Study of metabolism and identification of productive regions in filamentous fungi via spatially resolved time-of-flight secondary ion mass spectrometry.

Filamentous fungi are well-established production hosts that feature a strong interconnection between morphology, physiology, and productivity. For penicillin production in Penicillium chrysogenum, industrial processes frequently favor a pellet morphology comprising compact hyphal agglomerates. Inherently these tightly packed entanglements lead to inactive, degrading sections within the pellet's core because of limitations. Optimal process design requires detailed knowledge of the nature of the limitations and localization of productive zones in the biomass, which is generally obtainable through modeling and complex analytical methods such as oxygen microelectrode and histological investigations. Methods that combine physiological and morphological insight are crucial yet scarce for filamentous fungi. In this study, we used time-of-flight secondary ion mass spectrometry in combination with oxygen and glucose tracer substrates, requiring little effort for sample preparation and measurement. Our method is capable of analyzing oxygen and substrate uptake in various morphological structures by the use of 18O as a tracer. In parallel, we can assess productive biomass regions through identification of penicillin mass fragments to simultaneously study oxygen diffusion, substrate incorporation, and productive biomass sections.

Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide.

Perovskite oxide surfaces catalyze oxygen exchange reactions that are crucial for fuel cells, electrolyzers, and thermochemical fuel synthesis. Here, by bridging the gap between surface analysis with atomic resolution and oxygen exchange kinetics measurements, we demonstrate how the exact surface atomic structure can determine the reactivity for oxygen exchange reactions on a model perovskite oxide. Two precisely controlled surface reconstructions with (4 × 1) and (2 × 5) symmetry on 0.5 wt.% Nb-doped SrTiO3(110) were subjected to isotopically labeled oxygen exchange at 450 °C. The oxygen incorporation rate is three times higher on the (4 × 1) surface phase compared to the (2 × 5). Common models of surface reactivity based on the availability of oxygen vacancies or on the ease of electron transfer cannot account for this difference. We propose a structure-driven oxygen exchange mechanism, relying on the flexibility of the surface coordination polyhedra that transform upon dissociation of oxygen molecules.

Dislocations Accelerate Oxygen Ion Diffusion in La0.8Sr0.2MnO3 Epitaxial Thin Films.

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO3 (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO3 and SrTiO3 substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO3. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO3. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk.

Image-Based Chemical Structure Determination.

Chemical imaging is a powerful tool for understanding the chemical composition and nature of heterogeneous samples. Recent developments in elemental, vibrational, and mass-spectrometric chemical imaging with high spatial resolution (50-200 nm) and reasonable timescale (a few hours) are capable of providing complementary chemical information about various samples. However, a single technique is insufficient to provide a comprehensive understanding of chemically complex materials. For bulk samples, the combination of different analytical methods and the application of statistical methods for extracting correlated information across different techniques is a well-established and powerful concept. However, combined multivariate analytics of chemical images obtained via different imaging techniques is still in its infancy, hampered by a lack of analytical methodologies for data fusion and analysis. This study demonstrates the application of multivariate statistics to chemical images taken from the same sample via various methods to assist in chemical structure determination.

Large O2 Cluster Ions as Sputter Beam for ToF-SIMS Depth Profiling of Alkali Metals in Thin SiO2 Films.

A sputter beam, consisting of large O2 clusters, was used to record depth profiles of alkali metal ions (Me+) within thin SiO2 layers. The O2 gas cluster ion beam (O2-GCIB) exhibits an erosion rate comparable to the frequently used O2+ projectiles. However, because of its high sputter yield the necessary beam current is considerably lower (factor 50), resulting in a decreased amount of excess charges at the SiO2 surface. Hence, a reduced electric field is obtained within the remaining dielectric layer. This drastically mitigates the Me+ migration artifact, commonly observed in depth profiles of various dielectric materials, if analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) in dual beam mode. It is shown, that the application of O2-GCIB results in a negligible residual ion migration for Na+ and K+. This enables artifact-free depth profiling with high sensitivity and low operational effort. Furthermore, insight into the migration behavior of Me+ during O2+ sputtering is given by switching the sputter beam from O2+ to O2 clusters and vice versa. K+ is found to be transported through the SiO2 layer only within the proceeding sputter front. For Na+ a steadily increasing fraction is observed, which migrates through the unaffected SiO2 layer toward the adjacent Si/SiO2 interface.

The Chemical Capacitance as a Fingerprint of Defect Chemistry in Mixed Conducting Oxides.

The oxygen stoichiometry of mixed conducting oxides depends on the oxygen chemical potential and thus on the oxygen partial pressure in the gas phase. Also voltages may change the local oxygen stoichiometry and the amount to which such changes take place is quantified by the chemical capacitance of the sample. Impedance spectroscopy can be used to probe this chemical capacitance. Impedance measurements on different oxides ((La,Sr)FeO3-δ = LSF, Sr(Ti,Fe)O3-δ = STF, and Pb(Zr,Ti)O3 = PZT) are presented, and demonstrate how the chemical capacitance may affect impedance spectra in different types of electrochemical cells. A quantitative analysis of the spectra is based on generalized equivalent circuits developed for mixed conducting oxides by J. Jamnik and J. Maier. It is discussed how defect chemical information can be deduced from the chemical capacitance.

Water-Induced Decoupling of Tracer and Electrochemical Oxygen Exchange Kinetics on Mixed Conducting Electrodes.

Isotope exchange depth profiling and electrochemical impedance spectroscopy are usually regarded as complementary tools for measuring the surface oxygen exchange activity of mixed conducting oxides, for example used in solid oxide fuel cell (SOFC) electrodes. Only very few studies compared electrical (k(q)) and tracer (k*) exchange coefficients of solid-gas interfaces measured under identical conditions. The 1:1 correlation between k(q) and k* often made is thus more an assumption than experimentally verified. In this study it is shown that the measured rates of electrical and tracer exchange of oxygen may strongly differ. Simultaneous acquisition of k(q) and k* on La0.6Sr0.4FeO3-δ and SrTi0.3Fe0.7O3-δ thin film electrodes revealed that k* > 100 k(q) in humid oxidizing ((16)O2 + H2(18)O) and humid reducing (H2 + H2(18)O) atmospheres. These results are explained by fast water adsorption and dissociation on surface oxygen vacancies, forming two surface hydroxyl groups. Hence, interpreting experimentally determined k* values in terms of electrochemically relevant oxygen exchange is not straightforward.

The Sulphur Poisoning Behaviour of Gadolinia Doped Ceria Model Systems in Reducing Atmospheres.

An array of analytical methods including surface area determination by gas adsorption using the Brunauer, Emmett, Teller (BET) method, combustion analysis, XRD, ToF-SIMS, TEM and impedance spectroscopy has been used to investigate the interaction of gadolinia doped ceria (GDC) with hydrogen sulphide containing reducing atmospheres. It is shown that sulphur is incorporated into the GDC bulk and might lead to phase changes. Additionally, high concentrations of silicon are found on the surface of model composite microelectrodes. Based on these data, a model is proposed to explain the multi-facetted electrochemical degradation behaviour encountered during long term electrochemical measurements. While electrochemical bulk properties of GDC stay largely unaffected, the surface polarisation resistance is dramatically changed, due to silicon segregation and reaction with adsorbed sulphur.

The Effect of Acceptor and Donor Doping on Oxygen Vacancy Concentrations in Lead Zirconate Titanate (PZT).

The different properties of acceptor-doped (hard) and donor-doped (soft) lead zirconate titanate (PZT) ceramics are often attributed to different amounts of oxygen vacancies introduced by the dopant. Acceptor doping is believed to cause high oxygen vacancy concentrations, while donors are expected to strongly suppress their amount. In this study, La3+ donor-doped, Fe3+ acceptor-doped and La3+/Fe3+-co-doped PZT samples were investigated by oxygen tracer exchange and electrochemical impedance spectroscopy in order to analyse the effect of doping on oxygen vacancy concentrations. Relative changes in the tracer diffusion coefficients for different doping and quantitative relations between defect concentrations allowed estimates of oxygen vacancy concentrations. Donor doping does not completely suppress the formation of oxygen vacancies; rather, it concentrates them in the grain boundary region. Acceptor doping enhances the amount of oxygen vacancies but estimates suggest that bulk concentrations are still in the ppm range, even for 1% acceptor doping. Trapped holes might thus considerably contribute to the charge balancing of the acceptor dopants. This could also be of relevance in understanding the properties of hard and soft PZT.

Modified-Atmospheric Pressure-Matrix Assisted Laser Desorption/Ionization Identification of Friction Modifier Additives Oleamide and Ethoxylated Tallow Amines on Varied Metal Target Materials and Tribologically Stressed Steel Surfaces.

For many tasks in failure and damage analysis of surfaces deteriorated in heavy tribological contact, the detailed characterization of used lubricants and their additives is essential. The objective of the presented work is to establish accessibility of tribostressed surfaces for direct characterization via modified atmospheric pressure-matrix assisted laser desorption/ionization-mass spectrometry (m-AP-MALDI-MS). Special target holders were constructed to allow target samples of differing shape and form to fit into the desorption/ionization chamber. The best results of desorption and ionization on different target materials and varying roughnesses were achieved on smooth surfaces with low matrix/substrate interaction. M-AP-MALDI characterization of tribologically stressed steel surfaces after pin-on-disc sliding wear tests (SRV-tribotests) yielded positive identification of used friction modifier additives. Further structure elucidation by electrospray ionization mass spectrometry (ESI-MS) and measurements of worn surfaces by time-of-flight-secondary ion mass spectrometry (TOF-SIMS) accompanied findings about additive behavior and deterioration during tribological contact. Using m-AP-MALDI for direct offline examinations of worn surfaces may set up a quick method for determination of additives used for lubrication and general characterization of a tribological system.

Fast oxygen exchange and diffusion kinetics of grain boundaries in Sr-doped LaMnO3 thin films.

In this study, the contribution of grain boundaries to the oxygen reduction and diffusion kinetics of La0.8Sr0.2MnO3 (LSM) thin films is investigated. Polycrystalline LSM thin films with columnar grains of different grain sizes as well as epitaxial thin films were prepared by pulsed laser deposition. (18)O tracer exchange experiments were performed at temperatures from 570 °C to 810 °C and subsequently analyzed by secondary ion mass spectrometry (SIMS). The isotope concentration depth profiles of polycrystalline films clearly indicate contributions from diffusion and surface exchange in grains as well as in grain boundaries. Measured depth profiles were analyzed by finite element modeling and revealed the diffusion coefficients D and oxygen exchange coefficients k of both the grain bulk and grain boundaries. Values obtained for grain boundaries (Dgb and kgb) are almost three orders of magnitude higher than those of the grains (Dg and kg). Hence, grain boundaries may not only facilitate fast oxygen diffusion but also fast oxygen exchange kinetics. Variation of the A-site stoichiometry ((La0.8Sr0.2)0.95MnO3) did not lead to large changes of the kinetic parameters. Properties found for epitaxial layers without grain boundaries (Db and kb) are close to those of the grains in polycrystalline layers.

Apparent Oxygen Uphill Diffusion in La0.8Sr0.2MnO3 Thin Films upon Cathodic Polarization.

The impact of cathodic bias on oxygen transport in La0.8Sr0.2MnO3 (LSM) thin films was investigated. Columnar-grown LSM thin films with different microstructures were deposited by pulsed laser deposition. 18O tracer experiments were performed on thin film microelectrodes with an applied cathodic bias of -300 or -450 mV, and the microelectrodes were subsequently analyzed by time-of-flight secondary ion mass spectrometry. The 18O concentration in the cathodically polarized LSM microelectrodes was strongly increased relative to that in the thermally annealed film (without bias). Most remarkable, however, was the appearance of a pronounced 18O fraction maximum in the center of the films. This strongly depended on the applied bias and on the microstructure of the LSM thin layers. The unusual shape of the 18O depth profiles was caused by a combination of Wagner-Hebb-type stoichiometry polarization of the LSM bulk, fast grain boundary transport and voltage-induced modification of the oxygen incorporation kinetics.

Mapping electrochemically driven gas exchange of mixed conducting SrTi0.7Fe0.3O3 - δ and Ce0.8Gd0.2O1.9 thin films by 18O tracer incorporation under reducing atmosphere.

Thermally and electrochemically driven 18O tracer exchange experiments in H2/H218O atmosphere were performed on SrTi0.7Fe0.3O3 - Î´ and Ce0.8Gd0.2O2 - Î´ thin films on single crystalline YSZ substrates. Noble metal current collectors were deposited on both films and electrochemically polarized during the exchange experiment. The resulting tracer distribution was analyzed by spatially resolved secondary ion mass spectrometry. Increased tracer fraction near the current collectors was found under cathodic polarization and decreased tracer fraction under anodic polarization. High cathodic bias leads to enhanced n-type electronic conductivity, which increases the extent of the electrochemically active zone.

A novel ToF-SIMS operation mode for sub 100 nm lateral resolution: Application and performance.

A novel operation mode for time of flight-secondary ion mass spectrometry (ToF-SIMS) is described for a TOF.SIMS 5 instrument with a Bi-ion gun. It features sub 100 nm lateral resolution, adjustable primary ion currents and the possibility to measure with high lateral resolution as well as high mass resolution. The adjustment and performance of the novel operation mode are described and compared to established ToF-SIMS operation modes. Several examples of application featuring novel scientific results show the capabilities of the operation mode in terms of lateral resolution, accuracy of isotope analysis of oxygen, and combination of high lateral and mass resolution. The relationship between high lateral resolution and operation of SIMS in static mode is discussed.

Cation diffusion in La(0.6)Sr(0.4)CoO(3-δ) below 800 °C and its relevance for Sr segregation.

Cation diffusion was investigated in La0.6Sr0.4CoO3-δ (LSC) thin films on (100) yttria stabilized zirconia in the temperature range 625-800 °C. Isotopic ((86)Sr) and elemental tracers (Fe, Sm) were used to establish diffusion profiles of the cations in bi- and multi-layered thin films. The profiles were analyzed by time of flight-secondary ion mass spectrometry (ToF-SIMS). Grain and grain boundary diffusion coefficients of the cations were determined for LSC thin films with columnar grains - diffusion along grain boundaries is shown to be about three orders of magnitude faster than in grains. This could be verified for thin films with different grain size. A- and B-site cations showed very similar temperature dependencies with activation energies of ∼3.5 eV for bulk and ∼4.1 eV for grain boundary diffusion. The importance of cation diffusivities for surface segregation of Sr and thus for a major degradation mechanism of LSC cathodes in solid oxide fuel cells is discussed.

Electrochemical properties of La0.6Sr0.4CoO3 - δ thin films investigated by complementary impedance spectroscopy and isotope exchange depth profiling.

The oxygen exchange and diffusion properties of La0.6Sr0.4CoO3 - Î´ thin films on yttria stabilized zirconia were analyzed by impedance spectroscopy and 18O tracer experiments. The investigations were performed on the same thin film samples and at the same temperature (400 °C) in order to get complementary information by the two methods. Electrochemical impedance spectroscopy can reveal resistive and capacitive contributions of such systems, but an exact interpretation of the spectra of complex oxide electrodes is often difficult from impedance data alone. It is shown that additional isotope exchange depth profiling can significantly help interpreting impedance spectra by giving reliable information on the individual contribution and exact location of resistances (surface, electrode bulk, interface). The measurements also allowed quantitative comparison of electrode polarization resistances obtained by different methods.

Tensile lattice strain accelerates oxygen surface exchange and diffusion in La1-xSrxCoO3-δ thin films.

The influence of lattice strain on the oxygen exchange kinetics and diffusion in oxides was investigated on (100) epitaxial La1-xSrxCoO3-δ (LSC) thin films grown by pulsed laser deposition. Planar tensile and compressively strained LSC films were obtained on single-crystalline SrTiO3 and LaAlO3. 18O isotope exchange depth profiling with ToF-SIMS was employed to simultaneously measure the tracer surface exchange coefficient k* and the tracer diffusion coefficient D* in the temperature range 280-475 °C. In accordance with recent theoretical findings, much faster surface exchange (∼4 times) and diffusion (∼10 times) were observed for the tensile strained films compared to the compressively strained films in the entire temperature range. The same strain effect--tensile strain leading to higher k* and D*--was found for different LSC compositions (x=0.2 and x=0.4) and for surface-etched films. The temperature dependence of k* and D* is discussed with respect to the contributions of strain states, formation enthalpy of oxygen vacancies, and vacancy mobility at different temperatures. Our findings point toward the control of oxygen surface exchange and diffusion kinetics by means of lattice strain in existing mixed conducting oxides for energy conversion applications.

ToF-SIMS measurements with topographic information in combined images.

In 2D and 3D time-of-flight secondary ion mass spectrometric (ToF-SIMS) analysis, accentuated structures on the sample surface induce distorted element distributions in the measurement. The origin of this effect is the 45° incidence angle of the analysis beam, recording planar images with distortion of the sample surface. For the generation of correct element distributions, these artifacts associated with the sample surface need to be eliminated by measuring the sample surface topography and applying suitable algorithms. For this purpose, the next generation of ToF-SIMS instruments will feature a scanning probe microscope directly implemented in the sample chamber which allows the performance of topography measurements in situ. This work presents the combination of 2D and 3D ToF-SIMS analysis with topographic measurements by ex situ techniques such as atomic force microscopy (AFM), confocal microscopy (CM), and digital holographic microscopy (DHM). The concept of the combination of topographic and ToF-SIMS measurements in a single representation was applied to organic and inorganic samples featuring surface structures in the nanometer and micrometer ranges. The correct representation of planar and distorted ToF-SIMS images was achieved by the combination of topographic data with images of 2D as well as 3D ToF-SIMS measurements, using either AFM, CM, or DHM for the recording of topographic data.

Mutual Lewis acid-base interactions of cations and anions in ionic liquids.

Solute properties are known to be strongly influenced by solvent molecules due to solvation. This is due to mutual interaction as both the properties of the solute and of the solvent strongly depend on each other. The present paper is based on the idea that ionic liquids are cations solvated by anions and anions solvated by cations. To show this (in this system strongly pronounced) interaction the long time established donor-acceptor concept for solvents and ions in solution by Viktor Gutmann is extended to ionic liquids. A number of solvent parameters, such as the Kamlet-Abboud-Taft and the Dimroth-Reichardt E(T) scale for ionic liquids neglect this mutual influence, which, however, seems to be in fact necessary to get a proper description of ionic liquid properties. It is shown how strong such parameters vary when the influence of the counter ion is taken into account. Furthermore, acceptor and donor numbers for ionic liquids are presented.

The relevance of interfaces for oxide ion transport in yttria stabilized zirconia (YSZ) thin films.

Thin YSZ films were prepared on magnesia, sapphire and strontium titanate (STO) single crystals using pulsed laser deposition and, for comparison, by a sol-gel method on STO. The bulk and interfacial mass and charge transport properties of these films were investigated by complementary impedance spectroscopy and tracer diffusion measurements. In this context, a novel two-step tracer diffusion experiment is introduced. For YSZ films on sapphire and magnesia, grain bulk conductivities similar to those of polycrystalline samples were measured in most cases. Strongly blocking grain boundaries could be identified by impedance measurements. The films on sapphire and magnesia also exhibited good agreement between effective transport properties of impedance and tracer measurements. YSZ layers on strontium titanate single crystals, on the other hand, showed a strongly increased effective conductivity in impedance studies. However, in tracer diffusion experiments this could be unambiguously attributed to conduction in the substrate while the diffusion coefficient of YSZ on STO was comparable to that of YSZ films on other substrates. Moreover, the tracer diffusion experiments did not indicate any significant increase of oxide ion mobility on a free YSZ surface compared to a Pt|YSZ interface.