Anal. Chem. 1982, 5 4 , 322-324
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Acrylic Acid Polymer Film Chemically Modified Graphite Electrodes Sir: Plasma discharges have been recently used to modify the surface of a variety of materials. Bradley and Fales have used an electrical discharge plasma to activate the surface of materials such as Teflon, polyester, and polypropylene in order to graft a coating of acrylic acid polymer to these materials (1, 2). This procedure did not change the appearance or bulk properties of these materials but enabled them to be easily wetted, bonded with adhesives, colored by dyes, and made retentive for metal ions. Radio frequency (RF) plasmas have been employed in the construction of chemically modified electrodes. Evans and Kuwana have used an RF oxygen plasma to modify the surface of pyrolytic graphite electrodes (3,4).This procedure increases the number of bonding sites on the surface of the electrode by increasing the number of oxygen-containing functional groups. Anson et al. have employed an RF argon plasma to clean the surface of pyrolytic graphite electrodes (5). This treatment removes surface oxygen groups and some other species while activating the electrode surface for use in further reactions. Polymerization of species on electrodes has also been accomplished via R F plasma discharges. Murray et al. have polymerized electrochemically active substances on the surface of glassy carbon and platinum electrodes using an RF plasma (6, 7). In this work, acrylic acid was polymerized on graphite powder utilizing an RF plasma. This procedure increased the amount of carboxylic acid groups available on the surface for further covalent attachment. The advantages of this procedure are that it is simple and that either an aqueous or nonaqueous attachment scheme can be employed for further covalent attachment. Attachments via acid chlorides can be accomplished in nonaqueous systems (8,9), or a carbodiimide can be used for the attachment of solvent sensitive species, such as enzymes, in aqueous media (10-12).In addition, this procedure could be useful in the construction of polymer-film chemically modified electrodes that can be used as potentiometric sensors (13).
EXPERIMENTAL SECTION Apparatus. Plasma treatments were performed in a radio frequency plasma cleaner manufactured by Harrick Scientific (Ossining, NY).Titration of samples was accomplished through use of a Sargent Model C automatic rate buret with a 2.5-mL capacity and a Corning Model 12 pH meter connected via a differentiatorcircuit to a two-pen strip chart recorder, monitoring both the titration curve and its derivative. All electrochemistry was performed by using a potentiostat of conventional design, constructed in-house, with a saturated calomel electrode (SCE) as reference and a platinum wire as an auxiliary electrode. The carbon paste working electrode had a surface area of 0.32 om2. Materials. Graphite powder (Acheson no. 38) and o-tolidine (reagent grade) were obtained from Fisher Scientific. Acrylic acid (99%), 2-nitroso-1-naphthol (reagent grade), and mineral oil (IR grade) were obtained from Aldrich Chemical Co. Diphenyl carbazone (reagent grade) was obtained from MCB Chemicals and mercuric nitrate (reagent grade) was obtained from Mallinckrodt Chemicals. Distilled-deionized water was used for all solutions. All other chemicals were of reagent grade. Procedure. Graphite Pretreatment. Radio frequency (RF) acrylic acid plasma treatments were performed on graphite powder by placing the samples in the plasma chamber and evacuating to approximately 30 mtorr. Acrylic acid was placed in a flask which was connected to the chamber via Tygon tubing. Acrylic acid was introduced into the chamber through a needle valve and the preasure was maintained at 300 mtorr. An acrylic acid plasma was formed and the sample was reacted for 30 min, after which the plasma was quenched by isolating the chamber from vacuum 0003-2700/82/0354-0322$0 1.25/0
and opening the needle valve fully to allow acrylic acid vapors to fill the chamber. The sample was allowed to remain in the chamber (filled with acrylic acid vapors) an additional 30 min to quench any active sites remaining on the graphite, after which the needle valve was closed and the chamber was reconnected to vacuum. The sample was again evacuated for 30 min to approximately 30 mtorr. RF oxygen plasma treated samples were reacted in a similar manner, with oxygen substituted for acrylic acid and the pressure during the reaction decreased to 150 mtorr. Acrylic acid blank samples were prepared by exposing graphite samples to vapors of the acrylic acid under the same experimental conditions described above, but without forming a plasma. Degassed graphite samples (virgin blanks) were prepared by evacuating to approximately 30 mtorr for 1 h. Acid Group Analysis. Analysis of acidic groups present on the graphite powder was performed by back-titration. A sample of pretreated graphite (30-35 g) was placed in a flask and 300 mL of either sodium hydroxide or sodium carbonate solution was pipetted into the flask. The base solutions were prepared fresh for each analysis either by diluting a certain amount of sodium hydroxide pipetted from the surface of a 50% (v/v) mixture or by weighing an appropriate amount of reagent grade sodium carbonate and diluting with enough distilled-deionized water (which had been boiled to remove carbon dioxide) to make approximately a 0.02 N solution. Nitrogen, saturated with water vapor, was used to blanket the solution in the reaction flasks, and the solutions were stirred for 3 h. The graphite was allowed to settle and three 30-mL aliquots were back-titrated with 1.02 N HCl, which had been previously standardized with primary standard grade sodium carbonate. A blank was run on the base solution for each sample using the same reaction conditions. Chloride Analysis. A sample of pretreated graphite (approximately 10 g) was transferred directly from the plasma chamber to a round-bottom flask and was refluxed in 50-mL of thionyl chloride for 30 min. The graphite was filtered and washed with anhydrous benzene and dried under a vacuum of approximately 50 mtorr for 12 h. The sample was then refluxed in 100 mL of distilled-deionizedwater for 30 min, which hydrolyzed the acid chlorides, leaving free chloride ions in solution. The graphite was allowed to settle as the solution cooled to room temperature. Analysis for the chloride ions was performed by taking 20-mL aliquots from the supernatant solution and adding 2-3 drops of each indicator (1%solution of diphenylcarbazone in ethanol and 3% solution of 2-nitroso-1-naphthol in ethanol) and titrating with approximately 0.0250-0.0300 M mercuric nitrate solution which had previously been standardized with primary standard grade sodium chloride over a range of 0.6023-2.5592 mmol of chloride (14,15). Electrochemistry. The carboxylic acid groups on the acrylic acid treated samples were converted to the acid chlorides using the same method as used for the chloride analysis. The samples were then refluxed for 12 h with 100 mL of 20 mM o-tolidine in anhydrous benzene. The sample was then extracted with anhydrous benzene in a Soxhlet extraction apparatus for 24 h, followed by overnight drying under a vacuum of approximately 50 mtorr for 12 h (16). A carbon paste was prepared by degassing approximately 2 g of a graphite sample for 30 rnin at approximately 30 mtorr followed by mixing with a solution of 1 g of mineral oil in 10 mL of anhydrous benzene (17). Excess benzene was evaporated under a stream of nitrogen gas and then completely removed by drying under a vacuum of approximately 80 torr for 15 min. Surface Area Analysis. Nitrogen BET surface area analysis of the graphite powder was performed by Quantachrome Corp. (Syosset, NY).
RESULTS AND DISCUSSION After plasma treatment the surface of the graphite shows a "rainbown effect due to the interference patterns caused by the acrylic acid polymer film, and the interior of the plasma 0 1982 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
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Table I. Analysis of Graphite Samples"
carboxylic acid groupsb phenolic hydroxyl groups carboxylic acidb & phenolic hydroxyl groups acid chloride groupsc
acrylic acid plasma
acrylic acid blank
0.0250 i0.0054 0.0101 i0.0059 0.0351 k0.0023 0.0163 t0.0018
0.0244 ~0.0020 0.0104 i0.0083 0.0348 i0.0081
oxygen plasma 0.0128 i 0.0026
0.0069 i0.0032 0.0197 i0.0018 0.0104 *0.0007
virgin blank 0.0117 io.0009 0.0113 i0.0024 0.0230 i0.0022 0.0083 io.0001
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All acid titration analyses are averages and standard deviations of at least five runs. a All values are reported in mmol/g. All chloride titration analyses are averages and standard deviations of four runs. chamber is also covered with the film.The infrared spectrum of this film matches that of polyacrylic acid. This indicates that a film of polyacrylic acid is deposited on the surface of the graphite. Although polyacrylic acid is soluble in most solvents, Anson and Oyama have reported that when deposited on pyrolytic graphite the film resists dissolution by water (18). The film also seems to be insoluble in benzene when coated on a pyrolytic graphite disk, because the "rainbow" appearance remained on the disk even after 24 h of extraction with anhydrous benzene. Boehm et al. (19) have suggested that there are four different types of acidic groups present on carbon, and each can be quantitatively determined through use of titrations employing bases of various strengths. Analysis of the graphite samples was accomplished by a similar method. The total amount of carboxylic acid groups was determined by titrating with sodium carbonate. Sodium hydroxide titrated both the carboxylic acid and phenolic hydroxyl groups. The amount of phenolic hydroxyl groups present was determined by subtracting the sodium carbonate titration results from the results obtained in the sodium hydroxide titration. Table I lists the acidic and chloride group analyses for the graphite samples which were modified with acrylic acid and oxygen plasmas, as well as samples treated with acrylic acid vapors only (acrylic acid blank), and blank samples with no treatment other than degassing (virgin blank). The averages and standard deviations of at least five separate titrations are included for both the sodium hydroxide and sodium carbonate analyses. Although these results differ from some previously reported values (20), they compare well with results reported by Stulik et al. for titrations of graphite samples (21). The data show that the acrylic acid plasma greatly increases the number of carboxylic acid groups but does not increase the number of phenolic hydroxyl groups, which remain about the same as in the virgin blanks. This is what one would expect as the polymer coating would only add carboxylic acid groups to the surface. The acrylic acid blanks produced titration results similar to the acrylic acid plasma, which is due to the adsorption of acrylic acid monomer on the graphite surface. However, adsorption is not a useful technique for confining a linking agent to the surface. At some point during the attachment of o-tolidine to the acrylic acid vapor treated sample, the monomer was desorbed from the graphite surface. This was evident when an electrode prepared from this sample did not exhibit any peaks in the cyclic voltammogram. Polymerization of the acrylic acid on the graphite surface is a necessary condition for the formation of a stable linking agent for further chemical modification. The oxygen plasma treatment slightly lowered the overall oxygen content of the sample, slightly increasing the number of carboxylic acid groups while lowering the number of phenolic hydroxyl groups. This result agrees very well with results
obtained by using a completely independent technique (thermal analysis) (22). In a study by Kuwana and Evans (3), pyrolytic graphite disks which were treated with a RF oxygen plasma exhibited surface roughening and an increase in the amount of surface oxygen-containing functional groups. However, the pyrolytic graphite disks that were used in their study have a smooth surface containing a relatively low surface concentration of oxygen functional groups. These facts seem to imply that RF oxygen plasmas cannot be used indiscriminately in an attempt to increase oxygen-containing functional groups on carbon or graphite substances. The nature of the substrate must be considered if the desired results are to be obtained. If the substrate initially contains a low surface concentration of oxygen groups on a relatively smooth surface, such as pyrolytic graphite, then RF oxygen plasma treatment can be beneficial in increasing the oxygen groups. However, if the surface oxygen content of a substrate is relatively high to begin with (compared with pyrolytic graphite), as is the case with activated carbons or graphites, the value of a RF oxygen plasma treatment must be determined on a case by case basis. The values obtained for the chloride analysis should be the same as for the sodium carbonate titration; in other words, the number of carboxylic acid groups should equal the number of acid chloride groups. However, the lower value obtained for the chloride analysis compared to the carboxylic acid analysis could be due to the hydrolysis of the acid chlorides during some of the pretreatment procedures (before mass hydrolysis and titration), or the thionyl chloride reaction may not produce a 100% yield of acid chlorides under these conditions. BET surface area analysis of the graphite powder yielded a result of 12.5 m2/g. Using this value, a surface concentration of 2.00 X mol/cm2 of carboxylic acid groups was calculated for the acrylic acid plasma treated graphite which was approximately twice that obtained with an oxygen plasma treatment. For evaluation of the usefulness of the acrylic acid polymer film as a linking agent, o-tolidine was attached via the thionyl chloride reaction scheme and studied electrochemically. Figure 1 shows a series of cyclic voltammograms using an aqueous 0.1 M potassium chloride solution as the supporting electrolyte and a scan rate of 50 mV/s. Approximately the third cycle was recorded in each case. Curve A is an unmodified carbon paste working electrode in supporting electrolyte. Curve B is an unmodified carbon paste working electrode in supporting electrolyte that contains 0.1 mM 0tolidine. Curve C is a voltammogram of a carbon paste working electrode that has been modified by attaching o-tolidine and run in a solution that only contains supporting electrolyte. The peaks decrease to a steady-state height after about 10 cycles but their positions remain the same as in the solution phase, and the modified electrode is stable for at least
Anal. Chem. 1902, 54,324-326
ACKNOWLEDGMENT
Y
The authors wish to acknowledge H. J. Wieck for helpful discussions leading to this work.
15uA
LITERATURE CITED
I
112
1’0
I
0’6
0’0 V
v5
0’4
Bradley, A.; Fales, J. CHEMTECH 1971, April, 232. Bradley, A., CHEMTECH 1973, August, 507. Evans, J.; Kuwana, T. Anal. Chem. 1977, 49, 1632. Evans, J.; Kuwana, T. Anal. Chem. 1979, 51, 358. Oyama, N.; Brown, A. P.; Anson, F. C. J. Nectroanal. Chem. 1978, 87, 435. Nowak, R.; Schultz, F.; Umafia, M.; AbruRa, H.; Murray, R. J. Electroanal. Chem. 1978, 94, 219, Nowak, R.; Schultz, F.; UmaRa, M.; Lam, R.; Murray, R. Anal. Chem. 1980, 52, 315. Watklns, B.; Behllng, J.; Kariv, E.; Miller, L. J . Am. Chem. SOC.1975, 97, 3549. Lennox, J.; Murray, R. J. Nectroanal. Chem. 1977, 78, 395. Sheehan, J.; Hess, G. J. Am. Chem. SOC. 1955, 77, 1067. Evans, J.; Kuwana, T.; Henne, M.: Rover. G. J. Nectroanal. Chem. 1977, 80, 409. Line, W.; Kwong, A.; Weethall, H. Blochim. Biophys. Acta 1971, 242,
I
0’2
0’0
SCE
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Figure 1. Cycllc voltammograms of (A) unmodified carbon paste electrode in 0.1 M KCI supportlng electrolyte, (B) unmodified carbon paste electrode in 0.1 M KCI supportlng electrolyte containing 0.1 m M o-tolidine, and (c) carbon paste that has o-tolldine attached In a solution Containing only 0.1 M KCI supporting electrolyte. The scan rate was 50 m V / s in each case.
50 cycles between 0.0 and 1.0 V vs. SCE. The surface concentration of o-tolidine was calculated to be 3 X mol/cm2 by integrating the area under the peaks in curve C. This concentration is larger than the maximum value calculated from the titration and surface area analyses. The most likely reason for this is that the geometrical surface area of the electrode, which was used in the calculation, is smaller than the actual area due to surface roughness of the carbon paste.
CONCLUSION The determination of surface acidic groups by back-titration is a useful method of characterizing carbonaceous materials, which would probably give better results on substrates with higher surface areas. In fact, the surface area of graphite seems to be about the lower limit for this type of analysis. The acrylic acid polymer film seems to be a viable alternative to present attachment schemes in the construction of chemically modified electrodes. The advantages of this procedure are that the reactions used are relatively simple, electrodes constructed using this technique are stable, a large surface concentration can be obtained, and reaction schemes requiring either aqueous or nonaqueous environments can be used. Also, the modified electrodes have a long shelf life (room temperature in a capped vial); samples prepared 6 months earlier were electrochemically indistinguishable from freshly prepared samples.
194
Hilneman. W. R.; Wieck, H. J.; Yacynych, A. M. Anal. Chem. 1980, 52, 345. Domask, W.; Kobe, K. Anal. Chem. 1952, 24, 989. Belcher, R.; Wllson, C. “New Methods of Analytical Chemistry”, 2nd Ed. Relnhold New York, 1964; p 71. Dautartas, M.; Evans, J.; Kuwana, T. Anal. Chem. 1979, 51, 104. Olson, C.; Adams, R. Anal. Chlm. Acta 1960, 22, 582. Oyama, N.; Anson, F. J. Nectrochem. SOC. 1980, 127, 247. Boehm, H.; Diehl, E.; Heck, W.; Sappok, R. Angew Chem. 1984, 3 , 669. Kamln, R.; Wilson. G. Anal. Chem. 1980, 52, 1198. Stullk, J.; MaJer, V.; Vesely, J. J. Electroanal. Chem. Interfaclal Nectrochem. 1973, 113. Wleck, H. J.; Iannlello, R. M.; Osborn, J. A,; Yacynych, A. M., submltted for publication In Anal. Chim. Acta.
‘
Present address: Department of chemistry, Universlty of Massachusetts, Amherst, MA 01003.
George H. Heider, Jr. Mark B. Gelbert’ Alexander M. Yacynych’ Department of Chemistry Rutgers, The State University of New Jersey New Brunswick, New Jersey 08903 RECEIVED for review February 2,1981. Resubmitted October 16,1981. Accepted November 19,1981. A.M.Y. thanks the Rutgers Research Council, Biomedical Research Support Grants, the National Institutes of Health (Grant No. GM 28125-Ol), the National Science Foundation (Grant No. CHE 8022237) for research support, and Rutgers University for a summer fellowship. G.H.H. acknowledges the Rutgers B.A./Ph.D. program for summer fellowships. M.B.G. thanks the Rutgers undergraduate summer fellowship program for support during the course of this work.
Series Difference Detection for Reduction of Interferences in Chromatography Sir: The quantitation of components in complex mixtures by chromatography is frequently limited by the presence of interferences. The problem is particularly acute when the mixtures are of biological or environmental origin, and elaborate clean-up and/or derivitization methods are often required for these analyses. In this paper we described a simple and general technique for minimizing the effect of interferences which are present as broad bands. The technique (1) applies to a variety of chromatographic systems, and we will illustrate it through a HPLC-UV application. Consider the chromatogram shown in Figure l a which was obtained with the usual chromatographic ar-
rangement outlined in Figure 2a, where the output is the difference signal between detectors 1and 2, respectively, which are in parallel. The precision to which the peak of interest can be quantitated is directly dependent upon the precision to which its base line can be estimated, and in the present case it is unlikely to be very high. However, if the parallel arrangement is modified such that the eluant from detector 1flows into detector 2 via a cell (the series arrangement) as shown in Figure 2b, then the chromatogram in Figure l a appears as that in Figure lb, where the interference is considerably reduced. The method may be readily understood in terms of Figure
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