Anal. Chem. 1966, 58, 1803-1806
we measured the concentration of copper ions in the inner solution by AAS. The AAS results showed that the Cu(I1) ion concentration is 3.6 X lo4 M,which is amplified 3.6 times as compared with the sample Cu(II) concentration. Thus, the present electrode boosts the virtual concentration of Cu(I1) ions by uphill transport using redox energy. The electrode potential becomes more positive when the sample Cu(I1) concentration is increased further.
using a thinner membrane. The work is under way. Registry No. Cd, 7440-43-9;U, 7440-61-1;Cu, 7440-50-8.
LITERATURE CITED Rosenberg, T. Acta. Chem. S a n d . 1048, 2 , 14. Okahara, M.;Nakatsuji, Y. In Biomimtk and Bioinorganic Chemisfry, Topics in Current Chemistry 728; Boscheke, F. L., Ed.; Springer-Verlag: Berlin, 1985;p 37. Bloch, R.; Kedem, 0.;Vofsl, D. Nature (London) 1063, 799. 802. Kumano. A.; Nlwa, 0.;Kajlyama, T.; Takayanagi, M.;Kano, K.; Shinkal, s. Chem. Lett. 1083, 1327. Shlnkai, S.; Mlnaml, T.; Kusano, U.; Manabe, 0. J . Am. Chem. SOC. 1882. 704. 1967. Shlnkal, T.;’Ishlhara, M.;Ueda, K.; Manabe, 0. J . Chem. Soc., Pekin Trans. 2 1085. 511. Ohkl, A.; Takagi, M.;Ueno, K. Chem. Lett. 1080, 1591. Ohki. A.; Hinoshlta, H.; Takagi, M.;Ueno, K. Sep. Sci. Technd. 1083, 78, 969. Choy, E. M.;Evans, D. F.; Cussler, E. L. J . Am. Chem. SOC.1974, 96, 7085. Bloch, R. Membrane Science and Technology; -. Flinn, J. E., Ed.; Plenum: New York, 1970;p 171. Pribll, R.; Vesely, V. Collect. Czech. Chem. Commun. 1072, 3 7 , 13. Danesi, P. R.; Hcrwitz, E. P.; Rickert, P. Sep. Sci. Techno/. 1082, 17,
CONCLUSION The “built-in” uphill transport liquid membrane electrode described here seems to be an attractive method for modifying sensitivity and selectivity of the electrochemical response. The small volume of an inner filling solution plays a key role in signal enhancement. Permeability of ions across the membrane contributes to a selective response of the electrode. When both the mercury film and reference electrodes are dipped in the inner filling solution as is the case for the proposed Cu(I1) electrode, an advantage of eliminating an ohmic drop through the membrane results, although an explicit coupling, if any, of uphill and active transports with an electric potential gradient across the membrane would be expected for a separate three-electrode system like the Cd(I1) and uranyl electrodes (Figure 3). The response time of the present electrode is rather slow, which will be improved by
1603
1183. Danesi, P. R.; Chiarlria, R.; Castagnola, A. J . Memb. Sci. 1083, 14,
161. Florence, T. M.; Farrar. Y. Anal. Chem. 1963, 35, 1613.
RECEIVED for review August 28,1985. Resubmitted January 28, 1986. Accepted February 28,1986.
Polypyrrole Electrode as a Detector for Electroinactive Anions by Flow Injection Analysis Yoshihito Ikariyama and William R. Heineman* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172
A new electrochemlcal detector based on the repetltlve doping-undoplng of polypyrrole has been applled to the electrochemlcal determlnatlon of electrolnactlve anlons by flow Injection analysis. The electrode response for phosphate and carbonate was linear over the concentration range from 10 pM to 1 mM wtth a coemcid of variatbn of 1.2% for 250 pM phasphate and 2.5% for 1 mM carbonate. The electrode was stable for over 2 weeks In an anaeroblc atmosphere.
After its introduction by Nagy et a1 (l),flow injection analysis (FIA) has attracted considerable interest as a method for fast, repetitive, and reproducible analysis (2,3).Initially based on electroanalysis, the methodology has been further developed for this purpose (4,5)as well as extended to other modes of detection (6-8) such as absorbance (91, fluorescence (lo),and luminescence (11).Although amperometric detection for FIA and liquid chromatography (LCEC) has come into widespread use for the determination of electroactive subHtances (12-14), it is not directly applicable to the large inumber of compounds that do not undergo heterogeneous redox reactions at electrodes. This feature is advantageous in the sense that selectivity is improved by the absence of response to electroinactive interferences in the sample. Amperometric detection has been extended to some electroinactive substances by postcolumn reaction detectors (14), 0003-2700/S6/0358-1803$01.50/0
-
Scheme I
Doping
Scheme 11
Undoping
immunoassay (151,and a triple step potential wave form for certain adsorbates (16). In recent years electrodes coated with conductive polymer films have been the subject of considerable interest (17). Conductive polymers such as polypyrrole and polythiophene are an especially important class of polymers (18-23),since ionic substances are easily and repeatedly incorporated into (doping) and released from (undoping) the polymers. The general concept of the doping-undoping property of polypyrrole is shown in Schemes I and 11. Conductive polymer coatings of this type have been used as a charge-storage material in rechargeable batteries (24),an organic electrode material (25),a protecting film on semiconductor electrodes to prevent photocorrosion (26),an ion gate membrane (23, 0 1986 Amerlcan Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8,JULY 1986
In
A
I
RE
+ ......... w-
'...' .... ... @..' Time
Figure 1. Schematic representation of porypYrrole electrode detector response to (A) blank and (6)sample containing anions. The electrode is held at E, between samples for undoping and then stepped to E, before sample injection to detect anions by doping. and a presynaptic model with an electrochemical release of neurotransmitters (28). However, the doping-undoping property of these conductive polymers has not yet been applied as a detector for FIA. In this study, polypyrrole is used in an electrochemistrybased FIA as a detector that gives an electrochemical response to electroinactive ionic substances such as phosphate, carbonate, and acetate. The anion in a sample flowing through the electrochemical cell is doped into the polypyrrole electrode (Scheme I) at the doping potential with anodic current. The electrode is then regenerated by undoping a t the undoping potential (Scheme 11), before the next sample is analyzed. The doping current depends on anion concentration. The potential excitation signal and expected current response signal are shown in Figure 1.
EXPERIMENTAL SECTION Reagents. Pyrrole and tetraethylammonium perchlorate were purchased from Aldrich (St. Louis, MO) and Southwestern Analytical Chemicals, Inc. (Austin, TX), respectively. Sodium acetate, acetonitrile, and sodium bicarbonate were obtained from Fisher Scientific Co. (Fair Lawn, NJ). Sodium dihydrogen phosphate and glycine were the products of Matheson Coleman & Bell Manufacturing Chemists (Norwood, OH). These chemicals of reagent grade were used without further purification. Preparation of Polypyrrole Electrode. Polypyrrole electrodes were prepared by deposition resulting from the electrochemical oxidation of pyrrole at a platinum electrode. Pyrrole (10 mM) was dissolved in an acetonitrile solution containing 0.1 M tetraethylammonium perchlorate. The solution in the electrochemical cell was magnetically stirred and was deoxygenated with Nz-gas bubbling. In order to avoid contamination from chloride, the Ag/AgCl (saturated KCl) reference electrode was isolated with a d t bridge containing0.1 M (Et),NC104acetonitrile solution. Potential was applied with either a CV-1B cyclic voltammetry instrument or an LC-3A amperometric controller (BAS, West Lafayette, IN). The polypyrrole electrode was prepared on the Pt surface (diameter = 3 mm) of the working electrode in a thin-layer transducer cell for LCEC (BAS). A Pt plate (1 X 1 cm2)was used as the auxiliary electrode. Both the working and auxiliary electrodes had been cleaned in 0.1 M sulfuric acid until typical waves for oxygen and hydrogen adsorption/desorption were observed by cyclic voltammetry. A potential of 0.95 V vs. AgfAgCl was applied to the cell containing pyrrole, and the resulting current vs. time curve was recorded with a 5OOO Recordall Recorder (Fisher Scientific, Pittsburgh, PA). Polypyrrole was deposited for 3 min. The polypyrrole-coated working electrode was then attached tightly to the plastic top half of the thin-layer cell through a Teflon gasket as shown in Figure 2. The resulting thin-layer detector for FIA has three electrodes: polypyrrolecoated platinum working electrode, auxiliary electrode (stainless steel tube used for sample outlet), and AgfAgCl reference electrode. The latter two electrodes were positioned downstream from the cell. FIA. FIA was performed with an LCEC Analyzer (BAS) with the chromatographiccolumn removed. A 20-fiL sample loop was
Figure 2. Assembly of the electrochemical detector system. WE is
the polypyrrole-coated platinum working electrode, AE is a stainless steel tube auxiliary electrode, and RE is an Ag/AgCI reference elec-
trode.
1
1
I
1
0
1
2
3
TIME (min) Figure 3. Current vs. time profile for pyrrole polymerization at 0.95 V vs. Ag/AgCI electrode onto a platinum working electrode from 10 mM pyrrole in 0.1 M (Et),NCIO, acetonitrile solution.
-
1st scan
50th scan
Figure 4. Cyclic voltammograms of the polypyrrole electrode in 0.1 M (Et),NCIO, acetonitrile: scan rate, 100 mV/s.
used for all experiments. The length of the connecting tube (inner diameter = 0.013 cm) from the injection valve to the detector was 55 cm. The polypyrrole electrochemicaldetector was controlled by the LC-3A amperometric controller and current was recorded with the strip chart recorder. The electrode potential was set to 0.9 V vs. AgfAgCl and the anodic current was recorded as the electrode was doped by anion flowing through the cell. The electrode was undoped by applying -0.3 V after each analytical response, as shown in Figure 1. All electrode potentials are reported vs. the silver-silver chloride electrode.
RESULTS Preparation of the Polypyrrole Electrode. The polypyrrole electrode was prepared by the electropolymerization of pyrrole at a platinum electrode by oxidation at 0.95 V vs. Ag/AgCl. Film thickness was controlled by the electrolysis time during which a measured quantity of charge was allowed to pass. Figure 3 shows a typical current response. The initial current spike decayed rapidly to a steady level, which slowly increased as the film grew. After about 10 min the current began to gradually fall as the film got thicker. After its preparation, each polypyrrole-coated electrode was transferred to deoxygenated 0.1 M (Et)4NC104acetonitrile where its cyclic voltammetry was investigated. The fiist cycle,
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Blank
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Figure 5. Typical responses of polypyrrole detector to carbonate:
d o p i i potential, 0.9 V; undoping potential, -0.3 V; blank, 0.1 M glycine solution; S,, 1.0 X lo4 M carbonate; Sp, 2.5 X lo4 M carbonate; SB, 5.0 X M carbonate: flow rate, 9.3 mL/h. T a b l e I. Coefficients of V a r i a t i o n
anion
concn, mM
flow rate, mL/h
CV," %
N
carbonate phosphate acetate
1.00 0.25 2.00 0.25
9.3 9.3 13.7 13.7
2.5 1.2 1.1 6.9
10 10 13 12
Coefficient of variation. which is shown in Figure 4, shows anodic current due to doping with perchlorate during the positive scan and cathodic current due to undoping during the negative scan. On subsequent cycles the anodic peak and cathodic peak at +0.2 V gradually decreased. After ca. 50 scans the voltammograms showed little change with continued cycling. Oxidation (doping) and reduction (undoping) of the polypyrrole film require that perchlorate ions enter and leave the film to neutralize the charge in the polymer matrix. The color of the polypyrrole film changed from yellow (at -0.3 V) to bluish purple (at +0.9 V) repeatedly as the electrode was cycled (18). The bluish purple color is due to a charge-transfer (CT) complex with perchlorate in the doped state. The polypyrrole-coated electrode was assembled in the thin layer detection cell after such routine checking by cyclic voltammetry. Response of Polypyrrole Electrode to Anions. Carbonate, phosphate, and acetate were chosen as typical anionic species to show the response of the polypyrrole electrode to anions. In preliminary experiments the polypyrrole electrode was found not to respond to zwitterions such as neutral amino acids. Therefore, glycine was used as a mobile buffer for FIA. The results for carbonate detection are typical of those obtained for all of the anions. Sodium hydrogen carbonate was clissolved in a 0.1 M glycine solution and diluted to various concentrations. Both the mobile buffer (0.1 M glycine) and the samples were deoxygenated with N2-gasbubbling, since polypyrrole is irreversibly oxidized by exposure to dissolved oxygen. The electrode potential was set to +0.9 V before every sample injection. After a response was obtained, the potential was stepped to -0.3 V to regenerate the electrode by undoping the carbonate from the electrode. Figure 5 displays typical responses for the samples. Every sample gave a sharp anodic peak with a peak height depending on carbonate concentration. The FIA of carbonate was carried out with the conductive electrode at 0.9 V with a flow rate of 9.3 mL/h. Repeated stepping of the potential from 0.9 V to -0.3 V for several times preceded the FIA to confirm the stepwise response of the electrode as shown in Figure 1A. Figure 6 illustrates the calibration curve for carbonate. As shown in Figure 5 the detector did not respond to a blank sample. Good linearity was obtained in the concentration range from 10 pM to 1mM. The calibration curve for phosphate shows that micromolar levels of phosphate can be determined with the conductive film detector. Good linearity was observed in the range between 10 pM and l mM phosphate. In the case of acetate, the electrode responded linearly in the concentration ranging from 100 pM to 4 mM. Table I shows the coefficients of variation for the determination of carbonate, phosphate, and acetate.
-5
-3 -2 Log [concentration], (M) -4
Figure 6. Calibration curves for anions: (0) carbonate, 9.3 mLlh; (0) phosphate, 9.3 mL/h; (0)acetate, 13.7 mL/h. Peak height is the maximum doping current after Injection.
The stability of the polypyrrole electrode was studied by flowing deoxygenated glycine solution through the thin layer cell for 2 weeks. Although the base line of residual current for the conductive electrode decreased gradually, the doping current for injection of 1 mM carbonate was reproducibly observed. Although oxygen has been shown to damage the polypyrrole electrode, a functional electrode can be maintained for at least 2 weeks with the procedures described here.
DISCUSSION The repeated doping-undoping property of a conductive polymer for the electrochemical determination of electroinactive substances has been demonstrated. The polypyrrole electrode has been shown to respond to as low as M anion with a satisfactory coefficient of variation. The approach is based on the reversible entering-leaving of electroinactive ions at the polymer matrix upon oxidation-reduction of the polymer; i.e., the electronic state of the polymer plays a very important role. At the doping potential, the electroinactive anion causes electron flow from the polymer to the Pt base electrode. This electron flow depends on the anion concentration. Although the polypyrrole electrode has been shown to be less sensitive to bulkier anions (29),the electrode itself is not an ion-selective electrode. Therefore,the polypyrrole electrode is perhaps best suited as a detector for liquid chromatography for the analysis of anion mixtures for which a separation step is necessary. It is also promising as a detector of liquid chromatography based electrochemicalenzyme immunoassay (15) and as an electrode for an enzyme sensor (SO), since enzymes such as decarboxylases,phosphorylases, and esterases that generate electroinactive anions could be employed as labeling enzymes and immobilized enzymes for immunoassays and enzyme electrodes, respectively. The conductive electrode is unstable in oxygen atmosphere. During the assembly of the polypyrrole electrode into the detector cell, the electrode was exposed to air for some time (from 30 min to 1h). In spite of this handling in air, the film was stable.for at least 2 weeks. The sensitivity and the stability of the polypyrrole electrode should be improved by rigorously avoiding exposure of the film to air by polymerizing the film,assembling it in the detector system, and maintaining it under a N,-gas atmosphere. In the method described here zwitterions such as amino acids were shown to be usable as a mobile buffer, because they are not incorporated in the conductive polymer due to their
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Anal. Chem. 1086. 58. 1806-1810
inability to form a CT complex. The pH of the mobile buffer could be changed by using a zwitterion at ita appropriate PI. Also, large molecular suhtances such aa proteins and synthetic ionic polymers (polyelectrolyte) could be used for mobile buffers, since these bulky substances are too big to be doped into the polymer network. The conductivity of the polypyrrole film is dependent on the doping state (31). Consequently, film conductivity may be a contributing factor in the response mechanism of this electrode to anion concentration (29). Registry NO.Pt, 7440-06-4; phosphate, 14265-44-2; carbonate, 3812-32-6; acetate, 71-50-1; polypyrrole, 30604-81-0.
LITERATURE CITED (1) Nagy, G.; Feher, 2.; Pungor, E. Anal. Chim. Acta 1970, 52, 47. (2) Ruzicka. J.; Hansen, E. H. Anal. Chlm. Acta 1975. 78, 145. (3) Stewart, K. K.; Beecher, G. R.; Hare, P. E. Anal. Biochem. 1978, 70,
(4) (5) (6) (7) (6) (9) (10) (11) (12) (13)
167. Kissinger, P. T. Anal. Chem. 1977, 49, 447A. Rucki, R. J. Talnta 1080, 27. 147. Betterldge. D. Anal. Chem. 1978, 50, 632A. Moebach, K.; Danlelsson, 8. Anal. Chem. 1981, 5 3 , 83A. Ranger, C. B. Anal. Chem. 1981, 53, 21A. Slanla. J.; Bakker, F.; Bruijin-Hes, A. G. M.; Mols. J. J. 2. Anal. Chem. 1978, 289, 38. Basson, W. D. Flow Injection Analysis Seminar, AOAC Fail Meeting, 1980. Rule, G.; Seitz, R. Clin. Chem. (Winston-Salem, N . C . ) 1970, 2 5 , 1635. Ramsing, A. U.; Janata, J.; Ruzicka, J.; Levy, M. Anal. Chim. Acta 1980, 178, 45. Roston, D. A.; Shoup, R. E.; Kissinger, P. T. Anal. Chem. 1982, 54, 1417A.
Kissinger, P. T. I n Laboratory Technkpes in Ekctroana&&al Chemist ~ Kissinger, ; P. T., Heineman, W. R., E&.; Marcel Dekker: New York, 1984 Chapter 22. Heineman, W. R.; Halsall, H. B. Anal. Chem. 1985, 57, 1312A. Polta, J. A.; Johnson, D. C. Anal. Chem. 1985, 57, 1373. Murray, R. W. In €lecfroanalyfcal Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13. Dkz, A.; Kanazawa, K. K.; Gardini, G. P. J . Chem. Soc.,Chem. Commun. 1979. 635. Kanazawa, K. K.; Daz, A. F.; Geiss, B. H.; 0111, W. D.; Kwark, J. F.; Logan, J. A.; Rabolt, J. F; Street, G. B. J . Chem. Soc., Chem. Commun. 1979, 854. Watanabe, A.; Tanaka, M.; Tanaka, J. Bull. Chern. SOC.Jpn. 1981. 54, 2278. Bredas. J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J . Am. Chem. SOC. 1083, 705, 6555. Pickup, P. G.; Osteryoung, R. A. J . Am. Chem. SOC.. 1984, 706, 2294. Sato, M.; Tanaka, S.; Kaeriyama, K. J . Chem. SOC.,Chem. Commun. 1985, 713. Pickup, P. G.; Osteryoung, R. A. J . Nectrochem. SOC. 1983, 730, 1965. Diaz, A. A.; Vasquez, VaileJo,J. M.; Martinez, Duran, A. IBM J . Res. Dev. 1081. 25. 42. Frank, A. J.; Hdnda, K. J . Phys. Chem. 1982, 86, 1933. Burgmayer, P.; Murray, R. W. J . Am. Chem. SOC.1982. 704, 6139. Shinohara, H.;Aizawa, M.; Shirakawa, H. Chem. Lett. 1985, 179. Ikariyama, Y.; Heinemen, W. R., unpublished results. Updlke, S. J.; Hicks, G. P. Nature (London) 1987, 204, 986. Feldman, B. J.; Burgmayer, P.; Murray, R. W. J . Chem. SOC. 1985, 107, 872.
RECEIVED for review August 30, 1985. Resubmitted March 26,1986.Accepted March 26,1986. This work was supported by grants from the National Science Foundation (CHE8217045) and Bioanalytical Systems, Inc.
Serum Lithium Analysis by Coated Wire Lithium Ion Selective Electrodes in a Flow Injection Analysis Dialysis System Robert Y. Xie and Gary D. Christian* Department of Chemistry BG-IO, University of Washington, Seattle, Washington 98195
Lithium ion relectlvededrodsr incafpwalhgthe 14uown-4 ether 3-dodecyH-methyC1,5,8,12-tetraoxaoydotetrackcem, in PVC membranes were constructed In a microcondult clrcult, by depcn#kn on pbtbrum, rllver, or 0opp.r wires. values ol 0.0079-0.0083 or 0.012-0.014 were OMalned by use of the fbted Interferewe or the matchsd potential methods,respectively. Und#uted wood serum samples were analyzed by flow lnjedion analysls (FIA) by Incorporating a dialy&s Mmblane In the flow ckdgn. A 200-pL sample was Injected Into a ddonizd water cMekc (d0Mn)stream fbwlng at 0.4 mUmln, and the acceptor stream was 7 mM Na2B,0,.4H20 (pH 9.2) flowing at 0.3 mUmin. The Rhlum-to-sod i m transfmrt ratlo across the membrane was 0.57, wlth an overall SeleCrMty for WMUn wlth resped to sodhnn of 45 to 49 when wlng the matched potential methad wlth dlver or copper wlre electrodes. Serum samples amstyzed by the FIA/doctrodemothodudngtheoopp.rwireekch.ock, compared with atomic abeorptlon rewlte, showed an average error of -3.1% when wlng pooled serum standards and -8.8% when whg aqueous standards.
cb
Interest in lithium ion selective electrodes arises from the demand for improved clinical instrumentation. Clinical an0003-2700/88/035&1806$01.50/0
alyzers based on ion selective electrodes are available for analysis of sodium, potassium, calcium, and pH in blood serum. These instruments have advantages of small sample size consumption, short analysis time, and minimum operation cost without the necessity of flames and compressed gases. Lithium therapy is an efficient way to treat patients who are suffering from manic-depressive psychosis. The lithium must be maintained over a narrow range in the blood, and so monitoring of lithium concentration in serum is important. A number of promising lithium ion selective electrodes have been described. Among them, several diamide-based membrane electrodes were prepared by Zhukov et al. (I). The range of lithium over sodium (Li/Na) selectivities, an important criterion of lithium ion selective electrodes for blood measurements, was from 201 to 1001. Modification of one of these compounds was conducted by Xie et al. (2, 3 ) . However, while the response time for the modified membrane was improved, the electrode was not superior to its original form in selectivity. Metzger et al. ( 4 ) improved the Li/Na selectivity of the lipophilic diamide liquid membrane electrode to greater than 200:l. They also demonstrated a successful direct measurement of lithium added to blood serum ( 5 ) . However, no attempt was made to analyze patient samples. A lithium electrode based on a liquid membrane of a 14crown-4 ether, 3-dodecyl-3-methyl-1,5,8,12-tetraoxacyclotetradecane, is reported to have Li/Na selectivity comparable 0 1988 American Chemical Society
