The binding sites of carboxylic acid group contacting to Cu electrode

In this work, the binding sites of carboxylic acid binding to Cu electrode are explored by electrochemical jump-to-contact STM break junction. Single molecular conductance of benzene-based molecules with ending groups of carboxylic acid, carbonyl and hydroxyl are measured and compared. The conductance values of 1,4-benzenedicarboxaldehyde can be found in those of 1,4-benzenedicarboxylic acid, which shows that carboxylic acid can bind to Cu electrode through carbonyl group. Carboxylic acid can also bind to the electrode through carboxylate group, and gives out larger conductance values than those of carbonyl group. However, molecule with hydroxyl group is difficult to form single molecular junction with Cu. The current work demonstrates that the carboxylic acid can bind to the electrode through carbonyl and carboxylate groups, and a new anchoring group of carbonyl group can be used to form effective single molecular junction.     

Variability of conductance in molecular junctions

The conductance of molecular junctions, formed by breaking gold point contacts dressed with various thiol functionalized organic molecules, is measured at 293 K and at 30 K. In the presence of molecules, individual conductance traces measured as a function of increasing gold electrode displacement show clear steps below the quantum conductance steps of the gold contact. These steps are distributed over a wide range of molecule-dependent conductance values. Histograms constructed from all conductance traces therefore do not show clear peaks either at room or low temperatures. Filtering of the data sets by an objective automated procedure only marginally improves the visibility of such features. We conclude that the geometrical junction to junction variations dominate the conductance measurements.     

Linker dependent bond rupture force measurements in single-molecule junctions

We use a modified conducting atomic force microscope to simultaneously probe the conductance of a single-molecule junction and the force required to rupture the junction formed by alkanes terminated with four different chemical link groups which vary in binding strength and mechanism to the gold electrodes. Molecular junctions with amine, methylsulfide, and diphenylphosphine terminated molecules show clear conductance signatures and rupture at a force that is significantly smaller than the measured 1.4 nN force required to rupture the single-atomic gold contact. In contrast, measurements with a thiol terminated alkane which can bind covalently to the gold electrode show conductance and force features unlike those of the other molecules studied. Specifically, the strong Au-S bond can cause structural rearrangements in the electrodes, which are accompanied by substantial conductance changes. Despite the strong Au-S bond and the evidence for disruption of the Au structure, the experiments show that on average these junctions also rupture at a smaller force than that measured for pristine single-atom gold contacts.     

Effects of pipette modulation and imaging distances on ion currents measured with scanning ion conductance microscopy (SICM)

Local conductance variations can be estimated by measuring ion current magnitudes with scanning ion conductance microscopy (SICM). Factors which influence image quality and quantitation of ion currents measured with SICM have been evaluated. Specifically, effects of probe-sample separation and pipette modulation have been systematically studied for the case of imaging conductance variations at pores in a polymer membrane under transmembrane concentration gradients. The influence of probe-sample separation on ion current images was evaluated using distance-modulated (ac) feedback. Approach curves obtained using non-modulated (dc) feedback were also recorded to determine the relative influence of pipette-generated convection by comparison of ion currents measured with both ac and dc feedback modes. To better interpret results obtained, comparison to a model based on a disk-shaped geometry for nanopores in the membrane, as well as relevant position-dependent parameters of the experiment is described. These results advance our current understanding of conductance measurements with SICM.     

Pulstrodes: triple pulse control of potentiometric sensors

Ion-selective electrode membranes based on hydrophobic materials doped with chemically selective host molecules are an attractive sensing technology but normally suffer from a limited sensitivity, given by the Nernst equation, and a direct reliance on the reference electrode potential, which makes miniaturization difficult. These fundamental problems are addressed here by imposing a multipulse electrochemical excitation signal onto ion-selective membranes that lack ion-exchange properties. Current pulses are responsible for the generation of ion fluxes in the direction of the membrane, which give reproducible super-Nernstian response slopes that originate from depletion processes at the membrane surface. Membranes may also be measured at zero current after this pulse, giving super-Nernstian response regions at lower concentrations. Difference potentials obtained from subsequent pulses give about 10-fold higher sensitivities than predicted on the basis of the Nernst equation.     

Writing with DNA and protein using a nanopipet for controlled delivery

We present a new, general method for the controlled deposition of biological molecules on surfaces, based on a nanopipet operating in ionic solution. The potential applied to the pipet tip controls the flux of biological molecules from the pipet, allowing fine control of the delivery rate. We used the ion current to control the distance of the pipet from the surface of a glass slide and deposited the fluorescently labeled DNA or protein G at a defined location onto the surface. Features of 830 nm size were obtained by depositing the biotinylated DNA onto a streptavidin surface; 1.3 mum size spots were obtained by depositing protein G onto a positively charged glass surface.     

Electron transfer kinetics at polarized nanoscopic liquid/liquid interfaces

Rapid kinetics of electron transfer (ET) reactions across the interface between water and 1,2-dichloroethane were measured by steady-state voltammetry at nanopipet electrodes (50- to 400-nm orifice radius). The origins of previously reported imperfect voltammetric responses of ET reactions at micropipets were investigated. Several new experimental systems were explored, and two of them yielded high-quality voltammograms suitable for kinetic experiments. The determined standard rate constants were compared to those measured previously at polarized and nonpolarized liquid/liquid interfaces. The effect of the interfacial dimensions on the magnitude of the apparent ET rate constant is discussed. A new approach to ET kinetic measurements based on the use of the scanning electrochemical microscope with a nanopipet tip and a metallic substrate has been developed and employed to check the validity of determined kinetic parameters.     

Controlling Single Molecule Conductance by a Locally Induced Chemical Reaction on Individual Thiophene Units

Among the prerequisites for the progress of single‐molecule‐based electronic devices are a better understanding of the electronic properties at the individual molecular level and the development of methods to tune the charge transport through molecular junctions. Scanning tunneling microscopy (STM) is an ideal tool not only for the characterization, but also for the manipulation of single atoms and molecules on surfaces. The conductance through a single molecule can be measured by contacting the molecule with atomic precision and forming a molecular bridge between the metallic STM tip electrode and the metallic surface electrode. The parameters affecting the conductance are mainly related to their electronic structure and to the coupling to the metallic electrodes. Here, the experimental and theoretical analyses are focused on single tetracenothiophene molecules and demonstrate that an in situ‐induced direct desulfurization reaction of the thiophene moiety strongly improves the molecular anchoring by forming covalent bonds between molecular carbon and copper surface atoms. This bond formation leads to an increase of the conductance by about 50 % compared to the initial state.     

Redox activity and diffusion of hydrophilic, hydrophobic, and amphiphilic redox active molecules in a bicontinuous cubic phase

The objective was to examine how a bicontinuous cubic phase influences the diffusion and electrochemical activity of dissolved molecules. The cubic phase is a structure with three-dimensional continuous channels of water separated by an apolar membrane. A redox active molecule can dissolve in three different environments. A hydrophobic molecule will prefer the interior of the membrane, a hydrophilic molecule will prefer the water channels, and an amphiphilic molecule will be situated with its headgroup at the surface of the membrane and its tail in the interior. The electrochemical activity was measured with cyclic voltammetry and the transport behavior with chronocoulometry. All the molecules were redox active in the cubic phase; that is, all the molecules could reach the surface of the electrode and react. The cubic phase made the kinetics of the charge transfer slower, showing a quasi-reversible behavior. The reason may be that a layer of the membrane adheres to the hydrophobic electrode surface. The diffusion experiment showed that the diffusion was slower than in solution. The molecules that were restricted to diffuse within the membrane gave particularly low mass transport rates.     

Anisotropic Conductivity at the Single‐Molecule Scale

In most junctions built by wiring a single molecule between two electrodes, the electrons flow along only one axis: between the two anchoring groups. However, molecules can be anisotropic, and an orientation‐dependent conductance is expected. Here, we fabricated single‐molecule junctions by using the electrode potential to control the molecular orientation and access individual elements of the conductivity tensor. We measured the conductance in two directions, along the molecular plane as the benzene ring bridges two electrodes using anchoring groups (upright) and orthogonal to the molecular plane with the molecule lying flat on the substrate (planar). The perpendicular (planar) conductance is about 400 times higher than that along the molecular plane (upright). This offers a new method for designing a reversible room‐temperature single‐molecule electromechanical switch that controllably employs the electrode potential to orient the molecule in the junction in either “ON” or “OFF” conductance states.     

DNA sequencing with titanium nitride electrodes

We construct a hydrogen‐bond based metal–molecule–metal junction, which contains two identical “reader” molecules, one single DNA base as a bridged molecule, and two titanium nitride electrodes. Hydrogen bonds are formed between “reader” molecules and DNA base, whereas titanium–sulfur bonds are formed between “reader” molecules and titanium nitride electrodes. We perform electronic structure calculations for both the bare bridged molecule and the full metal–molecule–metal system. The projected density of states shows that when the molecule is connected to the titanium nitride electrode, the energy levels of the bridged molecule are shifted, with an indirect effect on the hydrogen bonds. This is similar to the case for a gold electrode but with a more pronounced effect. We also calculate the current–voltage characteristics for the molecular junctions containing each DNA base. Results show that titanium nitride as an electrode can generate distinct conductance for each DNA base, providing an alternative electrode for DNA sequencing. © 2013 Wiley Periodicals, Inc.     

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.     

In situ electrochemical distance tunneling spectroscopy of ds-DNA molecules

The electrical conductance of ds-DNA duplexes containing 8-14 base pairs modified at both ends with a -(CH(2))(6)-SH linker was measured in a buffered aqueous solution using electrochemically controlled distance tunneling spectroscopy. The tunneling experiment with self-complementary 5'-(GC)(n)()-3'-(CH(2))(6)-SH (n = 4-7) duplexes attached covalently to a gold STM tip and a Au(111) electrode shows a wide distribution of currents independent of the ds-DNA length. The voltage-induced horizontal orientation of ds-DNA within the junction results in decreased electrical conductance. The lower currents are also observed for ds-DNA molecules containing a single CA base mismatch.     

Bipolar pulse conductometric monitoring of ion-selective electrodes: Part 1. Method development with a calcium-selective electrode and elucidation of the basic principles involved

The potential generated by a plastic-membrane calcium ion-selective electrode (i.s.e.) is shown to be indirectly measurable by a non-zero current method based on bipolar pulse conductance. Linear current—voltage curves are obtained using 0–5-V pulses; the current axis intercept is related to the i.s.e. potential. A simple electrical contact (e.g., platinum or stainless steel) can be used instead of a poised reference electrode as the counter electrode in this two-electrode system. Long-term exposure of the i.s.e. to calcium solutions causes an upward drift in the measured current. This drift is minimized by avoiding long exposure times to solution, rinsing the electrode between measurements, and constructing current—voltage curves for determination of the current axis intercepts. Voltage pulses lasting 100 μs are optimum for this method. Shorter pulses are subject to error from capacitive charging currents, and longer pulses yield poorer precision, and degrade the electrode through faradaic reactions. The measured signal is dependent upon Ca²⁺ concentration (rather than activity), making ionic strength adjustment unnecessary. The concentration dependence is induced by application of voltage pulses greater than ~ 15 mV in amplitude. Selectivities of the potentiometric and conductometric methods are shown to be comparable for a variety of interfering monovalent and divalent cations. The conductometric method yields a fast i.s.e. response because of induced migration of Ca²⁺ into the membrane. Response time decreases as the pulse height increases. Pulses greater than 2 V in magnitude yield response times limited by the solution mixing time rather than by the electrode.     

Thin-film electrochemical sensor for diphenylamine detection using molecularly imprinted polymers

This work reports on the development of a new voltammetric sensor for diphenylamine based on the use of a miniaturized gold electrode modified with a molecularly imprinted polymer recognition element. Molecularly imprinted particles were synthesized ex situ and further entrapped into a poly(3,4-ethylenedioxythiophene) polymer membrane, which was electropolymerized on the surface of the gold electrode. The thickness of the polymer layer was optimized in order to get an adequate diffusion of the target analyte and in turn to achieve an adequate charge transfer at the electrode surface. The resulting modified electrodes showed a selective response to diphenylamine and a high sensitivity compared with the bare gold electrode and the electrode modified with poly(3,4-ethylenedioxythiophene) and non-imprinted polymer particles. The sensor showed a linear range from 4.95 to 115 μM diphenylamine, a limit of detection of 3.9 μM and a good selectivity in the presence of other structurally related molecules. This sensor was successfully applied to the quantification of diphenylamine in spiked apple juice samples.     

聚吡咯电化学固定化胆固醇氧化酶电极的电流响应及应用

用电极法测定总胆固醇量,可以通过制备胆固醇氧化酶和胆固醇脂酶的酶膜与氧电极偶联制成传感器或制备以过氧化氢检出方式的传感器来完成,本文研制了一种新型的酶电极,它不需选择电荷传递中间体代替氧分子的作用,本方法利用吡咯单体在水溶液中能电氧化聚合,并伴有对阴离子嵌入聚合物骨架的性质,使酶分子在聚合过程中作为阴离子而嵌     

Electrochemical investigation of melittin reconstituted into a mercury-supported lipid bilayer

The channel-forming peptide melittin was incorporated into a biomimetic membrane consisting of a mercury electrode coated with a thiolipid monolayer, with a lipid monolayer self-assembled on top of it. The thiolipid consisted of a hydrophilic tetraethyleneoxy chain terminated at one end with a disulfide group, for anchoring to the mercury surface, and covalently linked at the other end to two diphytanyl chains, which formed a lipid bilayer with the overhanging lipid monolayer. The conductance of the lipid bilayer in contact with aqueous 0.1 M KCl was measured by electrochemical impedance spectroscopy over a frequency range from 1 x 10(-2) to 1 x 10(5) Hz and a potential range of 0.7 V for different compositions of the outer lipid monolayer. The conductance increases abruptly above the background level at sufficiently negative applied potentials, attaining a maximum value that increases with the composition of the outer monolayer in the order PC/chol (60:40) < PC < PC/SM/chol (59:15:26) < PS, with PC = phosphatidylcholine, chol = cholesterol, SM = sphingomyelin, and PS = phosphatidylserine. The higher the maximum conductance, the less negative the applied potential at which it is attained. This behavior is also discussed using a model of the electrified interphase.     

A Micro Planar Ampehometric Glucose Sensor Using an Isfet as a Reference Electrode

Abstract

A very small glucose sensor has been realized, which consists of a gold working electrode with a glucose oxidase immobilized membrane on it, and a gold counter electrode, all made on a sapphire substrate. By using the pH sensitive ISFET as a reference electrode, the potential for a solution, whose pH is constant, can be measured and irreversible metal electrodes, such as gold or platinum, can be used as working electrode and counter electrode. The sensor is very suitable for miniaturizing and mass production, because the Integrated Circuit (IC) fabrication process can be applied. The glucose oxidase immobilized membrane was also deposited by a lift off method, one of the IC processes. A glucose concentration, from 1 to 100 mg/dl, was measured with good linear current output.     

Atomic force microscopy imaging and electrical recording of lipid bilayers supported over microfabricated silicon chip nanopores: lab-on-a-chip system for lipid membranes and ion channels

We describe a silicon chip-based supported bilayer system to detect the presence of ion channels and their electrical conductance in lipid bilayers. Nanopores were produced in microfabricated silicon membranes by electron beam lithography as well as by using a finely focused ion beam. Thermal oxide was used to shrink pore sizes, if necessary, and to create an insulating surface. The chips with well-defined pores were easily mounted on a double-chamber plastic cell recording system, allowing for controlling the buffer conditions both above and below the window. The double-chamber system allowed using an atomic force microscopy (AFM) tip as one electrode and inserting a platinum wire as the second electrode under the membrane window, to measure electrical current across lipid bilayers that are suspended over the pores. Atomic force imaging, stiffness measurement, and electrical capacitance measurement show the feasibility of supporting lipid bilayers over defined nanopores: a key requirement to use any such technique for structure-function study of ion channels. Online addition of gramicidin, an ion-channel-forming peptide, resulted in electrical current flow across the bilayer, and the I-V curve that was measured using the conducting AFM tip indicates the presence of many conducting gramicidin ion channels.     

A dielectric study of poly(ethylene-co-vinyl acetate)-poly(vinyl chloride) blends. III. Direct current conductivity and electrode polarization

The large direct current (d.c.) conductance observed at low frequencies and high temperatures in conjunction with a simple electrical circuit model was used to calculate specific conductivities in PVC and mixtures of PVC with a poly(ethylene-vinyl acetate) copolymer. Activation energies for d.c. conduction and dipole relaxation in PVC and the blends were found to be in reasonable correspondence. The effects of different electrode materials on the d.c. conductance as well as on electrode polarization are discussed.