Characterization of carboxylic acid-terminated self-assembled monolayers by electrochemical impedance spectroscopy and scanning electrochemical microscopy

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.     

Combinatorial electrochemical screening of fuel cell electrocatalysts

Combinatorial methods have been applied to the preparation and screening of fuel cell electrocatalysts. Hardware and software have been developed for fast sequential measurements of cyclic voltammetric and steady-state currents in 64-element half-cell arrays. The arrays were designed for the screening of high-surface-area supported electrocatalysts. Analysis software developed allowed the semiautomated processing of the large quantities of data, applying filters that defined figures of merit relevant to fuel cell catalyst activity and tolerance. Results are presented on the screening of carbon-supported platinum catalysts of varying platinum metal loading on carbon (and thus, particle size) in order to demonstrate the speed and sensitivity of the screening methodology. CO electro-oxidation, oxygen reduction, and methanol oxidation on a series of such catalysts reveal clear trends in characteristics and activities. Catalysts with smaller particle sizes reveal structure in the CO stripping voltammetry that can be associated with edge sites in addition to the closely packed planes, and this is concomitantly reduced as particle size is increased. Specific activity for steady-state methanol oxidation and oxygen reduction at room temperature in H(2)SO(4) electrolyte is found to be a maximum for the largest particle sizes, in agreement with the literature. These trends in activity are significantly smaller than the differences in activities of promoted platinum-based alloy catalysts for the same reaction.     

Fingerprint imaging by scanning electrochemical microscopy

An efficient strategy for visualizing human fingerprints on a poly(vinylidene difluoride) membrane (PVDF) by scanning electrochemical microscopy (SECM) has been developed. Compared to a classical ink fingerprint image, here the ink is replaced by an aqueous solution of bovine serum albumin (BSA). After placing the “inked” finger on a PVDF membrane, the latent image is stained by silver nitrate and the fingerprint is imaged electrochemically using potassium hexachloroiridate (III) (K₃IrCl₆) as a redox mediator. SECM images with an area of 5 mm × 3 mm have been recorded with a high-resolution using a 25-μm-diameter Pt disk-shaped microelectrode. Pores in the skin (40–120 μm in diameter) and relative locations of ridges were clearly observed. The factors relevant to the quality of fingerprint images are discussed.     

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.     

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.     

Characterization and manipulation of microscopic biochemically active regions by scanning electrochemical microscopy (SECM).

Preparation and characterization of microscopic biochemically active regions are important for the development of miniaturized bioanalytical systems with proteins, such as miniaturized enzyme electrode arrays. Scanning electrochemical microscopy (SECM) has emerged as an ideal tool for prototyping such systems. The technique is based on electrochemical conversions of dissolved species at a micrometer-sized probe electrode. It offers several mechanisms for local surface modifications under conditions compatible with conservation of protein functionality of enzymes and antibodies. The subsequent imaging of the immobilized activity provides direct information about local immobilized enzyme activities. The working modes of the techniques are illustrated by recent studies from this laboratory for the design and characterization of patterned enzyme layers covalently linked to gold surfaces via thiol self-assembly chemistry.     

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.     

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.     

Numerical analysis of Debye screening effect in electrode surface potential mapping by scanning electrochemical potential microscopy

We investigate the effects of the probe apex geometry, overlap of the electric double layers (EDLs) and Debye screening on surface potential mapping with scanning electrochemical potential microscopy (SECPM). The simulation consists of scanning a tip parallel to the electrode surface over a charged hemispherical nano-particle adsorbed on the electrode surface. As expected, a clear dependence of the apparent size of the imaged particle on the probe apex geometry has been noticed. The Debye screening has a significant effect on the probe sensitivity, while the electrolyte concentration affects the observed size of the imaged particles.     

Nanoscale surface modification via scanning electrochemical probe microscopy

Micro- and nanoscale surface modification using scanning probe microscopy techniques in combination with electrochemically induced surface structuring provides a maskless in situ fabrication strategy enabling deposition or etching of three-dimensional nanostructures. This current opinion article focuses on scanning electrochemical probe microscopy techniques highlighting recent progress in nanoscale 3D surface modification along with a spotlight on approaches of practical relevance.     

Photosystem I patterning imaged by scanning electrochemical microscopy

We report the first directed adsorption of Photosystem I (PSI) on patterned surfaces containing discrete regions of methyl- and hydroxyl-terminated self-assembled monolayers (SAMs) on gold. SAM and PSI patterns are characterized by scanning electrochemical microscopy (SECM). The insulating protein complex layer blocks the electron transfer of the SECM mediator, thereby reducing the electrochemical current significantly. Uniformly and densely packed adsorbed protein layers are observed with SECM. Pattern images correlate with our previous studies where we showed that low-energy surfaces (e.g., CH3-terminated) inhibit PSI adsorption in the presence of Triton X-100, whereas high-energy surfaces (e.g., OH-terminated) enable adsorption. Therefore, a SAM pattern with alternating methyl and hydroxyl surface regions allows PSI adsorption only on the hydroxyl surface, and this is demonstrated in the resulting SECM images.     

Microcontact printed diaphorase monolayer on glass characterized by atomic force microscopy and scanning electrochemical microscopy

Micropatterns of diaphorase (Dp) were fabricated on glass substrates by the microcontact printing (μCP) method and characterized with atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM). AFM images of the printed samples revealed that the mean height of the Dp patterns was 3–5 nm, indicating the formation of a monolayer pattern. The Dp molecules on the surface organized themselves into two-dimensional arrays. We used two kinds of inking solutions: Dp–phosphate buffer solution (PBS) (pH 7.0) and Dp–PBS (pH 7.0) with glutaraldehyde (GA, 1% v/v) as a cross-linking reagent. Although the AFM imaging showed high-quality Dp monolayer patterns in both cases, SECM measurements indicated that the enzymatic activity of Dp was almost lost when Dp–PBS with GA was used as the inking solution, whereas clear enzymatic activity was found when Dp–PBS was used.     

Electrochemical detection of receptor-mediated endocytosis by scanning electrochemical microscopy

We report a scanning electrochemical microscopy (SECM)-based receptor-mediated endocytosis detection method. Epidermal growth factor receptor (EGFR), which is one of the key membrane proteins associated with cancer, was used as a model for receptor-mediated endocytosis. EGFR molecules on the outer cell membrane were detected by SECM by using alkaline phosphatase (ALP) as a labeling enzyme. Since SECM detected the ALP activity on the outer membrane, the procedure helped discriminate the EGFR on the outer membrane from the intracellular EGFR involved in endocytosis. SECM showed a marked decrease in the current responses generated due to ALP activity by 93% on addition of the epidermal growth factor, indicating clearly that EGF triggered the endocytosis, which led to the withdrawal of most EGFRs from the outer membrane.     

扫描电化学显微镜在生物医学领域的应用研究进展

研究病变细胞和组织的异常表现可为理解重大疾病发生发展的病理机理和新型药物筛选提供重要参考.扫描电化学显微镜(Scanning electrochemical microscopy, SECM)是一种基于电化学原理的扫描探针显微镜,通过记录探针在样品表面扫描时的电流或电位等信息,对活细胞的形态和多种化学信息进行原位、实时...     

Nanostructuring and nanoanalysis by scanning electrochemical microscopy (SECM)

The generation and application of nanodes in SECM experiments are described. Nanodes are ultramicroelectrodes with an active disk diameter in the submicrometer range. We investigated the behaviour of these electrodes by testing their properties with SECM applications which were previously performed at the micrometer scale. The active diameter of the nanodes was determined using cyclic voltammetry and SECM. The nanoanalysis was conducted at two nano interdigitated arrays. The nanostructuring was demonstrated by galvanic and electroless silver deposition from solution and from the surface, respectively. Experiments with nanodes illustrate that they exhibit the same behaviour as ultramicroelectrodes, but are more sensitive to adsorption and dirt particles in the electrolyte solution.     

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.     

The cavity-microelectrode as a tip for scanning electrochemical microscopy

In this work, we present and discuss the use of cavity-microelectrodes (C-MEs) used as tip for the scanning electrochemical microscopy (SECM). Cavity-microelectrodes can be filled with a desired finely dispersed material thus compensating for the limited commercial availability of microwires. After discussing the possibility of filling and emptying a cavity-microelectrode with a desired tip shape, the consistency of negative and positive feedback approach curves obtained after filling a Au C-ME was verified. In addition, the tip/C-ME was tested under gas (oxygen) evolution condition, in order to demonstrate that the filling is stable in a wide range of gas fluxes thus extending the possible applications to tip generation/substrate collection mode. Finally, we introduce the use of the proposed system to quantify the rates of parallel reactions occurring at the material inserted in the tip under the tip generation/substrate collection mode.     

Quantitative characterization of shear force regulation for scanning electrochemical microscopy

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.     

Single-molecule conductance of redox molecules in electrochemical scanning tunneling microscopy

Experimental data and theoretical notions are presented for 6-[1'-(6-mercapto-hexyl)-[4,4']bipyridinium]-hexane-1-thiol iodide (6V6) "wired" between a gold electrode surface and tip in an in situ scanning tunneling microscopy configuration. The viologen group can be used to "gate" charge transport across the molecular bridge through control of the electrochemical potential and consequently the redox state of the viologen moiety. This gating is theoretically considered within the framework of superexchange and coherent two-step notions for charge transport. It is shown here that the absence of a maximum in the Itunneling versus electrode potential relationship can be fitted by a "soft" gating concept. This arises from large configurational fluctuations of the molecular bridge linked to the gold contacts by flexible chains. This view is incorporated in a formalism that is well-suited for data analysis and reproduces in all important respects the 6V6 data for physically sound values of the appropriate parameters. This study demonstrates that fluctuations of isolated configurationally "soft" molecules can dominate charge transport patterns and that theoretical frameworks for compact monolayers may not be directly applied under such circumstances.