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Although the two monosulfonates were not resolved with the 2% DEAE-dextran solution, as shown in Figure 3B, they were easily separated with the 0.3% PDDAC solution, as shown in Figure 3A. Use of the 0.3% PDDAC solution also permitted the separation of the five naphthalenedisulfonates shown in Figure 3B, but migration times were too long compared with those obtained with DEAE-dextran solutions. The reversed migration order of the monosulfonates and disulfonates between Figures 2 and 3B indicates that the disulfonates interact with the polymer cation more strongly than the monosulfonates. It is reasonable that dianions tend to form more stable ion pairs with polymer cations than monoanions, from the viewpoint of electrostatic interaction, but it seems difficult to reasonably explain the relative stability of ion pairs between the isomeric ions and with the polymer ions. The relative migration orders shown in Figure 3 suggests that the sulfonate group at the 1position of the naphthalene structure binds to the polymer cation more strongly than that a t the 2 position, because the stronger ion-pair formation causes a slower migration velocity. In conclusion, in order to increase selectivity in HPCE, electrophoretic mobilities can be manipulated through the ion-pair formation reaction of analyte ions with polymer ions. We have tried to modify mobilities in a similar manner with ionic micelles, such as dodecyltrimethylammonium chloride, instead of polymer ions, but this was not very successful. A long chain configuration of the polymer ion is probably more effective to discriminate isomers than the spherical and dynamic structure of the micelle. It may be possible to extend
this technique to the separation of polymer anions, such as oligonucleotides.
ACKNOWLEDGMENT We thank Professor S. H j e r t h for his suggestion of using DEAE-dextran for the technique described.
LITERATURE CITED Gordon, M. J.; Huang, X.; Pentoney, S. L., Jr.; Zare, R. N. Science 1988, 242, 224-220. Wailingford, R. A.; Ewing, A. G. A&. Chromtogr. 198% 3 0 , 1-76. Mosher, R. A.; Dewey, D.; Thormann, W.; Saviile, D. A.; Bier, M. Anal. Chem. 1989, 61, 362-366. Terabe, S.; Yashima, T.; Tanaka, N.; Araki, M. Anal. Chem. 1988, 60, 1673-1677. FuJiwara, S.; Honda, S. Anal. Chem. 1987, 59, 467-490. Gassmann. E.; Kuo, J. E.; Zare, R. N. Science 1985, 230. 813-814. Terabe, S . TrAC, Trends Anal. Chem. 1989, 8 , 129-134. Terabe, S.; Otsuka, K.; Ichikawa, K.; Tsuchiya, A,; Ando, T. Anal. Chem. 1984. 56, 111-113. Terabe, S.: Otsuka, K.; Ando, T. Anal. Chem. 1985, 57, 834-841. Terabe, S.; Ozaki, H.: Otsuka, K.; Ando, T. J . Cbromafogr. 1985, 332, 211-217.
Shigeru Terabe* Tsuguhide Isemura Department of Industrial Chemistry Faculty of Engineering Kyoto University Sakyo-ku, Kyoto 606, Japan RECEIVED for review November 3,1989. Accepted December 21, 1989. This work is partially supported by grants from Yokogawa Electric Corp. and Shimadzu Corp.
TECHNICAL NOTES Preparative Method for Fabricating a Microelectrode Ensemble: Electrochemical Response of Microporous Aluminum Anodic OxSde Film Modified Gold Electrode Kohei Uosaki,* Kentaro Okazaki, and Hideaki Kita Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan Hideaki Takahashi Analytical Chemistry Laboratory, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan INTRODUCTION Recently, much attention has been paid to microelectrode arrays (ensembles) because of their various advantages in many applications, including high signal to noise (S/N) ratio compared to conventional electrodes with the same electrode area (1). Various approaches have been taken to prepare microelectrode arrays. Hepel and Osteryoung constructed microelectrode arrays that are composed of disks of 0.375-pm radius, employing electron beam lithography (2). Martin and colleagues used microporous polycarbonate membranes (Nuclepore) as hosts and Pt ( r = 0.1-0.5 pm) or carbon ( r = 8 or 12 pm) as an electrode material (3, 4 ) . Morita and Shimizu prepared a microelectrode array employing platinized carbon fiber (5-7 wm) as an electrode and epoxy resin as a substrate (5). The smallest radius of an individual electrode reported is 0.1 wm. It is known that the smaller the active electrode area, the higher the S / N ratio (6). 0003-2700/90/0362-0652$02.50/0
In this paper, we propose a novel method for the preparation of a microelectrode ensemble with smallest electrode size reported by using micropores of aluminum anodic film as templates. Such oxide films, formed on A1 in acidic media, are known to possess micropores of 10-200 nm diameter normal to the surface with a barrier layer between the anodic oxide film and A1 substrate (7,8). Recently, Majda and his colleagues reported the electrochemical behavior of electroactive species confined within the micropores of the aluminum anodic oxide film (9-14). Tierney and Martin deposited “transparent” Au microcylinden within the micropores of the anodic oxide film (15). We have thought that these pores can be used as templates for microelectrode disks. Thus, after the anodic oxide film was removed from the Al substrate, gold was vacuum evaporated into the micropores of the oxide film, and the barrier layer was gradually removed. At a certain level of barrier layer removal, voltammograms of sigmoidal shape having a current independent of scan rate were observed, confirming the formation of an ensemble of microe0 1990 American Chemical Society
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Eleitropolishing
Anodic oxidation
a
Fbun 1. Schematic dingam of Um preparation of an aluminum anodic 0%- film nwdlfied Au electrode. lectrcde disks. The effecta of pore size, level of barrier layer removal, and gold deposition method on electrochemical properties are also discussed.
EXPERIMENTAL SECTION Materials. AI foil (0.3mm thick, 99.99%) was obtained from Nippon Chemical Condenser Co.,Ltd. Au wire (0.5 mm diameter, 99.99%) used for vacuum deposition was obtained from Tanaka Noble Metals Co., Ltd. All chemicals were reagent grade (Wako Pure Chemicals Ind., Ltd.) and were used as received. Preparation of Microporous Aluminum Anodic Oxide Film (7-14). Figure 1 shows the procedure for the preparation of the microporous aluminum anodic oxide thin film modified gold electrode. An aluminum plate (4 cm X 1.5 em) was electropolished in a mixed acid (780 mL of acetic acid and 220 mL with methanol. The of 60% perchloric acid) after being de& electropolishing was carried out with a hvc+electmdeconfiguration with aluminum as a counter electrode. The bias voltage was 30 V. and the temperature of the solution, which was continuously stirred, was kept at 10 OC. The electropolished aluminum plate was dried and stared in a desiccator after being washed by water. Anodic oxidation of the electropolished aluminum was carried out in 4% H,PO4 solution a t 65 V and 20 "C or in 10% H,S04 solution at 15 V and 5 OC by also employing a two-electrode configuration with aluminum as a counter electrode. The anodic oxidation time was 45 min, and the thickness of the anodic oxide films was 4 pm in both cases. For electropolishing and anodic oxidation of Al, a regulated de power supply (Takasago, Ltd., GP0110-3) and an electrometer (Takeda Riken Ltd., TR-8651) were used to control the bias voltage and monitor the current passed, respectively. The anodic oxide film was separated from the aluminum substrate by immersing the anodized AI in a saturated HgCll aqueous solution. The anodic oxide films thus obtained were washed with water and dried in air. The pore size of the oxide film was determined by SEM (Hitachi Ltd., S2100A) and TEM (Hitachi Ltd., H-700H). Vacuum Deposition of Au. To form an electrode, Au was vacuum deposited onto the open pore side of the film (see Figure 1). The deposition was carried out hy using a vacuum evaporation apparatus (Shimadzu Ltd., EA-400S) at a pressure of 5 X lo4 mmHg with two deposition angles, 30" and 90". against the surface. Usually the thickness of the Au was ea. 100 nm.
Figure 2. SEM image of a microporous anodic oxae film of Ai. obtained in 4 % HIPO, Solution at 65 V and 20 OC.
Figure 3 tamed in
TEM mage 01 a microporous ancaic oxide I Im 01 A , o b 10'0 H,SO, solution at 15 V and 5 OC
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 6, MARCH 15, 1990
Scan L
-200
"
I
10,20,30,4 0,5 0 m V s-'
rote 5OmVs-I 1
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I
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I
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0 E / m V vs. SSCE
I
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50 L
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-200
I
j
t
I
mVs-' l
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Flgwe 4. Cyclic voltammograms of an aluminum anodic oxide film (pore diameter, 15 nm) modified Au electrode, which was prepared by Au deposition with a deposition angle of 90°, in 50 mM Fe(CN),& aqueous solution at different degrees of dissolution of the barrier layer: (a) before dissolution, (b) after 2 min, and (c) after 10 min immersion in 10 mM NaOH.
Electrochemical Procedure. The gold-evaporatedoxide film was placed on a glassy carbon (GC) disk, which was used as a current collector, with the gold facing the GC. A copper wire was attached to the other side of the GC and the microporous oxide film/Au/GC/Cu assembly was placed in a Teflon electrode holder. A potentiostat (Hokuto Denko Co., Ltd., HA-301) was used to control the potential of the working electrode with respect to a sodium-saturated calomel electrode (SSCE) in electrochemical measurements. A Pt foil was used as a counter electrode. Extemal potential was provided by a function generator (Hokuto Denko Co., Ltd., HB-105). A three-electrode, three-compartment cell was employed for electrochemical measurements. Current-potential relations were recorded on an X-Y-t recorder (Rika Denki CO.,Ltd., RW-11T).
'b' 1
RESULTS The oxide film obtained in 4% H3P04solution at 65 V and 20 "C has pores of 100 nm diameter; that obtained in 10% H#04 solution at 15 V and 5 "C has pores of 15 nm diameter, as shown in Figure 2 and Figure 3, respectively. These pore sizes are in good agreement with the values that one of us reported before (7, 8). Figure 4 shows the cyclic voltammograms (CV) of a microporous oxide film (pore diameter, 15 nm) modified Au electrode, which was prepared by Au deposition with a deposition angle of go", in 50 mM Fe(CNI6" aqueous solution after different degrees of dissolution of the barrier layer. Before the dissolution of the barrier layer, the electrode showed no response to Fe(CN),*- as shown in Figure 4a. The electrochemical response appeared after the electrode was immersed in 10 mM NaOH solution for 2 min as shown in Figure 4b. In this case, the shape of the CV is sigmoidal and does not depend on the sweep rate. This response is similar to that observed a t a microelectrode (16,17). The limiting current is, however, very large compared with the value at a typical microelectrode (approximately nanoamperes). The sigmoidal shape gradually changed to peak-shaped CVs as the dissolution of the barrier layer proceeded, as shown in Figure 4c. The microporous oxide film modified Au electrode with pore diameter of 100 nm showed a CV very similar to that of Figure 4c, a t an even very early stage of barrier layer dissolution, as shown in Figure 5. Thus, the sigmoidal curve was obtained only at a low sweep rate and peak-shaped curves were observed at higher sweep rates. Figure 6 shows the CV of the microporous oxide film modified Au electrode with a pore diameter of 100 nm, which was prepared with a deposition angle of 30°,measured in 50 mM Fe(CN)64-solution. Figure 6a shows that the electrode did not respond to Fe(CN)64-before the barrier layer was dissolved, as was the case shown in Figure 4a. Figure 6b is the CV observed in the same solution of Figure 6a after the
-200
0
I
I
1
'
1
0 E / m V vs. SSCE
4
500
Flgure 5. Cyclic voltammograms of an aluminum anodic oxide film (pore diameter, 100 nm) modified Au electrode, which was prepared by Au deposition with a deposition angle of 90°, measured in 50 mM F~(CNL&aqueous sdution at various sweep rates. CVS were recorded after 1 min immersion in 10 mM NaOH.
electrode was immersed in 10 mM NaOH solution for 1min. Symmetric waves with a peak separation of 30 mV, showing exhaustive redox behavior, were observed. Figure 7a shows the scan rate dependence of the peak current at the electrode whose CV is shown in Figure 6b. The peak current is proportional to scan rate and it is confirmed that the electrochemical response of this electrode is like that of a thin layer cell (18). Figure 6c shows the CV of the electrode after 4-min immersion in a 10 mM NaOH solution, measured in 50 mM Fe(CN):- solution. The shape of the CV is now very close to that expected of the electrode for a linear diffusion-controlled situation. Figure 7b shows the scan rate dependence of the peak current at the electrode whose CV is presented in Figure 6c. The peak current is proportional to the square root of the scan rate.
DISCUSSION The electrochemical behavior during the barrier layer removal of the electrode on which Au is deposited a t an angle of 90" can be explained by a model schematically shown in Figure 8. In this case, Au is expected to enter deep into the pores and is deposited on their inner walls, forming columns of Au. At the early stage of dissolution when the sigmoidal curve is obtained, only a part of Au in the pores seems exposed to the solution. It is also likely that only a small fraction of the pores becomes open; otherwise, the separation between the individual pores is too small to give the sigmoidal curve. Since the exposed area of each Au column is very small, microelectrode behavior is expected, but because the number of the microelectrodes is large, the current observed is quite significant. It must be emphasized here that although various
ANALYTICAL CHEMISTRY, VOL. 62, NO. 6, MARCH 15. 1990 655
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Figun 6. Cyclic voitammogams of aluminum anodic oxide film ( w e diameter, 100 nm) mcdiRed Au electmde. which was prepared by AU dq" with deposition angle of 30'. in 50 mM Fe(CN)6G aqueous solution at different degrees of dissolution of the barrier layer: (a) before dissolution, (b) after 1 min, and (c) after 4 min immersion in 10 mM NaOH.
0
v / mVs-'
FWI* 7. (a)Scan rate dependence of the peak c u r " of Figure 6b. (b) Scan rate dependence of the peak current of Figure 6c.
,>
Flgure 8. Barrier layer removal process at the oxkie film (pore diam eter, 15 nm) modified Au electrode. which was prepared by Au depostion with a deposition angle of 90': (a) before dissolution, (b) at an early stage of dissolution at which a sigmoidal CV is observed. and (c) at a much later stage.
attempts have been made to obtain microelectrode arrays (ensembles),the perfect sigmoidal curves demonstrated in this paper have not been reported. One reason for such a perfect response in this case may he the very small active area of each Au electrode disk. Since the separation between micropores is relatively smal! compared to the diameter of the micropores, radial diffusion is not expected if the barrier layer is totally removed. Thus, the sigmoidal CVs can be observed only a t early stages of dissolution of the harrier layer and should change to peak-shaped CVs when dissolution proceeds, as experimentally observed. The results in Figure 4c show the
AlrOi
n A" Figure 9. Barrier layer removal process at the oxkie film (pore diam eter, 100 nm) modified Au electrode. which was prepared by AU deposition with a deposition angle of 30': (a) before dissolution, (b) at an early stage at which thin-layer-cell behavior IS observed. and (c) at a much later stage
transition from radial diffusion to linear diffusion. At the electrodes with larger pore diameters, it seems to be much more difficult to observe the perfect sigmoidal CV due to a much higher electrode-to-insulator ratio (Figure 3). The process of the barrier layer removal from the oxide film on which Au is deposited with a deposition angle of 30' is schematically shown in Figure 9. Before the dissolution of the barrier layer, the electrode cannot respond to Fe(CN)$because the Fe(CN)6P is blocked by the barrier layer (Figure 9a). If the small pore is opened by the barrier layer dissolution, Fe(CN)64-can enter into the micropores, reach the electrode surface, and give an electrochemical response. Since the transport of Fe(CN)k into the miocropore is still limited by the barrier layer at this stage, Fe(CN)64-in the micropore is exhaustively consumed in the oxidation process. Thus, the electrode shows the thin layer cell behavior (Figure9b). When the barrier layer is totally dissolved, the limitation for transport of Fe(CN)$'- is also removed and the response becomes the typical response of a Au macroelectrode, i.e., linear-diffusion-controlled, reversible behavior (Figure 9c). A more quantitative analysis of the electrochemical responses of these electrodes with different degrees of barrier layer dissolution is under way.
CONCLUSION A novel method for the preparation of an ultramicroelecM e ensemble is demonstrated. At the initial stage of barrier layer removal following the vacuum evaporation of Au into micropores of aluminum anodic oxide films, sigmoidal CVs, which are expected for the microdisk electrode, were observed. When Au was deposited with a deposition angle of 30°, to avoid filling the micropores with Au, the CV of a thin layer cell showing exhaustive redox behavior with a peak separation
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 6, MARCH 15, 1990
of 30 mV and symmetric waves, was observed.
ACKNOWLEDGMENT Thanks are due to Mr. Ueda for the TEM measurement and to Professor A. B. Ellis for the critical reading of the manuscript. LITERATURE CITED (1) (2) (3) (4) (5) (6)
Weber, S. G. Anal. Chem. 1989, 67,295, and references therein. Hepel, T.; Osteryoung, J. J. Electrochem. Soc. 1988, 133,752. Penner, M. R.; Martin, C. R . Anal. Chem. 1987, 59,2625. Cheng, I.F.; Martin, C. R. Anal. Chem. 1988, 6 0 , 2163. Morita, K.; Shimizu. Y. Anal. Chem. 1989, 6 1 , 159. Cassldy, J.; Sarfaraz, F.; Smith, J. J.; Pons, S. Nectrochim. Acta 1988, 37,629. (7) Ebihara, K.; Takahashi, H.; Nagayama, M. Kinzoku Hyomen Gizyufsu 1982, 33, 156. (8) Ebihara, K.; Takahashi, H.; Nagayama, M. Kinzoku Hyomen Gizyufsu 1983, 34,548.
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
Miiier, C. J.; Majda, M. J. Am. Chem. Soc. 1985, 707, 1419. Miller, C. J.; Majda, M. J. Nectfoanal. Chem. 1988, 207, 49. Miller, C. J.; Majda, M. J. Am. Chem. Soc. 1988, 108, 31 18. Miller, C. J.; Widrig, C. A,; Charych, D. H.; Majda, M. J. fhys. Chem. 1988, 92, 1928. Gass, C. A.; Miller, C. J.; Majda, M. J. Phys. Chem. 1988, 92, 1937. Miller, C. J.; Majda. M. Anal. Chem. 1988, 6 0 , 1168. Tierney, M. J.: Martin, C. R. J. Phys. Chem. 1989, 93,2878. Wightman, R. M. Anal. Chem. 1981, 53, 1125A. Pons, S.;Fieischmann, M. Anal. Chem. 1987, 59, 1391A. Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley 8 Sons Inc.: New York, 1980.
RECEIVED for review June 5,1989. Accepted December 6,1989. This work was partially supported by the Grant-in-Aid on Priority-Area Research on “Energy Conversion and Utilization with High Efficiency”, Ministry of Education, Science and Culture, Japan (No. 63603502, 01603502).
CORRECTION Manual Headspace Method To Analyze for the Volatile Aromatics of Gasoline in Groundwater and Soil Samples Valerie D. Roe, Michael J. Lacy, James D. Stuart, and Gary A. Robbins (Anal. Chem. 1989, 61, 2584-2585). Equation 2 should read
