Highly Selective Dopamine Sensor based on an ...

Procedia Chemistry Procedia Chemistry 1 (2009) 285–288 www.elsevier.com/locate/procedia

Proceedings of the Eurosensors XXIII conference

Highly Selective Dopamine Sensor based on an Imprinted SAM/Mediator Gold Electrode P.Y. Chena, P.C. Niena, and K.C. Hoa,b,* b

a Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

Abstract A new concept of molecular imprinted polymer (MIP) electrode was proposed for amperometric detection of dopamine (DA). The MIP electrode was fabricated based on a screen-printed gold electrode. Thioglycolic acid (TGA) and allyl mercaptan (AM) were bounded on the gold electrode by covalent bonding to control the TGA density. The mediator, quercetin (Q), was also covalently bonded to the carboxyl group of TGA by using the carboxyl activation agent: 1-ethyl-3-(3-dimethylamino-propyl) carbodiimide (EDC). Molecularly imprinted poly(methyl methacrylate) (PMMA) was polymerized by UV curing. Different surface distribution densities of TGA were studied for realizing the recognition abilities of MIP electrodes. The TGA density was well controlled on the gold surface and the DA recognition ability was correlated to it. Keywords: Dopamine; Molecularly imprinted polymer; Quercetin; Self-assembly monolayer; Sensor

1. Introduction Molecular imprinting technique is a very useful approach to fabricate a polymer matrix with molecular recognition sites, which are formed by the addition of template molecules during polymerization process. Molecularly imprinted polymers (MIPs) have been developed for more than two decades in many fields, such as chromatography [1], catalyst [2], drug delivery [3], artificial antibody [4], and sensing devices [5]. In the early years of MIP development, the applications of liquid chromatography were most concerned [6]. In this field, MIP particles were used as the stationary phase for liquid chromatography, and the function was to improve the separation efficiency, especially for the enantiomers [7]. The detection of the adsorbed molecules in imprinted polymers could be achieved by electrochemical [5], piezo-electric [8], and optical methods [9]. Among those methods, electrochemical method, especially the amperometric one, probably is the easiest and most economic way to fabricate a commercial sensor. In our previous study, molecularly imprinted sensor fabricated by a self-assembly monomer (SAM)/mediator system was fabricated successfully [10]. The selectivity of the MIP sensor was very sensitive to ascorbic acid but not

* Corresponding author. Tel.: +886-2-2366-0739; fax: +886-2-2362-3040. E-mail address: [email protected].

1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.071

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sensitivity for some structurally similar compounds, such as methyldopa (MDA) and isoproterenol (ISO). In this study, we introduce different thioglycolic acid (TGA) densities to enhance the selectivity of the MIP sensors toward these structurally similar compounds. 2. Experimental

2.1. Reagents and Apparatus Methyl methacrylate (MMA, 99%), methyldopa (MDA, 99%), isoproterenol (ISO, 98%), potassium chloride (KCl, ≧99%), thioglycolic acid (TGA, ≧98%), allyl mercaptan (AM, ≧98%), benzophenone (BPO, ≧99%), and quercetin (Q, ≧ 95%) were purchased from Aldrich (USA). Dopamine (DA, 99%), methanol and phosphate buffered saline (PBS) tablets were obtained from Sigma (USA). Deionized water (>18 MΩ) was produced by Purelab Maximum (ELGA). All chemicals were analytical reagent grade, and used as received. Cyclic voltammetry and amperometric measurements were carried out using potentiostat/galvanostat (CH Instruments, model CHI 440) and the compatible software. Long wavelength ultraviolet light (365 nm) was used to photopolymerize MMA monomer in this study. For photopolymerization, UV curing machine model SB-125 (Spectroline), combining with a UV lamp model BLE-125B (Spectroline), was used. The screen printed gold electrode was purchased from DropSens Company (Spain). 2.2. Fabrication of the MIP Au/TGA/Q electrode Bare screen printed gold electrodes were immersed in ethanol solutions which contained different AM to TGA ratio (0, 4, and 8) for 24 h. After the reaction, thiols were bonded covalently on the gold surface and the electrodes were then washed by deionized water and ethanol to remove the residues. TGA and AM were chosen simply because they have shorter carbon chain length, thus exhibiting better electron transfer rate for amperometric sensing than those of longer ones. For activating the carboxyl group of TGA monolayer, an ethanol solution containing 0.5 mM of 1-ethyl-3-(3-dimethylamino-propyl) carbodiimide (EDC) was used as the coupling agent for 1 h. Furthermore, 1.4 mM of Q was added to react with activated carboxyl groups of TGA for 24 h, and obtained an Au/TGA/Q electrode. The experimental conditions to prepare the immobilized Au/TGA/Q electrode were reported previously [11], and modified slightly by us in this study. It is generally observed that some analytes would adsorb on the surface of the conventional electrodes when performing the electrochemical detection, thus leading to the fouling of the electrode. However, in our case, fouling of Q is not a problem during the redox process since Q was already covalently immobilized on the electrode surface. The Au/TGA/Q electrode was stored in ethanol solution and controlled at 4°C when not in use. The MIP-Au/TGA/Q electrode was made by dipping the Au/TGA/Q electrode in a 5 mM of DA for 30 min during pre-adsorption, and then the MMA monomer and 3 wt% of BPO were spin-coated on the electrode at 6,000 rpm for 40 s in order to control the thickness of PMMA layer. The polymerization of MMA was done by applying the UV light (365 nm) on the electrode for 3 min. Finally, the template molecule (DA) was removed by exposing the electrode to a deionized water flux, since DA is highly dissolved in water. 3. Results and discussion

3.1. Amperometric detection based on MIP electrodes Before amperometric detection, the detection potential must be determined using a polarization curve. The linear sweep voltammetry (LSV) could obtain the potential range within which the electrochemical reaction is under the diffusion control. In this region, the current response would depend only on the concentration of the sensing target. All sensing potentials were set at 0.45 V, as determined by the LSV (not shown here). The selectivity against the ascorbic acid was studied in our previous work [10]. The amperometric detection data at bare gold electrode and MIP electrodes with different molar ratio of AM to TGA are shown in Table 1. The result reveals that the current response of DA decreases sharply when the molar ratio of AM to TGA was increased to the level of 8. The micro-

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cavities on the MIP electrodes become more obvious when the decreased currents of MDA and ISO were observed. Furthermore, the distance between reactive thiol, TGA, would be too close when the molar ratio of AM to TGA was too low. In this case, the current response of DA on the MIP electrode with smaller AM/TGA molar ratio would higher than that of the MIP electrode with larger AM/TGA molar ratio. On the other hand, the current responses of MDA and ISO were decreased with the increase of AM to TGA molar ratio. The evidence is solid that the selectivity would be improved by decreasing the density of TGA. According to the results, the MIP electrode show better selectivity. The definition for the selectivity and the sensing performance will be discussed in the next section.

(a)

(b)

(ii)

(i)

: AM ;

: TGA

Fig. 1. (a) The sketch of the fabrication process of MIP-Au/TGA/Q electrode; (b) Different densities of TGA. (i): High density of TGA leading to the loss of its recognition cavities. (ii): Low density of TGA, showing AM intervened between TGA.

Table 1. Current response for each compound detected by electrodes with different AM/TGA molar ratios. Compound MDA DA ISO

I (Au) (A/cm2) 48.4 52.4 44.3

I (MIP)(A/cm2) AM/TGA=0 44.3 50.5 12.0

I (MIP) (A/cm2) AM/TGA=4 22.2 58.6 ~0

I (MIP) (A/cm2) AM/TGA=8 12.3 37.4 ~0

3.2. Structural recognition ability The selectivity is defined as the sensitivity of target molecule divided by the sensitivity of interference corresponds to the same MIP-electrode. Higher selectivity reveals better performances in selection of target molecule against the interference. Fig. 2 shows the amperometric i-t curves for DA, MDA and ISO with increasing each concentration by using the MIP electrode at a molar ratio of AM to TGA of 8. The sensitivities for DA, MDA and ISO were 697.1, 188.2 and 4.7 mA cm-2 M-1, respectivitly. The limit of detection (LOD) was about 4.3 M (S/N ≧ 3). The selectivities of MDA and ISO were 3.7 and 147.4, respectively. The result shows that the MIP electrode possesses high selectivity against ISO, but lower against MDA. This result suggested that MDA can be partially recognized by the MIP modified electrode, because both the size and the functional group are similar to those of DA. On the other hand, the MIP modified electrode was able to exclude ISO, presumably due to its larger molecule size which is hard to fit into the cavities. 4. Conclusion The MIP-Au/TGA/Q electrode has been fabricated by a self-assembly monolayer technique and the effect of TGA density was discussed. The MIP electrodes were proposed for the amperometric detection. The selectivity of the MIP-Au/TGA/Q electrodes was highly related to the density of TGA. The MIP-Au/TGA/Q electrode possesses highest selectivity at the molar ratio of AM to TGA of 8. The sensitivity and the LOD of MIP electrode were 697.1 mA cm-2 M-1 and 4.3 M, respectively. As for the selectivity, the MIP-Au/TGA/Q electrode is more selective against ISO as compared to that of MDA.

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Current density (A/cm )

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MIP-Au/TGA/Q electrode (AM/TGA=8) DA MDA ISO

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Fig. 2. The amperometric i-t curves of MIP-Au/TGA/Q electrode at a AM to TGA molar ratio of 8.

Acknowledgements This work was supported by the National Research Council of the Republic of China (Taiwan) under grant numbers NSC 96-2220-E-006-015 and NSC 97-2220-E-006-008.

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