ATOMIC FORCE MICROSCOPY PROBES GO ELECTROCHEMICAL

A TOMIC FORCE

MICROSCOPY PROBES GO ELECTROCHEMICAL

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A N A LY T I C A L C H E M I S T R Y / N O V E M B E R 1 , 2 0 0 2

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he introduction of the scanning tunneling microscope (STM) in 1981 paved the way for

scanned probe microscopy (SPM). Developing STM put Gerd Binnig and Heinrich Rohrer on the fast track to the Nobel Prize in Physics and quickly led to an extended family of SPM techniques with applications across the sciences (1). These techniques gave researchers the opportunity to explore and manipulate the properties of DUAL FU NCTIONALITY R EQU I R E S I NTEG R ATI NG AN E LECTRODE I NTO

materials in ways that were unimaginable before the STM (2). This article de-

TH E AFM PROB E.

scribes developments in SPM methodology that

combine electrochemical and atomic force microscopy (AFM) probes, devices that were previously used only in isolation. The advantages, applications, and future directions of these new probes are also addressed.

Catherine E. Gardner Julie V. Macpherson University of Warwick (United Kingdom)

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Topographical imaging on the nanoscale Among SPMs, AFM—introduced in 1986 by Binnig, Calvin Quate, and Christoph Gerber—is one of the most versatile (3). The probe consists of a very sharp tip attached to a force-sensing cantilever. For imaging, the tip scans across the sample and the cantilever deflects in response to force interactions between the tip and substrate. These deflections are typically monitored by shining a laser off the back of the lever into a photosensitive detector, thus building a topographical map of the sample surface. A feedback loop is usually used, so the tip–substrate separation is continually adjusted to maintain a constant force. The first prototype AFM probes were constructed from tiny, beam-shaped gold foils with a diamond shard functioning as the tip (3). Today, AFM probes with various reproducible physical characteristics are routinely prepared from silicon and silicon nitride through microfabrication (4). An attractive feature of AFM is that the surface structure of a wide range of materials can be investigated, irrespective of sample conduc-

tivity. This versatility has proven especially important for highresolution topographical imaging of biological substrates under physiological pH conditions, such as living cells and DNA plasmids (5). In conventional form, AFM lacks chemical specificity, but molecular recognition is possible by tailoring the chemical functionality of the probe and substrate (6). Often termed chemical force microscopy (CFM), this genre measures the interaction between the modified tip and the substrate by recording force versus tip separation curves: as the tip is brought toward, into contact with, and then away from the surface. Alternatively, the lateral, frictional force exerted on the tip as it is scanned across the substrate also provides a means of mapping the chemical functionality of the surface (7 ). However, CFM has been used successfully to make binding-force (adhesion) and protein-unfolding measurements at the molecular level (8, 9). CFM is limited because tips are usually specific to one type of surface chemistry.

Imaging surface activity using electrochemistry (a) Scanning probe

Substrate

(b)

(c)

FIGURE 1. Schematic representations of common SPM imaging modes. (a) Fixed-height imaging: The probe images in a fixed plane above the surface, irrespective of changes in the underlying substrate topography. (b) Constant-distance imaging: A constant separation is maintained between the tip and the substrate. (c) Contactmode imaging: The probe remains in intimate contact with the surface throughout the imaging process.

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Electrochemistry represents an attractive approach for chemical analysis. Using amperometry as an example, the potential of an electrode can be tuned to detect a species of interest by virtue of current flow. Additionally, amperometry provides a means to rapidly alter the chemical composition of a solution adjacent to a surface. Therefore, scanned probe techniques based on electrochemical principles are being increasingly recognized as having a strength in imaging that AFM lacks (10). Scanning electrochemical microscopy (SECM) is the most widely used electrochemically based SPM technique (11–14). An ultramicroelectrode (UME) with micrometer dimensions usually serves as the imaging tip, but nanometer-sized electrodes are becoming more widespread (15–17 ). SECM is a solution-based technique, and the tip functions as either the working electrode in a dynamic electrochemistry circuit or as the indicator electrode in a potentiometric arrangement. For imaging, the tip is usually scanned at a fixed height (Figure 1a) above the interface of interest, without a feedback loop to maintain a constant tip–substrate separation. The interface (or second phase) is immiscible with the solution, and it may be a solid, biomaterial, liquid, or gas (18). The spatial resolution of SECM is determined by both the size of the electrode and the tip–substrate separation, which is usually less than the electrode radius. In the amperometric mode of operation, the SECM current signal is caused by the diffusional flux of a specific electroactive moiety to the electrode. Scanning ion conductance microscopy, an alternative type of electrochemistry-based SPM, measures a migration current between a submicrometer-sized capillary opening positioned close to a surface and a second electrode (19). This technique is frequently used to image live cells (20), but in conventional form it is nonspecific to ionic species.

(a)

SECM has provided considerable new quantitative insights into interfacial processes, as illustrated schematically in Figure 2a (11, 12, 18). In each of these examples, the tip is amperometric and the current flow provides information on the underlying surface process. Despite many successful applications, the spatial resolution of SECM is usually orders of magnitude worse than that of other SPM techniques, such as AFM and STM. Additionally, the UME response is convoluted by both topography and surface reactivity. Resolving these two components from a single electrochemical measurement can be difficult, particularly when the probe is scanned in the fixed-height mode (Figure 2b). Conventional open-loop SECM imaging experiments also have limitations for surfaces with significant topographical features or sample tilt, making tip crash a problem. Combining nonelectrochemical methods with SECM to maintain a constant separation between the tip and the sample during imaging has been a theme in recent research. For example, by measuring the shear force damping of a UME dithered laterally at resonant frequency (21, 22)—which is similar to regulating the substrate–probe separation in scanning near-field optical microscopy—it is possible to map the sample topography independent of the electrochemical measurement. Imaging proceeds by using a constant damping amplitude, which corresponds to a constant tip–substrate separation known as “constant distance” imaging (Figure 1b). The damping of the UME is normally monitored either by reflecting a laser off the UME onto a photodiode detector or by measuring the change in frequency of a tuning fork attached to the electrode. However, topographical resolution is determined by the overall size of the tip, which is composed of the active electrode and the insulating sheath. Submicrometer-sized probes have recently been used for imaging and as nonamperometric sensing tips (16, 23).

Electrochemical AFM probes Given the high spatial resolution and versatility of AFM, its combination with SECM is a particularly attractive route for electrochemical-topography imaging. Implementing this technology requires the integration of an electrode component into the AFM probe to confer dual functionality. In this configuration, the cantilever functions as the force sensor, which in conjunction with a sharp tip, provides high-resolution (nanometerscale) topographical measurements and precise control of the tip–substrate separation. The electrode, which may be located at various positions on the probe, produces a current response that serves as a quantitative, sensitive measure of the electrochemical properties of the interface akin to SECM. AFM probes with electrochemical capabilities are potentially powerful because they enable structural information to be correlated directly with chemical surface activity and topographical and structural changes to be mapped in response to electrochemically induced local perturbations where the probe is placed in the solution.

Hemispherical diffusion

Active site

Hindered diffusion

Imaging topography

Imaging permeability

Imaging transport

Permeable area

Transport

iperm > i hind

i trans > ihind

d

Inactive surface

i/i (∞) 1 iactive > i hind

i hind < i (∞)

i top ➝ i hind

Distance scanned by UME tip

(b)

i/i (∞)

Distance scanned by UME tip

Topography

Active site

Inactive surface

FIGURE 2. Examples showing SECM can be used to monitor a variety of interfacial processes. (a) The reactivity of the surface is reflected in the change in the tip current from that recorded in bulk solution [i()] as an amperometric tip approaches or is scanned over a surface. For example, as the tip scans over an inactive surface, the diffusional current for the electrolysis of an electroactive mediator is smaller than i(), because diffusion is hindered (i hind). As the tip scans over an active site, which is emitting an electroactive species, the current increases locally in the vicinity of the site (iactive). For a surface that contains defined transport pathways (e.g., pores) through which electroactive species can move, the current will once again increase in the vicinity of the site (i trans). In an interesting application, the tip can be used to perturb an equilibrium at an interface. In the example shown, an electroactive species partitions two phases (e.g., liquid–biomaterial). The tip is used to remove the species in the solution phase, causing a local concentration depletion. This induces movement of species from the second phase to the first for subsequent collection at the electrode (iperm). In this case, the permeability of the species of interest in the second phase can be determined without the electrode having to enter it. (b) The current flowing at the UME tip contains information on both the topography and the activity of the surface. For example, increases in the tip current are expected for diffusional feedback imaging if the UME scans over (bottom left) a depression in the surface of an inert material or over (bottom right) an active site such as a conductor. Without prior knowledge of the sample topography, it can be difficult to separate these two effects.

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Imaging conductivity and electrochemistry in air Designs for integrated electrochemical AFM probes depend on the application. For the highest-resolution electrochemical imaging, the electrode clearly needs to be as small as possible. In other applications, however, it may be desirable only to exert electrochemical control over the local solution conditions, in which case submicrometer-sized electrodes may not be necessary. The simplest way to produce an electrochemical AFM probe is by sputter-coating or evaporating a thin layer of a metal onto the surface of commercially available silicon or silicon nitride probes (24, 25). Sputter-coating is often preferred as it leads to probes with durable metal coatings (25). These probes can be used as conducting-AFM (C-AFM) tips in contact mode (Figure 1c) to simultaneously measure the local electrical properties and topography in air or a dry environment at high resolution. These probes are also suitable for electrochemical contact-mode imaging of surfaces in a humid environment. In this case, the spatial resolution of electrochemical imaging is determined by how the electrolyte solution of interest wets the apex of the tip. In turn, wetting depends on the relative humidity and geometry of the metal-coated tip.

(a) Laser

Detector

Platinumcoated probe

Membrane Ag/AgCl reference electrode

Analyte solution reservoir

Apply potential and measure current

(b)

(c)

1 pA

SECM measurements have previously been performed in humid environments (26, 27 ). The faradaic current signal at the UME tip—created by oxidation or reduction of a thin layer of electrolyte—provides a feedback signal for high-resolution topographical mapping (27). With SECM/AFM, the AFM component of the probe provides the feedback for topography so that the electrochemical response can be used to identify substrate activity. This type of SECM/AFM probe has recently been used to image and quantify the diffusional transport of target analytes through nanoscale pores (28). In this work, the substrate was a 10-µm-thick polycarbonate membrane with 100-nm-diam pores wetted and filled from below with an electrolyte solution containing a redox-active analyte, such as IrCl 63– or Fe(phenanthroline) 32+. Using a platinum-coated AFM probe as a moveable topographical and electrochemical sensor made it possible to address individual solution-filled pores. The second electrode placed beneath the membrane created a closed electrochemical cell when the tip came into contact with solution (Figure 3a). The current response at the electrode, measured as a function of position to create a current map (Figure 3b), was due to the diffusional transport of analyte through individual pores. A topography map of the surface was simultaneously recorded (Figure 3c), which clearly identifies the position of the pores. The currents at each pore showed some variability due to local imperfections in the internal pore structure (28). In general, the measured currents agreed well with the theoretical predictions for diffusion-limited mass transport through a single pore in this membrane. The smeared current appearance was attributed to slight displacement from dragging fluid as the tip moved across the opening of a pore.

100 nm 0

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Local electrochemical control of interfacial processes 2.0

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µm 2.5

1.0

2.0 1.5

0.5

1.0 0.5

1.5

2.5 2.0

1.0 1.5 0.5

1.0 0.5

FIGURE 3. SECM/AFM imaging of diffusional transport through single nanoscale pores. (a) Schematic of the experimental setup, in which a platinum-coated Si3N4 tip functions as the imaging probe. (b) Current and (c) topographical maps of the electroactive mediator IrCl 63– diffusing through pores in a wetted track-etched polycarbonate membrane.

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µm

Platinum-coated Si3N4 AFM tips were used in the initial development of combined SECM/AFM probes for studies in solution. The main body of these probes was coated in a manually applied thin layer of insulator, such as a nail varnish and superglue mixture or polystyrene. The electrode size was determined by the exposed surface area, which consisted of a 100– 200-µm-long platinum-coated cantilever with integrated tip. These probes

(b)

(a)

25 nm

B

12.5 nm

A+ B–

0 nm

(010) (001)

2 µm

FIGURE 4. SECM/AFM-induced dissolution of crystals. + –

(a) The ionic crystal of interest (A B ) is placed in a saturated solution of A+B–, so initially no dissolution takes place and the system is at equilibrium. Electrochemical removal of either A+ or B– in the tip–substrate gap results in a local undersaturation at the crystal–solution interface. This process causes ions to move from the crystal and through solution to the tip electrode, where they are collected. The topographical imaging capabilities of the probe allow the structure of the crystal surface to be monitored at ultrahigh resolution as dissolution proceeds. (b) The topographical image shows two oppositely rotating dislocation spirals on the (100) surface of potassium bromide, interacting as the dissolving crystal returns to a position of equilibrium with the solution.

clearly will not work for high-resolution electrochemical imaging, but they are attractive for exerting local electrochemical control of the solution properties and composition at a solid–liquid interface. For example, this type of integrated electrode can be used to locally generate a reactive flux of reagent or to perturb an interfacial equilibrium by electrolysis. The AFM component of the probe is then used to image any consequent changes in surface topography at high spatial resolution under dynamic conditions. Studies of induced dissolution from targeted regions of the surface of ionic single crystals are a good application of this type of SECM/AFM probe as shown in Figure 4 (29, 30). In practice, the topographical imaging capability of an SECM/AFM probe is used to locate an area of interest on the crystal surface. Dissolution in this zone is then promoted electrolytically by stepping the potential of the platinum-coated AFM tip from a value where no redox reactions occur and where the solution is saturated with respect to the crystalline material to a value for the electrolysis of one of the components in solution. The effect makes the solution locally undersaturated at the crystal–solution interface in the vicinity of the tip and cantilever, which provides the driving force for dissolution. The current response at the probe provides quantitative information on the dissolution kinetics. The first application of combined SECM/AFM examined dissolution from the (100) surface of KBr in acetonitrile (29). Initially, the solution was saturated with respect to KBr, and under these conditions the surface was essentially unreactive. A series of 1-s potential pulses was then applied to the SECM/ AFM probe to oxidize bromide ions to tribromide at different rates in the probe–substrate gap. Images were recorded after each pulse to observe the effect of the perturbation on dissolution from the crystal surface and to monitor changes in surface topography as the system returned to equilibrium. The surface

was shown to clearly dissolve via a spiral mechanism (Figure 4b), where steps of unit cell height unwound from screw dislocations emerging on the crystal surface. The extent and rate of dissolution are also linked to the driving force exerted by the electrochemical perturbation. The images obtained in this study provided the first direct experimental evidence for the operation of the spiral mechanism in the dissolution of an ionic crystal. SECM/AFM has subsequently found application in studies of dissolution from the (010) surface of potassium ferrocyanide trihydrate (30) and the natural cleavage plane of calcite (31), both in aqueous solution. In the case of calcite, the electrode was used to produce a reactive species, specifically H3O+, from the local oxidation of H2O, which induced dissolution of the surface through the following reaction: CaCO3 + H3O+ r Ca2+ + HCO3– + H2O Increasing the applied potential quantitatively increased the flux of protons arriving at the calcite crystal surface and made it possible to investigate this effect on dissolution (31). As the flux of protons arriving at the surface increased, the mechanism changed from dissolution initiated at point defects on the surface to faceting rhombohedral-shaped etch pits. Abruña and co-workers described a novel approach to generating protons electrochemically, in an AFM format, using a technique termed “redox probe microscopy” (32). Abruña’s group has dealt generally with electroactive polymer films immobilized on conducting AFM probes (33). The application of an oxidation potential to a goldcoated AFM probe modified with a thiohydroquinone monolayer liberated protons from the film. If the scanning tip was in contact with a surface coated by a pH-sensitive block copolymer, it was possible to “expose” the copolymer through the production of protons and generate patterns on the surface. Because the entire AFM probe was coated with the redox-active monolayer, proton generation occurred over the whole surface and so the resolution was not as good as with conventional AFM. Nonetheless, patterns produced on the ~100-nm level encourage future applications and developments.

High-resolution imaging in solution For high-resolution electrochemical imaging in solution with dual-functionality probes, it is necessary to minimize the area of the exposed electrode. Moreover, the insulation procedure should be selective and routinely leave one designated area of the probe free from coverage to act as the electrode. Prior work N O V E M B E R 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y

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of the cantilever spring on nanoscopic electrode constant, however, it is fabrication (17, 34) and nm 200 relatively simple to estithe production of tips mate these two paramefor electrochemical STM 40.0 100 ters (37 ). Overall, the (35) have led to several 30.0 fabrication procedure is insulation procedures 20.0 inexpensive and efficient that can be used rou0 10.0 0 enough to fabricate tinely to insulate sharp10.0 20.0 0 30.0 40.0 many devices in one day. ened microwires, except µm The probes can be used at the very apex of the in all commercially availtip. FIGURE 5. Mapping active surface sites. able AFM instruments. Solid-metal AFM (a) Image of a hand-fabricated SECM/AFM probe. (b) (Top) topographical (flattened) and The low resonant freprobes for the measure(bottom) fixed-height current maps for the diffusion-controlled tip detection of IrCl 62– genquencies of these SECM/ ment of the electrical, erated from the transport-limited oxidation of IrCl 63– at a 10-µm-diam substrate electrode. For topographical imaging, the tip was held in contact with the surface (unbiased), while AFM probes require topfrictional, and topoelectrochemical data were acquired with the tip imaging at a fixed height of 1 µm from ographical imaging to graphical properties of a the surface of the substrate. (Adapted from Ref. 38.) always be carried out in substrate have previouscontact mode (Figure ly been fabricated by 1c), but electrochemical hand from metal miimaging can be carried crowires (36). These probes can serve as an attractive alternative to AFM tips with out at either fixed height or constant distance (38) (Figures 1a thin metal coatings, particularly in C-AFM, where wear of the and 1b, respectively). For fixed-height imaging, the feedback metal coating and a loss of conductivity may be problematic. We loop is disabled and the stepper motor of the AFM instrument have shown also that probes for SECM/AFM can readily be retracts the tip a known distance from the surface of the sample. hand-fabricated (37 ). The starting material is a length of plat- For constant-distance electrochemical imaging, the tip scans inum wire 50 µm in diameter. The wire is bent and flattened to each line twice. In the first pass, sample topography is recordform the force-sensing cantilever and then etched to form the ed in contact mode; in the second pass, this information is used sharp imaging tip (Figure 5a). An insulating film is applied to to hold the tip at a constant, defined distance from the surface the probe in the form of an electrodeposited paint that retracts while electrochemical data are acquired. Known as lift mode, from the apex during curing so that only the very end of the tip this process has previously been used for magnetic and electrostatic AFM imaging. is exposed (17 ). SECM/AFM probes are particularly effective for the identiTypically, this procedure creates cone-shaped tip electrodes with effective radii in the range 10 nm–1 µm. As with conven- fication and chemical mapping of active surface sites, as illustional SECM measurements, the cone geometry is especially at- trated in Figure 5b (38). A 10-µm-diam platinum electrode tractive for noninvasive, quantitative imaging of concentration sealed in glass and immersed in an electrolyte solution served profiles at reactive surfaces. In this case, the integrated force- as a model active site. The substrate UME was biased at a posensing cantilever provides an independent means of obtaining tential sufficient to generate an electroactive species (IrCl 62–) topographical information as an added bonus. These probes from electrolysis of a solution mediator (IrCl 63–). The zone over can also be used to electrochemically perturb local solution which the tip collects IrCl 62– is clearly evident in the top part of conditions at submicrometer resolution and map the corre- Figure 5b (as a current flow at the SECM/AFM probe) and sponding change in sample topography. For both imaging and correlates well with the underlying location of the electrode, tip approach measurements, the force-sensing properties of identified by topographical imaging in the bottom part of FigSECM/AFM probes enable an unambiguous assignment of ure 5b. The magnitude of the current measured at the tip electhe tip–substrate contact point. Moreover, unlike SECM, the trode as a function of tip–substrate separation is quantitatively structural integrity of the probe is maintained after contact be- determined on the basis of models for diffusion from a diskcause as the tip pushes against the substrate, the probe goes shaped source (38). into constant compliance and the cantilever deflects. Finally, SECM/AFM has recently been used to correlate the electribecause the apex of the SECM/AFM tip is conducting, it is cal properties of a complex electrode material—a dimensionalalso possible to map the electrical properties of a surface in ly stable anode (DSA)—with its topographical, electrical, and C-AFM mode. electrochemical characteristics (39). The DSA comprises a ~1-µm Given the nature of the fabrication procedure, each probe TiO2 layer (grown on titanium) that contains pores ~50–200 differs in the size of the exposed electrode and the magnitude nm in diameter, some of which are filled with spherical plat582 A


inum deposits that are 0.15–1.2 µm in diameter. The key to the probe with a 100–300-nm-thick layer of gold, and then deanodic behavior of this type of electrode is the nature of the un- positing an ~800-nm-thick layer of silicon nitride using plasmaderlying contact between the platinum deposit and the titani- enhanced chemical vapor deposition (42). Successive millings using a focused ion beam sharpen and redefine the geometry of um substrate. The SECM/AFM probe operating in C-AFM mode was the AFM tip so that a gold microring electrode is formed first used to topographically identify individual platinum de- around the base of the AFM tip. Each tip is individually fabriposits. Then, with the probe held in a fixed position over dif- cated using this procedure. A similar electrode geometry has ferent deposits, stationary current–voltage curves were record- been adopted for the fabrication of pH-sensitive iridium oxide ed. These measurements demonstrated that only a small electrodes around the base of STM tips (43). In the Kranz tip design, the distance between the electrode minority of platinum deposits made intimate ohmic contact with the titanium surface. Most of the platinum sites connect- and the surface is governed by the constant height of the tip. ed to the titanium through an oxide layer. High-resolution Thus, with the tip imaging in contact mode, the electrode is electrochemical imaging studies with SECM/AFM provided maintained at a constant distance from the surface. As with SECM, the spatial resolution of this type of electrofurther evidence. An electroactive solute (IrCl 63–) chemical probe will depend on the electrode was added to the solution, and the DSA size and its distance from the surwas held at a potential to oxidize the face. The capabilities of this type solute while the SECM/AFM of SECM/AFM probe have probe was biased to collect Electrochemical AFM probes been examined using a test the product (IrCl 62–) just substrate comprising gold above the surface. The bands on an insulating resulting current images enable structural information to be gallium arsenide substrate demonstrated that only immersed in a solution a few of the platinum correlated with chemical containing Fe(CN)64–. deposits were capable of facile electron transport. With the probe held at a surface activity. The rest showed “sluggish” potential suitable to elecelectron transfer kinetics indicatrolyze Fe(CN) 64–, simultanetive of TiO2-contacted platinum ous images of the topography and electrochemical activity of the grid were sites. obtained. In the topography image, the raised gold bands on the substrate surface could be clearly identified and Microfabrication In the history of AFM development, microfabrication and lith- correlated with increases in the current detected at the probe as ographic techniques were introduced at an early stage to facili- the electroactive mediator was recycled by redox feedback in tate the production of probes with a high degree of re- the tip–substrate gap. producibility. Today, more advanced microfabricated probe designs are becoming increasingly commonplace and the appli- Looking to the future cations of AFM more adventurous. For example, electron beam Integrating electrodes into AFM probes is an attractive lithography can be used to write submicrometer and nanoscale prospect both for high-resolution manipulation of surface metal patterns on the end of AFM tips for magnetic and ther- processes, such as nanofabrication and selective etching, and in mal imaging (40). Gerber, Gimzewski, and colleagues have pro- imaging interfacial activity. Although hand-fabricated duced arrays of cantilevers on one probe body for electronic SECM/AFM probes have proven extremely effective, micronose applications (41). Probes consisting of eight linearly fabrication of SECM/AFM tips will make the technology more arranged cantilevers, each coated with a different polymeric widespread and lead to reproducible tip geometries, particularly sensor layer, enable the identification of up to eight different when batch fabrication strategies are used. We envisage the dechemical species. If an analyte interacts with the polymer, the velopment of a range of AFM probes with integrated electrodes film swells, resulting in a detectable nanomechanical motion of of different geometry, position, material, and size for different the cantilever. applications. For increased imaging speed and measurement caFor more widespread applications of SECM/AFM, probes pabilities, the production of multicantilever arrays, or “millimust have reproducible and defined characteristics. Thus, mi- pedes” (44), is particularly interesting, and the integration of crofabrication technologies represent an attractive way for- electrodes into this type of device would provide an extremely ward. Kranz and co-workers have made progress toward this attractive surface probe for multifunctional electrochemical goal by first sputter-coating a conventional silicon nitride AFM imaging. N O V E M B E R 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y

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The most sophisticated electron beam lithography techniques have a resolution on the order of several nanometers. Moreover, the single-walled carbon nanotube (SWNT) would provide a very powerful probe for simultaneous AFM and electrochemistry. Lieber’s group at Harvard has already demonstrated the increased topographical resolution offered by SWNT-AFM tips (45). It is only a matter of time before such probes are used for simultaneous electrochemical measurement as well.

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JVM thanks the Royal Society for a University Research Fellowship and the Engineering and Physical Sciences Research Council (EPSRC) for funding (GR/R34738/01). CEG thanks the EPSRC–Royal Society of Chemistry Analytical Partnership for a studentship. We both thank Pat Unwin for very helpful comments during the preparation of this article.

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Julie Macpherson is a Royal Society University research fellow in the department of chemistry at the University of Warwick. Her research interests are the development and application of high resolution electrochemical and electrical scanned probe techniques. Catherine Gardner is a doctoral student at the University of Warwick. Her research uses SECM/AFM to investigate transport processes. Address correspondence to [email protected].

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