Specific aims of this study are to investigate the mechanism that governs the surface stress generation with hybridization of single stranded DNA (ssDNA) molecules immobilized on micro-cantilevers. The hybridization of DNA on cantilever surfaces leads to configurational change, charge redistribution, and steric hindrance between neighboring hybridized molecules, which result in surface stress change and measureable cantilever deformation. Differential interferometer with two adjacent micro-cantilevers (a sensing/reference pair) was investigated to measure the cantilever deformation. The sensing principle is that binding/reaction of specific chemical or biological species on the sensing cantilever transduces to mechanical deformation. The differential bending of the sensing cantilever respect to the reference cantilever ensures that measured response is insensitive to environmental disturbances. In order to improve the sensitivity for sensing system, new approach of immobilization was utilized to enhance the deformation of the cantilever surface. Immobilization of receptor molecules was modified to use ssDNA with thiol-groups on both 3' and 5' ends; therefore both ends of the ssDNA molecules were immobilized to the gold surface and cause stronger surface interactions. To confirm the improvement of the sensitivity of the system, surface stress change associated with hybridization of ssDNA and malachite green-aptamer binding was measured.

To explore the mechanism under the surface stress change associated with ssDNA hybridization. A general beam bending model was established based on the minimization of the total energy of the system. The energy consisted of the bending energy of the cantilever and the in-film energy due to the hybridization of ssDNA on the surface. Different stages of immobilization were proposed according to the different immobilization densities and immobilization approaches, the in-film energy associated with each stage was investigated. Numerical predictions are carried out with different stages and compared to the experimental observations, and the findings confirmed the capability of the beam bending model to use in surface stress predictions.