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Talanta 78 (2009) 498505

Contents lists available at ScienceDirect

Talanta

journa l homepage: www.e lsev ier .com/ locate / ta lanta

ead(II) ion-selective electrode based on polyaminoanthraquinone particlesith intrinsic conductivity

in-Gui Lia,b,, Xiao-Li Maa, Mei-Rong Huanga,b,

Institute of Materials Chemistry, Key Laboratory of Advanced Civil Engineering Materials of the Ministry of Education, College of Materials Science and Engineering,ongji University, 1239 Si-Ping Road, Shanghai 200092, ChinaKey Laboratory of Molecular Engineering of Polymers of the Ministry of Education, Fudan University, Shanghai 200433, China

r t i c l e i n f o

rticle history:eceived 2 August 2008eceived in revised form7 November 2008ccepted 29 November 2008vailable online 6 December 2008

a b s t r a c t

A new polyvinylchloride membrane electrode was facilely prepared by using polyaminoanthraquinone(PAAQ) microparticles with an intrinsically electrical conductivity as a lead(II) ionophore. It is foundthat the electrode performance will significantly be improved with adding 1wt% PAAQ micropar-ticles and decreasing the membrane thickness. A 90m-thick membrane electrode consisting ofPAAQ(salt):polyvinyl chloride:dioctylphthalate:sodium tetraphenylborate of 1:33:66:1 (wt) but withoutany traditional lead(II) ionophore achieved the optimal performance and exhibited a good Nernstianresponse for Pb(II) ions over a wide concentration range from 2.5106 to 0.1M with a slope ofeywords:olyaminoanthraquinoneeadII) ion-selective electrodeonducting polymeread(II) ionophoreerformance optimization

28.9mV/decade and a detection limit down to 776nM. A reasonably short response time of 12 s wasrevealed together with a long lifetime over a period of around 4 months in a wide pH range between 2.8and 5.2. A fixed interference method indicated that the electrode has an excellent selectivity for lead(II)ion over alkali, alkaline earth and other heavy metal ions. The proposed electrode has been also found tobe a powerful indicator electrode for potentiometric titration of Pb(II) ions with EDTA. The electrode can

nitorembrane electrode be used to accurately mo

. Introduction

Lead(II)ISE is of great significance because the lead is ubiqui-ous in the environment and extremely hazardous to humanhealth.large number of lead(II)ISEs reported are mainly based on poly-ericmembranecontaining ionophores [16]. Thekeypart of theselectrodes is a highly sensing ionophore having strong affinity forparticular metal ion but giving low or even no response to others.normous efforts have been made to design and synthesize suit-ble materials that are highly selective to lead(II) ions. Macrocyclicrown ethers [24], calixarenes [57], and Schiff base [8,9], haveeen widely investigated for this purpose. Other ligands such asorphyrin [10,11], pyridinecarboximide [12], piroxicam [13], tetra-

enzyl pyrophosphate [14], capric acid [15], phenyl disulfide [16],ithiodibenzoic acid [17], and quinaldic acid derivatives [18] arelso served for the fabrication of the lead(II)ISEs. In spite of avail-bility of various lead(II)ISEs, the narrow working concentration

Corresponding authors at: Institute of Materials Chemistry, Key Laboratory ofdvancedCivil EngineeringMaterials of theMinistry of Education, College ofMateri-ls Science and Engineering, Tongji University, 1239 Si-Ping Road, Shanghai 200092,hina. Fax: +86 21 65980524.

E-mail addresses: adamxgli@yahoo.com (X.-G. Li), meironghuang@ymail.comM.-R. Huang).

039-9140/$ see front matter 2009 Published by Elsevier B.V.oi:10.1016/j.talanta.2008.11.045the Pb(II) pollution in environmental waters. 2009 Published by Elsevier B.V.

range, high detection limit, slow response rate and especially poorselectivity over interfering ions have restricted their widespreadapplication. In particular, the ionophores studied are almost all thelipophilic compounds that easily leak out from matrix. Only onereport on the polymer as ionophores with good anti-leaking abil-ity and then long lifetime was found [19]. However, the responsetime is long because of its intrinsically high membrane resistance.Therefore, it is of great challenge and significance to further search anew ionophore that possesses an intrinsically lowmembrane resis-tance and therefore high sensitivity and selectivity to lead(II) ionsso as to develop lead(II)ISEswith excellent detection performanceincluding quick responsibility and long duration.

In recent years, it is found that the conducting polymers espe-cially some polymers from aromatic diamines have a unique abilityto form stable complexeswith someheavymetal ions such as Pb(II),Hg(II) and Ag(I) ions [2026]. Polyphenylenediamine synthesizedin our laboratorywas found to possess a strong capability to adsorblead ions through complexation between Pb2+ ions and NH/ =Ngroups in the macromolecular chains [23]. Concerning these prop-erties, attempts have been made to employ these polymers to

extract and sense heavy metal ions. Carbon paste electrode mod-ified with poly(1,8-diaminonaphthalene) was successfully usedto determine lead(II) ions in a concentration range from 0.2 to10M [21]. The detection processwas complicated due to an indis-pensable combination of ion pre-enrichment and anodic-stripping

http://www.sciencedirect.com/science/journal/00399140http://www.elsevier.com/locate/talantamailto:adamxgli@yahoo.commailto:meironghuang@ymail.comdx.doi.org/10.1016/j.talanta.2008.11.045

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trodes based on PAAQ salt and base particles have been constructedseparately to study the effect of doping state on the response per-formance to Pb2+ ions. It is apparently observed from Fig. 1 that thePAAQ salt particles-based electrode performs better response prop-erties in all aspects, especially broaderworking concentration rangeX.-G. Li et al. / Tala

oltammogram, accompanying with a quite narrow work con-entration range. Nevertheless, these results have indicated theossibility of some aromatic conducting polymers with strongffinity to lead(II) ions to act as ionophores in lead(II)ISEs. Unfor-unately, no relevant reports have been found up to now.

A novel multifunctional polyaminoanthraquinone (PAAQ) wasuccessfully synthesized through a chemically oxidative polymer-zation [26]. PAAQexhibits a strong adsorbability to lead ions owingo its strong complexibilitywith lead ion throughNandO coordina-ion centers in thepolymer chains. It seems that thePAAQcouldbe aotential ionophore for construction of a newmembrane sensor foread ions. The ingredient and thickness of themembranewere opti-ized to provide unique Pb2+ISE that could result in reproducible,oiseless and stable potentials. The excellent performance of thelectrode containing PAAQ as the sensing ionophore to lead(II) ionsn thedetermination of lead(II) ions has been elaborated for thefirstime.

. Experimental

.1. Reagents

1-Aminoanthraquinone (AAQ), ammonium persulfate(NH4)2S2O8), HClO4 (70%), acetonitrile, high molecular weightolyvinyl chloride (PVC), dioctylphthalate (DOP), sodiumetraphenyl borate (NaTPB), tetrahydrofuran (THF) and leaditrate of analytical reagent grade were commercially obtainednd used as received. 0.1M Pb(NO3)2 stock solution was preparedy dissolving lead nitrate in distilled deionized water and stan-ardized whenever necessary. The working solutions of differentoncentrations were confected by gradually diluting the stockolutions.

.2. Synthesis of fine PAAQ microparticles as ionophore

PAAQ particles used as ionophore were simply prepared by ahemically oxidative polymerization of the AAQ monomer [26]. Aypical procedure for the preparation of PAAQ particles is as fol-ows: AAQmonomer (446mg, 2mmol) and HClO4 (11.8M, 0.17mL)ere added into acetonitrile (40mL) in a 100mL glass flask in aater bath at 20 C and the mixture was then stirred vigorously for0min. (NH4)2S2O8 (456mg, 2mmol) was dissolved separately ineionized water of 0.75mL to prepare an oxidant solution. The oxi-ant solutionwas then added dropwise into themonomer solutiont a rate of one drop (around 60L) every 3 s. The reaction mixtureas stirred continuously for 24h at 20 C. After reaction, the result-

ngpolymerparticlesasprecipitateswere isolated fromthe reactionixture by filtration andwashedwith ethanol to remove the resid-al oxidant, remainingmonomers and soluble oligomers. The PAAQarticles were left to dry in ambient air at 50 C for 3 days. Theluish black PAAQ particles obtained have nominal macromolecu-ar structural formula in Scheme 1 and bulk electrical conductivityf 5.0105 S/cm at 15 C.

.3. Electrode fabrication

To prepare a selective membrane, an appropriate amount ofAAQ particles were dispersed in 5mL THF by an intermittentlyltrasonic treatment. Proper amounts of PVC, DOP and NaTPBwereradually added into the PAAQ dispersion and subsequently stirredor 10min to ensure a uniform mix. The mixture obtained thus

as poured on a smooth plate glass and then allowed to evaporate

or 24h at room temperature. The translucent elastic membranesith different compositions and thicknesses were obtained after

he evaporation of THF. The thickness of the membrane was mea-ured by a roller type thickness gauge with the minimum scale of(2009) 498505 499

10m. A circular membrane of 16mm diameter was carefully cutout and glued to one end of plastic tube that would be filled with0.10M Pb(NO3)2 solution as internal reference solution. The pre-pared electrodes were conditioned in a 0.01M Pb(NO3)2 solutionfor 12h and finally washed by distilled deionizedwater until stablepotentials were reached before using.

2.4. Potential measurement

All potentiometric measurements were performed by a PHS-3C digital pH meter (Shanghai Kangyi Instruments Factory, China).A double-junction saturated calomel electrode (SCE) was used asthe external reference electrodewith the outer junction containing0.10M KCl and inner reference containing saturated KCl. The rep-resentative electrochemical cell for the electromotive force (EMF)measurement is as follows:

Ag/AgCl|Internal solution (0.10MPb(NO3)2)|Selective membrane|Sample solution|Salt bridge(0.10MKCl)|SCE

The performance of the electrodes was examined by measuringthe EMFs of the Pb2+ solutions in a concentration range of 108

to 101 M. The pH values were adjusted by HNO3 and NaOH whenconsidering the applicable pH range of the electrode. The responsetimeof the electrodewas determined based on the potentials at dif-ferent times for various concentrations of Pb(NO3)2 solution untilthe EMFs value kept constant in 5min.

3. Results and discussion

3.1. Doping state of PAAQ microparticles

For the PAAQ ionophore of Pb2+ in this study, it is certain that theN and O atoms could be soft coordination centers to complex Pb2+

ions. Besides, the as-prepared PAAQ salt particles contain a certainamount of doped anions just like ClO4 and SO42, which comefrom the HClO4 and reducing products of (NH4)2S2O8 used for thepolymerization. Both of them could be removed from the polymerchains through dedoping process in ammonia, accompanying withthe significant decline of conductivity. The reversible conversionbetween salt andbase states of thePAAQchains is auniquenatureofaromatic conducting polymers [2023]. Concerning this, the elec-Scheme 1. The macromolecular structural formula of PAAQ polymer together withthe exchange mechanism involved in producing potential signal.

500 X.-G. Li et al. / Talanta 78 (2009) 498505

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a deterioration of performance ultimately. To study the effect ofionophore content, three electrodes loading different amounts ofPAAQ ionophore were prepared and their potential responses areshown in Fig. 3 and Table 1. Interestingly, PAAQ:PVC:DOP:NaTPBig. 1. Potentiometric responses of Pb2+ selective electrodes based on PAAQicroparticles at two doping states of salt and base at PAAQ:PVC:DOP:NaTPB of:33:66:1 and membrane thickness of 160m.

rom 4.0106 to 0.1M, higher slope (28.1mV/decade), loweretection limit (1.86106 M), and shorter response time (14 s)han the PAAQ base particles-based electrode (working concentra-ion range: 1.0105 to 0.1M; slope: 27.3mV/decade; detectionimit: 6.67105 M; response time: 17 s). It should be noted thathe possibility of forming PbSO4 precipitate during detection pro-ess has been ruled out because possibly maximal SO42 and Pb2+

oncentrations are 1011 M and 0.1M, respectively, and the productf SO42 andPb2+ concentrationswouldbe1012 that ismuch lowerhan the solubility product constant of PbSO4 (1.6108). Theesidual SO42 and ClO4 ions existing along the PAAQ salt chainss the counter-anions are loose associated with protonated NH+ites on the macromolecular chains, forming a negatively chargedayer around PAAQ chains. The static attraction of the negativelyharged layer to Pb2+ ions, as well as complexation between N/Oites and Pb2+, could hold the Pb2+ ions within the membrane andherefore suppress the zero-current flux of the primary ions fromembrane into sample solution of low Pb2+ concentration, which

s considered as one important factor restricting the detection limit2729], ultimately leading to a low detection limit and broad lin-ar range. On the other hand, the presence of SO42 and ClO4 inhe PAAQ salts would prevent the extraction of anions and then therimary ions from the inner reference solution, i.e., the Pb2+ in thenner reference solutionwould not be extracted into themembranehase. As we know, the absence of primary ion flux is necessaryo achieve low detection limit. Generally, the primary ions alwaysend to transit from bulk membrane into the sample solution atlow Pb2+ concentration, leading to a local higher concentration

han the sensing concentration. Consequently, true concentrationf sample solution could not be satisfactorily detected. Vigorousgitation could improve thehomogeneity of the local concentrationo some extent. Additionally, much higher electrical conductivityf PAAQ(salt) (5.0105 S/cm) than PAAQ(base) (3.7108 S/cm)ould remarkably enhance the sensitivity of electrode aswell, con-ributing to a nearer Nernstian slope and quicker response.

Fig. 2 shows the FTIR spectra of the ionophores of PAAQ base

nd salt before and after the exposure of PAAQ in 0.001M Pb(NO3)2olution for 1h. The peaks at 3440, 1640 and 1270 cm1 due tohe NH, C O and CN, respectively, becomeweaker after contact-ng Pb2+ ions, indicating an interaction between NH/C O groupsnd Pb2+ ions, i.e., complexation between the Pb2+ ions and theFig. 2. FTIR spectra of PAAQ ionophores (base and salt) before and after the exposureof PAAQ in the 0.001M Pb(NO3)2 solution for 1h.

PAAQ. Furthermore, the PAAQ saltPb2+ complex illustrates weakerNH absorbance at 3440 cm1 than the PAAQ basePb2+ complex,suggesting stronger interaction between the PAAQ salt and Pb2+

ions. Therefore, the as-prepared PAAQ salt particles with strongerresponse to Pb2+ ions are selected as lead(II) ionophore in the fol-lowing study.

3.2. PAAQ microparticle content

It iswell knownthat the sensitivity andselectivityof ISEsdependsignificantly on the nature of ionophore and themembrane compo-sition. Thus the loading content of ionophore should be optimizedto produce the best performance of the proposed electrode. Thecomplexation function of ionophore with Pb2+ cannot be visual-ized sufficiently at low ionophore content. On the contrary, highionophore content is unfavorable to the transportation of ions inmembrane and may even cause pinholes in membrane, leading toFig. 3. Potentiometric responses of the Pb(II) ISEs based on different contents ofPAAQ in a 160m-thick matrix membrane.

X.-G. Li et al. / Talanta 78 (2009) 498505 501

Table 1Composition and performance characteristics of Pb(II) ISEs in Fig. 3.

PAAQ:PVC: DOP:NaTPB (weight ratio) Working concentration range (M) Working linear equation Slope (mV/decade) Detection limit (M) Response time (s)

1 129.61 126.11 112.6

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:25:50:1 1.0105 to 0.1 E=:33:66:1 4.0106 to 0.1 E=:50:100:1 1.0105 to 0.024 E=

f 1:33:66:1 (wt) exhibits the best performance involving theidest working concentration range from 4.0106 to 0.1M with

he largest Nernstian slope of 28.1mV/decade, the lowest detec-ion limit of 1.86106 M, and the shortest response time of4 s. This great improvement of electrode performance shouldriginate from the presence of PAAQ ionophore. In fact, theAAQ:PVC:DOP:NaTPB (0:33:66:1) membrane with the thicknessf 160mhas much lower potential stability and lower conductiv-ty (2.13108 S/cm) than the PAAQ:PVC:DOP:NaTPB (1:33:66:1)embrane (1.35107 S/cm). The PAAQ-free electrode might note suitable to detect potential response. It is reported that theddition of only a small amount of conducting polymers couldignificantly enhance the electrical and other properties of matrixolymers [24,25]. Excellent performance has also been realized athigh ionophore content for some PVC membrane ISEs based onexathia-18-crown-6-tetraone ionophore:PVC of 1:6(wt) [2] and,N-dimethylcyanodiaza-18-crown-6 ionophore:PVC of 1:5(wt)4]. However, the optimal weight ratio of PAAQ ionophore andVC is 1:33 for most electrodes involving the PAAQ-based elec-rode in this study. Much less PAAQ particles are required toroduce the best performance of the ISEs, i.e., PAAQmight be morefficient ionophore than hexathia-18-crown-6-tetraone and N,N-imethylcyanodiaza-18-crown-6.

.3. Nature of plasticizer

The plasticizer is considered to play an important role in opti-izing the performance of ISEs through influencing the dielectriconstant of themembranephase [57]. Fourplasticizers of different

olarity were used to study the effect of plasticizer on the selec-ive response of the Pb2+ISEs based on PAAQ. As shown in Fig. 4,he ISE with DOP as plasticizer obviously performs a higher Nern-tian response slope of 28.1mV/decade over a wider range from

ig. 4. Potentiometric responses of Pb(II) ISEs based on PAAQ microparticles withifferent plasticizers at PAAQ:PVC:plasticizer:NaTPB of 1:33:66:1 and membranehickness of 160m.+26.7 log[Pb2+] 26.7 3.29106 16+28.1 log[Pb2+] 28.1 1.86106 14+27.9 log[Pb2+] 27.9 2.92106 17

4.0106 to 0.1M. The ISEs based on the other three plasticizersonly give a diminished response slope (

502 X.-G. Li et al. / Talanta 78 (2009) 498505

Table 2Performance characteristics of Pb(II) ISEs in Fig. 6.

Membrane thickness (m) Working concentration range (M) Working linear e

220 1.0105 to 0.1 E=120.5 +28.7160 4.0106 to 0.1 E=126.1 +28.1 l90 2.5106 to 0.1 E=132.5 +28.9

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ig. 6. Potentiometric responses of the Pb(II) ISEs based on different membranehicknesses at PAAQ:PVC:DOP:NaTPB of 1:33:66:1 (wt).

etection limit, slope and response timewith decreasing themem-rane thickness. At the smallest membrane thickness of 0.09mm,he electrode exhibits an almost Nernstian slope of 28.9mV/decadend awidest working concentration range from 2.5106 to 0.1M,hichhas beenbroadenednearly by four times comparedwith thatt the thickness of 0.22mm. The electrode also possesses the lowestetection limit of 7.76107 M and the shortest response time of2 s. It seems that the optimal PAAQ membrane exhibits compara-le or even better performance than 10 representative lead(II)ISEsith PVC matrix in Table 3.It is reported that the membrane thickness of the PVC-basedb2+ ISEs is mostly between 0.2 and 0.5mm [1,10,11,27,28]. Theensing unit supported by a highly porous polymeric layer in theselectrodes is practically liquid state comprising a lipophilic organiconophore dissolved in oil phase such as DOP. The detection limitsf these electrodes commonly keep at the order of 106 or 107 M,

able 3omparison of performance characteristics of the proposed electrode with the PVC memb

onophore Working concentrationrange (M)

Slope (mV/decade)

,N-Dibenzyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane

8.2106 to 0.1 30.0

exathia-18-crown-6-tetraone 1.0106 to 0.008 29.0-Methoxyethoxylsym-dibenzo-16-crown-5-ether

5.0105 to 0.5 28.9

etrakis(p-carboxyphenyl)azo]-8-tetrahydroxycalix[4]arene

1.0106 to 0.1 29.4

eso-tetrakis(2-hydroxy-1-naphthyl)porphyrin

3.2105 to 0.1 29.2

hiral 2,6-bis-pyridinecarboximide 3.5106 to 0.01 27.9iroxicam 1.0105 to 0.1 30.0etrabenzyl pyrophosphate 1.0105 to 0.01 28.7apric acid 1.0105 to 0.01 29.0henyl disulfide 2.0106 to 0.01 29.3olid PAAQ particles 2.5106 to 0.1 28.9quation Slope (mV/decade) Detection limit (M) Response time (s)

log[Pb2+] 28.7 4.47106 15og[Pb2+] 28.1 1.86106 14log[Pb2+] 28.9 7.76107 12

beyondwhich the potential responsewould deviate from theNern-stian equation, because a zero-current flux of the primary ions fromthe membrane into sample solutions would result in ion activityof the primary ions maintaining as high as 106 or 107 M in thelocal domain near the membrane surface at the sample side. Theflux of primary ions is generated by a transmembrane concentra-tion gradient which occurs when the concentrations of the sampleand inner solutions are not the same. The detection limit could beimproved through reducing the transmembrane concentration gra-dient by adjusting the concentration of the inner solution to thevalue of sample solution, decreasing the total ion concentration,or increasing the thickness of the membrane [29]. However, theresult here is reverse to the above-mentioned circumstance. Thepotential response characteristics of theproposed electrodes canbeenhanced by decreasing the thickness of the membrane. This maybe relevant to the existing state of the ionophore. The fine PAAQparticle ionophore that is insoluble in DOP oil phase, disperses uni-formly in the PVC membrane as the solid state. The transportingrate of lead(II) ions in solid phase is much slower and more ardu-ous than that in liquid phase. Hence the flux of lead(II) ions is notencouraged in solid membrane. Nevertheless, the response is stillvery fast due to the relatively thin membrane, as discussed below.Considering the remarkable decrease of membrane strength, it isnot suggested to further reduce themembrane thickness.All furtherdetailed studies were carried out on the electrode with the com-position of PAAQ:PVC:DOP:NaTPB (1:33:66:1) and the membranethickness of 0.09mm.

Another result which disagrees with the theory that increas-ing membrane thickness might improve the detection limit wasalso reported in the investigation on the electrode based ondiphenylmethyl-N-phenylhydroxamic acid ionophore [30]. Amongthree styrene/acrylonitrile copolymer membranes with differentthicknesses of 0.14, 0.21, and 0.45mm, the 0.21mm-thick mem-brane demonstrates the lowest detection limit.3.6. Effect of pH

The dependence of the potentiometric response of the proposedISE on the pH value of the Pb2+ solution was tested at three Pb2+

rane lead(II) ISEs reported in literature.

Detection limit (M) Response time (s) Lifetime (month) Refs.

8.2106 10 3 [1]

8.0107 40 2 [2]1.0106 30 3 [3]

8.0107 20 1 [6]

3.5106 10 3 [11]

2.2106 25 0.5 [12]4.0106 45 3 [13]3.0106 10 0.7 [14]6.0106 15 3 [15]1.2106 45 1.7 [16]7.8107 12 4 This study

X.-G. Li et al. / Talanta 78 (2009) 498505 503

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ig. 7. The pH dependence of the PAAQ:PVC:DOP:NaTPB (1:33:66:1) membranelectrode with the thickness of 90m on the potentiometric response under threeonstant concentrations of lead ion.

oncentrations (1.0102, 1.0103 and 1.0104 M) over the pHange between 1 and 6. It is seen from Fig. 7 that the potentialesponse remains almost constant over the pH range from 2.8 to.2, beyond which a gradual change in potential can be observed.s a result, this range can be taken as the working pH range of theroposed electrode. The declined potential at higher pH valuesmaye ascribed to the formation of some hydroxy complexes of Pb2+

uch as Pb(OH)+ and Pb(OH)2, leading to a decreased Pb2+ concen-ration, while at lower pH, the abundant H+ ions can protonate theatoms of PAAQ and even cause the decomplexation of Pb2+PAAQomplex. The H+ ions itself can make interference to the electrodeesponse simultaneously. Both of them could result in the rise ofhe potential.

Thewideworking pH range of the proposed electrode is compa-able to the reported Pb2+ISEs based on the following ionophores:,N-dibenzyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane [1],exathia-18-crown-6-tetraone [2], phosphorylated calyx[4]arene31], and 5,11-dibromo-25,27-dipropoxycalix[4]arene [32]. Theower limit of the working pH range is expected to be furthermproved in case that the binding ability of Pb2+ onto PAAQhains is strengthened. The electrode based on 2,12-dimethyl-,17-diphenyltetrapyrazole with strong complex capability showsider working pH range of 1.66.0, whereas the electrode basedn 5,11-dibromo-25,27-dipropoxycalix[4]arene with a relativelyeaker complex capability has narrower pH range of 2.36.0 [32].

.7. Response and lifetime of the electrode

The response time of the PAAQ ISE was determined by mea-uring the time required to achieve a steady potential in Pb2+

olution with three concentrations of 1.0102, 1.0103 and.0104 M. It is found from Fig. 8 that the response times 12 s, approaching to the fastest response of Pb2+ISEs basedn N,N-dibenzyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane[1]ndN,N-bis(salicylidene)-2,6-pyridinediamine [33] as ionophores.his fast response is relevant to the fast kinetic process of complex-tion between Pb2+ and PAAQ ionophore in the doped state. Onhe other hand, the fine particles of the PAAQ salts, together with

heir higher intrinsic conductivity (5.0105 S/cm), may promotehe formation of a uniform, thin, and highly conducting compos-te membrane. All of these features are beneficial to the transitionf charges in membrane, resulting in the fast response of the elec-rode. There was no difference for the potential responses for eachFig. 8. Response time profile of the Pb(II) ISE based on PAAQ:PVC:DOP:NaTPB(1:33:66:1) membrane with the thickness of 90m.

concentration recorded from high concentration to low concentra-tion and from low concentration to high concentration, indicatingthat the ISE has an excellent reversibility. The standard deviationsof ten replicate potential measurements of 1mM and 0.1mM sam-ple solutions were 0.58 and 0.65mV, respectively, indicating thegood reproducibility of the prepared ISE. It is also observed thatthe electrode could be satisfactorily used over a period of 4monthswithout any significant loss of the performance characteristics suchas working concentration range, slope and response time. Dislikethe ordinary ionophore, PAAQ salts existing as fine solid particles donot leak from the matrix while contacting with aqueous solution,ensuring a reasonably long lifetime of the proposed electrode.

3.8. Electrode selectivity

The selectivity is one of the most important characteristics forselective electrodes, because it determines the feasibility and util-ity of the electrodes in real sample analysis. The potentiometricselectivity coefficient KPOT

Pb2+,Bhas been served to reflect the rela-

tive response of the membrane selective electrode for the primaryion over other interfering ions present in solution and can be usedto predict response functions in mixed samples. It can be deter-mined mainly through three different methods including separatesolution, fixed interference and matched potential methods. Thefixed interference method is preferable since it more closely mim-ics a practical application of the ISE. In the present study, the fixedinterference method was employed to assess the selectivity of thefabricatedPb2+ ISEover other commonly interfering ions. The selec-tivity coefficients were calculated through the equation:

KPOTPb2+,B

= aPb2+ (DL)/(aB)2/z

where Pb2+ (DL) is the detection limit of Pb2+ ion, B the activity

of the interfering ion, and z the charge of the interfering ion [34]. Itis noteworthy that the selectivity coefficients for all diverse cationslisted in Table 4 are in the order of 102 or 103, indicating thatthe proposed electrode is highly selective over all the interferingions studied. The interferences of alkalimetal ions (Na+ and K+) andalkaline-earthmetal ions (Ba2+ and Ca2+) are almost negligiblewiththe selectivity coefficients in the order of 103. Some heavy metal

ions and noble metal ions (Cu2+, Hg2+, Au3+ and Ag+) might cause aweak interference, which may arise from the complex capability ofPAAQ to some transition metal ions. It is discovered that aromaticamine polymers can complex Hg2+ and Ag+ ions as well as Pb2+

ions through the NH2 and NH groups in the chains [2225].

504 X.-G. Li et al. / Talanta 78 (2009) 498505

Table 4Potentiometric selectivity coefficients of the PAAQ salt particles-based Pb2+ISEobtained by the fixed interference method at an interfering ion concentration of1.0103 M.

Interfering ions Selectivity coefficient KPOTPb2+,B

Hg2+ 8.51102Ag+ 6.97102Cu2+ 5.82102Au3+ 4.68102NH4+ 7.59103Na+ 7.18103KBC

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Water sample Added Pb2+ (M) Detected Pb2+ (M) Recovery (%)

Tap water 1 50 48.9 97.8+ 4.27103a2+ 3.55103a2+ 3.09103

evertheless, with the selectivity coefficients in the order of 102,hese metal ions could not make an obvious disturbance to theetermination of Pb2+ ions. Even so, it should be noted that muchore severe interference from Ag+, Cu2+, Hg2+ and some otherations was always found in other Pb2+ ISEs [9,11,14,27,31].

.9. Application of the electrode

The analytical application of the electrode was investigateds an indicator electrode for the potentiometric estimation of.0103 M Pb2+ solution by titrating with two EDTA concentra-ions of 1.0103 M and 1.0102 M. The titration plots presentedn Fig. 9 do not show a standard sigmoid curve, probably due toome interference caused by Na+ ions from disodium EDTA salt.hen Pb2+ solution was titrated by EDTA, the Pb2+ concentrationecreased, accompanyingbyan increased concentrationofNa+ ionsrom EDTA with the titration proceeding. In the later period, themount of Na+ ions can greatly surpass the amount of Pb2+ ions,ausing interference to the potential. Similar titration plots werebserved when the ISEs based on N,N-dibenzyl-1,4,10,13-tetraoxa-,16-diazacyclooctadecane [1], 4-t-butylcalix[4]arene [35], and-t-butylcalix[6]arene [36]were served as indicator electrodes. The

2+toichiometry of Pb EDTA complex can be judged from the sharpreak point in Fig. 9. It must be appreciated that the potentiometricitration of Pb2+ also performs well when the EDTA concentrations in the same order of Pb2+. Under this circumstance, the errorf reading of volume of EDTA solution used could be diminished

ig. 9. Potentiometric titration plots of l.0103 M Pb(NO3)2 solution of 10mLith 1.0102 M and 1.0103 M EDTA, respectively by the ISE based onAAQ:PVC:DOP:NaTPB (1:33:66:1) membrane with the thickness of 90m.Tap water 2 200 205 102.5Rain water 1 50 51.5 103.0Rain water 2 200 195 97.5

to some extent as compared to the titration just consuming ca.1mL titrant of much higher EDTA concentration than the Pb2+ con-centration [3036]. Obviously, the prepared electrode in this studyshows the practical applicability as an indicator electrode in thepotentiometric titration to Pb2+ solutions.

Since no Pb2+ traces in tap water and rainwater samples weredetected by the prepared Pb2+ISE based on PAAQ, the electrodewas applied to detect Pb2+ concentration in the water samplesby standard addition method. To prepare the tap water samples,the collected tap water was first boiled for about 5min to removeCl2. The pH values of the tap water and rainwater samples wereboth adjusted to around 4.5 by HNO3. The five-replicate detectionand recovery results are presented in Table 5. It can be seen thatthe Pb2+ISE performs satisfactorily with the reasonable recovery,indicating the feasibility of the electrode in the detection of Pb2+

concentration in the environmental water like tap and rainwaters.

4. Conclusions

The fine particles of aromatic PAAQ in PVC membrane well per-form as a novel Pb2+ sensing ionophore with strong sensitivity andhigh selectivity because of their intrinsic electroconductivity andstrong affinity towards Pb2+ ions. The Pb2+ ISEs based on the parti-cles rather than traditional ionophores are high-performance ISEswithwideworking range from2.5106 to1.0101 M, lowdetec-tion limit down to 7.76107 M, and long lifetime. The responsetime of shorter than 12 s has approached to the fastest responserate of the existing electrodes. 1% PAAQ particles-containing ISEdemonstratesmuch better performance than the particles-free ISE.In particular, the proposed electrode shows an excellent selectivityfor Pb2+ over alkali, alkaline earth, and heavy metal ions. And thePAAQ ionophore is easily available since the PAAQ particles can befacilely and productively synthesized through chemically oxidativepolymerization of aminoanthraquinone. Thus the prepared elec-trode with a good combination of excellent performance, and longlifetime as well as low cost has a promising application in thedetermination of Pb2+ ions. The electrode can be used to accuratelymonitor the Pb(II) pollution in environmental waters just like tapand rain waters. The results in this report have indicated the greatpotential of other aromatic conducting polymers with strong abil-ity to chelate heavymetal ions to act as new ionophores in ISEs andthus the relevant research should be highly encouraged.

Acknowledgements

The project is supported by the National Natural Science Foun-dation of China (20774065) and the Foundation of Key Laboratoryof Molecular Engineering of Polymers, Fudan University, China.

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Lead(II) ion-selective electrode based on polyaminoanthraquinone particles with intrinsic conductivityIntroductionExperimentalReagentsSynthesis of fine PAAQ microparticles as ionophoreElectrode fabricationPotential measurement

Results and discussionDoping state of PAAQ microparticlesPAAQ microparticle contentNature of plasticizerLipophilic anion additive contentThickness of the membrane containing PAAQ microparticlesEffect of pHResponse and lifetime of the electrodeElectrode selectivityApplication of the electrode

ConclusionsAcknowledgementsReferences