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.     

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.     

Constant-distance mode scanning electrochemical microscopy (SECM)--Part I: Adaptation of a non-optical shear-force-based positioning mode for SECM tips

A non-optical shear-force-based detection scheme for accurately controlling the tip-to-sample distance in scanning electrochemical microscopy (SECM) is presented. With this approach, the detection of the shear force is accomplished by mechanically attaching a set of two piezoelectric plates to the scanning probe. One of the plates is used to excite the SECM tip causing it to resonate, and the other acts as a piezoelectric detector of the amplitude of the tip oscillation. Increasing shear forces in close proximity to the sample surface lead to a damping of the vibration amplitude and a phase shift, effects that are registered by connecting the detecting piezoelectric plate to a dual-phase analogue lock-in amplifier. The shear force and hence distance-dependent signal of the lock-in amplifier is used to establish an efficient, computer-controlled closed feedback loop enabling SECM imaging in a constant-distance mode of operation. The details of the SECM setup with an integrated piezoelectric shear-force distance control are described, and approach curves are shown. The performance of the constant-distance mode SECM with a non-optical detection of shear forces is illustrated by imaging simultaneously the topography and conductivity of an array of Pt-band microelectrodes.     

Topographic, electrochemical, and optical images captured using standing approach mode scanning electrochemical/optical microscopy

We developed a high-resolution scanning electrochemical microscope (SECM) for the characterization of various biological materials. Electrode probes were fabricated by Ti/Pt sputtering followed by parylene C-vapor deposition polymerization on the pulled optical fiber or glass capillary. The effective electrode radius estimated from the cyclic voltammogram of ferrocyanide was found to be 35 nm. The optical aperture size was less than 170 nm, which was confirmed from the cross section of the near-field scanning optical microscope (NSOM) image of the quantum dot (QD) particles with diameters in the range of 10-15 nm. The feedback mechanism controlling the probe-sample distance was improved by vertically moving the probe by 0.1-3 microm to reduce the damage to the samples. This feedback mode, defined as "standing approach (STA) mode" (Yamada, H.; Fukumoto, H.; Yokoyama, T.; Koike, T. Anal. Chem. 2005, 77, 1785-1790), has allowed the simultaneous electrochemical and topographic imaging of the axons and cell body of a single PC12 cell under physiological conditions for the first time. STA-mode feedback imaging functions better than tip-sample regulation by the conventionally available AFM. For example, polystyrene beads (diameter approximately 6 microm) was imaged using the STA-mode SECM, whereas imaging was not possible using a conventional AFM instrument.     

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.     

Optical characteristics of atomic force microscopy tips for single-molecule fluorescence applications.

Knowledge of the optical properties of atomic force microscopy (AFM) tips is relevant for the combination of optical and force spectroscopy. The luminescence properties of five commercial AFM tips were characterized using a combination of multiparameter fluorescence detection (MFD) and scanning confocal techniques. These include three Si3N4 tips, one silicon tip, and one high-density carbon (HDC) tip grown on top of a silicon tip. Time-decay histograms of the signal were analyzed to determine the strength of scatter, constant background, and fluorescence in the observed signal. Intensity and anisotropy images with optical resolution down to the diffraction limit were generated. The optical signal recorded from the apex of the Si3N4 tips ranged from 0.7 to 1.9 times the count rates from single Rhodamine 110 molecules under similar illumination conditions. The signal is predominantly composed of scatter and background (>85%), plus a small fluorescence component with lifetimes between 1 and 3 ns. The intensity of the recorded signal fell with increasing distance from the apex, and by 300 nm the signals fell below single-molecule levels for all Si3N4 cantilevers. Silicon cantilevers demonstrated very low count rates relative to single-molecule measurements under all conditions, and virtually no fluorescence. The high-density carbon tips also demonstrated low count rates, but the signal contained a short lifetime fluorescence component (0.7 ns). The intensity of the signals from each of the tips was geometry dependent, demonstrating the highest intensities at the edges and corners. Likewise, the anisotropy of all tip signals was observed to be geometry dependent, with the dependence varying on a case-by-case basis. The implications for using confocal illumination instead of total internal reflection are discussed.     

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.     

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.     

扫描电化学显微镜在电催化表面反应分析中的现代应用

能源和环境问题成为制约未来可持续发展的关键问题之一,因此,针对不同电催化反应设计电催化剂变得越来越重要.电催化剂因其能量效率高、制备简单和易操作等优点,而应用于可再生能源的相关反应(如水分解和人工光合作用)中.明确不同反应电催化剂的设计原理,深入理解其在相关反应中的催化机理,可进一步优化催化剂性能.本文综述了扫描电化学显微镜(SECM)应用于电催化反应的历程、关键方法以及一些代表性的工作,阐明了电催化剂的工作机理以推进电催化剂的设计.本文还介绍了为提高SECM的空间分辨率而尝试的纳米尺寸电极方面的新进展,分享了纳米电极在以前研究无法涉及的单一催化实体方面的应用.     

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.     

Nanometre resolution using high-resolution scanning electron microscopy corroborated by atomic force microscopy

The resolving power of high-resolution scanning electron microscopy was judged using topographical height data from atomic force microscopy in order to assess the technique as a tool for understanding nanoporous crystal growth.     

Direct imaging of meniscus formation in atomic force microscopy using environmental scanning electron microscopy

Environmental scanning electron microscopy was used to image meniscus formation between an AFM tip and a surface. At high relative humidity, 70%-99%, the meniscus formed is 100 to 1200 nm in height, orders of magnitude larger than predicted by the Kelvin equation using spherical geometry. The height of the meniscus also demonstrates hysteresis associated with increasing or decreasing relative humidity.     

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.     

Patterning of polystyrene by scanning electrochemical microscopy. Biological applications to cell adhesion

Polystyrene surfaces may be patterned by Ag(II), NO(3)(?), and OH(?) electrogenerated at the tip of a scanning electrochemical microscope. These electrogenerated reagents lead to local surface oxidation of the polymer. The most efficient surface treatment is obtained with Ag(II). The patterns are evidenced by XPS and IR and also by the surface wettability contrast between the hydrophobic virgin surface and the hydrophilic pattern. Such Ag(II) treatment of a polystyrene Petri dish generates discriminative surfaces able to promote or disfavor the adhesion of proteins and also the adhesion and growth of adherent cells. The process is also successfully applied to a cyclo-olefin copolymer and should be suitable to pattern any hydrogenated polymer.     

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.     

Characterization and theory of electrocatalysts based on scanning electrochemical microscopy screening methods

A strategy for finding new electrocatalysts for the oxygen reduction reaction (ORR) in acidic solutions is outlined and illustrated with results for Pd-Co catalysts. This is based on establishing guidelines for selecting test systems, rapid preparation of arrays, and rapid screening by scanning electrochemical microscopy. Promising candidates are further tested as supported electrocatalysts by larger scale electrochemical methods and in fuel cells, with optimization of the composition and structure. Those that emerge are characterized by a variety of methods, including X-ray diffraction, scanning electron microscopy, and X-ray photoemission spectroscopy. Finally, density functional theory is used for detailed calculations of oxygen adsorption and dissociation on the material and provides better guidelines for further testing.     

Scanning force microscopy based rapid force curve acquisition on supported lipid bilayers: experiments and simulations using pulsed force mode.

In situ pulsed force mode scanning force microscopy (PFM-SFM) images of phase separated solid-supported lipid bilayers are discussed with the help of computer simulations. Simultaneous imaging of material properties and topography in a liquid environment by means of PFM-SFM is severely hampered by hydrodynamic damping of the cantilever. Stiffness and adhesion images of solid-supported membranes consisting of cholesterol, sphingomyelin, and 1,2-dioleyl-phosphatidylcholine obtained in aqueous solution exhibit contrast inversion of adhesion and stiff. ness images depending on parameters such as driving frequency, amplitude, and trigger setting. Simulations using a simple harmonic oscillator model explain experimental findings and give a deeper insight into the way PFM-SFM experiments have to be performed in order to obtain interpretable results and hence pave the way for reliable material contrast imaging at high speed.