Anal. Chem. 1989, 61, 2193-2200
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Voltammetry in Supercritical Carbon Dioxide at Platinum Microdisk Electrodes Coated with Perfluorinated Ion-Exchange Membranes Adrian C.Michael a n d R. M a r k Wightman*-'
Department of Chemistry, Indiana University, Bloomington, Indiana 47401 I t has prevlously been shown that the resistance of supercritlcal COP Is too hlgh for voltammetry, even with mlcroelectrodes, unless electrolytes and/or polar modtflers are present in the fluld. I n a supercritical fluld flow system, such as Chromatography, the use of an added electrolyte Is not desirable. As an alternative, a film of an lonically Conducting polymer, Nafion, placed over an electrochemicalprobe consistlng of both a mlcrodlsk worklng electrode and a quaslreference electrode, has been used. Undlstorted voltammograms of ferrocene, 4-methylcatechol and 3,441hydroxybenzylamlne dlssdved In supercrltkal C02 In the presence of small quantities of H20 have been obtained without the use of added Supporting electrolyte. Experlments carried out In a supercritical fluld flow stream demonstrate the potential utility of membrane coated microelectrodes as chromatographic detectors.
INTRODUCTION The development of microvoltammetric electrode techniques over the past several years has broadened the scope of voltammetry to previously impractical areas such as low permittivity media, very short time scales, and spatially resolved measurements (1,2). These relatively new features of voltammetric methods are particularly useful for the development of detectors that can be coupled to a variety of chromatographic systems. Small electrode assemblies have been used effectively for on column detection in capillary liquid chromatography and electrophoresis (3, 4 ) and with conventional LC columns to spatially resolve regions of varying efficiency a t the column outlet (5). Strategies based on microelectrodes have also been used in gas chromatography. For example, Murray and co-workers (6), reported the use an electroactive substrate contained within an ionically conducting polymer membrane which responded to modification of transport phenomena caused by the sorption of solvent vapors. Pons and co-workers have used a microelectrode assembly for the direct detection of redox-active eluents (7). In the latter work, conduction involved the migration of ions across the surface of the insulator material separating the working and counter electrodes. These reports demonstrate that electrochemical detection can be used even if the chromatographic media are nonconductive. Similar strategies have been utilized for electrochemical studies in lubricants and other nonpolar solvents (8-10). The purpose of the experiments described in this paper is to characterize electrochemical detection in supercritical carbon dioxide (SC-C02)with Ndion-coated microelectrode probes. SC-C02 is frequently used as a chromatographic mobile phase and as an extraction solvent because it has a readily accessible critical point: T,= 31 "C; P, = 73 atm (11, 12). Despite the fact that conventional electrochemical techniques have been applied in various polar supercritical
* To whom correspondence should be addressed.
Present address: Department of Chemistry, University of North Carolina, Chapel Hill, NC 27514. 0003-2700/89/0361-2193$01.50/0
fluids (13-16), the extremely low dielectric constant of unmodified SC-C02 (6 < 1.8 at T = 50 "C and P < 1700 atm) precludes the use of voltammetry, even with a 5-pm radius microdisk electrode (17). Upon addition of water to the fluid, voltammograms of dissolved ferrocene can be recorded but they are distorted by ohmic drop and indicate that the ferricinium cation precipitates on the electrode. Addition of a supporting electrolyte, tetrahexylammonium hexafluorophosphate (THAPF& to the water-modified SC-C02causes the formation of a conductive molten salt layer over the electrodes (18) which results in voltammograms that are not resistively distorted and do not indicate precipitation of ferricinium. However, use of the molten electrolyte to permit undistorted voltammetry in a SC-C02flow system is not likely to be practical. As an alternative to the use of a supporting electrolyte dissolved in the SC-C02,we have investigated the use of an i o n i d y conducting Ndion film over an electrochemical probe consisting of a microdisk working electrode and a macrodisk quasi-reference electrode (QRE) (19). Electroactive substrates dissolved in the fluid are detected at the microelectrode after partitioning into the film. The film is useful for electrochemical detection in modified SC-C02because it provides permanent conductivity between the working and quasi-reference electrodes and responds in an interpretable fashion to various experimental parameters such as film thickness, fluid modifiers, voltammetric scan rate, and ionic nature of the redox substrate. The performance of the membrane-coated electrode in a SC-C02flow system is also described and has been considerably improved over that previously reported. EXPERIMENTAL SECTION Electrochemical Probe Fabrication. The electrochemical probes used for voltammetry in SC-C02 were similar to those described previously (19). Briefly, they consisted of a 5-rm radius Pt wire and a 26-gauge Pt wire heat sealed in concentric soft-glass tubes. Polishing the end of the probes produced two coplanar Pt disks imbedded in glass and separated by 300-500 pm. The microdisk served as the working electrode and the larger disk served as the QRE. Because of the small currents in these experiments, an auxiliary electrode was not required. The probe was particularly suitable for this work because (i) the flat face of the probe was easily modified with a polymer membrane and (ii) the Pt to glass seal was very robust and able to withstand repeated exposure to the elevated temperature and pressure of SC-C02. Although silver is usually a more suitable QRE, it was not used here because high-quality silver to glass seals could not be obtained. A 5-bm radius of the working electrode was used in all calculations. Nafion Film Preparation. Films were prepared with a casting procedure from a 0.5 wt % solution of Ndion (H' form) dissolved in isopropyl alcohol. Immediately before the coating procedure the solution was sonicated for 10 min. A Hamilton 1-pL syringe mounted in a micromanipulator was used to place a drop of solution on the probe face. The alcohol was evaporated at 80 "C for approximately 1 h and a vacuum was applied for 15 min to assure complete removal of the solvent. This resulted in adherent Ndion films covering the two Pt disks. In contrast to other reports (20),the films were not easily removed from the glass by water, even with agitation, and were smooth and free of cracks. Film 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 19. OCTOBER 1. 1989
thicknesses were estimated to be 1-2 pm by optical microscopy (in some instances aided by staining the film with orange Ru(bpy)32+).However, as previously noted (21),films prepared in this fashion were uneven as indicated by visible interference patterns that formed on the probe face. In some experiments, the films were converted to the Na+ form by soaking the coated probes for approximately 30 min in aqueous 1 M NaCl. Other techniques for preparing Nafion films were also investigated. To produce very thick films the probes were dip coated in a 10 wt % Nafion solution (22). The probes were immersed at an angle of approximately 50" from the vertical for several minutes and then slowly removed with a manipulator, allowing for as much drainage as possible from the electrode surface. The probe was then clamped vertically with the electrodes facing upward and cured as described above. The resultant films were 20-30 pm thick as estimated by optical microscopy. Finally, following the reports of Martin and co-workers (20),several films were prepared with a 0.5 wt % Nafion solution in DMF and a solvent evaporation temperature of 120 "C. These films gave voltammograms in SC-C02that were severely distorted by ohmic drop, an observation that is consistent with the rigid polymer morphology found by Martin (20). Because of the apparent distortion, films prepared in this fashion have not been characterized further. Voltammetric Experiments. For experiments in static SC-C02the probes were mounted in a locally constructed 10-mL stainless steel high-pressure cell (17) which also contained an electroactive substrate, a fluid modifier, and a stir bar. The fluid temperature was controlled with a GC oven and monitored with a thermocouple positioned near the probe tip. The cell was connected via stainless steel tubing to an HPLC syringe pump and brought to the desired C02 pressure and temperature while the cell contents were stirred. The fluid pressure was monitored with the transducer in the head of the syringe pump. Experiments were usually performed at 80 OC and 88 atm. At lower temperatures the amplitude of voltammetric currents were generally smaller, while at higher temperatures less stable responses were obtained. Voltammetry in flowing SC-C02was carried out by using the electrochemical flow cell previously described (19). The overall flow system, however, was modified slightly by the addition of a second sample vessel in parallel with the vessel used in our earlier work. The vessels were both connected to the syringe pump via a T-connector. One of the vessels contained water-modified SC-C02,while the other also contained ferrocene. The outlets of the vessels were connected to a valve that was used to select the path of fluid flow to the electrochemical detector. Thus, voltammograms in the presence of ferrocene could be compared with background voltammograms obtained in the presence of modifier only. This arrangement also allows examination of the temporal response of the membrane-coated electrodes to change in the concentration of analyte in the bathing fluid. Cyclic voltammetry was performed with an AT style personal computer equipped with an analog-to-digital interface (Scientific Solutions, Inc., Solon, OH). The electrode potential was controlled via a digital-to-analog converter (DAC). The microdisk electrode was connected to a Keithley 427 (Keithley Instruments, Cleveland, OH) picoammeter, the output of which was monitored by the computer via an analog-to-digitalconverter (ADC). All software was developed locally.
RESULTS A N D DISCUSSION Voltammetry in Static SC-C02 with Nafion-Coated Probes. When voltammetry is performed with the Nafioncoated probes in unmodified SC-C02,no current is observed, even in the presence of a dissolved electroactive substance such as ferrocene, because of the poor solvation of the counterions in the film by the nonpolar bathing fluid. Under such conditions, the counterions are essentially immobile as they cannot be dissociated from the fixed anionic sites of the film so that ionic conduction cannot occur (23). Additionally, this observation shows that the Nafion is not swollen by SC-C02 to a sufficient extent to permit conduction via segmental motions of the polymer chains. However, if the SC-C02 is modified with a small amount of water (0.1 M, 20 pL added
i I
1.2
0.6
0
-0.6
E (V vs Pt-QRE)
Figwe 1. Voltammetry in modified SC-CO,: (A) background voltammogram in SC-CO, with 0.1 M H 0 added (1 VIS; i = 6 nA); (B) background and fenocene (2 X 102M) voltammogram in SCCO, with 0.1 M H,O added (0.1 V/s; i = 2 nA); (C) voltammogram of ferrocene (2 X lo-' M) in SC-CO,containing 200 pL of acetonitrile (0.1 VIS; i = 2 nA).
to the 10-mL cell) a background voltammogram can be obtained (Figure 1A) that is quite similar to the background voltammograms usually obtained with Pt electrodes in aqueous media (24). The accessible potential range spans approximately 1.5 V and appears to be limited by the oxidation of water and the reduction of hydrogen ion. The anodic formation of surface oxides and the corresponding cathodic stripping wave are also observed. Frequently, the cathodic stripping process appears as multiple peaks in the background scan, as shown in Figure 1A by the small reduction wave a t a more negative potential than the larger stripping peak. As the potential of the surface reduction is well-known to be pH dependent, a likely explanation of this behavior is the presence of regions of differing pH on the electrode surface caused by heterogeneity of the recast Nafion film. If the positive potential scan is not extended to the surface oxidation region, the cathodic surface waves do not occur as shown by the background voltammogram of Figure 1B. When ferrocene (2 x lo4 M) is dissolved in the water-modified fluid, a distinct voltammogram for the ferrocene/ferricinium couple is observed (Figure 1B). At a scan rate of 0.1 V/s the anodic wave for to the oxidation of ferrocene is sigmoidal indicating radial diffusion of ferrocene to the microdisk electrode. Plots of E vs In [(id - i) / i] for the sigmoidal anodic waves have a slope of 43.1 f 5.5 mV (mean f standard deviation; scan rate = 0.1 VIS; n = 4), which is comparable to the 30.4 mV expected for a reversible redox couple a t 80 OC (25). The cathodic part of the voltammogram, corresponding to the reduction of the ferricinium cation, is peak shaped rather than sigmoidal. As microdisk voltammograms become more peak shaped when the diffusion coefficient of reactant decreases, this behavior indicates slower mass transport of ferricinium than ferrocene in the Nafion film. The slower mass transport of ferricinium is presumably a result of ionic interactions with the fixed anionic sites of the f i i , as will be discussed in greater detail below. As mentioned above, the accessible potential range is somewhat limited due to the use of water as a fluid modifier. Therefore, other modifiers were tested for their ability to promote conductivity in the Nafion film. When acetonitrile (200 pL added to the IO-mL cell) is used as the fluid modifier,
ANALYTICAL CHEMISTRY, VOL. 61,NO. 19,OCTOBER 1, 1989
the voltammogram shown in Figure 1C is obtained. The increased wave slope indicates severe ohmic distortion that could not be alleviated by further addition of acetonitrile. The voltammogram in Figure 1C is actually quite similar in form to the voltammogram of methylviologen dication at a Nafion-coated electrode in liquid acetonitrile, reported by Elliot (26).The apparent resistivity of the Nafion with acetonitrile as the modifier or solvent arises from the reduced mobility of ions in the film (23,26).Surprisingly, when methanol or DMF were used as the modifier, voltammograms of ferrocene could not be obtained. As Nafion has been reported to be conductive in both liquid methanol and DMF (23,26), this observation implies that they do not partition into the film from the fluid to a sufficient extent to provide conductivity. Voltammograms obtained by using the Na+ form rather than the proton form of Nafion also exhibit ohmic distortion, although not as severe as that shown in Figure 1C. This may be due to a lower mobility of Na+ relative to H+in Nafion. Thus, the voltammetric behavior of the Ndion-coated electrode depends on both the nature of the modifier and counterion present in the film. The best voltammetric response, i.e. with the least ohmic distortion, was obtained with the proton form of Nafion and water as the modifier. The increased resistivity of the film under the other conditions investigated outweighed the possible advantage of an increased potential range. Mass Transfer of Ferrocene to the Microdisk Electrodes. Cyclic voltammetry a t microdisk electrodes is conveniently characterized by the dimensionless quantity p (27,
28) p = ro(fu/D)1/2 where ro is the working electrode radius, D is the apparent diffusion coefficient, u is the scan rate, and f = nF/RT. At small values of p (i.e., p I11,microdisk voltammograms are sigmoidal due to convergent diffusion of reactant to the electrode surface. As p increases, diffusion of material to the electrode becomes increasingly linear and the voltammograms acquire the familiar peaked shape. An interesting property of voltammetry with microelectrodes is that the qualitative shape of a voltammogram is independent of the concentration of reactant, i.e. p does not contain a concentration term. With a microelectrode, therefore, it is possible to estimate the diffusion coefficient of a species without specifically knowing the concentration. This is particularly useful in these experimenta because the concentration of ferrocene in the Nafon film depends on an unknown partition coefficient. Usually, p is experimentally varied by changing the scan rate, but Bard has recently observed a shift from linear to radial diffusion conditions upon changing the temperature of a SOz fluid from subcritical to supercritical values (16). Figure 2 shows voltammograms of ferrocene recorded in water-modified SC-Cop at 1,10, and 100 V/s that can be used to estimate the diffusion coefficient operant in the mass transport of ferrocene to the Nafion-coated microdisk electrode. At each scan rate the anodic voltammograms appear sigmoidal. The scans shown have not been background corrected, however, and the voltammogram at 100 V/s may not be purely sigmoidal. Thus, it appears that the p value of 1 lies in the range of 10-100 V/s, which gives an estimated range for the ferrocene diffusion coefficient of (0.8-8)x lo4 cmz/s. This range overlaps that found for solutes in supercritical fluids ((0.5-3.3) X lo4 cm2/s) and is markedly different from that found in liquids ((0.5-2.0) X cm2/s) (12). The steady-state diffusion current at microdisk electrodes is given by id = 4nFrDC* (2) where C* is the bulk concentration of reactant. Using the
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I
.7
.2
-.3 .7
.2
E (V
-.3
.7
.2
-.5
vs Pt-ORE)
Flgure 2. Effect of scan rate on the voltammetry of ferrocene (2 X lo4 M) in SC-C02 with 0.1 M H,O added.
lower end of the estimated diffusion coefficient range and the steady-state current from the 1 V/s scan in Figure 2 in eq 2 yields an estimate of the bulk concentration of ferrocene of 2.07 X lo4 M, which is close to the actual concentration added to the fluid (2.0 X lo4 M). These results confirm that the film was sufficiently thin that the voltammograms shown in Figure 2 were controlled by the diffusion coefficient and concentration of ferrocene in the fluid rather than the Nafion. This is an interesting observation when compared with the voltammetry we previously reported in water-modified SC-COz containing THAPFB electrolyte in which the voltammograms were found to depend on the properties of a molten salt layer that formed on the electrodes (18). The large apparent diffusion coefficient found in the present study also implies that ferrocene diffuses freely through the Nafion film. It has been suggested that recast Nafion films, which imbibe large quantities of water, have an open internal structure and can easily incorporate neutral molecules (29)as well as co-ions (22, 23). Furthermore, it is also likely that a t the elevated temperature of the SC-C02,the fluidity of the polymer material is increased, facilitating the motion of solutes. Thus, with thin Nafion films the shape and amplitude of the voltammograms depend on the diffusion coefficient and concentration of ferrocene in the supercritical fluid. This concurs with the fact that the thickness of the Nafion films used is only a fraction of the disk radius while the diffusion layer extends well into the fluid (approximately 6ro for C/C* = 0.9 (28,30)). The film thickness that resulted from the recasting procedure was found to be variable, however. This was evident in the voltammetry in two significant ways. First, the amplitude of the steady-state limiting current was found to vary between experiments. Second, with purposely thick Nafion films the voltammograms at high scan rates reflect diffusion entirely within the film itself rather than through the fluid. Figure 3A is a voltammogram of ferrocene obtained at 100 V/s using a probe dip-coated with a thick (2&30 pm) Nafon film. The voltammogram is peak shaped on both the anodic and cathodic sweeps and no difference in the amplitude of the anodic and cathodic current is observed, as expected for linear diffusion of both forms of the redox couple. Furthermore, with these thick films, peak-shaped voltammograms are obtained at much slower scan rates (
