Sequential electrochemical oxidation and site-selective growth of nanoparticles onto AFM probes

In this work, we reported an approach for the site-selective growth of nanoparticle onto the tip apex of an atomic force microscopy (AFM) probe. The silicon AFM probe was first coated with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) through a chemical vapor deposition (CVD) method. Subsequently, COOH groups were selectively generated at the tip apex of silicon AFM probes by applying an appropriate bias voltage between the tip and a flat gold electrode. The transformation of methyl to carboxylic groups at the tip apex of the AFM probe was investigated through measuring the capillary force before and after electrochemical oxidation. To prepare the nanoparticle terminated AFM probe, the oxidized AFM probe was then immersed in an aqueous solution containing positive metal ions, for example, Ag+, to bind positive metal ions to the oxidized area (COOH terminated area), followed by chemical reduction with aqueous NaBH 4 and further development (if desired) to give a metal nanoparticle-modified AFM probe. The formation of a metal nanoparticle at the tip apex of the AFM probe was confirmed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA).     

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.     

Catalytic turnover of [FeFe]-hydrogenase based on single-molecule imaging

Hydrogenases catalyze the interconversion of protons and hydrogen according to the reversible reaction: 2H(+) + 2e(-) ? H(2) while using only the earth-abundant metals nickel and/or iron for catalysis. Due to their high activity for proton reduction and the technological significance of the H(+)/H(2) half reaction, it is important to characterize the catalytic activity of [FeFe]-hydrogenases using both biochemical and electrochemical techniques. Following a detailed electrochemical and photoelectrochemical study of an [FeFe]-hydrogenase from Clostridium acetobutylicum (CaHydA), we now report electrochemical and single-molecule imaging studies carried out on a catalytically active hydrogenase preparation. The enzyme CaHydA, a homologue (70% identity) of the [FeFe]-hydrogenase from Clostridium pasteurianum , CpI, was adsorbed to a negatively charged, self-assembled monolayer (SAM) for investigation by electrochemical scanning tunneling microscopy (EC-STM) techniques and macroscopic electrochemical measurements. The EC-STM imaging revealed uniform surface coverage with sufficient stability to undergo repeated scanning with a STM tip as well as other electrochemical investigations. Cyclic voltammetry yielded a characteristic cathodic hydrogen production signal when the potential was scanned sufficiently negative. The direct observation of the single enzyme distribution on the Au-SAM surface coupled with macroscopic electrochemical measurements obtained from the same electrode allowed the evaluation of a turnover frequency (TOF) as a function of potential for single [FeFe]-hydrogenase molecules.     

化学力显微镜针尖修饰技术研究新进展

评述了化学力显微镜的新成果。对自组装单分子膜修饰扫描探针显微镜针尖,生物分子修饰原子力显微镜针尖,电化学方法修饰扫描隧道显微镜针尖,纳米碳管材料修饰原子力显微镜针尖等作了介绍。     

扫描电化学显微镜原位表征pH值对核电蒸汽发生器合金腐蚀行为的影响

用扫描电化学显微镜(SECM)和电化学阻抗谱(EIS)原位表征了溶液pH值对核电蒸汽发生器800 合金溶液中的腐蚀行为的影响. 实验结果表明：在酸性氯化钠溶液中,SECM探针渐进曲线为正反馈,表明800合金为活化阳极溶解,腐蚀电位下的EIS 图呈现完整的单容抗弧特征;而在中性或者碱性溶液中,SECM探针渐进曲线为负反馈,表明800 合金为自钝化,不同阳极电位下的EIS 图均呈现不完整的容抗弧特征,但随着阳极极化电位的增加,EIS 谱容抗弧半径减小,表明钝化膜的耐蚀性下降;SECM二维扫描图像结果显示探针电流增加,表明电极表面活性增加,即钝化膜的溶解速度增加. 而在中性或者碱性溶液中的SECM二维图像中均可观察到若干活性点,这可能与晶界或者金属夹杂物等有关.     

Sub-micrometre imaging of metal surface corrosion by scanning Kelvin nanoprobe

The detection of the initial changes in the surface microstructure and local chemical properties connected to the corrosion process is possible using a high-resolution scanning Kelvin nanoprobe. This technique provides the simultaneous imaging of the topographical features and potential distribution across a surface at the sub-micrometer level. Nanoprobe measurements performed on samples of Al, Cu, Fe, Ni, Ag and Pt before and after exposure to different corroding solutions reveal the significant changes that take place during the first stages of the corrosion process. The similarities of the images obtained subsequent to corrosion are reflective of surface fractal behavior.     

Fabrication and investigation of nanostructures on transition metal dichalcogenide surfaces using a scanning tunneling microscope

Nanometer-scale holes have been fabricated on the surfaces of the semiconducting transition metal dichalcogenides (TMDCs) molybdenum ditelluride (MoTe2) and molybdenum disulfide (MoS2) by applying voltage pulses from the tip of a scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV). It was found that the tip geometry (tip shape and sharpness) influences the formation and structure of the atomic-scale nanostructures. Threshold voltage ranges for the surface modification of MoTe2 (3.0 +/- 0.3 V) and MoS2 (3.4 +/- 0.3 V) were determined. Negative sample voltage pulses applied to a p-type MoTe2 surface produced much larger and deeper nanometer-scale holes when compared with those produced by positive voltage pulses. The existence of threshold voltages and the pulse polarity dependence of nanostructure fabrication suggests that an electric field evaporation mechanism is applicable. Support for this mechanism was obtained by nanostructuring metallic TMDC NbSe2, where both the produced features and the threshold voltages (3.0 +/- 0.3 V) were similar for both positive and negative voltage pulses.     

Nanofabrication of the gold scanning probe for the STM-SECM coupling system with nanoscale spatial resolution

Scanning probe is the key issue for the electrochemical scanning probe techniques (EC-SPM) such as EC-scanning tunnel microscopy (STM), EC-atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM), especially the insulative encapsulation of the nanoelectrode probe for both positioning and electrochemical feedbacks. To solve this problem, we develop a novel fabrication method of the gold nanoelectrodes: firstly, a micropipette with nanomter-sized orifice was prepared as the template by a laser puller; secondly, the inside wall of micropipette apex was blocked by compact and conic Au nano-piece through electroless plating; thirdly, the Au nano-piece was grown by bipolar electroplating and connected with a silver wire as a current collector. The fabricated Au nanoelectrode has very good voltammetric responses for the electrodic processes of both mass transfer and adsorption. The advantage lies in that it is well encapsulated by a thin glass sealing layer with a RG value lowered to 1.3, which makes it qualified in the SECM-STM coupling mode. On one hand, it can serve as STM tip for positioning which ensures the high spatial resolution; on the other hand, it is a high-quality nanoelectrode to explore the local chemical activity of the substrate. The nanofabrication method may promote the SPM techniques to obtain simultaneously the physical and chemical images with nanoscale spatial resolution, which opens a new approach to tip chemistry in electrochemical nanocatalysis and tip-enhanced spectroscopy.     

STM针尖诱导铜表面纳米刻蚀与沉积

扫描隧道显微技术 ( STM)不但可在小至原子分辩的尺度上现场研究电极表面及其结构变化 ,还能对电极表面进行纳米尺度上的加工、修饰 [1～ 3] .由于 STM探针与被研究样品的表面仅相距～ 1 nm,探针附近区域电极 /溶液界面的结构和性质将不可避免地受到影响 .尽管人们已认识到针尖与样品表面不可忽视的相互作用 ,但利用该相互作用诱导纳米区域电化学反应的研究还很少 ,仅有半导体 Si[4～ 7] 和 Ga As[8] 表面基于强电场诱导或空穴注入的 STM针尖诱导纳米刻蚀等的报道 .本文在控制铜的电位负于其热力学平衡电位 ( Nernst电位 )的情况下 ,…     

Ion-specific forces between a colloidal nanoprobe and a charged surface

We investigate the effect of ion-specific potentials on the force between a nanoprobe attached to a cantilever tip, and a charged surface. The probe is treated as a spherical nanoparticle with constant charge. A modified Poisson-Boltzmann equation in bispherical coordinates is used to address this problem in a more quantitative way. We predict that the ion-specific series of measured forces depend on the sign and magnitude of surface charge densities.     

电化学石英晶体微天平研究界面电场对DNA杂交的影响

采用电化学石英晶体微天平, 现场监测不同界面电场下完全匹配的靶标DNA和不完全匹配的靶标DNA分别与寡聚核苷酸探针分子杂交的过程. 结果表明, 电极表面荷正电时DNA表观杂交效率比电极表面荷负电时高, 但假阳性比较显著; 而电极表面荷负电时能有效地抑制错配杂交. 探讨了引入界面电场后探针分子取向和微观作用力对DNA杂交的影响.     

Probing the hydrogen adsorption affinity of Pt and Ir by surface interrogation scanning electrochemical microscopy (SI-SECM)

Scanning electrochemical microscopy in the surface interrogation mode (SI-SECM) has been used to probe the strength of metal-adsorbed H bonds at Pt and Ir. The generally accepted view is that this technique can only give meaningful results for similar probe and substrate electrode dimensions. However, it is shown that SI-SECM can also provide valuable information for H adsorption even when the substrate is much larger than the microelectrode probe (of 500 and 25 μm diameter respectively), for properly chosen substrate and mediator combinations that minimize the tip currents under substrate open circuit potential conditions. Linear sweep voltammetry at a microelectrode positioned at a very small probe–substrate distance (2.5 μm) showed remarkable positive feedback in the presence of an oxidizable mediator (TMPD) when a full or partial H monolayer was electrochemically pre-formed on the Pt or Ir substrate. The magnitude and shapes of chronoamperometric responses at the tip were interpreted in terms of variation of H coverage with time and changes in the open circuit potential of the substrate. The higher affinity of Ir than Pt for adsorbed H has been used to validate the approach.     

Multidimensional electrochemical imaging in materials science

In the past 20 years the characterization of electroactive surfaces and electrode reactions by scanning probe techniques has advanced significantly, benefiting from instrumental and methodological developments in the field. Electrochemical and electrical analysis instruments are attractive tools for identifying regions of different electrochemical properties and chemical reactivity and contribute to the advancement of molecular electronics. Besides their function as a surface analytical device, they have proved to be unique tools for local synthesis of polymers, metal depots, clusters, etc. This review will focus primarily on progress made by use of scanning electrochemical microscopy (SECM), conductive AFM (C-AFM), electrochemical scanning tunneling microscopy (EC-STM), and surface potential measurements, for example Kelvin probe force microscopy (KFM), for multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers. Figure Electrochemical and electrical tools like scanning electrochemical microscopy, conductive atomic force microscopy, electrochemical scannig tunneling microscopy and Kelvin probe force microscopy (see figure) are powerful tools for the multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers.     

Fabricating and imaging carbon-fiber immobilized enzyme ultramicroelectrodes with scanning electrochemical microscopy.

The scanning electrochemical microscope (SECM) is used to image the activity of enzymes immobilized on the surfaces of disk-shaped carbon-fiber electrodes. SECM was used to map the concentration of enzymatically produced hydroquinone or hydrogen peroxide at the surface of a 33-microm diameter disk-shaped carbon-fiber electrode modified by an immobilized glucose-oxidase layer. Sub-monolayer coverage of the enzyme at the electrode surface could be detected with micrometer resolution. The SECM was also employed as a surface modification tool to produce microscopic regions of enzyme activity by using a variety of methods. One method is a gold-masking process in which microscopic gold patterns act as mask for producing patterns of chemical modification. The gold masks allow operation in both a positive or negative process for patterning enzyme activity. A second method uses the direct mode of the SECM to produce covalently attached amine groups on the carbon surface. The amine groups are anchors for attachment of glucose oxidase by use of a biotin/avidin process. The effect of non-uniform enzyme activity was investigated by using the SECM tip to temporarily damage an immobilized enzyme surface. SECM imaging can observe the spatial extent and time-course of the enzyme recovery process.     

Scanning electrochemical microscopy studies of glutathione-modified surfaces. An erasable and sensitive-to-reactive oxygen species surface

A surface sensitive to reactive oxygen species (ROS) was prepared by reduction of a diazonium salt on glassy carbon electrode followed by the chemical coupling of glutathione (GSH) playing the role of an antioxidant species. The presence of active GSH was characterized through spectroscopic studies and electrochemical analysis after labeling of the -SH group with ferrocene moieties. The specific reactivity of GSH vs ROS was evaluated with scanning electrochemical microscopy (SECM) using the reduction of O(2) to superoxide, O(2)(?-), near the GSH-modified surface. Approach curves show a considerable decrease of the blocking properties of the layer due to reaction of the immobilized GSH with O(2)(?-) and the passage of GSH to the glutathione disulfide (GSSG). The initial surface could be regenerated several times with no significant variations of its antioxidant capacity by simply using the biological system glutathione reductase (GR)/NADPH that reduces GSSG back to GSH. SECM imaging shows also the possibility of writing local and erasable micropatterns on the GSH surface by production of O(2)(?-) at the tip probe electrode.     

Study on Electrocatalytic Activity of Platinum for Hydrogen Oxidation in Acidic Solution by Scanning Electrochemical Microscopy

The electrocatalytic activity of platinum for hydrogen oxidation in 0.01 M H₂SO₄ + 0.1 MNa₂SO₄ solution has been investigated by scanning electrochemical microscopy (SECM) technique. The cyclic voltammogram (CV), approach curve, area scan imaging and chronoamperometric methods have been used. The results indicate that the imaging capability of the SECM feedback mode can be used more efficiently to visually identify materials' electrocatalytic activity, compared with the approach curve method for identification of the conductive or insulating nature of a surface. The SECM imaging method has demonstrated the effects of Pt substrate potential on the electrocatalytic oxidation of hydrogen under a constant tip potential. It is found that the more positive the Pt substrate potential, the lower the electrocatalytic activity of the Pt. Furthermore, the chronoamperometric results support the variation of the electrocatalytic activity with the Pt substrate potential as well.     

Synthesis and Characterization of Novel Polyamines Containing Different Substitute Groups Via Chemical Oxidative Polymerization

In this study, polymers of substitute aromatic amine compounds were synthesized by chemical oxidative polycondensation in aqueous alkaline medium using NaOCl as oxidant. The structures of synthesized compounds were confirmed by FT‐IR, UV‐Vis and NMR analyses. The characterizations of synthesized compounds were made by fluorescence, TG‐DTA, DSC, scanning electron microscopy (SEM) and Brunauer‐Emmett‐Teller (BET) surface area measurements. In addition, electrochemical and electrical conductivity values of compounds were carried out with cyclic voltammetry (CV) and four points probe technique measurements, respectively. Synthesized polymers viewed structural change, optical, electrochemical and thermal differences.     

Biocatalytic metal nanopatterning through enzyme-modified microelectrodes

The formation of narrow-size distribution nanomaterials on surfaces in defined patterns is a research area of great interest due to its relevance in many applications such as catalysis, optoelectronics, and sensing devices. Patterning surface with nanostructures has been achieved by numerous techniques including electron-beam lithography, microcontact printing, constructive lithography, and different scanning probe microscopy techniques. Here, we present a different approach by which gold patterns are formed by an enzyme-catalyzed reaction followed by a surface-catalyzed process. Our study takes the advantage of scanning electrochemical microscopy (SECM) where the tip is modified with an enzyme and generates a reductant. The latter participates in an electroless deposition reaction, where AuCl₄⁻ is reduced catalyzed by a Pd surface. The result is local deposition of gold patterns made of nanoparticles as soon as the reductant generated by the tip, i.e., hydroquinone, approaches the Pd surface. Two enzymes were used: glucose oxidase (GOx) and alkaline phosphatase (ALP). The entire process was carefully studied and optimized, which enabled a good control of the patterns formed.

     

Measuring the ionic flux of an electrochemically actuated conducting polymer using modified scanning ion conductance microscopy

We propose a modification of a scanning ion conductance microscope suitable for probing an electrode in an operating electrochemical cell. We demonstrate its use by measuring salt concentration variations near a conducting polymer electrode as the polymer is electrochemically oxidized and reduced. The electrochemical control circuit is opened to isolate the working electrode, at a frequency sufficiently high that the electrode capacitance maintains the electrode potential. The local solution conductivity variations are detected through the probe current during the open-circuit time. We demonstrate two-stage ion exchange during oxidation and reduction of poly(3,4-ethylenedioxythiophene) films that develops strongly with repeated cycling and is correlated with actuation changes. Spatial composition variations of the film, caused by redox current distribution over the surface, and electromigration to the probe tip, causing local solution composition changes, have clear and characteristic effects on the measured transients.     

Mechanics of soft interfaces studied with displacement-controlled scanning force microscopy

The development of scanning force microscopes that maintain precise control of the tip position using displacement control (DC-SFM) has allowed significant progress in understanding the relationships between the chemical and mechanical properties of soft interfaces. Here, developments in DC-SFM techniques and their applications are reviewed. Examples of material systems that have been investigated are discussed and compared to measurements with other techniques involving nanoprobe geometries to illustrate the achievements and promise in this area. Specifically discussed are applications to soft interfaces, including SAMs, lipid bilayers, confined fluids, polymer surfaces, ligand–receptor bonds, and soft metallic films.