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Lead Ruthenate Pyrochlore Formed in Clay for Sensitive Determination of Dopamine Jyh-Myng Zen,* Annamalai Senthil Kumar, Huey-Ping Chen Department of Chemistry, National Chung-Hsing University, Taichung 402, Taiwan *e-mail: [email protected] Received: August 20, 2002 Final version: October 18, 2002 Abstract Natural iron-intercalated clay (nontronite, SWa-1) was converted into a more efficient catalyst by formation of lead ruthenate pyrochlore (Py, Pb2Ru2 xPbxO7 y) directly inside the matrix. The new material can take advantage of the electrocatalytic properties of both Py and nontronite and can even generate new catalytic active sites due to interaction of Py and nontronite. The combination of these two interesting catalysts results in a more reactive electrode (designated as SWa-1/PyCME) for electroanalytical applications. The preparation and characterization of the SWa-1/PyCME as well as its electrocatalytic behavior toward the oxidation of dopamine are described in this study. Surface saturation kinetics in terms of Michaelis-Menten kinetics suited well the oxidation of dopamine on this modified electrode. Under optimized conditions, the detection limit (S/N ˆ 3) was 0.54 nM by square-wave voltammetry. Keywords: Lead ruthenate pyrochlore, Nontronite clay, Dopamine

1. Introduction Dopamine (3,4-dihydroxyphenyl ethylamine, DA) is an important neurotransmitter in mammalian central nervous system and plays an essential role in brain chemistry [1, 2]. It is also a drug for carcinogenic and endotoxic shock to improve the renal blood flow and septicemia shock as intravenous infusion in a dosage level of 2 ± 50 mg/min/kg body weight [3]. Ultrasensitive sensing of DA is thus important and is a pressing need in biomedical and clinical applications [4]. Recently, we have reported two sensitive chemically modified electrodes, i.e., Nafion/lead ruthenate pyrochlore chemically modified electrode (designated as NPyCME, Py ˆ Pb2Ru2 xPbxO7 y) and Nafion/nontronite modified electrode (designated as Nafion/SWa-1) for DA determination [5, 6]. Due to the similarity in cationic exchange property between Nafion and nontronite, we report here the application of nontronite as another useful host for Py. Note that montmorillonite is the most studied clay material for electrode modification and the cationic exchange capacity (CEC) of nontronite is higher (107 mmol/ 100 g) than that of montmorillonite (79.8 mmol/100 g) [7]. The in situ precipitation technique used in the preparation of the NPyCME was applied to nontronite in this study [7 ± 11]. By preparing the new material, we can take advantage the electrocatalytic properties of both Py and nontronite and may generate new catalytic active sites due to interaction of Py and nontronite. The combination of these two interesting catalysts was expected to make a more reactive modified electrode (designated as SWa-1/PyCME) for electroanalytical applications [12]. The intriguing aspects about preparation, characterization, and determinaElectroanalysis 2003, 15, No. 20

tion of DA on the SWa-1/PyCME were thoroughly studied. The electrochemical activity of the SWa-1/PyCME was further compared to the earlier reported systems of NPyCME and Nafion/SWa-1 [5, 6].

2. Experimental 2.1. Chemicals and Apparatus Nontronite (SWa-1, ferruginous smectite) was purchased from the Source Clay Minerals Repository (University of Missouri). Ruthenium trichloride (RuCl3 ¥ nH2O) and lead nitrate (Pb(NO3)2) were purchased from Aldrich. Dopamine (Sigma) and all other compounds (ACS-certified reagent grade) were used without further purification. Aqueous solutions were prepared with doubly distilled deionized water. Cyclic voltammetry (CV) and square-wave voltammetry (SWV) were performed on a BAS 50W electrochemical analyzer (West Lafayette, IN, USA). A BAS Model VC-2 electrochemical cell was employed for CV and SWV experiments. The electrode system consisted of an Ag/ AgCl reference electrode (Model RE-5, BAS), a platinum wire auxiliary electrode, and one of the following working electrodes: glassy carbon electrode (GCE), nontronitecoated GCE (designated as SWa-1/GCE), and the SWa-1/ PyCME. Since dissolved oxygen did not interfere with the working potential windows, no deaeration was performed for voltammetric measurements.

¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI: 10.1002/elan.200302736

Determination of Dopamine

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Fig. 1. Conceptional representation for the formation of the SWa-1/PyCME. SWa-1: Nontronite; Py: Lead ruthenate pyrochlore.

2.2. Procedure The GCE (0.07 cm2) was first polished with the BAS polishing kit and rinsed with deionized water, then further cleaned ultrasonically in 1 : 1 nitric acid and deionized water successively. For the SWa-1/GCE preparation, 10 mL of nontronite clay colloid (0.01 wt%) was carefully dipped on GCE surface and allowed for complete drying overnight [6, 13]. The electrode was thoroughly washed with double distilled water in order to remove the surface impurities for further applications. To prepare the SWa-1/PyCME with various values of x in Pb2Ru2 xPbxO7 y, different [Pb2‡/Ru3‡] ratio was first ionexchanged into the SWa-1/GCE. More precisely, [Ru3‡] was fixed at 10 mM and [Pb2‡] was varied as needed in the dipping solution. After the ion-exchanged procedure, the precipitation of hydroxides were carried out in ca. 6% KOH solution at 53 8C under constant purging of O2 for 24 h [10]. The coated clay layers on GCE can be pealed-off at strong alkaline and higher temperature conditions. We solved this problem by coating a thin layer of cellulose acetate (6 mL of 0.01 wt% in glacial acetic acid) before the precipitation of hydroxides. Figure 1 shows the typical schematic representation for the preparation of the SWa-1/PyCME. The cellulose acetate did not participate in either chemical or electrochemical reaction and was used merely to stabilize the exchanged ions and the clay layer on GCE. Due to the porous structure of cellulose acetate, the analytes can diffuse freely inside the layer. Cycling continuously in supporting electrolyte at a scan rate of 50 mV/s until the CV became constant pretreated the freshly prepared SWa-1/ PyCME. Normally it took 30 ± 50 cycles. Analytical characterization of DA on the SWa-1/PyCME was performed by SWV. In most cases, a pH 8 phosphate buffer solution (PBS) of ionic strength 0.1 M was used.

3. Results and Discussion 3.1. Electrochemical Behavior Figure 2A shows the typical SWV responses of 9 mM DA at GCE, SWa-1/GCE, and the SWa-1/PyCME in pH 8 PBS. Electroanalysis 2003, 15, No. 20

Fig. 2. A) Typical SWV response for 9 mM DA on different electrodes in pH 8 PBS. B) The forward and reverse SW voltammograms. SWV parameters: fHz ˆ 35 Hz; Es ˆ 4 mV; Ea ˆ 70 mV. Preconcentration for 20 s at open circuit.

The large increase in oxidation current together with the decrease in overpotential on the SWa-1/PyCME clearly indicates the effective participation of the intercalated Py inside the SWa-1 clay matrix. The calculated normalized current function, if ˆ A/(fHzEs[DA]), fHz ˆ frequency and Es ˆ step height, was 48 mA/(HzVmol/cm3) in pH 8 PBS. Compared to the if values of 0.69 and 6.04 mA/(HzVmol/cm3) for the NPyCME and Nafion/SWa-1 [5, 6], respectively, the SWa-1/PyCME shows enhanced electrochemical activity. Furthermore, the improvement in reversibility for DA over previous two systems provides an extra advantage in improving the sensitivity by SWV (Fig. 2B). Indeed, two order of increase in current signal was obtained from the SWa-1/PyCME as will be discussed later. Figure 3A shows the CV response of the SWa-1/PyCME with 0.4 mM DA in pH 8 PBS at various scan rates (v). The system obeys quasi-irreversible behavior with a slope of 0.51 in log(ipa) vs. log(v) plot indicating the diffusion-controlled characteristic of DA oxidation on the SWa-1/PyCME. Further experiments, under quasi-steady state condition (v ˆ 10 mV/s), showed a linear relationship with increasing ¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Scheme 1.

Fig. 3. CV at different scan rate (for 0.4 mM DA) (A) and concentration (B) at v ˆ 10 mV/s in pH 8 PBS on the SWa-1/PyCME.

concentration of DA up to 1.8 mM (Fig. 3B). This behavior indicates the operation of surface saturation kinetics in terms of the Michaelis-Menten (MM) mechanism [10, 14, 15]. Similar observation with first order (m ˆqlog(ipa)/ qlog(C), C ˆ [Analyte]) of reaction was noticed for hypoxanthine oxidation reaction on the NPyCME [10]. In the present case, the reaction was also found to be first order (m ˆ 0.91) and confirms the operation of surface saturation kinetics [14, 15]. Our previous studies successfully used PVC-Py composite electrodes as comparison to prove the assistance from the higher oxidation state of Run‡ in octahedral sites of Py for the electrocatalytic oxidation reactions on the NPyCME [9, 10]. Experiments also revealed the participation of PyRu(VI)/Ru(IV) redox group in DA electrocatalytic oxidation on the PVC-Py composite electrodes [9]. The same redox species was also believed to effectively participate in this study. Based on the surface saturation kinetics, the reaction pathway can be written as shown in Scheme 1 [10, 15]. In the above reaction pathway, step II with rate constant kc is the rate determination step for the overall oxidation reaction. The calculated surface excess (Gs) at the SWa-1/ PyCME in pH 8 PBS was 2.84  10 10 mol cm 2 for PyRu(VI)/Ru(IV) by chronocoulometry. Under this monolayer surface coverage of Py active site, the partition coefficient of DA in the solution phase/polymer film could Electroanalysis 2003, 15, No. 20

be taken as unity. Hence, the three-dimensional participation (according to Andrieux and Saveant model [16, 17]) can be restricted to a two-dimensional electrochemical behavior and thus obey the simple MM equation. Note that similar assumption was taken for the cases of Tosflex-Fe(CN)63 and Tosflex-Fe(CN)63 sol gel modified electrodes by our group recently [18, 19]. The heterogeneous kinetics parameters can be obtained from simple Lineweaver-Burke (LB) equation [15]: 1/ipa ˆ Km/nFAkcGsCDA ‡ 1/nFAkcGs k'e ˆ kcGs/Km where CDA is the dopamine concentration and other symbols have their usual significance. Based on Figure 3B data×s, typical LB plot (i.e., 1/ipa vs. 1/CDA) can be fitted into the equation of y ˆ 0.056 (mA 1) ‡ [18.1(mA mM) 1]x with R ˆ 0.9993. The kinetic parameter values observed are: Km ˆ 0.32 mmol dm 3, kc ˆ 4.63 s 1 and k'e ˆ 4.05  10 3 cm s 1. Compared to the calculated kinetics parameters for DA on the NPyCME of Km ˆ 1.85 mmol dm 3, kc ˆ 0.46 s 1 and k×e ˆ 0.201 cm s 1 in pH 7 PBS [10], the obtained kc value on the SWa-1/PyCME is about one order higher in magnitude. This result again verifies the enhanced electrochemical activity for the present case. ¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Determination of Dopamine

Fig. 5. A) SWV responses for increasing concentration of DA on the SWa-1/PyCME in pH 8 PBS. B) Calibration plot. Other conditions as in Figure 2.

Fig. 4. A) A plot of current function (if) against the [Pb2‡/Ru3‡] ratio in preparing the SWa-1/PyCME for 0.4 mM DA (v ˆ 50 mV s 1) by CV in pH 8 PBS. B) A plot of ipa and Epa against solution pH for the oxidation of 1 mM DA by SWV. Other SWV parameters are similar to Figure 2.

3.2. Analytical Characterization Necessary solution phase and instrumental parameters were first systematically optimized. Figure 4A shows the CV results for the electrodes prepared with various [Pb2‡/Ru3‡] ratios for the determination of 0.4 mM DA in pH 8 PBS. The best result was found for the SWa-1/PyCME prepared at [Pb2‡/Ru3‡] ˆ 1.6. This ratio is close to the optimized ratio of 1.5 in the preparation of the NPyCME indicating the similarity in ion-exchange capacities of Nafion and SWa-1 [7, 10]. Note that the ratio is not a necessary condition for the formation of Py inside the host matrix. The general formula for Py is Pb2Ru2 xPbxO7 y and the Py crystallites can also be prepared using solution phase precipitation procedure without any host by altering the [Pb2‡/Ru3‡] ratio in the precipitation bath [11]. Figure 4B shows the pH effect on ipa and Epa on the SWa-1/ PyCME for 1 mM DA. The Epa was found to shift with pH with a slope of 61 mV/decade indicating the exchange of equal number of electron with proton in the oxidation mechanism. The maximum ipa was found to be at ca. pH 8 indicating that the SWa-1/PyCME favors neutral DA (considering pKa ˆ 8.87 for DA) [5] in efficient electrocatalytic oxidation. This result actually induced another advantage of the SWa-1/PyCME for the application in biochemical and clinical analysis at physiological pH. Electroanalysis 2003, 15, No. 20

The SWV parameters were also systematically studied for the detection of 0.5 mM DA in pH 8 PBS. The optimized SWV parameters observed were: fHz ˆ 35 Hz; Es ˆ 4 mV; Ea (amplitude) ˆ 70 mV with an optimized preconcentration time (tp) of 20 s. Under the optimized conditions, the calibration plot showed two linear ranges of 0.1 ± 5 mM and 5 ± 9 mM with regression coefficients of 0.9991 and 0.9992, respectively (Fig. 5). The coefficient of variation of the response (n ˆ 10) for 0.1 mM DA was 1.56% with a fast response time revealing the good stability and reproducibility of the present system. The detection limit (DL) was 0.54 nM (S/N ˆ 3). Table 1 summarizes the analytical parameters for DA on the SWa-1/PyCME in comparison to earlier systems of NPyCME and Nafion/SWa-1 [5, 6]. Similar to the observation using the Nafion/SWa-1, the SWa1/PyCME can also tolerate up to 3-fold excess in concentration of ascorbic acid. Good sensitivity under mild preconcentration condition is the chief advantage of the SWa-1/PyCME over the other systems for future analytical applications.

4. Conclusions The in situ precipitation procedure of Py can be adoptable in the nontronite host matrix. The SWa-1/PyCME performs very well for DA electrocatalytic oxidation and has better sensitivity than the earlier systems of NPyCME and Nafion/ SWa-1. Surface saturation kinetics in terms of MichaelisMenten kinetic were well suited for the presence case. The calculated current function values (if) and first order heterogeneous rate constants (kc) gave supportive evidence for the enhanced electrochemical activity.

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Table 1. Comparative analytical parameters for the DA detection on various chemically modified electrodes. Electrodes

Parameters [a] if (  10 3A/ ( Hz V mol cm 3))

pH/PBS

Linearity

DL [b]

Conditions

References

NPyCME

0.69

7.0

0 ± 70 mM

180 nM

[5]

Nafion/ SWa-1

6.04

7.4

0 ± 25 mM

2.7 nM

Swa-1/ PyCME

48.00

8.0

0.1 ± 5 mM & 5 ± 9 mM

0.54 nM

preconcentration at 0.3 V (vs. Ag/AgCl) for 60 s preanodization at ‡ 1.8 V (vs. Ag/ AgCl) for 60 s followed by 20 s of preconcentration at open circuit preconcentration at open circuit for 20 s

[6]

This work

[a] Experiments with SWV. [b] S/N ˆ 3.

5. Acknowledgement The authors gratefully acknowledge financial supports from the National Science Council of the Republic of China.

6. Glossary GCE Py NPyCME SWa-1/GCE Nafion/SWa-1 SWa-1/PyCME Km kc k'e

Glassy carbon electrode Lead ruthenate pyrochlore Nafion/Py chemically modified electrode Nontronite clay coated GCE Nafion/nontronite modified GCE electrode Nontronite/Py chemically modified electrode Michaelis-Menten binding constant Catalytic rate constant Electrochemical rate constant

7. References [1] P. S. Whitton, Neurosci. Biobeh. Rev. 1997, 21, 481. [2] E. Pennisi, Science 1997, 276, 202. [3] Drug Index (Jan.±Mar. 1998) (Ed: R. Murali), Passi, New Delhi, India, Jan-Mar× 1998, p. 151.

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[4] J. Wang, Analytical Electrochemistry, 2nd ed., Wiley, New York 2000. [5] J.-M. Zen, I-L. Chen, Electroanalysis 1997, 9, 537. [6] J.-M. Zen, P.-J. Chen, Anal. Chem. 1997, 69, 5087. [7] J.-M. Zen, C.-B. Wang, J. Electroanal. Chem. 1994, 368, 251. [8] J.-M. Zen, A. Senthil Kumar, J.-C. Chen, Anal. Chem. 2001, 73, 1169. [9] J.-M. Zen, A. Senthil Kumar, J.-C. Chen, J. Mol. Catalysis A 2001, 165, 177. [10] J.-M. Zen, A. Senthil Kumar, Acc. Chem. Res. 2001, 34, 772 and references therein. [11] H. S. Horowitz, J. M. Longo, J. Lewandowski, U.S. Patent 4,129,525 1978. [12] W. F. Jaynes, J. M. Bigham, Clay & Clay Miner. 1987, 35, 440; A.Fitch., Clay & Clay Miner. 1990, 38, 391. [13] J.-M. Zen, C.-W. Lo, P.-J. Chen, Anal. Chem. 1997, 69, 1669. [14] M. E. G. Lyons, in: Advances in Chemical Physics Polymeric Systems (Eds: I. Prigogine, S. A. Rice), Wiley, New York 1996, p. 297. [15] A. Senthil Kumar, J.-M. Zen, Electroanalysis 2002, 14, 671. [16] C. P. Andrieux, J. M. Saveant, J. Electroanal. Chem. 1978, 93, 163. [17] C. P. Andrieux, J. M. Saveant, in Techniques of Chemistry Series, Vol. XXII, Molecular Design of Electrode Surfaces (Ed: R. W. Murray), Wiley, New York 1992, pp. 207 ± 270. [18] J.-M. Zen, D.-M. Tsai, A. Senthil Kumar, V. Dharuman, Electrochem. Commun. 2000, 2, 782. [19] J.-M. Zen, D.-M. Tsai, A. Senthil Kumar, Electroanalysis 15, 1171.

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