6818
J. Phys. Chem. 1994,98, 6818-6825
Spectrofluorimetric Hydrodynamic Voltammetry: Investigation of Reactions at Solid/Liquid Interfaces Richard C. Compton,' Jacob Winkler, D. Jason Riley, and Stephen D. Bearpark Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ,United Kingdom Received: February I , 1994; In Final Form: April 14, 1994'
The extension of spectrofluorimetric hydrodynamic voltammetry for the study of heterogeneous reactions occurring at solid/liquid interfaces is described. In particular the rate of migration of the cationic species H+ and K+ within a thin organic film containing the fluorescent probe N-(5-fluoresceinthiocarbamoy1)-1,2-dihexadecanoylsn-glycero-3-phosphoethanolamine (F362) as the triethylammonium salt dispersed in a large excess of 1-trimethylammonium 2,3-di [C14-C18 acyloxy] propane chloride (HEQCl) may be followed through the quenching of the F362 fluorescence by the cation. Two different types of experiment are described. In the first the organic film is supported on the silica wall of a channel flow cell, and the migration of H+cations into the film, induced through their stepwise formation at an electrode located upstream of the organic layer. In the second the film is coated onto the channel electrode surface, and migration of K+ ions induced by changes in the electrode potential. In each case the diffusion coefficient of the cation within the membrane may be inferred from the fluorescence transient.
Introduction We have recently introduced's2 an experimental approach"spectrofluorimetrichydrodynamic voltammetry" (SFV)-for the characterization and estimation of intermediates in electrolysis reactions by means of fluorescence spectroscopy. In particular the high sensitivity of the latter technique may be exploited by locating a specially designed channel electrode, shown in Figures 1 and 2 of ref 1, inside a spectrofluorimeter. The practical requirements of a channel flow cell (CFC) for this application were detailed previously,l and it was shown that in situ electrochemicalfluorimetry measurements could be performed with no significant loss of quality of either the voltammetric or luminescence measurements. Specifically the existence of otherwise unsuspected intermediates in electrochemical processes was demonstrated,' and the capability for identifying species formed at low concentrations and their mechanistic roles in electrode reaction mechanisms were realized.lg2 The aim of the work described in this paper is the extension of the SFV technique to the study of heterogeneous reactions at solid/liquid interfaces. In particular the use of SFV to probe the movement of charge in organic films was examined through the use of two distinct types of model experiment. In the first an insoluble film of the fluorescent material N-(5-fluoresceinthiocarbamoyl)- 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (F362) as the triethylammonium salt dispersed in a large excess of 2,3-bis[C14-C18-acyloxy]propyl-l-trimethylammonium chloride (HEQCl), was applied to a silica surface which formed part of one wall of a CFC located in a fluorescence spectrometer. An electrode was positioned immediately upstream of the film and used to electrolytically form protons through the oxidation of hydroquinone, HzQ (present in solution):
The protons so formed could diffuse into the organic film and react chemically with the F362, destroying its fluorescence. Galvanostatic steps on the electrode could be used to "turn on" Abstract published in Advance ACS Abstracts, June 1, 1994.
0022-365419412098-68 lSS04.50/0
0
OH
B
CHj-(CH,)=C--O-FH,
R
CHj--(CH,)~F--O-YH 0 H,C-O-P--OCH,CH,NH
A.
FSNH
E~NH'
n I15 (30%) and n = 17 (70%)
HEQCl
or "turn off" proton generation. It is shown how by monitoring the fluorescence signal, following such a galvanostatic step on the electrode, as a function of real time and electrolyte flow rate, information on proton transport through the organic film in this first model system may be obtained. In the second type of model, experimentalmeasurementswere made of fluorescent layers located at the electrode/solution interface. Preliminary experiments showed surfaceconcentrations of as little as 5.0 X 10-10 mol cm-2 of N-(Texas Red sulfonyl)1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt (T1395) could be detected using the experimental protocol outlined above without modification. This sensitivity was exploited in a further study of an organic layer of HEQCl and F362 (1OO:l)applied to the electrode surface and exposed to an aqueous solution of KCl. Strong fluorescence from F362 was observed but was found to be sensitive to the electrode potential applied. In particular for potentials negative of the point of zero charge the fluorescence was found to be greatest. If then the electrode was stepped to potentials positive of the point of zero charge, ion migration within the coat was induced. Specifically 0 1994 American Chemical Society
Spcarofluorimetric Hydrodynamic Voltammetry
The Journal oJPhysico1 Chemistry, Vol. 98. No. 27, 1994 6819
't
Irradiation r
Silica cover plate
z
-
Flow
T13%
K+ ions wererepelled from theelectrode, and thesequenched the fluorescence of Fxz. A diffusional model was developed to quantitatively describe the quenching transients experimentally observed, and it is again shown that information about ion transport is realizable in this sccond type of experiment.
Electrical contact through teflon Fisve 1. Teflon channel flow csll used for in situ SFV mcMurc"u at the elsctmde/solution interface. DETECIOR
Experimeatd Sectloo
The cell used for f l u o m n t measurements made from the electrode surface is illustrated in Figure 1. It is constructed in Teflon and has a 4 "X 4" platinum electrode cemented into the bottom of the cell. The cell and electrode were polished so that the bottom of the cell was smooth and entirely flat. The electrical connection was made to the rear of the electrode via a hole through theTefloncell. Theassembledcell wassupported within a Perkin-Elmer front surface accessory (Perkin Elmer Part No. 5212 3130) and u d in conjunction with a PerkinElmer Model LS50 luminescencespectrometer. Thecell position inside the spectrometer could be finely adjusted for optimum sensitivity with the front surface accessory so that the incident exciting beam analyzed a central proportion (4mm X 1 mm) of the covered electrode surface. The resulting luminescence was detected in a direction perpendicular to the incident light, as shown in Figure 2. For surface fluorescmce measurements made downstream of the electrode the flow cell described in ref 1 was utilized; in this case the electrode was supported on the silica cover plate of the cell shown in Figure 1 but was otherwise equivalent in design. The cover plate also carried the organic film under study. For preliminary measurements of the sensitivity of our experiments toward species located on electrode surfaces, N(Texas Red sulfonyl)-1 ,Z-dihexadecanoyI-sglycero-3-phosphoethanolamine, triethylammonium salt Molecular Probes, Eugene, OR) was dissolved in acetone in which the bulk concentrationwasdetermined by UVmeasurements. Thesolution was applied to the electrode and the solvent evaporated, leaving behind a homogeneous coat of T119r. For experiments involving ion transport within organic films, the latter were formed using a 2.0 g dm-3 aqueous solution of N-(5-fluoresainthiocarbamoyl)-l,2-dihexadecanoyl-sn-glyce~ 3-phosphathanolamine, triethylammoniumsalt (F362; Molecular Probe) and I-trimethylammonium-2,3-di[Cl~l8acyloxy] propanechloride (HEQCI; Hoechst) in the ratio 1:100 (wtlwt) (supplied by Unilever plc, Port Sunlight, United Kingdom). The solution was prepared by mixing aliquots of Fl6z and HEQCI dissolved in chloroform, evaporating, heating the resulting solid to 80 OC, and adding water. To form the organic layers, the platinum electrode or the appropriate area of the silica cover plate was covered with HEQCI ,,F solution which was subsequently evaporated off using a heat gun. Comparison of HEQCI with molecules of similar structure and known size34 allowed the molecular dimensions of HEQCl to beestimated and hence the film thicknesses employed to be determined from 1.6 X l ( r to 4.8 X l ( r cm. The cover plate was constructed in optical quality synthetic silica (Suprasil2) by HereausSilica and Metals Ltd.. Weybridge,
+
F l m 2 Top vim, of the channel flow cell depicted in Figure 1.
U.K. Cover plates. suitably coated with organic layers, were cemented to the channel unit using a low-melting wax (Vychem Ltd., Poole, U.K.). whichgaveexcellentadhesiontoTeflon.melted at 50 OC, dissolved in ethanol for cell dismantling/cleaning, and g a v e a g d seal f a water. Assembledflowcellswereincorporated intoa gravity fed all-glass flow system which permittedoutgassing of the solution. The latter was fed from a 250-cm' reservoir via one of two capillaries for flow range setting, the adjustment of flow within each range being accomplished by varying the height between the reservoir and the output tube through which spent electrolyte ran to waste. Flow rates between IO-' and 1 W cm' s-I were routinely used. A platinum gauzecounter electrode was placed downstream of the working electrode just outside the spectrometer, and a silverpseud~referenceelectrodewaslocated upstream. When assembled, the flow cell had cross-sectional dimensions of 6 mm X 0.4 mm and was approximately 45 mm long (all dimensions were measured accurately using a traveling microscope). The cell width was sufficiently large relative to the electrode width that "edge effects" could be neglected.' At the flow rates used, the flow was laminar (Reynolds number < 100). and,withalead-inlengthofmorethana fewmillimetersupstream of the electrode, a parabolic (Poisseuille) velocity profile was fully developedacross the short dimension of the cell in thevicinity of the electrode and downstream of it.6 The velocity profile across the width of the channel is, to an excellent approximation, that of plug flow. These hydrodynamic characteristics have been amply confirmed in nnmerous experiments with flow cells of the design specified.'.'.S Rotating disc electrode experiments w m conducted using platinum electrodesof approximately 7-mm diameter and Oxford Electrodes (Oxford, U.K.) equipment (motor controller and potentiostat). Potassium chloride (BDH Ltd.. AristaR). hydroquinone (Aldrich, 99%). and fluorescein (Aldrich. 98%) were used as received. All solutions were made up using triply distilled dbionizcd water (resistivity 18 MR cm) containing 0.5 M KCI as background electrolyte. Solutions were degassed with oxygenfree argon prior to use. AIlcomputationswerecamedoutonaSunSpar*ltationusing programs written in FORTRAN 77.
-
Compton et al.
6820 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994
1
600
500
400
i/
Wavelength / nm
Figure 3. Fluorescence spectrum of F362 bound in an organic film of HEQCI, measured on the' silica cover plate of a CFC downstream of its platinum electrode.
We first consider experiments in which protons are electrogenerated upstream of an organic film of HEQCl and F362, second report the sensitivity of measurements made directly on the electrode/solution interface as revealed through measurements with TI395 as outlined in the Introduction, and finally discuss experiments in which changes in the potential of an electrode induces ion migration within a coating formed directly on its surface. Experiments with Organic Films h t e d Downstream of the Electrode. Initially the fluorescence behavior of F362 in solution was investigated. The optimal excitation and emission wavelengths of F362 in aqueous solution were determined as 493 and 519 nm, respectively, and the signal intensity was found to be directly proportional to [F362] over the concentration range 3-16 pM. The intensity of the fluorescence signal of F362 in solution was found to be pH dependent. This was explained by the followingproton induced formationof the nonfluorescentlactonoid structure within F362.7 OH
HO
0
.' H v +
NOT Fluorescent
0
8
4
P i Thickness / 10 - 4cm
Figure 4. Fluorcsccnce intensity of the HEQCl/F,e2 film as a function of the thickness of the latter.
Results and Discussion
HO
0
H'
Fluorescent
This reaction mimics that seen in fluoresceinmolecules unattached to hydrocarbon chains for which a pKa value of 4.4 has been measured in aqueous solution.8 Fluorescence spectra (Figure 3) of F362 bound as the minor component (1%) in films of HEQCl on the silica cover plate of a CFC were next measured in 1.00 mM/0.5 M hydroquinone/ KCl aqueous solution, initially in the absence of electrolysis. Optimal spectra were obtained using an excitation wavelength of 493 nm which resulted in emission at 519 nm. These observations are in excellent agreement with the data reported above for solution phase F362 and in the literature* for F362 at pH ca. 7. The intensity of the F362 fluorescence was found to vary linearly with film thickness (Figure 4) over the range investigated, up to 7.6 pm. This suggested that the excitation beam sampled the entire film depth with no significant attenuation of the excitation beam. The stability of the fluorescent layers toward dissolution/desorption was examined by flowing an aqueous solution (of H2Q and KCl, as above) through the cell at a rate of 20 X 10-3 cm3 s-1 for 2 h. Monitoring of the fluorescencefrom the coat showed a reduction of fluorescence signal intensity of less than 2%. The rate dissolution of F362 from the silica surface was thus inferred to be negligibleon the time scale of the transient experiments described below.
50
o
1
!
I
0
600
I
Timela
1200
Figures. Transient response of the fluorescencesignal from the HEQCl/
Fxz coat due to electrochemicallyinduced proton injectioninto the organic
film located downstream of the CFC electrode. The solution flow rate used was 19.8 X l t 3 cm3 s-l.
Next we examined the behavior of the fluorescent coat of F362 when exposed to a flowing solution of the above composition (pH ca. 6.1) subjected to a stepwise injection of protons electrogenerated from hydroquinone at the upstream working electrode. Typically the current was jumped from 0.00 to 4.00 pA which represents 5-15% of the total transport limited current. This was found to result in the (partial) quenching of the fluorescence of F362. The transient behavior of the fluorescence intensity following proton injection into the system via the galvanostatic step on the working electrode is illustrated in Figure 5 . Also shown is the transient induced by switching 'off" the electrode current. It can be seen that the latter returns the coat to a fluorescent intensity essentially indistinguishable from that seen initially before the galvanostatic step showing that the loss of fluorescence is due to protonation of the insoluble coat rather than, for example, proton induced coat dissolution. We consider first the equilibrium signal intensity reached after the applicationof the galvanostaticstep. For a particular electrode current this was found to be dependent on the solution flow rate. By measurement of the initial and steady-state fluorescence intensities it was possible to calculate the average ratio ([F36~,k-~] / [Fjaz])under equilibriumconditionsover the entire area of the coat probed spectrofluorimetrically. It is expected that this ratio is given by the flowing equation
where Ka is the F36z acid dissociation constant, [H+]d. the concentration of protons at the solution/coat interface, and Kp the partition function of protons between the organic fluorescent layer and the solution:
The backwards implicit finite difference (BIFD) methods was used to compute the equilibrium H+ concentration profile
Spectrofluorimetric Hydrodynamic Voltammetry
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6821 film was cut along the x-direction parallel to the zy-plane into
K (typically 1000) small elements, each of length Ax, and the proton concentration at the film interface for each element was taken to have the constant value deduced from the BIFD modeling of the steady-state proton distribution in the CFC. Note that the latter is such that the surface concentration of protons decreases downstream of the electrode due to convective dilution effects. The assumption of instantaneous transport of protons in the aqueous phase relative to the organic coat permitted the analysis of the fluorescencetransients on the basis of two separate models. It was assumed that the transient response observed was due either to slow diffusion of protons through the coat (in the y-direction) or to a slow rate of proton induced transformation of F362 within the film into the nonfluorescent lactonoid form. The theory for both processes was developed and tested against experimental observations. First the case in which proton migration within the film is rate limiting was modeled. It was necessary to solve the onedimensional diffusion equation in the y-direction for each x-element (labeled k, where 1 C k C K):
-t
0.0
25
0
-3
3 -1
50
Flow Rate / 10 cm s
Figure 6. Theoretical (x) and experimental(+) values of ([F j 6 2 ~ d d ] / [Fad) as a function of flow rate. The numerical modeling assumed a ratio of K,/Kp 1.2 X l t J mol dm-3.
TABLE 1: Variation with Flow Rate of the Time Required for a Steady-State Proton Concentration To Be Achieved at a Point 8 mm Downstream of an Electrode Following a Potential Step flow rate/ flow rate/ (lo-' cm3s-I) time delay/s ( l t 3cm3 d) time delay/s 5.0
11.7
10.0
5.5 3.3
20.0
30.0 40.0 50.0
2.4 2.0 1.7
downstream of the electrode from a knowledge of the bulk concentration of HzQ, the CFC geometry, and the H+ diffusion coefficient9correspondingto the steady-state currenb of interest. Reference 5 contains full details of the necessary computational procedures which employed a Cartesian grid size defined by the following dimensions: the number of boxes in the y-direction = 500; the number of boxes in the x-direction over the electrode surface = 250. Programs took approximately 10 min of CPU time to run on a Sun Sparcstation. Theoretical values of ([F362,hawidI / [F362]) at equilibrium were obtained from eq 1 for different K./Kp ratios by using the calculated concentration profiles to give values of H+aol.averaged over the area of the coat probed by the fluorescence measurement. It was found that a value of Ka/Kp = 1.2 X le5 (fl.O X 1 V ) mol dm-3 resulted in the best fit between experimental and theoretical data over the wide range of flow rates (1.0 X le3to 45.0 X le3cm3 s-I) studied, as shown in Figure 6. The mean K./Kp ratio obtained from the analysis of three experiments was very similar to the values of K8 quoted in the literaturelo for Langmuir-Blodgett monolayer films of F362 exposed to acidified water. This suggested that if Ka were unchanged between the thick (3.8-pm) HEQCl film and the Langmuir-Blodgett film, then the partition function of protons between the HEQ/F362 layer and solution was close to unity. However, we consider the likely nature of the film in more detail below. In order to explain the fluorescence transients obtained in experiments such as those shown in Figure 5 , the concentration profiles for the evolution of H+ downstream of the electrode were determined for the CFC geometry used experimentally (channel width = 0.60 cm; electrode length = 0.400 cm; electrode width = 0.396 cm) using the BIFD method.11 The computational results showed that equilibrium proton surface concentrations were reached after a period of time which was flow rate dependent; at the slow flow rate of 5.0 X le3cm3 s-l the concentration of H+ on the same wall of the flow cell as the electrode, at a point 8 mm away from the downstream edge of the latter, reached 99% of its steady-state value within 12 s. The corresponding time for other flow rates is shown in Table 1. The times in Table 1 can be seen to be many times faster than the fluorescence response such as that shown in Figure 5 . Therefore in the followingkinetic analysis the proton concentration profile in the x-direction along the cell at the film/solution interface is considered to change effectively instantaneously. For the purposes of modeling, the
given a film thickness 1. The silica surface was assumed to be located at y = 0 and the solution/film interface at y = 1. An initial (t = 0) condition of zero proton concentration throughout the film was assumed, [H+]rhky,o= 0, and the following boundary conditions applied at t > 0:
The problem is isomorphic to that solved by Oglesby et a1.,12 so we merely quote the result:
from which the total number of protons in the sampled section of film at the time t (n(H+)fiI,,J can be deduced as
where w represents the width of the detector beam. A more useful form of (6) is
Similarly when diffusion of protons out of the film is considered, we analogously obtain
Compton et al.
6822 The Journal of Physical Chemistry, Vol. 98, No. 27, 1994
1
1 "\ where t = 0 now corresponds to the time at which generation of protons at the electrode ceased. We next examine the alternative model in which the chemical reaction forming the nonfluorescent lactonoid form is assumed to be rate determining and reversible. It was assumed that protonation was a second-orderprocess and deprotonation a firstorder process and that these processes were slow compared to proton diffusion through the coat: no
0
HO
COO
+
H'
-
k
'
-
~ "CSS m 3.8 pm
0
I
I
14'
OH
~
k >
R
R
where kl is a second-order rate constant and k-1 a first-order rate constant. The rate of reaction is given by the equation 0.0
which is easily solved, as [H+]sol.,k,,,r for each element in the x-direction is constant and known for the proton injection transient via the BIFD analysis given above, while [H+]so~.,k,/,fis equal to zero for the fluorescence transient induced when the electrode current is shut off. Also [F362,1actonoidlfilm,ky,f =
IF3621 film,ky,O -
LF362] fi1m.ky.f
(lo)
Integrating and summing over all elements gives
0 6oo Time/s l2Oo Figure 8. Function Ar) versus time as deduced experimentally from measurements made at four separate flow rates: 11.2 X 10-3, 19.6 X 10-3, 32.5 X 10-3, and 44.5 X 10-3 cm3 s-I.
'1 100 250 Time/s 443) Figure 9. Plots of 7 against time for proton uptake by the film located downstreamof the electrodesurface. The flow rate used experimentally was 19.6 X lk3cm3s-I.
for the forward transient and [F362.1actonoid] film,ky,r = exp(-k-,t) [F362,1actonoid]film,ky,O
(12)
for the reverse transient. Note that eq 6 predicts the time scale of the transient response to depend on the film thickness (I) in the case of slow proton diffusion, whereas eq 11 predicts it to be independent of 1 in the case of slow lactonoid formation. Experimentaltransients for different film thicknessesare shown in Figure 7. These permitted the calculation of the ratio (( [F362] - [F362]f=m)/[F362,1actonoid]f=m) at different pointson the transient. The values were found to be dependent on the film thickness; (([F362] - [Fsa2]r=m)/[F362.11ctonoi~]f-m) attains avaheof 0.5 after 32.0 s of proton generation for a film thickness of 3.8 fim and after 156 s for a film thickness of 7.6pm. It was concluded that the fluorescence transient did not reflect slow lactonoid formation within the coat. Accordingly further analysis was pursued using the slow proton diffusion model. The summations in eq 7 and 8 convergerapidly, so enabling FORTRAN computer programs to be written in order to calculate values off(t) andf'(t) for a range of T. Matching
of theoretical and experimental values off(r) andf'(r) allowed T to be calculated as a function of time. First, it was noted that although the raw transients did depend on flow rate through its influence on [H+]so~.,k,/,t, the values of f ( f ) and f'(t) deduced experimentally were independent of solution flow rate, as shown in Figure 8. This observation was consistent with the diffusion model proposed. Second,plots of T against time for proton uptake by the film were linear, as shown in Figure 9; again this is in agreement with the predictions of the diffusional model. Third, the average gradient, 0.015 f 0.003 s-1, of such plots for films formed by the evaporation of 0.5 cm3 of the parent solution was, within experimental error, approximately four times greater than that obtained, 0.005 f 0.001 s-1, when twice the amount (1.0 cm3) of parent solution was used. This again is exactly as predicted. Finally plots of T against time deduced from proton desorption transients made on films of the former thickness were linear (except at very long times) with a similar average gradient, 0.016 f 0.003 s-1, to that observed for proton adsorption (the deviation from the anticipated linear relationship at long times, > 1000s, was probably attributable to a two-dimensionaldiffusion effect). All data thus suggested slow proton diffusion in the coat
Spectrofluorimetric Hydrodynamic Voltammetry
0.00
0.25
p,,w
0.50
Flgure 10. Koutecky-Levich plot for the reduction of Fe(CN)6* at a platinum rotating disc electrode.
as the process controlling the rate of bleaching of the coat fluorescence. It was concluded that the rate determining step of the fluorescenceresponse to switching the proton generatingelectrode current on or off was the diffusion of protons into or out of the film. Since the gradient of a T versus time plot is equal to (0.21 412), the diffusion coefficient of protons within the coat could be inferred using the values of 1 estimated as described above. The following result was obtained: D = (1.04 f 0.1) X cmz s-1. This value is 4-5 orders of magnitude less than that of a free proton diffusingin aqueous solution. This suggests that the value reflects the movement of protons within the film as, for example, opposed to movement through pores or pinholes in the coating. The insoluble fluorescent coat is composed predominantly of the HEQCl species which comprises large, essentially immobile, cations together with charge compensating chloride ions. As such, the coat might be expected to act as an "anion exchange membrane", permitting the relatively rapid diffusion of anions but not cations. To confirm the possible anion exchange nature of the film, the reduction of the ferricyanide anion, Fe(CN)63-, at platinum rotating disc electrodes (RDEs) was investigated with the electrodes (i) "naked" and (5) coated with the organic film of interest. An aqueous 1 mM K3Fe(CN)6 solution at pH 13 containing 0.5 M KCl/KOH as supporting electrolyte was employed in the investigations. Modified platinum RDEs were prepared by the pipetting of 20 p L of the aqueous F362/HEQCl solution (see Experimental Section) on to the electrode surface and evaporating to dryness. The thicknesses of the films thus formed were estimated as 1.4 pm. Experiments using thicker films indicatedthat the electrodesurface was effectively insulated from solution and no voltammetry was visible. The results indicated that the transport limited current for the Fe(CN)6C was partially suppressed by the film. Such behavior may be analyzed via "Koutecky-Levich plots"10 to give values of an electrochemicalrate constant, ME, for the modified electrode:
where j is the flux of electroactive material, X,being discharged at the rotating electrode and [XI,,, denotes the concentration of X at the interface between the electrolyte and the outside of the layer which modifies the electrode surface. In particular
where kD is the mass-transfer rateconstant related to the frequency of rotation, f/Hz, of the electrode, and [XI, is the bulk concentration of X. Analysis in this manner using plots of reciprocal transport limited current, I-I, against cI)-1/2are shown in Figure 10 from which a value of 1.65 X 1 od cmz s-1 for diffusion of ferricyanidewithin the coat was calculated.13 The much more rapid rate of diffusion of ferricyanidewithin the coat as compared to H+tends to confirm the suggestion that anion migration within
The Journal of Physical Chemistry, Vol. 98, No. 27, 1994 6823
650 700 Wavelength / nm Figure 11. Emission spectrum from a layer of 5 X 1O-Io mol cm-* ofT!,s~ on the surface of a platinum electrode measured using an excitation wavelength of 580 nm.
600
the coat is fast since the anions can exchange with chloride ions and become transported through the coat. In contrast the latter effectively excludes cations, including protons, which can only move much more slowly with the fluorescein groups pendant on the F3,9 molecules acting as "proton sinks" within the film. Experiments with Organic Films Located on the Electrode Surface: Preliminary Measurements. First experiments were carried out to assess the minimum amount of material that might be readily visible in the fluorimetric examination of electrode surfaces. Aliquots of T1395, dissolved in acetone, were applied to the surface of the platinum electrode using a 10-rL syringe, and the solvent was allowed to evaporate. The cell was then incorporated into the fluorescence spectrometer, as described in the Experimental Section. Triply distilled, deionized water was flowed through the flow system, and the surface, investigated for fluorescenceby scanning through different excitation wavelengths and monitoring any emission. Optimum fluorescence was found for an excitation wavelength of 580 nm. This gave an emission maximum at 602 nm.14 Progressively smaller surface coverages were examined to identify the sensitivity of the technique toward the measurement; Figure 1 1 shows the excitaiton and emission spectra seen using a coverage of 5 X l0-lo mol cm-*. Adequate signal to noise is apparent. We estimate that this coverage corresponds to approximately 2-4 monolayers of the bulky fluorescent phospholipid probe. The latter has a high extinction coefficient for the absorption of light at 585 nm of approximately 10' M-1 cm-l.14 Nevertheless the possibility of the fluorescent monitoring of relatively low amounts of material located near the surface of electrodes is evident. Experiments with Organic Films Located on the Electrode Surface: Studies of Ionic Migration. Organic coats were made up by applying a solution of HEQCl and F362 to the platinum electrode and carefully evaporating the water with a heat gun. Coverages in the range (1.7-5.3) X lO-' mol cm-2 (of HEQCl/F362)were obtianed corresponding to estimated coat thicknesses of 1.6-4.8 pm. An aqueous 0.5 M KCl solution was passed through the flow system, and optimum fluorescencewas found for an excitation wavelength of 496 nm, which gave an emission maximum at 5 17 nm. The electrode potential was controlled by a standard three electrode potentiostat, and the fluorescence intensity, monitored as a function of electrodepotential measured relative to a pseudosilver reference electrode. Jumps from 0 V to potentials between 0.1 and 0.9 V were recorded. Figure 12 shows a typical result. Up to point a the potential was held at 0 V, after which it was stepped to 0.5 V. A marked decrease in the intensity of the fluorescence signal is observed. After a period of time a steadystate value was reached at the more positive potential. At point b, the potential was jumped back to 0 V and a regeneration of signal intensity was seen. The reverse transient reached a steadystate value at point c, the fluorescence intensity at this point comparable to that at point a indicating the near quantitative
Compton et al.
6824 The Journal ofphysical Chemistry. Vol. 98. No. 27. 1994
Zoo0
lo00
0
3wo
Timels
Fl-
12. Fluorescence signal intensity at a wavelength of 517 nm,
measured usingancrcitation wavelength of496nmata platinumelsVodc covered with 3.5 X I@' mol HEQCI/Fxz. Point a corresponds to a potential step from 0 to 0.5 V (vs Ag), point b to the revem step from 0.5 to 0.0 V (vs Ag), and point E to the Anal intensity recorded.
0.4
0.0
0.8
E I V (=.Ag)
Fbac 13. Cunmt/volte,gecurversorded for a platinumekctrcdecoated
Returning to a consideration of the fluorescence transients, a decrease in signal was observed as the potential of the platinum electrode was changed anodically from negative to positve of the PZC. Hence a model was developed where 'inversion" of the double layer injectscations into the organiccoat from thedouble layer. Subsequently diffusion transports the injected cation throughout the coat. The injected cation is most likely to be K+, and experiments on F362 free in solution showed that its fluorescence was quenched by the addition of KCI or KBO.. Modeling of the fluorescence transients initially followed an analogous approach to that used to describe the quenching of Fx2by protons diffusing into the coat and assumed a boundary condition akin to that formulated in eq I in which K+ cations in theaqueouslayerat theelectrode/coat boundary wereequilibrated with those immediatelyinside thecoat asdescribed by a (potential dependent) partition coefficient. However, agreement with experiment was poor regardless of the selected values for Kp and the diffusion Mefficient of K+ in the coat. Accordingly the following model was developed in which there is a kinetic barrier to the uptake of K+ into the organic layer. We assume that the flux of K+ entering the coat equals the rateat whichitcan transfer from theouter aqueouslayer adjacent to the electrode into the organic coat:
where &+ is the diffusion coefficient of K+ in the organic coat, [K+]film,ky., the concentration of K+ in the organic coat at time t and distance y from the electrode surface, [ K + ] D ~ , othe ,r concentration of K+ in the aqueous-like layer over the electrode surface containing the double layer at time t, and kH= the heterogeneous rate constant for K+ entering the organic coat from the latter. Considering the loss of K+ from the double layer
of HEQCI/Fxz measured in 0.5 M KCl/(aq).
with 3.5 X IO-' mol
[tkT 11J -
Layer
where h is the height of the aqueous-like layer over the electrode and A the area of the electrode. After integration
[K+]DL~,o,, = ([K+]DL.J.o.o - [ K + ] D ~ , o , & - ( ~ " * '+
Channels
.?e. :s: -4.
[K+]DL~.O.~ (17) Equations 15 and 17 together give
Teflon cell wall
Flgmre 14. lllustrationoftheinfmedstNCturcofthecloctrodc/solution interfacial region. Tiny pinholes in the mt 'feed" a thin layer of electrolyte immditely adjacent to the elstrode surface.
reversal of the F36zquenching proms. We return to the origin of the potential induced changes of fluorescence intensity subsequently. A current-voltage curve for the coated electrode in contact with the aqueous 0.5 M KCI solution was next recorded. This isshownin Figure 13.whichcloselyresembles thecorresponding curve measured at an uncoated electrode and solvent decomposition is evident at potentials anodic of + O h 5 V (vs Ag). This suggested thattheelectrode hadcontactwith theaqueoussolution, probably via tiny pinholes in the organic layer (Figure 14). A tbinlayerofelectrolyteisthereforesbownin Figure 14aslocated in the electrode/coat interphase. Reponed" measurements of the differential capacitance of platinum electrodes in contact with 0.5 M KCl(aq) suggest that the potential of zero charge (PZC)liesbetweenO.Oand0.5Vsotbatpotentialjumpsbetween these two potentials are likely to induce movement of K+ and CIions, in and out of the double layer.
where 6 = kHm/h and
F = ([K+l~~.,tp.o - [K+]DL.~,O.,)~HET/DK+(19) We assume that there is a one to one quenching between molecules of K+ and Fx2. probably as the result of the formation ofa tight ion pair. Thefluorsccncesignalduetotheunquenched F36zin the coat before the potential jump (Fo) and under the new equilibrium conditions (Fp) can be written start finish
YA/[F&~~T = Fo Y A / ( [ F & o T - [K+]fii,k,.-)
(20)
= F-
121)
where [F36z]mis theconcentration (molcm-3) ofthe fluorescent label in the organic coat. [K+]fildJ.. is the equilibrium concentration of K+ in the coat, and y is a proportionality constant. Rearranging eq 20 and 21 gives
Spectrofluorimetric Hydrodynamic Voltammetry
-= - F m Fo FO
[K+Itilm,ky,m
The Journal of Physical Chemistry, Vol. 98, NO. 27, 1994 6825
+
[F3621TOT
Figure 15 illustrates the concentration profile of K+ in the coat at time t and shows how the amount of unquenched F362 may be deduced. The corresponding fluorescence signal is given by
Unquenched
Ip=l left in layer
where I is again the height of the organic coat and
P = l for 1-
[K+Ifilm.ky,t, [F3621TOT
I
I
7
0 I Y Figure 15. Concentration of K+as a function of distance, y , into the coat. The coat thickness is 1. The shaded area is proportional to the amount of unquenched Fj62 in the coat.
Dividing by eq 20 gives
This equation permits the computation of the fluorescence transient provided [K+]Y,fcan be calculated for the transport mechanism of interest. This was achieved by the numerical integration of Fick's first law of diffusion for K+ inside the coat:
W+I -= at
a2[K+]
D,,
aY2
using a simple explicit finite difference method16 which utilized 500 boxes and required approximately 1 h of CPU time on a Sun Sparcstation. Equation 18 is used as a boundary condition at the pointy = 0. A no-flux condition was used at y = 1. The theoretical behavior calculated by this model showed good agreement with all experimental transients recorded if the following parameters were assumed: DK+= (2.30 i 0.20) X 1 0 - ~ 0 c m 2 s - ~ , f l - ( 1 . 5 f O . l O ) X 10-2s-*andF= 1.00f0.100 mol cm4. Coats of thicknesses in the range (1.6-4.8) X 1V cm were studied, and the level of agreement between theory and experiment shown in Figure 16 is typical. It may be concluded that the diffusional model suggested fully describes the observed potential induced quenching of the coat fluorescence. Comparative Remarks. We have seen that diffusional models describe the quenching of F362 inside HEQCl coats by both H+ and K+ions. In both cases the diffusion coefficient of the cation was 4-5 orders of magnitude less than that observed in aqueous solution. This may be rationalized on the likely basis of the coat functioning as an anion exchange membrane as discussed above. In the case of H+ this was found to exchange rapidly between solution and coat so that eq 1 was applicable, whereas for K+ a kinetic barrier to the uptake of cations at the electrode double layer/organic coat interface.
Conclusion We have demonstrated that the spectrofluorometric hydrodynamic voltammetry technique can be used as a sensitive probe of reactions taking place at solid/liquid interfaces.
0
500
loo0
Time / s
Figure 16. Typical fit between the experimental and simulated transient resulting from a potential step between 0.0 and 0.5 V at a platinum electrode coated with 3.5 X IO-' mol of HEQCI/F362. The coat thickness was estimated as 3.2 pm.
Acknowledgment. We thank Unilever plc and Solvay Interox Ltd. for financial support and SERC for a CASE studentship for D.J.R. References and Notes (1) Compton, R. G.;Fisher, A. C.; Wellington, R. G.;Winlder, J. J. Phys. Chem. 1992,96, 8153. (2) Compton, R. G.;Wellington. R. G. J. Phys. Chem. 1994.98.270. (3) C l a A n , P.; Carmona-F&iro, A. M.; Kurihara, K. J. Phys. Chem. 1989, 93, 917. (4) Mann, B.; Kuhn, H.J. Appl. Phys. 1971,42, 4398. ( 5 ) Compton, R. G.; Pilkington, M. B. G.; Steam, G. M. J. Chem. Soc., Faraday Tram. 1 1988,84,2155. (6) Levich, V. G. Physicochemical Hydrodynamics; Prentice-Hall: Englewood Cliffs, NJ, 1962. (7) Thelen, M.; Petrone, G.;O'Shea, P. S.; A d , A. Biochim. Biophys. Acta, 1984, 766, 161. ( 8 ) Perrin, D. D. Dissociation Constants of Organic Bases in Aqueous Solution: Supplement 2; Buttenvorth: London, 1972. (9) Mills, R.; Lobo,V. M. M. Sel/-Diffusion in Electrolyte Solutions; Elsevier: Amsterdam, 1989; p 208. (10) Soucaille, P.; Prats, M.; Tocanne, J.; Teissit, J. Biochim. Biophys. Acta 1988, 939, 289. (11) Fisher, A. C.; Compton, R. G. J. Phys. Chem. 1991, 95,7538. (12) Oglesby, D. M.; Omang, S.H.;Reilley, C. N. Anal. Chem. 1965,37, 1312. (13) Albery, W. 1.J. Eleclroanal. Chem. 1984,170,27. (14) Uster, P. S.; Pagano, R. E. J. Cell Biol. 1986, 103, 1221. (15) Uchida, I.; Ishida, A.; Matsue, T.; Itaya, K. J. ElectrwnaI. Chem. 1989, 266,455. (16) Britz, D. Digitial Simulation in Electrochemistry; Springer-Verlag: Berlin, 1988; p 24.
