Calibration-Free Analysis with Chronoamperometry at Microelectrodes.

Analytical methods are crucial for monitoring and assessing the concentration of important chemicals, and there is now a growing demand for methodologies that allow miniaturization, require lower sample volumes, and enable real-time analysis in the field. Most electroanalytical techniques depend on calibrations or standards, and this has several limitations, ranging from matrix interference, to stability problems, time required, cost and waste. Therefore, strategies that do not require standards or calibration curves greatly interest the analytical chemistry community.

Film before aggregates: an GISAXS study on electrochemically assisted surfactant assembly.

The process of electrochemically assisted surfactant assembly was followed in real time by grazing incidence small angle X-ray scattering with the aim to deconvolute the formation of mesoporous silica film and unwanted porous particles. The X-ray technique proved to be useful for the characterisation of this process, as it takes place at a very dynamic, solid/liquid interface. This paper shows the electrochemically driven onset and evolution of silica/surfactant structures.

Spatio-temporal detachment of homogeneous electron transfer in controlling selectivity in mediated organic electrosynthesis.

In electrosynthesis, electron transfer (ET) mediators are normally chosen such that they are more easily reduced (or oxidised) than the substrate for cathodic (or anodic) processes; setting the electrode potential to the mediator therefore ensures selective heterogeneous ET with the mediator at the electrode, rather than the substrate. The current work investigates the opposite, and counter intuitive, situation for a successful mediated electroreductive process where the mediator (phenanthrene) has a reduction potential that is negative to that of the substrate, and the cathode potential is set negative to both ( < M < ). Simulations reveal a complex interplay between mass transport, the relative concentrations of the mediator and substrate as well as the heterogeneous and homogeneous rate constants for multiple steps, which under suitable conditions, leads to separation of the homogeneous chemistry in a reaction layer detached from the electrode.

Cathodic Radical Cyclisation of Aryl Halides Using a Strongly-Reducing Catalytic Mediator in Flow.

Electro-reductive radical cyclisation of aryl halides affords the corresponding hetero- and carbo-cycles in an undivided flow reactor equipped with steel and carbon electrodes using an organic mediator. A dissolving metal anode is not needed, and the mediator can be employed in a sub-stoichiometric amount (0.05 equiv), increasing the practical utility of cathodic radical cyclisation.

The in situ electrochemical detection of microbubble oscillations during motion through a channel.

Bubble oscillation has many applications, from driving local fluid motion to cleaning. However, in order to exploit their action, a full understanding of this motion, particularly in confined spaces (such as crevices etc. which are important in ultrasonic decontamination) is important.

An Analytical Differential Resistance Pulse System Relying on a Time Shift Signal Analysis-Applications in Coulter Counting.

Improving the sensitivity and ultimately the range of particle sizes that can be detected with a single pore extends the versatility of the Coulter counting technique. Here, to enable a pore to have greater sensitivity, we have developed and tested a novel differential resistive pulse sensing (DiS) system for sizing particles. To do this, the response was generated through a time shift approach utilizing a "self-servoing regime" to enable the final signal to operate with a zero background in the absence of particle translocation.

Sampled-current voltammetry at microdisk electrodes: kinetic information from pseudo steady state voltammograms.

In sampled-current voltammetry (SCV), current transients acquired after stepping the potential along the redox wave of interest are sampled at a fixed time to produce a sigmoidal current-potential curve akin to a pseudo steady state voltammogram. Repeating the sampling for different times yields a family of sampled-current voltammograms, one for each time scale. The concept has been used to describe the current-time-potential relationship at planar electrodes but rarely employed as an electroanalytical method except in normal pulse voltammetry where the chronoamperograms are sampled once to produce a single voltammogram.

Nanostructured Pd hydride microelectrodes: in situ monitoring of pH variations in a porous medium.

In this study we report the exceptional potentiometric properties of pH microprobes made with nanostructured palladium hydride microelectrodes and demonstrate their application by monitoring pH variations resulting from a reaction confined in a porous medium. Their potentiometric response was found to be reproducible and stable over several hours but primarily Nernstian over a remarkably wide pH range, including alkaline conditions up to pH 14. Continuous operation was demonstrated by reloading hydrogen at regular intervals to maintain the correct hydride composition thereby alleviating the need for calibration.

Scanning electrochemical microscopy: using the potentiometric mode of SECM to study the mixed potential arising from two independent redox processes.

This study demonstrates how the potentiometric mode of the scanning electrochemical microscope (SECM) can be used to sensitively probe and alter the mixed potential due to two independent redox processes provided that the transport of one of the species involved is controlled by diffusion. This is illustrated with the discharge of hydrogen from nanostructured Pd hydride films deposited on the SECM tip. In deareated buffered solutions the open circuit potential of the PdH in equilibrium between its β and α phases (OCP(β→α)) does not depend on the tip-substrate distance while in aerated conditions it is found to be controlled by hindered diffusion of oxygen.

Electrochemical current-sensing atomic force microscopy in conductive solutions.

Insulated atomic force microscopy probes carrying gold conductive tips were fabricated and employed as bifunctional force and current sensors in electrolyte solutions under electrochemical potential control. The application of the probes for current-sensing imaging, force and current-distance spectroscopy as well as scanning electrochemical microscopy experiments was demonstrated.

Gold-gold junction electrodes:the disconnection method.

The formation of gold-gold junction electrodes for application in electroanalysis is described here based on electro-deposition from a non-cyanide gold plating bath. Converging growth of two hemispherical gold deposits on two adjacent platinum microelectrodes (both 100 µm diameter in glass, ca. 45 µm gap) followed by careful etching in aqueous chloride solution was employed.

Atomic force microscopy-scanning electrochemical microscopy: influence of tip geometry and insulation defects on diffusion controlled currents at conical electrodes.

Numerical simulations were performed to predict the amperometric response of conical electrodes used as atomic force microscopy-scanning electrochemical microscopy (AFM-SECM) probes. A simple general expression was derived which predicts their steady state limiting current as a function of their insulation sheath thickness and cone aspect ratio. Simulated currents were successfully compared with experimental currents.

Scanning electrochemical microscopy: approach curves for sphere-cap scanning electrochemical microscopy tips.

The parameters of functions used to predict diffusion-controlled scanning electrochemical microscopy approach curves under positive and negative (hindered diffusion) feedback for sphere-cap tips are reported. These functions were obtained by fitting approach curves simulated with an error-bounded adaptive finite element algorithm. Several geometries corresponding to different sphere-cap dimensions were considered including the effect of the tip insulating sheath.

Fabrication and characterization of nanostructured Pd hydride pH microelectrodes.

Novel pH microsensors were made by electrodepositing mesoporous Pd films onto Pt microdisks, electrochemically loading the films with hydrogen to form the alpha+beta Pd hydride phase, and then switching to the potentiometric mode to monitor pH. To create a nanostructure, the films were deposited within a molecular template formed by the self-assembly of surfactant molecules, a technique known as true liquid crystal templating. The films retain the micrometer size of the Pt microdisk but offer electroactive areas up to 900 times larger.

Mesoporous palladium-the surface electrochemistry of palladium in aqueous sodium hydroxide and the cathodic reduction of nitrite.

The voltammetry of nanostructured palladium layers electrodeposited from a hexagonal liquid crystal phase onto platinum microdiscs show well defined peaks for the adsorption/desorption of hydrogen and surface oxidation/reduction in 2 M NaOH. These peaks are more clearly resolved than at smooth palladium and reveal the complications associated with hydrogen adsorption/desorption on palladium in aqueous alkaline solutions. The reduction of nitrite at the nanostructured palladium is also reported and it is shown that it occurs via a mechanism involving a chemical reaction between adsorbed hydrogen and adsorbed nitrite ion.

Steady-state voltammetry of hydroxide ion oxidation in aqueous solutions containing ammonia.

An oxidation process observed in dilute aqueous solutions of ammonia was investigated under steady-state conditions with gold microelectrodes with radii in the range 2.5-30 microm. Over the ammonia concentration range 0.

Oxygen as redox mediator in scanning electrochemical microscopy. Application to the study of localised acid attack of marble.

Oxygen from air-saturated aqueous solutions was employed as redox mediator in SECM experiments. Accurate approach curves under negative-feedback conditions were obtained using platinum and gold microelectrodes. Imaging experiments were also carried out, using a 2.

Detection of hydrogen peroxide at mesoporous platinum microelectrodes.

Mesoporous (H(I)-ePt) platinum microelectrodes electrodeposited from the hexagonal (H(I)) lyotropic liquid crystalline phase are shown to be excellent amperometric sensors for the detection of hydrogen peroxide over a wide range of concentrations. Good reproducibility, high precision, and accuracy of measurements are demonstrated. Mesoporous microelectrodes retain the high rates of mass transport typical of conventional microelectrodes, and their high real surface area greatly enhances their catalytic activity.