Film Housing of Carbon Fibers Using Capillary-Pull Technology for

Anal. Chem. 1998, 70, 1646-1651

Poly(tetrafluoroethylene) Film Housing of Carbon Fibers Using Capillary-Pull Technology for One-Stage Fabrication of Carbon Disk Ultramicroelectrodes and Their Characterization Xueji Zhang† and Bozˇidar Ogorevc*

Analytical Chemistry Laboratory, National Institute of Chemistry, P.O. Box 3430, 1001 Ljubljana, Slovenia

Fabrication and characterization of carbon disk ultramicroelectrodes (CDUMEs), embodied in a pulled Teflon capillary, with overall tip dimension of ∼10 µm in diameter, are described. A CDUME was constructed by inserting a carbon fiber 7 µm in diameter into a commercial Teflon capillary, which was followed by pulling the capillary by means of a microelectrode puller employing appropriate heating and timing, to produce a self-sealing thin Teflon film insulation coating. Then, the so-coated carbon fiber was cut to expose a fresh carbon fiber disk. The proposed one-stage preparation method is fast (∼5 min), very simple, and inexpensive and eliminates the need for separate embodying, insulation, and sealing steps. It results in CDUMEs exhibiting excellent electrochemical behavior. Scanning electron and optical microscopy, voltammetry, and amperometry were employed to characterize these electrodes. Cyclic voltammograms of ferricyanide in aqueous media and of ferrocene in acetonitrile media displayed low-noise, low-background, sigmoidal responses with virtually no current hysteresis. To check the analytical applicability of these electrodes, a testing with adrenaline was performed by applying the differential pulse voltammetry mode. A linear calibration over the concentration range from 2.5 × 10-6 to 5.0 × 10-4 mol/L in pH 7.2 phosphate buffer solution was obtained with a detection limit of 7.0 × 10-7 mol/L. The proposed CDUMEs with flexible and nonfragile Teflon housing exhibit very low electrical noise and can be reproduced multiple times by simply recutting the tip.

and in extremely small volumes. For instance, UMEs were employed for detection of electroactive compounds in micro-,5 nano-,6 and picoliter7 sample volumes. Because of several specific properties, UMEs found their way to many attractive applications such as in vivo electrochemical detection of secretion from single cells,8,9 characterization of surface reactivities in scanning electrochemical microscopy,10 electrochemical detection in supercritical fluid chromatography,11 electrochemical detection in microcolumn separations,12 and stripping analysis in the absence of supporting electrolyte.13 Various geometries, sizes, and materials of UMEs have been reported to date.1,2,14-18 UMEs with tip diameters in the order of 1-5 µm,14 0.1-1 µm,15,16 several nanometer,17 and even 1 nm18 have been developed. However, most of them are housed in rather large support bodies which makes them unsuitable for measurements in ultramicroenvironments. The electrochemical advantages of micro- and ultramicroelectrodes over the conventionalsize electrodes are well-known.1,2 However, among the UMEs, the disk ultramicroelectrodes (DUMEs) have been particularly widely adopted18,19 due to the shape (geometry) and size (surface area) superiorities. To fabricate carbon or metal DUMEs, insulation of fibers is a central point. The most common procedure is to encase a fiber

† Present address: Centre for Chemical Sensors and Bioanalytical Chemistry, SFIT (ETH), Zurich, Switzerland. (1) Wightman, R. M.; Wipf, D. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, pp 267-353. (2) Bond, A. M. Analyst 1994, 119, R1-R21. (3) Wightman, R. M.; May, I. J.; Michael, A. C. Anal. Chem. 1988, 60, 769A779A. (4) Tur’yan, Y. I. Talanta 1997, 44, 1-13.

(5) Bratten, G. D. T.; Cobbold, P. H.; Copper, J. M. Anal. Chem. 1997, 69, 253-258. (6) Zhang, X.; Ogorevc, B.; Tavcˇar, G.; Grabec Sˇ vegl, I. Analyst 1996, 121, 1817-1822. (7) Clark, R. A.; Hietpas, P. B.; Ewing, A. G. Anal. Chem. 1997, 69, 259-263. (8) Ewing, A. G.; Strein, T. G.; Lau, Y. Y. Acc. Chem. Res. 1992, 25, 440-447. (9) Alvarez de Toledo, G.; Fernandez-Chacon, R.; Fernandez, J. M. Nature 1993, 363, 554-558. (10) Bard, A. J.; Denuault, G.; Lee, C.; Mandler, D.; Wipf, D. O. Acc. Chem. Res. 1990, 23, 357-363. (11) Wallenborg, S. R.; Markides, K. E.; Nyholm, L. Anal. Chem. 1997, 69, 439-445. (12) Ewing, A. G.; Mesaros, J. M.; Gavin, P. F. Anal. Chem. 1994, 66, 527A536A. (13) Daniele, S.; Mazzocchin, G. A. Anal. Chim. Acta 1993, 273, 3-11. (14) Saraceno, R. A.; Ewing, A. G. J. Electroanal. Chem. 1988, 257, 83-93. (15) Pendley, B. D.; Abrun ˜a, H. D. Anal. Chem. 1990, 62, 782-784. (16) Zhang, X.; Zhang, W.; Zhou, X.; Ogorevc, B. Anal. Chem. 1996, 68, 33383343. (17) Casillas, N.; Snyder, S. R.; White, H. S. J. Electrochem. Soc. 1991, 138, 641-642. (18) Penner, R. M.; Heben, M. J.; Longin, T. L.; Lewis, N. S. Science 1990, 250, 1118-1121. (19) Bond, A. M.; Mahon, P. T.; Oldham, K. B.; Zoski, C. G. J. Electroanal. Chem. 1994, 366, 15-27.

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S0003-2700(97)00833-0 CCC: $15.00

The introduction of micro- and ultramicroelectrodes (UMEs) has effectively revolutionized electrochemical measurements during the last few decades.1,2 The use of UMEs has grown dramatically in recent years, in particular for analytical measurements in ultramicroenvironments,3 in microvoltammetric cells,4

© 1998 American Chemical Society Published on Web 03/15/1998

into a glass capillary, to reduce the thickness of the glass support around the fiber by pulling it by means of a microelectrode puller, to seal the glass/fiber interface usually with an epoxy resin, and finally to polish the tip surface on a micropipet beveler.20 While glass housing preparation itself is relatively simple, the glass fragility may represent a serious inconvenience in both the fabrication and utilization period of such an electrode. In addition, the epoxy sealing may be inappropriate for measurements in nonaqueous media. Several other methods have also been reported, e.g., sealing fibers in a polypropylene matrix,21 fabrication of ceramic-coated carbon fiber DUMEs by vapor deposition technology, and preparation by depositing thin films of silica onto carbon fibers by resistive heating of fibers in the presence of a volatile precursor.22 These methods have been used successfully, yet all are relatively time consuming, require long training or significant skill, and/or need highly specialized equipment. Some other methods for insulation of carbon fibers were proposed, such as electrodeposition of phenolic polymer thin films.23,24 However, when the polymer-encased electrodes were electrochemically treated, the polymer film became unstable.25 This limits the utility, reproducibility, and lifetime of such electrodes. Recently, a method was reported for the fabrication of carbon fiber DUMEs in which anodic electrophoretic paint deposition was used.26 While this approach is simple, it appears (our observation) that, at high positive polymerization voltage, carbon fibers sometimes may get broken, in particular those with diameters of low-micrometer range. In this work, a novel method was introduced for the preparation of carbon disk ultramicroelectrodes (CDUMEs) utilizing commercial Teflon capillary tubes for both encasing the carbon fibers and self-sealing by employing a very simple capillary-pull technology. Teflon film formation, its insulation capability, and its sealing quality were inspected by optical and scanning electron microscopy and by voltammetry and amperometry measurements. The excellent electrochemical performance of the proposed CDUMEs is demonstrated by differential pulse voltammetric measurement of adrenaline. EXPERIMENTAL SECTION Apparatus. Electrochemical measurements were performed using either a modular electrochemical system Autolab (Eco Chemie, Utrecht, The Netherlands) equipped with PSTAT10 and ECD modules and driven by GPES software (Eco Chemie) or a potentiostat-galvanostat model 273 (EG&G PAR, Princeton, NJ) interfaced with a model 270 electrochemical analysis software (PAR). A three-electrode configuration consisted of a carbon fiber disk ultramicroelectrode, a platinum wire, and Ag/AgCl/KCl(satd) as the working, counter, and the reference electrodes, respectively. All potentials in this work are referred to Ag/AgCl/KCl(satd) as the reference. All electrochemical experiments were performed at room temperature (24 ( 2 °C). (20) Kelly, B. S.; Wightman, R. M. Anal. Chim. Acta 1986, 186, 79-87. (21) Nyholm, L.; Wikmark, G. Anal. Chim. Acta 1992, 257, 7-13. (22) Zhao, G.; Giolando, D. M.; Kirchhoff, J. R. Anal. Chem. 1995, 67, 25922598. (23) Strein, T. G.; Ewing, A. G. Anal. Chem. 1992, 64, 1368-1373. (24) Potje-Kamloth, K.; Janata, J.; Josowicz, M. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 1480-1485. (25) Cahill, P. S.; Wightman, R. M. Anal. Chem. 1995, 67, 2599-2605. (26) Schulte, A.; Chow, R. H. Anal. Chem. 1996, 68, 3054-3058.

For the fabrication of CDUMEs, an inverted optical microscope M40 (Wild, Heerbrugg, Switzerland) was used, while optical microscopy images were recorded by a M20 optical microscope (Wild). Scanning electron microscopy (SEM) images were recorded using a scanning electron microscope JSM-T220 (JEOL Ltd., Tokyo, Japan). Chemicals and Standard Solutions. Potassium ferricyanide was obtained from Fluka. Ferrocene and adrenaline [4-[1-hydroxy2-(methylamino)ethyl]-1,2-benzenediol] were purchased from Aldrich. All other chemicals were of analytical grade purity and were used as received. All aqueous solutions throughout the work were prepared with water that was first deionized and then further purified via a Milli-Q unit (Millipore, Bedford, MA). Working solutions of potassium ferricyanide and adrenaline were prepared with water. Ferrocene was dissolved in acetonitrile to desired concentration. Prior to electrochemical measurements, the solutions were deaerated by applying purified argon. Fabrication of CDUMEs. (A) Preparation of Carbon Fibers and Construction of Electrodes. Carbon fibers (7 µm in diameter and ∼15 mm in length, Goodfellow Co., Oxford, U.K.) were initially cleaned by sonication for 5 min first in acetone, then in nitric acid (1:1), and finally in Milli-Q water. Afterward, they were allowed to dry in air at room temperature. A single cleaned and dried carbon fiber was mounted on the end of a copper wire (0.2 mm in diameter and ∼10 cm in length) and fixed by means of high-purity silver conducting paint (SPI Supplies, West Chester, PA). A mounted carbon fiber was inserted into a ∼5-cm-long piece of a FEP Teflon capillary tube ZUP 1549 (Upchurch Scientific, Oak Harbor, WA), with an inner diameter of ∼500 µm (0.020 in.), and outer diameter of ∼1.5 mm (0.063 in.). After the carbon fiber was positioned in the middle part of a Teflon capillary piece, the copper wire was fixed at the end of the capillary by a fast-acting epoxy glue. (B) Capillary-Pull Thinning and Sealing. A Teflon capillary with inserted carbon fiber (as described above) was fixed at each end of the holder of a model PP-83 microelectrode puller (Narishige, Tokyo, Japan). The middle section of the capillary piece with the carbon fiber was adjusted in the center of the heating coil. A narrow segment of the capillary was heated (up to ∼300 °C) until the Teflon melted and the capillary stretched down for about 2-4 mm due to the weight applied. The Teflon material was allowed to cool for ∼30 s. Then, the described pulling procedure was repeated two to three times until the Teflon capillary was torn at the point of heating, exposing a part of the naked carbon fiber. In order to create a disk-shaped electrode, Teflon film-coated carbon fiber was cut off perpendicular to its longitudinal axis with a microsurgical scalpel blade. After each cutting, the tip was examined under a microscope at high magnification, and only if the cut appeared smooth was the electrode applied to electrochemical measurements. A Teflon-housed CDUME constructed in this way is schematically presented in Figure 1. After a certain period of usage, another very short part of the CDUME Tefloncoated tip can be cut off to produce a fresh carbon disk surface. Procedures. (A) Differential Pulse Voltammetric (DPV) Measurement of Adrenaline. The measurements were performed in pH 7.2 phosphate buffer solution by applying an anodic DPV scan from -0.1 to +0.4 V at a scan rate of 10 mV/s, with Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

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Figure 1. Schematic drawing of a Teflon-housed carbon fiber disk ultramicroelectrode (prior to cutting): (1) naked carbon fiber, (2) pulled Teflon film-coated carbon fiber, (3) carbon fiber, (4) copper-carbon fiber joint, (5) Teflon capillary tube, (6) epoxy resin, (7) copper wire, and (8) site of cutting to produce disk electrode. (D) disk electrode side view: (a) carbon fiber disk and (b) self-sealing Teflon insulation layer.

pulse height of 25 mV, potential step of 5 mV, and modulation time of 0.5 s. After each determination, the CDUME was immersed in a pure pH 7.2 phosphate buffer and anodically scanned in DPV mode from -0.1 to +0.7 V at a scan rate of 50 mV/s. The electrode was then washed with water, by which it was made ready for the next measurement. (B) Residual Current, Total Capacitance, and Electrode Noise Measurements. Residual current of the Teflon-insulated CDUMEs was measured by using linear scan voltammetry (LSV) at scan rates from 0.02 to 10.0 V/s. Scans were run between 0.0 and 0.8 V vs Ag/AgCl in a pure solution of 1 M KCl. The residual current was measured at +0.4 V. The total capacitance of a Tefloncoated CDUME was evaluated by integrating the current transients in response to 10-mV depolarization pulses which were divided by the amplitude of the voltage step and multiplied by response time. The rms current noise (low-pass filtered 1 kHz, -3 dB, four-pole Bessel) was determined by recording current with a high-gain-high-bandwidth patch-clamp amplifier (SWAM 2A), modified to allow variable dc voltage application in voltageclamp mode, in 0.1 M NaCl by employing a two-electrode configuration (CDUME and Ag/AgCl pellet as the reference) and with potential set to 0.65 V. RESULTS AND DISCUSSION Fabrication and Characterization of Teflon Film-Insulated CDUMEs. Beside simplicity of preparation, chemical inertness, physical adequacy, being fault-free, and the appropriate thickness of a carbon fiber insulation coating are the most important parameters in producing CDUMEs suitable for various ultramicroenvironment applications. Often, these parameters represent critical points in the fabrication of CDUMEs. The Teflon material chosen and the capillary-pull technology used in this work were found to provide a very simple way of fulfilling the above requirements. It is well-known that Teflon is a (bio)chemically inert material. In addition, the ability of the applied Teflon to soften and shrink at elevated temperatures enables the formation 1648 Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

Figure 2. Optical (A) and scanning electron (B) microscopy images of Teflon film-insulated CDUMEs; (A) exposed naked carbon fiber/ Teflon film interfacial region after pulling and prior to cutting; (B) a scalpel cut tip with exposed carbon fiber disk (7 µm in diameter).

of a relatively thin film of pulled Teflon (∼1-5 µm) that firmly seals the Teflon/fiber interface. In addition, since the Teflon capillary is transparent, this allows a direct observation in any phase of the fabrication process, which is particularly important for positioning a fiber and the heating coil. Since the insulation and sealing are critically important for the successful performance of insulated CDUMEs, an extensive investigation was devoted to checking the quality of insulation and sealing of the Teflon film-insulated CDUMEs. Figure 2A displays an optical microscopy image showing the interfacial region of a CDUME, to which the Teflon capillary-pull coating was applied. Evidently, a uniform film was achieved, manifesting no gaps or other defects on the insulation layer surface. Furthermore, the Teflon/fiber interface exhibits a good-quality seal, at least from an optical point of view. This is further demonstrated by an SEM image (Figure 2B) of a Teflon-coated CDUME tip after being cut, showing a smooth surface with no cracks or bubbles in the interface space between the carbon fiber and the insulation film. The average thickness of the Teflon coating is estimated to be ∼1.5 µm. Therefore, the overall tip diameter of such CDUMEs is typically ∼10 µm, which is close to the dimension of a small single biological cell. Thinner or thicker coatings can be obtained by varying the applied capillary-pull heating temperature, pull weight, and heating time.

The electrode capacitance can serve as an important indicator parameter for the estimation of the quality of the insulation and sealing at insulated electrodes. Any imperfections in sealing and insulation would lead to an increase in solution/electrode interface area, which would consequently result in an increase of the capacitance and also in higher residual current, electrode noise, and response time. Since the dielectric constant of the FEP Teflon material, used for the preparation of CDUMEs in this work, is known to be very low (∼2 within the whole frequency range), the magnitude of the observed capacitance was considered to be directly affected by the quality of the insulation and sealing between the carbon fiber and the Teflon film. The total capacitance of the Teflon-insulated CDUMEs was obtained using chronoamperometric measurements (see Experimental Section), and its value was calculated to be 31 ( 4 pF (n ) 8), which is reasonably low and comparable to the value obtained with paintcoated CDUMEs.26 Residual current was examined as a function of potential scan rate.27 The value of background current, read at 0.4 V, divided by a corresponding scan rate remained essentially constant over the range from 0.02 to 10.0 V/s, confirming the existence of a good seal at the Teflon/fiber interface. Some Teflon-insulated CDUMEs were checked for rms current noise employing patchclamp amperometric measurements. Typically, the electrode noise was evaluated as being less than 2 pA. It was found that this mainly depended on the quality of the cutting process. Such a low noise level assures that the Teflon film-insulted CDUMEs can be used for amperometric monitoring of single cell secretory activities, for example. The electrode-to-electrode performance reproducibility was examined by testing five CDUMEs produced by consecutive offcutting a tip of one single fabricated electrode. The steady-state limiting current from LSV recordings for the reduction of 1 mM ferricyanide in pH 7.2 phosphate solution, obtained at new electrode surfaces produced in this way, was found to be 1.2 ( 0.2 nA. The average value of the half-wave potential for ferricyanide at these electrodes was 0.214 ( 0.003 V. These results clearly indicate that a single fabricated Teflon-insulated CDUME can be successfully reproduced multiple times by simply recutting the tip of the electrode to expose a fresh surface. This substantially extends the useful lifetime of a single fabricated electrode without a need for polishing. The number of recuttings depends on the length of a tip and may be 5 or more. Voltammetric Behavior of Teflon-Insulated CDUMEs in Aqueous and Nonaqueous Media. Each electrode produced was initially tested by cyclic voltammetry measurements of potassium ferricyanide in 1 M KCl solution, pH 6. Figure 3 shows the sigmoidal-shaped cyclic voltammogram corresponding to the ferricyanide reduction, obtained at the CDUME displayed in Figure 2B. Notably, a minimal background charging current can be noticed. Moreover, practically no hysteresis, i.e., the difference in the current response along the forward and reverse scan, can be observed, indicating very good electrode surface and sealing characteristics. This conclusion can also be drawn from the nearly Nernstian behavior of ferricyanide reduction at the CDUME (Figure 3). A plot of log[(i1 - i)/i] vs potential (E - E1/2) showed (27) Wehmeyer, K. R.; Wightman, R. M. J. Electroanal. Chem. 1985, 196, 417421.

Figure 3. Cyclic voltammogram of 1.0 × 10-3 mol/L potassium ferricyanide in 1 mol/L KCl (pH 6) obtained at a Teflon film-insulated CDUME scan rate, 100 mV/s; initial and final potential, 0.5 V; vertex potential, -0.1 V.

a slope of 60 mV, thus being close to the predicted theoretical value, 59 mV at 25 °C, for a reversible one-electron transfer. Further evidence of good sealing and insulation, and excellent electrochemical characteristics of CDUMEs produced by the proposed method, was obtained by a comparison between the recorded limiting current of ferricyanide reduction in 1.0 M KCl solution and the calculated current using the given carbon fiber (electrode disk) diameter, employing a corrected equation for micro disk electrodes embedded in a thin insulation layer, accounting a nonlinear diffusion profile:22

io ) {1 + 0.379[ro/(ro + d)]2.342}4nFDCoro

where io is the limiting current, D and Co are the diffusion coefficient and the bulk concentration of the electroactive species, respectively, ro is the radius of the electroactive micro disk electrode, d is the thickness of the insulating layer, and n and F have their usual meaning. By inserting the parameters of CDUME presented in this work (Figure 2), with ro ) 3.5 µm and d ) 1.5 µm, into the above equation, and by taking that the diffusion coefficient of ferricyanide ion D ) 7.6 × 10-6 cm-2 s-1, the steady-state limiting current for the reduction of 1.0 × 10-3 mol/L ferricyanide was calculated to be 1.196 nA. The steadystate limiting current measured by LSV was nearly identical in a solution containing 1 mM ferricyanide. The average limiting current was 1.2 ( 0.1 nA (n ) 8), which is in good agreement with the calculated value. Since studies and measurements in nonaqueous (resistive) media represent a very important application area of the UMEs, the Teflon film-insulated CDUMEs were also examined under such conditions. In Figure 4, the cyclic voltammetry recordings of ferrocene in acetonitrile in the presence (curve a) and in the absence (curve b) of a supporting electrolyte (0.1 M NaClO4) are shown. Apparently, well-defined voltammograms again confirmed a perfect seal and appropriateness of the proposed CDUMEs for their utilization in nonaqueous media. In addition, as expected, a well-defined response in the absence of the supporting electrolyte clearly indicated practically no effect of the ohmic resistance of the measurement solution. A similar current response with Analytical Chemistry, Vol. 70, No. 8, April 15, 1998

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Figure 4. Cyclic voltammograms of 1.0 × 10-3 mol/L ferrocene in acetonitrile obtained at a Teflon film-insulated CDUME in (a) the presence and (b) the absence of a supporting electrolyte (0.1 M NaClO4): scan rate, 100 mV/s; initial and final potential, -0.15 V; vertex potential, 0.9 V.

only a small positive shift in the E1/2 value can be observed when CDUME behavior is compared in the absence to that in the presence of the supporting electrolyte. Insignificant ohmic losses, coupled with relatively high analytical sensitivity, seem promising for the utilization of these electrodes in resistive media, e.g., for detection in liquid chromatography and other continuous-flow systems. Differential Pulse Voltammetry of Adrenaline at the Teflon-Insulated CDUMEs. The practical applicability of the proposed Teflon film-insulated CDUMEs was tested by carrying out DPV measurements of the highly interesting adrenaline, as shown in Figure 5. Evidently, clearly defined low-noise DPV signals at low anodic peak potential were obtained. From the corresponding DPV data, the calibration plot for adrenaline in pH 7.2 phosphate buffer solution was linear over the range from 2.5 × 10-6 to 5.0 × 10-4 mol/L with a correlation coefficient of 0.997, having a slope of 5.5 pA‚L/µmol. Detection limit, based on an S/N ) 2 criterion, was estimated to be 7.0 × 10-7 mol/L. The relative standard deviations at adrenaline concentrations of 1.0 × 10-4 and 5.0 × 10-6 mol/L were found to be 2.3% and 2.9% (n ) 7), respectively. Significantly, this calibration range corresponds well with the estimated concentration levels of adrenaline and related substances in mammalians28 and chromaffin cells.29 From Figure 5, it is also noteworthy that the oxidation peak potential of adrenaline is 0.16 V vs Ag/AgCl, achieved at our Teflon film-insulated CDUME by using no pretreatment at all. This value differs from the peak potential of adrenaline (∼0.35 V vs SCE) at a fresh beveled CDUME25 but, on the other hand, is very close to the peak potential of adrenaline (0.18 V vs SCE) at a beveled CDUME after electrochemical pretreatment.25 It is assumed that this difference arises from the status, and possibly history, of the carbon disk surface: our CDUME was fresh cut while the other one25 was polished, prior to use. (28) Cooper, B. R.; Jankowski, J. A.; Leszczyszyn, D. J.; Wightman, R. M.; Jorgenson, J. W. Anal. Chem. 1992, 64, 691-694. (29) Kawagoe, K. T.; Jankowski, J. A.; Wightman, R. M. Anal. Chem. 1991, 63, 1589-1594.

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Figure 5. Differential pulse voltammograms of adrenaline in 0.2 mol/L phosphate buffer solution (pH 7.2) obtained at a Teflon filminsulated CDUME: (a) baseline; mol/L: (b) 1.0 × 10-5, (c) 2.0 × 10-5, (d) 3.0 × 10-5, (e) 4.0 × 10-5, and (f) 5.0 × 10-5 mol/L adrenaline; scan rate, 10 mV/s; pulse height, 25 mV.

CONCLUSIONS The fabrication route of applying a simple capillary-pull technology, in combination with the use of a Teflon capillary, to produce self-sealing thin Teflon film-insulated CDUMEs, results in a number of apparent advantages and attractive features. We point to several properties of the chosen insulation material that are very interesting with respect to CDUMEs fabrication: (i) the Teflon capillary tube can be pulled to a very thin film, while retaining excellent insulation and mechanical properties, which enables the fabrication of CDUMEs with very small overall tip size (∼10 µm), required in ultramicroenvironment measurements; (ii) the Teflon capillary is not fragile, as is the case with a glass capillary, providing a housing for easy handling and flexibility for use in special applications; (iii) the Teflon capillary is heat shrinkable, allowing direct high-quality sealing and thus no extra sealing is needed; (iv) Teflon material is chemically and biologically inert, enabling applications of Teflon-insulated CDUMEs in difficult and even in extreme media; (v) by simply recutting the Teflon film-coated electrode tip it is possible to produce several (∼5) electrodes, with new fresh surfaces, out of one single fabricated Teflon-insulated CDUME; (vi) the Teflon capillary is transparent, hence allowing direct observation in any phase of the fabrication process; (vii) the Teflon capillary used is an inexpensive, widely commercially available material, offering simple, safe, and fast single-stage fabrication of CDUMEs. It is assumed that the described fabrication route may also be applied to other electrode materials with thermal and other physical properties similar to carbon fiber. The experimental evidence of testing, characterization, and voltammetric measurements of

adrenaline, presented in this work, indicates that the Teflon filminsulated CDUMEs can be considered useful for a variety of in vitro and in vivo applications, e.g., for electrochemical monitoring of single-cell secretory activities.

Institute of Chemistry for financial support. The authors thank Prof. R. Zorec for providing patch-clamp rms noise measurements at the Institute of Pathophysiology, Medical School, University of Ljubljana.

ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of the Republic of Slovenia (Contract J1-8902-0104), which is gratefully acknowledged. X.Z. also thanks the National

Received for review August 1, 1997. Accepted January 31, 1998. AC970833G

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