Microscope à force atomique

thumb|350px|Le premier microscope à force atomique du monde, au musée de la Science de Londres. Le microscope à force atomique (AFM pour atomic force microscope) est un type de microscope à sonde locale permettant de visualiser la topographie de la surface d'un échantillon. Inventé en 1985, par Gerd Binnig, Calvin Quate et Christoph Gerber, ce type de microscopie repose essentiellement sur l'analyse d'un objet point par point au moyen d'un balayage via une sonde locale, assimilable à une pointe effilée. Ce mode d'observation permet alors, de réaliser la cartographie locale des grandeurs physiques caractéristiques de l'objet sondé (force, capacité, intensité de rayonnement, courant...), mais également de travailler dans des environnements particuliers tels que les milieux sous vide, liquides ou ambiants. Principe de fonctionnement La technique AFM exploite l'interaction (attraction/répulsion) entre les atomes de l'apex nanométrique d'une pointe et les atomes surfaciques d

Quasistatic and dynamic force microscopy of single antigen-antibody complexes and fibrin-fibrinogen systems

The molecular processes based on the specific receptor-ligand interactions (e.g. immune response to a foreign antigen, communication between nerve cells, etc.) are essential for a good and stable performance of living organisms. The aim of the present work is to study such molecular processes with the Atomic Force Microscope (AFM) operated in Force Spectroscopy (FS) mode. Information such as the force and length of the ligand-receptor bond rupture could be obtained from the standard quasistatic FS studies, whereas no external dithering applied to the AFM tips. Firstly, the antigen-antibody interactions were examined with the Bovine Serum Albumin (BSA) protein and its poly- and two different monoclonal antibodies. It was shown that the peak unbinding force depends on the type of antibody and the antibody concentration. The single potential barrier is dominant for the interaction between BSA and monoclonal antibodies and was revealed from the loading rate-dependent measurements. Secondly, we are interested in the process of fibrin gel formation, in particular in the forces involved in this process. It was demonstrated that the fibrin-fibrinogen interaction is the specific one. The unbinding force rupture for single fibrin-fibrin(ogen) complex was determined to be of about 320 pN at a loading rate of 3.5 nN/s. The single potential barrier is also dominant for this interaction. Moreover, a new method of direct and continuous measurement of the spring constant of single molecules or molecular complexes has been elaborated in our laboratory. To this end, the standard FS technique with functionalized tips and samples is combined with a small dithering of the AFM tip. The change of the dithering amplitude as a function of the pulling force is measured in order to extract the spring constant of the complex. The potentialities of this technique were demonstrated for the experiments with single BSA – polyclonal antibody to BSA (Ab-BSA) and fibrinogen – fibrinogen complexes.

EPFL2004

Cartographie et spectroscopie des propriétés mécaniques à l'échelle du nanomètre par spectrométrie acoustique locale à fréquence variable

This work aims at completing the development of local mechanical spectroscopy as initiated by F. Oulevey during his thesis. More precisely, the measurement of the amplitude and phase lag of the strain as a function of the applied stress frequency bas to be settled. Parallel to the development of the technical aspects making such an acquisition possible, the model used to analyze the measured quantities is modified in order to describe the studied system more realistically. The local mechanical spectrometer is based on an Atomic Force Microscope (AFM) using an optical method to detect the deflection of the lever. The stress is applied locally on the sample by a transducer fixed at the base of the cantilever. The bandwidth of the microscope' s electronics does not allow one to detect the cantilever movement at frequencies higher than the MHz. Two different approaches are followed to overcome this limit. The first approach exploits the nonlinearity of the contact to down-convert, via mechanical mixing, the high-frequency signals into low-frequency ones. This work demonstrates the feasibility of this technique, as well as its limits, mainly due to the difficulty to analyze confidently the obtained results. The second approach takes advantage of the stroboscopic detection of the movement of the cantilever. The intensity of the laser used to detect the deflection of the cantilever is modulated at a frequency close to the applied stress frequency. The amplitude of the signal recorded at the difference frequency corresponds then to the high-frequency amplitude of the cantilever movement. The frequency range attainable with this method is only limited by the bandwidth of the transducer providing the excitation. The model connecting the measured quantities (strain amplitude and phase lag) to the desired mechanical properties (elasticity and damping) consists in a beam clamped at one end, with a tip attached at some point along its length. The tip itself is connected to the sample via a spring and a dashpot in parallel, representing the normal interaction, and a spring and a dashpot in parallel representing the interaction in the plane of the sample. Combined with the stroboscopic detection, this model enables one to quantitatively determine the elastic modulus of stiff samples. Many results obtained on a wide range of samples exhibiting very diverse mechanical properties are presented. They demonstrate the validity of the stroboscopic approach, which combines simplicity of use and efficiency.

EPFL2000

Quasistatic and dynamic force microscopy of single antigen-antibody complexes and fibrin-fibrinogen systems

EPFL2004

Cartographie et spectroscopie des propriétés mécaniques à l'échelle du nanomètre par spectrométrie acoustique locale à fréquence variable

EPFL2000

Quasistatic and dynamic force microscopy of single antigen-antibody complexes and fibrin-fibrinogen systems

Cartographie et spectroscopie des propriétés mécaniques à l'échelle du nanomètre par spectrométrie acoustique locale à fréquence variable

Indentation-Type Scanning Force Microscopy, a Versatile Tool for Imaging, Measuring and Manipulating Soft Biological Tissues, for Example, Normal, Diseased and Engineered Cartilage

Indentation-Type Scanning Force Microscopy, a Versatile Tool for Imaging, Measuring and Manipulating Soft Biological Tissues, for Example, Normal, Diseased and Engineered Cartilage

Quasistatic and dynamic force microscopy of single antigen-antibody complexes and fibrin-fibrinogen systems

Cartographie et spectroscopie des propriétés mécaniques à l'échelle du nanomètre par spectrométrie acoustique locale à fréquence variable