Nanomolar detection of hydrogen peroxide at a nano-structured adducts of diorganotin dichlorides multiwall carbon nanotube modified glassy carbon electrode

Electrochimica Acta 78 (2012) 82–91

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Nanomolar detection of hydrogen peroxide at a nano-structured adducts of diorganotin dichlorides multiwall carbon nanotube modified glassy carbon electrode Azadeh Azadbakht a , Mohammad Bagher Gholivand b,∗ , Saeid Menati a a b

Department of Chemistry, Faculty of Science, Khorramabad Branch, Islamic Azad University, Khorramabad, Iran Department of Analytical Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran

a r t i c l e

i n f o

Article history: Received 7 January 2012 Received in revised form 1 May 2012 Accepted 8 May 2012 Available online 17 June 2012 Keywords: Hydrogen peroxide Sn-Me2 Cl2 (H2 cdsalen) Multiwall carbon nanotube Electrocatalytic reduction

a b s t r a c t This work describes the electrochemical behavior of Sn-Me2 Cl2 -Methyl 2-[2(salicylideneamino)ethylamino]cyclopent-1-ene-1-dithio carboxylate (H2 cdsalen) film immobilized on the surface of multiwall carbon nanotube glassy carbon electrode and its electrocatalytic activity toward the reduction of hydrogen peroxide. The surface structure and composition of the sensor were characterized by scanning electron microscopy (SEM). Electrocatalytic reduction of hydrogen peroxide on the surface of modified electrode was investigated with cyclic voltammetry and chronoamperometry methods and the results showed that the SnMe2 Cl2 (H2 cdsalen) film displays excellent electrochemical catalytic activities toward hydrogen peroxide reduction. The modified electrode indicated reproducible behavior and high level of stability during the electrochemical experiments. Also it has short response time, low detection limit, high sensitivity, and low operation potential, it can be used as an amperometric sensor for monitoring of H2 O2 . The proposed modified electrode was successfully used for determination of hydrogen peroxide in real sample such human serum. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The detection of hydrogen peroxide (H2 O2 ) is very important in various fields including clinic, food, pharmaceutical and environmental analysis, because H2 O2 is a chemical threat to the environment and the production of enzymatic reactions, at the same time, it has been recognized as one of the major factors in the progression of important diseases [1]. Accurate and reliable determination of H2 O2 has been widely investigated using chromatography [2], spectrophotometry [3] and electrochemistry [4–12] technologies. Among these methods, electrochemistry technique based on a simple and low cost electrode has been extensively applied for accurate determination of H2 O2 . Some inorganic materials modified electrode in determination of H2 O2 is attracting more and more attention owing to its stability and convenience of electron transfer. The materials include nanoparticles [13], perovskite-type oxide [14], some inorganic–organic composite materials [15], some inorganic-incorporated biology complex membranes [16].

∗ Corresponding author. Tel.: +98 831 4274557; fax: +98 831 4274559. E-mail addresses: Azadeh [email protected] (A. Azadbakht), [email protected] (M.B. Gholivand), S [email protected] (S. Menati). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.05.160

Meanwhile, metal complexes have recently emerged as some of the most promising materials used especially in the field of homogenous and heterogeneous catalysis including photocatalysis and electrocatalysis. Different transition metal derivatives such as iron(III) protoporphyrin and iron(III)–salen [17,18], metallophthalocyanine [19] and manganese complex [20] have been used for determination of H2 O2 . Literature survey showed that there is not any report about the use of tin or organotin complexes as a mediator for electrocatalytic reduction or oxidation processes. Organotin compounds have fascinated much attention owing to their potential biocidal activities, industrial and agricultural applications [21–23]. Several organotin-complexes have been found as affective antifouling [24], anti-microbial and antiviral agents [25]. Organotin complexes with schiff base ligands have been an area of focus owing to their anti-tumor activities [26,27]. In addition to this, the complexes belonging to this class also present interesting structural diversities [28]. Keeping in view all these points, opening a new field for application of organotin complexes such as using as a modifier in sensor construction is interested. The electrochemical properties of organotin in aqueous solution on mercury electrodes have been reported [29,30]. It was found that diorganotin cation or its hydrolysis products formed dialkyltin polymers via a single two-electron reduction. Carbon nanotubes (CNTs) are considered as a novel form of carbon materials in the last two past decades [31]. Recently, carbon

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nanotubes have also been incorporated into the electrochemical sensors. While they have many of the same properties as other types of carbon, they offer unique advantages including enhanced electronic properties, a large edge plane/basal plane ratio, and rapid kinetics of the electrode processes. Therefore, in comparison to the traditional carbon electrodes, CNT-based sensors generally have higher sensitivities, lower limits of detection, and faster electron transfer kinetics [32]. Furthermore, carbon nanotubes are new kinds of porous nanostructure carbon materials, which are promising as immobilization substances because of their significant mechanical strength, excellent electrical conductivity, high surface area and good chemical stability. Carbon nanotubes can be used to promote electron transfer reactions when used as electrode materials in electrochemical devices, electrocatalysis and electroanalysis processes due to their significant mechanical strength, high electrical conductivity, high surface area, good chemical stability, as well as relative chemical inertness in most electrolyte solutions and a wide operation potential window [33]. Both redox mediators and CNTs exhibited excellent electrochemical performance for sensor or biosensors fabrication. The synergistic effects in the enhanced current response when both CNTs and the redox mediator employed were observed [34,35]. Immobilization of molecules and bimolecules on CNTs has been pursued in the past, motivated by the prospects of using nanotubes as new types of sensor and biosensors. In the present study, we used CNT successfully for immobilization of a new synthesized organotin complex (Sn-Me2 Cl2 -Methyl 2[2-(salicylideneamino)ethylamino]cyclopent-1-ene-1-dithio carboxylate (H2 cdsalen)) film (Sn-Me2 Cl2 (H2 cdsalen) or Sn-MCH). The immobilization of organotin-complexes onto carbon nanotubes increased the catalytic activity of the modified surfaces. Due to chemical stability and high ability of CNTs to immobilize organotin complex, in this paper Sn-MCH/CNT/GC used for electrocatalytic reduction of hydrogen peroxide.

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nitrogen gas through them prior to the experiments. Double distilled water was used thoroughly. All experiments were carried out at ambient temperature. 2.2. Apparatus Electrochemical experiments were performed via using an Autolab modular electrochemical system (Eco Chem., Utrecht, The Netherlands) equipped with PSTA 20 model and driven by GPES software (Eco Chem.). A conventional three-electrode cell was used with an Ag|AgCl electrode (KCl 3 M) as the reference electrode, a Pt wire as counter electrode and a modified GCE as working electrode. The general morphology of the products was characterized by the scanning electron microscopy (SEM) (Philips XL 30). 2.3. Synthesis of schiff base: Methyl 2-[2-(salicylideneamino)ethylamino]cyclopent-1-ene-1-dithio carboxylate(H2 cdsalen) and its tin adduct [Sn-Me2 Cl2 (H2 cdsalen) or (Sn-MCH)] The schiff base H2 cdsalen was synthesized according to published procedure [36]. The Sn-Me2 Cl2 (H2 cdsalen)adduct was prepared by published method [37]. A solution of SnMe2 Cl2 (0.22 g, 1 mmol) in 20 ml of dry benzene was added to a solution containing (0.32 g, 1 mmol) H2 cdsalen in dry benzene. Then, the mixture was stirred for about 3 h and allowed to react at room temperature for about 24 h. The resulting yellow adduct was filtered, washed with benzene and n-hexane and dried in vacuum. Yield: 47%; m.p. 122–124C; Anal. Calc. For C18 H26 N2 OS2 Cl2 Sn: C, 40.0; H, 4.8; N, 5.1. Found: C, 39.5; H, 4.5; N, 4.7. 1 H NMR (500 MHz, CDCl3 ) 1.22 (6H, s, 2 J(119 Sn-H) = 71.0 Hz), 1.83 (2H, m), 2.57 (3H, s), 2.68 (2H, t), 2.75 (2H, t), 3.75 (2H,q), 3.85 (2H, t), 6.88 (1H, t), 6.95 (1H, d), 7.25 (1H, d), 7.34 (1H, t), 8.38 (1H, s, CH N), 12.38 (1H, s, NH), 12.90 (1H, s, br, OH); 119 Sn NMR (500 MHz, CDCl3): −251.92 ppm. The chemical structure of schiff base (H2 cdsalen) and the Sn-Me2 Cl2 (H2 cdsalen) adduct are depicted in Scheme 1.

2. Experimental 2.4. Electrode modification 2.1. Chemicals Multiwall carbon nanotubes with purity 95% (10–20 nm diameters) and 1 ␮m length were obtained from Nanolab (Brighton, MA). Sodium hydroxide, ammonia (25%), benzene (99.5%), n-hexane (99%), DMSO (99%), and hydrochloric acid (37%), were purchased from Merck (Germany) and Fluka. All other chemicals were of analytical-reagent grade and used without further purification. Solutions were deaerated by bubbling high purity (99.99%) of

To prepare a modified electrode, GCE was polished with emery paper followed by alumina (1.0 and 0.05 ␮m) and then thoroughly washed with twice-distilled water. This electrode was afterwards placed in ethanol container, and then a bath ultrasonic cleaner was used to remove the adsorbed particles. Then, 25 ␮L of DMSO–CNT solution (0.4 mg mL−1 ) was cast on the surface of GC electrode and dried in air to form a CNT film at electrode surface. Afterward, the electrode was thoroughly rinsed with water and

Scheme 1. Mechanisms for formation of Sn-Me2 Cl2 (H2 cdsalen) adduct.

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Fig. 1. Typical SEM image of (A) bare GCE, (B) CNT/GCE and (c) a nano structured Sn-MCH/GCE.

kept at room temperature for further use. The prepared CNT/GC electrode was placed in DMSO solution with 0.1 M tetrabuthyl ammonium perchlorate as the supporting electrolyte containing 0.01 M Sn-Me2 Cl2 (H2 cdsalen) and the electrode’s potential was cycled between −0.5 and 0.8 V at a scan rate of 50 mV s−1 for 50 cycles. The modified electrode (denoted as Sn-MCH/CNT/GCE) was thoroughly rinsed and cycled between −0.5 and 0.8 V in 0.25 M acetate buffer solution until a reproducible cyclic voltammogram (CV) was obtained. 3. Results and discussion The redox mediator that was used for modification of the CNT/GC electrode was an organotin adduct which was produced by Sedaghat et al. The mechanism of organotin adduct formation has been reported in the foregoing work [37]. They have reported that H2 cdsalen is coordinated as dangling form through oxygen atom and the hydrogen atom on phenolic oxygen in free ligand tautomerizes to the imine nitrogen due to the coordination of oxygen atom with the Sn, while an intramolecular hydrogen bond still exists between O and N (Scheme 1). A strong band attributable to (C N) which occurs at 1630 cm−1 in ligand shifts to higher frequency (1650 cm−1 ) consistent with adduct formation and the proton transfer from phenolic oxygen atom to the imine nitrogen atom. Further comparison with the spectrum of free ligand reveals the presence of a strong band in the spectra of the complex at 1540 cm−1 due to the (C O) providing evidence of participation of the phenolic oxygen in the metal–ligand bonding. The strong band at 570 cm−1 in the spectra of complex can be assigned to the Sn O bond [37]. The presence of electroactive sites in the structure of adduct makes it suitable for

construction of a sensor as a new modifier. Therefore, in this study this adduct was used for modification of the electrode. 3.1. Characterization of the Sn-MCH/CNT/GC electrode by SEM To investigate the surface structure and morphology of the modified electrode, we performed SEM. Fig. 1 shows the SEM images of bare GC electrode (A), CNT/GCE (B) and the nanostructure Sn-MCH/GC electrode (C). The result indicated that the film has a globular structure with relatively homogeneous distribution in the range 70–95 nm. The presence of smaller nanoparticles leads to an increase in the surface coverage for adsorption of more H2 O2 . Therefore, this surface morphology and porosity of Sn-MCH/CNT/GC electrode can result in an improved response for electroreduction of H2 O2 . 3.2. Properties of the nano-structured Sn-MCH/CNT/GC electrode In the preliminary experiments the electrochemical behavior of schiff base (H2 cdsalen) was studied at the surface of bare GC and GC electrode modified with CNT using cyclic voltammetric technique. In all experiments, the CV were recorded in mixed DMSO–water solution containing 0.1 M tetrabuthyl ammonium perchlorate as the supporting electrolyte. The results of CV of solution containing 0.1 mM schiff base (H2 cdsalen) at the GC electrode showed no redox peaks in the potential range of −0.2 to 1.2 V, indicating that H2 cdsalen was not electroactive in the scanned potential window at GCE, that may be due to high over potential associated with redox process. When the GC electrode was modified with CNT, the modified electrode gave one pair of redox peaks with a formal potential of 0.28 V versus Ag|AgCl electrode.

Fig. 2. Effect of pH on the electrochemical current of the CNT/GC electrode in solution containing 0.1 mM of H2 cdsalen. (B) Effect of pH on the potential of the anodic peak of 0.1 mM of H2 cdsalen at the surface of CNT/GC electrode.

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Scheme 2. Suggested mechanisms for the first and second redox peaks of Sn-Me2 Cl2 (H2 cdsalen) adduct.

The effect of pH on the electrochemical behavior of H2 cdsalen at the surface of CNT/GC electrode was studied in mixed DMSO and phosphate buffer solutions at pH values ranging from 1 to 7. A well defined redox peaks were observed at acidic media (pH < 6). At pHs greater than 6, the shape and the reproducibility of redox peak were poor .The pH dependence of the peaks current and peaks potential are shown in Fig. 2. As it is seen the maximum peak current was obtained at pH = 4. The potential of the anodic peak of H2 cdsalen was shifted linearly toward less positive potential values with increasing the pH between 1 and 6 by slope of 0.083 V/pH (Fig. 2B). The number of hydrogen ions participated in the rate determining step (ZH+ ) can be obtained from the slope value of the linear segment of the Ep –pH plot (59ZH+ /(1 − ˛)n). The slope of 0.083 V/pH for H2 cdsalen oxidation indicating that the number of protons involved in the reaction mechanisms is equal to number of electron precipitate in electrodic process. The behavior of modified electrode in the present of H2 cdsalen was evaluated in acetate buffer with pH = 4 and its results were compared with that obtained in phosphate buffer (pH = 4) at 50 mV s−1 . The better results (peak shape and peak current) were observed in acetate buffer. Thus, acetate buffer with pH = 4 was selected for further uses. The different behavior observed in the two buffers should be related to their composition and ionic strength, which can cause changes in the electron transfer kinetics in reversible reactions and the peak shape [38]. Furthermore, the effect of the scan rate on the CVs of the of schiff base (H2 cdsalen) was studied and from the plot of Ip versus v1/2 , the number of electrons (n) involved in the overall reaction was obtained according to the following equation [39]:

Ip = 2.99 × 105 n(1 − ˛)1/2 ACD1/2 1/2

(1)

where A is electrode surface area, Cb is the schiff base concentration and D is the diffusion coefficient. Considering (1 − ˛) = 0.5, A = 0.0037 cm2 , Cb = 0.1 × 10−6 mol dm−3 and D = 1.03 × 10−6 cm2 s−1 (D was calculated from chronoamperometry), from the slope of the Ip versus v1/2 plot, the total number of electrons (n) is about one. The suggested mechanism of redox reaction of the H2 cdsalen is presented in Scheme 2. According to the suggested mechanism, the group that undergo reduction or oxidation is thionyl (C S) group attached to the five member ring. The similar mechanism has been reported previously [40]. The electrochemical behavior of 1.0 × 10−2 M adduct of SnMCH was also investigated in solution of DMSO and acetate buffer containing 0.1 M tetrabuthyl ammonium perchlorate as the supporting electrolyte at the surface of CNT/GC electrode. The CV of the adduct showed two redox couples with formal potentials of 0.090 and 0.26 V versus Ag| AgCl electrode. The first peak in 0.090 V attributed to the Sn2/4+ as reported previously [41–43]. Since alone H2 cdsalen in the solution showed one pair of redox peaks with a formal potential of 0.28 V versus Ag|AgCl electrode, the second peak in 0.26 V may be attributed to (H3 cdsalen)/(H2 cdsalen). The effect of pH on the electrochemical behavior of the adduct was studied and the results showed that the peaks corresponding to Sn2/4+ were pH independent as reported previously [41,44] while the anodic and cathodic peaks of the second redox shifted by pH variation (as expected for H2 cdsalen). Based on the obtained results the following mechanisms were suggested for the first and second redox peaks (Scheme 2). The immobilization of the Sn-MCH films at the surface of CNT/GCE was carried out with CV. Fig. 3 shows consecutive CVs using the CNT/GC electrode in DMSO–acetate buffer solution with 0.1 M tetrabuthyl ammonium perchlorate as the supporting electrolyte containing 1.0 × 10−2 M

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2

1

I/μA

0

-1

-2

-3 -0.4

-0.2

0 0.2 E vs (Ag|AgCl)/ V

0.4

0.6

Fig. 3. Consecutive CVs of CNT/GC electrode in DMSO–acetate buffer solution with 0.1 M tetrabuthyl ammonium perchlorate as the supporting electrolyte containing 1.0 × 10−2 M Sn-MCH adduct at a scan rate of 50 mV s−1 : 10th to 50th cycles (every 5 cycle).

Sn-MCH adduct. As it is seen, two anodic and two cathodic peaks correspond to the Sn(IV)/Sn(II) and(H2 cdsalen)/(H3 cdsalen) redox couples were observed. Film growth is accompanied by increasing current of both peaks, indicating a progressive deposition of the electroactive material, which forms a film as a result of the electrodeposition of the Sn-MCH adduct. By increasing the number of scans, the anodic and cathodic peaks shifted to more positive and more negative potentials respectively, that it may be due to increase in the electrical resistance of the polymer film [45]. Thus, to overcome the resistance, more overpotential is needed. The anodic and cathodic peaks current did not alter upon further potential cycling (more than 50 cycles). Therefore, 50 cycles were used for deposition of the Sn-MCH film at the surface of the CNT/GC electrode. Fig. 4 shows CVs of GCE, CNT/GCE, Sn-MCH/GCE and SnMCH/CNT/GC electrode in 0.25 M acetate buffer solution at a scan rate of 50 mV s−1 . No electrochemical response was observed at surface of GC and CNT/GC electrodes (curves a and b). Two ill redox

b

6

I/ μA

d

C a

0

-6 -0.4

0

0.4

E vs (Ag/AgCl)/ V Fig. 4. (A) CVs of GCE (curve a), CNT/GCE (curve b), Sn-MCH/GCE (curve c) and SnMCH/CNT/GCE (curve d) in 0.25 M acetate buffer solution at a scan rate of 50 mV s−1 .

peaks observed when Sn-MCH/GC electrode was used (curve c). Modification of mentioned electrode by CNT on the bare GC electrode and then deposition of Sn-MCH film at the surface of CNT/GCE (Sn-MCH/CNT/GC electrode) enhances the sensor response about 16 times (curve d). The presence of CNT supplied a larger surface area to allow more deposition of Sn-MCH adduct. Furthermore, according to the Randles–Sevcik equation [46], the surface area of the Sn-MCH/CNT/GC electrode was calculated and found to be 0.0629 cm2 , being 6.6 times larger than that of Sn-MCH/GC electrode (0.0093 cm2 ) and the surface area of CNT/GC (0.021) electrode is 2.2 times larger than that of Sn-MCH/GC electrode (0.0093 cm2 ). The voltammetric behavior of Sn-MCH/CNT/GCE was characterized at various pHs by CV. Based on this study, the peak potentials and peak currents of the foregoing electrode in 0.25 M acetate solutions (pH 1–8) exhibit negligible variations in peaks potential of Sn2/4+ redox system but the peaks potential of (H3 cdsalen)/(H2 cdsalen) couple was found to be pH dependent. On the other word, the results are similar to those obtained for Sn-MCH adduct at the CNT/GC electrode. In this investigation the sharper and well-defined peaks were obtained at pH 4, but in solutions of pH higher than 7 the stability of the modified electrode decreases and the peak currents decrease. The CVs of the Sn-MCH/CNT/GCE in 0.25 M acetate buffer solution (pH 4) at different scan rates are shown in Fig. 5A. The peaks current of the voltammograms are linearly proportional to the scan rate () up to 70 mV s−1 for the first redox couples (Fig. 5B). According to the following equation this result confirms a surface type reactions [47]: Ip =

n2 F 2 AC 4R

(2)

For scan rates higher than 70 mV s−1 , because of semi-infinite linear diffusion behavior, the peaks current of first redox couples are proportional to 1/2 , usually associated with a diffusional process (Fig. 5C). The surface coverage can be evaluated from the equation  = Q/nFA, where Q is the charge obtained by integrating the anodic peak under the background correction, at a low scan rate of 5 mV s−1 , and other symbols have their usual meanings. Several parameters such as the electron transfer kinetics between the adsorbed compound and the electrode, the stability of the sensor and the nature of limitation of the produced current (diffusion or kinetic limited) are directly or indirectly affected by the degree of the surface coverage [48]. When the population of redox molecules exceeds the current is not any more proportional to the redox population, since overlapping effect results in a decrease of the rate of increase of the current versus the surface coverage. This phenomenon is more profound at high scan rates [49]. Therefore the calculation of surface coverage was carried out at low scan rate. The surface coverage value for the Sn-MCH/CNT/GC electrode was 9.7 × 10−8 mol cm−2 , which corresponds to the presence of multilayer of surface species. As can be seen in Fig. 5A, the peak-to-peak separation (Ep = Epa − Epc ) increases with the scan rate, indicating the limitation arising from charge transfer kinetics. 3.3. Electrocatalytic reduction of H2 O2 at Sn-MCH/CNT/GC electrode Evaluation of the modified electrode in reduction of H2 O2 was conducted by CV. In Fig. 6, the CVs responses of GC, CNT/GC, SnMCH/GC and Sn-MCH/CNT/GC electrodes in 0.25 M acetate buffer solution without and with H2 O2 in solution illustrated. As can be observed, H2 O2 did not undergo reduction at GC and CNT/GC electrodes (curves b and d). When GC electrode was modified with Sn-MCH film (Sn-MCH/GC) and then inserted into the same H2 O2 containing electrochemical cell, a small electrocatalytic activity was observed for H2 O2 reduction (curve f).

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6

I / μA

A

6

87

B

0

2

-6

I/μA

0

50

100

150

ν/ mV s -1 6

C

I / μA

-2

-6 0

-0.4

0

0.4

E vs(Ag/AgCl)/ V -6 2

5 8 ν1/2 /mV1/2 S-1/2

11

Fig. 5. (A) CVs of the Sn-MCH/CNT/GCE electrode recorded in 0.25 M acetate buffer solution at different scan rates: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120 mV s−1 , respectively. (B) and (C) represent the variation of the anodic (a) and cathodic (b) peak current of the same electrode versus potential scan rate and square root of potential scan rate, respectively.

Modification of the GC electrode with CNT and then by SnMCH film improved the electroactivity of modified electrode (Sn-MCH/CNT/GC) for reduction of H2 O2 (curve h). When Sn-MCH film deposited on multiwall carbon nanotube and was used as a modified electrode for reduction of H2 O2 in the same solution a large cathodic peak was observed with a small anodic counterpart. The observed current is associated with H2 O2 reduction and is demonstrated by comparing the current in Fig. 6B (curve h) with that in curve g, which shows the cyclic voltammetric behavior of an electrode modified with Sn-MCH/CNT/GC in a H2 O2 free electrolyte (0.25 M acetate buffer solution). It is apparent that the cathodic current associated with the surface attached materials is significantly less than that obtained in the solution containing H2 O2 . At the surface of GC and CNT/GC electrodes, H2 O2 was not reduced. As can be seen, electroactivity toward H2 O2 on modified electrode (Sn-MCH/CNT/GC) was significant (Fig. 6B, curve h).

Thus, a decrease in overpotential and enhancement of peak current for H2 O2 reduction are achieved with the modified electrodes. Such behavior is indicative of an EC mechanism. On the other hand, the increased current in the cathodic region is due to the fact that the H2 O2 present in solution diffuses toward the electrode and oxidizes the Sn(II) which is produced electrochemically. As Sn(IV) is regenerated by H2 O2 during the potential sweep, there is a resultant increase in the cathodic current. From the above results the catalytic reduction of H2 O2 at the surface of the proposed modified electrode can be written as follows: [SnIV -Me2 Cl2 (H2 cdsalen)]2+ + 2e− → SnII [Me2 Cl2 (H2 cdsalen)] (3) SnII [Me2 Cl2 (H2 cdsalen)] + H2 O2 + 2H+ → [SnIV -Me2 Cl2 (H2 cdsalen)]2+ + 2H2 O

B

A

f

10

10

c

d

b

I/ μA

I/ μA

(4)

0

-10

e

g a h 10

-30 0.2

0

0.2

E vs(Ag/AgCl)/ V

0.4

-0.4

-0.1

0.2

0.5

E vs(Ag/AgCl)/ V

Fig. 6. (A) CVs of GCE (a and b), CNT/GCE (c and d), (B) Sn-MCH/GCE (e and f) and Sn-MCH/CNT/GCE (g and h) in 0.25 M acetate buffer solution at scan rate of 50 mV s−1 in the absence (a, c, e, and g) and the presence (b, d, f, and h) of 7 mM H2 O2 , respectively.

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3.4. Chronoamperometric studies

20

The chronoamperometry (CA) method, as well as other electrochemical methods, is employed for the investigation of electrode processes at chemically modified electrodes. Fig. 9A shows a series of well-defined chronoamperograms for the Sn-MCH/CNT/GCE in the absence and presence of different concentrations of H2 O2 at an applied potential of 0.01 versus Ag|AgCl that was selected from mass transfer controlled region. (The initial potential for H2 O2 reduction at this modified electrode was about 0.07 V versus Ag|AgCl. At this potential, H2 O2 was not yet electroactive. By setting the working electrode potential toward negative values such 0.01 V, H2 O2 was reduced and its surface concentration reduced to zero and maximum peak current was achieved.) For an electroactive material with diffusion coefficient of D, the corresponding current of the electrochemical reaction (under diffusion control) is described by Cottrell’s law [46]:

I/μA

-10

-40

I = nFAD1/2 C−1/2 t −1/2

-70 -0.3

0.1

0.5

E vs(Ag/AgCl)/ V Fig. 7. CVs of the Sn-MCH/CNT/GCE in the presence of differentH2 O2 concentration: 2, 3.5, 5.4, 7.5, 10, 12, 15, 18.1, 25, 33, 49, 60, 75, 84, 100, 120, 150 mM respectively.

where D and C0 are the diffusion coefficient and bulk concentration, respectively. The average value of D obtained from the slopes of I versus t−1/2 plots (Fig. 9B) for different concentrations of H2 O2 is 1.03 × 10−6 cm2 s−1 . Chronoamperometry can be used for the evaluation of the catalytic rate constant. At intermediate times, the catalytic current (Icat ) is dominated by the rate of electrocatalyzed reduction of H2 O2 and the rate constant for the chemical reaction between H2 O2 and redox sites of surface-confined Sn-MCH/CNT is determined according to the method described in the literature [51]:



Icat =  1/2 [1/2 erf( 1/2 )] + exp IL As can be seen from Fig. 6 the presence of H2 O2 in the solution has no effect on the peaks associated with H2 cdsalen and thus, for simplicity its redox mechanism was omitted from the above mechanism. CVs of different concentrations of H2 O2 (ranging from 2 to 150 mM) at the modified electrode in 0.25 M acetate buffer solution were obtained and the results showed that, upon the addition of H2 O2 an enhancement in the cathodic peak current was observed and the anodic peak decreased and finally disappeared; this indicated that, the electrogenerated Sn(II) is consumed during a chemical step (Fig. 7). The pH dependence of the H2 O2 response on Sn-MCH/CNT/GC was investigated in a pH range of 1–7. It was found that the electrocatalytic reduction of H2 O2 was more favored under acidic conditions; we chose pH 4 for the next experiments because of well-behaved electrochemistry of Sn-MCH/CNT/GC at this pH. The CVs of 5 mM H2 O2 at different scan rates were studied (Fig. 8A). It was found that the peak current of H2 O2 reduction is proportional to the square root of the scan rate (Fig. 8B), indicating that the electrode process is controlled by mass diffusion. Moreover, a plot of the scan rate normalized current (I/v1/2 ) versus the scan rate exhibits the typical shape of an electrochemical (EC ) catalytic process (Fig. 8C) [50]. (In the EC mechanism, when the chemical reaction rate constant (k) is high or the scan rate () of the electrochemical reaction is low, the reagent would be produced effectively. Thus, the values of Ipc are higher than those predicted from the Randless equation. As the scan rate decreases, the (Ipc /v1/2 ) values would increase; and the peak current would be less detectable. Finally, the peak current disappears and platform independent of scan rate appears instead of it. The EC mechanism can easily be detected, since it is the only mechanism, through which the values of (Ipc /v1/2 ) significantly increase by decreasing the scan rate.)

(5)



−



 1/2

(6)

where Icat and IL are the currents of the Sn-MCH/CNT/GCE electrode in the presence and absence of hydrogen peroxide, and  = kC0 t is the argument of the error function, k is the catalytic rate constant, C0 is the bulk concentration of hydrogen peroxide, erf is error function and t is the elapsed time (s). In such cases where  > 1.5, erf( 1/2 ) is almost equal to unity, and the above equation can be reduced to: Icat 1/2 =  1/2 1/2 = 1/2 (kc0 t) IL

(7)

From the slope of the Icat /IL versus t1/2 plot, we can simply calculate the value of kcat for a given concentration of hydrogen peroxide. Some such plots, constructed from the chronoamperograms of Sn-MCH/CNT/GCE in the absence and presence of different concentrations of hydrogen peroxide, are shown in Fig. 9C. The average value of kcat was found to9.87 × 102 M−1 s−1 . 3.5. Amperometric detection of H2 O2 at the modified electrode Since amperometry under stirred conditions is more sensitive than CV, it was used to estimate the lower limit of detection. Fig. 10A displays a typical steady-state catalytic current time response of the rotated modified electrode (2000 rpm) with successive injection of H2 O2 , at a fixed potential of 0.01 V versus reference electrode. As shown, during the successive addition of H2 O2 a well-defined response was observed, demonstrating stable and efficient catalytic ability of the Sn-MCH immobilized on the CNT/GCE. The response current is linear in the range of 10–170 nM of H2 O2 (Fig. 10B). The calibration plot has a correlation coefficient of 0.998 and the detection limit of 3 nM at signal to noise ratio of 3. Detection limit and linear calibration range of the proposed modified electrode were compared with those previously reported, and the results are summarized in Table 1. As can be seen, the analytical parameters are comparable or better than the results reported for

A. Azadbakht et al. / Electrochimica Acta 78 (2012) 82–91

89

B

0

Ipc / μA

A

I/ μA

5

-4

-8 0

10

ν

-15 -0.4

0

Ipc ν-1/2/ μA mV-1/2 S1/2

-5

0.4

E vs (Ag/AgCl)/ V

1/2

/

20

mV1/2

S1/2

-0.3

C

-0.5

-0.7 30

80

ν / mV S-1 Fig. 8. (A) CVs of the modified electrode in the presence of 5 mM H2 O2 in 0.25 M acetate buffer solution at various scan rates: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120 mV s−1 , respectively. (B) Plots of Ip versus v1/2 and (C) Ipc /1/2 versus  derived from CVs of Sn-MCH/CNT/GCE in the presence of 5 mM H2 O2 at different scan rates.

H2 O2 determination at the surface of other modified electrodes [4–6,52–58]. 3.6. Stability study and effect of electroactive interferences By repetitive CV of the Sn-MCH/CNT/CCE for approximately 100 times in 0.25 M acetate buffer solution (pH 4) at a scan rate of 50 mV s−1 , the peak current value decreases less than 5%, indicating good stability. The modified electrode retained its initiate activity for more than 30 days when kept in air at ambient conditions. A decrease of 4% was observed in the current response of the electrode at the end of 30th day. The Sn-MCH/CNT/GCE impart higher stability onto amperometric measurements of H2 O2 .

To confirm the existence of the film stability on the electrode surface after it has been used in hydrodynamic amperometry measurements of H2 O2 , the CVs of the modified electrode before and after rotation for 600 s were recorded. No significant change was observed in peaks current and potential of the modified electrode after rotation (Fig. 11A). These results indicate remarkable stability of the modified electrode. Since the detection of H2 O2 is an important task in many biological, medical and clinical studies then interferences of some electroactive compounds commonly present in physiological samples can prevent accurate determination of H2 O2 concentration. Fig. 11B shows the current responses of the Sn-MCH/CNT/GCE to successive additions of 15 nM H2 O2 and 15 nM of common

0 0

B

-6

I/ μA

A

-20

-12

a b c d e

-18 0.3

I/ μA

0

0.6

t-1/2 / S-1/2 12

C

I cat /IL

-40

-60 0

10

20

t /s

30

e d c b a

8 4

40 0 2

3

4

5

t1/ 2 /S1/2 Fig. 9. (A) Chronoamperograms obtained on Sn-MCH/CNT/GCE in the absence (a) and presence of H2 O2 at various concentrations. (B) Plots of I versus t−1/2 and (C) (Icat /IL ) versus t1/2 derived from the data of the main panel for various concentrations of H2 O2 0.1 mM (a) 3 mM (b), 7 mM (c), 10 mM (d), and 15 mM (e).

90

A. Azadbakht et al. / Electrochimica Acta 78 (2012) 82–91

Table 1 Comparison of the performances of various hydrogen peroxide sensors. Dynamic rang (mol dm−3 )

Electrode a

−5

Hb/SA -MWCNTs /GC NAD+ /SWCNT/GCE PTHc /Nafion/HRP/GC Ti(III)-TNTsd /Hb (PSSe /HRP)5 /ZnO/Au (PDDAf /Fe3 O4 )5 /ITOg HRP-Nafion-SPEh ZnO/Au/Nafion/HRP/GC Cu2 S/OMCsi /Nafion/GC Ag NPs/ATP/GC This work a b c d e f g h i

−4

4.0 × 10 –2.0 × 10 Up to 1.0 × 10−7 Up to 1.0 × 10−3 4.9 × 10−6 –11 × 10−4 5.0 × 10−6 –17 × 10−4 4.1 × 10−6 –8.0 × 10−4 5.9 × 10−6 –35.3 × 10−6 15 × 10−6 –11 × 10−4 1.0 × 10−6 –3030 × 10−6 1.0 × 10−5 –2.1 × 10−5 1.0 × 10−8 –170 × 10−9

b

Limit of detection (mol dm−3 )

Reference

16.4 × 10−6 1.0 × 10−11 0.06 × 10−6 1.5 × 10−6 2.1 × 10−6 1.4 × 10−6 0.48 × 10−6 9.0 × 10−6 0.2 × 10−6 2.4 × 10−6 3.0 × 10−9

[52] [53] [54] [55] [56] [57] [58] [5] [6] [4]

Sodium alginate. Multiwall carbon nanotubes. Poly thionine. TiO2 nanotubes in situ self-doped with Ti(III). Poly(sodium 4-styrenesulfonate). Poly(diallyldimethylammonium chloride). Tin-doped indium oxide. Screen-printed electrode. Ordered mesoporous carbons.

Table 2 Determination of hydrogen peroxide in serum sample solutions. Sample

Found (10−3 mol dm−3 )

Added (10−3 mol dm−3 )

After added (10−3 mol dm−3 )

Recovery (%)

1 2

0.014 0.020

0.05 0.15

0.067 0.165

104.6 97

0.009

II// μA μA

A 0.008

interfering species (ascorbic acid, uric acid, and dopamine) under the optimized experimental conditions. Compared to the response to hydrogen peroxide, the current generated due to the interfering species are negligible, indicating high selectivity of the sensor. Main reason for this behavior can be related to the use of 0.01 V as detection potential in this study that greatly reduced the responses of common interferences.

B

0.006 0.003 0

I/ μA

0

100

200

10-9 mol dm-3 0.005

3.7. Real sample analysis The analysis of real world sample was also performed. The sensor was used to determine the concentration of H2 O2 in serum samples. The samples were diluted 1000 times with 0.25 M acetate buffer solution. Using a standard addition method, the recoveries of H2 O2 samples with concentrations of 0.05 mM (sample 1) and 0.15 mM (sample 2) were obtained as shown in Table 2. The results show that this sensor might have a potential use in detection of H2 O2 .

0.002 20

90

160

t /s Fig. 10. (A) Amperometric response of rotating modified GC electrode (rotation speed 2000 rpm) held at 0.01 V in 0.25 M acetate buffer solution for successive addition of 10 nM H2 O2 . (B) Plot of current response versus H2 O2 concentration.

0.006

5

A

B

a

H2O2 H2O2

I/ μA

I/ μA

2

b

0.005

-1

AA DA

UA

0.004

-4 -0.3

0

0.3

E vs (Ag/AgCl)/ V

0.6

0

20

40

60

80

t /s

Fig. 11. (A) CVs of the modified electrode before (a) and after (b) rotation for 600 s. (B) Current responses obtained at the Sn-MCH/CNT/GCE for the additions (indicated by arrows) of 15 ␮M H2 O2 , AA, DA, UA and H2 O2 (left to right).

A. Azadbakht et al. / Electrochimica Acta 78 (2012) 82–91

4. Conclusion We have demonstrated the suitability of Sn-MCH/CNT/GCE as an ideal catalyst for low-potential determination of H2 O2 with a high sensitivity. The experimental results reported above demonstrate that (i) the Sn-MCH nanoparticles can be firmly deposited on the CNT/GCE by an electrochemical method; (ii) the Sn-MCH/CNTmodified GCE can catalyze the reduction of H2 O2 at pH 4; (iii) the kinetics of catalytic reaction is fast; and (iv) it is stable and has short response time, low detection limit, high sensitivity, and low operation potential, it can be used as an amperometric sensor for monitoring of H2 O2 . The proposed modified electrode was successfully used for determination of H2 O2 in real sample such human serum. Acknowledgement The authors gratefully acknowledge the financial support of this work by the Razi University. References [1] S. Yao, J. Xu, Y. Wang, X. Chen, Y. Xu, S. Hu, Analytica Chimica Acta 557 (2006) 78. [2] M. Tarvina, B.M. Cord, K. Mount, K. Sherlach, M.L. Millerd, Journal of Chromatography A 1217 (2010) 7564. [3] D. Herberth, O. Baum, O. Pirali, P. Roy, S. Thorwirth, K.M.T. Yamada, S. Schlemmer, T.F. Giesen, Journal of Quantitative Spectroscopy & Radiative Transfer 113 (2012) 1127. [4] H. Chen, Z. Zhang, D. Cai, S. Zhang, B. Zhang, J. Tang, Z. Wua, Talanta 86 (2011) 266. [5] C. Xiang, Y. Zou, L.X. Sun, F. Xu, Sensors and Actuators B 136 (2009) 158. [6] X.J. Bo, J. Bai, L.X. Wang, L.P. Guo, Talanta 81 (2010) 339. [7] V. Mishin, J.P. Gray, D.E. Heck, D.L. Laskin, J.D. Laskind, Free Radical Biology and Medicine 48 (2010) 1485. [8] F. Deyhimi, F. Nami, Journal of Molecular Catalysis B: Enzymatic 68 (2011) 162. [9] S. Liu, L. Wang, J. Tian, Y. Luo, X. Zhang, X. Sun, Journal of Colloid and Interface Science 363 (2011) 615. [10] R. Ning, W. Lu, Y. Zhang, X. Qin, Y. Luo, J. Hu, A.M. Asiri, A.O. Al-Youbid, X. Sun, Electrochimica Acta 60 (2012) 13. [11] W. Lu, Y. Luo, G. Chang, F. Liao, X. Sun, Thin Solid Films 520 (2011) 554. [12] W. Lu, F. Liao, Y. Luo, G. Chang, X. Sun, Electrochimica Acta 56 (2011) 2295. [13] Y. Liu, D. Wang, L. Xu, H. Hou, T. You, Biosensors and Bioelectronics 26 (2011) 4585. [14] G.L. Luque, N.F. Ferreyr, A.G. Leyva, G.A. Rivas, Sensors and Actuators B 142 (2009) 331. [15] M. Ammam, E.B. Easton, Sensors and Actuators B 161 (2012) 520. [16] G.P. Bienert, J.K. Schjoerring, T.P. Jahn, Biochimica et Biophysica Acta 1758 (2006) 994. [17] G.L. Turdean, I.C. Popescu, A. Curulli, G. Palleschi, Electrochimica Acta 51 (2006) 6435. [18] V. Mirkhani, M. Moghadam, S. Tangestaninejad, I. Mohammadpoor-Baltork, N. Rasouli, Inorganic Chemistry Communications 10 (2007) 1537. [19] P. Mashazi, T. Mugadzab, N. Sosiboa, P. Mdlulia, S. Vilakazia, T. Nyokong, Talanta 85 (2011) 2202.

91

[20] A. Salimi, M. Mahdioun, A. Noorbakhsh, A. Abdolmalekic, Raoof Ghavami, Electrochimica Acta 56 (2011) 3387. [21] Y. Nakamaru, S. Uchida, Journal of Environmental Radioactivity 99 (2008) 1003. [22] J. Paulitsch, M. Schenkel, Th. Zufra, P.H. Mayrhofer, W.D. Munz, Thin Solid Films 518 (2010) 5558. [23] A. Husain, S.A.A. Nami, K.S. Siddiqi, Journal of Molecular Structure 970 (2010) 117. [24] I. Omae, Applied Organometallic Chemistry 17 (2003) 81. [25] M. Nath, S. Goyal, Metal Based Drugs 2 (1995) 297. [26] M. Gielen, A.G. Davies, K. Pannell, E. Tiekink, Tin Chemistry Fundamentals Frontiers and Applications, Wiley, 2008. [27] A.G. Davies, Organotin Chemistry, VCH, Weinheim, Germany, 2004. [28] B.B. Tushar, S.M. Cheerfulman, W. Willem, B. Monique, et al., Journal of Organometallic Chemistry 690 (2005) 3080. [29] M.D. Morris, Journal of Electroanalytical Chemistry 16 (1968) 569. [30] Y.Q. Long, S.S. Huang, J.P. Liao, B.F. Li, Microchimica Acta 142 (2003) 263. [31] S. Iijima, Nature 354 (1991) 56. [32] C.B. Jacobs, M.J. Peairs, B.J. Venton, Analytica Chimica Acta 662 (2010) 105. [33] M. Inagaki, K. Kaneko, T. Nishizawa, Carbon 42 (2004) 1401. [34] Y.L. Yao, K.K. Shiu, Electrochimica Acta 52 (2007) 278. [35] M. Zhang, W. Gorshi, Analytical Chemistry 77 (2005) 3960. [36] S.B. Kumar, S. Bhttacharyya, S.K. Dutta, E.R.T. Tiekink, M. Chaudhury, Journal of the Chemical Society – Dalton Transactions (1995) 2619. [37] T. Sedaghat, S. Menati, Inorganic Chemistry Communications 7 (2004) 760. [38] P.T. Kissinger, W.R. Heineman, Laboratory Techniques in Electroanalytical Chemistry, Marcel Dekker, New York, 1984. [39] S. Antoniadou, A.D. Jannakoudakis, E. Theodoridou, Synthetic Metals 30 (1980) 295. [40] J.A. Joule, K. Mills, Heterocyclic Chemistry, fourth ed., 2000. [41] H. Razmi, E. Habibi, Electroanalysis 21 (2009) 867. [42] F. Li, J. Song, F. Li, X. Wang, Q. Zhang, D. Han, A. Vaska, L. Niu, Biosensors and Bioelectronics 25 (2009) 883. [43] N. Muhammad, A. Shah, Z. Rehman, S. Shuja, S. Ali, R. Qureshi, A. Meetsma, M.N. Tahir, Journal of Organometallic Chemistry 694 (2009) 3431. [44] H. Razmi, E. Habibi, Analytical Biochemistry 392 (2009) 126. [45] T.F. Otero, E.D. Larreta-Azelain, Polymer 29 (1988) 1522. [46] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, John Wiley and Sons, New York, 2000. [47] R.W. Murray, A.J. Bard, Electroanalytical Chemistry, Marcel Dekker, New York, 1984. [48] B. Persson, L. Gorton, Journal of Electroanalytical Chemistry 292 (1990) 115. [49] M.I. Prodromidis, A.B. Florou, S.M. Tzouwara-Karayanni, M.I. Karayannis, Electroanalysis 12 (2000) 1498. [50] F. Pariente, E. Lorenzo, F. Tobalina, H.D. Abruna, Analytical Chemistry 67 (1995) 3936. [51] Z. Galus, Fundamentals of Electrochemical Analysis, Horwood, New York, 1976. [52] H.Y. Zhao, W. Zheng, Z.X. Meng, H.M. Zhou, X.X. Xu, Z. Li, Y.F. Zheng, Biosensors and Bioelectronics 24 (2009) 2352. [53] A. Salimi, L. Miranzadeh, R. Hallaj, H. Mamkhezri, Electroanalysis 20 (2008) 1760. [54] A. Salimi, A. Korania, R. Hallaj, R. Khoshnavazi, H. Hadadzadeh, Analytica Chimica Acta 635 (2009) 63. [55] M. Liu, G. Zhao, K. Zhao, X. Tong, Y. Tang, Electrochemistry Communications 11 (2009) 1397. [56] B.X. Gu, C.X. Xu, G.P. Zhu, S.Q. Liu, L.Y. Chen, M.L. Wang, J.J. Zhu, Journal of Physical Chemistry 113B (2009) 6553. [57] L. Zhang, Y. Zhai, N. Gao, D. Wen, S. Dong, Electrochemistry Communications 10 (2008) 1524. [58] Y.J. Teng, S.H. Zuo, M.B. Lan, Biosensors and Bioelectronics 24 (2009) 1353.