Stability of mercury-coated platinum microelectrodes upon touching a solid surface in scanning electrochemical microscopy (SECM) experiments

The features of experimental approach curves, recorded in scanning electrochemical microscopy measurements, and acquired with a series of sphere-cap tips while approaching a solid insulating substrate, are examined in detail. Sphere-caps are prepared by electrodeposition of liquid mercury onto a platinum microdisk substrate 10 μm radius, and having RG=R/a=10 (R is total tip dimension, which includes the shield thickness and the electrode radius a). From the data obtained, it is established that sphere caps with aspect ratio h/a>0.3 (h is the sphere cap height) are able to touch, or even be squeezed against the surface of the solid substrate. Under these latter conditions, for the sphere caps, the negative feedback process at low tip–substrate distances, is more pronounced than that of the corresponding naked microdisk. A good tip stability and no mercury loss is observed upon touching the substrate for sphere caps with h/a<1.3. This circumstance allows one to perform “contact” voltammetric measurements of lead ions adsorbed onto a thin layer chromatography plate.     

Effects of the mercury ion concentration on the preparation of mercury deposits on platinum micro-disk electrodes

In this paper, the effect of mercury ion concentration on the preparation of mercury microelectrodes fabricated on a platinum microdisk of 10 microns radius was studied. The preparation of the mercury microelectrodes was followed by chronoamperometry, and the measurements were performed in Hg2(NO3)2 solutions with concentrations over the range 0.1-12.5 mM. The mercury microelectrode size was referred to as h/a, where h is the height of the mercury deposit and a is the radius of the inlaid microdisk. The mercury microelectrodes investigated had h/a values over the range 0-2. The results obtained indicated that from concentrated mercury ion solutions (> 0.5 mM) the mercury growth was somehow erratic due to the coalescence of small mercury droplets. On the other hand, from dilute solutions (< 0.5 mM), the mercury deposits grew smoothly. Under the latter conditions the geometric coefficient k, which characterises the steady state diffusion limiting current expression at a mercury microelectrode, was determined with good accuracy (within 5% error) from the chronoamperometric curves recorded during the mercury microelectrode preparation. In general, the coefficient k was calculated from theoretical expressions derived for a sphere cap and spheroidal geometry, which may model the shape of the mercury deposits. The comparison between theoretical and experimental k values suggested that both geometries equally modelled the experimental mercury deposit obtained. Finally, for the sphere cap geometry an algebraic expression relating k and h/a was derived for an easier k evaluation from experimental h/a values.     

Scanning electrochemical microscopy imaging of rhodochrosite dissolution using gold amalgam microelectrodes

Gold/mercury amalgam (Au/Hg) microelectrodes with a diameter of 25 microm were developed for the detection of environmentally relevant analytes such as manganese and iron by scanning electrochemical microscopy (SECM), and applied to investigate the controlled dissolution of manganese carbonate (MnCO(3); rhodochrosite) in acidic conditions. Characterization of the amalgam electrode geometry via approach curves recorded during SECM experiments revealed Au/Hg microelectrodes with sphere cap geometry. Quantitative determination of Mn(2+) has been achieved by calibration of the Au/Hg microelectrode in bulk solution experiments. Subsequent SECM imaging experiments confirm the applicability of amalgam microelectrodes for imaging of Mn(2+) production during dissolution of MnCO(3) at pH 3.9. This study confirms feasibility and provides the fundamental basis of SECM imaging with amalgam microelectrodes to address biogeochemically relevant questions.     

Application of thin-shielded mercury microelectrodes in anodic stripping voltammetry

The performance in anodic stripping voltammetry (ASV) of hemispherical mercury microelectrodes, fabricated by electrodeposition of liquid mercury on the surface of Pt microdisks which were surrounded by a rather thick or thin insulating shield, was compared. The Pt microdisks were produced by sealing a wire of 25 μm diameter into a glass capillary, and by coating the cylindrical length of the Pt wire with a cathodic electrophoretic paint. The ratio of the overall tip radius b, to the basal radius of the electrode a, so-called RG = b/a, was equal to 110 ± 10 and 1.52 ± 0.01 for the thick- and thin-shielded microdisk, respectively. The mercury microelectrodes were characterized by cyclic voltammetry at 1 mV s⁻¹, in 1 mM Ru(NH₃)₆³⁺ aqueous solution. The steady-state voltammogram recorded with the thin-shielded mercury microelectrode displayed less hysteresis, while the steady-state current was about 30% higher than that of the thicker one. This was a consequence of the additional flux due to diffusion from behind the plane of the electrode. The flux enhancement, which was operative at the thin-shielded mercury microelectrode during the deposition step in the ASV experiments, allowed recording stripping peaks for Cd and Pb, which resulted about 32% larger than those recorded at the thicker shielded mercury microelectrode, under same experimental conditions.The usefulness of the thin-shielded mercury microelectrode for ASV measurements in real samples was verified by determining the content of heavy metal ions released in the pore water (pH 4.5) of a soil slurry.     

Ex Situ Scanning Electrochemical Microscopy (SECM) Investigation of Bismuth‐ and Bismuth/Lead Alloy Film‐Modified Gold Electrodes in Alkaline Medium

Scanning electrochemical microscopy (SECM) in feedback mode was employed to characterise the reactivity and microscopic peculiarities of bismuth and bismuth/lead alloys plated onto gold disk substrates in 0.1 mol L^?1 NaOH solutions. Methyl viologen was used as redox mediator, while a platinum microelectrode was employed as the SECM tip. The metal films were electrodeposited ex situ from NaOH solutions containing either bismuth ions only or both bismuth and lead ions. Approach curves and SECM images indicated that the metal films were conductive and locally reactive with oxygen to provide Bi³⁺ and Pb²⁺ ions. The occurrence of the latter chemical reactions was verified by local anodic stripping voltammetry (ASV) at the substrate solution interface by using a mercury‐coated platinum SECM tip. The latter types of measurements allowed also verifying that lead was not uniformly distributed onto the bismuth film electrode substrate. These findings were confirmed by scanning electron microscopy images. The surface heterogeneity produced during the metal deposition process, however, did not affect the analytical performance of the bismuth coated gold electrode in anodic stripping voltammetry for the determination of lead in alkaline media, even in aerated aqueous solutions. Under the latter conditions, stripping peak currents proportional to lead concentration with a satisfactory reproducibility (within 5 % RSD) were obtained.     

Electrochemical and microscopic studies of surface-confined DNA

We have studied the micropatterning and characterization of the organic monolayers using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM), atom force microscopy, and AC impedance, and have determined the electrochemical parameters, i.e., the apparent reaction rate constant (K _f) and the coverage of the electrode surface (θ). CV and SECM experiments demonstrated that the surface of the modified electrode represents an insulating substrate for ferricyanide. Using the high sensitivity of the electron transfer of ferricyanide to the modification of the gold surface with DNA, we selected this reaction as a probe to study the different modification stages at this modified electrode. SECM images obtained from bare, partially modified, and totally modified electrodes showed very good resolution with different topographies or null according to the extent of modification. Based on a comparison with the results of the experiments, a reasonable agreement can be obtained, which means a conjunction of these techniques.     

Investigation of ion-bombarded conducting polymer films by scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) was used to investigate the effect of ion bombardment on thin films of the conducting polymers poly[3-ethoxy-thiophene] (PEOT) and poly[ethylenedioxy-thiophene] (PEDT). Bombardment with Ar⁺-ions converts the topmost 30 nm thick layer to an essentially insulating material. SECM approach curves as well as two dimensional scans prove the existence of regions of different conductivity within the irradiated regions that did not show a significant dependence on ion dosage. PEDT layers patterned by ion bombardment through microscopic masks are investigated as prototypes of miniaturized printed circuit boards that can be formed by galvanic copper deposition onto conducting PEDT. Defects in conducting polymer patterns were analyzed by SECM imaging before any deposition of copper. Appropriate representations of SECM images for the evaluation of this technologically important question are discussed.     

Electrochemistry in deep eutectic solvents

We report the cyclic voltammetry, chronoamperometry, and scanning electrochemical microscopy of ferrocene dissolved in deep eutectic solvents (DES), consisting of choline chloride (ChCl) and either trifluoroacetamide (TFA) or malonic acid as the hydrogen-bond donor. Despite the use of ultramicroelectrodes, which were required due to the modest conductivities of the DES employed, linear diffusion behavior was observed in cyclic voltammetric experiments. The high viscosity of 1:2 ChCl/TFA relative to non-aqueous electrochemical solvents leads to a low diffusion coefficient, 2.7 x 10(-8) cm2 s(-1) for ferrocene in this medium. Because of the difficulties in achieving steady-state conditions, SECM approach curves were tip velocity dependent. Under certain conditions, SECM approach curves to an insulating substrate displayed a positive-feedback response. Satisfactory simulation of this unexpected behavior was obtained by including convection terms into the mass transport equations typically used for SECM theory. The observance of positive-feedback behavior at an insulating substrate can be described in terms of a dimensionless parameter, the Peclet number, which is the ratio of the convective and diffusive timescales. Fitting insulator approach curves of ferrocene in 1:2 ChCl/TFA shows an apparent increase in the diffusion coefficient with increasing tip velocity, which can be explained by DES behaving as a shear thinning non-Newtonian fluid.     

Electrostatic Interaction between Two Dissimilar Spheres with Constant Surface Charge Density

Explicit exact analytic expressions are obtained in the form of infinite series for the potential energy of the electrostatic interaction for the system of two dissimilar hard spheres with constant surface charge density in an electrolyte solution on the basis of the linearized Poisson-Boltzmann equation. The effects of the particle polarization, that is, the internal fields induced within tim interacting spheres, which are found to be of the order of instead of 1/κa (where κ is the Debye-Hückel parameter and a is the sphere radius), are taken into account. As in the case of the interaction at constant surface potential, the zeroth-order approximation to the interaction energy corresponds to the interaction energy that would be obtained if both spheres were ion-penetrable spheres ("soft" spheres) and to that obtained by the linear superposition approximation. The first-order approximation corresponds to the interaction energy that would be obtained if either sphere were a soft sphere, with the other being a hard sphere with constant surface charge density. The first-order correction term can be interpreted as the image interaction between the soft sphere and its image with respect to the hard sphere.     

The dielectric response of a colloidal spheroid

In this article, we present a theory for the dielectric behavior of a colloidal spheroid, based on an improved version of a previously published analytical theory [C. Chassagne, D. Bedeaux, G.J.M. Koper, Physica A 317 (2003) 321–344]. The theory gives the dipolar coefficient of a dielectric spheroid in an electrolyte solution subjected to an oscillating electric field. In the special case of the sphere, this theory is shown to agree rather satisfactorily with the numerical solutions obtained by a code based on DeLacey and White's [E.H.B. DeLacey, L.R. White, J. Chem. Soc. Faraday Trans. 2 77 (1981) 2007] for all zeta potentials, frequencies and κa1 where κ is the inverse of the Debye length and a is the radius of the sphere. Using the form of the analytical solution for a sphere we were able to derive a formula for the dipolar coefficient of a spheroid for all zeta potentials, frequencies and κa1. The expression we find is simpler and has a more general validity than the analytical expression proposed by O'Brien and Ward [R.W. O'Brien, D.N. Ward, J. Colloid Interface Sci. 121 (1988) 402] which is valid for κa1 and zero frequency.     

Self-healing characteristic of praseodymium conversion coating on AZNd Mg alloy studied by scanning electrochemical microscopy

Application of rare earth conversion coatings as a surface treatment for magnesium has been the subject of several studies revealing the potential to act as an effective passivating layer. Herein a mechanistic study is presented on the formation of a rare earth conversion layer based on Pr(NO₃)₃ on AZ80X magnesium alloy in simulated biological (buffered) solution. Scanning electrochemical microscopy (SECM) was used to investigate the insulating properties and degradation behaviour of the Pr conversion layer. The self-healing properties of the conversion layer in the presence of Pr^3 + were also studied using SECM. Results revealed the self-healing characteristic of the Pr conversion film in the presence of active, Pr^3 +, species. The Pr conversion layer provided passivation in the short term by producing an electrochemically inert and insulating layer. SECM results in potentiometric mode elucidated the role of near surface pH in the formation of the conversion coating.     

Imaging the stomatal physiology of somatic embryo-derived peanut leaves by scanning electrochemical microscopy

The stomatal physiology, chlorophyll distribution and photosynthetic activity of somatic embryo (SE)- and seedling-derived peanut plants grown in vitro (test tube-grown) and extra vitrum (soil-grown) are investigated using scanning electrochemical microscopy (SECM). This SECM imaging is performed in two different feedback modes, corresponding to oxygen evolution and chlorophyll distribution. More specifically, the oxygen evolution profiles of the in vitro leaves indicate important differences in leaf anatomy between the SE- and seedling-derived leaves. On the other hand, the chlorophyll distribution images show individual stomata of size ca. 27 ± 5μm. Further studies on senescing (aged) leaves reveal interesting voltammograms that vary widely over the stomatal complexes and the surrounding tissues, probably due to the release of electroactive metabolites during chlorophyll breakdown when the leaves turn yellow. Thus, the present investigation could open up new opportunities for characterizing botanical systems using electroanalytical techniques. In addition, it could provide further insights into various areas of current relevance, including signal transduction, cell fate/differentiation and developmental biology. Schematic representation of SECM imaging used in this investigation. The SECM probe is a Pt UME disk (25 μm diameter) embedded in an insulating glass sheath so that the ratio of the diameter of the death to that of the electrode surface (RG) is 7. RE denotes the reference electrode Ag/AgCl, sat. KCl and CE refers to the counter electrode, a Pt wire. Oxygen evolving from the leaf surface during photosynthesis diffuses into the electrolyte (0.1 M KCl) and gets reduced at the Pt UME, biased to a potential of −0.5 V, at a diffusion-limited rate to produce a change in the tip-current     

The effect of anions on hydrogen chemisorption and oxide formation on Pt in aqueous acids

An approximate treatment shows that the superficial area of a mercury pool overlain by an aqueous electrolyte solution and contained within a cylindrical vessel of radius R (not less than about 12 mm) is given by πR²+2.603 aR+0.26 a², where a is a constant in the range 2.4±0.2 mm.     

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.     

Micro ring–disk electrode probes for scanning electrochemical microscopy

The construction and characterisation of ring–disk (RD) microelectrodes suitable for use in scanning electrochemical microscopy (SECM) is reported. Such RD electrodes are proposed as probes for novel generator–collector SECM experiments. In this case, the interaction of both the reactants and products with the substrate under investigation can be followed simultaneously from a single approach curve to the substrate. Examples of such approach curves to conducting and insulating substrates are given to demonstrate the potential of this new mode of SECM operation.     

Modifying the 5′‐Cap for Click Reactions of Eukaryotic mRNA and To Tune Translation Efficiency in Living Cells

The 5′‐cap is a hallmark of eukaryotic mRNAs and plays fundamental roles in RNA metabolism, ranging from quality control to export and translation. Modifying the 5′‐cap may thus enable modulation of the underlying processes and investigation or tuning of several biological functions. A straightforward approach is presented for the efficient production of a range of N7‐modified caps based on the highly promiscuous methyltransferase Ecm1. We show that these, as well as N²‐modified 5′‐caps, can be used to tune translation of the respective mRNAs both in vitro and in cells. Appropriate modifications allow subsequent bioorthogonal chemistry, as demonstrated by intracellular live‐cell labeling of a target mRNA. The efficient and versatile N7 manipulation of the mRNA cap makes mRNAs amenable to both modulation of their biological function and intracellular labeling, and represents a valuable addition to the chemical biology toolbox.     

SECM study of the pH distribution over Cu samples treated with 2-mercaptobenzothiazole in NaCl solution

This paper reports the effect of treatment with 2-mercaptobenzothiazole (2MBT) (a corrosion inhibitor) on pH distribution over Cu samples monitored in situ using scanning electrochemical microscopy (SECM) in potentiometric mode. Line scans were conducted over two copper wires embedded in insulating resin, one of which had been pre-treated with 2MBT. It was found that the treated Cu sample had a less acidic character than the non-treated sample. Furthermore, the pH above the resin areas is considerably more acidic than that above either Cu wire. This feature is attributed to the consumption of OH⁻ ions on the non-treated Cu wire during the formation of atacamite as a corrosion product, and the tautomeric equilibrium established by the detached 2-mercaptobenzothiazole molecules in the bulk electrolyte. These pH distributions are spatially resolved in the SECM mapping above both Cu wires.     

Investigation of ion-bombarded conducting polymer films by scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) was used to investigate the effect of ion bombardment on thin films of the conducting polymers poly[3-ethoxy-thiophene] (PEOT) and poly[ethylenedioxy-thiophene] (PEDT). Bombardment with Ar+-ions converts the topmost 30 nm thick layer to an essentially insulating material. SECM approach curves as well as two dimensional scans prove the existence of regions of different conductivity within the irradiated regions that did not show a significant dependence on ion dosage. PEDT layers patterned by ion bombardment through microscopic masks are investigated as prototypes of miniaturized printed circuit boards that can be formed by galvanic copper deposition onto conducting PEDT. Defects in conducting polymer patterns were analyzed by SECM imaging before any deposition of copper. Appropriate representations of SECM images for the evaluation of this technologically important question are discussed.     

Surfactant monolayers on a clean mercury surface: Apparatus and experiments

A Langmuir trough for studying monolayers on a mercury surface was constructed usingT. Smith's design. The surfactant (long-chain alkyl-trimethylammonium compounds) in aqueous solution were spread on a clean mercury surface in an atmosphere of helium, and the surface pressure re-areaA and thickness of surface filmd-areaA curves were obtained. The-A curves were characterized by the appearance of multiple inflection points and plateaus, being explained as stepwise dense surface packing of molecules, and the formation of multilayers by film compression with long axes of molecules lying flat on the mercury surface.     

The lateral electric force on an off-axis sphere in a circular cylinder

The electrophoretic velocity of a sphere within a liquid-filled circular cylinder in a direction parallel to the cylinder axis has been studied by Yariv and Brenner (Phys. Fluids 2002, 14, 3354–3357; SIAM J. Appl. Math. 2003, 64, 423–441). We use their analyses of the electric field in order to determine the electrical force on the sphere along the cylinder radius (i.e., perpendicular to its axis) when either the radius of the sphere is small compared to that of the cylinder, or when the radius of the sphere is only slightly smaller than that of the cylinder. In both cases the force acts towards the centreline of the cylinder, and hence this force tends to stabilize electrophoresis of the sphere along the cylinder axis.