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Sensors and Actuators B 124 (2007) 1–5

An electrochemical sensor for Cd2+ based on the inducingadsorption ability of I−

Gang Li a,1, Chidan Wan b,1, Zhiming Ji a, Kangbing Wu a,∗a Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, PR China

b General Surgery, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, PR China

Received 4 October 2006; received in revised form 20 November 2006; accepted 20 November 2006Available online 18 December 2006

bstract

Herein, an electrochemical sensor for sensitive and simple determination of Cd2+ was fabricated based on the excellent properties of acetylenelack (AB) and strong inducing adsorption ability of I−. In pH 3.6 HAc–NaAc buffer containing 5.0 × 10−3 mol L−1 I−, Cd2+ was induced toccumulate at AB paste electrode surface under the inducing adsorption of I−, and then reduced to Cd at −1.10 V. During the following potential

weep from −1.10 to −0.40 V, reduced Cd was oxidized, resulting in a sensitive and well-shaped stripping peak at −0.83 V. Based on this, a newlectrochemical method was developed for Cd2+ analysis. The linear range is found to be 5.0 × 10−8 to 2.0 × 10−6 mol L−1, and the detection limits 2.0 × 10−8 mol L−1. Finally, this sensor was successfully used to detect Cd2+ in water samples.

2006 Elsevier B.V. All rights reserved.

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eywords: Cadmium; Electrochemical determination; Acetylene black; Induci

. Introduction

Cadmium, a common industrial pollutant, is a toxic metalhat is a trace constituent of fossil fuels, electrodes in batter-es, sludges, zinc smelting and municipal wastes [1]. It easilyccumulates in the body via crops or water. In modern society,he water pollution caused by Cd2+ usually occurs and brings

any adverse effects. For instance, the pollution in Beijiangiver, Guangdong province of China, is just caused by Cd2+.esearches indicate that Cd2+ is hard to be expelled from bodynd produces many toxic effects such as damaging nephrid-um, causing sugar urine, bone loosen, bone atrophy and boneistortion. What is more, according to a recent study [2], envi-onmental exposure to cadmium increases the risk of cancer, sohe international agency on cancer research has recently classi-ed cadmium as a carcinogen. Therefore, developing sensor for

apid and sensitive determination of Cd2+ is of great importance.

Among various sensors, electrochemical sensor has attracteduch attention since it possesses high sensitivity, good selec-

∗ Corresponding author. Fax: +86 27 8754 3632.E-mail address: kbwu@mail.hust.edu.cn (K. Wu).

1 Authors contributed equally to this work.

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.11.033

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ivity, low cost, simplicity and easy data read-out. To date, areat number of electrochemical sensors have developed ford2+ utilizing all kinds of working electrodes [3–10]. However,

o the best of our knowledge, electrochemical determinationf trace levels of Cd2+ using acetylene black (AB) paste elec-rode and the inducing adsorption ability of I− has not beeneported.

Paste electrode (also called carbon paste electrode) inventedy Adams at the end of the 1950s, is a mixture of an elec-rically conducting carbon powder and a pasting liquid. Now,aste electrode attracts increasing attention and widely used inlectroanalysis or electrochemistry due to the following advan-ages: easy preparation, porous surface, wide potential range,ow residual current, low cost and convenient surface renewal.cetylene black, a special type of carbon black, is made by the

ontrolled combustion of acetylene in air under pressure. It isell-known that AB possesses many extraordinary properties

uch as large specific surface area, excellent electric conductiv-ty and strong adsorptive ability. Otherwise, previous reports11–13] have proven that I− can induce some metal ions to

dsorb at mercury electrode surface, and proper mechanism haseen given.

Therefore, the main objective of current work is to developn electrochemical sensor for sensitive and rapid determination

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pgH−sweep from −1.10 to −0.40 V (Fig. 2b). However, when adding5.0 × 10−3 mol L−1 KI into 0.1 mol L−1 HAc–NaAc (pH 3.6)buffer, the stripping peak current increases significantly, and thepeak potential shifts negatively to −0.83 V (Fig. 2d). From the

G. Li et al. / Sensors and

f Cd2+ utilizing the excellent properties of AB and inducingdsorption ability of I−. To achieve this goal, AB paste electrodeas fabricated as working electrode and low concentration of KIas added into determining medium to induce Cd2+ accumulate

t AB paste electrode surface. Thus, the accumulation efficiencynd surface amount of Cd2+ were remarkably improved in theresence of low concentration of I−. As a result, both the anodictripping peak current of Cd2+ and the sensitivity of this sensorre significantly improved. In brief, this new sensing and deter-ining system possesses following advantages: high sensitivity,

apid response, extreme simplicity, free of mercury and low cost.

. Experiment

.1. Reagents

Stock solution of 1.00 × 10−2 mol L−1 Cd2+ was prepared byissolving Cd(NO3)2 (Sinopharm Group Chemical Reagent Co.td., China) into redistilled water, and then diluted to workingolution at desired concentration with bidistilled water. Otherhemicals used are analytical reagents and water used is bidis-illed.

Acetylene black (AB, purity > 99.99%) was purchased fromTREM Chemicals (USA). Paraffin oil was purchased frominopharm Group Chemical Reagent Co. Ltd.

.2. Apparatus

All the electrochemical measurements were carried out usingVersaStatTM II Potentiostat/Galvanostat (Princeton Appliedesearch, USA), which was controlled by a PC using the Pow-rsuit Software.

A conventional three-electrode system, consisting of an ABaste working electrode, a saturated calomel reference electrodeSCE) and a platinum wire auxiliary electrode, was employed.he body of working electrode was a polytetrafluoroethylene

PTFE) cylinder (Fig. 1) that was tightly packed with AB paste.copper wire inserts into the AB paste providing electrical

ontact.Atomic absorption spectrometric measurements were con-

ucted with AA 6300 atomic absorption spectrophotometerSHIMADZU, Japan).

.3. Preparation of AB paste electrode

The AB paste electrode was prepared by mixing 100.0 mgB, 100.0 �L paraffin oil and several drops of ethanol in a smallortar to form a homogeneous AB paste, and then dried under

n infrared lamp. After that, the paste was pressed into the end

Fig. 1. The diagram of an AB paste electrode.

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ators B 124 (2007) 1–5

avity (3 mm in diameter, 1 mm in depth) of working electrodeody, and the electrode surface was smoothed against a weighingaper. It is important to note that the amount of paraffin oil muste carefully controlled because excessive paraffin oil will lowerhe conductivity, while insufficient paraffin oil is not beneficialo obtain uniform AB paste.

.4. Analytical procedure

Unless otherwise stated, 0.1 mol L−1 HAc–NaAc (pH 3.6)ontaining 5.0 × 10−3 mol L−1 KI was used as determiningedium for Cd2+ analysis. The analytical procedure includes

ccumulation step and stripping step. Firstly, Cd2+ was accu-ulated at AB paste electrode surface and then reduced to Cd

nder −1.10 V for a desired time (2 min in this work) whiletirring solution. Finally, reduced Cd was oxidized to Cd2+ dur-ng the anodic potential sweep from −1.10 to −0.40 V after aest period of 15 s, resulting in a stripping peak at −0.83 V. Thetripping peak current is measured as analytical signal for Cd2+.

After each measurement, the used AB paste was carefullyemoved from the end cavity and a new AB paste electrode wase-fabricated as above procedure. That is to say, each AB pastelectrode was used one time to achieve better reproducibility.

. Results and discussion

.1. Electrochemical behavior of Cd2+

The anodic stripping voltammetric behaviors of Cd2+ at ABaste electrode in the absence and presence of I− were investi-ated. After 2 min accumulation under −1.10 V in 0.1 mol L−1

Ac–NaAc (pH 3.6) buffer, an anodic stripping peak appears at0.74 V for 2.0 × 10−7 mol L−1 Cd2+ during the square wave

ig. 2. Square wave anodic stripping voltammograms of Cd2+ in 0.1 mol L−1

Ac–NaAc (pH 3.6) at AB paste electrode. (a) Blank voltammograms; (b).0 × 10−7 mol L−1 Cd2+; (c and d) 1.0 × 10−7 and 2.0 × 10−7 mol L−1 Cd2+

n presence of 5.0 × 10−3 mol L−1 KI. Accumulation potential = −1.10 V, accu-ulation time = 2 min, pulse amplitude = 25 mV, frequency = 25 Hz and potential

ncrement = 4 mV. Arrow means potential sweep direction.

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Fig. 5 shows the influence of accumulation time on the anodicstripping peak current of 2.0 × 10−7 mol L−1 Cd2+. As seen, thestripping peak current gradually increases as increasing accu-

G. Li et al. / Sensors and

omparison of curves (b and d), it is very clear that low concen-ration of I− can remarkably enhance the stripping peak currentf Cd2+. According to previous reports [11–13], I− exhibitsbvious inducing adsorption ability towards some metal ions,nd subsequently induces them to adsorb at mercury electrodeurface. Moreover, the proper mechanism has been proposed toccount for this. Due to O’Dom and Murray [12], it is assumedhat one particular complex species is formed in the solution,nd then this complex species adsorbs on mercury electrodeurface to produce an adsorbed species. In the current system,d2+ forms a special complex with I− and then this complex isasily and efficiently accumulated at AB paste electrode surfacender the strong inducing adsorption ability of I−. Therefore, theurface amount of Cd2+ increases greatly and undoubtedly thetripping peak current also increases remarkably in the presencef I−.

Additionally, the stripping peak of Cd2+ in the presencef 5.0 × 10−3 mol L−1 KI was examined as a function ofd2+ concentration. When the concentration of Cd2+ lowers to.0 × 10−7 mol L−1, the stripping peak current also decreases by0% (Fig. 2c); if the concentration of Cd2+ further decreases toero (without Cd2+), the anodic stripping peak current will van-sh (Fig. 2a). These phenomena reveal that the stripping peak at

0.83 V corresponds to Cd2+ and what is more, the peak currentas good linearity with the concentration of Cd2+.

.2. Choice of supporting electrolyte

Generally speaking, supporting electrolyte (also called deter-ining medium) determines the electrochemical response of

esearch target to some extents. Therefore, selecting suitableupporting electrolyte is very important in electrochemicaletermination. Here, the anodic stripping responses of Cd2+

n various supporting electrolytes, such as pH 3.6, 4.0, 4.6,.0 and 5.6 HAc–NaAc buffer, HCl, KCl, pH 5.5, 6.0, 6.5nd 7.0 phosphate buffer (each 0.1 mol L−1) containing differ-nt concentrations of KI from 0 to 1.0 × 10−2 mol L−1 wereetailedly examined. It is found that the stripping peak currents highest in pH 3.6, 0.1 mol L−1 HAc–NaAc buffer containing.0 × 10−3 mol L−1 KI. What is more, the stripping peak shapes well-defined, and the background current is relatively low.herefore, 0.1 mol L−1 HAc–NaAc (pH 3.6) buffer containing.0 × 10−3 mol L−1 KI is selected for Cd2+ analysis.

.3. Optimization of I− concentration

From Fig. 2, we know that the stripping peak current of Cd2+

emarkably enhances in the presence of low concentration of−. Therefore, it is necessary to investigate the effect of concen-ration of I− and to obtain the optimized concentration. To doo, the anodic stripping peak currents of 2.0 × 10−7 mol L−1

d2+ in the presence of different concentrations of I− wereompared in Fig. 3. It is found that the stripping peak cur-

ent increases linearly with I− concentration over the rangerom 0 to 4.0 × 10−3 mol L−1, then remains unchangeable overhe range from 4.0 × 10−3 to 7.0 × 10−3 mol L−1. Finally, theeak current begins to decline when I− concentration exceeds

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ig. 3. Influence of KI concentration on the stripping peak current (ip) of.0 × 10−7 mol L−1 Cd2+. Other conditions are the same as in Fig. 2d.

.0 × 10−3 mol L−1. Thus, the concentration of I− was chosens 5.0 × 10−3 mol L−1 in this work.

.4. Accumulation potential and time

As to anodic stripping voltammetry (ASV), accumulationotential and time must be considered since they partially deter-ine the sensitivity. Herein, the anodic stripping peak currents

f Cd2+ under different accumulation potentials were comparedn Fig. 4, suggesting that the anodic stripping peak current gradu-lly increases when accumulation potential shifting from −0.90o −1.30 V. At more negative potential, Cd2+ adsorbed at ABaste electrode is reduced more completely, resulting in thetripping peak current enhancement. However, when the accu-ulation potential is more negative than −1.10 V, the anodic

tripping peak current increases very slightly and meanwhile,he background current increases greatly. What is more, moreegative accumulation potential will make other metal ions to beeduced, introducing interference for the determination of Cd2+.herefore, the accumulation potential was fixed at −1.10 V in

ig. 4. Effect of accumulation potential on ip of 2.0 × 10−7 mol L−1 Cd2+. Otheronditions are the same as in Fig. 2d.

4 G. Li et al. / Sensors and Actu

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ytstfpwas measured for Cd2+ (Fig. 6a). Additionally, known amountof Cd2+ standard solution was added into the sample solu-tion, and the same procedure was conducted (Fig. 6b). Fromthe comparison of these two peak currents, the concentration

ig. 5. Influence of accumulation time on ip of 2.0 × 10−7 mol L−1 Cd2+. Otheronditions are the same as in Fig. 2d.

ulation time. When extending accumulation time, more andore Cd2+ was induced to accumulate at AB paste electrode

urface. Therefore, the surface amount of Cd2+ increases andnally the stripping peak current shows obvious enhancement.n order to shorten analysis time and achieve higher sensitivity,he accumulation time is selected as 2 min.

.5. Analytical properties of this sensor

.5.1. ReproducibilityAfter each measurement, the AB paste was carefully removed

rom the end cavity and another new AB paste electrodeas remade as above-mentioned procedure. The reproducibilityetween multiple electrode preparations was estimated via com-aring the anodic stripping peak current of 2.0 × 10−7 mol L−1

d2+. The relative standard deviation (R.S.D.) is 5.6% for 10B paste electrodes, revealing that this method possesses good

eproducibility and great potentials.

.5.2. InterferenceTo evaluate the interferences of some foreign cations on

he determination of Cd2+, a systematic study was carried out.he stripping peak currents of 2.0 × 10−7 mol L−1 Cd2+ aftermin accumulation in the absence and presence of various con-entrations of foreign cations were measured. Based on this,he peak current change can be achieved. Herein, when the

eak current change exceeds 10%, it is considered that thisation causes obvious interference, and the corresponding cationoncentration was defined as tolerance level. The results areisted in Table 1, suggesting that 10,000-fold concentration

able 1nterferences of foreign cations on the stripping peak current of.0 × 10−7 mol L−1 Cd2+

oreign cations Tolerance level (mol L−1)a

a2+, Mg2+, Fe3+, Al3+, Ni2+, Zn2+ and Mn2+ 2.0 × 10−3

b2+, As3+, Cr3+ and Sn4+ 1.0 × 10−4

g2+ 2.0 × 10−5

u2+ 2.0 × 10−6

a For 10% error.

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ators B 124 (2007) 1–5

f Ca2+, Mg2+, Fe3+, Al3+, Ni2+, Zn2+ and Mn2+; 500-foldoncentration of Pb2+, As3+, Cr3+ and Sn4+; 100-fold concen-ration of Hg2+, 10-fold concentration of Cu2+, almost have nonfluence.

.5.3. Linear rangeThe relationship between the anodic stripping peak current

nd the concentration of Cd2+ was studied under the optimizedonditions. The stripping peak current (ip) is proportional tohe concentration of Cd2+ over the range from 5.0 × 10−8 to.0 × 10−6 mol L−1 (r = 0.995).

.5.4. Limit of detectionThe limit of detection was estimated utilizing the method

f gradually decreasing concentration of Cd2+. When theoncentration of Cd2+ decreases to 2.0 × 10−8 mol L−1, anbservable stripping peak was observed after 2 min accumu-ation. When further decreasing the concentration, the strippingeak almost disappears. So, the limit of detection is evaluatedo be 2.0 × 10−8 mol L−1 for 2 min accumulation. Additionally,he limit of detection will lower as increasing accumulationime. If the accumulation time extends to 4 min and 8 min,he limit of detection are 5.0 × 10−9 and 1.0 × 10−9 mol L−1,espectively.

.6. Determination of Cd2+ in water samples

In order to verify its application in practical sample anal-sis, this new sensing and determining system was employedo detect Cd2+ in some water samples. Five milliliters of waterample was added into 5.00 mL pH 3.6 HAc–NaAc buffer con-aining 0.01 mol L−1 I−, and then accumulated under −1.10 Vor 2 min with stirring, after that, the square wave anodic strip-ing voltammogram was recorded and the stripping peak current

ig. 6. Illustration for the determination of Cd2+ in water samples. Curve (a):quare wave anodic stripping voltammograms of Cd2+ in water samples, curveb): curve (a) + 10.0 �L of 1.00 × 10−4 mol L−1 Cd2+ standard solution. Otheronditions are the same as in Fig. 2d.

G. Li et al. / Sensors and Actu

Table 2Determination of Cd2+ in water samples

Samples Detected byAAS (mol L−1)

Detected bythis method(mol L−1)

R.S.D. (%) Recovery (%)

A 1.24 × 10−7 1.39 × 10−7 5.4 98.2B 2.48 × 10−7 2.27 × 10−7 5.3 99.2C −7 −7

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f Cd2+ in water samples can be easily obtained. The resultsre listed in Table 2. Otherwise, the R.S.D. of each sampleor three times parallel detections was evaluated, and the values below 6%, revealing good reproducibility. Finally, in ordero testify the accuracy and feasibility of this method, atomicbsorption spectrometry (AAS) was also used to detect Cd2+.t is very clear that the results obtained by this method are inood agreement with those obtained by AAS, indicating thathis method has good accuracy. From these data, conclusion cane made that this sensor has great potential for practical samplenalysis.

. Conclusions

In this work, a simple and valuable electrochemical sensoras developed for the sensitive determination of Cd2+ based on

he combination of excellent properties of AB with strong induc-ng adsorption ability of I−. In the presence of low concentrationf I−, Cd2+ in bulk solution is effectively induced to accumulatet AB paste electrode surface. As a result, the anodic strippingeak current and determining sensitivity significantly increased.

cknowledgement

The authors are grateful to the financial support from theational Natural Science Foundation of China (No. 20507006).

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