Anal. Chem. 1995, 67, 1491-1495
Carbon Ring-Disk Ultramicroelectrodes Gang Zhao, Dean M. Giolando,* and Jon R. Kirchhoff* Department of Chemistry, The University of Toledo, Toledo, Ohio 43606-3394
Navel ring-disk ultramicroelectrodes (RD-UMEs) with analytical tip diameters as small as 25-30 pm were fabricated. Carbon RD-UMEs were reproducibly prepared by the chemical vapor deposition of altemating concentriclayers of silica and carbon on resistivelyheated 10 pm carbon fibers. High-quality films with excellent adhesion at the interfaces between the carbon and silica layers were shown by electrochemical and scanning electron microscopy measurements. Electrochemical measurementsof a solution of 1.0 mM ferrocene with 200 mM LiC104 in CRCN were used to characterize the single- and dual-electroderesponse of the RD-UME. The electrochemical responses of the ring and the disk are sigmoidal in shape and indicated that radial diffusion is the primary mode of mass transport at each electrode at slow scan rates. Diffusion-controlledgeneration-collection experiments showed that the concentric dual-electrode configuration exhibits high collection efficiencies at the ring electrode with a 2-5 pm separation between electrodes and a 2-4 pm ring thickness. Close proximity of the ring and disk electrodes led to enhanced detection sensitivity due to back diffusion of regenerated molecules of a reversible redox couple from the collector to the generator electrode. Recent trends directed toward the miniaturization of electrodes have produced ultramicroelectrode (UME) devices.'s2 Interest in UMEs is due to the considerable improvement in the quality of the electrochemical information that is obtained as the dimensions of the electroactive surface decrease.3 As a result, UMEs have been used in applications for in vivo measurements in biological envir0nments4-~including withiin single cells,8-11 as detectors in capillary separation methods,12J3 and as electrochemical probes in scanning electrochemical micro~copy.~~ (1) He&, J. Angew. Chem., Inf. Ed. Engl. 1993,32,1268-1288. (2) Wightman, R M. Science 1988,240, 415-420. (3) Wightman, R M.; Wipf, D. 0. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984, pp 267-353. (4) Broderick, P. A Electroanalysis 1990,2, 241-251. (5) Wightman, R M.; May, L.J.; Michael, A. C. Anal. Chem. l988,60,769A779A (6) Wightman, R M. Anal. Chem. 1981,53, 1125A-1134A (7) Adams, R N. Anal. Chem. 1976.48, 1126A-1138A (8) Ewing, A G.; Strein, T. G.; Lau,Y.Y. Acc. Chem. Res. 1992, 25,440-447. (9) Abe, T.;Lau, Y.Y.; Ewing, A G. Anal. Chem. 1992, 64, 2160-2163. (10) Bailey, F.; Malinski, T.; Kiechle, F. Anal. Chem. 1991, 63, 395-398. (11) Meulemans, A; Poulain, B.; B a n , G.; Tauc, L.;Henzel, D. Anal. Chem. 1986,58,2088-2091. (12) Ewing, A G.; Memos, J. M.; Gavin, P. F. Anal. Chem. 1994, 66, 527A537A. (13) OShea, T. J.; Lunte, S. M. Anal. Chem. 1993, 65, 247-250. (14) Bard, A. J.; Denuault, G.; Lee, C.; Mandler, D.; Wipf, D. 0. Acc. Chem. Res. 1990,23, 357-363. Q 1995 American Chemical Society 0003-2700/95/0367-1491$9.00/0
The pursuit of these applications has led to advancements in the fabrication of many types of disk UMEs. The most common fabrication procedure for disk UMEs is to encase a carbon fiber or metal wire into a pulled glass capillary and to seal the tip with epo~yY.3J5J6Several other methods for encapsulation of carbon fibers or metal wires have been developed to establish reproducible fabrication procedures, to improve the regularity of the electrode geometry, and to enhance the durability and quality of the seal between the electrode and the insulating sheath. These include the electrodeposition of a thin layer of organic polymer on a fiber or wire17J8and the heat sealing of a polymer matrix about a fiber or wire.19-22 We have recently described a facile new method for the reproducible fabrication of disk UMEs by the chemical vapor deposition of silica h s onto resistively heated carbon f i b e r ~ . aThis ~ ~ technique of combining chemical vapor deposition and resistive heating (CVD-RH) produces disk UMEs with a thin layer of insulating silica concentricallydistributed about the carbon fiber. An additional advantage of this method is the excellent adhesion of the silica hto the fiber, thereby precluding the need for sealants. In this report, we have utilized the CVD-RH technique for the fabrication of a carbon ring-disk ultramicroelectrode 0 - U M E ) . The RD-UME was prepared by the chemical vapor deposition of alternatingconcentric layers of silica and carbon on 10pm carbon fibers. Although carbon ring UMEs have been d e s ~ r i b e d , to ~~-~~ our knowledge this is the first report of the preparation and electrochemical characterization of a carbon RD-UME. Such a multiplelayered electrode is desirable in UME applications where enhanced sensitivity and selectivity of dual-electrode analysis are required. For such purposes, CVD methods are useful because they provide the ability to reproducibly design multiplelayered electrode systems for a particular UME application. Ponchen, J. L;Cespuglio, R; Gonan, F.; Jouvet, M.; Pujol, J. P. Anal. Chem. 1979,51,1483-1486. Kelly, R S.; Wightman, R M. Anal. Chim.Acta 1986, 187, 79-87. Strein, T. G.; Ewing, A G. Anal. Chem. 1992, 64, 1368-1373. Potje-Kamloth, K; Janata, J.; Josowicz, M. Ber. BunsengesPhys. Chem. 1989, 93, 1480-1485. Ramos, B. L;Blubaugh, E. A; Ridgway, T. H.; Heineman, W. R Anal. Chem. 1994, 66,1931-1935. Nyholm, L.; W h a r k , G. Anal. Chim. Acta 1992,257, 7-13. Baranski, A. S.; Quon, H. Anal. Chem. 1986,58,407-412. Golas, J.; Osteryoung, J. Anal. Chim. Acta 1986, 181, 211-218. Zhao, G.; Giolando, D. M.; Kirchhoff, J. R j . Electroanal. Chem. 1994, 379, 505-508. Zhao, G.; Giolando, D. M.; Kirchhoff, J. R, submitted for publication in Anal. Chem. Kim, Y.-T.; Scarnulis, D. M.; Ewing, A G. Anal. Chem. 1986, 58, 17821786. MacFarlane, D. R; Wong, D. K Y.1. Electroanal. Chem. 1985,185,197202. Fleischmann, M.; Bandyopadhyay, S.; Pons, S.j . Phy. Chem. 1985, 89, 5537-5541.
Analytical Chemistty, Vol. 67, No. 8, April 15, 1995 1491
EXPERIMENTAL SECTION Instrumentation. An optical microscope (Reichert, Austria)
with magnifications from 3 x 16 to 63 x 16 and a scanning electron microscope (JEOL, JSM-6100) were used for visual inspection of the coated fibers. Prior to scanning electron microscopy (SEM) and electrochemical measurements, the RDUME was embedded into an epoxy matrix and polished perpendicular to the fiber axis with 1.0, 0.3, and 0.05 pm y-AlzO3. After polishing, the exposed RD-UME surface was cleaned with a sonicator (Branson 1210) to remove the polishing compound and epoxy residues. Further, the polished surface was coated with a 200 A Au film for SEM examination. Fiber temperatures during the CVD procedure were estimated with an 8630 Series optical pyrometer (Leeds and Northrup Co.). Electrochemical measurements were conducted with a Bioanalytical Systems, Inc. (BAS) lOOB electrochemical analyzer, which was interfaced to a Gateway 386PC, a BAS PA-1 preamplifier, and a Houston Instruments DMP-40 digital plotter. The electrochemical cell consisted of a RD-UME working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl wire reference electrode arranged in a three-electrode configuration. Dualelectrode measurements were conducted with a BAS CV-27 potentiostat in addition to the BAS 100B. A laboratory-built bipotentiostat with a computer interface was also employed for the generation-collection experiments. Both experimentalsetups provided similar results. All electrochemical measurements were conducted at 21 & 1 "C in a grounded laboratory-built Faraday cage. Materials and Methods. Solutions (1.0 mM) of ferrocene (Aldrich) in CH3CN (Burdick and Jackson) with 200 mM LiClO4 (G. F. Smith) were freshly prepared before use. Silicon tetraethoxide (Si(OEt)4) was synthesized by the reaction of Sic4 and dehydrated ethanol.% All other chemicals were reagent-grade and used without further purification. No significant electroactive impurities were found in the solvent and supporting electrolyte. Acetone was stored over 4 A molecular sieves. All solutions were purged with a stream of Ar prior to the voltammetric characterization. Preparationof Coated Fibers. The CVD-RH apparatus was previously described for the fabrication of silicacoated carbon disk UMES.~~>~* Either two or four carbon fibers (10 pm, Amoco, Thornel P55S) were attached in series with silver epoxy (EPOTEK, H20E) to a U-shaped copper metal holder in either a V or W configuration. After the silver epoxy dried, the fiber holder was transferred to a reaction chamber which was then purged with a stream of Ar. The total Ar flow was adjusted to 500 mL min-1. Part of the total flow (200 mL min-1) was diverted to a heated precursor vessel (100 "C) containing Si(OEt)4 to transport the precursor to the reaction chamber. The temperature of the fibers was raised to -700 "C (27-30 mA fiber1) with a powerstat variable transformer (Superior Electric Co., 3PN116B), which supplied the ac current for resistive heating. This temperature is just below the detectability of the optical pyrometer but is signified by the point where the fiber begins to glow. Under these conditions, a silica film was deposited onto the fiber surface at a growth rate of -0.2 pm min-l. Deposition was allowed to continue for 10 min. Then, the gas stream was changed to 5%H2 in Ar (500 mL min-I), a part of which (40-60 mL min-') was passed (28) Bradley, D. C.; Mehrotra, R C.; Gaur, D. P. In Metal Alkoxides; Academic Press: New York. 1978.
1492 Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
Figure 1. Preparation of a coated fiber for making electrical contacts: (A) a ring-disk coated fiber; (8)the coated fiber after HF etching; (C) the exposed carbon fiber after careful removal of part of the exposed carbon ring with a scalpel.
through a bottle of Sic& held at 25 "C. After these flow rates were adjusted, 02 (60 mL min-') was added to the total precursor gas flow prior to the reaction chamber. The ac current used for resistive heating was carefully increased until the ignition temperature for Hz and 02 was reached (-1000 "C, 37-40 mA fiber-').% Silica deposition rates of -5 pm min-l were achieved. The heated fibers were exposed to the Sic&,Hz,and 02 precursor mixture for a given deposition time that depended on the desired film thickness. The coated fibers were cooled to -700 "C by slowly decreasing the ac current. The gas stream was changed to Ar (500 mL min-l), and a portion (200 mL min-I) was passed through a bottle of acetone held at 25 "C. After 2 min, the temperature of the coated fibers was increased to loo0 "C, which resulted in the deposition of pyrolytic carbon onto the silica surface at a growth rate of -0.05 pm min-1. Once the desired ring thickness was obtained, the outer carbon surface was coated with a silica film by repeating the procedure described above. The coated fibers were removed from the holder by cutting the coated fiber with a diamond fiber-optics cleaver (Aurora Instruments, Inc., Model FCD) to within 1 mm of the Ag epoxy connection. Typically, the coated fibers are -20 mm in length with a uniform film thickness except at the ends where the fibers are conically shaped due to lower deposition temperatures near the electrical As a result, 1-2 mm of the coated fibers are tapered to bare fiber. Finally, each coated fiber was cut into two sections of nearly equal lengths. Preparation of a RD-UME.A RD-UME was fabricated from a coated fiber by fist cutting the conical end with a diamond fiberoptics cleaver. Aided by an optical microscope, the position of the cut along the conical taper was chosen to produce the desired outside diameter for the analytical tip. About 3 mm of the opposite end was immersed into an aqueous solution of 30%HF to remove the silica coatings (Figure 1). With the assistance of an optical microscope, 2 mm of the exposed carbon ring was carefully stripped off with a scalpel. Then, the coated fibers were washed with deionized water and dried in an oven at 70 "C for 30 min. The exposed portions of the carbon fiber and the carbon ring were independently connected to a 70 mm length of 0.2 mm diameter copper wire with Ag epoxy. Lastly, the RD-UME was sealed into a 1.5 mm glass capillary pulled to a tip diameter slightly greater
-
than the coated fiber. The glass capillary was pulled after resistively heating 18 gauge nichrome wire, which encircled the capillary. The capillary was iked to the coated fiber with epoxy resin (Elmers). The RD-UME extends 1-3 mm beyond the capillary sleeve. RESULTS AND DISCUSSION
RD-UMEPreparation. We have previously described a CVD approach for the preparation of disk UMEs, which consists of the deposition of uniform and concentric micrometer-scale silica films on 10pm carbon fibers from a precursor system of SiCb, 02, and H2F3 Film thickness, film adhesion to the fiber substrate, and the overall dimensions of the silicacoated carbon fiber UME are controlled by the choice of precursor, fiber diameter, deposition time, and temperature. The fabrication of the RD-UME exploits the high-temperature stability of the silicacoated carbon fiber, which can be maintained at temperatures sufficient for further precursor decomposition procedures. Minor modification in the CVD precursor delivery system permits multiple layering of concentric films with this technique. We have recently discovered that improved adhesion of the silica film to carbon is obtained by pyrolysis of Si(OEt)4, while Sic&is a better precursor for obtaining thicker and stronger films." Hence, for superior adhesion at the carbon-silica interfaces and reproducible film deposition, the decomposition of Si(0Et)a is used to precoat the fiber with a 1-2 pm film after which the precursor gas stream is changed to SiCb, 02, and H2 for more rapid film growth rates. When the desired thickness of silica is deposited, the CVD reactor is purged with Ar and a layer of pyrolytic carbon is then deposited from acetone, similar to the CVD coating of Ni meshes with pyrolytic carbon.29 Lastly, the silica coating process is repeated to insulate the carbon ring layer. A RD-UME is prepared by cutting the conical end of a coated fiber at a position that produces the desired outside diameter for the analytical tip. This is an advantageous feature of the CVDRH technique for preparing a RD-UME from a single fiber because tip diameters ranging from 15 to 500 pm can be obtained. Figure 2 shows a typical SEM of the tip of a RD-UME. The 10pm carbon disk, the silica ring, and the carbon ring are distinct and uniformly concentric. The thin white ring about the carbon fiber disk is due to A1203 particles at the interface between the fiber and silica. Noteworthy, there are no gaps at any of the interfaces between the carbon and silica layers. This is consistent with the better adhesion characteristics for the Si(OEt)4 precursor system. Furthermore, the CVD-RH technique directly provides quality adhesion without an epoxy sealant, which could potentially degrade in a sample solution and limit an analysis. Voltammetric Characterization of the RD-UME. Cyclic voltammograms of an unstirred solution of 1.0 mM ferrocene (Fc) with 200 mM bclo4 in CH3CN at a RD-UME are shown in Figure 3 and reveal the individual electrochemical responses for the disk and the ring portions of the RD-UME. Both voltammograms exhibit the characteristic sigmoidal response for UMEs at slow scan rates with similar E112 values (A, 413 mV; B, 410 mv) and wave slopes (A, 60 mV; B, 63 mv). This indicates that the electrochemical response at both the ring and the disk is well behaved with radial diffusion as the primary mode of mass transport under these experimental conditions. The limiting currents are determined to be 4.85 and 19.0 nA for the disk and (29) Kummer, M.;Kirchhoff, J. R Anal. Chem. 1993,&, 3720-3725.
Figure 2. Scanning electron micrograph of a polished coated fiber with the dimensions a = 10.0, 6 = 2.5, and c = 2.2 pm. The outer silica layer is indistinguishable from the epoxy matrix; magnification, 15OOx (reproduced at 73% of original size for publication purposes); working distance, 27 mm. 5
A 0 -
-10 -5
-
-15
-
-20 r
II
h
I
I
I
t
I
800
600
400
200
0
&
a
u
2-
B
-6
t
1000
-200
E (mV)vs AglAgCl Figure 3. Cyclic voltammograms of 1.O mM ferrocene with 200 mM LiClOs in CH&N at a RD-UME: (A) at the ring electrode; (B) at the disk electrode. The electrode dimensions are a = 10.0, 6 = 13.4, and c = 5.3 with an outer silica layer of 11.0 pm. Scan rate, 100 mV s-l.
the ring electrodes, respectively. These measured limiting currents are in good agreement with the limiting currents30of 4.63 and 19.4 nA, which were calculated using a diffusion coefficient Analytical Chemisfry, Vol. 67, No. 8, April 15, 1995
1493
Table I.Current Responses for Qeneratlon-Collection, Feedback, and Shielding Experimentsat the RD.UMEs* a
1 2 3 4 5 6 7 8 9 10 11 12
Ocm)
b Ocm)
c (um)
ig,disk
kriw
kdisk
ic.ring
i0,disk
Z0,rins
is,disk
is,*
10.8 8.8 10.1 9.0 8.0 8.0 9.7 9.8 9.8 8.0 9.4 9.8
2.2 8.7 10.7 13.1 15.1 15.6 17.5 19.2 24.7 34.3 36.4 42.2
4.6 3.1 2.0 0.7 2.2 1.1 2.0 2.8 4.0 1.4 1.9 2.7
7.80 4.88 5.17 4.68 5.56 5.10 5.49 4.80 5.24 4.39 4.21 4.10
16.5 15.5 16.8 13.1 18.2 19.6 22.2 24.6 30.3 27.4 30.0 43.9
7.30 2.60 2.99 1.98 2.54 2.10 2.70 2.48 3.00 1.63 1.38 1.73
7.40 2.52 2.74 1.76 2.76 2.20 2.87 2.41 2.70 1.66 1.57 1.55
5.40 4.36 4.56 4.41 5.27 4.90 5.34 4.70 4.97 4.18 4.20 3.90
11.7 13.5 15.5 12.3 17.2 19.2 21.2 21.4 32.5 26.8 29.5 43.1
0.50 1.90 2.02 2.74 3.32 3.00 2.69 2.32 2.11 2.56 2.76 2.37
9.1 12.3 14.1 10.7 14.9 17.4 19.6 22.2 27.6 24.6 28.3 42.3
For RD-UMEs embedded in an infinite insulation plane. a, b, and c are the disk electrode diameter, distance between the disk and ring electrodes, and the ring electrode thickness, respectively. is and ic are the generator and collector currents, respectively; io is the limiting current at the generator electrode when the collector is at open circuit; i. is the shielded current. Current units are nanoamperes and are measured from the limiting current of cyclic voltammograms of 1.0 mM ferrocene in CHBCNwith 200 mM IdClOs using the two potentiostat system. Electrode dimensions were determined by SEM. Table 2. Collection Efficiencies, Feedback Factors, and Shielding Factors at the RPUMEs.
1 2 3 4 5 6 7 8 9 10 11 12
&,diskb
&,ringb
&,diskc
4if,ringc
&.diskd
0.440 0.167 0.178 0.151 0.129 0.107 0.122 0.101 0.100 0.059 0.046 0.039
0.950 0.516 0.530 0.376 0.496 0.430 0.523 0.502 0.515 0.378 0.373 0.378
0.444 0.119 0.134 0.061 0.055 0.041 0.028 0.021 0.054 0.050 0.002 0.051
0.410 0.148 0.084 0.065 0.058 0.021 0.047 0.015 -0.068 0.022 0.017 0.019
0.907 0.564 0.557 0.379 0.370 0.388 0.496 0.506 0.575 0.388 0.343 0.392
Calculated with the current data from Table 1.
&,disk
= ic,disk/ig,ring, and
for ferrocene31of 2.4 x cm2s-l and the electrode dimensions determined from SEM measurements. In addition, a small charging current is observed. These findings further support the visual SEM observation that the CVD-RH technique produces highquality h s with excellent adhesion at the interfacesbetween the carbon and silica layers. The RD-UME is a microscopic static electrode analogue of the rotating ring-disk electrode O E ) . However, since the RDUME is a static working electrode system, further electrochemical characterization of the shielding factor (#$, feedback factor (&, and collection efficiency (q5J of the ring-disk combination is better described by the treatment of Bard et al.32for closely spaced microelectrode arrays (MEA) rather than the RRDE. The & is defined as the current ratio between the collector and generator electrodes, while & and & take into account the influence of overlapping diffusion layers on the current response. Tables 1 and 2 summarize the electrochemical data and the I#J~, &, and dc values for 12 different RD-UMEs and are organized from the smallest to the largest gap between electrodes. Figure 4 shows the steady-state responses from a typical diffusioncontrolled generation-collection experiment at a RD-UME of dimensions u = 9.7 pm, b = 17.5 pm, and c = 2.0 pm (electrode 7), where a is (30) Based on the theoretical treatment for ring and disk electrodes from ref 3. Bublitz, D. E.; Hoh, G. /. Am. Chem. SOC.1960,82,5811(31) Kuwana, T.; 5817. (32) Bard, h J.; Crayston, J. A; Kittlesen, G. P.; Varco Shea, T.;Wrighton, M. S. Anal. Chem. 1986,58,2321-2331.
1494 Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
&ring
= ic,ring/ig,disk.
& = ig/io
@S,*d
0.222 0.089 0.090 0.130 0.134 0.094 0.075 -0.037 0.152 0.082 0.041 0.019
- 1. & = 1- is/io.
the disk diameter, b is the separation between the disk and the ring, and c is the ring thickness. The oxidation of Fc is monitored as a function of potential at the generator, while the current at the collector is obtained by the reduction of the electrogenerated Fc+ at a fixed potential of 0.0 V. The & of this electrode is determined to be 0.523 for the ring as collector (i, = 2.87 I& ig = 5.49 nA) and 0.122 for the disk as collector (i, = 2.70 I& ig = 22.2 nA). Although the dimensions of the fabricated electrodes do not provide a rigorous systematic evaluation, some general trends are evident. The @c is clearly a function of the ring thickness and the gap between the ring and the disk electrodes. When the ring is used as the collector, relatively high collection efficiencies are obtained by maintajning a small separation distance (2-5 pm) and a large ring thickness (2-4 pm). These geometric parameters for the RD-UME yield efficient collection of Fc+ as it radially diffuses from the disk to the ring due to the concentric dual-electrode codguration. The close proximity of the two electrodes in the RD-UME configuration has important implications on the observed electrochemical response due to the overlap of the adjacent diffusion profiles at each electrode. In the generation-collection experiments, back diffusion from the collector to the generator electrode results in amplification of the current recorded at the generator electrode. The value of & determined at both the disk and the ring electrodes increases as the separation distance is reduced. The extent of overlap between the diffusion layers of the ring and
5
A iit
c-
0-
-5
'D
A
.-
1000
5
800
600
400
200
0
.200
10
between the RD-UME and the RRDE is observed. Dual-electrode analysis with the RRDE exploits the hydrodynamic mass transport of electrogenerated species from the disk to the outer ring electrode. Hydrodynamic mass transport is necessary at the RRDE because the large separation distance between electrodes would severely limit detection at the ring electrode if mass transport was solely by diffusion. In contrast, the electrodes in the RD-UME are separated by only a few micrometers, and electrogenerated species can easily diffuse from one electrode to the other within the time frame of a typical experiment. For example, an electrogenerated molecule with a typical diffusion coefficient of 5 x cm2 s-l will radially diffuse in only 0.05 s from the center of a 10 pm disk to the ring of a RD-UME with a 2 pm separation di~tance.3~ Under these conditions, a large flux of the electrogenerated species will arrive at the ring and provide a high faradaic response with diffusion as the only means of mass transport. Therefore, generation-collection experiments are easily accomplished at the RD-UMEunder static conditions with either the disk or the ring acting as the generator electrode.
0
CONCLUSIONS -10
.20
--
1000
800
800
400
200
0
-200
E (mV)YS Ag/AgCI Flgure 4. Steady-state currents from the diffusion-controlled generation-collection experiments of 1.O mM ferrocene with 200 mM LiCl04 in CH3CN at a RD-UME: (A) with the ring electrode as collector; (B) with the disk electrode as collector. The electrode dimensions are a = 9.7, b = 17.5, and c = 2.0 pm. The collector potential was held at 0.0 V. Scan rate, 20 mV s-l.
the disk was assessed by the magnitude of dS. The dSvalues for the ring and the disk electrodes of Figure 4 are 0.075 and 0.496, respectively. In all the RD-UMEs, the disk is shielded to a greater extent than the ring. These results are consistent with the observation at MEA electrodes that the inner generator electrode is shielded to a greater extent than the outer electrode^.^^ Similar to the trend for &, the shielding influence decreases as the separation distance increases. The observations described above for the experimental data contained in Tables 1and 2 are further supported by the results from a detailed digital simulation study of d,, A, and dCat the RD-UME.33 Upon miniaturization of the ring-disk configuration to UME dimensions, a fundamental change in the electrochemical response
Functional carbon RD-UMEs have been fabricated by the deposition of alternate insulating silica and conducting carbon rings on 10 pm carbon fibers by CVD techniques. CVD yields concentric carbon and silica rings, which are high quality, mechanically stable, and free of cracks. Most importantly, SEM and electrochemical measurements indicate that excellent adhesion is obtained at the interfaces between the silica and carbon surfaces. Voltammetric characterization shows the RD-UME is a viable dual-electrode system that functions under conditions where diffusion is the only means of mass transport. High shielding and collection efficiencies have been achieved with detection sensitivity greatly increased by the back diffusion of regenerated molecules of a reversible redox couple. The small overall size of the RD-UME in combination with the short diffusion distance between electrodes makes the RD-UME a promising dualelectrode biosensor for selective in vivo measurements. ACKNOWLEDGMENT
Financial support from The University of Toledo Biomedical Research Support Grant Program sponsored by the National Institutes of Health is gratefully acknowledged. We are indebted to the College of Arts and Sciences for support of the scanning electron microscopy facility. We also thank Drs. Susan M. Lunte and Craig E. Lunte, Mr. Song-Ryoul Park, and Mr. Min Zhong of the University of Kansas for the use of the bipotentiostat and assistance in the generationcollection experiments. G.Z. was the recipient of the CARI Fellowship at The University of Toledo. Received
for
review
September
9,
1994.
Accepted
January 25, 1995.@ (33) Zhao, G.; Giolando, D. M.; Kirchhoff, J. R, manuscript in preparation. (34) Kissinger, P. T.In Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T.;Heineman, W. R, Eds.;Marcel Dekker: New York, 1984; p 13.
AC940900W @Abstractpublished in Advance ACS Abstracts, March 1, 1995.
Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
1495
