Scanning Electrochemical Microscopy Probe Research Articles

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.

Scanning electrochemical microscopy investigation of nitrate reduction at activated copper cathodes in acidic medium

The cathodic reduction of nitrate at copper electrodes was investigated by using scanning electrochemical microscopy (SECM) in the stripping-mode SECM (SM-SECM). This electrochemical process is facilitated at surfaces activated by a potential protocol involving anodic dissolution and subsequent reduction of copper cations, which causes an increase in the surface area. In the presence of chloride, a much more significant current enhancement was noticed, and a mechanism based on the formation of CuCl was proposed to explain the increased rate of nitrate reduction at this experimental condition.

Combined high resolution Scanning Kelvin probe—Scanning electrochemical microscopy investigations for the visualization of local corrosion processes

An integrated SKP–SECM system was successfully optimised with respect to improved lateral resolution. An aluminum alloy was synthesised by solidification of a liquid melt of pure Al, Cu and Mg metal powders in order to visualize single S-phase intermetallic particles (IMPs) using a newly proposed “glass free” SKP–SECM tip. The obtained IMPs were randomly distributed in the solid solution matrix of the alloy and their average chemical composition was in agreement with that of S-phase IMPs in commercially available AA2024-T351 alloys. The S-phase IMPs were localized in the SKP mode of the SKP–SECM system. The increased electrochemical activity of the S-phase IMPs was visualized using the feedback mode of SECM and the in situ consumption of O2 on the surface of a single S-phase IMP was visualized in the redox-competition mode of the SECM using the same tip. Thus, the local Volta potential difference obtained in the SKP mode could be overlaid with the local electrochemical activity for O2 reduction.

Active Sites for Outer-Sphere, Inner-Sphere, and Complex Multistage Electrochemical Reactions at Polycrystalline Boron-Doped Diamond Electrodes (pBDD) Revealed with Scanning Electrochemical Cell Microscopy (SECCM)

The local rate of heterogeneous electron transfer (HET) at polycrystalline boron-doped diamond (pBDD) electrodes has been visualized at high spatial resolution for various aqueous electrochemical reactions, using scanning electrochemical cell microscopy (SECCM), which is a technique that uses a mobile pipet-based electrochemical cell as an imaging probe. As exemplar systems, three important classes of electrode reactions have been investigated: outer-sphere (one-electron oxidation of ferrocenylmethyltrimethylammonium (FcTMA(+))), inner-sphere (one-electron oxidation of Fe(2+)), and complex processes with coupled electron transfer and chemical reactions (oxidation of serotonin). In all cases, the pattern of reactivity is similar: the entire pBDD surface is electroactive, but there are variations in activity between different crystal facets which correlate directly with differences in the local dopant level, as visualized qualitatively by field-emission scanning electron microscopy (FE-SEM). No evidence was found for enhanced activity at grain boundaries for any of the reactions. The case of serotonin oxidation is particularly interesting, as this process is known to lead to deterioration of the electrodes, because of blocking by reaction products, and therefore cannot be studied with conventional scanning electrochemical probe microscopy (SEPM) techniques. Yet, we have found this system nonproblematic to study, because the meniscus of the scanning pipet is only in contact with the surface investigated for a brief time and any blocking product is left behind as the pipet moves to a new location. Thus, SECCM opens up the possibility of investigating and visualizing much more complex heterogeneous electrode reactions than possible presently with other SEPM techniques.

Scanning Electrochemical Microscopy Study of Ion Annihilation Electrogenerated Chemiluminescence of Rubrene and [Ru(bpy)3]2+

Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)(3)](2+) was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (k(ann) > 10(7) M(-1) s(-1)). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)(3)](2+) to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.

Quantitative characterization of shear force regulation for scanning electrochemical microscopy

Abstract The quest for higher spatial resolution in scanning electrochemical microscopy (SECM) calls for the application of smaller probe electrodes. When electrodes are to be used in the feedback mode, smaller electrodes require higher intrinsic kinetics at the sample. The fabrication of nanoelectrodes, as well as their use as SECM probes at constant distance, are reported. The properties of shear force regulation system are characterized quantitatively. Simultaneous topography and reactivity imaging were demonstrated using gold microstructures on a glass substrate.

Mapping of Electrocatalytic Sites on a Single Strand of Carbon Fiber Using Scanning Electrochemical Microscopy (SECM)

A novel use of scanning electrochemical microscopy (SECM) to study electrocatalytic heterogeneities present on a single strand of carbon fiber (SSCF) is presented. The SECM investigations on an SSC...

Pathways for Hydrogen Absorption into Zircaloy-2 under Cathodic Polarization Assessed by Scanning Electrochemical Microscopy, Scanning Electron Microscopy, and Electrochemical Impedance Spectroscopy

Hydrogen absorption by Zircaloy-2 has been studied in neutral sodium sulfate solutions using steady-state polarization measurements, electrochemical impedance spectroscopy (EIS), and scanning electrochemical microscopy (SECM). For applied potentials < −1 V (vs. saturated calomel electrode, SCE), water reduction occurs in faults in the passive oxide covering the alloy. For potentials ≤−1.3 VSCE, SECM detects a distribution of reactive locations on the electrode surface. Matched SECM and SEM images of the same electrode surface show more reactive sites to be located in the β-phase grain boundaries than on the α-grains. The reactivity of surface locations was determined using SECM probe approach curves. Secondary phase particles incorporating impurities such as Fe, Ni, and Cr as Zr(Fe, Cr)2 and Zr2(Fe, Ni) are particularly reactive spots, which may act as “windows” for hydrogen absorption into the alloy. This claim is supported by the observation that the surface of the α-grains remains passive even at potentials as low as −2.0 VSCE. The need to include a Warburg impedance element in modeling EIS recorded at potentials ≥−1.3 VSCE suggests that H2O reduction is confined to tight flaws in the oxide at grain boundary locations.

Fabrication of soft gold microelectrode arrays as probes for scanning electrochemical microscopy

Soft linear gold microelectrode arrays for high throughput scanning electrochemical microscopy (SECM) imaging were fabricated using the Aerosol Jet® printing technology. Nanoparticulate gold ink was printed on polyimide Kapton HN® thin films. After sintering, a 200 nm thick Parylene C coating was deposited to cover and seal the gold tracks. A cross-section of the array of microelectrodes was exposed by laser cutting using an ArF excimer laser beam directed onto a metallic mask. Cyclic voltammograms, approach curves and SECM images in feedback mode demonstrate the capability of the arrays as SECM probes. Reactivity imaging of a platinum band structure on glass was performed with Parylene C coating facing the substrate providing an almost constant working distance. The softness of the array leads to a bending and allows scanning in contact mode like brushing the sample surface. For hard surfaces such as array electrode structures and similar materials, this occurs without detectable damage to the sample.

Electrochemical Scanning Probe Microscopy Based Diagnostics for In situ Monitoring of Charge Storage Processes in Oxide Nanoparticles

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Atomic force microscope imaging and force measurements at electrified and actively corroding interfaces: Challenges and novel cell design

We report the design of an improved electrochemical cell for atomic force microscope measurements in corrosive electrochemical environments. Our design improvements are guided by experimental requirements for studying corrosive reactions such as selective dissolution, dealloying, pitting corrosion, and∕or surface and interface forces at electrified interfaces. Our aim is to examine some of the limitations of typical electrochemical scanning probe microscopy (SPM) experiments and in particular to outline precautions and cell-design elements, which must necessarily be taken into account in order to obtain reliable experimental results. In particular, we discuss electrochemical requirements for typical electrochemical SPM experiments and introduce novel design features to avoid common issues such as crevice formations; we discuss the choice of electrodes and contaminations from ions of reference electrodes. We optimize the cell geometry and introduce standard samples for electrochemical AFM experiments. We have tested the novel design by performing force-distance spectroscopy as a function of the applied electrochemical potential between a bare gold electrode surface and a SAM-coated AFM tip. Topography imaging was tested by studying the well-known dealloying process of a Cu(3)Au(111) surface up to the critical potential. Our design improvements should be equally applicable to in situ electrochemical scanning tunneling microscope cells.

Minuscule device for hydrogen generation/electrical energy collection system on aluminum alloy surface

Cogeneration of hydrogen and electrical energy in a single system is still a challenging issue. In this work, in a micro scale, a novel miniaturized system is introduced to capture the electrical energy of produced hydrogen on aluminum alloy by using an ultra-microelectrode based on scanning electrochemical microscopy (SECM). Sophisticated nanosize atomic force microscopy (AFM) based SECM probe could collect the electrochemical current close proximity distance from the aluminum surface to attain the highest possible current efficiency. Various collected current levels were associated to the aluminum microstructure constituents. It is expected that future development in instrumentation could principally facilitate SECM as a tool for hydrogen economy.

Scanning electrochemical microscopy in the 21st century. Update 1: five years after

This Perspective is an update to our more extensive survey of scanning electrochemical microscopy (SECM) published in 2007. During this time, the SECM field retained its momentum by expanding into new areas and meeting the emerging scientific and technological challenges. Here we focus on most prominent developments such as high-resolution imaging, investigation of structures and processes on the nanoscale, alternative energy applications, and new approaches to solving "real world" problems. The fabrication of novel SECM probes and related theoretical advances are also discussed.

Electrochemical scanning probe microscope (EC-SPM) for the in situ corrosion study of materials: an overview with examples

Electro-chemical scanning probe microscope (EC-SPM) is one of the potential surface investigation techniques to study the electrode-electrolyte interface and over, the years, it has significantly contributed to the understanding of the physico-chemical phenomenon of electrochemical process in different electrochemical environments. The ability to probe the local character at the sub-microscopic level of electrode-electrolyte interfaces with potentiostatic control has advanced its applications in various fields of fundamental, and applied, research such as material science, biological science, physical and chemical science, in order to study the structure and dynamic properties of materials and interfaces. Included in such studies are the structure-reactivity of electrode surface, electro-deposition, surface reconstructions, absorption process, process of corrosion initiation, the study of biological substances in nearby physiological conditions and other interfacial surface morphological studies. Out of several interesting applications of EC-SPM, one of the important and interesting fields of study is the process of corrosion initiation of different materials in simulated environments. To date, its applications are wide spread for understanding passive film property, surface dissolution, surface morphological changes prior to corrosion initiation, the efficiency of inhibitors, and many other branches of corrosion science. As EC-SPM continues to play a significant role in comprehending the process of corrosion, it is important to keep up-to-date on the progress made in the recent past. Thus, the basic aim and scope of the present review is to provide the trends and status of corrosion research of various technologically important materials using EC-SPM. The review includes, an introduction to the fundamentals of EC-SPM, instrumententation in commercial equipments, and applications in the field of corrosion research of various technologically important materials in different electrochemical environments. Illustrative examples have been provided from several investigations reported in the field of electrochemical scanning probe microscopy towards addressing the most critical aspects of corrosion. Major emphasis has been given towards passivation, corrosion initiation, material protection using coatings and associated phenomenon. The review mostly focuses on morphological features of various investigations made so far; for more explanation and other allied experimental techniques readers are requested to go through the respective articles.

Polished Nanopipets: New Probes for High-Resolution Scanning Electrochemical Microscopy

Nanometer-sized pipets pulled from glass or quartz capillaries have been extensively used as probes for scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM). A small separation distance between such a probe and the sample, which is required for high-resolution SECM measurements, may be hard to attain because of considerable roughness of the pipet tip. In this Letter, we report the preparation and characterization of polished nanopipet SECM probes with a much smoother tip edge. Using polished pipets, quantitative SECM measurements were performed at extremely short tip/substrate distances (e.g., d ≈ 1 nm).

High‐Resolution Imaging of Nanostructured Si/SiO2 Substrates and Cell Monolayers Using Scanning Electrochemical Microscopy

AbstractHigh‐resolution scanning electrochemical microscopy (SECM) in the constant‐height mode was applied to image details of a nanostructured porous substrate as well as a confluent epithelial cell monolayer. Platinum disk ultramicroelectrodes (UMEs) with radii down to 100 nm were fabricated by sealing electrochemically etched Pt wires into soda‐lime glass capillaries. A subsequent thermal treatment was used to improve the quality of electrode sealing. The size and shape of the SECM probes were adapted to the requirements of the inorganic and biological surfaces to enable high‐resolution SECM imaging and to ensure stability and reliability during image acquisition. A mesoporous substrate (Si/SiO2) with 800 nm rectangular pores was imaged with sub‐µm resolution. In addition, confluent epithelial cell monolayers derived from kidney were investigated. Independent measurements with ferrocene methanol and hexamine ruthenium (III) chloride led to equivalent SECM results. High quality SECM images with well separated cells within the monolayer were obtained. Resolved regions of cell‐cell contacts extending about 1–3 µm in lateral direction were imaged.

An efficient method for fabrication of disk-shaped scanning electrochemical microscopy probes with small glass-sheath thicknesses

This work describes a method for fabrication of disk microelectrodes for scanning electrochemical microscopy (SECM) tips. A sharpened platinum wire is sealed into a borosilicate glass capillary and is thermally projected out of the capillary as a tip. The disk microelectrode is obtained by mechanical polishing of the protruding glass-shrouded wire apex. What make this method distinctive are the following details: (1) The Pt-containing capillary is embedded into a polymer support to carry out a secure polishing of the tip apex; once the disk is exposed, the polymer is dissolved. (2) Disk exposure is revealed by an electrical detection method. (3) As no manual sharpening of the glass tip is required, very small glass-sheath thicknesses can be obtained ( R g = sheath radius/microelectrode radius < 3). The fabrication of tips with radii larger than 0.3 μm is relatively straightforward (success rate near 90%) with this method. SECM approach curves (positive and negative feedback), SEM and optical microscopy were used to characterize the described tips.

One Pot Hemimicellar Synthesis of Amphiphilic Janus Gold Nanoclusters for Novel Electronic Attributes

A one-pot hemimicellar synthesis of oriented, amphiphilic, and fluorescent Janus gold clusters, establishing the Janus character in terms of ligand asymmetry and distribution, has been demonstrated. The method was based on the efficient Langmuir strategy, where the in situ two-dimensional (2D) reduction of Au(3+) in the sprayed micellar electrostatic complex, TOA(+)-AuCl(4)(-), was accomplished by subphase tryptophan that acted as the hydrophilic protecting ligand on one hemisphere of the spherical gold cluster. In contrast to the reported micelle-assisted Janus cluster formation, here the cluster growth occurred inside the surface pressure driven hemimicelles, which rapidly formed 2D cluster arrays without any interfacial reorientation. The Janus structure was validated using angle dependent polarized Fourier Transform Infrared Reflection-Absorption Spectroscopy (FT-IRRAS), where orientation dependent vibrational changes in the adsorbed ligand functionalities were detected. Electrochemical impedance measurements of the transferred Janus layers onto hydrophobized ITO revealed the heterogeneous electron transfer rate constant k(ET) to show a clear orientational odd-even parity effect with the odd layers showing much higher rates. Isobaric area relaxation investigations further evidenced toward a hemispherical instantaneous nucleation with edge growth mechanism of the nanoclusters formed at the tryptophan subphase. Surface pressure as a thermodynamic variable effectively controlled the interparticle separation; intercluster electron coupling exhibited insulator-metal transition in the Janus cluster monolayers through scanning electrochemical microscopy investigations.

Redox reactions of reactive oxygen species in aqueous solutions as the probe for scanning electrochemical microscopy of single live T24 cells

The redox reactions of two main components of reactive oxygen species (ROS), superoxide and hydrogen peroxide, along with oxygen in aqueous solutions were investigated using a conventional electrochemical technique, differential pulse voltammetry (DPV). Superoxide undergoes oxidation at a Pt working electrode biased at 0.055 V versus Ag/AgCl, while hydrogen peroxide can be oxidized and reduced at 0.817 and –0.745 V, respectively. Oxygen in the solutions is reduced at the electrode with an applied potential of –0.455 V. Based on these results, hydrogen peroxide and superoxide released from live cells can be successfully monitored, identified, and mapped using scanning electrochemical microscopy (SECM) at different potentials. Single human bladder (T24) cells were imaged using a 5 μm diameter SECM probe biased at –0.400, –0.600, and –0.800 V. Oxygen reduction that seems an interference can be discriminated from that of hydrogen peroxide by means of SECM.

Lithography by Scanning Electrochemical Microscopy with a Multiscaled Electrode

A multiscaled electrochemical probe is presented for Scanning Electrochemical Microscopy (SECM) experiments. It is fabricated by wet chemical etching followed by sputter-coating of an ordered optical fiber bundle. Owing to the optical fiber bundle preparation, the global electrode may present different shapes. After the chemical etching step, each one of these shapes is conserved and finally decorated with 6000 nanotips. Numerical simulations and approach curves are used to study the probe properties and the influence of the global shape and of the nanotips. The numerical simulations show that the approach curves do not depend on the shape of the electrode but rather on the total height of the protuberance of its electroactive part. Such new SECM probes are then used to pattern a Teflon surface. Indeed, by controlling the time scale of the applied potential pulses, the thickness of the reaction layer is confined at each nanotip, and the nanotip pattern is electrochemically transferred onto the non-conductive surface. Both scales (i.e., global electrode shape and nanotip array) thus show distinct and complementary features for positioning the probe and for the subsequent electrochemical patterning.