Materials Science and Engineering C 33 (2013) 721–726

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Nano-scale islands of ruthenium oxide as an electrochemical sensor for iodate and periodate determination Fatemeh Chatraei, Hamid R. Zare ⁎ Department of Chemistry, Yazd University, Yazd, 89195-741, Iran

a r t i c l e

i n f o

Article history: Received 7 May 2012 Received in revised form 2 August 2012 Accepted 28 October 2012 Available online 2 November 2012 Keywords: Iodate Periodate Nano-scale islands of ruthenium oxide Electrocatalytic determination Amperometry

a b s t r a c t In this study, a promising electrochemical sensor was fabricated by the electrodeposition of nano-scale islands of ruthenium oxide (ruthenium oxide nanoparticles, RuON) on a glassy carbon electrode (RuON–GCE). Then, the electrocatalytic oxidation of iodate and periodate was investigated on it, using cyclic voltammetry, chronoamperometry and amperometry as diagnostic techniques. The charge transfer coefficient, α, and the charge transfer rate constant, ks, for electron transfer between RuON and GCE were calculated as 0.5 ± 0.03 and 9.0 ± 0.7 s−1 respectively. A comparison of the data obtained from the electrocatalytic reduction of iodate and periodate at a bare GCE (BGCE) and RuON–GCE clearly shows that the unique electronic properties of nanoparticles definitely improve the characteristics of iodate and periodate electrocatalytic reduction. The kinetic parameters such as the electron transfer coefficient, α, and the heterogeneous electron transfer rate constant, k′, for the reduction of iodate and periodate at RuON–GCE surface were determined using cyclic voltammetry. Amperometry revealed a good linear relationship between the peak current and the concentration of iodate and periodate. The detection limits of 0.9 and 0.2 μM were calculated for iodate and periodate respectively. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Iodine is a necessary micronutrient playing an important role in cell growth and brain function. Its deficiency is a threat to human body health. Iodine deficiency disorders are caused by insufficient iodine intake and, in some cases, goitrogenous factors in the diet [1]. A deficiency of thyroid hormones can result in a serious delay in neurologic development [2]. Also, periodate is an important oxidant which can oxidize metalloids [3,4], polyhydroxylated compounds [5] and has catalytic applications [6] at trace levels. On the other hand, an excess of periodate or iodate can cause goiter and hypothyroidism as well as hyperthyroidism [7]. Thus, sensitive and selective determination of iodine compounds, particularly iodate and periodate, is important in a variety of areas such as food production and processing [8], clinical and biological sciences [9,10], and industrial applications [11]. Several methods have been reported to determine periodate and iodate in mixtures. They include spectrophotometric [12–14], chromatography [15], and capillary electrophoresis [16]. However, it must be pointed out that most of these methods entail complicated processes and expensive instruments. To be employed, they also need well-trained persons. Owing to their rapid response, cheap, safe, and its simple usage, electrochemical methods have often been employed for the detection of iodate and periodate [17–21]. To the best of our knowledge, among the electrochemical methods for the determination of iodate and periodate, nanoparticle-modified electrodes are the least extensively studied. ⁎ Corresponding author. Tel.: +98 351 8122669; fax: +98 351 8210991. E-mail address: [email protected] (H.R. Zare).

Due to the electrochemical reversibility and the different oxidation states exhibited by ruthenium oxide derivatives — these materials can be used as an efficient electron transfer mediators to modify different electrode surfaces. Nanoparticle-modified electrodes have several advantages, including high electrical, conductivity, high surface area, and chemical stability [22,23]. Ruthenium oxide nanoparticles (RuON) are also well recognized for their excellent electrocatalytic properties for the detection of important analytes [22,24]. As a sequel to our previous research on the preparation of sensors and biosensors [22,24–30], this study is conducted on GCE modified with RuON (RuON–GCE) which has been characterized by cyclic voltammetry and used for the electrocatalytic reduction of iodate and periodate. Our findings indicate that this modified electrode offers several distinct advantages including extraordinary stability, high surface charge transfer rate constant, and good ability of detection for the determination of iodate and periodate. Amperometry was used to evaluate the analytical performance of the modified electrode in the quantitation of iodate and periodate in a micromolar or lower concentration range. 2. Experimental 2.1. Reagents, equipment, and electrode modification NaIO3, NaIO4, RuCl3·xH2O, and other reagents were purchased from Merck and used without purification. The buffer solutions (0.1 M) were made from H3PO4 + NaH2PO4, and the pH was adjusted with 0.1 M H3PO4 or 2.0 M NaOH. The pH was measured with a Metrohm model

0928-4931/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.10.024

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F. Chatraei, H.R. Zare / Materials Science and Engineering C 33 (2013) 721–726

691 pH mV−1 meter. The electrochemical experiments were performed with an Autolab PGSTAT 30 (Eco Chemie, Utrecht, Netherlands) and a conventional three-electrode cell. The working electrode was a bare glassy carbon electrode (BGCE) or ruthenium oxide nanoparticles modified glassy carbon electrode (RuON–GCE), and the reference electrode was a saturated calomel electrode (SCE). Prior to the modification, the BGCE (2 mm in diameter) was polished successively with alumina on a polishing cloth and then rinsed with doubly distilled water. After being cleaned, RuON–GCE was fabricated as previously described [22]. 3. Results and discussion 3.1. Characterization of the surface morphology of RuON–GCE Scanning electron microscopy was used to characterize the surface morphologies of different electrodes. The scanning electron microscopy (SEM) of bare GCE and RuON−GCE are given in Scheme S1. As previously described [22] and it can be seen in Scheme S1, when the RuON are electrodeposited on the GCE surface (Scheme S1A), the nanoparticles of the ruthenium oxide with size distribution approximately 70 to 100 nm are observed clearly on the GCE surface (Scheme S1B). 3.2. Electrochemical properties of RuON–GCE Fig. 1 shows the cyclic voltammograms of the RuON–GCE in a 0.1 M phosphate buffer solution (pH 3.0) at various potential scan rates. As it can be seen, the voltammogram peaks are typical of a surfaceimmobilized redox couple with a formal potential (E 0′) value of 137 mV. The formal potentials are obtained from the equation E0′ =

Ep.a − α(Ep.a − Ep.c) [31], considering α = 0.5 (see below). The anodic peak currents are directly proportional to the potential scan rate, v, as shown in inset A of Fig. 1. The linear regression equation is expressed as Ipa = 0.0368 v − 0.3045 and Ipc = −0.0347 v + 0.3775 with a correlation coefficients of Rpa = 0.997 and Rpc = 0.997, respectively. In addition, E 0′ is almost independent of v for potential scan rates below 150 mV s −1, suggesting facile charge transfer kinetics over this range of potential sweep rates. In other words, in the mentioned range of potential scan rates, the modified electrode has a Nernstian (reversible) behavior. Fig. 1, inset B, shows the magnitudes of the peak potentials (Ep) as a function of logarithm v. The results show that the values of the anodic and cathodic peak potentials are proportional to log v and nΔEp > 200 mV for scan rates higher than 500 mV s−1 (Fig. 1, inset C). The linear regression equation is expressed as Epa = 0.1106 logv − 0.1035 and Epc = −0.1103 logv + 0.4079 with a correlation coefficients of Rpa = 0.995 and Rpc = 0.995, respectively. Under these conditions, the surface electron transfer rate constant, ks, and the charge transfer coefficient, α, for the electron transfer between the GCE surface and the RuON can be estimated from the linear variations of the oxidation and reduction peak potentials with the log v according to the Laviron theory [32]. Using these linear plots at pH 3.0, the mean values 0.5 ± 0.03 and 9.0 ± 0.7 s −1 were obtained for α and ks respectively. The cyclic voltammogram of the RuON–GCE in a 0.1 M phosphate buffer solution in a pH range of 2.0–7.0 is shown in Fig. 2A. The pH dependence of the conditional formal potentials of the ruthenium oxide nanoparticles, E 0′, is given by the following equation: 00

0

E ¼ E −ð2:303mRT=nFÞ pH

ð1Þ

A

4

5 Ip / µA

Ipa

0 Ipc

10

-4 0 0.3

I / µA

Ep vs SCE / V

2

60 -1 v / mV s

120

B Epa

0.15 Epc

0

1

0.6

-1 Ep vs SCE / V

0.36

2.1 -1 log v (v / mV s )

3.6

C Epa

0.13 Epc

-0.1 2.5

-4 -0.2

0.2

3.1 -1 log v (v / mV s )

0.6

3.7

1

E vs SCE / V Fig. 1. Cyclic voltammograms of RuON–GCE in a 0.1 M phosphate buffer solution (pH 3.0) at different potential scan rates: numbers 1–10 correspond to 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV s−1 respectively. Insets: (A) variation of peak currents vs. potential scan rate, (B) variation of peak potential vs. the logarithm of the potential scan rate. (C) Magnification of the plot in inset B for high potential scan rates.

F. Chatraei, H.R. Zare / Materials Science and Engineering C 33 (2013) 721–726 5

0.9

1

723

1

A

A

f

0.3

I / µA

a g

-2

-0.9 -0.25

-0.01

0.23

b c

I / µA

-0.3

0.47

9

d

Ip / µA

E vs SCE / V -5

B

5.5 2

e

0.14

0 -8 -0.4

0.02

1.5 [IO3 ] / mM

3

0.5

1.4

E vs SCE / V -0.1 1

3

5

1

7

pH

B

Fig. 2. (A) Cyclic voltammograms (at 100 mV s−1) of RuON–GCE at various buffered pHs. Numbers 1–5 correspond to 2.0, 3.0, 4.0, 5.0 and 7.0 pHs respectively. (B) Plot of formal conditional potential, E0′, vs. pH.

i

a

-1

3.3. Electrocatalytic reduction of iodate and periodate at RuON–GCE In order to test the electrocatalytic activity of RuON–GCE in the reduction of iodate and periodate, the cyclic voltammograms of RuON–GCE and BGCE were obtained in the absence and the presence of iodate in pH 3.0 (Fig. 3A) and pH 7.0 (Fig. 3B). Fig. 3A shows the cyclic voltammograms of RuON–GCE in the buffer solution of pH 3.0 in the absence (voltammogram a) and the presence of iodate different concentrations (voltammogram b–e). A comparison is made of the voltammograms after the addition of different concentrations of iodate. There is a drastic enhancement of the cathodic peak current but virtually no current in the anodic sweep. In addition, the cathodic peak potential for the reduction of iodate at RuON–GCE in the buffer solution of pHs 3.0 and 7.0 are 113 and −100 mV, while at BGCE iodate was not reduced until potential of −200 and −350 mV, respectively (Fig. 3A and B). The plots of the catalytic current versus the iodate concentration in pHs 3.0 and 7.0 are linear in the concentration ranges 0.3–2.8 mM and 1.0–6.5 mM respectively (insets of Fig. 3A and B). The linear regression equation is expressed as Ip = 1.9681 C + 2.0805 and Ip = 0.5072 C + 0.6007 with a correlation coefficients of R = 0.997 and R = 0.997, in insets of Fig. 3A and B respectively. The electrocatalytic reactions above can be explained by the possible reaction scheme: þ

Rux Oy þ 2nH þ 2ne→Rux Oy−n þ nH2 O

ð2Þ

j

I / µA

where E 0 is the standard redox potential, m and n the number of protons and electrons involved in the redox reactions, and R, T and F the gas constant, temperature, and Faraday constant respectively. Based on Eq. (1), for a Nernstian redox system, the variation of E 0′ versus pH is linear and its theoretical slope value is −59.2 mV pH−1 when m = n. As shown in Fig. 2B, the conditional formal potential (E 0′) of the surface redox couple is pH-dependent with a slope of − 52.6 per unit of pH, which is close to the Nernstian slope value, when the transferred protons and electrons in the redox processes of RuON are equal in number. The linear regression equation is expressed as E0′ = −0.0526 pH + 0.3125 with a correlation coefficients of R = 0.999.

b c d

4.5

e f g

-3

h

Ip / µA

E0' vs SCE / V

0.26

2.5 0.5 0

-5 -0.4

3.5 [IO3 ] / mM

0.2

7

0.8

E vs SCE / V Fig. 3. (A) Cyclic voltammograms of RuON–GCE in a 0.1 M phosphate buffer solution (pH 3.0) containing different concentrations of iodate at the scan rate of 30 mV s−1 in (a) the absence and the presence of (b) 0.3, (c) 0.8, (d) 1.5 and (e) 2.8 mM iodate. (f) as (a) and (g) as (b) at a BGCE. Inset shows the plot of the catalytic peak current vs. iodate concentration. (B) Cyclic voltammograms of RuON–GCE in a 0.1 M phosphate buffer solution (pH 7.0) containing different concentrations of iodate at the scan rate of 30 mV s−1 in (a) the absence and the presence of (b) 1.0, (c) 2.0, (d) 3.0, (e) 4.0, (f) 5.0, (g) 5.5 and (h) 6.5 mM iodate. (i) as (a) and (j) as (b) at a BGCE.

–

–

3Rux Oy−n þ nIO3 →3Rux Oy þ nI

ð3Þ

where RuxOy and RuxOy−n represent the oxidized and reduced forms of the ruthenium oxide. The similar catalytic scheme can be proposed for the electroreduction of periodate at RuON–GCE in the buffer solution of pHs 3.0 and 7.0 (Fig. 4A and B). The electrocatalytic cathodic peak potential of periodate in the buffer solutions of pHs 3.0 and 7.0 appears at 72 and −100 mV at RuON–GCE (Fig. 4A and B) while at BGCE, periodate was not reduced until potential of −200 and −350 mV, respectively. A decrease in the overpotential and the enhancement of the reduction peak current for iodate and periodate indicates a strong catalytic activity of RuON–GCE toward the reduction of both analytes. Although, the modified electrode shows a good electrocatalytic activity for iodate and periodate in both pHs of 3.0 and 7.0, the results suggest that the sensitivity of species determination decreases with an increase in the pH. In addition, the stability and the electrochemical

F. Chatraei, H.R. Zare / Materials Science and Engineering C 33 (2013) 721–726

0.6

1

Ac

B

1.2

v / mV s-1

A

2

9

16 1.75

6

Ip / µA

a

d

I / µA

I / µA

d

-0.2

a

log I (I / µA)

-0.6

a f -1.4

-1.8

0.9

1.5

4

b 1.25

2 1.6

2.8 1/2

v

Ip v-1/2/ µA (mV s-1)-1/2

724

4 -1 1/2

/ (mV s )

0.6

b

1

b c 0.1

-2.6 -0.4

0.6

E vs SCE / V

-0.1

5

0.3 0.26

0.2

Ip ¼ 3:01  10 n½ð1−α Þnα 

1=2 1=2

AC b D

v

:

ð4Þ

Considering (1 − α)nα = 0.79 (see below), D = 1.88 × 10−6 cm2 s−1 (obtained by chronoamperometry), Cb = 0.5 mM of iodate, and A = 0.0314 cm2, it is estimated that the total number of electrons involved in the cathodic reduction of iodate is n = 6.1 ≅ 6. This value is the same as that previously reported for the electrocatalytic reduction of iodate at electrodes modified with other mediators [18,21]. Under the above conditions for an ErCi′ mechanism, the theoretical model of Andrieux and Saveant [33] can be used to calculate the catalytic rate constant of electron transfer, k′, between the modifier and the analytes. Based on the Andrieux and Saveant theoretical model [33], the following relationship (Eq. (3)) exists between the peak current and the square root of potential scan rate, v1/2. Their relationship holds true in the case of a slow scan rate and a large catalytic rate constant, k′, between an analyte and a modifier:

1=2

Icat ¼ 0:496nFAC b ðnFDv=RTÞ

:

ð5Þ

-1

v / mV s 2

B

9

16 1.9

4.6

Ip / µA

0.82

log I (I / µA)

reversibility of RuON–GCE increase in a buffer solution of pH 3.0. Therefore, the subsequent studies were carried out in a buffer solution of pH 3.0. In another experiment on a 0.1 M phosphate buffer solution (pH 3.0) containing 0.5 mM iodate or 0.1 mM periodate at different scan rates (not shown), the cyclic voltammograms of RuON–GCE were used to get kinetic information about the rate-determining step in the electrocatalytic reduction of iodate and periodate at the modified electrode surface. As shown in the inset of Fig. 5A, plot a, the reduction peak currents, Ip, of iodate increase linearly with the square root of the scan rate, v 1/2. The linear regression equation is expressed as Ip = 1.1084 v1/2 + 0.8702 with a correlation coefficients of R = 0.997. This behavior indicates that the nature of redox process is diffusion-controlled. The number of electrons in the overall reaction of iodate electrocatalytic reduction can be obtained from the slope of the IP versus v 1/2 plots (inset of Fig. 5A, plot a). Also, the plot of the scan rate-normalized current (Ipv −1/2) versus v exhibits a characteristic shape typical of an ErC′i process (inset of Fig. 5A, plot b). According to the following equation for totally irreversible diffusion-controlled processes [33]: 1=2

0.32

0.3

E vs SCE / V

Fig. 4. (A) Cyclic voltammograms of RuON–GCE in a 0.1 M phosphate buffer solution (pH 3.0) at the scan rate of 30 mV s−1 in (a) the absence and (b) the presence of 0.5 mM periodate. (c) as (a) and (d) as (b) at a BGCE. (B) Cyclic voltammograms of RuON–GCE in a 0.1 M phosphate buffer solution (pH 7.0) at the scan rate of 30 mV s−1 in (a) the absence and the presence of (b) 0.6 and (c) 1.0 mM periodate. (d) as (a) and (f) as (b) at a BGCE.

5

0.28

E vs SCE / V

a 1.45

4 b

0.58

1

3.4 1.5

2.75

1

4

Ip v-1/2/ µA (mV s-1)-1/2

-3 -0.4

v1/2 / (mV s-1)1/2

5

0.34

0.1 0.22

0.25

0.28

0.31

E vs SCE / V Fig. 5. (A) Tafel plots drawn from the cyclic voltammograms of RuON–GCE in a 0.1 M phosphate buffer solution (pH 3.0) containing 0.5 mM iodate recorded at different scan rates. Numbers 1–5 correspond to 4, 6, 8, 10 and 14 mV s−1 respectively. Inset shows (a) plots of the electrocatalytic peak currents vs. the square root of sweep rate, and (b) variation of the sweep rate-normalized current (Ip v−1/2) vs. the potential scan rate, v. (B) Tafel plots drawn from the cyclic voltammograms of RuON–GCE in a 0.1 M phosphate buffer solution (pH 3.0) containing 0.1 mM periodate recorded at different potential scan rates. Numbers 1–5 correspond to 4, 6, 8, 10 and 14 mV s−1 respectively. Inset shows (a) plots of the electrocatalytic peak currents vs. the square root of sweep rate, and (b) variation of the sweep rate–normalized current (Ip v−1/2) versus the potential scan rate, v.

Where Cb is the bulk concentration (mol cm−3) of the analyte. Small values of k′ result in a prefactor lower than 0.496 [33]. The slope of plot a in the inset of Fig. 5A was used to evaluate the prefactor corresponding to iodate. For low potential scan rates (4 to 14 mV s −1), the average value of this prefactor was found to be 0.32 for a RuON–GCE in the presence of 0.5 mM iodate. Based on extensive computations, a working curve is drawn to show the relationship between the numerical values of the prefactor, Icat(RT)1/2 / nFACb(nFDv)1/2, and log [kT (RT)1/2 / (nFDv) 1/2] [33]. An average value of k′ = 3.6 × 10−3 cm s−1 was calculated from such a working curve for the catalytic rate constant of the electron transfer between iodate and electrodeposited ruthenium oxide nanoparticles. Fig. 5A shows Tafel plots that were drawn from the data of the rising part of the current-voltage curves recorded at potential scan rates of 4 to 14 mV s −1 in a 0.1 M phosphate buffer solution (pH 3.0) containing 0.5 mM iodate. The results of polarization studies for the electroreduction of iodate at RuON–GCE show that, for all potential sweep rates, the average Tafel slope is −3.63 V −1 (Fig. 5A). An average Tafel slope of −3.63 V−1 agrees well with the involvement of one electron in the rate-determining step of the electrode process, assuming a charge transfer coefficient of α = 0.21. Also, the value i0 is obviously readily accessible from the intercept of the Tafel

F. Chatraei, H.R. Zare / Materials Science and Engineering C 33 (2013) 721–726

at 80 mV. In the chronoamperometric studies, we determined the diffusion coefficient, D, of iodate and periodate. Figs. S1A and S1B show the experimental plots of I versus t −1/2 with the best fits for different concentrations of iodate and periodate employed. From the slopes of the resulting straight lines and using the Cottrell equation, [34] we calculated the average values of 1.88 × 10 −6 cm 2 s −1 and 5.0 × 10 −6 cm 2 s −1 for the diffusion coefficients of iodate and periodate respectively. The calculated value of diffusion coefficient is in good agreement with 2.1 × 10 −6 cm 2 s −1, which was previously reported for iodate [17].

plots [34]. The average value i0 of iodate at RuON–GCE is found to be 34.73 μA. As depicted in Fig. 5B, plot a, a similar behavior is observed for the electrocatalytic reduction of periodate at RuON–GCE. The linear regression equation is expressed as Ip = 0.4766 v 1/2 + 2.7263 with a correlation coefficients of R = 0.998. From the inset of Fig. 5B and considering (1 − α)nα = 0.83 (see below), D = 5.0 × 10−6 cm2 s−1 (obtained by chronoamperometry), Cb = 0.1 mM periodate, and A = 0.031 cm2, the total number of electrons involved in the reduction of periodate is found to be n = 7.8 ≅ 8. Also, based on the model of Andrieux and Saveant [33] for low potential scan rates (4–14 mV s −1) and 0.1 mM periodate, the average value of the catalytic rate constant between periodate and the modifier, k′, was calculated as 6.6 × 10−3 cm s−1. Fig. 5B shows Tafel plots that were drawn from the data of the rising part of the current–voltage curves recorded at the scan rate of 4 to 14 mV s−1, in a 0.1 M phosphate buffer solution (pH 3.0) containing 0.1 mM periodate. An average Tafel slope of 2.83 V−1 indicates that a one-electron process was involved in the rate-determining step, assuming a charge transfer coefficient of α = 0.2. Also, the value i0 was found to be 12.38 μA from Tafel plots. The catalytic reduction of iodate and periodate by a RuON–GCE was also studied by chronoamperometry. Chronoamperometric measurements of different concentrations of iodate and periodate at RuON–GCE were done by setting the working electrode potential

3.4. Amperometric detection of iodate and periodate Fig. 6A and B show a typical amperometric response obtained by successfully adding iodate and periodate to a continuously stirred modified electrode (rotation speed 3000 rpm) in a solution of pH 3.0 at 78 mV and 100 mV applied potentials respectively. As illustrated, during the successive additions of 1.0 mM of iodate and periodate, a well-defined response is observed, demonstrating a stable and efficient catalytic property of RuON for both analytes. The reduction currents at RuON–GCE are proportional to the iodate concentration in the range of 1.5–518.3 μM (Fig. 6A, inset I). The linear regression equation is expressed as Ip = 0.0104 C + 0.0999 with a correlation coefficients of

II 8

-0.035 -0.16 0

350

700

t/s

0

I / µA

A

I / µA

0.09

4

725

I

4

I / µA

0 0.0

-0.1

300.0

600.0

I / µA

[IO3-] / µM

-4

-0.475

-0.85 350

2000

1175

t/s

-8 300

1600

2900

4200

t/s 4

B I / µA

1.5

I

0.75

-4

0 0

0.2

I / µA

20 -0.7

I / µA

I / µA

15

30

[IO4-] / µM

-12

II

10

0

-1.6 20

1110

0

2200

125

250

-

[IO4 ] / µM

t/s

-20 0

1700

3400

5100

t/s Fig. 6. (A) Amperometric response at the rotating RuON–GCE surface (rotation speed 3000 rpm) held at 78 mV in a 0.1 M phosphate buffer solution (pH 3.0) for successive additions of 1.0 mM iodate. Insets: (I) plot of amperometric currents vs. iodate concentrations in the linear range of 1.5–518.3 μM. (II) The recorded amperogram for 38.2 μM of iodate during 400 s. (B) Amperometric response at rotating RuON–GCE (rotation speed 3000 rpm) held at 100 mV in a 0.1 M phosphate buffer solution (pH 3.0) for successive additions of 1.0 mM periodate. Insets (I) and (II) show plot of amperometric currents vs. periodate concentrations in the ranges of 1.0–24.6 μM and 24.6–215.8 μM respectively.

726

F. Chatraei, H.R. Zare / Materials Science and Engineering C 33 (2013) 721–726

Table 1 Comparison of some analytical parameters of different modified electrodes for iodate and periodate determination. Linear range (μmol dm−3) Detection limit (μmol dm−3)

Modifiera

Iodate Nano-Au/P3MT 5.0–500.0 Thionin–MWCNTs 2.0–1000.0 AMMO 1.0–200.0 Fe(III)P–MWCNTs 10.0–4000.0 Os(III)–SWCNTs 1.0–2500.0 V-Schiff base–MWCNTs 0.5–500.0 Tungsten oxide films 5.0–5000.0 Keggin-type cobalt 2.0–280.0 tungstate anion RuON 1.5–518.0

Periodate

Iodate Periodate

– 1.0–1000.0 – – 1.0–2800.0 1.0–100.0 – –

1.4 1.0 0.5 2.5 0.038 0.35 1.2 0.8

1.0–24.6 0.9 24.6–215.8

Reference

– 0.4 – – 0.036 0.5 – –

[17] [18] [19] [21] [35] [36] [37] [38]

0.2

This work

a Nano-Au/P3MT: gold nanoparticles/poly(3-methylthiophene), Thionin–MWCNTs: Thionin/multi-walled carbon nanotubes, AMMO: amorphous mixed-valent molybdenum oxide films, Fe(III)P–MWCNTs: iron − porphyrin/multi-walled carbon nanotubes, Os(III)– SWCNTs: osmium (III) complex/ single-wall carbon nanotubes, V-Schiff base–MWCNTs: vanadium-Schiff base complex/multi-walled carbon nanotubes, RuON: ruthenium oxide nanoparticles.

amounts of periodate or iodate, and their recovery was determined by amperometric measurements at the RuON–GCE surface. The results are presented in Table 2. As they suggest, the relative standard deviations (RSD%) and the recovery rates of the spiked sample are acceptable, and the matrix of the water sample does not make any interference in the determination of periodate or iodate at the proposed sensor. 4. Conclusions The results of this paper indicate that nano-scale ruthenium oxide exhibits a good electrocatalytic activity toward the reduction of iodate and periodate. The catalytic reaction rate constant, k′, for the reduction of iodate and periodate at a RuON–GCE surface is found to be 3.6 × 10 −3 and 6.6 × 10 −3 cm s −1 respectively. Also, the detection limit of iodate and periodate determination at a modified electrode surface is obtained as 0.9 and 0.2 μM, respectively. Good reproducibility, high stability, low detection limit, fast amperometric response time, technical simplicity, and possibility of rapid sensor preparation are the great advantages of this modified electrode. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.msec.2012.10.024.

R = 0.999. Also, insets I and II of Fig. 6B clearly show that the plot of the peak current versus the periodate concentration is constituted from two linear segments (1.0–24.6 μM and 24.6–215.8 μM) with different slopes. The linear regression equation is expressed as Ip = 0.0541 C + 0.0778 and Ip = 0.0855 C − 1.1123 with a correlation coefficients of R = 0.999 and R = 0.999 in insets I and II of Fig. 6B respectively. The lower detection limits of iodate and periodate, Cm, were found to be 0.9 and 0.2 μM respectively, using the equation Cm = 3 sbl/m [35], where sbl is the standard deviation of the blank response ( 0.0034 μA) and m is the slope of the calibration plot of iodate and periodate (0.010 and 0.054 μA μM−1 respectively). The average amperometry peak current and the precision estimated in terms of the coefficient of variation for 12 repeated measurements (n = 12) of 20.0 μM of iodate at RuON–GCE were 0.32 μA and 3.6% respectively. Also, inset II of Fig. 6A shows the amperometric response of 38.2 μM iodate during prolonged 400 s experiment. The response remained stable throughout the experiment, indicating no inhibiting effect of analyte and its reduction products on the RuON–GCE surface. As inferred, RuON–GCE is an excellent and stable electrode which can facilitate the low potential amperometric measurement of iodate and periodate. In Table 1, some of the analytical parameters obtained for iodate and periodate in this study are compared with those previously reported by others [17–19,21,35–38]. As it can be seen, the responses of the proposed modified electrode are superior in some cases, especially in linear concentration ranges, as compared to the previously reported modified electrodes. In addition, the proposed electrode has been also applied to the recovery of periodate and iodate in drinking water. In this experiment, 5.0 mL of water sample was diluted to 10 mL with a 0.1 M phosphate buffer solution (pH 3.0). Then, the sample was spiked with different

Table 2 Results of recoveries of periodate in drinking water sample at the RuON–GCE surface. Analytes

Added (μmol dm−3)

Found (μmol dm−3)a

Recovery %

Periodate

8.0 13.0 10.0 50.0 50.0 100.0

8.1 ± 0.2 20.7 ± 1.5 30.7 ± 0.9 50.5 ± 1.2 102.2 ± 4.5 199.5 ± 9.2

101.0 98.6 99.0 101.0 102.2 99.8

Iodate

a

Mean ± standard deviation (n = 4).

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