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Electrochemical Detector for Liquid Chromatographic Determination of Carbohydrates Javad M. Zadeii and Juan Marioli'
Shimadzu-Kansas Research Laboratory, Center for Bioanalytical Research, The University of Kansas, 2095 Constant Avenue, Lawrence, Kansas 66047
Theodore Kuwana*
Center for Bioanalytical Research, The University of Kansas, 2095 Constant Avenue, Lawrence, Kansas 66047
A highly sensitive and selective electrochemical (LCEC) detector has been developed for the llquld chromatographic (LC)analysis of carbohydrates. This detector consists of copper/copper oxide particles dispersed in a Nafion f h that is cast onto the surface of a glassy carbon electrode. The copper is electrochemically deposited into the Nafion by a constant potential method. The response of the detector is based on the catalytic oxidation of various sugars in alkaline solutions at a pH greater than 13. The copper-Naflon-modIfled electrode exhibits high sensitivity while Improving the stability. I t allows the convenient quantlflcation of monosaccharides and disaccharides at micromolar concentrations. The response to carbohydrates is assessed with respect to the degree of Nafion coating, copper loading, concentration dependence, time stabillty, and other variables.
extensively as permselective coatings ( 7 ) ,sensors for incorporating cations (8), and charge exclusion membranes (9). Controlled electrodeposition of copper in the Nafion film results in catalytic sites containing copper/copper oxide, which oxidize various carbohydrates in alkaline solutions a t p H 13. The mechanical stability of copper is greatly improved by incorporating it in the Nafion polymeric film. Nafion also prevents poisoning and passivation of catalytic sites when analytes containing surfactants and other interferents are present. The optimization of parameters affecting the analytical performance of the electrode, such as the Nafion thickness and the amount of copper loading, in the fabrication of a sensitive and stable electrode is discussed herein.
INTRODUCTION
Reagents. A 5% solution of Nafion perfluorinated ionomer (1100 EW) was obtained from Solution Technology, Inc. (Mendenhall, PA), and was diluted with ethanol as required. Solutions of a-D(+)-glucose, D-sorbitol, D-mannitol, inositol, D-arabinose, sucrose, xylitol (Aldrich), a-lactose, maltose, a-L-rhamnose (Sigma),and fructose (Matheson Coleman and Bell) were prepared daily in Nanopure water. A solution of carbonate-free NaOH was prepared as the mobile phase for high-performance liquid chromatography (HPLC) and flow injection analysis (FIA). Nanopure water and the NaOH solutions were used to dilute analytes for HPLC and FIA analysis, respectively. Cupric nitrate, potassium nitrate, bovine albumin (Sigma),L-ascorbic acid, sodium dodecyl sulfate (SDS) (Baker),and uric acid (Aldrich) were used as received. Apparatus. Cyclic voltammetric experiments were recorded with either a Bioanalytical Systems Model CVlB potentiostat interfaced with a Houston Instruments Model 2000 recorder or a Cypress Systems computer-aided potentiostat coupled to an Everex System 1800 A T computer. In the latter case, the graphic output was recorded on a Hewlett-Packard Model HP-7440A plotter. The voltammetric cell was a 20-mL glass vial. The cell was joined to the glassy carbon working electrode (Bioanalytical Systems Model MF-2012), a Ag/AgCl reference electrode (BioandyticalSystems Model RE-I),and a platinum wire auxiliary electrode. The flow-injection and chromatographic systems (Shimazdu Scientific Instruments, Co., Columbia, MD) consisted of pumps (LC-6A)equipped with high-sensitivitypulse dampeners, a column oven (CTO-GA),an electrochemical detector (L-ECD-GA),and a recorder (CR-601). A glassy carbon electrochemical flow cell (BioanalyticalSystems Model MF-1000) was used as the substrate for surface modification. For chromatographic separations, a 25-cm-long,4-mm-i.d.Dionex HPIC-AS6 anion-exchange column with a polymeric RP guard column (Brownlee Labs Model GPP-013) was used. The scanning electron microscopy (SEM) photomicrographs were obtained with a Hitachi Model S-570 scanning electron microscope.
A recent chemically modified electrode (CME) for carbohydrate detection was reported by Prabhu and Baldwin (I, 2). They used a glassy carbon (GC) electrode onto which a layer of crystalline CuCl, was deposited. The lifetime of this electrode was 2-3 days, which is a marked improvement over their earlier cobalt phthalocyanine (COPC) CME (3). The Cu-coated modified electrode offered enhanced sensitivity for carbohydrate detection. However, the selectivity (aside from chromatographic resolution) and the long-term stability of such a catalytic electrode need further improvement if it is to serve as a practical carbohydrate detector. We describe herein a CME consisting of copper particles and copper particles coated with copper oxide dispersed in a Nafion film that has been coated on a glassy carbon electrode. Polymer-coated CMEs have been the subject of many on-going investigations ( 4 ) . Earlier, Bartak and co-workers (5, 6) demonstrated that polymeric films such as conducting polyaniline and poly(4-vinylpyridine) can serve as anchoring (nucleation) sites for platinum and other metals. Metals electrodeposited in thin polymer films appear as microparticles three-dimensionally dispersed in the polymer matrix. The advantages that can be accrued from this type of electrode are mechanical stability and improved selectivity, which can be achieved by the controlled electrodeposition of metal into the polymer. Films made from Nafion, a polyelectrolyte cation-exchange polymer, are quite inert in most electrolytes and adhere very strongly to the electrode substrate. They have been used 'On leave from the Rio Cuarto National University, 5800 Rio
Cuarto, Argentina.
EXPERIMENTAL SECTION
0003-2700/91/0363-0649$02.50/00 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
Electrodes. Glassy carbon electrodes for the batch and flow cells were polished successively with 1.0-,0.3-, and 0.05-pm alumina, thoroughly washed with distilled water, sonicated, washed with Nanopure water, and then dried in an oven at 60 "C. Modified electrodes were prepared by casting 5 pL of different concentrations of Nafion solution on the surface of the face-up electrode and drying at room temperature. The electrode was then placed in an electrochemical cell containing 0.1 mM cupric nitrate and 50 mM potassium nitrate. A potential of -0.5 V or -0.7 V vs Ag/AgCl was applied to the electrode for various deposition times while the solution was stirred at 400 rpm. After the copper deposition was completed, the electrode was placed in a cell containing 0.15 M NaOH, and cyclic voltammograms were performed to determine the response characteristics of the CME in the presence of 1.0 mM glucose. This electrode was incorporated in a chromatographicflow cell to serve as the LCEC detector.
RESULTS AND DISCUSSION Deposition of Copper Particles on the Nafion Film. A Nafion film can be produced on a solid surface by spreadcoating (10, 11) or dip-coating (12) or by electrochemical methods (13). A film of Nafion was prepared by spreading 5 p L of 0.25% Nafion on a 3.0-mm-diameter glassy carbon electrode. The SEM photomicrograph of this electrode showed a smooth, flat morphology. The thickness of the Nafion film can be roughly estimated taking into account the area over which the polymer was deposited and the density (1.58 g/cm3) of the Nafion film (14). The Nafion thickness, calculated to be 0.5 Mm, was consistent with that observed by SEM. The SEM photomicrograph of a thicker film of Nafion reveals a porous structure. The electrodeposition of copper in the Nafion film was accomplished in a solution of 1 X lo-* M copper nitrate by a single potential step electrolysis ranging from 5 to 100 s. A SEM photomicrograph of a typical Nafion-coated electrode containing electrodeposited copper is shown in Figure 1. The copper particles are not dispersed uniformly on the carbon surface. Within the Nafion film, nucleation sites that are very close have agglomerated particles, and the clusters of aggregated copper are noncrystalline. However, on a bare GC, the copper particles exhibit a three-dimensional growth behavior that results in a crystallite form. The amount of copper loaded into the Nafion-coated electrode is a function of applied potentials and electrolysis time. The amount of copper deposited into the polymer is related to the charge consumed during the electrodeposition through
Q Ncu = nF where Ncu is the amount of copper in moles and Q is the charge in coulombs. n and F have their regular electrochemical significance. When the deposition potential was changed to a more negative value up to -0.7 V, the amount of electrodeposited copper increased in a linear fashion. The duration of the applied potential is a second parameter that affects the amount of copper loading into the polymer. The amount of copper in the Nafion affects the background current and noise level of the modified electrode. Table I summarizes the analytical data for the Cu-Nafion electrodes as evaluated by batch and LC experiments. In both cases, the background current and noise level increase with an increase in the amount of copper electrodeposited into the Nafion. Although higher backgrounds are usually unfavorable in most analytical situations, the detection limit a t a SIN ratio of 1:l is not seriously affected. There is some indication that the limit is lowered when the copper loading is less. The noise
c-----)
25um
Flgure 1. SEM photomicrograph of a copper-Nafion-coated GC electrode. The modified electrode was prepared by casting 4 pL of
0.25% Nafion on a 3.0-"diameter
GC electrode into which copper was deposited by constant potential electrolysis at -0.7 V for 2 min from 1 X lo-* M Cu(NO& solution.
Table I. Analytical Performance of Copper-Nafion Electrodes" Cu loadings LC flow batchb 0.06 wmol 0.17 pmol 0.04 pmol 0.26 Fmol
bkgdCcurrent noise
signald dl'
50.0 nA 1.0 nA 1.9 PA 0.55
200.0 nA 6.0 nA 3.4 pA 1.8
100.0 nA 1.0 nA 7.4 nA 1.3
200.0 nA 3.0 nA 16.0 nA 1.9
'Thickness of Nafion 7 pm. *Solution stirred with stirring bar rotating at 400 rpm. 'Background. dApplied potential was 0.48 V for both the batch and flow experiments. Obtained steady-state current in case of batch, peak current for LC; 1.0 mM solution of glucose for the batch experiment and 20 wL of 0.010 mM glucose solution injected for LC. eConcentration in pM and detection limit calculated at a S / N ratio of 1:l.
level was determined by measuring the peak-to-peak variation in the ac fluctuations of the detector baseline. Voltammetry. In order to determine which copper species participate in the oxidative process of carbohydrates, cyclic voltammetry was used in an attempt to elucidate the mechanism of the oxidation a t copper-Nafion/GC electrodes in alkaline solutions. Prabhu and Baldwin ( I , 2) had attributed the carbohydrate oxidation a t the CuCI2-modified electrode to the redox couple Cu(II)/Cu(III),where copper acts as an electrocatalyst. In their later work ( 1 5 ) ,they suggested that the active surface state might be a Cu(I1) species. The cyclic voltammetric (CV) response for two types of copper-based electrodes (e.g., Nafion coated with copper and bare GC with copper) were examined and compared in a NaOH solution (pH 13). Figure 2 shows a series of cyclic voltammograms with the copper-Nafion electrode in 0.15 M NaOH following electrodeposition of copper from a 1 X IO-* M copper nitrate solution at -0.7 V. The first cyclic voltammogram (labeled A) exhibits well-defined anodic peaks a t -0.412, -0.166, and 0.600 V vs Ag/AgCI. These peaks are
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
+ C 01 L
0
0.8 0.5 02 -01 -04 -0.7 Potential ( V I Figure 2. Cyclic voltammograms for a copper-Nafion-coated electrode recorded (A) for a solution containing 0.15 M NaOH, (B) after five M glucose folcontinuous cycles following (A), and (C) for 1 X lowing (6).Scan rate 20 mV/s. Supporting electrolyte 0.15 M NaOH. Other conditions as in Figure 1.
assigned to the oxidation of copper to Cu(I), Cu(II), and Cu(III), respectively. Miller (16)has observed similar anodic transitions of copper in alkaline solutions at a split ring-disk electrode at about -0.43, -0.18, and 0.50 V vs SCE. Pyun and Park ( I 7) employed in situ spectroelectrochemical methods to identify Cu(1) and Cu(I1) species a t a copper-plated platinum electrode in 0.1 M KOH solution. They assigned -0.47 V to the formation of Cu(1) and -0.2 V to the formation of Cu(l1). Their potentials were referenced vs an Ag/Ag,SO, electrode. There is close agreement between the reported values for the Cu(I)/Cu(O) and Cu(II)/Cu(I) transitions in all these studies. The three oxidation states of copper, Cu(I), Cu(II), and Cu(III), are reported to have the following standard potentials in alkaline media:
Eozg8,V ref
(3)
+ 2e- = 2Cu0 + 2 0 H Cu(OH), + 2e- = Cuo + 20H2Cu(OH), + 2e- = CuzO + 2 0 H - + HzO
(4)
Cu0,-
(1) (2)
or
Cu20 + H20
+ 2 H z 0 + e- = Cu(OH), + 2 0 H -
CuzO,
+ HzO + 2e- = 2CuO + 2 0 H -
-0.58 18 -0.44 18 -0.30 18 -0.58
19
+0.52 20
651
The half-cell potentials tabulated in eqs 1-4 are the standard values but referenced with respect to a standard Ag/AgCl potential. Reactions 1 and 3 are within 100 mV of the experimental values for the Cu(I)/Cu(O) and Cu(II)/Cu(I) reactions at pH = 13. Since these are surface-confined reactions with the formation of passivating films, it is not surprising that quantitation is difficult. Others (15,21) have reported on the complexity of the reactions since both soluble and insoluble products are involved. It should be noted that the oxidation of Cu(0) to Cu(OH)*is thermodynamically favored (reaction 2 in Table I) to occur prior to the oxidation of CuzO to Cu(OH),. Thus, disproportionation can occur between Cu(OHIZand Cuo at the metal to oxide/hydroxide interface to form Cu20. The occurrence of the disproportionation reaction and the continued oxidation of Cuo to C U ( O H )may ~ explain why a small oxidative wave is observed from -0.1 to +0.5 V. Pyun and Park have suggested that the Cu(1) and Cu(I1) species in their hydroxide forms convert to oxides upon aging. In the cathodic scan direction from +0.8 V, a small reverse wave appears a t +0.56 V corresponding to the reduction of Cu(II1) to Cu(I1) and a larger, well-defined wave at -0.55 V for Cu(II)/Cu(I). Continued scanning in the negative direction produces a third wave with a peak potential of -0.85 V (not shown in Figure 2A) which is assigned to the Cu(I)/Cu(O) transition (17). Baldwin (15) and Miller (16) also detected similar cathodic peaks a t copper-based electrodes. The CV profile for copper without Nafion on the glassy carbon surface is very similar to that with Nafion. Figure 2B shows a cyclic voltammogram (after continuous cycling for 3 min) obtained with the Nafion-coated copperbound electrode in a NaOH solution. The CV scan results in the disappearance of the copper oxidation peaks of Cu(1) and Cu(II1) and a sharp decrease in the Cu(I1) peaks. This indicates that the copper deposits are covered with a layer of copper oxides particles, which hampers the continued dissolution of copper from the “bulk”. The morphology of this electrode is very similar to that of a freshly deposited copper-Nafion/GC electrode as seen by SEM at the same resolution. The addition of 1.0 mM glucose to the NaOH solution yielded a drawn-out anodic peak at 0.5 V associated with the catalytic oxidation of glucose (Figure 2C). Although the exact mechanism for the oxidation of carbohydrates at a copper electrode is not known at this time, Cu(I1) or Cu(II1) species have been suggested to act as an electrontransfer mediator (15) or as a complexing agent [Cu(II)] to solubilize the oxide layer (22). As seen in Figure 2, the oxidation of glucose occurs at or very near to the oxidative waves for Cu(II)/Cu(III) on the copper-Nafion-coated electrode (compare Figure 2A with 2C). In our case, Cu(II)/Cu(III) oxides seem to actively participate in the catalytic oxidation of the carbohydrates as suggested from the redox potentials. The electrocatalytic oxidation of sugar requires an alkaline solution with pH 13 or greater. There is a sharp decrease or complete absence of a voltammetric response, for example, for glucose, when the pH of the NaOH solution is decreased or when a sodium borate buffer of pH 7.0-9.0 or a phosphate buffer solution at physiological pH is used. Thus, a 0.15 M NaOH solution of pH 13 was selected for these carbohydrate experiments. The effects of organic surfactants such as albumin and sodium dodecyl sulfate on glucose flow injection response were compared with those of the copper-Nafion/GC electrode and M glucose response bare copper/GC electrode. The 1 X employing the Nafion-coated electrode decreased by 8% and 6% upon addition of 500 ppm albumin and SDS, respectively. In contrast, the glucose response of the bare copper/GC electrode decreased by 75% and 18% with the addition of 500 ppm albumin and SDS, respectively. In addition, successive
-
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injections of sample containing 1 X M glucose and 500 ppm albumin resulted in a rapid loss of electrode activity at the bare electrode, with up to 50% decrease in response for the 30 repetitive injections. The Nafion-coated electrode, on the other hand, shows a highly stable response and is resistant to electrode fouling. Thus, the elimination of interferences with respect to surface-active compounds, which cause “poisoning”, is another important feature of the film-coated detectors. Analytical Utility. The copper-Nafion CME seems to be a viable amperometric detector for the on-line, flow-through analysis, including LC and FIA. Figure 3 shows the chromatograms for a sample mixture of sugars containing both mono- and disaccharides. The effectiveness of the chromatographic separation is indicated by the excellent resolution at a low flow rate of 0.3 mL/min. At a higher flow rate of 1.0 mL/min, these sugars are coeluted, producing unresolved peaks. In addition, a substantial decrease in the sensitivity (signal-to-noise (SIN) ratio) of the peaks is recorded at high flow rates. The analytical usefulness of copper-Nafion electrodes for carbohydrate analysis is based primarily upon the linearity and reproducibility of the response. Such an electrode exhibits a concentration-dependent profile that is well-defined and reproducible. The chromatographic responses of sorbitol (So), arabinose (Ar), and glucose (GI) were evaluated with a series M. of six concentration increments from 5 X lo4 to 1 X The current vs concentration plots yielded linear responses with slopes (log FA/log M) of 1.29 ( S o ) ,1.24 (Ar), and 1.22 (GI); intercepts (log PA) of 5.46 (So), 5.39 (Ar), and 4.95 (GI); and correlation coefficients of 0.998 (So, G1) and 0.992 (Ar). The fact that different intercepts are observed may be related to the degree of ionization of each carbohydrate. These sugars have pK, values of 13.6 (So), 12.34 (Ar), and 12.28 (Gl). With the copper-Nafion/GC electrode, very low detection limits (LODs) were obtained following on-column injection of various Carbohydrates. The LODs estimated for arabinose, glucose, fructose, rhamnose, maltose, and lactose at a S I N ratio of 3 were 10, 20, 30,60, 100, and 120 pmol, respectively. The catalytic oxidation of carbohydrates, in general, and of glucose, in particular, results in reproducible chromatographic responses. For 100 successive on-column measurements of 1 X M glucose over 18 h, the relative standard deviation (RSD) was 4.5%. The response of the copperNafion electrode to carbohydrates remained stable for many hours. The lifetime (longevity) of the electrode was determined by loading different amounts of copper on the electrode as monitored by the electrodeposition period. For example, an electrode (in a chromatographic flow cell) prepared by accumulating Cuo from a 0.3 M Cu(N0J2 solution for 10 s at -0.7 V gave a reproducible response for glucose at 90-95% of its original (first hour) response (over 60 injections made per day) for many days. After 6 days, the response slowly decreased to approximately 60% of its original response. Although the response decreased, a well-defined signal could be attained with a linear calibration plot. On the other hand, with low copper loading, a deposition for 2 s from 0.3 M C U ( N O ~resulted )~ in a reproducible signal (510% RSD) for only 2 days. The analysis of the chromatographic eluant after passing through the detector for copper by atomic absorption spectrometry showed a low level of copper (2 ppm). It is presumed that the copper is being slowly lost from the electrode. Thus, the amount of copper remaining on the electrode determines its useful lifetime. The loss of copper from the polymer matrix is probably the main reason for the decrease in the sensitivity and the stability
-1
0
10
20
Time [minl Figure 3. Chromatograms of mixtures of carbohydrates containing (1)
inositol, (2)sorbitol, (3)rhamnose, (4)arabinose, (5)glucose, (6)ribose, (7) lactose, and (8)sucrose. Carbohydrate concentrations: 3 X M (3, 5, 7, 8) and 5 X M (6). Injection M (1, 2, 4), 2 X volume 20 pL. Flow rate 0.3 mL/min. Applied potential 0.48 V. Mobile phase 0.15 M NaOH. of the electrode. Although the presence of Nafion decreases the rate of copper loss, it does not completely eliminate it. Other studies are being conducted in our laboratory to minimize the loss rate of copper from the electrode and to develop modification methods that stabilize the copper. Preliminary chromatographic experiments have shown that ascorbic acid (an acidic sugar) can be detected along with other carbohydrates, while uric acid is not detected. Thus, a new dimension of selectivity is achieved for the detection of carbohydrates without interference from uric acid based on the catalytic activity of copper oxides and the Nafion coating. One of the problems in the use of a polymer-coated electrode for flow-through systems is the finite loss or gradual change in the mechanical integrity of the film. It is difficult to maintain an invariant electrode structure, chemically or physically, for extended periods of time. However, the addition of an internal standard periodically for calibration purposes will correct for any changes in the electrode sensitivity to the analyte carbohydrates.
CONCLUSIONS Carbohydrates are not easily oxidized and, hence, are not easily detected at solid electrodes, such as platinum, gold, or glassy carbon, by constant potential (amperometric) methods due to slow electron-transfer kinetics. A novel electrode described herein, consisting of copper particles dispersed in Nafion on GC, overcomes the kinetic limitation by a catalytic process that substantially lowers the overpotential for the oxidation reaction. This catalysis provides sensitivity and specificity. Nafion, a polyelectrolytic anionic polymer, is spread-coated on a GC electrode, and copper particles are dispersed within the Nafion by electrodeposition. The electrode is used for the LCEC analysis of carbohydrates. The mechanical stability of the copper is greatly improved by such incorporation into the polymeric film. Nafion also provides a barrier to surface-active species reaching the catalytic copper sites, thereby adding a new dimension to selectivity in LCEC analysis. Other advantages are the low cost and relative ease in preparation of the detector.
Anal. Chem. 1991, 63,653-656
ACKNOWLEDGMENT We thank Rosalind Mitchell and Nancy Harmony for assistance in the preparation of the manuscript.
LITERATURE CITED Prabhu, S.;Baldwin. R. Anal. Chem. 1989, 6 1 , 852-856. Prabhu, S.;Baldwin, R. Anal. Chem. 1989, 6 1 , 2258-2263. Santos, L. M.; Baldwin, R. P. Anal. Chem. 1987, 59, 1766-1770. Murray, R. W.; Ewing, A. G.; Durst, R. A. Anal. Chem. 1987, 59, 379A-390A. Kost, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1988, 6 0 , 2379-2384. Kost, K . M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1990, 62, 151-157. Hoyer, B.; Florence, T. M.; Batley, G. Anal. Chem. 1987, 59, 1608-16 14. Nagy, G.; Gerhardt, G. A.; Oke, A.; Rice, M. E.; Adams, R. N.; Moore, R. B.;Szentirmav. M. N.: Martin. C. R. J. Electroanal. Chem. 1985. 188, 85-94. Wang, J.; Tuzhi, P.; Golden, T. Anal. Chim. Acta 1987, 194, 129-138 . -. . -.
Martin, C. R.; Rhoades, T. A.; Ferguson, J. A. Anal. Chem. 1982, 54, 1639- 1641. Martin, C. R.; Dollard. K. A. J. Electroanal. Chem. 1983, 159, 127-1 35. Kristensen. E. W.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1987, 59, 1752-1757.
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(13) Brazell, M. P.; Kasser, R. J.; Renner, K. J.; Feng, J. X.; Moghaddam, B.;Adams, R. N. J. Neurosci. Methods 1987, 2 2 , 167-172. (14) Harrison, D. J.; Turner, R. F. B.; Baltes. H. P. Anal. Chem. 1988, 6 0 , 2002-2007. (15) Luo, p.; Prabhu, s. V.; Baldwin, R. P. Anal. Chem. 1990, 6 2 , 752-755. (16) Miller, B. J . Electrochem. SOC. 1969, 116, 1675-1680. (17) Pyun, C . H.; Park, S. M. J. Nectrochem. SOC. 1986. 133, 2024-2030. (18) CRC h'andbook of Chemistry and Physics, 64th ed.; CRC Press: Boca Ration, FL, 1983-1984; Table 1 of Electrochemical Series, D-156. (19) Latimer, W. M. The Oxidation States of the Elements and their Potentials in Aqueous Solutions: Prentice-Hall: New York, 1938. (20) Lange's Handbook of Chemistry, 13th ed.; Dean, J. A.. Ed.; McGrawHill Book Co.: New York, 1985; Table 6-2. (21) Burke, L. D.; Ahern, M. J. G.; Ryan, T. G. J . Electrochem. SOC.1990, 137, 553-561. (22) Kok, W. Th.; Hankamp, H. B.; Bos, P.; Frei, R. W. Anal. Chim. Acta 1982, 142, 31-45.
RECEIVED for review April 27,1990. Accepted December 10, 1990. This work was supported by the Shimadzu Corporation, Kyoto, Japan, and the Kansas Technology Enterprise Corporation under their Applied Research Matching Grants Program. Juan Marioli acknowledges the support of the National Research Council of Argentina.
CORRESPONDENCE Enhanced Poly(chlorotrifluoroethy1ene) Composite Electrodes Sir: Poly(chlorotrifluoroethy1ene)composite electrodes were first introduced in 1978 (I). The first such electrode, affectionately known as the Kelgraf electrode, consisted of graphite compression molded with the inert binder poly(ch1orotrifluoroethylene) or Kel-F (3M Company tradename). These composite electrodes have been characterized electrochemically, as well as with electron microscopy and surface chemical methods, and have been shown to behave as microelectrode ensembles ( 2 , 3 ) .The improved signal-to-noise ratios observed with these electrodes are caused by the enhanced current densities due to the contribution of nonlinear diffusion. These electrodes have proven to be most useful as voltammetric detectors for liquid chromatography and flow injection analysis (4-7) where an additional enhancement effect contributes to the analytical signal, namely the depletion layer recharging effect (8). In more recent work, Kel-F has been used as an inert binder with other conductors. The Kelsil, Kelgold, and Kelplat electrodes are made with the precious metals silver, gold, and platinum, respectively (9, IO). Although work has been done to optimize the performance of these composite electrodes by controlling the weight percent of the conductor, little attention has been given to the active site radii, which have the potential of drastically affecting the behavior of these electrodes. Previous studies have controlled the size of the Kel-F resin and the graphite particles used prior to the compression molding step. However, independent studies have yielded similar results for the active site radii that are on the order of 25 wm ( 2 , I I ) . Note that, in all instances, the conductor particles have been considerably smaller than that of the plastic. Therefore, the active sites have been considered to be made of consolidated conductor between the particles of the inert binder. In fact, these electrodes have been classified as consolidated composites consisting of a network of conducting particles within the inert polymer (IO). In this work, we described a procedure for reducing the
active site radii of the Kel-F graphite. As will be shown, a reduction in the active site radii for an electrode with a given percentage of graphite significantly enhances the current. The grinding procedure used to accomplish this particle size reduction has the additional advantage of providing a more easily polished electrode and allows electrodes to be made with higher percentages of graphite than have been possible in the past.
EXPERIMENTAL SECTION Reagents and Materials. All chemicals used were of reagent grade. The Kel-F 81 resin was obtained from the 3M Commercial Chemicals Division, St. Paul, MN. The powdered graphite (UCP-2-325) was obtained from Ultra Carbon Corp., Bay City, MI. The Kel-F rod used in the electrode fabrication was from Plastic Profiles Inc., East Hanover, NJ. Instrumentation Procedures. The Kel-F graphite electrodes were prepared with an apparatus and procedure described previously (11). The only change in the procedure involved grinding the particular Kel-F graphite mixtures prior to the compression molding step. The mixtures were ground in a micronizing mill obtained from McCrone Accessories and Components, Chicago, IL, for 60 min using corundum grinding elements. The mixture was then compression molded. The resulting Kel-F graphite pellets were then machined to a diameter of 3 mm and press fit into a hole in a pure Kel-F rod. Epoxy was placed around the Kel-F graphite pellet prior to the press fitting to ensure that no seepage of solution occurred between the Kel-F rod and the electrode material. Electrical contact was made with a brass rod through a hole at the opposite end of the Kel-F rod. The electrodes were polished with successively finer suspensionsof alumina down to 0.05-gm particle size. It was found that less time was required to obtain a mirrorlike surface than with the electrodes fabricated without grinding. The potential step experiments were performed by using a homemade potentiostat interfaced to an Apple IIe computer. The interface consisted of a 12-bit analog-to-digital converter board (TM-AD213) and a 12-bit digital-to-analog converter board (TM-DAlOl), both from TecMar,
0003-2700/91/0363-0653$02.50/00 1991 American Chemical Society
