Ulrich Guth

Recent developments in electrochemical sensor application and technology—a review

The review describes different developments of electrochemical sensors, which are used for measurements in a broad range of temperatures and concentrations of gases and dissolved ions in situ. The current approaches in sensor technology are targeted on miniaturization of electrochemical cells, application of multi-sensor setups, extensions of sensor application respecting pressure, temperature and aggressive media and on the use of dynamic methods.

Selectivity of HC-sensitive electrode materials for mixed potential gas sensors

Abstract In potentiometric zirconia-based gas sensors, electrodes with medium catalytic activity show a high sensitivity for hydrocarbons (HCs) when the measurements take place in non-equilibrated oxygen containing gas mixtures at temperatures ≤800 °C. This behaviour, explained by mixed potential theory, depends strongly on preparation and particularly on measuring conditions. To learn more about the processes at the electrode surface and their influence on the potential behaviour of the electrodes, we investigated composite electrodes Au-oxide/YSZ with Ga2O3, In2O3 and Nb2O5 as oxides and 80 mass% Au and perovskite electrodes made of La1−xSrxCr1−yGayO3−δ in N2-diluted gas mixtures containing O2 and different combustibles like C3H6, C3H8, CO and C7H8. As compared with perovskite electrodes, gold composite electrodes using Nb2O5 as oxide show enhanced sensitivities especially to C7H8. The temperature dependence of the catalytic activity for the HC combustion correlates very well with the temperature/voltage response.

Determination of dissolved CO2 concentration and CO2 production rate of mammalian cell suspension culture based on off-gas measurement

The determination of dissolved CO(2) and HCO(3)(-) concentrations as well as the carbon dioxide production rate in mammalian cell suspension culture is attracting more and more attention since the effects on major cell properties, such as cell growth rate, product quality/production rate, intracellular pH and apoptosis, have been revealed. But the determination of these parameters by gas analysis is complicated by the solution/dissolution of carbon dioxide in the culture medium. This means that the carbon dioxide transfer rate (CTR; which can easily be calculated from off-gas measurement) is not necessarily equal to carbon dioxide production rate (CPR). In this paper, a mathematical method to utilize off-gas measurement and culture pH for cell suspension culture is presented. The method takes pH changes, buffer and medium characteristics that effect CO(2) mass transfer into account. These calculations, based on a profound set of equations, allow the determination of the respiratory activity of the cells, as well as the determination of dissolved CO(2), HCO(3)(-) and total dissolved carbonate. The method is illustrated by application to experimental data. The calculated dissolved CO(2) concentrations are compared with measurements from an electrochemical CO(2) probe.

The measurement of dissolved and gaseous carbon dioxide concentration

In this review the basic principles of carbon dioxide sensors and their manifold applications in environmental control, biotechnology, biology, medicine and food industry are reported. Electrochemical CO2 sensors based on the Severinghaus principle and solid electrolyte sensors operating at high temperatures have been manufactured and widely applied already for a long time. Besides these, nowadays infrared, non-dispersive infrared and acoustic CO2 sensors, which use physical measuring methods, are being increasingly used in some fields of application. The advantages and drawbacks of the different sensor technologies are outlined. Electrochemical sensors for the CO2 measurement in aqueous media are pointed out in more detail because of their simple setup and the resulting low costs. A detailed knowledge of the basic detection principles and the windows for their applications is necessary to find an appropriate decision on the technology to be applied for measuring dissolved CO2. In particular the pH value and the composition of the analyte matrix exert important influence on the results of the measurements.

Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La1−aCaaCr0.2Ti0.8O3−δ

Abstract A series of perovskite-type compounds La 1− a Ca a Cr 0.2 Ti 0.8 O 3− δ ( a =0.6–1.0) was synthesized by a ceramic technique in air (final heating 1350 °C). For 0.4 a R- 3 c , and for a >0.6 as orthorhombic in space group Pbnm . The lattice constants for a >0.8 decrease noticeably with increasing Ca content and for a 2 at 20–1400 °C, and in 5% H 2 /Ar ( p H 2 O/ p H 2 =0.01) at 850–1000 °C. Oxygen stoichiometries and electrical conductivities of the solid solutions with a =0.6 to 1.0 were investigated. Increasing Ca-contents decrease the stability of the oxides in respect to the thermal dissociation with oxygen release. At 900 °C within the oxygen partial pressure region 1×10 −15 to 0.21×10 5 Pa the compounds with a >0.8 are p-type and with a 0.2 Ca 0.8 Cr 0.2 Ti 0.8 O 3− δ changes its conductivity type from n-type at p O 2 −10 Pa to p-type in more oxidizing atmosphere. For the compound with a =0.6 in 5% H 2 /Ar flow at 1000 °C, a maximum conductivity of about 50 S/cm has been observed. In air the composition with a =0.95 has the highest conductivity of about 1 S/cm. In an oxidizing environment the maximal conductivities correspond to the ratio [Cr 3+ ]/[Cr 4+ ]=1 (corresponding to an oxygen stoichiometry near 3.0), and in reducing environment to the ratio [Ti 3+ ]/[Ti 4+ ]=1 (corresponding to an oxygen stoichiometry near 2.95).

Gold-composite electrodes for hydrocarbon sensors based on YSZ solid electrolyte

In potentiometric zirconia based sensors gold electrodes show a high sensitivity for hydrocarbons (HCs) when the measurements are carried out in non equilibrated oxygen containing gas mixtures at temperatures <700 °C. This behaviour explained by mixed potential theory is not stable and depends strongly on preparation and particularly on measuring conditions. To modify the electrode behaviour composites consisting of gold and gallium oxide were investigated. Gold pastes with different amount of Ga2O3 were prepared and screen printed on YSZ pellets. After sintering at defined temperatures between 900 and 950 °C the cells were tested regarding the electrode behaviour in a C3H6, O2 gas mixture using a platinum air reference electrode. These composite electrodes show as compared with pure gold an enhanced sensitivity at low propylene concentrations and a time-independent characteristic at high concentrations of C3H6. The optimal composition is found to be at 20 mass-% Ga2O3. This electrode can be treated in reducing gases at temperatures 850 °C without changing its characteristics.

Synthesis, crystal structure, and transport properties of La1−xCaxCr0.5Ti0.5O3−δ

A series of perovskite-type compounds La1−xCaxCr0.5Ti0.5O3−δ (x=0…1.0) was synthesized by a ceramic technique in air (final heating 1350 °C). The crystal structures of the compounds were characterised at room temperature for 0.0 0.2 as orthorhombic in space group Pbnm. The lattice constants with increasing Ca contents for x>0.5 decrease noticeably and for x<0.5 remain nearly constant. All compositions were stable at pO2=21,000 to 6×10−16 Pa at 850…1000 °C. Oxygen stoichiometries and electrical conductivities of the solid solutions were investigated. The stability of the oxides against thermal dissociation evaluated by the oxygen release decreases with increasing Ca-contents. The compounds with x≥0.6 are p-type and with x<0.4 n-type semiconductors at 900 °C and oxygen partial pressures between 1×10−15 and 0.21×105 Pa. La0.5Ca0.5Cr0.5Ti0.5O3−δ and La0.6Ca0.4Cr0.5Ti0.5O3−δ change their conductivities from n-type in the range pO2<10−10 Pa and pO2<10−5 Pa, respectively, to p-type in more oxidizing atmosphere. For the compound with x=0.5 in Ar/H2/H2O gas flow (pH2O/pH2=0.53) at 900 °C a maximum conductivity of about 1 S/cm has been observed. In air the composition with x=0.75 has highest conductivity of about 40 S/cm. Partial substitution of calcium by lanthanum leads to a decrease of the average thermal expansion coefficients (TEC) from 13.1×10−6 K−1 (CaCr0.5Ti0.5O3) to 10.5×10−6 K−1 (La0.25Ca0.75Cr0.5Ti0.5O3) and 9.5×10−6 K−1 (La0.5Ca0.5Cr0.5Ti0.5O3). No simple correlations between catalytic activity, electrical conductivity, and crystal structure parameters were found.

Ionic and electronic conduction in stoichiometric and sub-stoichiometric perovskites

Abstract A-site stoichiometric and sub-stoichiometric (La,Sr) 1− y (Ga,Cr)O 3− δ perovskites, with y =0 or 0.05 and variable Cr content, were prepared by the ceramic route. Structural characterisation was carried out by X-ray diffraction (XRD). Electrical conductivity of samples was determined by impedance spectroscopy and dc measurements, as a function of temperature and oxygen partial pressure, in order to study the role of composition on the electrical properties. Cr content affects the activation energy of the electrical conductivity, and a transition from dominant ionic conduction to electronic conduction is observed. p-type conductivity is believed to dominate in air for materials with Cr content equal or higher than 20%. A-site sub-stoichiometry had no effect on improving the electrical properties.

Investigation of corrosion phenomena on chemical microsensors

Compared to normally sized sensors, chemical microsensors are exposed to a higher corrosion risk. Corrosive damage can be caused by both the direct attack of the measuring medium on the chemically sensitive layer and the corrosion of the substrate materials or the electrical contacts due to absorption or penetration of moisture through the encapsulating material. Corrosion phenomena on chemical sensors have been investigated by using electrochemical methods like the measurement of electrode potentials and sensor output signals or the measurement of the electrochemical noise, by resistance and impedance measurements as well as by gravimetric methods and by microscopic or REM observation of the specimen under test. In the paper, results of corrosion measurements on pH glass electrodes, ISFET pH sensors, semiconductor-based miniaturised oxygen sensors and zirconia-based potentiometric gas sensors by utilising the above listed methods are presented and discussed.

Protein folding forces.

We investigate the average inter-residue folding forces derived from mutational data of the 15 proteins: barstar, barnase, chymotrypsin inhibitor 2 (CI2), Src SH3 domain, spectrin R16 domain, Arc repressor, apo-azurin, cold shock protein B (cspB), C-terminal domain of ribosomal protein L9 (CTL9), FKBP12, alpha-lactalbumin, colicin E7 immunity protein 7 (IM7), colicin E9 immunity protein 9 (IM9), spectrin R17 domain, and ubiquitin. The residue-specific contributions to folding in most of the 15 protein molecules are highly non-uniformly distributed and are typically about 1piconewton (pN) per interaction. The strongest folding forces often occur in some of the helices and strands of folding nuclei which suggests that folding nucleation-condensation is partially directed by formation of some secondary structure interactions. The correlation of the energy changes of mutants with inter-residue contact maps of the protein molecules provides a higher resolution than assigning the mutant data to certain positions in the polypeptide strand alone. In contrast to previous Phi-value analysis, we now can partially resolve folding motions. Compaction of at least one alpha-helix along its axis mediated by internal hydrogen bonds and stabilized by diffuse tertiary structure interactions appears to be one important molecular event during early folding in barstar, CI2, spectrin R16 domain, Arc repressor, alpha-lactalbumin, IM7, IM9, and spectrin R17 domain. A lateral movement of at least two strands neighbored in sequence towards each other appears to be involved in early folding of the SH3 domain, cspB, CTL9, and FKBP12.