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An Ultrasensitive Voltammetric Method for Dopamine and Catechol Detection Using Clay-Modified Electrodes Jyh-Myng Zen* and Ping-Jyh Chen Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan Received: September 4, 1997 Final version: October 10, 1997 Abstract Electrochemically preanodized nontronite clay-modified electrodes were found to be very sensitive in the detection of dopamine and catechol by square-wave voltammetry. When preanodized in a neutral medium at þ1.8 V (vs. Ag/AgCl) for 60 s, the clay-modified electrodes exhibit a marked enhancement of the current response for dopamine and catechol. The sensitivity of dopamine can be further improved at the preanodized claymodified electrode with the addition of triethylamine. After 20 s preconcentration in phosphate buffer (pH 7.4) solution, linear calibration plots are obtained over 0–0.7 mM and 0.7–15 mM ranges with a detection limit (3j) of 0.10 nM for catechol. As to dopamine, in the presence of 30 mM triethylamine, linear calibration plots are obtained over 0–1 mM and 1–10 mM ranges and the detection limit is 0.056 nM. Keywords: Nontronite, Dopamine, Catechol, Clay-modified electrode

1. Introduction Many electrochemists have extensively studied clay-modified electrodes (CMEs) for the purpose of photoelectrochemical applications since the first report by Ghosh and Bard in 1983 [1]. The well-defined layered structure, flexible adsorption properties, and potential as catalyst and/or catalyst support makes clay an interesting material for modifying electrode surfaces. The spatial constraints and surface chemical effects induced by clay that can lead to new patterns of reactivity and selectivity makes the utilization of the material promising. Indeed, CMEs can also be useful in analytical detection. For example, montmorillonite CMEs were used to the detection of inorganic cations such as Fe3þ and Ru(NH3)63þ [2, 3]. A laponite CME was applied to the detection of neutral and cationic organometallic compounds [4]. A Nafion/ nontronite CME was developed for the determination of paraquat by square-wave (SW) cathodic stripping voltammetry [5]. Excellent catalytic activity for the electroreduction of hydrogen peroxide by nontronite CME with incorporated methyl viologen as electron mediator was also observed [6]. A glucose sensor was further constructed by immobilizing glucose oxidase between two nontronite clay coatings on glassy carbon electrode (GCE) with methyl viologen as mediator [7]. Another interesting nontronitebased glucose sensor based on the unique adsorption and regeneration properties among dopamine, glucose oxidase, and nontronite was reported recently [8]. In fact, due to colloidal clays’ appreciable surface area, intercalation properties, low cost, high stability, and high cation exchange capacity, their uses in analytical purpose certainly deserve an extensive study. Our group recently reported a sensitive and selective voltammetric method for the determination of dopamine and uric acid in the presence of a high concentration of ascorbic acid by using electrochemically preanodized nontronite CMEs [9]. The CMEs were prepared on the surface of GCEs to prevent the swelling of electrode material after repetitive measurements. Nontronite is a clay in which at least half of the aluminum ions within the clay are replaces by iron [10]. Lighter transition metals in higher oxidation states such as Fe3þ is a hard acid, while dopamine is apparently a hard base. According to Pearson’s principle, hard acids prefer to bind to hard bases, and soft acids prefer to bind to soft bases [11]. Therefore, a preanodized procedure can convert the iron in nontronite into higher oxidation states and results in a very strong Electroanalysis 1998, 10, No. 1

complexing force with dopamine. The present study further extends the previous observation to develop an ultrasensitive voltammetric method for the detection of dopamine. The essential idea is that, in addition to the preanodized procedure, the analyte can also be converted into a harder base simply by adding an electron donor, such as triethylamine, into the test solutions. By doing so, an even stronger complexing force between the preanodized nontronite CME and dopamine can be achieved and hence an even better sensitivity. In this article, catechol was also chosen to be studied for comparison in order to elucidate the function of triethylamine in the proposed system. The optimal experimental conditions in the detection of both organic analytes are also thoroughly investigated. Furthermore, we described electrochemical studies of the preanodized nontronite CME and address the transport characteristics in the detection of dopamine.

2. Experimental Standard clay mineral, nontronite (SWa-1, ferruginous smectite), was purchased from the Source Clay Minerals Repository (University of Missouri, MO). Catechol (Sigma), dopamine (Sigma), and all the other compounds (ACS-certified reagent grade) used in this work were prepared without further purification in double distilled deionized water. Electrochemistry was performed on a Bioanalytical Systems (West Lafayette, IN) BAS-50W electrochemical analyzer. A BAS Model VC-2 electrochemical cell was employed in these experiments. The three-electrode system consisted of a nontronite CME working electrode, a Ag/AgCl reference electrode (Model RE-5, BAS), and a platinum wire auxiliary electrode. Since dissolved oxygen did not interfere with the anodic voltammetry, no deaeration was performed. The nontronite CME was prepared by dropping 6 mL of a clay colloid (0.5 g/L) on a clean GCE and dried at 40 8C for 10 min. Unless otherwise stated, a 0.1 M, pH 7.4 phosphate buffer was used as the supporting electrolytes for catechol and dopamine determination. Solutions of dopamine and catechol were prepared daily using deionized water and used directly for detection under open air at room temperature. The accumulation step was proceeded in constantly stirred (200 rpm) solution, and the voltage-scanning step was performed after 2 s of quiet time.

q WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998

1040-0397/98/0101-0012 $ 17.50þ.50/0

Dopamine and Catechol Detection using Clay-Modified Electrodes

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Fig. 1. Cyclic voltammograms for 10 mM (A) dopamine and (B) catechol in 0.1 M, pH 7.4 phosphate buffer at the nontronite CME with (a) and without (b) electrochemical pretreatment at þ1.8 V for 60 s. Scan rate was 220 mV/s.

3. Results and Discussion Figure 1 compares the cyclic voltammograms for 10 mM dopamine and 10 mM catechol recorded at a nontronite CME with (Fig. 1, curve a) and without (Fig. 1, curve b) electrochemical pretreatment. Evidently, the application of an electrochemical pretreatment at þ1.8 V for 60 s shows an obvious increase in cyclic voltammetric peak response and a more reversible behavior for both analytes. As shown in Table 1, the DEp is only as small as 30 mV. A preanodized procedure can in effect convert the iron in nontronite into higher oxidation forms and results in a strong complexing force with dopamine and catechol. Meanwhile, it is well known that the sensitivity of SW voltammetry of adsorbed species is proportional to the degree of reversibility of the electrochemical reaction [12, 13]. Since both the redox dopamine and catechol couple exhibit a reversible behavior at the preanodized nontronite CME, a clear advantage is expected in using the SW mode with respect to the sensitivity of dopamine and catechol detection. For a reversible wave, a convenient constant in linear scan voltammetry of ip/v1/2C¬o (often called the current function), can be used to estimate n for an electrode reaction as shown below [14] jEp ¹ Ep=2 j ¼ 22:5RT=nF ¼ 56:5=n mV at 25 8C

preanodized nontronite CME. The most proper oxidation reactions at the preanodized nontronite CME for dopamine and catechol are therefore concluded in Figure 2. The effect of triethylamine to dopamine and catechol detection was studied next. Figure 3A compares the effect of triethylamine for 10 mM dopamine and 10 mM catechol recorded at a preanodized CME. As can be seen, there is a clear increase in current response with the addition of the first equal molar of triethylamine (i.e., 10 mM). Afterwards, the additional triethylamine has only negligible effect on the current response. Interestingly, no such phenomenon was observed for catechol. The difference between dopamine and catechol can be explained as follows. As shown in Figure 2, dopamine possesses a functional group of –NHþ 3 at pH 7.4. The addition of one equivalent of triethylamine apparently causes the deprotonation of –NHþ 3 . Evidently, –NH2 is more nucleophilic than –NHþ 3 . Dopamine can therefore turn into a harder base with the addition of triethylamine and hence achieving an even stronger binding force between the preanodized nontronite CME and dopamine. As to catechol, since no –NH2 functional group is available for deprotonation, the addition of triethylamine has only negligible effect in improving the sensitivity. More evidence to support the explanation was provided by the

As shown in Table 1, n ¼ 2 were observed for both the electrochemical oxidation of dopamine and catechol at the Table 1. The steady-state cyclic voltammetric data for dopamine and catechol at the preanodized nontronite CME; [dopamine] ¼ 10 mM; [catechol] ¼ 10 mM.

Epa [mV] Epc [mV] Ep/2 [mV] ipa [mA] ipc [mA] DEp [mV] n

Dopamine

Catechol

158 140 128 4.7 4.2 30 1.9 (< 2)

168 142 138 3.8 3.9 30 1.9 (< 2)

Fig. 2. Proposed electrochemical reaction schemes of dopamine and catechol at a preanodized nontronite CME. Electroanalysis 1998, 10, No. 1

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Fig. 3. Effect of triethylamine to A) SW current response and B) intercept in chronocoulometry for 10 mM dopamine (B) and 10 mM catechol (•) recorded at a preanodized nontronite CME. Preconcentration time was 20 s at open circuit. SW parameters: modulation amplitude 45 mV; modulation frequency 55 Hz; step height 4 mV.

investigation of the transport characteristics of dopamine and catechol when using chronocoulometry. Based on Anson plot, the intercept of Q vs. t1/2 can be used to evaluate the surface excess. Interestingly, as shown in Figure 3B, the trend of change is similar to that in Figure 3A. Obviously, the increase in binding force between the preanodized nontronite CME and dopamine cause an increase in the surface excess and the current response.

J.-M. Zen, P.-J. Chen In the presence of 30 mM triethylamine, Figure 4 shows the SW voltammograms obtained for 1–200 nM dopamine with a 20 s preconcentration time at open circuit. Linear calibration plots are obtained over the 0–1 mM and 1–10 mM in buffer solution (pH 7.4) with slopes (mA/mM) and correlation coefficients of 169.0, 0.9986 and 12.0, 0.9998, respectively. The detection limit (3j) is 0.056 nM. This value is significantly lower compared to a detection limit of 2.7 nM for DA reported in our previous study using a Nafioncoated CME with a 20 s preconcentration time [9]. Note that the detection limit is at least 2 order of magnitude lower than most of other methods with a much shorter preconcentration time [15–21]. Similarly, Figure 5 shows the dependence of the SWV peak current for catechol with concentrations ranging from 0 to 15 mM without the addition of triethylamine. Linear calibration plotw are obtained over the 0–0.7 mM and 0.7–15 mM in pH 7.4 buffer solution with slopes (mA/mM) and correlation coefficients of 128.1, 0.9995 and 5.4, 0.9996, respectively. The detection limit (3j) is 0.10 nM. To characterize the reproducibility of the CME, repetitive measurement-regeneration cycles were carried out in 10 mM dopamine with the addition of 30 mM triethylamine. The CME can be easily renewed by cleaning at þ1.8 V for 60 s in the 0.1 M, pH 7.4 phosphate buffer solution. In other words, the preanodization procedure itself can result in a renewed electrode. The results of 20 successive measurements showed a 0.27 % coefficient of variation. A 0.37 % coefficient of variation was obtained for 20

Fig. 4. Calibration plot (A) and SW voltammograms obtained for (B) 1–200 nM and (C) 0.2–15 mM dopamine in the presence of 30 mM triethylamine. Other conditions as in Figure 3.

Fig. 5. Calibration plot and SW voltammograms obtained for catechol without the addition of triethylamine. Other conditions as in Figure 3. Electroanalysis 1998, 10, No. 1

Dopamine and Catechol Detection using Clay-Modified Electrodes successive measurements of 10 mM catechol. Thus, the electrode renewals give good reproducible surfaces.

4. Conclusions The present study demonstrates the preanodized nontronite CMEs can effectively improve electrochemical monitoring of organic analytes such as catechol and dopamine. The addition of triethylamine can further increase the sensitivity in the detection of dopamine. The CME can be easily regenerated and the detection can be achieved without deoxygenating. The modification procedure is reproducible and the resultant attachment is stable. The method can also be applied to other catecholamines. Research along this direction is currently underway.

5. Acknowledgements

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6. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

The authors gratefully acknowledge financial support from the National Science Council of the Republic of China under Grant NSC 87-2113-M-005-021. The authors also express their thanks to Drs. Yao-Jung Chen and Han-Mou Gau for valuable discussion.

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