Nanomechanics of Organic Layers and Biomenbranes

Abstract

The main objective of this work was to quantitatively measure the nanotribological properties or organic layers and rationalize them from a physicochemical point of view. To accomplish that, a wide array of samples was selected, ranging from technological coatings as alkanethiols to biological coatings as phospholipid bilayers, which can be used to functionalize miniaturized electromechanical systems. Although the studied molecules are different in nature and functionality, the intermolecular forces that determine the layers structure are the same: van der Waals, electrostatic and water-related interactions. We are interested in quantifying these interactions as a function of the intermolecular distances, the nature of the medium the molecules are immersed in (liquid or air) and also assess external factors that have specific effects on the samples. These external factors were overall tested in biological membranes (phospholipid bilayers), where temperature and the presence of ions play a decisive role in their structure. <br/><br/>In order to achieve our objectives, two experimental techniques were mainly used. The first one is the Lateral Force Microscopy, which was implemented in our lab during the execution of this project. As the nature of the measurements we wanted to perform was quantitative, it was also necessary to implement the methods to calibrate Atomic Force Microscopy probes both vertically and laterally.<br/><br/>Besides, as we wanted to test the biological samples in liquid environment, a new methodology to perform Lateral Force Microscopy measurements in liquid had to be implemented. The second main technique is Force Spectroscopy, which was used to complement the nanomechanical information obtained from the friction measurements and that gave us a more detailed picture of the mechanical response of layers. Our technical goal was to obtain fast, reproducible and consistent Force Spectroscopy results, fact that implied the development of a dedicated software routine and the systematic control of the probes shape.

<I>En esta tesis se han usado varias técnicas nanométricas con el objetivo de estudiar diversas monocapas orgánicas. El tipo de superficies estudiadas se puede clasificar en dos tipos; Primeramente, monocapas aplicadas a reducir la fricción en dispositivos de tipo MEMS y NEMS (Micro y NanoElectroMechanical Systems). Las moléculas estudiadas han sido ácido araquídico, ácido alcanofosfónico completamente hidrogenado y parcialmente fluorado y capas de tipo DLC (Diamon-Like Carbon films). El segundo tipo de muestras comprende recubrimientos adecuados para el desarrollo de MEMS y NEMS para aplicaciones biológicas, esto es, dispositivos que necesitan recubrimientos biocompatibles. Como primera aproximación se han estudiado bicapas fosfolipídicas en media acuoso desde el punto de vista nanomecánico.<br/><br/>Las técnicas empleadas han sido la microscopia de fuerzas atómicas(AFM) en el modo topográfico, la espectroscopia de fuerzas para comprimir las capas y estudiar su respuesta nanomecánica en condiciones de comprensión vertical y la microscopia de fuerzas laterales o de fricción (LFM), con la cual se pueden obtener las propiedades nanotribológicas de las muestras de estudio afectando áreas tan pequeñas como 25 X 25 nanómetros cuadrados. Mediante el uso de la microscopia de fuerzas a temperatura variable ha sido posible estudiar in-situ las transiciones de fase de bicapas fosfolipídicas soportadas sobre mica en medio acuoso y su combinación con la espectroscopia de fuerzas ha permitido la obtención de la fuerza de rotura de dichas capas a temperatura variable. </I>