiment were prepared by diluting the stock solutions to the required concentration with pure water. Pure water (18.2 M) from a Millipore Autopure WR600A system (Millipore Ltd., USA) was used throughout the experiments. Potato chip samples were collected from local supermarkets in Xiamen. All the solutions mentioned above were sealed and stored at 4 ◦ C.
2.1. Instruments A commercial manual sampling SPME device and two commercial SPME fibers, 100 m PDMS and 85 m CAR/PDMS, were purchased from Supelco (Bellefonte, PA, USA). The determination of nerolidol, amphetamine, ametryn and dodecane were carried on a Shimadazu GC-2010 gas chromatograph (GC) system coupled with a flame ionization detector (FID). In the determination of deltamethrin, an electron capture detector (ECD) was selected and used. Nerolidol, amphetamine, ametryn, dodecane and deltamethrin were separated by a 30 m × 0.25 mm I.D. DB-5 capillary column. Derivated acrylamide was separated by a RtxWax capillary column (30 m × 0.32 mm I.D., 0.25 m) and detected by an ECD. The column temperature program used for the separation of nerolidol, amphetamine, ametryn and dodecane was set as follows: an initial temperature of 80 ◦ C, and stayed 3 min, then increased to 120 ◦ C at 20 ◦ C min−1 , and held 2 min, then raised to 240 ◦ C at 10 ◦ C min−1 and maintained 2 min, finally ramped to 280 ◦ C at 30 ◦ C min−1 and maintained 2 min. The detector temperature was held at 300 ◦ C and the temperature of the injector was set at 240 ◦ C. In the separation of deltamethrin, the column temperature program was set as: an initial temperature of 60 ◦ C, stayed 5 min, then increased to 170 ◦ C at 30 ◦ C min−1 , and held 2 min, then increased to 260 ◦ C at 30 ◦ C min−1 , and held 1 min, and finally ramped to 280 ◦ C at 5 ◦ C min−1 and maintained 4.5 min. The detector temperature was at 300 ◦ C and the temperature of the injector was set at 240 ◦ C. As for the separation of derivated acrylamide, the column temperature program was: an initial temperature of 80 ◦ C, stayed 2 min, then increased to 240 ◦ C at 10 ◦ C min−1 , held 2 min. The detector temperature was at 280 ◦ C and the temperature of the injector was set at 230 ◦ C. The phase identification of g-C3 N4 power was obtained by X-ray diffraction (XRD) on a Rigaku Ultima ˚ Fourier IV XRD with Cu K␣ radiation (35 kV, 15 mA, = 1.54051 A). transform infrared spectroscopy (FT-IR) spectra were recorded on a Nicolet 380 spectrophotometer. Thermogravimetric analysis (TGA) of g-C3 N4 was performed on a SDT Q600 TG/DTA thermogravimetric analyzer, the morphology of the fiber coating was observed
by an S4800 scanning electron microscope (SEM, Hitachi, Tokyo, Japan). 2.2. Synthesis of g-C3 N4 The bulk g-C3 N4 was synthesized by pyrolysis of melamine molecules under high temperature. In the synthesis, 20 g melamine was heated at 550 ◦ C for 4 h in air. The obtained yellow product was the g-C3 N4 , then the bulk g-C3 N4 was grinded into powder, which was used for the g-C3 N4 fiber coating [34]. 2.3. Preparation of g-C3 N4 SPME Fiber Before the coating fabrication, the stainless steel (17.5 cm) was sequentially washed with acetone and pure water for 10 min with ultrasonication, and then dried in an oven at 60 ◦ C. A length of 1.5 cm from the end part of stainless steel wire was immersed into a ca 0.3 g mL−1 silicone SE-30 hexane solution for 30 s, and then g-C3 N4 was stuck to the stainless steel. Finally, the g-C3 N4 coated fiber was heated in an oven at 90 ◦ C overnight. 2.4. SPME procedure The g-C3 N4 coated fiber was installed into a 5 L microsyringe. Before use, the fiber was aged at 230 ◦ C for 30 min under nitrogen to avoid any contamination or carryover of analytes. The extraction was conducted in a 20 mL amber glass vial with a PTFE-lined septum cap. During the extraction, the coated fiber was exposed and directly dipped into middle of the sample solution, and the working solution was agitated by a magnetic stirring bar with a constant stirring rate (800 rpm) at room temperature. After the extraction, the fiber was withdrawn back and immediately inserted into the GC injector for the thermal desorption and analysis. 2.5. Sample preparation Potato chip samples were grinded into powder, and 1.000 g of sample was added into a 50 mL centrifuge tube. After addition of 10 mL n-hexane, the sample was homogenized with ultrasonication for 10 min, then the supernatant n-hexane was removed. The degreasing process was repeated as the above mentioned procedure. 10 mL methanol/H2 O (1/1, v/v) solution was added into the residue to exact acrylamide from the sample through ultrasonication for 10 min. In order to get clarified aqueous layer, the tube was centrifuged at 10,000 rpm for 10 min. The residue was extracted once more. The liquor was merged and transferred into a 50 mL quantitative flask for further use. In the derivation of acrylamide, 1.0 mL H2 SO4 (10%, v/v) was added into the flask. The mixture was then placed in a refrigerator at 4 ◦ C for 10 min. 1 mL 0.1 mol L−1 KBrO3 and 1.0 g KBr power were sequentially added into the precooled solution. The mixture was mixed homogeneously and taken into a refrigerator at 4 ◦ C for 1 h. 0.1 mol L−1 Na2 S2 O3 was added into the mixture until the solution color turned to colorless. Lastly, the mixture was diluted with water to a constant volume (50 mL) for SPME experiments. 3. Results and discussion 3.1. Characterization of g-C3 N4 As shown in Fig. 1a, the phase identification of g-C3 N4 revealed that two characteristic peaks in the sample could be identified clearly. The strong diffraction peak at 27.5◦ (0 0 2) was an interlayer stacking peak of aromatic systems (d002 = 0.324 nm) of g-C3 N4 . The small peak at 13.0◦ (1 0 0) was an in-planar peak (d100 = 0.676 nm) [38]. The FT-IR spectrum of g-C3 N4 as shown in Fig. 1b displayed
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Fig. 1. (a) XRD pattern of g-C3 N4 ; (b) FT-IR spectra of g-C3 N4 ; (c) TGA curve of g-C3 N4 in air.
that a range of absorption peaks could be observed in the range of 1200–1600 cm−1 , corresponding to the stretching mode of C N heterocycles. The broad peaks in the range of 3000–3700 cm−1 were attributed to the adsorbed H2 O molecules and NH stretching vibration. Additionally, the absorption peak at 810 cm−1 was assigned to the characteristic breathing mode of the triazine units [39]. The results indicate that the aromatic g-C3 N4 owns specific structure leading to its unique absorption ability. TGA curve of powder was shown in Fig. 1c. It is clear to find that the sample began to decompose when the temperature was over 600 ◦ C, and the total decomposition occurred at the temperature of 750 ◦ C [31]. Generally, the applied temperature for a GC injector is lower than 500 ◦ C, indicating that g-C3 N4 is applicable as SPME coating.
3.2. Characterization of the g-C3 N4 coated fiber for SPME The SEM images as shown in Fig. 2 exhibited the morphology of a stainless steel wire modificated g-C3 N4 . The g-C3 N4 coating possessed a rough and hard surface from the low-magnification image of Fig. 2a. In the high magnification SEM image (Fig. 2b), typical slate-like, stacked lamellar texture of milled g-C3 N4 could be observed. The loose structure provided g-C3 N4 fiber with excellent extraction surfaces or points, and the extraction efficiency of a thick coating was better than that of a thin one (Fig. S1). The coating
thickness was approximately 140 m (Fig. 2c), which insured the enough extraction amounts for analytes.
3.3. Extraction ability of the g-C3 N4 coated fiber In order to investigate the extraction ability of the g-C3 N4 coated fiber, deltamethrin, nerolidol, amphetamine, dodecane, ametryn and acrylamide were selected as model analytes. In order to determine acrylamide rapidly and sensitively, acrylamide was brominated. The polarity of brominated acrylamide was similar to acrylamide. Since these compounds have different polarity and electron polarizabilities, it is helpful to understand the extraction ability of the fiber. In the comparison experiments, commercial 100 m PDMS and 85 m CAR/PDMS fibers were chosen. As shown in Fig. 3, the g-C3 N4 fiber exhibited practically outstanding extraction ability towards the first five analytes comparing with those of commercial fibers, and slightly higher extraction ability for acrylamide than that of CAR/PDMS. For the first five compounds, using the g-C3 N4 fiber, higher GC signal responses (3 to 19 folds) could be obtained than those using CAR/PDMS, and 2 to 4 fold higher than those obtained from PDMS. It is clear to see that the first five compounds are low-polar, while acrylamide is a kind of small molecule with high polarity. Generally, a SPME coating is hardly to be applied to extract both low- and high- polarity analytes. The high
Fig. 2. Scanning electron micrographs of a g-C3 N4 coated fiber with magnifications of (a) 110×; (b) 2500×; (c) the cross-section image with magnification of 150×.
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N. Xu et al. / J. Chromatogr. A 1364 (2014) 53–58 Table 1 Recoveries (%) of acrylamide spiked at different levels in potato chip samples by SPME-GC-ECD method. Spiking level (g g−1 )
Recoveries (%)
Intra day (n = 3)
10 50
Fig. 3. Comparison of extraction performance of six analytes by g-C3 N4 fiber with two commercial fibers. Error bars show the standard deviation of the mean (n = 3).
extraction ability of the g-C3 N4 coated towards the first five lowpolar analytes was due to the hydrophobicity of g-C3 N4 [23]. Moreover, the aromatic structure of g-C3 N4 would be helpful to increase the extraction ability of the g-C3 N4 fiber towards deltamethrin, nerolidol, amphetamine and ametryn. Hence, the synergistic effects including hydrophobic interaction, strong – stacking interaction and hydrogen bonding between g-C3 N4 and the four analytes obviously increased the extraction ability of the g-C3 N4 coating. In the extraction of acrylamide, the g-C3 N4 fiber revealed slightly higher extraction ability than that using commercial CAR/PDM fiber owing to its high affinity for acrylamide [40]. This interesting phenomenon was mainly occurred by the unique structure of g-C3 N4 since the hydrogen bonding and dipole-dipole interaction with acrylamide contributed by the C N heterocycles of the triazine units in g-C3 N4 . Furthermore, the loose structure of the fiber enhanced the available adsorptive sites of the fiber and increased the mass transfer rate between the analytes and the coating. Additionally, in this study, silicone SE-30 had few influence on the extraction ability of the new fiber coating. It mostly played a role as glue to stick g-C3 N4 to the stainless steel.
3.4. Optimization of SPME parameters for acrylamide In order to evaluate the application ability of the g-C3 N4 fiber in matrix samples, the fiber was applied to determinate acrylamide in potato chip samples. Before the applications, several parameters for SPME including extraction time, extraction temperature, desorption temperature and desorption time were investigated and optimized. 20 mL aqueous solution spiked with 0.5 g mL−1 acrylamide was used in the study. As SPME is an equilibrium-based procedure, the extraction time is one of the key factors influencing the extraction amount, which was investigated by varying the time from 10 to 50 min in the experiment. The profile of the extraction time vs. signal response of acrylamide was exhibited in Fig. 4a. The result showed that the maximum signal could be achieved when the extraction time was increased to 30 min. Therefore, in the following study, the extraction time of 30 min was chosen. In SPME applications, a fiber generally needs much time to reach its extraction equilibrium, and non-equilibrium mode is commonly applied in a SPME process, which limits the extraction efficiency. Although the g-C3 N4 coating thickness was about 140 m, total extraction equilibrium could be achieved in a short extraction time (30 min), indicating the high diffusion speed between the g-C3 N4 coating and analytes.
Inter day (n = 3)
Sample 1
Sample 2
Sample 1
Sample 2
100.9 ± 4.2 108.4 ± 1.6
83.3 ± 4.4 95.3 ± 8.5
100.7 ± 3.2 109.9 ± 8.6
80.2 ± 7.7 98.2 ± 3.4
The mass transfer rate of acrylamide from water to the g-C3 N4 fiber was affected by the extraction temperature. The effect of the extraction temperature from 15 ◦ C to 60 ◦ C was investigated. As shown in Fig. 4b, a suitable extraction temperature was found to be in the range of 15 ◦ C to 30 ◦ C. The higher extraction temperature caused the signal decrease. Thus, room temperature (about 25 ◦ C) was preferred as the extraction temperature. The desorption temperature and time affected the desorption of the target analytes from the fiber coating. The target analytes cannot be desorbed entirely at a low desorption temperature, while they may be decomposed at a high desorption temperature. In the experiment, the effect of the desorption temperature in the range of 210 ◦ C to 250 ◦ C was investigated. The profile as shown in Fig. 4c indicates that the peak area reaches the maximal value at 230 ◦ C, and decreases slightly with the increase of desorption temperature. In addition, the effect of the desorption time was investigated at 0.1, 0.5, 1, 2, 3 min at 230 ◦ C in this work. As shown in Fig. 4d, the peak area of acrylamide reached its maximal value at the desorption time of 2 min. In the following experiments, the desorption temperature was set at 230 ◦ C, and the desorption time of 2 min was selected. 3.5. Application of the g-C3 N4 fiber Before the determination of acrylamide in potato chip samples using the g-C3 N4 fiber, the method validation including linear range, limit of detection (LOD), precision and accuracy were studied under the optimized conditions. A good linear correlation of the GC peak area to the acrylamide concentration in the range of 0.5–250 g g−1 with a correlation coefficient of 0.999 could be obtained. The detection limit for acrylamide was found to be 0.018 g g−1 (S/N = 3), which was comparable with those of other similar methods (Table S1). The repeatability for each single fiber was investigated at triplicate analyses (n = 3), and the reproducibility for fiber to fiber was evaluated by using three different fibers fabricated in the same way (n = 3). RSD of single fiber repeatability was 3.46% and fiber to fiber reproducibility was 8.53%. Moreover, the g-C3 N4 fiber could be used more than 100 times without any obvious damage of the coating (Fig. S2). To assess the method accuracy, as shown in Table 1, the detection recoveries of acrylamide were detected and calculated by extracting acrylamide from the spiked potato chip samples. The spiked amount level was selected 10 or 50 g g−1 . The experimental results revealed that the intra day recoveries was in the range from 83.3% to 108.4% at the different spiked amount levels. Meanwhile, the inter day recoveries was in the range from 80.2% to 109.9% during three days. Under the optimized extraction and GC conditions, the g-C3 N4 fiber was applied to the determination of acrylamide in potato chip samples. The typical GC chromatograms for sample 1 (a), sample 1 spiked at 50 g g−1 level (b) and 50 g g−1 standard solution (c) were exhibited in Fig. 5. The amount of acrylamide in two samples was found to be 3.87 and 3.07 g g−1 , respectively, indicating the applicability of the g-C3 N4 fiber for the determination of acrylamide. The extraction performance of home-made g-C3 N4 fiber was a little better than that of commercial CAR/PDMS fiber on the
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Fig. 4. (a) The effect of extraction time on extraction efficiency. Conditions: extraction temperature, room temperature; desorption time, 3 min. (b) The effect of extraction temperature on extraction efficiency. Conditions: extraction time, 30 min; desorption time, 3 min. (c) The effect of desorption temperature on peak area. Conditions: extraction time, 30 min; extraction temperature, room temperature; desorption time, 3 min. (d) The effect of desorption temperature on peak areas. Conditions: extraction temperature, room temperature; extraction time, 30 min. Concentrations of acrylamide was 0.5 g mL−1 . Error bars show the standard deviation of the mean (n = 3).
matrix. Coupled with GC-ECD analysis, the g-C3 N4 fiber was successfully applied to analyze acrylamide in potato chip samples. Under the optimized experimental conditions, the SPME-GC-ECD method showed low detection limit, wide linear range and good recoveries in the determination of acrylamide. Moreover, g-C3 N4 presents great potential in the extraction of different polar analytes from complicated samples.
Acknowledgements The research was financially supported by the National Nature Scientific Foundation of China (No. 21105084), the Fujian Key Projects of Science and Technology (No. 2011YZ0001-1) and NFFTBS (No. J1310024), which are gratefully acknowledged.
Fig. 5. Chromatograms of extraction acrylamide from (a) potato chips (sample 1); (b) spiked potato chips (sample 1) at 50 g g−1 level; (c) 50 g g−1 standard solution.
acrylamide determination in potato chips at optimal conditions (Fig. S3). 4. Conclusions In this study, g-C3 N4 was used as a coating material for SPME fiber. The extraction efficiency of the fiber towards deltamethrin, nerolidol, amphetamine, dodecane, ametryn and acrylamide was compared with 100 m PDMS and 85 m CAR/PDMS fibers. Extraction efficiency for the first five compounds was better than those of two commercial fibers and slightly higher extraction efficiency for acrylamide compared with CAR/PDMS. The g-C3 N4 fiber was of ease to fabricated, low cost, high extraction efficiency, good selectivity, long lifespan and good tolerance to complex sample
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma. 2014.08.081.
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