Fabrication and characterization of probes for combined scanning electrochemical/optical microscopy experiments

A technique that combines scanning electrochemical microscopy (SECM) and optical microscopy (OM) was implemented with a new probe tip. The tip for scanning electrochemicaVoptical microscopy (SECM/OM) was constructed by insulating a typical gold-coated near-field scanning optical microscopy tip using electrophoretic anodic paint. Once fabricated, the tip was characterized by steady-state cyclic voltammetry, as well as optical and electrochemical approach experiments. This tip generated a stable steady-state current and well-defined SECM approach curves for both conductive and insulating substrates. Durable tips whose geometry was a ring with < 1 microm as outer ring radius could be consistently fabricated. Simultaneous electrochemical and optical images of an interdigitated array electrode were obtained with a resolution on the micrometer scale, demonstrating good performance of the tip as both an optical and an electrochemical probe for imaging microstructures. The SECM feedback current measurements were successfully employed to determine tip-substrate distances for imaging.     

Combined scanning electrochemical/optical microscopy with shear force and current feedback

A technique that combines scanning electrochemical microscopy (SECM) and scanning optical microscopy (OM) was developed. Simultaneous scanning electrochemical/optical microscopy (SECM/OM) was performed by a special probe tip, which consists of an optical fiber core for light passage, surrounded by a gold ring electrode, and an outermost electrophoretic insulating sheath, with the tip attached to a tuning fork. To regulate the tip-substrate distance, either the shear force or the SECM tip current was employed as the feedback signal. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force allowed simultaneous topographic, along with SECM and optical imaging in a constant-force mode. The capability of this technique was confirmed by obtaining simultaneously, for the first time, topographic, electrochemical, and optical images of an interdigitated array electrode. Current feedback from SECM also provided simultaneous electrochemical and optical images of relatively soft samples, such as a polycarbonate membrane filter and living diatoms in a constant-current mode. This mode should be useful in mapping the biochemical activity of a living cell.     

Combined scanning electrochemical-atomic force microscopy

A combined scanning electrochemical microscope (SECM)-atomic force microscope (AFM) is described. The instrument permits the first simultaneous topographical and electrochemical measurements at surfaces, under fluid, with high spatial resolution. Simple probe tips suitable for SECM-AFM, have been fabricated by coating flattened and etched Pt microwires with insulating, electrophoretically deposited paint. The flattened portion of the probe provides a flexible cantilever (force sensor), while the coating insulates the probe such that only the tip end (electrode) is exposed to the solution. The SECM-AFM technique is illustrated with simultaneous electrochemical-probe deflection approach curves, simultaneous topographical and electrochemical imaging studies of track-etched polycarbonate ultrafiltration membranes, and etching studies of crystal surfaces.     

Fabrication and characterization of a nanometer-sized optical fiber electrode based on selective chemical etching for scanning electrochemical/optical microscopy

We have already reported a method for fabricating ultramicroelectrodes (Suzuki, K. JP Patent, 2004-45394, 2004). This method is based on the selective chemical etching of optical fibers. In this work, we undertake a detailed investigation involving a combination of etched optical fibers with various types of tapered tip (protruding-shape, double- (or pencil-) shape and triple-tapered electrode) and insulation with electrophoretic paint. Our goal is to establish a method for fabricating nanometer-sized optical fiber electrodes with high reproducibility. As a result, we realized pencil-shaped and triple-tapered electrodes that had radii in the nanometer range with high reproducibility. These nanometer-sized electrodes showed well-defined sigmoidal curves and stable diffusion-limited responses with cyclic voltammetry. The pencil-shaped optical fiber, which has a conical tip with a cone angle of 20 degrees , was effective for controlling the electrode radius. The pencil-shaped electrodes had higher reproducibility and smaller electrode radii (r(app) < 1.0 nm) than those of other etched optical fiber electrodes. By using a pencil-shaped electrode with a 105-nm radius as a probe, we obtained simultaneous electrochemical and optical images of an implantable interdigitated array electrode. We achieved nanometer-scale resolution with a combination of scanning electrochemical microscopy SECM and optical microscopy. The resolution of the electrochemical and optical images indicated sizes of 300 and 930 nm, respectively. The neurites of living PC12 cells were also successfully imaged on a 1.6-microm scale by using the negative feedback mode of an SECM.     

Design and analysis of an integrated optical sensor for scanning force microscopies

In this paper, a novel probe for displacement sensing will be introduced. It is based on a conventional GaAs cantilever, integrated with a Bragg grating as a photo-elastic strain sensor. The deflection of the cantilever is measured directly from the intensity modulation of the reflected light. The principle of the experimental setup and the sensor, as well as the theoretical investigation of the force and displacement sensitivity of the probe, is presented. Finite-element method simulations were performed to get the optimum sensor design. Transfer matrix method simulation of the waveguide grating have been described in detail. In order to enhance the sensitivity, different types of grating structures are discussed. Using this new design, it should be possible to achieve sensitivities, defined as the fractional change in detected optical power per unit displacement of the cantilever, as high as 10/sup -4/ /spl Aring//sup -1/ of cantilever deflection.     

Scanning electrochemical microscopy. Theory of the feedback mode for hemispherical ultramicroelectrodes: steady-state and transient behavior

This contribution represents the first comprehensive attempt to treat complex geometry configurations of the scanning electrochemical microscope (SECM) using the alternating direction implicit finite difference method (ADIFDM). Specifically, ADIFDM is used to simulate the steady-state as well as the transient (chronoamperometric) behavior of a hemispherical ultramicroelectrode (UME) tip of the SECM. The feedback effect in this configuration is less pronounced as compared with a disk-shaped UME system. The differences between the two systems are discussed. Analytical approximations for the steady-state behavior and for characteristic features of the transient behavior are suggested. Finally, experimental feedback currents measured above a conductor and an insulator are in excellent agreement with the theory.     

Optical actuation of micromechanical tunneling structures with applications in spectrum analysis and optical computing

An efficient method for optically actuating a micromechanical cantilever is presented for the first time to our knowledge. Measurable responses can be obtained for moderate light sources if electron tunneling occurs between the cantilever tip and a metallic contact below it. The small deflection of the cantilever that is due to light pressure is sufficient then to produce large tunneling current variations. On the basis of this effect several applications such as a miniaturized spectrum analyzer and one-step optical computing units for addition, integration, or differentiation of one-dimensional or two-dimensional optical signals are presented.     

Optical fiber sensor for electric field and electric charge using low-coherence, Fabry-Perot interferometry

An optical fiber sensor for electric field and electric charge, based on the deflection of a small cantilever, has been developed. When the sensor head is placed in an electric field, induced charging produces deflection of the cantilever, which is measured using low-coherence, Fabry-Perot interferometry. The sensor has been used to measure the electric field in the vicinity of a Van de Graaff generator, in the range 135-650 V/cm. The measured deflections are in good agreement with the predictions of a simple model equating the electrostatic and mechanical forces acting on the cantilever.     

Fabrication of nanometer-sized electrodes and tips for scanning electrochemical microscopy.

A novel method for fabrication of nanometer-sized electrodes and tips suitable for scanning electrochemical microscopy (SECM) is reported. A fine etched Pt wire is coated with polyimide, which was produced by polymerization on the Pt surface initiated by heat. This method can prepare electrodes with effective radii varying from a few to hundreds of nanometers. Scanning electron microscopy, cyclic voltammetry, and SECM were used to characterize these electrodes. Well-defined steady-state voltammograms could be obtained in aqueous or in 1,2-dichloroethane solutions. This method produced the nanoelectrodes with exposed Pt on the apex, and they can also be employed as the nanotips for SECM investigations. Different sizes of Pt nanotips made by this method were employed to evaluate the kinetics of the redox reaction of Ru(NH3)6(3+) on the surface of a large Pt electrode by SECM, and the standard rate constant kappa0 of this system was calculated from the best fit of the SECM approach curve. This result is similar to the values obtained by analysis of the obtained voltammetric data.     

Potential of interferometric cantilever detection and its application for SFM/AFM in liquids

We have developed an optical cantilever deflection detector with a spot size <3?μm and fm?Hz(-1/2) sensitivity over a>10?MHz bandwidth. In this work, we demonstrate its potential for detecting small-amplitude oscillations of various flexural and torsional oscillation modes of cantilevers. The high deflection sensitivity of the interferometer is particularly useful for detecting cantilever oscillations in aqueous solutions, enabling us to reach the thermal noise limit in scanning or atomic force microscopy experiments with stiff cantilevers. This has resulted in atomic-resolution images of solid-liquid interfaces and submolecular-resolution images of native membranes.     

Selective insulation with poly(tetrafluoroethylene) of substrate electrodes for electrochemical background reduction in scanning electrochemical microscopy

We describe a wet process for the fabrication of poly(tetrafluoroethylene) (PTFE)-covered electrodes in which arrays of holes ( approximately 200 microm) are formed. The PTFE coating provides electrical insulation of most of the electrode surface with selected regions exposed for electrochemical experiments. The arrays of microholes can be controllably patterned and filled with precursor solutions using a piezoelectric dispenser. A micrometer spot of electrocatalyst is produced after reduction of the precursor. The application is tested for scanning electrochemical microscopy (SECM) in the tip generation-substrate collection (TG-SC) studies of electrocatalysts. The method is shown to reduce the substrate background currents that are included in the electrochemical signal read from the local perturbation induced with the SECM tip to the substrate in the TG-SC mode of SECM. This background current reduction is consistent with the decrease in the exposed area of the electrode. The general methodology for the fabrication of the substrate electrodes and two proof-of-concept applications in the TG-SC SECM modality are described.     

A Model for the Hybridization Exothermic Effect in Label-Free Biodetections by a Nanomechanical Cantilever-DNA Chip

The influence of the hybridization exothermic effect on nanomechanical deflections of DNA chips in label-free biodetections is investigated. First, from the related experimental curves, the thermal variation of the biolayer during the linkage of DNA base pairs is estimated by Breslauer’s method and the Langmuir adsorption isotherm. Second, the temperature field of the chip is obtained by the lumped parameter model and the classical Fourier’s method. Third, the nanomechanical deflection of the chip is predicted by an alternative model for thermoelastic problems of laminated cantilever beams. The effect of a DNA base sequence on thermal deflection of chips is also investigated. In the case of adiabatic conditions, numerical results show that the theoretical predicted value of 1.5 nm to 2 nm deflection is within the scope of the optical-beam-deflection readout system’s accuracy.     

Simple and clear evidence for positive feedback limitation by bipolar behavior during scanning electrochemical microscopy of unbiased conductors

On the basis of an experimentally validated simple theoretical model, it is demonstrated unambiguously that when an unbiased conductor is probed by a scanning electrochemical tip (scanning electrochemical microscopy, SECM), it performs as a bipolar electrode. Though already envisioned in most recent SECM theories, this phenomenon is generally overlooked in SECM experimental investigations. However, as is shown here, this may alter significantly positive feedback measurements when the probed conductor is not much larger than the tip.     

Scanning electrochemical microscopy 50. Kinetic study of electrode reactions by the tip generation-substrate collection mode

A scanning electrochemical microscopy (SECM) methodology for localized quantitative kinetic studies of electrode reactions based on the tip generation-substrate collection (TG-SC) operation mode is presented. This approach does not use the mediator feedback required in typical kinetic SECM experiments. The reactant is galvanostatically electrogenerated on a tip placed in proximity to the substrate. It diffuses through the tip-substrate gap and undergoes the reaction of interest on the substrate surface. The substrate current is monitored with time until it reaches an apparent steady-state value. The process was digitally simulated using an explicit finite difference method, for an irreversible first-order electrode reaction at the substrate. Transient responses, steady-state polarization curves, and TG-SC approach curves can be used to obtain substrate kinetics. The effects of the experimental parameters were analyzed. The possibility of easily changing the experimental conditions with the SECM is an attractive approach to obtain independent evidence that can be used for a strict test of reaction mechanisms. The technique was applied for a preliminary simplified kinetic examination of the oxygen reduction reaction in phosphoric acid.     

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.     

Noncontact electrochemical imaging with combined scanning electrochemical atomic force microscopy

Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-microm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface.     

具有溯源性的原子力显微镜探针法向弹性常数标定系统

原子力显微镜(AFM)悬臂梁探针的弹性常数在微纳米尺度力学测试中十分重要,其准确程度直接影响力学测量结果的可靠性,故需对其进行精确标定.因天平法的测量结果可溯源,本文在已有天平法的基础上,研制了一套新型标定系统.该系统将AFM测头与超精密电磁天平相结合.微悬臂梁在精密位移台的带动下接触天平并产生弯曲,接触力由天平测得,微悬臂梁的弯曲量由光杠杆检测,并通过反馈系统进行精确控制,最后根据胡克定律计算出弹性常数.利用本系统对4种不同型号商用微悬臂梁探针的法向弹性常数进行了标定,标定结果表明本系统具有良好的测量重复性.通过进行不确定度分析,得到测量结果的相对标准不确定度优于2%.     

Stochastic surface effects in nanobeam sensors

This work analyzes the statistical properties of nanobeam deflections due to stochastic surface stresses, induced by the surface adsorption/desorption of surrounding particles. A mechanical model for a heterogeneous nanobeam is first introduced. The model considers combined axial forces and bending moments due to non-uniform surface effects. Then, local surface interactions are statistically derived from the Langmuir interaction model and their corresponding stochastic surface stresses are introduced. Two types of nanobeam sensor are studied: a cantilever beam with pure surface bending effect and a clamped beam with mixed surface force and bending moment effects. The advantages of each type are discussed. The deflection statistics are found analytically and validated by Monte Carlo simulations. An analytical relation between the adsorption/desorption rates and the maximum deflection variance is found.     

Design of a Novel Substrate-Free Double-Layer-Cantilever FPA Applied for Uncooled Optical-Readable Infrared Imaging System

This paper describes the design and performances of a novel focal-plane array (FPA) containing pixels of double bimaterial-layer cantilevers without silicon (Si) substrate for being applied in the uncooled optical-readable infrared (IR) imaging system. The top layer of the cantilever pixels is made of two materials with large mismatching thermal expansion coefficients: silicon nitride (SiN_x) and gold (Au), which convert IR heat into mechanical deflection. The bottom layer is SiN_x cantilever, which partially serves thermal isolation legs. The top and bottom pads form the resonant cavity, which can dramatically enhance the absorption of incident IR irradiation, and the substrate-free configuration enables reducing the loss of incident IR energy. Responding to the IR source with spectral range from 8 to 14 mum, the IR imaging system may receive an IR images through visible optical readout method. A thermal-mechanical model for such cantilever microstructure is proposed, and the thermal and thermal-mechanical coupling field characteristics of the cantilever microstructure are optimized through numerical analysis method and simulation by using the finite-element method. The thermal-mechanical deflection simulated is 7.2 mum/K, generally in good agreement with what the thermal-mechanical model and numerical analysis forecast. The analysis suggests that the detection resolution of current design is 0.03 K, whereas the noise analysis from FPA indicates the current resolution to be around 100 muK and the limit noise-equivalent temperature difference (NETD) of the IR imaging system can reach to 7 mK.     

Carbon nanofiber electrodes and controlled nanogaps for scanning electrochemical microscopy experiments

The electrochemical behavior of electrodes made by sealing carbon nanofibers in glass or with electrophoretic paint has been studied by scanning electrochemical microscopy (SECM). Because of their small electroactive surface area, conical geometry with a low aspect ratio and high overpotential for proton and oxygen reduction, carbon nanofiber (CNF) electrodes are promising candidates for producing electrode nanogaps, imaging with high spatial resolution and for the electrodeposition of single metal nanoparticles (e.g., Pt, Pd) for studies as electrocatalysts. By using the feedback mode of the SECM, a CNF tip can produce a gap that is smaller than 20 nm from a platinum disk. Similarly, the SECM used in a tip-collection substrate-generation mode, which subsequently shows a feedback interaction at short distances, makes it possible to detect a single CNF by another CNF and then to form a nanometer gap between the two electrodes. This approach was used to image vertically aligned CNF arrays. This method is useful in the detection in a homogeneous solution of short-lifetime intermediates, which can be electrochemically generated at one electrode and collected at the second at distances that are equivalent to a nanosecond time scale.