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ORIGINAL ARTICLE
9 (
2_suppl
); S1563-S1568
doi:
10.1016/j.arabjc.2012.04.009

Voltammetric determination of resorcinol on the surface of a glassy carbon electrode modified with multi-walled carbon nanotube

, , ,
 Department of Analytical Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Islamic Republic of Iran

⁎Corresponding author. Tel.: +98 3615552930; fax: +98 3615552935. s.m.ghoreishi@kashanu.ac.ir (Sayed Mehdi Ghoreishi)

Received: , Accepted: ,
© The Author(s) 2012
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

A multi-walled carbon nanotubes (MWCNTs) film coated glassy carbon electrode (GCE) was fabricated, and the electrochemical oxidation of resorcinol (RS) was studied in Britton–Robinson (BR) buffer (pH 6.0) using cyclic, square wave, and differential pulse voltammetry (CV, SWV, and DPV). The results revealed that the modified electrode shows an electrocatalytic activity toward the oxidation of RS by a marked enhancement in the current response in buffered solution. The oxidation of RS at this nano-structured film coated electrode was irreversible and diffusion-controlled. Under the optimum conditions, the anodic peak current showed a linear relation versus RS concentration in the range of 1.2 × 10−6 to 1.9 × 10−4 M with detection limits of 4.9 × 10−7 and 1.1 × 10−6 M (signal-to-noise = 3) for SWV and DPV, respectively. Moreover, the modified electrode demonstrated good reproducibility (RSD = 2.4%, n = 10) and long-term stability. This method has been applied to the determination of RS in wastewater, and the recoveries were from 93% to 104%.

Keywords

Multi-walled carbon nanotubes
Resorcinol
Glassy carbon electrode
Voltammetry
Determination
1

1 Introduction

Phenolic compounds are highly toxic environmental pollutants, and seriously threaten human’s health. They are widely used in many fields, such as tanning, cosmetic, dye, chemical and pharmaceutical industries (Wang et al., 2007). Phenolic compounds in environment come from different sources, including industrial wastewater, solid castoff of coal tar, coking factory, gasworks, paper mill, chemical plants and pharmaceutical industry (Ware, 1992). Some of the phenolic compounds have been listed as control targets because of their toxicities in many countries. Phenolic compounds are also poisonous organic pollutants. Therefore, many governments have spent a lot of effort in their detection and control.

Resorcinol (RS) is one kind of phenolic compounds with high toxicity. It can be easily absorbed through the gastric tract and human skin, which can cause dermatitis, catarrh, convulsion, cyanopathy, and even death (Guardia et al., 1995). At present, various methods have been employed for RS determination, including spectrophotometric (Kang et al., 2000), high-performance liquid chromatography with diode array detection (Yang et al., 2006), microchip capillary electrophoresis with end channel amperometric detection (Wu and Lin, 2006), quartz crystal microbalance (Mirmohseni and Oladegaragoze, 2004), flow injection chemiluminescence (Du et al., 2001), surface plasmon resonance (Wright et al., 1998), fluorescence (del Olmo et al., 2000; Pistonesi et al., 2006) and spectrofluorimetric (Fan et al., 2007; Wenxiang and Dan, 2007).

Electrochemistry has always provided analytical techniques characterized by instrumental simplicity, moderate cost, and portability. These techniques have introduced the most promising methods for specific applications (Goyal et al., 2006, 2007a, 2009a; Gupta et al., 2010, 2011a). But these methods normally have low sensitivity at conventional electrodes. However, low sensitivity and poor selectivity of electrochemical methods can be overcome by electrode modification (Goyal et al., 2005, 2007b, 2007c, 2008a, 2008b; Gupta et al., 2011b).

Carbon nanotubes (CNTs) are a novel nanoscale material, mainly consisting of SWCNTs and MWCNTs (Bethune et al., 1993; Fernandez-Abedul and Costa-Garcia, 2008). These nanoscale materials have attracted considerable interest owing to their extraordinary mechanical and unique electrochemical properties (Goyal et al., 2008c). The subtle electronic behavior of CNTs demonstrates that they have the ability to promote electron-transfer reactions when used as electrode materials (Nugent et al., 2001). Several types of CNT electrodes have been reported, including CNT paste electrodes (Davis et al., 1997; Valentini et al., 2003), CNT film-coated electrodes (Luo et al., 2001; Guo et al., 2004), CNT powder microelectrodes (Zhao et al., 2002) and CNT paper electrodes (Barisci et al., 2000). CNT-modified GCEs provide a stable and sensitive electrochemical response for phenols (Wang et al., 2003), NADH (Musameh et al., 2002), estrogenic phenolic compounds (Vega et al., 2007a,b; Agui et al., 2007), ascorbic acid, dopamine (Wu and Hu, 2004), uric acid (Wu and Hu, 2004) and tetracycline (Vega et al., 2007a).

The main objective of this work was to develop a simple and sensitive electrochemical sensor for the determination of RS. Based on the good electrocatalytic activity of MWCNT modified GCE toward the electrochemical oxidation of RS, a sensitive electrochemical method has been proposed for the determination of RS.

2

2 Experimental

2.1

2.1 Chemicals and reagents

Resorcinol was purchased from the MERCK Company. BR buffer solution was made up of phosphoric acid (MERCK), boric acid (MERCK), and ice acetic acid from the MERCK and its pH value was adjusted with NaOH (MERCK). MWCNTs with purity of >95% (40–60 nm in diameter) were obtained from the Chinese Academy of Sciences. Generally, pristine MWCNTs contained 3% amorphous carbon, SWCNTs, and graphite requiring some purification pre-treatments before electro-analytical characterization. MWCNTs refluxed under stirring for 5 h in concentrated nitric acid. It has been reported that this treatment could purify the CNTs and cause carboxylation at their termini (Tsang et al., 1994; Hiura et al., 1995). A 1.0 × 10−3 M RS standard solution was simply prepared by dissolving RS in deionized water. Working solutions were prepared with this standard solution and using suitable dilution with a 0.1 M BR buffer solution. BR buffer solution was also used as the supporting electrolyte.

2.2

2.2 Apparatus

Electrochemical measurements were carried out on a M273 electrochemical workstation (EG&G Corporation, USA) with a conventional three-electrode system. This equipment was equipped with a platinum plate (Metrohm) as the counter electrode, a saturated calomel electrode (SCE) from Metrohm as the reference electrode and a MWCNT/GCE as the working electrode. Solution pH values were determined using a 691 pH meter (Metrohm Swiss made) combined with glass electrode (Metrohm). Deionized water was formed with an ultrapure water system (smart 2 pure, TKA, Germany). MWCNT was dispersed with an ultrasonic bath (SONOREX DIGITAL, 10P, BANDELIN). Data were collected at room temperature.

2.3

2.3 Fabrication of the modified electrode

Using ultrasonic agitation, 0.5 mg of purified MWCNTs was dispersed into 5 ml of redistilled water for 30 min. Prior to modification, the GCE was polished with 0.05 mm alumina slurry and then cleaned ultrasonically in double-distilled water. Subsequently, the GCE was coated with 20 μL of MWCNT suspension and the solvent was evaporated in air.

2.4

2.4 Analytical procedure

Twenty milliliters of 0.1 M BR buffer solution (pH 6.0) containing a specific amount of standard solution of RS was added to an electrochemical cell. Electrochemical measurements were carried out by CV and recorded in the potential range of 0.1–1.0 V at a scan rate of 20 mV s−1. DPV employed the following parameters: Einitial = 0.3 V, Efinal = 0.9 V, scan rate = 20 mV s−1. SWV was recorded from 0.3 to 0.9 V and the current peak at 0.6 V was measured. The SWV parameters were as follows: SWV frequency = 80 Hz, pulse height = 100 mV, amplitude = 10 mV.

3

3 Results and discussion

3.1

3.1 Voltammetric behavior of RS at MWCNT/GCE

Fig. 1 shows the cyclic voltammograms (CVs) of 5.0 × 10−5 M RS on a bare GCE and MWCNT–GCE in 0.1 M BR buffer solution (pH 6.0). Compared to the bare electrode, the anodic peak current of RS increased significantly at the modified electrode. Also, the oxidation peak potential shifted to the negative values (Fig. 1a) in contrast to those at the bare electrode (Fig. 1b), which indicated that the modified electrode has a catalytic effect on the oxidation of RS. The following reasons might explain the electrocatalytic response of RS on the surface of MWCNT/GCE. First, the treatment of HNO3 introduced many active groups on the surface of the CNTs, e.g. —COOH, —C⚌O and —OH. These active groups could form hydrogen bonds with the —OH groups of the RS, which weakened the —OH bond energies, and the electrons would be transferred through O⋯H—O. Second, due to the unique electronic structures of the CNTs, they can act as a promoter to enhance the electrochemical reaction. The huge specific surface area of CNTs can also increase the effective area of the electrode. So the peak current increased and the oxidation potential shifted negatively (Ding et al., 2005).

Cyclic voltammograms of 5.0 × 10−5 M RS in 0.1 M BR buffer (pH 6.0) at scan rate of 20 mV s−1 (a) on a MWCNT/GCE; (b) on bare electrode; (c) absence of RS on a modified electrode.
Figure 1
Cyclic voltammograms of 5.0 × 10−5 M RS in 0.1 M BR buffer (pH 6.0) at scan rate of 20 mV s−1 (a) on a MWCNT/GCE; (b) on bare electrode; (c) absence of RS on a modified electrode.

Fig. 1c shows the cyclic voltammogram of the modified electrode in the absence of RS. The electro-oxidation of RS is an irreversible process, because no corresponding reductive peak observed on the cathodic branch.

3.2

3.2 Effect of scan rate

The oxidation peak current of 4.0 × 10−4 M RS was measured with CV at different scan rates from 20 to 200 mV s−1 (Fig. 2). By increasing the scan rate, the heights of oxidation peak were increased. Also, peak potential shifted toward more positive values for oxidation process. Oxidation peak current varied linearly with the square root of scan rate (ν1/2), demonstrating the diffusion-controlled process of RS on the surface of MWCNT/GCE.

Cyclic voltammograms of MWCNT/GCE in 0.1 M BR buffer (pH 6.0) containing 4.0 × 10−4 M RS at different scan rates. Inset: The relationship between anodic current and scan rates.
Figure 2
Cyclic voltammograms of MWCNT/GCE in 0.1 M BR buffer (pH 6.0) containing 4.0 × 10−4 M RS at different scan rates. Inset: The relationship between anodic current and scan rates.

The linear regression equation was ipa = 0.9647 + 0.0038 v1/2 (ipa: A, v: mV s−1) with the correlation coefficient of 0.9982 (Fig. 2, inset).

3.3

3.3 Effect of pH

The electrochemical behavior of RS was investigated over the pH range from 4.0 to 9.0 in 0.1 M BR buffer solution. Fig. 3 shows the square wave voltammograms of 5.0 × 10−5 M RS in 0.1 M BR buffer solution. The peak potential shifted to the negative values by increasing the pH. The negative shift in anodic peak potential (Epa) with pH can be described by the following equation:

(1)
E pa ( V versus SCE ) = 0.985 - 0.07 pH r = 0.9972
The relationship between the pH values (from 4 until 9) and the anodic peak potentials obtain from SWV at scan rate of 20 mV s−1.
Figure 3
The relationship between the pH values (from 4 until 9) and the anodic peak potentials obtain from SWV at scan rate of 20 mV s−1.

The slope of the regression Eq. (1) is close to the theory value of 58.5 mV pH−1 for two electrons and two protons process (Sun et al., 2010; Wu et al., 2010; Yin et al., 2011), indicating that the electrochemical redox of RS at MWCNT/GCE should be a two electrons and two protons process (scheme 1). The relationship between the peak current and pH was investigated (Fig. 4). As can be seen the peak current in pH 6.0 is maximum, therefore pH 6.0 was chosen as the optimum pH and this pH was used in all the following experiments. After that, the oxidation potential was observed to decrease by increasing the pH and above pH 9.0, the oxidation peak current no longer appeared. Therefore a weak acidic solution was better for the experiments.

Mechanism of RS oxidation at MWCNT/GCE.
Scheme 1
Mechanism of RS oxidation at MWCNT/GCE.
The relationship between the pH values (from 4 until 9) and the anodic peak current obtain from SWV at scan rate of 20 mV s−1.
Figure 4
The relationship between the pH values (from 4 until 9) and the anodic peak current obtain from SWV at scan rate of 20 mV s−1.

3.4

3.4 Calibration curve

The relationship between the oxidation peak current and RS concentration was examined using DPV and SWV methods. The oxidation peak current was found to be proportional to RS concentration over the ranges of 1.2 × 10−6 to 1.9 × 10−4 M. Fig. 5 shows the differential pulse voltammograms obtained for RS at various concentrations in 0.1 M BR buffer solution (pH 6.0). Inset in Fig. 5 shows the linear relationship between ipa and CRS. The linear regression equation is expressed as ipa (μA) = 0.0033c (μM) + 0.024 (r = 0.9988). When the signal-to- noise ratio was 3, the detection limit was 1.1 × 10−6 M.

Differential pulse voltammograms of RS at various concentrations (1.2 × 10−6 to 1.9 × 10−4 M) MWCNT/GCE in 0.1 M BR (pH 6.0) at electrode with different concentrations. (Inset) linear relationship between ipa and CRS.
Figure 5
Differential pulse voltammograms of RS at various concentrations (1.2 × 10−6 to 1.9 × 10−4 M) MWCNT/GCE in 0.1 M BR (pH 6.0) at electrode with different concentrations. (Inset) linear relationship between ipa and CRS.

Since SWV is one of the popular techniques for carrying out the analysis in various samples, due to its high sensitivity and multi-analysis capability, it has been utilized in the present investigation to estimate the detection limit of RS (Goyal et al., 2009b). Fig. 6 shows the SWV results obtained for the oxidation of different concentrations of RS at the modified electrode. The linear regression equation is expressed as ipa (μA) = 0.0211c (μM) + 0.154 (r = 0.9994). When the signal-to- noise ratio was 3, the detection limit was 4.9 × 10−7 M, which is lower than the DPV value of 1.1 × 10−6 M. Which values of detection limit is comparable to values reported by other research groups for electrocatalytic oxidation of RS at the surface of modified electrodes (Table 1).

SWV of RS at MWCNT/GCE in 0.1 M BR (pH 6.0) with different concentrations (1.2 × 10−6 to 1.9 × 10−4 M). (Inset) linear relationship between ipa and CRS.
Figure 6
SWV of RS at MWCNT/GCE in 0.1 M BR (pH 6.0) with different concentrations (1.2 × 10−6 to 1.9 × 10−4 M). (Inset) linear relationship between ipa and CRS.
Table 1 Comparison of the efficiency of some modified electrodes used in the electro-analysis of RS.
Modified electrodes Detection limit (μmol l−1) Dynamic range (μmol l−1) Reference
Graphene–chitosan/GCE 0.75 1–550 Yin et al. (2011)
SWNTs/GCE 0.3 0.4–10 Wang et al. (2007)
40–100
MWNTs/GCE 1 5–80 Ding et al. (2005)
MWNTs/multielectrode array 0.6 6–100 Zhang et al. (2009)
MWNTs/GCE 0.49 1.2–190 This work

3.5

3.5 Stability of the MWCNT/GCE

The reproducibility of MWCNT/GCE was examined by repeating the measurement of 4.0 × 10−4 M RS. After each determination, the used modified electrode underwent five successive cyclic voltammetric sweeps between 0.1 and 1.0 V at a scan rate of 20 mV s−1 in 0.1 M BR (pH 6.0) to remove any adsorbents and yield a reproducible electrode surface. The relative standard deviation (RSD) of ten measurements was 2.4%. The long-term stability of the MWCNT/GCEs was also tested by measuring the oxidation peak current at a fixed RS concentration of 4.0 × 10−4 M over a period of 3 weeks. After 3 weeks, the current response only decreased by 4.4%. These results indicate that the electrode has excellent reproducibility and long-term stability, making it attractive for the fabrication of chemical sensor.

3.6

3.6 Interferences

The effect of possible interference was investigated by the addition of other species to a solution containing 1.0 × 10−5 M RS in 0.1 M BR electrolytes. Each possible contaminant was first added to yield a concentration identical to that of RS. Subsequently, another addition was made so that the interfering concentration was 10 times more than RS. Ascorbic acid, uric acid, penicillin, aspirin, and amoxicillin were tested. Uric acid, penicillin, aspirin and ascorbic acid were found to cause less than a 9% increase in the RS current peak height, even when present in 10-fold excess. The presence of amoxicillin in 10-fold excess of RS was sufficient to cause a 6% decrease in RS signal.

3.7

3.7 Recovery test

We prepared an artificial wastewater by mixing the different phenolic compounds and drug and the experiments were carried out under optimal conditions. Different concentrations of RS were added to this artificial wastewater and the RS contents were determined by SWV. Then the recoveries of five measurements and RSD were calculated and the results are summarized in Table 2.

Table 2 Recovery data observed for RS at different concentrations. Analysis carried out by SWV.
Sample no. Added (μM) Founded (μM) Recovery (%)
1 4.00 3.89 97.1
2 6.00 5.81 97.3
3 8.00 7.42 93.0
4 9.00 9.46 104.0

4

4 Conclusions

In this work, the MWCNTs-modified GCE was used for the determination of RS by SWV and DPV methods, quantitatively. The modified electrode exhibited electrocatalytic activity for the oxidation of RS associated with negative shift in anodic peak potential. The method provided a simple, convenient and fast route to detect the concentration of RS.

Acknowledgements

The authors are grateful to University of Kashan for supporting this work by Grant No. 1591959.

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