Anal. Chem. 1888, 60, 1639-1642
experiment by a factor of 200 such that it should now be possible to determine 5 X low5pg/mL Ca in the presence of lo3 pg/mL Rb. Figure 4 illustrates LEI calibration curves for aqueous solutions of Ca ranging in concentration from 3 x IO4 pg/mL to 10 pg/mL. Curve 1 depicts the experimental observations in the case of samples containing only Ca. Curves 2 and 3 compare the results of conventional (2) and differential (3) two-color LEI measurements for the same concentrations of Ca in the presence of lo3 pg/mL Rb. Inspection of these calibration curves amply demonstrates the superiority of the differential LEI technique.
5t
1-X 2- e 3-A
-3
-4
-2
-1
1639
ACKNOWLEDGMENT We are extremely grateful to Robert J. Miller for valuable help during the preparation of this manuscript. Registry No. Ca, 7440-70-2; Rb, 744-17-2. 1
0
LITERATURE CITED Figure 4. Calibration curves for the determination of Ca in aqueous sokrtion: 1, conventional LEI signal detection (R3 = 20 kQ); 2, differntial scheme for the determination of Ca in the presence of io3 pglmL Rb. The flame volume near one of the detection electrodes was irradiated by the lasers (A, k2);3, differential scheme for the determination of Ca in the presence of lo3 pg/mL Rb, as shown In Figure 1 (the laser beam at A, was directed to the first detection electrode and the laser beams at A, and A, were directed to the second one).
+
the two laser beams and their respective detection electrodes was the same. Also the distance between each laser beam and the high-voltage electrode was the same. In the differential LEI experiment, one of the two interaction regions (see Figure 1) can be irradiated by a second laser beam tuned to resonance with the Ca 4p lPo 6d lDz transition at 468.5 nm. Rb has no transition in this wavelength region. The nonselective ionization signals resulting from 422.67-nm excitation at both interaction regions cancel in the transformer leaving only a Ca signal resulting from both 422.67- and 468.5-nm excitation. The extent of cancellation depends on symmetrical positioning of the interaction regions relative to their respective detection cathodes and on their being equal 422.67-nm power densities a t both interaction regions. The scheme improves the selectivity of the LEI
-
(1) Green, R. B.; Keller, R. A.; Schenck, P. K.; Travis, J. C.; Luther, G. C. J . Am. Chem. SOC. 1978, 98, 8517-8516. (2) Turk, G. C.; Travis, J. C.; De Voe, J. R.; O'Haver, T. C. Anal. Chem. 1978, 50, 817-820. (3) Turk, G. C.; Travis, J. C.; De Voe, J. R.; O'Haver, T. C. Anal. Chem. 1979, 5 1 , 1890-1896. (4) Gonchakov, A. S.; Zorov, N. B.; Kuzyakov, Yu. Ya.; Matveev, 0.1. Anal. Lett. 1979, 12, 1037-1048. (5) Travls, J. C.; Turk, G. C.; Green, R. B. Anal. Chem. 1982, 5 4 , 1008A10.11A. ... (6) Zorov, N. 8.; Kuzyakov, Yu. Ya.; Matveev, 0.I. Zh. Anal. Khim. 1982. 37. 520-523. (7) Travis, J.C.; Turk, G. C.;De Voe, J. R.; Schenck, P. K.; Van Dijk, C. A. Prog. Anal. A t . Spectrosc. 1984, 7 , 199-241. (8) Axner, 0.; Magnusson, I. Phys. Scr. 1985, 3 1 , 587-591. (9) Chaplygin, V. I.; Kuzyakov, Yu. Ya.; Novodvorsky, 0.A,; Zorov, N. B. Talanta 1987, 3 4 , 191-196. (10) Axner, 0.; Magnusson, I.; Petersson, J.; Sjostrom, S. Appl. Spectrosc. 1987, 41, 19-26. (1 1) Axner, 0.; Berglind, T.; Heully, J. L.; Lindgren, I.; Rubinsztein-Dunlop, H. J . Appl. PhyS. 1984, 55, 3215-3225. (12) Turk, G. C.; De Voe, J. R.; Travis, J. C. Anal. Chern. 1982, 5 4 , 643-648. (13) Magnusson, T.; Axner, 0.; Rubinsztein-Dunlop, H. Phys. Scr. 1988, 33, 429-433. (14) Turk, G. C.; Ruegg, F. C.; Travis, J. C.; De Voe, J. R. Appl. Spectrosc. 1988, 40, 1146-1152. (15) Chaplygin, V. I.; Zorov, N. B.; Kuzyakov, Yu. Ya. Talanta 1983, 30,
505-508.
RECEIVED for review November 30, 1987. Accepted March 3, 1988.
Simple Technique for Constructing Thin-Layer Electrochemical Cells Chaojiong Zhang and Su-Moon Park*
Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 Thin-layer electrochemical cells (TLECs) have been used for rapid bulk electrolysis as well as for other purposes (1,2). The cell thickness typically ranges between 2 and 300 pm, smaller than the semiinfiiitive electrochemical diffusion layer thickness (2Dt)lI2,for a given experimental time. The theory for the electrochemical behavior in thin-layer cells is well established (1, 2). An optically transparent thin-layer electrode (OTI'LE) has an added feature in its ability to take spectra of electrogenerated species (3). OTl'LEs are constructed by using the gold minigrid, which is sandwiched between two microscope slides with counter and reference electrodes usually located outside 0003-2700/88/0360-1639$01.50/0
the sandwich ( 4 ) . These gold minigrids are partially transparent ( 6 0 4 0 % ) depending on the number of wires woven per inch. Other transparent electrodes used for assembling thin-layer cells include sputtered or evaporated platinum thin film (5, 6) and tin oxide doped with antimony oxide (7), all coated glass plates. Thin-layer electrochemical cells using these transparent electrodes are not always straightforwardly assembled, and they need to be reassembled after each experiment. In our current communication, we wish to report a very simple method of constructing TLECs employing a disk electrode and a piece of a glass pate. This cell can be used for elec0 1988 American Chemical Society
1640
ANALYTICAL CHEMISTRY, VOL. 60, NO. 15, AUGUST 1, 1988
rived by realizing the relation
Q = l t i d t = nFS
AR 2*1000.€R
Here F is the Faraday constant, S is the area in square centimeters, and eR is molar absorptivity in liters per mole per centimeter. The factor 2 is used, since the probing beam travels through the absorbing medium twice. For a reversible electron transfer, the cyclic voltammetric (CV) peak in the TLEC has the form (1,2, 9) ZP
Ljl b
a
Flgure 1. Thin-layer cell used for both electrochemical and spectroelectrochemical studies: (@ bulk electrolysis cell and (b) thin-layer cell. Key: 1, llght from monochromator; 2, optlcal fiber probe: 3, utility port for purging, filling, and dralning solutions: 4, Teflon stopper; 5, reflected light to PMT; 6, reference electrode: 7 and 9, Teflon stopper: 8, working electrode: 10, fine fritted glass; 11, counter electrode: 12, optical fiber bundle: 13, quartz tubing: 14, quartz plate: 15, reference electrode tip: 16, luggin capillary; 17, heat-shrinkable Teflon tubing: and 18, working electrode.
trochemical as well as spectroelectrochemical measurements. The cell is the thin-layer analogue of the nv-normal incidence reflectance spectroelectrochemical (NNIRS) cell (8). EXPERIMENTAL SECTION The chemicals, which were all of reagent grade, were used as received. Doubly distilled deionized water was used for preparing the solutions. The electrochemical and spectroelectrochemical experiments were performed according to procedures previously described (8). The TLEC was assembled on a platinum disk electrode (Sargent-Welch S-30101-20,diameter = 6.5 mm), polished to a mirror finish down to 0.3 pm with alumina slurries (Fischer) (Figure 1). A short sleeve was attached, protruding slightly above (-2 mm) the plane of the disk electrode. This sleeve keeps the quartz plate from slipping as well as minimizing solution convection. The solution confined between the quartz plate and the platinum disk electrode behaves like that inside the TLEC. With the exception of these modifications, the whole cell is identical with the one used in the NNIRS experiments (8). The cells can be easily deaerated if necessary by bubbling solvent-saturated nitrogen or argon gas. The glass plate can then be placed on or removed from the disk electrode with the help of a small pair of forceps. The reference and counter electrodes were located such that a minimum amount of iR drop would be experienced. RESULTS AND DISCUSSION Before the results are presented, equations relating electrochemical signals with those in optical units for TLEC measurements are described. Consider a reversible electrochemical reaction involving oxidized (Ox)and reduced (Red) species:
Ox + ne- =, Red
(1) The absorbance of the electrogenerated Red, AR, can be de-
- n2FLvVCo* 2RT
(3)
where v is the scan rate in volts per second, Vis the volume in cubic centimeters, Co* is the bulk concentration of Ox in moles per cubic centimeter, and other symbols have their usual meanings. Substituting (3) into (2), followed by a differentiation with respect to t, the expression for derivative cyclic voltabsorptometry (DCVA) is
(g)p~Fn e R U E C o * 38.92n€RvlCo* =
=
(4)
Here 1 is the thickness of the cell. Note here that Co* has now a unit of moles per liter. A similar equation can be derived for irreversible electron transfer by using a proper current expression (9). DCVA was first described by Bancroft et al. (10,11), in which study the authors used the dAldE vs E plot, whereas we use the dAldt vs E plot. By using the dA/dt vs E plot, we have the same response functions as expected in current (see below, Figure 4). When both the CV and the corresponding DCVA curves are obtained, eq 3 and 4 may be combined, yielding nFS(dA/dt), CR
=
2000ip
(5)
Thus the determination of C Rcan be made if the electrode area S is known. With C Ron hand, the thickness of the TLEC can be determined from eq 4. All these methods of data analysis are self-contained. The only information necessary is the electrode area and the n value. The performance of the TLEC assembly employing the NNIRS system was evaluated with the methyl viologen (l,l'-dimethyl-4,4'-bipyridiniumchloride) redox system. The methyl viologen dication (MV2+)is known to undergo a reversible reduction to its radical cation, MV+', which can be further reduced to the neutral form, MVo (12). This redox system has been extensively studied, both electrochemically and spectroelectrochemically (12). An advantage of this redox system for spectroelectrochemical studies is that the oxidized form, i.e., dication (MV2+),does not show a significant absorption at wavelengths greater than 300 nm, but its reduced form, MV+', exhibits broad absorption bands between 300 and 800 nm. The MV" is also known to undergo a dimerization reaction; the absorbance spectrum of the dimer in the UV-vis region is different from that of the monomer (13, 14). We chose to use a rather concentrated solution of MVZ+to see the effect of the dimerization reaction. Shown in Figure 2 are two in situ spectra of electrogenerated MV+' recorded under different electrolysis conditions. The differences shown indicate that the reaction products in a completely electrolyzed solution in the TLEC and the partially electrolyzed solution in a normal cell are different. The spectrum in Figure 2b, recorded in the TLEC, shows that the reduction product is predominantly the dimer (13). However, when the spectrum is recorded from a bulk solution (Figure 2a), bands corresponding to both the monomer and the dimer are seen. The spectrum taken in the bulk solution is in ex-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 15, AUGUST 1, 1988
a Q,
0.6
1641
E-5
l
25
a 2 0,
0
E
rn
n L
4
0.4
E 15
:.
0
E
v)
s
0.2
(0
10
51
5.'
E
I,.,.''
-3
0.3
5
18
15
28
5
16
15
20
._
I
.
I : :
s
i
1 ,.:*
wavelength, nm
E-3
Flgure 2. I n situ spectra of electrogenerated MV+ at an applied potential of -0.60 V vs Ag/AgCI from (a)the bulk solution and (b) the
thin-layer cell. The bulk concentration of MV2+ was 20 mM In 3.0 M KCI.
scan rate, Vis
Flgure 4. Dependencies of (a) the CV peak current and (b) the peak
DCVA signal on the voltage scan rate.
--E
time, s
-0.5
h
-1.0
Li
3
0
-1.5
0.25-
Flgure 3. Cyclic voltammogram (a) and DCVA (b) recorded at scan rate of 1 mV/s from the same solution as in Figure 2, employing the thin-layer cell.
cellent agreement with ones reported in the literature (8,15). In the bulk solution the electrogenerated MV+' diffuses out to the bulk of the solution, maintaining a low concentration of MV+' at the diffusion front. As a result, absorption bands corresponding to the monomeric form are observed at 396 and 606 nm. In the TLEC, approximately 70% of the MV+' generated is expected to be in the dimeric form when calculated from the dimerization constant of 377 (13),assuming that the solution has been electrolyzed completely. Figure 3 shows a CV(a) and a DCVA(b) recorded from the TLEC. The two curves show characteristics of those signals obtained for a reversible electron transfer in a TLEC (1,2, 9). Further, the CV peak current and the DCVA peak intensity are directly proportional to the scan rate, as shown in parts a and b of Figure 4, respectively. From the comparison of the CV peak current (Figure 3a) and the (dA/dt) (Figure 3b), we obtain the molar absorptivity of 2.59 X 10E
z2
0
-
b
'..
I
0 -0.25 -0.25
......
_:.'
1
....
2
I
I
I
3
4
5
.
.
time, s
Flgure 5. Chronoamperogram (a) and derivative absorptometry (b) recorded from the thin-layer cell. Other experimental condltions were the same as in Figure 2.
L.mol-l.crn-' at 545 nm, employing eq 5. Using this as tR in eq 4, we calculate the cell thickness to be 29.3 wm. The cell thickness was reproducible within an experimental error of about 5% when the same piece of the quartz cover was used. This thickness (29.3 hm) determined from eq 4 compares well with 24.8 pm, which was obtained from the slope of the (dA/dt), vs Y plot (Figure 4b). . The latter is probably more reliable, since it is obtained from five measurements. We believe that the cell thickness can be controlled by changing the weight of the quartz cover; we do not have any quantitative results to support this supposition. For experiments as long
1842
Anal. Chem. 1988, 6 0 , 1642-1645
as about 10 min, we did not experience any effect due to the diffusion through the edge. Finally, it is interesting to observe spectroelectrochemically that the TLEC experiences a large solution resistance voltage drop. This is illustrated in Figure 5, where the potentiostatic current (a) and its corresponding dA/dt plot are shown. While the current follows the pattern characteristic of TLEC measurements, the dA/dt curve shows a maximum rate of MV" production at about 1s. This is because the optical fiber probe is smaller than the electrode; thus the effect of the iR drop is shown in the form of the time delay. In the case of the current, the total current is recorded, while the dA/dt curve records the response shown in the center of the electrode. This observation indicates that spatial mapping is possible, provided the optical probe is small enough. In conclusion, we have constructed a thin-layer electrochemical cell with which both electrochemical and spectroelectrochemical measurements can be made. It has been demonstrated that the CV currents are characteristic of those recorded from a typical thin-layer cell. The cell is simple to assemble reproducibly. Also, equations relating electrochemical and optical parameters in thin-layer cells have been derived. The DCVA signal is directly proportional to the cell thickness, which allows a straightforward determination of the thickness. Registry No. Pt, 7440-06-4; quartz, 14808-60-7; methyl viologen, 1910-42-5.
LITERATURE CITED Hubbard, A. T.; Anson, F. C. I n Electroanalytical Chemistty; Bard, A. J., Ed.; Marcel Dekker: New York, 1970; Vol. 4, Chapter 2. Hubbard, A. T. CRCCrit. Rev. Anal. Chem. 1973, 2 , 201. Winograd, N.; Kuwana, T. I n Nectroanalytical Chemistry; Bard, A. J., Ed.; Marcel1 Dekker: New York, 1974; Vol. 7, Chapter 1. Heineman, W. R.; Norris, 8. J.; Goelz, J. F. Anal. Chem. 1975, 45, 79. Kobayashi, T.; Yoneyama, H.; Tamura, H. J. Nectroanal. Chem. Interfacial Electrochem. 1984. 161, 419. Kobayashi, T.; Yoneyama, H.; Tarnurla. H. J. Nectroanal. Chem. I n terfacial Electrochem. 1984, 177, 293. Szentrlmay. R.; Yeh, P.; Kuwana, T. I n Nectrochemical Studies of Biologcal Systems; Sawyer, D. T., Ed.; ACS Symposium Series 38, American Chemical Society: Washlngton, DC, 1977; Chapter 9. Pyun, C.-H.; Park, S.-M. Anal. Chem. 1986, 58, 251. Bard, A. J.; Faulkner, L. R. Nectrochemical Methods; Wlley: New York, 1980; pp 406-413. Bancroft, E. E.; Sldwell, J. S.;Blount, H. N. Anal. Chem. 1981, 5 3 , 1390. Bancroft, E. E.; Blount, H. N.; Hawkridge, R. M. I n Nectrochemical and Spectrochemical Studies of Biological Redox Components ; Kadlsh, K. M., Ed.; Advances in Chemistry 201; American Chemical Society: Washlngton, DC, 1962. Bird, C. L.; Kuhn, A. T. Chem. Sac. Rev. 1981, 1 0 , 49. Schwarz, W. M., Jr. Ph.D. Dissertation. 1961, University of Wisconsin. Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 8 6 , 5524. Watanabe, 1.;Honda, K. J. Phys. Chem. 1982, 86, 2617.
RECEIVED for review November 4,1987. Accepted March 9, 1988.
Cobalt Phthalocyanine/Cellulose Acetate Chemically Modified Electrodes for Electrochemical Detection in Flowing Streams. Multifunctional Operation Based upon the Coupling of Electrocatalysis and Permselectivity Joseph Wang,* Teresa Golden, and Ruiliang Li Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003 Flow-through amperometric detectors have proven themselves as highly sensitive means for measuring electroactive species and are the instrumentation of choice in numerous laboratories around the world (I). Particularly successful has been the combination of the resolution of a chromatographic column with electrochemical detectors ( 2 ) . Electrochemical detectors offer excellent sensitivity, wide linear range, discrimination against nonelectroactive species, low dead volume, and low cost. However, improvements in the stability, selectivity, and scope of flow detectors are highly desirable to meet new challenges posed by clinical and environmental samples. The utility of solid-electrode-based detectors is often hampered by a gradual fouling of the surface due to adsorption of large organic surfactants or of reaction products. Amperometric detection lacks the ability to discriminate between solutes possessing similar redox characteristics. Finally, the detection of many important solutes is often hindered by their slow electron-transfer kinetics at the commonly used electrode materials. One field that offers great potential for alleviating the above problems, and hence for enhancing the power of electrochemical flow detectors, is that comprising chemically modified electrodes (CMEs) ( 3 , 4 ) . Although modified electrodes have frequently been used for voltammetric measurements in batch systems, the sophistication currently available in tailoring surfaces has not been widely utilized for flow analysis.
So far, the main approach for using CMEk in flowing streams has been electrocatalysis. This scheme exploits the ability of certain surface-bound redox mediators to enhance electron-transfer kinetics and thus lower the operating potential (5-8). In particular, modified carbon paste electrodes containing added cobalt phthalocyanine (CoPC) were shown to decrease by several hundreds millivolts the potential required for electrooxidation of several irreversibly oxidizable species (5, 7). Because of its electrocatalytic capability toward a wide variety of redox systems, CoPC is expected to play an important role in future detection schemes. Permselectivity is another promising avenue for utilizing CMEs in flowing streams. The size or charge exclusion characteristics of polymeric coatings such as cellulose acetate (9), Nafion (IO), or poly(viny1pyridine) (II) have offered substantial improvements in the selectivity and stability of amperometric measurements. For example, surface fouling problems are eliminated, as the cellulose acetate (CA) forms an effective barrier for the transport of organic surfactants. We have demonstrated that the permeability of cellulose acetate coatings can be controlled and manipulated by hydrolyzing them in alkaline media for different time periods (9). In the work reported here, we describe the incorporation of the catalyst cobalt phthalocyanine into the cellulose acetate domain. At this new microstructure, the cellulose acetate
0003-2700/88/0360-1642$01.50/00 1988 American Chemical Society
