Pressure Sensitive Behavior Of Polymer Composite Films Based On The Orientations Of Gold And Silver Nanoparticles

In recent years, plasmonic noble metal nanoparticles have received considerable attention due to their broad applications in optoelectronics, sensing, catalysis and photothermal therapy. In the field of sensing, chemicals and biological molecules can be detected based on localized surface plasmon resonance(LSPR), but pressor sensing based on this phenomenon has been rarely studied. Moreover, sensing applications based on LSPR only used the change of plasmon resonance with respect to dielectric environment, morphology of nanoparticles and assembly behaviors. Its change with other factors, such as orientation of nanoparticles, was not utilized. In this work, pressure sensitive films have been designed and fabricated by taking advantage of the orientational dependence of LSPR of noble metal nanoparticles. The pressure sensitive behavior of these polymer composite films were evaluated using the change of intensity ratio of resonance peaks, and the pressure sensitive behavior was analysized by numerical simulation.The plasmon resonance behaviors of rod-like and plate-like noble metal nanoparticles with different orientations were simulated using discrete dipole approximation. Then a model of pressure sensing based on the orientational change of noble metal nanoparticles was proposed based on the simulation results. During the deformation of noble metal nanoparticle-polymer composite films, the orientation of nanoparticles changes from random distribution to a new distribution which is more inclined towards the direction perpendicular to the external force, resulting the change of intensity ratio of resonance peaks, which can be utilized to measure the applied pressure.A series of noble metal nanostructures were synthesized by seeded growth method and hot-injection method, including gold nanorods, silver nanoplates, silver@gold composite nanoplates and gold nanoparticles. The aspect ratios of Au nanorods with different aspect ratios obtained by seeded growth method are 2.06, 2.31 and 2.60, and their longitudinal resonance peaks are located at 651, 688 and 726 nm. The average edge and thickess of Ag nanoplates which is triangular is 135 nm and 15 nm, and the in-plane dipole resonance peak position is 793 nm. The thicknesses of gold shell of Ag@Au composite nanoplates are 3, 4 and 5 nm, respectively, and their in-plane dipole resonance peaks are located at 758, 786 and 819 nm. Au nanorod-PVA composite films and Ag@Au composite nanoplate-PVP composite films that are pressure sensitive are fabricated by solution casting. Various noble metal nanostructures are dispersed well in the films, and their LSPR peaks show red shifts due to the increase in the refractive index of the surrounding medium.By analyzing the change of extinction spectra of noble metal nanoparticlepolymer composite films, how to use this change to represent the pressure was discussed. Based on defining the intensity of different resonance peaks, the change of intensity ratio of resonance peaks were introduced. Then the influence of the intensity and duration of applied pressure, the morphology parameters of nanoparticles and the proportion of PEG in the films on the pressure sensitive behavior of polymer composite films is studied in the experiments. The magnitude of the change of intensity ratio of resonance peaks depends on the intensity and duration of applied pressure, indicating that the orientational change of noble metal nanoparticles driven by the deformation of the surrounding polymer matrix under pressure could be utilized to construct pressure sensitive films for measuring the applied static pressure. The measuring ranges of Au nanorod-polymer films and Ag@Au composite nanoplate-polymer films are 6.93~27.72 MPa and 13.86~55.44 MPa, respectively, with accuracy of about ?8%. The sensitivity of pressure sensitive films can be adjusted through the proportion of PEG in the films, as well as the aspect ratio of Au nanorods. The change of intensity ratio of resonance peaks which indicates the applied pressure rises with the increase of the proportion of PEG or the aspect ratio of Au naonorods.From mechanical analysis, a simple planar extensional flow is expected in the polymer matrix when the polymer composite films deforms with the application of external force. The mathematical expression, which describes the orientational change of particles, was deduced from Jeffery theory explaining the orientational change of rod-like particles(or plate-like particles) in a fluid, indicating the orientational change of particles depends on the polymer deformation and the aspect ratio of particles. By combining the intensity representations of different resonance peaks of noble metal nanoparticles, the orientational change of particles in different deformation conditions and the change of intensity ratio of resonance peaks with the intensity and duration of applied pressure, we obtained the relationship between the polymer deformation and the intensity and duration of applied pressure. The mathematical expression of polymer deformation was obtained from the simulation of deformation data using Burgers model, and then the simulation results were acquired which describes the change of intensity ratio of resonance peaks with the intensity and duration of applied pressure. The proposed model in this thesis can simulate the change of intensity ratio of resonance peaks with the intensity and duration of applied pressure, and the errors are less than 10% except the situation of shorter duration.