Materials Science and Engineering C 32 (2012) 1323–1330

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Direct electrodeposition of gold nanotube arrays of rough and porous wall by cyclic voltammetry and its applications of simultaneous determination of ascorbic acid and uric acid Guangming Yang a,⁎, Ling Li a, Jinhe Jiang b, Yunhui Yang b,⁎ a b

Department of Resources and Environment, Baoshan University, Baoshan 678000, PR China College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650092, PR China

a r t i c l e

i n f o

Article history: Received 5 December 2011 Received in revised form 25 March 2012 Accepted 1 April 2012 Available online 6 April 2012 Keywords: Gold nanotubes array Electrodeposition Ascorbic acid Uric acid Anodic aluminum oxide template Polycarbonate membrane

a b s t r a c t Gold nanotube arrays of rough and porous wall has been synthesized by direct electrodeposition with cyclic voltammetry utilizing anodic aluminum oxide template (AAO) and polycarbonate membrane (PC) during short time (only 3 min and 2 min, respectively). The mechanism of the direct electrodeposition of gold nanotube arrays by cyclic voltammetry (CV) has been discussed. The morphological characterizations of the gold nanotube arrays have been investigated by scanning electron microscopy (SEM). A simultaneous determination of ascorbic acid (AA) and uric acid (UA) by differential pulse voltammetry (DPV) was constructed by attaching gold nanotube arrays (using AAO) onto the surface of a glassy carbon electrode (GCE). The electrochemical behavior of AA and UA at this modified electrode has been studied by CV and differential pulse voltammetry (DPV). The sensor offers an excellent response for AA and UA and the linear response range for AA and UA were 1.02 × 10 − 7–5.23 × 10− 4 mol L − 1 and 1.43 × 10 − 7–4.64 × 10 − 4 mol L− 1, the detection limits were 1.12 × 10− 8 mol L − 1 and 2.24 × 10 − 8 mol L− 1, respectively. This sensor shows good regeneration, stability and selectivity and has been used for the determination of AA and UA in real human urine and serum samples with satisfied results. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Ascorbic acid (AA) and Uric acid (UA) are electroactive compounds of great biomedical interest, playing a potential role in human metabolism. AA, a water-soluble vitamin, is widely present in many biological systems and in multivitamin preparations. At the same time, AA is commonly used to supplement inadequate dietary intake and as an antioxidant [1]. Its deficiency leads to the development of a wellknown syndrome called scurvy. It is administered in the treatment of many disorders, including Alzheimer's disease, atherosclerosis, cancer, and infertility as well as some clinical manifestations of HIV infections [2]. UA is the major final product of purine metabolism in human body. High levels of UA are indicative of gout and represent a risk factor for cardiovascular diseases [3]. The simultaneous detection and assay of AA and UA remain of critical interest, not only for biological researches but also for routine analysis, as they coexist in several physiological liquids (such as serum, urine, tears, and cerebrospinal fluids, et al.,). Therefore, the development of a sensitive and selective method for their simultaneous determination is highly desirable for analytical application and diagnostic research.

⁎ Corresponding authors. Tel./fax: + 86 875 3115198. E-mail address: [email protected] (G. Yang). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.04.004

Many instrumental methods including chromatography [4,5], spectrophotometric [6], electrochemical [7–17], and radiometric analysis [18] have been reported to determine different compounds, ions and protein molecules et al. in biological liquids. Electroanalytical methods plays a critical role in the development of determination of analytes, especially in the biological fluids because of the simple treatment of samples, simple experimental protocols, short time analysis and good accuracy high sensitivity, excellent detection limit and low cost materials comparing with t other methods[19–23]. The surface of the electrodes and the determination methods are the crucial problems in the electroanalytical analysis. During the past decades, modern electrochemical techniques and various modified electrodes have been widely used for the determination of electrochemically active compounds [24–31] and other ion analytes [32–35]. AA and UA are electrochemically active compounds and can be determined using electrochemical techniques. However, the electrochemical detection of AA and UA on unmodified electrodes results in poor selectivity since their oxidation takes place at very close high overpotentials [36]. To overcome this problem, numerous works have been devoted to the design of the modified electrodes combing different electrochemical measurements (CV [37], DPV [38–54] linear sweep voltammograms [55,56] and square wave voltammetry [57]). These interfaces introduce electrocatalytic properties or specific molecular interactions and electrochemical measurements which proved

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deacetylation) and chloroauric acid tetrahydrate (HAuCl4.4H2O) purchased from Sigma (St. Louis, MO, USA) used as received without further purification. Unless otherwise stated, all other reagents were of analytical grade and used as received. Porous anodic aluminum oxide (AAO, 0.2 μm) template and polycarbonate membrane (PC, 0.2 μm) were purchased from Whatman. Human urine(without any treatment) and serums samples were obtained from Second Affiliated Hospital of Kunming Medical College without any other treatment and stored at 4 °C (Ethical Committee License Number: s-20101678, which was approved by Affiliated Hospital of Kunming Medical College Ethical Committee). The serum samples were diluted to one tenth with buffer solution without any treatment and stored at 4 °C before used. The supporting electrolyte was 0.1 mol L − 1 phosphate buffer solution (PB), which was prepared with KH2PO4 and Na2HPO4.

to be efficient to reduce overpotentials and to separate AA and UA signals. Many modified electrodes including carbon nanotube [37,45,50,52,54], organic compound [39,46], carbon nanofibers [40], metal nano-particle [38,41], polymer film [42,43,45,51,57] nano-hybrid film [44,48,49] and carbon nanohorn [56] have been reported for simultaneous determination of AA and UA. However, the surface of above-mentioned electrodes is disorder. Recently, nanostructured gold modified electrodes in particular have been the subject of intensive research due to its high electrocatalytic activity [58–64]. At the same time, many researches have indicated that the ordered, rough porous modified and other electrodes with high surface areas can enhance electrochemical characteristics, improve capability of catalyst and signal-to-noise ratio, high Faradaic current density, fast electron-transfer rate, enhanced sensitivities and better detection limit [61,62]. Gold nanowire arrays electrodes have outstanding ability to mediate fast electron transfer kinetics for a wide range of electroactive species and show electro-catalytic activity towards important compounds such as hemoglobin[58] organophosphorous pesticides [64], DNA oligonucleotides [65]and glucose [66], et al. Highly ordered gold nanotube arrays electrodes have attracted growing attention due to their hollow nanostructures which can provide more surface and electroactive sites comparing with nanowire arrays electrodes and nanorodes electrodes. However, there are few reports to study the application of gold nanotube arrays electrodes [67–71] because the method of synthesis of gold nanotube arrays is usually complex and timeconsuming. For instance, a biosensor has been constructed by modifying the glucose oxidase within gold nanotube arrays [67]. Electro-catalytic H2O2 amperometric detection using gold nanotubes electrode ensembles has been reported by Marc Delvauxa et al. [68] and Zhang [69]. Bienzyme horseradish peroxidase- glucose oxidase-modified gold nanotubes has been reported to detect glucose at low overpotentials [70]. Gold nanotube arrays electrode has been successfully utilized to fabricate electrochemical sensor for the determination of MP [71] and a reagentless amperometric immunosensor for human chorionic gonadotrophin based on gold nanotube arrays electrode [72]. In this paper, we synthesis the gold nanotube arrays by utilizing CV to directly deposit onto an anodic alluminum oxide (AAO) template or polycarbonate membrane (PC). At the same time, the mechanism of the direct electrodeposition of gold nanotube arrays has been discussed. Furthermore, we successfully utilize these gold nanotube arrays (using AAO) to fabricate an electrochemical sensor for simultaneous determination of AA and UA by DPV. Due to the excellent electro-catalytic activity, electron transfer capability of gold nanotube arrays and determination method of DPV, this sensor shows high sensitivity, rapid response, long-term stability and excellent detection limit. To the best of our knowledge, it is the first time to use PC membrane to synthesize gold nanotubes by CV, this method is the most efficient and time-saving comparing with previous reports [67,73–79] and this is the first time to simultaneous determination of AA and UA using gold nanotube arrays. This, therefore, would open up a novel approach for effectively synthesizing gold nanotube arrays and detecting AA and UA with high sensitivity, rapid response and long lifetime.

A glassy carbon electrode (GCE, 3 mm diameter) was first polished with emery paper and alumina slurry, successively rinsed thoroughly with absolute alcohol and distilled water in ultrasonic bath, and finally dried in air. With a 0.2% (w/w) chitosan solution, gold nanotubesgrown AAO with different electrodepositing times was attached down on the electrode surface of the GCE, followed by sealing the edge of the AAO template by epoxy resin. After dried for 12 h, the AAO template was dissolved by 1 mol L − 1NaOH for 1 h (Specifically, when a short deposition time of 3 min was used for synthesizing gold nanotubes, the substrate AAO template could be completely dissolved with 1 mol L − 1NaOH for 10 min, if the dissolved time become long, the array will be damaged.), followed by washing the resultant electrode successively with water and then allowing for dry in air. This way, gold nanotube arrays can be readily modified onto the surface of a glassy carbon electrode. The gold nanotube arrays/GCE was stored at 4 °C and a wet environment when it was not used. At the same time, the gold nanotube arrays using PC membrane was dissolved by chloroform for 2 min. The schematic diagram of the stepwise procedure of the sensor was shown in Scheme 1.

2. Experimental

2.4. Experimental measurements

2.1. Apparatus and reagents

The electrochemical measurement was based on a DPV method. Before measurement, the sensor was rinsed doubly distilled water. A three-electrode cell (10 mL PB) was used, consisting of the gold nanotube arrays/GCE as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum foil electrode as the counter electrode. The electrochemical characteristics of the gold nanotube arrays/GCE were characterized by CV and DPV. CV and measurements were carried out at room temperature without stirring from −300 mV to +800 mV (versus SCE). The parameter DPV of measurements were as follows: init E (V) = 0.8; Final E (V) = −0.3;

All electrochemical experiments were performed with a CHI 660C electrochemistry workstation (Shanghai CH Instruments Co., China). Scanning electron microscopy (SEM) analysis was performed using a XL30ESEM-TMP microscope (Philips, Holand). A three-electrode cell (10 mL) was used, consisting of the modified glassy carbon electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum foil electrode as the counter electrode. Ascorbic acid, uric acid, Chitosan (MW-1 × 10 6; 75–80%

2.2. Synthesis of gold nanotubes For the electrodeposition of gold nanotubes, a thin film of Au (30 nm) was first sputtered onto one side of an AAO template or PC membrane to make the membrane conductive. Using a graphite conductive adhesive, a copper wire (5 cm) was connected to AAO template or polycarbonate membrane (PC) on the sputter-coated gold film, followed by drying the graphite adhesive at room temperature for 12 h. In a solution containing 1% (w/w) HAuCl4 and 0.1 M perchloric acid, electrodeposition was performed by CV at the scan rate of 100 mV s − 1 and in the potential range between −300 mV and 500 mV. Gold nanotube arrays (using AAO) were synthesized with varied deposition times of 3 min, 5 min, 7 min, 8 min, 9 min, and 10 min, respectively. At the same time, the gold nanotube arrays (using PC membrane) was only deposited for 2 min. 2.3. Preparation of gold nanotube arrays /GCE

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Scheme 1. The schematic diagram of the stepwise procedure of the sensor.

Incr E (V) = 0.004; Amplitude (V) = 0.025; Pulse Width (s) = 0.05; Sample Width (s) = 0.0167; Pulse Period (s) = 0.2; Quiet Time (s) = 20. The electrochemical detection is based on the change of the DPV response (ip) to samples. 3. Results and discussion 3.1. The mechanism of the direct electrodeposition of gold nanotube arrays by cyclic voltammetry Many investigators have reported electrodeposition of nanowires within templates in which the preparation conditions appear to exhibit a strong influence on the microstructures of the nanowires. However, there are only a few reports on the direct electrodeposition of nanotubes. An explanation of the nanotubes growth mechanism

must remain speculative. Almost researchers hold that the nucleation and growth of gold deposits take place on the existing gold film which has been sputtered onto one face of template, after that the Au crystallites extend gradually into the channels and the concentration polarization and electrochemical polarization at cathode is the critical matter of hollow nanostructure of gold nanotubes [70,72]. Here, we determinate this view is true by observing morphological characterization of rear positions of gold nanotube arrays using SEM, indicating that the initial nucleation and growth of gold deposits on the pre-sputtered gold layer of template form a thin film and after that the gold crystallites extend gradually into the channels (see Fig. 1(a–d). According previous reports, constant cathode current density [70] or standing deposition set-up [73] which can make concentration polarization and electrochemical polarization at cathode, which is the critical factor to make the morphologies of the gold

a

b

c

d

Fig. 1. SEMs of the rear of gold nanotube arrays with different electrodeposition times: (a) 0 min; (b) 2 min, (c) 7 min.

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Scheme 2. The schematic diagram of formation of Au nanotube arrays. (a) Sputtering Au film onto template; (b) Growth of gold deposits on the pre-sputtered gold layer; (c) Extension of the Au deposits into the nanochannels and formation of Au nanotube; (d) When the nanochannels were blanked off, the nanotube is easy to become to solid nanotube.

nanotubes has hollow nanostructure. CV is a type of potentiodynamic electrochemical measurement. In a CV experiment the working electrode potential is ramped linearly versus time, which means the change of potential of working electrode is periodic and wellproportioned. At this stage, we think when the potential scan from positive to negative potential makes the speed of deposition of gold changing from slow to quick and when the potential scan from negative to positive potential, the result is adverse, because of this change results the concentration polarization of C1 and C2 (in Scheme 2) and electrochemical polarization of cathode so that gold nanotubes has the hollow constructer. Furthermore, the electrodeposition of gold nanotubes by CV is similar to a pulsed electrodeposition technique [74], which makes the wall of gold nanotube has the rough and porous construction so that this nanotube arrays has more electrocatalytic activities than the smooth. Additionally, we believe when the pore of template is not banked off, the growth of direction are including the thickness of gold films, thickness of gold nanotube and length of gold nanotube. Adversely, main direction is the thickness of gold nanotube. Accompanying with electrodepositing time increased the morphologies of the gold nanotube change to solid nanotube. The formation mechanism of the gold nanotube arrays is schematically presented in Scheme 2.

3.2. Morphological characterization of gold nanotube arrays The morphology of gold nanotubes prepared with different electrodepositing times (3 min, 5 min, 7 min and 10 min) has been investigated in our previous works [71]. The morphology of the top and lateral gold nanotube arrays has been shown in the Fig. 1 (electrodepositing for 7 min), respectively. From Fig. 2(a) and (b), it can be seen that gold nanotubes are hollow, vertically oriented, the average inner diameter is about 130 nm and the length is about 2.0 μm. Importantly, it can be seen that the wall of gold nanotubes is rough and porous. Furthermore, it also can be observed that the rear position of gold nanotube arrays (Fig. 1(d)) is accidented which indicates the gold nanotubes array can be attached tightly onto the surface of GCE by chitosan solution. Additionally, it demonstrated that the nucleation and growth of gold deposits on the pre-sputtered gold layer of the AAO template form a thick gold film that is strong enough to support the gold nanotubes upon the dissolution of the AAO template from the surface of GCE. Importantly, the gold nanotube arrays (Fig. 2(c–e)) has been synthesized by this method using PC membrane. It can be seen that the bareness of gold nanotubes increased with dissolved time increasing. However, when the dissolved time exceeds 10 min, the gold nanotube arrays was not vertically oriented and destroyed.

a

c

b

d

e

Fig. 2. SEMs of different positions of gold nanotube arrays (electrodepositing for 7 min using AAO): (a–b): a. The top; b. The lateral; (c–e): SEMs of gold nanotube arrays (electrodepositing for 2 min using PC) with different dissolved time by chloroform: c. 3 min; d. 5 min; e. 10 min.

G. Yang et al. / Materials Science and Engineering C 32 (2012) 1323–1330

Fig. 3. Responses of a gold nanotube arrays /GCE obtained in 0.1 mol L− 1 PB buffer solution (pH 7.0) containing 2.5× 10− 5 mol L− 1 AA and 5.0× 10− 5 mol L− 1 UA. Electrodeposition times for gold nanotubes of the gold nanotube arrays /GCE: (a): 3 min; (b):5 min; (c):7 min; (d): 8 min; (e): 9 min; (f): 10 min.

3.3. Effect of electrodeposition time of gold nanotubes on the response of gold nanotube arrays /GCE to AA and UA Fig. 3 describes DPV response of AA and UA at gold nanotube arrays /GC electrodes prepared with various electrodeposition times. It is noteworthy that the response of the modified electrode constructed by hollow gold nanotubes (electrodepositing for 5 min and 7 min) was stronger than that of the modified electrode constructed by solid gold nanotubes (electrodepositing for 8, 9 and 10 min). The larger surface areas available with the hollow nanotubes (to catalyze the oxidation of AA and UA) than that of the solid nanotubes should be responsible for this significance. Upon optimization, an electrodepositing time of 7 min was identified to be the best for preparing gold nanotube arrays for AA and UA sensor. At the same time, Fig. 4 describes CV response of AA and UA at gold nanotube arrays /GC electrodes prepared with various electrodepositing times in 0.1 M PB buffer solution (pH 7.0) containing 2.5 × 10 − 5 mol L− 1 AA and 5.0 × 10 − 5 mol L− 1 UA at 100 mVs− 1. The result also indicates that the gold nanotube arrays/GC electrode (electrodepositing for 7 min) has best catalysis to AA and UA. 3.4. Cyclic voltammetry characterization Electrochemical behavior of AA and UA at a gold nanotube arrays / GC electrode was characterized with CV. In a blank buffer solution

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Fig. 5. Cyclic voltammograms of a gold nanotube arrays /GCE obtained in (a) 0.1 mol L− 1 PB buffer solution (pH 7.0) and (b) 0.1 mol L− 1 PB buffer solution (pH 7.0) buffer solution containing 2.5× 10− 4 mol L− 1 AA and 5.0×10− 4 mol L− 1 UA.

(0.1 mol L − 1 PB), the gold nanotube arrays / GC electrode showed a small background current (Fig. 5 (a)). Upon the addition of 2.5 × 10 − 4 mol L − 1 AA and 5.0 × 10 − 4 mol L − 1 UA in the buffer solution, an irreversible oxidation peak appeared at 0.415 V, 0.640 V, respectively (Fig. 5 (b)), which should be attributed to the oxidation of AA and UA due to the catalysis by gold nanotube. Furthermore, varying scan rate from 20 to 100 mVs − 1 resulted in a positive shift of the oxidation peak potential and an increase of peak current for AA and UA (data not shown). The good linear relationship between the peak current of AA and UA and the square root of the scan rate indicates a typical diffusion-controlled electrochemical behavior for AA and UA at the gold nanotube array/GCE electrode (data not shown). 3.5. Optimization of experimental variables The experimental variables that could affect the oxidation response of AA and UA, including the type and pH of buffer solutions, were then investigated. Response of the electrochemical sensor to 2.5 × 10 − 5 mol L− 1 AA and 5.0 × 10 − 5 mol L − 1 UA was studied in different buffer solutions including borate buffer (a), disodium hydrogen phosphate-citric acid (b), phosphate buffer (c), and acetate buffer (d). The results showed that the strongest current response was achieved in phosphate buffer solution (data not shown). Furthermore, pH value of the supporting electrolyte is an important parameter in determining the performance of electrochemical sensors. The effect of the pH of PB buffer solution on the response of the sensor to 2.5 × 10 − 5 mol L − 1 AA and 5.0 × 10− 5 mol L − 1 UA showed that pH 7.0 was optimal (data not shown). Thus, PB buffer solution at the pH of 7.0 was employed in the rest of the present work. Additionally, according to the reports [37] the electrocatalytical oxidization pathway of AA (Eq. (1)) and UA (Eq. (2)) at the electrochemical sensor can be shown as follows, respectively.

Eq. (1) OH

OH O

O

O

O

OH

OH HO

OH

O

+

2H + + 2e -

O

Eq. (2) O

Fig. 4. Cyclic voltammograms of a gold nanotube arrays/GCE with various electrodepositing times in 0.1 mol L− 1 PB buffer solution (pH 7.0) buffer solution containing 2.5 × 10− 4 mol L− 1 AA and 5.0 × 10− 4 mol L− 1 UA. Electrodeposition times for gold nanotubes of the gold nanotube arrays /GCE: (a): 3 min; (b): 5 min; (c):7 min; (d): 10 min.

HN

H N

O

NH

N H

O +

O NH2

2 H 2O O

N H

H N NH

O

+

CO2

+ 2H + 2e

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G. Yang et al. / Materials Science and Engineering C 32 (2012) 1323–1330 Table 1 Characteristics of calibration graph for the determination of AA and UA. The standard sample

AA

UA

Number Concentration(μmol L− 1) ip(μA) RSD%(n = 5) ip(μA) RSD%(n = 5) 1 2 3 4 5 6 7 8 9

0.1100 1.500 10.00 25.00 50.00 75.00 100.00 200.00 300.00

0.60 1.10 3.54 5.54 17.50 27.00 37.00 68.00 95.00

2.60 2.70 3.00 3.40 2.45 3.50 4.40 4.45 3.50

0.10 0.50 2.87 6.70 15.00 23.00 30.00 60.00 85.00

2.52 2.73 3.12 3.46 2.48 3.43 4.51 4.44 3.52

3.7. Selectivity Fig. 6. Differential pulse voltammograms of a gold nanotube arrays /GCE obtained for AA and UA at different concentrations (a: 10 μmol L− 1, b: 25.0 μmol L− 1, c: 50 μmol L− 1, d: 75 μmol L− 1, e: 100 μmol L− 1).

3.6. The optimal response characteristics of gold nanotube arrays/ GC electrodes Under optimal experimental conditions, DPV of the electrochemical sensor was studied for different concentrations of AA and UA. As expected, the response increased upon the increase of AA and UA concentration (Fig. 6). The calibration curve for the detection of AA and UA with the studied electrochemical sensor is shown in Fig. 7 and the Characteristics of calibration graph for the determination of AA and UA is shown in Table 1 The resulting electrochemical sensor offers an excellent response for AA and UA and the linear response range for AA and UA were 1.02 × 10 − 7–5.23 × 10 − 4 mol L − 1 with a regression equation of Δip (μA) = 1.02 + 0.323C (mol L − 1) and 1.43 × 10− 7–4.64 × 10− 4 mol L − 1 with a regression equation of Δip (μA) = 0.446 + 0.288C (mol L− 1), respectively, the detection limits were 1.12 × 10− 8 mol L − 1 and 2.24 × 10− 8 mol L − 1 at 3σ, respectively. The correlation coefficients were 0.995 and 0.999, respectively. The linear range and the detecting limit of this sensor are better than those reported previously [38,39,41,43,44,51,52,54,55], indicating the excellent catalytic behavior of the gold nanotube arrays on the GCE surface of the present work.

Fig. 7. Calibration curve of a gold nanotube arrays /GCE obtained in 0.1 mol L− 1 PB buffer solution (pH 7.0).

As a successful protocol for the detection of AA and UA in practical clinical applications, good selectivity and high sensitivity are two most important requirements. To assess the selectivity of the gold nanotube arrays /GC electrode, seven potential interferents were investigated in the presence of 2.5 × 10 − 5 mol L − 1 AA and 5.0 × 10 − 5 mol L − 1 UA. The results indicate that glucose, calamine, oxalic acid, glutamic acid and cocaine have no significant interference to AA and UA. At the same time, the DPV curve of the simultaneous determination of AA, dopamine and uric acid was shown in Fig. 8. Although AA (5.0 × 10 − 5 mol L − 1), dopamine (5.0 × 10 − 5 mol L − 1) and AA (5.0 × 10 − 5 mol L − 1) can be oxidized at the gold nanotube arrays /GC electrode, their peak potentials are 0.41 V, and 0.51 V and 0.62 respectively. Therefore, the interferences of dopamine can be readily eliminated by differential pulse voltammetry (Table 2). 3.8. Reproducibility and stability The repeatability of the response current of the gold nanotube arrays/GC electrode was studied with 2.5 ×10− 5 mol L− 1 AA and 5.0 × 10− 5 mol L− 1 UA, showing a low variation coefficient of 5.13% and 4.82% for five successive assays, respectively. The stability of the electrode was studied by measuring its response to 2.5× 10− 5 mol L− 1 AA and 5.0 ×10− 5 mol L− 1 UA over 60 days. When not in use, the sensor was stored at 4 °C and a wet environment. After 10 days, the current response of the sensor still remained up to 98.2% (RSD =2.45%, n =5). These results demonstrated the good repeatability and high stability of the sensor.

Fig. 8. Differential pulse voltammetry curves of gold nanotube arrays/GCE of AA (5.0 × 10− 5 mol L− 1), dopamine and UA (5.0 × 10− 5 mol L− 1) in 0.1 mol L− 1 PB buffer solution (pH 7.0).

G. Yang et al. / Materials Science and Engineering C 32 (2012) 1323–1330

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Table 2 Detection of real samples in real human urine and serum samples. Sample

Urine 1 Urine 2 Serum 1 Serum 2

The sensor (μmol L− 1)

RSD (%, n = 5)

Spectrophotomety (μmol L− 1)

RSD (%, n = 5)

F

t

AA

UA

AA

UA

AA

UA

AA

UA

AA

UA

AA

UA

9.21 15.12 19.14 28.25

10.49 13.43 16.03 19.21

5.13 4.87 5.91 4.52

5.02 5.23 5.98 5.11

8.99 14.32 20.18 27.79

11.00 13.59 15.75 20.31

5.61 5.21 5.03 5.14

4.89 5.32 4.94 5.07

1.14 1.03 1.24 1.25

1.04 1.05 1.51 0.90

1.43 1.21 1.56 1.12

1.67 1.87 1.90 1.78

Note: The F-value and t -value are taken at 95% confidence level and the 8 degrees of freedom.

3.9. Detection of real samples Finally, validation of the proposed electrochemical sensor was evaluated by performing recovery tests for AA and UA in real human urine (The electrolyte was constituted with 1.0 mL urine and 9.0 mL PB(pH 7.0)) and serum (The electrolyte was constituted with 0.5 mL serum and 9.5 mL PB(pH 7.0)) samples without any other treatment. Results were compared with those determined by Second Affiliated Hospital of Kunming Medical College using spectrophotometry (Tabel1) and evaluated by F-tests and t-value which indicates that the two methods agree well with each other. At the same time, Fig. 9 describes differential pulse voltammetry (DPV) response of AA and UA in real human urine samples (Curve (a)) and Curve (b)) and real serum samples (Curve(c) and Curve (d)) without any other treatment at gold nanotube arrays /GC electrodes. This result demonstrates that the proposed method can provide a useful tool for determining AA and UA in human serum and urine. 4. Conclusions

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

In this paper, we developed an electrodeposition approach for directly synthesizing gold nanotube arrays with CV. The mechanism of the direct electrodeposition of gold nanotubes array has been further discussed. Modifying the gold nanotube arrays onto the surface of a GCE, we obtained an electrochemical sensor that showed sensitive and selective response to AA and UA. This work provides a useful platform for rapidly detecting AA and UA with high sensitivity and excellent selectivity. Acknowledgements This work was jointly supported by the Foundation of Baoshan University (grants No. 11B005K) and National Natural Science Foundation of China (Grant No. 20865006).

[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54]

Fig. 9. Differential pulse voltammograms of a gold nanotubes arrays /GCE in real samples (a: urine sample 1, b: urine sample 2, c: serum sample 1, d: serum sample 2).

[55]

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