Anish Khan*,Aftab Aslam Parwaz KhanMahmoud A.Hussein,Bernaurdshaw Neppolian,Abdullah M.Asiri

1Chemistry Department,King Abdulaziz University,Jeddah 21589,Saudi Arabia

2Center of Excellence for Advanced Materials Research,King Abdulaziz University,P.O.Box 80203,Jeddah 21589,Saudi Arabia

3Polymer Chemistry Laboratory,Chemistry Department,Faculty of Science,Assiut University,71516 Assiut,Egypt

4SRM Research Institute,SRM University,Kattankulathur,Chennai 603203,Tamil Nadu,India

Keywords:Conducting polymers Electrochemistry Ion-exchangers Electrode Cd2+Poly(2-anisidine)

A B S T R A C T Itisnecessarytosynthesizenewmaterialfortheadvancementsofthetechnology.Inthisstudy,newandnovelpoly(2-anisidine)@zirconium tungstate(P2A/ZrW2O8)was synthesized by simple so-gel method.Physicochemical characterization of P2A/ZrW2O8was done by thermogravimetric analysis(TGA),scanning electron microscopy(SEM),X-ray powder diffraction(XRD),Fourier transform infrared spectroscopy(FTIR),ion exchange and simultaneous four probe dc conductivity studies.The conductivity study revealed its highly semiconducting nature,in the range of 10?1-10?2S·cm?1.Ion-exchange capabilities of the composite make it applicable for cation-exchange studies.The result of distribution studies(Kd)revealed its selectivity towards Cd2+compared to other metal ions.This property of the composite was utilized for designing Cd2+selective membrane electrode.Several important physical parameters of the ion-selective electrode were determined,such as Nernstian slope(32.32 mV·decade?1),working pH range was 2.0-4.0 and response time was found ～17 s.The analytical utility of this wave like composite membrane electrode was as,indicator electrode in various potentiometric titrations.

1.Introduction

Even though,there are number of electrically conducting polymer systems and their broad study has been done,commercialization of the new,improved materials still needed.The reason is that there are property boundaries,such as limited mechanical strength,high cost and poor flexibility that have prevented these materials from being strongly commercialized.To improve the conducting polymers for the best use for the scientific community in the present form so many efforts have been done over the past few years.

Conducting and semiconducting composites have opened a wide range of applications in various fields,such as light-emitting displays as well as photovoltaic.For instance,polythiophene-quantum dot(CdSe nanorod)organic-inorganic composite with tailored ligands was described for improving the morphology of the nanocomposite[1].Similarly,composite of a conducting polymer and metal nanoparticles have been suggested to provide exciting possibilities for sensing of hazardous gas and catalysis[2].The synthetic methodology of the nanocomposite is dependent on the physical as well as the chemical nature of host polymer and guest inorganic filler.Guest should be soluble in some solvent system either it is miscible or not in the water[3,4].Polymer composite has also been used for the separation of heavy metal ions by various methods,including direct or indirect separation or in the form of ion exchange[5-8].

Sensor technology particularly,their design and preparation,has been a research topic for several decades[9,10].To be a new competitive sensor,it is necessary to improve the transfer rate of ions by modifying the membrane with suitable functionalities[11,12].In this regard,hetropolyacids(HPAc)have received much attention in the last two decades[13-16].However,the high solubility of hetropolyacids leads to limitations in their use in aqueous media because leaching of polycations occurs through the membrane and results in a drop of overall membrane electrochemical performance.However,if the hetropolyacids are entrapped in a polymer,it leads to minimization of the leaching of cations due to the interaction of the polymer with the hetropolyacids;in addition poor solubility of the conducting polymer adds to the stability by keeping the integrity of the individual components simultaneously contributing its own nature of the polymer to the composite.

In this regard,we prepared P2A/ZrW2O8nanocomposite with the involvement of 2-anisidine monomer and zirconium tungstate.This composite was characterized and ion-exchange studies were done.On the basis of ion-exchange studies it was used for the fabrication of a membrane by incorporating it in the form of an electro-active agent with a polyvinylchloride(PVC)binder for use as a selective indicator electrode for the identification and determination of heavy metal ions in solutions.

2.Materials and Method

2.1.Reagents and instruments

Zirconium chloride was purchased from Chemical Drug House Pvt.Ltd.,India(CDH);2-anisidine was purchased Merck Chemicals Ltd.UK,and sodium tungstate was purchased from Loba Chemie Pvt.Ltd.,India.Mercuric nitrate,sodium nitrate,hydrochloric acid,lead nitrate,potassium nitrate and sulfuric acid were all of analytical grade.The pH and potential(EMF)were all measured with a pH/ion meter(Orion,USA,model 720A)connected to a reference electrode(calomel electrode).Distillation water(18.6 MΩ·cm?1)was prepared by a Millipore Milli-Q Plus Ultra Pure Water Purifier,USA and all solutions were prepared in the standard method by dilution as with distilled water.For surface study of the prepared materials,a scanning electron microscope(SEM)JEOL JSM 6300,Japan was used that was linked with an Oxford-Isis(UK)X-ray microanalysis system(EDX).For the analysis of the various functional groups we used an FTIR spectrophotometer model 2000(PerkinElmer,Inc.USA).

2.2.Preparation of P2A/ZrW2O8composite

A sol-gel method was adopted for the preparation of the P2A/ZrW2O8nanocomposite.P2A was obtained by oxidation of 2-anisidine with ammonium persulfate in a conical flask,having a magnetic bar,at 0-5°C temperature while being held overnight in a refrigerator.An inorganic precipitate of ZrW2O8was prepared from zirconium nitrate(0.1 mol·L?1in 1 mol·L?1HCl)and sodium tungstate(0.1 mol·L?1aq.medium)in several volume ratios in a separateconicalflaskandcontinuedstirringwasdoneformore2handafter that it was kept overnight for digestion.The polymer solution and ppt were mixed together by stirring under the same condition(0-5°C temperature),for 24 h.The resultant of the above reaction was purified by precipitation in methanol.After digestion,it was filtered and repeatedly washed by CH3OH,HCl(0.1 mol·L?1),Demineralized water(DMW)and acetone.The composite obtained,having a greenish color was vacuum dried for 24 h at room temperature.

The composite obtained above was converted to the H+form by dippingin1mol·L?1HNO3solutionfor24handreplacingthesupernatant liquid intermittently.The excess of the acid that remained following the H+form conversion of the composite was removed by washing with DMW completely and dried at 40°C in an oven and later it was checked for its ion-exchange capacity(1.8 meq·g?1)and stored for further studies.

2.3.Electrical conductivity measurements

The composite was doped by treating with HCl.1 mol·L?1aqueous HCl was used for the treatment of the composite,followed by washing with DMW to remove excess acid.It was then dried in an oven overnight at about 40-50°C.200 mg portions of the material were ground by a mortar and pestle and made into a pellet by applying hydraulic pressure for 25 min at 25 kN,and the thickness of the pellets was measured by a screw gauge.A DC electrical conductivity measuring technique was used for the measurement of the conductivity of the pressed pellet with increasing temperature from 40 to 200°C with the help of a four probe machine(SES Instrument Pvt.Ltd.India).

2.4.Preparation of P2A/ZrW2O8composite membrane

The method for the preparation of the membrane was discussed in our previous studies[17].To study the optimum membrane composition it was necessary to prepare membranes having different amounts of composite and PVC with 10 ml Tetrahydrofuran(THF).In this study,we prepare three different membranes,in every case we took 0.1,0.2 and 0.3 g amount of P2A/ZrW2O8separately with fixed amount of PVC(0.2 g)and THF(10 ml).First,PVC was dissolved in THF solvent properly and then P2A/ZrW2O8was mixed to make a suspension.This suspension was poured into a 10 cm length and 5 mm wide circular glasstubeandleftforevaporationovernight.Threesheetshavingdifferent thicknesses of 0.16,0.19 and 0.23 mm were prepared by this way with the desired area of the membrane being cut using a sharp edge blade.

2.5.Characterization of membrane

The membrane properties were characterized in terms of three parameters,water content,porosity and thickness[18,19].

2.5.1.Water content(%,total wet mass)

In this case,themembranes werefirst dipped into thewater to elute any diffusible salts,wrapped in filter paper to dry any surface moisture and at once weighed,they were then further dried over P2O5for 24 h in vacuumat45°Candthenwatercontentwascalculatedbythefollowing Eq.(1)

where Wd=mass of the dry membrane and Ww=mass of the soaked/wet membrane.

2.5.2.Porosity

The volume of water per unit membrane volume inside the cavity,i.e.,the porosity(ε)was determined:

A=area of the membrane,L=thickness of the membrane and ρw=density of water.

2.5.3.Thickness and swelling

Theaveragethicknessofthemembranewasmeasuredwiththehelp of a screw gauge.The swelling of the membrane is the difference in the average thicknesses of the wet and dry membranes;it was obtained after being equilibrated in 1 mol·L?1NaCl over 24 h.

2.6.Fabrication of ion-selective membrane electrodes

The selected membrane sheet,on the basis of the best membrane performance,was 0.16 mm thick;it was cut in the form of a disc and mounted by araldite at the lower end of a Pyrex glass tube having OD 0.8cmandID0.6cmandthenleftfor24htodry.0.1mol·L?1cadmium nitrate solution was filled in the tube and,for electrical contact,a saturated calomel electrode was inserted within the tube while another was used as an external reference.The performance of the electrode was determined on the basis of the following parameters:lower detection limit,electrode response curve,working pH range and response time.Standard solutions of different molarities of Cd2+ranging from 10?1mol·L?1to 1?10mol·L?1were prepared for the determination of the electrode response.For the determination of potential both an external reference and the membrane electrode were plugged into a potentiometer and the reading was recorded.

Before taking the potential of the electrode it was necessary to soak,(even it was stored in Cd(NO3)2)the electrode membrane in a higher concentration solution of Cd(NO3)2for 5-7 days to maintain equilibrium and then 1 h before use,it was soaked in 0.1 mol·L?1Cd(NO3)2.The electrode must also be stored in 0.1 mol·L?1Cd(NO3)2.The results of the potential obtained were plotted against the respective ion concentration in the aqueous solution.pH measurements were done on the Cd(NO3)2solutions,over the 1-10 pH range having constant ion concentration(1× 10?3Mb Cd(NO3)2)and at each pH potential was recorded and plotted against each other.For response time determination,themembrane potentialwasfirstrecordedafterimmersion in a 1 × 10?3mol·L?1solution then it was shifted to a 10 times higher concentration and then potentials were immediately recorded(1 × 10?2mol·L?1)and plotted time vs.potential as recorded.

3.Results and Discussions

The nanocomposite,P2A/ZrW2O8wasprepared bypolymerizingthe conducting monomer o-anisidine as matrix and adding inorganic ZrW2O8as a nanofiller.This was prepared with the belief that it works with thecombined properties of electrical conduction with high surface to volume ratio.The reaction for the synthesis is explained in Fig.1.

3.1.Characterization

SEM images,at several magnifications of P2A/ZrW2O8nanocomposite are shown in Fig.2(A),(B),(C)and(D).Large smooth flakes areseen in the images of Fig.2(A)and(B)at low magnifications about 10 μm while at higher resolution in Fig.2(C)and(D)there are wave like morphologies of the composite embedded in the main body of the material that represent the wave composite morphology of the current material.The change of the morphology of the composite is because of the interaction of the inorganic matrices with the organic in close combination.

IntheTGAcurve(Fig.2)oftheP2A/ZrW2O8,theinitialmasslosswas observed at temperatures up to 125°C that is loss of mass equal to 9.5%for the removal of surface water reside in the composite in the form of hydrated water molecule[20].The second mass loss occurred from 150 to 330°C in the TGA measurement;the loss of mass was about 10%and is attributed to the slow decomposition of the P2A of the material.Degradation of theinorganic contents ofthecomposite,on the basis of thecurve,took place at 400°C onwards.In theDTGA curve only two largepeakswere found,atabout～100 and 250 °C,representingthe exothermic reactions in the composite(Fig.3).

Fig.1.Reaction mechanism for the preparation of the P2A/ZrW2O8composite.

Fig.2.ScanningelectronmicroscopicimagesofP2A/ZrW2O8nanocompositeatdifferentmagnificationsshowingflakesatlowresolutionandwavelikemorphologyathighmagnifications.

IntheFTIRspectraofthecomposite(Fig.4),therewasalargeintensity peak in the range of 3100-3200 cm?1representing the characteristic aromatic C--H stretching vibrations and also a peak in the spectra was at 1637 cm?1for the C＝C aromatic groups[21,22].The absorption in this region(600-1500 cm?1)is attributed to the presence of the bipolaron(Supplementarymaterial)inthedopedP2A.Thefingerprintregionof theP2Aisaround 600-1500 cm?1.The peaks foundin thisregion are attributed to the ring stretching(aromatic),the C--H in plane deformation P2A and out of plane deformation for C--H 786 cm?1[23,24].Polaron-polaron interaction is due to the presence of planar P2AandZrW2O8moietyandmay result in theshiftingofthe ringdeformation mode.This information showed that some of ZrW2O8does not completely interact with the bulky P2A molecules.

Fig.3.TGA spectra of P2A/ZrW2O8up to 1000°C in the nitrogen atmosphere.

Fig.4.Fourier transform infra-red spectroscopic spectrum of the P2A/ZrW2O8 nanocomposite.

Todeterminethechemicalandelectronicstatesaswellastoknowthe elemental composition and empirical formula,the X-ray photoelectron spectroscopy(XPS)method was used.It is a quantitative method for material.In this method,irradiation of the materials is done by X-rays and the number of electrons that pass through the material,about 1.0 to 10.0 nm thick,is analyzed in terms of their kinetic energy.In this study we analyzed the P2A/ZrW2O8composite by XPS measurement for the presence of C,N,Zr,O and W elements.XPS was used to determine the chemical state of the P2A/ZrW2O8nanocomposite.The full XPS spectrum of P2A/ZrW2O8nanocomposite is presented in Fig.5(a).In Fig.5(b),the spin-orbit peak of the W4f7/2binding energy for the samples appeared at around 35.9 eV,which is in good agreement with the reference data for tungsten oxide[25].The C1s spectrum showed peaks at 285.2 eV,and 291 eV presented in Fig.5(c).The C1s peaks indicated the presence of carbon in the nanocomposite[26].In Fig.5(d),the binding energy of N1swasat399.3and408eVinthenanocompositemaybeformultiphase[27].TheO1sspectrumshowedapeakat532.6eVinFig.5(e).Thepeakat 532.6 eV is assigned to oxygen,which indicated the presence of oxygen(i.e.,O2?)in the P2A/ZrW2O8nanocomposite[28].In Fig.5(f),the presence of Zr in the form of 3d5/2 was shifted to around 185 and 190 eV in the composite.Therefore,it was concluded that the P2A/ZrW2O8nanocomposite had these five elements.

3.2.Electrical conducting behavior of P2A/ZrW2O8wave like nanocomposite

Fig.5.X-rayphotoelectron spectroscopyoftheP2A/ZrW2O8nanocomposites.(a)FullspectrumofP2A/ZrW2O8nanocomposites,(b)W4f7/2level,(c)C1s level,(d)N1s level,(e)O1slevel,and(f)Zr3d5/2level;acquired with MgKα1radiation.The X-ray beam-spot size was 300.0 μm;pass-energy was 200.0 eV and pressures less than 10?8Torr.

The conductivity of the P2A/ZrW2O8composite pellets was determined by a four probe technique used for semiconductors[29].It is a more satisfactory method compared to the two probe(conventional method)conductivity measurement method.The current and voltage,asmeasuredbythefourprobeson thecompositematerialforthedetermination of electrical conductivity,were fitted for thefollowingEq.(3):

whereρisthecorrectedresistivity(Ω·cm),ρ0=uncorrectedresistivity(Ω·cm),andG7(W/S)isthecorrectionfactorusedforthecaseofanonconductingbottomsurface,whichisafunctionofW,thethicknessofthe sample under test(cm)and S,the probe spacing(cm),i.e.

where I is the current(A),V is the voltage(V)and σ=electrical conductivity(S·cm?1).

The protonic conductivity,due to the presence of the moisture,needs to be reduced from the consideration as the measurement was doneinambientconditionsandthecompositepelletsweredriedbefore taking the measurement of the samples.

In the present study the conductivity of the composite is because of the presence of the conducting polymer,P2A.The conducting properties of a material having more than one phase depend on the percolation behavior.In our composite the conductivity was because of the oxidized form of the organic conducting phase of P2A and it was excessively(oxidized form)maintained by its counter ion P2A/ZrW2O8.The change in conductivity((σ))with increasing temperature(35 to 200°C)of composite(prepared with 7 vol%of 2A)was investigated.The conductivity of composite increased with increasing temperature from 10?2to 10?1S·cm?1.Plot of the lg(conductivity)versus 1000/T(Fig.6)shows an Arrhenius behavior similar to other semiconductors[30].

Fig.6.Arrhenius plot for the lg(conductivity)of polymer-ZrW2O8composite for increasing temperature at 25 °C to 200 °C.

The conductivity of the P2A/ZrW2O8was higher than for its constituents in the literature[31].This may be because of the property of electron donation by the inorganic ZrW2O8.Because of the‘thermal activated behavior'working in the composite,as the temperature was increased the conductivity increased also[32].It may also be possible that the exchange of charge transfer took place between the dopant and the composite chains as the temperature increases[33].Because of thermal curing effect,the conjugation length of the polymer chain increases,resulting in chain alignment of the composite and ultimately it leads to the increase of the conductivity.

3.3.Cd2+ion-selective membrane electrode

TheP2A/ZrW2O8wasusedforthepreparationofaheterogeneousISE.It is necessary for the ISE that it has appropriate physicochemical properties and electroactive material properties and composition in the membrane determines the selectivity and sensitivity of the electrode.A number of samples of the P2A/ZrW2O8membrane were prepared with different ratios of PVC and composite and checked for the uniformity,cracks,mechanical strength,composite distribution,thickness,etc.On the basis of these properties membrane having 33 wt%PVC and M-1 mm thick was suitable for further studies as shown in Table 1.

Table 1 Characterization of the membrane of P2A/ZrW2O8composite

The heterogeneous Cd(II)ion selective membrane electrode obtained by the above process was checked for response and showed a linear range from ～1 × 10?1mol·L?1to 5 × 10?5mol·L?1for Cd2+(Fig.6).Suitable concentrations were chosen from the sloping portion of the linear curve.The limit of detection that was found was 5 × 10?5mol·L?1,obtained as the intersection of the extrapolated segments of the obtained graph[34].So we can conclude the working concentration for this electrode for Cd(II)cations having the Nernstian slope of 32.32 mV·decade?1change of Cd2+concentration[35]was 1 × 10?1mol·L?1to 5 × 10?5mol·L?1as is shown in Fig.7.

It was also necessary to determine the potential change for different pH with fixed(1 × 10?2mol·L?1)metal concentration solutions.On the basis of the results(Fig.8)it is clear that within the pH range of 2.0-4.0 the potential was stable;above pH 4 it oscillates.This is the working pH range for this membrane.Promptness of the response[36],the time at which the potential becomes stable,was also determined for this membrane and it was found to be～17 s.Life span of the membrane was found to be 65 days without any change in the potential by more than±1 mV per concentration decade without any notable potential drift.However,it was necessary to re-equilibrate the electrode when we noticed any drift in the potential in the metal solution of 0.1 mol·L?1after four to five days.

A mixed solution method was used to determine the selectivity coefficients,forvarious differingcations fortheCd(II)ionselective P2A/ZrW2O8composite membrane electrode[37].The selectivity coef ficientindicateshowmuchaforeignion(Mn+)interactswiththedetermination of the primary ion(Cd(II))response on P2A/ZrW2O8membrane electrode.On the basis of the results of the selectivity coefficient(Table 2)it is obvious that this electrode was selective for Cd(II)in the presence of interfering cations.Thus,this study shows that this electrode could be used as an indicator electrode for the titration of Cd(II)against ethylenediaminetetraacetic acid(EDTA).During titration a potential decrease took place as we added EDTA into the salt solution due to the metal chelate complexes of the metal ions with EDTA.The result shows that the amount of Cd(II)ions can be simply and easily determined by this titration curve favoring that the titration curve should be a sharp rise at the point of equivalence(Fig.9).According to the theory if the slope value is 32.32 mV·decade?1close to the standard Nernstian slope((29±3)mV·decade?1)value,as in the current study,the electrode is more accurate than the other having a deviated value[38-42].

Fig.7.Calibration curve for the Cd2+ion selective P2A/ZrW2O8membrane indicator electrode at different concentrations;the inset shows the slope of the curve.

Fig.8.Effect of metal solution on the pH of the cadmium ion selective P2A/ZrW2O8 membrane indicator electrode from pH 1 to 13.

Table 2 The selectivity coefficients()of various interfering cations(Mn+)for the P2A/ZrW2O8ion-selective membrane electrode electrodes

Table 2 The selectivity coefficients()of various interfering cations(Mn+)for the P2A/ZrW2O8ion-selective membrane electrode electrodes

Metal ions Selectivity coefficient values Mg2+ 0.003 Sr2+ 0.008 Zn2+ 0.011 Hg2+ 0.011 Pd2+ 0.032 Cu2+ 0.041 Ca2+ 0.041

Fig.9.EDTA titration curve for the determination of Cd2+ions in the real samples.

4.Conclusions

WesynthesizedP2A/ZrW2O8organic-inorganicnanocompositebya very simple sol-gel method.The P2A sol of the aniline polymer derivative was prepared by oxidation in the presence of APS while the gel of theinorganic ZrW2O8wasobtained by theinorganic reaction of sodium tungstate and zirconium nitrate in the appropriate volume concentration.The TGA studies showed higher thermal stability of the composite thantheP2A.Electricalconductivitystudieswerealsodoneonthecompositebyafourprobemethodanditwasfoundtobeasemiconductorin nature.The conductivity of composite increased with increasing temperature from 10?2to 10?1S·cm?1from 35 °C to 200 °C.Due to the cation-exchange nature of the composite,it was applied for the preparation of an ion selective membrane electrode(ISE)for Cd2+.P2A/ZrW2O8ion selective membrane electrodes had a very near Nernstian slope of 32.32 mV·decade?1change of selective metal ion(Cd2+)concentration,good response time(～17 s),high selectivity range,and good life span(65 days)with a wide working pH range,2.0-4.0.As an improvement,various physical properties were found in the composite in comparison to the individual component,suggesting its application potential without chemically hampering the polymer.

Acknowledgments

We are thankful to the Centre of Excellence for Advanced Materials Research,KingAbdulaziz University,SaudiArabia for theability to carry out the experiments to this work.