Anal. Chem. 1999, 85,251-254
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Sensor Array for Carbohydrates and Amino Acids Based on Electrocatalytic Modified Electrodes Qiang Chen, Joseph Wang,' Gary Rayson, Baomin Tian, and Yuehe Lin Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
The operation and advantages of a fourslement chemical sensor array, comprising carbon paste electrodes doped with dmerent metal oxide Catalysts, are described. The Cu&, RuOr, N W , and CoOmodtfied surfaces exhibit distinctly different catalytic properties toward carbohydrates or amino acids. By coupling the unlque sensor array response patterns with various dattstlcalregresdon techniques, it Is possible to determine indlvlduai carbohydratesor amino acids in different sample mixtures. For two- and three-component mixtures, the partial least squares (PLS) callbration method yields an average relative prediction error of 2.3%. Such muitlcomponent quantitatlon Is accomplkhed in amperometric flow inJectbnexperiments. The responseIs highlystable and linear, as desired by the mathematical models.
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INTRODUCTION Sensor arrays, in which each element is only partially selective for different analytes, are of considerable interest for practical analytical applications.'-* The resulting sensor array response pattern can be coupled with an appropriate chemometric (pattern recognition) approach, hence leading to greatly improved chemical information. Arrays based on different sensing principles have thus been developed and applied to multicomponent analyses of environmental or clinicalsample mixtures. Particular attention has been given to the use of different coatings (adsorbates) in connection with arrays of piezoelectric crystals, for detecting hazardous gases and vapors.'-3 Electrochemical devices can also benefit from the operation of sensor arrays. Most publications on arrays of electrodesdeal with potentiometric measurementa.4-6 In particular, arrays of sparingly selective potentiometric electrodes have been used for determining individual cation activities within mixtures. Arrays of amperometric electrodes have also been designed, on the basis of the use of different electrode materials7 or permselective coatings.8 Enhanced selectivity accrued from the differences in the redox and transport properties, respectively, of the array elements. This paper describes the operation and advantages of a novel amperometric sensor array, based on the use of catalytic chemically modified electrodes. Our approach involves the incorporation of different metal oxide catalysts in an array of carbon paste electrodes. Because each modifier exhibits a different electrocatalytic behavior to a given analyte, the information content (and hence the selectivity) are greatly improved. The operation of the new modified-electrodearray
Fig& 1. Schematic vlew of the large-volume wall-jet detector: (A) Inlet; (B) electrodearray: (C) counter electrode; (D)referenceelectrode; (E)outlet. Also shown (right): bottom view of the electrodearray. See text for details. r
A
1
E it u
10 mln
B
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1
2
3
4
1 22 33 44 r l
C I
10inln
D 1 2
3 4
1
2
3
4
TIME Flgure 2. Flow Injection peaks for 0.6 mM glucose (I),lactose (2), ribose (3),and maltose (4)at the Cup0 (A), NiO (B), flu02 (C), and Coo (D) modified carbon paste electrodes. Conditions: fbw rate, 1.5 mL/ mln; applied potential, +0.45 V; carrier solution, 0.5 M KOH.
is demonstrated through the anodic flow detection of amino acids and carbohydrates, and data reduction is accomplished by means of partial least squares (PLS) and multiple linear regression (MLR)statistical methods.
EXPERIMENTAL SECTION
Apparatus. The four-electrode array, based on the largevolume wall-jet detector, is shown in Figure 1. For this purpose, (1)Carey,W.P.;Beebe,K.R.;Kowalski,B.R.;Illman,D.L.;Hirschfeld, four cavities (3-mmdiameter) were equally spaced at the bottom T.A n d . Chem. 1986,56,149. of a 12-mm diameter Plexiglas cylinder (3 mm from the center) (2) Carey, W. P.; Beebe,K. R.; Kowalski, B. R. A n d . Chem. 1987,59, and were filled with the modified carbon pastes (B).Electrical 1529. contact was provided by copper wires, inserted from the opposite (3) Katrizky, A. R.; Savage, G. P.; Pilarska, M. Tulunta 1991,38,201. (4) Otto, M.; Thomas,J. D. R. Anal. Chem. 1985,57, 2647. direction. The cell body was made of a 70-mL glass beaker (5-cm (5) Beebe, K.; Ven, D.; Dandifer, J.; Kowalski, B. A n d . Chem. 1986, diameter, 6.5-cm height). The working-electrodearray, the Ag/ 60,66. AgCl reference electrode (Model RE1,Bioanalytical Systems (6) Forster, R. J.; Regan, F.; Diamond, D. Anal. Chem. 1991,63,876. and the platinum wire auxiliary electrode (C) were (BAS)) (D), (7) Glass, R.S.;Perone, S. P.; Ciarlo, D. R. Anal. Chem. 1990,62,1914. placed in the cell through holes in ita Plexiglas cover, with 1-cm (8)Wang,J.; Rapon, G.; Lu,Z.; Wu,H.Anal. Chem. 1990,62, 1924. 0003-2700/93/0365-0251$04.00/0
Q 1003 Amcwlcan Chemicel Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 3. FEBRUARY 1. 1993 800
threonine
I
SENSORS 1
2
3
4
1
2
3
4
I
F b u n 5. Sen= anay response panem lor piycine. bucha. methionlm. and threonine. Sensors: Cu10 (1). NIO (2). Coo (3).and Ru02(4)modiM carbon paste electrodes. Condnionsareas In Figure
3.
TIME
a. Flow iniection peaks lw
1 mM glycine (I), leucine (2), methionine (3).and mreonlne (4)allheCu20(A). NIO (B). COO(C),and RuOp(D)modiiicarbon pasleelectrodes. Ccmdltions: carriersolution. 1.0 M NaOH other conditions, as in Figure 2.
Flgura
TIME 6. Time deprdance of the electrode sensHMty over a 120minpwbd. FlowinJectionresponsof~N~(A)andCoo(B)modlRed electrodes to injections of 2 X lo3 M glycine (a)and &lonine (b), as well as 1 X M glucose (c)and maltose (d) solutlons. The t l m lnlewal between evety sei of injections is 15 mln. Omer conditions are as In Figure 2. -re
0.5
1
SENSORS Fbum 4. Sensor anay response panan lor gluwse. lactose, rbose. andmaltose. Sensors: COO( 1). Cu20(2).NiO(31.andRuOA4)modiffed carbon paste electrodes. Conditions are as in Figure 2.
separation from each other. Solution inflow was maintained through a vertical glass tube sealed tm the cell bottom (A). The tube w a s narrowed (to 0.8-mm i.d.), toward ita upper end, to fnrm the jet nozzle. The nozzle was kept 1 mm away from the center of the working-electrode array. Solution outflow was maintained through a 2-mm-i.d. glass tube, branching from the cell wall at a right angle (about 4 cm above the bottom) (E). The flow injectiun system consisted of a 200-mL sample reservoir, a Rainin Model 5041 sample injection valve (20-uL loop),interconnecting tuhing. and the wall-jetsensor array. Flow ofthecarriersolution was maintained bygravity. Allexperiments were performed with a BAS CV-27 voltammograph and a BAS X-Y-trecorder. An IHM .Model 5SSX personal computer was used for the PLS and M1.H calibration and concentration predictions. Bothprogramswerewritten in theBASIClanguage. Themodifiedcarbnnpastes wereprepared by thoroughly hand mixing (with a mortar and pestle) the desired amnunt of the metaloxide (25O; wt), with Acheson 38graphitepowder (Fisher) (45'; wt) and mineral oil (Aldrich) (3OPA wt). The elertrode surfaces were smoothed on a piece of weighing paper. Reagents. Deionized water was used to prepare all solutions. Glucose. lactone, ribose, maltose. leucine, methionine, threonine
(Sigma). and glycine (Fisher) were used without further purifieation. Carriersolutionsforthemeasurementaof m h h y d r a t m and amino acids were 0.5 M KOH and 1.0 M NaOH, respectively. The various metal oxides, used for the surface modifications, were purchased from Aldrich. Proeedures. Flow injection analysis, with equipotential operation and a flow rate of 1.5 mL/min, was used for demonstrating the menta of the array. The operatingpotential (usually +0.45 V) was applied sequentially at the individual electrodes. Some experiments were performed by recording the complete hydrodynamic voltammograms at the individual sensors.
RESULTS AND DISCUSSION Theuseofsensors basedon chemically modifiedelectdea asarray elements appearsideal,dueto thediversityofpossible modifiers available. The partial selectivity of electrocatalytic surfaces holds a particular promise in this direction. The concept of catalytic electrode arrays is illustrated in the following aections for flow injection assays of mixtures of carbohydrates or amino acids. Different metal oxide electrocatalytic centers, known to accelerate the redox process of these compounds and hence to offer their stable detection at low, constant potentials,"* are employed in connection with a wall-jet array flow detector. The incorporation of these
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(9) Cox, J. A.; Jaworski, R.K.:Kd-
P. J. Electmamlysis 1991.3,
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(10) Wang, J.; Tsha. 2. A w l . Chem. 1990.62. 1413. (11) Xie. Y.; Huber. C.0.A w l . Chom. lwl, 63. 1714. (121 Taha. 2.; Wang. J. EIeclrwwl>sis 1991, 3, 215.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 3, FEBRUARY 1, 1993
Table I. Multivariate Prediction for Two-Component Mixtures PLS predicted cone, mM actual cone, mM sampleno. glucose lactose maltose ribose glucose lactose maltose ribose 1 2 3 4
0.200
0.800 0.800 0.200
0.800
0.206
0.782 0.817 0.205
0.200 0.798 0.200
MLR predicted conc, mM glucose lactose maltose ribose 0.210
0.202 0.797
0.800
0.200
0.760 0.838 0.208
0.185
0.800
0.218
0.796
PLS predicted conc, mM actual cone, mM MLR predicted conc, mM sample no. glycine leucine methionine threonine glycine leucine methionine threonine glycine leucine methionine threonine 1 2 3 4
0.500
1.OOO
0.505 0.500 1.OOO
1.OOO
1.OOO 0.500 0.500
1.020
0.491 0.493 1.OOO
0.983
0.971 0.488 0.527
0.499 1.OOO 1.044
Table 11. Multivariate Prediction for Three-Component Mixtures actual cone, mM PLS predicted cone, mM sample no. glycine methionine threonine glycine methionine threonine 1 2 3 4
0.250 0.500 0.750 0.750
0.750 0.750 0.250 0.500
0.500 0.250 0.500 0.250
0.265 0.472 0.743 0.774
e.g. between sensors 2 and 3 in Figure 5. The relative PLS predictive error ranges from 0 to 11 % ,with an average value of 2.3 % as compared to an average error of 2.9 % for the MLR prediction. Similar improvements in the prediction ability of the PLS method (due to reduced errors created by collinearity) were reported for arrays of piezoelectric crystal sensors.2 Overall, the data of Tables I and I1 indicate that the array/chemometric approach successfully addresses the lack of separation power of the flow injection system. Further improvements in the information content can be achieved by applying different potentials at the individual channels. The resultingarray response, consistingof complete hydrodynamic voltammograms a t each modified electrode (Figure 81,offers unique qualitative information based upon the potential dependence of the catalytic response. Notice, in particular, the different shape of the voltammograms for the different amino acids a t each of the tailored surfaces. Since each of the four electrodes is operated at seven different potentials, 28 channels of data are obtained for each analyte. Further improvements could be achieved by covering the individual catalytic surfaces with different permselective films, in a manner analogous to that reported for the array of unmodified electrodes. From the results mentioned above, it can be seen that the unique coupling of tailored (catalytic) electrode surfaces, sensor arrays, flow injection operation, and a chemometric approach provides a high-power analytical tool. Such a combination can enhance the information content of amperometric sensing. Additional improvements can be achieved by increasing the number of sensor elements, through minaturization of the individual electrodes, by coupling with liquid chromatographic detection, or via the use of advanced chemometric approaches. While the concept is presented here within the framework of metal oxide catalysts, other electrocatalytic moieties, e.g. phthalocyanines or porphyrins, could be similarly employed (provided that they can offer a stable response). In addition to enhanced chemical infor-
0.742 0.756 0.256 0.499
0.3
0.996 0.501 0.417
MLR predicted conc, mM methionine threonine
glycine
0.487 0.278 0.503 0.226
L
0.911
0.275 0.471 0.746 0.771
I
0.736 0.756 0.254 0.503
0.478 0.275 0.503 0.225
/ m
0.5
POTENTIALN Figwo 8. Hydrodynamlc voltammograms for 1 X
M methlonine
M, threonine (01, and glycine (A)at IndMduai channels: Coo (A), CunO (B), Ru02 (C), and NIO (D) carbon paste electrodes. Other condltbns are as in Figure 3.
mation, such sensor array design can greatly benefit the field of tailored electrodes, as it would allow reliable comparison of different modifiers under identical conditions. There is no doubt that, due to the diversity of possible surface modifiers, the field of chemicallymodified electrodescan lead to the development of new and unique sensor arrays.
ACKNOWLEDGMENT This work was supported by the Lawrence Livermore NL (Subcontract B160617).
RECEIVEDfor review September 24, 1992. Accepted November 6,1992.
