Bioelectrochemistry 98 (2014) 64–69

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Bioelectrochemistry journal homepage: www.elsevier.com/locate/bioelechem

Prussian blue-modified nanoporous gold film electrode for amperometric determination of hydrogen peroxide Seyran Ghaderi, Masoud Ayatollahi Mehrgardi ⁎ Department of chemistry, University of Isfahan, Isfahan, 81746-73441, Iran

a r t i c l e

i n f o

Article history: Received 9 December 2013 Received in revised form 8 March 2014 Accepted 17 March 2014 Available online 25 March 2014 Keywords: Nanoporous gold film Prussian blue Hydrogen peroxide Amperometric sensor Electrocatalysis

a b s t r a c t In this manuscript, the electrocatalytic reduction of hydrogen peroxides on Prussian blue (PB) modified nanoporous gold film (NPGF) electrode is described. The PB/NPGF is prepared by simple anodizing of a smooth gold film followed by PB film electrodeposition method. The morphology of the PB/NPGF electrode is characterized using scanning electron microscopy (SEM). The effect of solution pH and the scan rates on the voltammetric responses of hydrogen peroxide have also been examined. The amperometric determination of H2O2 shows two linear dynamic responses over the concentration range of 1 μM–10 μM and 10 μM–100 μM with a detection limit of 3.6 × 10−7 M. Furthermore, this electrode demonstrated good stability, repeatability and selectivity remarkably. © 2014 Published by Elsevier B.V.

1. Introduction Hydrogen peroxide is the main product of enzyme-catalyzed reactions and an essential mediator for the analysis of biological reactions. Furthermore, hydrogen peroxide is used in industrial processes such as pharmaceutical, food and plastic processing industries, intensively. High concentration of H2O2 can cause serious damages in the skin and human health. Therefore, highly sensitive, accurate, rapid and economical determination of H2O2 is very important in both biomedical and environmental studies [1,2]. Although many techniques have been reported for the determination of H2O2 including volumetric titration [3], fluorescence [4–6], chemiluminescence [7,8] and spectrophotometry [9,10]; but most of them have some limitations such as low sensitivity, time-consuming, susceptibility to interferences, and in some cases, these techniques require complex and expensive instrumentations [11]. Electrochemical techniques are the powerful methods for the detection of analytes due to their particular characteristics [12,13]. These methods are preferred compared to other techniques for monitoring of H2O2. Most of the employed electrodes in the fabrication of electrochemical H2O2 sensors are based on enzymes [14,15]. By the way, these electrodes have some practical restrictions related to the use of enzyme; enzymes are relatively expensive and unstable [16,17]. Moreover, enzyme-based electrodes are the electrochemical sensors that generate anodic current during electrooxidation of hydrogen peroxide. The redox reaction of hydrogen peroxide has a relatively high potential at these ⁎ Corresponding author. Tel.: +98 311 7932710; fax: +98 311 6689732. E-mail address: [email protected] (M.A. Mehrgardi).

http://dx.doi.org/10.1016/j.bioelechem.2014.03.007 1567-5394/© 2014 Published by Elsevier B.V.

electrodes. However, electrochemically active interfering species, which are usually present in real samples, are easily oxidized at that potential and influence the biosensor sensitivity dramatically by producing an interfering current [18–20]. Consequently, the main problem of these analytical devices is their sensitivity to the interferences, present in analyte solution [21]. By considering these aspects, it is necessary to develop a simple and effective non-enzymatic sensor for measurement of hydrogen peroxide. An efficient approach to lower over-potential and selective reduction of H2O2 is its amperometric detection on the modified electrode by Prussian blue [22]. Prussian blue (PB) or potassium iron (III) hexacyanoferrate (II) is an inorganic polycrystalline complex with well-known electrochromic [23], electrochemical [24], photophysical [25], magnetic [26] and especially electrocatalytic properties [27]. Prussian blue shows the electrocatalytic activity for the reduction of hydrogen peroxide at relatively low potentials with considerable high activity and selectivity [28–30]; therefore, it is usually considered as an “artificial peroxidase” and has been extensively used in the fabrication of electrochemical biosensors [31]. The development of amperometric sensors on the basis of Prussian blue modified electrodes was reported by Karyakin group for the first time in 1994 [32]. Selective determination of H2O2 by electroreduction in the presence of O2 allows a remarkable decrease in the electrode potential, avoiding the influence of interfering species [33]. Recently, increasing attention has been paid on the nanoscale materials (nanoparticles and nanoporous metals) due to their unique characteristics, such as the catalytic activities, optical, electronic and magnetic properties that cannot be observed by their bulk complement [2,34]. Compared with the modified electrodes by metal nanoparticle, the metal electrodes with nanoporous structure have much higher surface

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area and better electron transport [35]. Furthermore, the complexity of adsorbing methods can affect the reproducibility of electrode preparation, the electrochemical behavior and mechanical stability of these electrodes [36]. Metal nanoporous films (NPFs) could be directly formed on the surface of an electrode and overcome these drawbacks [37]. The hydrogen peroxide reduction has been studied with nanoscale gold materials such as gold nanowires [38], gold nanorods [39] and gold nanoparticles systemically [30,40]. Although nanoparticles could be attached onto the electrodes for subsequent application by simple spin coating or drop casting method the adhesion is normally restricted to weak physical or chemical interaction. The complicated nature of these processes can influence the reproducibility of electrode preparation and consequently, the electrochemical behavior and mechanical stability of electrodes prepared in this manner. Growing nanostructured materials directly onto the electrode surface can overcome this problem, while simplifying the fabrication process. In this manuscript, the electrocatalytic activity of the Prussian blue modified nanoporous gold film electrode (PB/NPGF) toward the electrochemical reduction of H2O2 with emphasis on the high sensitivity and selectivity and simplicity and reproducibility of the electrode is presented. Cyclic voltammetry (CV) and amperometry techniques were used in the investigation of H2O2 reduction on PB/NPGF. Moreover, the detection limit, linear range, selectivity and stability are further investigated. 2. Experimental 2.1. Apparatus All electrochemical studies were performed using the potentiostat/ galvanostat μ-Autolab PGSTAT101. Experiments were carried out using a conventional three electrode system containing a PB modified nanoporous gold CDtrode as the working electrode; a platinum wire and a Ag/AgCl/3.0 M KCl as the counter electrode and the reference electrode, respectively. All potentials throughout the manuscript are referred to the Ag/AgCl/3.0 M KCl electrode. The morphology of the electrodes was characterized using scanning electron microscope (SEM) (Seron, Model AIS-2100). A Metrohm pH/mV meter model 827 was used for pH measurements. All the experiments were done at room temperature. 2.2. Reagents and solutions All inorganic salts and reagents were received from commercial resources in analytical grade or better and were used without further purification. The concentration of diluted H2O2 was determined by the classic potassium permanganate titration method. Phosphate buffer solutions (0.1 M) were prepared from NaH2PO4 in the pH range of 2–8 and pH of these solutions was adjusted using HCl and NaOH. Deionized-double distilled water was used throughout the manuscript.

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2.3. Preparation of the electrodes The working electrodes were prepared using small pieces of recordable disks (CD-R) made of gold (CDtrode) as previously reported in the literature [36,41,42]. Briefly, a piece of CD was cut and the protective layer was removed by putting it in the concentrated HNO3. Then it was washed with water thoroughly. Now the gold surface is exposed and it could be used as a working electrode. Usually, the chemical attack to the protective films requires just a few minutes and after that, the remaining material can be easily removed with water. The NPGF electrode was prepared in two steps. In the first step, the gold CDtrode is anodized in a phosphate buffer solution (pH 7.4) for three minutes, by applying a potential step from the open-circuit potential (OCP) to 4 V. The progression of oxide film was accompanied by gas production at all time until anodizing process was finished. In the second step, the reduction of gold oxide to metallic gold was performed using ascorbic acid as a non-toxic and inexpensive reducing agent. The anodized gold substrate incubated in a solution of ascorbic acid (1.0 M) for 3 min. The color of the gold surface became dark due to its high surface area and nanometer crystal size [36,37]. The modification of the NPGF electrode surface using Prussian blue film was accomplished by repetitive potential cycling over − 0. 1 to + 0.5 V (10 cycles) at a scan rate 20 mV/s in an oxygen-free solution containing 0.1 mM FeCl3 and 0.1 mM K3Fe(CN)6. A solution of 0.1 M HCl and 0.1 M KCl was used as supporting electrolyte. After film deposition, electrochemical activation of PB film was performed by cycling at a scan rate 50 mV/s in the same supporting electrolyte until a stable voltammogram was obtained [22].

3. Results and discussion 3.1. Characterization of PB/NPGF electrode The morphology of the smooth gold film, nanoporous gold film and PB modified NPGF was characterized using SEM. The images of SEM are shown in Fig. 1. The surface micrograph of NPGF reveals a nanoporous film, while that of the gold film substrate showed a smooth film. Furthermore, it is very clear that PB/NPGF electrode surface is coated by a uniform PB film with tiny and regularly PB grains. Moreover, the cyclic voltammetric experiments in 0.5 M sulfuric acid solution demonstrates that NPGF electrode roughly has a six-fold surface area rather than smooth gold electrode following electrode processing (Fig. S-1). Cyclic voltammetry (CV) is a powerful method in the investigation of electrochemical modification of electrodes and has been used in the present study to characterize the modified electrodes. Fig. S-2 implies the typical cyclic voltammograms of the electrodeposition of PB film on the surface of NPGF in PB solution containing 0.1 mM FeCl3 and 0.1 mM K3Fe(CN)6. As it can be seen, pair of redox peaks grows gradually as the cycles increase. The current increases incessantly that indicates PB film is accumulating on the modified electrode. After 10 cycles, the currents of voltammograms don't increase more, which demonstrates

Fig. 1. SEM images of (a) the smooth gold film (b) the nanoporous gold film and (c) the PB modified NPGF.

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that the electrodeposition of PB is almost complete. Therefore, in this study, 10 cycles are selected as an optimum condition for PB deposition on NPGF. The anodic and cathodic peak potentials are located at +0.23 V and + 0.18 V, respectively. The formal potential (E0) is calculated + 0.20 V and the peak potential separation is 10 mV. These values elucidate high conductivity of the film and high reversibility of the redox reaction which occurred at the modified electrode surface and also they are comparable to those which were reported previously [43,44]. The effects of scan rates and pH changes as well have been discussed in supporting information. 3.2. Electrocatalytic reduction of H2O2 at the PB/NPGF electrode

The cyclic voltammograms of PB/NPGF in the presence of 1 mM H2O2 in 0.1 M PBS (pH 2) at different scan rates over range of 10–200 mV/s were recorded (Fig. 3A). These voltammograms demonstrate that, the reduction peak currents are directly proportional to the square root of the scan rates (Fig. 3B). Therefore, the process of H2O2 reduction is controlled by diffusion, which is a suitable behavior for quantitative application. Furthermore, a plot of Ipc/Ipa versus scan rate exhibits an indicative shape typical of an EC catalytic process [43] and shows that the electrocatalytic process performs better at the lower scan rates (Fig. 3C).

A

In order to study the electrocatalytic activity of the modified electrodes toward H2O2 reduction, their voltammetric response was recorded at various pHs. The effect of pH on the voltammetric behavior of PB modified NPGF electrode was studied by cyclic voltammetry in 0.1 M phosphate buffer solutions at pH values changing from 2 to 8 (Fig. S-4). In all cases, the formal potential of the surface redox couple remains unchanged, but the peak currents decreased with increasing pH. Although, stable and reproducible cyclic voltammograms were observed at pH 2–8, the most well-defined voltammograms were obtained at pH 2, so this value was chosen as an optimum pH for further studies. Also the electrocatalytic behavior of the modified electrode has been investigated in the absence and presence of different concentration of H2O2 as well. Fig. 2 illustrates the cyclic voltammograms of the PB/ NPGF electrode in the 0.1 M PBS, pH 2, and in the presence and absence of hydrogen peroxide. By gradual addition of H2O2, the reduction peak current increased gradually and the oxidation peak current for PB decreased. This behavior is exactly in-line with an electrocatalytic reduction and indicating the catalytic properties of the modified electrodes by PB to the reduction of H2O2. Note that on a bare electrode, a current for the reduction of H2O2 is not observed in this potential window.

Increase of scan rate

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Current /µA

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Potential/V (vs Ag/AgCl)

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y = 14.57x - 45.31 R² = 0.994

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2 1

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0

0.2

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0.6

0

50

100

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250

scan rate/ (mV/s)

potential /V (vs. Ag/AgCl) Fig. 2. Cyclic voltammograms of the NPGF electrode (curve a) in the presence of 10 mM H2O2 and the PB/NPGF electrode in the absence (curve b) and presence (curve c and d) of 1 and 10 mM H2O2. (Supporting electrolyte 0.1 M PBS and 0.1 M KCl solution pH 2, scan rate: 50 mV/s).

Fig. 3. (A) Cyclic voltammograms of the PB modified NPGF electrode in the presence of 1 mM H2O2 in 0.1 M PBS (pH 2) at different scan rates over a range of 10–200 mV/s. (10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180, 200 mV/s). (B) The plot of anodic (a) and cathodic (b) peak currents vs. square root of scan rate. (C) The plot of Ipc/Ipa vs. scan rate.

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3.3. Amperometric studies

reduction current of H2O2 at the modified electrode in the presence of dissolved oxygen does not show significant changes. Therefore, the PB/NPGF electrode shows good selectivity for the electrocatalytic reduction of H2O2 in the presence of dissolved oxygen in the operating conditions. Furthermore, the interferences of five biological compounds such as glucose, fructose, sucrose, ascorbic acid (AA) and uric acid (UA) were studied at the PB/NPGF electrodes by amperometry method. The amperometric response of the modified electrode to sequential additions of these compounds and H2O2 was measured. The current responses generated due to these interfering species are negligible. This behavior could be attributed to the difference in molecular weight of biological compounds and H2O2. PB has the zeolithic nature with a cubic unit cell, that allows the diffusion of low molecular weight molecules such as H2O2, but compounds with large molecular weight such as biological compounds cannot diffuse into the cubic unit cell [27]. Therefore, this sensor has high selectivity and strong anti-interference ability.

The analytical performances of the sensor toward H2O2 reduction were more investigated using amperometric technique under the optimum conditions (0.1 M PBS, pH 2, applied potential of − 0.05 V vs. Ag/AgCl). Fig. 4A shows a typical current versus time curve for the sequential addition of H2O2. The amperometric experiments were used to draw calibration curves and two linear dynamic ranges (1 × 10−6 −1 × 10−5 M and 1 × 10−5 −1 × 10−4 M) were obtained with a correlation coefficient of 0.992 and 0.994, respectively (Fig. 4B). The existence of two linear dynamic ranges is indicating different mechanism at these ranges [45]. At low concentrations of hydrogen peroxide, the electrocatalytic mechanism is dominant, but at higher concentrations, the direct reduction of hydrogen peroxide on the surface can play an important role in the analytical signal. The detection limit of the electrode for hydrogen peroxide was found to be 3.67 × 10− 7 M (at a signal-to-noise ratio of 3). This value is comparable with corresponding values reported by the other existing sensors using Prussian blue prepared from chemically or electrochemical deposition, as presented in Table 1.

3.5. Stability and repeatability study The stability of the NPGF modified electrode was examined using cyclic voltammetry in the 0.1 M PBS solution containing 0.1 M KCl (pH 2). Cyclic voltammograms were recorded for 2 months, every day. The plot of the reduction and oxidation peak current vs. time is shown in Fig. S-6. As it can be seen, there are no significant changes in the peak current during the time. The stability of electrode response demonstrates the long-term applicability of the PB/NPGF electrode. The repeatability of the construction of the electrode (anodization and modification by PB) was also investigated. Fig. S-6B displays the recorded voltammograms of five PB/NPGF electrodes in the 0.1 M KCl (pH 2). The relative standard deviation (RSD) for the reduction peak

3.4. Effect of electroactive interferences Selectivity is an important factor which affects accurate determination of the sensor. In this paper, the interference effect of molecular oxygen and some biological compounds was studied. Cyclic voltammetry was used for investigation of interference effect of molecular oxygen on H2O2 reduction. The cyclic voltammograms of 2 mM H2O2 solution at the PB modified NPGF electrode was recorded in the presence (a) and the absence (b) of dissolved O2 (Fig. 5). As it can be seen, the

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Fig. 4. (A) The current-time profiles obtained at the PB/NPGF electrode under the conditions (0.1 M PBS (pH 2), applied potential of − 0.05 V vs. Ag/AgCl) during the successive addition of H2O2 in the range 1–10 μM and 10–100 μM. (B) The calibration graphs derived from the current-time profiles. (peak current/μA = 0.096[H2O2]/μM + 1.614, R2 = 0.992 and peak current/μA = 0.022[H2O2]/μM + 3.023, R2 = 0.994).

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Table 1 Comparison of different PB film modified electrodes for H2O2 determination. Electrode

Linear range (mM)

Detection limit (μM)

Sensitivity (A/Mcm2)

Reference

PBNPs/CCEa,b PBNPs/CS/ITOc,d PB@Ptnano/PCNTs/GCEe PB/GO/GCE PB/MPS/Auf GC/RGO/PB/PTBOg,h PB/RTIL/CNTs/GC PB/NPGF

1–0.26 0.01–0.4 2.5 × 10−4 − 1.5 0.005–1.2 0.002–0.2 0.005–0.6 4.9 × 10−4 − 0.7 0.001–0.01 and 0.01–0.1

700 0.27 0.15 0.122 1.8 1.5 0.49 0.36

0.7546 3.79 0.85 0.4087 – 0.420 0.1859 0.8

[43] [46] [47] [48] [31] [49] [50] Our work

a b c d e f g h

Prussian blue nanoparticles. Carbon ceramic electrode. Chitosan. Indium tin oxide. Poly(diallyldimethylammonium chloride) modified carbon nanotubes. (3-mercaptopropyl)-trimethoxysilane. Reduced graphene oxide, Poly(toluidine blue O).

current and potential was obtained 6.57% and 9.1%, respectively. Thus, the proposed PB-modified electrode was found to exhibit excellent stability and reproducibility.

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.bioelechem.2014.03.007.

4. Conclusions In summary, a new electrochemical sensor based on PB electrocatalyst using nanoporous gold electrode has been introduced for the rapid and accurate determination of hydrogen peroxide. Cyclic voltammetric experiments demonstrated that Prussian blue on NPGF platform shows an effective electrocatalytic behavior for hydrogen peroxide reduction. Amperometric experiments were used for investigation of analytical performances of the electrode. This sensor shows high selectivity and sensitivity and offers several advantages such as simplicity, rapidity and low cost.

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Current /µA

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0.15

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Potential /V (vs Ag/AgCl) Fig. 5. Cyclic voltammograms of 2 mM H2O2 in PB modified NPGF electrode in the presence (a) and the absence of dissolved oxygen (b) in 0.1 M PBS (pH 2).

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