polyaniline modified electrode and application for ascorbic acid detection

Journal of Electroanalytical Chemistry 755 (2015) 39–46

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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jeac

Preparation in-situ of carbon nanotubes/polyaniline modified electrode and application for ascorbic acid detection Yuan Fang, Qi Jiang ⁎, Min Deng, Yan Tian, Qi Wen, Mingfei Wang Key Laboratory of Advanced Technologies of Materials (Ministry of Education of China) and Superconductivity and New Energy R&D Centre, Southwest Jiaotong University, Chengdu 610031, PR China

a r t i c l e

i n f o

Article history: Received 10 February 2015 Received in revised form 22 July 2015 Accepted 23 July 2015 Available online 29 July 2015 Keywords: Ascorbic acid Carbon nanotubes/polyaniline modified electrode Preparation in-situ

a b s t r a c t A novel grown in-situ carbon nanotube/polyaniline chemically modified electrode (GSCNT/PANI-CME) was synthesized for the electrochemical detection of ascorbic acid (AA). First, nickel catalyst was obtained by direct current electrochemical deposition on a graphite electrode (GE) surface. Second, CNTs were grown in-situ to obtain CNTs chemically modified electrode (GSCNTs-CME) by catalytic chemical vapor deposition. Third, the grown CNTs were acid treated to carboxylate and remove nickel particles. Finally, PANI was prepared in-situ by electrochemical polymerization on the GSCNTs-CME to obtain GSCNTs/PANI-CME. The obtained electrodes were characterized by scanning electron microscopy and cyclic voltammetry. The results showed that CNTs grew uniformly on the GE surface and the original tubular structure was remained. PANI was uniformly coated on the surface of CNTs in the obtained composite, which was a typical three-dimensional network structure. The GSCNTs/PANICME exhibited an excellent electrocatalytic activity to AA. The oxidation peak current was increased linearly with the concentration of AA in the range from 1.0 × 10−6 mol L−1 to 4.5 × 10−4 mol L−1, with a detection limit of 1.0 × 10−7 mol L−1 (S/N = 3). The experimental data showed that the obtained electrode was selective, stable and reproducible. The recoveries were between 97.4% and 102.1%. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ascorbic acid (AA, Vitamin C) is an effective reducing agent and a powerful antioxidant in food and human organ [1]. Thus, it is important for the food and medicine safety to develop a simple and rapid method to determine accurately the AA. Compared with conventional methods: fluorimetry, chromatography, spectrophotometry and redox titration, electrochemical methods have the obvious advantages of low cost, high sensitivity, short response time and simple operation [2,3]. Although AA is an effective reductive compound, it cannot be detected directly by the electrochemical methods using the conventional electrode such as glassy carbon electrode, graphite electrodes and Pt electrode for its high overpotential, low selectivity, low reaction sensitivity and poor reproducibility on these electrodes [4,5]. In order to solve these problems, many kinds of modified electrodes were prepared to replace the conventional electrodes [6–8]. Polyaniline (PANI), an organic conducting polymer, has attracted much attention as the modified electrode material for its efficient electrocatalytic activity and high selectivity in the AA detection [9]. Carbon nanotubes (CNTs), discovered in 1991 [10], also have attracted considerable attention as the modified material to electrochemical sensors and biosensors for their unique properties such as high surface area, ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Q. Jiang).

http://dx.doi.org/10.1016/j.jelechem.2015.07.039 1572-6657/© 2015 Elsevier B.V. All rights reserved.

strong adsorptive ability, high electrical conductivity and good chemical stability [11–14]. There is π–π conjugation between the benzenoid rings of CNTs and PANI quinoid rings, which can urge the signal transmission between the two modified materials and thus CNT/PANI composite modified electrode may have better electrochemical detection performances. In fact, the researchers have proved that there is a good synergy between the CNTs and PANI, and the modified electrodes made from the two modified materials have excellent electrochemical detection performances [15–18]. According to current research reports [15–26], the CNTs/PANI chemically modified electrodes (CNTs/PANI-CMEs) can be prepared by two ways. One is fixing functionalized CNTs on the surface of the bare electrode by dropping and dipping method, then growing PANI on the CNTs by chemical oxidation or electrochemical polymerization [19–22]. The other is that the composite of CNTs/PANI is prepared in advance. Subsequently, the composite is fixed directly on the surface of the bare electrode by dropping and dipping method [23–26]. However, these ways may change the original novel nanometer hollow tube structure of the CNTs due to adding electrode materials (for example, the binder), which may decrease the utilization efficiency of the CNTs and increase the resistance of the composite. In addition, the binding between modified materials and electrode surface may be fragile, which could affect the stability and reutilization of the modified electrode. In order to avoid these defects mentioned above, we put forward to a new way to prepare the CNTs/PANI-CME: the CNTs were grown in-situ on the

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surface of the graphite electrode (GE) by catalytic chemical vapor deposition (CCVD) without binder. Then the PANI was prepared in-situ on the surface of the CNTs by electrochemical polymerization. So the CNTs/PANI-CME was prepared without any binder, named as the GSCNTs/PANI-CME. The obtained modified electrode was used for electrochemical detection of the AA in aqueous solution and showed a broader linear range of 1.0 × 10−6 mol L−1–4.5 × 10−4 mol L−1 and the detection limit was 1 × 10−7 mol L−1. The detection limit is lower than those of the similar electrodes obtained by other ways [27–30]. 2. Experimental 2.1. Materials and reagent All chemicals used in this paper were analytical reagent. The purity of the gases (Ar, H2 and C2H2) used in the experiment was 99.99%. The GEs (5.3 mm in diameter, 7.0 mm in length) were produced by Sichuan Hengli Electrical Carbon Co. Ltd. The phosphate buffer solution (PBS) was prepared by Na2HPO4 and NaH2PO4, which was used as the supporting electrolyte during all measurements. Deionized water was used to rinse the materials and prepare the solutions.

2.3. Characterization and instrumentations A conventional three-electrode system was assembled by using the saturated calomel electrode (SCE) as the reference electrode, platinum electrode as the counter electrode and the modified electrodes as the working electrode and employed to electrochemical measurements. Cyclic voltammetry (CV) was used in the electrochemical measurement. And differential pulse voltammetry (DPV) was used to verify the testing results' reliability from the CV. Optical micrograph was used to observe the morphology of the GE surface by using a DMRX-type polarization microscope (Leica Microsystems, Germany). A JEOL JSM-7001F field emission scanning electron microscope (SEM) was used for surface image measurements of the modified electrodes. All the cyclic voltammetry (CV) measurements were carried out on a galvanostatic-voltage instrument (ZF-9, Shanghai, China) using Ar gas to move the air of the solutions. The electrochemical impedance spectroscopic (EIS) was carried out with supporting electrolyte solution (5.0 mmol L− 1 [Fe(CN)6]3−/4− in pH = 6 PBS). The AC amplitude was 5 mV and the testing frequency ranged from 10 kHz to 0.01 Hz. The applied potential was 0.172 V.

2.2. Preparation of modified electrodes

3. Results and discussion

2.2.1. Electrochemical deposition of Ni catalyst on the GE The GE was polished to a mirror-like surface and fastened into polytetrafluoroethylene electrode set with silicone; only the polished surface was exposed to the air. To prepare Ni catalyst, a conventional twoelectrode system was assembled by using Ni slice (99.99%) and the polished GE as the auxiliary electrode and working electrode, respectively. The electrochemical deposition solution was composed of nickel sulfate (NiSO4·7H2O, 300 g L−1), nickel chloride (NiCl2·6H2O, 45 g L−1), boric acid (H3BO3, 40 g L−1), saccharin sodium (C6H4COSO2NNa·2H2O, 5 g L−1), sodium dodecyl sulfate (CH3(CH2)11OSO3Na, 0.1 g L−1) and deionized water [31]. The direct current electrochemical deposition of Ni catalyst was carried out at a constant potential of 2.05 V for 5 min. Finally, the GE was carefully removed from the electrode set and washed with deionized water.

Fig. 1A and B is the CV curves of the PANI electrochemical polymerization on the GE and GSCNTs-CME with 20 mV s−1 scan rate, respectively. It can be seen that the two CV curves both have three couples of redox peaks (marked as a, b and c) with the potential from −0.01 V to 0.85 V. The oxidation peak a corresponds to the transformation of PANI from leucoemeraldine form (fully reduced state) to emeraldine salt. The oxidation peak b is due to the oxidation of head to tail dimers (intermediate). The oxidation peak c is related to the state transformation from emeraldine to pernigraniline (fully oxidized state), which indicates that PANI has successfully grown on the surface of the electrode [33]. Moreover, with the increase of scan cycle numbers, there is a significant increase of the redox peak current, which is due to the fact that the PANI electrochemical polymerization is an autocatalytic process. Compared with Fig. 1A and B, we can find that the peak a current value of the GSCNTs-CME reaches 1.17 mA after 10 cycles, which is five times more than that of the GE. The peak b current value of the GSCNTs-CME is up to 0.85 mA, which is five times more than that of the GE. The peak c current value from the GSCNTs-CME reaches 1.14 mA, which is seven times more than that from the GE. These phenomena, we consider, should be attributed to the excellent electron transfer rate and high specific surface area of CNTs, indicating that grown in-situ CNTs could improve the modified electrode electrochemical performances. Fig. 1C shows the CV curves of the four obtained electrodes in 5.0 mmol L− 1 [Fe(CN)6]4 −/3 − solution containing 0.1 mol L−1 KCl with 20 mV s− 1 scan rate. Oxidation/reduction characteristics of [Fe(CN)6]4−/3− could be seen in all the CV curves. However, the oxide peak current (210.1 × 10− 6 A) based on the GSCNTs/PANI-CME is much higher than those based on the GE, GSCNTs-ME (121.3 × 10−6 A) and PANI-CME (69.7 × 10−6 A). So the electroactive areas of the electrodes can be obtained by the Randles–Sevcik equation [34]. The electroactive areas of PANI-CME, GSCNTs-CME and GSCNTs/PANI-CME are 4.1, 7.1 and 12.4 cm2, respectively. The electroactive area of the GSCNTs/PANI-CME is the biggest and which is bigger than the total of the PANI-CME and GSCNTs-CME, indicating that there is a synergistic effect between the PANI and CNTs. If the oxide peak currents observed by CVs in the presence of AA are normalized with their electroactive areas, we can have the current densities 17.0 × 10−6 (PANI-CME), 17.1 × 10− 6 (GSCNTs-CME) and 16.9 × 10−6 A·cm−2 (GSCNTs/PANI-CME). The differences of the current densities are very small, indicating that there is a close relationship between the electroactive area and its oxide peak current.

2.2.2. Preparation of the GSCNTs-CME The CNTs were grown by CCVD in the tubular resistance furnace, which has been reported in our previous papers [32]. The pretreated GE was placed in a quartz boat, and the quartz boat was put into the quartz tube (ϕ = 60 mm, l = 1000 mm) of the tubular resistance furnace. Then, a flow rate of 50 sccm of Ar gas was introduced into the quartz chamber for eliminating the air in the quartz tube, meanwhile the tube furnace was heated up to 800 °C from room temperature at 10 °C per minute. The temperature was kept at 800 °C for 30 min and at the same time, H2 gas was introduced into the tube at the flow rate of 50 sccm instead of the Ar gas. After that, the temperature was cooled down to 700 °C with Ar gas flowing at the flow rate of 50 sccm. Then the temperature was held at 700 °C for 20 min while a mixture gas composed of C2H2 and Ar (C2H2/Ar, 20/160) flew into the quartz tube to grow CNTs. Finally, the system was cooled down to ambient temperature under the protection of Ar gas at a flow rate of 25 sccm. After that, the electrode was taken out and treated by concentrated nitric acid for 1 h. The electrode was rinsed with deionized water and dried. 2.2.3. Preparation of the GSCNTs/PANI-CME and the PANI-CME The GSCNTs/PANI-CME was prepared by in-situ electrochemical polymerization of aniline on the surface of the GSCNTs-CME. PANI was prepared by cyclic voltammetric scanning between −0.2 and 1.0 V for 10 cycles at a scan rate of 0.02 V s−1 in the 50 mL supporting electrolyte solution containing aniline (0.1 mol L− 1) and sulfuric acid (0.5 mol L−1). As a contrast electrode, the PANI-CME was prepared by directly depositing PANI on the surface of the GE as the same operations.

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Fig. 1. CV curves of the PANI electrochemical polymerization on the GE (A) and GSCNTs-CME (B), CV curves based on the obtained electrodes in 5.0 mmol L−1 [Fe(CN)6]4−/3− solution containing 0.1 mol L−1 KCl (C) with 20 mV s−1 scan rate and EIS based on the obtained electrodes in 5.0 mmol L−1 [Fe(CN)6]3−/4− in pH = 6 PBS (D, from 0.01 to 10 kHz).

Fig. 1D is the EISs of GE (a), PANI-CME (b), GSCNTs-CME (c) and GSCNTs/PANI-CME (d). The electron transfer resistance (RCT) values for PANI-CME, GSCNTs-CME, GSCNTs/PANI-CME are 250, 190 and 130 Ω, respectively. Amongst which, the RCT of the GSCNTs/PANI-CME is the lowest, indicating its low resistance and high electron transfer efficiency. Fig. 2 is the morphology maps of the samples (A, optical micrograph of the GE surface; B, optical micrograph of the blank GE surface after electrodepositing Ni catalyst; C, SEM image of the GSCNTs-CME; D, SEM image of the GSCNTs/PANI-CME with 5000 times magnification; E, SEM image of the GSCNTs/PANI-CME with 50,000 times magnification; F, SEM image of the PANI-CME). From Fig. 2A and B, it can be seen that the blank GE surface is smooth and flat before the deposition, but the GE surface becomes uneven after the deposition, indicating that the Ni deposition on the surface of the GE is successful. As seen from Fig. 2C, the grown CNTs are smooth with about 50 nm outer diameter. At the same time, the CNTs are uniformly distributed and the hollow tubular structure is well preserved. From Fig. 2D, the obtained CNT/PANI composite looks like fibers with a three-dimensional network structure and has a uniform distribution. The thickness of the CNTs/PANI film is about 200 μm by an optical microscope (not shown in this paper). Moreover, the diameter of the fibers is obviously bigger than that of the CNTs from Fig. 2C. The fiber diameter of the composite is about 70 nm, which can be obviously seen at larger magnification times from Fig. 2E. Compared to Fig. 2E and C, the diameter of the fibers/ tubes significantly increases, indicating that PANI has been successfully prepared and uniformly coated on the surface of the CNTs. Fig. 2F shows that the PANI on the PANI-CME is a granular form and agglomeration with each other. Compared to Fig. 2D and F, we can see that the CNT/ PANI composite shows a clear three-dimensional network structure with lots of pores. Amongst which, the CNTs are used as the mechanical skeleton to support the composite and conductive skeleton to accelerate the charge transfer, which is good for the electrochemical detection performances of the modified electrode. Fig. 3 is the CVs of the GE (A), GSCNTs-CME (B), PANI-CME (C) and GSCNTs/PANI-CME (D) in substrate solution (a, pH = 6.0 PBS) and

detection solution (b, pH = 6.0 PBS containing 5.0 × 10−5 mol L−1 AA) with a scan rate of 20 mV s−1. As seen from Fig. 3A, there is no obvious redox peak both in substrate solution and detection solution, indicating that the GE has no electrocatalytic behavior to the AA. As shown in Fig. 3B and C, there are obvious oxidation peaks appeared in b curve at 0.36 V and 0.45 V, respectively. These peaks are corresponded to the AA electrochemical oxidation responses. The electric current of the oxidation peaks is 4.5 × 10−2 mA and 5.6 × 10−2 mA, respectively, indicating that the GSCNTs-CME and PANI-CME both have excellent electrocatalytic responses to AA. Compared with the results of the GE from Fig. 3A, we can find that it is just the CNTs/PANI grown on the electrode surface that endows the electrode with electrocatalytic activity to AA. There is also an obvious oxidation peak of AA at about 0.38 V in b curve from Fig. 3D. The peak current is about 8.5 × 10− 2 mA and the oxidation peak potential has a negative shift relative to the PANI-CME, indicating that the electrocatalytic activity to AA of the GSCNTs/PANI-CME is better than those of the GSCNTs-CME and PANI-CME. Namely, there is a synergistic effect between the PANI and CNTs in the composite and the synergistic effect is good for the electrode electrocatalytic activity to AA. Amongst the three modified electrodes, the GSCNTs/PANI-CME has the largest oxidation peak current and the smallest redox peak potential difference (0.14 V), indicating that the GSCNTs/PANI-CME has the most excellent electrochemical performances to AA. Fig. 4 shows the relationships between scan rate, pH and electrochemical response to AA (5.0 × 10−5 mol L−1) on the GSCNTs/PANICME (A, effect of pH on the peak potential and peak current for AA oxidation; B, CVs with different scan rates: 1.67 mV s− 1, 5 mV/s, 10 mV s−1, 20 mV s−1, 50 mV s−1, 100 mV s−1 and 200 mV s−1; C, relationship between oxidation peak current and square root of the scan rate). The effect of pH on the peak potential and peak current for AA oxidation is studied by CV experiments with different pH values from 2.0 to 7.0 in PBS containing 5.0 × 10−5 mol L−1 AA. As the results are shown in Fig. 4A, it can be seen that the oxidation peak potential begins to negatively shift continuously and the oxidation peak current increases up to a certain extent, then also begins to decrease with increasing the pH

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Fig. 2. Morphology maps of the samples (A, optical micrograph of the blank GE surface; B, optical micrograph of the GE surface after electrodepositing Ni catalyst; C, SEM image of the GSCNTs-CME; D, SEM image of the GSCNTs/PANI-CME with 5000 times magnification; E, SEM image of the GSCNTs/PANI-CME with 50,000 times magnification; F, SEM image of the PANI-CME).

value. So in order to find larger oxidation peak current and lower oxidation peak potential, we choose the pH = 6.0 as our experimental pH value. From Fig. 4B, it can be seen that the AA oxidation peak currents are increasing with the increase of scan rate. Meanwhile, oxidation peak voltages also have a slight shift to high voltage with increasing the scan rate, indicating that the redox reactions between the modified electrode and AA are controlled by kinetics [22]. As seen from Fig. 4C, there is a good linear relationship between the oxidation peak currents and the square root of scan rate when the scan rate changes in between 1.67 mV s−1 and 200 mV s−1. The linear regression equation is Ipk = 20.92 v1/2 − 0.157 with a correlation coefficient of 0.99904. So, we can have the results that the electrochemical response between AA and GSCNTs/PANI-CME is a diffusion controlled process. In order to detect AA, the calibration curve is necessary. So, the CV and DPV are tested to obtain the peak currents of the oxidation peak with different AA solutions. Fig. 5 is the analysis of the AA detection

based on the GSCNTs/PANI-CME (A, CVs in different AA solutions (from inner to outer): 0.5, 1.0, 5.0, 10.0, 50.0, 100.0, 200.0, 400.0, 450.0 and 500.0 × 10− 6 mol L− 1 in pH = 6.0 PBS with 20 mV s− 1 scan rate. The inset is an enlarged view; B, relationship between the oxidation peak current and AA concentrations; C, the fitting curve of the relationship between the oxidation peak current and AA concentrations based on CV (namely, calibration curve) from 1.0 × 10− 6 to 450.0 × 10−6 mol L−1. D, DPV in different AA solutions (from inner to outer): 1.0, 5.0, 10.0, 50.0, 100.0, 200.0, 400.0 and 450.0 × 10−6 mol L−1 in pH = 6.0 PBS with 20 mV s− 1 scan rate; F, the fitting curve of the relationship between the oxidation peak current and AA concentrations based on DPV). As shown in Fig. 5A and the inset, we can obtain the corresponding data and draw the relationship between the oxidation peak current and AA concentrations, which are listed in Fig. 5B. It is very easy to find that there is an excellent linear relationship during the range of 1.0 × 10−6 and 4.5 × 10−4 mol L−1, as shown in Fig. 5C. Moreover, the linear equation is I(μA) =

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Fig. 3. CVs of the GE (A), GSCNTs-CME (B), PANI-CME (C) and GSCNTs/PANI-CME (D) in substrate solution (a, pH = 6.0 PBS) and detection solution (b, pH = 6.0 PBS containing 5.0 × 10−5 mol L−1 AA) with a scan rate of 20 mV s−1.

0.347C(μM) + 70.193 with a correlation coefficient of 0.9997. The relevant detection limit is about 1.0 × 10−7 mol L−1 at a signal to noise ratio of 3. As seen from Fig. 5D and F, the electrochemical response also has a

linear response in the range from 1 to 450 × 10−6 mol L−1, in accordance with the following equation: I(μA) = 0.347C(μM) + 12.175 with a correlation coefficient of 0.9998. The results show that the testing

Fig. 4. Relationship between scan rate, pH and electrochemical response to AA (5.0 × 10−5 mol L−1) on the GSCNTs/PANI-CME (A, effect of pH on the peak potential (●) and peak current (■) for AA oxidation; B, CVs with different scan rates: 1.67 mV s−1, 5 mV/s, 10 mV s−1, 20 mV s−1, 50 mV s−1, 100 mV s−1 and 200 mV s−1; C, relationship between oxidation peak current and square root of the scan rate).

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Fig. 5. Analysis of the AA detection based on the GSCNTs/PANI-CME (A, CVs in different AA solutions (from inner to outer): 0.5, 1.0, 5.0, 10.0, 50.0, 100.0, 200.0, 400.0, 450.0 and 500.0 × 10−6 mol L−1 in pH = 6.0 PBS with 20 mV s−1 scan rate. The inset is an enlarged view; B, relationship between the oxidation peak current and AA concentrations; C, the fitting curve of the relationship between the oxidation peak current and AA concentrations based on CV (namely, calibration curve) from 1.0 × 10−6 to 450.0 × 10−6 mol L−1; D, DPV in different AA solutions (from inner to outer): 1.0, 5.0, 10.0, 50.0, 100.0, 200.0, 400.0 and 450.0 × 10−6 mol L−1 in pH = 6.0 PBS with 20 mV s−1 scan rate; F, the fitting curve of the relationship between the oxidation peak current and AA concentrations based on DPV.).

results from the DPV are similar to those from the CV, the slope and linear range, indicating that the testing results from the CV are reliable. Table 1 is the electrochemical determination performances of different methods for the AA. Compared with the listed AA sensors shown in Table 1, the GSCNTs/PANI-CME has shown an obvious improvement of electrochemical determination performance including linear response range and detection limit.

To test the reliability of the GSCNTs/PANI-CME in the real sample, recovery experiments are performed. The obtained data is listed in Table 2. Table 2 is the data of the AA detection results based on the GSCNTs/PANICME. From Table 2, we can see that the average concentration from four parallel measurements is about 1.054 × 10−5 mol L−1 (RSD = 1.9%), which is very close to the real value of 1.0 × 10−5 mol L−1. At the same time, different concentration AA solutions are added into the sample for

Table 1 Electrochemical determination performances of different methods for the AA. Modified electrode PAn-p-aminobenzene sulfonic acid Molecularly imprinted PAn Poly(acriflavine) modified electrode DBSA doped polyaniline nanoparticles modified electrode Poly(3,4 ethylenedioxy thiophene) SWCNTs/ascorbate oxidase/nafion

Linear range

Detection limit −5

−1

3.5–17.5 × 10 mol L 5.0–40.0 × 10−5 mol L−1 3.0–20.0 × 10−5 mol L−1 0.3–8.0 × 10−6 mol L−1 1.0–18.0 × 10−6 mol L−1

Interferences

Ref.

7.5 × 10 mol L 1.8 × 10−5 mol L−1 1.5 × 10−6 mol L−1 8.3 × 10−6 mol L−1

Dopamine Glucose Dopamine, uric acid Dopamine, acetaminophen, uric acid

[35] [36] [37] [38]

7.0 × 10−7 mol L−1

Dopamine

[39]

−6

−1

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Table 2 Data of the AA detection results based on the GSCNTs/PANI-CME. Samples

Detected (10−6 mol L−1)

Added (10−6 mol L−1)

Found (10−6 mol L−1)

Recovery (%)

RSD (%, n = 6)

1 2 3 4

10.54

20.00 40.00 60.00 80.00

30.02 51.37 69.86 91.48

97.4 102.1 98.9 101.2

2.5 1.9 0.8 1.0

recovery tests. The results show that the recoveries are between 97.4% and 102.1% with lower RSD (less than 2.5%), indicating that the GSCNTs/PANI-CME can be used efficiently for the electrochemical detection of AA in some real samples. As we know, to AA electrochemical detection, the main interfering compounds are glucose, dopamine and uric acid. The interference analysis experiment of the obtained GSCNTs/PANI-CME has been carried out. The results are listed in Fig. 6. Fig. 6 is the CVs of 2.0 × 10−4 mol L−1 AA in pH 6.0 PBS (black line) and with the addition of glucose, dopamine and uric acid (red line, the concentration is 2 × 10−3 mol L− 1). As shown in Fig. 6, the oxidation peak current does not show obvious differences between before and after adding the interfering compounds, indicating that the interfering compounds do not cause interference on the detection of AA based on the prepared GSCNTs/PANI-CME, which demonstrates that the GSCNTs/PANI-CME can be used effectively for the AA electrochemical detection. The reproducibility and stability of electrochemical detection performance are important to an electrode. So the reproducibility and stability experiments are carried out on the GSCNTs/PANI-CME by the following operations: the obtained modified electrode has been stored in a dry state for ninety days. Meanwhile, CV testing has been measured once every ten days with 2.0 × 10−4 mol L−1 AA solution in pH 6.0 PBS at 20 mV s− 1 scan rate. The oxidation peak currents are recorded and listed in Fig. 7. Fig. 7 is the relationship between the oxidation peak current and the storing time of the GSCNTs/PANI-CME with 2.0 × 10−4 mol L−1 AA solution in pH 6.0 PBS at 20 mV s−1 scan rate. The diagram showed the variation of oxidation peak current based on the GSCNTs/PANI-CME in nine measurements. As seen from Fig. 7, the electrode has retained about 92% of its initial response current after 90 days of use. In addition, experiments (not listed in this paper) also prove that there is no loss of electroactivity after the continuously cyclical sweep for 30 cycles in 2.0 × 10−4 mol L−1 AA, which is superior to

Fig. 6. CVs of 2.0 × 10−4 mol L−1 AA in pH 6.0 PBS (black line) and with the addition of 2 × 10−3 mol L−1 glucose, dopamine and uric acid (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

most of the reported CNT/PANI electrodes [40–42], suggesting that the GSCNTs/PANI-CME has a relatively high stability. 4. Conclusions Several conclusions can be drawn from the experimental results and discussion mentioned above, including: Firstly, the GSCNTs/PANI-CME can be prepared successfully by insitu growing CNTs on the GE surface and in-situ electrochemical polymerizing PANI on the GSCNT-CME surface. The CNTs grow uniformly on the GE surface and remain their original tubular structure. The PANI has been coated uniformly on the surface of CNTs and the composite is of a typical three-dimensional network structure. Secondly, the obtained GSCNTs/PANI-CME has exhibited an excellent electrocatalytic activity to AA. The linear range of the calibration curve is from 1.0 × 10−6 to 4.5 × 10−4 mol L−1 and the detection limit is about 1.0 × 10−7 mol L−1 (S/N = 3). Thirdly, the obtained modified electrode has excellent selective, stable and reproducible. The recoveries are from 97.4 to 102.1% with less than 2.5% RSD. These results show that the GSCNTs/PANI-CME can be used effectively for the AA electrochemical detection. Acknowledgments The work was supported by the National Natural Science Foundation of China (50907056), Sichuan Province Science and Technology Innovation Young Plant Project (2014-071) and Chengdu Science and Technology Huimin Project (2014-HM01-00073-SF).

Fig. 7. Relationship between the oxidation peak current and the storing time of the GSCNTs/PANI-CME with 2.0 × 10−4 mol L−1 AA solution in pH 6.0 PBS (20 mV s−1 scan rate).

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