Revealing The Interfacial Structures And Dynamics Of Sevreal Soft Matters Using Sum Frequency Generation Vibrational Spectroscopy

Soft matter refers to the substance between solid and ideal fluid.Common soft matters include colloids,polymers,proteins,deoxyribonucleic acid(DNA),liquid crystals and so on.As the development of science and technology,researchers gradually realize the importance of soft matter in practical application,such as packaging materials,detergents,adhesives,cosmetics,food additives,lubricants and tire rubber.Simultaneously,in terms of application,soft matter is also of vital importance in biomedical engineering,such as replacement of tissues and organs,repair of defective organs,storage of blood for purification,and control of drug delivery.However,the advent of nanotechnology has allowed people to shift their focus from the macro-scale to the micro-scale.Various micro-scale processing techniques have emerged.As the proportion of the interfaces in material increases,the relationship between the interfacial structure and the material property becomes important.This importance is mainly reflected in the following aspects:1.The orientation distribution of certain molecular groups can change the interfacial physicochemical properties,such as the hydrophobicity and the acid-base property;2.The dynamic evolution of the molecular chain at the interface can affect the bulk structure of the material,i.e.,wetting and dewetting;3.The formation of special structures at the interface(such as helix,folding,etc.)needs specific conditions,such as the concentration,interfacial hydrophobicity or hydrophilicity,acid-base condition.In the recent decades,the development of the sum frequency generation vibrational spectroscopy,SFG,has realized the possibility of detecting the interfacial structures.As a second order nonlinear optical technique,SFG possesses the interfacial selectivity and submonolayer sensitivity,which can be applied to various interface detections,such as solid/liquid interface,solid/solid interface,solid/gas interface,liquid/liquid interface,gas/liquid interface.SFG can be conceived as a coherent process of infrared absorption and anti-stokes Raman.Therefore,the detected molecular groups have to possess both the infrared and Raman activity.In this study,SFG was applied as the main technique and a Fresnel coefficient model was applied for investigating the buried interface.The interfacial structures and dynamical evolution behaviors of various soft matters(such as polymer,protein and DNA)were investigated and analyzed.The selected soft matters are either model materials commonly applied in theoretical and experimental studies,or materials for practical application in biomedical engineering.The applied Fresnel coefficient model can be applicable to thin film materials.The main results are summarized as follows:1.Firstly,the transformation matrix and coefficients of the molecular hyperpolarizabilities(from the molecular based coordinate system to the laboratory coordinate system)were calculated respectively.The Fresnel coefficients of the two interfaces were calculated for the thin-film samples.In the calculation,the multiple reflection and refraction of light beams within the thin film was considered.By adjusting the thickness of the thin film,this model can change the relative intensity of the optical electric field at the two interfaces,so as to achieve the selective detection of one single interface.2.By combining SFG and laser scanning confocal microscopy,the mechanism of the protein-antifouling of Poly(2-hydroxyethyl methacrylate)(PHEMA)was analyzed from a molecular level and also the correctness of the Fresnel coefficient model was proved.The relationship between the thickness of the film and the SFG intensity of the two interfaces(PHEMA surface,PHEMA/substrate interface)was determined by theoretical calculation of Fresnel coefficient.Then thin films of different thicknesses were prepared and studied.The experiments results showed that SFG spectra from the two interfaces were different,which demonstrates the correctness of the Fresnel coefficient model.Furthermore,the results showed that there existed a water hydration layer at PHEMA surface,which is believed to effectively resist the adsorption of protein molecules.This hydration layer might partially account for the biocompatibility of PHEMA.3.Study the interfacial structures of biomacromolecules by using chiral SFG.a).The factors influencing the formation of secondary structure of protein at the hydrophobic interface were studied by chiral and achiral SFG.Silk fibroin was chosen as the sample,which has been widely used in biomedical engineering and has good biocompatibility.In this study,the concept of critical overlapping concentration in the polymer physics was introduced.After theoretical calculation,this value was determined and then two concentrations were chosen,one above this value and another below.It was found that the antiparallel?-sheet structure can only be formed when the inducing agent methanol was added at high concentration.While at the low concentration,the antiparallel?-sheet structure has been formed before adding methanol.It was further found that,at low concentration,methanol would promote the formation of an extended helical structure at the interface.All the acquired results indicate that the interactions among the protein molecules will prominently affect the formation of the protein secondary structure.b).Study the water structure inside the DNA minor groove.In the experiment,the oligonucleotide with the 3'-end modified by cholesterol was chosen as the sample.In this way,the oligonucleotide can be inserted into the lipid bilayer through the hydrophobic interaction.The calcium ion(Ca²⁺)solutions with different concentrations were added as the perturbation.Then the experimental results showed that both the chiral and achiral water signals were dependent on the concentration of the Ca²⁺solution when the concentrations of the Ca²⁺ions were in the range from?0.006 m M to?60 m M.However,when the concentration of the Ca²⁺ion solution was matched with the normal level of the human body,the achiral water signal still exhibited the concentration-dependent phenomenon,but the chiral water signal showed independence.Therefore,it can be concluded that water molecules within the minor groove are stable in structure and have the ability to resist the outer perturbation of the Ca²⁺ions to some extent and sustain the structural stability of DNA.4.Study the dynamics of the interfacial polymer chains by suing SFG.By annealing methodology,different adsorbed states of polystyrene(PS)layers were prepared on substrates,i.e.weakly adsorbed layer and irreversibly adsorbed layer.Besides,good solvent and non-solvent of PS were adopted to affect the polymer chain structure.The collected results indicated that polymer chain within the weakly adsorbed layer is relatively soft and can be affected by the quality of the solvent.However,polymer chain inside the irreversibly adsorbed layer is rigid and cannot be altered by the surrounding environment,which is believed to be correlated with its multi-adsorption.Further study on the interfacial irreversibly adsorbed layer of PS showed that there existed evolution for the polymer chains within the layer.According to the data,the evolution can be divided into two processes;one is the replacement process among different chains and the other is the local relaxation process.Furthermore,the size and number of the dewetting hole at the surface of the film were quantified by laser scanning confocal microscopy.By comparing the acquired data with the SFG spectra,it can be concluded that the evolution of the irreversibly adsorbed layer can prominently promote dewetting of the polymer thin film.Furthermore,If the transition time point between the two processes of the irreversibly adsorbed layer is extracted and defined as the critical evolution time,the plot of it as a function of the molar mass of the polymer will be an asymptotic increase.The reason is supposed to be attributed to the power law distribution relationship of the firmly-occupied chain fraction with the molar mass.While for the loosely adsorbed layer,the evolution process only contained the relaxation process.The spectra were fitted and then the relaxation time was extracted.By plotting the relaxation time as a function of the molar mass,it was found that the increase also showed an asymptotic increase.It is believed that this phenomenon should be correlated with the interfacial selectivity of SFG and the long chain nature of polymer.In summary,the interfacial structures and dynamical evolution behaviors of soft matters including polymer,protein and DNA were investigated in this thesis.The developed Fresnel coefficient model can be expanded to more complex systems.In the research of polymer,the investigation of interfacial structures and dynamical evolution behaviors is of great significance in understanding the interfacial adsorption,anti-fouling,wetting,dewetting,stability of colloids and composite materials.In the research of protein and DNA,the structural information of the interface has important application value for designing new materials.