poly-l -lysine modified glassy carbon electrode as an electrochemical sensor for the determination of dopamine in the presence of ascorbic acid

Journal of Electroanalytical Chemistry 759 (2015) 113–121

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Graphene oxide-Ag/poly-L-lysine modified glassy carbon electrode as an electrochemical sensor for the determination of dopamine in the presence of ascorbic acid Zhuo Guo a,⁎, Guo-qing Huang a, Jian Li b, Ze-yu Wang a, Xian-feng Xu a a b

Department of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA

a r t i c l e

i n f o

Article history: Received 31 July 2015 Received in revised form 29 October 2015 Accepted 1 November 2015 Available online 4 November 2015 Keywords: Graphene oxide Ag nanoparticles Poly (L-lysine) (PLL) Dopamine Ascorbic acid

a b s t r a c t The graphene oxide (GO) and Ag hybrid matrix GO-Ag were coated onto the glassy carbon electrode (GCE) surface, then, poly-L-lysine films (PLL) were prepared by electropolymerization with cyclic voltammetry (CV) method to prepare GO-Ag/PLL modified glassy electrode (GO-Ag/PLL/GCE). The electrochemical sensor based on GO-Ag/PLL/GCE was used to sensitively determine dopamine (DA) in the presence of ascorbic acid (AA). Electrochemical behaviors of DA and AA mixture were investigated on GO-Ag/PLL/GCE by CV, which gave a baseline separation of the oxidation peak potential by 232 mV. So the modified electrode was suitable for determine of DA in the presence of AA. The GO-Ag/PLL/GCE electrode improved the DA electrochemical catalytic oxidation, which demonstrates that GO-Ag/PLL/GCE has a remarkable electrocatalytic activity towards the oxidation of DA. Moreover, PLL modified electrodes have good stability, excellent permselectivity, more active sites and strong adherence to electrode surface, which enhanced electrocatalytic activity. Under the optimized conditions, there were linear relationships between the peak currents and the concentrations in the range of 0.1–10 μM for DA, with the limit of detection (LOD) (based on S/N = 3) of 0.03 μM for DA. The GO-Ag/PLL/GCE sensor was successfully applied to the determination of DA in bovine serum samples and showed high selectivity, sensitivity, and reproducibility. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Dopamine (DA) is a kind of import catecholamine neurotransmitters in the mammalian central nervous system [1–3]. It implicates many human behaviors for example motivation, motor function, reward, and cognition, and plays an important role in memory and learning [4,5]. Many research studies show that abnormal levels of DA may cause neurological disorder such as senile dementia, HIV infection, schizophrenia and Parkinson's disease [6–8]. So the accurate and rapid determination of DA concentration in body fluid is a significant issue to conveniently trace and diagnose such diseases in clinical practice. However, ascorbic acid (AA) usually coexists with DA in real systems. Therefore, reliable analytical procedures with high sensitivity are required for determination of DA in the presence of AA in various matrices. So far, many methods have been established for simultaneous determination of DA and AA in their mixture, such as surface plasmon resonance [9], capillary electrophoresis [10], liquid chromatography [11,12], and fluorescence [13]. However, most of the above-mentioned methods are costly and time consuming, complicate pretreatment and ⁎ Corresponding author. E-mail address: [email protected] (Z. Guo).

http://dx.doi.org/10.1016/j.jelechem.2015.11.001 1572-6657/© 2015 Elsevier B.V. All rights reserved.

are low in sensitivity. Because AA and DA are electroactive compounds, electrochemical methods are commonly used for their determination in body fluids and pharmaceuticals. Furthermore, electrochemical methods have more advantages such as fast analysis and automation, reduction of costs, high sensitivity and selectivity, wide linear dynamic, low power requirement, and no requirement for previous separation [14–17]. However because of the homogeneous catalytic effect, the similarity of oxidized potentials of DA and AA at traditional electrodes results in overlaps of voltammetric responses, which makes determination of DA in the presence of AA very difficult [18]. To address the problem, modified electrodes have been achieved for separate their signal potentials with enhanced intensity. Recently, many advanced materials including polymers [19,20], metal complexes [21,22], metal oxide [23], multi-walled carbon nanotube [18,24] and graphene-based nanomaterials [23,25] have been reported to effectively detect DA and AA. Among these electrode materials, graphene oxide (GO) has attracted significant interest since experimentally produced in 2004 [26–29]. GO's excellent properties make it suitable for applications in highly sensitive and selective determination [30–32]. It can be used as an excellent electrode material for analytical applications and diagnostic research purpose. Considering the characteristic of the graphene oxide, GO could be used as an electrochemical sensor to study the oxidation of DA. On the other hand, the properties of GO are similar to graphene,

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with graphite structures of a single atomic plane and high surface areas. These unique structures could promote target molecules substantially adsorbed on the surface to give a higher current signal [33]. Furthermore, GO has a lot of oxygen-containing functional groups, such as –OH, –COOH and C–O–C, which will hydrogen bonding increase the attachment of DA to the surface of GO by the effective groups (−OH,– NH2). Recently, GO-based composite materials with metallic nanoparticles have been intensively studied with regard to potential applications in the area of energy storage, catalysis, and chemical sensors [34–36]. The GO combined with metallic composite materials can improve the hybrids electronic and thermal conductivity [37]. Ag nanoparticles have excellent catalytic capabilities owing to their large active surface area and favorable lattice planes compare to bulk metal. In practical usage, Ag nanoparticles are generally dispersed into a conducting support to avoid their agglomerations and keep their activities [38]. Polymer modified electrodes have been paid increasing attention in electrochemical analysis due to their more active sites, excellent permselectivity, good stability and strong adherence to electrode surface. Poly (L-lysine) (PLL) film can be easily formed on electrode surface by electropolymerization of L-lysine. PLL has been receiving much attention owing to its good biocompatibility, versatility and easiness of the preparation, relatively good solubility in water, a flexible structure framework and a good deal of active amino groups [39,40], which make them suitable for applications in electrochemical determination. Combining the advantageous features of metal hybrids GO and PLL, we herein present directly GO-Ag complex with PLL as a linker through a covalent amide group for further attaching bioactive molecules for the determination of DA in the present of AA. These materials not only have the inherited advantages from each component material, but also improve properties due to synergetic effect. The results show that the sensitivity of the proposed method is better or comparable than the previous ones. The proposed electrochemical sensor can be used for determination of DA in bovine Serum samples with satisfactory results. 2. Experimental 2.1. Apparatus The phases of the samples were identified by powder X-ray diffraction (XRD) analysis using a SIEMENS D5005 diffractometer with Cu Kα radiation at 40 kV and 30 mA. Raman spectra were obtained on a J-Y T64000 Raman Spectrometer with 514.5 nm wavelength incident laser light. Transmission electron microscopy (TEM) micrographs were recorded on a Hitachi 600 transmission electron microscopy (Japan) operating at 200 kV. The sample for TEM characterization was prepared by placing a drop of colloidal solution of the sample dispersed in ethanol over a carbon-coated copper grid. Scanning electron microscopy (SEM) images were obtained from HITACHI SU8010 operating at 20.0 kV, equipped to perform elemental chemical analysis by energy dispersive X-ray spectroscopy (EDX). Because the sample has good electrical conductivity, the sample was not metal-coated prior to measurements to eliminate any charging effects for SEM images. Electrochemical measurements were carried out with an AutolabPGSTAT302 electrochemical workstation (Metrohm). A conventional three-electrode system, including a bare or modified glassy carbon electrode (GCE, 3 mm in diameter) as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum (Pt) wire as counter electrode was used in this work. All electrochemical experiments were carried out under highly pure nitrogen at room temperature.

KMnO4, H2O2 (30 wt.%), NaNO3 and N, N-dimethylformamide (DMF) were purchased from Beijing Chemical Reagent Co. (Beijing, China). All chemicals were used as received without further purification. All solutions were freshly prepared with deionized water. 2.3. Synthesis of graphene oxide (GO) GO was synthesized from graphite powder by a modified Hummers method [41,42]. In a typical synthesis, graphite powder (1 g) and H2SO4 (25 mL, 98 wt.%) were mixed in a 250 mL round-bottom flask and stirred at room temperature for 12 h. Afterwards, of NaNO3 (500 mg) was added into the mixture and stirred at 0 °C for 30 min. Subsequently, KMnO4 (3.65 g) was slowly added to prevent the temperature to exceed 20 °C and the suspension was stirred for 2 h. After that, the temperature of the system was kept at 35 ± 3 °C for 30 min, then, 46 mL of water was slowly added into the suspension within 25 min, and the temperature of the system was increased to 98 °C and maintained for 15 min. At last, 27.5 mL of water and H2O2 (3.5 mL, 30%) were added, followed by water washing and filtration. The as-prepared graphite oxide was exfoliated in water by bath ultrasonication for 2 h, to form GO. Finally, the as purified GO was dispersed well in water (1 mg/mL). 2.4. Synthesis of GO-Ag nanocomposite The GO-Ag nanocomposite was prepared by reducing silver ions directly on GO with glucose as reducing and stabilizing agent. The typical procedure for GO-Ag nanocomposite synthesis was described as follows [43]. Firstly, GO powder (15.0 mg) was dispersed in water (15.0 mL) by sonication for 1 h to form a stable GO colloid and then glucose was added to this solution under stirring. Second, ammonia (0.55 mol.L−1) was added slowly to a solution of AgNO3 (5.0, 8.0, 10.0, 12.0, and 15.0 mL, respectively, 0.06 mol.L−1) until the AgOH/Ag2O precipitate dissolved to form Ag (NH3)2OH solution. Subsequently, the Ag(NH3)2OH solution was mixed with the GO and glucose-containing solution. After being stirred for 0.5 h, the mixture was kept undisturbed at room temperature for 2 h. The slurry-like product was centrifuged and washed with excess water repeatedly to remove any impurities. Finally, the obtained product was dried overnight in an oven at 60 °C, and then GO-Ag nanocomposites with different Ag loading were obtained. By thermogravimetric analysis (TG), content of Ag in the GO-Ag nanocomposites were 8.5, 13.7, 21.6, 29.6 and 34.8 wt.%, respectively. 2.5. Preparation of GO-Ag/PLL/GCE A glassy carbon electrode (GCE) was first polished stepwise with 1.0, 0.3 and 0.05 μm alumina powders, and then washed with ethanol-water (1:1, V/V) and double distilled water in an ultrasonic bath for 30 min. Different Ag loading of GO-Ag hybrids materials (5.0 mg) were dispersed in DMF (10 mL) with the aid of ultrasonic bath for 30 min to give a 0.5 mg.mL−1 black suspension. Then the prepared suspension (5.0 μL) was coated onto the fresh GCE surface using a micropipette, followed by evaporating the solvent under an infrared lamp. The obtained electrode was denoted as GO-Ag/GCE. The electropolymerization of L-lysine on GO-Ag/GCE was performed by dipping the above GO-Ag/GCE into PBS buffer solution (pH 7.5) containing 1.0 × 10−3 mol/L L-lysine by cyclic potential scanning from −1.0 to 0.4 V for 30 cycles with a scan rate of 100 mV s−1. The final obtained electrode was denoted as GO-Ag/PLL/ GCE. The schematic representation of the immobilization and hybridization of DA on the GO-Ag/PLL is shown in Scheme 1. 3. Results and discussion

2.2. Reagents

3.1. Characterizations of GO-Ag nanocomposites

Graphite powder (320 mesh, spectrum pure), L-lysine, DA, AA were purchased from Aladdin Reagent Co (Shanghai, China). AgNO3, H2SO4,

Typical XRD patterns of the as-prepared samples are shown in Fig. 1. After chemical oxidation, it was found that the peak at 26.6° for graphite

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Scheme 1. Schematic illustration of the fabrication process of electrochemical sensors for the determination of DA.

corresponding to the (002) disappeared, and GO exhibited a strong peak at 10.1° corresponding to the (001) inter-planar, implying that graphite has been successfully oxidized by Hummers' method. Compared with the XRD pattern of GO, the XRD pattern of GO-Ag nanocomposites obviously changed. Some characteristic peaks at 38.3°, 44.4°, 64.6°, and 77.6° are assigned to the (111), (200), (220), and (311) crystallographic planes of face-centered cubic (fcc) silver nanoparticles, respectively [JCPDS no. 04-0783]. The XRD results indicate that the final product is composed of crystalline Ag [44]. The SEM images of the GO film (a) and GO-Ag composite (b, c) at different magnifications were shown in Fig. 2. It is shown in Fig. 2a that the GO sheets displayed a typical wrinkled and crumpled surface of graphene, and the sheets stacked together to form a typical multi-layer structure. After the reduction of silver ions on the surface of GO, Ag nanoparticles were completely distributed on GO sheets (Fig.2b and c). Ag nanoparticles were completely decorated on the surface of GO sheets indicating a strong interaction between GO support and Ag nanoparticles. Because of opposite charges of GO and Ag ions, Ag nanoparticles can interact with GO by electrostatic binding, physisorption or charge transfer interactions. The geometric wrinkling arising from π–π interaction within sheets of GO not only minimizes the surface energy but also serves as

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scaffold for Ag modification due to its large rough surface [45]. Highly dispersed Ag nanoparticles on GO supports with larger surface area would improve the catalytic activity and sensor's sensitivity. The EDX spectrum of GO-Ag (Fig. 2d) shows element peaks of C, O and Ag, confirming the existence of Ag nanoparticles on the surface of GO nanosheets. The content of Ag in GO-Ag nanocomposite is 20.16 wt.%, which is consistent with the TG result of 21.6 wt.%. The existing oxygen element peak confirmed that there are a lot of oxygen-containing groups on the GO surface, which were generated in preparation of GO. From the TEM images of the GO and GO-Ag composites (Fig. 3a and b), it can be seen that a large number of Ag nanoparticles are attached onto the surface of GO nanosheets and the size of Ag nanoparticles is between 50 and 100 nm. Fig. 3b also shows that Ag nanoparticles are distributed onto the surface of GO sheets with slightly aggregation. FT-IR spectra of graphite (a), GO (b) and GO-Ag (c) are shown in Fig S1 (in the Supporting information). In Fig. S1 (b), the FT-IR spectrum of GO (b) exhibits the characteristic absorptions from oxygen-containing functional groups. In detail, the absorption band at 3415 and 1379 cm−1 can be assigned to the stretching vibration and deformation vibration of O–H, respectively. The band at 1731 cm−1 belongs to the C_O stretching vibration of COOH groups, while the band at 1617 cm−1 derives from the vibration of the adsorbed water molecules and/or the contribution of the skeletal vibration of unoxidized graphitic domains. The stretching vibration peaks of C–O (epoxy) and C–O (alkoxy) are observed at 1184 and 1024 cm−1, respectively [46,47]. From the comparison of curve (b) and (c), several characteristic absorption bands in both curves are similar and their intensities have changed just a little, which indicated that oxygen-containing functional groups also remained after Ag nanoparticles were reduced onto the surface of GO. The interaction between GO and DA can be hydrogen bonds and some hydrophobic force, since the –COOH and –OH groups on GO can form hydrogen-bonding interaction with –OH and –NH2 groups on DA. Raman spectroscopy can give the further information about the structure and topology of the carbon clusters. In Fig. 4a, the prominent D peak (1350 cm− 1) with intensity comparable to the G peak (1600 cm−1) along with their large band width are indicative of significant structural disorder in GO. After Ag nanoparticles were deposited on GO (Fig. 4b), both the Raman intensities of the D and G bands increase obviously, and this phenomenon is ascribed to the surfaceenhanced Raman scattering (SERS) activity of Ag nanoparticles [45]. 3.2. The electropolymerization of L-lysine The L-lysine polymer film modified electrode was synthesized by electropolymerization of L-lysine on the GO-Ag/GCE surface. The experiment was performed in PBS buffer solution (pH 7.5) containing 1.0 × 10−3 mol/L L-lysine with cyclic voltammetric sweeps in the potential range from −1.0 to + 0.4 V. As shown in Fig. 5, the anodic peak (a) and the cathodic peak (b) are observed at −0.184 and −0.279 V on continuous scanning, reflecting the continuous growth of the PLL film on the GO-Ag/GCE surface. After 30 cycles, the electropolymerization procedure was completed, and electrode was washed with distilled water to remove any physically adsorbed material. After dried in air, a blue film (PLL film) could be observed on the surface of electrode. At a higher positive potential, L-lysine monomer can be oxidized to form αamino free radical, and the free radical can be linked to electrode surface [48]. The scan cycles and the monomer concentration can affect the electropolymerization process and the thickness of films. The results showed that stable polymer film was achieved under a scan cycle of 30 with the monomer concentration of L-lysine in PBS buffer solution 1.0 × 10−3 mol/L. 3.3. Electrochemical characterization of the modified electrodes

Fig. 1. X-ray diffraction pattern of graphite, GO and GO-Ag composite.

The electrochemical performance of modified GCE was compared with bare GCE in 1.0 mM Fe(CN)3−/4− and 0.5 M KCl mixture solution, 6

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Fig. 2. The SEM images of GO film (a), GO-Ag composite (b, c) at different magnifications and EDX spectra of GO-Ag composite (d).

and the cyclic votammograms were shown in Fig. 6. At bare GCE (curve a), a couple of well-defined redox peaks were observed with peak-topeak separation (ΔEp) of 148 mV. When the electrode was coated with GO, (curve b), the peak currents of redox peaks decreased obviously, which could be attributed to the negatively charged carboxyl groups on GO surface blocked the diffusion of Fe(CN)36 −/4− from solution to electrode surface. Moreover, the decrease of ΔEp (105 mV) was observed at GO/GCE, which indicated that GO film could improve the reversibility of the electrode reaction process. On PLL/GCE and GO-PLL/ GCE (curve c and d), the peak currents of redox peaks increased with the ΔEp value decreased, which could be attributed to attraction of the amino group cations on the PLL to Fe(CN)36 −/4 −. On GO-Ag/GCE (curve e), the redox peak currents increased with the ΔEp value decreased to 96 mV, indicating that the electron transfer of ferricyanide was enhanced on the surface of GO-Ag/GCE. The results were attributed to the Ag nanoparticles with excellent electrical conductivity and fast

electron transfer rate presenting on the surface of electrode. Moreover, the synergistic effects of GO with large surface and Ag nanomaterials with excellent electrical conductivity on the electrode surface improved the whole interfacial conductivity by the increased effective area [21, 49–51]. On GO-Ag/PLL/GCE (curve f), the biggest redox peak currents appeared with the smallest ΔEp value of 58 mV, which was almost the same as the theoretical value of 59 mV for the redox probe of Fe(CN)36 −/4 −. The results could be attributed to the attraction of the amino group cations to Fe(CN) 36 −/4 − leading to more easily redox of Fe(CN)36 −/4 − [48]. Electrochemical impedance spectroscopy (EIS) was also used for investigation of the capability of electron transfer on the surface of different electrodes. The semicircle diameter of EIS is equal to the electron transfer resistance, depending on the insulating and dielectric features at the electrode and electrolyte interface. In Fig. S2, the semicircle portion at high frequencies corresponds to the charge transfer limiting

Fig. 3. The TEM images of GO film (a) and GO-Ag composites (b).

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Fig. 4. Raman spectra of GO (a) and GO-Ag (b).

process and the charge transfer resistance (Rct) refers to the semicircle diameter. A semicircle of about 385 Ω in diameter for bare GCE (curve a) at higher frequencies indicates characteristically a diffusion limited step of the electrochemical process. When GO was deposited on the surface of GCE (curve b), the impedance value (499 Ω) was larger than that at GCE, which could be attributed to the negative charges on GO films introduced an electrostatic repulsion into the electrode/solution system, leading to a lower rate of the electron transfer on [Fe(CN)6]3−/4−. This phenomenon also demonstrated that GO was successfully immobilized on the GCE surface. After modified by PLL and GO-PLL(curve c and d), the semicircle diameter was decreased to about 344 and 273 Ω. The curve of PLL/GCE and GO-PLL/GCE exhibited gradually smaller radius of semicircles compared with bare GCE, which showed that assembled PLL molecules with positive charges are able to attract [Fe(CN)6]3−/4− electrostatically due to containing plentiful active amino groups [40]. After modified by GO-Ag nanomaterials and GO-Ag/PLL (curve e and f) respectively, the semicircle diameter was remarkably decreased to about 161 and 115 Ω, suggesting that a significant acceleration of Fe(CN)36 −/4 − redox reaction occurred on the surface of GO-Ag and GO-Ag/PLL. These results demonstrated Ag and GO-Ag nanomaterials could reduce the electron transfer resistance to the flow of electrons, which is consistent to the CV results above. Moreover, assembled PLL

Fig. 5. Repetitive cyclic voltammograms of 1.0 × 10−3 mol/L L-lysine in PBS solution (pH 7.5) at the GO-Ag/GCE: (a) oxidation peak of lysine; (b) reduction peak of lysine. Scan rate: 100 mV/s.

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Fig. 6. CVs of bare GCE (a), GO/GCE (b), PLL/GCE(c), GO-PLL/GCE(d), GO-Ag/GCE (e) and GO-Ag/PLL/GCE (f) in 1.0 mmol/L [Fe(CN)6]3−/4− solution containing 0.5 mol/L KCl.

molecules with positive charges are able to attract [Fe(CN)6]3−/4− electrostatically. And the smallest radius of GO-Ag/PLL/GCE showed that its complex is able to synergistically accelerate the electron transfer on the electrode surface. 3.4. Electrocatalytic oxidation of DA and AA at GO-Ag/PLL/GCE The individual electrochemical behaviors of DA and AA at GO-Ag/ PLL/GCE were investigated by CV, as shown in Fig. 7. In the case of AA, the oxidation peak corresponds to the oxidation of hydroxyl groups to carbonyl groups in furan ring of AA on GO-Ag/PLL/GCE. For DA, the appearance of a well-defined redox couple with a 98 mV peak separation shows the reversibility of DA on the GO-Ag/PLL/GCE is good. This couple of redox peaks corresponds to double-electron oxidation of DA to dopamine quinone and the subsequent reduction of dopamine quinone to DA [23]. 3.5. Electrochemical behavior of DA and AA in a mixture The excellent electrocatalytic activity of GO-Ag/PLL/GCE towards the oxidation of DA is also investigated by Differential pulse voltammograms (DPVs) (Fig. 8) with a mixture of DA and AA in PBS (pH 7.5). For the bare GCE and PLL/GCE (curve a and c), the oxidation peaks of DA and AA were overlapped in potential range of − 100-500 mV, so

Fig. 7. Cyclic voltammograms at GO-Ag/PLL/GCE in PBS containing AA (1000 μM) and DA (10 μM), respectively. Scan rate: 50 mV/s.

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that the determination of the DA in the present of AA is impossible. From DPV of two analytes on GO/GCE, GO-PLL/GCE and GO-Ag/GCE (curve b, d and e), peak currents (Ip) of AA and DA increased gradually and ΔEp for AA-DA was 219, 195 mV and 198 mV, respectively. However, the electrochemical response to DA was low on these modified GCEs. The electrochemical behaviors of the two analytes at GO-Ag/PLL/GCE were shown in curve f. The oxidation potentials peaks of AA and DA appeared at approximately − 4 mV and 228 mV, and the ΔEp of AA-DA was 232 mV. Furthermore, the peak currents for GO-Ag/PLL/GCE for DA increased 21.6 times as compared with those of GO/GCE, and 2.4 times compared with those of GO-Ag/GCE. This result demonstrates that GO-Ag/PLL/GCE has a remarkable electrocatalytic activity towards the oxidation of DA, because there are several interactions between them. (1) The interaction between GO and DA can be hydrogen bonds and some hydrophobic force, because the -COOH and -OH groups on GO can form hydrogen-bonding interaction with -OH and -NH2 groups on DA. (2) π-π stacking exists between GO and DA because of their aromatic structure. (3) There may be electrostatic interaction between GO and DA, because the amino group on DA molecules could be positively charged when pH is equal of higher than the pKa of DA (8.93). (4) The effect of Ag nanomaterials improved the whole interfacial conductivity. (5) PLL modified electrodes have good conductibility, excellent permselectivity, more active sites and strong adherence to electrode surface, which can enhance the electrocatalytic activity.

potentials of the DA shift to negative values. This is a consequence of a deprotonating step involved in all oxidation processes facilitated at higher pH values. Linear relationships of the peak potential of DA as a function of solution pH with slopes of 35.0 mV/pH. From Fig. S3, it is also found that for DA, the current response increases from pH 6.0 to 7.5, and reaches the maximum at pH 7.5. Then it decreases from pH 7.5 to 9.0. Considering the pKa of DA is 8.93, when pH of the solution increases from 6 to 9, the amino group on DA molecule will be gradually protonated to amino cation. The interaction of positive charged DA molecules and GO-Ag/PLL-GCE will therefore be considerably improved, exhibiting sensitive electrochemical response to the change of pH. To maintain the physiological environment, and obtain high sensitivity and good separation effect, PBS with pH 7.5 was selected for further experiment. 3.6.3. Effect of scan rate The effect of scan rate on the CV responses of DA at the GO-Ag/PLL/ GCE was studied. As shown in Fig. S4, the oxidation peaks currents of DA increase with increasing scan rate, while their oxidation peak potentials gradually shift to positive values. The oxidation peak currents of DA at the GO-Ag/PLL/GCE are directly proportional to the scan rates, indicating that the electrode process for DA is surface controlled redox process. The linear equations for DA is obtained as represented in Eq. (1). Ipa ¼ 0:047 þ 0:023ν ðR ¼ 0:998Þ

ð1Þ

3.6. Optimization of experimental conditions 3.6.1. The influence of Ag loading The effect of Ag loading on the sensor performance was investigated here. Different sensors were prepared with Ag loading 8.5, 13.7, 21.6, 29.6 and 34.8 wt.%, respectively, and ampermetric responses were determined for DA. Fig. S6 shows that the largest value was achieved when Ag loading is 21.6 wt.%. This is probably due to that Ag nanoparticles will aggregation with the increase of Ag loading. Based on the results, the optimum Ag loading was chosen to be 21.6 wt.%. Therefore, GO-Ag nanoparticles with Ag loading about 21.6 wt.% were used as characterizations and electrochemical performance test experiments in this paper. 3.6.2. The influence of pH The influence of solution pH on the peak potentials (Ep) and peak currents (I) of the electrochemical detection of DA at GO-Ag/PLL/GCE was investigated by DPVs in a pH range of 6.0–9.0. It is well known that pH value has a profound effect on the amperometric responses. In Fig. S3, it is clear that with the increase of pH the oxidation peak

where Ipa is the anodic peak current (μA), ν is the potential scanning rate (mV.s−1) and R is the correlation coefficient. 3.6.4. Effect of the electropolymerization cycles and rate The optimization of the PLL film thickness was also investigated (Fig. S5). With increasing the polymerizing cycles, the current response of DA increased at first, which may be attributed to the fact that more negatively charged groups could be immobilized on the electrode. However, after 30 cycles, PLL covers the electrode surface completely and active area cannot change significantly thereafter. Any further increase in the scan cycle will result in the decrease in the redox peak current. This could be explained that an increase in thickness of the film would prevent the electron transfer. Hence, we selected 30 cycles as the optimum scan number for the film formation process in this study. It has been reported that the morphology of polymer films depends on the electropolymerization rate [52]. The film formed on the surface of electrode may be compact and smooth in a slow polymerization process, while becoming rough and porous when its growth rate is increased, which can facilitate the electron transfer. So we choose 100 mV/s as the optimal scan rate. 3.7. DPV response of GO-Ag/PLL/GCE for the application to the detection of DA in the presence of AA DPV was employed for determination of DA in the presence of AA at the GO-Ag/PLL/GCE because of its higher current sensitivity and better resolution compared with CV. Fig. 9 shows DPV curves of different concentrations of DA in PBS (pH 7.5) with the presence of a fixed concentration of AA (500 μM). With the increase of concentration, the anodic peak currents of DA were enhanced and were proportional to the concentration from 0.1 to 10 μM. The linear regression equations can be expressed in the concentration range as follows:
Ipa ¼ 1:48CDA −0:44 Ipa : μA; C : μM ð0:1–10 μMÞ ðR ¼ 0:997Þ

Fig. 8. Differential pulse voltammograms (DPVs) for AA and DA at bare GCE (a), GO/GCE (b), PLL/GCE(c), GO-PLL/GCE(d), GO-Ag/GCE (e) and GO-Ag/PLL/GCE (f).

ð2Þ

and the detection limit (LOD) of DA is 0.03 μM (n = 10). The LOD was calculated according to 3σ/s criterion, where s is the slope of the linear portion in the calibration graph, and σ is the standard deviation of the intercept of regression line. The sensitivity of GO-Ag/PLL/GCE for DA is estimated to be 1.48 μA/μM, which is much better than that of

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3.9. Interference

Fig. 9. DPV of solutions containing 500 μM AA and with concentrations varying in the range of 0.1–10 μM DA at GO-Ag/PLL/GCE in 0.1 M PBS (pH 7.5). The inset shows the corresponding calibration curve of oxidation peak currents with analyst concentrations.

previous graphene-based DA sensors, such as graphene modified electrode (0.0659 μA/μM) [53], nitrogen doped graphene(NG) modified electrode (0.03195 μA/μM) [54], tryptophan-functionalized graphene modified electrode (0.5202 μA/μM) [55], overoxidized polypyrrole/ graphene modified electrode (0.015 μA/μM) [56], and overoxidized polyimidazole/graphene oxide copolymer modified electrode (0.211 μA/μM) [57]. The concentration range for DA is adequate for determining UA in serums samples according to serum concentrations. The analytical parameters including detection limit and linear range using GO-Ag/PLL/GCE are better or comparable to the results reported for determination of DA at different modified electrode surface, as displayed in Table 1.

3.8. Reproducibility and stability To evaluate the reproducibility of the sensor, a series of five electrodes were prepared for the detection of 5 μM DA. The relative standard deviations (RSD) of the measurements for the five electrodes were 2.8% for DA, which suggested that the reproducibility of the proposed sensors were quite good. In addition, the sensor was stored in a dry location at room temperature. After 5 days, the sensor retained 93% of its original response. After 10 days, the response of the sensor maintained 90% of its initial responses. The above results indicated that the electrochemical sensor possessed acceptable stability.

Table 1 Comparison of some characteristics of the different modified electrodes for the determination of DA. Electrode materials GO-Ag/PLL/GCE MWCNT/GCE Chitosan–graphene PtNPs-MWCNT/GCE CDP–GS–MWCNTs PDDA/graphite OMC/Nafion Fe3O4-NH2@GS/GCE PDDA@HCNTs/GCE e-FGPE Pd3Pt1/PDDA-RGO

Detection limit (μM) 0.03 13 1 0.05 0.05 0.2 0.5 0.126 0.08 0.01 0.04

Linear range (μM)

Reference

0.1–10 1.5–7.8 1–24 0.06–2.03 0.15–21.65 5–250 1–90 0.2–38 2.5–10 0.5–35 4–200

This work [58] [59] [60] [61] [62] [63] [23] [18] [25] [21]

UA (uric acid) coexist with AA and DA in biological samples and are electroactive. AA and DA have oxidation potentials close to UA. Therefore, it is essential to investigate the mutual interferences of these biomolecules in the sensitive and selective detection of DA on GO-Ag/ PLL/GCE. We had carried out two sets of experiments to ensure that the quantification of DA at GO-Ag/PLL/GCE was free from interferences. In the first set of experiments, we had varied the concentration of DA while keeping the concentration of AA and UA constant as depicted in Fig. S7. The linear regression equation for the variation of concentration of DA in this case was Ipa = 1.61CDA + 1.9 (Ipa: μA, C: μM) (0.1–10 μM) (R = 0.997). This implies the selectivity of GO-Ag/PLL/GCE towards DA in the presence of AA and UA. In the second set of experiments, as shown in Fig. S8, the GO-Ag/PLL/GCE shows clear response towards the addition of 5 μM DA, while the successive addition of AA and UA with the same concentration exhibits no response. This suggests applicability of this method for real sample analysis in the simultaneous quantification of DA in the presence of AA and UA at GO-Ag/PLL/GCE. 3.10. Sample analysis In order to verify the reliability of the method for analysis of DA on physiological environment,the fetal bovine Serum was diluted 10 times with 0.1 M PBS (pH 7.5) to be the real samples with DPV. The analytical results are summarized in Table 2. The relative standard deviation (R.S.D.) of them was below 4%, indicating the potential usefulness of the GO-Ag/PLL modified GCE for the determination of DA in real biological samples with satisfactory results. 4. Conclusion An electrochemical analysis method with high sensitivity and selectivity for measurement of DA in the presence of AA was developed by using GO-Ag/PLL as electrode modification material. The excellent electrocatalytic activity of GO-Ag/PLL/GCE towards the oxidation of DA is attributed to the synergistic effects of GO with large surface and Ag nanomaterials with excellent electrical conductivity on the electrode surface improved the whole interfacial conductivity by the increased effective area. Moreover, PLL modified electrode has good stability, excellent permselectivity, more active sites and strong adherence to electrode surface, which can enhance its electrocatalytic activity. Under the optimum condition, the modified electrode showed good selectivity, excellent linear relation from 0.1 to 10 μM for DA, high sensitivity with a detection limit of 0.03 μM for DA from their mixture solution. Acknowledgments The present work was supported by the Outstanding Young Scientist Foundation of the Education Department of Liaoning Province, China (LJQ2012033). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jelechem.2015.11.001. Table 2 Determination of DA in bovine serum samples (n = 3). Sample

Added (μM)

Found (μ A)

Recovery (%)

RSD (%)

Serum 1 Serum 2 Serum 3

3.0 5.0 10.0

2.9 5.2 9.6

96.7 104.0 96.0

2.01 2.11 2.96

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