Influence of disordered packing pattern on elastic modulus of single-stranded DNA film on substrate.

Determining mechanical properties of single-stranded DNA film grafted on gold surface is critical for analysis and design of DNA-microcantilever biosensors. However, it remains an open issue to quantify the relations among the disordered packing patterns of DNA chains, the mechanical properties of DNA film and the resultant biodetection signals. In this paper, first, the bending experiment of microcantilever is carried out to provide the basic data for a refined multi-scale model of microcantilever deflection induced by ssDNA immobilization. In the model, the complicated interactions in DNA film (consisting of DNA, water molecules and salt ions) are simplified as effective interactions among coarse-grained soft cylinders, which can reveal the varieties of DNA structure in the circumstances of different lengths and salt concentrations; Ohshima's distribution of net charge density is employed to incorporate compositional variations of salt ions along the thickness direction into the Strey's mesoscopic empirical potential on molecular interactions in DNA solutions, and the related model parameters for ssDNA film on substrate are obtained from the curve fitting with our microcantilever bending experiment. Second, the effect of nanoscopic distribution of DNA chains on elastic modulus of ssDNA film is studied by a thought experiment of uniaxial compression, and the disordered patterns of DNA chains are generated by Monte Carlo method. Simulation results point out that nanoscale ssDNA film shows size effect, gradient and diversity in elastic modulus and can achieve maximum stiffness by preferring a disordered and energetically favorable packing pattern collectively induced by electrostatic force, hydration force and configurational entropy.