Cesare Ascoli

Dynamical friction coefficient maps using a scanning force and friction microscope

A novel method to analyze and to distinguish the non-dissipative component from the dissipative frictional component of the lateral force in a Scanning Force and Friction Microscope (SFFM) is worked out and applied to images acquired on various samples. We have determined the twisting spring constants of the cantilevers by computer statical analysis, since they are essential for the quantitative determination but are not provided by the manufacturer. Quantitative results for the μD dynamical friction coefficient images are reported. Comparison with the forward-backward lateral force image subtraction model is made.

Longitudinal patterns similar to G-banding in untreated human chromosomes: evidence from atomic force microscopy

The structure of human metaphase chromosomes, fixed according to standard procedures for optical microscopy but not treated for banding, was exammed by atomic force microscopy (AFM). The images show that chromosomes display a banding pattern very similar to G-banding, detected by the AFM as a variation in the thickness of chromatin. This similarity allows the identification of individual chromosomes.

Monitoring of an atomic force microscope cantilever with a compact disk pickup

In the present study we test a compact disk pickup as the cantilever position sensor in an atomic force microscope (AFM). The pickup is placed on top of the optical microscope used for the visual inspection and alignment of the specimen. The AFM is also equipped with its own cantilever movement sensor system. Both the built-in and the new detection devices are simultaneously active for comparison purposes. Two different measurements are performed in sequence on the same sample each using one sensor at a time as the error signal source for the AFM feedback loop. The pickup has demonstrated good sensitivity as well as excellent performance in terms of compactness, reliability, and cost.

Measurements of Euglena motion parameters by laser light scattering.

Measurements of Euglena gracilis motion parameters have been performed by the spectral analysis of the scattered laser light. Samples were oriented by a radiofrequency field to obtain easily interpretable spectra. Cell rotation frequency and flagellar beating frequency distributions were obtained from the homodyne spectra, whereas the Doppler lines obtained at small observation angles by heterodyne detection yielded the swimming speed distributions. We discuss the broadening of the heterodyne spectra at large angles of observation. An application of this method to the study of the photo-kinetic effect is also described.

Laser thermal effects on atomic force microscope cantilevers

Abstract The effects of laser radiation on cantilevers used in atomic force microscopes have investigated. Optical parameters of the cantilevers, deflection due to radiation pressure and thermal effects have been measured on cantilevers freely moving in air. For cantilevers with a gold coating we have found thermal effects at least two orders of magnitude larger than for uncoated cantilevers.

Weak hydrostatic forces in far-scanning ion conductance microscopy used to guide neuronal growth cones

Scanning ion conductance microscopy (SICM) is currently used for high resolution topographic imaging of living cells. Recently, it has been also employed as a tool to deliver stimuli to the cells. In this work we have investigated the mechanical interaction occurring between the pipette tip and the sample during SICM operation. For the purpose, we have built a setup combining SICM with atomic force microscopy (AFM), where the AFM cantilever replaces the sample. Our data indicate that, operating in far-scanning mode with current decrease values below 2%, no force can be detected, provided that the level of the electrolyte filling the pipette is equal to that determined by the capillary tension. A filling level different from this value determines a hydrostatic pressure, a flux through the pipette tip and detectable forces, even in far-scanning mode. The absolute value of these forces depends on the pipette tip size. Therefore, a possible pitfall when using SICM for cell imaging is to imply zero-force working conditions. However the hydrostatic forces can be exploited in order to deliver weak mechanical stimuli and guide neuronal growth cones. Evidences of the effectiveness of this approach are herein given.

Normal and lateral forces in scanning force microscopy

With an atomic force/friction force microscope operating in the constant force mode and with an optical lever technique as a deflection sensor, we have investigated the total force acting on the cantilever tip during the raster scanning of the sample surface. A model including the normal and lateral components of the force has been worked out. The normal force is related to the cantilever loading. The lateral force has two components, dissipative and nondissipative, having opposite symmetry with respect to the scanning direction. Within our model, the nondissipative component, which is related to the topography, can be distinguished from the friction component in two different ways, both leading to ‘‘pure friction’’ images. The first method is based on the comparison of two images acquired in the forward and backward scanning direction, respectively. The second method is based on the comparison of the topographic and lateral force images acquired in the same scanning direction. This latter way does not need correction for the nonlinear behavior of the piezoelectric transducer. Results from various samples are reported.

Effects of electromagnetic fields on the motion of Euglena gracilis.

The orientation behavior of Euglena gracilis cultures in electromagnetic fields is shown to agree with the predictions of a model involving only a passive mechanism. The increase in Euglena motor activity with increasing field intensity is demonstrated by measuring various motion parameters by the laser scattering technique. The effect of electric field on the speed of Euglenas is compared with that of temperature. We conclude that the electric field warms up the culture, thus inducing an increase in cell motility.

Fabrication of hybrid superconductor–semiconductor nanostructures by integrated ultraviolet-atomic force microscope lithography

Hybrid superconductor–semiconductor (S–Sm) nanostructures were fabricated by integrating standard ultraviolet photolithography and direct patterning of photoresist with an atomic force microscope (AFM). This novel technology was used to fabricate Nb–InAs–Nb weak links comparable in length to the coherence length. These structures exhibit high critical currents up to 10 μA/μm in planar geometry at 0.3 K. The fabrication protocol is based on the modification of photolithographically defined patterns by AFM static ploughing of the photoresist. Wet chemical etching is subsequently used for the definition of nanoscale S–Sm–S bridges. Additionally Lift-off procedures allowed the fabrication of submicron superconducting bridges. Successful fabrication of the nanostructures was verified by electrical characterization and by AFM and scanning electron microscope structural characterization.

Nanometer biodevice fabrication by electron beam lithography

A conventional electron beam lithography machine operated at 50 kV is used in this work to fabricate devices and structures for biophysical and molecular electronics applications featuring critical dimensions down to the nanometer region. Such nanostructures are used for deposition and manipulation of organic molecules; fabricated devices include fine pitch self-standing meshes for laser deposition of molecules, a bimetallic miniaturized glucose sensor and nanogaps for molecular trapping and probing. The developed process is described in detail and by means of Monte Carlo simulation, the various electron scattering processes are modelled. Critical issues, such as fabrication of nanogaps with dimension down to 5 nm, are also addressed.