Up-scaling a Sol-Gel Process for the Production of a Multi-Component Xerogel Powder.

A sol-gel process for the synthesis of a multi-component oxide material from the system SiO2-ZrO2-Al2O3underwent optimization and up-scaling. Initially, on a laboratory scale, components including precursors, catalysts, and additives were methodically evaluated to ensure a safe and efficient transition to larger volumes. Subsequently, the equipment for the whole setup of the sol-gel process was strategically selected. Leveraging insights from these optimizations, the process was successfully scaled-up to pilot-scale operation, conducting hydrolysis, condensation reactions, gelation, aging, and drying within a single, integrated conical dryer system for an 80 L batch. A visual test and FTIR spectroscopy were applied for process control and monitoring.

Self-Powered Electrochromic Readout of Potentiometric pH Electrodes.

An absorbance-based colorimetric sensor array that is self-powered by an ion-selective electrode (ISE) in a short-circuited cell is presented. As the cell voltage is maintained at zero, the potential at the ISE serves as the power generator to directly transfer its power to a potential-dependent Prussian blue (PB) film in contact with an electrolyte solution in a separate detection compartment. This allows one to activate the color change of the PB film without the need for an external power supply. The potential of the PB detection element is optimized to change color between 50 and 250 mV (vs Ag/AgCl). Because the potential originates at the ISE, it is proportional to the ion activity in the sample in agreement with the Nernst equation. In this way, a higher cation activity in the sample generates a more positive potential, which enhances the PB absorbance that serves as the analytical signal. A self-powered optical sensor array coupled to poly(vinyl-chloride)-based pH electrodes based on two different ionophores is utilized here as a model. The measuring range is tuned chemically by varying the pH of the inner filling solution of each ISE, giving a measuring range from pH 2 to 10.5. As the optical sensor is driven by a potentiometric probe, the sensor output is independent of solution ionic strength. It is successfully applied for quantitative analysis in unmodified turbid/colored samples that included red wine, coke, coffee, baking soda, and antacid. The colorimetric output correlates well with the reference method, a calibrated pH electrode. Compared to earlier systems where the cell potential is dictated by an external power source, the PB film exhibits excellent reproducibility and a rapid response time of about 44 s.

Optical Sensing with a Potentiometric Sensing Array by Prussian Blue Film Integrated Closed Bipolar Electrodes.

The simultaneous optical readout of a potentiometric sensor array of ion-selective electrodes (ISEs) based on PVC membranes is described here for the first time. The optical array consists of electrochromic Prussian Blue (PB) films in multiple closed ion-selective bipolar electrodes (BPEs), which gives a physical separation between the optical detection and sample compartments. The potential-dependent turnover of PB generates Prussian White (PW). A near-Nernstian response of the PB film is confirmed by colorimetric absorbance experiments as a function of applied potential. In the combined bipolar electrode cell, the overall potential is kept constant with a single potentiostat over the entire array where each PB spot indicates the potential change of an individual connected potentiometric probe. For cation-selective electrodes, the absorbance or blue intensity of the connected PB film is enhanced with increasing target cation activity. The colorimetric absorbance changes are simultaneously followed by a digital camera and analyzed by Mathematica software. A multiple cation-BPE array allows one to achieve simultaneous quantitative analysis of potassium, sodium, and calcium ions, demonstrated here in highly colored fruit juices. Mass transport at the PB thin film is shown not to be rate-limiting. The measuring ranges can be tuned in a wide range by potential control. The PB film exhibits greatly improved reproducibility and stability as compared to previous work with a ferroin redox probe confined in a thin solution layer.

Tunable Optical Sensing with PVC-Membrane-Based Ion-Selective Bipolar Electrodes.

We show here that the response of ion-selective membrane electrodes (ISEs) based on traditional PVC membranes can be directly translated to a colorimetric readout by a closed bipolar electrode (BPE) arrangement. Because the resulting optical response is based on the turnover of the redox probe, ferroin, dissolved in a thin layer compartment, it directly indicates the potential change at the ISE in combination with a reference electrode. This class of probes measures ion activity, analogous to their ISE counterparts. Unlike other ion optodes, the response is also fully tunable over a wide concentration range by the application of an external potential and occurs in a compartment that is physically separate from the sample. To allow for the electrical charge to pass across the ion-selective electrodes, the membranes are doped with inert lipophilic electrolyte, ETH 500, but otherwise have an established composition. The observed response behavior correlates well with theory. A wide range of ion-selective membranes are confirmed to work with this readout principle, demonstrating the detection of potassium, sodium, calcium, and carbonate ions. The corresponding sigmodal calibration curve is used for quantitative analysis in a range of samples including commercial beverages and river and lake samples. The data are successfully correlated with atomic emission spectroscopy and direct potentiometry.

Colorimetric Readout for Potentiometric Sensors with Closed Bipolar Electrodes.

We present here a general strategy to translate potential change at a potentiometric probe into a tunable color readout. It is achieved with a closed bipolar electrode where the ion-selective component is confined to one end of the electrode while color is generated at the opposite pole, allowing one to physically separate the detection compartment from the sample. An electrical potential is imposed across the bipolar electrode by solution contact such that the potentiometric signal change at the sample side modulates the potential at the detection side. This triggers the turnover of a redox indicator in the thin detection layer until a new equilibrium state is established. The approach is demonstrated in separate experiments with a chloride responsive Ag/AgCl element and a liquid membrane based calcium-selective membrane electrode, using the redox indicator ferroin in the detection compartment. The principle can be readily extended to other ion detection materials and optical readout principles.

Electrochemical ion transfer mediated by a lipophilic Os(ii)/Os(iii) dinonyl bipyridyl probe incorporated in thin film membranes.

A new lipophilic dinonyl bipyridyl Os(ii)/Os(iii) complex successfully mediates ion transfer processes across voltammetric thin membranes. An added lipophilic cation-exchanger may impose voltammetric anion or cation transfer waves of Gaussian shape that are reversible and repeatable. The peak potential is found to shift with the ion concentration in agreement with the Nernst equation. The addition of tridodecylmethylammonium nitrate to the polymeric film dramatically reduces the peak separation from 240 mV to 65 mV, and the peak width to a near-theoretical value of 85 mV, which agrees with a surface confined process. It is suggested that the cationic additive serves as a phase transfer catalyst.

Alkalinization of Thin Layer Samples with a Selective Proton Sink Membrane Electrode for Detecting Carbonate by Carbonate-Selective Electrodes.

Potentiometry is known to be sensitive to so-called free ion activity and is a potentially valuable tool in environmental speciation analysis. Here, the direct detection of free and total carbonate is demonstrated by alkalinization of a thin layer sample (∼100 µm), which is electrochemically triggered at a pH responsive membrane placed opposite a carbonate-selective membrane electrode. The concept may serve as a promising future methodology for in situ environmental sensing applications where traditional sampling and pretreatment steps are no longer required. The possibility of increasing the pH of the sample was demonstrated first with a proton selective membrane (pH readout at zero current) placed opposite the thin layer gap. An optimal applied potential (600 mV) for 300 s resulted in a pH increase of 4 units in an artificial sample, with a relative standard deviation (RSD) of ∼2%. The pH probe was subsequently replaced by a solid contact carbonate selective electrode for the determination of carbonate species (4.17 µM) in a sample of 1 mM NaHCO3. Increasing the pH to 12.1 by the electrochemically controlled proton sink allowed one to convert bicarbonate to the detectable carbonate species. Initial bicarbonate concentration (∼1 mM) was obtained as the difference between the converted bicarbonate and the initial carbonate concentration. An initial application of this concept was illustrated by the speciation analysis of an unfiltered sample from the Arve river (12.3 ± 0.2 µM and 22.5 ± 0.3 mM carbonate and bicarbonate, respectively). The values were confirmed by volumetric titration.

Phenytoin speciation with potentiometric and chronopotentiometric ion-selective membrane electrodes.

We report on an electrochemical protocol based on perm-selective membranes to provide valuable information about the speciation of ionizable drugs, with phenytoin as a model example. Membranes containing varying amounts of tetradodecylammonium chloride (TDDA) were read out at zero current (potentiometry) and with applied current techniques (chronopotentiometry). Potentiometry allows one to assess the ionized form of phenytoin (pKa~8.2) that corresponds to a negatively monocharged ion. A careful optimization of the membrane components resulted in a lower limit of detection (~1.6 µM) than previous reports. Once the pH (from 9 to 10) or the concentration of albumin is varied in the sample (from 0 to 30 g L(-1)), the potentiometric signal changes abruptly as a result of reducing/increasing the ionized concentration of phenytoin. Therefore, potentiometry as a single technique is by itself not sufficient to obtain information about the concentration and speciation of the drug in the system. For this reason, a tandem configuration with chronopotentiometry as additional readout principle was used to determine the total and ionized concentration of phenytoin. In samples containing excess albumin the rate-limiting step for the chronopotentiometry readout appears to be the diffusion of ionized phenytoin preceded by comparatively rapid deprotonation and decomplexation reactions. This protocol was applied to measure phenytoin in pharmaceutical tables (100mg per tablet). This tandem approach can likely be extended to more ionizable drugs and may eventually be utilized in view of pharmacological monitoring of drugs during the delivery process.