MOLLIQ-04364; No of Pages Journal of Molecular Liquids xxx (2014) xxx–xxx Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq A highly sensitive electrode modified with graphene, gold nanoparticles, and molecularly imprinted over-oxidized polypyrrole for electrochemical determination of dopamine Do Phuc Tuyen a, Do Phuc Quan a,⁎, Nguyen Hai Binh b, Van Chuc Nguyen b, Tran Dai Lam b,⁎⁎, Le Trong Huyen c, Le Huy Nguyen c, Pham Hung Viet a, Nguyen Thai Loc b, Tran Quang Huy d a Research Centre for Environmental Technology and Sustainable Development, Hanoi University of Science, 334, NguyenTrai Road, Hanoi, Viet Nam Institute of Materials Science, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet Road, Hanoi, Viet Nam c School of Chemical Engineering, Hanoi University of Science and Technology, 1, Dai Co Viet Road, Hanoi, Viet Nam d National Institute of Hygiene and Epidemiology, 1, Yersin Street, Hanoi, Viet Nam b a r t i c l e i n f o Article history: Received March 2014 Received in revised form 20 July 2014 Accepted 21 July 2014 Available online xxxx Keywords: Dopamine Over-oxidized polypyrrole Graphene Gold nanoparticles a b s t r a c t A glassy carbon electrode (GCE), modified with graphene (Gr) films, molecularly imprinted (MIP) over-oxidized polypyrrole (OPPy), and electrochemically de posited gold nanoparticles (AuNP) was used for rapid detection of dopamine (DA) The as-prepared electrode (AuNP/Gr/OPPy-MIP/GCE) was characterized by Raman spectroscopy and atomic force microscopy (AFM) Electrochemical determination of DA was conducted via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) The results showed that the modified electrodes exhibited high electrocatalytic activity towards the oxidation of DA in 0.1 M phosphate buffer solution (pH 6.9) Linear dependency of electrocatalytic oxidation current on DA concentrations was observed from 0.5 μM to μM and detection limit was estimated to be 0.1 μM (S/N = 3) The developed electrode could serve as a viable platform for further studies on in vivo detection of DA © 2014 Elsevier B.V All rights reserved Introduction Dopamine (DA) is a neurotransmitter which belongs to the family of the catecholamine and phenethylamine Dopamine plays an important role in the functions of the central nervous, cardiovascular, renal and hormonal systems and Parkinson's disease [1] Several techniques have been developed for DA determination [1,2], but their sensitivity and selectivity are still not satisfactory Generally, the performances of these methods are adversely affected by interferences of other active biomolecules such as ascorbic acid (AA) and uric acid (UA) which coexist in biological fluids Recently, electrochemistry-based analyses have been attempted for determination of DA Nevertheless, these approaches are hindered by the similar electrochemical behaviors of DA, AA and UA [3] For this reason, research challenges are to obtain clear separation of the electrochemical signals of these three compounds The concept of modified electrodes using different materials is an exciting development in electroanalytical chemistry Combination of various materials such as organic redox mediators, nanoparticles, polymers, self-assembled monolayers and graphene have been ⁎ Corresponding author Tel.: +84 38588152; fax: +84 438587964 ⁎⁎ Corresponding author Tel.: +84 37564129; fax: +84 438360705 E-mail addresses: doquan@vnu.edu.vn (P.Q Do), lamtd@ims.vast.ac.vn (D.L Tran) employed in the modification of electrodes for DA determination [4–11] Among these materials, graphene (Gr) has attracted considerable attention due to high electrical conductivity, surface-tovolume ratio, electron transfer rate and exceptional thermal stability [12] The unique and superior properties of Gr have been extensively exploited for the fabrication of electrochemical sensors and biosensors [13] Gold nanoparticle (AuNP) is another material which has been widely used in electrochemical analyses due to its excellent catalytic activity, mass transport and high effective surface area [14,15] The technology of molecular/ionic imprinting in networked polymers has provided a new generation of recognition materials, capable of competing with biological specific receptors [16] Molecularly imprinted polymer (MIP) is formed by the copolymerization of functional monomers and crosslinkers in the presence of a target molecule The extraction of molecular template creates a complementary cavity with chemical affinity for the original molecule Mechanistically, this is similar to antigen–antibody or ligand–receptor interactions of key-lock recognition Polypyrrole (PPy) is one of the conducting polymers which has many attractive features as a molecular recognition system [17,18] PPy can be over-oxidized at high positive potential and when the doping ions are expelled, PPy lost its conductivity due to introduction of oxygen-containing groups into its backbone [19,20] Such over-oxidation may result http://dx.doi.org/10.1016/j.molliq.2014.07.029 0167-7322/© 2014 Elsevier B.V All rights reserved Please cite this article as: P.T Do, et al., A highly sensitive electrode modified with graphene, gold nanoparticles, and molecularly imprinted overoxidized polypyrrole for e , Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.029 P.T Do et al / Journal of Molecular Liquids xxx (2014) xxx–xxx in carbonyl and carboxylic groups being incorporated into the PPy backbone which may attract electropositive groups (\NH+ ) of DA and repel anionic molecules like AA Consequently, it can be expected that OPPy film obtained by MIP technique has better cation exchange and molecular sieve properties as compared to conventional PPy This would enhance selectivity and sensitivity of the material towards DA detection The decreased conductivity of OPPy modified electrodes is to be compensated by the combination of AuNP and Gr layers which possibly helps improve the sensitivity of the electrochemical sensor for DA detection The objectives of this study were a) to investigate a new design of the glassy carbon electrode modified by graphene, gold nanoparticles, and particularly molecularly imprinted overoxidized polypyrrole and b) to apply the as-prepared electrode for electrochemical determination of dopamine using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) Cu substrate Graphene/Cu PMMA/Graphene/Cu Etching Cu Experimental 2.1 Chemicals and reagents Pyrrole (Py) was purchased from Fluka (Switzerland) Ascorbic acid (AA), dopamine hydrochloride (DA.HCl), uric acid (UA) and other chemicals were purchased from Sigma (Germany) and used without further purification All other reagents were of analytical reagent grade All solutions were de-aerated with purified nitrogen prior to use Removing PMMA Electrochemical sensor 2.2 Apparatus and measurement Electropolymerization of PPy and electrochemical measurements were performed using Autolab PGSTAT-30 Potentiostat/Galvanostat with GPES software (EcoChemie, The Netherland) The three-electrode system was employed in which reference electrode was based on Ag/ AgCl/saturated KCl; auxiliary electrode was Pt wire and working electrodes were either bared GCE or modified GCE Electrochemical detection for DA was performed in 0.1 M phosphate buffer solution (PBS, pH 6.9) by using CV and DPV techniques The DPV parameters were 50 mV pulse amplitude, a scan rate of 50 mV/s and the potential range from −0.2 to 0.5 V All aqueous solutions were prepared with DI water of 18 MΩ·cm resistivity Prior to the electropolymerization of PPy-MIP, the surface of the GCE was polished with 15 μm and 0.3 μm alumina slurry and cleaned by sonication in DI water 2.3 Preparation of modified electrodes 2.3.1 Preparation of Gr/GCE Graphene films (Gr) were prepared by CVD method at growth temperature of 1000 °C in Ar environment The Gr film was then transferred to glassy carbon electrode by chemical etching (Fig 1) Briefly, a thin layer of polymethyl methacrylate (PMMA) was pre-coated on top of obtained Gr films on Cu tapes Subsequently, the samples were annealed at 180 °C in air for The Gr/PMMA films were released from the Cu tapes by chemical etching of the underlying Cu in Fe(NO3)3 solution Then, suspended films were transferred to deionized water to remove the residuals of Cu etching stages Next, Gr/PMMA films were transferred onto electrode surfaces Gr transfer process was improved by using a second PMMA coating step after the Gr/ PMMA being placed on the substrate [21] Finally, the PMMA films were dissolved by acetone and the samples were cleaned by rinsing several times in deionized water The crystalline quality and thickness of Gr layers were characterized by Raman spectroscopy and atomic force microscopy, respectively Fig Schematic representation of Gr transfer process onto GCE 2.3.2 Preparation of AuNP/Gr/OPPy-MIP/GCE DA-imprinted PPy was electrochemically synthesized on Gr/GCE in aqueous solution consisting of mM DA, 10 mM Py and 0.1 M KCl using cyclic voltammetry The polymerization was performed in five cycles with potential ranging from 0.0 to 0.80 V, versus Ag/AgCl (sat KCl) and scan rate of 10 mV/s The template molecules DA were removed by immersing the MIP-PPy in 95% ethanol and DI water for 60 The over-oxidation of DA-imprinted PPy films was conducted using ten cycles of cyclic voltammetry from 0.0 to 1.0 V (vs Ag/AgCl), at a scan rate of 10 mV/s in 0.5 M KOH Gold nanoparticles (AuNP) were electrochemically deposited on the OPPy surface using 10 potential scans from 0.0 to 1.1 V in 2.5 mM HAuCl4 and 0.1 M KNO3 solution at a scan rate of 50 mV/s The preparation of AuNP/Gr/OPPy-MIP/GCE electrodes, involving four main steps (except Gr transfer onto GCE) is shown in Fig Results and discussion 3.1 Characterization of modified electrodes The Gr films were characterized with Raman spectroscopy, a powerful method to determine the crystalline quality of Gr layers (Fig 3) It was evidenced that I2D/IG depended on the number of Gr layers (monolayer: the ratio I2D/IG ~ 2–3; bilayer: b I2D/IG b 2; multilayer: I2D/IG b 1) [12,13] In this study, the ratio I2D/IG obtained was 0.44, indicating that the Gr film grown on the Cu tape had multilayered structure Raman spectra (632.8 nm excitation laser) of Gr transferred on the electrode exhibit three peaks at ~ 1333, ~ 1582 and ~2660 cm−1, in which the latter is assigned to 2D characteristic peak of Gr It is worth mentioning that the intensity of the D-band at ∼ 1333 cm− 1, an important Please cite this article as: P.T Do, et al., A highly sensitive electrode modified with graphene, gold nanoparticles, and molecularly imprinted overoxidized polypyrrole for e , Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.029 P.T Do et al / Journal of Molecular Liquids xxx (2014) xxx–xxx indicator of defects in the Gr, is quite low, demonstrating the successful transfer of Gr onto the given electrode surface One of the main drawbacks of the Gr transfer process is the incomplete contact between Gr and substrate surface The unattached regions tend to break easily when the PMMA coating is dissolved When an appropriate amount of liquid PMMA was dropped on the cured PMMA layer, dissolution of the pre-coated PMMA was expected to minimize formed cracks/defects As a result, the contact of Gr with the electrode surface was improved [12,13,20] AFM image shows that the thickness of the Gr film is about 5–6 nm (Fig 4) which meant that the grown Gr film consists of about 15 layers DA-imprinted PPy was synthesized by electrochemical polymerization (Fig 5A) The affinity of template molecules DA and OPPy network were previously ascribed to interactions of hydrogen bonds [16,17,19] The over-oxidation of DA-imprinted PPy films is illustrated in Fig 5B During the over-oxidation of the OPPy film, the formation of carbonyl and carboxylic groups could provide a better permselectivity for DA with the electrostatic force [19] The CVs describing deposition of AuNP on OPPy surface are given in Fig 5C The cathodic peak at ~0.3 V exhibits the deposition of AuNP onto the electrode surface The voltammetric response of AuNP-modified electrode in 50 mM K4Fe(CN)6 and 0.1 M KCl solution presented the strongest reversible redox peak current of ferrocyanide/ferricyanide couple after 10 cycles It suggests that AuNP electrodeposited onto the OPPy surface rendered OPPy better conductive On the other hand, compared with the Gr/GCE, the values of ΔEp decreased significantly at the modified AuNP/Gr/ OPPy-MIP/GCE (from 155 to 84 mV) This downshifting demonstrated that the introduction of AuNP in AuNP/Gr/OPPy-MIP/GCE played an important role in providing the conducting bridges for the electron transfer of ferrocyanide/ferricyanide couple, leading to enhanced electron transfer kinetics of AuNP/Gr/OPPy-MIP/GCE as compared to the Gr/GCE 3.2 Electrochemical detection of DA Cyclic voltammograms of AuNP/Gr/OPPy-MIP/GCE in 0.1 M PBS containing DA were subjected to various scan rates and were shown in Fig 6A Both the values of anodic peak current (Ipa) and cathodic peak current (Ipc) exhibited linear relationship with scan rate over the range of 10–125 mV/s The linear regression equations, Ipa (μA) = − 5.53 + 2.3v 1/2 (mV/s)1/2 (R = 0.975) and I pc (μA) = + 4.77 − 1.7v 1/2 (mV/s) 1/2 (R = 0.981) were obtained (inset in Fig 6A) These results imply that the electrocatalytic reaction is diffusion controlled, which is ideal for quantitative analysis in practical applications The typical DPV calibration curve at the AuNP/Gr/OPPy-MIP/GCE for DA detection in the concentration range from 0.54 to μM is shown in Fig 6B The current response of the modified electrode showed a linear dependence on DA concentration with ipa (μA) = − 0.16 + 1.76 × C DA (μM), (R = 0.990) It is well known that DA, AA and UA can be oxidized at practically the same potential at bare electrodes, resulting in the peak overlapping as well as poor Fig Schematic fabrication process of AuNP/Gr/OPPy-MIP/GCE and its recognition for DA Fig Raman spectra of the synthesized Gr films Please cite this article as: P.T Do, et al., A highly sensitive electrode modified with graphene, gold nanoparticles, and molecularly imprinted overoxidized polypyrrole for e , Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.029 P.T Do et al / Journal of Molecular Liquids xxx (2014) xxx–xxx Fig The AFM image of the Gr films response resolution in DA determination Therefore, in this study, UA and AA were chosen as the interfering molecules to evaluate the DA selectivity of the electrodes The CVs of 0.01 mM DA at the Gr/OPPy-MIP/GCE and AuNP/Gr/OPPy-MIP/GCE in the presence of mM AA and 50 μM UA were studied (Fig 7A) There were only electrochemical signals of DA and UA in the cyclic voltammograms at 0.135 V and 0.281 V (vs Ag/AgCl), respectively The oxidation peaks of DA and UA were clearly separated from each other for about 146 mV, signifying that anions such as AA can be rejected by OPPy-MIP films The CV results indicated that OPPy-MIP was selective 120 100 140 A 80 oxidation 1st cycle 100 PPy(DA) I / A Py(DA) I / A B 120 60 40 80 60 40 20 20 2nd - 10th cycles -20 0.0 0.2 0.4 0.6 0.8 0.0 0.2 E / V vs Ag/AgCl 500 C 60 100 0.8 1.0 Gr/GCE AuNP/Gr/OPPy-MIP/GCE 40 200 Au D cycle Au I / A I / A nd 3rd cycle 300 0.6 E / V vs Ag/AgCl 1st cycle 400 0.4 3+ 20 0 -20 -100 Au -200 3+ Au -40 -0.4 0.0 0.4 E / V vs Ag/AgCl 0.8 1.2 -0.4 -0.2 0.0 0.2 0.4 0.6 E / V vs Ag/AgCl Fig (A) Electrochemical preparation of DA-imprinted PPy by CVs in Py(DA) solution (B) Over-oxidation of DA-imprinted PPy by CVs in 0.5 M KOH solution (C) Electrochemical deposition of AuNP on OPPy film (D) CVs of 50 mM K4Fe(CN)6 and 0.1 M KCl at the Gr/GCE (black curve), and AuNP/Gr/OPPy-MIP/GCE (red curve), respectively The scan rate is 50 mV/s Please cite this article as: P.T Do, et al., A highly sensitive electrode modified with graphene, gold nanoparticles, and molecularly imprinted overoxidized polypyrrole for e , Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.029 P.T Do et al / Journal of Molecular Liquids xxx (2014) xxx–xxx A 10 mV/s 30 mV/s 50 mV/s 70 mV/s 90 mV/s 125 mV/s 30 = -5.53 + 2.37v pa 1/2 R2 = 0.975 A R = 0.981 Ipa = -4.77 + 1.70v1/2 -10 10 12 v1/2 / (mV/s)1/2 10 0 -2 -4 -10 -6 -20 -8 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 AuNP/Gr/OPPy-MIP/GCE Gr/OPPy-MIP/GCE -10 -0.4 0.5 -0.2 E /V vs Ag/AgCl Ip / A 8,00 M 4.70 M I / A 18 Ip = -0.16 + 1.76 CDA 0.8 0.6 0.8 2.0 M DA 14 2.60 M CDA / M 1.10 M 12 10 0.54 M 50 M UA -0.4 0.6 3.5 M DA 16 1.60 M 0.4 5.0 M DA B R2 = 0.990 15 12 0.2 16 B 12 18 0.0 E / V vs Ag/AgCl I /A 21 UA DA I /A I /A 20 I 20 10 I /A 40 -0.2 0.0 0.2 0.4 0.6 -0.4 -0.2 E / V vs Ag/AgCl Fig (A) Electrochemical response of 10 μM DA in 0.1 M PBS at AuNP/Gr/OPPy-MIP/GCE with different scan rates (10, 30, 50, 70, 90 and 125 mV/s) Inset: Ipa-v1/2 and Ipc-v1/2 plots (B) DPVs of mixture solution of different concentration of DA in 0.1 M PBS at AuNP/Gr/ OPPy-MIP/GCE Inset: The linear regression curve of peak current vs DA concentration 0.0 0.2 0.4 E / V vs Ag/AgCl Fig (A) CVs for mM AA, 50 μM UA and 10 μM DA, in 0.1 M PBS at AuNP/Gr/OPPy/GCE The scan rates is 50 mV/s (B) DPVs of mixture solution of different concentrations (2.0, 3.5 and 5.0 μM) of DA, mM AA, 50 μM UA in 0.1 M PBS at AuNP/Gr/OPPy-MIP/GCE Conclusion for DA in the presence of AA and UA Furthermore, the current responses of the AuNP/Gr/OPPy-MIP/GCE were much better than those of the Gr/ OPPy-MIP/GCE, which confirmed the role of AuNP in the overall performance of the modified AuNP/Gr/OPPy-MIP/GCE electrodes DPV responses of electrochemical signals to different DA concentrations in the presence of mM AA and 50 μM UA are demonstrated in Fig 7B While Ipa increased linearly with varying concentration of DA, no obvious changes in the peak current of UA were observed Therefore, with the modified electrode AuNP/Gr/OPPy-MIP/GCE, interferences of AA and UA during DA determination were minimized In order to examine the matrix effect in biological fluid samples such as rabbit serum and urine, the standard addition method has been used The rabbit serum and urine samples were diluted with 0.1 M PBS solution (ratio 1:1) and were analyzed for DA determination using AuNP/Gr/OPPy-MIP/GCE The obtained results, summarized in Table 1, clearly indicated that DPV method with AuNP/Gr/ OPPy-MIP/GCE was reliable for determining DA in the real sample matrix The selectivity of developed electrode was also studied by comparing the electrochemical responses of DA-imprinted PPy-MIP modified electrodes with those of non-imprinted polymer (PPy-NIP) modification The current responses of the AuNP/Gr/OPPy-MIP/GCE were considerably higher than those of the AuNP/Gr/OPPy-NIP/GCE Thus, imprinting effect produced in the presence of DA helped impart excellent selective recognition capacity of AuNP/Gr/OPPy-MIP/GCE towards the template molecule (Fig 8) In this study, DA-imprinted modified electrodes of AuNP/Gr/OPPyMIP/GCE have been successfully produced and characterized The electrochemical behaviors of the above electrodes were fully investigated The prepared electrode showed a good recognition capacity for template molecule (DA) in the presence of other structurally similar molecules (AA, UA) A linear relationship between the DA concentration (from 0.5 μM to μM) and the current response was obtained with good reproducibility and a low detection limit of 10−7 M Acknowledgments This work was supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant no 104.07.108.09 Table Determination of DA in rabbit serum and urine samples (n = 5) Sample DA added (μM) Found (μM) R.S.D (%) Recovery (%) Rabbit serum 3.0 5.0 3.0 5.0 2.818 4.492 3.114 4.881 4.2 4.8 1.9 2.1 93.9 89.8 103.8 97.6 Rabbit urine Note: R.S.D: relative standard deviation Please cite this article as: P.T Do, et al., A highly sensitive electrode modified with graphene, gold nanoparticles, and molecularly imprinted overoxidized polypyrrole for e , Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.07.029 P.T Do et al / Journal of Molecular Liquids xxx (2014) xxx–xxx 10 M DA 50 M UA I / A AuNP/Gr/OPPy-MIP/GCE AuNP/Gr/OPPy-NIP/GCE Electrodes Fig Comparison of the DPV peak currents for AuNP/Gr/OPPy-MIP/GCE and AuNP/Gr/ OPPy-NIP/GCE References [1] J Kulisevsky, Drugs Aging 16 (5) (2000) 365–379 [2] B.W Dunlop, C.B Nemeroff, Arch Gen Psychiatry 64 (3) (2007) 327–337 [3] X Cao, L Luo, Y Ding, X Zou, R Bian, Sensors Actuators B Chem 129 (2) (2008) 941–946 [4] S.-Y Yi, H.-Y Chang, H.-H Cho, Y.C Park, S.H Lee, Z.-U Bae, J Electroanal Chem 602 (2) (2007) 217–225 [5] J Huang, Y Liu, H Hou, T You, Biosens Bioelectron 24 (4) (2008) 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were clearly separated from each other for about 146 mV, signifying that anions such as AA can... R.S.D: relative standard deviation Please cite this article as: P.T Do, et al., A highly sensitive electrode modified with graphene, gold nanoparticles, and molecularly imprinted overoxidized polypyrrole