Combined scanning electrochemical/optical microscopy with shear force and current feedback

A technique that combines scanning electrochemical microscopy (SECM) and scanning optical microscopy (OM) was developed. Simultaneous scanning electrochemical/optical microscopy (SECM/OM) was performed by a special probe tip, which consists of an optical fiber core for light passage, surrounded by a gold ring electrode, and an outermost electrophoretic insulating sheath, with the tip attached to a tuning fork. To regulate the tip-substrate distance, either the shear force or the SECM tip current was employed as the feedback signal. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force allowed simultaneous topographic, along with SECM and optical imaging in a constant-force mode. The capability of this technique was confirmed by obtaining simultaneously, for the first time, topographic, electrochemical, and optical images of an interdigitated array electrode. Current feedback from SECM also provided simultaneous electrochemical and optical images of relatively soft samples, such as a polycarbonate membrane filter and living diatoms in a constant-current mode. This mode should be useful in mapping the biochemical activity of a living cell.     

Alternating current impedance imaging of membrane pores using scanning electrochemical microscopy

Alternating current impedance imaging of a 6-microm thick membrane containing conical-shaped pores (60-nm and 2.5-microm diameter openings) using scanning electrochemical microscopy (SECM) is described. Impedance images of the pore openings were obtained by rastering a glass-sealed conically shaped Pt tip (approximately 1-microm radius) above the membrane surface, while measuring the total impedance between the tip and a large area Pt electrode located on the opposite side of the membrane. Individual pore openings in the high pore density membrane (approximately 8 x 10(4) pores/cm2) are observed in the SECM impedance image. The image contrast is due to the decrease in tip and membrane resistance, in the vicinity of the pore opening. An equivalent circuit for the SECM cell and membrane is proposed and evaluated against the measured SECM imaging impedance. Criteria for employing SECM in impedance mode to image membranes are discussed.     

Alternating current impedance imaging of high-resistance membrane pores using a scanning electrochemical microscope. Application of membrane electrical shunts to increase measurement sensitivity and image contrast

Whether an individual pore in a porous membrane can be imaged using scanning electrochemical microscopy (SECM), operated in ac impedance mode, is determined by the magnitude of the change in the total impedance of the imaging system as the SECM tip is scanned over the pore. In instances when the SECM tip resistance is small relative to the internal pore resistance, the total impedance changes by a negligible amount, rendering the pore invisible during impedance imaging. A simple solution to this problem is to introduce a low-impedance electrical shunt (i.e., a salt bridge) across the membrane. This principle is demonstrated by imaging polycarbonate membranes (6-12-microm thickness) containing between 1 and 2000 conical-shaped pores (60-nm- and 2.5-microm-diameter openings) using an approximately 1-microm-radius Pt tip. Theory and experiments show that image contrast (the change in ac current measured as the probe is scanned over the pore) is inversely proportional to the total resistance of the membrane and can be increased by a factor of approximately 50x by introducing a low-resistance electrical shunt across the membrane. Remarkably, SECM images of membranes containing a single high-resistance (approximately 1 G Omega) pore can only be imaged by short-circuiting the membrane. Image contrast also becomes independent of membrane resistance when an electrical shunt is used, allowing for more quantitative comparisons of the features in ac impedance images of different membranes.     

Fabrication and characterization of probes for combined scanning electrochemical/optical microscopy experiments

A technique that combines scanning electrochemical microscopy (SECM) and optical microscopy (OM) was implemented with a new probe tip. The tip for scanning electrochemicaVoptical microscopy (SECM/OM) was constructed by insulating a typical gold-coated near-field scanning optical microscopy tip using electrophoretic anodic paint. Once fabricated, the tip was characterized by steady-state cyclic voltammetry, as well as optical and electrochemical approach experiments. This tip generated a stable steady-state current and well-defined SECM approach curves for both conductive and insulating substrates. Durable tips whose geometry was a ring with < 1 microm as outer ring radius could be consistently fabricated. Simultaneous electrochemical and optical images of an interdigitated array electrode were obtained with a resolution on the micrometer scale, demonstrating good performance of the tip as both an optical and an electrochemical probe for imaging microstructures. The SECM feedback current measurements were successfully employed to determine tip-substrate distances for imaging.     

Scanning electrochemical microscopy of model neurons: imaging and real-time detection of morphological changes

Living PC12 cells, a model cell type for studying neuronal function, were imaged using the negative feedback mode of a scanning electrochemical microscope (SECM). Six biocompatible redox mediators were successfully identified from a large pool of candidates and were then used for imaging PC12 cells before and after exposure to nerve growth factor (NGF). When exposed to NGF, cells differentiate into a neuron phenotype by growing narrow neurites (1-2 microm wide) that can extend > 100 microm from the cell proper. We demonstrate that carbon fiber electrodes with reduced tip diameters can be used for imaging both the cell proper and these neurites. Regions of decreased current, possibly resulting from raised features not identifiable by light microscopy, are clearly evident in the SECM images. Changes in the morphology of undifferentiated PC12 cells could be detected in real time with the SECM. After exposure to hypotonic and hypertonic solutions, reversible changes in cell height of <2 microm were measured.     

Scanning electrochemical microscopy. 55. Fabrication and characterization of micropipet probes

The fabrication and characterization of novel micropipet probes for use in scanning electrochemical microscopy (SECM) are described. These can be used to dispense small (pL) amounts of a solution while monitoring the electrochemical response at a substrate and at a ring electrode tip on the micropipet probe. The probes were constructed by insulating gold-coated borosilicate micropipets with electrophoretic paint and exposing a ring electrode at the tip by heat treatment. Characterization of the probes was performed using scanning electron microscopy, cyclic voltammetry, and SECM approach curve experiments. Routine construction of tips with diameters of the order of 3 microm was possible using this technique. The probes exhibited stable steady-state currents and positive and negative feedback approach curves that agreed with those predicted by theory. Demonstrative SECM imaging experiments were performed using a picodispenser to continuously dispense an electroactive solution (ferrocenemethanol) to the SECM cell while the probe was located within a few micrometers of a Pt substrate surface. Oxidation of the dispensed electroactive solution was performed at the substrate, and feedback currents were measured at the probe tip by holding the gold ring at a reducing potential. This mode of tip-dispensing SECM was used to obtain images of a platinum substrate electrode while monitoring both the substrate current and the feedback current at the probe.     

Impedance feedback control for scanning electrochemical microscopy.

A new constant-distance imaging method based on the relationship between tip impedance and tip-substrate separation has been developed for the scanning electrochemical microscope. The tip impedance is monitored by application of a high-frequency ac voltage bias between the tip and auxiliary electrode. The high-frequency ac current is easily separated from the dc-level faradaic electrochemistry with a simple RC filter, which allows impedance measurements during feedback or generation/collection experiments. By employing a piezo-based feedback controller, we are able to maintain the impedance at a constant value and, thus, maintain a constant tip-substrate separation. Application of the method to feedback and generation/collection experiments with tip electrodes as small as 2 microm is presented.     

Disk-generation/ring-collection scanning electrochemical microscopy: theory and application

The potential of ring-disk ultramicroelectrodes (RD UMEs) as probes for scanning electrochemical microscopy (SECM) was investigated both theoretically and experimentally. In particular, the disk-generation/ring-collection (DG/RC) mode of operation was considered. In this case, the interaction of two species with the substrate under investigation can be followed simultaneously from single tip current-distance measurement (approach curve) to the substrate. Theoretical approach curves for DG/RC were calculated by numerical methods. Such approach curves to both insulating and conducting substrates indicate a strong tip response dependence on the ring radius while the response was relatively insensitive to ring thickness and overall tip radius. The RD tip was characterized by fitting experimental approach curves recorded at insulating and conducting substrates to simulated curves for a given tip geometry. DG/RC SECM was then applied to investigate the partitioning of iodine across a liquid-liquid interface.     

A positionable microcell for electrochemistry and scanning electrochemical microscopy in subnanoliter volumes

Positionable voltammetric cells with tip diameters of < 50 microm were constructed from theta glass capillaries. One channel of the pulled glass capillary contains a carbon fiber microelectrode sealed in epoxy while the other houses a Ag/AgCl reference electrode that makes electrical contact to the analyte solution via a salt bridge at the tip. The device can be operated as a two-electrode cell and can therefore make measurements in droplets of solution that are similar in size to the tip. Alternatively, if the droplet of solution is larger than the tip, spatially resolved measurements of a substrate in solution can be made. Voltammetric experiments and feedback imaging with the scanning electrochemical microscope (SECM) were accomplished in microdroplets with solution volumes of less than 1 nL. pH images of a substrate immersed in 70-microL-thick films of solution were obtained in the generator-collector mode of SECM using an iridium oxide-modified microcell. This type of microcell is particularly useful for making electrochemical measurements in very small droplets of solution where a mobile working electrode could easily collide with a separately positioned reference electrode.     

An instrument for simultaneous EQCM impedance and SECM measurements

A novel combination of an electrochemical quartz crystal microbalance (EQCM) and a scanning electrochemical microscope (SECM) has been built. Unlike conventional EQCMs, the instrument described here allows rapid in situ measurement of the modulus of the quartz crystal's transfer function. Data analysis in the complex plane for the Butterworth-Van Dyke (BVD) equivalent circuit yields the real and the imaginary components R (damping resistance) and XL (reactive inductance) of the crystal's electroacoustic impedance around its resonant frequency of 10 MHz. The influence of different tip shapes of an approaching microelectrode on the electroacoustic impedance of the quartz crystal was studied and found to be minimal for certain geometries. The capability of the EQCM/SECM instrument was tested in cyclic voltammetric plating/stripping experiments using a copper(I) chloride solution of high concentration in 1 M HCl. Four parameters, XL, R, the substrate, and the tip current, can be recorded simultaneously. Depletion layer effects were observed and could be corrected for to yield accurate current efficiencies for potentiodynamic and potentiostatic copper plating. The amperometric response of the SECM tip positioned closely to the substrate reflects the concentration changes of electroactive ions in the diffusion layer of the substrate electrode.     

Chemical imaging with combined fast-scan cyclic voltammetry-scanning electrochemical microscopy

Fast-scan cyclic voltammetry (FSCV) is applied to the tip of a scanning electrochemical microscope (SECM) for imaging the distribution of chemical species near a substrate. This approach was used to image the diffusion layer of both a large substrate electrode (3-mm-diameter glassy carbon) and a microelectrode substrate (10-microm-diameter Pt). Additionally, oxygen depletion near living cells was measured and correlated to respiratory activity. Finally, oxygen and hydrogen peroxide were simultaneously detected during the oxidative burst of a zymosan-stimulated macrophage cell. These results demonstrate the utility of FSCV-SECM for chemical imaging when conditions are chosen such that feedback interactions with the substrate are minimal.     

Constant-distance mode scanning potentiometry. 1. Visualization of calcium carbonate dissolution in aqueous solution

Constant-distance mode scanning potentiometry was established by integrating potentiometric microsensors as ion-selective scanning probes into a SECM setup that was equipped with a piezoelectric shear force-based tip-to-sample distance control. The combination of specially designed micrometer-sized potentiometric tips with an advanced system for tip positioning allowed simultaneous acquisition of both topographic and potentiometric information at solid/liquid interfaces with high spatial resolution. The performance of the approach was evaluated by applying Ca(2+)-selective constant-distance mode potentiometry to monitor the dissolution of calcium carbonate occurring either at the (104) surface of calcite crystals or in proximity to the more complex surface of cross sections of a calcium carbonate shell of Mya arenaria exposed to slightly acidic aqueous solutions. Micrometer-scale heterogeneities in the apparent calcium activity profiles have successfully been resolved for both samples.     

Scanning electrochemical microscopy 50. Kinetic study of electrode reactions by the tip generation-substrate collection mode

A scanning electrochemical microscopy (SECM) methodology for localized quantitative kinetic studies of electrode reactions based on the tip generation-substrate collection (TG-SC) operation mode is presented. This approach does not use the mediator feedback required in typical kinetic SECM experiments. The reactant is galvanostatically electrogenerated on a tip placed in proximity to the substrate. It diffuses through the tip-substrate gap and undergoes the reaction of interest on the substrate surface. The substrate current is monitored with time until it reaches an apparent steady-state value. The process was digitally simulated using an explicit finite difference method, for an irreversible first-order electrode reaction at the substrate. Transient responses, steady-state polarization curves, and TG-SC approach curves can be used to obtain substrate kinetics. The effects of the experimental parameters were analyzed. The possibility of easily changing the experimental conditions with the SECM is an attractive approach to obtain independent evidence that can be used for a strict test of reaction mechanisms. The technique was applied for a preliminary simplified kinetic examination of the oxygen reduction reaction in phosphoric acid.     

Use of catechol as selective redox mediator in scanning electrochemical microscopy investigations

The use of catechols, and more specifically of dopamine, as a specific redox mediator for scanning electrochemical microscopy (SECM) investigations was evaluated in the challenging situation of an ultrathin layer deposited on a conductive substrate (carbon materials). Experiments show that dopamine is a well-adapted redox system for SECM in feedback mode and in unbiased conditions. Used as a redox mediator, catechol permits the investigations of modified surfaces without an electrical connection of the sample thanks to fast charge transfer kinetics but with a surface selectivity that does not exist in classical outer-sphere redox mediators. The interest of catechol in SECM as a sensitive redox mediator is exemplified by monitoring several modification steps of an ultrathin (<1 nm) hierarchically porous organic monolayer deposited on carbon substrates. For quantitative analysis, the SECM approach curves using dopamine could simply be characterized with an irreversible electron transfer kinetics model in a large range of pH.     

Visualization of chlorine evolution at dimensionally stable anodes by means of scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) has been used to detect and visualize the local electrocatalytic activity of dimensionally stable anodes (DSA) for Cl(2) evolution from brine. The sample generation-tip collection (SG-TC) mode of SECM shows limitations arising from complications connected with the reduction of Cl(2) at the SECM tip due to the presence of a significant amount of nondissolved Cl(2) gas. Because only dissolved Cl(2) can be electrochemically reduced at the tip, a large amount of the Cl(2) gas which is produced at active spots of the DSA is not detected. Additionally, a decrease of the cathodic current at the tip may occur owing to the adhesion of gas bubbles and blocking of the electrode surface. To overcome this limitation, the redox competition mode of SECM was extended and applied to the local visualization of Cl(2) evolution from highly concentrated brine solutions. High concentrations of Cl(2) produced at the sample can cause inhibition of the same reaction at the tip by accumulation of Cl(2) in the proximity of the SECM tip. In this way the tip current is decreased, which can be used as a measure for the catalytic activity of the sample underneath the tip.     

Noncontact electrochemical imaging with combined scanning electrochemical atomic force microscopy

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-microm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface.     

Feedback-independent Pt nanoelectrodes for shear force-based constant-distance mode scanning electrochemical microscopy

A new generation of platinum nanoelectrodes for constant-distance mode scanning electrochemical microscopy (CD-SECM) has been prepared, characterized, and used for high spatial resolution electrochemical measurements and visualization of electrochemically induced concentration gradients in microcavities. The probes have long (1-2 cm), narrow quartz tips that were conically polished and have a Pt nanoelectrode that is slightly offset from center. Because of the size and location of the electrode on the probe, it does not exhibit SECM feedback while approaching the analyzed sample surfaces even to distances within a few hundred nanometers. The probe was positioned near the surface while scanning and performing electrochemical measurements through use of nonoptical shear force control of the tip-to-sample distance. Test structures consisted of cylindrically shaped microcavities that are 50 microm in diameter with three individually addressable electrodes: a gold disk at 8-microm depth, a crescent-shaped gold ring at 4-microm depth along the wall, and a top gold electrode at the rim. Different electrodes within the microcavity were used to reduce and oxidize redox species in 250 microL of a solution of 5 mM hexaamineruthenium(III) chloride and 0.1 M potassium chloride, protected from evaporation by mineral oil, while the SECM tip followed the topography of the structures and monitored the current from the oxidation of [Ru(NH3)6]2+. Electrochemically generated concentration profiles were obtained from these complex test structures that are not possible with any other SECM technology at this time.     

Probing heterogeneous electron transfer at an unbiased conductor by scanning electrochemical microscopy in the feedback mode

The theory of the feedback mode of scanning electrochemical microscopy is extended for probing heterogeneous electron transfer at an unbiased conductor. A steady-state SECM diffusion problem with a pair of disk ultramicroelectrodes as a tip and a substrate is solved numerically. The potential of the unbiased substrate is such that the net current flow across the substrate/solution interface is zero. For a reversible substrate reaction, the potential and the corresponding tip current depend on SECM geometries with respective to the tip radius including not only the tip-substrate distance and the substrate radius but also the thickness of the insulating sheath surrounding the tip. A larger feedback current is obtained using a probe with a thinner insulating sheath, enabling identification of a smaller unbiased substrate with a radius that is approximately as small as the tip radius. An intrinsically slow reaction at an unbiased substrate as driven by a SECM probe can be quasi-reversible. The standard rate constant of the substrate reaction can be determined from the feedback tip current when the SECM geometries are known. The numerical simulations are extended to an SECM line scan above an unbiased substrate to demonstrate a "dip" in the steady-state tip current above the substrate center. The theoretical predictions are confirmed experimentally for reversible and quasi-reversible reactions at an unbiased disk substrate using disk probes with different tip radii and outer radii.     

Scanning electrochemical microscopy. 47. Imaging electrocatalytic activity for oxygen reduction in an acidic medium by the tip generation-substrate collection mode

The oxygen reduction reaction (ORR) in acidic medium was studied on different electrode materials by scanning electrochemical microscopy (SECM) operating in a new variation of the tip generation-substrate collection mode. An ultramicroelectrode tip placed close to the substrate electrode oxidizes water to oxygen at a constant current. The substrate is held at a potential where the tip-generated oxygen is reduced and the resulting substrate current is measured. By changing the substrate potential, it is possible to obtain a polarization (current-potential) curve, which depends on the electrocatalytic activity of the substrate material. The main difference between this mode and the classical feedback SECM mode of operation is that the feedback diffusion process is not required for the measurement, allowing its application for studying the ORR in acidic solutions. Activity-sensitive images of heterogeneous surfaces, e.g., with Pt and Au electrodes, were obtained from the substrate current when the x-y plane was scanned with the tip. The usefulness of this technique for imaging electrocatalytic activity of smooth metallic electrodes and of highly dispersed fuel cell-type electrocatalysts was demonstrated. The application of this method to the combinatorial chemical analysis of electrode materials and electrocatalysts is discussed.     

Carbon nanofiber electrodes and controlled nanogaps for scanning electrochemical microscopy experiments

The electrochemical behavior of electrodes made by sealing carbon nanofibers in glass or with electrophoretic paint has been studied by scanning electrochemical microscopy (SECM). Because of their small electroactive surface area, conical geometry with a low aspect ratio and high overpotential for proton and oxygen reduction, carbon nanofiber (CNF) electrodes are promising candidates for producing electrode nanogaps, imaging with high spatial resolution and for the electrodeposition of single metal nanoparticles (e.g., Pt, Pd) for studies as electrocatalysts. By using the feedback mode of the SECM, a CNF tip can produce a gap that is smaller than 20 nm from a platinum disk. Similarly, the SECM used in a tip-collection substrate-generation mode, which subsequently shows a feedback interaction at short distances, makes it possible to detect a single CNF by another CNF and then to form a nanometer gap between the two electrodes. This approach was used to image vertically aligned CNF arrays. This method is useful in the detection in a homogeneous solution of short-lifetime intermediates, which can be electrochemically generated at one electrode and collected at the second at distances that are equivalent to a nanosecond time scale.