Article

Reference

Transport and accumulation of ferrocene tagged poly(vinyl chloride)

at the buried interfaces of plasticized membrane electrodes

SOHAIL, Manzar, et al.

Abstract

Cyclic voltammetry (CV), synchrotron radiation-X-ray photoelectron spectroscopy (SR-XPS)

and near edge X-ray absorption fine structure (NEXAFS) show that oxidation of ferrocene

tagged PVC induces an accumulation of high molecular weight polymer at the buried interface

between the substrate electrode and the plasticized membrane.

SOHAIL, Manzar, et al. Transport and accumulation of ferrocene tagged poly(vinyl chloride) at

the buried interfaces of plasticized membrane electrodes. The Analyst, 2013, vol. 138, no. 15,

p. 4266

DOI : 10.1039/c3an00464c

Available at:

http://archive-ouverte.unige.ch/unige:31509

Disclaimer: layout of this document may differ from the published version.

1 / 1

Transport and accumulation of ferrocene taggedpoly(vinyl chloride) at the buried interfaces ofplasticized membrane electrodes†

Manzar Sohail,a Roland De Marco,*ab Muhammad Tanzirul Alam,a Marcin Pawlakc

and Eric Bakkerc

Cyclic voltammetry (CV), synchrotron radiation-X-ray photoelectron

spectroscopy (SR-XPS) and near edge X-ray absorption fine structure

(NEXAFS) show that oxidation of ferrocene tagged PVC induces an

accumulation of high molecular weight polymer at the buried

interface between the substrate electrode and the plasticized

membrane.

Electroactive polymers are utilized widely in electrochemistryand materials science, and common features of all thesematerials are their semi-rigid mechanical properties, theirability to carry electrical currents and their ability to be oxidisedor reduced via the application of electric elds.1

Applications of surface modied electrodes in electro-catalysis, electroanalysis, electrochemical sensors and energystorage are enriched by the chemical and electrochemicalmodication of polymer backbones by pendant groups, and thishas been a research focus in recent times.2,3

Plasticized PVC with an added electrolyte is oen used in ion-selective electrodes (ISEs). Semi-rigid polymer electrolytes basedon PVC are attractive because they possess a relatively high ionicconductivity, attractivemechanical properties, compatibility withelectroactive components and a wide electrochemical window.4

Owing to the limited electron exchange capacities of conductivepolymers and a need for doping with redox active mediators,Langmaier et al.5 suggested an attractive alternative of incorpo-rating lipophilic ferrocene derivatives into PVC, providingmembranes with high redox capacities. Recently, Pawlak et al.6

tagged high molecular weight PVC with 6 mol% redox ferrocene

groups to the PVC backbone via click chemistry, showing thatthis polymer may be used as a solid polymer electrolyte in ISEs.The main advantage of tagging of PVC with ferrocene is thesignicant improvement in exchange current densities and redoxcapacities of the FcPVC lm, which is a key factor with solid-contact (SC)-ISEs in electrochemically active modes such as thecoulometric ion sensing of important analytes.6

Conventional wisdom suggests that polymers covalentlymodied with redox active species lead to xed redox centresthat do not migrate under the inuence of moderate electricelds due to the high molecular weights and low diffusion ratesof the polymers. This thesis is given strong credence by the workof Puntener et al.7 who demonstrated that, although PVC withcovalently attached ionophore is not totally immobile in plas-ticized PVC, detrimental transmembrane ion uxes and aconcomitant degradation of potentiometric or electrochemicalresponse is suppressed substantially due to the extremelysluggish diffusion of the large ionophore modied PVC mole-cule. Accordingly, it is generally believed that only small mole-cules can migrate across polymer membranes duringelectrochemical polarization.6–8 It is also thought that a mixedvalency of redox active species is required for transfer of chargeby electron hopping or an electron self-exchange and electro-chemical reactivity in the lm.9

In this study, we have used cyclic voltammetry (CV),synchrotron radiation-X-ray photoelectron spectroscopy (SR-XPS) and near edge X-ray absorption ne structure (NEXAFS) tostudy the interfacial electrochemistry of a FcPVC membrane incontact with an aqueous electrolyte. For the rst time, we haveobserved signicant mass transport of a high molecular weightredox active polymer, even though this polymer comprises asingle redox species. It is also demonstrated that the redoxactive polymer diffused to and accumulated at the buriedinterface between the substrate electrode and plasticizedmembrane at applied potentials higher than the oxidationpotential of FcPVC. This unexpected diffusional and electro-chemical behaviour of single species redox active polymer wasthe focus of the present study.

aFaculty of Science, Health, Education and Engineering, University of the Sunshine

Coast, 90 Sippy Downs Drive, Sippy Downs, Queensland 4556, Australia. E-mail:

rdemarc1@usc.edu.au; Fax: +61 7 5456 5544; Tel: +61 7 5430 2867bDepartment of Chemistry, Curtin University, GPO Box U1987, Perth, Western

Australia 6109, AustraliacDepartment of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-

Ansermet 30, CH-1211 Geneva, Switzerland

† Electronic supplementary information (ESI) available. See DOI:10.1039/c3an00464c

Cite this: Analyst, 2013, 138, 4266

Received 7th March 2013Accepted 17th May 2013

DOI: 10.1039/c3an00464c

www.rsc.org/analyst

4266 | Analyst, 2013, 138, 4266–4269 This journal is ª The Royal Society of Chemistry 2013

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In Fig. 1a, the CV of a FcPVC lm in contact with the aqueous0.1 M KSCN shows the characteristic peaks of the Fe2+/Fe3+

redox couple associated with the ferrocene functionality in thecovalently tagged PVC. It is evident that oxidation of the ferro-cene moiety on the PVC backbone occurs at +150 mV againstthe Ag/AgSCN wire reference electrode. If there were no ion sitesor a lipophilic salt that is incapable of ion-exchange with theaqueous electrolyte such as ETH500 within the membrane,oxidation of ferrocene tagged PVC would allow a deep pene-tration (previous ATR-FTIR studies with plasticized PVCundergoing ion transfer with KSCN revealed a minimum depthof penetration of several microns10,11) of the counter ion due tothe diffusivity of hydrophobic SCN� in the plasticized PVCmembrane; however, this system comprising NaTFPB dopantis expected to maintain electroneutrality by expelling Na+ intothe aqueous electrolyte as FcPVC+ is generated within themembrane. Scan rate dependent CV data (see the inset inFig. 1a) ts the Cottrell equation12 demonstrating that oxidationof FcPVC is a diffusion-controlled process (possibly diffusion ofFcPVC from the membrane surface to the electrode substrate).

Aer washing of the oxidized electrode (chronoamperometryat 500 mV for 180 s) in MQ water to remove excess electrolyte, itwas subjected to SR-XPS and NEXAFS surface analysis aersuccessive argon ion sputtering to monitor the depth distribu-tion of FcPVC. Surprisingly, as compared to a control samplewhere Fe(II) and Cl were homogeneously distributed throughoutthe membrane (results not shown), neither Fe(II), Fe(III) or Clfrom FcPVC were found at the oxidized electrode surface. Asshown in Fig. 2a, the total electron yield (TEY) L-edge Fe(2p)NEXAFS spectra which are representative of the surface states ofFe (in a few atomic layers), revealed a total absence of iron at the

outermost surface of themembrane (0min sputtering) and littleiron down to a depth of 90 mM or 180 min of sputtering.

XPS survey scans of the membrane at different sputteringtimes did not reveal any discernible changes in membranecomposition (e.g., exchange of K+ by Na+) other than the loss ofFe at the surface. The relative heights and positions of the wellresolved doublet of the Fe(2p3/2) and Fe(2p1/2) L-edge NEXAFSpeaks at 708.5 and 711.5 eV as well as 721.5 and 724.5 eV arecharacteristic of the Fe(II)‡ species.13 Furthermore, XPS narrowscans of the C(1s) and O(1s) levels (results not shown) revealedthat only plasticiser was present at the membrane surface. Aer180 minutes of argon ion sputtering or 90 mm in depth, theL-edge Fe(2p) spectra revealed the presence of signicantlevels of iron with a gradual increase in Fe concentration to adepth of 185 mm (close to the nominal membrane thickness ofapproximately 200 mm) or 370 min of sputtering as the buriedinterface of the electrode substrate was exposed.

The uorescence yield (FY) L-edge Fe(2p) NEXAFS presentedin Fig. 2b, which are representative of the material bulkchemistry since the uoresced X-rays have a long escape depthin the solid-state, revealed a presence of Fe at a sub-surface levelof several microns beneath the membrane surface. The FYL-edge or Fe(2p) NEXAFS spectra are consistent with a mixtureof Fe(II) and Fe(III), as evidenced by new and well resolvedshoulders for Fe(III) at 710 eV and 723.1, respectively,13 inaddition to the Fe(II) peaks at 708.5 and 711.5 eV as well as 721.5and 724.5 eV. Importantly, the intensity of Fe L-edge NEXAFSspectra increased as a function of argon ion sputtering or depth,which is consistent with the accumulation of FcPVC to the

Fig. 1 (a) CV of FcPVC at the GC electrode surface in contact with 0.1 M KSCN;(b) CV of a FcPVC membrane doped with ETH 500 in the all-solid-state or sand-wich membrane configuration. Scan rate was 25 mV s�1.

Fig. 2 (a) TEYor surface sensitive Fe L-edge NEXAFS spectra of oxidized FcPVC atdifferent sputtering times; and (b) FYor bulk sensitivity Fe L-edge NEXAFS spectraat the same sputtering times.

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buried interface between the substrate electrode and the plas-ticized membrane.

CV experiments on the sandwich membrane (see Fig. 1b)yielded oxidation and reduction peak shapes similar to those in0.1 KSCN (cf. Fig. 1a). The peak separation and long tails in CVpeaks imply a diffusion-controlled electrochemical reaction.14

The formal potential (E1/2) for ferrocene in FcPVC at a scan rateof 25 mV s�1 is +50 mV against the GC pseudo reference issignicantly lower than E1/2 for ferrocene in the KSCN aqueouselectrolyte (viz., 150 mV vs. the Ag/AgSCN wire reference elec-trode). This variation is ascribable to differences in the refer-ence potential (Ag/AgSCN wire and GC pseudo referenceelectrodes) and omission of the aqueous solution in the lattercase. It is important to note that a fully reduced ferrocene-basedmembrane, as used in this work, may not pass charge effectivelysince oxidation at one electrode must be accompanied by areduction process at the counter electrode, and this would beoptimized if the membrane contained equal amounts ofreduced and oxidized forms. Nevertheless, this experimentaimed to demonstrate that ferrocene oxidation at the glassycarbon buried interface is possible, so the oxidation process inthe sandwich membrane is probably limited by the accompa-nying reduction of dissolved oxygen to hydroxide under neutralconditions as the plasticized membrane system is known tocontain a water layer comprising dissolved oxygen at the buriedinterface. In any event, these sandwich membrane CV resultsprovide unequivocal evidence for a diffusion-controlled oxida-tion or reduction of FcPVC at the buried interface between theGC substrate electrode and the semi-rigid and plasticizedFcPVC membrane, otherwise this electrochemical reactivitywould not be possible in the sandwich membrane.

It is probable that the presence of a mixture of Fe(II) andFe(III) oxidation states is due to the fact that not all of theferrocene moieties are electrochemically accessible for oxida-tion to ferrocenium.6 Narrow XPS scans of the Fe(2p) levels (notshown) also revealed the presence of Fe deep within themembrane and at the buried interface between the electrodesubstrate and plasticized membrane. These unexpected resultsdemonstrate that FcPVC diffuses to and accumulates at theburied interface as the ferrocene moiety in FcPVC is oxidized toferrocenium-PVC during electrolysis. Furthermore, the Cl(2p)

XPS spectra of the FcPVC membrane as a function of depth (seeFig. 3) also revealed an absence of Cl associated with PVC in theoutermost surface layers of the oxidized FcPVC membrane dueto a lack of Cl(2p3/2) and Cl(2p1/2) peaks at 302.9 eV and 304.2 eVin FcPVC, while high amounts of Cl in FcPVC were evident at adepth of 90 mm or 180 minutes of argon ion sputtering.Signicantly, the trend for Cl(2p) with argon ion sputteringmirrored exactly the behaviour of the L-edge Fe NEXAFS spectra,thereby conrming unequivocally that FcPVC had diffused toand accumulated at the buried interface of the FcPVCmembrane electrode. It is important to note that, as FcPVC+ isgenerated electrochemically at the buried glassy carbon surface,this process must be accompanied by the migration of TFPB�

dopant to the buried interface and the expulsion of Na+ at themembrane/electrolyte interface, so as to maintain electro-neutrality in the membrane. The resultant FcPVC+TFPB� ionassociation complex (probably in a highly ordered state) willprobably have a much lower mobility in the plasticizedmembrane phase, and will be captured at the buried interfacedue to its relative immobility.

These unexpected results demonstrate that, if electrolysis isperformed over a sufficient period of time (chronoamperometryfor 180 s) to allow for oxidation or reduction of redox activespecies in the polymer membrane, irrespective of the size of theelectrochemically active molecule, the redox active speciesmigrates to and reacts at the surface of the electrode substrate.This seemingly unexpected result with redox active polymer(molecular weight of FcPVC of 80 kDa) can be likened to themigration of high molecular weight proteins (up to 100 kDa(ref. 15)) across membranes under the inuence of mildelectric elds.15

These outcomes suggest that voltammetric experiments canbe carried out in solvent-free and two-electrode semi-rigidmembranes without a need for satisfying the electron hopping/mixed valency mechanism traditionally assumed with thesesystems.8 From the late 1980's, there have been few papers onrigid and semi-rigid solid-state electromaterials, with thedevelopment of solvent-free systems in voltammetry hamperedby restrictions such as poor ionic conductivities and complextransportation dynamics in solid-state electromaterials. Genget al.16 showed that the amount of plasticiser in the polymerplays a crucial role in regulating the mass transport dynamicsin solid-state polymer electrolytes. In this context, an elevationin plasticiser content increases polymer chain exibility andreduces intermolecular interactions by increasing the freevolume of the polymer membrane. Hence, in a sandwichmembrane conguration, the use of applied potentialsexceeding the formal potentials for oxidation or reduction ofredox active species should induce diffusion and reactivity ofelectroactive species at the surfaces of the electrode substrates.Accordingly, the reduced form is accumulated at the negativeelectrode, while the oxidised form is accumulated at the posi-tive electrode (in this case, ferrocenium accumulates at theanode).

Although recent research by Bakker and co-workers17 withETH500 doped and plasticized FcPVC using a comparablemembrane composition to the present one demonstratedFig. 3 Cl(2p) XPS spectra of FcPVC at different sputtering times.

4268 | Analyst, 2013, 138, 4266–4269 This journal is ª The Royal Society of Chemistry 2013

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surface electrochemical reactivity of the membrane to ferricya-nide in the electrolyte, it is still not clear why FcPVC is under-going a diffusion-controlled oxidation at the GC substrate viamass transport through the membrane, while electrochemicaloxidation of FcPVC at the membrane/electrolyte interface is notobserved. Although the careful study of Bakker and co-workers17

at varying dopant levels of Fc demonstrated that there is suffi-cient electron transportation via electron hopping or electronself-exchange to allow electrons to reach the membrane surfaceand participate in the concomitant electrochemical reactivity ofsolution ferricyanide, we have either failed to detect this surfaceconned electrochemical reactivity due to an insufficient levelof FcPVC at the membrane surface of this concentrationpolarized membrane for detection by SR-XPS and NEXAFS (<1mol%), and/or the signicantly diminished electrical conduc-tivity of the membrane in the absence of ETH500 dopant hassuppressed substantially the electrochemical reactivity of themembrane/electrolyte interface. Furthermore, the work ofBakker and co-workers17 demonstrated that, below a criticalloading of FcPVC (viz., 15 wt% FcPVC with 6 mol% Fc, 75 wt%DOS and 10 wt% ETH500), electron transportation in themembrane and the concomitant surface redox reactivity of themembrane is not observable. Accordingly, this Fc dilutioneffect17 can be harnessed in a singling-out of the diffusion-controlled redox reactivity of the membrane, thereby allowingthe utilization of this new membrane reaction chemistry inpotential applications such as the creation of SC ISEs. In anyevent, further work is needed to elucidate this unusual reactionmechanism.12

This new phenomenon may have important ramicationsfor the preparation of innovative nanoscale and microscaleelectrochemical devices based on layering of electromaterialsvia electrochemically controlled reactions at the buried inter-faces between substrate electrodes and semi-rigid and plasti-cized polymeric membranes. It may be possible toelectrochemically tune the properties and layering of electro-materials in novel electrochemical devices such as SC ISEspossessing an interwoven structure of redox buffering materialand semi-rigid and plasticized polymeric membrane at theburied interface between the parent materials. Also, there isimportant scope to examine a plethora of redox active materialsand polymer matrices in the creation of a new generation of SCISEs, as well as to explore the underlying electrochemical reac-tion mechanism in full detail.

This work was undertaken on the so X-ray spectroscopybeamline at the Australian Synchrotron (AS), Victoria, Australia.We are very grateful to Dr Bruce Cowie and Dr Anton Tadich atthe AS for assistance and advice in the running and

interpretation of SR-XPS and NEXAFS spectra. Also, we thank DrMargaret Marshman at USC for assistance with the collection ofthe SR-XPS and NEXAFS data. We also acknowledge the nan-cial support of the Australian Research Council (ARC) throughprojects LX0776015 and DP0987851 and the Swiss NationalScience Foundation.

Notes and references

‡ The presence of Fe(II) only in the surface layers is due to surface beam damage,as noted elsewhere in the literature with ferrocene-based self assembledmonolayers.18

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6 M. Pawlak, E. Grygolowicz-Pawlak and E. Bakker, Anal.Chem., 2010, 82, 6887.

7 M. Puntener, M. Fibbioli, E. Bakker and E. Pretsch,Electroanalysis, 2002, 14, 1329.

8 E. F. Dalton, N. A. Surridge, J. C. Jernigan, K. O. Wilbourn,J. S. Facci and R. W. Murray, Chem. Phys., 1990, 141, 143.

9 P. J. Kulesza and J. A. Cox, Electroanalysis, 1998, 10, 73.10 R. Kellner, G. Gotzinger, E. Pungor, K. Toth and L. Polos,

Fresenius' J. Anal. Chem., 1984, 319, 839.11 R. Kellner, G. Fischbock, G. Gotzinger, E. Pungor, K. Toth,

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12 M. Sohail, R. De Marco, M. T. Alam, M. Pawlak andE. Bakker, Anal. Chem., in preparation.

13 A. Braun, Q. Chen, D. Flak, G. Fortunato, K. G. Schrantz,M. Gratzel, T. Graule, J. Guo, T.-W. Huang, Z. Liu,A. V. Popelo, K. Sivula, H. Wadati, P. P. Wyss, L. Zhangand J. Zhu, ChemPhysChem, 2012, 13, 2937.

14 S. W. Feldberg, Anal. Chem., 2011, 83, 5851.15 M. Laustsen, US Patent, no. 5437774, 1995.16 L. Geng, R. A. Reed, M. H. Kim, T. T. Wooster, B. N. Oliver,

J. Egekeze, R. T. Kennedy, J. W. Jorgenson, J. F. Parcherand R. W. Murray, J. Am. Chem. Soc., 1989, 111, 1614.

17 M. Pawlak, E. Grygolowicz-Pawlak and E. Bakker, Pure Appl.Chem., 2012, 84, 2045.

18 F. Zheng, V. Perez-Dieste, J. L. McChesney, Y. Y. Luk,N. L. Abbott and F. J. Himpsel, Surf. Sci., 2005, 587, L191.

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