a drastic decrease in its diameter and a simultaneous tight seal of the metal within the glasscapillary. Both radii of the metal and surrounding glass can be controlled by changing theparameters of the pulling program, e.g., the temperature of heating and the strength of pulling.

Immediately after pulling, the platinum core may be covered with glass and must beexposed. In order to obtain disk-shaped UMEs, the pulled UME can be polished using amicropipette beveller (e.g., Sutter BV-10) equipped with a micromanipulator. The micro-manipulator is used to move the pulled UME toward a slowly rotating abrasive disk cov-ered with 0.05-�m alumina. The axis of the pulled UME is kept perpendicular to the planeof the abrasive rotating disk. The diameter of the exposed platinum wire increases with thelength of polishing. Optical microscopy is used to check the polished UME and the pol-ishing is stopped when a desired size is obtained. UMEs with diameter of 1–3 �m can beobtained using this procedure. Nanometer-sized electrodes can also be obtained if thepolishing is carefully controlled (see Section 6.3.3).

Once fabricated, the UMEs can be characterized using SEM and steady-state voltam-metry as described in Section 6.3.1. If the UMEs are used as SECM tips, the size and theshape of the tips can be evaluated using SECM (6) as described in Chapter 12.

REFERENCES

1. R. M. Wightman, D. O. Wipf, in Electroanalytical Chemistry, A. J. Bard, Ed., Marcel Dekker:New York, 1989, Vol. 15, p. 267.

2. A. J. Bard, F.-R. Fan, M. V. Mirkin, in Electroanalytical Chemistry, A. J. Bard, Ed., MarcelDekker: New York, 1994, Vol. 18, p. 243.

3. B. D. Pendley, H. D. Abruna, Anal. Chem. 62, 782 (1990).4. Y. H. Shao, M. V. Mirkin, G. Fish, S. Kokotov, D. Palanker, A. Lewis, Anal. Chem. 69, 1627 (1997).5. B. B. Katemann, W. Schuhmann, Electroanalysis 14, 22 (2001).6. A. J. Bard, M. V. Mirkin, Eds., Scanning Electrochemical Microscopy, Marcel Dekker: New

York, 2001.

6.3.3 Laser-pulled ultramicroelectrodes

Janine Mauzeroll1 and Robert J. LeSuer 2

1Laboratoire d’Electrochimie Moléculaire, Université Paris, 7-UMR CNRS 7591, France

2Department of Chemistry and Physics, Chicago State University, Chicago, IL 60628

6.3.3.1 Introduction

This section discusses the fabrication of microelectrodes with diameters of a fewmicrometers to tens of nanometers using a laser-pulled technique. The methods dis-cussed focus on the fabrication of platinum (Pt) ultramicroelectrodes (UMEs) sealed in

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quartz, although the concepts can be readily applied to other glass/metal combinations.A general review of UME fabrication has been presented (1) and several publicationshave demonstrated the utility of laser-pulled tips (2, 3). This section provides a compre-hensive description of laser-pulled UME fabrication, highlighting key steps and provid-ing insight into trouble spots. The many parameters used in making laser-pulled UMEsadd a significant amount of complexity but provide a comparable amount of flexibilityin the final shape of the UME.

6.3.3.1 Microelectrode fabrication

(a) Equipment requirements (4)The most important tool for fabricating pulled microelectrodes is a micropipette puller. Apopular micropipette puller used for scanning electrochemical microscopy (SECM) micro-electrodes is the Sutter Instrument (Novato, CA, http://www.sutter.com/) P-2000 CO2 laserpuller. A laser puller, which in 2004 cost approximately $13k, has several advantages overresistance-based pulling systems. The most relevant of them to microelectrode fabricationare the ability to heat quartz and better control the heat distribution. Metal wire (Goodfellow)typically used is 25 �m in diameter though other diameters can be used. Platinum wire canbe purchased in two different forms: hard or annealed. Annealed wire is generally softerand can be pulled at lower temperatures than hard Pt wire. However, hard Pt wire is eas-ier to manipulate and is less likely to be damaged by mechanical stress. The parametersgiven below were designed for pulling annealed wire, but optimizations for hard wire con-figurations will be discussed as well.

Quartz capillaries (Sutter Instrument, Co.) with an outer diameter (OD) of 1 mm andinner diameter (ID) of 0.3 mm are preferred. Softer glass can be used, and must be used,if the microelectrode puller is based on resistance heating. The ID of the capillary willinfluence both the ability to position the wire properly and the thickness of the insulatingglass sheath surrounding the metal surface of the UME. Sutter claims that their capillariesmaintain a constant OD/ID ratio throughout the pull when making micropipettes. It is notclear how this translates to the construction of a UME as the wire is sealed into the glasscapillary prior to pulling. Ideally, one would prefer a small OD/ID ratio as this means lessinsulating sheath will need to be removed during the polishing and beveling steps.However, for capillaries with the same OD, a thicker wall provides extra electrode stabil-ity as well as a smaller hole through which to align the axis of the microwire with that ofthe capillary.

Additional equipment includes a vacuum pump for evacuating the capillary prior to seal-ing along with vacuum tubing of appropriate size. After the electrode is pulled, contact canbe made between the microwire and an electrode lead with either premixed silver epoxy,mercury, or a concentrated electrolyte solution. Additional larger capillaries as well asquick setting epoxy may be used to reinforce the microelectrode prior to exposing and pol-ishing. To expose the UME wire with either sharpening or beveling, either commercialinstruments (WPI or Sutter) or home built designs can be used. The basic necessities forpolishing and beveling UMEs include: a method to rotate the electrode; a flat surface toapply polishing cloths of varying abrasiveness (600 or higher grit sandpaper, micropolish-ing cloths, 0.05 �m alumina slurries and diamond pastes, all of which can by supplied by

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Buehler, Lake Bluff, IL); a method to manipulate the plane of the polishing surface; and amicroscope or other method to facilitate viewing the UME tip. An optical microscope is anecessity for monitoring the various stages of pulled UME construction.

(b) Capillary and wire preparationAll materials should be handled with gloves and kept free of dust and debris to eliminatesealing of contaminants into the glass along with the microwire. Capillaries can be cleanedwith a dilute (10% v/v) nitric acid solution, rinsed with water, and allowed to dry thoroughlyin an oven. Pt wire can be cleaned with either dilute nitric acid or organic solvents (ace-tone and hexanes) followed by a rinse with high purity (18 M� cm) water and a thoroughdrying. The next step is to straighten the microwire to be inserted into the center of the cap-illary. One to two centimeters of microwire should be carefully straightened by rubbingalong the axis of the microwire. One should avoid twisting the microwire or introducingother mechanical stress that may result in breaks in the pulled microelectrode. Hard wiretwists and bends less but requires more power in the pulling step to generate a useableUME. Using a stiff wire small enough to fit into the capillary, the Pt microwire is thenpositioned into the middle of the capillary so that, ideally, two symmetric microelectrodescan be produced from a pull.

(c) Wire sealingAfter positioning the microwire into the center of a capillary, it is placed into the pullerand a vacuum is attached to each end of the capillary (Figure 6.3.3.1). A vacuum isapplied to the capillary for up to 30 min to remove residual moisture and to minimizebubble formation in the glass during the sealing. The safety shield of the P-2000 is notdesigned to support the vacuum tubes that are connected to the end of the capillary to bepulled. However, it is possible to apply a vacuum to the capillary without interfering withthe safety mechanisms of the pipette puller and it is strongly encouraged that the readercontacts the manufacturer before diverging from recommended instrument use. To insurethat the instrument exerts no pulling force on the capillary during the sealing step, stop-pers (which can be home built or purchased from Sutter) are placed on each of the pullingsleds (Figure 6.3.3.2a). When the capillary is sufficiently evacuated, a heating program isapplied to seal several millimeters of the microwire into the quartz. Of the five variablesavailable on the P-2000 laser puller, only the heating power (H) and filament (F) areimportant for the sealing step. Two sets of parameters have been used successfully by theauthors. With a heating power set to 900, the filament size is set to 4; or heating power andfilament can be set to 925 and 15, respectively. The difference between the two programslies in the amount of wire that will be sealed in the glass. For filament 15, the laser scansslowly over 8 mm of the capillary. A filament of 4 scans the laser over 6.5 mm of the cap-illary more quickly. The amount of metal sealed into glass will influence the length of thetaper formed during the pull. In both cases, the heating program was applied for 50 sec,followed by a 25-sec cooling time. A total of five heating/cooling cycles were performedto insure a proper seal (Figure 6.3.3.2b). Schuhmann (3) reports that much lower heatingcan be used successfully (H: 775; F: 5) with five cycles of 40 sec heating and 20 sec cool-ing. The sealing and pulling parameters are influenced by the wear and tear of individualinstruments. Thus, when optimizing the sealing step, one should monitor the extent of a

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202 6. Ultramicroelectrodes

Stopper StopperQuartz tube

Vacuum

Vacuum Vacuum

Reinforcing Glass

Hg

Vacuum

25 µm Pt wire

(a)

(b)

(c)

(d)

Figure 6.3.3.2 Diagram of the sealing and pulling protocol for the fabrication of Pt laser-pulledUME. (a) The straight Pt wire is inserted into the quartz capillary. The capillary is installed in thepuller and connected to a vacuum. The sleds have been secured using stoppers. (b) The sealing pro-gram is run five times. (c) The pulling program is applied following the removal of vacuum and stop-pers. (d) The pulled tips are electrically connected and strengthened with a larger diameter glasscapillary.

Laser

vacvac

stopperstopper

Glass capillaryFlexible rubberjoint

Metal joint tovacuum

Laser

vacvac

stopperstopper

Figure 6.3.3.1 Diagram of the P-2000 laser-puller setup for the fabrication of Pt laser-pulled UME.The secured quartz capillary is connected via a rubber joint to a machine metal tube that is connectedto a vacuum pump with a Y-joint. Two homemade stoppers are added to the sleds of the puller.

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seal after each cycle to minimize the amount of heating time required, because prolongedheating cycles could result in damage to the puller.

(d) Wire pullingThe P-2000 uses five parameters to conduct a pull. The heating power, H, is the amountof heat applied to the capillary. Filament, F, can be 1 of 16 values that determines thelength of capillary heated as well as the speed at which the laser is scanned (Table 6.3.3.1).For a given H, slower scan speeds and smaller scan lengths result in greater heating of thecapillary. Velocity, V, determines the point at which the glass has reached the desired temperature based on changes in glass viscosity. Once the velocity is achieved, the laser isshut off. On the P-2000, the magnitude of this value is counterintuitive; a higher value forvelocity means the laser will shut off when the glass reaches a lower temperature. Delay, D,is the time between the laser shutting off and the hard pull. A delay of 127 indicates thatthe hard pull will trigger when the laser is shut off. Values above 127 (to a maximum of255) indicate the millisecond delay between shutting off the laser and the hard pull. Valueslower than 127 allow the user to initiate the pull with the laser still on. Pull, P, is the forceexerted on the capillary to create the UME. With the exception of the delay, all of theparameters have units that are not easily translated into physical quantities. Each parame-ter has an important effect on the shape of the microelectrode and these are summarized inTable 6.3.3.2.

Optimization of a pulling program requires patience, practice, and a bit of luck. The effi-ciency of UME fabrication using this technique is rarely more than 60% and when every stepis taken into consideration, is typically much less. Under ideal conditions, one capillaryshould produce two identical microelectrodes. However, small imbalances in the pullingsolenoids or imprecise calibration of the laser scanning results in less than ideal behavior.

6.3 UME Fabrication/Characterization Basics 203

Table 6.3.3.1

Filament number corresponding to the scan length and scan speed of the laser for the Sutter P-2000

Speed Length (mm)

1.5 1.9 4.5 6.5 8

Fast 1 2 3 4 5medium 6 7 8 9 10slow 11 12 13 14 15

Table 6.3.3.2

Effect of pulling parameter on the shape of the pull

Parameter Increase Decrease

Heat Smaller tip; longer taper Larger tip; shorter taperVelocity Smaller tip Larger tipPull Smaller tip; longer taper Longer tip; shorter taper

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When attempting to optimize a pulling program, one should start with one of the programslisted here or a program suggested by the manufacturer that closely resembles the desireddimensions. Only one parameter should be adjusted at a time (for the P-2000, Sutter rec-ommends changing values other than F in intervals of 5). The P-2000 reports the timerequired to pull the UME, and this value can be diagnostic of the success of the pull. Tomaximize the reproducibility of a parameter set, the values should be adjusted such that apull takes between 4 and 6 sec to complete.

Two very similar procedures have been used by the authors with similar success. Thefirst performs the pull within 25 sec of the final heating step, such that the quartz has notcompletely cooled prior to the pull. The laser is turned off, the vacuum tubes removedfrom the capillary and the stoppers removed from the pulling sleds followed by a pullusing the parameters: Heat: 875; Filament: 2; Velocity: 130; Delay: 150; Pull: 200. Thisprocedure results in electrode tapers of 3–5 mm and metal disk diameters from 200 nm to4 �m. Alternatively, the sealed capillary can be cooled to room temperature prior to exe-cuting a pull using the parameters: Heat: 875; Filament: 2; Velocity: 50; Delay: 120; Pull:200. This method results in electrodes with tapers of roughly 1 cm and electrodes as smallas 3 nm diameter have been fabricated.

It is important to inspect a pulled UME to insure continuity of the Pt wire. Oftentimes,small cracks from 1 to 100 �m will occur. These cracks typically occur either where theglass begins to taper or at the very tip of the UME and are most likely due to a section ofPt wire being pulled when it has cooled too much. Cracks at each of these locations canbe avoided by systematically optimizing the pulling parameters (Figure 6.3.3.3). When acrack is at the start of a taper, the glass surrounding that portion of wire likely absorbsmuch of the heat from the laser and the Pt microwire does not melt sufficiently. Either asmaller velocity or higher heat value can be used to supply more heating to the capillaryat this stage of the pull. When the crack occurs at the tip of the UME, the wire has cooledtoo much toward the end of the pull, and the delay between the hard pull and the laser shutoff trigger should be lowered. Note that by changing these values, the shape of the UMEwill change as well, so parameters should be changed only enough to eliminate the breakin the Pt wire.

(e) Electrical connection and reinforcementConnection of an electrode lead to the unsealed Pt wire can be made with mercury, sil-ver epoxy, or a concentrated electrolyte solution. Mercury is the easiest material for

204 6. Ultramicroelectrodes

Figure 6.3.3.3 Optical micrographs of a break in the Pt wire, which occurred during the pullingstep. Small cracks likely appear where the pulling force is exerted on a cooled portion of the electrode.

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making electrical contact but can easily be displaced and results in loss of contact. An elec-trolyte solution can be used for making electrical contact but problems related to a shift inelectrode potentials and the possibility of solution evaporation over time can occur. Silverepoxy provides a solid contact with the least amount of capacitance. However, delivery ofthe epoxy into a small capillary is challenging. To deliver mercury into a 0.3-mm ID cap-illary, a long syringe was custom made from HPLC-like needles that could fit onto a typ-ical disposable syringe (SGE Inc., Austin, TX). We have had the most success with Hgcontacts and Schuhmann (3) reports good results with silver epoxy. To provide additionalstability, the UME can then be sealed in a larger, reinforcing glass tube using 5-min epoxy.This step is sometimes avoided since it is critical that the axis of the UME be parallel tothe reinforcement tube. However, rotation of the UME/reinforcement tube while the epoxydries gives a satisfactory result. Figure 6.3.3.2 summarizes the four steps of UME fabri-cation using a laser puller.

( f ) Exposing the metal surfaceThe authors used a home built beveller to expose the Pt disk and sharpen the UME; how-ever, commercial bevellers are available (WPI or Sutter). Generally, solid-surface bev-ellers use a high quality lapping film that is widely used in the fiber optics industry andcan be easily replaced if the abrasive is damaged or saturated with glass particles. Othermodels rely on optically-flat mirrored glass disks, wetted with an abrasive slurry to bevelfluid-filled microelectrodes that are commonly used in microinjection applications. In theformer case, the beveller spins at 4000 rpm to provide sufficient cutting force to producea sharp, uniform tip in a very short time. The latter model spins at 60 rpm and is said to bepreferred for pipettes that are 1 �m or less. Fluid-filled pipettes are hollow and much moredelicate than pulled tips and so in designing our home-built system, we elected to workwith high spin rates so that a minimum amount of time would be needed to expose andshape the tips. Only one touch to the polishing cloth is necessary to change the surface ofthe electrode and the least amount of manipulation is desirable.

The home-built beveller consisted of a rotating-disk electrode combined with a flat sur-face that could be translated up and down and tilted (Figure 6.3.3.4a). To expose the Pt sur-face, the UME is wrapped in Teflon tape, placed in a rotating-disk electrode setup (PineInstrument, Co., Grove City, PA) and rotated at 5000 rpm. The desired polishing materialis applied onto a flat surface, which is brought in contact with the rotating UME. Touchingthe tip of the UME to the abrasive surface can be observed as an oscillation of the longquartz end. This procedure is repeated until a reasonable CV response is obtained. The pro-cedure is repeated with various polishing materials, typically starting with wet 1200 gritsandpaper and finishing with 0.05 �m alumina on a polishing cloth, until the CV responseof the UME is ideal (Figure 6.6.3.4b).

Alternatively, the exposing step can be performed using hydrofluoric acid (2).Unlikepolishing, HF etching will produce a conical or cylindrical UME. Although this may causecomplications in the quantitative analysis of data, a protruding metal tip minimizes the dif-ficulties of crashing the insulating sheath into a surface, and precise knowledge of zerotip/substrate distance can be obtained. Very small UMEs have been obtained by dipping apulled tip in 49% HF solution for 3 sec three times (Figure 6.3.3.5). Concentrated HF solu-tions dissolve quartz very quickly, especially on UMEs with taper thicknesses less than

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1 �m. Very little work has been done with HF etching of UMEs. However, as the SECMtheory for conical tips has been developed (5), this method should be explored as a viablealternative to mechanical polishing.

(g) Sharpening and etchingThe final step in making an UME for SECM techniques is also the most challenging. Thegoal is to remove enough of the insulating glass sheath from the tip of the electrode suchthat the diameter of the sheath is (ideally) less than 10 times the diameter of the metal disk.Like sharpening a pencil, quartz surrounding the exposed Pt disk can be removed by pol-ishing the UME at an angle. A flat surface with fine grit (600+) sandpaper is tilted andbrought into contact with the UME. Unlike exposing the metal surface, where a UMEslightly off the rotation axis was advantageous, UME sharpening requires the tip to be onthe rotation axis in order to avoid asymmetry in the sharpening and, worse, breaking ofthe UME. Quartz tapers are surprisingly flexible and can withstand a small amount ofbending, thus allowing contact between the tip and the abrasive surface to be observed(Figure 6.3.3.6a). UMEs sharpened in this way result in insulating sheath diameters thatare approximately 10–20 times larger than the disk diameter (Figure 6.3.3.6b).

6.3.3.2 Microelectrode characterization

(a) VoltammetryVoltammetric techniques are used in analyzing the qualities of a new UME as discussedin Section 6.3.1. Slow scan (�100 mV sec�1) linear sweeps should result in a scan rate

206 6. Ultramicroelectrodes

Figure 6.3.3.4 (a) Exposing the Pt surface using a homebuilt beveller. (b) Voltammetric responseof exposed tips in 1 mM ferrocenemethanol/0.1 M KCl.

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6.3 UME Fabrication/Characterization Basics 207

Figure 6.3.3.5 Scanning electrochemical micrograph (SEM) of an etched HF UME. Etching of asealed Pt UME in concentrated HF for several seconds can provide electrodes with very small RGs.However, the etching process is not easily controlled and can yield nonideal results.

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independent sigmoidal curve with a current height equal to nFr0DC* where n is thenumber of electrons transferred, F is the Faraday constant, r0 is the radius of the disk, D isthe diffusion coefficient, and C* is the concentration of the electroactive component insolution. Ideal voltammetric behavior consists of a steady-state plateau in the forwardportion of the potential sweep and a reduced capacitive hysteresis on the return portion ofthe sweep. As seen in Figure 6.3.3.4b, pulled-laser UMEs behave like ideal UMEs in termsof retraceability, reduced capacitance, and stability of the steady-state current. In electro-chemical studies where small analyte concentrations are of interest, these tips are wellsuited (6). These electrodes maintain a well-shaped sigmoidal behavior even at high scanrates and the lack of background current indicates that the electrodes are tightly sealed.

Measuring the resistance between a UME and an indium/gallium alloy is an alternativemethod of estimating the electrode surface area of a UME (2). This method has beenincorporated into some commercial bevellers for monitoring exposure of the UME diskduring polishing. However, this method has the disadvantage of contaminating the elec-trode with the alloy.

Neither voltammetry nor resistance methods can easily differentiate between planar andnonplanar geometries, and other techniques need to be used in order to fully characterizea UME. Two other techniques, SECM (see Chapter 12) and scanning electron microscopy(SEM), help to differentiate between an inlaid disk, recessed, and conical-shaped UMEs.

208 6. Ultramicroelectrodes

Figure 6.3.3.6 (a) Sharpening of the exposed Pt UME using the homebuilt beveller. (b) Opticalmicrographs of a sharpened UME.

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6.3 UME Fabrication/Characterization Basics 209

Feedback mode SECM experiments (Figure 6.3.3.7a) can distinguish between differentUME geometries of conventional size (7–13). In the case of laser-pulled tips, presentSECM theory can also distinguish between inlaid and noninlaid geometry (Figure 6.3.3.7b).The feedback current response for a recessed and a convex geometry (hemispherical, con-ical, and spherical) will both result in a lower normalized current response than that of theinlaid geometry (2). In positive feedback mode, the recessed geometry will yield a posi-tive feedback response when slightly recessed and what appears to be kinetically con-trolled behavior when deeply recessed. It will also have a maximum current related to thecontact of the glass opening with the metal surface and the sealing of the microcavity. Theconvex geometries will yield positive feedback and present a short circuit current when putin contact with the conductive substrate.

SEM can be used to “visually” inspect UMEs (Figure 6.3.3.8). Images allow one todetermine the approximate diameter and geometry of a UME as well as to determine theelectrode RG and observe if the disk is centered in the quartz. The biggest difficulty inobtaining suitable SEM images is that the large amount of insulating material surroundinga small, conductive disk sometimes results in a significant amount of distortion due tocharging of the quartz sheath.

6.3.3.3 Microelectrode maintenance and storage

Maintenance of UMEs is a misnomer because the time it takes to “repair” a nonfunc-tioning electrode can be better spent fabricating a new UME. However, broken or fouled

Figure 6.3.3.7 (A) Steady-state current–distance curves for mercury/Pt (25 �m diameter) hemi-spherical tip (line) as compared with theoretical behavior of a planar disk (�) and hemispherical (●).[Reproduced with permission from J. Mauzeroll, E. A. Hueske, A. J. Bard, Anal. Chem. 75,3880–3889 (2003). Copyright 2003, American Chemical Society.] (B) Steady-state current–distancecurves for a “lagooned” tip over a planar conductive substrate corresponding to different values ofthe parameter l/a (where l is the depth of metal recession and a is the tip radius) and the analogousworking curve for a disk-shaped tip. l/a: 10 (�), 5 (�), 1 (●), 0.5 (�), and 0.1 (�). The upper curvewas computed for a disk-shaped tip from equation S11 of reference (2). [Reproduced with permis-sion from Y. Shao, M. V. Mirkin, G. Fish, S. Kokotov, D. Palanker, A. Lewis, Anal. Chem. 69, 1627(1997). Copyright 1997, American Chemical Society.]

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UMEs can be cut to larger diameters (2–5 �m) and used as conventional electrodes formany months. Electrodes can be stored dry, typically on a glass slide or taped to someother surface to protect the fragile tip, or they can be stored in distilled water to mini-mize contamination from air pollutants. Unlike larger (25 �m diameter) microelec-trodes, polishing is not a suitable cleaning method unless an increase in the electrodearea is acceptable. UMEs on the order of 1 �m diameter and larger can, in principle, bepolished and sharpened by hand without increasing the electroactive area. FouledUMEs can sometimes be cleaned electrochemically via cycling through the potentialwindow of 0.5 M H2SO4 for 20 min (14). It is also possible to renew a UME by bathingit in the vapor of refluxing HNO3 followed by electrochemical reduction of the PtO sur-face (15). One should note, however, that with very small UMEs, the formation of anoxide layer and its subsequent removal can result in the active surface becomingrecessed.

6.3.3.4 Conclusions

This section has described the fabrication of sub 100 nm electrodes based on a laser-pullingtechnique. Because laser-pulled UMEs with an inlaid disk geometry require a mechanicalsharpening step, micrometer to submicrometer dimensions might be the practical limit forelectrodes fabricated with this technique. If an inlaid disk is not a requirement, the lowerlimit of electrode size can be extended to the 10 nm region by using HF-etching techniquesto expose a conical surface.

The push towards nanometer sized electrodes brings with it challenges in some of thefundamental assumptions made in electrochemistry as discussed in reference (16).

210 6. Ultramicroelectrodes

Figure 6.3.3.8 Scanning electron micrographs of 50-nm radius conductive Pt disk somewhat dis-placed toward the edge of the 1-�m radius glass ring. [Reproduced with permission from Y. Shao,M. V. Mirkin, G. Fish, S. Kokotov, D. Palanker, A. Lewis, Anal. Chem. 69, 1627 (1997). Copyright1997, American Chemical Society.]

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REFERENCES

1. A. J. Bard, M. V. Mirkin, Scanning Electrochemical Microscopy, Marcel Dekker, Inc.: NewYork, 2001.

2. Y. Shao, M. V. Mirkin, G. Fish, S. Kokotov, D. Palanker, A. Lewis, Anal. Chem. 69, 1627(1997).

3. B. B. Katemann, W. Schuhmann, Electroanalysis 14, 22 (2002).4. The companies listed are those that have been used by the authors, but their inclusion in this

chapter is neither an endorsement nor a statement that they are the only companies that providesuch services.

5. C. G. Zoski, B. Liu, A. J. Bard, Anal. Chem. 76, 3646 (2004).6. J. Mauzeroll, A. J. Bard, Proc. Natl. Acad. Sci. USA 101, 7862 (2004).7. M. V. Mirkin, F. R. F. Fan, A. J. Bard, J. Electroanal. Chem. 328, 47 (1992).8. C. Demaille, M. Brust, M. Tionsky, A. J. Bard, Anal. Chem. 69, 2323 (1997).9. Q. Fulian, A. C. Fisher, G. Denuault, J. Phys. Chem. B 103, 4387 (1999).

10. Q. Fulian, A. C. Fisher, G. Denuault, J. Phys. Chem. B 103, 4393 (1999).11. Y. Selzer, D. Mandler, Anal. Chem. 72, 2383 (2000).12. C. G. Zoski, M. V. Mirkin, Anal. Chem. 74, 1986 (2002).13. N. J. Gray, P. R. Unwin, Analyst 125, 889 (2000).14. B. Liu, A. J. Bard, M. V. Mirkin, S. E. Creager, JACS 126, 1485 (2004).15. R. N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, Inc.: New York, 1969.16. C. Amatore, in Physical Electrochemistry: Principles, Methods and Applications, I. Rubinstein,

Ed., Marcel Dekker, Inc.: New York, 1995, p. 131.

6.3.4 Platinum conical ultramicroelectrodes

Biao Liu

Department of Chemistry and Biochemistry, The University of Texas atAustin, Austin, Texas 78712-0165, USA

6.3.4.1 Introduction

Conical-shaped ultramicroelectrodes (UMEs) are of special interest in connection with theimaging of surfaces, in kinetic studies, in probing thin films, and in probing minute envi-ronments, such as single cells. The most common fabrication procedure is by the etchingof platinum wire or carbon fibers followed by coating with an insulating material exceptat the apex of the electrode. In this section, the fabrication of both blunt and sharp conicalelectrodes are discussed.

6.3.4.2 Blunt conical UMEs

Pt conical UMEs are constructed from 25 �m or 50 �m diameter Pt or Pt–Ir wires. A 2-cmlength of Pt wire is connected to a conductive wire with silver epoxy (Epotek, H20E, EpoxyTechnology, Inc., Billerica, MA). The ensemble is then enclosed in a glass capillary, which

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