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Feature Article

Stripping Voltammetric Analysis of Heavy Metals at NitrogenDoped Diamond-Like Carbon Film Electrodes

A. Zeng,a E. Liu,*a S. N. Tan,b S. Zhang,a J. Gaoa

a School of Mechanical & Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798;e-mail: mejliu@ntu.edu.sg

b Academic Group of Natural Sciences, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk,Singapore 637616

Received: October 1, 2001Final version: January 14, 2002

Abstract

Conductive nitrogen doped diamond-like carbon (N-DLC) film electrodes were used to investigate the possibility ofdetecting heavy metals such as lead, copper and cadmium by differential pulse anodic stripping voltammetry(DPASV) in the absence of mercury film. The preconcentration conditions (deposition potential, deposition time) andsolution pH were optimized for the determination of lead in aqueous solution. A linear dependence of lead strippingcurrent peak within the concentration (5 107 to 2106 M Pb2) and deposition time (30 to 300 s at 1.00 V vs.

SCE) was obtained. A multi-elemental analysis (Pb2, Cd2 and Cu2) illustrated that the N-DLC film electrodeprovided a significant stripping response for determination of multi-metals simultaneously. The present novelelectrode showed great promise for the analysis of heavy metals.

Keywords: Conductive nitrogen doped diamond-like carbon (N-DLC) film, Heavy metals, Stripping voltammetry

1. Introduction

Anodic stripping voltammetry (ASV) has been widely usedfor detection of heavy metals in various samples because ofits remarkably low (nanogram per liter) detection limits [1 2]. Other advantageous features of stripping voltammetry

include the capability of simultaneous multi-elementsdetermination, and relatively inexpensive instrumentationas compared with spectroscopic techniques used for tracemetal analysis. In addition, its low operating power makesthem attractive as portable and compact instruments for on-site monitoring of trace metals.

In the past, mercury-coated carbon-based electrodes suchas graphite and glassy carbon (GC) have been used for ASVextensively [3 4]. Mercury film electrodes are consideredto be the choice for heavy metal stripping analysis, butdisposal problems and cost are the major drawbacks.Moreover, mercury toxicity has been a great concern ofenvironmentalists and has motivated the sensor technolo-gists to develop mercury-free electrodes.

In recent years, bare boron-doped chemical vapordeposited (CVD) conductive diamond film electrodeshave been proposed as suitable electrodes for the strippinganalysis of heavy metals [5 6]. They have favorablecharacteristics, such as excellent chemical inertness, a lowbackground current [7 8], and a large potential rangebetween the onset of oxygen and hydrogen evolution [7,9 12]. There is also a report about metal strippinganalysis using bare carbon films obtained from polymerprecursors through pyrolyzing process [13] and screen-printing [14].

However, the high film procession temperature (800 100 8C of substrate temperature for CVD diamond filmdeposition [15], and 1100 8C for pyrolyzing process [13])limited their application of diamond film and pyrolyzedcarbon films. For the high hardness as diamond makes themechanical durability of diamond-like carbon (DLC) films

much harder than that of screen-printed carbon films. DLCfilms have the similar chemical characteristics to that ofdiamond, such as excellent chemical inertness, high foulingresistance and large potential window [16]. DLC can also bedoped into electrically conductive films and be deposited inroom temperature. DLCs amorphous structure makes theDLC film surface much more smoother than that ofpolycrystalline diamond film, resulting in considerabledecrease in residual current due to the lower capacitance[17]. As such, DLC thin films could be used for electrodematerials in stripping analysis. As demonstrated in thisarticle, differential pulse ASV (DPASV) will be used todetect heavy metals with nitrogen doped DLC film electro-des.

2. Experimental

Nitrogen doped diamond-like carbon (N-DLC) films weredeposited on conductive silicon (111) substrate with DCmagnetron sputtering, and annealed in the chamberimmediately following deposition. The film structure wascharacterized with micro-Raman spectroscopy, and electricconductivity was determined with a four-probe electricresistance meter. The film deposition process and physical

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characterization were reported elsewhere [16]. Siliconwafers of 2 inches in diameter coated with N-DLC filmswere cut into 8 8 mm2 square pieces, and were examinedfor porosity using SEM at a magnification of 1500. Thepieces without detectable pores by SEM were chosen to beconfirmed nano-porous free with electrochemical impe-dance spectroscopy (EIS) in 0.1 M KCl (pH 1.0) solution,and the impedance variability was lower than 0.2% in theimmersion period of 2 h. A specially designed O-ring toolwas used to seal DLC film electrode [18], with an exposedarea of 4-mm diameter.

A three-electrode electrochemical cell was used for theelectrochemical measurements, where a platinum plate wasused as the counter electrode and a saturated calomelelectrode (SCE) as the reference. All the electrochemicalmeasurements were taken using an EG&G potentiostat(173A). Before the first electrochemical testing, the N-DLCelectrode was cleaned with acetone and distilled water.After each striping analysis experiment, the electrode washeld at 0.50 V (vs. SCE) for 2 min, to clean away the metal

deposited on the electrode, and then the electrode waswashed with distilled water. The supporting electrolyte was0.1 M KCl for the voltammetric measurements, and the pHvalue was adjusted with HCl and KOH solution. Allchemicals employed were analytical reagent grade. Withdifferential pulse voltammetry, the sweep rate used was50 mV/s, the pulse amplitude 100 mV, and the sampling time10 ms. All the voltammograms were compensated with theohmic drop of 3.4 W, which was measured using EIS. Toensure good hydrodynamic mass transport, a magneticstirring bar was used to stir electrolyte at the bottom of thetesting cell at 200 rpm.

3. Results and Discussion

3.1. Detection of Heavy Metals: Lead, Cadmium, Copper

For a 0.1 M KCl acid electrolyte (pH1.0), containing 0.5 mMCd2,0.5 mM Pb2,and3 mM Cu2, at depositionpotentialof1.0 V for 120 s, the stripping voltammogram is measuredas Figure 1, where sharp and well-defined stripping peakshave been obtained. Cadmium, which has a current peakpotential of0.63 V, is the first metal stripped on the N-DLC film electrode during the potential scan. This peak isfollowed by the peaks for lead at 0.46 V and copper at0.18 V, respectively. Evidently the current response of theelectrode is significant to differentiate all the tested metalsand demonstrates that the three ions can easily bedetermined simultaneously with good peak separation.This sensitivity is comparable with that of mercury-filmelectrode reported in literature [19].

The standard deviations of peak heights for ten strippingcycles of Cd2, Pb2,andCu2 were measured as 2.8%, 2.0%and 1.7%, respectively. It was reported that the standarddeviations of Pb2 variability for mercury coated diamondfilm was in the range from 0.5%to 5% [15], and for mercurycoated sputtered carbon film was about 4% [19]. The

variability of Pb2 on screen-printed graphite/ carbon blackwas 5.8% [14]. This demonstrates that the measurementrepeatability of N-DLC film electrode is at less comparablewith the one of mercury coated diamond film [15] andsputtered carbon film [19]electrodes, and better thanthat ofbare screen-printed graphite/carbon black electrodes [14].On another account, there is no evidence of pore formationin the sputtered DLC film electrode, whereas as reported[20], cylindrical pores wider than 15 mm were sometimesformed in mercury film electrode, which inevitably deter-iorated the measurement repeatability.

3.2. Optimization of Stripping Parameters

Figure 2 and Figure 3 illustrate the effect of the depositionpotential (Fig. 2) and deposition time (Fig. 3) on thestripping response on the N-DLC film electrode.

3.2.1. Deposition Potential

Figure 2 shows the effect of the deposition potential appliedto the N-DLC film electrode in the 0.5 mM Pb2 and 0.5 mMCd2 under acid condition. The stripping peak position oflead is at about 0.46 V, and that of cadmium is at about0.63 V. It is evident from the height of the lead andcadmium peaks that an increase in the maximum platingefficiency has been obtained when decreasing the deposi-tion potential from 0.90 V to 1.30 V. Further reductionof the deposition potential (to 1.40 V) resulted in drasticreduction in peak heights. This negative response is mostlikely caused by hydrogen evolution, which can be referredto the behavior of N-DLC films in acid medium [16].

Figure 2 shows that the Cd peak is significantly enhancedwhereas the Pb peak is enhanced slightly upon applying

Fig. 1. Stripping voltammograms obtained at N-DLC filmelectrode in a nondeaereted 0.1 M KCl (pH 1.0) with 5 107 MPb2 5 107 M Cd23106 M Cu2. The sweep rate was50 mV/s, the pulse amplitude was 100 mV, the sampling time was10 ms, and the preconcentration was 120 s at 1.00 V.

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more negative deposition potentials in the range of0.90 Vto1.30 V. A deposition potential of about1.30 V resultsin the greatestsensitivity toward Cd2 ionsin acidic solution.However at potentials more negative than 1.30 V, hydro-gen evolution causes disruption at the N-DLC film surfaceto some extent, and the stripping current peak is stillsignificant. This observation again confirms the high qualityof the N-DLC films. Further details of the behavior of theDLC films in acid medium could be referred to literature[16]. The Cd peak is continuously shifted to more negativepotential values upon applying more negative depositionpotentials, whereas the height of the Pb peak is affected in arelatively smaller scale. This may be due to the largerincrease in the concentration of Cd atoms at the N-DLCfilm, leading to a more negative reduction and oxidationpotential for reversible charge-transfer, which would besimilar to the behavior of Cd2 on mercury coated sputteredcarbon film electrode [19].

3.2.2. Deposition Time

Figure 3 shows the resultsobtained whenvaryingdeposition

time for DPSV analysis of a 0.5 mM Pb2

and 0.5 mM Cd2

solution. The sharpness of the stripping peaks indicates thatthe transport of metal ions in the solution remainsunrestricted. Increasing the deposition time results in theincreased signal intensities (peak current heights) for bothlead andcadmium, due to theincreased amount of metals atthe electrode surface. In addition, it is evident that adeposition time of over 30 s is sufficient to obtain a well-defined stripping current profile for lead at this solutionconcentration. Usually, an excessive deposition time wouldresult in interference due to formation of intermetalliccompounds and should therefore be avoided, unless theconcentration of the ions is extremely dilute. Fortunately,

such phenomenon has not been observed in this study.As replotted in Figure 4, a linear relationship with

deposition time in the range 30 to 300 s has been observed.Obviously, the amount of metals plated intothe N-DLC filmis directly proportional to deposition time. The correlationcoefficients for Cd and Pb are 0.9949 and 0.9962, respec-tively. The deposition time sensitivities (slope of the peakcurrent) of Pb is higher than that of Cd by a factor of about5.6, and the different sensitivity for the two ions needsfurther investigate.

3.3. Concentration Dependence

DPSV was carried out at submicromolar Pb2 concentra-tions at N-DLC electrodes in 0.1 M KCl (pH 1). Preconcen-tration was carried out by holding the potential at 1.00 V(vs. SCE) for 120 s. Figure 5 displays the plots for voltam-metric stripping analyses of Pb over the concentration rangefrom 5 107 M t o 2 106 M. A monotonic increase(Fig. 6) in the stripping peak current is observed withincreasing Pb2 concentration. For the concentration used,the linearity is 0.9984.

The detection limit of Pb2 for N-DLC film electrode (S/N 3) is 76 nM, which is comparable with the value on bare

Fig. 2. Effect of deposition potential on the potentiometricstripping response of the N-DLC film electrode. Concentration:5 107 M Pb2 5107 M Cd2 in 0.1 M KCl (pH 1.0). Depo-sition potential: a) 0.90 V, b) 1.00 V, c) 1.10 V, d) 1.20 V, e)1.30 V, and f) 1.40 V. Other conditions were as in Figure 1.

Fig. 3. Potentiometric stripping response as a function ofdeposition time. Concentration: 5107 M Pb25 107 MCd2 in 0.1 M KCl (pH 1.0). a) 30 s, b) 60 s, c) 90 s, d) 120 s, e)180 s, and f) 300 s. Deposition potential 1.00 V. Other conditionswere as in Figure 1.

Fig. 4. Dependence of stripping peak currents on depositiontime. The other conditions were as in Figure 3.

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diamond film electrode, as described as low ppb [6]5 nM corresponding to 500 nM. Lower detection concen-tration could be expected, when a deposition time longerthan 120 s, deposition potential more negative than1.00 V, and KCl concentration lower than 0.1 M areapplied. It is possible to expect lower detection limits on N-DLC electrodes coated with mercury film electrodes. Fordiamond film electrodes coated with mercury, the detectionlimit increased to 0.3 ppb corresponding to 1.4 nM [15].The sensitivity could also be improved with the use of, forexample, the rotating disk electrode or rotating ring-diskelectrode techniques, as has been done for mercury films onglassy carbon electrodes [21 22].

3.4. Effect of pH

The effect of solution pH is shown in Figure 7 for 0.50 mMPb2 with 0.1 M KCl as supporting electrolyte. A depositionpotential of 1.00 V was applied to the N-DLC filmelectrode for a period of 120 s. In the media more basic

than pH 3.0, the peak current for the lead stripping peaks is

found to diminish in magnitude. The onset of current occursat a more negative potential when the electrolyte becomesmore basic. Conversely, in more acid solutions, leadstripping peaks become increasingly sharper. Thesechanges are attributed to the lead hydroxide formation inthe more basic media. Each lead hydroxide species has adifferent reoxidation half-wave potential, causing differentstripping peak position. Similar phenomenon has beenreported at bare screen-printed graphite/carbon film elec-trode[14].WhenpHislowerthan3.0,influenceofpHonthestripping sensitivity is not as pronounced as for the morebasic solutions.

4. Conclusions

DPASV analysis utilizing the mercury-free N-DLC filmelectrodes for determination of heavy metals has beendemonstrated in this work. The preconcentration param-eters (deposition potential and deposition time) andsolution pH have been optimized for the determination oflead in aqueous solution. The deposition potential of0.90 V to 1.30 V is identified to be suitable in determi-nation of lead with the N-DLC film electrodes. A linearcurrent peaks height dependence within the concentration(5 107 to 2 106 M Pb2) and deposition time (30 to300 s at1.00 V vs. SCE) have been obtained. In more acidsolutions, the lead stripping peaks became increasinglysharper and more intense. When the pH is lower than 3.0,influence of the pH on the stripping sensitivity is not aspronounced as for the more basic solutions. The multi-elemental analysis (Pb2, Cd2 and Cu2) has demonstratedthat the mercury-free N-DLC film electrodes could providestripping response that can be used for the determination ofmulti-metals simultaneously.

The main focus of this study was to demonstrate thepossibility of applying bare N-DLC film electrodes inmercury-free detection of heavy metal with DPASV. The

Fig. 5. Dependence of voltammograms on concentration of Pb2

in 0.1 M KCl (pH 1.0). a) 5107 M, b) 8 107 M, c) 1.2 106

M, d) 1.5 106 M, and e) 2106 M. Deposition time was 120 s.Other conditions were as in Figure 1.

Fig. 6. Stripping peak current response as a function of concen-tration of lead nitrate. The other conditions were as in Figure 5.

Fig. 7. Stripping voltammograms of 5107 M Pb2 in 0.1 MKCl response as a function of pH. a) 1.0, b) 2.0, c) 3.0, d) 4.0and e) 5.0. Other conditions were as in Figure 1.

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development of mercury-free trace metal detection is achallenging and important area in environmental chemistry.The unique qualities of the N-DLC electrode (extremestability and low background current) are highly favorablefor the type of ASVanalysis. This technique shows promisefor applications such as online monitoring of Pb in drinkingwater. A more detailed investigation of this technique,including the measurement of Pb in drinking water andanalysis of various other metals, will be carried out in thefuture.

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