Polymer films studied by scanning tunneling microscopy

Polymer films with thicknesses up to 300 nm were investigated by scanning tunneling microscopy. It was demonstrated that the films contain areas whose images change depending on the scan parameters, which can be explained by the emission processes.     

Screening of photocatalysts by scanning electrochemical microscopy

A method for rapid screening of photocatalysts employing a form of scanning electrochemical microscopy (SECM) is described. A piezoelectric dispenser was used to deposit arrays composed of approximately 300-microm-size photocatalyst spots with different compositions onto conducting glass, fluorine-doped tin oxide substrate. The scanning tip of the SECM was replaced by a fiber optic connected to a xenon lamp and was rapidly scanned over the array. In this arrangement, the photocatalytic performance of the spots was evaluated by measuring the photocurrent at the substrate of the array. A fiber optic with a ring electrode can also be used to electrochemically detect products of the photoreaction. Several iron oxide-based bimetallic oxide combinations were found to exhibit enhanced photocatalytic activity, when compared to pure alpha-Fe2O3. These combinations included iron-palladium, iron-europium, and iron-rubidium in specific ratios. A trimetallic bismuth-vanadium-zinc oxide combination was also found to show a higher photocurrent, by approximately 40%, compared to BiVO3.     

Microimaging of algal bioconvection by scanning electrochemical microscopy

Local bioconvection generated by algal flagellar movement was imaged by scanning electrochemical microscopy. As a microelectrode probe vertically approached an individual multicellular flagellate alga, Volvox carteri, an oxidation current of a coexisting redox marker ([Fe(CN)6]4-) increased gradually, due to bioconvective enhancement of mass transport, and eventually decreased because the algal body blocked the diffusion of the marker. Two-dimensional imaging of the bioconvection of an individual alga was also possible. The bioconvective enhancement of the current was hindered by a toxic compound that inhibits the flagellar movement.     

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

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

Distinction of the two phases of CuTCNQ by scanning electrochemical microscopy

The two known phases of CuTCNQ and TCNQ (TCNQ = 7,7',8,8'-tetracyanoquinodimethane) have been probed by scanning electrochemical microscopy (SECM) in the feedback mode. The first use of this technique for distinguishing differences in the electronic properties of semiconductor phases exploits the large differences in conductivity that exist between CuTCNQ and the parent TCNQ material and also between the CuTCNQ phases I and II. However, the packing density of the individual CuTCNQ crystals in a film structure also is shown to influence the SECM feedback response. Finally, it is shown that films of pure phase II material or mixtures of the phases can be mapped using feedback mode SECM. The SECM method provides valuable insights for elucidating properties of semiconducting solids that are mounted on insulating substrates.     

GaAs/AlGaAs nanowire heterostructures studied by scanning tunneling microscopy

We directly image the interior of GaAs/AlGaAs axial and radial nanowire heterostructures with atomic-scale resolution using scanning tunneling microscopy. We show that formation of monolayer sharp and smooth axial interfaces are possible even by vapor-phase epitaxy. However, we also find that instability of the ternary alloys formed in the Au seed fundamentally limits axial heterostructure control, inducing large segment asymmetries. We study radial core-shell nanowires, imaging even ultrathin submonolayer shells. We demonstrate how large twinning-induced morphological defects at the wire surfaces can be removed, ensuring the formation of wires with atomically flat sides.     

Slow-crack propagations through bimaterial interfaces studied by scanning electron microscopy

The scanning electron microscope (SEM) is used for the study of slow crack propagation through a bimaterial interface. This work is concerned with the variation of crack velocity, the variation of crack tip opening angle (CTOA) and the stress intensity factor (K) at the crack tip, and the investigation of crack arrest phenomena at the bimaterial interface. It was observed that the crack accelerates to a maximum velocity as the crack tip approaches the interface and then decreases rapidly to a minimum value at the interface. The interface acts as a decelerator to crack propagation. The position and the value of the maximum velocity depends on the mechanical properties of two phases and specimen configuration. The crack propagates at a constant CTOA until it arrests at the interface. During the crack-arrest time the CTOA increases rapidly to a limiting value. Then the crack passes across the interface and propagates in the next phase with almost the same CTOA as the initial crack in phase I. The stress intensity factor,K, increases to a maximum value near the bimaterial interface.     

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.     

Oxygen consumption of single bovine embryos probed by scanning electrochemical microscopy

Oxygen consumption of individual bovine embryos was noninvasively quantified by scanning electrochemical microscopy (SECM). A probe microelectrode was used to scan near a single embryo surface in a culture medium to monitor the oxygen reduction current at 37 degrees C, under a water-saturated atmosphere of 5% CO2 and 95% air. The oxygen concentration profiles near the embryos were in good agreement with the theoretical spherical diffusion. When an embryo reached the stage of a morula with a 74-microm radius on day 6 after in vitro fertilization, the oxygen concentration difference (deltaC) between the bulk solution and the morula surface was 6.90 +/- 1.35 microM. The oxygen consumption rate (F) of the single morula was estimated to be (1.40 +/- 0.27) x 10(-14) mol s(-1). After the SECM measurement, the embryo was continuously cultured for another 2 days and grew to the stage of a blastocyst with a 100-microm radius. For the blastocyst, the deltaC values for the inner cell mass side and the trophoblast side were 16.40 +/- 1.83 and 9.14 +/- 1.68 microM, respectively. The oxygen consumption rate of the blastocyst was found to be in the range of (2.50 +/- 0.46) x 10(-14) mol s(-1) < F < (4.49 +/- 0.50) x 10(-14) mol s(-1). We have carried out SECM measurements for 19 embryos, and the results were compared in detail with these from an optical microscopic observation. The deltaC values for the morulae on day 6 after in vitro fertilization were strongly related to the morphological embryo quality. The morulae showing a larger deltaC value developed into blastocysts of a larger size, and the deltaC value after the subsequent 2 days of cultivation was found to be increased.     

Generalized theory for nanoscale voltammetric measurements of heterogeneous electron-transfer kinetics at macroscopic substrates by scanning electrochemical microscopy

Here we report on a generalized theory for scanning electrochemical microscopy to enable the voltammetric investigation of a heterogeneous electron-transfer (ET) reaction with arbitrary reversibility and mechanism at the macroscopic substrate. In this theory, we consider comprehensive nanoscale experimental conditions where a tip is positioned at a nanometer distance from a substrate to detect the reactant or product of a substrate reaction at any potential in the feedback or substrate generation/tip collection mode, respectively. Finite element simulation with the Marcus-Hush-Chidsey formalism predicts that a substrate reaction under the nanoscale mass transport conditions can deviate from classical Butler-Volmer behavior to enable the precise determination of the standard ET rate constant and reorganization energy for a redox couple from the resulting tip current-substrate potential voltammogram as obtained at quasi-steady state. Simulated voltammograms are generalized in the form of analytical equations to allow for reliable kinetic analysis without the prior knowledge of the rate law. Our theory also predicts that a limiting tip current can be controlled kinetically to be smaller than the diffusion-limited current when a relatively inert electrode material is investigated under the nanoscale voltammetric conditions.     

Real-time pH microscopy down to the molecular level by combined scanning electrochemical microscopy/single-molecule fluorescence spectroscopy

A new technique combining scanning electrochemical microscopy (SECM) and single-molecule fluorescence spectroscopy was developed to accomplish locally and temporally defined pH adjustments in buffer solutions and on surfaces monitored by fluorescence alteration of pH-sensitive fluorophores in real time. Local pH gradients were created by electrochemical generation of H(+) or OH(-) during redox reactions at ultramicro- or nanoelectrodes with radii from 5 microm to 35 nm. Ratiometric fluorescence measurements were performed with a confocal laser microscope using two detectors for different spectral regions. Time-resolved pH measurements were carried out with freely diffusing SNARF-1-dextran. For pH measurements on surfaces, total internal reflection fluorescence microscopy was used in combination with a CCD camera. The fluorophore SNAFL-succinimidyl ester was bound to amino-terminated octadecylsilane-coated coverslips. Local pH determinations could be accomplished with an accuracy of 0.2 unit. The measured pH profiles showed a strong dependence on the tip diameter, the buffer/mediator concentration ratio, and the tip-surface distance. As an application for bionanotechnology using SECM-induced pH changes on the molecular level, the proton-driven ATP synthesis by single membrane-bound F(0)F(1)-ATP synthases was investigated. ATP synthesis resulted in stepwise subunit rotation within the enzyme that was monitored by single-molecule fluorescence resonance energy transfer.     

Detection of hydrogen peroxide produced during electrochemical oxygen reduction using scanning electrochemical microscopy

The substrate-generation/tip-collection mode of scanning electrochemical microscopy was used to detect hydrogen peroxide formed as an intermediate during oxygen reduction at various electrodes. The experiment is conceptually similar to rotating ring-disk experiments but does not require the production of a ring-disk assembly for the specific electrode material in question. In order to limit the extension of the diffusion layer above the sample, the sample electrode potential is pulsed while the Pt ultramicroelectrode probe (UME) is held at a constant potential for oxidative amperometric detection of hydrogen peroxide. The signal at UME is influenced by the sample region within the diffusion length of hydrogen peroxide during the pulse of 2.5 s. The method is tested with three model electrodes showing different behavior with respect to the oxygen reduction reaction (ORR) in acidic solution. Simple analytical models were used to extract effective rate constants for the most important reaction paths of ORR at gold and palladium-cobalt samples from the chronoamperometric response of the UME to a reduction pulse at the sample electrode.     

Detection of microspotted carcinoembryonic antigen on a glass substrate by scanning electrochemical microscopy

Microspots of carbinoembryonic antigen (CEA) on glass substrates were characterized by scanning electrochemical microscopy (SECM). CEA was immobilized via a sandiwch method using horseradish peroxidase (HRP)-labeled anti-CEA. The reduction current of the oxidized form of ferrocenylmethanol generated by the HRP reaction was monitored to view SECM images. This method detects as low as ～10(4) CEA molecules in a single 20-μm-radius spot.     

Study of electron transfer across the liquid/ice-like matrix interface by scanning electrochemical microscopy

In this work, we report the findings of a study on scanning electrochemical microscopy (SECM) to investigate the interfacial electron-transfer (ET) reaction between the 7,7,8,8-tetracyanoquinodimethane radical anion (TCNQ*-) in 1,2-dichloroethane and ferricyanide in an ice-like matrix (a mixture of insulting ice and conductive liquid) under low temperatures. Experimental results indicate that the formed liquid/ice-like matrix interface is superficially similar in electrochemical characteristics to a liquid/liquid interface at temperatures above -20 degrees C. Furthermore, imaging data show that the surface of the ice-like matrix is microscopically flat and physically stable and can be applied as either a conductive or an insulting substrate for SECM studies. Perchlorate ion was selected as the common ion in both phases, the concentrations of which controlled the interfacial potential difference. The effect of perchlorate concentration in the DCE phase on interfacial reactions has been studied in detail. The apparent heterogeneous rate constants for TCNQ*- oxidation by Fe(CN)6(3-) in another phase under different temperatures have been calculated by a best-fit analysis, where the experimental approach curves are compared with the theoretically derived relationships. Reaction rate data obey Butler-Volmer formulation before and after the freezing point, which is similar to most other known cases of ET reactions at liquid/liquid interfaces. However, there is a sharp change observed for heterogeneous rate constants around the freezing point of the aqueous phase, which reflects the phase transition. At temperatures below -20 degrees C, surface-confined voltammograms for the reduction of ferricyanide were obtained, and the ice-like matrix became an insulating one, which indicates that the aqueous phase is really a frozen phase.     

Assemblies of fluorine containing bent-shaped liquid crystal molecules studied by using scanning tunneling microscopy

In this work we prepared a fluorine containing bent-shaped liquid crystal from biphenyl as the central core and rod-like azobenzene mesogens as the side arms, namely 4',3-biphenyl bis[4-(4'-hexadecanloxy-3-fluorophenylazo)benzoate] (L104). The self-assembly behaviors of L104 molecules on graphite surface were investigated by using scanning tunneling microscopy (STM) under ambient conditions. The high-resolution STM images of L104 assemblies revealed three kinds of structures showing the joint effects of dipole-dipole interactions originated from the fluorine and the bent-core alignments for maximizing pi-pi interactions. These observations may be beneficial for understanding the assembly mechanism and designs for novel banana-shaped liquid crystal molecules.     

Screening of oxygen evolution electrocatalysts by scanning electrochemical microscopy using a shielded tip approach

Oxygen evolution electrocatalysts in acidic media were studied by scanning electrochemical microscopy (SECM) in the substrate generation-tip collection (SG-TC) imaging mode with a 100 microm diam tip. Pure IrO2 and Sn(1-x)Ir(x)O2 combinatorial mixtures were prepared by a sol-gel route to form arrays of electrocatalyst spots. The experimental setup has been developed to optimize screening of electrocatalyst libraries under conditions where the entire array is capable of the oxygen evolution reaction (OER). The activity of individual spots was determined by reducing the interference from the reaction products of neighboring spots diffusing to the tip over the spot of interest. A gold layer deposited on the external wall of the SECM tip was used as a tip shield. In this study the shield was kept at a constant potential to reduce oxygen under mass transfer controlled conditions. The tip shield consumes oxygen coming from the neighbor spots in the array and enables the tip to correctly detect the activity of the spot below the tip. Simulations and experimental results are shown, demonstrating the effectiveness of the tip shield with the SG-TC setup in determining the properties of the composite materials and imaging arrays.     

Spatially addressed deposition and imaging of biochemically active bead microstructures by scanning electrochemical microscopy

A new procedure is described to deposit paramagnetic beads on surfaces to form microscopic agglomerates. By using surface-modified beads, microscopic structures with defined biochemical activity are formed. The shape and size of agglomerates were characterized by scanning electron microscopy (SEM), and the biochemical activity was mapped with scanning electrochemical microscopy (SECM). This approach is demonstrated using beads modified with anti-mouse antibodies (Ab). After allowing them to react with a conjugate of mouse IgG and alkaline phosphatase (ALP), the beads were deposited as agglomerates of well-defined size and shape. The biochemical activity was recorded in the generation-collection SECM mode by oxidizing 4-aminophenol formed in the ALP-catalyzed hydrolysis of 4-aminophenyl phosphate at the surface of the beads. The signal height correlated with both the amount of beads present in one agglomerate and the proportion of Ab binding sites saturated with the ALP mouse IgG conjugate. The feedback mode of the SECM was used to image streptavidin-coated beads after reaction with biotinylated glucose oxidase.     

In situ spatial and time-resolved studies of electrochemical reactions by scanning transmission X-ray microscopy

The first in situ measurements with scanning transmission X-ray microscopy (STXM) of an active electrochemical cell are reported. An electrochemical wet cell, consisting of an electrodeposited polyaniline thin film on a thin Au film covered by an overlayer of 1 M HCl solution sitting between two X-ray transparent silicon nitride windows, was assembled. X-ray absorption images and spatial and time-resolved spectra of this system under potential control were examined using the beamline 5.3.2 STXM at the Advanced Light Source. The chemical state of the polyaniline film was reversibly converted between reduced (leucoemeraldine) and oxidized (emeraldine chloride) states by changing the applied potential. The electrochemical changes were monitored by spatially resolved C 1s and N 1s X-ray absorption spectroscopy and chemical-state selective imaging. Comparison of differences between images at two energies at different potentials provided electrochemical contrast with a resolution better than 50 nm, thereby monitoring that component of the polyaniline film that was electrochemically active. Kinematic studies in the subsecond regime are demonstrated.     

Examining ultramicroelectrodes for scanning electrochemical microscopy by white light vertical scanning interferometry and filling recessed tips by electrodeposition of gold

In this paper, we present a technique to rapidly and directly examine ultramicroelectrodes (UMEs) by white light vertical scanning interferometry (VSI). This technique is especially useful in obtaining topographic information with nanometer resolution without destruction or modification of the UME and in recognizing tips where the metal is recessed below the insulating sheath. Two gold UMEs, one with a metal radius a = 25 μm and relative insulating sheath radius RG = 2 and the other with a = 5 μm and RG = ～1.5, were examined, and the average depth of the gold recessions was determined to be 1.15 μm and 910 nm, respectively. Electrodeposition of gold was performed to fill the recessed hole, and the depth was reduced to ～200 nm. With the electrodeposited gold electrode and a conventional microelectrode (a = 25 μm) as a tip and substrate, respectively, a tip/substrate distance, d, of 600 nm was achieved allowing scanning electrochemical microscopy (SECM) in positive feedback mode at a close distance, which is useful for measuring fast kinetics.