Study On Thermal Effect Of DNA-microelastic Structures

The microcantilever-based experiment, scaling theory of polymer brushes, liquid crystaltheory of DNA solution, two-variable method for laminated beams, etc., are utilized to investigatethe influences of thermal effect and intermolecular interactions on the nanomechanical responsesof DNA-microelastic structures in label-free biodetections. The relation between the deflection,resonance frequency shift of a DNA-microelastic structure and DNA molecular structure feature,ionic strength of buffer solution, mechanical properties of non-biological layers, temperaturechange, etc., is established. Main works are as follows:(1) Using optical lever technique based on a microcantilever sensor, the real-time deflections of aDNA-microcantilever during the immobilization and hybridization processes are measured.The influences of nucleotide number and temperature change on the deflection signals of aDNA(ssDNA or dsDNA)-microcantilever are investigated. This result shows the influence oftemperature change on the deflections of a DNA-microcantilever cannot be ignored.(2) Based on de Gennes’s scaling theory for polymer brushes and Zhang’s two-variable methodfor laminated beams, an energy model for nanomechanical motion of assDNA-microcantilever is presented. The effects of nucleotide number, grafting density, andtemperature change on deflections are discussed. The comparisons of numerical predictionsand experimental data from Chapter2suggest that, although the conformational entropy is animportant factor, it is necessary to investigate the influence of other intermolecularinteractions (e.g. electrostatic force) on deflections.(3) By means of Strey’s liquid crystal theory for DNA solutions, the contributions of threemicroscopic interactions including electrostatic force, hydration force, and conformationalentropy to the deflections of a DNA-microcantilever are discussed. The physico-chemicalparameters for a DNA-microcantilever during the immobilization and hybridizationprocesses are accomplished by curve fitting with the experimental data from Chapter2underisothermal conditions. This theoretical model elucidates the experimental phenomenon ofupward or downward motions induced by DNA hybridization. The proposed model alsopredicts that there exists a failure phenomenon, which will make the differential deflectiontechnique invalid. A failure analysis induced by interactions between DNA molecules andchange of ionic strengths is discussed.(4) Considering thermoelastic energy of non-biological layers and free energy of DNA biofilm,an analytical model for the deflections of a DNA-microcantilever is presented under thecombination of mechanical and thermal loadings. The predicted deflections are comparedwith the experimental data from Chapter2to validate the applicability of this model. Under different conditions (e.g. grafting density, nucleotide number, ionic strength of solution, andsubstrate material), the controlling temperature is obtained. And the thermal correlation ofDNA-microcantilevers is discussed. At the same time, the macroscopic thermal expansioncoefficient of a DNA biofilm on the basis of continuum mechanics viewpoints is obtained.The difference between the near-surface system (DNA-microcantilever) and the osmoticpressure solution system is discussed.(5) An analytical model is presented to predict the dynamical response of aDNA-microcantilever according to Hamilton’s principle. The effects of the sort of thematerial of non-biological substrate layer, DNA molecular structure features, and temperaturechange on the resonant frequency of a DNA-microcantilever are discussed. Results indicatethat the influences of substrate materials and types of DNA molecules on the resonantfrequency shift of a DNA-microcantilever are prominent. In most cases, the resonantfrequency of a DNA-microcantilever decreases with DNA adsorption. However, the resonantfrequency of a dsDNA-SU8-microcantilever exists increase or decrease. At the graftingdensity of0.25#/nm2, the resonant frequency of a dsDNA-SU8-microcantilever keeps almostno change, which makes the dynamical detection fail. In addition, compared to the staticmode of operation, the dynamic mode of a DNA-microcantilever has a higher thermalstability.(6) A variational method is presented to formulate an analytical model for the axisymmetricbending of a DNA-thin-circular-plate under the combination of axisymmetric mechanical andthermal loadings. The comparisons of the proposed four-layered plate model and the reducedtwo-layered plate model are discussed. The effect of temperature change on deflections of aDNA-plate is studied. Results show that the contribution of PDMS and Ti layers tonanomechanical deflections of a DNA-plate should be considered. As the influence oftemperature change on deflections of a DNA-plate cannot be ignored, the temperature changeshould be carefully controlled, especially at a low grafting density, with a small nucleotidenumber, and at a high ionic strength of solution.