stabilization of the redox polymer [os(bipy)2(pvp)10cl]cl by in situ chemical cross-linking

Electroanalpis, 6( 1994) 5 53- 560

Stabilization of the Redox Polvmer [Os(bipy),(PVP)&I]CI by In Shu Chemical Cross4 in king

Andrew I? Dohwty7 Tim Bucklq, David M. Kelly7 and Johannes G. vos' School of Chemical Sciences, Dublin CiQ? Linitw-sig: Dublin 9, Ireland Ueceziied rlpril21, 1993.

ABSTRACT The redox polymer [O~(bipy)~(P~TP),~CllCl, where bipy is 2,2'-bipyridyl and PLT is poly-4-vinylpyridine, has been physically stabilized by chemical cross-linking at various levels via the solid-state reaction with the cross-linking agents p-dibromobenzene, 1 ,j-dibromopentane, and 1 ,lo-dibromodecane. The stability of the resulting polymer films under the severe hydrodynamic conditions of thin-layer flow cells and at rotating disk electrodes (RDEs) was greatly increased compared to the uncross-linked polymer. The charge transport properties of the redox polymer were maintained or improved by cross-linking. The surface and Nernstian behavior of the cross-linked redox materials are not found to be substantially affected by cross-linking.

KEY WORDS: Modified electrodes, Redox polymer, Stabilization.

INTRODUCTION

Since the first examples of modified electrodes, much criticism concerning the physical and chemical stability of the modifving species has been voiced and, in many cases, justifiably so [l-31. Through the course of their development, many examples of modified electrodes have been reported; however, studies are seldom concerned with improving stability, rather exerting control over mass/charge transport properties or the electrocatalytic activity of the redox polymer 14.51. Consequently, strat- egies for polymer film stabilization have no systematic theme, and the procedures adopted hitherto include a diversity of approaches mainly based on principles bor- rowed from polymer science.

The early examples of adsorbed or covalently attached monolayers on electrode surfaces were subject to severe stabiliv problems owing to rapid desorption or hydrolysis of the covalently attached redox moiety [ 21. Adsorbed polyelectrolytes provided an attractive and more stable means of immobilizing redox species at electrode surfaces by electrostatic incorporation or by coordina- tion [6.7]. This is due to the slow desorption of poly- meric materials from the electrode surface [8]. Electro- statically incorporated redox sites are, however, subject to chemical instability because of loss of the immobilized redox sites to the electrolyte that is normally de- void of that species [6]. However, redox polymers con-

taining coordinatively bonded redox centers, such as those studied here, exhibit good chemical stability owing to the strong nature of the metal-ligand bond. Unfortu- nately, layers of these materials are subject to physical instability because of dissolution from the surface owing to strong interaction with the contacting electrolyte. In a previous study [9], the half-life (ill?) of the redox polymer [R~(bipy)~(PVp),Cl]Cl, where bipy is 2,2'-bipyridyl and PVP is poly-4-vinylpyridine, in thin-layer flow cells was found to be 8 hours. Overlaying this polymer with electropolymerized poly(3-methylthiophene) or poly(N- ethyltyramine) resulted in electrodes with t,,? greater than 48 hours. However, reduced electrode response as a result of mass transport restrictions was evident. Photo-in- duced cross-linking of the ruthenium-containing polymer was also investigated: and similar behavior was observed. Barisci and Wallace studied the stability of the analogous material, electropolymerized [ Ru(bipy),( \;P).)~ ]'+ (vpy = vinyl pyridine), and found that stability was increased, but the sensitivity of the modified electrode was significantly reduced [ 101. The manipulation of the polymer backbone by incorporation of styrene residues into PW-based redox polymers was found to increase the op- erational stability of the polymer films, and no decreases in electrode responses were observed over 14 days of

'To whom correspondence should be addressed.

6 199-1 YCH Publishers. Inc

554 Dohem- et a1

Br‘

continuous use in flow-injection analysis (FW) [l l] . The use and stability of chemically cross-linked redos polymer films have been reported. Facci and Murray demonstrated a copolymer comprised of the monomers 4- vinylpyridine and y-methac~lox?propyltrirnetho.ul.silane where the latter forms cross-links in the presence of moisture [ 121. Lindholm and Sharp used 1,6-dibromoh- esane, 1.1 O-dibromodecane, and cucu’-dibromo-p-xTlene to cross-link PIT [13]. This resulted in polymer films with greatly improved retention of electrostaticall!- incorporated [ IrC6)’-’3-.

In order to stabilize the redox polymer films, the dissolution/stripping processes must be prevented. Sta- bilization procedures must also be fast, simple, and compatible with mass production techniques such as lichog- raphy and also produce films with rapid-current carrying capacity. In this article. the physical stabilization of the redox polymer [ Os( bipy),(PVP),oC1 ]C1 by in situ chemical cross-linking with 1 ,j-dibromopentane, 1 ,lO-dibromodecane, and p-dibromobenzene at room temperature is demonstrated. The advantages to this approach to electrode stabilization are considered.

E X P r n M r n M Materials and Reagents The synthesis of [ Os(bipy),( P\T),,CI]CI is described else- where 1141. The molecular weight of the PVP back bone was 600,000 g mol-’ as determined bp viscometry. The redos polymer was cross-linked directly on the electrode surface via a solid-state reaction with the cross- linking agents 1, j-dibromopentane, Ll0-dibromodecane, and p-dibromobenzene (Aldrich). This was accomplished bJ- the preparation of separate solutions containing known amounts of redox polymer and cross- linking reagent. The preparation of redox polymer solutions of exact composirion is difficult as the redox polymer retains water even after rigorous dFing. h 1.0% weight/volume (w/v) solution of the redox polymer in methanol was prepared; this solution was then accurately diluted serially to obtain 0.1% w/v and 0.01% w/v solutions. The original 1.0% solution was then “calibrated“ by coating known volumes of the polymer solution onto electrodes and measuring the surface coverage by coulometry during slow-sweep rate (1.0 mV s-’) cyclic voltammetry (0’). The entire film is electroactive in the 0.1 rnol dm-3 H2S0, electrolyte used. The volume of polymer solution to produce a particular surface coverage can then be accurately evaluated. Solutions of the cross-linking agents were prepared in methanol. Appro- priate volumes (typically 10 pL), dispensed from a Ham- ilton 25 pL syringe, of polymer solution and cross-linking agent solution to give the desired surface coverage and level of cross-linking were then placed on the electrode surface and mixed gently and allowed to d n slowly in air. The electrodes were then allowed to cure in vacuo for 24 hours before use. The level of cross-linking used was 1, 5 , 10, and 20 mol% of the monomer units present per chain. As the stoichiornet? of the reaction is 2/1!

- 0 s -N

.N N

=. \.= . \

FIGURE 1. Structure of cross-linked redox polymer where X = 8, Y = 1, and Z = 1 for a 10 mol YO cross-linked polymer and N-N is bipy and R = pentyl, decyl, and benzyl groups.

this results in 2! 10, 20, and 40 mol % quaternization of the total quantity of pyridine groups. It is assumed that interchain reaction occurs, but the possibility of intra- chain cross-linking exists. The precision of the coating procedure is well within the range ? 10% of the desired surface coverage, assuming equal precision for the ap- plication of the cross-linking agent; then the levels of cross-linking (cross-links/vpy per chain) lie in the range 1/(100 * 5 ) , 1/(20 It l), 1/(10 ? O.j), and 1/(5 ? 0.25). The structure of the cross-linked material is shown in Figure 1.

All electrolyte solutions were prepared from water obtained from the Milli-Q water purification system. An amount of 0.1 mol dm-’ H 2 S 0 4 was used as the electrolyte, as the redox polymer is soluble in mineral acid. This electrolyte presents sufficiently arduous conditions to test the stabilit) of the cross-linked films under op- erational conditions.

PROCEDURES CV; Rotating Disk Electrode Voltammety, and Coulometry The 0’ and rotating disk electrode (RDE) voltammetry were carried out using a conventional three-electrode electrochemical cell. The potentiostat was the EG&G Princeton Applied Research Model 362 potentiostat. The rotating disk assembly was the Metrohm Model 629-10.

Stabilization of the Redos Polymer [ Os(b~py)~( PW),,CI]CI by In Situ Chemical Cross-linking 555

Voltammograms were recorded on the Linseis Model 1'100 X-Y recorder. The reference electrode was the sat- urated calomel electrode (SCE). All potentials are quoted without regard to the liquid junction potential. The counter electrode was 1.0 cm' platinum gauze placed parallel to the working electrode at a distance of 1 1 cm.

Coulometn was carried out using the EG&G Prince- ton Applied Research Model 379 digital coulometer placed in series between the working electrode and the potentiostat return. The charge owing to the oxidation of Os(I1) to Os(II1) within the polymer film was measured during slow sweep rate 0' (1.0 mV s-') from 0.0 to 0.6 V (versus SCE). Results were corrected for background charg- ing currents.

Flow-Injection Apparatus The flowinjection apparatus consisted of a Gilson hfiniPlus 3 peristaltic pump, a six port Rheodyne injection valve fitted with a 20 WL fixed volume sample loop. The detector was the EG&G Princeton Applied Research Model 400 Electrochemical Detector connected with Teflon HPLC tubing. Responses were recorded on a Phillips PIM 8252 X-t chart recorder. In the thin-layer flow cell, the .4g/ AgCI electrode acted as the reference electrode and the stainless steel cell body acted as the counter electrode. The working electrodes were 3 mm glassy carbon shrouded in Teflon. All potentials are quoted following numerical conversion to the SCE scale. The carrier electrolyte was 0.1 mol drn-3 H,SOj at a flow rate of 1.0 ~ m - ~ m i n - ' .

Infrared Spectroscopy Solutions of PW were made in methanol in an identical fashion to the redox polymer solutions described ear- lier. The solutions were calibrated by casting thin films of the PVP from methanolic solution onto a Teflon block. Accurate weighing of the films allowed the determina- tion of the exact concentrations of the polymer solutions. A methanolic solution of p-dibromobenzene was also prepared. This cross-linking agent was chosen to examine the reaction, as it is solid at room temperature and is therefore least likely to be mobile in the polymer film matrix. Appropriate volumes of the polymer solution and the p-dibromobenzene solution to give a 12% quaternization of the vpy residues (6% cross-linlung) were transferred to the Teflon block, mixed thoroughly and allowed to dry slowly in air. On evaporation of the solvent, the polymer films were removed from the Teflon block and a series of transmission infrared (IR) spectra were recorded over a 22 hour period. The cast films were stored in a vacuum desiccator between measurements to prevent adsorption of moisture. After 22 hours, the films were heated to 85°C for a further 24 hours, and the IR spectra were again recorded.

RESULTS AND DISCUSSION IR Analysis of the Cross-linking Reaction The previously reported cross-linking of PW-based polymers using the solid-state reaction with dibromoalkanes involved elevated temperatures, typically 60°C to 120°C

[13,15.16]. Such conditions are undesirable for a number of reasons. Commercial and in-house electrodes con- structed from glassy carbon are normally shrouded in Teflon or glass. The glassy carbon disks are immobilized by heat-shrinking of the shrouding material around the electrode to produce a carbon disk coplanar with the shrouding material. Maintaining these electrodes at elevated temperatures causes disruption of the heat seal and. consequently, leaking of electro1)te to the side of the disks and. therefore, invalidating measurements: and furthermore. this prevents thorough cleaning of the ac- tive electrode surface. In addition, the normally used cross- linking agents such as 1,j-dibromopentane and 1 .lO-dibromodecane are liquid at room temperature, although their partial pressures are low; and it is envisaged that as only small quantities of the cross-linking agent are used and distributed across a considerable area compared to volume. loss due to volatilization at elevated temperatures may be problematic. In this study, PVP was used to study the solid-state cross-linl;mg reaction in thin films that can be easily cast from methanolic solution: since films cast from the redox polymer on Teflon are fragile and not amenable for IR analysis. As the glass transition temperature (Ts) of P W is 140°C and ca. 300°C for the redox polymer [ 141, long-range polymer motion is likely to be absent for both the PW and the redox polymer at room temperature. Similar cross-linking reaction behavior is therefore expected to occur for both the redox polymer and PVP.

It has been reported that a stretching frequency of the pyridine nioie5 in PVP occurs at 1600 cm-' and that upon quaternization of the aromatic nitrogen, a frequency shift to 1640 cm-' occurs [lj]. The relative intensin. of these frequencies can be used to give a measure of the extent of quaternization of the pyridine residues. The IR results assuming equal extinction coefficients have been found to agree closely with elemental analysis [ 151. In Figures 2(A), and (B), IR spectra for the PVP and cross-linked PW (partially quaternized), re- spectively, can be seen. After a time period of 1 hour, the ratio of absorption at the frequencies 1640 cm-'/ 1600 cm-' was 0.11 2 0.01, which indicates 11 2 1% quaternization. As the target level is 12%, it appears that much of the reaction has been completed after 1 hour at room temperature. The films were allowed to dry slowly, ca. 20 minutes for complete solvent evaporation. It is possible that the reaction proceeded quickly while the reactarm were in solution (or highly solvated) at high local concentration on the Teflon surface. No further dis- cernible change in the intensin. ratio occurred up to 3 hours. The films were then left for a further 19 hours. At this time point, the relative intensities of absorption was 0.12 * 0.01, which indicated 12 I 1% quaternization of the vpy units, indicating completion of the reaction. To examine the extent of completion of the reaction and the need for elevated temperatures, the films were heated to 85°C for a further 24 hours. The extent of quaternization following heat treatment was found to be 13 k 1%. These results indicate that the cross-linking reaction is favorable at room temperature and that the

556 Dohem et al.

1 2 - 2000 1500 cm-l b

-$ _--_---- .+--- I I

E

\ 0 4 -

0 2 -

10 20 30 40 50 60 70

Tim / h

FIGURE 3. Plot of polymer surface coverage in the thin- layer flow cell for (a) uncross-linked film and (b) cross- linked with 10 rnol % p-dibromobenzene. Electrolyte was 0.1 rnol dm-3 H,SO, at a flow rate of 1.0 cm3 min-'.

FIGURE 2. IR spectra of (A) pyridine-stretching frequency and (B) quaternized pyridine-stretching frequency as thin films of PVP.

reaction proceeds to within 90% completion after 22 hours at room temperature. This is of particular importance as the stabilization procedure can be carried out at room temperature.

Stability of Cross-linked and Uncross-linked 1 O~(b@!Y)*(PVP)IOC~lC~ In Figure 3(a). a plot of surface coverage (0 versus time can be seen for a modified electrode placed in a thin- layer flow cell. The initial surface coverage was 1.0 x

mol cm-'. It can be seen that the polvmer surface coverage slowly decreases with time. After a period of 24 hours, the surface coverage was reduced to 7.4 X lo-' mol cm-' which represents a loss of 26% of the initial coverage. After 46 hours, the total surface coverage decreased b>- 50% ; therefore, the t,,? of the uncross- linked material is 46 hours. This is a considerable improvement compared to the ruthenium analogue compound studied by Wallace et al. [9]. This is likely due to the high molecular weight material used in this study. As the redox polymer is soluble in mineral acid, the linear decrease in surface coverage is likely to represent a slow solvation process of the polymer and the resulting physical removal of the polymer from the electrode surface. The initial rate of polymer removal (0 to 48 hours) is slow, -1.1 * 0.2 x lo-'' rnol cm-' h-'. Between 48 and 70 hours, the rate of polymer removal is significantly slower, 4 . 2 t 0.2 X lo-'' mol cm-' h-'. At 48 hours, the surface coverage is 4.0 x lo-' mol ern-?. As a change in the rate of polymer removal occurs at this surface coverage, it appears to suggest a change in the stability and, therefore, physical nature of the polymer chse to the electrode surface. It has been postulated that the structure of redox polymers close to the polyner/electrode interface is different (more compact) compared to polymer regions at the electrolyte interface [ 171. This is likely a result of increased adsorption sites for polymer mol- ecules close to the electrode surface compared to polymer chains at the electrolyte interface. The stability of the redox polymer under the hydrodynamic conditions of the RDE were also examined. The t lIz of the uncross- linked polymer was -24 hours at a rotation speed of 1000 rpm. The t,,2 is considerably shorter than in thin- layer flow cells and is a reflection of the more severe conditions experienced during RDE voltammetry. The rate of polymer stripping was 2.1 2 0.2 x 10-l' mol cm-' h-'. A stable surface coverage of 2.0 X lov9 mol cm-2 was observed, which again indicates a more strongly adsorbed layer of polymer at the electrode surface.

Stabilization of the Redox Polvnier [ Os(blpy)?( P\T),,CI]CI by In Siru Chemical Cross-linking 557

It is recognized that electrocatalysis at redos pol!-- mer-modified electrodes is most efficient when the me- diated reaction occurs in considerable regions of the polymer [IS]. This has been demonstrated for several substrates at the uncross-linked redox polymer studied here [ 19.301. In these situations, the characteristic kinetic current ik defined by Saveant [21,22] for a through-film reaction is of the following form:

iL, = nFANb&, (1) Where n is the number of electrons, F is the Faraday constant, A is the electrode area, k is the second-order rate constant for the cross-exchange reaction, K is the partition coefficient of the substrate, bo is the concentration of electroactive sites within the polymer film, L is the layer thickness, and j y T is substrate concentration. Us- ing this equation and the fastest rate of polymer removal in the thin-layer flow cell, a time period of 40 minutes is estimated to cause a 1% change in the sensitivity of the electrode. Although important, the loss of surface coverage is sufficiently slow to ensure no significant change in the catalytic properties of the modified electrode over short time scales where direct calibration is possible such as in FLA. In situations where direct or frequent calibration is impossible such as in remote sensing or in line process analysis. polymer loss would significantly alter the sensitivity of the electrode over protracted time periods. For this reason, stabilization of the electrocatalytic layer is of great importance.

The compound [0~(bipy)~(PVP),,C1]C1 was cross- linked with 1, j-dibromopentane, 1,lO-dibromodecane. and p-dibromobenzene at 1, 5, 10, and 20 rnol % using polymer surface coverages of 5.0 x lo-’’, 1.0 x and 1.0 X rnol cm-’. The stabilit\. of the cross-linked films was assessed in an identical fashion to the uncross- linked material. All cross-linking agents, at all levels of cross-linking resulted in highly stable polymer films. For the thin-layer flow cell, the surface coverage was maintained within 95% of the initial value after 72 hours of continuous electrolyte flow (Figure 3(b)). This is a significant improvement compared to the uncross-linked material. Similarly for the RDE experiments, the surface coverage was maintained within 90% of the initial level over 24 hours of continuous rotation at 1000 rpm. In Figure 4, the initial CV (A) and the CV after 24 hours (B) for a 1.0 X rnol cm-’ polymer film cross-linked with 20% cross-linking with p-dibromobenzene can be seen. All cross-linking agents at each level of cross-linking produced stabilized polymer films with no apparent differences in the level of stability between cross-linking agents or between different levels of cross-linking. This indicates that simple anchoring of the outer-laver chains is sufficient to effect polymer stabilin even at levels as low a 1 mol %. When it is considered that the polymer at the laver/electrode interface is strongly adsorbed, it is not surprising that an “all-or-nothing” result is obtained, that is, either cross-linking stabilizes the film or it does not.

I \

E / V vs. SCE

0.7 0.7 0.1 0.1

FIGURE 4. Cyclic voltammograms for a 1.0 x lo-* rnol cm-‘ polymer film cross-linked with 20 rnol o/o p- dibromobenzene, (A) is the initial CV and (B) is the CV after a 24 h rotation at 1000 rpm. The electrolyte was 0.1 rnol dm-3 H,SO,.

Electrochemisty and Charge Transport - Properties of Cross-L inked

It has been observed that conditions for optimum-modified electrode stability are mutually opposed to those for optimum electrode performance [ 231. This is because strong interaction between the redox polymer and the contacting electrolyte is needed to ensure rapid charge transport rates and mass transport; however, this strong interaction also promotes dissolution of the polymer layer. In order to investigate the effect of cross-linking on the electrochemical behavior of the redox polymer, the surface behavior and charge transport parameters for the cross-linked materials were estimated. In Table 1, the charge transport diffusion coefficients (D,) for films cross- linked with each cross-linking agent and at each mol% level is shown along with the D,, value for the uncross- linked material. These values have been estimated by 0’ using the Randles-Sevcik equation under conditions of semi-infinite linear diffusion [3-4). Since for these coar-

1 O s ( b i P Y M ~ ~ ) ~ O C ~ I ~ ~

558 Dohem et a1

TABLE 1 Electrolyte was 0.1 mol d N 3 H2S04

Cross- Linking D,,/cn? s - ' ~ Agent (mol "/a) 1,5-dibromopentane 1,70-dibromodecane p-dibromobenzene

1 4.8 x lo-'' 2.8 x 10-9 1.3 X lo-''

5 8.5 x lo-''

Charge Transport Diffusion Coefficients for Each Cross-Linking Agent at Each Level of Cross-Linking. The

20.2 x 10-l0 50.2 x lo-''

10.1 x lo-'' 50.2 x 10-l0 51.0 x lo-"

20.3 x lo-'' 20.2 x 10-l0 51.0 x lo-" 20 6.4 x lo-'' 2.9 x 1 0 - ~ 2.0 x lo-''

20.1 x 10-'O 50.2 x 10-l0 22.0 x lo-"

Homopolymer 1.8 x lo-'' 20.2 x 10-l0

20.3 X

2.7 x 1 0 - ~ 0.7 x lo-''

1.5 x 10-9 2.9 x 0.7 x lo-'' 10

"Estimated by CV with c = 0.7 x bError estimated from measurements for at least two electrodes.

mol ~ m - ~ .

TABLE 2 Surface Behavior Parameters. The Electrolyte Was 0.1 rnol dm-3 H2S0, and the Surface Coverages were 1.0 x 1 0 - ~ mol cm-*

Cross Linking Ei13bImV Agent Versus FWHM~ (mol "/o) SCE dEpb/mV ipal ipcb / m v DC"/mV s-'

p-dibromobenzene 1 250 5 5 0.0 1 5 0.01 90 " 5 100 5 250 5 5 0.0 1 2 0.01 90 5 5 100

10 250 5 5 0.0 1 2 0.01 90 2 5 100 20 250 -t 5 0.0 1 ? 0.01 90 2 5 100

1 255 5 5 20 2 5 1 5 0.01 90 i 5 50 5 255 2 5 10 5 5 1 r 0.01 90 -t 5 50

10 255 2 5 30 2 5 1 i 0.01 90 If: 5 50 20 255 i 5 10 2 5 1 5 0.01 90 i 5 50

1 250 2 5 0.0 1 i 0.01 90 2 5 100 5 250 5 5 0.0 1 2 0.01 90 '' 5 100

10 250 2 5 0.0 1 i 0.01 90 2 5 100 20 250 2 5 0.0 1 2 0.01 90 * 5 100

7,5-dibromopentane

1,lO-dibromodecane

"Potential of change over to semi-infinite diffusional control. bEstimated at 1.0 mV s-'.

ings infinite diffusion conditions are maintained up to scan rates of about 100 mV/s, cyclic voltammograms obtained at 200 mV/s were used to calculate the diffusion coefficients. The D, value for the uncross-linked material in 0.1 mol dm-j H2S0, is 1.8 X 10-" cm' s-' which is comparable to that found previously [ 171. The D, values for polymer films cross-linked with p-dibromobenzene are in the range 0.7 x lo-'' to 2.0 x lo-'' cm2 s-'. These values are close to that found for the homopolymer. It appears that cross-linking with this reagent does not affect the charge transport rates of the polymer films significantly at any level. For 1,lO-dibromodecane, the rate of charge transport was found to be constant at 2.8 ? 0.1 X lo-' cm' s-l berween levels of cross-linking. Again, no systematic trend in D, with level of cross-linking was observed. This is of particular ad- vantage as accurate levels of cross-linking is not critical

to produce films of constant current-carrying capacity. The D, value for 1,lO-dibromodecane is, however, about one order of magnitude greater than for the homopolymer, indicating considerable improvements in the charge transport characteristics of the redox material. Again, this is advantageous for sensor applications. The D,, values for the 1,j-dibromopentane cross-linked polTmer films were found to increase from 4.8 X lo-'' cm- s-' to 1.5 x cm' s-l with increasing cross-linking from 1 to 10 mol %. At 20 rnol %, cross-linking the D,, value decreased to 6.4 X lo-'' cm2 s-l. These results indicate that chemical cross-linlung with the dibromoalkanes used here results in improvements in the charge transport characteristics of the redox materials.

The charge transport properties of [ Os( bipy),( pvP),,CI]C1 have been extensivelv studied. and it has been suggested that the overall rate of charge

Stabilization of the Redos Polvnier [ Os(bipy)J PI’P),,CI]CI by In Sit11 Chemical Cross-llnking 559

transport is controlled by counterion motion [ 1-2 j]. It has been observed that cross-linking of redox polymers results in compaction of the polymer films and hindered short-range ion motion [26] . iissuniing counterion motion is rate limiting, cross-linking can only maintain or decrease the magnitude of D , as the rate of self-es- change appears faster than counterion transport. From the results presented here, it is evident that improvements in D, are observed for 1.10-dibromodecane and 1.5-dibromopentane. This can be explained by assuming that cross-linking of the polymer film is likely to result in decreased inter-redos site distances with a corre- sponding increase in the effective redox site concentration (0,) resulting in increased values for D,, according to the following equation in a charge transport mem- brane where the rate of electron self-exchange is the rate- determining process [27] :

7 6 0

4 D, = rkc$ - ( 2 )

Where k , is the self-exchange rate constant, and 6 is the effective interelectroactive site distance at the moment of electron exchange. Alternatively, it is possible that cross- linking results in well-defined polymer morphology that facilitates counterion motion. The reason for the increase in D , values is at present under further investi- gation.

In Table 2 . the parameters describing the surface behavior of rhe cross-linked redox material are given. These parameters are useful in understanding the redox behavior of the electroactive materials [28]. For 1 ,lo-dibromodecane and p-dibromobenzene, the values for peak- to-peak separation (&,) are 0.0 mV, ratio of anodic to cathodic peak currents (iprr/ipc) are 1.0 2 0.01, and peak- widths at half-maximum height (FWHM) are 90 -+ 5 mV. These observations are consistent with rapid-reversible surface behavior of the redox couple. Finite diffusional behavior is observed up to potential-sweep rates of 100 mV s-’. These features are indicative of ideal surface behavior. The results for 1,5-dibromopentane are similar; however, the is slightly higher (0.255 V versus SCE) compared to the homopolymer, and the onset of semi- infinite linear df i s ion occurs at a lower potential-sweep rate (50 mV s-I). These results appear to indicate additional diffusional constraints at films cross-linked with 1, j-dibromopentane. Examination of the Nernstian behavior of the cross-linked redox material also supports this vim. The Xernstian plots for each cross-linlung agent at 20 mol % cross-linking are shown in Figure 5. The Nernstian slope for both p-dibromobenzene and 1 , l O - dibromodecane is 58 5 2 mV decade-’ and the plots are linear over the range - 2 2 log [ Os(II)]/[ Os( III)] 5 f 2 . This indicates ideal Nernstian behavior under the conditions used here. Identical behavior is observed for the uncross-linked polymer in sulfuric acid electrolyte. This suggests sufficient polymer void volume to accom- modate incoming charge-compensating counterions. The Nernstian slope for 1,5-dibromopentane up to 50% oxidation is 57 ? 2 mV decade-’ again indicating sufficient void volume for charge compensation. While at >jO% oxidation, super Nernstian behavior is observed with a

4 4 0 1

340 - 8

Ln

9 > E \ w

240

140

-2.5 0- 0 2-5

FIGURE 5. Nernstian plots for polymers cross-linked with 20 mol Yo of p-dibromobenzene (O), 1,lO-dibrornodecane (W), and 1 ,5-dibromopentane (6). The electrolyte was 0.1 mot dm-3 H,SO,, and the surface coverages were 1.0 x lo-’ moi cmP.

slopes of 83 2 3 mV decade-’. This suggests insufficient void volume to incorporate incoming counterions at this level of film oxidation and additional over potential is required for the enforced incorporation of counterions [391.

CONCLUSIONS

The objective of this study was the development of a simple procedure for the physical stabilization of the PVP- based redox materials and to investigate the effect of the stabilization procedure on the electrochemical properties of the redox material. It is clear from the results presented here that cross-linking with 1, j-dibromopentane, 1 ,lo-dibromodecane, and p-dibromobenzene is effective for the physical stabilization of the polymer films and that the exact level of cross-linking for 1,lO-dibromodecane and p-dibromobenzene is not critical in pro- ducing films of near-ideal electrochemical properties. The level of stabilization achieved for all cross-linking agents

560 Dohert) et a1

and at all levels is sufficient for long-term operation of the modified electrode under severe hydrodynamic conditions such as in FM and RDE experiments. The cross- linking approach used here is 3 simple one-step procedure that is carried out at room temperature under solid-state conditions. This approach is compatible with mass production technolos as the stabilization reagent need only be added to the redox material prior to or during the deposition step. There is no requirement for further processing such as heat or radiation treatment. The effect of cross-linking also improves the charge transport properties of the redox polymer, thus. extend- ing the upper linear range of sensors developed from these materials. As charge transport rates depend on the npe of cross-linking agent used. it is also possible that “fine control’’ of the current-carning capacin of the redos polymers may be achieved by selection of appropriate cross-linking agents and level of cross-linking.

ACMVO WLEDGMENT The authors ulould like to thank EOLAS (the I m h Science and Technolo83 Agenq) for fiiulncial assstance for tbs work

REFERENCES 1. S. Dong and Y. Wang, Electroam@sis 1 (1989) 99. 2. R. E‘. Murray. Acc. Chon. Res. 1.3 (1980) 135. 3. R. A. Durst and E. A. Blubaugh, in Furzdainentals and 4-

plications of Chemical Sensors. vol. 309, D. Schuetzle and R. Hammond, Eds.. .4CS Symp. Ser., 1984. p. 245.

4. K. Sumi and F. C. Anson. J . PITS. Chem. 90 (1986) 3845. 5 . D. M. Montgomery and F. C. Anson,./. Am. Chetn SOC. 307

6. S. Oyama and F. C. hson , J , Am. CImn. SOC. 101 (1979)

-. J. M. Clear, J. M. Kelly. C. M. O’Connell, and J. G. Vos.

(1985) 3431.

1634.

J . C l m x Rex fill) (1981) 3037.

8. S. E. Creager and 31. A. Fox, J , Electroanal. C/~nrz. 158 (1989) 431.

9. G. G. \Xallace, A l . Meaney, hl. R. Srn).rh. and J. G. Vos, Elec- hoana(1sis I (1989) 3

10. J. N. Barasci and G. G Ilace, Anal. Lett. 24 (1991) 2059. 11. D. P. Leech, Ph.D. Thesis, Dublin Cit) University. Dublin,

12. J. Facci and R. v”. R1urray.J P/?I~.$. Clxnz. 85 (1981) 2870. 13. B. Lindholm and hl. Sharp. J. Elect?-oaiull. Chem 198 (1986)

14. R. J. Forster and J. G. \.os. Macromolecules 2-3 (1990) 4372. 15. H. Nishide, J. Deguchi. and E. Tsuchida. Bull. Cl~em. SOC.

Jpn 49 (19’6) 3498. 16. J. F. Cassidy and K. Tol;uda,./. Electroaml. Chem. 285 (1990)

28-. 1’. R. J. Forster. A. J. Kelly. J. G. Vos, and hl. E. G. Lyons,

J. Electroanal. CIJenz. 270 (1989) 365. 18. W. J. Aben and A . R Hillman. -4nn. Rep. Prog. C l ~ r n 7 N C )

(1981) 3--. 19. R. J. Forster and J. G. Vos. J. Cikm SOC. Faraday Tram. 87

(1991) 1863. 20. A. P. D o h e m and J. G. Vos, J. Chem. SOC. Faradq, Trans

88 (1992) 2903. 21. C. P. h d r i e u x , J. M. Dumas-Bouchiat, F. M’Halla, and J. hl.

Saveant, J. Electi*oanul. Clienz. 113 (1980) 19. 22. C. P. h d r i e u x , J. hl. Dumas-Bouchiat, and J. M. Saveant,

J. Electroaizal. Chem 1.3 I (1982) 1. 23. R. 1. Forster and J. G. Vos, in Comprehensiue Anu&tical

Chonkitq,, vol. XXVII. 51. R. Sm!.rh and J. G. Vos. Eds.: El- sevier, Amsterdam. 1992. Ch. -.

2’f. A. Sevcik, Collect. Czech. Chew?. Commun. 44 (1948) 327. 25. R. J. Forster. 11. E. G. Lyons, and J. G. Vos, J. Clwnz. SOC.

26. S . 31. Oh and L. R. Faulkner, J. Electroanal. Chem. 269 (1989)

2-. B. Lindholm and >I. Sharp, J. Electyoanal. Chem. 198 (1986) 3’.

28. A. R. Hillman. in Electrochemical Science and Technology of Po(smers. R. G. Linford. Ed.. Elsevier, Amsterdam. 1987. Ch. 5.

29. E. F. Bowden! M. F. Dautartas, and J. F. Evans, J. Elec- troanal. Chon. -319 (1987) 49.

Ireland. 1991

3‘.

Faraday Tram. 87 (1991) 3761. --

stabilization of the redox polymer [os(bipy)2(pvp)10cl]cl by in situ chemical cross-linking

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