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Anal. Chem. 1885, 57, 1913-1910
Electroanalytical Properties of Band Electrodes of Submicrometer Width Kenneth R. Wehmeyer, Mark R. Deakin, and R. Mark Wightman* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Platinum, gold, and carbon electrodes have been constructed with a band geometry. Thin films of the electrode material were obtained commercially or were formed on insulating substrates by conventlonai sputtering techniques, and an insulating overlayer was placed on each of the films. The edge of this assembly has been employed as a voltammetric electrode which is characterized by a microscopic width (5-2300 nm) and a macroscopic length (centimeters). These electrodes exhibit sigmoidal, rather than peak-shaped, voitammehic curves because of nonlinear diffusiorl. The limiting current at band eiedrodes is shown to depend on the eiectrode width in an inverse logarithmic manner. Therefore, extremely large current densities are observed as the eiectrode radius is decreased. The large current densities lead to a greater Sensnivlty of the vdtammetrlc curve to the effects of slow heterogeneous charge transfer kinetics. The combination ot small area and high current denslty results in an electrode that has a much larger faradalc to residual current ratio than Is obtained at an electrode of conventional size. This Is demonstrated by the determination of ruthenium hexaammine by conventional linear scan voltammetry with a lowest detectable concentration of 7 X lo-' M.
In recent years increasing attention has been drawn to the area of microvoltammetric electrodes. Electrodes of small dimensions (10 pm or less) possess unique properties which can make their use in many applications preferable to electrodes of conventional size. For example, in vivo measurements of neurotransmitters require electrodes of small size to minimize damage to neuronal tissue (I,2). Due to their small size, microvoltammetric electrodes generate extremely small currents and, thus, i R drop is minimized. This allows electrochemistryto be performed in highly resistive media (3, 4 ) arid cyclic voltammetry to be extended to scan rates of greater than 10000 V s-l without need for instrumental corrections (3). The double layer capacitance on such electrodes is also reduced due to the small area. This results in an electrochemicalcell with a small RC time constant, which has allowed potentiostatic experiments to be performed at submicrosecond time scales (5). The advantage of microvoltammetric electrodes has been demonstrated in nucleation studies where, due to their small area, a single nucleus of metal atoms can be grown (6, 7). Another unique property of microvoltammetric electrodes is the sigmoidal current response obtained in cyclic voltammetry. The sigmoidal response is a result of enhanced mass transport due to nonlinear diffusion (8). The enhanced transport obtained with disk electrodes of 0.6-25 pm diameter allows the determination of the kinetics of homogeneous reactions coupled to electrode processes (9, 10). The enhanced flux at microvoltammetric electrodes has also been demonstrated to be useful in the study of electrochemical kinetics since reaction rates can be observed under steady-state conditions (6, 11). The enhanced flux at microvoltammetric electrodes should also lead to an increase in the sensitivity of electroanalytical measurements (1). This is because the faradaic current density 0003-2700/85/0357-1Q13$01.50/0
increases with decreasing electrode dimensions, while many contributors to the residual current are proportional to the electrode area. However, the small magnitude of the currents obtained with disk-shaped microvoltammetric electrodes can pose instrumental difficulties in the measurement of dilute concentrations. Therefore, some investigators have examined the use of arrays of microvoltammetric electrodes. Digital simulation has been employed to examine the optimum dimensions for designing ensembles of microvoltammetric electrodes for high sensitivity (12,13). An array of 10-pm carbon disks has been shown to have sufficient sensitivity for trace analysis following liquid chromatographic separation (14). An ensemble of randomly spaced microvoltammetric electrodes formed from a combination of reticulated vitreous carbon and epoxy also has been investigated (15). It has been proposed that the favorable signal to noise properties of composite electrodes are a result of this phenomena (16). Microvoltammetric electrodes with a band geometry can provide larger currents than disk microvoltammetric electrodes, while maintaining the properties of nonlinear diffusion (17). The band electrode can be designed to be microscopic in one dimension (10 pm or less), but macroscopic in length, thereby resulting in larger currents. Band electrodes with thicknesees of 5-20 pm have been constructed from thin sheets of conductors (In,or by vapor or chemical deposition techniques (18,19). An assembly of several band electrodes has been fabricated with photolithography techniques, arid when covered by a polypyrrole layer the assembIy was shown to have properties similar to a solid-state field effect transistor (20). At band electrodes of micrometer dimensions, and under conditions of nonlinear diffusion, we have shown that the faradaic current can be predicted by the equation for the current at a hemicylinder of equivalent area (I 7)
i = 27rnFDCZ[l/(ln 40)] (1) In eq 1,i is the electrode length (cm), 8 = Dt/?, r is the radius of the equivalent hemicylinder (cm), and the other terms have their usual electrochemical meaning. We have previously shown that this equation predicts the current obtained at cylinders for 0 > 10 (17). This equation leads to the interesting prediction that the faradaic current is relatively insensitive to the radius of the electrode, while the residual current with surface origins will decrease in a linear manner with a decrease in radius. The detection capabilities of conventional solid electrodes in voltammetric techniques are limited by residual current due to double layer charging and to surface oxides found on the electrodes (21). Thus, band-shaped microvoltammetric electrodes should provide a decrease in voltaminetric detection limits. This paper provides an experimental realization of this prediction, and also provides a voltammetric characterization of band electrodes of submicrometer dimensions. EXPERIMENTAL SECTION Reagents. All chemicals were of reagent grade, and solutions were prepared in doubly distilled water (Mega-Pure System MP-3A and D2,Corning G h s Works, Corning, NY).Ruthenium hexaammine trichloride (ICN Pharmaceuticals, Plainview, NY) was dissolved in in 0.1 M potassium phosphate buffer (pH 7.0) and 0.1 M KNOs. Potassium ferricyanidesolutionswere prepared 0 lQS5 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
daily in aqueous 0.1 or 1.0 M KCl. Construction of Band Electrodes. Glass microscope slides 2.5 cm long, 1.9 cm wide, and 1mm thick (Corning Glass Works, Corning, NY) were cleaned with detergent and water, dried in an oven, cleaned with isopropyl alcohol, and redried. A portion of each slide was masked with adhesive tape so that a strip of glass 0.6 to 1.0 cm wide remained exposed in the center of the slide. The slides were then covered with gold or platinum by a sputtering process using a Polaron Model E 5100 Series I1 Cool Sputter Coater (Polaron, Inc., Hatfield, PA). The tape was carefully removed from the coated slides and the slides were heated in an oven at approximately 200 OC for about 2 h. Metal films with thicknesses ranging from 30 nm to 2300 nm could be prepared in this manner. The thickness of these films was determined by scanning electron microscopy. The length of the bands was measured by optical microscopy. The films obtained from commerical sources and those prepared locally were insulated from solution contact on the large, exposed face by applying a drop of epoxy (Epon 828 with 14% mphenylenediamine, Miller Stephenson, Chicago, IL) to the conductor surface and placing a clean glass slide (1.9 cm x 1.9 cm) over the epoxy. One edge of this assembly forms the band electrode. Electrical contact was made at the other end with a wire attached by silver epoxy (Epo-Tek H20E, Epoxy Technology Inc, Billerica, MA) to the remaining exposed gold/glass surface. The electrical contact and a portion of the wire were then completely covered with silicon rubber. The end of the assembled electrodewas exposed by using 320 sandpaper. The exposed band was then ground flat with 1000 grit carborundum and 30 pm sandpaper. The electrode surface was subsequently polished with 6.0 and 1.0 pm diamond paste (Buehler, Ltd., Evanston, IL) and 0.05 pm alumina (Fisher Scientific, Cincinnati,OH). Before daily use, each electrode was polished briefly with 0.05 pm alumina and washed consecutivelywith distilled water, acetonitrile,and distilled water. The limiting current normally was reproducible to within 10% after each polish. Polishing with diamond paste in the final step led to irreproducible limiting currents, presumably because the paste is difficult to remove from the electrode surface. Excessive polishing with 0.05 pm alumina was avoided since the current response decreases and becomes peak-shaped, indicating that the metal becomes recessed into the insulating material. Gold films on a Mylar support were purchased from Goodfellow Metals, Cambridge,UK. The thickness of the film was given by the manufacturer as 5 nm, and the nominal thickness was used in all calculations. The Mylar supported gold film was sealed between two glass slides by the same process described above. Films of 1-nmnominal thickness were found to be nonconductive. Glass microscope slides (2.5 X 7.5 cm) coated with a 30-nm carbon film were purchased from a commerical source (Lebow Co., Goleta, CA). A resistance of 7500 Q was reported by the manufacturer for each slide. The slides were cut into smaller pieces and the carbon removed from the edges of the slide so that a strip of carbon 0.6-1.0 cm in length remained in the center of each piece. Carbon band electrodes were prepared as described above. An alternate method of preparing platinum band electrodes involved in use of a soft glass tube (3.0 mm 0.d.) which was cleaned with isopropyl alcohol and coated with a -30 nm film of platinum by sputtering. The coated tube was then sealed inside a slightly larger soft glass tube using an air/gas flame. Electrical contact and polishing were carried out as described above. Electrochemical Apparatus. A homebuilt potentiostat was used to obtain voltammograms from 5 mV s-l to 200 V s-l (3). A PARC 174A polarograph analyzer (Princeton Applied Research Corp., Princeton, NJ) was used for all other electrochemical experiments. Voltammograms were recorded on a flat bed X-Y recorder (Houston Instruments, Austin, TX) or if scan rates exceeded 500 mV s-l on a 10MHz storage oscilliscope (Tektronix, Inc., Beaverton, OR). Conventionally sized gold and platinum electrodes (2.0 mm diameter disks) in an insulating Teflon sheath were purchased from BioanalyticalSystems, Inc., West Lafayette, IN. The electrodes were polished with 0.05-pm alumina before each run. The use of the same polishing techniques as used with band electrodes facilitates comparisons between electrodes of different sizes. The electrochemical cell was a 50-mL beaker which was covered with Parafilm through which holes were made for electrodes, A platinum wire auxiliary and a saturated calomel
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Figure 1. Cyclic voltammograms for the reduction of 1.3 mM (Ru(NH3)2+in 0.1 M phosphate buffer at a 5-nm gold band electrode at various scan rates.
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Figure 2. Dependence of the maximum current on scan rate for the reduction of 1.3 mM Ru(NH3)2+ in 0.1 M phosphate buffer at two different gold band electrodes.
electrode (SCE) were employed. For reductions, solutions were purged with argon which was passed through an ammonium vanadate scrubber and distilled water. Trace determination of ruthenium(II1)hexaammine was performed inside a Faraday cage. The diffusion coefficient for this compound in 0.1 M phosphate buffer is 6.0 X lo4 cm2 s-l as determined from the limiting steady-state current at a 10 pm diameter carbon disk (3). RESULTS AND DISCUSSION Voltammetric Behavior. Voltammetric electrodes with a band geometry exhibit significant contributions from nonlinear diffusion ( I 7). Therefore, voltammograms at these electrodes have a sigmoidal shape at slow scan rates. As the scan rate is increased linear diffusion predominates and the voltammograms become peak shaped (Figure 1). The contribution from linear diffusion is greatest for thick band electrodes, as can be seen from a plot of the maximum current as a function of scan rate (Figure 2). This is because transport of molecules in a radial direction at a thin band electrode provides a larger proportion of the total flux than at a larger electrode. Limiting Current. The enhanced diffusive mass transport at band electrodes results in very large current densities. For example, the steady-state current density obtained a t the 5
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
‘t
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I 0.2
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0 -7
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Flgure 4. Cyclic voltammograms for the reduction of 3.0 mM FHCN),” in (a, c) 1.0 M KCI and (b, d) 0.1 M KCI at gold band electrodes: scan rate, 50 mV/s.
log w(cm)
Flgwe 3. Maximum voltammetric cunents for the reductbn of 9.7 mh4 Ru(NH,)t+ in 0.1 M phosphate buffer at gold band electrodes (current m l k e d by electrode length)as a function of the electrock bandwidth (circled points). Voltammograms recorded at 20 mV s-’scan rate. The sdld line was calculated from eq 1 using r = w l r , t = 15 8, and D = 8.0 X lo-’ cm2/s. The C denotes a carbon band electrode.
nm band electrode is 0.25 A cm-2 (Figure l),as calculated from the manufacturer’s stated thickness and the measured length. In contrast, the current density at a conventional rotating disk electrode in the same solution would be 0.0005 A cm-2, at a rotation speed of 20 000 rpm. It has been shown previously that the limiting voltammetric current at band electrodes of 5 to 20 pm thickness could be predicted by eq 1 when the radius of the hemicylinder is replaced with w / r , where w is the thickness of the band (17). The voltammetric limiting currents at band electrodes of thicknesses from 30 to 2000 nm show good agreement with the currents calculated from eq 1(Figure 3). Due to the inverse logarithmic relationship the current is largely independent of the electrode radius-a decrease in the electrode radius by a factor of lo3 results in only about a 3-fold decrease in the voltammetric current. For the electrode of 5 nm nominal width less current is observed than is expected. This may occur because eq 1is inappropriate for an electrode of this small size or because a portion of the electrode surface is blocked. Since an electrode of this size approaches molecular dimensions, unusual phenomena may occur. Electrode Kinetics. As thG current density increases because of increased mass transport, finite rates of electron transfer become more apparent in the electrochemical response (22). This forms the basis for a variety of methods used to measure heterogeneous rate constants (23). It has been demonstrated that the increase in diffusional transport caused by a decrease in the radius of a microdisk electrode can be used for such measurements (24,25). Although exact equations are not available for band electrodes, it would be anticipated that the effect of electrode kinetics would be more apparent for these electrodes than for electrodes of conventional size. The effect of electron transfer rates on the voltammetric shape at band electrodes was investigated with ferricyanide solutions containing different amounts of KCI. The heterogenous rate constant for the ferrilferrocyanide couple has been shown to increase linearly with KCl concentrations (26). At a gold disk electrode of conventional size the standard heterogeneous rate constant for the reduction of Fe(CN)63was measured to be 3.6 X cm s-l in 0.1 M KCl and 2.8 X cm s-l in 1.0 M KC1 (rate constants were determined using the method of Nicholson, ref 27). At a scan rate of 50 mV s-l this corresponds to a separation of the anodic and cathodic waves of 95 and 66 mV, respectively. Voltammograms in these solutions were also obtained with the 36 nm and 2300 nm bold band electrodes (Figure 4). A dramatic change in shape of the voltammogram is seen for the thinnest
electrode, indicative of its sensitivity to the rate of heterogenous electron transfer. Electrical Properties. The resistivity of vacuum deposited thin films of gold or platinum is in the range of 10-25 D cm (28,29),and thus does not contribute significantly to the overall cell resistance. The expression for the solution resistance of a line electrode has been approximated by considering the case of two concentric hemicylindrical electrodes (30) P
R, = - In ( r 2 / r 1 ) Tl
where p is the specific resistance, r2 is the radius of the large hemicylinder, and rl is the radius of the smaller hemicylinder. Combination of eq 1 with eq 2 indicates that the iR drop at hemicylindrical electrodes is relatively insensitive to the dimensions of the electrode under conditions where eq 1 is appropriate. Thus is appears that a decrease in thickness of the electrode should not increase the iR drop, in accord with the experimental data. The residual current (composed of capacitive current and other contributions) was evaluated by comparing the current density obtained at gold and platinum electrodes of conventional size in 0.1 M KN03 solution with that obtained at the band electrodes. The residual current density was 2.2 X and 1.8 X A cmW2(at 0.4 V vs. SCE) at the large platinum and gold electrodes, respectively. The gold and platinum films prepared locally, as well as the commercial 30-nm carbon film, showed an inverse relationship between thickness and residual current density. The residual current density with the smallest thickness (-40 nm) examined was 100 times greater than that at the large electrode under the same conditions. Close inspection of the data indicated a resistive component in the voltammograms, possibly because of an imperfect seal between the electrode and the insulating material. As the electrode thickness is decreased, the area to perimeter ratio is also decreased, and such problems would be expected to be more severe. Electron micrographs of the surface show regions where an imperfect seal exists along the length of the band. Defects or porosity in the locally constructed films could also lead to the observed results (31,32). The effect of the insulating material or surface roughness caused by polishing on the double layer capacity at very small thicknesses may also play a role. Voltammograms obtained at the electrode fabricated from the commercial 5-nm gold film and the platinum film sealed in glass (-30 nm thickness) did not exhibit as large of a resistive behavior. For these electrodes the residual current density was -30 times greater than that at the electrodes of conventional size. Trace Analysis. Cyclic voltammetry is regarded as a qualitative rather than a quantitative tool because of ita poor detection limits. The detection limit for all voltammetric methods is ultimately limited by the magnitude of the residual
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CONCLUSIONS Band electrodes with geometries that are microscopic in one dimension and macroscopic in another dimension have been shown to have many of the properties of disk shaped microelectrodes due to a significant amount of nonlinear diffusion. However, due to their macroscopic length, measurable currents are obtained at ultrasmall bandwidths. These electrodes provide a useful tool to study the effect of extremely high current density. Furthermore, they show properties of great utility for electroanalysis because of their small area and high current density.
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ACKNOWLEDGMENT The technical assistance of Susan Richardson and Frank Qualls is gratefully acknowledged.
E ( V v s SCE)
Flgure 5. Cyclic voltammograms for the reduction of (a) 45 pM Ru(NH3)t+ in 0.1 M KNO, at a 2 mm diameter platinum disk (scan rate, 5 mV s-‘) and (b) 0.37 p M Ru(NH,):’ in 0.1 M KNO, at a -30-nm platinurn band electrode sealed in soft glass (scan rate, 2 mV s-’),
current. Residual or background currenta have several sources: electrolysis of the electrolyte or solvent, double layer charging, and redox processes of the electrode surface. Pulse techniques employing various potential wave forms have been used to provide a discriminationagainst double layer charging current with a resulting decrease in the detection limit to the lo-’ to M range (33). However, Evans (34) has pointed out that the detection limit of pulse techniques is determined by the oxidation and reduction of surface groups. The data in this paper show that the faradaic current at a band electrode is only slightly dependent on the electrode thickness. In contrast, the current arising from the double layer charging and the electrolysis of surface groups should be directly proportional to the electrode area. Thus,a decrease in the width of the band electrode should result in a decrease in the residual current with a concurrent improvement in the detection limits. As has been shown, some of the band electrodes have approached the values given by these predictions. Platinum films sealed in glass exhibited the best performance and thus these were used for trace determinations. In Figure 5 voltammograms for 0.37 pM and 43 pM solutions of ruthenium hexaammine at the platinum line and a conventional 2 mm diameter platinum disk electrode are compared. The platinum line electrode gave a well-defined voltammogram whereas the voltammogram from the conventional platinum electrode is largely composed of background current. Limiting currents from the band electrode were linear with the concentration of ruthenium hexaammine up to 12 p M , the highest concentration examined (r = 0.9999, n = 8). The lowest concentration examined was 7.1 X M ruthenium hexaammine, and at this concentration the residual current was twice the value of the faradaic current at a scan rate of 2 mV s-l. The range of concentration detectable by cyclic voltammetry with the platinum band electrode compares favorably with the detection limits obtained with conventionally sized electrodes using the more exotic pulse potential wave forms. It should be noted that these results are only an indication of the potential capabilities of line electrodes. If a means of fabricating band electrodes which give the expected values for the residual current is achieved, it can be easily seen that detection limits in the IO” to M range should be possible by linear scan voltammetry.
LITERATURE CITED (1) Wlghtman, R. M. Anal. Chem. 1981, 53, 1125A-1134A. (2) Ewlng, A. 0.; Dayton, M. A.; Wlghtman, R. M. Anal. Chem. 1981, 53, 1842-1847. (3) Howell, J. 0.; Wlghtman, R. M. Anal. Chem. 1984, 56, 524-529. (4) Lines, R.; Parker, V. D. Acta Chem. Scand., Ser. B 1977, 831, 369-374. (5) Robinson, R. S.; McCreery, R. L. J. Electroanal. Chem. 1985, 182, 61-72. (6) Hills, G.; Pour, A. K.; Scharikfer, B. flectrochim. Acta 1983, 28, 891-898. (7) Gunawardena. 0.; Hills, G.; Scharlfker, B. J . €/echoanal. Chem. 1981. 130. 99-112. (8) Dayton, M: A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 52, 946-950. (9) Fleischmann, M.; Lasserre, F.; Robinson, J.; Swan, D. J. Electroanal. Chem. 1984, 177, 97-114. (IO) Flelschmann, M.; Lasserre, F.; Robinson, J. J. E/ecfroana/. Chem. 1984, 177, 115-127. (11) Scharifker, 6.; Hllls, G. J. flectroanal. Chem. 1981, 130, 81-97. (12) Reller, H.; Klrowa-Elsner, E.; Gileadl, E. J. €/ectroanal. Chem. 1982, 138, 65-77. (13) Reller, H.; Klrowa-Eisner, E.; Gileadl, E. J. Electroanal. Chem. 1984, 161, 247-268. (14) Caudill, W. L.; Howell, J. 0.; Wlghtmann, R. M. Anal. Chem. 1982, 54, 2532-2535. -- - - - - - -
(15) Sleszynskl, N.; Osteryoung, J.; Carter, M. Anal. Chem. 1984, 56, 130-135. (16) Welsshaar, D. E.; Tallman, D. E. Anal. Chem. 1983, 55, 1146-1151. (17) Kovach, P. M.; Caudill, W. L.; Wlghtman, R. M. J. flectroanal. Chem. 1985, 185, 285-295. (18) Saito, Y. Rev. Polarogr. 1968, 15, 177-186. (19) Symanskl, J. S.; Bruckenstein, S. J. flectrochem. SOC. 1984, 731,
IlOC. (20) Klttlesen, 0. P.; White, H. S.; Wrlghton, M. S. J. Am. Chem. SOC. 1984, 106. 7389-7396. (21) Adams, R. N. “Electrochemistry at SolM Electrodes”; Marcel Dekker: New York, 1969; pp 36-37. (22) Albery, J. “Electrode Kinetics”; Clarendon Press: Oxford, 1975; p 59. (23) Sarangapani, S.; Yeager, E. I n “Comprehensive Treatise of Electrochemistry”; Yeager, E., Bockrls, J. O’M., Conway. B. E., Sarangapanl, S., Eds.; Plenum Press: New York, 1984; Chapter 1. (24) Blndra, P.; Brown, A. P.; Flelschmann, M.; Pletcher, D. J. flectroanal. Chem. InterfacM flectrmhem. 1975, 58, 31-37. (25) Swan, D. Ph.D. Thesis, University of Southampton, 1979. (26) Peter, L. M.; Dun, W.; Blndra, P.; Gerlscher, H. J. Nectroenal. Chem. 1976, 71, 31-50. (27) Nicholson, R. S. Anal. Chem. 1985, 37, 1351-1355. (28) von Benken, W.; Kuwana, T. Anal. Chem. 1970, 42, 11 14-1 116. (29) Holland, L. “Vacuum Deposltlon of Thin Films”; Chapman and Hall: London, 1956; pp 232-239. (30) Kasper, C. Trans. flectrochem. SOC.1940, 77, 365-384. (31) Thun, R. E. In “Physics of Thin Films; Advances In Research”; Hass, Q., Ed.; Academlc Press: New York, 1963: pp 187-228. (32) Lewls, W. “Thin Films and Surfaces”; Chemlcal Publishing Co.: New York, 1950. (33) Bond, A. M. “Modern Polarographic Methods In Analytical Chemistry”; Marcel Dekker: New York, 1980. (34) Sokol, W. F.; Evans, D. H. Anal. Chem. 1981, 53, 578-580.
RECEIVED for review February 21, 1985. Accepted April 9, 1985. This research was supported by USARO and NSF. R.M.W. is an Alfred P. Sloan Fellow.
