Differential pulse anodic stripping voltammetric determination of Pb ion

Differential pulse anodic stripping voltammetric determination of Pb ion at a montmorillonites/polyaniline nanocomposite modi?ed glassy carbon

electrode

YongPing Dong a ,?,Yong Ding a ,Ying Zhou a ,Jun Chen a ,?,ChengMing Wang b

a School of Chemistry and Chemical Engineering,Anhui University of Technology,Maanshan 243002,China

b

Hefei National Laboratory for Physical Science at the Microscale,University of Science and Technology of China,Hefei 230026,China

a r t i c l e i n f o Article history:

Received 13November 2013

Received in revised form 31December 2013Accepted 10January 2014

Available online 5February 2014Keywords:

Montmorillonites Polyaniline

Modi?ed electrode

Differential pulse anodic stripping voltammetry Pb 2+

a b s t r a c t

In this paper,Na +–montmorillonites/polyaniline nanocomposite modi?ed glassy carbon electrode (PANI/MMT/GCE)was prepared and characterized by scanning electron microscopy and electrochemical tech-niques.It was found that when PANI/MMT ?lm was modi?ed on the electrode,electrochemical responses could be improved signi?cantly.The modi?ed electrode could be applied in sensitive detection of Pb 2+by differential pulse anodic stripping voltammetry (DPASV).The 50wt%feeding ratio of MMT to aniline of the nanocomposite exhibited the best electrochemical response to Pb 2+.In 0.2mol L à1acetate buffer solution (pH 3.5),the stripping peak currents after enrichment at à1.20V for 120s showed good linear relationships with Pb 2+at concentration ranges of 4–100nM with the correlation coef?cient of 0.998,and the detection limit is 1nM.The proposed method is applicable in the determination of trace Pb 2+in real water samples with the relative standard deviations less than 5.2%and the recovery rates of 94–103%.

ó2014Published by Elsevier B.V.

1.Introduction

Lead is one of the most serious environmental contaminants which are highly toxic to nervous,immune,reproductive and gas-trointestinal systems of both humans and animals [1–3].Addition-ally,the accumulation of Pb 2+in the human body exhibits severe deleterious effects on neurobehavioral development in children,increases blood pressure,and causes kidney injury and anemia.Therefore,the development of a highly sensitive method for the determination of trace amounts of lead has received a considerable attention.Over the past decades,many techniques such as spectro-photometry,inductively coupled plasma optical emission spec-trometry,atomic absorption spectrometry,atomic ?uorescence spectrometry,and neutron activation analysis have been employed for the determination of heavy metals [4–9].However,most of the above methods require several time consuming manipulation steps,sophisticated instruments and special training.Electrochem-ical stripping voltammetry has been widely recognized as a power-ful technique for the determination of trace metal ions due to its unique ability to preconcentrate target metal ions during the accumulation step [10].The mercury electrodes including hanging drop mercury electrode,dropping mercury electrode and thin mer-cury ?lm electrode have been widely used as the working electrode in stripping voltammetry.However,the toxicity of mercury re-stricts its use.Ordinary electrochemical solid electrodes,such as glassy carbon,platinum,gold,and graphite electrodes are also not suitable for the determination of metal ions because of their poor sensitivity [11].Thus,it is very important to develop a sensi-tive,selective,and non-toxic electrode for the determination of heavy metals by stripping methods.Chemically modi?ed electrode (CME)might be the most suitable substitute for the traditional electrodes in the stripping voltammetry.Various CMEs have been exploited for anodic stripping voltammetric determination of trace metals,such as functionalized mesoporous silica electrode,carbon nanotubes modi?ed electrode,ordered mesoporous carbon,nano-crystalline diamond thin-?lm electrode,graphene modi?ed elec-trode,and graphite nano?ber–Na?on modi?ed bismuth ?lm electrode [12–19].Though CMEs have been widely used in the detection of metal ions,new materials still needed to improve the detection sensitivity and reproducibility in analysis of trace metals.

Recently,organic–inorganic nanocomposites have received con-siderable attention and many hybrid composite materials have been synthesized and studied [20].Conducting polymers and clay

https://www.360docs.net/doc/7814169751.html,/10.1016/j.jelechem.2014.01.0141572-6657/ó2014Published by Elsevier B.V.

?Corresponding authors.Tel.:+865552311807;fax:+865552311822.

E-mail addresses:

dongyp@https://www.360docs.net/doc/7814169751.html,

(Y.

Dong),

junchen@https://www.360docs.net/doc/7814169751.html,

(J.Chen).

are two frequently-used organic and inorganic materials which are widely used in the electrode modi?cation.Among the conducting polymers,polyaniline(PANI)is most famous and often used in the electrode modi?cation due to its simple synthesis,high con-ductivity,and excellent environment stability[21].When a PANI coating modi?ed glassy carbon electrode is immersed in a solution containing trace heavy metal ions,interactions occur between electron-rich sites and positively charged metal species and thus the metal ions are adsorbed onto the electrode surface and further separated from the bulk solution[22,23].However,the poor ther-mal stability and dif?cult processability of PANI should be over-come for the application of this electrically conducting polymer. Many reports have focused on the preparation and new property studies of novel nanocomposites consisting of PANI with various layered materials[24–29].Montmorillonite(MMT)is a layered sil-icate known for good swelling and ion-exchange properties[30]. MMT is also very popular inorganic matrix because of its layered nanostructure,natural origin,and availability at low cost.MMT has already been employed in electrochemical analysis because of its low cost and remarkable improvement in sensitivity,which attained attention for voltammetric determination of organic com-pounds and traces of heavy metal ions[31–36].In the last decade, the growth of the interest in PANI composites with montmorillon-ite has been observed.The composites of PANI and MMT were synthesized by mechanochemical reaction[37],in emulsion poly-merization[38],in situ deposition of PANI on MMT[39–41].The composite of MMT and conducting polymer has been applied in gas sensor and corrosion protection[42,43].However,to the best of our knowledge,PANI/MMT nanocomposite has not been re-ported in the detection of trace heavy metals.Therefore,in the present paper,PANI/MMT nanocomposite was synthesized and used to modify glassy carbon electrode(GCE).The modi?ed elec-trode was used to detect Pb2+in water samples by differential pulse anodic stripping voltammetry.

2.Experimental

2.1.Apparatus and reagents

All electrochemical experiments were performed with a model 760D CHI electrochemical workstation(Chenhua Instruments Co., Shanghai,China).A conventional three-electrode cell con?guration was employed for the voltammetric measurements.A PANI/MMT/ GCE(3-mm diameter disk)was used as the working electrode,with a saturated calomel reference electrode(SCE)and a platinum wire electrode for the reference and the counter electrode,respectively. Scanning electron microscopy(SEM)images were obtained on a Hitachi S-4800scanning electron microscope.

All chemicals employed in this work were of analytical-reagent grade and used without further puri?cation and solutions were prepared with doubly distilled water.Stock solutions of0.1mol Là1 Pb2+was prepared by dissolving Pb(NO3)2(Shanghai Reagent Cor-poration,China)into redistilled water,and then diluted to working solution at desired concentration with redistilled water.Aniline (99%),ammonium peroxydisulfate(APS),glycidyl methacrylate (GMA),diethanol amine(DEA)and2-Acrylamido-2-Methyl Pro-pane Sulfonic Acid(AMPS)were purchased from Sinopharm Chem-ical Reagent Co.,Ltd.Sodium montmorillonite(Na+-MMT)was obtained from Kunipia-F(Kunimine Co.).

2.2.Synthesis of PANI–MMT composites

The organic modi?cation of Na+–MMT clay with GMA–DEA is carried out according to the Ref.[44].The typical process is as follows:15g of Na+–MMT clay were fully dispersed in450mL redistilled water by stirring overnight at room temperature.An ex-cess of GMA–DEA(6.45g)was dissolved in50mL of redistilled water,then continuously and drop-wisely added to the clay disper-sion.The pH value of the mixture was adjusted to3by using hydrochloric acid.The mixture was stirred for6h.Afterward,the modi?ed MMT clay(hereinafter referred to as GMA–DEA–MMT) were?ltrated and washed several times to remove excess GMA–DEA and hydrochloric acid.Then,put150mL of2wt% GMA–DEA–MMT dispersion into a250mL round-bottom?ask equipped with a mechanical stirrer,a condenser,and nitrogen in-let.3.0g AMPS was then added and stirred under a nitrogen atmo-sphere by purging for30min.The initiator,0.03g APS(dissolved in 5mL of redistilled water),was?nally introduced in the reactor and the polymerization allowed to proceed at70°C for4h under a slow stream of nitrogen.The resulted dispersion was centrifuged and washed repeatedly with redistilled water to remove the un-grafted poly(AMPS).The obtained modi?ed MMT clay was dried at50°C in an oven.

The PANI/MMT is prepared by the chemical oxidation method as follows[44]:the certain amount of the PAMPS-g-MMT was well-dispersed in80mL of0.5mol Là1hydrochloric acid.Then, 0.8g aniline monomer was added and stirred for30min.The poly-merization started by introduction of APS solution(1.96g APS in 20mL of0.5mol Là1hydrochloric acid)drop-wisely.An overnight reaction was allowed to ensure completion of polymerization.The resultant precipitate was?ltered and sequentially washed with copious amounts of0.1mol Là1hydrochloric acid and industrial alcohol until the?ltrate was clear.Finally,the product was dried at50°C in an oven.

According to the aforementioned approach,numerous samples of the PANI/MMT nanocomposites with different MMT/aniline feeding ratio are obtained.The feeding ratio of MMT/aniline is de-?ned using following equation:

MMT=Aniline feeding ration?

W MMT

W Aniline

?100wt%

2.3.Preparation of the modi?ed GCE

Before modi?cation,the bare glassy carbon electrode was care-fully polished to a mirror with1.0l m,0.30l m and0.05l m alu-mina slurry on a polishing cloth,respectively.Then,rinsed thoroughly with doubly distilled water between each polishing step and washed successively with doubly distilled water,1:1(v/ v)HNO3aqueous solution and doubly distilled water in an ultra-sonic bath and dried in air,and used for the following experiments. The modi?er suspension was prepared by dispersing25mg of PANI/MMT in5.0mL of water under sonication for30min,giving a quite stable black suspension.The PANI/MMT modi?ed GCE(de-noted as PANI/MMT/GCE was prepared by casting10l L of the sus-pension on GCE surface using a micropipette and letting water evaporate in air.

2.4.Electrochemical detection of Pb(II)

DPASV was used for the observation of electrochemical behav-ior under optimized conditions.Pb were deposited at the potential ofà1.2V for120s by the reduction of Pb(II)in0.1M NaAc-HAc (pH 3.5).The Solid-phase extraction(SPE)procedures were performed by stirring the solution with a magnetic stirrer,which is included in the electrodeposition step.Following the preconcen-tration step,the stirring was stopped.After10s,DPASV was per-formed by potential scan fromà1.0V toà0.1V with amplitude of50mV,pulse width of50ms and potential step of4mV vs SCE.Aliquots of the Pb(NO3)2standard solution were introduced after the background signal was recorded.Prior to the next cycle,

Y.Dong et al./Journal of Electroanalytical Chemistry717-718(2014)206–212207

a 120s conditioning step at +0.3V (with solution stirring)was used to remove the previous residual P

b from the electrode sur-face.All the experiments were carried out at room temperature.3.Results and discussion

3.1.Characterization of PANI/MMT/GCE

The SEM image of PANI/MMT composite modi?ed glassy car-bon electrode (PANI/MMT/GCE)is depicted in Fig.1.The typical SEM image reveals that the obtained PANI/MMT is highly uni-form ?ake-like structure.The surface of ?ake-like PANI/MMT is rough,suggested that nanoscale PANI was formed on the MMT substrate.The homogeneous distribution of PANI particles on the MMT bring high roughness of nanocomposite and will increase the absorption ability of PANI ?lm in the detection of metal ions [44].

The PANI/MMT/GCE is also characterized by electrochemical methods.Fig.2shows the cyclic voltammetric response of a bare GCE,a MMT modi?ed GCE (MMT/GCE),a PANI modi?ed GCE (PANI/GCE)and a PANI/MMT/GCE in 0.1mol L à1phosphate buffer solution (PBS,pH 7.0).It could be found that currents of the bare GCE and the MMT/GCE in the PBS are extremely low and no elec-trochemical peaks are observed.One pair of electrochemical peaks are observed at the PANI/GCE,which should be related to the elec-trochemical reaction of the modi?ed PANI ?lm.There electrochem-ical peaks could also be observed at the PANI/MMT/GCE with the intensity increased greatly,suggesting that the PANI/MMT ?lm ex-hibit more electroactivity than that of pure PANI ?lm.It could be found from SEM image that well-dispersed nanoscale PANI parti-cles were deposited on the MMT layers.Therefore,it could be de-duced that electrochemical behavior of PANI could be suf?ciently improved through the deposition on the MMT substrate.From the electrochemical result and IR results reported in our previous work,it is obvious that PANI in the composite is its emeraldine salt [44].

The electrochemical impedance spectrum (EIS)is also used to characterize electron transfer ability of the modi?ed electrode

using Fe eCN T3à=4à

6redox couple as an indicator.The results are MMT/GCE is much smaller than

that of the bare GCE suggesting that the charge transfer is more facile on the modi?ed electrode.It is because that well-dispersed nanostructured PANI could greatly improve diffusion of target molecules [46].The result is in good accordance with that of cyclic voltammetry.

3.2.Electrochemical responses of the bare GCE and the PANI/MMT/GCE Fig.4shows the stripping performance the bare GCE and the PANI/MMT/GCE in 0.1M NaAc-HAc (pH 3.5)solution in presence and absence of 8l M Pb 2+.No stripping peak is observed in NaAc-HAc buffer solution either on the bare GCE or on the PANI/MMT/GCE,revealing that the PANI/MMT ?lm will not generate electrochemical response in NaAc-HAc solution.A very small and distorted stripping peak of Pb is observed at the bare GCE in 0.1M NaAc-HAc (pH 3.5)solution containing 8l M Pb 2+,suggest-ing that Pb 2+could not be sensitively detected at the bare GCE.The stripping peak of Pb at the PANI/MMT/GCE is strong,well-shaped,and symmetric.The not-zero background current at the modi?ed electrode is probable due to the charging current of nano-composite.The intensity of stripping peak increases nearly 5times compared with that of the bare GCE,revealed that Pb 2+could be detected sensitively at the PANI/MMT/GCE.

SEM image of the surface of PANI/MMT nanocomposite modi?ed electrode.

208Y.Dong et al./Journal of Electroanalytical Chemistry 717-718(2014)206–212

3.3.Effect of feeding ration of MMT to aniline and the amount of PANI/ MMT suspension

It has already proven that electroanalysis of heavy metal ions at the conducting polymer modi?ed electrode has received consider-able attention because the conducting polymer could enrich the target heavy metals and improve the analytical performance[47]. Among many layered materials,MMT have been extensively inves-tigated because of their large surface area and ion-exchange prop-erties[30].In the present work,MMT was used as the substrate for the deposition of PANI.It was found in our previous work that the MMT/aniline feeding ratio could affect the morphology of PANI/ MMT composites[44].Therefore,in the present study,different MMT/aniline feeding ratio of PANI/MMT composites(0.0%,12.5%, 25%,50%,75%,100%,125%)were prepared and the effects of these PANI/MMT composites on electrochemical response of Pb2+are comparatively examined and shown in Fig.5.It could be found that with the increase of MMT/aniline feeding ratio,the electrochemical signals of Pb2+increase correspondingly and reach the maximum at feeding ratio of50wt%.A further increase in the feeding ratio weakens the electrochemical signals.In the pure PANI?lm modi-?ed electrode,the aggregation of PANI particles will hamper its conductivity and result in low electrochemical signal.When PANI particles was deposited on MMT?lm,the adsorption ability to me-tal ions and the electrochemical behavior of the composite increase correspondingly.The reason is that MMT is a good substrate for the deposition of PANI particles and could prevent PANI particles from aggregation,which will improve the electrochemical response of metal ions.However,when the amount of MMT in the composite is too large,the conductivity of the modi?ed electrode will de-crease,as a result,the electrochemical signal decrease correspond-ingly.Therefore,the50%feeding ratio of MMT to aniline is chosen to synthesize MMT/PANI composite and used in the electrode modi?cation.

The voltammetric response of Pb2+is closely related to the thickness of PANI/MMT?lm which is determined by the volume of PANI/MMT suspension cast onto the GCE surface.The relation-ship between the volume of PANI/MMT suspension and the strip-ping peak current is examined and shown in Fig.5.The stripping peak current gradually increases as improving the volume of PANI/MMT suspension from0.0to10.0l L.However,the peak cur-rent conversely decreases with further increase of the volume of PANI/MMT suspension.As increasing the amount of PANI/MMT onto GCE surface,the active sits for Pb2+accumulation also in-crease.So,the stripping peak current shows enhancement.When the PANI/MMT?lm is too thick,the mass transport and charge transfer rate maybe conversely decrease,resulting in the decline of the peak current.Therefore,the optimized amount of PANI/ MMT is chosen as10l L.

3.4.Effect of electrochemical technologies

Among various anodic stripping voltammetry methods,square wave anodic stripping voltammetry(SWASV),linear sweep anodic stripping voltammetry(LSASV)and differential pulse anodic strip-ping voltammetry(DPASV)are often used.In order to obtain high-est sensitivity,these electrochemical techniques are comparatively used in the detection of Pb2+on the PANI/MMT/GCE as shown in Fig.6.It could be found that electrochemical signal of SWASV is similar with that of DPASV and is larger than that of LSASV.How-ever,the baseline of SWASV varies signi?cantly with the increase of applied potential while the baseline of DPASV changes a little under the same condition.Therefore,DPASV technique is used in the following electrochemical experiments.

Y.Dong et al./Journal of Electroanalytical Chemistry717-718(2014)206–212209

3.5.Effect of pH and acetate concentration

The effect of various supporting electrolytes including phos-phate,acetate,NH3–NH4Cl on the stripping peak currents of Pb2+ is investigated.The best electrochemical response is obtained in acetate buffer solution.Then,the effect of concentration of acetate buffer solution on the stripping peak current of Pb2+is investigated and the result is shown in Fig.7.As it can be seen,the optimal con-centration of acetate buffer solution is0.2mol Là1.Therefore, 0.2mol Là1acetate buffer solution is chosen as the supporting electrolytes in the following study.

The in?uence of pH on the determination of Pb2+is studied at a pH range from3.0to5.0.As shown in Fig.7,the peak current in-creases with increasing pH values and reaches a maximum value at pH3.5.The continuous increase in pH value results in a decrease in peak current.To achieve maximum sensitivity,the pH of acetate buffer solution is controlled to3.5in subsequent experiments. 3.6.In?uences of deposition time and deposition potential of Pb2+

The dependence of stripping peak current on accumulation time is studied and shown in Fig.8.As increasing the accumulation time,more Pb2+will be exchanged and adsorbed onto the surface of the PANI/MMT/GCE,so the stripping peak current will increase. As depicted in Fig.8,the stripping peak current of1l M Pb2+grad-ually increases with the accumulation time,indicating that the sensitivity of determination of Pb2+will increase as extending accumulation time.When accumulation time is longer than 120s,the curves of peak current vs time began to leave off.This can be attributed to saturation adsorption of Pb2+on the PANI/ MMT?lm.Therefore,120s is chosen to achieve high sensitivity and save analysis time.

The effect of deposition potential on metal stripping signals with the deposition time of120s in the range fromà0.8to à1.4V was also shown in Fig.8.When the accumulation potential shifts fromà0.80V toà1.20V,the stripping peak current in-creases greatly.As the accumulation potential becoming more neg-ative,Pb2+is reduced more completely,and consequently the stripping peak current increases.When the accumulation potential is negative thanà1.20V,the stripping peak current changes slightly,meanwhile,the background current increases greatly. Therefore,à1.20V is selected to obtain high sensitivity and better response.3.7.Calibration data

Calibration curves for the determination of Pb2+at the PANI/ MMT/GCE are achieved by DPASV under the optimal conditions. The DPASVs of different concentration of Pb2+are illustrated in Fig.9.The resulting calibration plots are linear over the range from4to100nM.The linear regression equation of Pb2+is Ip=1.08092Cà0.55133(Ip:l A,C:nM)with a correlation coef?cient of0.998.The detection limit of1nM is obtained at a sig-nal-to-noise ratio of3.

The comparison results of the proposed sensor with some reported sensors for the determination of Pb2+are given in Table1. It could be concluded that analytical performance of the PANI/ MMT/GCE in the detection of Pb2+is satisfactory.

3.8.Interferences

Under the optimal conditions,the interference study is carried out by adding various foreign substances into a standard solution which contained 1.0?10à6mol Là1of Pb2+and0.2mol Là1 NaAc-HAc(pH3.5).The ratios of interference for a±5.0%signal change relative to1.0?10à6mol Là1of Pb2+are listed as follows: 3000for K+,Na+,Clà,NO3à;600for Mg2+,Zn2+,Ca2+;300for

210Y.Dong et al./Journal of Electroanalytical Chemistry717-718(2014)206–212

Ba2+and Fe3+;100for Co2+,Cd2+and Cr3+;20for Cu2+,Ni2+.The cat-ionic components can compete for the binding sites on the elec-trode surface and the anions may act as either complexing or oxidation agents.It is found that Ni2+and Cu2+have signi?cant in?uence on the stripping response at low mass ratio.This is be-cause Ni2+and Cu2+can form intermetallic compounds with Pb2+ and prevent its accumulation on the electrode.The interference of Cu2+and Ni2+could be released by the addition of ferrocyanide as suggested in Ref.[45].

3.9.Analysis of a real sample

The PANI/MMT/GCE is employed for determination of Pb2+in three real samples from local environment including tap water, water from Yushan lake and Lian lake.All water samples are used without?ltered in advance,and then added to0.2mol Là1ace-tate buffer(pH 3.5)and determined under the optimal condi-tions.The experimentally results are shown in Table2.Pb2+is not found in these water samples.The results con?rm the good application potential of the present method for real water samples.

4.Conclusions

In this study,PANI/MMT nanocomposite modi?ed electrode was used to detect heavy metals in real water samples by differen-tial pulse anodic stripping voltammetry.The result revealed that trace lead ions in water samples could be sensitively detected at the modi?ed electrode.The high sensitivity of this modi?ed elec-trode for analysis of Pb2+opens the possibility to consider the PANI/MMT/GCE as a good alternative to mercury-based electrodes. Owing to the environmentally friendly decorate material,inexpen-sive instrumentations,and simple operation,the methods will of-fer potential applications for monitoring of Pb2+in environmental samples.Acknowledgements

This work is?nancially supported by National Natural Science Foundation of China(No.61271156,No.21207001),China Post-doctoral Science Foundation(2013M541637)and Innovation Team Project of AHUT(No.TD201204).

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Table1

Comparison of some characteristics of the different modi?ed electrodes reported for the determination of Pb2+.

Electrode Modi?er Method LOD(l mol Là1)Linear range(l mol Là1)Ref

CPE Kaolinite DPASV0.0840.3–20[48] CMCPE IL MWCNT-Schiff base SWASV 1.2?10à4 1.9?10à3–5.3[49] Graphene Bi NPs/PANI SWASV 3.3?10à40.1–1.1[50] GCE Thiol-PANI/MWCNTs DPASV 1.9?10à4 4.8?10à4–0.14[51] CNT Gold NPs SWASV 2.6?10à30.016–0.11[52] CPE RGO/Bi DPASV 2.7?10à30.097–0.58[53] Gold Graphene OSWV– 4.0?10à4–0.2[54] Graphene Bi SWASV–0.3–1.7[55] CPE MWCNT/Schiff base SWASV 6.4?10à4 2.5?10à3–0.7[56] GCE BiF/GNFs-Na?on DPASV9.7?10à59.7?10à4–0.24[19] GCE PANI/MMT DPASV 1.0?10à3 4.0?10à3–0.1Present work

CPE:carbon paste electrode,DPASV:differential pulse anodic stripping voltammetry,SWASV:square-wave anodic voltammetry,OSWV:Osteryoung square wave voltam-metry,MWCNT:multi-wall carbon nanotube,RGO:reduced graphene oxide,NPs:nanoparticles.

Table2

Analytical results of lead in water samples and recovery studies.

Sample Added(nM)Found a(nM)RSD(%)Recovery(%)

Tap water0–––

4038.4 3.596.0

Yushan lake0–––

4041.3 4.3103.2

Lian lake0–––

4038.6 5.296.5

a Mean of three determinations.

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