Anal. Chem. 1989, 6 1 , 2599-2602
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Electroanalysis of Aromatic Aldehydes with Modified Carbon Paste Electrodes Katherine E. Liu and HBctor D. Abruiia*
Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301
The utility of carbon paste electrodes modified with aniline, 4-amlnobenzoic acid, or [Fe(CN),LI3- (L = 4-amlnopyridine) for the determlnatlon of aklehydes in soiutlon is demorrstraied. The method Is based on the reactivity of an Immobilized aromatic primary amlne toward aldehydes to yleld the correspondlng imine whose redox response is used as the analytlcai signal. The effects of paste composition and time of reaction on the analytical signal were assessed for the three modifiers mentioned above. Electrodes modified with [Fe(CN),L]’- gave the best resuits In terms of signal to noise ratlo, reproducibility, and electrode stability. Submlcromoiar amounts of benzaldehyde could be determined by employing this approach. The approach is also applicable to the determination of ailphatic aidehdyes (butyraldehyde) although with dimlnlshed sensltlvlty. The presence of other nonaldehyde carbonyl species (e.g. benzophenone) had no detrlmentai effects on the anaiytlcai signal.
INTRODUCTION The use of chemically modified electrodes (CME) in analytical applications (1,4) continues to be an area of vigorous research activity. A broad range of approaches has been pursued including electrostatic accumulation (5-12), coordination effects (12-20), precipitation (21, 22), and others (23-29). Electrode modification has also been employed to prevent electrode fouling or to enhance selectivity (or both) in analytical determinations of dopamine and related biogenic amines (30-33). In addition, many electrode materials have been modified for use in analytical determinations. Of these, the use of carbon paste electrodes (CPE) appears especially advantageous because of the ease of electrode preparation and regeneration as well as low background currents. In fact a number of studies on the use of CPE for analytical determinations have been previously reported. These include the use of a CPE containing dimethylglyoxime for the determination of nickel (19),1,9-dimethyl-9,10-phenanthroline for the determination of copper (ZO),and 1,lO-phenanthroline for the determination of cobalt and iron. Phthalocyaninemodified carbon paste electrodes have also been employed in the determination of sulfhydryl groups (34)and a-keto acids (35, 36). We recently demonstrated the utility of chemically modified CPE for the determination of primary amines (37). The approach was based on the reaction of the amine with an aromatic aldehyde, incorporated into a polymer film to generate the corresponding imine whose electrochemical response was used as the analytical signal. By use of this approach, submicromolar sensitivity as achieved as well as a dynamic range of about 3 orders of magnitude. We have extended this approach to the determination of aldehydes by incorporating an aromatic amine group into the carbon paste electrode to make an imine and again use its redox response as the analytical signal. We present a thorough study of such an analysis system including the use of three different reagents containing the
aromatic amine group, carbon paste composition in terms of reagent loading, and preconcentration time.
EXPERIMENTAL SECTION Reagents. Aniline and benzaldehyde (Aldrich) were vacuum distilled twice and stored under nitrogen in amber bottles. 9Anthraldehyde, 1-naphthaldehyde, and 4-aminobenzoic acid (Aldrich) were recrystallized from ethanol. 4-Aminopyridineand pentacyanoaminoferroate, [Fe(CN)5NH3]“(Aldrich), were used as received. Water was purified by passing through a Hydro water purification train or a Millipore Milli Q system. Acetonitrile (Burdick and Jackson distilled in glass) was dried over 4-A molecular sieves. Tetra-n-butylammonium perchlorate (TBAP; G. F. Smith) was recrystallized 3 times from ethyl acetate and dried under vacuum for 72 h. [Fe(CN),LI3- (L = 4-aminopyridine) referred to in the text as the “iron complex” was prepared by dissolving the ammonium disodium salt of pentacyanoamino ferroate in water and adding (dropwise) a stoichiometric amount of 4-aminopyridine. Substitution of the amino group by the 4-aminopyridinewas immediately apparent from the color change (from yellow-green to purple). After being stirred for 30 min at room temperature the solution was filtered. The complex was isolated by removing the water with gentle heating under vacuum. The isolated complex was washed with ether and dried under vacuum overnight. Instrumentation. Electrochemical experiments were performed on either a BAS 100 electrochemical analyzer or an IBM EC 225 voltammetric analyzer. Data were recorded on a Soltec X-Y recorder. Differential pulse voltammograms (DPV) were obtained by using a 50-mV pulse width and a scan rate of 10 mV/s. Electrochemical cells of conventional design were employed. All potentials are referenced to the sodium-saturatedcalomel electrode (SSCE) without regards for the liquid junction. Electrodes. Platinum (sealed in glass) and glassy carbon (sealed in Teflon) disk electrodes were polished with 1-pm diamond paste (Buehler) and rinsed with water and acetone prior to use. Carbon paste electrodes were prepared by mixing graphite powder (Fisher) (typically 1g) into a solution of ceresin wax (0.1 g) in heptane. The components were thoroughly mixed and the solvent was allowed to evaporate. The amine-containingreagent (i.e. aniline, 4-aminobenzoicacid, or the iron complex) was dissolved (the amine-reagent/graphite powder ratio was varied from 10% to 80%) in ethanol and added to the carbon/wax mixture. It was thoroughly mixed and the solvent was evaporated until a free-flowing powder was obtained. Paraffin oil (0.3-0.4 mL) was added and the mixture was again thoroughly mixed to a smooth consistency paste. The paste was placed in the cup of a locally designed and built carbon paste electrode assembly made from Kel-F with a platinum wire contact. The body and plunger of the electrode assembly were threaded so that carbon paste could be extruded by simply turning the plunger. The surface of the carbon paste electrode was smoothed by gently rubbing on a piece of filter paper with graphite powder. This procedure was also employed in regenerating the surface of the carbon paste electrode. Procedures. The carbon paste electrode was immersed in 5 mL of ethanol or 50/50 ethanol/water solution of the aldehyde to be determined (e.g. benzaldehyde) and HCl was added (imine formation is acid catalyzed) to a concentration of 0.02 M. The electrode was allowed to contact the solution for different time periods (vide infra) while stirring. The electrode was rinsed with water and acetone and placed in a degassed (with nitrogen or argon for 20 min) acetonitrile solution/O.l M TBAP and the redox response of the imine formed was obtained by DPV. The height
0003-2700/89/0361-2599$01.50/00 1989 American Chemical Society
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E vs. SSCE ~1.50
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V Flgure 1. Differential pulse voltammograms in acetonitrile/O.1 M TBAP and in the oxidative region for carbon paste electrodes modified with (A) hminobenzoic acid (35% loading), (E) aniline (50% loading),and (C) the iron complex (10% loading) after imine formation with benzM. Current scale S = 25 aldehyde at a concentration of 1.0 X nA for A and B and 50 nA for C.
of this wave was employed in obtaining calibration curves. All analytical measurements represent the average of at least four replicate determinations. The surface of the carbon paste electrode was regenerated prior to each determination.
RESULTS AND DISCUSSION The effects of a number of parameters on the analytical response were investigated. These included (a) source of incorporated aromatic amine, (b) loading level of reagent into the carbon paste, and (c) incorporation time. Benzaldehyde was used as a representative analyte in order to determine the optimal conditions for analysis. The results are presented by considering the various reagents and the effects of loading and incorporation time. Preliminary Voltammetric Characterization. The redox responses of the carbon paste electrodes containing the various amine reagents were obtained after imine formation (with benzaldehyde) in acetonitrile/O.l M TBAP in order to determine the optimal redox wave to be employed on analytical determinations. Because of the inherent advantage of employing oxidative processes (no degassing is required) in the analytical determination, such processes were investigated first. Figure 1shows anodic scans (DPV) for the carbon paste electrodes modified with 4-aminobenzoic acid (A), aniline (B), and the iron complex (C) after imine formation with benzaldehyde. For carbon paste electrodes modified with aniline, the peak potentials for oxidation of the amine and the imine were +0.91 and +1.0 V, respectively. For electrodes modified with 4-aminobenzoic acid the corresponding values were +1.23 and +1.13 V. It is clear that in these two cases the large overlap of the two voltammetric waves would make analytical quantification difficult. On the other hand, the redox responses for the amine and imine for carbon paste electrodes modified with the iron complex were very well resolved (Figure
le).
Because of the significant overlap of voltammetric waves for the amine and imine oxidations for electrodes modified with aniline or 4-aminobenzoic acid, the reduction processes were investigated. We found that whereas the amines did not exhibit any reduction peak in the region from 0.0 to -1.50 V, the imines derived from aniline and 4-aminobenzoic acid with benzaldehyde had very well defined redox responses at 4 . 8 3 (Figure 2A) and -0.94 (Figure 2C) V, respectively. An analogous behavior was observed for electrodes modified with the iron complex. In this case in imine had a very well defined reduction a t -0.63 V (Figure 2B). The results of these preliminary studies are summarized in Table I. For studies with electrodes modified with aniline or 4aminobenzoic acid, the reduction waves were employed as the
; -0.50
-1.00
-1.50
E vs. SSCE Flgure 2. Representative DPV responses for the determination of benzaldehyde with carbon paste modified electrodes. (A) Electrode modified with aniline at 50% loading. Benzaldehyde concentrations: a, 5 X b, 1 X c, 1.5 X d, 1.8 X S = 50 nA. (E) Electrode modified with the iron complex at 10% loading. Benzb, 1 X c, 1.5 X lo-'; d, aldehyde concentrations: a, 5 X 2.0X S = 100 nA. (C) Electrode modified with 4aminobenzoic acid at 35% loading. Benzaldehyde concentrations: a, 1.5 X b, 2.0 X c. 3.0 X d, 4.0 X S = 50 nA. Table I. Peak Potential Valuesn for the Redox Responses of Amine Containing Reagents and the Imines Derived from Benzaldehyde* reagent
oxid
red.
1
ani 1in e imine
+0.91 +1.0
-0.83
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[Fe(CN)SL]-3
+1.23 +1.13 +0.38
imine
+0.95
-0.64
imine 3
"In volts vs SSCE.
acetonitrile/O.l M TBAP.
analytical signal. For electrodes modified with the iron complex, both the oxidative and the reductive processes were studied and similar results were obtained. However, the use of the oxidative wave is preferred since no degassing is required. A. Effects of Modifer, Reaction Time, and Reagent Loading. We investigated the effects of using three different sources of the aromatic amine: aniline, 4-aminobenzoic acid, and the iron complex containing coordinated 4-aminopyridine. These were chosen because they represented three different types of interaction with the carbon paste. The aniline was chosen because of its high solubility and partitioning into the pasting medium. The iron complex was chosen as an example of a very highly charged (-3) species and the 4-aminobenzoic acid as representing an intermediate case. For each of these modifiers we investigated the effects of loading on the carbon paste and time of incorporation on the analytical response. Our initial studies were with aniline as a modifier followed by 4-aminobenzoic acid and the iron complex. 1. Aniline. From our previous study on the determination of amines with modified carbon paste electrodes (37) we had determined that a 50% reagent loading gave optimal results. Thus, our initial investigations with aniline as a modifier were at this loading level. We studied the effect of the time of
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Flgure 3. (A) Calibration curves for the determination of benzaldehyde
with an aniline modified carbon paste electrode (50% loading) as a function of reaction time. (B) Analytical response for a 50% loading aniline modified,carbon paste electrode as a function of time of incorporation for benzaldehyde at a solution concentration of 2.5 X lo4 M.
reaction for 5, 10, 15, and 20 min. In all cases (Figure 3A) linear calibration curves were obtained with slopes that generally increased with increasing time of reaction. Although the signal amplitude increased with reaction time, the relative increase in the signals a t higher exposure times was much smaller than at shorter times of exposure, suggesting a leveling of the response with time. In fact, a plot of peak current vs time a t a constant concentration of benzaldehyde exhibited such a trend (Figure 3B). Although the magnitude of the signal was largest at 20 min, there was also an accompanying increase in the background current so that the signal to noise and reproducibility were in fact lower than a t 10 min. Although we are not certain as to the origin of the background, we believe that it might be associated with decomposition of the aniline since the background increases further with aging of the paste. Thus, for aniline as a modifier, a 10-min reaction time was considered optimal. After establishing the optimal reaction time, we studied the effect of reagent loading. In addition to the 50% paste, measurements were carried with pastes a t 26%, and 79% loading. The results, shown in Figure 4A, show that for a 10-min reaction time all loadings gave linear calibration curves with slopes (and thus sensitivities) that increased with loading. However, a t the 79% loading level, there was a significant increase in the background. At even higher loadings, the background was still higher and, in addition, there were problems with the mechanical integrity of the paste. However, as Figure 4B shows there appears to be an asymptotic response as a function of loading. From these results, a 50% loading was deemed adequate. Representative responses for the determination of benzaldehyde at various concentrations with aniline modified carbon paste electrodes (at 50% loading) are presented in Figure 2A. 2. 4-Aminobenzoic Acid and [Fe(CN),LI3- (L = 4Aminopyridine). Similar experiments were performed for
% Loading
Flgure 4. (A) Calibration curves for the determination of benzaldehyde with an aniline modified carbon paste electrode as a function of paste composition using a 10-min reaction time. (6)Analytical response for a l0-min incorporation time for aniline modified carbon paste electrodes
as a function of reagent loading for benzaldehyde at a solution concentration of 2.5 X M.
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Figure 5. Calibration curves for the determination of benzaldehyde with carbon paste electrodes modified with aniline, 4-aminobenzoic acid,
or [Fe(CN),LI3-under optimal experimental conditions. carbon paste electrodes modified with 4-aminobenzoic acid and the iron complex. In the case of 4-aminobenzoic acid, the optimal conditions were a 35% loading level and a 10-min reaction time, whereas for the iron complex these were 25% loading and again a 10-min reaction time. In the case of the iron complex, the 25% loading represented the solubility limit so that higher loadings could not be investigated. In addition, the analtyical responses for the carbon paste electrodes were significantly higher and the background levels much lower than for the other two modifiers. Representative responses for the determination of benzaldehyde at different concentrations using carbon paste electrodes modified with the iron complex (at 10% loading) or 4-aminobenzoic acid (at 35% loading) are presented in parts B and C, respectively, of Figure 2.
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3. Comparison between Modifiers. Figure 5 shows the calibration curves for the determination of benzaldehyde with the three different reagents a t the optimal operating conditions. What is immediately apparent is that the response for electrodes modified with the iron complex was about 4 times larger (notice different scales) than those modified with either aniline or 4-aminobenzoic acid. In addition the response of the 4-aminobenzoic acid modified electrodes was about 60% higher than that for the aniline modified electrodes as determined by the ratio of the slopes of the calibration curves. The reasons for the very large differences in response between electrodes modified with the iron complex and those modified with aniline and 4-aminobenzoic acid are not clear. One could speculate that the iron complex helps in the propagation of charge; however, we have no direct evidence to this effect. Another potential explanation might be the chemical stability of the reactants. For example, electrodes modified with aniline could not be stored (in air) for more than 2 days without severe degradation of performance. Electrodes modified with 4-aminobenzoic acid were stable for about a week and those modified with the iron complex were indefinitely stable. This trend follows the anticipated ease of oxidation of the various reagents and may also pay a role in dictating the observed responses. It is clear that out of the three materials used for modification, the iron complex exhibited the best performance in terms of signal to noise ratio and electrode stability. Since electrodes modified with the iron complex exhibited the best properties, we have performed some additional studies with it. Specifically, we tried to determine the lower limit of detection and found that even at submicromolar concentration (0.2 p M ) a discernible peak, associated with the incorporated imine, could be detected. However, the reproducibility degraded significantly (rt30%) over that obtained a t 10 pM (rtll%). However, these experiments point to the very high sensitivity that can be achieved. In addition, due to the nature of the chemical reaction involved (only aldehydes will form imines) a very high degree of specificity was obtained. For example, the presence of ketones (e.g. acetophenone) in 100-fold excess did not have any effect on the determination of benzaldehyde. Although aliphatic aldehydes (butyraldehyde) could also be determined (albeit with diminished sensitivity), the method appeare to be most sensitive to aromatic aldehydes. For example the presence of butyraldehyde at 100-fold excess had a relatively small effect on the determination of benzaldehyde, and this probably reflects differences in the rate of imine formation. However, similar detection limits to those obtained for benzaldehyde were obtained for 9-anthraldehyde and 1-naphthaldehyde, consistent with our previous assertion. It should be mentioned that an analogous analytical scheme had been previously presented by Baldwin and co-workers (23) where allylamine was adsorbed onto a platinum electrode and subsequently reacted with ferrocene carboxaldehyde to yield the surface-immobilized ferrocenylimine. The redox response of the surface-immobilized ferrocene was then used as the analytical signal. Although that approach is analogous to the one presented here (both depend on imine formation), we employ a redox process associated with the imine itself as the analytical signal so that our approach would be generally applicable to the determination of aldehydes by imine formation. In addition, most methods for the determination of aldehydes (38) are based on the preparation of derivatives suitable for spectrophotometric determination and are gen-
erally very time-consuming. There was also a recent report (39)on the determination of picolinaldehyde by reduction of its Girard-P derivative at a mercury electrode. The reported limit of detection was 1.5 x 10" M and the linear range was found to be from 1.5 X to 2.9"' M. Thus, the procedure presented here compares very favorably with these methods. It is clear that reagents based on the iron complex employed here for the determination of aldehydes and previously for the determination of aromatic amines represent a very versatile group of reagents for the selective and sensitive determination of organic species and we continue to explore their potential analytical applications.
LITERATURE CITED Murray, R. W. I n Electroanalytical Chemisfry; Bard, A. J.. Ed.; Marcel Dekker: New York, 1983; Vol. 13, pp 191-368. Murray. R. W.: Ewino. A. G.: Durst. R. A. Anal. Chem. 1987. 59. 379A: Abrufia, H. D. "Electrode Modification with Polymeric Reagents". I n Electroresponsive Mleculer and Polymeric Systems; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1988 Vol. 1, pp 97-171. Abrufia, H. D. Coord. Chem. Rev. lS88, 86, 135. Oyama, N.; Anson, F. C. J. Am. Chem. Soc. 1979, 101, 3450-3456. Oyama, N.; Anson, F. C. J. Electrochem. SOC. 1980, 127, 247-248. Espencheid, M. W.; Ghatak-Roy, A. R.; Moore. R. B., 111; Penner, R. M.; Szentirmay, M. N.; Martin, C. R. J. Chem Soc ., Faraday Trans. 1 1988, 82,1051-1070. Whitley, L. D.; Martin, C. R. Anal. Chem. 1987, 59, 1746. Cox, J. A.; Kulesza, P. J. Anal. Chim. Acta 1983, 154, 71. Wang, J.; Greene, B.; Morgan, C. Anal. Chim. Acta 1987, 158, 15. Guadalupe. A. R.; Abrufia, H. D. Anal. Lett. 1986, 19(15&16), 1613-1632. Guadalupe, A. R.; Abrufia, H. D. Anal. Chem. 1985, 57, 142-149. Wler, L. M.; Guadalupe, A. R.; Abruiia, H. D. Anal. Chem. 1985, 57, 2009-201 1. Guadalupe, A. R.; Wier, L. M.; Abrufia, H. D. Am. Lab. 1988, la@), 102-107. McCracken, L L.; Wier, L. M.; Abrufia, H. D. Anal. Lett. 1987, 20, 1521. Hurrell, H. C.;Abrufia, H. D. Anal. Chem. 1988, 60,245. Kasem, K. K.; Abrdna, H. D. J. Electroanal. Chem. 1988, 242, 87. Gehron, M. J.; Brajter-Toth, A. Anal. Chem. 1986, 58, 1488-1492. Baldwin, R. P.; Chrlstensen, J. K.; Kryger, L. Anal. Chem. 1980, 58, 1790. Prabhu. S.V.; Baldwin, R. P.; Kryger, L. Anal. Chem. 1987, 59, 1074. Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, 11, 393-402. Cox, J. A.; Majda, M. Anal. Chem. 1980, 5 2 , 861-864. Price, J. F.: Baldwin, R. P. Anal. Chem. 1980. 5 2 , 1940-1944. Pham, M.C.; Tourillon, G.; Lacaze, P.-C.; Dubois, J.-E. J. Electroanal. Chem. 1980, 11 1 , 385-390. Pham, M.4.; Dubois, J . I . ; Lacaze, P.C. J. Electrochem. SOC.1983, 130, 346-35 1. Cox, J. A.; Kulesza, P. J. J. Electroanal. Chem. 1983, 159, 337-346. Willman, K. W.; Murray, R. W. J. Electroanal. Chem. 1982, 133, 211-231. Lubert, K. H.; Schnurrbush, M.; Thomas, A. Anal. Chim. Acta 1982, 144, 123-136. Ikaniyama, Y.; Heineman, W. R. Anal. Chem. 1986, 58, 1803. Nagy, F.; Gerhardt, G. A.; Oke, A. F.; Rice, M. E.; Adams, R. N.; Moore, R. B.; Szentirmay, M. N.; Martin, C. R. J. Electroanal. Chem. 1985, 188,85. Gerhardt, G. A.; Oke, A. F.; Nagy, F.; Mcghaddam, B.; Adams, R. N. Brain Res. 1984, 290, 390. Wang, J.; Tuzhi, P. Anal. Chem. 1986, 58,3257. Wang, J.; Tuzhi, P.; Golden, T. Anal. Chim. Acta 1987, 194, 129. Halbert, M. K.; Baldwin, R. P. Anal. Chem. 1985, 57, 591. Santos, L. M.; Baldwin, R. P. Anal. Chem. 1988, 58,848. Santos, L. M.; Baldwin, R. P. J. Chromatogr. 1987, 414, 161. Guadalupe, A. R.; Jhaveri, S. S.;Liu, K. E.; Abrufia, H. D. Anal. Chem. 1987, 59, 2436. Siggta, S.;Hanna, J. G. Quantitative Organic Analysis, 4th ed.; Wiiey: New York, 1979. Barragin de la Rosa, F. J.; Callejdn Mochdn, M.; Gulrafim PCez. A. Anal. Chim. Acta 1985, 172, 65 I
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RECEIVED for review July 13, 1989. Accepted September 8, 1989. This work was generously funded by the National Science Foundation. H.D.A. is a recipient of a Presidential Young Investigator Award (1984-1989) and an A. P. Sloan Fellow (1987-1991). K.E.L. acknowledges support by the REU Program a t NSF.
