Ádám Mechler

Calcium-dependent Open/Closed Conformations and Interfacial Energy Maps of Reconstituted Hemichannels

Using an atomic force microscope, we have studied three-dimensional molecular topography and calcium-sensitive conformational changes of individual hemichannels. Full-length (non-truncated) Cx43 hemichannels (connexons), when reconstituted in lipid bilayer, appear as randomly distributed individual particles and clusters. They show a lack of preferential orientation of insertion into lipid membrane; in a single bilayer, connexons with protrusion of either the extracellular face or the large non-truncated cytoplasmic face are observed. Extracellular domains of these undocked hemichannels are structurally different from hemichannels in the docked gap junctional plaques examined after their exposure by force dissection or chemical dissection. Calcium induced a reversible change in the extracellular pore diameter. Hemichannels imaged in a physiological buffer with 1.8 mm Ca+2 had the pore diameter of ∼1.8 nm, consistent with the closed channel conformation. Reducing Ca+2 concentration to ∼1.4, 1, and 0 mm, which changes hemichannels from the closed to open conformation, increased the pore diameter to ∼2.5 nm for ∼27, 74, and 100% of hemichannels, respectively. Thus, open/close probability of the hemichannel appears to be [Ca2+]-dependent. Computational analysis of the atomic force microscopy phase mode imaging reveals a significantly higher interfacial energy for open hemichannels that results from the interactions between the atomic force microscope probe and the hydrophobic domains. Thus, hydrophobic extracellular domains of connexins regulate calcium-dependent conformational changes.

Raman spectroscopic and atomic force microscopic study of graphite ablation at 193 and 248 nm

Experimental results are presented for the excimer laser ablation of highly oriented pyrolytic graphite at 193 and 248 nm for both single pulses and pulse trains in the fluence range of ∼1–15 J/cm2. The morphology and the depth of the ablated pits are monitored by atomic force microscopy, while the material characterization is performed by micro-Raman spectroscopy. A shift from ∼1.12 to ∼2.23 J/cm2 laser fluence is found in the single shot ablation threshold for the 248 nm laser wavelength compared to that at 193 nm. Broad D and G peaks in the Raman spectra indicate the formation of amorphous carbon layers as a result of laser irradiation with 193 and 248 nm pulses. This amorphous layer is present at lower fluences (several J/cm2) and after the very first shots. The modified layer created at 193 nm, compared to 248 nm, consists of optically denser material having more turbostratical/glassy character. The spectra do not show significant changes for fluences exceeding 6–7 J/cm2. A several hundred nanometers-high ring-like structure can be observed around the ablated pits. For laser fluences in excess of the estimated threshold at ∼6 J/cm2 (close to the aforementioned limit), the diameter of this structure increases with laser fluence. One hypothesis to explain the ring formation and the saturation of the Raman spectra supposes that the graphite melts and squirts on the laser irradiation. The ring, debris material and the amorphous layers disappear after heat treatment of the samples at 650°C, most probably by oxidative etching.

Nanoscale velocity–drag force relationship in thin liquid layers measured by atomic force microscopy

The relationship between velocity and drag force acting on a nanoprobe has been measured with an atomic force microscope (AFM). A special nanoprobe “whisker” was partially submerged in thin layers of glycerol–water mixtures and moved by using the AFM in scanning mode. The viscous drag force-caused torsion of the cantilever probe was recorded as a function of scanning speed and submersion depth. A linear drag force–velocity function was determined for cylindrical bodies with diameters of the order of 50nm. The experimental results were supported by calculations for the torsional force exerted on an AFM probe dragged through a viscous medium. The viscosity was calculated for each experiment assuming no slip conditions and was in agreement with the macroscopically determined values. With some refinements, this offers a possible means of determining viscosity in thin liquid layers.

Nanoscale resolution microchannel flow velocimetry by atomic force microscopy

The velocity of a microchannel flow was determined by atomic force microscopy (AFM) using a 50nm wide “whisker,” which was partially submerged and scanned transverse to the flow while drag was recorded. A peaked, near parabolic, flow velocity profile was found. Particle image velocity (PIV) measurements using 70nm diameter quantum-dot-coated polystyrene spheres confirmed the shape of the AFM-measured velocity profile. AFM-based nanometer resolution velocimetry confirms that the drag-velocity relationship for the whisker remains consistent over a wide range of shear values and appears to successfully resolve submicron scale flows, which are beyond the limits of conventional PIV measurements.

Excimer laser irradiation induced formation of diamond-like carbon layer on graphite

Abstract Highly oriented pyrolytic graphite (HOPG) was irradiated by an ArF excimer (λ=193 nm) laser above the ablation threshold, at approx. 0.45 and 2 J/cm2. The surface morphology and the quality of the remaining material was investigated by atomic force microscopy (AFM) and area-selective Raman spectroscopy. At the lower fluence a material removal rate of several monolayers per laser pulse was detected, without changing the quality of the remaining material. Irradiation at the higher fluence resulted in ablation rates of the order of 10 nm/pulse and the formation of an approx. 300 nm thick diamond-like carbon (DLC) film with approx. 50% concentration of the sp3 hybrid-states of carbon. In the surroundings of the ablated hole a narrow ring of mechanically soft, nanocrystalline and turbostratic carbon was observed. Upon annealing the irradiated surfaces in air at 650°C for 30 min, the graphite structure of the laser-modified layer was perfectly recovered with the disappearance of the surrounding ring.

Effect of step function-like perturbation on intermittent contact mode sensors: a response analysis

Abstract The dynamics of the intermittent contact mode (ICM) probe was investigated. In the experimental study we applied a step function signal to the Z piezo drive and recorded the amplitude signal of the probe while the probe was engaged with the surface. Transient overshoots appear at the edges of the steps. These transients are absent from the control contact force measurements, that is, they are proper to the ICM operation. The phenomenon was investigated by numerical calculations, focused on the effect of change of the drive frequency and the quality factor. We concluded, that the low value of the quality factor results in small transients and short settling time, which are necessary for fast atomic force microscopic operation. Simultaneously, the interaction force increases. Our calculations indicate that the tip–sample force can be lowered by setting the drive frequency slightly below the resonance.

Gas-deposited WO3 nanoparticles studied by scanning force microscopy

WO3 nanoparticles were generated by gas deposition. Deposits on Al substrates were studied by scanning force microscopy operated in the intermittent-contact (tapping) mode. At low surface coverage (< 0.5 %), we observed single nanoparticles with a mean size of ~ 1.5 nm. An increase of the amount of particles led to agglomerates, which appeared at surface coverages as low as 2 to 4 %. At full coverage the mean agglomerate size was ~ 5 nm. This value did not change as the sample was annealed at temperatures up to 250 °C. The size distribution of the agglomerates was found to be log-normal, i.e., similar to the size distribution of the gas-phase nanoparticles forming the deposit. For explaining the obtained log-normal size distribution of the agglomerates simulations of the agglomeration process were also carried out.

Laser-assisted chemical vapor deposition of diamond microstructures

High quality diamond has been deposited area selectivity onto thin tungsten films layered onto fused silica substrates in a hot filament CVD reactor. The deposition method uses a hot filament process which is combined with localized heating by a focused Nd-YAG laser beam.