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Anal. Chem. 1990, 62, 1924-1927
Coated Amperometric Electrode Arrays for Multicomponent Analysis Joseph Wang,* Gary D. Rayson, Ziling Lu, and Hui Wu Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
This paper deocrlbes a sensor array of several amperometrlc electrodes, each coated wRh a dlfferent permselectlve fllm. Coatlngs wRh dlfferent transport properties, based on size (cellulose acetate), charge (Naflon, poly(vlnyipyrldlne), poly( ester sulfonic acld)), and poiarlty (phospholipid) are employed In connection with a four-electrode thln-layer flow detector. WRh equlpotentlal operation, the array’s response pattern of each analyte provides a unique characterization of the lndlvldual components. Multlcomponent analysls is obtained by taking advantage of the partial selectlvity of the indlvldual sensors and using a pattern recognition (multiple linear regression) method. Additional informatlon Is obtained by recording the complete hydrodynamic voltammogram at the Individual sensors. The merits of the new array are illustrated for the quantification of neurologically slgnlflcanl catechol compounds.
INTRODUCTION The use of chemically modified electrodes as chemical sensors is receiving considerable attention. The ability to deliberately control the manipulate the surface properties can lead to a variety of attractive effects. One promising avenue is coverage of the surface with an appropriate permselective membrane barrier. Coatings with different discriminative properties, such as size (1-31, charge (4-6), or polarity (7,8) have been developed to yield significant improvements in in vivo monitoring of primary neurotransmitters (9),amperometric detection for liquid chromatography or flow injection systems (I-6,10, I I ) , enzymatic and nonenzymatic sensing of glucose (12-15), or anodic stripping measurements of trace metals (16). The additional separation step, performed in situ at the surface, thus adds new dimensions of information that enhance the power of amperometric devices. This paper describes the characterization and analytical utility of a novel array of partially selective amperometric electrodes based on coverage with different permselective coatings. Because of their great potential for practical analytical applications, sensor arrays are of considerable recent interest. The formation of such arrays brings an additional degree of information. Arrays based on different sensing principles have thus been described in past years. In particular, Kowalski and co-workers (17, 18) developed and characterized arrays of piezoelectric crystals, coated with different adsorbates, for detecting gaseous vapors in connection with environmental monitoring. An amperometric sensor array for detecting hazardous gases was developed by Stetter et al. (19). This array consisted of four uncoated electrodes, operated at different potentials and preceded by different heated filaments to treat the passing samples. Sparingly selective ion-selective electrode arrays were described by Otto and Thomas (20) and Beebe et al. (21). The coupling of these arrays with a chemometric (pattern recognition) approach has opened the door to multicomponent analysis. The partial selectivity of the individual sensing elements is particularly attractive for such use of multivariate
calibration methods. Similar advantages accrue in the present study from the use of partially selective amperometric electrode arrays. Permselective coatings, with widely diverse transport properties, are used to obtain the pattern ampercmetric response of the array and hence to identify the analyte. We wish to report these features and advantages in the following sections in connection with flow-injection detection of neurochemically important compounds.
EXPERIMENTAL SECTION Apparatus. The four-electrode thin-layer flow cell is shown in Figure 1. The body consisted of two dual electrode (glassy carbon) half cells (Model MF 1O00, Bioanalytical Systems (BAS)). One of the blocks was dried to accept the solution inlet and outlet tubings. The two blocks were separated by two Teflon gaskets (TG-15M, BAS). The Ag/AgCl (3 M NaC1) reference electrode and stainless steel auxiliary electrode were positioned downstream in the conventional manner. The flow injection system was described previously (2); a 20-pL sample loop was used. The electrodes were connected to an IBM instruments Model EC 220 voltammetric analyzer, the output of which was displayed on a Houston Omniscribe strip-chart recorder. The flow injection response of each electrode was recorded sequentially. €&agents. Poly(viny1pyridine) (PVP, Polysciences, Inc.), Nafion (5% solution, Solution Technology, Inc.), cellulose acetate (Aldrich), Eastman AQ55D (28% solution, Eastman Kodak Co.), ascorbic acid (Baker),and L-a-phosphatidylcholine(Type XI-E), cholesterol, catechol, epinephrine, norepinephrine, dopamine, (3,4-dihydroxyphenyl)aceticacid (DOPAC), and promethazine (Sigma) were used without further purification. All measurements were performed in 0.05 M phosphate buffer solution (pH 5.5). Solutions were prepared with doubly distilled water. Surface Modification. Prior to their coating, the four glassy carbon surfaces were polished with 0.05-fim a-alumina particles, rinsed with doubly distilled water, and sonicated in a water bath for 5 min. The different coatings were applied at the individual electrodes by placing 5 p L of the corresponding solutions to cover the active disk and allowing to air-dry. The individual films did not overlap. The different solutions were prepared by (a) dissolving PVP in methanol (0.5% solution), (b) diluting the Nafion solution 10-fold in ethanol, (c) dissolving cellulose acetate in 1:l acetone:cyclohexanone solution (2 % solution), (d) adding water (at 1%)to the Eastman AQ 55D solution and diluting it 20-fold in methanol, and (e) adding 12 mg of cholesterol to 1 mL of chloroform solution containing 10 mg of phosphatidylcholine. The cellulose acetate coating was hydrolyzed in 0.07 M KOH, prior to coverage of the other disks. Procedure. Flow injection analysis, with equipotential operation and flow rate of 1.0 mL/min, was used for demonstrating the merits of the arrays. Working potentials (in the plateau regions) were applied sequentially at the individual electrodes. Some experiments were performed by recording the complete hydrodynamic voltammograms at the individual sensors. CALCULATIONS The current response, iI, of a coated electrode can be generalized in the following equation (22):
p,
iI = nFA-C b where n is number of electrons in the reaction, F is the value of Faraday, A is the electrode area, P, is the film permeability
0003-2700/90/0362-1924$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
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Cl c2
Figure 1. Expanded view of the thin-layer flow cell: (A, B) solution inlet and outlet; (12,-C,) working electrodes: (D,, D,) spacers.
for the electroactive species, b is the film thickness, and C is the bulk concentration of this species. Consider the case in which several oxidizable species (A, B, etc.) are present in the same solution, so that the redox processes RA OA + nAe-, RB OB + nBe-, etc. occur at the operating potential. The measured current is thus given by
-
-
Multiple linear regression (MLR) is employed to predict a component’s concentration in a sample mixture. By use of several coated electrodes for every calibration mixture, a set of linear equations is obtained that can be expressed in matrix notation as follows:
(3) where I is the matrix of transformed current valued with n I=CR
-- - - - - - - - - -
j al
b
----
c--
C& e
TIME Figure 2. Flow-injection peaks for 2 X lo4 M dopamine (a),DOPAC (b), and catechol (c) at the 25-mln hydrolyzed cellulose acetate (A), PVP (B), and Eastman-AQ55D (C) coated electrodes: flow rate, 1.0 mL/min; applied potential, +0.95 V; 0.05 M phosphate buffer (pH 5.5).
rows (number of mixtures) and p columns (number of electrodes), C is the matrix mixture concentration having n rows and m columns (number of analytes), and R is the response parameter matrix with m rows and p columns. In the most general case of overdetermined systems, the response parameters are obtained according to
R
= (C’C)-WI
(4)
where the superscript t indicates the transpose of a matrix. The matrix of response parameters, R,is then used to analyze unknown mixtures by measuring their current values and substituting the transformed values into c = eRt(R.Rt)-’
(5) where c is the vector of the m sought-for concentrations and e is the vector of current values at the p different electrodes. RESULTS The concept of partially selective electrode arrays is illustrated in the following sections for assays of mixtures of catechol compounds, of great relevance to neurochemical studies. Figure 2 shows flow injection peaks recorded at a three-channel coated amperometric electrode array for dopamine (a), DOPAC (b), and catechol (c). Despite the structural similarities of these analytes and the use of the same operating potential, the different coated electrodes exhibit distinctly different response peaks. The size-exclusion cellulosic film (A) exhibits higher permeability toward the smaller catechol molecules. These neutral molecules readily transport through the cationic PVP film (B), that also yields enhanced response toward the anionic DOPAC. On the other hand, facile transport through the negatively charged poly(ester sulfonic acid) Eastman-AQ layer (C) is observed for the cationic dopamine; this film effectively excludes the DOPAC anions. All coatings thus exhibit the expected permselective response, as well as fast response to dynamic changes in concentrations. As will be illustrated in the following sections, the partial (rather than total) selectivity of the individual coatings is advantageous for the use of such arrays for multicomponent analysis.
I
SENSORS Figure 3. Sensor array response pattem for 2 X
lod
M promethezine
(crosshatch),dopamine (open),and catechol (hatch): sensors, 2O.mh hydrolyzed celkrlose-acetate (l),PVP (2), Nafion (3),and lipid (4) coated electrodes; flow injection operation wlth detection at +0.80 V. Other conditions are given in Figure 2. The multielectrode operation, based on different transport properties, can be used to generate the array’s response of each analyte. For example, Figure 3 shows the array response pattern for dopamine, promethazine, and catechol at a four-channel array with permeabilities based on size, charge, or polarity. For each analyte, the amperometric array response is unique from the response pattern of the other species. For instance, the large hydrophobic promethazine molecules are readily detected at the hydrophobic lipid- and Nafion-coated electrodes, while being excluded from the cellulose acetate and PVP coated elements. The structurally similar dopamine and catechol exhibit a relatively similar response at the cellulose acetate and lipid-coated electrodes but differ largely in their behavior a t the charged PVP and Nafion coated surfaces.
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15,
1990
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3 0
u v e 0.7
1.0 POTENTIAL, V
Flgure 5. Hydrodynamic voltammograms for 2 X M dopamine (0),DOPAC (A)and catechol (MI at the 25-min hydrolyzed cellulose acetate (A), PVP (B), and Eastman-AQ 55 (C)coated electrodes. Other conditions are given in Figure 2.
SENSORS Figure 4. Sensor array response pattern for 2 X M epinephrine (crosshatch),norepinephrine (open),and dopamine (hatch): sensors, 30-min hydrolyzed cellulose acetate (l), Nafion (2), and Eastman AQ55D (3)coated electrodes; flow injection operation with detection at +0.90 V. Other conditions are given in Figure 2. Table I. Multivariate Prediction for Two-Component Mixtures 1. ascorbic acid + epinephrineo 2. ascorbic acid epinephrinea 3. catechol + DOPAC*
+
6.0 + 2.0 2.0 + 6.0 2.0 + 6.0
//
6.09
1.48
+ 1.97 + 5.62
2.35 + 5.02
Sensors: Nafion, PVP, and 30-min hydrolyzed cellulose acetate coated electrodes, held at +0.8 V. *Sensors: as in Figure 2, held at
+o.a v.
Another response pattern of great significance for neurochemical studies is shown in Figure 4. While negatively charged coatings (e.g. Nafion) allow discrimination against anionic species (e.g., DOPAC, ascorbic acid), they lack the desired resolving power between cationic neurotransmitters with similar redox properties, e.g. dopamine, epinephrine, and norepinephrine. As shown in Figure 4, small variations in the transport of these compounds through charge- and size-exclusion coatings can be combined and exploited for generating a characteristic array’s response for each cationic neurotransmitter. For transport through ion-exchange polymers the permeability of the electroactive species (P,in eq 1) is given by P, = &Df, where Kd is the distribution constant and Df the diffusion coefficient in the film. Differences in the KD and D, values of cationic neurotransmitters in Nafion films have been elucidated recently (11). For the array shown in Figure 4,the sulfonated Nafion and Eastman AQ coatings exhibit different trends in the permeability (Ndion, dopamine > norepinephrine > epinephrine, vs Eastman AQ, dopamine > epinephrine > norepinephrine). In the case of the cellulosic film the trend is dopamine (molecular weight 153) > epinephrine (molecular weight 183) > norepinephrine (molecular weight 169). The latter indicates that molecular weight is not the sole factor affecting the transport through the cellulosic layer. Overall, the characteristic signature patterns as shown in Figures 3 and 4 can be used to “fingerprint” and identify the individual components and to predict their concentrations in simple mixtures. (Because of the increased similarity of cationic neurotransmitters, additional sensors may be needed to increase the prediction capability.) The ability of the amperometric array/chemometric approach to perform multicomponent analysis is illustrated in Table I, using different sample sets. The MLR prediction results follow closely the actual concentration of ascorbic acid, epinephrine, catechol, and DOPAC. The relative predictive
errors range from 1 to 26%, with an average value of 11% . In addition to the use of additional sensors, the predictive ability should be improved by employing a partial leastsquares calibration technique (which addresses more successfully errors created by collinearity (I 7)). Further improvement in the power of the partially selective electrode array can be achieved by providing an additional (third) dimension of information to the sensor array pattern. In particular, it is possible to apply different potentials to each electrode of the array and to register the corresponding individual currents (Le. recording the complete hydrodynamic voltammogram). An example of such a three-dimensional (potential-current-sensor) display of data is illustrated in Figure 5, using cellulose acetate (A), PVP (B), and Eastman AQ 55D (C) coated electrodes and dopamine, DOPAC, and catechol as model analytes. Since each of the three electrodes is operated at eight different potentials, 24 channels of data are obtained for each analyte. Although beyond the scope of the present study, these provide extremely useful qualitative information, based on the coupling of redox and transport properties of the individual solutes. An even further strengthening of the qualitative information could be achieved by adding a fourth (time) dimension-in connection with liquid chromatographic detection-based on the retention properties of the analytes. Such ability to obtain multipledimension array patterns can be beneficial for more complex samples. Since the individual points are obtained at constant potential, no loss in sensitivity is observed (as compared to potential scanning schemes, that suffer from charging-background current contributions).
DISCUSSION We have demonstrated that an array of partially selective coated electrodes, in connection with a chemometric approach, can greatly enhance the power of amperometric sensing. One important requirement for the chemometric MLR approach is linear additivity with respect to analyte concentration. In accordance with eq 1, the amperometric response of each coated electrode is expected to be linear. For all analytes tested, the concentrations used in this study were within the dynamic range of the individual electrodes. However, deviation from linearity may occur due to factors such as surface passivation (via adsorption of reactants, reaction products or electroinactive surfactants), chemical reactions between the individual solutes, and ohmic losses (associated with the resistance of the thin-layer cell). For example, most analytes tested exhibit a curvature in their calibration plots a t concentrations higher than 1 X M. Such deviations from linearity, which are independent of coexisting constituents,
Anal. Chem. 1990, 62,1927-1934
may be addressed by other mathematical treatments. For example, curved concentration dependences may be modeled by quadratic or mixed concentration terms with alternative chemometric approaches. The individual sensors must also be highly stable. The more reproducible the results are, the more reliable is the mathematical model. While (as expected) the film-to-film reproducibility is 3 4 % (in terms of relative standard deviation of the response), the individual films maintain their permselective transport properties over long periods and thus offer excellent reproducibility ( 1 4 % relative standard deviation). This was indicated from repetitive flow injection analysis measurements, performed in 15-min intervals over several (3-5) hours. (In addition to their permselective properties, these coatings possess antifouling properties which further enhance the sensor lifetime.) While the concept is presented within the framework of biomedical (neurochemical) applications, similar amperometric-electrode arrays could be beneficially employed for environmental monitoring or industrial process controI. The array also provides a rapid estimate of the discriminative capability of each element (as desired for developmental work) and obviates the needs for developing totally selective coatings. The use of pattern recognition techniques can facilitate the selection of coatings for amperometric arrays, in a manner analogous to the selection of adsorbates for piezoelectric crystal arrays (23). Additional improvement can be achieved by expansion of these arrays to include additional partially selective electrodes (based on different discriminative properties, e.g. shape, or various molecular weight cutoffs). The latter may be accomplished from size-exclusion electropolymerized films (e.g., polyaniline, polyphenol (3))deposited for different times. Improvements in the collection and displaying of the individual current signals can be achieved by using a multipotentiostat with individual working-electrode terminals.
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LITERATURE C I T E D (1) Shmpaiam, 0.; Wilson, G. S. Anal. Chem. 1983, 55, 1608. (2) Wang, J.; Hutchins, L. D. AM/. Chem. 1985. 57, 1536. (3) Wang, J.; Chen, S. P.; Lin, M. S. J . Elechoanal. Chem. Interfaclel Elechochem. 1989, 273, 231. (4) Steck, A.; Yeager, H. L. Anal. Chem. 1980, 52, 1215. ( 5 ) Wang, J.; Tuzhi, P.; Oolden. T. Anal. Chim. Acta 1987, 794, 129. (6) Wang, J.; Golden, T.; Tuzhi, P. Anal. Chem. 1987, 59, 740. Kauffmann, J. M.; Patrlache, G. J.; Christian, G. Anal. (7) Chastei, 0.; Chem. 1989, 67, 170. (8) Garcia, 0. J.; Quintela, P. A.; Kaifer, A. E. AM/. Chem. 1989, 67, 979. (9) @mer&, G. A.; Oke, A. F.; Nagy, G.; Moghaddam, B.; Adams, R. Brain Res. 1984, 290, 390. (10) Hutchins-Kumar, L. D.; Wang, J.; Tuzhi, P. Anal. Chem. 1988, 58, 1019. (1 1) T W s , A. J.; Ozinga, W. J. J.; Poppe, H.; Kok, W. Th. Anal. Chem. 1990, 62, 367. (12) Harrison, D. J.; Turner, R. F. 6.; Baltes, H. P. Anal. Chem. 1988, 6 0 , 2002. (13) Amine, A.; Kauffmann, J. M.; Patrlarche, G. J.; Guilbault. G. G. AM/. Lett. 1989. 22. 2403. (14) Gorton, L.; Karen, H. E.; Hale, P. D.; Inagaki, T.; Okamoto, Y.; Skotheim. T. A. Anal. Chim. Acta 1990, 228. 23. (15) Bindra, D.; Wilson, G. S. AM/. Chem. 1989, 67, 2566. (16) b y e r , 6.; Florence, T. M.; Batley, G. E. Anal. Chem. 1987, 59, 1608. (17) Carey, W. P.; Beebe, K. P.; Kowalski, E. R. Anal. Chem. 1987, 59, 1529. (18) Carey, W. P.; Kowalski, B. R. Anal. Chem. 1988, 60, 541. (19) Stetter, J. R.: Jur, P. C.; Rose, S . L. Anal. Chem. 1988, 58, 860. (20) Otto, M.; Thomas, J. D. R. Anal. Chem. 1985, 57, 2647. (21) Beebe, K.; Uerz, D.; Sandifer, J.; Kowalski, 8. R. A M I . Chem. 1988, 60, 66. (22) Smart, R. 6.; Dormond-Herrera, R.; Mancy, K. H. Anal. Chem. 1979. 57. 2315. (23) Carey, W. P.; Beebe, K. P.; Kowalski, B. R.; Illman. D.L.; Hirschfeid, T. Anal. Chem. 1988, 58, 149.
RECEIVED for review March 27,1990. Accepted June 19,1990. This work was supported by the donors of the Petroleum Research Fund, administrated by the America1 Chemical Society, and by the National Institutes of Health (Grant No. GM 30913-06).
Role of Selective Sorption in Chemiresistor Sensors for Organophosphorus Detection J a y W. Grate,* Mark Klusty, William R. Barger, and A r t h u r W. Snow
Chemistry Division, Naval Research Laboratory, Washington,D.C. 20375-5000
Nlckei, palladium, platinum, and copper tetrakis( cumyiphenoxy)phthaiocyanlnes were combined with an elastomeric, oligomeric fiuoropoiyoi material In mixed Langmuir-Blodgett films on chemlreslstor sensors for organophosphorus vapors. The phthalocyanine carried the electronic current, while the fiuoropoiyoi Improved the sorption characterlstics of the film. This strategy produced sensors with improved response and recovery times and high sensitivity. Factors lnfiuenclng the selectivity of the sensor responses were analyzed In terms of two steps: sorption and transduction. Sorption was shown to be the primary determinant of selectlvity among the organic vapors tested.
INTRODUCTION The use of Langmuir-Blodgett (LB) films on chemical sensors is now well established (1-12). This technique is used
to apply thin films of the materials which must interact selectively with the chemical environment. Upon interaction with the species to be detected, the film material undergoes a change that is detected by the transducer to which the film has been applied. This could be a mass change, a change in electrical conductivity, or a change in optical properties, to name just three. Changes in electrical conductivity are the basis for the chemiresistor sensors illustrated in Figure 1 (5-8). A set of interdigital electrodes on an insulating substrate is covered by a thin film of a weakly conducting organic material. A constant bias voltage is applied and the current through the sensor is measured. Changes in current measure the change in film conductivity which occurs in response to interactive vapors. The advantage of the LB technique in preparing films for these and other sensors is the ability to prepare very thin films which are continuous. In addition, application of the film one layer at a time affords precise control of film thickness. These
This article not subject to US. Copyright. Published 1990 by the American Chemical Society
