Electrochemistry Communications 13 (2011) 174–177

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Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m

Screen-printed ionic liquid/preanodized carbon electrode: Effective detection of dopamine in the presence of high concentration of ascorbic acid Jen-Lin Chang a, Guor-Tzo Wei b, Jyh-Myng Zen a,⁎ a b

Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan Department of Chemistry and Biochemistry, National Chung Cheng University, 168 University Road, Min-Hsiung, Chiayi 62102, Taiwan

a r t i c l e

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Article history: Received 11 November 2010 Received in revised form 2 December 2010 Accepted 2 December 2010 Available online 13 December 2010 Keywords: Ionic liquid Dopamine Screen-printed electrode

a b s t r a c t Screen printing technology was utilized to print a thin layer (~1 μm) of hydrophobic room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), onto a disposable screenprinted carbon electrode (SPCE) to prepare a relatively easy to mass produce RTIL-modified electrode. In contrast to other RTIL composite electrodes using RTIL as the binder, our method can highlight the extraction ability of RTIL to improve the detection performance in the monitoring of biomolecules. In this study, we demonstrate that the screen-printed [BMIM][PF6]/preanodized SPCE (designated as SPIL–SPCE*) possesses high sensitivity and good selectivity for the detection of dopamine in the presence of high concentration of ascorbic acid. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Recently, room temperature ionic liquid (RTIL)-modified electrodes have been widely investigated due to their great applications in many fields. Generally, RTIL is used as a binder to form a gel with graphite, carbon nanotube, carbon fiber or graphitic sheets through grinding or sonicating for mixing RTIL and carbon materials to prepare the modified electrodes [1–12]. To act as a binder, the amount of RTIL on the surface of these electrodes is actually rather limited. It is well-known that RTIL is promising as an alternative of volatile organic solvents for use in liquidliquid extraction [13–17] and also as membrane solution [18,19]. To highlight this advantage of RTIL and also for ease of mass production, we present here a simple method of fabricating a new type of RTIL-modified electrode. Screen printing technology is utilized to print a thin layer of hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) onto a screen-printed carbon electrode (SPCE). The [BMIM][PF6] was reported as a better dressing agent to improve sensitivity, linearity and stability in carbon paste electrode [20,21]. This thin RTIL layer is expected to improve the detection performance in monitoring of biomolecules. Our previous work reported the application of a disposable preanodized screen-printed carbon electrode (SPCE*) for simultaneous determination of dopamine (DA), uric acid and ascorbic acid (AA) [22]. Improved detection selectivity and sensitivity is achieved by the substantial increase in surface bound carbon–oxygen functional groups and the generation of edge plane sites through surface

⁎ Corresponding author. Tel.: +886 4 22850864; fax: +886 4 22854007. E-mail address: [email protected] (J.-M. Zen). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.12.006

reorientation at the SPCE*. Note that, sensitive and selective detection of DA in the extracellular fluid of the central nervous system has received much attention due to its importance in brain chemistry [23,24]. However, the main problems are the basal DA concentration is very low (0.01–1 μM) while the concentration of interfering anions such as AA is much higher (about 0.1 mM) and AA and DA are oxidized at overlapping potential at traditional solid electrodes [25– 31]. In this study, we demonstrate that simply by screen-printing an [BMIM][PF6] thin film on the SPCE* can increase the sensitivity and selectivity for detection of DA in presence of high concentration of AA. 2. Experimental DA and AA were obtained from Sigma and used as received without further purification. All other chemicals used are of ACS-certified reagent grade. Aqueous solutions were prepared with doubly distilled deionized water. A 0.1 M, pH 6 phosphate buffer solution (PBS) was used in all electrochemical studies. The [BMIM][PF6] was prepared by slowly adding hexafluorophosphoric acid (1.3 mol) to a mixture of 1-butyl-3-methylimidazolium chloride (1 mol) in 500 mL of water, and then the mixture was stirring for 12 h [16,32]. After stirring, the upper acidic aqueous layer was decanted and the lower IL portion was washed with water until the washings were no longer acidic. Finally, the IL was heated under vacuum at 70 °C to remove any excess water. Electrochemical measurements were performed with a CHI900 electrochemical workstation in a three-electrode cell assembly. A bare SPCE, SPCE*, SPIL–SPCE or SPIL–SPCE* working electrode, an Ag/AgCl, 3 M KCl reference electrode and a platinum auxiliary electrode were used to complete the cell setup. The SPCE with a working area of 0.2 cm2 and a conductive track radius of 2.5 mm was purchased from

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Fig. 1. Typical cyclic voltammograms of 1 mM DA at a bare SPCE (dashed line) and the SPIL–SPCE (solid line) in 0.1 M, pH 6 PBS at a scan rate of 50 mV/s. The insert shows the dependence of log(peak current) vs. log(scan rate).

Fig. 2. Schematic representation of SPIL–SPCE (a), SPCE* (b) and SPIL-SPCE* (c). A) SWV responses of detecting 100 μM DA in the presence of 2 mM AA at a bare SPCE, SPCE* and SPIL– SPCE*. Cyclic voltammograms for the detection of 1 mM DA (B) and 1 mM AA (C) in 0.1 M, pH 6 PBS at a scan rate of 50 mV/s at SPIL–SPCE, SPCE* and SPIL–SPCE*, respectively.

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Fig. 3. (A) SWV responses of 0 (a), 3 (b), 5 (c), 7 (d), 10 (e), 30 (f), 50 (g), 70 (h), 100 (i), 300 (j) and 500 μM (k) DA in 0.1 M, pH 6 PBS in the presence of 1 mM AA. (B) Calibration plot. (C) Reproducibility of SWV responses for continous detection of 3 μM DA in the presence of 1 mM AA.

Zensor R&D (Taichung, Taiwan). The measured average resistance was 85.64 ± 2.10 Ω/cm. The SPCE* was prepared by applying a potential of 2.0 V for 200 s in 0.1 M PBS (pH 6) and subsequently washed gently with deionized water and then air-dried [33]. The RTIL-modified electrode was fabricated by screen-printing an appropriate volume of [BMIM][PF6] on a bare SPCE or SPCE* and then placed into the oven at 70 °C in the air for 20 min. The thickness of the [BMIM][PF6] layer on the RTIL-modified electrode was measured as ~ 1 μm deep by optical microscopy. 3. Results and discussion Fig. 1 compares the cyclic voltammetric responses of DA at a bare SPCE and the SPIL–SPCE. It is interesting that, in addition to the redox peak a2/c2 observed at a bare SPCE, an extra redox peak (i.e., a1/c1) of DA at ~0.24 V was observed only at the SPIL–SPCE. This extra redox peak (a1/c1) should have something to do with the extraction of DA into the [BMIM][PF6] thin film through its liquid–liquid extraction ability. To prove this extraction behavior, we conduct an easy experiment by studying the scan rate effect to the charge transfer process of each peak. Also shown in Fig. 1, the slopes in the plot of log(peak current) vs. log (scan rate) are indeed different for a1/c1 and a2/c2 with values of 0.89/ 0.80 and 0.35/0.51, respectively. These results clearly indicate that, due to the extraction of DA into the RTIL thin film, the electron transfer process is more of adsorption-controlled for the redox peak a1/c1 and diffusion-controlled for the redox peak a2/c2 and provides a strong support for the proper extraction behavior of the [BMIM][PF6] thin film. As mentioned earlier, the goal of this study is to develop an electrode with high sensitivity and good selectivity for the detection of DA in the presence of high concentration of AA. Fig. 2A compares the square wave voltammetric (SWV) responses at a bare SPCE, SPCE* and SPIL–SPCE* for detecting 100 μM DA in the presence of 2 mM AA. As can be seen, AA is oxidized at a potential close to that of DA resulting in an overlapping voltammetric response at a bare SPCE. Although the peaks for DA and AA were well-resolved from each other at the SPCE* [22], it is still very difficult to detect DA in the presence of at least 100 time in excess concentration of AA in real world application. This situation, however, is

greatly improved at the SPIL–SPCE*. The reason for the sensitive sensing of DA at the SPIL–SPCE* is obvious. At pH 6 PBS, AA exists in the anionic form (pKa = 4.10) while DA is in the cationic form (pKb = 8.87). Consequently, the formation of the ion-pair between the cation of DA and the anion of the [BMIM][PF6] facilitates only the extraction of DA to the SPCE*. This advantage of using [BMIM][PF6] to improve the selectivity can be further verified by comparing the cyclic voltammograms of DA and AA at the SPIL–SPCE, SPCE* and SPIL–SPCE*. As shown in Fig. 2B and 2C, a clear increase in peak current of DA and almost invariable peak current of AA was attained at the SPIL–SPCE* compared to those of SPCE*. It should be noted that Nafion-modified electrodes have been of particular interest for DA determination in the presence of AA [34,35]. However, this kind of modified electrode suffers from slow response due to low diffusion coeffcients of analytes in the films. Due to the reversible peak characteristics of DA, the SWV method was adopted to increase the sensitivity for the detection of DA. By scanning the potential from 0.1 to 1.0 V vs. Ag/AgCl at a SW frequency of 2.5 Hz, amplitude of 25 mV and step height of 4 mV, a linear range up to 100 μM with a regression coefficient of 0.990 was obtained. To prove the proposed method is valid for the detection of DA in the presence of high concentration of AA, Fig. 3 illustrates the variation of SWV responses with various concentrations of DA in the presence of 1 mM AA. We obtain a linear calibration plot up to 100 μM with a regression coefficient of 0.993. For 11 determinations of 3 μM DA, a coefficient of variation of 2.94% further indicates good repeatability of the proposed method. The detection limit was calculated as 0.26 μM (S/N= 3). Note that our linear range is broader and detection limit is lower than recently published boron-doped carbon nanotube (0.02–75 μM) and graphene-related electrodes (2.64 μM, 3–100 μM) [36–38]. 4. Conclusion In this investigation, we improve the performance of DA in the presence of high concentration of AA at the SPIL–SPCE*. Compared to those of IL-carbon paste electrodes, the fabrication process of the SPIL– SPCE* is simple and easy for mass production. The electron transfer process is adsorption-controlled for DA at the SPIL–SPCE* indicating a proper extraction behavior of the [BMIM][PF6] thin film. The SPIL–SPCE* highlights the extraction ability of RTIL to improve the detection performance in monitoring of biomolecules. Acknowledgement The authors gratefully acknowledge financial support from the National Science Council of Taiwan. This work was supported in part by the Ministry of Education, Taiwan, under the ATU Plan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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