Tip current/positioning close-loop mode of scanning electrochemical microscopy for electrochemical micromachining

Scanning electrochemical microscopy (SECM) has been approved as a prospective electrochemical micromachining (ECMM) technique soon after its birth. However, it still remains challenge for SECM to fabricate arbitrary three-dimensional (3D) microstructures because of the limitation of positioning system. To solve this problem, we proposed a tip current signal/positioning close-loop mode in which the tip current signal is fed back to the positioning system in order to program the motion trial of SECM tip. Both the triedge-cone and sinusoidal microstructures were obtained by the close-loop positioning mode. The static-state etching process was demonstrated not to be disturbed by the slow motion rate of SECM tip. The unique positioning mode would be significant for both ECMM and electrochemical imaging.     

Simultaneous detection of uric acid and glucose on a dual-enzyme chip using scanning electrochemical microscopy/scanning chemiluminescence microscopy

Scanning electrochemical microscopy (SECM) and scanning chemiluminescence microscopy (SCLM) were used for imaging an enzyme chip with spatially-addressed spots for glucose oxidase (GOD) and uricase microspots. For the SECM imaging, hydrogen peroxide generated from the GOD and/or uricase spots was directly oxidized at the tip microelectrode in a solution containing glucose and/or uric acid (electrochemical (EC) detection). For the SCLM imaging, a tapered glass capillary (i.d. of 1∼2 μm) filled with luminol and horseradish peroxidase (HRP) was used as the scanning probe for generating the chemiluminescence (CL). The inner solution was injected from the capillary tip at 78 pl s⁻¹ while scanning above the enzyme-immobilized chip. The CL generated when the capillary tip was scanned above the enzyme spots was detected using a photon-counter (CL detection). Two-dimensional mapping of the oxidation current and photon-counting intensity against the tip position affords images of which their contrast reflects the activity of the immobilized GOD and uricase. For both the EC and CL detections, the signal responses were plotted as a function of the glucose and uric acid concentrations in solution. The sensitivities for the EC and CL detection were found to be comparable.     

Correlating surface microstructures with reactivity on commercially pure zirconium using scanning electrochemical microscopy and scanning electron microscopy

Scanning electrochemical microscopy and scanning electron microscopy were employed to correlate the surface microstructures with surface reactivity of commercially pure zirconium. It was found that heightened reactivity was associated with iron impurities lying beneath the oxide surface. This could result in failure of nuclear reactor components fabricated using zirconium alloys due to hydrogen ingress and corrosion. COMSOL multiphysics software was used to quantify the electrochemical kinetic constants associated with the differences in surface reactivity.     

Facilitated tip-positioning and applications of non-electrode tips in scanning electrochemical microscopy using a shear force based constant-distance mode

In scanning electrochemical microscopy (SECM) a microelectrode is usually scanned over a sample without following topographic changes (constant-height mode). Therefore, deconvolution of effects from distance variations arising from non-flat sample surface and electrochemical surface properties is in general not possible. Using a shear force-based constant distance mode, information about the morphology of a sample and its localized electrochemical activity can be obtained simultaneously. The setup of the SECM with integrated constant-distance mode and its application to non-flat or tilted surfaces, as well as samples with three-dimensional surface structures are presented and discussed. The facilitated use of non-amperometric tips in SECM like enzyme-filled glass capillaries is demonstrated.     

Monitoring of glucose and glutamate using enzyme microstructures and scanning electrochemical microscopy

Glucose oxidase and glutamate oxidase lines, with typical width of 100 µm, were patterned on gold surfaces using a micro-dispensing system, by shooting 100 pl droplets of the corresponding enzyme solutions. The probe of a scanning electrochemical microscope (SECM) was then carefully positioned in the close proximity of the enzyme microstructure and poised to + 600 mV vs. Ag/AgCl, KCl 0.1 M. The H₂O₂, generated by the enzyme lines at different concentrations of glucose and glutamate in the surrounding solution, was sequentially monitored. Reproducible calibration curves for glucose and glutamate were obtained in one single experiment, proving that the combination of enzyme microstructures with SECM can provide a new way of achieving multianalyte detection.     

Local surface patterning by chitosan-stabilized gold nanoparticles using the direct mode of scanning electrochemical microscopy (SECM)

An approach for patterning surfaces with prepared nanoparticles is described. Chitosan-stabilized gold nanoparticles (Au/chitosan NPs) were locally deposited on stainless steel (StSt), indium tin oxide (ITO), and highly-ordered pyrolytic graphite (HOPG). Deposition was driven by local pH gradient formed between a surface and a scanning electrochemical microscopy tip set in the direct mode. The pH at the substrate was increased upon biasing the surface by negative potentials, which caused the reduction of water. As the pH on the surface exceeded that of $ {\mathrm{pK}}_{{\mathrm{chitosanH}}^{+}}\sim 6.3 $ deprotonation of the amino groups of chitosan caused the irreversible deposition of the chitosan/AuNPs. The effect of different parameters, such as tip–surface distance and time, on deposition was studied. While the potential duration showed no clear influence, smaller tip–substrate distance and more negative potentials applied to the surface caused larger deposits. The overpotential needed for the deposition of nanoparticles on HOPG was the highest while that for StSt was the lowest. On the former, the sluggish kinetics caused the deposition of ring-shaped structures while disk-shaped deposits were formed on the other surfaces.     

Generation and detection of single metal nanoparticles using scanning electrochemical microscopy techniques

Different pathways towards the generation and detection of a single metal nanoparticle (MNP) on a conductive carbon support for testing as an electrocatalyst are described. Various approaches were investigated including interparticle distance enhancement, electrochemical and mechanical tip-substrate MNP transfer onto macroscopic surfaces, scanning electrochemical microscopy (SECM)-controlled electrodeposition, and the use of selective binding monolayers on carbon fiber electrodes (CFEs) for solution-phase-selective adsorption. A novel SECM technique for electrodepositing MNPs on CFE tips immersed 100-200 nm below the electrolyte level was developed and used to generate single Pt and Ni nanoparticles. Following their generation, we demonstrate electrocatalytic detection of Fe3+ on individual Pt particles with the CFE in a Fe3+/H2SO4 solution. We also describe an approach of attaching MNPs to CFEs by controlling the composition of monolayers bonded to the CFE. By employing a monolayer with a low ratio of binding (e.g., 4-aminopyridine) to nonbinding molecules (e.g., aniline) and controlling the position of the CFE in a colloidal Pt solution with a SECM, we attached a single 15 nm radius Pt nanoparticle to the CFE. Such chemisorbed Pt particles exhibited a stronger adhesion on surface-modified CFEs and better mechanical stability during proton reduction than MNPs electrodeposited directly on the CFE.     

Novel strategy for constant-distance imaging using alternating current scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) still lacks reliable means for performing constant-distance imaging experiments. We demonstrate, for the first time, that the same negative alternating current (AC) feedback can be observed on approach to an insulator and an unbiased conductor at optimal experimental conditions. This leads to a novel constant-distance imaging mode. To perform AC-SECM imaging, only minor modifications of the existing SECM set-up were necessary. The new constant-distance AC-SECM imaging was conducted to provide topographical information not affected by variations in sample conductivity and reactivity. Furthermore, simultaneous AC and DC SECM measurements were carried out to demonstrate that both topographical and chemical information could be revealed.     

Detection of hydrogen peroxide produced at a liquid/liquid interface using scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) was used to monitor in situ hydrogen peroxide (H₂O₂) produced at a polarized water/1,2-dichloroethane (DCE) interface. The water/DCE interface was formed between a DCE droplet containing decamethylferrocene (DMFc) supported on a solid electrode and an acidic aqueous solution. H₂O₂ was generated by reducing oxygen with DMFc at the water/DCE interface, and was detected with a SECM tip positioned in the vicinity of the interface using a substrate generation/tip collection mode. This work shows unambiguously how the H₂O₂ generation depends on the polarization of the liquid/liquid interface, and how proton-coupled electron transfer reactions can be controlled at liquid/liquid interfaces.     

Sub-micrometer polarimetry of chiral surfaces using near-field scanning optical microscopy

In this work we have studied the optical activity of chiral crystal surfaces with polarized near-field scanning optical microscopy (NSOM); our studies clearly demonstrated that polarized NSOM can be utilized to determine chirality at crystal surfaces.     

Recent advances of scanning electrochemical microscopy and scanning ion conductance microscopy for single-cell analysis

Single-cell analysis is important for understanding fundamental biological processes and mechanisms. Scanning electrochemical microscopy and scanning ion conductance microscopy as two kinds of scanning probe microscopy, with high temporal and spatial resolutions as well as in situ and noninvasive characterization capabilities, emerge as strong tools for single-cell analysis. In this review, we introduce the latest advances of scanning electrochemical microscopy and scanning ion conductance microscopy for single-cell analysis, including characterizations of cell morphology dynamics, membrane properties and mechanics, and monitoring cell surface charge, extracellular pH, and intracellular substances.     

Investigation of stress-enhanced surface reactivity on Alloy 800 using scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) was used to simultaneously investigate the impact of tensile and compression stress on reactivity of the same Alloy 800 sample in ferrocenemethanol solution containing thiosulfate. Strong surface reactivity was observed on the stressed regions. Both tensile (the exterior edge) and compressed (inner edge) regions showed higher reactivity than the region between these. Hence both tensile stress and compression stress increased the localized surface reactivity, indicating potential for acceleration of corrosion susceptibility of Alloy 800 under stress in thiosulfate-contained chemistries.     

Enhanced feature analysis using wavelets for scanning probe microscopy images of surfaces

In this work we develop wavelet theory for the analysis of surface topography images obtained by scanning probe microscopy (SPM) such as atomic force microscopy (AFM). Wavelet transformation is localized in space and frequency, which can offer an advantage for analyzing information on surface morphology and topography. Wavelet transformation is an ideal tool to detect trends, discontinuities, and short periodicities on a surface. Additionally, wavelets can be used to remove artifacts and noise from scanning microscopy images. In terms of 3-D image analysis, discrete wavelet transform can capture patterns at all relevant frequency scales, thus providing a level of image analysis that is not possible otherwise. It is also possible to use the methodology for analyzing surface structures at the molecular level. The results demonstrate superior capabilities of wavelet approach to scanning probe microscopy image analysis compared to traditional analysis techniques.     

Application of scanning electrochemical microscopy and scanning electron microscopy for the characterization of carbon-spray modified electrodes

Different gold surfaces modified by carbon-spray have been investigated by scanning electron microscopy (SEM) and scanning electrochemical microscopy (SECM). A transformation of the SECM image to a distance-location profile is proposed which assists the correlation of both images. The structures found in the transformed SECM images of carbon-spray layers on gold substrates can be explained by the topographic features visible in the SEM pictures. Tempering the carbon spray results in an increased density of electrochemically reactive carbon particles which could be confirmed by cyclic voltammetric investigations. Gold minigrids modified with carbon spray expose some areas of especially large currents which could not be predicted from their SEM images. This effect may result from particles located at the edge of a wire intersection having relatively large active surfaces per particle. They contribute significantly to the total current of the minigrid.     

The effect of electrochemical surface treatments of carbon fibers using scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been applied to the study of the topography of polyacrylonitrile (PAN)-based carbon fibers before and after electrochemical surface treatments. The results show that the electrical anodic oxidation only changes the surface aspects; at nanometric resolution; the nanostructure and nanotexture such as the step-like crystallite stacking are decreased with increasing electric current densities.     

Fabrication of Prussian Blue modified ultramicroelectrode for GOD imaging using scanning electrochemical microscopy

A Prussian Blue (PB) film modified disk ultramicroelectrode (UME) was fabricated by electrochemical deposition technique on a Pt-disk UME. The electrocatalytical reductions of hydrogen peroxide derived from glucose oxidase (GOD) on this modified UME were investigated. The enzymatic biochemical reactivity was imaged by scanning electrochemical microscopy (SECM) utilizing the PB film modified UME. It is evident that sensitivity and spatial resolution for hydrogen peroxide measurement were improved obviously. SECM images obtained clearly revealed the concentration profile of the reaction products around the enzymes. The PB film modified microelectrode is in the nature of simple preparation, high catalytic activity on hydrogen peroxide and substrate selectivity for SECM etc.     

Interrogation of living cells using alternating current scanning electrochemical microscopy (AC-SECM)

In this paper we present the application of alternating current scanning electrochemical microscopy (AC-SECM) to the study of living cells. Commercial AFM instrumentation was modified to allow for performing robust AC-SECM measurements. Constant height AC imaging of the Cos-7 cells, performed directly in cell culture medium without the addition of a redox mediator, provided topographical information of the cell. Stationary tip measurements on the AC current were carried out to investigate the cellular activity of a single cell. The dependence of AC current magnitude on tip-to-sample separation distance was used to monitor real time changes in cell height of individual Cos-7 cells. Furthermore, AC-SECM was employed to observe changes in metabolic cellular activity stimulated by ethanol and phorbol-1,2-myristate-acetate-3. The effect of changing cellular activity on constant height AC-SECM imaging was also studied.