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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
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Transport and accumulation of ferrocene tagged poly(vinyl chloride) at the buried interfaces of plasticized membrane electrodes †
Manzar Sohail,aRoland De Marco,*abMuhammad Tanzirul Alam,aMarcin Pawlakc and Eric Bakkerc
Cyclic voltammetry (CV), synchrotron radiation-X-ray photoelectron spectroscopy (SR-XPS) and near edge X-ray absorptionfine 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 electrochemistry and materials science, and common features of all these materials are their semi-rigid mechanical properties, their ability to carry electrical currents and their ability to be oxidised or reducedviathe application of electricelds.1
Applications of surface modied electrodes in electro- catalysis, electroanalysis, electrochemical sensors and energy storage are enriched by the chemical and electrochemical modication of polymer backbones by pendant groups, and this has 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 based on PVC are attractive because they possess a relatively high ionic conductivity, attractive mechanical properties, compatibility with electroactive components and a wide electrochemical window.4 Owing to the limited electron exchange capacities of conductive polymers and a need for doping with redox active mediators, Langmaieret al.5suggested an attractive alternative of incorpo- rating lipophilic ferrocene derivatives into PVC, providing membranes with high redox capacities. Recently, Pawlaket al.6 tagged high molecular weight PVC with 6 mol% redox ferrocene
groups to the PVC backboneviaclick chemistry, showing that this polymer may be used as a solid polymer electrolyte in ISEs.
The main advantage of tagging of PVC with ferrocene is the signicant improvement in exchange current densities and redox capacities of the FcPVClm, which is a key factor with solid- contact (SC)-ISEs in electrochemically active modes such as the coulometric ion sensing of important analytes.6
Conventional wisdom suggests that polymers covalently modied with redox active species lead toxed redox centres that do not migrate under the inuence of moderate electric
elds due to the high molecular weights and low diffusion rates of the polymers. This thesis is given strong credence by the work of P¨unteneret al.7who demonstrated that, although PVC with covalently attached ionophore is not totally immobile in plas- ticized PVC, detrimental transmembrane ion uxes and a concomitant degradation of potentiometric or electrochemical response is suppressed substantially due to the extremely sluggish diffusion of the large ionophore modied PVC mole- cule. Accordingly, it is generally believed that only small mole- cules can migrate across polymer membranes during electrochemical polarization.6–8It is also thought that a mixed valency of redox active species is required for transfer of charge by electron hopping or an electron self-exchange and electro- chemical reactivity in thelm.9
In this study, we have used cyclic voltammetry (CV), synchrotron radiation-X-ray photoelectron spectroscopy (SR- XPS) and near edge X-ray absorptionne structure (NEXAFS) to study the interfacial electrochemistry of a FcPVC membrane in contact with an aqueous electrolyte. For therst time, we have observed signicant mass transport of a high molecular weight redox active polymer, even though this polymer comprises a single redox species. It is also demonstrated that the redox active polymer diffused to and accumulated at the buried interface between the substrate electrode and plasticized membrane at applied potentials higher than the oxidation potential of FcPVC. This unexpected diffusional and electro- chemical behaviour of single species redox active polymer was the 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:
[email protected]; Fax: +61 7 5456 5544; Tel: +61 7 5430 2867
bDepartment of Chemistry, Curtin University, GPO Box U1987, Perth, Western Australia 6109, Australia
cDepartment 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 2013 Accepted 17th May 2013 DOI: 10.1039/c3an00464c www.rsc.org/analyst
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In Fig. 1a, the CV of a FcPVClm in contact with the aqueous 0.1 M KSCN shows the characteristic peaks of the Fe2+/Fe3+
redox couple associated with the ferrocene functionality in the covalently tagged PVC. It is evident that oxidation of the ferro- cene moiety on the PVC backbone occurs at +150 mV against the Ag/AgSCN wire reference electrode. If there were no ion sites or a lipophilic salt that is incapable of ion-exchange with the aqueous electrolyte such as ETH500 within the membrane, oxidation of ferrocene tagged PVC would allow a deep pene- tration (previous ATR-FTIR studies with plasticized PVC undergoing ion transfer with KSCN revealed a minimum depth of penetration of several microns10,11) of the counter ion due to the diffusivity of hydrophobic SCN in the plasticized PVC membrane; however, this system comprising NaTFPB dopant is expected to maintain electroneutrality by expelling Na+into the aqueous electrolyte as FcPVC+ is generated within the membrane. Scan rate dependent CV data (see the inset in Fig. 1a)ts the Cottrell equation12demonstrating that oxidation of FcPVC is a diffusion-controlled process (possibly diffusion of FcPVC from the membrane surface to the electrode substrate).
Aer washing of the oxidized electrode (chronoamperometry at 500 mV for 180 s) in MQ water to remove excess electrolyte, it was subjected to SR-XPS and NEXAFS surface analysis aer successive argon ion sputtering to monitor the depth distribu- tion of FcPVC. Surprisingly, as compared to a control sample where Fe(II) and Cl were homogeneously distributed throughout the membrane (results not shown), neither Fe(II), Fe(III) or Cl from FcPVC were found at the oxidized electrode surface. As shown in Fig. 2a, the total electron yield (TEY) L-edge Fe(2p) NEXAFS spectra which are representative of the surface states of Fe (in a few atomic layers), revealed a total absence of iron at the
outermost surface of the membrane (0 min sputtering) and little iron down to a depth of 90mM or 180 min of sputtering.
XPS survey scans of the membrane at different sputtering times did not reveal any discernible changes in membrane composition (e.g., exchange of K+by Na+) other than the loss of Fe at the surface. The relative heights and positions of the well resolved doublet of the Fe(2p3/2) and Fe(2p1/2) L-edge NEXAFS peaks at 708.5 and 711.5 eV as well as 721.5 and 724.5 eV are characteristic of the Fe(II)‡species.13Furthermore, XPS narrow scans of the C(1s) and O(1s) levels (results not shown) revealed that only plasticiser was present at the membrane surface. Aer 180 minutes of argon ion sputtering or 90 mm in depth, the L-edge Fe(2p) spectra revealed the presence of signicant levels of iron with a gradual increase in Fe concentration to a depth of 185mm (close to the nominal membrane thickness of approximately 200mm) or 370 min of sputtering as the buried interface of the electrode substrate was exposed.
Theuorescence yield (FY) L-edge Fe(2p) NEXAFS presented in Fig. 2b, which are representative of the material bulk chemistry since theuoresced X-rays have a long escape depth in the solid-state, revealed a presence of Fe at a sub-surface level of several microns beneath the membrane surface. The FY L-edge or Fe(2p) NEXAFS spectra are consistent with a mixture of Fe(II) and Fe(III), as evidenced by new and well resolved shoulders for Fe(III) at 710 eV and 723.1, respectively,13 in addition to the Fe(II) peaks at 708.5 and 711.5 eV as well as 721.5 and 724.5 eV. Importantly, the intensity of Fe L-edge NEXAFS spectra 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) TEY or surface sensitive Fe L-edge NEXAFS spectra of oxidized FcPVC at different sputtering times; and (b) FY or bulk sensitivity Fe L-edge NEXAFS spectra at 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 in 0.1 KSCN (cf.Fig. 1a). The peak separation and long tails in CV peaks imply a diffusion-controlled electrochemical reaction.14 The formal potential (E1/2) for ferrocene in FcPVC at a scan rate of 25 mV s 1 is +50 mV against the GC pseudo reference is signicantly lower thanE1/2for ferrocene in the KSCN aqueous electrolyte (viz., 150 mVvs.the Ag/AgSCN wire reference elec- trode). This variation is ascribable to differences in the refer- ence potential (Ag/AgSCN wire and GC pseudo reference electrodes) and omission of the aqueous solution in the latter case. It is important to note that a fully reduced ferrocene-based membrane, as used in this work, may not pass charge effectively since oxidation at one electrode must be accompanied by a reduction process at the counter electrode, and this would be optimized if the membrane contained equal amounts of reduced and oxidized forms. Nevertheless, this experiment aimed to demonstrate that ferrocene oxidation at the glassy carbon buried interface is possible, so the oxidation process in the sandwich membrane is probably limited by the accompa- nying reduction of dissolved oxygen to hydroxide under neutral conditions as the plasticized membrane system is known to contain a water layer comprising dissolved oxygen at the buried interface. In any event, these sandwich membrane CV results provide unequivocal evidence for a diffusion-controlled oxida- tion or reduction of FcPVC at the buried interface between the GC substrate electrode and the semi-rigid and plasticized FcPVC membrane, otherwise this electrochemical reactivity would not be possible in the sandwich membrane.
It is probable that the presence of a mixture of Fe(II) and Fe(III) oxidation states is due to the fact that not all of the ferrocene moieties are electrochemically accessible for oxida- tion to ferrocenium.6Narrow XPS scans of the Fe(2p) levels (not shown) also revealed the presence of Fe deep within the membrane and at the buried interface between the electrode substrate and plasticized membrane. These unexpected results demonstrate that FcPVC diffuses to and accumulates at the buried interface as the ferrocene moiety in FcPVC is oxidized to ferrocenium-PVC during electrolysis. Furthermore, the Cl(2p)
XPS spectra of the FcPVC membrane as a function of depth (see Fig. 3) also revealed an absence of Cl associated with PVC in the outermost surface layers of the oxidized FcPVC membrane due to a lack of Cl(2p3/2) and Cl(2p1/2) peaks at 302.9 eV and 304.2 eV in FcPVC, while high amounts of Cl in FcPVC were evident at a depth of 90 mm or 180 minutes of argon ion sputtering.
Signicantly, the trend for Cl(2p) with argon ion sputtering mirrored exactly the behaviour of the L-edge Fe NEXAFS spectra, thereby conrming unequivocally that FcPVC had diffused to and accumulated at the buried interface of the FcPVC membrane electrode. It is important to note that, as FcPVC+is generated 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 the membrane/electrolyte interface, so as to maintain electro- neutrality in the membrane. The resultant FcPVC+TFPB ion association complex (probably in a highly ordered state) will probably have a much lower mobility in the plasticized membrane phase, and will be captured at the buried interface due to its relative immobility.
These unexpected results demonstrate that, if electrolysis is performed over a sufficient period of time (chronoamperometry for 180 s) to allow for oxidation or reduction of redox active species in the polymer membrane, irrespective of the size of the electrochemically active molecule, the redox active species migrates 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 the migration of high molecular weight proteins (up to 100 kDa (ref. 15)) across membranes under the inuence of mild electricelds.15
These outcomes suggest that voltammetric experiments can be carried out in solvent-free and two-electrode semi-rigid membranes without a need for satisfying the electron hopping/
mixed valency mechanism traditionally assumed with these systems.8From the late 1980's, there have been few papers on rigid and semi-rigid solid-state electromaterials, with the development of solvent-free systems in voltammetry hampered by restrictions such as poor ionic conductivities and complex transportation dynamics in solid-state electromaterials. Geng et al.16showed that the amount of plasticiser in the polymer plays a crucial role in regulating the mass transport dynamics in solid-state polymer electrolytes. In this context, an elevation in plasticiser content increases polymer chain exibility and reduces intermolecular interactions by increasing the free volume of the polymer membrane. Hence, in a sandwich membrane conguration, the use of applied potentials exceeding the formal potentials for oxidation or reduction of redox active species should induce diffusion and reactivity of electroactive species at the surfaces of the electrode substrates.
Accordingly, the reduced form is accumulated at the negative electrode, while the oxidised form is accumulated at the posi- tive electrode (in this case, ferrocenium accumulates at the anode).
Although recent research by Bakker and co-workers17with ETH500 doped and plasticized FcPVC using a comparable membrane composition to the present one demonstrated
Fig. 3 Cl(2p) XPS spectra of FcPVC at different sputtering times.
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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 substratevia mass transport through the membrane, while electrochemical oxidation of FcPVC at the membrane/electrolyte interface is not observed. Although the careful study of Bakker and co-workers17 at varying dopant levels of Fc demonstrated that there is suffi- cient electron transportationviaelectron hopping or electron self-exchange to allow electrons to reach the membrane surface and participate in the concomitant electrochemical reactivity of solution ferricyanide, we have either failed to detect this surface conned electrochemical reactivity due to an insufficient level of FcPVC at the membrane surface of this concentration polarized membrane for detection by SR-XPS and NEXAFS (<1 mol%), and/or the signicantly diminished electrical conduc- tivity of the membrane in the absence of ETH500 dopant has suppressed substantially the electrochemical reactivity of the membrane/electrolyte interface. Furthermore, the work of Bakker and co-workers17 demonstrated that, below a critical loading of FcPVC (viz., 15 wt% FcPVC with 6 mol% Fc, 75 wt%
DOS and 10 wt% ETH500), electron transportation in the membrane and the concomitant surface redox reactivity of the membrane is not observable. Accordingly, this Fc dilution effect17 can be harnessed in a singling-out of the diffusion- controlled redox reactivity of the membrane, thereby allowing the utilization of this new membrane reaction chemistry in potential applications such as the creation of SC ISEs. In any event, further work is needed to elucidate this unusual reaction mechanism.12
This new phenomenon may have important ramications for the preparation of innovative nanoscale and microscale electrochemical devices based on layering of electromaterials viaelectrochemically controlled reactions at the buried inter- faces between substrate electrodes and semi-rigid and plasti- cized polymeric membranes. It may be possible to electrochemically tune the properties and layering of electro- materials in novel electrochemical devices such as SC ISEs possessing an interwoven structure of redox buffering material and semi-rigid and plasticized polymeric membrane at the buried interface between the parent materials. Also, there is important scope to examine a plethora of redox active materials and polymer matrices in the creation of a new generation of SC ISEs, as well as to explore the underlying electrochemical reac- tion mechanism in full detail.
This work was undertaken on the so X-ray spectroscopy beamline at the Australian Synchrotron (AS), Victoria, Australia.
We are very grateful to Dr Bruce Cowie and Dr Anton Tadich at the AS for assistance and advice in the running and
interpretation of SR-XPS and NEXAFS spectra. Also, we thank Dr Margaret Marshman at USC for assistance with the collection of the SR-XPS and NEXAFS data. We also acknowledge thenan- cial support of the Australian Research Council (ARC) through projects LX0776015 and DP0987851 and the Swiss National Science 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 assembled monolayers.18
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