Full Paper

Simultaneous Voltammetry Determination of DihydroxybenzeneIsomers by Nanogold Modified Electrode

Lu Han, Xiaoli Zhang*

School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China*e-mail: zhangxl@sdu.edu.cn

Received: July 29, 2008Accepted: October 21, 2008

AbstractA convenient electrochemical deposition method to prepare nanogold/glassy carbon modified electrode (nano-Au/GCE) is adopted. In 0.1 mol/L HAc-NaAc buffer solution (pH 4.61), the nano-Au/GCE shows an excellentelectrocatalytical behavior for the redox of dihydroxybenzene. A simple, rapid and highly selective voltammetry forsimultaneous determination of dihydroxybenzene isomers, hydroquinone, catechol, and resorcinol, is developed usingthe nano-Au/GCE. This method has been applied to the direct determination of the three dihydroxybenzene isomersin artificial wastewater.

Keywords: Nanogold modified electrode, Dihydroxybenzene isomers, Voltammetry, Simultaneous determination

DOI: 10.1002/elan.200804403

1. Introduction

Dihydroxybenzene as an important chemical material andsynthetic intermediate is widely used in many fields, such astanning, dye, cosmetic, chemical, pesticide and pharma-ceutical industries. The isomers of dihydroxybenzene widelyexist in environment as a kind of important environmentalpollutant because they are toxic to humans and difficult todegrade in the ecological environment [1]. Furthermore,they usually coexist in products. Based on comprehensiveconsideration of industrial production and pollution pre-vention, it is very necessary to develop a simple and rapidanalytical method for detecting isomers of dihydroxyben-zene.

At present, the main determination methods for dihy-droxybenzene are chromatography [2, 3], spectrophotom-etry [4] and electrochemical methods [5, 6]. There are somelimitations in chromatography, such as complicated oper-ation, expensive instruments and long testing period.Though electrochemical methods have a unique character-istic of low maintenance costs, high accuracy and excellentsensitivity, the oxidation-reduction peak potentials of theisomers (hydroquinone and catechol) are so close thatsimultaneous determination of the isomers using conven-tional electrodes is difficult and one has to determine theirtotal amount.

In recent years, various chemical modified electrodeshave attracted considerable attention for simultaneousdetermination of dihydroxybenzene isomers [7 – 9]. How-ever, the preparation of the modified electrode andcalibration process of these methods are tedious. Nano-materials show higher electrocatalytic activity, as a kind ofspecial sensing material of chemically modified electrodes.

Particularly, the gold nanoparticles have attracted muchattention in recent decades, owing to their excellent opticaland electronic properties, as well as biocompatible ability.And they have been used to enzyme analysis [10 – 12],immune analysis [13 – 15], nucleic acid analysis [16, 17], druganalysis [18, 19] and environment analysis [20].

In this paper we got the nanogold modified electrode(nano-Au/GCE) with high sensitivity and selectivity usingelectrochemical deposition method at low potential anddeveloped a new method for simultaneous determination ofthree dihydroxybenzene isomers using nano-Au/GCE,which was not reported.

2. Experimental

2.1. Reagents

Catechol (CC), resorcinol (RC) and hydroquinone (HQ)(obtained from J&K Chemical Reagents Ltd., Beijing,China) was used without further purification, and their stocksolutions were stored at�4 8C away from light. HAuCl4 waspurchased from Shanghai Chemical Reagents Co. Ltd.(Shanghai, China). All the chemicals used were analyticalreagent grade. Doubly distilled water was used to preparethe solutions.

2.2. Apparatus

A CHI 802 electrochemical analyzer (Shanghai ChenhuaInstrument Company, China) was used to perform electrodecharacterization and voltammetric measurements. A con-

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ventional three-electrode system, including a saturatedcalomel electrode (SCE) as reference electrode, a platinumplate as counter electrode and a bare or modified glassycarbon electrode (GCE, 3 mm diameter) as workingelectrode, was used in this work. All potentials in the textwere against SCE.

2.3. Preparation of the Modified Electrode

Typically, prior to electrochemical modification, the bareGCE was polished to a mirror-like surface with 0.50 mm and0.05 mm alumina slurries. After rinsing with water, the GCEwas ultrasonicated in absolute ethanol and doubly distilledwater for 5 min, successively, in order to remove anyadsorbed substances on the electrode surface. Then theclean electrode was immersed into 0.1 mol/L KNO3 con-taining 0.4 g/L HAuCl4 and electrochemical deposition ofgold nanoparticle was conducted for 60 s at �0.2 V (vs.SCE). The electrode modified with nano-Au particles, i.e.,nano-Au/GCE, was taken out and rinsed with water. Finally,the nano-Au/GCE was activated by several successivevoltammetric cycles from 0 to 1.2 V with a scan rate was50 mv/s in acetate buffer solution (pH 4.61) until a steadyvoltammogram was obtained.

2.4. Analytical Procedure

The solution was degassed with N2 and kept under a N2

blanket. Then, the linear sweep of voltammogram ofdihydroxybenzene isomers was recorded. Finally, the vol-tammetric curves were treated using a semiderivativetechnique provided by the CHI-802 electrochemical ana-lyzer.

3. Results and Discussion

3.1. SEM Image of GCE and Nano-Au/GCE

Nano-Au/GCE can be fabricated by depositing Au onto thepolished GCE in 0.1 mol/L KNO3 containing 0.4 g/LHAuCl4. The size and morphology of deposited goldnanoparticles can be controlled to some extent throughchoosing deposition potential, solution concentration anddeposition time. It is found that when deposition time of 60 sand deposition potential of �0.2 V were used, gold nano-particles could form on the surface of GCE. Figure 1 showsthe typical morphology of bare GCE (a) and nano-Au/GCE(b) characterized by scanning electron microscopy (SEM).The surface of bare GCE is smooth. The gold nanoparticleson the nano-Au/GCE are spheroidal with well-distributedsize, and its average diameter is about 50 nm.

3.2. Electrochemical Characterization of Bare Au andNano-Au/GCE

Figure 2a shows the cyclic voltammograms of K3[Fe(CN)6]at bare Au electrode and nano-Au/GCE. It can be seen thatthere are a pair of redox peak of K3[Fe(CN)6] and the peakpotential difference, DEp between the anodic peak potential(Epa) and the cathodic peak potential (Epc) is 98 mV at thebare Au electrode, whereas at the nano-Au/GCE, the DEp is72 mV. The small DEp indicates that the electrochemicalreversibility of K3[Fe(CN)6] at the nano-Au/GCE is muchimproved and the peak current increased. The responsecurrent of K3[Fe(CN)6] at the nano-Au/GCE is 1.2-foldlarger than corresponding one at the bare Au electrode.

Figure 2b shows the cyclic voltammograms of HQ re-spective at bare Au and nano-Au/GCE. From the dash line,it can be seen that at the bare Au electrode, the oxidationand reduction of HQ result in broad peaks. Hence it showspoor reversible behavior with DEp, 371 mV. However, at thenano-Au/GCE, the reversibility of HQ is significantly

Fig. 1. SEM image of GCE (a) and nano-Au/GCE (b).

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improved together with the current signal increase. The DEp

is 42 mV and the peak current is 3.8-fold larger thancorresponding one at the bare Au electrode, so it can beestimated that the average rough factor of nano-Au/GCE is3.8 (calculated by the ratio of peak current at the nano-Au/GCE and the bare Au electrode). This suggests that the nanogold can accelerate the electronic transmission speedeffectively.

3.3. The Cyclic Voltammograms of Three Isomers ofDihydroxybenzene

The cyclic voltammograms of a mixed solution containingHQ, CC, and RC (1.25� 10�4 mol/L for each) at GCE, Auand nano-Au/GCE electrode are shown in Figure 3. At thebare GCE electrode, there are two broad oxidation peaksand an almost invisible reduction peak on the cyclevoltammogram. In the anodic branch of the voltammogram,the oxidation peaks of HQ and CC overlap to form a widepeak at about 413 mV. The oxidation peak of RC appears at788 mV. The similar phenomenon is observed from the Auelectrode. However, three well-defined oxidation peakswith the peak potentials of 212, 312, and 711 mV corre-sponding to the three compounds are displayed at the nano-Au/GCE. The fact that potential difference betweenoxidation peaks of HQ and CC is larger than 100 mV, aswell as the oxidation peak potential of RC is far away fromthat of both HQ and CC allows simultaneous determinationof the three dihydroxybenzene isomers. When the semi-derivative technology is used to treat the voltammogram,the resolution of the peaks can be enhanced (Fig. 3, inset). Inaddition, two well-separated reduction peaks correspondingto HQ and CC can be observed at the nano-Au/GCE. It isnoted that for the three compounds the oxidation peakpotentials at the nano-Au/GCE shift towards negativedirection compared with those at the Au electrode or bare

GCE and the peak currents at the nano-Au/GCE are higherthan those at the Au electrode or bare GCE. It can beconcluded that the nano-Au particles on the nano-Au/GCEhave electrocatalytic activity for redox behavior of the threecompounds.

3.4. Optimization of the Experimental Conditions

In order to optimize the response of HQ, CC, and RC atnano-Au/GCE, some supporting electrolytes, such as KCl,Na2HPO4-NaH2PO4, HAc-NaAc and borate buffers weretested. The results indicate that in 0.1 mol/L HAc-NaAcbuffer solution, the peak current of HQ, CC, and RC islarger than that in the others. With increase of pH value, thepeak current of HQ, CC, and RC increases and thendecreases after pH 4.61. At the pH value exceeds 7.92, theoxidation peak current decreases rapidly.

Figure 4 shows the relationship between the pH value andthe oxidation peak potentials of HQ, CC, and RC. The peakpotentials of the three dihydroxybenzene isomers shifttowards more negative values with the increase of pH valueand the peak potential, Epa, vs. pH has a good linear relationin the range of pH 3.74 – 6.33. The three lines are almostparallel, which means that the peak potential differencebetween HQ, CC, and RC is constant. Meanwhile, theresults indicate that the uptake of electrons is accompaniedby an equal number of protons for each of them.

The influence of scan rate on oxidation of the threedihydroxybenzene isomers at the nano-Au/GCE was inves-tigated by the cyclic voltammetry. The oxidation peakpotentials shift to more positive value with the increase ofscan rate. In addition, the peak currents exhibit a linearrelation to the square root of the scan rate in the range of20 – 200 mV/s. These suggest that the oxidation of threedihydroxybenzene isomers at the nano-Au/GCE is a dif-fusion-controlled process.

Fig. 2. Cyclic voltammograms of a) K3[Fe(CN)6] and b) 1.0� 10�4 mol/L HQ respective at bare Au electrode (radius of 1.5 mm) andnano-Au/GCE (basement GCE electrode with radius of 1.5 mm); scan rate was 50 mV/s.

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3.5. Simultaneous Determination of Three Isomers ofDihydroxybenzene

Figure 5 represents the semiderivative voltammetric re-cordings of the background-subtracted anodic linear sweep-ing voltammograms of the solutions containing HQ, CC, andRC. It can be found that the peak height of each componentis proportional to its concentration in the presence of theothers. The linear relationships between the peak height andthe concentration are obtained in the range of 2.5� 10�6 –8.5� 10�4 mol/L for HQ, 1.0� 10�6 – 6.5� 10�4 mol/L forCC and 3.0� 10�6 – 4.0� 10�4 mol/L for RC, respectively.The linear regression equation and correlation coefficientsare as follow:

HQ: e (mA s�1/2)¼ 0.2492þ 0.3981 C (10�5 mol/L), corre-lation coefficient, 0.9996.

CC: e (mA s�1/2)¼ 0.2322þ 0.4010 C (10�5 mol/L), correla-tion coefficient, 0.9989.

RC: e (mA s�1/2)¼ 0.0250þ 0.3928 C (10�5 mol/L), correla-tion coefficient, 0.9977.

The limit of detection (LOD) is 5.0� 10�7 mol/L for HQ,6.5� 10�7 mol/L for CC and 9.0� 10�7 mol/L for RC (at asignal-to-noise ratio of 3).

The possible interferences of some inorganic salts andother organic compounds were also tested with 1.0�10�4 mol/L of HQ, CC, and RC for each. The results indicatethat a thousand-fold excess of Kþ, Naþ, Mg2þ, NO3

�,HPO4

2�, SO42�, Ac�, Cl� or H2PO4

�, and a hundred-foldexcess organic compounds such as ethanol, methanol andacetone have no influence on the signals of the dihydrox-ybenzene with deviations below 5%.

To ascertain further the reproducibility of the nano-Au/GCE, four GCE were modified to obtain the nano-Au/GCEand their responses towards the HQ, CC, and RC weretested. The separation between the voltammetric signals ofHQ, CC, and RC and the sensitivities remained the same atall four nano-Au/GCE electrodes, confirming that theresults are reproducible. When one nano-Au/GCE contin-uously measured fifty times for 8.0� 10�5 mol/L HQ, itsrelative standard deviation (RSD) is 0.57%, suggesting thatthe nano-Au/GCE can be used continuously without beingpolluted. In addition, nano-Au/GCE also shows highstability and there is no obvious change in current responseafter the nano-Au/GCE stored in 0.1 M acetate buffer atroom temperature for 10 days. But, the electrode should be

Fig. 3. Cyclic voltammograms and semiderivative voltammogram (insert) of HQ, CC, and RC (1.25� 10�4 mol/L for each). 0.1 mol/LHAc-NaAc buffer solution (pH 4.61); scan rate was 50 mV/s. The radius of Au electrode and GCE were all 1.5 mm, and the radius ofbasement GCE for nano-Au/GCE was also 1.5 mm.

Fig. 4. Relationship between pH value and oxidation peakpotentials of HQ and CC. Experimental conditions were sameas in Figure 3, except for the pH.

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activated by successive scanning from 0 to 1.2 V in acetatebuffer solution for about 10 cycles before each determina-tion.

3.6. Application to Artificial Wastewater Samples

Synthetic samples consisting of 2.0� 10�5 mol/L HQ, 2.0�10�5 mol/L CC, and 2.0� 10�5 mol/L RC in local tap waterwere used to verify the possibility of the method. The

quantitative determination was performed by standard-addition method. The results are listed in Table 1. The RSDof this method is 2.2 for HQ, 2.7 for CC, and 2.3 for RC,respectively. To examine the recovery, a known amount ofdihydroxybenzene was added to the sample. The recoveriesare summarized in Table 2.

4. Conclusions

It has been demonstrated that the nano-Au/GCE describedherein offers following advantages:

1) The fabrication approach is simple and the success rate ishigh.

2) When the nano-Au/GCEs are used, the oxidation peaksof hydroquinone and catechol can be separated.

3) The nano-Au/GCE can be used to simultaneouslydetermine three dihydroxybenzene isomers without anypretreatment.

Fig. 5. The semiderivative voltammograms of three dihydroxybenzene isomers. a) CC 5.5� 10�5 mol/L, RC 5.5� 10�5 mol/L, HQ (1 –5) at 1.5, 2.5, 3.5, 4.5, 5.5� 10�5 mol/L; b) HQ 5.5� 10�5 mol/L, RC 5.5� 10�5 mol/L, CC (1 – 5) at 1.5, 2.5, 3.5, 4.5, 5.5� 10�5 mol/L, andc) HQ 5.5� 10�5 mol/L, CC 5.5� 10�5 mol/L, RC (1 – 5) at 1.5, 2.5, 3.5, 4.5, 5.5� 10�5 mol/L. Experimental conditions were same as inFigure 3, except for concentration of dihydroxybenzene isomers.

Table 1. Determination of dihydroxybenzene isomers in syntheticsamples.

Sample 1 Sample 2

HQ CC RC HQ CC RC

Content (10�6 mol/L) 20 20 20 40 40 40Determined (10�6 mol/L) 20.4 19.6 21.4 40.1 40.5 39.7RSD (%) 1.6 3.0 2.1 2.7 2.3 2.5

[a] Average values of four measurements

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4) By studying, we have got some useful message that it ishopeful to develop a kind of microsensor, owing to themicrostructure of nano-Au, applying in on-line detectingof industrial production and real-time monitoring ofenvironment.

5. Acknowledgement

This project was supported by the National Basic ResearchProgram of China (973 Program) (No. 2007CB936602).

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Table 2. The recoveries of dihydroxybenzene isomers in synthetic samples.

Sample 1 Sample 2

HQ CC RC HQ CC RC

Added (10�6 mol/L) 20 20 20 40 40 40Found [a] (10�6 mol/L) 19.6� 0.4 19.8� 0.6 20.3� 0.5 39.8� 0.8 39.5� 0.6 40.5� 0.5Recovery (%) 96 – 100 96 – 102 99 – 104 97 – 102 97 – 100 100 – 103

[a] Average values of three measurements

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