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Sensitive and rapid determination of catechol in tea samples using mesoporous Al-doped silica modified electrode
Food Chemistry 113 (2009) 701–704
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Sensitive and rapid determination of catechol in tea samples using mesoporous Al-doped silica modified electrode Huogang Lin, Tian Gan, Kangbing Wu * Department of Chemistry, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
a r t i c l e
i n f o
Article history: Received 1 April 2008 Received in revised form 6 July 2008 Accepted 19 July 2008
Keywords: Catechol Porous silica Electrochemistry Determination Tea
a b s t r a c t A mesoporous Al-doped silica (Al/SiO2) was synthesised according to the published method, and then used to modify the carbon paste electrode (CPE). The electrochemical behaviour of catechol was investigated. Compared with the unmodified CPE, the resulting mesoporous Al/SiO2 modified CPE remarkably increases the peak currents of catechol, and greatly lowers the peak potential separation. Therefore, the mesoporous Al/SiO2 exhibits catalytic activity to catechol and significantly improves the determining sensitivity. Based on this, a sensitive, rapid and convenient electrochemical method was proposed for the determination of catechol. The linear range is between 5.0 10 7 and 5.0 10 5 mol L 1 with a correlation coefficient of 0.998. The limit of detection is as low as 1.0 10 7 mol L 1. Finally, this novel method was employed to determine catechol in tea samples, which testified by high-performance liquid chromatography. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Catechol (o-benzenediol, 1,2-benzenediol or 1,2-dihydroxybenzene) is a natural polyphenolic compound that widely exists in higher plants such as teas, vegetables, fruits, tobaccos and some traditional Chinese medicines (Sun, Cui, Li, & Lin, 2000). Catechol has been widely studied due to the biological importance such as antioxidation, antivirus and affecting the activities of some enzymes. Therefore, it is important to establish a sensitive, rapid and convenient method for the determination of catechol. Catechol contains phenolic hydroxy group and possesses excellent electrochemical activity, so various electrochemical methods using different modified electrodes have been reported for the determination of catechol. For example, a mesoporous platinum electrode with limit of detection (LOD) of 10 6 mol L 1 (Ghanem, 2007), a penicillamine modified electrode with LOD of 7.5 10 7 mol L 1 (Wang et al., 2007), a multi wall carbon nanotubes-modified electrode with LOD of 2.5 10 7 mol L 1 (Qi & Zhang, 2005), a nano Au/alkanedithiol self-assembled gold electrode with LOD of 10 6 mol L 1 (Su & Mao, 2006), and a nano-TiO2 modified electrode (Lunsford, Choi, Stinson, Yeary, & Dionysiou, 2007), were employed. However, electrochemical determination of catechol using mesoporous Al-doped silica has not been reported.
* Corresponding author. Fax: +86 27 8754 3632. E-mail address: [email protected] (K. Wu). 0308-8146/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2008.07.073
Mesoporous material is a kind of porous materials with regular and tunable pore from 2 to 50 nm. Since its discovery (Beck et al., 1992), mesoporous material has attracted increasing attention and widely used in different fields such as catalysis, energy storage, energy conversion, adsorption and separation (Kim et al., 2006; Landskron & Ozin, 2004; Li et al., 2007; Tolbert, Firouzi, Stucky, & Chmelka, 1997). The aim of this work is to develop a sensitive and rapid electrochemical method for the determination of catechol utilising the excellent properties of mesoporous material. To accomplish this, a mesoporous Al-doped silica (denoted as Al/ SiO2) was synthesised according to the published work (Galarneau, Cangiotti, Renzo, Fajula, & Ottaviani, 2006), and then mixed homogenously with graphite powder and paraffin oil to give a mesoporous Al/SiO2 modified carbon paste electrode (CPE). The electrochemical behavior of catechol was examined at the unmodified and mesoporous Al/SiO2 modified CPEs. Owing to the specific and regular mesopores, large surface area, high adsorption ability and numerous active sites, the mesoporous Al/SiO2 modified CPE remarkably enhances the oxidation signal of catechol, and obviously lowers the oxidation overpotential. Therefore, the mesoporous Al/SiO2 shows efficient catalytic activity toward catechol and significantly improves the determining sensitivity. In addition, the electrochemical behaviour of hydroquinone was also studied. Hydroquinone yields an oxidation peak before catechol. The oxidation peaks of hydroquinone and catechol are very separated and do not interfere with each other at the modified CPE. Compared with the published electrochemical methods, determining catechol using mesoporous Al/SiO2 possesses higher sensitivity, rapider response and better simplicity.
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H. Lin et al. / Food Chemistry 113 (2009) 701–704
2. Experimental 2.1. Reagent All the chemicals were of analytical grade and used without further purification. Catechol, hydroquinone (p-benzenediol) and resorcinol (m-benzenediol) were obtained from Aldrich. Cetyltrimethyl ammonium bromide ammonia (CTAB), tetraethyl orthosilicate (TEOS), sodium aluminate (NaAlO2), graphite powder (spectral reagent) and paraffin oil were purchased from the Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Instruments All the electrochemical measurements were performed using an 830B electrochemical analyser (CH Instruments, USA). A conventional three-electrode system, consisting of a mesoporous Al/SiO2 modified carbon paste working electrode, a saturated calomel reference electrode, and a platinum wire auxiliary electrode, was employed.
Fig. 1. Cyclic voltammograms of 1.0 10 4 mol L 1 catechol in 1.0 mol L at CPE (a) and mesoporous Al/SiO2 modified CPE (b). Scan rate = 100 mV s
1
HClO4 .
1
2.3. Synthesis of mesoporous Al/SiO2
3. Results and discussion
CV images of catechol standard at the unmodified CPE (curve a) and mesoporous Al/SiO2 modified CPE (curve b). At the unmodified CPE, an oxidation peak is observed at 0.64 V during the anodic sweep from 0.00 to 1.00 V. On the reverse scan, a corresponding reduction peak appears at 0.35 V. So, the peak potential separation (DEP = Epa Epc) is as large as 290 mV at the unmodified CPE, indicating that the electron transfer is slow. Under the identical conditions, the response of catechol shows great difference at the mesoporous Al/SiO2 modified CPE. Firstly, the oxidation peak potential shifts negatively to 0.56 V, while the reduction peak potential shifts positively to 0.51 V. Therefore, the DEp is 50 mV at the mesoporous Al/SiO2 modified CPE, suggesting that the electrode process of catechol becomes more reversible. Compared with that at the unmodified CPE, the DEp decreases by 240 mV at the mesoporous Al/SiO2 modified electrode, indicative of highly-efficient catalytic ability. Secondly, the oxidation and reduction peak currents significantly increases, revealing that the mesoporous Al/SiO2 modified CPE can greatly improve the determining sensitivity of catechol. The electrochemical response of low concentration of catechol was investigated using differential pulse voltammetry (DPV). Fig. 2 shows the DPV images of catechol at different electrodes in 1.0 mol L 1 HClO4. At the unmodified CPE (curve a), an oxidation peak is observed for 1.0 10 5 mol L 1 catechol. The peak potential is 0.58 V and the peak height is very low, suggesting that the activity of unmodified CPE is very poor. Fig. 2b depicts the DPV image of mesoporous Al/SiO2 modified CPE in 1.0 mol L 1 HClO4 without catechol. No oxidation peak is observed. Upon addition of 1.0 10 5 mol L 1 catechol, a highly-sensitive and well-shaped oxidation peak appears at 0.51 V (curve c). From the comparisons of curves (a) and (c), it is clearly found that the oxidation peak current of catechol significantly increases, and the peak potential shifts negatively from 0.58 to 0.51 V at the mesoporous Al/SiO2 modified CPE. Mesoporous Al/SiO2 possesses specific pore channels, large surface area, high adsorption ability and numerous active site, therefore, the mesoporous Al/SiO2 modified CPE is more active and exhibits higher accumulation efficiency to catechol. Without a doubt, the oxidation signal of catechol is remarkably improved at the mesoporous Al/SiO2 modified CPE.
3.1. Electrochemical behaviour of catechol
3.2. Effect of supporting electrolyte
The electrochemical behaviour of catechol was studied using cyclic voltammetry (CV) in 1.0 mol L 1 HClO4. Fig. 1 shows the
The electrochemical responses of catechol in different supporting electrolytes were examined. The supporting electrolytes
Mesoporous Al/SiO2 was synthesised as the published work (Galarneau et al., 2006) using CTAB as the template. A solution of CTAB in NaOH mixed with the alumina source (NaAlO2) was prepared and stirred at 298 K. After that, the silica source (TEOS) was added to this solution under stirring to give a gel mixture with the molar compositions: 1 SiO2/0.03 NaAlO2/0.25 NaOH/0.1 CTAB/100 H2O. After 30 min of stirring at 298 K, the mixture was sealed and heated at 343 K for 24 h under static conditions. The resulting solid precipitate was recovered by filtration, washed with deionised water and then dried at 80 °C overnight. Finally, the dried solid precipitate was calcined at 823 K for 8 h to remove CTAB and form mesopores. 2.4. Preparation of mesoporous Al/SiO2 modified CPE Mesoporous Al/SiO2 (100.0 mg) was homogeneously mixed with graphite powder (400.0 mg) and paraffin oil (150.0 lL) in a carnelian mortar, giving a uniform mesoporous Al/SiO2 modified carbon paste. After that, the resulting carbon paste was tightly pressed into the end cavity (3 mm in diameter) of working electrode. Finally, the electrode surface was polished on a smooth paper. 2.5. Sample preparation Various tea samples were purchased from local market and treated as follows (Figueiredo, Tarley, Kubota, Rath, & Arruda, 2007). The tea sample (about 0.20 g) was exactly weighed and the catechol was extracted with 60 mL of 20% (v/v) methanol solution for 20 min at 80 °C. The mixture was filtered and the volume made up to 100.0 mL for further measurement. 2.6. Analytical procedure Unless otherwise stated, 1.0 mol L 1 HClO4 was used as the supporting electrolyte for the determination of catechol. The differential pulse voltammograms were recorded, and the oxidation peak current was measured as an analytical signal of catechol.
H. Lin et al. / Food Chemistry 113 (2009) 701–704
Fig. 2. DPV images of 1.0 10 5 mol L 1 catechol at the unmodified CPE (a) and mesoporous Al/SiO2 modified CPE (c). (b) DPV image of mesoporous Al/SiO2 modified CPE without catechol. Pulse amplitude 40 mV, pulse width 20 ms, scan rate 40 mV s 1.
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Fig. 3. Cyclic voltammograms of 1.0 10 4 mol L 1 catechol and 2.0 10 4 mol L 1 hydroquinone in 1.0 mol L 1 HClO4 at unmodified CPE (a) and 1 mesoporous Al/SiO2 modified CPE (b). Scan rate = 100 mV s .
include pH 5.5, 6.0, 6.5, 7.0, 7.5, 8.0 phosphate buffer (0.1 mol L 1), pH 3.6, 4.0, 4.6, 5.0 HAc–NaAC buffer (0.1 mol L 1), 0.01 mol L 1 HClO4, 0.1 mol L 1 HClO4 and 1.0 mol L 1 HClO4. It was found that the oxidation peak current of catechol is highest in 1.0 mol L 1 HClO4. Therefore, 1.0 mol L 1 HClO4 was employed in the following studies. 3.3. Electrochemical response of catechol and hydroquinone Catechol and hydroquinone are phenol isomers with similar molecular structure. Thus, their oxidation peaks usually overlapped in many cases. Herein, the electrochemical response of catechol and hydroquinone were studied using CV, which shown in Fig. 3. At the unmodified CPE (curve a), a broad oxidation peak is observed during the anodic sweep from 0.00 to 1.00 V. On the reverse scan, just one reduction peak appears. Obviously, the electrochemical signals of catechol and hydroquinone interfere with each other at the unmodified CPE. However, two pairs of independent redox peaks are observed at the mesoporous Al/SiO2 modified CPE (curve b). The peak potential difference between hydroquinone and catechol is as large as 80 mV. Therefore, the mesoporous Al/SiO2 modified CPE exhibits better resolution ability to hydroquinone and catechol. Furthermore, the electrochemical response of catechol under different concentrations of hydroquinone was investigated, which shown in Fig. 4. In the absence of hydroquinone, a sensitive and well-defined oxidation peak is observed at 0.51 V for catechol (curve a). As gradually increasing the concentration of hydroquinone (curve b–f), another oxidation peak appears at 0.41 V. The peak current at 0.41 V increases linearly with the concentration of hydroquinone, while the peak current at 0.51 V almost keeps unchanged. The results suggest that the oxidation of hydroquinone and catechol at the mesoporous Al/SiO2 modified CPE takes place independently. Thus, the interference of hydroquinone is slight when using the mesoporous Al/SiO2 modified CPE. 3.4. Amount of mesoporous Al/SiO2 Although mesoporous Al/SiO2 possesses excellent properties and considerably enhances the oxidation signal of catechol, its electric conductivity is poor. Therefore, large amount of mesopor-
Fig. 4. DPV images of 2.0 10 5 mol L 1 catechol at mesoporous Al/SiO2 modified CPE without (a) and with 2.0 10 5 (b), 4.0 10 5 (c), 6.0 10 5 (d), 8.0 10 5 (e) and 1.0 10 4 mol L 1 (f) hydroquinone. Other conditions as in Fig. 2.
ous Al/SiO2 certainly increases the background current and lowers the conductivity of modified CPE. Fig. 5 demonstrates the oxidation peak current of catechol under different amount of mesoporous Al/SiO2. When the content of mesoporous Al/SiO2 gradually increases from 0% to 20%, the oxidation peak current of catechol remarkably enhances. As the amount of mesoporous Al/SiO2 is improved, the accumulation efficiency to catechol is higher. Thus, the oxidation peak current of catechol increased. Upon further increasing the amount from 20% to 30%, the oxidation peak current conversely decreases and the charging current obviously increases, prohibiting determination of trace levels of catechol. Therefore, the content of mesoporous Al/SiO2 is controlled at 20%. 3.5. Reproducibility The reproducibility between multiple mesoporous Al/SiO2 modified CPEs was estimated by determining the response of 2.0 10 6 mol L 1 catechol at 10 different mesoporous Al/SiO2 modified CPEs. The RSD is calculated to be 4.8%, revealing that this method for the determination of catechol has excellent reproducibility.
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H. Lin et al. / Food Chemistry 113 (2009) 701–704
determined in triplication, and the RSD is below 5.0%. The sample was also analysed by HPLC (Li et al., 2001) to confirm the catechol content. The results obtained by two methods were in good agreement. In addition, the recovery for catechol standard was found to be in the range from 94.7% to 103.0%. Therefore, this new method is reliable and effective for the determination of catechol. 4. Conclusion Due to the regular mesopores, numerous active sites, large surface area and strong adsorption ability, the mesoporous Al/SiO2 modified electrode exhibits catalytic activity to catechol and significantly improves the oxidation signal. Based on this, a reliable, sensitive, rapid and simple electrochemical method was proposed for catechol. Acknowledgement Fig. 5. Effect of amount of mesoporous Al/SiO2 on the oxidation peak current of 1.0 10 5 mol L 1 catechol.
This work was supported by the Chenguang Project of Wuhan (200850731352). References
Table 1 Determination of catechol in tea samples Sample
By HPLC (g g
A B C D E
0.216 0.251 0.186 0.174 0.184
1
)
By this method (g g
1
)
0.227 0.237 0.204 0.192 0.168
RSD (%)
Recovery (%)
4.2 3.7 4.1 4.0 4.4
94.7 97.7 103.0 95.2 96.8
3.6. Linear range and limit of detection The linear range and limit of detection were examined using DPV under the optimised conditions. The linear range is over the range from 5.0 10 7 to 5.0 10 5 mol L 1 with a correlation coefficient of 0.998. Owing to the larger active surface area and higher accumulation capability, the limit of detection at the mesoporous Al/SiO2 modified CPE is as low as 1.0 10 7 mol L 1 based on a three signal-to-noise ratio, which lower than that of the published electrochemical methods. 3.7. Interference The interferences of other species on the determination of catechol were examined. It was found that 2.0 10 3 mol L 1 Cd2+, Cu2+, Pb2+, Fe3+, Mn2+, Ni2+ and Zn2+; 2.0 10 4 mol L 1 4-nitrophenol, 2-chlorophenol, phenol, xanthine, hypoxanthine, uric acid, ascorbic acid and resorcinol; 4.0 10 5mol L 1 hydroquinone, virtually did not interfere with the oxidation signal of 2.0 10 6 mol L 1 catechol at the mesoporous Al/SiO2 modified CPE (peak current change <5%). 3.8. Determination of catechol in tea samples This method was used to determine catechol in several tea samples. The content of catechol was determined by the standard addition method, and the results are listed in Table 1. Each sample was
Beck, J. S., Vartuli, J. C., Roth, W. J., Leonowicz, M. E., Kresge, C. T., Schmitt, K. D., et al. (1992). A new family of mesoporous molecular sieves prepared with liquid crystal templates. Journal of the American Chemical Society, 114, 10834–10843. Figueiredo, E. C., Tarley, C. R. T., Kubota, L. T., Rath, S., & Arruda, M. A. Z. (2007). Online molecularly imprinted solid phase extraction for the selective spectrophotometric determination of catechol. Microchemical Journal, 85, 290–296. Galarneau, A., Cangiotti, M., Renzo, F., Fajula, F., & Ottaviani, M. F. (2006). Synthesis of micelle templated silico-aluminas with different alumina contents. The Journal of Physical Chemistry B, 110, 4058–4065. Ghanem, M. A. (2007). Electrocatalytic activity and simultaneous determination of catechol and hydroquinone at mesoporous platinum electrode. Electrochemistry Communications, 9, 2501–2506. Kim, J., Lee, J. E., Lee, J., Yu, J. H., Kim, B. C., An, K., et al. (2006). Magnetic fluorescent delivery vehicle using uniform mesoporous silica spheres embedded with monodisperse magnetic and semiconductor nanocrystals. Journal of the American Chemical Society, 128, 688–689. Landskron, K., & Ozin, G. A. (2004). Periodic mesoporous dendrisilicas. Science, 306, 1529–1532. Li, H., Bian, Z., Zhu, J., Huo, Y., Li, H., & Lu, Y. (2007). Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity. Journal of the American Chemical Society, 129, 4538–4539. Li, Z., Wang, L., Yang, G. Y., Liu, W., Shi, G. L., & Jiang, C. Q. (2001). Determination of phenol and catechol in tobacco leaf by solid phase extraction and high performance liquid chromatography. Chinese Journal of Analytical Chemistry, 29, 1409–1411. Lunsford, S. K., Choi, H., Stinson, J., Yeary, A., & Dionysiou, D. D. (2007). Voltammetric determination of catechol using a sonogel carbon electrode modified with nanostructured titanium dioxide. Talanta, 73, 172–177. Qi, H. L., & Zhang, C. X. (2005). Simultaneous determination of hydroquinone and catechol at a glassy carbon electrode modified with multiwall carbon nanotubes. Electroanalysis, 17, 832–838. Su, L., & Mao, L. Q. (2006). Gold nanoparticle/alkanedithiol conductive films selfassembled onto gold electrode: electrochemistry and electroanalytical application for voltammetric determination of trace amount of catechol. Talanta, 70, 68–74. Sun, Y. G., Cui, H., Li, Y. H., & Lin, Q. X. (2000). Determination of some catechol derivatives by a flow injection electrochemiluminescent inhibition method. Talanta, 53, 661–666. Tolbert, S. H., Firouzi, A., Stucky, G. D., & Chmelka, B. F. (1997). Magnetic field alignment of ordered silicate-surfactant composites and mesoporous silica. Science, 278, 264–268. Wang, L., Huang, P. F., Bai, J. Y., Wang, H. J., Zhang, L. Y., & Zhao, Y. Q. (2007). Covalent modification of a glassy carbon electrode with penicillamine for simultaneous determination of hydroquinone and catechol. Microchimica Acta, 158, 151–157.
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
Sensitive and rapid determination of catechol in tea samples using mesoporous Al-doped silica modified electrode Huogang Lin, Tian Gan, Kangbing Wu * Department of Chemistry, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
a r t i c l e
i n f o
Article history: Received 1 April 2008 Received in revised form 6 July 2008 Accepted 19 July 2008
Keywords: Catechol Porous silica Electrochemistry Determination Tea
a b s t r a c t A mesoporous Al-doped silica (Al/SiO2) was synthesised according to the published method, and then used to modify the carbon paste electrode (CPE). The electrochemical behaviour of catechol was investigated. Compared with the unmodified CPE, the resulting mesoporous Al/SiO2 modified CPE remarkably increases the peak currents of catechol, and greatly lowers the peak potential separation. Therefore, the mesoporous Al/SiO2 exhibits catalytic activity to catechol and significantly improves the determining sensitivity. Based on this, a sensitive, rapid and convenient electrochemical method was proposed for the determination of catechol. The linear range is between 5.0 10 7 and 5.0 10 5 mol L 1 with a correlation coefficient of 0.998. The limit of detection is as low as 1.0 10 7 mol L 1. Finally, this novel method was employed to determine catechol in tea samples, which testified by high-performance liquid chromatography. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Catechol (o-benzenediol, 1,2-benzenediol or 1,2-dihydroxybenzene) is a natural polyphenolic compound that widely exists in higher plants such as teas, vegetables, fruits, tobaccos and some traditional Chinese medicines (Sun, Cui, Li, & Lin, 2000). Catechol has been widely studied due to the biological importance such as antioxidation, antivirus and affecting the activities of some enzymes. Therefore, it is important to establish a sensitive, rapid and convenient method for the determination of catechol. Catechol contains phenolic hydroxy group and possesses excellent electrochemical activity, so various electrochemical methods using different modified electrodes have been reported for the determination of catechol. For example, a mesoporous platinum electrode with limit of detection (LOD) of 10 6 mol L 1 (Ghanem, 2007), a penicillamine modified electrode with LOD of 7.5 10 7 mol L 1 (Wang et al., 2007), a multi wall carbon nanotubes-modified electrode with LOD of 2.5 10 7 mol L 1 (Qi & Zhang, 2005), a nano Au/alkanedithiol self-assembled gold electrode with LOD of 10 6 mol L 1 (Su & Mao, 2006), and a nano-TiO2 modified electrode (Lunsford, Choi, Stinson, Yeary, & Dionysiou, 2007), were employed. However, electrochemical determination of catechol using mesoporous Al-doped silica has not been reported.
* Corresponding author. Fax: +86 27 8754 3632. E-mail address: [email protected] (K. Wu). 0308-8146/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2008.07.073
Mesoporous material is a kind of porous materials with regular and tunable pore from 2 to 50 nm. Since its discovery (Beck et al., 1992), mesoporous material has attracted increasing attention and widely used in different fields such as catalysis, energy storage, energy conversion, adsorption and separation (Kim et al., 2006; Landskron & Ozin, 2004; Li et al., 2007; Tolbert, Firouzi, Stucky, & Chmelka, 1997). The aim of this work is to develop a sensitive and rapid electrochemical method for the determination of catechol utilising the excellent properties of mesoporous material. To accomplish this, a mesoporous Al-doped silica (denoted as Al/ SiO2) was synthesised according to the published work (Galarneau, Cangiotti, Renzo, Fajula, & Ottaviani, 2006), and then mixed homogenously with graphite powder and paraffin oil to give a mesoporous Al/SiO2 modified carbon paste electrode (CPE). The electrochemical behavior of catechol was examined at the unmodified and mesoporous Al/SiO2 modified CPEs. Owing to the specific and regular mesopores, large surface area, high adsorption ability and numerous active sites, the mesoporous Al/SiO2 modified CPE remarkably enhances the oxidation signal of catechol, and obviously lowers the oxidation overpotential. Therefore, the mesoporous Al/SiO2 shows efficient catalytic activity toward catechol and significantly improves the determining sensitivity. In addition, the electrochemical behaviour of hydroquinone was also studied. Hydroquinone yields an oxidation peak before catechol. The oxidation peaks of hydroquinone and catechol are very separated and do not interfere with each other at the modified CPE. Compared with the published electrochemical methods, determining catechol using mesoporous Al/SiO2 possesses higher sensitivity, rapider response and better simplicity.
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H. Lin et al. / Food Chemistry 113 (2009) 701–704
2. Experimental 2.1. Reagent All the chemicals were of analytical grade and used without further purification. Catechol, hydroquinone (p-benzenediol) and resorcinol (m-benzenediol) were obtained from Aldrich. Cetyltrimethyl ammonium bromide ammonia (CTAB), tetraethyl orthosilicate (TEOS), sodium aluminate (NaAlO2), graphite powder (spectral reagent) and paraffin oil were purchased from the Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Instruments All the electrochemical measurements were performed using an 830B electrochemical analyser (CH Instruments, USA). A conventional three-electrode system, consisting of a mesoporous Al/SiO2 modified carbon paste working electrode, a saturated calomel reference electrode, and a platinum wire auxiliary electrode, was employed.
Fig. 1. Cyclic voltammograms of 1.0 10 4 mol L 1 catechol in 1.0 mol L at CPE (a) and mesoporous Al/SiO2 modified CPE (b). Scan rate = 100 mV s
1
HClO4 .
1
2.3. Synthesis of mesoporous Al/SiO2
3. Results and discussion
CV images of catechol standard at the unmodified CPE (curve a) and mesoporous Al/SiO2 modified CPE (curve b). At the unmodified CPE, an oxidation peak is observed at 0.64 V during the anodic sweep from 0.00 to 1.00 V. On the reverse scan, a corresponding reduction peak appears at 0.35 V. So, the peak potential separation (DEP = Epa Epc) is as large as 290 mV at the unmodified CPE, indicating that the electron transfer is slow. Under the identical conditions, the response of catechol shows great difference at the mesoporous Al/SiO2 modified CPE. Firstly, the oxidation peak potential shifts negatively to 0.56 V, while the reduction peak potential shifts positively to 0.51 V. Therefore, the DEp is 50 mV at the mesoporous Al/SiO2 modified CPE, suggesting that the electrode process of catechol becomes more reversible. Compared with that at the unmodified CPE, the DEp decreases by 240 mV at the mesoporous Al/SiO2 modified electrode, indicative of highly-efficient catalytic ability. Secondly, the oxidation and reduction peak currents significantly increases, revealing that the mesoporous Al/SiO2 modified CPE can greatly improve the determining sensitivity of catechol. The electrochemical response of low concentration of catechol was investigated using differential pulse voltammetry (DPV). Fig. 2 shows the DPV images of catechol at different electrodes in 1.0 mol L 1 HClO4. At the unmodified CPE (curve a), an oxidation peak is observed for 1.0 10 5 mol L 1 catechol. The peak potential is 0.58 V and the peak height is very low, suggesting that the activity of unmodified CPE is very poor. Fig. 2b depicts the DPV image of mesoporous Al/SiO2 modified CPE in 1.0 mol L 1 HClO4 without catechol. No oxidation peak is observed. Upon addition of 1.0 10 5 mol L 1 catechol, a highly-sensitive and well-shaped oxidation peak appears at 0.51 V (curve c). From the comparisons of curves (a) and (c), it is clearly found that the oxidation peak current of catechol significantly increases, and the peak potential shifts negatively from 0.58 to 0.51 V at the mesoporous Al/SiO2 modified CPE. Mesoporous Al/SiO2 possesses specific pore channels, large surface area, high adsorption ability and numerous active site, therefore, the mesoporous Al/SiO2 modified CPE is more active and exhibits higher accumulation efficiency to catechol. Without a doubt, the oxidation signal of catechol is remarkably improved at the mesoporous Al/SiO2 modified CPE.
3.1. Electrochemical behaviour of catechol
3.2. Effect of supporting electrolyte
The electrochemical behaviour of catechol was studied using cyclic voltammetry (CV) in 1.0 mol L 1 HClO4. Fig. 1 shows the
The electrochemical responses of catechol in different supporting electrolytes were examined. The supporting electrolytes
Mesoporous Al/SiO2 was synthesised as the published work (Galarneau et al., 2006) using CTAB as the template. A solution of CTAB in NaOH mixed with the alumina source (NaAlO2) was prepared and stirred at 298 K. After that, the silica source (TEOS) was added to this solution under stirring to give a gel mixture with the molar compositions: 1 SiO2/0.03 NaAlO2/0.25 NaOH/0.1 CTAB/100 H2O. After 30 min of stirring at 298 K, the mixture was sealed and heated at 343 K for 24 h under static conditions. The resulting solid precipitate was recovered by filtration, washed with deionised water and then dried at 80 °C overnight. Finally, the dried solid precipitate was calcined at 823 K for 8 h to remove CTAB and form mesopores. 2.4. Preparation of mesoporous Al/SiO2 modified CPE Mesoporous Al/SiO2 (100.0 mg) was homogeneously mixed with graphite powder (400.0 mg) and paraffin oil (150.0 lL) in a carnelian mortar, giving a uniform mesoporous Al/SiO2 modified carbon paste. After that, the resulting carbon paste was tightly pressed into the end cavity (3 mm in diameter) of working electrode. Finally, the electrode surface was polished on a smooth paper. 2.5. Sample preparation Various tea samples were purchased from local market and treated as follows (Figueiredo, Tarley, Kubota, Rath, & Arruda, 2007). The tea sample (about 0.20 g) was exactly weighed and the catechol was extracted with 60 mL of 20% (v/v) methanol solution for 20 min at 80 °C. The mixture was filtered and the volume made up to 100.0 mL for further measurement. 2.6. Analytical procedure Unless otherwise stated, 1.0 mol L 1 HClO4 was used as the supporting electrolyte for the determination of catechol. The differential pulse voltammograms were recorded, and the oxidation peak current was measured as an analytical signal of catechol.
H. Lin et al. / Food Chemistry 113 (2009) 701–704
Fig. 2. DPV images of 1.0 10 5 mol L 1 catechol at the unmodified CPE (a) and mesoporous Al/SiO2 modified CPE (c). (b) DPV image of mesoporous Al/SiO2 modified CPE without catechol. Pulse amplitude 40 mV, pulse width 20 ms, scan rate 40 mV s 1.
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Fig. 3. Cyclic voltammograms of 1.0 10 4 mol L 1 catechol and 2.0 10 4 mol L 1 hydroquinone in 1.0 mol L 1 HClO4 at unmodified CPE (a) and 1 mesoporous Al/SiO2 modified CPE (b). Scan rate = 100 mV s .
include pH 5.5, 6.0, 6.5, 7.0, 7.5, 8.0 phosphate buffer (0.1 mol L 1), pH 3.6, 4.0, 4.6, 5.0 HAc–NaAC buffer (0.1 mol L 1), 0.01 mol L 1 HClO4, 0.1 mol L 1 HClO4 and 1.0 mol L 1 HClO4. It was found that the oxidation peak current of catechol is highest in 1.0 mol L 1 HClO4. Therefore, 1.0 mol L 1 HClO4 was employed in the following studies. 3.3. Electrochemical response of catechol and hydroquinone Catechol and hydroquinone are phenol isomers with similar molecular structure. Thus, their oxidation peaks usually overlapped in many cases. Herein, the electrochemical response of catechol and hydroquinone were studied using CV, which shown in Fig. 3. At the unmodified CPE (curve a), a broad oxidation peak is observed during the anodic sweep from 0.00 to 1.00 V. On the reverse scan, just one reduction peak appears. Obviously, the electrochemical signals of catechol and hydroquinone interfere with each other at the unmodified CPE. However, two pairs of independent redox peaks are observed at the mesoporous Al/SiO2 modified CPE (curve b). The peak potential difference between hydroquinone and catechol is as large as 80 mV. Therefore, the mesoporous Al/SiO2 modified CPE exhibits better resolution ability to hydroquinone and catechol. Furthermore, the electrochemical response of catechol under different concentrations of hydroquinone was investigated, which shown in Fig. 4. In the absence of hydroquinone, a sensitive and well-defined oxidation peak is observed at 0.51 V for catechol (curve a). As gradually increasing the concentration of hydroquinone (curve b–f), another oxidation peak appears at 0.41 V. The peak current at 0.41 V increases linearly with the concentration of hydroquinone, while the peak current at 0.51 V almost keeps unchanged. The results suggest that the oxidation of hydroquinone and catechol at the mesoporous Al/SiO2 modified CPE takes place independently. Thus, the interference of hydroquinone is slight when using the mesoporous Al/SiO2 modified CPE. 3.4. Amount of mesoporous Al/SiO2 Although mesoporous Al/SiO2 possesses excellent properties and considerably enhances the oxidation signal of catechol, its electric conductivity is poor. Therefore, large amount of mesopor-
Fig. 4. DPV images of 2.0 10 5 mol L 1 catechol at mesoporous Al/SiO2 modified CPE without (a) and with 2.0 10 5 (b), 4.0 10 5 (c), 6.0 10 5 (d), 8.0 10 5 (e) and 1.0 10 4 mol L 1 (f) hydroquinone. Other conditions as in Fig. 2.
ous Al/SiO2 certainly increases the background current and lowers the conductivity of modified CPE. Fig. 5 demonstrates the oxidation peak current of catechol under different amount of mesoporous Al/SiO2. When the content of mesoporous Al/SiO2 gradually increases from 0% to 20%, the oxidation peak current of catechol remarkably enhances. As the amount of mesoporous Al/SiO2 is improved, the accumulation efficiency to catechol is higher. Thus, the oxidation peak current of catechol increased. Upon further increasing the amount from 20% to 30%, the oxidation peak current conversely decreases and the charging current obviously increases, prohibiting determination of trace levels of catechol. Therefore, the content of mesoporous Al/SiO2 is controlled at 20%. 3.5. Reproducibility The reproducibility between multiple mesoporous Al/SiO2 modified CPEs was estimated by determining the response of 2.0 10 6 mol L 1 catechol at 10 different mesoporous Al/SiO2 modified CPEs. The RSD is calculated to be 4.8%, revealing that this method for the determination of catechol has excellent reproducibility.
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determined in triplication, and the RSD is below 5.0%. The sample was also analysed by HPLC (Li et al., 2001) to confirm the catechol content. The results obtained by two methods were in good agreement. In addition, the recovery for catechol standard was found to be in the range from 94.7% to 103.0%. Therefore, this new method is reliable and effective for the determination of catechol. 4. Conclusion Due to the regular mesopores, numerous active sites, large surface area and strong adsorption ability, the mesoporous Al/SiO2 modified electrode exhibits catalytic activity to catechol and significantly improves the oxidation signal. Based on this, a reliable, sensitive, rapid and simple electrochemical method was proposed for catechol. Acknowledgement Fig. 5. Effect of amount of mesoporous Al/SiO2 on the oxidation peak current of 1.0 10 5 mol L 1 catechol.
This work was supported by the Chenguang Project of Wuhan (200850731352). References
Table 1 Determination of catechol in tea samples Sample
By HPLC (g g
A B C D E
0.216 0.251 0.186 0.174 0.184
1
)
By this method (g g
1
)
0.227 0.237 0.204 0.192 0.168
RSD (%)
Recovery (%)
4.2 3.7 4.1 4.0 4.4
94.7 97.7 103.0 95.2 96.8
3.6. Linear range and limit of detection The linear range and limit of detection were examined using DPV under the optimised conditions. The linear range is over the range from 5.0 10 7 to 5.0 10 5 mol L 1 with a correlation coefficient of 0.998. Owing to the larger active surface area and higher accumulation capability, the limit of detection at the mesoporous Al/SiO2 modified CPE is as low as 1.0 10 7 mol L 1 based on a three signal-to-noise ratio, which lower than that of the published electrochemical methods. 3.7. Interference The interferences of other species on the determination of catechol were examined. It was found that 2.0 10 3 mol L 1 Cd2+, Cu2+, Pb2+, Fe3+, Mn2+, Ni2+ and Zn2+; 2.0 10 4 mol L 1 4-nitrophenol, 2-chlorophenol, phenol, xanthine, hypoxanthine, uric acid, ascorbic acid and resorcinol; 4.0 10 5mol L 1 hydroquinone, virtually did not interfere with the oxidation signal of 2.0 10 6 mol L 1 catechol at the mesoporous Al/SiO2 modified CPE (peak current change <5%). 3.8. Determination of catechol in tea samples This method was used to determine catechol in several tea samples. The content of catechol was determined by the standard addition method, and the results are listed in Table 1. Each sample was
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