Anal. Chem. 1988, 6 0 , 553-560
553
y-Irradiated Polymer-Modified Graphite Electrodes with Enhanced Response to Catechol Louis A. Coury, Jr., Eileen M. Birch, and William R. Heineman* Edison Sensor Technology Center, Biomedical Chemistry Research Center, and Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 -01 72
The immobilization of poly(N-vlnylpyrrdklone) In the presence of Its corresponding monomer by y-irradlatlon of films cast on graphlte electrodes Is reported. The polymer has the abillty to preconcentratephenolic compounds from aqueous solutlon, and the electrochemical response due to several catechol compounds of varlous charges that partition into films on electrode surfaces is examined. The response of modified electrodes for catechol relative to ascorblc acid Is Increased by a factor of 2-5 compared to bare electrodes. Response to ascorblc acM is dimlnlshed by the polymer coatlng, and the attenuation appears to be independent of the charge on ascorbate. The sultabliity of spectroscopic graphite as a substrate for polymer immobllization Is explored. Methods of correcting for the characteristically high and variable background charges encountered in chronocoulometric determlnations of electrode areas are discussed.
The determination of phenolic compounds is an important process in many different areas of analytical chemistry. Phenols are found in biological systems as tocopherols, estrogens, lignins, and catecholamine neurotransmitters ( I ) . Because of the ubiquity of phenol derivatives in the synthesis of dyes and plastics, the determination of naphthol, cresol, and resorcinol in wastewater effluents is often necessary (2). In the beverage industry, phenols are of interest because they are essential components of fruit juices, beer, and wines in the form of vanillic, ferulic, and sinapinic acids (3). Oxidative electrochemical detection of many of these compounds after separation by liquid chromatography electrochemical detection (LCEC) is often the analytical method of choice (4). Coating electrode surfaces with polymers is an attractive means of increasing the selectivity of voltammetric analyses (5). Complexation of amlytes by a polymer can increase the effective concentration and hence the response due to species of interest at the electrode surface while the electrochemical response due to interferents which do not interact with the polymer is attenuated, most likely due to a combination of decreased concentration near the electrode surface and hindered mass transport through the polymer. The water-soluble polymer poly(N-vinylpyrrolidone)(PNVP) is known to form complexes with a variety of phenolic compounds, and numerous binding isotherms in aqueous solution have been measured (6). The nature of the complexation has been reported to be a pH-dependent, hydrogen-bonding interaction between the amide centers in the polymer and the phenolic hydroxyl group (7). In this study, PNVP films coated on graphite electrodes were immobilized with y radiation, and the voltammetric responses due to the neutral species catechol, the cationic species dopamine and norepinephrine, and the anionic species 3,4-dihydroxyphenylaceticacid (DOPAC) and caffeic acid were evaluated. Each of these compounds contains an o-dihydroxybenzene functional group. The response due to ascorbic acid at these electrodes was also investigated since ascorbate is a commonly occurring interferent in the in vivo determination of catecholamines (8).
The solution used to coat the electrodes also contained N-vinylpyrrolidone monomer (NVP) since it was determined that less fragile gels were formed at lower dosages in its presence. The irradiation process is known to generate radicals along the polymer chain, the net effect of which can result reticuin further polymerization with monomer present (9), lation of polymer chains by linking of radicals, or polymer degradation (10). If the irradiated polymer undergoes a net increase in molecular weight ( 1 1 ) or if polar, hydrophilic groups are altered by the irradiation process, the resulting polymeric material may, in some cases, be rendered insoluble. The advantages of y irradiation include the ability to generate higher populations of radicals in polymer f i s simply by increasing the applied dosage, the ability to generate radicals inside of films without the addition of initiators or cross-linking agents, and the ability to simultaneously sterilize electrodes during the immobilization process. The latter feature could be of importance in future applications involving the fabrication of implantable electrochemical in vivo sensors by this method. The irradiated polymer films reported here are insoluble in aqueous solutions, but do swell considerably, permitting supporting electrolyte as well as solvent to reach the electrode surface. The irradiation process apparently does not disrupt the complexation behavior of the polymer. The electrochemical characteristics of the graphite substrate were also examined in order to compare results obtained at different electrodes. EXPERIMENTAL SECTION Reagents. The PNVP and NVP monomer were obtained from Polysciences, Inc., and were used as received. The manufacturer reports a molecular weight of 360 000 as determined by viscosity measurements. All solutions were prepared by using distilled, deionized, photolyzed water from a Sybron/Barnstead ORGANICpure system (Fisher Scientific)unless otherwise noted. Water resistivity was typically 16.5 MQ cm or greater. Glassware was washed in a KOH/ethanol bath for 48 h, rinsed in an HCl neutralizing bath, and then rinsed thoroughly with deionized water. Solutions of catechol, dopamine hydrochloride, and DOPAC (all from Sigma Chemicals), caffeic acid and norepinephrine (both from Aldrich Chemical Co.), and ascorbic acid (Fisher) were prepared immediately before use in aqueous 0.5 M KC1/0.02 M phosphate media (pH 3 or 7 ) ,which had been deoxygenated with Ar . Instrumentation. Cyclic voltammetry, square-wave voltammetry, and chronocoulometry were conducted by using a BAS-100 electrochemical analyzer (Bioanalytical Systems) and results were plotted with a Houston Instruments DMP-2 digital plotter. Coated electrodes were examined with a Nikon Optiphot metallurgical microscope (Jaynes-Elkro, Columbus, OH). The electrochemical cell included a Pt auxiliary electrode and a BAS RE-1 Ag/AgC1(3 M NaCl) reference electrode. All potentials reported in this paper are referenced to this electrode. Solutions were deoxygenated with Ar or purified N2 gas (Wright Bros., Inc., Cincinnati, OH) passed through a saturation tower filled with electrolyte solution for at least 15 min prior to each experiment. Working Electrodes. Spectroscopic graphite rods (type FXI-365T) of dimensions 0.46 cm X 3.81 cm were obtained from Poco Graphite, Inc., Decatur, TX. After loose graphite dust was removed from the rods by cleaning briefly in water in a sonic bath (Bransonic, Inc.) and drying in air, each electrode was sprayed
0003-2700/88/0360-0553$01 .50/0 0 1988 American Chemical Society
554
ANALYTICAL CHEMISTRY, VOL. 60. NO. 6. MARCH 15, 1988
C
A Flgure 1. Irradiation apparatus: (A) securing bob: (B) lid: (C) 0-rlng: (D) chamber; (E) wooden
vial rack (F) vial containing electrodes.
with Krylon Crystal Clear (Borden Chemical Co.) to insulate the side of the rod from solution. The Krylon product is an aerosol of methyl and n-butyl methacrylates in toluene, which dries to form a clear, hydrophobic, electrically insulating film (surface resistivity = 2 X lo8 Ma em) (12). Krylon coatings on graphite seem to be relatively resistant t o y radiation at the dosages employed in this study as demonstrated by no detectahle change in electrochemical response to ferri-/ferrocyanide at control electrodes before and after irradiation. After being coated with Krylon, electrodes were thoroughly abraded with K-622 emery paper to remove the spray from the electrode surface and then returned to the sonic bath for several minutes to remove polishing grit, loose graphite particles, and Krylon granules. The electrodes were next placed in a drill press and polished with 600A emery paper and then sonicated again. The effective area of each electrode was then calculated from electrochemical measurements on potassium ferricyanide as described below. Polymer Immobilization. The polymer solution used to coat electrodes was 8% (w/v) PNVP and 5% (v/v) NVP in HPLCgrade methanol (Fisher). A 20-rL aliquot of this solution was delivered via Eppendorf pipet to the surface of each electrode and allowed to dry. If the coverage was determined to be nonuniform from visual inspection, a 10-rL aliquot of methanol was sometimes added to redissolve the polymer, and the coating was again allowed to dry. The electrodes were sealed in Ar-purged, 2-dram vials, generally with two or three coated electrodes and an uncoated control electrode in each vial. The controls were examined to confirm the integrity of the Krylon coating after irradiation. Irradiation Procedure. The vials containing electrodes were placed in the specially designed wooden holder for irradiation shown in Figure 1. The rack was placed inside the irradiation chamber, which was then sealed and lowered into the %o y radiation source at the University of Cincinnati Nuclear Engirods held vertically neering Facility. The source consists of 18 in a ring and submerged in a concrete pool filled with water. The chamber was placed in the center of the rods during irradiation. The radiation fluxin the chamber varied as a function of position in the source. The resultant dosage received by each electrode, which is the product of the flux and the total irradiation time, could he varied by placing the electrodes at different heights in the chamber or at different distances from the chamber center. Part A in Figure 2 shows a plot of the dosage from a 95-h irradiation at a given height (6.25 in.) from the chamber base for various X,Ycoordinates. Part B shows the variation in dosage at a fixed radius (1.5 in.) as a function of height in the chamber, These dosage profiles were calculated from dosimetry data supplied by personnel at the University of Cincinnati B°Cofacility.
RESULTS AND DISCUSSION Characterization of Graphite Substrate. Graphite was chosen as the electrode material for this study because i t is inexpensive and radiation resistant, has a useful potential range for the oxidation of organic compounds, and is somewhat
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4
8
12
16
20
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porous. The last feature results in improved adhesion of the polymer coating in that polymer-filled pores may serve to anchor the film to the graphite surface. We have found that spectroscopic graphite is generally more easily modified than smoother substrates like Pt to which the polymer films are not as strongly anchored. One disadvantage of spectroscopic graphite is its large and variable residual current to which the high porosity is undoubtedly a contributing factor (13). This feature can adversely affect the reproducibility of voltammetric measurements. Since background subtraction of voltammetric data obtained in supporting electrolyte solutions is one means of increasing the reliability of measurements obtained at these electrodes (141,two different methods of background correction were evaluated during the initial stages of this study. Chronocoulometry was used to evaluate electrodes in terms of their effective areas (or “projected geometric areas”) by employing two different methods. For each electrode examined in this part of the study, a double potential step experiment was implemented by stepping the potential from +0.4 V vs. Ag/AgCl reference to +0.15 V and then hack to +0.4 V, using a 250-ms step time. This experiment was conducted for each electrode in 1.0 M KNO, and then again in 1.0 M KN03/1.00 mM K,Fe(CN), and the data (1 point/ms) were analyzed in two different manners. First, the data collected in supporting electrolyte solutions were subtracted from the data obtained in the ferricyanide solution (point-by-point subtraction method). The last 80% of the subtracted charge data from the forward step, Q, was regressed onto the square root of time, t’12 (15),using a linear, least-squares fit. As will he explained below, if left uncorrected, the background charge monitored in experiments with these electrodes greatly influences the slope as well as the
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
Table I. Electrode Areas Calculated by Subtraction of Charge Points
electrode
point-point subtraction slope,” pC/ms1/2
1
2 3 4 5 6 7
mean
RSDb
corr coeff
555
T TnR
calcd area, cm2
1.745 1.398 1.514 2.672 1.427 1.429 1.553
0.9998 0.9999 0.9999 0.9992 0.9999 0.9999 0.9999
0.189 0.151 0.164 0.289 0.154 0.154 0.168
1.677 27.1%
0.9998 0.0261 %
0.181 27.1%
Slope from regression of point-by-point subtracted Q values onto PI2. bRelativestandard deviation.
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Table 11. Electrode Areas Calculated by Subtraction of Q vs t Slopes back-
analyte slope,” electrode 1
2 3 4 5 6 7
mean RSD
pC/ms1/2
ground slope,b pC/ms1/2
difference slope, pC/msl/*
2.294 (0.9998)‘ 1.932 (0.9998) 1.816 (0.9999) 3.087 (0.9991) 1.518 (0.9999) 1.529 (0.9999) 1.783 (0.9999)
0.547 (0.9996) 0.533 (0.9989) 0.301 (0.9986) 0.414 (0.9982) 0.090 (0.9909) 0.099 (0,9900) 0.229 (0.9984)
1.746
0.189
1.399
0.151
1.515
0.164
2.674
0.289
1.428
0.154
1.430
0.154
1.554
0.168
1.994 27.5%
0.316 60.0%
1.678 27.1%
0.181 27.1 %
calcd area, cm2
“Obtained in 1.00 mM K,Fe(CN)&.O M KNOB. bObtained in 1.0 M KN03. ‘Correlation coefficient is listed in parentheses under each slope. intercept of such plots (“Anson plots”). The calculated slope was then used to determine a background-corrected area for each electrode by using a diffusion coefficient for potassium ferricyanide of 7.19 X lo4 cm2/s (16). These data are presented in Table I. The disadvantage of this method is that a large number of points must be manually entered into the regression program (400 per electrode) which is a time-consuming process and prone to data entry errors. We have found that identical results are obtained in every case when the slope from the regression calculated from chronocoulometric data obtained in supporting electrolyte is subtracted from the slope obtained in ferricyanide solution, and the difference in slopes subsequently used to calculate background-corrected areas. This is a much simpler procedure since the BAS-100 automatically calculates such slopes on-board in an identical manner (viz. a linear, least-squares regression of the last 80% of the data points) (17). Results obtained by using this method for the same set of electrodes are presented in Table 11. The results imply that the background charge monitored in these experiments obeys an approximately t1I2dependence, a situation usually restricted to solution species diffusing to the electrode surface (15). However, this background charge is probably not a result of charge transfer to electroactive impurities in the electrolyte solution but rather is a consequence of slow charge transfer to electroactive groups on the
Figure 3. Squarawave voltammogram of bare graphite electrode in 1.0 M KNO,: frequency, 15 Hz; pulse amplitude, 25 mV; step increment, 4 mV.
graphite surface. A prominent peak due to electron transfer to an unknown redox species on the graphite surface is obtained by square wave voltammetry at a freshly polished electrode in 1.0 M KNOBas shown in Figure 3. The background scans of all of the graphite electrodes studied displayed evidence of this redox process, usually occurring at about 0.25 V vs Ag/AgCl reference, although the magnitude of the response varied widely. The variability in the magnitude of this peak is perhaps responsible for the large relative standard deviation for the background slopes listed in Table 11. Cyclic voltammograms a t these electrodes suggest that this redox process is quasi-reversible, which is consistent with the observation that the peaks persist upon repetitive scanning. A redox couple at a similar potential has been reported elsewhere and postulated to be a 1,2-naphthoquinone-like functionality present at the carbon surface (13).The mediated reduction of solution species by surface groups during these potential step experiments could be possible, and similar effects have been observed for radio frequency plasma treated pyrolytic graphite electrodes (18). It is interesting to note that this sort of faradaic background might be expected to grow with increasing rotation rate in a hydrodynamic voltammetry experiment if it were due to some electroactive impurity present at low concentration. This effect was not observed, however, in other work conducted employing rotating graphite disk electrodes, which seems most consistent with the explanation of a slow charge transfer to surface groups. Solution Irradiation Studies. The solvent system, concentration of polymer, relative concentration of monomer, and absorbed y-ray dosage all determine the extent of gel formation which will occur upon irradiation ( 9 , I I ) . In order to obtain a rough idea of the conditions necessary to induce gelation, glass vials containing polymer solutions were purged with Ar, sealed, and irradiated. It was found that for a solution of 8% (w/v) polymer and 5% (v/v) monomer in methanol, gel formation occurred at around 10 Mrad. At dosages above 20 Wad, a hard, rubbery solid was produced. When monomer was absent, an 8% (w/v) methanolic polymer solution did not gel until 20 Mrad, and the solid produced at this and at higher dosages was fragile and easily disrupted with a spatula. Although solution irradiations are useful for evaluating a wide variety of polymer/monomer combinations, the results are not directly applicable to solid polymer/monomer coatings on electrode surfaces. The indirect effect on the polymer from radicals arising from residual solvent will be much less or absent in the solid film. In addition, the probability of encounters between generated radicals resulting in new bond formation is lower in a solid film than in solution due to
556
ANALYTICAL CHEMISTRY. VOL. BO. NO. 6. MARCH 15. 1988
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diminished segmental chain motions (9,191. Furthermore, graphite itself has been reported to inhibit y-radiation-induced polymerization of monomers impregnated in spectroscopic graphite rods (20). For these reasons, higher dosages are generally required to immobilize films on surfaces than to cause gelation in solution. Any quinone- or phenollike species present on the grapbite could ala0 inhibit gel formation near the surface since molecules of this type are used to protect irradiated polymer fibers from becoming brittle by preventing cross-linking and/or degradation (21). Quinones are also routinely added to monomers and polymer solutions to inhibit free-radical polymerization during shipping and storage (22). Earlier attempts a t electrode modification in the course of this study involved irradiation of polymer solutions confined in tubing placed over the end of inverted electrodes. Although gelation was attained, the gels often formed a t the top of the tube leaving a layer of solvent between the electrode and the gel, despite the fact that the gel was denser than the solvent and should have formed at the bottom of the tube. Thii may suggeat inhibition of radical formation or radical scavenging a t the electrode surface. Polymer-Modified Electrodes. In Figure 4, an electrode prepared a t 25 Mrad (left), shown after swelling in pH 7.0.5 M KCI/O.OZ M phosphate buffer for about 20 min, is compared to an electrode (right) with a similar f h before swelling. The mwoUen film is not discernible in this photo, but appears as a shiny, uniform coating when viewed from the bottom. Although exact swollen film dimensions were not measured for each electrode, the films prepared in this study were all quite thick (at least 1 mm after swelling). One consequence of using films with these dimensions is that the diffusion layer created during an electrochemical experiment never extends beyond the polymer phase to the bulk phase. resulting in conditions of mass transport dictated solely by the swollen polymer film. The swollen coating8 were observed to be flared at the solution side for all dosages studied. This is perhaps attributable to reduced swelling near the graphite surface due to the physical constraints of the polymer being anchored by pores. It is also possible that the polymer became grafted to the methacrylate (Krylon) coating a t the electrode edge. The radiation-induced grafting of PNVP onto methacrylate blocks has been previously demonstrated (23), and such grafted anchoring would also be expected to reatrict polymer swelling. Cyclic Voltammetry of Catechol And Ascorbic Acid. In order to evaluate the electrochemical behavior of these modified electrodes, the voltammetric response in a 1 mM
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catechol solution was compared to the response in a 10 mM ascorbic acid solution for electrodes prepared a t different dosages as well as for bare electrodes. Figure 5 shows cyclic voltammograms (CVs) for ascorbate (part A) and catechol (part B) in pH 7 phosphate media. The top voltammogram in each case is for a bare graphite electrode, while the lower voltammogram in each set is for the swollen, 25 Mrad, PNVP-coated electrode shown in Figure 4. These CVs were recorded after each electrode had been in solution for 50 min. Experiments were performed in ascorbate first, then the electrodes were soaked in phosphate buffer until no signal for the oxidation of ascorbate was observed (ea. 1 h for the polymer-coated electrode), and then the CVs in catechol s o lutions were obtained. Shown in Figure 6 (top) are plots of peak current density (calculated from the area before modification) versus time for ascorbic acid in pH 3 phosphate solution as determined by cyclic voltammetry. In these experiments, CVs were run periodically, and the peak current densities increased with time as the ascorbic acid partitioned into the film. The straight line H is for an unmodified electrode and curve G is for a PNVP-coated electrode which was not irradiated. In the latter case, the polymer could clearly be observed to dissolve from the electrode during the experiment. The limiting response in plot G,however, is more or less the same as that of the unmodified electrode (H). I t should be pointed out that the bare,unirradiated electrode used in this instance was prepared a t the m e time as the modified electrodes and remained in a vial under AI while the other electrodes underwent irradiation, as did the coated, unirradiated control electrode. Plots A 4 and E-F of Figure 6 are virtually indistinguishable from one another, indicating that variation in radiation daeage has little effect on the response of the electrode to ascorbic acid. Curve D for the 30.8-Mrad electrode shows a current level which is inexplicably higher than for the other electrodes. Since the current levels for a 40.1-Mrad and a
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988 1.B 2.61
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Figure 6. Response curves for irradiated and control electrodes: (A) 45.0 Mrad, (B) 41.8 Mrad, (C) 40.1 Mrad, (D) 30.8 Mrad, (E) 25.6 Mrad, (F) 11.2 Wad, (G) 0 Wad, (H) bare electrode. Top, IO mM ascorbate at pH 3; bottom, 1 mM catechol at pH 3.
25.6-Mrad electrode (curves C and E, respectively) are both lower and very similar, it seems likely that this particular 30.8-Mrad electrode is anomalous in its behavior for some reason other than the dosage (e.g., uneven polymer coating or residual O2in the vial during irradiation, which could induce polymer degradation). With this exception, the irradiated electrodes are remarkably similar in their response toward ascorbic acid, each showing a response roughly 50% that of the bare electrode. The plot at the bottom of Figure 6 displays CV peak current density versus time data in 1 mM catechol at pH 3 for the same electrodes used in the plot at the top of the figure. Again, line H is for the unmodified electrode, curve G is for an unirradiated, coated electrode, and the other curves are for irradiated, polymer-modified electrodes. The peak current densities are 2-3 times higher at the modified electrodes than a t the bare electrode, presumably due to the higher concentration of catechol a t these electrode surfaces arising from complexation with the polymer. This enhancement is observed in spite of the fact that the diffusion coefficient for catechol in the polymer film is undoubtedly lower than in bulk solution. The selectivity for catechol relative to ascorbic acid is even greater, however, since responses for apparently uncomplexed species such as ascorbate are attenuated by the polymer coating. Figure 7 presents similar data obtained with a different set of electrodes at pH 7 . In the top set of curves for ascorbate, the unirradiated electrode again gave a current response higher than that of the bare electrode, but the majority of the modified electrodes showed almost identical, attenuated responses. The plots in the bottom of the figure show the peak current densities for catechol at pH 7, and enhanced response is evident for all dosages studied. Adsorption of catechol or species formed upon its oxidation may be responsible for the negative slope observed for the unmodified electrode (line H), since the peaks measured grew broader and diminished in
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Table 111. Voltammetric Selectivity Ratios"
normalized selectivities* dosage, Mrad
PH 3
PH 7
45.0 41.8 40.1 30.8 25.6 11.2
4.1 3.1 3.5 2.6 3.5 3.2
5.8 2.5 5.4 4.3 4.2
bare electrode
1
1
3.5
"Determined by square-wave voltammetry at time = 50 min. bPeak current for 1 mM catechol divided by peak current for 10 mM ascorbic acid, normalized to results for a bare electrode. height upon repetitive scanning of the potential. Voltammetric Selectivity. In order to evaluate the enhanced response of the electrodes for catechol in comparison to ascorbic acid (selectivity), the ratios of the peak currents obtained by square-wave voltammetry for each of the species at a given electrode, normalized to the responses obtained at a bare electrode were calculated and are presented in Table 111. The experimental parameters chosen were the same as for the voltammograms shown in Figure 3. These data were collected after the electrodes had been in the analyte solution for 50 min to ensure that analyte concentrations in the polymer coatings were unchanging. No trend is apparent in the data with respect to dosage, but the results clearly indicate that the modified electrodes are from 2 to 5 times more responsive to catechol than ascorbate when compared to an unmodified electrode. Attenuation of ascorbate signals at physiological pH with increased signals for catechols could be of analytical importance for potential application of similar electrodes to the construction of neurochemical sensors because the relative concentration
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
558
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