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(16) Miller, E.; Bruckenstein, S. Anal. Chem. 1974, 46, 2026-2033. (17) Mlller, E.; Bruckensteln, S. J. Necfrochem. Soc. 1974, 727, 1558-1562. (18) Tokuda, K.; Bruckenstein, S.; Miller. E. J . Necfrochem. Soc. 1975, 722, 1316-1322. (19) Bruckenstein, S.; Miller, E. Acc. Chem. Res. 1977, IO, 54-61. (20) Tokuda, K.; Bruckenstein, S. J . Necfrochem. Soc. 1979, 726, 431-436. (21) Kansaki, Y.: Bruckenstein, S. J . Necfrochem. Soc. 1979, 726, 437-441. (22) Rosamllla, J. M.; Miller, E. Anal. Chem. 1983, 5 5 , 1142-1145. (23) Miller, E.; Rosamilia, J. M. Anal. Chem. 1983, 5 5 , 1281-1285. (24) Rosamllla, J. M.; Miller, E. Anal. Chem. 1984, 56, 2410-2413. (25) Rosamilia, J. M.; Miller, E. J . Elechoanal. Chem. Inferfaciel Nectrochem. 1984, 160. 131-140. (26) Blaedel, W. J.; Boyer, S. L. Anal. Chem. 1971, 43, 1538-1540. (27) Blaedel, W. J.; Iverson, D. G. Anal. Chem. 1977, 49, 1563-1566. (28) Blaedel, W. J.; Yim, 2. Ana/. Chem. 1980, 52, 564-566. (29) Blaedel. W. J.; Wang, J. Anal. Chem. 1981, 5 3 , 78-80. (30) Wang, J.; DewaM, H. D. Anal. Chlm. Acta 1982. 736,77-84. (31) Blaedel, W. J.; Engstrom, R. C. Anal. Chem. 1978, 5 0 , 476-479.
(32) (33) (34) (35) (36) (37)
Blaedel, W. J.; Wang. J. Anal. (2”. 1980, 52, 1697-1700. Blaedel, W. J.; Wang. J. Anel. CMm. Acta 1980. 116, 315-322. Wang, J. Anal. Chem. 1981, 5 3 , 1528-1530. Wang, J.; Frelha, E. A. Analyst (London) 1983, 708, 685-690. Wang, J. Anal. Chlm. Acta 1981, 7 2 9 , 253-257. Pratt, K. W.. Jr.: Johnson, D. C. Electrochlm. Acta 1982, 2 7 , 1013-1021. (38) Schuette, S. A.; McCreery, R. L. Anal. Chem. 1986, 5 8 , 1778-1782. (39) Schuette, S. A.; McCreery, R. L. Anal. Chem. 1987, 5 9 , 2692-2699. (40) Frumkin, A. N.; Petrii, 0. A.; Damaskln, E. E. I n comprehensive Treatise of €lecfrochemlsfty; Bockrls, J. O’M., Conway, E. E., Yeager, E., Eds.: Plenum: New York, 1980; Vol. 1, Chapter 5.
RECEIVED for review November 27,1989. Accepted January 8, 1990. This work was supported in part by the Ohio
University Research Committee and Research Challenge programs. H.D.D. acknowledges support from the Grasselli-Day summer faculty research fellowship.
Constructlon of Submicrometer Voltammetric Electrodes Bradford D. Pendley and H6ctor D. Abrufia* Department of Chemistry, Baker Laboratory, Cornell University, Zthaca, New York 14853-1301
INTRODUCTION As a result of several unique advantages of microelectrodes, there has been a great deal of interest in their development for electrochemical measurements. The enhanced rate of mass transport results in a steady-state current response a t sufficiently slow sweep rates and the reduced capacitive charging current allows increased temporal resolution of electrochemical experiments. The small currents produced at microelectrodes lead to a reduction in the iR drop so that electrochemical measurements in highly resistive media can be performed. In addition, an improved faradaic to nonfaradaic current ratio enhances the signal to noise ratio. And finally, extremely small environments can be examined with microelectrodes whose total tip size is of the order of a few micrometers ( I ) . Structurally small electrodes can be used for electroanalysis in minute environments such as single cells. A variety of approaches for the construction of microelectrodes have been reported including electropolished platinum (2,3) or carbon ( 4 ) sealed in glass pipets, electropolished tungsten dipped in lacquer (5),gold-filled micropipet tips (6),carbon ring (71, carbon fiber (8), bevelled carbon fiber (9),and platinumiridium conical and hemispherical (IO)electrodes. Although all of these procedures yield electrodes with diameters of several micrometers or less, in many cases the construction process is quite difficult and the success rate low. We are interested in the fabrication of small metal disk electrodes that can be chemically modified with a multifunctional polymer and used as selective electrochemical probes for some species of interest (e.g. metal ions) in small environments (11). We require that these electrodes have a total tip diameter (i.e. electrode and insulating material) of a few micrometers or less and that they be easily and reproducibly made. We describe a procedure for the fast and reliable construction of platinum microelectrodes sealed in glass. The procedure involves pulling annealed platinum wire (75 pm) placed inside a borosilicate pipet to give microelectrodes of 1-5 pm in total tip diameter.
VP-6423s X-Y recorder. Chronocoulometrywas performed with a BAS 100 electrochemical analyzer. Electrochemical cells of conventional, three-compartment design were employed. All experiments utilized a three-electrode configuration and all potentials are referenced to the sodium-saturated calomel electrode (SSCE) without regard for the liquid junction. All experiments were performed inside a Faraday cage. Microscopy. Scanning electron micrographs were obtained by use of a JOEL JSd-35CF scanning electron microscope with an acceleratingvoltage of 25 kV. Light microscopy was done with a Nikon Optiphot Biological Microscope. Reagents. Acetonitrile (Burdick and Jackson Distilled in Glass), methylene chloride (Fisher ACS certified, spectroscopic grade), and N,N-dimethylformamide(Fisher ACS certified) were dried over 4-A molecular sieves. Tetrahydrofuran (Fisher ACS certified) was distilled from sodium/benzophenone. Tetra-nbutylammonium perchlorate (TBAP) (G. F. Smith) was recrystallized 3 times from ethyl acetate and dried under vacuum. Water was purified with a Millipore Milli-Q system. All other reagents were of at least reagent grade quality and were used without further purification. Microelectrode Fabrication. A 3-4-cm piece of annealed platinum wire (75 pm diameter) (Engelhard Industries) was cut and placed inside a 6-7-cm section of a borosilicate pipet (25 pL) (Fisher) so that about 1 mm of the wire protruded from one end. This end was then folded over the edge of the pipet to secure the wire. The pipet/wire assembly was placed through four loops of nichrome wire (18 gauge) on a pipet puller (Narishige, Type PF-2) and adjusted so that the distance between the chucks was precisely 1.9 cm. Pulling was carried out at a current of 15 A and a magnet setting of 6.7. The resulting two microelectrodes were inspected by light microscopy to determine whether the platinum wire extended to the very end of the tip and was less than 10 pm in diameter. If this was the case and no cracks were present in the glass, electrical contact to the platinum wire was established with a copper wire and silver paint. Although some microelectrodes worked at this stage, more reproducible behavior was observed when the electrode tip was manually snapped back to expose a new platinum surface. This resulted in the total outer diameter of the tip being larger than originally made, but still less than 10 pm. The electrodes were rinsed with water and acetone prior to use.
EXPERIMENTAL SECTION Electrochemistry. Cyclic and differential pulse voltammetry (DPV) were performed with an IBM EC 225 voltammetric analyzer. For DPV, the pulse amplitude and scan rate were 50 mV and 10 mV/s, respectively. Data were recorded on a Soltec
RESULTS AND DISCUSSION A scanning electron micrograph of a typical platinum microelectrode is shown in Figure l. The image is a composite of secondary and backscattered electrons to show both the tip morphology and the platinum. The micrograph shows the
0003-2700/90/0362-0782$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 62. NO. 7. APRIL 1. 1990
789
Table I. Analysis of Steady-State Voltammogramsa solvent
wave slope? mV
COrI mff
acetonitrile methylene chloride tetrahydrofuran Nfl-dimethylfomamide
60i3 61 i 2 68 3 59 i 1
0.99,
0.93, 0.99, 0.9q
@Forferrocenein 0.1 M TBAP/solvent. Wave slopes obtained from Sean rates including 50,100,and 200 mV/s and are reported as +1 standard deviation Table 11. Comparison of Measured and Predicted Limiting Currents outside diameter, electrode
-
Flgurs 1. Scanning electron micrograph of the tip of a phtinum disk microelectrode.
y< J
I
4.8
I
I
4.6
I
I
I
4.4
I
I
4.2
E vs. SSCE Flgum 2. Cyclic voltamnmgram at a scan rate of 100 mV/s of (A) 3.4 mM fenocaw, h 0.1 M TBAP/CH&b at a 4.6 #m total tip. 2.8 pm platinum disk electrode. and (E) blank.
tip surface to be relatively defect free. T h e total outer diameter of the electrode is 2.1 pm with a 0.48 #m diameter platinum disk located in the center. T h e electrode can therefore be described as a disk in a finite insulating plane. The integrity of the glass-platinum seal was investigated by using chronocoulometry. Under steady-state conditions, a time-independent chronocoulometric response was observed after application of a potential step, indicative of a good seal between the glass and the metal (12). The microelectrodes are electrochemically well behaved. Figure 2 shows a cyclic voltammogram of ferrocene in methylene chloride and a very well developed sigmoidal voltammogram is observed. Similar steady-state voltammograms were also observed over the range of 50-1000 mV/s sweep rates in acetonitrile, tetrahydrofuran, and Nfl-dimethylformamide. The results of the analysis of these steady-state voltammograms are shown in Table 1. The wave slopes were calculated from plots of E vs log (i/i, - i) and are expected to be 59 mV at 25 “C for a reversible, one-electron reaction (13). Reversible behavior was observed in all solvents except tetrahydrofuran, where the kinetics appear sluggish. Peaks in differential pulse voltammograms were well-defined and symmetric, with A &,, between 105 and 110 mV. The steady-state or limiting current for microelectrodes of a disk geometry is expected to follow the equation ( 1 )
i, = 4nFDCr
(1)
where r is the electrode radius and all other symbols have their usual meaning. Table I1 compares the predicted and measured limiting currents for five microelectrodes. The predicted current was calculated from eq 1,using D = (2.4 i 0.1) X 10” cm2/s (14). In addition, the predicted current also takes into account that the platinum disk is present in a finite insulating plane of glass, and therefore, there is an enhancement in the observed cur-
1 2 3 4 5
current, nA measureda predicted* 4.11 i 0.02 1.84 i 0.08 2.39 0.01 3.21 i 0.05 2.13 i 0.02
*
4.78 i 0.50 1.46 i 0.29 2.04 i 0.30 5.05 0.87 2.22 f 0.25
Nm
glass
platinum
6.9 5.1 4.6 4.6 9.2
2.3 2.3 3.0 2.8 4.6
Measured from steady-state currents of voltammograms offer“ne (1.3-3.5mM) in 0.1 M TBAP/acetonitrile for three scan8 at rates from 50 to loo0 mV/s. Values are reported as i1 standard deviation. “he predicted limiting current includw enhancement resulting from the platinum disk not being embedded in an infinite insulating plane (15). The uncertainty in the value reflects uncertainties in the measurement of the radius and in the value of the diffusion wfficient. rent. Shoup and Szabo have used an explicit hopscotch algorithm to simulate the limiting current at a disk-shaped electrode that is not imbedded in an infinite insulating plane (15). Measurements of both the metal electrode and the outer glass diameters were made by using light microscopy. This allowed for correction of the predicted limiting current using the results of Shoup and Szabo. There is a large uncertainty in the predicted limiting currents listed in Table I1 and this is due to the fact that for electrodes of diameters such as presented here, the limit of resolution of conventional light microscopy is being approached. Even so, however, with the exception of electrode 4, agreement is quite good. To ensure success in electrode preparation, it is essential that the platinum wire be annealed and not hard drawn. Hard-drawn platinum wire can be used to make microelectrodes, but there is no reduction in the wire’s diameter upon pulling (16). Presumably, the increased ductility of the annealed platinum wire enables it to be drawn to very small diameters. This general idea has also been used in this laboratory to fabricate similar microelectrodes using gold wire instead of annealed platinum wire, although this has not been pursued extensively. The method of microelectrode fabrication detailed here offers three significant advantages. Firat, it is extremely quick and easy to fabricate electrodes. Second, the method allows for the construction of very small microelectrodes (typically with total outer diameters between 1 and 5 pm) with smooth taper and excellent metal to glass seal. I t should also be mentioned that platinum disk electrodes of extraordinarily small diameters can be prepared. For example, Figure 3 show a cyclic voltammogram for a platinum electrode in contact with a 48.2mM solution of ferrocene in 0.5 M TBAP/acetonitrile. From the magnitude of the limiting current, we estimate that the platinum disk has a diameter of about 27 A. It should be noted that Morris e t al. (17) have performed voltammetric measurements at platinum band electrodes with widths approaching molecular dimensions and have found that the limiting currents obtained at electrode widths less than
784
ANALYTICAL CHEMISTRY, VOL. 62,
NO. 7,
APRIL 1, 1990 Silver, A. M e d . E k t r o n . Blol. Eng. 1965, 3, 377-387. Ballintijn, C. M. Experienf/a 1961, 77, 523-528. Meulemans, A.; Poulain, B.; Baux, G.; Tauc, L.; Henzel. D. Anal. Chem. 1966, 58,2088-2091. Hubel, D. H. Science 1957, 725,549-550. Whalen, W. J.; Riley, J.; Nair, P. J . Appl. Physbl. 1967, 23(5),
A
k
P----
B
798-801,
Kim, Y.-T.; Scarnulis, D. M.; Ewing, A. G. Anal. Chem. 1966. 58,
I
I +0.80
I
1782-1786.
I I I I I I +0.60 4 . 4 0 +0.20
Ponchon, J.-L.; Cespuglio, R.; Oonon, F.; Jouvet, M.; Pujol, J.-F. Anal. Chem. 1979, 57, 1483-1488. Kelly, R. S.;Wlghtman, R. M. Anal. Chlm. Acta 1866, 187, 79-87. Penner, R. M.; Heben, M. J.; Lewis, N. S. Anal. Chem. 1969, 6 1 ,
E vs. SSCE Figure 3. (A) Cyclic voltammogram of 48., mM ferrocene in 0.5 M TBAP/acetonitrile at a 1 p m total tip, 27-A platinum disk electrode at a scan rate of 50 mV/s. (B) Blank at a scan rate 100 mV/s.
200 8, are lower than theoretically predicted. They proposed that the diffusion coefficient of the electroactive species near the electrode may be lower than that in the bulk solution. Since the bulk value of the diffusion coefficient was used to estimate the electrode radius, it is likely that the true radius is actually larger. However, we believe the estimation to be good within a factor of 2 or 3. This represents the smallest platinum disk electrode ever reported and illustrates the capability of the method presented. Third, the success to attempts ratio of constructing electrodes whose total outer diameter is less than 10 pm has been about 42%.
LITERATURE CITED (1) Wightman, R. M.; Wipf, D. 0. I n Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1989;Vol. 15, pp 267-353.
1830-1636. Hurrell, H. C.; AbruRa, H. D. Anal. Chem. 1968, 60, 254-258. Thormann, W.; Bond, A. M. J. Electroanal. Chem. InterfeclelElectrochem. 1967, 218, 187-196. Bard, A. J.; Faulkner, L. R. Electrochemlcal Methods; Wiiey: New York, 1980. Kuwana, T.; Bublitz, D. E.; Hoh, G. J. Am. Chem. Soc. 1960, 82,
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Shoup, D.; Szabo, A. J Electroanal. Chem . Interf.acle1 Electrochem. 1964, 760, 27-31. Johnson, M. W.; Manhoff, L. J., Jr. Science 1951, 173, 182-184. Morris, R. B.; Franta. D. J.; White, H. S. J. Phys. Chem. 1967, 97,
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RECEIVED for review November 10, 1989. Accepted January 3, 1990. This work was generously funded by the National Science Foundation. H.D.A. is a recipient of a Presidential Young Investigator Award (1984-1989) and an A. P. Sloan Fellow (1987-1991). B.D.P. acknowledges support as a Shell Doctoral Fellow (1989-1990).
C ORRECTI 0N Random-Walk Theory of Nonequilibrium Plate Height in Micellar Electrokinetic Capillary Chromatography Joe M. Davis (Anal. Chem. 1989, 61, 2455-2461). On page 2458, second column, line 44, the text should read as follows: Quantity t o / & for the nucleic acids was equated to 0.47, the ratio of the elution times of the void peak and the final peak.
