979
Anal. Chem. 1986, 58,979-981
Cationic Perfluorinated Polyelectrolyte as an Electrode Modifier Sir: The irreversible adsorption of polyelectrolytes onto electrode surfaces has become a familiar and often preferred approach for construction of polymer-coated electrodes since this strategy was first reported for poly(4-vinylpyridine) ( I ) . This is because the adsorption procedure is usually simplified compared with other approaches for electrode coatings ( 2 , 3 ) (e.g., electrosorption, glow-discharge polymerization, covalent binding, and electropolymerization). The polyelectrolyte coatings can usually be conducted either by dip coating via direct immersion of bare electrode into a polyelectrolyte SOlution or by evaporation of aliquots of the polyelectrolyte solution placed onto the electrode surface. A number of papers have dealt with the polyelectrolyte-coated electrodes prepared by the adsorption procedure, that is, their electrochemical behaviors and a variety of practical applications (2-5). However, the number of useful polyelectrolyte systems that are presently available is limited. The most extensively used polyelectrolyte films are those of protonated or quaternized poly(4-vinylpyridine) ( I , 6-9), protonated poly(L-lysine) (IO-I3), sulfonated or carboxylated fluoropolymer (14-25), poly(styrenesu1fonate) (4), and a few other materials. In this paper, we report a new polyelectrolyte coating which is cationic perfluoropolymer. The dissolution procedure of the cationic perfluoropolymer membrane produced by Toyo Soda Manufacturing Co., Ltd. (Kanagawa, Japan), and a brief electrochemical characterization of films prepared (by the above-mentioned adsorption procedure) from the dissolved polymer are described.
EXPERIMENTAL SECTION The cationic perfluoropolymer membrane specimen (abbreviated as CPFP in this paper) was a gift of the Toyo Soda Co. Its structure is shown in Figure 1. The semitransparent membrane (0.15 mm thick) was obtained as being soaked in distilled water. Finely cut particles (ca. 0.17 g) of the membrane were placed in a flask with 30 mL of the mixed solvent of HzOCH30H-2-propanol (the volume of each solvent was 10 mL). Then, the particles did not seem to significantly dissolve in the mixed solvent. The solution containing the CPFP particles was refluxed with stirring at ca. 70 OC for about 36 h. The resulting viscous solution, which looked like a translucent gel, was, after cooling to 25 "C, filtered out by a filter paper. The resultant filtrate was taken as a stock solution of the CPFP. The concentration of the CPFP in the stock solution thus prepared was estimated as follows: Aliquots of the stock solution (for example 1 mL) were cast and spread onto a glass slide, the weight (wl) of which was previously measured by a microbalance (its precision is on the order of 0.001 mg). After that, the entire glass slide was dried at ca. 50 "C overnight and then under vaccum for several hours. After that, the total weight (wz)of the entire slide glass (Le., the slide glass and the CPFP film) was measured. From the weight difference of w p- wl,the concentration of the CPFP stock solution was determined to be 21 mg ~ m - i.e., ~ , 2.1 w/v %, which may approximately correspond to the saturated concentration of the CPFP in the H20CH30H-2-propanol (volume ratio; 1:l:l) at 25 "C. The density of the dry CPFP film was estimated to be 1.7 f 0.4 g cm-3 from the weight and the volume (calculated by the measured surface area and thickness) of the given dry film that was prepared from the dissolved CPFP, as mentioned above. For preparing the CPFP-coated electrodes,aliquots of the stock solution were spread by a microsyringe on the freshly cleaved disk surface of the basal-plane pyrolytic graphite (BPG) (Union Carbide Corp.) electrode or Pt electrode, the surface of which was pretreated as described previously (26),and then air-dried to remove the solvent a t room temperature. The supporting electrolyte material, sodium trifluoroacetate (CF3COONa)was reagent grade. The BPG electrodes (area, 0.19 cm2)were prepared and mounted into a glass tube with a heat-shrinkable polyolefin tube 0003-2700/86/0358-0979$01.50/0
(1,6,7).All other chemicals were reagent grade and were used as received. The electrochemical characterization of the films prepared from the dissolved CPFP was carried out by examining the cyclic voltammetric responses of the CPFP film-coated electrodes in the 0.2 M CF3COONa-CF3COOH aqueous solution (pH 1.0) containing 0.2 mM redox species (Le., Fe(CN)e3-, Fe(CN):-, Mo(CN)*~-,W(CN)s4-, RU(CN)~*-, IrCle2-,R u ( N H ~ ) ~or~ +hy, droquinone) as well as in the m e electrolytic solution containing no redox species. Potentials were measured and are reported with respect to a sodium chloride saturated calomel electrode (SSCE). Solutions were degassed with prepurified N2 prior to the electrochemical measurements at 25 "C.
RESULTS AND DISCUSSION The electrochemical responses of Pt and BPG electrodes coated with the CPFP film in a 0.2 M CF3COONa-CF3COOH aqueous solution (pH 1.0) are shown in Figure 2, where those of.uncoated Pt and BPG electrodes are shown for comparison. It is apparent from this figure that the CPFP film is well swollen in an aqueous solution. Since the CPFP itself contains no easily oxidizable or reducible moieties, the CPFP-coated electrodes can be expected to exhibit no characteristic redox behavior when simply immersed in a pure supporting electrolyte. However, in addition to usually observed redox responses due to the surface reactions a t the uncoated Pt electrode (27),the additional anodic peak was observed at ca. 1.36 V vs. SSCE (see Figure 2B). This was found to correspond to the oxidation of C1- ions, which are initially present as the counterions of the quaternized ammonium sites in the CPFP film. As the potential scan was cycled, this anodic peak current decreased gradually. This is due to the fact that the Clz molecules produced come out of the film domain toward the bulk of the solution. The voltammetric response obtained after ca. 20 cycles was almost the same as that obtained with the uncoated Pt electrode. Of course, the CPFP film remains water insoluble as shown below. The oxidation of the C1- ion present in the CPFP film was also observed a t the CPFPcoated BPG electrode (Figure 2D). The observed behavior is essentially the same as that at the CPFP-coated Pt electrode. The cathodic waves a t ca. 0.0 and 0.75 V vs. SSCE in Figure 2D may be due to the reductions of the produced Clz molecule and the compounds produced by the follow-up chemical reaction. Figure 3 shows the typical cyclic voltammograms demonstrating the incorporation of Fe(CN)63-into the CPFP film coated on a BPG electrode in a 0.2 M CF3COONa aqueous The incorsolution (pH 1.0) containing 0.2 mM Fe(CN):-. poration of Fe(CN)B3-by anion exchange with the C1- ions initially present in the film continued for about 1 h. No further increases in peak current were observed with scanning times longer than 1h. The peak current at the coated electrode was about 10 times that a t an uncoated electrode, and the concentration of the incorporated Fe(CN):- was 0.15 M. This indicates that the quantity of incorporated Fe(CN)63corresponds to ca. 10% of the total amount (volume concentration, 1.7 M) of the quaternized ammonium site in the CPFP film. When the CPFP-coated electrode was removed from the incorporating solution, washed, and replaced in the pure supporting electrolyte, the Fe(CN)63-anions leached gradually from the CPFP film, and thus the peak current decreased with the scanning time. I n this case, the Fe(CN)63- anions were lost from the coating at about the same rate as they were incorporated. However, the wave was still evident after 60 min of cycling. When the coated electrode soaked in the pure electrolyte for ca. 2 h was returned into the Fe(CN)l- in0 1986 American Chemical Soclety
980
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 +CF~-CF~------+CF,-CF~+ I O I
75 F3C-CF -0-C F,-CH,-N n/rn=65
Flgure 1.
CH3 I+ -CH, I CH3
C.i? 2ndscon
B
: 1st scan
+
2nd %an\
E
t
scan
,1st J
IlOP
Structure of the cationic perfluoropolymer used.
n
, L
v
1st scan,
0
1.0 E / V vs.SSCE
0.5
I
15
I I 0 0.5 E / V vs. SSCE
I
-0.5
I 1.0
11
i 1.5
Cyclic voltammetric responses of uncoated and CPFP-coated and BPG electrodes in a 0.2 M CF,COONa-CF,COOH aqueous solution (pH 1.0);electrodes: (A) uncoated Pt, (B) CPFP-coated Pt, (C) uncoated BPG, and (D) CPFP-coated BPG. Electrode area is as foliows: (A, B) 7.8 X lo-, cm2,(C, D)0.19 cm2. Quantity of the CPFP coated on electrodes was 2.7 X lo3 g cm-2. Thickness of the CPFP film was 1.5 X lo-, cm. Surface concentration of the quaternlzed ammonium site in the CPFP film was 2.6 X lo-' mol cm-2(volume concentration, 1.7 M). Scan rate was 500 mV s-'. Flgure 2.
Pt
~ 6 0 r n i n
-
I
/--
I TlOLlA
I
I 0
I 0.4 E / V vs I
I
i 08
SSCE
I
I 0
I 04
I
E /
V
I
I 08
vs SSCE
Figure 3. (A) Cyclic voltammograms demonstrating the
Incorporation into the CPFP film coated on a BPG electrode in a 0.2 M CF,COONa aqueous solution (pH 1.O) containing 0.2 mM Fe(CN);-. g cm-2. Thickness of Quantity of the CPFP coated was 5.4 X the CPFP film was 3.0 X cm. Surface concentration of the quaternized ammonium site in the CPFP film was 5.2 X lo-' mol cm-2 (volume concentration, 1.7 M). (B) Cyclic voltammograms obtained when the electrode used in A was washed and transferred to a 0.2 M CF,COONa solution (pH 1.O). (C) Cyclic voltammogram obtained at an uncoated BPG electrode in the same solution as used in A. Immerslon time is Indicated on the voitamrnograms. In every case, scan rate was 50 mV s-I. of Fe(CN):-
corporating solution, the peak heights increased gradually with the scanning time and reached the steady-state currents, which are almost the same as those of Figure 3A. This clearly indicates that the CPFP film coated on electrodes remains water insoluble and does not substantially leach from the electrode surface. It was found that other highly charged anionic redox species besides Fe(CN)63- (e.g., M O ( C N ) ~ ~Fe(CN)64-, -, W(CN)s4-, Ru(CN):-, IrCls2-)are also incorporated into the CPFP film.
0.4 0 E / V vs. SSCE
I
I
0
0.4 E / V vs. SSCE
0.8
Figure 4. (A) Cyclic voltammograms for the Ru(NH,),~+'~+ redox couple recorded at the CPFP-coated BPG electrode in a 0.2 mM Ru(NH,):+ aqueous Solution. (B) Cyclic voltammograms obtained when the electrode used in A was washed and transferred to the pure supporting electrolyte. (C) Cyclic voltammogram obtained at an uncoated BPG electrode in the Same solution as used in A. (D) Cyclic voltammograms for the hydroquinone/quinone redox couple recorded at the CPFPcoated BPG electrode in a 0.2 mM hydroquinone aqueous solution. (E) Cyclic voltammograms obtained when the electrode used in D was washed and transferred to the pure supporting electrolyte. (F) Cyclic voltammogram obtained at an uncoated BPG electrode in the same solution as used in D. Quantity and thickness of the CPFP film on BPG electrodes and surface and volume concentrations of the quaternized ammonium site in the CPFP film are the same as in Figure 3. Supporting electrolyte was 0.2 M CF,COONa-CF,COOH (pH 1.0). Immersion time is indicated on the voltammograms. In every case, scan rate was 50 mV s-I.
Further, more than two kinds of these anionic redox species were found to be simultaneously incorporated into the CPFP film. In this case, the degree of the incorporation depended on the kind of the redox species. The electrochemical responses of highly charged cationic species and of a neutral one at the CPFP-coated electrode were also examined. A typical example of such examination is shown in Figure 4. The redox responses of R U ( N H ~ ) ~ ~ + / ~ + and hydroquinone/quinone couples can be observed a t the CPFP-coated electrodes. However, the peak heights are smaller than those at an uncoated electrode, and in addition, the peak separation between anodic and cathodic peak potentials is larger, probably because of the film resistance for the movement of the reactants toward the electrode surface. Further, it can be seen from parts B and E of Figure 4 that R U ( N & ) ~ ~and + hydroquinone are not substantially incorporated into the CPFP film. Thus, from the results shown in Figures 2, 3, and 4,the incorporation and binding of the highly charged anionic redox compounds such as those mentioned above into the CPFP film on electrodes can be considered to be essentially due to the electrostatic interaction between the anionic compounds and the cationic sites (Le., quaternized ammonium sites) of the CPFP film. In conclusion, the CPFP film prepared from the polymer solution has a potential as a new electrode modifier with its anion-exchange properties because of (i) its strong adhesion to electrode surface, (ii) its good swelling in aqueous solutions, (iii) its water insolubility, (iv) its reasonable ion-exchange capacity, and (v) its reasonable chemical and mechanical stability. Further, it should be noted that the CPFP film has an anion-exchange character irrespective of the pH of a SOlution, as previously reported for the electropolymerized poly(N,N-dimethylaniline)film (28).
ACKNOWLEDGMENT We thank the Toyo Soda Manufacturing Co., Ltd., for samples of the CPFP membranes. Registry No. Pt, 7440-06-4; Clz, 7782-50-5; Fc(CN)e3-, 13408-62-3; R u ( N H ~ ) ~18943-33-4; ~+, RU(NH&'+, 19052-44-9; W(CN)84-, 18177-17-8; Fe(CNk4-, M O ( C N ) ~ ~17923-49-8; -,
981
Anal. Chem. 1986, 58, 981-982
13408-63-4;Ru(CN):-, 21029-33-4; IrCh2-, 16918-91-5; graphite, 7782-42-5; hydroquinone, 123-31-9; quinone, 106-51-4.
LITERATURE CITED Oyama, N.; Anson, F. C. J . Electrochem. Soc. 1980, 727, 247-250. Snell, K. D.; Keenan, A. G. Chem. SOC. Rev. 1979, 8 , 259-282. Murray, R. W. Electroanal. Chem. 1984, 73, 191-387. Majda, M.; Faulkner, L. R. J . Electroanal. Chem. 1984, 769, 77-95. Faulkner, L. R. Chem. Eng. News 1984, 27, 28-45. (6) Oyama, N.; Shlmomura, T.; Shlgehara, K.; Anson, F. C. J . Electroanal. Chem. 1980, 112, 271-280. (7) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 5 2 , 1192-1198. (8) Oyama, N.; Sato, K.; Matsuda, H. J . Electroanal. Chem. 1980, 115, 149-155. (9) Shigehara, K.; Oyama, N.; Anson, F. C. Inorg. Chem. 1981, 2 0 , 518-522. (10) Anson, F. C.; Ohsaka, T.; SavBant, J. M. J . Phys. Chem. 1983, 87, 640-647. (11) Anson, F. C.; Ohsaka, T.; SavBant, J. M. J . Am. Chem. Soc. 1983, 105, 4883-4s90. (12) Anson, F. C.; SavOant, J. M.; Shlgehara, K. J . Electroanal. Chem. 1983, 145, 423-430. (13) Anson, F. C.; SavOant, J. M.; Shigehara, K. J . Am. Chem. Soc. 1983, 105, 1096-1106. (14) . . Rublnstein. I.: Bard, A. J. J . Am. Chem. Soc. 1980, 102, 6641-6642. (15) Henning. T. P.; Whlte, H. S.; Bard, A. J. J . Am. Chem. Soc. 1981, 703, 3937-3938. (16) Rubinsteln, I . ; Bard, A. J. J . Am. Chem. Soc. 1981, 703, 5007-5013. - - .. .- . -. (17) Buttry, D. A.; Anson, F. C. J . Electroanal. Chem. 1981, 730, 333-338. (18) Whlte, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Suc. 1982, 704, 4sii-4816. (1) (2) (3) (4) (5)
(19) Martln, C. R.; Rubinsteln, I.; Bard, A. J. J . Am. Chem. SOC. 1982, 104,4817-4s24. (20) Buttry, D. A,; Anson, F. C. J . Am. Chem. Soc. 1982, 704, 4824-4829. (21) Martin, C. R.; Rhoades, T. A,; Ferguson, J. A. Anal. Chem. 1982, 5 4 , 1639-1641. (22) Buttry, D. A.; Anson, F. C. J . Am. Chem. Soc. 1983, 705, 685-689. (23) Ovama, N.: Ohsaka, T.; Sato, K.; Yamamoto, H. Anal. Chem. 1983, 55, 1429-1431. Rublnstein, I. J . Electroanal. Chem. 1985, 188, 227-244. Tsou, Y.-M.; Anson, F. C. J . Electrochem. Soc. 1984, 137, 595-801. Ohnuki, Y.; Ohsaka, T.; Matsuda, H.; Oyama, N. J . Electroanal. Chem. 1983, 158, 55-67. Delahay, P. "Double Layer and Electrode Kinetics"; Wlley: New York, London, and Sydney, 1965. Oyama, N.; Ohsaka, T.; Shimizu, T. Anal. Chem. 1985, 5 7 , 1526-1532.
Noboru Oyama* Takeo Ohsaka Takeyoshi Okajima Department of Applied Chemistry for Resources Tokyo University of Agriculture & Technology Koganei, Tokyo 184, Japan RECEIVED for review October 29, 1985. Accepted December 30, 1985. The present work was partially supported by Grant-in-Aid for Scientific Research No. 60211011 for N. Oyama, from the Ministry of Education, Science, and Culture, Japan.
Flow- Injection Analysis of Volatile, Electroinactive Organic Compounds at a Platinum Gas Diffusion Membrane Electrode by Use of a Redox Mediator Sir: Pneumatoamperometric analyses described to date have involved a volatile, electroactive species. This species either is the original sample component or is generated by a chemical reaction. It can either be in a gas (1-6) or liquid (7, 8) carrier stream and is determined after it passes through a gas-permeable, hydrophobic membrane to an electrode. This scheme is shown in Figure 1 of ref 7), where Y is the volatile species. The usual detection technique a t the porous electrode of the gas-permeable membrane electrode, GPME, is amperometry at constant potential (1-5, 7, 8). Cyclic voltammetry has also been employed for the cathodic stripping analysis of silver sulfide films produced by reaction of hydrogen sulfide with a porous silver electrode (6). In this report we describe the determination of a volatile, nonelectroactiue species through its reaction with one component of a redox couple in contact with the porous electrode. One realization of this scheme is shown in Figure 1 for the case where the volatile, nonelectroactive species, Z, reacts with Ox to form Red and products, P. By use of the appropriate electrode potential, Red is oxidized back to Ox at the electrode surface. The current required for this redox process becomes a direct measure of the concentration of the volatile species. The Ox/Red redox couple acts as mediator in the determination of Z. Its concentration and solution environment can be optimized for the chemical and electrochemical steps because the electrochemical cell is isolated from the carrier stream by the hydrophobic gas porous membrane. The Os(VIII)/Os(VI) redox couple in alkaline media reacts rapidly with a number of organic compounds and has excellent redox behavior. For this reason, we illustrate the scheme in Figure 1using the latter redox couple as mediator in the determi0003-2700/86/0358-098 1$01.50/0
nation of acetone, methanol, and ethanol.
EXPERIMENTAL SECTION All apparatus (pump, flow injection assembly, detector cell, electronics, and recorder) have been described previously (7,8). Reagent grade chemicals were used throughout and water was prepared by use of a Millipore Mill-& system. Os04 (0.5 g) was dissolved in 250 mL of 1 M KOH to prepare a 0.0078 M Os(VII1) solution. This solution was used as the supporting electrolyte in the electrochemical cell. The porous platinum electrode on the electrochemical cell side of the GPME was held at +0.3 V vs. the saturated calomel electrode. At this potential Os(VI), produced by the reaction of Os(VII1) with acetone, methanol, or ethanol, is quantitatively reoxidized back to Os(VII1). This potential was chosen on the basis of cyclic voltammetric studies and previous literature (9).
RESULTS AND DISCUSSION Twenty-microliter volumes of aqueous acetone solutions were injected into the carrier stream (air saturated, deionized water) flowing at a rate of 0.5 mL/min. Anodic current peaks proportional to the concentration of acetone studied (4.3 X lo4 M to 3.4 X M) were obtained. The amounts of acetone corresponded to 5-4000 ng. The slope of this linear calibration line was 1.85 nA/ng of acetone. Twelve successive injections of 157-ng samples of acetone yielded peak heights with a relative standard deviation of 0.8%. Typical peak responses are shown in Figure 2 for sample injections made at 1.5-min intervals. A sample analysis rate of 40 samples/h is easily achieved. Analogous studies were performed with methanol (1.58-158 pL) and ethanol (1-500 pg) and a linear relationship between peak current response and sample size was found. The slopes 0 1986 American Chemlcal Soclety
