Journal of Solid State Electrochemistry

, Volume 16, Issue 3, pp 1099–1104

Electrocatalytic oxidation of glucose on nanoporous gold membranes

Authors

  • Qianqian Li
    • School of Chemistry and Chemical EngineeringShandong University
  • Songzhi Cui
    • School of Chemistry and Chemical EngineeringShandong University
    • School of Chemistry and Chemical EngineeringShandong University
    • School of Chemistry and BioscienceYili Normal University
Original Paper

DOI: 10.1007/s10008-011-1501-x

Cite this article as:
Li, Q., Cui, S. & Yan, X. J Solid State Electrochem (2012) 16: 1099. doi:10.1007/s10008-011-1501-x

Abstract

With characteristic of structural integrity and high surface area, nanoporous gold (NPG) prepared by dealloying method is proposed to be a highly sensitive catalyst for glucose electrooxidation. It can be found that a-NPG which obtained by electrochemical corrosion method has the highest sensitivity for glucose electrooxidation among the three studied samples. Under alkaline conditions, the catalytic current density of a-NPG is over 1.5 times and 17 times higher than that of f-NPG (prepared by free corrosion) and poly-Au electrode, respectively. Using a-NPG sample for glucose detection, the obtained minimum sensible concentration are 413 nM in alkaline media and 1 μM in neutral solutions. The a-NPG electrode also shows stable recovery and reproducibility characteristics. These results indicate that NPG may work as an efficient electrode material for electrochemical sensors and a promising catalyst for alkaline glucose fuel cells.

Keywords

Glucose oxidationNanoporous goldElectrocatalysisDetectionNonenzymatic sensor

Introduction

Electrocatalytic oxidation of glucose, as a key reaction in the fields of sensors [25, 29] and fuel cells [13, 19], has been widely investigated during the past decades. Nanomaterials with special surface structures and properties provide great opportunities for this oxidation reaction [4]. Besides the widely used nanoparticles [30, 33, 34], the reported nanostructures also include porous materials [9, 15, 22]. Due to the three-dimensional and continuous pore channels, porous metallic materials possess excellent performances for catalysis, sensing, and biotechnology [6, 8, 10, 18]. Among various porous metallic materials, the nanoporous gold (NPG), a unique class of functional materials for catalytic applications [21, 27, 28], have been extensively studied since Au would not be self-poisoned [14, 24, 26], and its activity could be regenerated after removing the oxidation products on the surface [2, 3]. Although there are so many approaches to achieve this desired material [16, 17], the simple dealloying process [7] represents a facile method to fabricate NPG with extremely clean surface, which is a well-known significant factor for electrochemistry.

As have been proved in previous works, NPG shows superior activity toward a series of important electrochemical reaction including methanol oxidation [12, 32], formic acid oxidation [11], thus are regarded as promising electrocatalysts in glucose electrooxidation. Recently, Ma et al. prepared nanoporous Au–Ag alloy with different Ag content, and they found the existence of a tiny amount of silver can obviously enhanced catalytic activity for the electrooxidation of glucose [23]. In this work, our interest is focused on the use of NPG membranes in glucose detection and further in nonenzymatic biosensors. Systematical studies demonstrate that NPG shows much higher electrocatalytic activity and sensitivity toward glucose oxidation in alkaline and neutral solutions than polycrystalline gold. Additionally, glucose detection under physiological condition is also monitored. The results indicate that NPG is a potential material to be used in nonenzymatic glucose biosensor.

Experimental

Reagents and apparatus

All chemicals were of analytical grade and used as purchased without further purification. d-Glucose, NaOH, HNO3 (67%), Na2HPO4·12H2O, and NaH2PO4·2H2O were obtained from Sinopharm Chemical Reagent Co., Ltd. Ultra pure water (18.2 MΩ) was used throughout the experiments, and 0.1 M phosphate buffer solutions (PBS) were prepared with pH 7.4. Au/Ag alloy (50:50, wt.%) leaves with thickness of 100 nm (Sepp Leaf Products, New York) were used for NPG fabrication. The surface structure of NPG was observed use JSM-6700F SEM. All electrochemical measurements were performed at room temperature in a three-electrode electrochemical cell with a CHI 760C electrochemical workstation (Shanghai). Saturated calomel electrode (SCE) was selected as reference electrode in all electrochemical measurements and a pure Pt foil as the counter electrode. Both NaOH and PBS solutions were purged with high pure nitrogen (99.999%) for 20 min prior to measuring.

Preparation of NPG electrodes

NPG with 35-nm pore size was made by dealloying commercial 12-carat white gold (Au50Ag50 wt.%) membrane in concentrated (67%) nitric acid (free corrosion) for 30 min at 303 K (mark as f-NPG). The NPG with 12-nm pore size was obtained by electrochemical dealloying under anodic potential (600 mV) for 25 s at 303 K on a CHI 760C (mark as a-NPG). Subsequently, these two NPG samples were immediately transferred to ultra-pure water and repeatedly washed to remove Ag+ and NO3-. Then, the membranes were affixed onto a clean glassy carbon electrode and fixed with 2 μL dilute Nafion solution (0.5 wt.%). The as-prepared NPG electrodes were dried at room temperature before measuring.

Results and discussion

Surface structure of the NPG

Figure 1 shows the SEM images of NPG samples, which exhibits unique structure of free-supporting, three-dimensional, bi-continuous, and highly porous network with even pore size. Figure 1a shows NPG sample prepared under free corrosion conditions. Dealloying Au/Ag foil is a competition process between silver atoms dissolution which induced surface roughening and gold atoms diffusion which induced surface smoothing. This competition makes the three-dimensional porous structure. The sample obtained under this condition is named f-NPG with pore size about 35 nm. While in Fig. 1b, instead of free corrosion, electrochemical dealloying technique is employed to control the pore size [27]. The sample obtained under this condition is named a-NPG with pore size about 12 nm (Fig. 1b).
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Fig. 1

SEM images of f-NPG (a) and a-NPG (b) samples with different pore sizes

Electrocatalytic properties of NPG for glucose oxidation in alkaline media

The f-NPG and a-NPG samples were further characterized by means of cyclic voltammetries (CV) tests in 0.1 M NaOH aqueous solution with (curve b) and without (curve a) 10 mM glucose solution (Fig. 2). Poly-Au electrode was also used here for comparison. The detailed description of glucose oxidation peaks in Fig. 2 had been listed in Table 1. During the positive scan, the peaks in the potential region between ∼ −1.0 and −0.1 V (column 1, Table 1) were the results of glucose oxidation on active Au surface, producing a layer of adsorbed glucose intermediates on electrode surface [5]. These intermediate species were then oxidized at a positive potential (column 2, Table 1) [1]. During the negative scan, the glucose was re-oxidized on the partially oxidized Au surface followed by the partial reduction of surface gold oxides, giving an extend oxidation peak (column 4, Table 1). Meanwhile, compared with poly-Au and f-NPG, the obvious negative shift of the onset potential and enhancement of peak current density (column 3, Table 1) of the a-NPG indicated a superior catalytic activity toward glucose oxidation [23].
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Fig. 2

Cyclic voltammograms (CVs) of different electrodes in a N2-satutated 0.1 M NaOH solutions (curve a) and 0.1 M NaOH + 10 mM glucose (curve b) at 293 K. Scan rate was 20 mV s-1

Table 1

A detailed description of glucose oxidation peaks in Fig. 2

Electrode

Ep1

Ep2

Ip2

En

Poly-Au

−0.6 ∼ −0.16 V

−0.15 ∼ 0.3 V

74.8 mA cm-2

−0.6 ∼ 0.1 V

f-NPG

−0.8 ∼ −0.5 V

−0.5 ∼ 0.3 V

831.2 mA cm-

−0.8 ∼ 0.1 V

a-NPG

−0.9 ∼ −0.5 V

−0.5 ∼ −0.24 V

1270.2 mA cm-2

−0.9 ∼ −0.01 V

Ep1 the potential of first oxidation peak during positive scan

Ep2 the potential of second oxidation peak during positive scan

Ip2 the current density of the second oxidation peak

En the potential of oxidation peak during negative scan

The chronoamperometric (CA) measurements were carried out at −0.2 V for 300 s on these three catalysts in 0.1 M NaOH solution containing 10 mM glucose. The results in Fig. 3 also supported above findings that a-NPG was the most active catalyst among these three samples. The current became relatively stable after a sharp drop during the first few minutes due to double layer charge and discharge. With the reaction going on, the adsorbed intermediate species known as the competitor with the glucose adsorption rapidly formed and occupied the active surface sites to reach dynamic balance. This proved that the active surface area of a catalyst material directly affected the catalytic capability toward oxidation of glucose.
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Fig. 3

Chronoamperometric (CA) measurements carried out on poly-Au, f-NPG, and a-NPG electrodes at −0.2 V for 300 s

The a-NPG possessed a higher number of electrochemically active surfaces than poly-Au, which implied that the structure of a-NPG is beneficial for more glucose to achieve active sites. Linear sweep voltammetry was carried out to integrate the anodic peak charges for glucose oxidation at different scan rates as shown in Fig. 4. Qv represented the integrated charge of the anodic peak for glucose oxidation a different scan rates. Qm represented the charge obtained at the scan rate of 2 mV s-1. It demonstrated that the Qv/Qm ratios of both electrodes decreased with the increase of scan rate. However, the decrease on a-NPG was slower than poly-Au, indicating that the oxidation of glucose on a-NPG catalyst was controlled by activation polarization at lower scan rates because the nanoporous structure provided more active sites for glucose oxidation. This was further proven by plotting the peak current density against the square root of the scan rate as shown in the inset of Fig. 4. It showed a straight line on poly-Au electrode while a-NPG deviated from the line, which can be attributed to the gradient concentration effect in the nanoporous structure of a-NPG.
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Fig. 4

Curve of normalized charge of anodic peak of glucose oxidation on poly-Au and a-NPG electrodes against the scan rate. Inset presents the plots of the peak current density against the square root of the scan rate for tow electrodes

To evaluate the catalytic performance of a-NPG catalyst in alkaline solution, the amperometric response toward the oxidation of glucose was tested on rotating disk electrode (1,600 rpm). Figure 5 shows the typical steady-state current time response of the rotated a-NPG electrode. A quick response (response time less than 3 s) was observed during the successive glucose (1 μM) injection under a constant applied potential of −0.2 V (vs. SCE). The calibration plot for glucose current vs. concentration is linear in the range from 1 μM to 10 μM with a slope of 0.32242 μA μM-1. According to the equation: CL = 3 Sd/k [31], where CL is the detection limit, Sd (0.09439 μA) is the standard deviation of the blank response under rotating conditions (1,600 rpm) and k is the slope of the calibration plot, the detection limit of glucose in alkaline solution was calculated as low as 413 nM. This proves that NPG is highly sensitive to oxidation of glucose.
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Fig. 5

Current-time record of a-NPG affixed onto rotating disk electrode (1,600 rpm) by successively injection 1 μM glucose into 0.1 M NaOH solution at a regular interval time of 60 s. Applied potential is −0.2 V. Inset: the corresponding calibration plot of glucose oxidation current vs. concentration

Electrocatalytic properties of a-NPG for glucose oxidation in neutral media

The catalytic performance of a-NPG catalyst in neutral media was tested on rotating disk electrode (1,600 rpm) in 0.1 M pH 7.4 PBS. Figure 6a showed the CVs of a-NPG in PBS with (curve b) and without (curve a) 10 mM glucose at 20 mV s-1. The onset potential of glucose oxidation was about −0.4 V and an obvious oxidation wave was observed at about 0.32 V in positive potential scan process and the re-oxidation peak in negative scan process could be found at about 0.29 V. Therefore, the selected potential for amperometric measurement on a-NPG towards glucose oxidation is 0.3 V in neutral media (Fig. 6b). The linear dependence (R2 = 0.9839) of the amperometric response can be seen in the range of 0 ∼ 10 mM. The low detection limit of 1 μM makes a great possibility for a-NPG catalyst material to be used as enzyme-free glucose sensor in clinical condition.
https://static-content.springer.com/image/art%3A10.1007%2Fs10008-011-1501-x/MediaObjects/10008_2011_1501_Fig6_HTML.gif
Fig. 6

a CVs of a-NPG in N2-satutated 0.1 M PBS (pH 7.4) (curve b) and 0.1 M PBS (pH 7.4) + 10 mM glucose (curve a). The potential scan rate is 20 mV s-1. b Current-time record of a-NPG affixed onto rotating disk electrode (1,600 rpm) by successively injection 1 mM glucose into 0.1 M PBS (pH 7.4) at a regular interval time of 60 s. Applied potential is 0.3 V. Upper left inset: the current response of a-NPG towards 1 μM glucose; lower right inset: the corresponding calibration plot of glucose oxidation current vs. concentration

As we all know, though the real clinical samples normally contains some other species which has similar electroactivities with glucose, such as AA, urea, and sucrose, the physiological level of glucose is much higher than that of those mentioned matters. So, the anti-interferents study of a-NPG was performed in 0.1 M PBS (pH 7.4) with successively injected 5 mM glucose, then 0.1 mM AA, urea, and sucrose which were used as the co-exist samples to disturb the current response of glucose. From Fig. 7a, we can clearly see that not only the stability but also the linear of the amperometric response were not affected by urea and sucrose. The wave on the stability induced by AA was not enough to interfering the detection of glucose that this could be neglected. As shown in Fig. 7b, the recovery characteristics of this electrode toward different concentrations of glucose (0, 5, and 10 mM) were illustrated by current-time records. The tests were carried out by successive injections of glucose at 0 (i.e., PBS without glucose), 5, and 10 mM at 60-s intervals, and then the experiment was repeated using the same electrode. For the states of 0, 5, and 10 mM glucose, the corresponding recovery rates were calculated to be 95%, 90.8%, and 95.6%.
https://static-content.springer.com/image/art%3A10.1007%2Fs10008-011-1501-x/MediaObjects/10008_2011_1501_Fig7_HTML.gif
Fig. 7

a Current-time record of a-NPG affixed onto rotating disk electrode (1,600 rpm) by successively injection 5 mM glucose and 0.1 mM AA, urea, and sucrose into 0.1 M PBS (pH 7.4) at a regular interval time of 60 s; b The recovery characteristics of the a-NPG with 0, 5, 10 mM glucose concentration. Applied potential is 0.3 V

To be used in glucose sensors, it is necessary to evaluate whether all the a-NPG samples formed with this simple method could show the same excellent properties. Six electrodes were independently used to carry out the amperometric response of 5 mM glucose solution in order to reveal the reproducibility. The results were shown in Table 2. The relative standard deviation value was calculated to be 8.15%, which is compatible to the previous reports [20].
Table 2

The data of reproducibility on a-NPG electrodes

Electrode number

1

2

3

4

5

6

Current density, μA cm-2

4.2

4.8

4.6

3.9

4.3

4.8

Conclusions

NPG membranes, a type of porous nanostructures, were fabricated by dealloying and were studied for glucose electrooxidation and detection. Taking advantage of the smallest pore size and largest active surface area, a-NPG exhibits the best activity and sensitivity toward the reaction both in neutral and alkaline conditions. The detection results in neutral media also promised that a-NPG exhibited excellent anti-interference from AA, urea, and sucrose at normal physiological levels and reproducibility characteristics. The a-NPG is thus expected to be a promising electrocatalyst for application in the fields of glucose electrochemical sensors and fuel cells.

Acknowledgment

We thank Prof. Y. Ding and Houyi Ma for valuable discussions and sharing their nanomaterials and facilities.

Copyright information

© Springer-Verlag 2011