Studies On Interfaces Of Liquid Crystal Based Biosensors By Sum Frequency Generation Spectroscopy

Liquid crystal(LC)are widely used in display equipment.Nowadays,because of its high sensitivity to the environment,LC molecules are used to build the LC-biosensor.LC-biosensor have advantages such as fast,simple,low-cost,high sensitivity and low limit of determination.For the development of LC-biosensor,stability,sensitivity and reliability are the key factors,and all these are directly related to the LC molecules alignment and orientation responses to the around environment.In this thesis,we applied sum frequency generation vibrational spectrum(SFG),coherent anti Stokes Raman scattering spectrum(CARS),polarizing microscopy imaging techniques studied LC molecules arrangement at different conditions such as,different temperature,humidity and surface molecular adsorption of the LCs film.LC nanofilms have been prepared on CaF2 and their physical and chemical properties have been investigated.The main results are as follows:1)The influence of the thickness on the orientation of LC molecules has been studied.It is found that the overall liquid crystal alignment tends to be perpendicular to the surface of CaF2 substrate with the increase of the thickness of LC film.2)LC nanofilms have sensitive response to water molecules in air.There is strong coupling between-CN and water molecules.Water molecules just adsorbs on the surface and does not penetrate into the LC film,and the binding of water molecules will cause the LC molecules in the liquid crystal film flip,which rearranged very fast.3)The excitation of-CH2 asymmetric stretching vibration of liquid crystal molecules will weaken the coupling of-CH2 between LC molecules in thicker LC films,thus resulting in the change of the arrangement of liquid crystal molecules.We use a polarizing microscope construct a simple biosensor basing on LC loaded phospholipid molecules to detect peptides,DNA and other large biological molecules interact with phospholipids.These experiment can combine with other molecular structure measurement techniques such as SFG to confirm and complement each other's results and make a more reasonable explanation for the mechanism of bio-molecules and lipid interaction.The specific work is the use of molecular assembly method,using N,N-Didecyl-N-methyl-N-(3-trimethoxysilylpropyl)ammonium chloride(DMTAC)to make LCs a determined orientation in glass substrate.The DMTAC self-assemble-monolayers(SAMs)are loaded with LC molecules with assemble phospholipid monolayer on the other interface of the LC film.The alkane chains at two hydrophobic interfaces extend into the LC film,which makes the molecular arrangement of LC molecules to be completely perpendicular to the substrate.It is completely opaque under the polarizing microscope.Binding of biological macromolecules and phospholipid molecules and molecular structure changes will affect the overall arrangement of LC molecules in a long length range.Using this LC-biosensor system we can directly get some biological information at the interfaces through the polarizing microscopy imaging.We applied this biosensor system to observe the interaction of peptides with phospholipid monolayer and the structural changes of DNA chain.In this thesis,the mechanism of LC based biosensing has been studied at the molecular level.The experimental results are expected to help the technical improvement of the LC-biosensors.