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%SiNMR has found considerable use in studies of siloxanes groups (10)give compounds such as I after, e.g., alkali fusion. since the chemical shift ranges for different types of silicon However, in routine work silylated artifacts are usually not resonances are well separated (4). The observation of only expected and may cause serious problems for the unaware one signal at 6 7.22 ppm originating from the magnetically interpreter when they appear in HPLC fractions. identical Si atoms thus rigorously excludes any other possiRegistry No. I, 34214-91-0; 1,1,3,3-tetramethyl-1,3-bis(nobility than a disiloxane structure for I (range for siloxanes 7-10 nadecyl)disiloxane, 109719-36-0;dimethyloctadecylsilyl fluoride, ppm) and is close to the range for the Me2Si0 group 80054-55-3;dimethyloctadecylsilyl chloride, 18643-08-8. .. - reported (6.7-7.1 ppm). This result was of crucial importance in deLITERATURE CITED ducing the structure (I) for the unknown compound. Welinder, B. S.; Linde, S.; Hansen, B.; Sonne, 0. J. Chromatogr. The noise-decoupled and off-resonance-decoupledI3C NMR 1984, 298,41. spectra confirmed that branching was absent. By analogy to Anthoni, U.; Christophersen, C.;Larsen, C.; Nielsen, P. H. J. Org. Chern ., in press. the spectra of paraffins and alkylsilanes ( 5 ) the long alkyl Pfefferkorn, R.; Grutzmacher, H. F.; Kuck, D. I n t . J. Mass Spectrom. chains linked to Si were assigned as follows (6 values given Ion Phys. 1983, 47,515. Williams, E. A. Annu. Rep. NMR Spectrosc. 1983, 15, 235. for CH3-Si and CH3a-CHzb-CHzc-CHzd-(CHz)~-CH2~Schraml, J.; Chvalovsky, V.; Magi, M.; Lippmaa, E. Collect. Czech. CHzP-CHz”-Si): 0.4 (CH,), 14.1 (CH3a),22.8 (CHzb),29.5 Chem , Commun . 1979, 44 854. (CH2c),32.1 (CHZd),29.8 (CH2,very strong), 33.5 (CH,T), 23.4 Jancke, H.; Engelhardt, G.; Kriegsmann, H.; Volkova, L. M.; Deiazari, N. W.; Andrianov, K. A. 2.Anorg. A/@. Chem. 1973, 402,97. (CH,”, and 18.5 (CHza). Consistent with literature values (6, Pinazzi, C. P.; Soutif, J. C.;Brosse, J. C. Bull. SOC.Chim. F r . 1974, 7) the ‘H NMR spectrum could be assigned as follows (6 2166. Onyszchuk, M. Can. J. Chem. 1961, 39,808. values): 0.05 (s, CH3Si),0.51 (t, 2CHzSi),0.89 (t, 2CH3C),and Lavygin, I. A.; Izmailov, B. A.; Zhdanov, A. A.; Myakushev, V. D.; 1.28 (br s, assumed 33CHz). Skorokhodov, I . I.; Mialina, V. M. Zh. Prikl. Khim. (Leningrad) 1980, Any ambiguity concerning the molecular weight was re53, 1155. Chem. Abstr. 1981, 94,47399t. Luellmann, C.; Genieser. H. G.; Jastorff, B. J. Chromatogr. 1985, 323, ( moved by reaction with BF3 (8)and liquid hydrogen fluoride 273. to give, in both cases, dimethyloctadecylsilyl fluoride (idenUffe Anthoni tified by mass spectrometry) together with ca. 2 % diCharles Larsen methylnonadecylsilyl fluoride arising from the impurity. Per Halfdan Nielsen Zinally, authentic I was prepared by hydrolysis of diCarsten ChristoDhersen* methyloctadecylsilyl chloride (9) and was proven to be idenMarine Chemistry Section tical with the isolated material in physicaland spectroscopic The H. C. 0rsted Institute properties thus definitely proving the proposed formula. University of Copenhagen These results suggest to users of commercially available Universitetsparken 5 reversed-phase silicas that they pay attention to the properties DK-2100 Copenhagen, Denmark of the material. Since the artifact was obtained from a column used many times before, the appearance can hardly be asRECEIVED for review February 24, 1987. Accepted June 15, cribed to insufficient washing, a “bad” batch, etc. It is well-known that hydrophobic derivatized silicas with octadecyl 1987. I
Electroanalysis of Primary Amines with Chemically Modified Carbon Paste Electrodes Sir: As the field of chemically modified electrodes (1-5) continues to develop in scope and sophistication, many of the projected applications continue to be realized. One such area is the use of these modified interfaces in analytical determinations. Although the analytical utility of modified electrodes was recognized early, only recently have researchers focused on this area. Approaches for analytical determinations with chemically modified electrodes are quite varied and include electrostatic accumulation (6-9), coordination effects (IO),precipitation (11,12), and other (13-18). Carbon paste electrodes have also been employed by Baldwin and co-workers in the determination of nickel and copper with carbon paste electrodes containing dimethylglyoxime (19) and 1,9-dimethyl-1,10phenanthroline (20), respectively. Also related to these approaches are the use of adsorptive (21,22) and extractive accumulation (23,24) at mercury and carbon paste electrodes, respectively. We recently proposed and demonstrated (25-27) an approach for metal ion electroanalysis based on the use of electrodes modified with functionalized polymer films that carry a coordinating group as well as an internal redox couple. The former is chosen so as to exhibit high affinity for the particular metal ion of interest and the latter serves in the deposition of the polymer and as an internal standard as well.
We have demonstrated the applicability of this approach to the determination of various metal ions. In addition, we have also recently (28) expanded the applicability of this approach to the determination of aromatic amines incorporated by ion exchange into copolymer films of styrenesulfonate and vinylferrocene. This particular approach, however, had poor selectivity for different aromatic amines. We have now devised an approach based on the reactivity of specific functional groups attached to functionalized polymer films. By the appropriate choice of reactants and reaction conditions, such an approach can be made very selective and sensitive toward a particular functionality. We demonstrate the applicability of this approach to the determination of primary amines by reaction with an aromatic aldehyde, incorporated into a polymer film, to generate the corresponding imine whose electrochemical response is used as the analytical signal. This is analogous to the work of Price and Baldwin (13)who employed platinum electrodes modified with an adsorbed layer of allylamine for the determination of ferrocenecarboxaldehyde via imine formation. In this case the aldehyde group was incorporated by coordination of pyridine 4-carboxaldehyde to a pentacyanoferroate center to generate [Fe(CN),(L)I3- (L is pyridine 4carboxaldehyde) (This material will be referred to as the “iron
0003-2700/87/0359-2436$01.50/0 6 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 19, OCTOBER 1, 1987
complex" in the text.) Analogous to the well-studied ferro/ ferricyanide couple (6, 7), this material can be easily incorporated into films of quaternized poly(viny1pyridine) with the iron complex retaining its well-behaved electrochemical response. This signal can be used t o ascertain the coverage of the complex as well as the number of aldehyde sites since there is a one-to-one correspondence. This is important since it allows the determination of a saturation response. Alternatively, the aldehyde group was also incorporated by employing benzaldehyde 4-carboxylic acid a t p H values where the carboxylic acid is deprotonated. The quaternized poly(viny1pyridine) having the iron complex (or the deprotonated benzaldehyde 4-carboxylic acid) as counterion was incorporated in the formulation of a carbon paste electrode. The electrode is immersed in the solution to be analyzed where imine formation (from the condensation of the primary amine with the aldehyde) takes place. The electrochemical response of the imine is used as the analytical signal and related to the solution concentration of the amine. Since the potential for oxidation of the imine is well-removed from that of the iron complex (typically peak potentials are about +1.0 and +0.3 V, respectively) there is no interference. This approach yields good sensitivity and excellent selectivity for the determination of primary amines.
EXPERIMENTAL SECTION Synthesis. Linear poly(4-vinylpyridine) of 50 000 molecular weight (Polysciences) was purified by recrystallization from methanol/ether. Quaternization was effected in refluxing dry methanol and under N2 (3-5 h) with either methyl iodide or benzyl chloride with the latter giving rise to more stable polymer films. From elemental analysis, the degree of quaternization was 95%. The halide salts of the quaternized polymer were converted to the correspondinghexafluorophosphate (PF6-)salts with aqueous (NH,)(PF,). [ F e ( C N ) , ( p y r i d i n e - 4 - c u r ~ o ~ u ~ d e ~ y ~The e ) 1 3ammonium -. disodium salt of pentacyanoaminoferroate(Aldrich)was dissolved in water and a stoichiometric amount of pyridine-4-carboxaldehyde (Aldrich) was added dropwise. Complex formation was immediately apparent by the color change (from yellow-green to purple). The solution was allowed to stir for 30 min at room temperature afterwhich it was filtered. The complex was isolated by removing the water with gentle heating under vacuum. The quaternized poly(viny1pyridine) (hexafluorophosphatesalt) dissolved in methanol/water (1:l)was added to a vigorously stirred aqueous solution of [Fe(CN),(L)I3-(NH4)(Na), M) with immediate precipitation of the polymer incorporating the iron complex. The precipitate was collected by filtration, rinsed with water and ether, and dried under vacuum. Reagents. All reagents were of at least reagent grade quality and were used without further purification except as noted below. 1-Aminonaphthalene was recrystallized three times from petroleum ether, dried under vacuum, and stored in an amber bottle. Aniline and dimethylaniline were distilled under vacuum and stored in amber bottles. Water was purified by a Milli-Q purification train. Instrumentation. Cyclic voltammetric 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. Square wave voltammograms were performed on the BAS 100 with a 25-mV pulse amplitude with a 4-mV step and at a frequency of 15 Hz. Electrochemical cells were of conventional design. All potentials are reported against the sodium-saturated calomel electrode (SSCE) without regards for the liquid junction. Unmodified carbon paste electrodes or glassy carbon electrodes sealed in Teflon were employed in cyclic voltammetric studies. Glassy carbon electrodes were polished with 1-pm diamond paste (Buehler) prior to use and were rinsed with water, acetone, and methanol. Procedures. Carbon paste electrodes were prepared from graphite powder (Fisher) with ceresin wax and paraffin oil as pasting medium and with 10, 25, 50, or 60% by weight (relative to the weight of graphite powder) of the polymer. The smallest background currents were obtained when the following procedure
2437
Figure 1. (A) Cyclic voltammogram at 200 mV/s at a carbon paste electrode for the iron complex in pH 2.1 buffer. (B) Cyclic voltammogram at 200 m V / s at a glassy carbon electrode for the imine derived from benzaldehyde and 1-aminonaphthalene in acetonltrile/O. 1 M tetra-n -butylammonium perchlorate.
was employed 0.1 g of ceresin wax dissolved in hot hexane was added to 1.7 g of graphite powder and the mixture was stirred and the hexane allowed to evaporate until a free flowing powder was obtained. Polymer (0.85 g) dissolved in methanol was added and again the mixture was stirred until all of the methanol had evaporated. The mixture was further dried under vacuum (ca. 1 h) until a free flowing powder was again obtained. Paraffin oil (1.5 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 surface of the carbon paste electrode was smoothed by gently rubbing on a piece of weighing paper with graphite powder. In analytical determinations, the carbon paste electrode was contacted with the analysis solution (5 mL of pH 2.1 buffer; trifluoroacetate/sodium hydroxide) for a period of 5 min with stirring. The electrode was removed and rinsed with water and placed in pH 2.1 buffer where a square wave voltammogram was run between 0.0 and +1.4 V. The electrode was regenerated by removing a small amount of the paste by forcing out some of the paste (with a piston which is part of the electrode assembly) and polishing on a piece of weighing paper.
RESULTS AND DISCUSSION The initial studies were geared a t determining the electrochemical response of the various components in solution and incorporated in the quaternized poly(viny1pyridine) films. Figure 1A shows a cyclic voltammogram, a t an unmodified carbon paste electrode, of the iron complex in aqueous p H 2.1 buffer solution, and a well-developed wave with an E"' value of +0.26 V can be observed. No other process could be observed at potentials up to +1.4 V. A glassy carbon electrode was coated with a thin film (film thickness of about 0.5 pm) of the quaternized poly(viny1pyridine) and the iron complex was incorporated by ion exchange. Analogous to ferricyanide, large amounts of material could be incorporated. Such a modified electrode showed a reversible redox process that we associate with the iron complex; however, it should be noted that the potential of this wave shifted over the range of +0.05 to +0.30 V. Although we are not certain of the origin of this effect, Anson (29, 30) has reported shifts in the formal potentials for species incorporated into polyelectrolytes such as protonated and quaternized poly(viny1pyridine). The imine derived from benzaldehyde and l-aminonaphthalene was prepared and its electrochemical response obtained. I t exhibited an irreversible oxidation with a peak potential value of +0.92 V. (Figure 1B) A similar value was obtained for the imine derived from 1-aminonaphthalene and the iron complex. These preliminary results established that the redox activity of the iron complex is retained upon incorporation and that its redox potential is well-removed from that of the imine derived from 1-aminonaphthalene in accordance with our estimates.
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ANALYTICAL CHEMISTRY, VOL. 59, NO.
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+”* +’ i +05
E vs SSCE Figure 2. Square wave voltammogram for a modified carbon paste M solution of l-aminoelectrode after contacting with a 4 X naphthalene for 5 min.
Once this was established, studies on the determination of 1-aminonaphthalene at various concentrations were performed by using the protocol described in the Experimental Section. The effects of paste composition and reaction time on the analytical determination were assessed. In the former case carbon paste electrodes with polymer loading levels of 10, 25, 50, and 60% were prepared and investigated. Of these, the paste with a 50% polymer loading gave the best results in terms of reproducibility and low background and was employed in all subsequent work. In terms of reaction time it was found that maximal responses were obtained for reaction times of 3 min or longer and thus reaction times of 5 min were typically employed in analytical determinations. A typical experimental determiantion is presented in Figure 2. It shows two well-developed waves at peak potentials of +0.34 and +0.94 V and these values correlate very well with those previously determined for the iron complex and the imine, respectively. It is worth noting that no redox process for the protonated amine could be detected, implying that the protonated amine is not incorporated into the polymer film. This is not surprising since the polymer film is polycationic; however, it is worth noting because this represents an added advantage of the approach since the protonated amine (and any protonated amine in general) will not be incorporated and thus will not interfere in the analytical determination. The peak current value for the imine oxidation wave normalized to the wave for the iron complex was used as the analytical signal and from it calibration curves could be constructed. In here, the solution concentration of the amine was varied from 3 X to 1.4 X and a t these concentration levels, very good correlations were typically encountered (r = 0.998). In a series of seven replicate determinations of naphthylamine at a concentration of 1.2 X lo4, the standard deviation was of the order of f12%. Signals down to the micromolar level could be detected, but in this concentration region the correlations and reproducibility were not as good ( r values of about 0.89). Similar results were obtained when using carbon paste electrodes modified with benzaldehyde 4-carboxylic acid as the source of aldehyde. However, in this case we do not have an internal redox couple to serve as internal standard. We have also assessed the selectivity of this procedure for the determination of primary amines. In the presence of a 10-fold excess of dimethylaniline no significant deviations were observed confirming the noninterfering nature of the species. Studies employing aniline as the primary amine yielded results very similar to those outlined above for l-aminonaphthalene. Although these studies should be considered preliminary, they serve to illustrate the applicability of this approach to organic electroanalysis. The fact that imine formation is
specific to primary amines makes this an exceedingly selective probe and this is borne by experimentation. Furthermore, this approach can be easily extended to the determination of other organic functionalities where by the choice of appropriate reagents one can exploit the selectivity of particular reactions. For example, the use of a primary amine will yield a material selective toward aldehydes, again via imine formation. (Essentially the same reaction studied here where the roles of the reactant and analyte have been reversed.) The incorporation of a hydrazine group will give rise to a material that would react with carbonyl groups in general via hydrazone formation. Thus, it is clear that by making use of very simple chemical transformations and through the use of the appropriate reagents, very sensitive and exquisitely selective probes can be designed. We are currently in the process of extending this approach to these and other determinations. Registry No. 1-Aminonaphthalene, 134-32-7. LITERATURE CITED Murray, Royce W. I n Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York. 1983; Vol. 13, pp 191-368. Murray, R. W. Acc. Chem. Res. 1980, 13, 135-141. Snell, K. D.; Keenan, A. G. Chem. SOC. Rev. 1979, 8, 259-282. Albery, W. J.; Hillman, A. R. Annu. Rev. C . , R . SOC.London 1981, 337-437. Fujihira, M. I n Topics in Organic Electrochemistry; Fry, A. J., Britton, W. E., Eds.; Plenum: New York, 1986; pp 255-294. Oyama, N.; Anson, F. C. J . Am. Chem. SOC.1979, 101, 3450-3456. Oyarna, N.; Anson, F. C. J . Nectrochem. 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. 7 , 1986, 8 2 , 1051-1070. Nagy, G.; Gerhardt, G. A.; Oke, A. F.; Rice, M. E.; Adams, R. N.; Moore, R. B., 111; Szentirmay, M. N.; Martin, C. R. J . Nectroanal. Chem. 1985, 188, 85-94. Gehron, M. J.; Brajter-Toth, A. Anal. Chem. 1986, 5 8 , 1468-1492. Cheek, G. T.; Nelson, R . F. Anal. Lett. 1978, 7 1 , 393-402. Cox, J. A.; Majda, M. Anal. Chem. 1980, 52, 861-864. Price, J. F.; Baldwin, R. P. Anal. Chem. 1980, 52, 1940-1944. Pham, M.4.; Tourillon. G.; Lacaze, P.-C.; Dubois, J.-E. J . Nectroanal. Chem. 1980, 7 1 1 , 385-390. Pham, M.-C.; Dubois, J.-E.; Lacaze, P . 4 . J . Electrochem. SOC. 1983, 130 346-35 1. Cox, J. A.; Kulesza, P. J. J. Electroanal. Chem. 1983, 759, 337-346. Willman. K. W.; Murray, R. W. J . €lectroanal. Chem. 1982, 133, 2 11-23 1. Lubert, K. H.; Schnurrbush, M.; Thomas, A. Anal. Chim. Acta 1982, 144, 123-136. Baldwin, R. P.; Christensen, J. K.; Kryger, L. Anal. Chem. 1986, 5 8 , 1790-1798. Prabhu, S. V . ; Baldwin, R. P.; Kryger, L. Anal. Chem. 1987, 5 9 , 1074- 1078. Wang, J. Am. Lab. (Fairfield. Conn. 1985, 17(5), 41-50. Kalvoda, R. Anal. Chim. Acta 1982, 138, 11-18. Wang, J.; Freiha, B. A. Anal. Chim. Acta 1983, 154, 87-94. Wang, J.; Freiha, 8. A. Anal. Chem. 1984, 5 6 , 849-852. Guadalupe, A. R.; Abrufia, H. 0. Anal. Chem. 1985, 5 7 , 142-149. Wier, L. M.; Guadalupe, A. R.; AbruAa, H. D. Anal. Chem. 1985, 57, 2009-201 1. Guadalupe. A. R.; Wier, L. M.; AbruRa, H. D. Am. Lab. 1986, 18(8), 102-107. Guadalupe, A. R.; AbruRa, H. D. Anal. Lett. 1986, 79(15&16), 1613-1 632. Shigehara, K.; Oyarna, N.; Anson, F. C. J . Am. Chem. SOC. 1981, 103, 2552-2558. Oyama, N.; Shimornura, A.; Shigehara, K.; Anson, F. C. J . Electroanal. Chem. 1980, 112, 271-280. I
A. R. Guadalupe S. S. J h a v e r i K. E. Liu H. D. Abruiia* Department of Chemistry Cornel1 University Ithaca, New York 14853 RECEIVED for review February 6, 1987. Accepted June 25, 1987. This work was funded by the National Science Foundation (CHE-8605097). H.D.A. is a recipient of a Presidential Young Investigator Award and acknowledges matching funds by the Dow Chemical Co., Eastman Kodak Co., C. S. Johnson & Son Co., ARCO, and Honeywell. K.E.L. was supported by the REU program at NSF (CHE 8712498).
