Scanning electrochemical microscopy. 40. Voltammetric ion-selective micropipet electrodes for probing ion transfer at bilayer lipid membranes

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

Herein, we feature our recent efforts toward the development and application of nanoelectrochemistry at liquid/liquid interfaces, which are also known as interfaces between two immiscible electrolyte solutions (ITIES). Nanopipets, nanopores, and nanoemulsions are developed to create the nanoscale ITIES for the quantitative electrochemical measurement of ion transfer, electron transfer, and molecular transport across the interface. The nanoscale ITIES serves as an electrochemical nanosensor to enable the selective detection of various ions and molecules as well as high-resolution chemical imaging based on scanning electrochemical microscopy. The powerful nanoelectroanalytical methods will be useful for biological and material applications as illustrated by in situ studies of solid-state nanopores, nuclear pore complexes, living bacteria, and advanced nanoemulsions. These studies provide unprecedented insights into the chemical reactivity of important biological and material systems even at the single nanostructure level.

In Situ Monitoring of Extracellular K+ Using the Potentiometric Mode of Scanning Electrochemical Microscopy with a Carbon-Based Potassium Ion-Selective Tip

The expression of potassium channels can be related to the occurrence and development of tumors. Their change would affect K⁺ outflow. Thus, in situ monitoring of extracellular K⁺ shows a great significance. Herein, the dual-functional K⁺ ion-selective electrode as the scanning electrochemical microscopy (SECM) tip (K⁺-ISE SECM tip) has been developed for in situ monitoring of the extracellular K⁺. Based on multi-wall carbon nanotubes as a transduction layer, the K⁺-ISE SECM tip realizes both the plotting of approach curves to position the tip for in situ detection and the recording of potential responses. It shows a near Nernstian response, good selectivity, and excellent stability. Based on these characteristics, it was used to in situ monitor K⁺ concentrations ([K⁺]_o) of three breast cancer cell lines (MCF-7, MDA-MB-231, and SK-BR-3 cells) at 3 μm above the cell, and [K⁺]_o of MDA-MB-231 cells show the highest value, followed by MCF-7 cells and SK-BR-3 cells. K⁺ outflow induced by electrical stimulation or pH changes of the culture environment (Δ[K⁺]_o) was further determined, and the possible mechanism of K⁺ outflow was investigated with 4-aminopyridin (4-AP). MCF-7 cells present the largest value of Δ[K⁺]_o, followed by MDA-MB-231 cells and SK-BR-3 cells at all the stimulation potentials, and pH 6.50 shows the greatest impact on K⁺ outflow of the three cell lines. The pretreatment of 4-AP changed K⁺ outflow, probably due to the regulation of voltage-gated channels. These findings provide insight into a deep understanding of the microenvironment influence on K⁺ outflow, thereby reflecting the possible mechanism of potassium channels.

PEDOT/PVC-modified amperometric carbon electrodes for acetylcholine detection

Selective detection of acetylcholine (ACh) with PEDOT/PVC-modified amperometric carbon electrodes.

Biological imaging with scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) is a powerful and versatile technique for visualizing the local electrochemical activity of a surface as an ultramicroelectrode tip is moved towards or over a sample of interest using precise positioning systems. In comparison with other scanning probe techniques, SECM not only enables topographical surface mapping but also gathers chemical information with high spatial resolution. Considerable progress has been made in the analysis of biological samples, including living cells and immobilized biomacromolecules such as enzymes, antibodies and DNA fragments. Moreover, combinations of SECM with comple-mentary analytical tools broadened its applicability and facilitated multi-functional analysis with extended life science capabilities. The aim of this review is to present a brief topical overview on recent applications of biological SECM, with particular emphasis on important technical improvements of this surface imaging technique, recommended applications and future trends.

Focused-Ion-Beam-Milled Carbon Nanoelectrodes for Scanning Electrochemical Microscopy

Nanoscale scanning electrochemical microscopy (SECM) has emerged as a powerful electrochemical method that enables the study of interfacial reactions with unprecedentedly high spatial and kinetic resolution. In this work, we develop carbon nanoprobes with high electrochemical reactivity and well-controlled size and geometry based on chemical vapor deposition of carbon in quartz nanopipets. Carbon-filled nanopipets are milled by focused ion beam (FIB) technology to yield a flat disk tip with a thin quartz sheath as confirmed by transmission electron microscopy. The extremely high electroactivity of FIB-milled carbon nanotips is quantified by enormously high standard electron-transfer rate constants of ≥10 cm/s for Ru(NH3)63+. The tip size and geometry are characterized in electrolyte solutions by SECM approach curve measurements not only to determine inner and outer tip radii of down to ~27 and ~38 nm, respectively, but also to ensure the absence of a conductive carbon layer on the outer wall. In addition, FIB-milled carbon nanotips reveal the limited conductivity of ~100 nm-thick gold films under nanoscale mass-transport conditions. Importantly, carbon nanotips must be protected from electrostatic damage to enable reliable and quantitative nanoelectrochemical measurements.

Voltammetric Ion Selectivity of Thin Ionophore-Based Polymeric Membranes: Kinetic Effect of Ion Hydrophilicity

The high ion selectivity of potentiometric and optical sensors based on ionophore-based polymeric membranes is thermodynamically limited. Here, we report that the voltammetric selectivity of thin ionophore-based polymeric membranes can be kinetically improved by several orders of magnitude in comparison with their thermodynamic selectivity. The kinetic improvement of voltammetric selectivity is evaluated quantitatively by newly introducing a voltammetric selectivity coefficient in addition to a thermodynamic selectivity coefficient. Experimentally, both voltammetric and thermodynamic selectivity coefficients are determined from cyclic voltammograms of excess amounts of analyte and interfering ions with respect to the amount of a Na(+)- or Li(+)-selective ionophore in thin polymeric membranes. We reveal the slower ionophore-facilitated transfer of a smaller alkaline earth metal cation with higher hydrophilicity across the membrane/water interface, thereby kinetically improving voltammetric Na(+) selectivity against calcium, strontium, and barium ions by 3, 2, and 1 order of magnitude, respectively, in separate solutions. Remarkably, voltammetric Na(+) and Li(+) selectivity against calcium and magnesium ions in mixed solutions is improved by 4 and >7 orders of magnitude, respectively, owing to both thermodynamic and kinetic effects in comparison with thermodynamic selectivity in separate solutions. Advantageously, the simultaneous detection of sodium and calcium ions is enabled voltammetrically in contrast to the potentiometric and optical counterparts. Mechanistically, we propose a new hypothetical model that the slower transfer of a more hydrophilic ion is controlled by its partial dehydration during the formation of the adduct with a "water finger" prior to complexation with an ionophore at the membrane/water interface.

A polarized liquid–liquid interface meets visible light-driven catalytic water oxidation

A nanostructured BiVO₄photocatalyst assembled at a chemically polarized liquid–liquid interface generates an efficient amount of O₂with a [Co(bpy)₃]³⁺scavenger in the organic phase.

Scanning Electrochemical Microscopy (SECM): Fundamentals and Applications in Life Sciences

In recent years, scanning electrochemical microscopy (SECM) has made significant contributions to the life sciences. Innovative developments focusing on high-resolution imaging, developing novel operation modes, and combining SECM with complementary optical or scanning probe techniques renders SECM an attractive analytical approach. This chapter gives an introduction to the essential instrumentation and operation principles of SECM for studying biologically-relevant systems. Particular emphasis is given to applications aimed at imaging the activity of biochemical constituents such as enzymes, antibodies, and DNA, which play a pivotal role in biomedical diagnostics. Furthermore, the unique advantages of SECM and combined techniques for studying live cells is highlighted by discussion of selected examples.

Amperometric Ion Sensing Approaches at Liquid/Liquid Interfaces for Inorganic, Organic and Biological Ions

Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) has become an invaluable tool for the selective and sensitive detection of cationic and anionic species, including charged drug molecules and proteins. In addition, neutral molecules can also be detected at the ITIES via enzymatic reactions. This chapter highlights recent developments towards creating a wide spectrum of sensing platforms involving ion transfer across the ITIES. As well as outlining the basic principles needed for performing these sensing applications, the development of ITIES-based detection strategies for inorganic, organic, and biological ions is discussed.

Electrochemical nanoprobes for the chemical detection of neurotransmitters

Neurotransmitters, acting as chemical messengers, play an important role in neurotransmission, which governs many functional aspects of nervous system activity. Electrochemical probes have proven a very useful technique to study neurotransmission, especially to quantify and qualify neurotransmitters. With the emerging interests in probing neurotransmission at the level of single cells, single vesicles, as well as single synapses, probes that enable detection of neurotransmitters at the nanometer scale become vitally important. Electrochemical nanoprobes have been successfully employed in nanometer spatial resolution imaging of single nanopores of Si membrane and single Au nanoparticles, providing both topographical and chemical information, thus holding great promise for nanometer spatial study of neurotransmission. Here we present the current state of electrochemical nanoprobes for chemical detection of neurotransmitters, focusing on two types of nanoelectrodes, i.e. carbon nanoelectrode and nano-ITIES pipet electrode.

Nanopipet-Based Liquid-Liquid Interface Probes for the Electrochemical Detection of Acetylcholine, Tryptamine, and Serotonin via Ionic Transfer

A nanoscale interface between two immiscible electrolyte solutions (ITIES) provides a unique analytical platform for the detection of ionic species of biological interest such as neurotransmitters and neuromodulators, especially those that are otherwise difficult to detect directly on a carbon electrode without electrode modification. We report the detection of acetylcholine, serotonin, and tryptamine on nanopipet electrode probes with sizes ranging from a radius of ≈7 to 35 nm. The transfer of these analytes across a 1,2-dichloroethane/water interface was studied by cyclic voltammetry and amperometry. Well-defined sigmoidal voltammograms were observed on the nanopipet electrodes within the potential window of artificial seawater for acetylcholine and tryptamine. The half wave transfer potential, E1/2, of acetylcholine, tryptamine, and serotonin were found to be -0.11, -0.25, and -0.47 V vs E(1/2,TEA) (term is defined later in experimental), respectively. The detection was linear in the range of 0.25-6 mM for acetylcholine and of 0.5-10 mM for tryptamine in artificial seawater. Transfer of serotonin was linear in the range of 0.15-8 mM in LiCl solution. The limit of detection for serotonin in LiCl on a radius ≈21 nm nanopipet electrode was 77 μM, for acetylcholine on a radius ≈7 nm nanopipet electrode was 205 μM, and for tryptamine on a radius ≈19 nm nanopipet electrode was 86 μM. Nanopipet-supported ITIES probes have great potential to be used in nanometer spatial resolution measurements for the detection of neurotransmitters.

Nanopipettes: probes for local sample analysis

Nanopipettes are demonstrated as probes for local mass spectrometric analysis with potential for small-scale extraction of analytes from single cells, tissue and organisms.

Nanopipettes (pipettes with diameters <1 μm) were explored as pressure-driven fluid manipulation tools for sampling nanoliter volumes of fluids. The fundamental behavior of fluids confined in the narrow channels of the nanopipette shank was studied to optimize sampling volume and probe geometry. This method was utilized to collect nanoliter volumes (<10 nL) of sample from single Allium cepa cells and live Drosophila melanogaster first instar larvae. Matrix assisted laser desorption/ionization-mass spectrometry (MALDI-MS) was utilized to characterize the collected sample. The use of nanopipettes for surface sampling of mouse brain tissue sections was also explored. Lipid analyses were performed on mouse brain tissues with spatial resolution of sampling as small as 50 μm. Nanopipettes were shown to be a versatile tool that will find further application in studies of sample heterogeneity and population analysis for a wide range of samples.

Quad-barrel multifunctional electrochemical and ion conductance probe for voltammetric analysis and imaging

The fabrication and use of a multifunctional electrochemical probe incorporating two independent carbon working electrodes and two electrolyte-filled barrels, equipped with quasi-reference counter electrodes (QRCEs), in the end of a tapered micrometer-scale pipet is described. This "quad-probe" (4-channel probe) was fabricated by depositing carbon pyrolytically into two diagonally opposite barrels of a laser-pulled quartz quadruple-barrelled pipet. After filling the open channels with electrolyte solution, a meniscus forms at the end of the probe and covers the two working electrodes. The two carbon electrodes can be used to drive local electrochemical reactions within the meniscus while a bias between the QRCEs in the electrolyte channels provides an ion conductance signal that is used to control and position the meniscus on a surface of interest. When brought into contact with a surface, localized high resolution amperometric imaging can be achieved with the two carbon working electrodes with a spatial resolution defined by the meniscus contact area. The substrate can be an insulating material or (semi)conductor, but herein, we focus mainly on conducting substrates that can be connected as a third working electrode. Studies using both aqueous and ionic liquid electrolytes in the probe, together with gold and individual single walled carbon nanotube samples, demonstrate the utility of the technique. Substrate generation-dual tip collection measurements are shown to be characterized by high collection efficiencies (approaching 100%). This hybrid configuration of scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) should be powerful for future applications in electrode mapping, as well as in studies of insulating materials as demonstrated by transient spot redox-titration measurements at an electrostatically charged Teflon surface and at a pristine calcite surface, where a functionalized probe is used to follow the immediate pH change due to dissolution.

Improved ion-selective detection method using nanopipette with poly(vinyl chloride)-based membrane

Ion-selective electrodes (ISEs) are widely used to detect targeted ions in solution selectively. Application of an ISE to a small area detection system with a nanopipette requires a special measurement method in order to avoid the enhanced background signal problem caused by a cation-rich layer near the charged inner surface of the nanopipette and the selectivity change problem due to relatively fast saturation of the ISE inside the nanopipette. We developed a novel ion-selective detection system using a nanopipette that measures an alternating current (AC) signal mediated by saturated ionophores in a poly(vinyl chloride) (PVC) membrane located at the conical shank of the nanopipette to solve the above problems. Small but reliable K(+) and Na(+) ionic current passing through a PVC membrane containing saturated bis(benzo-15-crown-5) and bis(12-crown-4) ionophore, respectively, could be selectively detected using the AC signal measurement system equipped with a lock-in amplifier.

Ion permeability of the nuclear pore complex and ion-induced macromolecular permeation as studied by scanning electrochemical and fluorescence microscopy

Efficient delivery of therapeutic macromolecules and nanomaterials into the nucleus is imperative for gene therapy and nanomedicine. Nucleocytoplasmic molecular transport, however, is tightly regulated by the nuclear pore complex (NPC) with the hydrophobic transport barriers based on phenylalanine and glycine repeats. Herein, we apply scanning electrochemical microscopy (SECM) to quantitatively study the permeability of the NPCs to small probe ions with a wide range of hydrophobicity as a measure of their hydrophobic interactions with the transport barriers. Amperometric detection of the redox-inactive probe ions is enabled by using the ion-selective SECM tips based on the micropipet- or nanopipet-supported interfaces between two immiscible electrolyte solutions. The remarkably high ion permeability of the NPCs is successfully measured by SECM and theoretically analyzed. This analysis demonstrates that the ion permeability of the NPCs is determined by the dimensions and density of the nanopores without a significant effect of the transport barriers on the transported ions. Importantly, the weak ion-barrier interactions become significant at sufficiently high concentrations of extremely hydrophobic ions, i.e., tetraphenylarsonium and perfluorobutylsulfonate, to permeabilize the NPCs to naturally impermeable macromolecules. Dependence of ion-induced permeabilization of the NPC on the pathway and mode of macromolecular transport is studied by using fluorescence microscopy to obtain deeper insights into the gating mechanism of the NPC as the basis of a new transport model.

Chemical communication between liposomes encapsulating a chemical oscillatory reaction

Electrochemical measurements and numerical simulations are employed to understand the chemical communication between liposomes prepared in microfluidics and encapsulating a chemical oscillator.

Fabrication, characterization, and functionalization of dual carbon electrodes as probes for scanning electrochemical microscopy (SECM)

Dual carbon electrodes (DCEs) are quickly, easily, and cheaply fabricated by depositing pyrolytic carbon into a quartz theta nanopipet. The size of DCEs can be controlled by adjusting the pulling parameters used to make the nanopipet. When operated in generation/collection (G/C) mode, the small separation between the electrodes leads to reasonable collection efficiencies of ca. 30%. A three-dimensional finite element method (FEM) simulation is developed to predict the current response of these electrodes as a means of estimating the probe geometry. Voltammetric measurements at individual electrodes combined with generation/collection measurements provide a reasonable guide to the electrode size. DCEs are employed in a scanning electrochemical microscopy (SECM) configuration, and their use for both approach curves and imaging is considered. G/C approach curve measurements are shown to be particularly sensitive to the nature of the substrate, with insulating surfaces leading to enhanced collection efficiencies, whereas conducting surfaces lead to a decrease of collection efficiency. As a proof-of-concept, DCEs are further used to locally generate an artificial electron acceptor and to follow the flux of this species and its reduced form during photosynthesis at isolated thylakoid membranes. In addition, 2-dimensional images of a single thylakoid membrane are reported and analyzed to demonstrate the high sensitivity of G/C measurements to localized surface processes. It is finally shown that individual nanometer-size electrodes can be functionalized through the selective deposition of platinum on one of the two electrodes in a DCE while leaving the other one unmodified. This provides an indication of the future versatility of this type of probe for nanoscale measurements and imaging.

Nanoscale mechanism of molecular transport through the nuclear pore complex as studied by scanning electrochemical microscopy

The nuclear pore complex (NPC) is the proteinaceous nanopore that solely mediates molecular transport across the nuclear envelope between the nucleus and cytoplasm of a eukaryotic cell. Small molecules (<40 kDa) diffuse through the large pore of this multiprotein complex. A passively impermeable macromolecule tagged with a signal peptide is chaperoned through the nanopore by nuclear transport receptors (e.g., importins) owing to their interactions with barrier-forming proteins. Presently, this bimodal transport mechanism is not well understood and is described by controversial models. Herein, we report on a dynamic and spatially resolved mechanism for NPC-mediated molecular transport through nanoscale central and peripheral routes with distinct permeabilities. Specifically, we develop a nanogap-based approach of scanning electrochemical microscopy to precisely measure the extremely high permeability of the nuclear envelope to a small probe molecule, (ferrocenylmethyl)trimethylammonium. Effective medium theories indicate that the passive permeability of 5.9 × 10(-2) cm/s corresponds to the free diffusion of the probe molecule through ~22 nanopores with a radius of 24 nm and a length of 35 nm. Peripheral routes are blocked by wheat germ agglutinin to yield 2-fold lower permeability for 17 nm-radius central routes. This lectin is also used in fluorescence assays to find that importins facilitate the transport of signal-tagged albumin mainly through the 7 nm-thick peripheral route rather than through the sufficiently large central route. We propose that this spatial selectivity is regulated by the conformational changes in barrier-forming proteins that transiently and locally expand the impermeably thin peripheral route while blocking the central route.

Chiral interactions of the drug propranolol and α1-acid-glycoprotein at a micro liquid-liquid interface

The investigation of chiral interactions of drugs with plasma proteins is of fundamental importance for drug efficacy and toxicity studies. In this paper, we demonstrate a simple liquid-liquid interface procedure for investigating chiral interactions. Chiral discrimination of the enantiomers of a basic drug, propranolol, was achieved at a micro liquid-liquid interface, using α(1)-acid-glycoprotein (AGP) as a chiral acute phase plasma protein. When the protein is added to an aqueous phase containing the enantiomers of propranalol hydrochloride, the binding of (S)- and (R)-propranolol hydrochloride to the protein results in a decrease in the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) current responses corresponding to the decrease in transfer of propranolol at an aqueous-1,2-dichloroethane interface. This decrease is a consequence of the complexation of the drug and the protein. The complex drug-protein does not transfer across the interface nor changes the transfer potential of the uncomplexed form of propranolol enantiomers. The bound concentration of propranolol enantiomers in the presence of AGP was found to be greater for (S)-propranolol than (R)-propranolol for solutions containing constant concentrations of AGP (50 μM). Scatchard analysis yielded association constants of 2.7 and 1.3 × 10(5) M(-1) for (S)- and (R)-propranolol, respectively.

Ion-selective permeability of an ultrathin nanoporous silicon membrane as probed by scanning electrochemical microscopy using micropipet-supported ITIES tips

We report on the application of scanning electrochemical microscopy (SECM) to the measurement of the ion-selective permeability of porous nanocrystalline silicon membrane as a new type of nanoporous material with potential applications in analytical, biomedical, and biotechnology device development. The reliable measurement of high permeability in the molecularly thin nanoporous membrane to various ions is important for greater understanding of its structure-permeability relationship and also for its successful applications. In this work, this challenging measurement is enabled by introducing two novel features into amperometric SECM tips based on the micropipet-supported interface between two immiscible electrolyte solutions (ITIES) to reveal the important ion-transport properties of the ultrathin nanopore membrane. The tip of a conventional heat-pulled micropipet is milled using the focused ion beam (FIB) technique to be smoother, better aligned, and subsequently, approach closer to the membrane surface, which allows for more precise and accurate permeability measurement. The high membrane permeability to small monovalent ions is determined using FIB-milled micropipet tips to establish a theoretical formula for the membrane permeability that is controlled by free ion diffusion across water-filled nanopores. Moreover, the ITIES tips are rendered selective for larger polyions with biomedical importance, i.e., polyanionic pentasaccharide Arixtra and polycationic peptide protamine, to yield the membrane permeability that is lower than the corresponding diffusion-limited permeability. The hindered transport of the respective polyions is unequivocally ascribed to electrostatic and steric repulsions from the wall of the nanopores, i.e., the charge and size effects.

Kinetic study of rapid transfer of tetraethylammonium at the 1,2-dichloroethane/water interface by nanopipet voltammetry of common ions

Steady-state voltammetry at the pipet-supported liquid/liquid interface has previously been used to measure kinetics of simple and facilitated ion transfer (IT) processes. Recently, we showed that the conventional experimental protocol and data analysis produce large uncertainties in kinetic parameters of rapid IT processes extracted from pipet voltammograms. Here, we used a new mode of nanopipet voltammetry, in which a transferable ion is initially present as a common ion in both liquid phases, and improved methodology for silanization of the outer pipet wall to investigate the kinetics of the rapid transfer of tetraethylammonium (TEA(+)) at the 1,2-dichloroethane/water interface. This reaction was often employed as a model system to check the IT theory. The determined standard rate constant and transfer coefficient of the TEA(+) transfer are compared with previously reported values to demonstrate limitations of conventional nanopipet voltammetry with a transferrable ion present only in one liquid phase.

Reverse current pulse method to restore uniform concentration profiles in ion-selective membranes. 1. Galvanostatic pulse methods with decreased cycle time

The applications of ion-selective electrodes (ISEs) have been broadened through the introduction of galvanostatic current pulse methods in potentiometric analysis. An important requirement in these applications is the restoration of the uniform equilibrium concentration profiles in the ISE membrane between each measurement. The simplest restoration method is zero-current relaxation, in which the membrane relaxes under open-circuit conditions in a diffusion-controlled process. This paper presents a novel restoration method using a reverse current pulse. An analytic model for this restoration method is derived to predict the concentration profiles inside ISE membranes following galvanostatic current pulses. This model allows the calculation of the voltage transients as the membrane voltage relaxes back toward its zero-current equilibrium value. The predicted concentration profiles and voltage transients are confirmed using spectroelectrochemical microscopy (SpECM). The reverse current restoration method described in this paper reduces the voltage drift and voltage error by 10-100 times compared to the zero-current restoration method. Therefore, this new method provides faster and more reproducible voltage measurements in most chronopotentiometric ISE applications, such as improving the detection limit and determining concentrations and diffusion coefficients of membrane species. One limitation of the reverse current restoration method is that it cannot be used in a few applications that require background electrolyte loaded membranes without excess of lipophilic cation exchanger.

Spatially resolved detection of a nanometer-scale gap by scanning electrochemical microscopy

Nanowires with nanometer-scale gaps are an emerging class of nanomaterials with potential applications in electronics and optics. Here, we demonstrate that the feedback mode of scanning electrochemical microscopy (SECM) allows for spatially resolved detection of a nanogap on the basis of its electrical conductivity. A gapped nanoband is used as a model system to describe a mechanism of a unique feedback effect from a nanogap. Interestingly, both experiments and numerical simulations confirm that a peak current response is obtained when an SECM tip is laterally scanned above an insulating nanogap formed in an unbiased nanoband. On the other hand, no peak current response is expected for a highly conductive nanogap, which must be extremely narrow or filled with highly conductive molecules for efficient electron transport.

Scanning Electrochemical Microscopy of One-Dimensional Nanostructure: Effects of Nanostructure Dimensions on the Tip Feedback Current under Unbiased Conditions

Scanning electrochemical microscopy (SECM) is developed as a powerful approach to electrochemical characterization of individual one-dimensional (1D) nanostructures under unbiased conditions. 1D nanostructures comprise high-aspect-ratio materials with both nanoscale and macroscale dimensions such as nanowires, nanotubes, nanobelts, and nanobands. Finite element simulations demonstrate that the feedback current at a disk-shaped ultramicroelectrode tip positioned above an unbiased nanoband, as prepared on an insulating substrate, is sensitive to finite dimensions of the band, i.e., micrometer length, nanometer width, and nanometer height from the insulating surface. The electron-transfer rate of a redox mediator at the nanoband surface depends not only on the intrinsic rate but also on the open-circuit potential of the nanoband, which is determined by the dimensions of the nanoband as well as the tip inner and outer radii, and tip-substrate distance. The theoretical predictions are confirmed experimentally by employing Au nanobands as fabricated on a SiO(2) surface by electron-beam lithography, thereby yielding well defined dimensions of 100 or 500 nm in width, 47 nm in height, and 50 μm in length. A 100 nm-wide nanoband can be detected by SECM imaging with ∼2 μm-diameter tips although the tip feedback current is compromised by finite electron-transfer kinetics for Ru(NH(3))(6) (3+) at the nanoband surface.

Nanoelectrochemistry of mammalian cells

There is a significant current interest in development of new techniques for direct characterization of the intracellular redox state and high-resolution imaging of living cells. We used nanometer-sized amperometric probes in combination with the scanning electrochemical microscope (SECM) to carry out spatially resolved electrochemical experiments in cultured human breast cells. With the tip radius approximately 1,000 times smaller than that of a cell, an electrochemical probe can penetrate a cell and travel inside it without apparent damage to the membrane. The data demonstrate the possibility of measuring the rate of transmembrane charge transport and membrane potential and probing redox properties at the subcellular level. The same experimental setup was used for nanoscale electrochemical imaging of the cell surface.

Scanning electrochemical microscopy

This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.

Review: Recent applications of scanning electrochemical microscopy to the study of charge transfer kinetics

Scanning electrochemical microscopy (SECM) has been proven to be a valuable technique for the quantitative investigation and surface analysis of a wide range of processes that occur at interfaces. In particular, there is a great deal of interest in studying the kinetics of charge transfer characteristics at the solid/liquid and liquid/liquid interface. This overview outlines recent advances and applications of SECM to the investigation of charge transfer reactions at the solid/liquid interface and liquid/liquid interface.

Scanning Electrochemical Microscopy with Slightly Recessed Nanotips

Slightly recessed nanoelectrodes were prepared by controlled etching of nanometer-sized, flat Pt electrodes. By using high-frequency (e.g., 2 MHz) ac voltage, the layer of Pt as thin as greater, approximately >3 nm was removed to produce a cylindrical cavity inside the insulating glass sheath. The etched electrodes were characterized by combination of voltammetry and scanning electrochemical microscopy (SECM) to determine the radius and the effective depth of the recess. The theory was developed for current versus distance curves obtained with a recessed tip approaching either a conductive or an insulating substrate. Good agreement between the theoretical and experimental approach curves indicated that recessed nanotips are suitable for quantitative feedback mode SECM experiments.

Scanning electrochemical microscopy for direct imaging of reaction rates

Not only in electrochemistry but also in biology and in membrane transport, localized processes at solid-liquid or liquid-liquid interfaces play an important role at defect sites, pores, or individual cells, but are difficult to characterize by integral investigation. Scanning electrochemical microscopy is suitable for such investigations. After two decades of development, this method is based on a solid theoretical foundation and a large number of demonstrated applications. It offers the possibility of directly imaging heterogeneous reaction rates and locally modifying substrates by electrochemically generated reagents. The applications range from classical electrochemical problems, such as the investigation of localized corrosion and electrocatalytic reactions in fuel cells, sensor surfaces, biochips, and microstructured analysis systems, to mass transport through synthetic membranes, skin and tissue, as well as intercellular communication processes. Moreover, processes can be studied that occur at liquid surfaces and liquid-liquid interfaces.

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.

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.