The Effect of the Electrostatic Field on the Deflection of Microcantilever Biosensors

Abstract

Microcantilever biosensors offer the capability to detect specific molecular binding like complementary DNA fragment hybridization or specific antibody-antigen binding. The cantilever deflection, which can be optically detected, is caused by the adsorption of biological molecules like DNA fragments upon the microcantilever surface and the subsequent specific binding to the complementary species. The cantilever deflection is due to the surface stress induced by the free energy variation on the cantilever surface. Contributions to the free energy variation come from a number of interactions within the molecules, such as electrostatic interactions, biomolecule conformational entropy and internal energy variation, hydration forces. In the present work the effect of the electrostatic field within DNA biomolecules on the cantilever deflection is investigated. The electrostatic field within double strand DNA molecules is studied by means of a Finite Element (FE) analysis aimed at numerically solving the non linear Poisson Boltzmann equation (PBE) in the domain representing the biomolecule system. The electrostatic analysis has been coupled to a FE structural analysis in order to evaluate the influence of the electrostatic field on the cantilever deflection. The double strand DNA molecules are modelled as a periodic disposition of cylinders negatively charged at the surface. The hexagonal and square DNA molecule patterns were compared, and the Manning condensation hypothesis was discussed. The results are shown for different operating conditions and compared with experimental data from literature.