Resumen de Studying Electrostatic Polarization Forces at the Nanoscale. Dielectric constant of supported biomembranes measured in air and liquid environment
Georg Gramse
The objective of my thesis was to develop novel techniques and methods to probe the dielectric properties of biomembranes in air and their natural environment - liquid solution. The dielectric constant ?(r) of biomembranes is a parameter especially important in cell electrophysiology as it ultimately determines the ion membrane permeability, the membrane potential formation or the action potential propagation velocity, among others. However, no technique is able to provide this quantity with the required nanoscale spatial resolution and in electrolyte solution. AFM is an extremely versatile tool to investigate electric properties at the nanoscale, and hence constitutes a good candidate technique to approach the quantification of the nanoscale dielectric properties of biomembranes. Although a few AFM techniques exist, capable of investigating polarization properties, it remains difficult to extract quantitative values of ?r from the measurements, especially in liquid environment. One reason for this is on the instrumental side, since for studies at the nanoscale very small quantities have to be measured, that can be easily overwhelmed by electronic noise as it maybe for example the case in current sensing based techniques. Electrostatic Force sensing techniques may in principle have an advantage here, since the used cantilevers for force detection are extremely sensitive and naturally, undesired nonlocal electrical signals from the cantilever are suppressed. Another important aspect is attributed to a lack of sufficiently precise quantitative models to relate measured force with the dielectric constant value of the sample. Indeed, for measurements on insulating substrates like mica or glass that are sometimes required for biological samples, still no quantitative model is available. Moreover, successful measurements of dielectric properties in liquid media, that is fundamental for the functionality of some biological samples, has not been shown until now. As consequence of the existing limitations for quantitative dielectric imaging the objectives of this work were to extend the quantitative capabilities of Electrostatic Force Microscopy to image the dielectric constant of biomembranes with nanoscale spatial resolution. In particular, the three objectives I addressed in the work are: 1. To evaluate the possibility to perform quantitative dielectric measurement on biomembranes on metallic substrates and in air with Electrostatic Force Microscopy that may offer higher precision with respect to current sensing techniques. 2. To extend the applicability of quantitative dielectric measurement to the case of thick insulating substrates in order to facilitate its use with biomembranes that cannot be prepared on metallic substrates. 3. To develop a setup for dielectric imaging in liquid environment based either on current detection or on the principles of electrostatic force microscopy. Finally to perform nanoscale dielectric measurements on bio-membranes in their natural liquid environment. How each of these objectives could be reached is detailed in my thesis.
