Stanton Ching, Ray Dudek, and Elie Tabet Connecticut College, New London, CT 06320
The development of ultramicroelectrode techniques is arguably the most important contribution to electroanalytical chemistry in the past 20 years. As their name implies, ultramicroelectrodes are extremely small, with dimensions on the order of micrometers or less. This small size, and the electrode characteristics that come with it, can be exploited in a number of unique applications. Indeed, nltramicroelectrodes provide access to cyclic voltammetry experiments previously considered impossible with conventionally-sized electrodes. New research includes measurements in highly resistive media (nonpolar solvents, polymers, gaseous interfaces, supercritical fluids), high-speed voltammetry (scan rates over one million volts per second), and analyses in small volumes or at microscopic locations (single brain cells, capillary chmmatography detectors, electrochemical microscopes) ( 1 4 . This article presents a basic introduction to the theory and use of nltramicroelectrodes in cyclic voltammetry. Although numerous undergradnate-level experiments are available for cyclic voltammetry (5-15) none have addressed the topic of ultramicroelectrode techniques. The laboratory exercises and discussions provided here are designed to complement existing experiments on conventional cyclic voltammetry (6,8-10).
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Journal of Chemical Education
Cyclic Voltammetly at Ultramicroelectrode Disks
In experiments with normal time scales, cyclic voltammograms (CV's) obtained with ultramicroelectrodes differ significantly from those obtained using electrodes of conventional size. This is due to differences in mass transnort within the difliusion layer, the reeon adjacent to the electrode in which elcctrolvsis takes nlace.The influence ofthe diffusion layer on the gppearance of a CV is shown in Figure 1. At large electrodes, mass transport occurs mostly perpendicular to the surface (planar diffision), Figure la. The result is a typical peak-shaped voltammogram. For a reversible redox process, the peak current follows the Randlessevcik relationship (16,17), eq 1,in which i, is peak current (A), n is electron stoichiometry, A is electrode area (cmZ),D is the diffision coefficient (cmz/s),d is bulk concentration of electroactive substance, and u is scan rate (Vls). i, = (2.69 x ~ O ~ ~ ~ A D ~ C ' (1) U ~
By contrast, mass transport a t nltramicroelectrodes takes on a hemispherical profile (radial diffusion),Figure lb. This produces a sigmoidal, steady-state voltammogram (1-4). Unlike the situation for a standard size electrode, currents generated a t ultramicroelectrodes are dependent on their geometry For a disk-shaped electrode, the limitingplateau current from a CVis given by eq 2, in which &,
P-
copper wire
P t microwire sealed in glass
Figure 2. Construction diagram for a Pt disk ultramicroelectrode.The length of the glass shaft can range from one to several inches depending on the thickness and strength of the capillary tubing used.
I
0.0 V
t0.5 E
vs. Ag
wire
Figure 1. D.flosion layer prof^ es and representat ve cyclic voltammograms tor (a) panar diff~sonand CV from a conventona y-size0 elenrode (2 mm diameter);(b] radal dilfuson an0 CV from an ,Itramicroeiectrode (I 0 pm diameter).Solution: 1 mM ferrocene in 0.1 M TBAPFdCH,CN. Scan rate: 50 mVls.
-
is limiting current (A), F is Faradafs constant, and r is electrode radius (cm) (18).For a conventionally-sized electrode, the current is proportional to electrode area, but is not geometry dependent (eq 1). ilim= 4 n ~ r ~ ~ *
(2)
Radial diffiion also greatly enhances mass transport to and from the electrode surface. As a result, the current density a t an ultramicroelectrode under conditions of radial diffision is much higher than that of an electrode experiencing planar diffision. Since current density determines the ratio of Faradaic to non-Faradaic current for a CV, backmund currents are tvuicallv small or unobserved for'cV's ibtained with ultrami~~oele&rodes operating at or near steady-state conditions. This phenomenon occurs due to the fact that the current density (and therefore the ratio of Faradaic to non-Faradaic current) increases as the smallest dimension of the electrode decreases; whereas, the overall background current decreases with diminishing electrode area.
0.0 v
+0.5
E
VS.
Ag wire
Figure 3. Cyclic voltammograms of 1 m M ferrocene in 0.1 M TBAPF$CH,CN solutions obtained with (a) 10; (b) 25;(c)50 pmdiameter Pt disks. The scan rate in 50 mVls. gram of a platinum disk ultramicroelectrode is shown in Figure 2. Experimental Reagents
All chemicals are reagent grade and can be used as received. Ferrocene and tetrabutylammonium hexafluorophosphate (TBAPF6)were obtained from Aldrich. Acetonitrile and dichloromethane were obtained from Fisher Scientific. Apparatus
Ultramicroelectrode Construction
A wide variety of methods are available for constructing ulrramicmelec~rodes( I ) . Among the common are disks, rings, and bands. The ultramicmelectrode disk is easiest to fabricate and can be made readily by encapsulating metal microwire in a matrix of glass or epoxy ( I ) . Carbon electrodes can be ~reuareds i d a r l v with carbon microfiber. Once t6e tip of the insulated wire serves as the electrode disk. Electrical contact is made to a longer, thicker wire with silver epoxy or mercury. A dia-
Apparatus for cyclic voltammetry has been described previously (8). Most commercial and home-built potentiostats are suitable for these experiments. The systems used here are a Bioanalytical Systems model CV-1B and an Electron Dansfer Technologies PCStat. Analog output was followed with a Yokogawa 3025 x-y recorder. Due to the small currents observed, the potentiostathecorder system must be capable of delivering relatively noise free W s in the 2-5 nA current range. A Faraday cage for the electrochemical cell is highly re~mmendedto suppress backVolume 71 Number 7 July 1994
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-
+0.5
0.0 V
E
vs. Ag wire
F~gure4 Cycllc vo tammograrns of 1 mM ferrocene In 0 1 M TBAPF, CH,CN solut ons obtamed wlth a 25-pm d ameter PI dlsk The scan rates are (a)20. (0) 100, (c)500 mV s ground noise. Sensitivity can be raised by increasing the fermcene concentration, but precipitation a t the electrode may be a problem for concentrations approaching 10 mM. Electrodes Ultramicroelectrodes can be constructed as shown in Figure 2 (1).Platmum micmwire (10, 25, and 50 pm diameter, Goodfellow)is flame-sealed in one end of glass capillary tubing. Silver epoxy (epo-tek HZOE, parts A and B from Epoxy Technology, Inc.) is then syringed into the open end of the tube and copper or stainless steel wire is inserted to make electrical contact. The epoxy is cured for two- hours C and the back end of the electrode is - - ~ at ~ -70 - ~ O sealed with heat-shrink tubing. Sanding and polishing the electrode tip exposes the Pt disk. The Wglass seal can be examined under a microscope for imperfections, although the oualitv of the electrode should ultimatelv be assessed elec&och&cally. The shaft of the electrode'can be made with different leneths depending on the wall thickness of the capillary tubhg. ~l&amic&electrodesare available commerciallv from most companies specializingin electrochemical apparatus, but the east is cokparativay high and variable electrode diameters mav not be available. Cyclic voltammetry was performed using a standard 3electrode cell with Pt auxiliary and Ag wire quasi-reference electrodes. A 2-mm diameter Pt disk was used as a conventionalsize electrode.All electrodes were freshly polished prior to each measurement with l pm alumina or diamond paste (Buehler) followed by sonication in water and rinsing in CH3CN. ~
~~
Procedure Prepare a solution containing 1mM femcene and 0.1 M TBAPFEin 10 mL CHCN. Record CVs usim a scan rate of 50 m ~ /with i (a) a co&entionally-sized ~telectrodeof 1mm diameter or lareer and (b) a 10-um diameter Pt ulof ferrocene is tramicroelectmde. T K ~oxidation -0.25 V versus silver wire, but this varies since the silver wire electrode is only a quasi-reference (16,17). Cyclic voltammomams in all cases were recorded within a potential windowranging from 0.6 to 1.0 V Use the same solution to examine the ~lectrochemicalresponse of ferrocene with different sized ultramicroelec604
Journal of Chemical Education
I
I
I
0.5 0.0 E vs. Ag wire F~g~re 5 Cychc voltammograms of 1 mM ferrocenein Ch2CI, w In 1 mM TBAPF, (a) 1 0 - ~ r ndarneler R Lllramlcroe ecirode. (b) 2mm o arneler Pt o sk electrode The scan rate (s50 mV s. +1 .O
tmdes. Record CVs using 10,25, and 50-pm diameter Pt electrodes a t a scan rate of 50 mV/s. Again, with the same solution, observe how variable scan rates affect the appearance of ferrocene CVs for 10and 25-pm diameter ultramicroelectrodes. Record CVs with each electrode at scan rates of 20,100, and 500 mV/s. Prepare a 5-mL solution of CH7C17containing ferrocene and TBAPF,, both in only 1 m ~ - c o k ~ t r a t i oOhtain n a CV with a 10-umdiameter ultramicroelectrode.Rewat the measurement with an electrode of conventional size. Results and Discussion Perhaps the most fascinating aspect of ultramicroelectrodes from an undergraduate perspective is their extremely small size. Taking time initially for visual inspection of the electrodes is, therefore, worthwhile. The 50- and 25-um disks can be seen with the naked eve close . upon . examination. However, a magnifying glass or microscope oRen is needed to detect the 10-um disk. Bv virtue of this small size, ultramicroelectrodek can be used for experiments that require electrochemistri in small sample volumes or at mi&oscopic locations. ~ b example, r they have been used extensively to study the dynamics of dopamine, an electroactive neumtrans&tter. in and around ihe cells of brain tissue (19). The effects of diffusion on cyclic voltammetry are illustrated from a comparison of CVs obtained for ferrocene with a Pt electrode of conventional size, and a 10-pm diameter Pt ultramicroelectrode, Figure 1. Planar diffusion at the large electrode gives a standard peak-shaped CV. By contrast, radial diffision at the ultramicroelectrode gives a siemoidal. steadv-state response. As ex~ected.the hieher ., current density at the ultramicroelectrodr manifests itself in a smaller backmound current. The diffusion coefficient of ferrocene can becalculated with each electrode by applying eqs 1 and 2. With a conventional electrode, D can be determined from the slope of a plot of i, versus di2(8). For an ultraminoelectrode, D is calculated much more easily by measuring the limitingplateau current from the CV.
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A question commonly asked of ultramicroelectmdes is *How small do they have to be?" Since the current for radial diffusion depends on electrode geometry, the answer is not straightforward. In general, however, radial diffusion (and, therefore, ultramicroelectmde character) is established when the value ofDt lr: is greater than one. In this exoression. t is the electrolvsis time (6) and rnis the smallThus, . foia given set of esi dimension of the electride (a) conditions. there is a trade-offbetween the size of the electrode and 'the time scale of the experiment. Radial diffusion is favored with small electrodes and slow scan rates; whereas, planar diffusion is favored with large electrodes and fast scan rates. The interdependence of electrode size and experimental time scale is demonstrated by experiments with results presented in Figures 3 and 4. Figure 3 shows what happens to CV's of ferrocene when they are recorded with electrodes of lo-, 25-, and 50-pn diameter. With a 50-mV/s scan rate, peak-shape characteristics become more prevalent as the-size of the electrode increases. This arises due to concurrent enlarpement of the hemispherical diffusion layer needed to support radial mass transport (Fig. lb). Consequently, radial diffusion takes longer to establish with larger electrodes and planar diffusion becomes a more likely contributor to mass transport. This is substantiated in Fieure 3. which shows more orominent ~ e a k shape in t h & & &corded &th the large;. electrodes. Despite the greater influence of planar diffusion,limiting plateau currents can he measured and diffusion coefficients calculated from each of the CVs in Fieure 3. Limiting currents for the lo-, 25-, and 50pm ele&odes fall in a k:10 ratio, and all three electrodes yield similar values for D. These results are in agreement with the radial diffusion expression given by eq 2, suggesting that radial diffusion is still a major mode of mass transport with these electrodes. Variations in the cyclic voltammetric time scale also can influence the characteristics of ultramicroelectmdes. depending on their size. For a 10-pm diameter disk, s i h a r siemoidal CVb (see Fia. 1)are obtained with scan rates UD to-500 mV/s, indicating that radial diffusion is maintain& within this time frame. These results support the lack of scan rate de ndence for radial diffusion (eq 2) and contrasts the d 2dependence found for planar diffusion (eq 1).However, the CV's obtained with a 25-)lm electrode exhibit peak shape character at faster scans due to the onset of planar diffusion, Figure 4. Here it is apparent that the time scales for adequately establishing radial diffusion at an electrode of one size can be inadequate for another, larger electrode. Ultramicroelectrodeshave redefmed high speed cyclic voltammetry as a result of their much smalikr uncompensated resistance and background capacitance values (20). Such experiments, as much as lo6 faster than what ispossible at electmdes, have aided the understandine of fast chemical Drocesses that accomDanv " electrode rea&ons. One of the most useful properties of ultramicroelectrodes is that they are much less susceptible to the severe distortion caused bv uncomoensated resistance. or ohmic droo. This has openkd new dpportunities for cyckc voltammetr& studies in poorly conductive media, such as nonpolar solvents, polymers, and other rigid matrices. The effects of hiah solution resistance can be observed bv limiting the ~
~
.
~~
P"
.
the migration of electroactive species), it is typically used in large excess relative LO the substance being studied. In a strict sense, cyclic voltammetry is impossible in the complete absence of supporting electrolyte. Voltammograms of 1mM ferrocene recorded in CH2C12 with just 1mM TBAPFBare shown in Figure 5. This extremely low concentration of supporting electrolyte has essentially no influence on the CV obtained with a 10 pm ultramicmelectrode, Figure 5a. However, substantial peak separation and broadening are observed in the CV obtained using a conventionally-sized electrode, Figure 5b. The distortion is caused by large uncompensated solution resistance (R,), which creates a substantial ohmic drop (iRJ in the cell between the applied and working electrode potentials Ultramicmelectrodes, by contrast, are much less sensitive to large solution resistances because the value of iR, (and, therefore, the ohmic drop) is relatively insimificant given the extremely small currents that are ge"ernted. Thus, even at very low electrolyte concentrations there is still sufficienti o ~ conductionfor c a successful experiment. Voltammetry with ultramicroelectrodes has even been recorded in solutions without deliberatelv added su~oortine " electrolyte. In such cases, small amounb of impurities are an adequate source of electrolyte. Theoretical analysis has shown that the effective solution resistance is inversely proportional to the smallest dimension of the electrode (21). This point is noteworthy because it indicates that a nepli~bleohmic drop can be observed even with l a z e currents-as long as one-of the electrode dimensions issmall, such as with a ring or band geometry.
..
Conclusion Ultramicroelectrodeshave made a tremendous i m ~ a cin t electroanalytical chemistry and their use conti&es to mow. The exercises presented here illustrate basic ~rinciples of ultramicroele&odes and compares their properties with those of conventional electrodes. Comparisons include radial versus linear diffusion, sigmoidal versus peak-shaped voltammograms, electrode-size dependence, scan rate dependence, and effects of solution resistance. Literature Cited I. ~ i g ~ t ~ . ~ . ~ . ; ~ i ~ Dekker: New York. 1989:Vol.15. p 267.
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2, Wightman,R, M, 888,240, 415, s.;Fleiphmann, M,AuI, Chrm, 188,,59, 4. wight-, R. ~ . hcham. d 1e81,s3,112s~. s. E V S ~ ~ . DH . ; o % o ~ ~ ~KI IM, . ; P ~ ~ ~ - ~ , R . A . ; K ~ ~J.~ ~c h. oMm. .J~ d u eIQQS,~~, . ~~~
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7. ~i~smger, P T.: in^^^^, R. J. cham. E&C. 1883,60,702. 8. vanB ~ ~ s c ~ o J.~J ;PL~ ~, W J.~ Y; ~ , in^^^, w R.: ~ o s tD. ~A,; , ~ ~ a i nPgT.~ ~ , J. Cham. Educ 1W.60,772. 9. B a l d ~ nR. , P.:Raviehandran. K.; Johnson, R. K. J Chem. E&e. 1884,62,820. 10. Bdh.. . E.:. G ~ .O J.A . . . : &- ~ ~ u ~~ z .~R .~ J. J.~ mom. ~ E=~ U h C1m7. . M. . ~M.: 189. 11. Plszczek. L:hatoulcz.A.:Kielbasa.J. J. Cham. Edue. 1988.65.171,
.
17. Heheman, W. R.; Kis8ir.w.P T. in Loboratory lkhnigues in Ekdmondyti61 ChmiBry. Kisainger, 1984:Chanter 3. D 82
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T.; Heinemsn, W. R.,Eds.; Marcel Deltker: N w York.
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