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

Electrochemical and Optical Sensors for Ion Sensing

The main goal of this thesis has been the development and fabrication of integrated sensors by combining optical redox indicators (ferroin and Prussian blue films) and potentiometric sensing systems into a closed bipolar electrode platform. This approach allows one to overcome some major fundamental limitations of traditional optical sensors. The simultaneous optical detection of an array of potentiometric sensors has been demonstrated using parallel closed bipolar electrodes. Finally, we were able to realize a self-powered optical sensing array based on potentiometric pH electrodes without a use of an external power source where the power required to affect the color change is directly drawn from the sensing probe. In other work, a new lipophilic molecular redox probe is explored as an ion-to-electron transducer in thin layer ion-transfer voltammetry. It was shown to give improved electrochemical reversibility relative to other common transducing materials

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 into 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 if 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 are otherwise of established composition. The observed response behavior correlates well to 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 the 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

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

The simultaneous optical readout of a potentiometricsensor array of ion-selective electrodes (ISEs) based on PVCmembranes is described here for thefirst time. The optical arrayconsists of electrochromic Prussian Blue (PB)films in multiple closedion-selective bipolar electrodes (BPEs), which gives a physicalseparation between the optical detection and sample compartments.The potential-dependent turnover of PB generates Prussian White(PW). A near-Nernstian response of the PBfilm is confirmed bycolorimetric absorbance experiments as a function of applied potential.In the combined bipolar electrode cell, the overall potential is keptconstant with a single potentiostat over the entire array where each PBspot indicates the potential change of an individual connectedpotentiometric probe. For cation-selective electrodes, the absorbance or blue intensity of the connected PBfilm is enhancedwith increasing target cation activity. The colorimetric absorbance changes are simultaneously followed by a digital camera andanalyzed by Mathematica software. A multiple cation-BPE array allows one to achieve simultaneous quantitative analysis ofpotassium, sodium, and calcium ions, demonstrated here in highly colored fruit juices. Mass transport at the PB thinfilm is shownnot to be rate-limiting. The measuring ranges can be tuned in a wide range by potential control. The PBfilm exhibits greatlyimproved reproducibility and stability as compared to previous work with a ferroin redox probe confined in a thin solution layer

Electrochemical ion transfer mediated by a lipophilic Os(<scp>ii</scp>)/Os(<scp>iii</scp>) 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

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 (100 mg 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

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

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 <i>in situ</i> 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