Method for Preparation of a Sol− Gel-Derived Carbon Ceramic

Anal. Chem. 2009, 81, 3660–3664

Method for Preparation of a Sol-Gel-Derived Carbon Ceramic Electrode Using Microwave Irradiation Abdolkarim Abbaspour* and Ali Ghaffarinejad Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71456-85464, Iran A fast and new method for preparation of a sol-gel carbon ceramic electrode (CCE) by microwave (MW) irradiation is introduced. In comparison to previous preparation methods which require a very long time (mostly 48 h for ceramic completion and drying in air), this method requires only a few minutes. Furthermore, before MW irradiation an ultrasonic wave was applied to influence the gelation time and dispersion of particles in the sol-gel. The composition of the proposed carbon ceramic was characterized by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy, which was very similar to the air-dried sol-gel carbon ceramics. The proposed electrode was used for determination of dopamine (DA); the results show that this method has a wider linear range (LR) and lower detection limit (DL) than the air-dried CCE and exhibits a greater sensitivity for determination of DA compared to a recently reported CCE. Interest in the sol-gel processing of inorganic ceramic and glass materials began as early as the mid-1800s with Ebelman and Graham’s studies on silica gels.1 The sol-gel process is a flexible method for preparing transparent optical materials at ambient processing conditions and also enables entrapment of numerous inorganic,2 organic, organometallic, and biological molecules (proteins and enzymes) within the microporous network of the sol-gel-derived matrix.3 Sol-gel technology involves the fabrication of material through the low-temperature hydrolysis of a suitable monomeric precursor, followed by condensation and polycondensation to yield a polymeric oxo-bridged SiO2 network. In 1994, Tsionsky et al.4 introduced a carbon ceramic electrode (CCE) on the basis of the sol-gel process, which consists of a dispersion of carbon (graphite) powder into the sol-gel solution. This new kind of electrode has been utilized to a great extent for electrochemical sensors, whose electrode surfaces could be renewed by a simple polishing step.5-9 Beside graphite particles, * To whom correspondence should be addressed. Phone: +98-711-2284822, +98-711-6137357. Fax: +98-711-2286008. E-mail: [email protected]. (1) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33–72. (2) Wang, J.; Pamidi, P.V. A. Anal. Chem. 1997, 69, 4490–4494. (3) Gupta, R.; Kumar, A. Biotechnol. Adv. 2008, 26, 533–547. (4) Tsionsky, M.; Gun, G.; Giezer, V.; Lev, O. Anal. Chem. 1994, 66, 1747– 1753. (5) Razmi, H.; Habibi, E.; Heidari, H. Electrochim. Acta 2008, 53, 8178–8185. (6) Razmi, H.; Heidari, H. Electroanalysis 2008, 20, 2370–2378. (7) Arguello, J.; Magosso, H. A.; Landers, R.; Gushikem, Y. J. Electroanal. Chem. 2008, 617, 45–52.

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gold,2 glassy carbon,10 and multiwalled carbon nanotube11,12 particles also have been used as conducting materials for ceramic composite electrodes. Heating and driving chemical reactions by microwave (MW) irradiation has been an increasingly popular method in the scientific community for synthesis of various polymers,13 organic14 and inorganic15 materials, and in biotechnologies16 and nanotechnology.17 In comparison to the conventional heating process (a) MW has efficient energy, enabling increased production speeds and decreased production costs; (b) MW energy is precisely controllable and can be turned on and off instantly, eliminating the need for warm-up and cool-down.17 Because of these benefits, in recent years MW irradiation also has gained increasing attention in ceramic18 and carbon nanotube composite production19 and in the processes based on gels such as carbon gels and xerogels,20-22 sol-gel mesoporous particles,23 and sol-gel films.24 Despite the advantages of CCE such as ease of preparation and surface renewability, its preparation is very time-consuming (mostly 48 h8,25). For this reason in this study we applied MW irradiation for solving this major problem in carbon ceramic (8) Shul, G.; Nogala, W.; Zakorchemna, I.; Niedziolka, J.; Opallo, M. J. Solid State Electrochem. 2008, 12, 1077–1084. (9) Abbaspour, A.; Shamsipur, M.; Siroueinejad, A.; Kia, R.; Raithby, P. R. Electrochim. Acta, in press DOI: 10.1016/j.electacta.2008.11.005. (10) Sun, D.; Zhu, L.; Zhu, G. Anal. Chim. Acta 2006, 564, 243–247. (11) Gavalas, V. G.; Andrews, R.; Bhattacharyya, D.; Bachas, L. G. Nano Lett. 2001, 1, 719–721. (12) Zhu, L.; Tian, C.; Zhai, J.; Yang, R. Sens. Actuators, B 2007, 125, 254–261. (13) Hoogenboom, R.; Schubert, U. S. Macromol. Rapid Commun. 2007, 28, 368–386. (14) Bruckmann, A.; Krebs, A.; Bolm, C. Green Chem. 2008, 10, 1131–1141. (15) Wang, W.; Zhu, Y. Curr. Nanosci. 2007, 3, 171–176. (16) Sandoval, W. N.; Pham, V. C.; Lill, J. R. Drug Discovery Today 2008, 13, 1075–1081. (17) Krishnakumar, T.; Jayaprakash, R.; Pinna, N.; Singh, V. N.; Mehta, B. R.; Phani, A. R. Mater. Lett. 2009, 63, 242–245. (18) Das, B. P.; Panneerselvam, M.; Rao, K. J. J. Solid State Chem. 2003, 173, 196–202. (19) Xu, H.; Zeng, L.; Xing, S.; Shi, G.; Xian, Y.; Jin, L. Electrochem. Commun. 2008, 10, 1839–1843. (20) Zubizarreta, L.; Arenillas, A.; Mene´ndez, J. A.; Pis, J. J.; Pirard, J. P.; Job, N. J. Non-Cryst. Solids 2008, 354, 4024–4026. (21) Zubizarreta, L.; Arenillas, A.; Domı´nguez, A.; Mene´ndez, J. A.; Pis, J. J. J. Non-Cryst. Solids 2008, 354, 817–825. (22) Tonanon, N.; Wareenin, Y.; Siyasukh, A.; Tanthapanichakoon, W.; Nishihara, H.; Mukai, S. R.; Tamon, H. J. Non-Cryst. Solids 2006, 352, 5683–5686. (23) Inada, M.; Nishinosono, A.; Kamada, K.; Enomoto, N.; Hojo, J. J. Mater. Sci. 2008, 43, 2362–2366. (24) Wang, J.; Binner, J.; Pang, Y.; Vaidhyanathan, B. Thin Solid Films 2008, 516, 5996–6001. (25) Sheng, Q. L.; Yu, H.; Zheng, J. B. Electrochim. Acta 2007, 52, 4506–4512. 10.1021/ac802690s CCC: $40.75  2009 American Chemical Society Published on Web 03/31/2009

Table 1. Cyclic Voltammetric Parameters for MWCCEs Prepared in Various Powersa 150 W

300 W

500 W

700 W

900 W

parameter

av

RSD (%)

av

RSD (%)

av

RSD (%)

av

RSD (%)

av

RSD (%)

Epa Epc Ipa Ipc ∆Ep Ipa/Ipc

382.68 292.42 37.96 39.73 90.25 0.96

0.33 0.43 24.32 24.12 2.75 1.21

382.84 292.59 37.09 38.48 91.08 0.96

0.26 0.61 2.35 3.10 3.19 1.31

384.02 289.68 41.78 42.74 94.35 0.98

0.85 1.55 32.79 33.08 8.18 1.08

385.98 291.43 31.18 31.98 94.55 0.97

0.37 0.80 5.02 4.59 3.97 0.66

381.65 294.90 32.77 33.04 86.75 0.99

0.13 0.10 7.69 8.11 0.92 0.50

a All data were obtained in solution containing 5 mM K3Fe(CN)6 in 0.1 M H3PO4 at a scan rate of 50 mV/s (average of 12 measurements (n ) 12)).

Figure 1. FT-IR spectra of carbon ceramic dried in air for 48 h (A) and carbon ceramic prepared by MW irradiation (B).

preparation. Use of this method causes to decrease the ceramic formation time to a few minutes. In our knowledge this is the first report for preparation of carbon ceramics by MW irradiation. Furthermore, in order to affect mainly the gelation time26 and also the dispersion of particles in the sol-gel27 we applied ultrasonic before MW irradiation. The X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and electrochemical properties of the carbon ceramic prepared with the proposed method were comparable with those of the carbon ceramic prepared with the conventional method (48 h). EXPERIMENTAL SECTION Apparatus. Voltammetric studies were accomplished using a Metrohm electroanalyzer model of 757 VA Computrace. The software of this device was 757 VA Computrace version 2.0. A 780 Metrohm pH meter was used for pH measurements. The three-electrode system consists of the bare CCE as working (26) Ocotla´n-Flores, J.; Saniger, J. M. J. Sol.-Gel Sci. Technol. 2006, 39, 235– 240. (27) Kielbasa, J. E.; Liu, J.; Ucer, K. B.; Carroll, D. L.; Williams, R. T. J. Mater. Sci.: Mater. Electron. 2007, 18, S435–S438.

Figure 2. XRD spectra of (A) carbon ceramic prepared by MW irradiation and (B) conventional air-dried carbon ceramic.

electrode, Ag|AgCl|3 M KCl as a reference electrode, and a Pt wire as a counter electrode. The body of the working electrode was a Teflon cylinder (2.0 mm i.d.) which was tightly packed with carbon ceramic. A stainless steel rod was inserted into the Teflon tube containing carbon ceramic to establish the electrical contact. For MW irradiation a domestic Moulinex oven type Y87 was used. FT-IR spectra were recorded with a JASCO FT-IR 680 plus spectrometer. XRD studies were performed at room temperature (25 °C) by an X-ray diffractometer model of Bruker D8 advance, with a Cu KR radiation source (λ ) 1.5418 Å) generated at 40 kV and 25 mA. The step time was 0.050°/s and 5° e 2θ e 70°. All voltammetric experiments were performed at room temperature (25 ± 0.5 °C). Materials and Solutions. All materials and reagents were analytical grade and were used without further purification. Graphite fine powder (extrapure, particle size e50 µm) was purchased from Merck. Methyltrimethoxysilane (MTMOS) and dopamine (DA) hydrochloride were obtained from Fluka and Sigma, respectively. Dopamine hydrochloride and hexacyanoferrate solutions were prepared fresh daily. All solutions were prepared with deionized water. Analytical Chemistry, Vol. 81, No. 9, May 1, 2009

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ceramic particles in the packing step. Before applying the MWCCE for the electrochemical studies the surface of the electrode was polished and smoothed easily on a weighing paper and then was washed with deionized water. Finally, conventional carbon ceramic with the same composition was prepared, and instead of MW irradiation the mixture was dried at room temperature for 48 h. RESULTS AND DISCUSSION

Figure 3. Cyclic voltammogram of 0.10 mM DA in ABS5 with a scan rate of 25 mV/s.

Preparation of Electrodes by MW Irradiation (MWCCE) and by the Air Drying (CCE) Method. The procedure for sol-gel preparation was as our previous study,9 but with a minor change. In summery, 50 µL of methanol, 33 µL of MTMOS, and 10 µL of HCl (4 M) catalyst were injected in a test tube, respectively. The test tube spout was closed with a flexible film (Parafilm), and the mixture was sonicated for 3 min until a clear and homogeneous solution was obtained (sol-gel). Then 125 mg of graphite powder was ultrasonically mixed with the sol-gel for 3 min. The resultant was hand-mixed with a spatula and then sonicated again for 9 min. As mentioned before in the preparation of the MWCCE an alternative experimental setup involves an ultrasonic bath. The first outstanding effect of ultrasound on the gelation process is the drastic decrease in the gelation time in comparison with the classic techniques (it is well-known that the sonication of liquid mixtures results in the formation of active radical species and recombination of these radicals), and at constant temperature, the time for gelation decreases with increasing ultrasonic dose.26,28 The second effect is the improvement of dispersion of graphite powder in the sol-gel. The homogenized graphite (by sonication) in the sol-gel was irradiated with MW for a few minutes to produce carbon ceramic. Finally, the produced carbon ceramic was well ground in an IR mortar. The carbon ceramic powder was pressed into the Teflon tube as a working electrode, and a stainless steel rod in the tube for electrical connection and also as a piston for removing a thin layer of carbon ceramic for the electrode surface renewing process was inserted into the ceramic. The ceramic grinding in the IR mortar creates smaller and more homogenized ceramic particles, which improves reproducibility and better connection between (28) Blanco, E.; Esquivias, L.; Litra´n, R.; Pine˜ro, M.; Ramı´rez-del-Solar, M.; de la Rosa-Fox, N. Appl. Organomet. Chem. 1999, 13, 399–418.

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Optimization of Conditions for Carbon Ceramic Preparation with MW Irradiation: Effect of Irradiation Time and Mixture Vessel Shape. The initial ceramic mixture’s vessel and the irradiation time have important effects on the carbon ceramic formation by MW. The shape of the vessel for insertion of the carbon and sol-gel mixture in the MW oven has an interesting effect. For example, if the mixture is put in a flat dish, it will be burned immediately at the initial seconds. This may be due to the presence of two residual flammable materials, i.e., methanol and MTMOS. It is notable that talking about burning does not mean a huge flame can be obtained, because in this process only a little amount of flammable material exists. However, when a flammable material is applied it is recommended that in applying MW for preparing ceramics one should be careful. For this reason test tubes are suitable as the vessel, noting that the length and diameter of the test tubes also have an effective role in the required time for the MW irradiation. In addition, when a short test tube was used, the mixture was sparked in the first seconds, which may be a sign of slight burning of the mixture. On the other hand, when a very much larger or longer test tube was used, the mixture needed longer times for the drying process. Accordingly, after examining some available test tubes it was concluded that a Pyrex test tube with 14 mm × 125 mm dimensions was suitable and was used in the rest of the experiments. In addition the method of mixing the sol-gel with carbon has also a notable effect on the prevention of slight burning in the MW oven, i.e., if the mixture of the sol-gel and carbon powder becomes more homogenized, the burning probability will be less. For this reason and improving the dispersion of carbon powder applying the ultrasonic wave was useful. In order to examine the effect of MW irradiation time, four irradiation times (5, 10, 20, and 60 min) were chosen. The results showed that at all irradiation powers (i.e., 150, 300, 500, 700, and 900 W) an irradiation time g10 min is sufficient for carbon ceramic formation. In shorter times the carbon ceramic was not completely dried; therefore, an irradiation time of 10 min was chosen for the rest of the experiments. Optimization of MW Irradiation Power. For this purpose the production of carbon ceramic was performed at powers of 150, 300, 500, 700, and 900 W. In each irradiation power three separate carbon ceramics were prepared and then were packed in the body of the three separate Teflon tubes (i.e., totally 15 electrodes). For study of the electrochemical properties of these MWCCEs, the cyclic voltammetry technique was performed in solution containing 5 mM K3Fe(CN)6 in 0.1 M H3PO4 at a scan rate of 50 mV/s (Warnings HCN production is possible from ferri/ferrocyanide in stronger acids, so precautions to ensure good ventilation should be taken).

Figure 4. Cyclic voltammetry of DA at various concentrations of DA in ABS5 at a scan rate of 25 mV/s (A) on the MWCCE, (B) on the CCE; each inset is the average of three calibration plots in various days. Table 2. Comparison of Calibration Plots of DA on MWCCE and CCEsa electrode

LR

DL (µM)

MWCCE 4 µM to 1 mM 1.5 (exptl)b CCE 6 µM to 1 mM 4.0 (exptl) CCEc 0.1-0.8 mMd 20

sensitivity (µA/mM)

R2

12.87 11.46 0.9

0.997 0.999 0.998

a Conditions: ABS5 as supporting electrolyte and scan rate of 25 mV/s. b The minimum amount of DA stock solution which was added until a peak was observed. c Ref 30. d For ref 30 the supporting electrolyte was 0.1 M phosphate buffer (pH 5), and the scan rate was 20 mV/s.

This experiment was repeated four times for each electrode, and after each experiment the electrode surface was renewed by removing a very thin layer of the electrode surface with the stainless steel electrical connection rod and polishing again this new surface. Table 1 summarizes the cyclic voltammetric parameters of these electrodes. As this table shows, although peak currents were greater for the 500 W MWCCE and the value of ∆Ep was smaller for the 900 W MWCCE, but for these electrodes the amount of relative standard deviation (RSD) was not reasonable for some of their parameters which exhibit that these electrodes are not reproducible. In comparison to other electrodes, the MWCCE which was prepared at 300 W had reasonable reproducibility, and also the values of its peak currents and ∆Ep were close to those of the 500 and 900 W MWCCEs. Therefore, 300 W was chosen as the optimum power for the rest of the experiments. A similar study was performed at the same conditions but on a conventional CCE. For four repetitions of this experiment the average of ∆Ep was 92.57 mV (RSD ) 2.14%). These results exhibit that ∆Ep of the CCE is similar to the value of the MWCCE prepared at 300 W. FT-IR and XRD Studies. In order to verify the structure of the carbon ceramic prepared by MW irradiation, FT-IR and XRD spectra of two types of carbon ceramics (applying MW and the

conventional method) were compared with each other. FT-IR spectra of the two ceramics are illustrated in Figure 1. As is obvious in this figure the positions of the peaks are very similar to each other in both spectra, which indicates the similarity of the two compositions. The peaks that appear at 3450 cm-1 are related to Si-OH final groups, and the bands related to the methyl (CH3) group appear at 775 and 1273 cm-1. The Si-O-Si bands appeared in the range of ∼1000-1150 cm-1. Figure 2 also shows the XRD spectra of these ceramics. Panels B and A of Figure 2 show the XRD spectra of conventional carbon ceramic and carbon ceramic produced with MW irradiation, respectively. According to the XRD patterns, the strong peaks of the graphite correspond to the (002), (100), and (101) planes of graphite. Also the peaks at around 2θ ) 30° are related to the polysiloxane polymer. As illustrated in this figure the two spectra are completely similar to each other. This observation also reveals that the compositions of the two ceramics are similar. Determination of DA with MWCCE and CCE. DA is a neurotransmitter known to play a very important role in the central nervous system, which when present in low concentration is likely to give rise to neurodegenerative diseases such as Parkinson’s and Alzheimer’s among others.29 To examine the MWCCE performance the oxidation of DA was investigated by the cyclic voltammetric technique. Figure 3 shows the cyclic voltammogram of 0.10 mM DA in 0.1 M acetate buffer solution at pH 5 (ABS5). As this figure illustrates the cyclic voltammogram of DA on the MWCCE is reversible with ∆Ep of 32 mV and Ipa/Ipc is close to unity, and according to the equation ∆Ep ) 59 mV/n for reversible electrochemical reactions, it can be deduced that DA oxidation on the MWCCE is a two-electron process. (29) Corona-Avendano ˜, S.; Alarco´n-Angeles, G.; Ramı´rez-Silva, M. T.; RosquetePina, G.; Romero-Romo, M.; Palomar-Pardave´, M. J. Electroanal. Chem. 2007, 609, 17–26.

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For this oxidation the effect of pH in the range of 1.4-8 on the Epa was also examined. Solutions of 0.1 M phosphate buffers were used for pHs of 1.4, 2, 3, 6, 7, and 8, and solutions of 0.1 M acetate buffer were used for pHs of 4 and 5. The slope of variation of Epa versus pH indicates the ratio of proton numbers to number of electrons which are involved in the reaction. For this study this slope was -57.17 mV/pH unit (R2 ) 0.999), which implies that the oxidation mechanism of DA is a twoproton two-electron process. The minus slope is due to proton generation in the electro-oxidation of DA on the electrode surface. In addition, increasing the concentration of OH- leads to consumption of more H+, and therefore, the electro-oxidation became easier and Epa will be decreased. For comparison the variation of Ipa versus DA concentration on the MWCCE and CCE surfaces, the cyclic voltammetric method was used. Figure 4A shows the cyclic voltammograms of MWCCE in various concentration of DA solution in ABS5, and the inset of Figure 4A shows the average of three calibration plots in different days. Figure 4B shows the cyclic voltammograms of CCE in the various concentrations of DA at the same conditions, and again the inset of Figure 4B illustrates the average of three calibration plots in different days. Results of these plots are summarized in Table 2 and compared with the previous report for determination of DA with a bare CCE at pH 5.30 As these results show, in comparison to CCE, MWCCE has wider LR

(linear range) and lower DL (detection limit) and has a greater sensitivity for determination of DA.

(30) Salimi, A.; MamKhezri, H.; Hallaj, R. Talanta 2006, 70, 823–832.

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CONCLUSIONS Ease of preparation and surface renewability of CCEs make them more applicable in electrochemical measurements. The main problem in making a CCE is the long time for preparation of carbon ceramic (mostly 48 h). In this paper the required time for preparation of carbon ceramic by MW irradiation was decreased to a few minutes. The CCE which was made by MW irradiation was compared with the conventional CCE using XRD, FT-IR, and cyclic voltammetry. Results indicate that they were similar to each other in composition. By applying the MWCCE in determination of DA, this electrode showed a moderately wider linear range and lower detection limit. Also, we are trying to apply this method for preparation of sol-gel derived by multiwalled carbon nanotube ceramic, which now is in the initial stage of study, and its primary results are very promising. The completed study will be reported later. ACKNOWLEDGMENT We gratefully acknowledge the support of this work by the Shiraz University Research Council. Received for review December 19, 2008. Accepted March 7, 2009.