Dual microband electrodes: current distributions and diffusion layer ‘titrations’. Implications for electroanalytical measurements

The simulation of transport to double microband electrodes in generator–collector mode is reported focusing especially on the ‘titration curve’ approach to electroanalysis in which a titrant is electrogenerated from a redox active precursor on the generator electrode and reacts homogeneously with the target analyte. The current on the detector electrode reflects the amount of titrant ‘surviving’ passage between the two electrodes. The form of the titration curve – plots of detector current as a function of generator current – is shown to be highly sensitive to the electrode kinetics of the redox couple driven at the generator electrode. Accordingly the naïve use of such methodology for analysis without accompanying simulation and kinetic analysis is fraught with danger. Use of the conformal mapping approach in combination with the ADI method for investigation of the ‘titration’ current distributions at the double band system gives fast and precise simulation of this and similar problems. Convergence analysis is described which allows for the automatic selection of the simulation grid size so as to obtain a chosen accuracy (for example 1%) of the current for all experimentally meaningful values of the geometrical and physico-chemical parameters of the system to be investigated.     

Characteristics of Subtractive Anodic Stripping Voltammetry of Lead,Cadmium and Thallium at Silver‐Gold Alloy Electrodes

Silver‐gold alloy electrodes have been studied for the purpose of the quantitative determination of heavy metals by subtractive anodic stripping voltammetry, (SASV). The results have been compared with those obtained with the silver and gold electrodes. The 50/50 a/o Ag/Au alloy electrode is the most suitable for quantifying thallium in the presence of lead and cadmium. The separation of its peak from those of lead and cadmium is 200 mV, which is about twice the separation obtained on the pure metal electrodes and is also better than on mercury. The silver electrode is suitable for the simultaneous determination of thallium, lead and cadmium. The peaks of lead and cadmium overlap on the 50/50 alloy. Pure silver or pure gold can be used for simultaneous quantification of these two metals. The use of gold for quantifying lead and cadmium is more limited because the peak potential of cadmium is shifted in the negative direction as its concentration increases and at [Cd²⁺]>200 nM, the two peaks merge. SASV enables correction for background currents and is of utmost importance for obtaining well‐defined peaks. The peaks of lead, cadmium and thallium appear over a relatively narrow potential range (ca. 200 mV) on all the electrodes presented in this work. For this reason, the quantifying of a peak is based on the derivative at the inflection point of only one of its branches (ascending or descending). All SASV measurements were carried out without removal of oxygen.     

Spectroelectrochemical Measurements at Colloidal Au Multilayer Optically Transparent Electrodes

Application of thin colloidal Au multilayers on glass substrates as optically transparent electrodes is reported herein. Evaluation of the electrode by scanning electron microscopy and as an optically transparent thin layer electrode (OTTLE) illustrate its use for spectroelectrochemical applications. A spectroelectrochemical Nernst plot acquired for the oxidation of o‐tolidine yielded a value of E°=0.605 V (vs. Ag/AgCl) and n=1.88. Both of these values are in excellent agreement with previously published values obtained using a traditional Au minigrid OTTLE. The colloidal Au multilayer electrode is expected to offer many advantages over the minigrid OTTLE in specific applications. These advantages include direct control over microscopic surface topography, large electrochemical surface area, applicability to many different electrode geometries, and increased microscopic surface roughness for SERS and SPR applications.     

Recognition of Monohydrogen Anions of Geometrical and Positional Isomers of Dicarboxylic Acids Based on Polymeric Liquid Membranes Incorporating Polyamine Host

Macrocyclic lipophilic polyamines were applied as the sensory elements of polymeric liquid membrane electrodes. These hosts gain the anion receptor functionality upon protons uptake from the aqueous solutions. The electrodes studied were able to distinguish potentiometricaly isomers of monocharged and doublecharged forms of ethylene and benzene dicarboxylic acids. The selectivity of interaction between protonated hosts and anionic guests relays not only on the electrostatic interactions, but on hydrogen bounds formations as well. The influence of lipophilicity of protonated hosts on the potential response generation was discussed.     

Development of an Amperometric Sensor Highly Selective For Dopamine and Analogous Compounds Determination Using Bis(2,2′‐Bipyridil) Copper(II) Chloride Complex

The use of [Cu(bipy)₂]Cl₂?6H₂O as a biomimetic catalyst in the construction of an amperometric sensor for dopamine determination is reported. The sensor was prepared modifying a glassy carbon electrode with a Nafion membrane doped with [Cu(bipy)₂]Cl₂?6H₂O complex. The sensor presented a higher response in 0.25 mol L^?1 phosphate buffer solution (pH 7.0), with an applied potential of ?50 mV (vs. SCE). In the optimized operational conditions, a linear response range between 35 and 240 μmol L^?1, with a sensitivity of 2.02±0.07 nA l μmoL^?1 cm^?2 and detection limit of 8.0 μmol L^?1 were typically observed for the sensor. The response time presented for this sensor was 0.5 s, presenting the same response for at least 40 successive measurements, with good repeatability (3.0%) expressed as relative standard deviation for n=6. The difference of the response between four sensor preparations was 4%. A detailed investigation about the sensor response for other sixteen phenolic compounds and interfering species were carried out. The sensor was applied in the determination of dopamine in pharmaceutical preparation with success.     

The Electrochemically Initiated Reaction of Sulfide with N,N‐Diethyl‐p‐phenylenediamine in Dimethylformamide. Part II: Implications for Sensing Strategies

The electrochemically initiated reaction of p‐phenylenediamines with sulfide in aqueous media is well documented. We now report the adaptation of this chemistry into nonaqueous media. This is critically appraised as a means of detecting sulfide. The electrochemically initiated reaction of N,N‐diethyl‐p‐phenylenediamine with sulfide is shown at both macro‐ and platinum microdisk electrodes with quantitative detection of sulfide produced by means of the enhanced currents observed upon its addition. The linear detection range for sulfide is dependent on the concentration of N,N‐diethyl‐p‐phenylenediamine present with a linear range from 28–3290 μM and a limit of detection of 22 μM achievable. This represents a large increase compared to that found previously in aqueous media and offers the prospect of more ready applications in high temperature systems.     

Sensitive voltammetric determination of gallium in aluminium materials using renewable mercury film silver based electrode

Simple cyclic renewable silver amalgam film electrode (Hg(Ag)FE), applied for the determination of gallium(III) using differential pulse anodic stripping voltammetry (DP ASV), is presented. The effects of various factors such as: preconcentration potential and time, pulse height, step potential and supporting electrolyte composition are optimised. The calibration graph is linear from 5?nM (0.35?µg?L^?1) to 80?nM (5.6?µg?L^?1) for a preconcentration time of 60?s, with correlation coefficient of 0.995. For a Hg(Ag)FE with a surface area of 9.9?mm² the detection limit for a preconcentration time of 120?s is as low as 0.1?µg?L^?1. The repeatability of the method at a concentration level of the analyte as low as 3.5?µg?L^?1, expressed as RSD is 3.2% (n?=?5). The proposed method was successfully applied by studying the synthetic samples and simultaneously recovery of Ga(III) from spiked aluminium samples.     

Tetralactam-modified gold electrodes for amperometric detection of acrylic acid

The presence of toxic acrylamide in a wide range of food products such as potato crisps, French fries or bread has been confirmed by Swedish scientists from Stockholm University. The neurotoxicity and possible carcinogenicity of this compound and its metabolites compel us to control them by quantitative and qualitative assays. Exposing acrylamide to pH extremes results in its hydrolysis to acrylic acid or its salt. In this work, we present the use of gold electrodes coated with self-assembled monolayers (SAMs) containing tetralactam molecules and its precursor as active elements for voltammetric detection of acrylic acid in water solution. The host molecules have been immobilised on the electrode surface by covalent Au–S bond or by embedment method into the thiol layer via hydrophobic and van der Waals interactions. Interactions with analytes were confirmed by Osteryoung square-wave voltammetry.     

Towards an automated, standardized protocol for determination of equilibrium potential of ion-selective electrodes

An automated real-time method for determination of ISE steady state value and response time is developed, following most recent IUPAC recommendations. Specifically, detection of the ‘steady state’ is related to (1) the time derivative of the emf as it reaches a limiting value (ΔE/Δt_limit, e.g., 0.1–1.0 mV min⁻¹) and (2) the duration of time for which the absolute value of the time derivative remains less than this limiting value (stability window, denoted win_st). A suite of representative ISEs, including glass, solid state, and polymer-based electrodes, is examined to determine sensitivity of results to parameterization choice. Measurements taken over a wide range of concentration values and in un-processed samples (i.e., without use of ionic strength adjustment) provide insight into behavior of ISEs in applications where analyte concentrations span a wide range and/or sample pre-processing may not be an option, e.g., use of sensors for in situ environmental sampling. Results show that declared steady state emf is strongly sensitive to variations in ΔE/Δt_limit but relatively unaffected by changes in the stability window when win_st ≥30 s. Linearity of calibration curves produced, quantified by root mean squared error (RMSE) against a linear fit, improves as ΔE/Δt_limit decreases, however the percentage of measurements which reach a declared steady state within the prescribed sample window (∼6.5 min) falls with corresponding decreases in the ΔE/Δt_limit parameter. Response time, defined as the time required to reach declared steady emf, is also a strong function of parameterization. Dependence of response times on sample composition and/or ISE membrane composition and type are also discussed; results for ISEs in samples comprised exclusively of interfering ions are included. In general, limiting emf derivatives of {0.25–0.4 mV min⁻¹} and stability windows of {30–40 s} achieve both good analytical accuracy and compliance with potentially short sampling window requirements. Methodology based on use of these parameters can improve sampling speed and accuracy as well as promote inter-comparison of data and ISE characterizations among research teams.