Volume 84, Issue 10 - 7. Kurt-Schwabe-Symposium / Winfried Vonau

This work describes the development and application of different lance-based monitoring systems coupled to an integrated multiparameter sensor probe for the spatial investigation of the liquid phase in biotechnological processes. Miniaturized electrochemical sensors are integrated into a housing, which is fixed at the head of a lance, both made of stainless steel. It can be moved inside a reactor in horizontal direction by the lance itself and in vertical direction by a flexible rope, which is connected to the sensor probe. Inserting the sensor unit directly into the liquid phase of a fermenter, e.g. in laminar processes like biogas and beer production, coupled with multiposition measurements allows to obtain space and time resolved data. The monitored parameters are the pH value, the redox potential, the temperature, and the concentration of dissolved carbon dioxide. Wireless data transfer is realized by means of radio technology with the virtual control center RAMOC (Radio Aided Monitoring) of the company innotas GmbH. Results of the real-time determination in the liquid phase in an industrial, continuously operated biogas plant during several weeks are presented as a case study.

In this study, polyelectrolyte-modified field-effect-based electrolyte-insulator-semiconductor (EIS) devices have been used for the label-free electrical detection of double-stranded deoxyribonucleic acid (dsDNA) molecules. The sensor-chip functionalization with a positively charged polyelectrolyte layer provides the possibility of direct adsorptive binding of negatively charged target DNA oligonucleotides onto the SiO 2 -chip surface. EIS sensors can be utilized as a tool to detect surface-charge changes; the electrostatic adsorption of oligonucleotides onto the polyelectrolyte layer leads to a measureable surface-potential change. Signals of 39 mV have been recorded after the incubation with the oligonucleotide solution. Besides the electrochemical experiments, the successful adsorption of dsDNA onto the polyelectrolyte layer has been verified via fluorescence microscopy. The presented results demonstrate that the signal recording of EIS chips, which are modified with a polyelectrolyte layer, can be used as a favorable approach for a fast, cheap and simple detection method for dsDNA.

Electrodes with different microstructures made of platinum and yttria stabilized zirconia (YSZ) were investigated. They were characterized by means of impedance measurements and electrode charging experiments in the temperature range of 450 ℃ to 800 ℃ and in oxygen partial pressures of 0.2 bar down to 10 −5  bar. Samples were characterized before and after an ageing step at 690 ℃ in water vapour containing atmosphere. Microstructural data were determined by a 3D microstructure reconstruction from SEM image stacks. Results indicate that a large platinum surface or a high triple phase boundary (TPB) density both lead to a lower electrode impedance and stabilize the electrode against degradation in humid atmosphere.

In the present study some of the methods developed for the experimental determination of (interfacial or film) stress changes in electrochemical systems containing solid-liquid interfaces are briefly reviewed, as well as the kind and quality of information that can be obtained using these methods are discussed. The “bending beam” (“bending cantilever”, “laser beam deflection”, “wafer curvature”) method and related techniques, the piezoelectric method, the extensometer method, and the method based on the measurement of contact angle are discussed in detail. Special attention has been paid to problems related to the use of optical methods for position sensing.

An automatic pH-measurement directly on-site on the farmland requires robust and reliable sensor systems. The massive antimony oxide electrode offers the possibility to determine the acidity in freshly sampled soils within the required range of pH 4 to pH 7 in a short time and without elaborate preparation steps. Some essential steps of the development of an antimony oxide sensor system applicable for the on-site measuring are presented in detail. The introduced set-up is a promising fundament for the establishment of a rapid pH-determination direct on the field.

The characterization of the redox behavior of analytes is a very important aspect for many applications. Pure electrochemical approaches can provide useful information on electroactive species, but are of limited use regarding the identification of generated species. The hyphenation of electrochemistry and mass spectrometry (EC-MS) is a powerful method to investigate redox systems. In the present work, we show a simple approach to on-line EC-MS based on the application of electrochemical flow cells with implemented disposable electrodes. They are connected to electrospray ionization mass spectrometry (ESI-MS) via fused silica capillary tubing. The modularity of the flow cells offers a high flexibility of experimental setup and settings, so that a fast detection of oxidation or reduction products can be achieved. The usage of disposable electrodes guarantees a high level of quality assurance for EC-MS measurements.

When studying electrochemical processes, one of the most widely used methods of determining the reaction pathway is an electrochemical assay of products using a rotating ring–disk electrode (RRDE). An RRDE tip consists of two electron conducting parts: a centrally located disk and a ring around it. When brought into contact with an electrolyte solution, the disk and the ring both form electrodes, the potentials of which can be independently controlled by a bi-potentiostat. When the tip is rotated, reactants from the solution arrive at the disk where they undergo an electrode reaction (oxidation or reduction) at a given rate, depending on the rotation speed. The formed products leave the disk surface and due to forced convection make their way towards the ring electrode where they can undergo another electrode reaction and can thus be detected. Normally, one applies potentiostatic control to at least one of the electrodes when carrying out an RRDE experiment; albeit the simultaneous potentiodynamic perturbation of the electrodes offers an increased applicability range. This paper presents the construction of a measuring system capable for the “bi-potentiodynamic” perturbation of two working electrodes, and demonstrates the use of such methods in case of a few chosen example systems.