- Email: [email protected]
liquid interface
465
J. Electronnal. Chem., 266 (1989) 465-469 Elsevier Sequoia S.A., Lausanne - Printed
Preliminary
in The Netherlands
note
Steady state current for ion transfer reactions liquid/liquid interface
at a micro
J.A. Campbell and H.H. Girault Department of Chemistry, Scotland (Great Britain) (Received
University
of Edinburgh, Wesr Mains Road, Edinburgh EH9 3JJ,
22 May 1989)
INTRODUCTION
The advantages of microelectrodes in electrochemistry, both for the study of fast charge transfer reactions, and for investigations in resistive media have been widely reported [ 11. Charge transfer reactions across the Interface between Two Immiscible Electrolyte Solutions (ITIES) which include ion transfer, assisted ion transfer and electron transfer reactions are generally fast (10e3 < k” < 10-l cm s-‘) and the measurement of reliable kinetic data for these processes is largely hampered by the high resistivity of the organic phase. For this reason, the use of micro ITIES offers advantages in circumventing most of the difficulties generally encountered with larger interfaces where planar diffusion mass transport dominates. The first method proposed to obtain a micro ITIES was to support the interface at the tip of a micropipette similar to those used in electrophysiology for intracellular measurements. In this case, however, it was shown that an asymmetric diffusion field prevailed comprising a linear component inside the pipette and a radial component outside [2]. This first approach permitted the study of charge transfer with solvents of low permittivity such as 1,2-dichloroethane but kinetic studies had to rely on a model derived from the computer simulation of this asymmetric mass transport regime [3]. The only kinetic studies at micropipettes which could be analysed following the methodology generally used at solid microelectrodes were pseudo-first order bimolecular reactions where the concentration of the reactant inside the pipette was in excess compared to that of the reactant outside. With this approach the mass transport was controlled by the radial diffusion of the reactants and products outside the pipette. In this way it was possible to study ion/ionophore interfacial complexation reactions [4] such as M+(aq)
+ C(org)
0022-0728/89/$03.50
+ CM+(org) 0 1989 Elsevier Sequoia
S.A.
466
and interfacial electron transfer reactions [5] of the type 0, (as) + B, (erg) = B, (as) + 0, (erg) in cells where the aqueous phase was located inside the pipette. The micropipette approach generates steady state currents and is thus potentially very useful for analytical applications especially for the in vivo amperometric deter~nation of ionic species but because of the large resistance of the electrolyte in the micropipette tip (R > lo4 Q) this technique does not permit the use of very fast sweep cyclic voltammetry. The purpose of the present note is to show that micro ITIES can be formed in a microhole in a treated thin polymer film. Both the hole drilling and the polymer film treatment are relatively new processes based on laser technology. This new approach to micro ITIES fabrication allows the establishment of a diffusion field with radial contributions on both sides of the interface thereby enabling all the methodology developed for microdisc electrodes to be transposed for use with micro ITIES. EXPERIMENTAL
The cell consisted of a 20 pm diameter hole formed in a 12 pm thick polyester film (MeIynex.ICI) using a laser micro-machining technique developed by Exitech Ltd (Oxford, U.K.) in which bursts of UV laser photons are directed onto the surface using image projection techniques to define a pattern. The resolution provided by this technique is well below 1 pm and the microholes obtained are circular and very well defined. Because the process is based on UV irradiation by excimer lasers, the drilling occurs by laser photo ablative decomposition of the polymer and thermal effects such as melting, flowing and debris formation are eliminated. Previous attempts to use microholes, in membranes such as nucleopore, for the support of micro ITIES have hitherto proved useless due to the hydrophilic, or hydrophobic, nature of the membranes which leads to the creep of one or the other solvent through the holes thereby wetting the whole surface of the membrane. This difficulty is herewith overcome by “hydrophilising” one side of the normally hydrophobic polyester film using a laser process especially developed by Exitech Ltd. The success of this treatment was verified by comparing the wetting properties of water and 1,Ldichloroethane on the treated and untreated film. The film was mounted on the end of a glass tube (2 mm i.d., 6 mm o.d.) and the cell used in a conventional two electrode mode (Fig. 1). As a model system, the transfer of the acetylcholine cation (ACh) across the water/ 1,2-dichloroethane (DCE) interface was studied using Cell (I): 10 mM LiCl 10 mM TBATPBCI 10 mM TBACl ; AgCl 2 mM AChCl AgCl Ag DCE water water
467
/41 I
--I ’
?J!2zd 2’
i
Fig. 1. Cell for ITIES at a microhole in a thin polymer AChCl. (2) Organic electrolyte 10 mM TBATPBCl. The reference/counter electrodes are Ag/AgCI.
film. (1) Aqueous electrolyte 10 mM LiCl+ 2 mM (3) Organic reference electrolyte 10 mM TBACI.
A voltage ramp was applied at the organic reference electrode using a PPRl waveform generator (Hitek, U.K.) and the current through the aqueous reference electrode measured with a current follower based on a high-input impedance FET operational amplifier (Burr Brown OAP 104). Experiments were conducted at room temperature, i.e. 22°C _t - 1°C. RESULTS
AND
DISCUSSION
Figure 2 shows the cyclic voltammograms obtained from Cell (I). The potential window is limited at the negative end by Li+ crossing from water to dichloroethane and at the positive end by TBAf crossing from dichloroethane to water.
(EOil
_ Ewater’ ’ ”
Fig. 2. Cyclic voltammogram
for the transfer
of ACh+
from water to 1,2-dichloroethane
at 50 mV s- ‘.
-0.5
-0.6
tEoil- ‘her)
‘”
d
%il-
-0.4
-0.3
-0.2
%at*r )‘”
c
Fig. 3. Sweep rate dependence. (a) IO. {b) 20, (c) 100, (d) 20
mV s- I.
Figure 3 shows that at sweep rates of 10 and 20 mV s-l steady state waves are observed with a limiting current of 4.65 x 1W9 A. Using the equation for the diffusion limited current, i,, at a microdisc: i, = 4FDcr
where F is the Faraday constant,
D and c are the diffusion coefficient
and bulk
469
concentration respectively of the ion transferred and r the radius of the electrode, coefficient of the we obtain a value of 6.06 X lop6 cm’ s- ’ for the diffusion acetylcholine cation in water. This compares with a value of 5.2 x 10P6 cm2 s- ’ obtained from cyclic voltammetry at a large interface at 20 o C. At scan rates of 100 and 200 mV s-’ the current begins to peak as linear diffusion becomes more significant. The peaking with increasing sweep rate appears to be slightly more pronounced when scanning negatively, with respect to the aqueous phase, than positively. This may be due to the interface being disposed asymmetrically in the microhole or an artefact arising from the masking of the peak by the end of the potential window. Ideal microdisc behaviour requires a small film thickness to hole radius ratio and the present approach will be limited by the availability of pinhole free super thin films. This inherent limitation is currently under investigation. A further very interesting aspect of the laser technology involved in this work is the possibility of fabricating well defined microhole arrays. The results reported herein illustrate that micro ITIES can be fabricated in such a way that the methodology developed for microdisc electrodes can be extended to the study of charge transfer across liquid/ liquid interfaces. ACKNOWLEDGEMENTS
The authors wish to thank Dr. J. McAleer and Medisense U.K. for financial support and Exitech for stimulating discussions regarding the laser technology. One of us (J.A.C.) received an Information Technology postgraduate studentship from SERC. REFERENCES 1 R.M. Wightman and D.O. Wipf in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 15, Marcel Dekker, New York, 1989, p. 267. 2 G. Taylor and H.H. Girault, J. Electroanal. Chem., 208 (1986) 179. 3 G. Taylor, J. McAleer and H.H. Girault, in preparation. 4 J.A. Campbell, A.A. Stewart and H.H. Girault, J. Chem. Sot.. Faraday Trans. 1, 85 (1989) 843. 5 J.A. Campbell and H.H. Girault, in preparation.
J. Electronnal. Chem., 266 (1989) 465-469 Elsevier Sequoia S.A., Lausanne - Printed
Preliminary
in The Netherlands
note
Steady state current for ion transfer reactions liquid/liquid interface
at a micro
J.A. Campbell and H.H. Girault Department of Chemistry, Scotland (Great Britain) (Received
University
of Edinburgh, Wesr Mains Road, Edinburgh EH9 3JJ,
22 May 1989)
INTRODUCTION
The advantages of microelectrodes in electrochemistry, both for the study of fast charge transfer reactions, and for investigations in resistive media have been widely reported [ 11. Charge transfer reactions across the Interface between Two Immiscible Electrolyte Solutions (ITIES) which include ion transfer, assisted ion transfer and electron transfer reactions are generally fast (10e3 < k” < 10-l cm s-‘) and the measurement of reliable kinetic data for these processes is largely hampered by the high resistivity of the organic phase. For this reason, the use of micro ITIES offers advantages in circumventing most of the difficulties generally encountered with larger interfaces where planar diffusion mass transport dominates. The first method proposed to obtain a micro ITIES was to support the interface at the tip of a micropipette similar to those used in electrophysiology for intracellular measurements. In this case, however, it was shown that an asymmetric diffusion field prevailed comprising a linear component inside the pipette and a radial component outside [2]. This first approach permitted the study of charge transfer with solvents of low permittivity such as 1,2-dichloroethane but kinetic studies had to rely on a model derived from the computer simulation of this asymmetric mass transport regime [3]. The only kinetic studies at micropipettes which could be analysed following the methodology generally used at solid microelectrodes were pseudo-first order bimolecular reactions where the concentration of the reactant inside the pipette was in excess compared to that of the reactant outside. With this approach the mass transport was controlled by the radial diffusion of the reactants and products outside the pipette. In this way it was possible to study ion/ionophore interfacial complexation reactions [4] such as M+(aq)
+ C(org)
0022-0728/89/$03.50
+ CM+(org) 0 1989 Elsevier Sequoia
S.A.
466
and interfacial electron transfer reactions [5] of the type 0, (as) + B, (erg) = B, (as) + 0, (erg) in cells where the aqueous phase was located inside the pipette. The micropipette approach generates steady state currents and is thus potentially very useful for analytical applications especially for the in vivo amperometric deter~nation of ionic species but because of the large resistance of the electrolyte in the micropipette tip (R > lo4 Q) this technique does not permit the use of very fast sweep cyclic voltammetry. The purpose of the present note is to show that micro ITIES can be formed in a microhole in a treated thin polymer film. Both the hole drilling and the polymer film treatment are relatively new processes based on laser technology. This new approach to micro ITIES fabrication allows the establishment of a diffusion field with radial contributions on both sides of the interface thereby enabling all the methodology developed for microdisc electrodes to be transposed for use with micro ITIES. EXPERIMENTAL
The cell consisted of a 20 pm diameter hole formed in a 12 pm thick polyester film (MeIynex.ICI) using a laser micro-machining technique developed by Exitech Ltd (Oxford, U.K.) in which bursts of UV laser photons are directed onto the surface using image projection techniques to define a pattern. The resolution provided by this technique is well below 1 pm and the microholes obtained are circular and very well defined. Because the process is based on UV irradiation by excimer lasers, the drilling occurs by laser photo ablative decomposition of the polymer and thermal effects such as melting, flowing and debris formation are eliminated. Previous attempts to use microholes, in membranes such as nucleopore, for the support of micro ITIES have hitherto proved useless due to the hydrophilic, or hydrophobic, nature of the membranes which leads to the creep of one or the other solvent through the holes thereby wetting the whole surface of the membrane. This difficulty is herewith overcome by “hydrophilising” one side of the normally hydrophobic polyester film using a laser process especially developed by Exitech Ltd. The success of this treatment was verified by comparing the wetting properties of water and 1,Ldichloroethane on the treated and untreated film. The film was mounted on the end of a glass tube (2 mm i.d., 6 mm o.d.) and the cell used in a conventional two electrode mode (Fig. 1). As a model system, the transfer of the acetylcholine cation (ACh) across the water/ 1,2-dichloroethane (DCE) interface was studied using Cell (I): 10 mM LiCl 10 mM TBATPBCI 10 mM TBACl ; AgCl 2 mM AChCl AgCl Ag DCE water water
467
/41 I
--I ’
?J!2zd 2’
i
Fig. 1. Cell for ITIES at a microhole in a thin polymer AChCl. (2) Organic electrolyte 10 mM TBATPBCl. The reference/counter electrodes are Ag/AgCI.
film. (1) Aqueous electrolyte 10 mM LiCl+ 2 mM (3) Organic reference electrolyte 10 mM TBACI.
A voltage ramp was applied at the organic reference electrode using a PPRl waveform generator (Hitek, U.K.) and the current through the aqueous reference electrode measured with a current follower based on a high-input impedance FET operational amplifier (Burr Brown OAP 104). Experiments were conducted at room temperature, i.e. 22°C _t - 1°C. RESULTS
AND
DISCUSSION
Figure 2 shows the cyclic voltammograms obtained from Cell (I). The potential window is limited at the negative end by Li+ crossing from water to dichloroethane and at the positive end by TBAf crossing from dichloroethane to water.
(EOil
_ Ewater’ ’ ”
Fig. 2. Cyclic voltammogram
for the transfer
of ACh+
from water to 1,2-dichloroethane
at 50 mV s- ‘.
-0.5
-0.6
tEoil- ‘her)
‘”
d
%il-
-0.4
-0.3
-0.2
%at*r )‘”
c
Fig. 3. Sweep rate dependence. (a) IO. {b) 20, (c) 100, (d) 20
mV s- I.
Figure 3 shows that at sweep rates of 10 and 20 mV s-l steady state waves are observed with a limiting current of 4.65 x 1W9 A. Using the equation for the diffusion limited current, i,, at a microdisc: i, = 4FDcr
where F is the Faraday constant,
D and c are the diffusion coefficient
and bulk
469
concentration respectively of the ion transferred and r the radius of the electrode, coefficient of the we obtain a value of 6.06 X lop6 cm’ s- ’ for the diffusion acetylcholine cation in water. This compares with a value of 5.2 x 10P6 cm2 s- ’ obtained from cyclic voltammetry at a large interface at 20 o C. At scan rates of 100 and 200 mV s-’ the current begins to peak as linear diffusion becomes more significant. The peaking with increasing sweep rate appears to be slightly more pronounced when scanning negatively, with respect to the aqueous phase, than positively. This may be due to the interface being disposed asymmetrically in the microhole or an artefact arising from the masking of the peak by the end of the potential window. Ideal microdisc behaviour requires a small film thickness to hole radius ratio and the present approach will be limited by the availability of pinhole free super thin films. This inherent limitation is currently under investigation. A further very interesting aspect of the laser technology involved in this work is the possibility of fabricating well defined microhole arrays. The results reported herein illustrate that micro ITIES can be fabricated in such a way that the methodology developed for microdisc electrodes can be extended to the study of charge transfer across liquid/ liquid interfaces. ACKNOWLEDGEMENTS
The authors wish to thank Dr. J. McAleer and Medisense U.K. for financial support and Exitech for stimulating discussions regarding the laser technology. One of us (J.A.C.) received an Information Technology postgraduate studentship from SERC. REFERENCES 1 R.M. Wightman and D.O. Wipf in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 15, Marcel Dekker, New York, 1989, p. 267. 2 G. Taylor and H.H. Girault, J. Electroanal. Chem., 208 (1986) 179. 3 G. Taylor, J. McAleer and H.H. Girault, in preparation. 4 J.A. Campbell, A.A. Stewart and H.H. Girault, J. Chem. Sot.. Faraday Trans. 1, 85 (1989) 843. 5 J.A. Campbell and H.H. Girault, in preparation.