Electropolymerization of Methylene Blue on Carbon Ionic Liquid Electrode and Its Electrocatalysis to 3,4-Dihydroxybenzoic Acid

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1 158 Journal of the Chinese Chemical Society, 2009, 56, Electropolymerization of Methylene Blue on Carbon Ionic Liquid Electrode and Its Electrocatalysis to 3,4-Dihydroxybenzoic Acid Rui-JunZhao( ),QiangJiang( ), Wei Sun* ( ) and Kui Jiao ( ) College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao , P. R. China In this paper an ionic liquid modified carbon paste electrode (CILE) was prepared and methylene blue (MB) was electropolymerized on the CILE by using the cyclic voltammetric technique in the potential range from -1.0 V to 0.8 V (vs. SCE). A stable polymer film was obtained and exhibited a pair of redox peaks. The morphology and characteristics of poly(methylene blue) (PMB) film was studied by the techniques such as scanning electron microscopy and electrochemical impedance spectroscopy. This PMB modified CILE (PMB/CILE) showed excellent electrocatalytic response to 3,4-dihydroxybenzoic acid with the increase of the electrochemical responses. The oxidation peak current had a linear relationship with 3,4-dihydroxybenzoic acid concentration in the range of ~ mol L -1 and the detection limit was mol L -1 (3 ). Keywords: Methylene blue; 3,4-Dihydroxybenzoic acid; Ionic liquid modified carbon paste electrode; Electropolymerization; Electrocatalysis. INTRODUCTION Electropolymerization is a powerful tool in the development of modified electrodes with the advantage of selective modification of multi-layer electrodes that couldn t be obtained by the usual method; it also can change the properties of polymers. The polymers got from electropolymerization show some unique properties that are peculiar to the corresponding monomer. Many conducting polymers, such as polyaniline, 1 polypyrrole, 2 and polythiophene, 3 have received a great deal of interest since they have high conductivity, high redox reversibility and high stability. Different electroactive dyes can also be electropolymerized on the electrode surface and the thus polymers formed exhibit excellent applications in electroanalysis and bioelectrochemistry. For example, Hianik 4 et al. electropolymerized phenothiazine dyes onto a glassy carbon electrode and used it as a DNA biosensor for a DNA aptamer specific to human -thrombin. Yang 5 et al. prepared poly (safranine T) films on a graphite electrode and investigated the electrocatalytic effect on dopamine. Zhou 6 et al. electropolymerized nile blue on a glassy carbon electrode for the amperometric determination of hemoglobin. Carbon ionic liquid electrode (CILE), which is made of graphite powder and different kinds of ionic liquids (ILs), has aroused great interest in recent years. Due to the advantages of ILs such as negligible vapor pressure, wide potential windows, good ionic conductivity, extraction and catalytic ability, CILE has shown many specific characteristics including simple preparation procedure, wide potential windows, high rates of electron transfer and good antifouling ability. ILs can act not only as binders to connect graphite powder, but also as inherent catalysts present on the electrode surface. The direct electrochemistry of electroactive molecules such as proteins and ssdna has been achievedoncile Maliki 11 et al. also electrodeposited Pd nanoparticles on CILE, which showed good electrocatalytic ability for dopamine and ascorbic acid. In this paper, methylene blue (MB) was electropolymerized on the surface of CILE. MB is a kind of electroactive cationic dye and has been extensively used as an indicator and mediator in the field of bioelectrochemistry A sensor for hemoglobin based on poly(methylene blue) (PMB) electrodeposited on a glassy carbon electrode has also been reported. 15 To our knowledge, no reports have described the electropolymerization of MB on the surface of CILE. The conditions for electropolymerization were optimized and the PMB modified CILE was characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The electrocatalytic behaviors for 3,4-dihy-

2 Poly(methylene blue) on CILE and Its Electrocatalysis J. Chin. Chem. Soc., Vol. 56, No. 1, droxybenzoic acid (3,4-DHBA) were further investigated on PMB/CILE. 3,4-DHBA, a kind of phenolic compound which is present principally in fruits, legumes, nuts and vegetables, has functions such as antioxidition, lowering blood pressure and anti-cancer effects. 3,4-DHBA is also used as an intermediate chemical in the preparation of dyes and medicines. Several analytical methods have been used for the determination of 3,4-DHBA, such as high performance liquid chromatography with UV detection or fluorimetric detection, 16,17 flow injection analysis, 18 capillary electrophoresis, 19 GC-MS, 20 chemiluminescence 21 and electrochemistry. 22,23 Up to now, methods based on polymer modified carbon ionic liquid electrode have not been reported for the study of 3,4-DHBA. The experimental results indicated that the fabricated PMB/CILE showed good electrocatalytic ability for the oxidation of 3,4- DHBA. EXPERIMENTAL SECTION Apparatus and reagents A LK2005 electrochemical workstation (Tianjin Lanlike Instrument, China) was used for all the electrochemical experiments with a traditional three-electrode system composed of a CILE or PMB/CILE as working electrode, a platinum wire as auxiliary electrode and a saturated calomel electrode (SCE) as reference electrode. Scanning electron microscopy (SEM) was obtained on a JSM-6700F scanning electron microscope (Japan Electron Company). Methylene blue (MB, Shanghai Reagent Company), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM] BF 4, Hangzhou Chemer Chemical Limited Company), 3,4-dihydroxybenzoic acid (3,4-DHBA, Shanghai Kefeng Chemical Reagent Limited Company) and graphite powder (average particle size 30 m, Shanghai Colloid Chemical Plant) were used as received. 0.1 mol L -1 phosphate buffer solution (PBS) was used as the supporting electrolyte. All the other chemicals were of analytical reagent grade, and double-distilled water was used in all experiments. Preparation of polymer modified electrode The CILE was prepared as follows: 1.6 g of graphite powder, 100 Lof[EMIM]BF 4 and 400 L of liquid paraffin were mixed in a mortar by hand until a homogeneous paste was obtained. The prepared carbon paste was tightly packed into a glass tube and a copper wire was introduced into the other end for the electrical contact. Prior to use, the surface of the well-prepared electrode was smoothed on a weighing paper. For MB polymerization, the CILE was immersed into 0.1 mol L -1 ph 7.0 PBS containing 1.0 mmol L -1 MB and 0.1 mol L -1 KCl. The cyclic voltammetry was performed between the potential range from -1.0 V and 0.8 V for 50 cycles with a scan rate of 50 mv s -1. An electrode with electropolymerized layers was rinsed with PBS, distilled water and dried at room temperature. RESULTS AND DISCUSSION Electropolymerization of methylene blue on CILE The solution for the electrolysis consisted of 1.0 mmol L -1 MB, 0.1 mol L -1 ph 7.0 PBS and 0.1 mol L -1 KCl. Fig. 1 shows the cyclic voltammograms of polymer film growth in electrolytic solution. From the first cycle it can be seen that a pair of well-defined redox peaks was obtained with the anodic peak and cathodic peak appearing at V and V, respectively, which was attributed to the oxidation and reduction of MB. With the increased number of potential cycles, a new pair of redox peaks was obtained with the anodic peak and cathodic peak at 0.0 V and V, respectively, and the currents of the new peaks increased. The result indicated that the polymer film was formed on the CILE and its thickness increased with the number of potential cycles. According to reference 24, electropolymerization of MB is due to the oxidative condensation of aromatic phenothiazine fragments and partial demethylation of dimethylamino side groups. In this paper CILE was used as the basal electrode for the polymeriza- Fig. 1. Cyclic voltammograms of MB electropolymerization in a mixture solution of 1.0 mmol L -1 MB and 0.1 mol L -1 KClin0.1molL -1 PBS with a scan rate of 50 mv s -1.

3 160 J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 Zhao et al. tion. CILE had been demonstrated to have advantages such as wide potential windows, high rates of electron transfer and good anti-fouling ability. A layer of ILs also formed on the surface of CILE, which had inherent catalytic ability. As indicated in reference 25, the presence of IL in the carbon paste electrode exhibited not only the high adherence of graphite particles, but also the -electron interaction of the layers of the ionic liquids with the -electron system of graphite. So the presence of IL as modifier in the carbon paste electrode provided an inherent catalytic interface for MB polymerization to take place. The optimal conditions for the growth of PMB were investigated. Electropolymerization of MB requires a high potential for the irreversible oxidation of the monomer. If the potential is too high, over-oxidation of the polymer will take place. So the potential range was optimized and the redox peak currents of PMB reached the maximum in the potential range from -1.0 V to 0.8 V. The redox peak currents of PMB increased with the increase in number of the potential cycles. But when the number of potential cycles was up to 50, there were no obvious increases of the redox peak currents. From the experiments it was found that the redox peak currents of PMB decreased with the increase of scan rate in the range from 30 mv s -1 to 200 mv s -1. After considering the factor of time in experiments, the number of potential cycles and scan rate were selected as 50 and 50 mv s -1, respectively. Cyclic voltammograms of modified electrodes in buffer solution The cyclic voltammograms of modified electrodes in ph 7.0 PBS were further investigated with the results shown in Fig. 2. Curves a and b were that of CPE and CILE in PBS; no electrochemical response appeared with a little increase of the background current, which may be due to that the addition of IL affected the capacitance of the electrode. Curve d was that of PMB/CILE in PBS with a pair of redox peaks appeared, which was due to the oxidation and reduction of PMB on the electrode surface. The electropolymerization of MB on the traditional carbon paste electrode was also carried out and the cyclic voltammogram of PMB/CPE in PBS is shown as curve c. It can be seen that a pair of rather broad redox peaks appeared with a small electrochemical response. The experimental results testified to the superiority of CILE to CPE, indicating the addition of IL facilitated the electron transfer between PMB and electrode. Surface morphologies of modified electrodes Scanning electron microscopy (SEM) was used to characterize and compare the morphology of different kinds of modified electrodes. Fig. 3 compared the morphological features of the CILE and PMB/CILE. The SEM image of CILE was uniform and no separated carbon layers could be observed (Fig. 3a). Since [EMIM]BF 4 was a hydrophilic ionic liquid with higher viscosity, it could fill into the layer of graphite powder and bridge the isolated carbon flask, so a flat surface could be obtained. Fig. 3b shows a SEM image of PMB/CILE with porous, loose, multi-layer composite films topography. The significant differences in the surface structure of the two electrodes exhibited that the methylene blue was successfully electropolymerized on the surface of CILE. Electrochemical impedance spectroscopy of modified electrode The interfacial characteristic of PMB/CILE was studied by electrochemical impedance spectroscopy (EIS) in a Fig. 2. Cyclic voltammograms of (a) CPE, (b) CILE, (c) PMB/CPE and (d) PMB/CILE in ph 7.0 PBS with the scan rate of 50 mv s -1. Fig. 3. Surface morphologies of CILE (a) and PMB/ CILE (b).

4 Poly(methylene blue) on CILE and Its Electrocatalysis J. Chin. Chem. Soc., Vol. 56, No. 1, mol L -1 KCl solution containing the redox couples of 1.0 mmol L -1 K 4 Fe(CN) 6 and 1.0 mmol L -1 K 3 Fe(CN) 6 at the applied potential of 0.0 V (vs. SCE). The applied perturbation amplitude was 5 mv and the frequencies swept from 10 5 to 0.05 Hz. In EIS the semicircle diameter is equal to the electron transfer resistance (R et ), which is related to the electron transfer kinetics of the redox probe at the surface of the electrode. Fig. 4 records the electrochemical impedance spectra of CILE (curve a) and PMB/CILE (curve b). It could be seen that on CILE a flat semicircle was produced at a high frequency with the R et value of 3000, which showed that the electrode/solution surface was difficult for the electron transfer. While on PMB/CILE a straight line was produced (curve b), indicating that the electron transfer process was controlled by a mass transport finite-diffusion at low frequency. The results showed that the PMB film exhibited good conductivity and increased the ability of the electron transfer at the electrode/ solution surface. Electrochemical behaviors of 3,4-DHBA on PMB/ CILE Fig. 5 shows cyclic voltammograms of 1.0 mmol L -1 3,4-DHBA in 0.1 mol L -1 PBS (ph 7.0) on CILE and PMB/ CILE, respectively. A pair of quasi-reversible redox peaks at the surface of CILE was produced with the anodic peak potential and the cathodic peak potential appearing at V and 0.0 V. The peak-to-peak separation ( E p ) was 537 mv (vs. SCE). While on the surface of PMB/CILE, a pair of well-defined quasi-reversible redox peaks appeared with the increased currents and the decrease of peak-topeak separation. The anodic peak potential and the cathodic peak potential appeared at V and V, respectively, with the E p value of 77 mv. It is wellknown that the increase of peak current and the decrease of overpotential are the characteristics of electrocatalytic reactions. The experimental results testified to the superiority of PMB/CILE over CILE, indicating that PMB film on the surface of the electrode increased the ability of electron transfer at the interface of electrode/solution and catalyzed the redox reaction of 3,4-DHBA. The interaction mechanism may be that the anodic groups N(CH 3 ) 2 + of PMB bind the cathodic groups COO - of 3,4-DHBA with the electrostatic interaction and then increase the speed of electron transfer rate. Fig. 6 shows cyclic voltammograms of 3,4-DHBA on the PMB/CILE with different scan rates ( ) in the 0.1 mol L -1 ph 7.0 PBS. It can be seen that the redox peak currents increased with the increase of scan rate in the range from 10 mv s -1 to 100 mv s -1. The relationship of redox peak currents (Ip) of 3,4-DHBA and scan rate were plotted and good linear relationships were got with the linear regression equations as Ipa ( A) = (V s -1 ) (n = 10, Fig. 4. Electrochemical impedance spectroscopy of CILE (a) and PMB/CILE (b) in a 0.1 mol L -1 KCl solution containing the redox couple of 1.0 mmol L -1 K 4 Fe(CN) 6 and 1.0 mmol L -1 K 3 Fe(CN) 6 with the applied perturbation amplitude as 5 mv and the frequencies swept from 10 5 to 0.05 Hz. Fig. 5. Cyclic voltammograms of 3,4-DHBA on CILE (a) and PMB/CILE (b) with the scan rate of 50 mv s -1 in ph 7.0 PBS.

5 162 J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 Zhao et al. = 0.997) and Ipc( A) = (V s -1 ) (n = 10, = 0.993), respectively, which illustrated a surface-controlled redox process. The peak-to-peak separation also increased with the scan rate, indicating the electrode process turned to be more irreversible. The electrochemical parameters of 3,4-DHBA on the PMB/CILE were estimated with the following Laviron s equations: 26 0' 23. RT Epc E log nf Fig. 6. Cyclic voltammograms of 3,4-DHBA on PMB/ CILE with different scan rates (from 1 to 8: 10, 30, 50, 60, 70, 80, 90,100 mv s -1 ). RT Epa E 23 0 '. log ( 1 ) nf logk s log( 1 ) ( 1 )log RT ( 1 ) nf Ep log nf 23. RT where is the charge transfer coefficient, n is the number of electrons transferred and k s is the electron transfer rate constant. The relationships of redox peak potentials with ln were E pa (V) = ln (n = 8, = 0.995) and E pc (V) = ln (n = 8, = 0.990). According to the above equations, the values of, nandk s were calculated as 0.57, 2.1 and s -1, respectively. The k s value indicated a quasi-reversible redox process of 3,4-DHBA at the surface of PMB/CILE. Fig. 7(A) shows the cyclic voltammgrams of PMB/ CILE at different concentrations of 3,4-DHBA. With the increase of 3,4-DHBA concentration, the redox peak currents increased gradually. A linear relationship could be established between the oxidation peak current and the concentration of 3,4-DHBA in the range of to mol L -1 (Fig. 7B). The linear regression equation was calculated as I pa ( A) = C (mmol L -1 ) (n = 13, = 0.998) with a detection limit of mol L -1 (3 ). CONCLUSION Electrochemical polymerization of methylene blue on the surface of the carbon ionic liquid electrode was performed and the properties of the resulting films were inves- Fig. 7. (A) Cyclic voltammograms of different concentrations of 3,4-DHBA on PMB/CILE in ph 7.0 PBS (from 1 to 7: 0.5, 0.8, 0.9, 1.0, 1.2, 1.5, 1.8 mmol L -1 ); (B) The calibration curve between oxidative peak currents with 3,4-DHBA concentrations.

6 Poly(methylene blue) on CILE and Its Electrocatalysis J. Chin. Chem. Soc., Vol. 56, No. 1, tigated. The experimental results of SEM and EIS exhibited that methylene blue was polymerized on the surface of the electrode. The polymer film on the electrode showed good electrocatalytic ability for 3,4-dihydroxybenzoic acid. The linear relationship of the oxidation peak current and the concentration was obtained with the concentration range from to mol L -1. ACKNOWLEDGEMENTS We are grateful for the financial support of the National Natural Science Foundation of China (Nos , ) and the Doctoral Foundation of QUST ( ). Received August 4, REFERENCES 1. Maeikienè, R.; Malinauskas, A. Electrochim. Acta 1999, 41, Genies, E.-M.; Bidan, G.; Diaz, A.-F. J. Electroanal. Chem. 1999, 149, Tanaka, K.; Shichiri, T.; Yamabe, T. Synth. Met. 1986, 16, Evtugyn, G.-A.; Porfireva, A.-V.; Hianik, T.; Cheburova, M.-S.; Budnikov, H.-C. Electroanalysis 2008, 20, Yang, Q.-X.; Zhang, M.-F.; Li, G.-B.; Zhang, Y. Chinese J. Anal. Lab. 2007, 26, Zhou, D.-M.; Chen, H.-Y. Electroanalysis 1997, 9, Sun, W.; Wang, D.-D.; Gao, R.-F.; Jiao, K. Electrochem. Commun. 2007, 9, Sun, W.; Gao, R.-F.; Jiao, K. J. Phys. Chem. B 2007, 111, Sun, W.; Li, X.-Q.; Jiao, K. J. Chin. Chem. Soc. 2008, 55(5), Sun, W.; Li, Y.-Z.; Yang, M.-X.; Liu, S.-F.; Jiao, K. Electrochem. Commun. 2008, 10, Safavi, A.; Maleki, N.; Tajabadi, F.; Farjami, E. Electrochem. Commun. 2007, 9, Yang, J.; Yang, T.; Feng, Y.-Y.; Jiao, K. Anal. Biochem. 2007, 365, Karyakin, A.-A.; Karyakina, E.-E.; Schuhmann, W.; Schmidt, H.-L. Electroanalysis 1994, 6, Tonlé, I.-K.; Ngameni, E.; Tcheumi, H.-L.; Tchiéda, V.; Carteret, C.; Walcarius, A. Talanta 2008, 74, Brett, C.-M.; Inzelt, G.; Kertesz, V. Anal. Chim. Acta 1999, 385, Garcia-Parrilla, M.-C.; Camacho, M.-L.; Heredia, F.-J.; Troncoso, A.-M. Food Chem. 1994, 50, Lores, M.; Garicia, C.-M.; Cela, R. J. Chromatogr. A 1994, 683, Haghighi, B.; Gorton, L.; Ruzgas, T.; Jönsson, L.-J. Anal. Chim. Acta 2003, 487, Deng, Y.; Wellons, A.; Bolla, D.; Krzyaniak, M.; Wylie, H. J. Chromatogr. A 2003, 1013, Franca, A.; Nicola, D.; Panajotis, K.; Luciana, D.-G. J. Agric. Food Chem. 1995, 43, Xie, C.-G.; Li, H.-F. Chin. J. Metall. Anal. 2004, 24, Long, H.; Li, Y.-H. Chin.J.Anal.Lab.1999, 18, Moghaddam, A.-B.; Kobarfard, F.; Davarani, S.-S.; Nematollahi, D.; Shamsipur, M.; Fakhari, A.-R. J. Electroanal. Chem. 2006, 586, Karyakin, A.-A.; Karyakina, E.-E.; Schmidt, H.-L. Electroanalysis 1999, 11, Maleki, N.; Safavi, A.; Tajabadi, F. Anal. Chem. 2006, 78, Laviron, E. J. Electroanal. Chem. 1979, 101, 19.