Silver nanowire array sensor for sensitive and rapid detection of H2O2

Electrochimica Acta 104 (2013) 439–447

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Silver nanowire array sensor for sensitive and rapid detection of H2 O2 E. Kurowska a,∗ , A. Brzózka b,1 , M. Jarosz a , G.D. Sulka a,1 , M. Jaskuła a a b

Department of Physical Chemistry and Electrochemistry, Jagiellonian University, Ingardena 3, 30060 Krakow, Poland AGH University of Science and Technology, Faculty of Non-Ferrous Metals, Mickiewicza 30, 30059 Krakow, Poland

a r t i c l e

i n f o

Article history: Received 31 August 2012 Received in revised form 5 January 2013 Accepted 15 January 2013 Available online 23 January 2013 Keywords: Silver Nanowires Hydrogen peroxide Sensor

a b s t r a c t A novel amperometric sensor based on silver nanowire (Ag NW) array was fabricated and used for a rapid and sensitive detection of hydrogen peroxide (H2 O2 ). The 50-nm in diameter nanowires were synthesized by electrodeposition of silver in the nanopores of anodic aluminum oxide (AAO) template. At an applied potential of −0.2 V vs. SCE, the developed Ag NW array electrode exhibited electrocatalytic response toward the reduction of H2 O2 at a very wide range of H2 O2 concentrations and three significantly linear sections in a calibration plot were observed. The determination of H2 O2 with the highest sensitivity of about 0.0266 ␮A cm−2 ␮M−1 is possible in the concentration range from 0.1 mM to 3.1 mM with a detection limit of 29.2 ␮M. Moreover, the proposed sensor exhibited relatively high sensibility, good reproducibility, and satisfying long-term stability. The developed Ag NW array sensor excluded the interference from substances typically present in the biological samples such as ascorbic acid (AA), uric acid (UA), ethanol (EtOH), glucose (G) and oxalate ions. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction In recent years has been a considerably increase of interest in the design and development of new methods for the determination of hydrogen peroxide (H2 O2 ). A rapid, accurate and reliable determination of hydrogen peroxide traces is an important issue due to the fact that H2 O2 plays a crucial role in many fields including foods and agriculture products [1,2], cosmetic and pharmaceutical components [3], chemical and biochemical industry [4–7], clinical control [8–10], environmental protection [11,12] and fuel cells [13,14]. Furthermore, hydrogen peroxide as strong oxidant is used for medical and pharmaceutical sterilizations, and paper bleaching [15,16]. Over the past years, the detection of hydrogen peroxide has been recognized as a very important task in many biological, medical and clinical studies [5–9,17]. For example, it has been found that reactive oxygen species (ROS) – also H2 O2 – can be considered as the mediators of the biochemistry of cellular pathology and may be involved in etiology of several neurodegenerative diseases, including Parkinson’s disease [5]. Hydrogen peroxide is also a final product of various enzymatic reactions, and its concentration may be used as a direct indicator of the reaction progress. One of the most commonly studied enzymatic reactions is catalytic oxidation of glucose to hydrogen peroxide and gluconolactone by glucose oxi-

∗ Corresponding author at: Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30060 Krakow, Poland. Tel.: +48 12 663 22 66; fax: +48 12 634 05 15. E-mail address: [email protected] (E. Kurowska). 1 ISE member. 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.01.077

dase. By monitoring the production of H2 O2 , glucose level in blood can be precisely determined [10]. Among various methods used for detection and determination of hydrogen peroxide, such as titration [18], spectrophotometry [19], fluorometry [20] and chemiluminescence [21], chromatography [22], electrochemical techniques [17,23–26] allow to perform direct and real-time measurements even in biological samples. The electrochemical sensors with an excellent detection limit toward H2 O2 are commonly based on enzymes [e.g., 17,25,26]. Unfortunately, enzymes as biologic macromolecules, cannot provide a complete long-term stability of the sensors due to their inherent instability causing denaturation. Moreover, the enzyme-based sensors for H2 O2 determination gradually lose their catalytic activity during measurements and there are not selective when used in biological systems containing other electroactive compounds such as ascorbic acid, uric acid, 3,4-dihydroxyphenylacetic acid, paracetamol, bilirubin, dopamine, and catecholamines [24,25,27]. These reductants oxidize at similar potentials and affect significantly selectivity of the sensor. The way to avoid these interferences is to perform the detection of hydrogen peroxide by cathodic reduction at low potentials. In order to avoid many disadvantages of native enzyme-based sensors for the electrochemical determination of H2 O2 , a considerable attention has been focused on development of novel non-enzymatic electrodes based on nanomaterials with enhanced enzyme-mimetic properties. The use of nanostructured materials is one of the most promising strategy to replace macroscopic electrodes. The presence of nanosized objects (e.g., nanopores, nanoparticles and nanorods/nanowires) in the electrochemical sensor can decrease the overpotential of many analytes and

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Table 1 Comparison of non-enzymatic hydrogen peroxide sensors based on nanomaterials. Where: GCE – glassy carbon electrode, CPE – carbon paste electrode, AA – ascorbic acid, AP – acetaminophen, DA – dopamine, EtOH – ethanol, G – glucose, HOAc – acetic acid, MeOH – methanol, UA – uric acid, n.a. – not available data. L.p.

Sensor type

Detection potential (V)

Linear range (M)

Detection limit [␮M]

Interfering substance studied

Ref.

1 2 3 4 5

MWCNTs-PEDOT NPs/GCE CMnO/CPE Co3 O4 nanowalls/Co Mn-NTA NWs-Nafion/GCE Cu NPs-PPy NWs/Au

−0.5 V vs. Ag/AgCl +0.3 V vs. Ag/AgCl −0.2 V (vs. Ag/AgCl +0.7 V vs. SCE −0.3 V vs. SCE

0.1–9.8 × 10−3 1.0–6.9 × 10−4 0–5.35 × 10−3 5 × 10−6 –2.5 × 10−3 7.0 × 10−6 –4.3 × 10−3

174 2 10 0.2 2.3

[24] [30] [23] [31] [32]

6

PB-MWCNTs-Au NChs/Au

0.0 V vs. SCE

1.75 × 10−6 –1.14 × 10−3

0.5

7

Nanoporous Au/GCE Nanoporous Au/GCE Nanorough Ag

(a) 0.2–2.5 × 10−3 (b) 0.2–2.2 × 10−3 1.0 × 10−5 –8.0 × 10−3 10 × 10−6 –22.5 × 10−3

n.a.

8 9

(a) +0.4 V vs. SCE (b) −0.15 V vs. SCE −0.4 V vs. SCE −0.3 V vs. SCE

AA, UA, DA n.a. n.a. n.a. HOAc, EtOH, MeOH, G, glycine, l-cysteine AA, HOAc, EtOH, G, l-tyrosine, l-cysteine AA, UA

[34] [35]

Porous Au–Pt NPs PVP-Ag NWs/GCE Ag NPs-PVA/Pt Ag NPs-PAYR/GCE Ag NPs-ZnO/GCE AgNPs-rGO/GCE Au/Ag NPs-MTMOS/GC Ag NPs-PMo12-PANI/GCE Ag-DNA NPs/GCE Ag NPs-MWCNTs/Au Ag NPs-CNT/Au Ag NW array

+0.1 V vs. Ag/AgCl −0.3 V vs. Ag/AgCl −0.5 V vs. SCE −0.4 V vs. SCE −0.25 V vs. Ag/AgCl −0.3 V vs. Ag/AgCl −0.65 V vs. SCE 0.0 V vs. Ag/AgCl −0.45 V vs. Ag/AgCl −0.2 V vs. Ag/AgCl −0.45 V vs. Ag/AgCl −0.2 V vs. SCE

0.3–10 × 10−3 0.02–3.62 × 10−3 45 × 10−6 –6.0 × 10−3 1.0–450 × 10−6 2 × 10−6 –5.5 × 10−3 0.1–60 × 10−3 10–70 × 10−6 2–20 × 10−6 2 × 10−6 –2.5 × 10−3 0.05–17 × 10−3 0.05–0.5 × 10−3 0.1–3.1 × 10−3

50 2.3 10 0.32 0.42 1.8 1 0.75 0.6 0.5 1.6 29.2

AA, G, EtOH, MeOH AA, UA, SO4 2− , CO3 2− , Fe3+ , ClO3 − , Cl− AA, AP n.a. n.a. AA, DA, UA AA, AP, DA AA, DA, UA, G n.a. n.a. AA, AP, UA AA, AP, UA n.a. AA, OA, SO, KO, EtOH, G, UA

10 11 12 13 14 15 16 17 18 19 20 21

improve the sensitivity of electroanalytical method due to a large surface to volume ratio and prolonged entrapment of analyte in nanostructured electrodes [28]. According to Han et al. [29], electrodes with a nanoporous structure offer better environments for electrochemical reactions where interactions between the analyte and electrode pore walls occur more frequently (extended time of analyte entrapment). Furthermore, a small distance between analyte and the electrode surface inside pores significantly accelerates electron transfer between the electrode and analyte and, consequently, enhances rapid current response for the target molecule. A great variety of nanostructured materials with enhanced enzyme-mimetic properties have been recently employed for electrochemical detection of hydrogen peroxide, e.g., multiwalled carbon nanotubes with poly(3,4-ethylenedioxythiophene) nanoparticles on the glassy carbon and indium tin oxide electrodes (MWCNTs-PEDOT NPs/GCE, ITO) [24], nanostructured cryptomelane-type manganese oxide on the carbon paste electrode (CMnO/CPE) [30], cobalt oxide (Co3 O4 ) nanowall electrodes synthesized by a direct heating of Co foils [23], Mn-nitrilotriacetate acid nanowires in nafion on the glassy carbon electrode (Mn-NTA NWs-Nafion/GCE) [31], copper nanoparticles on the Au electrode modified with polypyrrole nanowires (Cu NPs-PPy NWs/Au) [32], and Prussian blue nanorods at multiwalled carbon nanotubes and gold nanochains on the Au electrode (PB-MWCNTs-Au NChs/Au) [33]. In addition, nanostructured noble metals have been used for electrochemical detection of H2 O2 [28,34–36]. Very recently, the nanoporous gold electrode prepared by dealloying of the Au–Ag alloy [28,34], electrochemically roughened silver electrode [35] and Au electrode with a very rough surface arising from electroplated Pt nanoparticles [36] have been proposed for a nonenzymatic detection of H2 O2 . Various hybrid materials, consisting of noble metal nanoparticles or nanowires incorporated in inorganic/organic matrices, are commonly employed to modify electrodes used for the electrocatalytic reduction of H2 O2 . For example, polyvinylpyrrolidone-silver nanowires randomly distributed on the glassy carbon electrode (PVP-AgNWs/GCE) [37], Ag nanoparticles immobilized in a polyvinyl alcohol film on the Pt electrode (Ag NPs-PVA/Pt) [38], Ag nanoparticles incorporated in poly(alizarin

3.26 6

[33] [28]

[36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] This work

yellow R) on the glassy carbon electrode (Ag NPs-PAYR/GCE) [39], Ag nanoparticles on the glassy carbon electrode modified with ZnO (Ag NPs-ZnO/GCE) [40], Ag nanoparticles-decorated reduced graphene oxide on the glassy carbon electrode (Ag NPsrGO/GCE) [41], core–shell Au/Ag nanoparticles embedded in the methyltrimethoxysilane sol–gel network on the glassy carbon electrode (Au/Ag NPs-MTMOS/GCE) [42], Ag nanoparticle-incorporated phosphomolymdate-polyaniline on the glassy carbon electrode (Ag NPs-PMo12-PANI/GCE) [43], Ag-DNA hybrid nanoparticles electrodeposited on the glassy carbon electrode (Ag-DNA NPs/GCE) [44], and multi-wall carbon nanotube/silver nanoparticle nanohybrids on the gold electrode (Ag NPs-MWCNTs/Au) [45,46] have been already used for this purpose. Comparing to nanoporous metals, main disadvantages of those hybrid materials are complex structure, lower stability in time and often sophisticated fabrication procedures. Some examples of non-enzymatic H2 O2 sensors based on nanostructured materials together with their most important characteristics are collected in Table 1. One dimensional (1-D) metallic nanorod/nanowire arrays are expected to display improved electrocatalytic performance toward H2 O2 reduction due to their high conductivity, direct and real-time electrical signal transduction, superior stability and large surface to volume ratio that is mandatory for fast reaction kinetics. Similarly to nanoporous metals, the enhanced residence time of reactants near the electrode surface should be observed in nanowire arrays. Therefore, in the current study, we report for the first time on the fabrication of H2 O2 sensor based on the silver nanowire (Ag NW) array. 2. Experimental 2.1. Chemicals and materials A high-purity aluminum foil (0.50 mm thick, 99.999%, Goodfellow), sulfuric acid (Merck), and NaOH (POCh S.A.) were used for the fabrication of porous anodic aluminum oxide (AAO) templates. A commercially available solution containing Ag+ ions at concentrations 28.7 g dm−3 (Alfa Aesar) was used for the synthesis of silver

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nanowires. The following reagents and chemicals were used for the electrochemical measurements and interference tests: hydrogen peroxide (30%, Lach-Ner), Na2 HPO4 (POCh S.A.), NaH2 PO4 ·2H2 O (Chempur), ascorbic acid (C6 H8 O6 , 99% in purity, Aldrial), d-(+)glucose (POCh S.A.), sodium oxalate (POCh S.A.), potassium oxalate (POCh S.A.), and ethanol (POCh S.A.). All chemicals and solvents were of analytical grade (unless it is otherwise noted) and all the solutions were prepared with four time distilled water. 2.2. Fabrication of nanoporous AAO templates Aluminum coupons with dimensions of 0.5 cm × 2.5 cm were degreased in acetone and ethanol, and subsequently electropolished in a mixture of perchloric acid (60 wt.%) and ethanol (1:4 vol.) at the constant potential of 20 V and 0 ◦ C for 30 s. The anodic porous alumina (AAO) films were prepared by a two-step anodization under the constant cell voltage of 25 V in 0.3 M H2 SO4 at 1 ◦ C, as described previously [47–49]. The duration of the first and second anodizing steps was 8 h and 12 h, respectively. The alumina layers formed after the first step of anodization were chemically removed by etching in a mixture of 6 wt.% H3 PO4 and 1.8 wt.% H2 Cr2 O4 at 45 ◦ C for 12 h. After the oxide removal, the second anodization was carried out under the same conditions as used in the first step. All anodizations were performed in a 0.5 dm3 glass beaker cooled by a circulating system (Thermo Haake, DC10-K15) with energetic magnetic stirring. A working surface of the sample was 0.6 cm2 and the rest of aluminum stripe was insulated with an acid-resistant paint. A Pb plate was used as a cathode and the distance between both electrodes was about 3 cm. In order to obtain free-standing AAO templates, remaining Al was removed in a saturated HgCl2 solution. After degreasing in ethanol, a chemical etching of the barrier layer was performed in a 5 wt.% H3 PO4 solution [49]. 2.3. Fabrication of Ag nanowire array electrodes Ag nanowires (Ag NWs) were fabricated by DC electrodeposition into the pores of alumina membranes according to a schematic representation shown in Fig. 1. The home-made 60-␮m thick AAO membranes with a pore diameter of about 50 nm were used for electrodeposition (Fig. 1A). After a sputtering deposition of Ag conductive layer on the bottom surface of the AAO template (Fig. 1B), silver nanowires were electrodeposited in a typical two-electrode cell in the electrolyte containing 28.7 g dm−3 Ag+ (Fig. 1C). The AAO template with the sputtered Ag layer served as the cathode, and a Pt stripe as the anode. The electrodeposition was carried out at room temperature under the constant voltage of −1.1 V for 30 s. After the electrochemical fabrication of nanowires, the AAO membrane with silver nanowires inside the pores was adhered to a copper plate

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(Fig. 1D). A conductive paste containing micro-sized silver particles was used as an adhesive. After about 24 h, when the adhesive dried, the supporting Cu plate was insulated with the non-conductive paint (Fig. 1E). To obtain free-standing Ag nanowire array electrode, the AAO template was dissolved in 1 M NaOH at room temperature for 30 min (Fig. 1F). 2.4. Apparatus and measurements The morphology and chemical composition of silver nanowire arrays were studied using a field-emission scanning electron microscope (FE-SEM/EDS, Hitachi S-4700 with a Noran System 7). The electrochemical measurements were performed on a potentiostat (Reference 3000, Gamry). All the electrochemical experiments were carried out using a conventional three-electrode cell consisting of the Ag nanowire (Ag NW) array electrode as the working electrode, a Pt counter electrode and a saturated calomel reference electrode (SCE). All potential values given below refer to SCE. For comparison of the electrocatalytic performance, different working electrodes, such as a macroscopic Ag plate electrode, Ag adhesive paste-based electrode (Ag microparticle-based conducting paste used for Cu support fixing), and glassy carbon electrode (GCE) were tested as well. All electrochemical measurements were carried out at 25 ◦ C in a 0.1 M phosphate buffer solution (PBS, pH 7.4) deaerated by bubbling pure argon for 30 min prior to the experiments, and maintained under argon atmosphere during measurements. Cyclic voltammograms were obtained in the potential range from 0.0 to −0.6 V vs. SCE at different scan rates ranging from 5 mV s−1 to 200 mV s−1 . Amperometric experiments were carried out in a stirred system by applying a potential step of −0.2 V vs. SCE to the working electrode. Typically, the current–time curves were recorded after the successive additions of 10 ␮L aliquots of 0.1 mM H2 O2 in 100 ml of PBS. To evaluate the catalytic rate constant, chronoamperograms were also recorded in the non-stirred solutions in the absence and presence of hydrogen peroxide over the concentration range of 0.25–2.54 mM. Between measurement, the free-standing Ag NW array electrodes were stored in 0.1 M PBS (pH 7.4). 3. Results and discussion 3.1. Characterization of Ag NW arrays The morphology and composition of the silver nanowire array were examined by SEM observation and EDS analysis. The crosssection view of electrodeposited Ag nanowires inside the AAO template is shown in Fig. 2A. Fig. 2B presents the top-view SEM image of the prepared free-standing Ag NW array after the template removal. The diameter and length of Ag nanowires is about 50 nm and 300 nm, respectively. It can be seen that the nanowire dimensions are highly uniform. The chemical composition of the nanowires was determined by EDS (Fig. 2C) and confirms the existence of Ag element. Other peaks originate from the residues of NaOH used for AAO dissolution and from graphite tape used for the sample fixing to a SEM holder. 3.2. Electrochemical response of the Ag NW array electrode to H2 O2 Hydrogen peroxide is reduced on a silver cathode via the well known mechanism as follows [34,50]:

Fig. 1. Schematic representation of the procedure for fabrication of metallic nanowire arrays.

H2 O2 + e− → OHads + OH−

(1)

OHads + e− → OH−

(2)

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Fig. 2. (A) SEM image of the cross section of Ag NWs in the AAO template; (B) SEM top view image of released free-standing Ag NW array; (C) the corresponding EDS spectrum of Ag NWs.

The net reduction of H2 O2 in alkaline solutions is given by: H2 O2 + 2e− → 2OH−

(3)

To explore potential application of the Ag NW array, the electrocatalytic reduction of hydrogen peroxide on the electrode was studied in by cyclic voltammetry within the potential range of 0.0 to −0.6 V vs. SCE in a deaerated 0.1 M PBS solution (pH 7.4) containing 100 mM H2 O2 at a scan rate of 50 mV s−1 (Fig. 3A). The electrocatalytic performance of Ag NW electrode was compared with the Ag plate, Ag adhesive-based and glass carbon electrodes (GCE). In the presence of hydrogen peroxide, the Ag NW electrode exhibits the strongest catalytic reduction current peak about 2.1 mA cm−2 centered at −0.50 V vs. SCE. The weaker electrocatalytic behavior was observed for the macroscopic Ag plate electrode and for the electrode based on the Ag adhesive paste. In contrast, no response toward the reduction of H2 O2 was observed for GCE. The electrocatalytic activity of the Ag NW array electrode was also examined at different scan rates ranging from 5 to 200 mV s−1 in the presence and absence of hydrogen peroxide (Fig. 3B). When H2 O2 was added into the PBS, a considerable increase of the cathodic peak current was observed for both scan rates 5 and 50 mV s−1 . As shown in Fig. 3B, the cathodic peak current increases with scan rate. The peak current increases in a linear relationship with the square root of the scan rate in the range of 5–30 mV s−1 (Fig. 3C). It suggests a diffusion controlled process occurring at low scan rates. The linear relationships were observed even after 14 and 56 days of the sensor storage. This observation indicates that the free-standing Ag NW array exhibits relatively long-term catalytic performance toward H2 O2 reduction. The enhanced sensor sensitivity after 7 and 14 days of the storage can be attributed to the changes in a specific surface area of the working electrode resulting in increased surface roughness of NWs due to their partial dissolution in the aqueous solution used for conditioning of the sensor. 3.3. Amperometric response and stability of the sensor The amperometric response to hydrogen peroxide at the Ag NW array electrode was investigated by successive addition of 0.1 mM

Fig. 3. (A) Cyclic voltammograms (CVs) of different electrodes (Ag NWs, Ag plate, Ag adhesive paste-based and GCE) in the deaerated 0.1 M PBS (pH 7.4) in the presence of 100 mM H2 O2 at the scan rate of 50 mV s−1 ; (B) CVs of Ag NWs in the deaerated 0.1 M PBS in the absence and presence of 100 mM H2 O2 at different scan rates; (C) plot of cathodic peak current density against the square root of the scan rate.

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H2 O2 to a continuous stirred and deaerated 0.1 M PBS solution at −0.2 V vs. SCE. The optimum electrode potential was selected as −0.2 V in order to avoid a possible oxygen reduction and interferences from some electroactive species, e.g., ascorbic acid, uric acid and glucose. For comparison, Fig. 4A presents also the corresponding current–time responses of the Ag plate, Ag adhesive-based and GCE electrodes. The results clearly demonstrate that the best electrocatalytic response possess the Ag NW array electrode and the reducing current density decreases stepwise after addition of H2 O2 . No response was observed at the GCE, while much weaker current responses were obtained at Ag plate and Ag adhesive paste-based electrodes. For Ag plate electrode, instead of the stepwise response, the gradual decrease in the current density was observed upon successive addition of small amounts of H2 O2 . When the concentration of H2 O2 in the solution was too high, the measured amperometric responses of all electrodes were very noisy. For the Ag NW array sensor, three sections of the linear response with high correlation coefficients were found in a calibration curve (Fig. 4B) but, the most reliable results with a low noise level were observed for the first linear range corresponding to low concentrations of H2 O2 . In this concentration range, the calibration graph is linear over the concentrations 0.1–3.1 mM, and the sensitivity is 0.0266 ␮A cm−2 ␮M−1 H2 O2 (correlation coefficient of 0.9996). The detection limit was estimated to be 29.2 ␮M. The sensor was examined for their storage and operational stability. A long-term stability of Ag NW array sensor was examined after various time intervals (Fig. 4C). Fig. 4C shows calibration curves of the freshly prepared Ag NW array sensor, after 14 days, and after 42 days of its storage. For all the cases, the similar linear response range of the sensor to H2 O2 concentration was observed. For the concentration range from 0.1 mM to about 3.1 mM of H2 O2 , the sensitivity of the stored sensor varies between 0.0215 and 0.0329 ␮A cm−2 ␮M−1 . We further investigated the amperometric response of Ag NW array sensor to successive addition of small and high amounts of hydrogen peroxide. Fig. 5A shows the typical current–time response on the addition of 11 samples of 0.01 mM and 0.1 mM H2 O2 under the optimized experimental conditions. As can be seen, the sensor responds to the 0.1 mM H2 O2 and sharp decreases in current density were observed after about 6 s from the injection moments (Fig. 5B). The steady-state current densities were typically achieved within 11–15 s. For the lower concentration of H2 O2 added to the solution, a very weak sensor activity toward reduction of H2 O2 was found. Although the current density decreases with increasing concentration of hydrogen peroxide, the stepwise response was not clearly observed. The corresponding calibration plot of average current densities against concentration of H2 O2 , depicted in Fig. 5C, shows a linear response range from 0.01 mM to 0.1 mM with a relatively poor sensitivity of 14.3 ␮A cm−2 mM−1 and the correlation coefficient of 0.9797. Fig. 6A shows the amperometric response of the Ag NW array electrode to successive addition of 3.26 mM H2 O2 . When the concentration of H2 O2 in the solution is high, the sensor showed a noisily performance and the amperometric response became nonlinear after addition of a few samples of hydrogen peroxide (Fig. 6B). The current response and the concentration of H2 O2 have a linear relationship with the concentration range from 3.3 mM to 19.6 mM and the correlation coefficient of 0.9988. The chronoamperometric measurements clearly demonstrated that the fabricated Ag NW array sensor exhibits satisfying longterm stable analytical performance with a relatively fast response and high sensitivity proportional to the H2 O2 concentration in a range from 0.1 mM to 3.1 mM. The analytical performances of the proposed sensor were compared with other sensors based on nanostructured materials reported in the literature (Table 1). As can be observed, the proposed Ag nanowire array sensor exhibits

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Fig. 4. (A) Amperometric response of the Ag NW array, Ag plate, Ag adhesive pastebased and GCE sensors to successive addition of 0.1 mM H2 O2 at −0.2 V vs. SCE in a continuously stirred and deaerated 0.1 M PBS solution (pH 7.4); inset: scale-up amperometric responses of first H2 O2 additions during 90–500 s; (B) plot of H2 O2 concentration vs. catalytic current density with the marked three linear ranges; (C) calibration curves for the freshly prepared and stored Ag NW array sensor.

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Fig. 6. (A) Amperometric response of the fabricated sensor to the successive addition of 3.26 mM H2 O2 in the 0.1 M PBS solution (pH 7.4) at the applied potential of −0.2 V vs. SCE; (B) corresponding calibration plot between the current density and the concentration of H2 O2 .

excellent performance in terms of good sensitivity and wide linear range. Chronoamperometry can be also used for the evaluation of the catalytic rate constant according to the method described in the literature [51]. At intermediate times (0.2–4 s), the catalytic current (Icat ) is dominated by the rate of electrocatalyzed reduction of hydrogen peroxide and the rate constant for the chemical reaction between hydrogen peroxide and redox sites of the Ag NWs surface can be determined from the following equation: Fig. 5. (A) Amperometric response of the fabricated sensor to the successive addition of different concentrations of H2 O2 in the 0.1 M PBS solution (pH 7.4) at the applied potential of −0.2 V vs. SCE; (B) scale-up sensor response to the five first sample of 0.1 mM H2 O2 injected to the solution; (C) calibration plot between the current density and the concentration of H2 O2 for the test with addition of 0.01 mM H2 O2 .

Icat 1/2 = 1/2 (kCo t) Ibuff

(4)

where Icat and Ibuff are the currents in the presence and absence of hydrogen peroxide, k is a catalytic rate constant, Co is a bulk concentration of hydrogen peroxide, and t is time elapsed. From the slope of Ical /Ibuff vs. t1/2 plot, the value of k can be simply calculated for a given concentration of hydrogen peroxide. The chronoamperograms for the Ag NW array electrode recorded under the optimized experimental conditions in the absence and presence of hydrogen peroxide over the concentration range of 0.25–2.54 mM are shown

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Fig. 8. Two sets of amperometric responses of the fabricated sensor to H2 O2 and different interfering substances at the applied potential of −0.2 V vs. SCE in a continuously stirred and deaerated 0.1 M PBS solution (pH 7.4).

3.4. Interference study

Fig. 7. (A) Chronoamperograms of the Ag Nw array electrode in the absence and the presence of different concentrations of hydrogen peroxide in the 0.1 M PBS solution (pH 7.4) at the applied potential step of −0.2 V vs. SCE; (B) dependence of Icat /Ibuff on t1/2 derived from the data of the chronoamperograms.

in Fig. 7A. Fig. 7B shows the plots, constructed from the chronoamperograms of the Ag NW array electrode. The average value of k was found to be 2.1 × 104 cm3 mol−1 s−1 , demonstrating that the catalytic reaction proceeds at a moderate rate at the applied potential step of −0.2 V.

To investigate the selectivity of the Ag NW array sensor toward H2 O2 reduction, several possible electroactive interferents were examined using the chronoamperometric measurements in the optimized experimental conditions. Two sets of inference tests (A and B) were performed in a continuous stirred and deaerated 0.1 M PBS solution at −0.2 V vs. SCE. Fig. 8A (test A) shows the current responses of the Ag NW array electrode to successive additions of 1.5 mM H2 O2 and 1.5 mM of interfering species, commonly present in physiological samples, such as ascorbic acid, uric acid (UA), and glucose under the optimized experimental condition. During the test B, the amperometric response was studied upon the successive addition of 0.1 mM H2 O2 (three additions), 0.6 mM H2 O2 (three additions) and interfering substances such as 0.2 mM

Table 2 Interference from other substances for H2 O2 determination at the Ag NW array electrode. Where: IR is a selectivity ratio. Interfering substances

Signal change Test A, [H2 O2 ] = 1.5 mM

AA OA SO PO EtOH G UA

Test B, [H2 O2 ] = 2.1 mM

IR

Current change (%)

IR

Current change (%)

1.0 – – – – 1.0 1.0 1.1

0.5 – – – – 1.1 1.9 (first addition) 1.1 (last addition)

1.0110 0.9820 1.0020 0.9837 0.9928 0.9962 –

1.1 1.8 0.2 1.6 0.7 0.4 –

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ascorbic acid (AA), 1 mM oxalic acid (OA), 0.8 mM sodium oxalate (SO), 2 mM potassium oxalate (PO), 1 mM ethanol (EtOH), and 0.5 mM glucose (G). As can be seen from Fig. 8B, an obvious current response was observed with the addition of 0.1 mM and 0.6 mM H2 O2 , whereas the response current was kept almost at the same value after the addition of various interfering substances. The effect of the interfering substances on the Ag NW array response was illustrated in Table 2. The selectivity ratio (IR ), being a ratio of the current density before and after the addition of the interferents, was collected in Table 2. A reinjection of hydrogen peroxide to the solution containing interfering substances produces a further stepwise decrease of the reduction current. In Fig. 8B, the response current of the final additions of 0.6 mM H2 O2 was the same with the one of second additions, suggesting that those interferents did not affect the reduction of H2 O2 at the Ag NW array electrode. The average value of the current density change is 2.39 and 2.44 ␮A cm−2 for the first three and the final three additions of 0.6 mM H2 O2 , respectively. The above result show that the detection of H2 O2 is not influenced by these potential interference compounds. 4. Conclusions In this paper, a novel H2 O2 sensor based on the silver nanowire array was fabricated using the nanoporous home-made AAO template. The experimental results showed that the proposed sensor possesses an excellent electrocatalytic activity toward the reduction of H2 O2 in a 0.1 M PBS solution (pH 7.4). The active response of the sensor covers the wide range of H2 O2 concentrations and three different linear sections are observed in the calibration plot. The best linear range for the determination of H2 O2 with the highest sensitivity of about 0.0266 ␮A cm−2 ␮M−1 was from 0.1 mM to 3.1 mM and the detection limit was 29.2 ␮M. The Ag NW array sensor exhibits satisfying long-term stability and even after 42 days of the storage the sensor retains its sensitivity. The prepared Ag NW array sensor showed a relatively rapid and reproducible electrocatalytic response toward H2 O2 reduction and provided a very promising platform for the development of sensors. Although the proposed Ag NW array electrode showed electrocatalytic activity toward hydrogen peroxide reduction, it did not respond to other electroactive interferents typically present in the biological samples such as ascorbic acid, uric acid, ethanol, glucose and oxalate ions. Acknowledgements The research was partially carried out with the equipment purchased thanks to the financial support of the European Regional Development Fund in the framework of the Polish Innovation Economy Operational Program (contract no. POIG.02.01.00-12-023/08). The SEM imaging was performed in the Laboratory of Field Emission Scanning Electron Microscopy and Microanalysis at the Institute of Geological Sciences, Jagiellonian University, Poland. References [1] C-P. Lu, C-T. Lin, C-M. Chang, S-H. Wu, L-C. Lo, Nitrophenylboronic acids as highly chemoselective probes to detect hydrogen peroxide in foods and agricultural products, Journal of Agriculture and Food Chemistry 59 (2011) 11403. [2] E. Recio, M.L. Álvarez-Rodríguez, A. Rumbero, E. Garzón, J.J.R. Coque, Destruction of chloroanisoles by using a hydrogen peroxide activated method and its application to remove chloroanisoles from cork stoppers, Journal of Agriculture and Food Chemistry 59 (2011) 12589. [3] L. Campanella, R. Roversi, M.P. Sammartino, M. Tomassetti, Hydrogen peroxide determination in pharmaceutical formulations and cosmetics using a new catalase biosensor, Journal of Pharmaceutical and Biomedical Analysis 18 (1998) 105. [4] Y. Usui, K. Sato, M. Tanaka, Catalytic dihydroxylation of olefins with hydrogen peroxide: an organic-solvent- and metal-free system, Angewandte Chemie International Edition 42 (2003) 5623.

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