Electron microscopy and diffraction of radiation-sensitive nanostructured materials.

Soft matter research of natural organic and synthetic nanomaterials is an area in nanoscience and technology that has been growing particularly quickly in recent years. The materials under investigation are sensitive to high-energy electrons. Any structure characterization using electron microscopy thus requires special care. First, we illustrated this on naturally grown nanotubes observed by normal and cryogenic scanning electron microscopy. Second, we studied the ordering and orientation of the mesophase in template-grown nanotubes and nanorods containing discotic liquid crystals without and with doping, as desired. For these studies, we mainly used transmission electron diffraction and microscopy at low-dose conditions, high-efficiency image acquisition, and cryoprotection of the structures at liquid helium temperature. Additional analytical information was obtained by electron energy filtering observations.

Structure and phase transitions in Ca2CoSi2O7-Ca2ZnSi2O7 solid-solution crystals.

While the incommensurability in melilites is well documented, the underlying atomic configurations and the composition-dependent phase behavior are not yet clear. We have studied the transition from the incommensurate phase to the high-temperature normal phase (IC-N), and to the low-temperature commensurate phase (IC-C) of selected members of the Ca(2)Co(1 - x)Zn(x)Si(2)O(7) system using X-ray and single-crystal electron diffraction, as well as calorimetric measurements. The space group of the unmodulated normal phase and of the basic structure of the incommensurate phase is P42(1)m; the commensurate lock-in superstructure was refined as a pseudomerohedral twin in the orthorhombic space group P2(1)2(1)2. We found that the commensurate modulation is mainly connected with a sawtooth-like periodicity of rotations of the T(1) tetrahedra in the 3 x 3 superstructure. In this structure, the clustering of the low-coordinated Ca(2+) ions is not complete so that only imperfect octagons were detected. Generally, the effect of increasing substitution of Co by Zn was a continuous reduction of the IC-N and IC-C transition temperatures.

Cationic microparticles consisting of poly(lactide-co-glycolide) and polyethylenimine as carriers systems for parental DNA vaccination.

Cationic microparticles for DNA adsorption were formulated by blending poly(lactide-co-glycolide) (PLGA) (50:50), with different cationic agents, either PEI 25 kDa (polyethylenimine) or CTAB (cetyl-trimethyl-ammonium-bromide). The aim was to create adjuvant delivery systems increasing the efficiency of DNA vaccines. Microparticles formulated with 10% PEI exhibited a highly positive zeta-potential, small particle sizes, in contrast to particles prepared with CTAB, which revealed highly aggregated structures in scanning electron micrographs. PEI 10% microparticles efficiently adsorbed DNA and protected DNA from enzymatic degradation. Microparticles with up to 10% PEI did not affect membrane integrity whereas CTAB particles showed higher LDH release. Transfection efficiencies were assessed using a luciferase reporter gene assay compared to naked DNA and PEI/DNA polyplexes. DNA adsorbed onto microspheres with 10% or 50% PEI generally had higher transfection efficiencies than CTAB but reached lower expression levels than PEI/DNA polyplexes alone. This documented the intact release of DNA. The mechanism of gene delivery to non-phagocytic cells was studied via covalent fluorescence labeling of both the DNA and PEI by confocal microscopy and suggested uptake of DNA. Immunization of mice was performed using plasmids encoding immunodominant antigens of Listeria monocytogenes adsorbed onto RG 502 H+PEI 10% microparticles. The efficiency was tested by intravenous challenge with an otherwise lethal dose of L. monocytogenes. PLGA+PEI microspheres can be used as adjuvant delivery systems for DNA but further optimization is necessary to exploit their full potential.

Transition from the incommensurately modulated structure to the lock-in phase in Co-åkermanite.

The adaptation of the incommensurate structure modulation in Ca(2)CoSi(2)O(7) (dicalcium cobalt disilicate) single crystals to decreasing temperature has been examined using in situ high-resolution transmission electron microscopy and electron diffraction. The transition from the incommensurate to the commensurate lock-in phase of Co-åkermanite exhibits a pronounced hysteresis of a highly strained metastable state with a characteristic microdomain morphology. A network of domain walls surrounding single orientation domains develops out of the room-temperature tartan pattern, the domains increase in size and their alignment changes from crystallographic to random. At 100 K the phase transition becomes almost complete. In parallel, the evolution of the modulation structure can be described by a change from a loose arrangement of octagonal tilings into a close-packed configuration of overlapping octagons in the commensurate low-temperature lock-in phase. Thereby, the octagon represents the ordered distribution of low-coordinated Ca clusters within a nanodomain extending over 4 x 4 subunits, on average [Riester et al. (2000). Z. Kristallogr. 215, 102--109]. The modulation wavevector was found to change from q(1,2) = 0.295 (a* +/- b*) at 300 K to q(1,2) = 0.320 (a* +/- b*) at 100 K.