Atomic force microscope study of presmectic modulation in the nematic and isotropic phases of the liquid crystal octylcyanobiphenyl using piezoresistive force detection

Using a temperature controlled atomic force microscope (AFM), we have studied surface induced pre-smectic order in the nematic and isotropic phases of 4-cyano- 4'-n -octylbiphenyl. A modified AFM head with piezoresitive cantilevers has been used to measure the structural force between a flat BK7 glass plate and a 10 microm glass sphere, both being treated to induce homeotropic alignment of the confined liquid crystal layer in between. We have observed surface-induced presmectic force not only in the isotropic, but also in the nematic phase. We have measured the temperature dependencies of the presmectic force, the smectic correlation length xi and the smectic order parameter psi at the surface. The correlation length xi(T) shows a power-law temperature dependence with a critical exponent of nu=0.67 +/- 0.03 and the bare correlation length of xi(0) = (0.39 +/- 0.08) nm, in good agreement with x-ray data. The smectic density at the surface is psi(2)(S) =0.4 in the nematic phase and decreases in the isotropic phase.

Structures and phase transitions in thin free standing films of an antiferroelectric liquid crystal

The premises of a discrete mean-field model for polar smectic liquid crystals are tested by analyzing the ellipsometric experiments on two, three, and four-layer freestanding films of MHPOBC. The measured temperature dependences of the ellipsometric parameters in a weak dc external field are compared to the predictions of a simple clock model. A very good quantitative agreement is found indicating an odd-even effect: XY structures are stable for odd and Ising-like structures for an even number of layers.

Laser trapping of small colloidal particles in a nematic liquid crystal: clouds and ghosts

We show that, contrary to intuition, small (< or =1 microm) transparent particles can be trapped and manipulated in a nematic liquid crystal using an intense laser beam, although their index of refraction is lower than both refractive indices of the surrounding birefringent fluid. Two mechanisms are identified that are responsible for this anomalous trapping: (i) surface-induced distortion of the birefringent media around the particle, creating a high-index "cloud" around the colloid, and (ii) laser-induced distortion or (partial) melting of a nematic, creating a ghost colloid.

Different propensity to form amyloid fibrils by two homologous proteins-Human stefins A and B: searching for an explanation

By using ThT fluorescence, X-ray diffraction, and atomic force microscopy (AFM), it has been shown that human stefins A and B (subfamily A of cystatins) form amyloid fibrils. Both protein fibrils show the 4.7 A and 10 A reflections characteristic for cross beta-structure. Similar height of approximately 3 nm and longitudinal repeat of 25-27 nm were observed by AFM for both protein fibrils. Fibrils with a double height of 5.6 nm were only observed with stefin A. The fibril's width for stefin A fibrils, as observed by transmission electron microscopy (TEM), was in the same range as previously reported for stefin B (Zerovnik et al., Biochem Biophys Acta 2002;1594:1-5). The conditions needed to undergo fibrillation differ, though. The amyloid fibrils start to form at pH 5 for stefin B, whereas in stefin A, preheated sample has to be acidified to pH < 2.5. In both cases, adding TFE, seeding, and alignment in a strong magnetic field accelerate the fibril growth. Visual analysis of the three-dimensional structures of monomers and domain-swapped dimers suggests that major differences in stability of both homologues stem from arrangement of specific salt bridges, which fix alpha-helix (and the alpha-loop) to beta-sheet in stefin A monomeric and dimeric forms.

Structural Forces near Phase Transitions of Liquid Crystals

The structure of the fragile liquid-crystalline phases has a strong impact on the forces between bodies immersed in a liquid crystal (LC). We have equipped an atomic force microscope with a precise temperature control and measured various liquid-crystalline structural forces at temperatures close to the phase transitions. The observed forces agree well with predictions of Landau--de Gennes phenomenological theory of LCs, even at a nanoscale length. In addition to this, we have observed a molecular layer, adsorbed on the surfactant-covered glass surface, and determined its thickness and elastic properties.

Capillary forces in a confined isotropic-nematic liquid crystal

We have investigated nematic capillary condensation in the isotropic phase of nematic liquid crystals 5CB (4-cyano-4(')-n-pentylbiphenyl) and 8CB (4-cyano-4(')-n-octylbiphenyl) confined to nanometer thick layers between two orienting surfaces. The capillary condensation was induced by decreasing the liquid crystal layer thickness using an atomic force microscope, and the onset of condensation was detected by monitoring the structural force on a confining surface. Very strong and long-ranged capillary forces were observed at temperatures close to the isotropic-nematic transition. We have analyzed the temperature dependence of the thickness of the liquid crystal layer, at which the condensation occurs, with a thermodynamic Kelvin equation and determined the interfacial tension between the isotropic and nematic phases. The separation dependence of capillary forces was analyzed within the Landau-de Gennes approach, including electrostatic interaction due to surface charging. The quantitative agreement between the measured and calculated force profiles is very good, and a single set of parameters is needed to describe a set of measured force profiles at different temperatures. Surface charge density, surface potential, and Debye screening length were determined directly from the observed surface forces.

Structure and polarity of 8CB films evaporated onto solid substrates

Brewster-angle reflection ellipsometry and surface optical second harmonic generation were used to study the growth of 4'-n-octyl-4-cyanobiphenyl (8CB) films evaporated in air onto polymeric and quartz glass substrates. The layer-by-layer growth of the films terminates after formation of two distinctive interfacial layers. Both of these two layers are polar and tilted. In the first layer the molecules lie nearly flat on the surface, while in the second layer they point on average about 50 degrees toward the surface normal. The dipole moment of the second layer has a lower magnitude and an opposite direction with respect to the dipole moment of the first layer.

Structure and dynamics of freely suspended film of the smectic-C*alpha phase in an external transverse electric field

We present a theoretical analysis of static and dynamic properties of freely suspended films of the smectic-C*alpha phase in an external electric field, applied along the smectic layers. The analysis is performed within the "clock" model, where the interactions up to next-nearest neighbors are considered. The calculated critical electric field for the unwinding of the smectic-C*alpha phase is of the order of several 100 V/mm and strongly depends on the interlayer interactions. The calculated relaxation rates of the eigenfluctuations of a system of N layers are in the kilohertz range for phase fluctuations and several megahertz for amplitude fluctuations.

Amyloid fibril formation by human stefin B in vitro: immunogold labelling and comparison to stefin A

The mechanism by which proteins form amyloid fibrils is of high interest to the scientific community as its understanding could resolve questions relevant to conformational diseases. The structural and energetic basis of the process is still largely unknown. The main controversial issue is the co-existence of several protein conformations. Three models for the mechanism of protein fibrillogenesis have been proposed which need to be tested by experiments. In this report, amyloid fibrils grown from human stefin B (type I cystatin) are described. This physiologically relevant protein readily forms fibrils in vitro, in contrast to the homologue--human stefin A--which forms fibrils under extreme conditions only. In order to specifically label stefin B fibrils in vitro, rabbit polyclonal antibody and mouse monoclonal antibody A6/2 against human stefin B were used for immunogold labelling. Samples were examined by transmission electron microscopy. Fibrils of stefin B were strongly labelled using polyclonal antibody and Protein A gold, whereas no positive reaction was observed with monoclonal antibody A6/2.

Observation of an electrostatic force between charged surfaces in liquid crystals

We report on the atomic force microscope observation of an electrostatic force between glass surfaces immersed in cyanobiphenil liquid crystals. The measured force is repulsive and decays exponentially with increasing surface separation. A mean field description of the electrostatic interaction in liquids has been used to determine the Debye screening length, the concentration of dissolved ions, and the surface electric potential. The effect of the observed interfacial electric field on the liquid crystal orientation at the surface has been discussed. It has been found that the coupling between the liquid crystal order and the surface electric field does not contribute considerably to the surface orienting action.

Surface-induced nematic and smectic order at a liquid-crystal-silanated-glass interface observed by atomic force spectroscopy and Brewster angle ellipsometry

We have investigated nematic and smectic surface-induced ordering in the isotropic phase of the 4-cyano-4'-n-octylbiphenyl liquid crystal on silanated BK7 and LaSF glass using atomic force spectroscopy and Brewster angle ellipsometry. We have observed complete wetting of the silanated glass surfaces with the nematic phase when approaching the isotropic-nematic phase transition from above. Using the atomic force spectroscopy at the same interfaces, we have detected a significant presmectic ordering that is enhanced at the nematic-isotropic transition. We have observed the first, strongly adsorbed layer of liquid-crystalline molecules underneath this presmectic ordering. This first molecular layer is laterally inhomogeneous with typically 100 nm voids and covers approximately 70% of the surface. It is stable far above the clearing point and is responsible for the surface-memory effect. The results have been analyzed using Landau-de Gennes theory. The surface coupling energies of the nematic and smectic order have been determined, as well as the coupling energy between the nematic and smectic order.

Human stefin B readily forms amyloid fibrils in vitro

Human stefin B (cystatin B) is an intracellular cysteine proteinase inhibitor broadly distributed in different tissues. Here, we show that recombinant human stefin B readily forms amyloid fibrils in vitro. It dimerises and further oligomerises, starting from the native-like acid intermediate, I(N), populated at pH 5. On standing at room temperature it produces regular (over 4 microm long) fibrils over a period of several months. These have been visualised by transmission electron microscopy and atomic force microscopy. Their cross-sectional diameter is about 14 nm and blocks of 27 nm repeat longitudinally. The fibrils are smooth, of unbranched surface, consistent with findings of other amyloid fibrils. Thioflavin T fluorescence spectra as a function of time were recorded and Congo red dye binding to the fibrils was demonstrated. Adding 10% (v/v) trifluoroethanol resulted in an increased rate of fibrillation with a typical lag phase. The finding that human stefin B, in contrast to the homologue stefin A, forms amyloid fibrils rather easily should promote further studies of the protein's behaviour in vivo, and/or as a model system for fibrillogenesis.

Forces in the isotropic phase of a confined nematic liquid crystal 5CB

Using a temperature controlled atomic force microscope, we have measured the temperature dependence of the force between a flat silanated glass surface and a silanated glass microsphere, immersed in the isotropic phase of the nematic liquid crystal 5CB (4'-n-pentyl 4-cyanobiphenyl). At separations of several nanometers, we observed a weak, short range attractive force of the order of 100 pN, which was increased by decreasing the temperature. The temperature dependence of the amplitude and the range of this attractive force can be described by a combination of van der Waals and a mean-field prenematic force due to the surface-induced nematic order. This is supported by ellipsometric study and allows for the determination of the surface coupling energy of 5CB on a silanated glass surface.

Optical rotation and structure of ferrielectric smectic phases

We analyze the measured optical rotation in three- and four-layer smectic ferrielectric phases within the matrix approach to the light propagation. We show that "perfect" three- and four-layer structures with 120 degrees and 90 degrees phase rotation of the director in neighboring layers give negligible optical rotation of polarized light travelling along the normal to the smectic layers. Significant optical rotation is obtained in deformed three- and four-layer smectic phases. The analysis of the measured optical rotatory power clearly shows that three-layer ferrielectric phases of [4-(1-methylheptyloxycarbonylphenyl)-octylbiphenyl-4-carboxylate] and 4-[(4-[[1(*)-methyl]heptycarboxy]phenyl)carboxy]phenyl-4'-decyloxy-1-benzencarbatioate (10OTBBB1M7) are deformed with the deformation angle of 35 degrees-45 degrees. The deformation angle in the four-layer smectic phase of 10OTBBB1M7 is 70 degrees-90 degrees. This is in reasonable agreement with other experiments and suggests the validity of the "deformed clock model."

Capillary condensation of a nematic liquid crystal observed by force spectroscopy

We have observed capillary condensation in nanometer-thin films of nematic liquid crystals using force spectroscopy. The liquid crystal was confined to a submicron gap between a flat substrate and a microsphere, attached to the cantilever of an atomic force microscope. A long-ranged and strongly attractive force due to capillary condensation of a nematic phase was observed at temperatures close to the bulk isotropic-nematic phase transition. The critical point, terminating the first-order confined isotropic-nematic transition line, was determined for the first time.

Investigation of the felodipine glassy state by atomic force microscopy

The glassy state of felodipine was prepared by melting crystals of felodipine on a clean glass slide and cooling to room temperature. It has been confirmed that glassy felodipine is a metastable state, and undergoes transformation to the more stable crystalline form. Crystallization occurred slowly and spontaneously at room temperature, below the glassy state transition temperature (Tg). The contact mode of atomic force microscopy was used for topographical imaging of the glassy and crystalline states of felodipine. When the glassy felodipine region next to the recrystallized zone was exposed to controlled mechanical stress through the tip, rapid additional crystallization was observed. This crystallization process can be induced and imaged in real time by atomic force microscopy.

Atomic force microscope evidence for the existence of smecticlike surface layers in the isotropic phase of a nematic liquid crystal

Using a temperature controlled atomic force microscope we have observed presmectic layering in the isotropic phase of 4-cyano-4(')-n-octylbiphenyl (8CB) on silanated glass. The first molecular layer shows a smecticlike compressibility modulus of B approximately 10(7) N/m(2) and is stable more than 20 K beyond the bulk clearing point. It is followed by a presmectic modulation that increases when cooling towards the isotropic-nematic transition. In the bulk isotropic phase, the layers cover approximately 70% of the glass surface, indicating a clusterlike organization.

AFM imaging of surface adsorbed polymeric 3-alkylpyridinium salts from the marine sponge Reniera sarai

Bioactive 3-alkylpyridinium polymers (poly-APS) have recently been isolated from the marine sponge Reniera sarai. Previous results have shown that these molecules in aqueous solutions form supramolecular aggregates with an average hydrodynamic radius of 23 +/- 2 nm. To obtain additional evidences about the shape and the dimensions of poly-APS aggregates, we used atomic force microscopy (AFM) operating in tapping mode. The images clearly showed adsorbed aggregates with a lateral dimension of approximately 40 nm and a thickness of the order of approximately 1 nm. The distribution of volumes of the adsorbed aggregates is very similar to the distribution of hydrodynamic radii as obtained from the dynamic light scattering experiments. The volume distribution of these aggregates shows a maximum at 1750 nm3, which corresponds to a sphere with a radius of 7.5 nm.

Slow mode of the smectic-A-smectic-C(*)(alpha) phase transition

Unusual slow fluctuations as revealed recently by dynamic light scattering close to the Sm-A-Sm-C(alpha)(*) phase transition in the antiferroelectric liquid crystal 4-(1-methylheptyloxy-carbonyl)phenyl 4(')-octyloxy biphenyl-4-carboxylate can be explained by the electrostatic coupling between impurity ions and director fluctuations. Within the vicinity of Sm-A-Sm-C(alpha)(*) transition, the relaxation rate of the slow mode depends linearly on temperature, but with a different slope in each phase. The square root of its intensity shows a clear Curie-Weiss divergence at the phase transition, which is a direct confirmation of the electrostatic coupling mechanism.