Nano-Mechanical Properties of Modified Poly(Methyl Methacrylate) Films Studied by Atomic Force Microscopy

Atomic force microscopy (AFM) has been used to study the effect of various photoinitiators doped into poly(methyl methacrylate) (PMMA) on the mechanical properties of PMMA films at the nanometer scale. Pure and modified PMMA films (containing four different photoinitiators) were exposed to a mercury vapor lamp in air atmosphere. Force–distance curves for hardness, Young modulus, and adhesion forces were obtained using different AFM modes (tapping or contact-mode) and different tips (diamond or silicon nitride). The results revealed that the added photoinitiators slightly changed the nanomechanical properties of PMMA as a result of alterations in the photochemical reactions and physical processes occurring in the studied systems. tert-Butyl peroxybenzoate had the most efficient effect on the measured parameters in UV-irradiated PMMA, whereas benzoyl peroxide was less active. The mechanism of the observed processes is discussed.     

Electron spin resonance of FeVO4

We report on electron spin resonance (ESR) investigations of a FeVO₄ single crystal. Temperature and angular dependences of ESR resonance positions were measured and calculated in temperature range of 35–100 K. The spectra show rich angular dependences of the linewidth, the shape and the resonance field. They consist of a single broad line with asymmetric distortion. Due to the low symmetry of the crystal lattice this distortion can be explained by taking into account the influence of non diagonal dynamic susceptibility.     

Application of the three-valence model of photorefraction to rhodium-doped barium titanate

Abstract

Small signal and light induced absorption data taken at 633 nm and 1.06 μm for different samples of rhodium-doped barium titanate (Rh : BaTiO₃) have been analysed. These data have been used in conjunction with the three-valence model theory of photorefraction to determine N _D, the total amount of rhodium in each sample. It is shown that the values of N _D calculated at the two wavelengths for individual samples are inconsistent with each other, although no evidence for other impurities in the samples has been observed. This indicates that a more complex theory than the three-valence model is necessary to explain the photorefractive processes in Rh : BaTiO₃, and that an additional photorefractive centre may not be sufficient to account for the observed discrepancies in N _D.     

Sequential interval motif search: unrestricted database surveys of global MS/MS data sets for detection of putative post-translational modifications

Tandem mass spectrometry is the prevailing approach for large-scale peptide sequencing in high-throughput proteomic profiling studies. Effective database search engines have been developed to identify peptide sequences from MS/MS fragmentation spectra. Since proteins are polymorphic and subject to post-translational modifications (PTM), however, computational methods for detecting unanticipated variants are also needed to achieve true proteome-wide coverage. Different from existing "unrestrictive" search tools, we present a novel algorithm, termed SIMS (for Sequential Motif Interval Search), that interprets pairs of product ion peaks, representing potential amino acid residues or "intervals", as a means of mapping PTMs or substitutions in a blind database search mode. An effective heuristic software program was likewise developed to evaluate, rank, and filter optimal combinations of relevant intervals to identify candidate sequences, and any associated PTM or polymorphism, from large collections of MS/MS spectra. The prediction performance of SIMS was benchmarked extensively against annotated reference spectral data sets and compared favorably with, and was complementary to, current state-of-the-art methods. An exhaustive discovery screen using SIMS also revealed thousands of previously overlooked putative PTMs in a compendium of yeast protein complexes and in a proteome-wide map of adult mouse cardiomyocytes. We demonstrate that SIMS, freely accessible for academic research use, addresses gaps in current proteomic data interpretation pipelines, improving overall detection coverage, and facilitating comprehensive investigations of the fundamental multiplicity of the expressed proteome.     

Influence of annealing on the structure and stoichiometry of europium-doped titanium dioxide thin films

This work presents the influence of annealing on the structure and stoichiometry of europium (Eu)-doped titanium dioxide (TiO₂). Thin films were fabricated by magnetron sputtering from a metallic Ti-Eu target in oxygen atmosphere and deposited on silicon and SiO₂ substrates. After deposition the selected samples were additionally annealed in air up to 1070 K.Film properties were examined by means of X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) and the results were analyzed together with the undoped TiO₂ thin films prepared under similar technological conditions.XRD results showed that depending on the Eu content, as-deposited thin films consisted of the TiO₂-anatase or TiO₂-rutile.An additional annealing will result in the growth of anatase crystals up to 35 nm, but anatase to rutile phase transformation has not been recorded. AFM images display high quality and a dense nanocrystalline structure. From the XPS Ti2p spectra the 4+oxidation state of Ti was confirmed. The O1s XPS spectra displayed the presence of an O²⁻ photoelectron peak accompanied by an additional broader peak that originates from hydroxyl species chemisorbed at the sample surface. It has been found that Eu dopant increases the OH⁻ content on the surface of prepared TiO₂:Eu thin films. The calculated O/Ti ratio was in the range of 1.85-2.04 depending on the sample.     

Microanalysis of Pd and V-doped TiO2 thin films prepared by sputtering

Thin films of TiO₂ doped with vanadium and palladium, prepared by the magnetron sputtering method, were studied by means of X-ray diffraction (XRD), Scanning Electron Microscopy with Energy Disperse Spectrometer (SEM-EDS) and Atomic Force Microscopy (AFM). Investigations have brought important information about microstructure due to dopant incorporation in the TiO₂ host lattice. Directly after deposition thin films were XRD-amorphous and SEM investigations did not reveal details on the microstructure. Analysis of the topography of prepared thin films required application of Atomic Force Microscope. The AFM images show that as-deposited sample was dense with grain sizes varied in the range of 5.5 nm-10 nm, that indicated high quality nanocrystalline behavior. Additional annealing results in the formation of three phases in the thin film, e.g. (Ti,V)O₂ — solid solution, PdO and metallic inclusions of Pd. SEM-EDS system allowed analysis of the elemental composition, especially the V one, which lines have not been evidenced in the XRD diffraction pattern. EDS maps show homogenous distribution of elements Ti, O, V, Pd in prepared thin films.     

Protein kinase C regulates a vesicular class of calcium channels in the bag cell neurons of aplysia

Protein kinase C (PKC) acutely increases calcium currents in Aplysia bag cell neurons by recruiting calcium channels different from those constitutively active in the plasma membrane. To study the mechanism of PKC regulation we previously identified two calcium channel alpha1-subunits expressed in bag cell neurons. One of these, BC-alpha1A, is localized to vesicles concentrated primarily in somata and growth cones. We used antibodies to BC-alpha1A to analyze its expression in the bag cell neurons of juvenile Aplysia at a developmental stage at which PKC-sensitive calcium currents have previously been shown to be low. We find that vesicular BC-alpha1A staining is generally reduced in juvenile bag cell neurons but that its expression level can vary among juvenile animals. In 17 bag cell clusters examined, the percentage of neurons that displayed punctate alphaBC-alpha1A staining ranged from 0 to 85%. Sampling of calcium currents from cells of the same clusters by whole cell patch-clamp techniques revealed that the PKC-sensitive calcium current density is significantly correlated with the degree of vesicular staining. In contrast, no correlation of basal calcium current levels with aBC-alpha1A staining was found. These results strongly suggest that BC-alpha1A, a member of the ABE-subfamily of calcium channels, carries the PKC-sensitive calcium current in bag cell neurons. They are consistent with a model in which PKC recruits channels from the vesicular pool to the plasma membrane.     

Heterologous expression of the Kv3.1 potassium channel eliminates spike broadening and the induction of a depolarizing afterpotential in the peptidergic bag cell neurons

The bag cell neurons of Aplysia are a cluster of cells that control egg laying behavior. After brief synaptic stimulation, they depolarize and fire spontaneously for up to 30 min. During the first few seconds of this afterdischarge, the action potentials of the bag cell neurons undergo pronounced broadening. Single bag cell neurons in culture also show spike broadening in response to repeated depolarizations. This broadening is frequency-dependent and associated with the induction of a depolarizing afterpotential lasting minutes. In some neurons the depolarizing afterpotential is sufficient to trigger spontaneous firing. To test the possibility that spike broadening during stimulation is required to trigger the depolarizing afterpotential, we eliminated frequency-dependent broadening by heterologous expression of the Kv3.1 potassium channel. This channel has rapid activation and deactivation kinetics and no use-dependent inactivation. Expression of Kv3.1 prevented spike broadening and also eliminated the depolarizing afterpotential. Measurements of the integral of calcium current during voltage commands, which simulated the action potentials of the control neurons and those expressing Kv3.1, indicate that spike broadening produces up to a fivefold increase in calcium entry. Manipulations that limit calcium entry during action potentials or chelation of intracellular calcium using BAPTA AM prevented the induction of the depolarizing afterpotential. We conclude that spike broadening is essential for the induction of the depolarizing afterpotential probably by regulating calcium influx and suggest that one of the physiological roles of spike broadening may be to regulate long-term changes in neuronal excitability.