Electrochimica Acta 54 (2008) 868–875

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Electrochimica Acta

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Immobilisation of the polyoxometallate cluster,K6NaH[Sb2W20Fe2O70(H2O)6]·29H2O, in a polypyrrole film

Kevin Foster, Lihua Bi, Timothy McCormac ∗,1

Centre for Research in Electroanalytical Technology “CREATE”, Department of Science, Institute of Technology Tallaght Dublin, Dublin 24, Ireland

a r t i c l e i n f o

Article history:Received 1 November 2007Received in revised form 11 March 2008Accepted 15 March 2008Available online 30 March 2008

Keywords:PolyoxometallateKrebsPolypyrroleCation insertion–expulsion

a b s t r a c t

The conducting polymer, polypyrrole was successfully employed to surface immobilise the Krebstype polyoxometallate, K6NaH[Sb2W20Fe2O70(H2O)6]·29H2O, through cyclic voltammetry. The resultingK6NaH[Sb2W20Fe2O70(H2O)6]·29H2O doped polypyrrole films were found to exhibit the redox activityassociated with both the Fe(III) and W–O redox centres within the POM. The former was found to besituated at potentials within the conducting part of the polymer whilst the latter redox process was inthe insulating domain of the polypyrrole. The Fe(III/II) POM based redox process was found to be pHdependent. Upon redox switching of the polymer through this Fe(III/II) redox system, a process of cationinsertion and expulsion into the polypyrrole matrix was observed with both the nature and concentrationof the supporting electrolyte having a substantial effect upon the potential values at which this processoccurred. This cation insertion–expulsion process was investigated through the application of the Elec-trochemical Quartz Crystal Microbalance (EQCM) technique. The result of which indicated that it wasboth the passage of alkali metal cations and protons from the background electrolyte into the polymerfilm which maintained electroneutrality within the POM polypyrrole films upon redox switching through

the Fe(III/II) redox system. Finally the POM doped polypyrrole films exhibited a clear catalytic propertytowards the reduction of hydrogen peroxide (H2O2) with a sensitivity of 131.8 (±3.5) �A cm−2/mM with

al pH

1

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a LOD of 16.6 �M at neutr

. Introduction

Polyoxometallates (POMs) possess many attractive propertieshich have allowed them to yield applications across a broad spec-

rum, from medicinal chemistry to applications within catalysis1–10]. The ability to surface immobilise these POMs onto a range ofurfaces such as carbon, platinum, indium tin oxide and silicon is ofmportance in order that their interesting solution electrochemicalnd photophysical properties can be maintained in the immobilisedtate. To this end, researchers have utilised various immobilisationtrategies for POMs onto the traditional electrode materials, such as,elf assembly, electrodeposition, and conducting polymer entrap-ent [11–21]. Our group has been concerned with the employment

f conducting polymer films as suitable immobilisation templates

or POMs for some time. Various groups through the past fewecades have employed various conducting polymers, such as,olypyrrole and polyaniline [12,22,23]. These POM doped con-ucting polymer films have then been utilised in electrocatalysis

∗ Corresponding author. Tel.: +353 1 4042814; fax: +353 1 4042700.E-mail address: tim.mccormac@ittdublin.ie (T. McCormac).

1 ISE member.

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013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2008.03.058

.© 2008 Elsevier Ltd. All rights reserved.

17,18,24], as electrochemical supercapacitors [25], molecular con-uctors [26], for the synthesis of “micro-morphological materials”27] and catalysis [28–31]. The application of electrochemical andurface based techniques has been applied to elucidate the proper-ies of these surface bound materials. The Electrochemical Quartzrystal Microbalance (EQCM) has been specifically employed toain some insight into the associated mass changes which occururing the redox switching of such POM – conducting polymerybrid materials [19,32,33] with particular information regardinghe role of both the solvent and electrolyte being attained. In theast decade researchers have successfully expanded the range ofossible POMs. Krebs co-workers [34] were the first to synthesisend characterise the transition metal substituted “heteropolymeta-ate clusters” of SbIII, namely K6NaH[Sb2W20Fe2O70(H2O)6]·29H2O,

hich contains the ˇ-B-SbW9O33 building unit. The main impetusor investigating this Krebs type POM has been the well known cat-lytic properties of Krebs type POMs but also the general lack inhe literature of detailed electrochemical and surface modification

tudies with such POMs. In this article we present results attainedor the immobilisation of the Fe(III) substituted Krebs POM within aonducting polypyrrole film. The resulting polymeric film exhibitedlear electrochemical redox activity associated with the Fe(III) cen-res of the POM, which were found to be located within a potential

imica Acta 54 (2008) 868–875 869

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K. Foster et al. / Electroch

egion where the polypyrrole remains in the doped or conduct-ng state. The role of both the nature and the concentration of theackground electrolyte upon the redox switching of this hybridaterial was investigated along with its possible application

owards the electrocatalytic reduction of H2O2.

. Experimental

.1. Materials

K6NaH [Sb2W20Fe2O70(H2O)6]·29H2O was synthesised andharacterised according to the literature [34]. Elemental analysisesults were: calcd (found) H 1.184 (1.188); K 3.882 (3.890); Na 0.3800.384); Fe 1.848 (1.852). All chemicals were of reagent grade andsed as received unless otherwise stated. Pyrrole (99% ultra pure)as purchased from Acros Organics and purified before use by pass-

ng through a neutral Al2O3 column to obtain a colourless liquid.ydrogen peroxide (H2O2) was obtained as a 30% solution. Aqueous

olutions were prepared using deionised water.

.2. Apparatus and procedures

Electronic spectra were recorded on a Shimadzu UV-160AV–visible recording Spectrophotometer using quartz cells of path

ength 10 mm. NMR spectra were recorded at room temperature onJNM LA FT NMR system Joel 300 MHz apparatus. Scanning Elec-

ron Microscopy (SEM) was performed using a Hitachi S-3000Nystem. For SEM investigations films were formed on 3 mm radiusarbon disks. The oscillating frequency of the EQCM and currentensity were recorded by a personal computer interfaced to auartz crystal analyser (Model CHI 400A EQCM, f0 = 8 MHz) and aodel 400 CHI potentiostat.A CHI 660A potentiostat was employed for all electrochemi-

al experiments. Electrochemical experiments were performed insingle compartment three-electrode cell. For electrochemistry

arried out in organic solvent the reference electrode employedas a silver wire in contact with an acetonitrile solution of AgNO3

0.01 M) and 0.1 M of the same supporting electrolyte as employedn the cell. For electrochemistry carried out in aqueous solu-ion the reference employed was a Ag/AgCl (3 M KCl) electrode.he working macroelectrode, vitreous carbon (d = 3 mm), was pol-shed with 1.0, 0.3 and 0.05 �m alumina, sonicated in deionisedater for 5 min after each grade, after which it was washed

horoughly with deionised water and acetone prior to use. The aux-liary electrode material was a platinum wire for all experiments.ll electrolyte solutions were degassed with argon for 15 minrior to the electrochemical investigations at room temperature tonsure an oxygen free environment. Conducting polypyrrole dopedith [Sb2W20Fe2O70(H2O)6]8− were electrochemically deposited

rom a deionised aqueous solution of 25 mM pyrrole and 1 mMSb2W20Fe2O70(H2O)6]8− by chronocoulometry and cyclic voltam-

etry. Subsequent studies were carried out and it was found thatlectrosynthesis mode employed (potentiostatic or potentiody-amic) has no real effect on the electrochemical behaviour andtability of the polymer.

. Results and discussion

.1. Solution phase electrochemical behaviour

The solution phase redox behaviour of the K6NaHSb2W20Fe2O70(H2O)6]·29H2O POM is shown in Fig. 1(a).What islearly seen are three redox couples with E1/2 values of + +0.227,0.410 and −0.519 V (versus Ag/AgCl). The latter two are associated

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ig. 1. (a) Cyclic voltammogram of a 0.5 mM of [Sb2W20Fe2O70(H2O)6] in pH 2uffer (0.5 M Na2SO4 + 0.5 M H2SO4) at a bare carbon electrode (A = 0.0707 cm2). Scanate = 10 mV s−1. (b) Cyclic voltammogram of 0.5 mM of [Sb2W20Fe2O70(H2O)6]8− inH 5 buffer at a bare carbon electrode (A = 0.0707 cm2). Scan rate = 10 mVs−1.

ith the redox switching of the tungsten-oxo framework and areelieved to be two and four electron processes, respectively. Theuasi-reversible couple centred around +0.227 V is believed toe a bielectronic redox process associated with the two Fe(III)entres within the POM. A common characteristic of POMs ishat their electrochemical behaviour is influenced by solutionH [3]. Fig. 1(b) shows the electrochemical behaviour of theSb2W20Fe2O70(H2O)6]8− in a pH 5 buffer. It is clearly seen that theedox couple associated with the Fe(III) centres is clearer in nature.n addition the second tungsten-oxo process is no longer visible

ithin the electrochemical window of the buffer system, due tots negative shift in potential. The pH shift in both the Fe(III/II) andrst W–O redox processes was 72 and 77 mV per decade change inH, respectively. These results are in agreement with our group’secently published results [35].

.2. Polypyrrole – [Sb2W20Fe2O70(H2O)6]8− films

.2.1. Electrochemical characterisationAs mentioned previously our main aim was to successfully

mmobilise the Krebs Fe(III) POM within a conducting poly-eric film. The polymer of choice was polypyrrole due its well

nown ability to act as a polymeric support for POMs, its easef electropolymerisation and the detailed knowledge about its

870 K. Foster et al. / Electrochimica

Fig. 2. (a) Cyclic voltammogram of immobilised Ppy/Fe HPA film in pH6, on a car-b[(

emiroat−tigttirat−wwttr

oaiWaatrtwpPtbm[

cttodpitrlTspwFc

wsbsc

[

wcbtobp

fiectotattcprevious exposure of a polypyrrole film to other electrolytes has an

on electrode (A = 0.0707 cm2), scan rate 5 mV s−1. (b) Cyclic voltammogram of aSb2W20Fe2O70(H2O)6]8− doped polpyyrole film deposited upon a carbon electrodeA = 0.0707 cm2), in pH 6 buffer (i) and pH 2 buffer (ii), scan rate = 5 mV s−1.

lectrochemical behaviour. Fig. 2(a) illustrates the cyclic voltam-ogram attained for a polypyrrole – [Sb2W20Fe2O70(H2O)6]8− film,

n pH 6 buffer solution. What is evident is the presence of a largeedox couple with an E1/2 of −0.086 V (versus Ag/AgCl) on topf a significant background current. This couple is thought to bessociated with the redox switching of the Fe(III) centres withinhe polymer bound POM. Within the potential limits of +0.2 to0.2 V the conducting polymer remains in the conducting state, and

he presence of the large non-faradaic current component, whichs characteristic of conducting polymers, to the cyclic voltammo-ram within these limits confirms this. At potentials more negativehan −0.2 V, the presence of a reduction and associated oxida-ive shoulder, centred around point (a) in Fig. 2(a) is seen. Thiss believed to be the redox activity associated with the polypyr-ole backbone, when it is switched between the conducting (Ppy+)nd non-conducting states (Ppy0). At further negative potentials,he presence of a weak redox couple with an E1/2 of approximately0.750 V (versus Ag/AgCl), is observed. The latter being associatedith the first bielectronic redox process of the POM’s W–O frame-

ork as it is comparable in potential to the POM in solution under

he same pH conditions. Unfortunately it was not possible to viewhe second W–O redox process with any of the conducting polypyr-ole films formed, the reason for this is still yet unclear. However,

erea

Acta 54 (2008) 868–875

ne factor is the narrow electrochemical window available in suchqueous electrolyte solutions as in Fig. 2(a). In addition, it is onlyn buffer pH 6 that any redox activity associated with the POM’s

–O cage can be seen. The reasons for this particular observationre unclear at present. However it should be stressed that a distinctdvantage of this Krebs POM doped polypyrrole film is the abilityo electrochemically switch the polymer through the oxidised andeduced states without leaching of the incorporated POM out ofhe polymer matrix. This leaching phenomenon has been observedith various POM doped conducting polymers, in that, when theolymer is reduced electrochemically there is a slow release of theOM out of the polymer film with replacement of the POM withinhe polymer with an anion from the contacting electrolyte. This haseen one of the drawbacks for the employment of conducting poly-er films as suitable immobilisation matrixes for POMs in general

36].For our polypyrrole Krebs POM hybrid films, we decided to con-

entrate on elucidating further the electrochemical behaviour ofhe Fe(III) centres within the Krebs when it is immobilised withinhe polymer, primarily due to the poor electrochemical responsebtained for the tungsten-oxo framework within the insulatingomain of the polypyrrole. The first step was to elucidate if theH dependent nature of the Fe(III/II) couple, observed in solution,

s maintained in the immobilised polymeric state. Fig. 2(b) showshe overlap of two cyclic voltammograms obtained for a polypyr-ole – [Sb2W20Fe2O70(H2O)6]8− film in pH buffers 2 and 6, whilstooking at the potential domain encompassing the Fe(III/II) couple.his negative shift with more alkaline pH agrees with the POM’solution behaviour with a shift of 60 mV per decade change in theH being observed, thus indicating the addition of two protons forhat is believed to be a bielectronic wave for the two equivalent

e(III) centres. This agrees well with the value of 72 mV per decadehange in pH obtained for the solution phase species.

To elucidate what are the processes occurring within the filmhen it is switched between the Fe(III/II) states is of interest. As this

witching does not lead to the reduction of the conducting polymerackbone, it is thought that a process of cation insertion must occuro as to maintain electroneutrality within the film when the Fe(III)entres are reduced, as shown in Eq. (1) below:

PP+FeIIIHPA] + X+ → [PP + FeIIHPA]2−

[X+]2(in film) (1)

here PP+ is the oxidised form of the polymer and X is the counteration whether in the film or supporting electrolyte solution. It iselieved that two cations of unit positive charge would be requiredo balance the bielectronic process associated with this reductionf the Fe(III) sites within the polymer bound Krebs POM. The role ofoth the nature and concentration of the electrolyte cation upon theolymer voltammetry were obvious investigations to undertake.

The response for the so called “breaking in” process, that is therst time redox cycling of a polymeric film in contact with a solvent-lectrolyte system, is of importance as it can point to any significanthanges in the polymer’s morphology. Table 1 shows the results ofhe breaking in process in solutions containing 0.1 M Li+, Na+, K+, Cs+

r H+ chloride based background electrolytes. What is apparent ishat there are only shifts of up to approximately 15 mV in either thenodic peak potential, Ep,a or cathodic peak potentials, Ep,c, betweenhe initial (Ei

p) and last voltammetric scan (Efp), thereby indicating

hat the film’s morphology is essentially unchanging upon redoxycling. The memory effect of a conducting polymer film, whereby

ffect upon the subsequent film redox cycling, was of interest asesidual cations may still exist within the film. This is somewhatvident from Table 1, in that the E1/2 for the KCl electrolyte prior tond after redox cycling in HCl was −70.5 mV and −41 mV, respec-

K. Foster et al. / Electrochimica Acta 54 (2008) 868–875 871

Table 1Redox behaviour of film in various background electrolytes

Electrolyte 0.1 M Eipa (mV) versus

Ag/AgClEf

pa (mV) versusAg/AgCl

Eipc (mV) versus

Ag/AgClEf

pc (mV) versusAg/AgCl

E1/2 (mV) versusAg/AgCl

�Ep (mV) versusAg/AgCl

LiCl −113 −122 −160 −160 −141 38NaCl −105 −117 −141 −151 −129 24KCl −48 −56 −85 −91 −70.5 29HCl 166 161 144 144 152.5 17KC

C

tais

titdppc

Ffi(c

fbecctuf

Cld 6 −37 −45sCl −36 −51 −85

arbon electrode (A = 0.0707 cm2).

ively. The E1/2 values associated with this cation insertion processs a function of cationic nature show that the Cs+ cation is easier toncorporate than the Li+ cation. This is expanded upon in the nextection.

The effect of the cation concentration in the supporting elec-rolyte upon the redox behaviour of our polymer film has beennvestigated, with the work herein described being based onhe previous work by Cassidy and co-workers’ on a [Fe(CN)6]4−

oped polypyrrole film [37]. Anson and co-workers [38] alsoerformed similar experiments upon Nafion films containingositively charged redox sites. The effect of increasing cation con-entration on the Fe(III/II) redox activity is seen in Fig. 3(a), as a

ig. 3. (a) Cyclic voltammogram of a [Sb2W20Fe2O70(H2O)6]8− doped polpyyrolelm deposited upon a carbon electrode (A = 0.0707 cm2) in 0.2 M LiCl (i) and 2 M LiClii), scan rate = 100 mV s−1. (b) Plots of E1/2 versus log [C]. E1/2 was measured fromyclic voltammograms run at 100 mV s−1 in chloride salts.

apitCw[iTfsfd

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−69 −41 8−94 −68 34

unction of LiCl concentration. It can be seen that Ep,c seems toe more sensitive to electrolyte concentration, this is somewhatxpected as it is the process of cation insertion which occurs uponathodic scanning. Fig. 3(b) shows a plot of E1/2 for the Fe(III/II) pro-ess as a function of Log[C], where [C] is the cation concentration inhe bulk solution. From the plot it can be seen that linearity existsp to 2 M, for concentrations greater than this a sharp deviationrom linearity was noted. The literature suggests that this may bettributed to polymer swelling [37,38]. The slopes of the lines ofeak potential versus Log[C] obtained for the different electrolytes

n Fig. 3(b) can be seen in Table 2. What is interesting to mention ishat, a trend exists between E1/2 and hydrated cation size, with thes+ cation being able to move into and out of the film more easilyhen compared to the other cations, this has been seen previously

37,39]. This is confirmed from the slopes for the Li+ and Cs+ cationsndicating that it is easier for the Cs+ to move in and out of the film.his perhaps can be explained by the fact that it is the hydratedorm of the cations, which enters the film matrix [37]. From thelopes it was also evident that the anodic and cathodic peaks wereound to be affected differently, with the reduction peak being moreependent on the nature of the supporting electrolyte employed.

The stability of these Krebs POM polypyrrole hydrid systemsowards redox cycling, through the Fe(III/II) redox process, as aunction of both solution pH and polymer surface coverage, � ,he latter defined as Q/nFA, where n is the number of electronsransferred (two for the FeIII/II system), F is Faraday’s constant96,485 C mol−1) A is the electrode area (cm2) and Q is the chargeassed in the reduction or oxidation cycle of the FeIII/II redox cycleor the film, was investigated. For a film with a surface coverage,

= 2.3 × 10−8 mol cm−2, after cycling in pH 5 buffer only a 6.4%oss of electroactivity after 100 redox cycles was observed. For alm with a surface coverage, � = 3.4 × 10−9 mol cm−2, a 14.3% loss

n electroactivity was observed for the same number of cycles in pHbuffer. Scan rate studies were also performed on films of varying

hickness. From thin films plots of current versus scan rate, linearityxisted up to 400 mV s−1. For films of surface coverage of magni-ude (×10−8 mol cm−2) linear relationships could only be seen up to00 mV s−1, beyond this there is an increase in peak splitting even-

ually leading to a loss of peak symmetry. For a surface confinededox active species a fwhm of 45.3 mV for a two electron processre not seen for any of the films investigated. However a typicalrend of peak splitting and fwhm increasing with increasing scan

able 2lopes of the plots of Epa, Epc and E1/2 versus Log [C]

ation Atomic radius(pm)

Hydratedradius (nma)

Slope of Ep versus Log[C]

Epa (V) Epc (V) E1/2 (V)

i+ 152 0.6 65.6 105.8 86.3a+ 186 0.4–0.45 54 85.6 70.6+ 227 0.3 57.8 73.2 66.3s+ 265 0.25 40.4 66.8 54.4

a Values from ref. [31].

872 K. Foster et al. / Electrochimica Acta 54 (2008) 868–875

Table 3Behaviour of films with varied surface coverage at different scan rates showing peak separation and anodic and cathodic peak fwhm in pH 2 buffer solution at bare carbonelectrode (A = 0.0707 cm2)

pH Surface coverage � (mol cm−2) × 10−9 Scan rate (mV s−1) Peak splitting (mV) Anodic peak fwhm (mV) Cathodic peak fwhm (mV)

2 11.3 10 20 122 14240 57 150 190

100 34 150 180300 84 150 202500 121 180 210

2 4.3 10 20 112 14240 14 114 178

100 19 122 1843753

rfi

3[

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bibeen extended, two slopes would be likely to have been seen dueto the initial polymerisation mechanism followed by the crosslink-ing that can sometimes take place through the �-position of thepyrrole unit [32].

300500

ate is seen throughout. This is illustrated in Table 3 for a range oflm thicknesses investigated in pH 2 buffer.

.2.2. EQCM measurements of polypyrrole –Sb2W20Fe2O70(H2O)6]8− films

The EQCM technique has been used to study in situ the masshanges that take place during the electrochemical growth andedox cycling of our Krebs doped polypyrrole films. The polymerlms have been electrochemically grown onto a Au quartz elec-rode from aqueous solutions containing 25 mM Pyrrole and 1 mMe(III) Krebs POM. Changes in the mass of the quartz crystal werealculated from the changes in resonance frequency by using theell known Sauerbrey equation:

f = − 2f 20√

��

�m

A(2)

here f0 is the resonant frequency of the fundamental mode of therystal, A the area of the gold disk coated onto the crystal (0.205 cm2

or the given crystal), � is the density of the quartz (2.648 g/cm−3),the shear modulus of the quartz (2.947 × 1011 g/cm s−2).Fig. 4(a) and (b) shows the current and frequency changes during

he electrochemical formation of our polypyrrole film. In Fig. 4(a)hat is evident is that upon cycling there is a current increase

ssociated with the electrodeposition of the polymer film, withhe appearance and subsequent increase in the redox activity ofhe Fe(III) sites within the Krebs POM as it is incorporated at thelectrode surface within the polymer film. In Fig. 4(b) it can bebserved that there is no change in frequency until a potentialf +0.60 V is reached, after which it remains constant and onlyecreases at around +0.55 V on the reverse cyclic scan. These fre-uency decreases correspond to mass increases at the electrodeurface. This has been explained previously [40], that firstly theurrent is consumed whilst generating monomer radicals and shorthain oligomers which are formed. Once the concentration of theonger chain oligomers is sufficient, precipitation onto the electrodeurface occurs, thereby leading to a subsequent mass increase. Itas possible to calculate the doping level within the polypyyrolelm based on the methods in the literature [41]. This involved util-

sing the value of the slope (�m/�Q) from the plot of M versus Q,uring the electropolymerisation step. A doping value (�) of 18%as calculated for the POM based doped polypyrrole film.

Galvanostatic methods have also been employed to electro-hemically deposit the conducting polymer films. If the frequencynd polymerisation charge possess a linear variation with time,

his is said by some authors to be the indication of homogeneouslm growth [40]. Although a contradiction to this has been seenhereby other authors believe that the electrochemical synthesis

f hybrid materials PPy/POMs, always yield a “two layer system”42]. From our results only one slope was observed, it can therefore

F(ot

132 188140 190

e assumed that only one type of polymerisation mechanism is tak-ng place with a regular polymer formation. If the experiment had

ig. 4. (a) Electrochemical polymerisation of pyrrole: [Sb2W20Fe2O70(H2O)6]8−

25/1 mM) by potential cycling between −0.3 and 0.8 V (10 scans) at a scan ratef 100 mV s−1. (b) The corresponding frequency changes associated with the elec-rodeposition of the polymer onto the electrode surface.

K. Foster et al. / Electrochimica

Fc

fi

�

wt[

afirtifiomdvpp

tTr

t

M

Dcfcedsia

c(

M

Ft0atttacwaAtwvt3fwcK

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fgiHgetcFttpautwismaller in current magnitude than the catalytic wave observed in

ig. 5. (a) Current changes and (b) mass changes of resultant polymer film redoxycled through the Fe(III/II) couple in 0.1 M NaCl. Scan rate = 20 mV s−1.

The total frequency change that takes place upon cycling of alm in an electrolyte can be expressed as:

ft = �fc + �fa + �fs (3)

here �fc, �fa and �fs (H2O) are the frequency changes caused byhe transport of cationic, anionic and solvent species, respectively43].

The resultant polymer film in 0.1 M NaCl, with changes in massnd current during the oxidation and reduction of the PPy/FeHPAlm can be seen in Fig. 5(a) and (b). The mass increases duringeduction and decreases on oxidation. This is due to the Na+ cationhat is transported into the film during reduction and expelled dur-ng oxidation in order to maintain charge neutrality in the polymerlm. The mass change does not however return to its original valuen the completion of the cycles. One of the possible reasons for thisass increase with every scan is that some of the cations inserted

uring the reduction process may remain trapped in the film. Sol-ent molecules are also transferred during the potential cyclingrocess and may also have an effect on the mass increase of theolymeric film.

During the reduction process, charge neutrality, as already men-ioned can take place either by anion expulsion or cation insertion.he apparent mass of the ions involved can be determined by theatio of the mass variation over the charge variation according to

Fett

Acta 54 (2008) 868–875 873

he equation below:

′ =∣∣∣

�m

�Q

∣∣∣ F (4)

epending on the experimental conditions, mass/frequency valuesan differ. When using potentiostatic/galvanostatic conditions therequency is independent. Whereas when using potentiodynamiconditions, results can be varied. This has been seen by Vantankhaht al. where by the deposition of silver under potentiodynamic con-itions varies from −23.3 × 107 to −16 × 107 Hz cm2 g−1 when thecan rate varies between 0.5 and 15 mV s−1 [44]. In the present stud-es a range of slow scan rates (4, 5, 10 mV s−1) have been appliednd in all cases the mass changes are the same.

For supporting electrolytes of a pH higher than 1, at least twoations are present in both H+ and the cation from the electrolytesB+) [33] yielding M′ to be expressed as follows:

′ = xMH+ + (1 − x)M′B+ (5)

ig. 5(a) and (b) shows the initial cyclic voltammogram cycle andhe corresponding mass changes obtained at a PPy/Fe POM film in.1 M NaCl aqueous solution. �m was found to be equal to 0.863 �g,nd the corresponding cathodic charge for the first cycle was foundo be 4.56 mC leading to M′ = 18.41 g mol−1 according to Eq. (5). Ashe actual mass of a Na+ cation is 22.99 g mol−1 it is believed thathe charge compensation into the polymer film takes place via Na+

nd H+ insertion. In using Eq. (5) the total percentage of chargeompensation anions can be calculated. Using the above values itas calculated that 79.2% of the inserted cations are Na+ and 20.8%

re H+, with the assumption that no water molecules are involved.PPy/Fe POM film was also investigated in 0.1 M KCl aqueous solu-

ion, as seen in Fig. 6(a) and (b), and �m was found to be 1.01 �gith a cathodic charge for the first scan being 3.72 mC. Using these

alues M′ was found to be 26.24 g mol−1. Again using Eq. (5) theotal charge compensation cations were made up of 66.5% K+ and3.5% H+. This dual-cation insertion effect has also been reportedor a similar system by Vieil et al. [33]. In this study a polymer filmas synthesised using a Keggin type POM as the dopant anion for

harge neutrality. The authors investigated a PPy/SiMo12 film in aCl solution.

.2.3. Electrocatalytic reduction hydrogen peroxideA range of polyoxometallates [7,8,45,46] have been investigated

or their ability towards the electrocatalytic reduction of hydro-en peroxide. Our modified electrode has been used to determinef it would be suitable to act as an efficient electrocatalyst for

2O2 reduction. This section focuses on the reduction of hydro-en peroxide at a PPy/Fe POM film in acetate buffer (pH 6, 6.5). Thelectrocatalysis phenomenon can be seen in the Fig. 7(a) where byhe addition of H2O2 leads to an increased reduction current whenompared to the polymer film in the absence of the H2O2. Fromig. 7(a) it is readily observed that the reduced form of the iron cen-res (i.e. Fe(II)) within the polymer bound POM readily react withhe added H2O2. So as to rule out a possible role of the conductingolypyrrole backbone, the effect of H2O2 upon the voltammetry forpolypyrrole film doped chloride (PPy/Cl) film was investigated

nder the same experimental conditions. Only a slight change inhe polymer’s voltammtery was observed with a slight reductionave at around −0.25 V (versus Ag/AgCl) being observed, as seen

n Fig. 7(b). This being both further negative by about 150 mV and

ig. 7(a). In addition, the interaction of H2O2 with a bare carbonlectrode produces no discernible reaction. This points clearly tohe role of the polymer bound reduced form of the Krebs HPA inhe reduction of the H2O2.

8 imica Acta 54 (2008) 868–875

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74 K. Foster et al. / Electroch

Upon investigating the effect of scan rate over a range of0–100 mV s−1 the resulting dependence of Ipc on �1/2 is linearndicating that the reduction reaction is diffusion controlled. Sub-equent tests have been carried out in pH 6.5 buffer, this was doneith consideration for the application of a sensor that could besed under physiological conditions. An Eapp = −0.165 V was appliedo the Fe POM polymer modified electrode under hydrodynamiconditions for approximately 15 min prior to the injection of therst addition of the hydrogen peroxide. The standard calibrationurve obtained from performing the three separate I–t transientsesponses for the polymer films of similar surface coveragesn = 3) is shown in Fig. 7(c). This figure represents averaged thicklms, with surface coverage’s in the range 1–1.8 × 10−8 mol cm−2,here a sensitivity of 131.8 (±3.5) �A cm−2 /mM is seen withlimit of detection (LOD) for our system in a buffer of pH

.5, based on three times the S/N ratio, found to be 16.6 �M.n addition, a response time of approximately 20 s was foundn the initial stages of the calibration curve. For thinner films,

ith surface coverage’s in the range 3.4–5.0 × 10−9 mol cm−2, aensitivity of 110.5 (±5.9) �A cm−2 /mM is seen with a LOD of.4 �M. What can be seen from their respective sensitivities ishat thicker films possess the highest sensitivity, probably dueo the higher number of electrocatalytic Fe(II) sites within thelm.

The LOD for our sensor was found to be superior to some of

he values quoted in the literature. Modified electrodes consist-ng of films of heme proteins with clay nanoparticles possess aOD of 120 �M. [47]. LOD values between 5 �M and 6 mM werechieved using a poly(0-phenyllenediamine) film combined with

ig. 6. (a) Cyclic voltammogram initial cycle in 0.1 M KCl aqueous solution, (b) cor-esponding mass changes for the initial cycle in 0.1 M KCl.

Fig. 7. (a) Cyclic voltammograms of a [Sb2W20Fe2O70(H2O)6]8− doped polypyrrolefilm, deposited upon a carbon electrode (A = 0.0707 cm2), in the absence (i) andpresence (ii) of 2 mM H2O2 in pH 6 buffer. (b) Cyclic voltammograms of a chlo-ride doped polypyrrole film, deposited upon a carbon electrode (A = 0.0707 cm2),in the absence (i) and presence (ii) of 2 mM H2O2 in pH 6 buffer. (c) Standardco(

pmOsoA[

alibration plot representing amperometric data (n = 3) for the polymer film of, 3,ver the range of 0.1–1 mM Hydrogen peroxide in pH 6.5 buffer. Surface coverages1–1.8 × 10−8 mol cm−2).

latinum microparticles [48]. A phosphomolybdate-polypyrroleodified electrode has shown to possess an LOD of 50 �M [49].

ne method for the detection of hydrogen peroxide which is

uperior to the others mentioned is the use of a functionalisedrmosil – modified electrode which possesses a LOD of 0.5 �M.s can be seen our LOD (6.4 �M) is consistent with many others

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. Conclusions

The Fe(III) Krebs POM, K6NaH [Sb2W20Fe2O70(H2O)6]·29H2O,as successfully been immobilised, by electrochemical means, intopolypyrrole matrix. The resulting polymer exhibited clear redox

ctivity associated with the Fe(III) sites from the Kreb’s structure,ithin the conducting region of the polypyrrole. It was found that

edox switching the polymer through this Fe(III/II) couple leads toprocess of cation insertion into the polymer film. Investigations

nto effect of both the nature and concentration of the contactinglectrolyte point to Cs+ entering and exiting the film easier thani+, pointing to the cations entering the film with their hydrationpheres intact. Catalytic activity was observed for the reduction ofydrogen peroxide in both pH 6 and 6.5 buffer solutions. In the moreeutral solution a series of films were examined and it was foundhat films of thick surface coverage (1–1.8 × 10−8 mol cm−2) had aensitivity of 131.8 (±3.5) �A cm−2 /mM with a LOD of 16.6 �M.or thin films (3.4–5.0 × 10−9 mol cm−2) sensitivity was found toe 110.5 (±5.9) �A cm−2/mM with a LOD of 8.4 �M. EQCM stud-

es were carried out, to study in situ the mass changes that takelace during the electrochemical growth and redox cycling ofPy/HPa film. Electrochemical growth was carried out employingoth chronoamperometry and cyclic voltammetry. The resultinglms were investigated in various electrolytes. When a film wasycled in 0.1 M NaCl the resulting mass changes associated withation insertion were found to be due to 79.2% Na+ and 20.8% are+. When the film was cycled in KCl mass changes were found toe made up of 66.5% K+ and 33.5% H+.

cknowledgement

The authors would like to acknowledge funding from ITT Dublinhrough the 2006 PhD continuance fund.

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