Thin ion-selective membrane films deposited on solid electrode substrate are useful tools to study ion transfer processes. This is because the experimental conditions may be chosen such that diffusion processes within the membrane and contacting aqueous solution are not rate limiting. In an ideal case, therefore, equilibrium considerations may be used to describe the resulting ion transfer voltammograms. For example, the electrochemical oxidation of an electrically neutral redox molecule in the membrane results in a cationic oxidized form. To preserve electroneutrality, a cation is transferred out of the membrane into solution, freeing the cation-exchanger of the membrane to become the counterion of the oxidized redox molecule. This work describes a model system that agrees well with thermodynamic theory, using the lipophilic (1-dodecyl-1H-1,2,3-triazol-4-yl)ferrocene as redox molecule and a monovalent reference cation for ion transfer. The full peak width at half maximum was found as 0.110 V, in agreement with theory, and with peak current proportional to scan rate supporting thin layer behavior. The charge passed during the voltammetric scan was related to ion-exchanger concentration available for ion extraction as a function of potential. Subtraction of the ion transfer potential using the reference ion from the experimental one for each charge increment gave the potential change for the electrochemical ion-to-electron transducer. In one application, the potential change of the polymeric transducing layer poly(3-octylthiophene) (POT) film upon electrochemical oxidation within the membrane was characterized. A non-linear potential–charge curve was observed, in contrast to earlier assumptions.
This paper presents the very first direct structural evidence for the formation of a 100 +/- 10 A water layer in coated-wire polymeric-membrane ion-selective electrodes (ISEs).
Apart from the optimal real-time electricity price to buy the electricity, the optimal, time dependent, capacity contracted with the DSO is of crucial importance for concerted charging of electric vehicles in a parking garage. The battery management system, on its turn, imposes constraints on the sequence of steps in which power is transmitted. Maximum power in individual charging steps has to vary as a function of the state-of-charge to keep an optimal state-of-health of the battery. Finally, the mobility wishes of the car user, given by the desired departure time and SOC will vary. In the PowerMatchingCity Smart Grid living lab a strategy has been developed to optimize the charging strategies of a collection of cars by using a combination of agent-based optimization, using the PowerMatcher, and constrained, combinatorial optimization. In this article, this solution approach, the algorithms and the configuration are described. Furthermore, the implementation in the PowerMatchingCity virtual power plant configuration with a fleet of 10 vehicles is discussed. First simulation results of constrained optimization for forecasting are analysed.
A new principle for electrochemical enzyme immunoassays is proposed where the enzyme substrate is selectively delivered by an electrochemical excitation pulse to the location where the biomolecular interaction takes place. This is achieved by covalent attachment of the capture antibody on the surface of a polymeric ion-selective membrane, which also serves to monitor the enzyme activity over time by open circuit chronopotentiometry. The membrane biofunctionalization was accomplished by using a click chemistry protocol in addition to N-hydroxysuccinimide ester crosslinking and was characterized using confocal microscopy and electrochemical impedance spectroscopy. To demonstrate the principle, a choline-oxidase labelled antibody served as the detection reporter in a sandwich immunoassay for the determination of human lysozyme in saliva in the range of 10–100 μg mL−1 with a detection limit of 0.7 μg mL−1 (3σ). The enzyme-immunocomplex at the sensing surface catalyzes the oxidation of choline, an ion marker to which the membrane is selective and that is released from the membrane by a pulstrode protocol (galvanostatic pulse of 1 s duration and 10 μA magnitude). The enzyme turnover is interrogated by measuring the open circuit potential of the electrode over time after the release pulse. A model based on diffusion and enzyme kinetics is developed to rationalize the sensing principle and to guide the experiments. The results with this integrated immunosensor compare well to that of a conventional enzyme-linked immunoassay.
Constant potential capacitive readout of ion-selective membranes offers better sensitivity than traditional potentiometry by giving an easily identifiable transient current spike, which can be integrated to give a charge proportional to logarithmic of ion activity. Some concerns of this technique include complex fluidic handling, longer measurement times, baseline drifts and memory effects arising from excess charge accumulation on the capacitor. This work describes an electronic circuit strategy to automate the switching procedure, resulting in rapid and reproducible measurements. In contrast to earlier work, alternating to a reference solution is no longer required for establishing a new current baseline. The open-circuit potential is measured before performing chronoamperometry and its value is applied directly to the sample solution. The intermittent discharging of the capacitor by electronic circuit assures that the potential difference across the capacitor returns again to zero. This electronic capacitive readout system was applied to measure sodium concentration using a 10 μF capacitor to amplify the signal. The precision was found to be improved over direct potentiometry, corresponding to 0.11 mmol L−1 NaCl and 0.13 mmol L−1 NaCl for standard solutions and pooled serum sample, respectively. Using the automated electronic system for ion measurement makes the system more robust while resulting in fast and reliable quantitative measurements.
Coulometry belongs to one of the few known calibration-free techniques and is therefore highly attractive for chemical analysis. Titrations performed by the coulometric generation of reactants is a well-known approach in electrochemistry, but suffers from limited selectivity and is therefore not generally suited for samples of varying or unknown composition. Here, the selective coulometric release of ionic reagents from ion-selective polymeric membrane materials ordinarily used for the fabrication of ion-selective electrodes is described. The selectivity of such membranes can be tuned to a significant extent by the type and concentration of ionophore and lipophilic ion-exchanger and is today well understood. An anodic current of fixed magnitude and duration may be imposed across such a membrane to release a defined quantity of ions with high selectivity and precision. Since the applied current relates to a defined ion flux, a variety of non-redox active ions may be accurately released with this technique. In this work, the released titrant's activity was measured with a second ionophore-based ion-selective electrode and corresponded well with expected dosage levels on the basis of Faraday's law of electrolysis. Initial examples of coulometric titrations explored here include the release of calcium ions for complexometric titrations, including back titrations, and the release of barium ions to determine sulfate.
We report on a paper-based analytical device (PAD) for the exhaustive, and therefore absolute, determination of halides in a range of diverse water samples and food supplements. A mixture of chloride, bromide, and iodide ions is assessed in a wide range of concentrations, specifically, from 10(-4.8) to 0.1 M for bromide and iodide and from 10(-4.5) to 0.6 M for chloride, with a limit of detection of 10(-5) M. As a result of a careful optimization of the electrochemical cell, a thin layer made of cellulose paper (75-μm thickness), a cation-exchange Donnan exclusion membrane (FKL), and a silver-foil working electrode were selected as optimum materials. Cyclic voltammetry (from 0 to 0.8 V) was chosen as the interrogation technique to impose the exhaustive oxidative plating and re-reduction of halides on the silver element, accompanied by outward and inward counterion fluxes. The scan rate plays an important role in the ability of the technique to resolve mixtures of ions. Moderate scan rates (10 mV s(-1)) provide a suitable compromise between sensitivity, limit of detection, and resolution. This paper-based microfluidic device is extremely simple in terms of manipulation, cost, and contamination risk. Paper is an excellent basis for the establishment of a confined thin aqueous layer, the construction of disposable halide sensors, and portability for measuring outside the controlled laboratory environment. A discussion of the relevant analytical characteristics is presented herein, followed by a demonstration of halide assessment in water samples (sea, tap, river, and mineral waters) and food supplements enriched with iodide and chloride as early examples.
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