Piezoelectric and flexoelectric effects of DNA adsorbed films on microcantilevers

DNA-based biosensors have played a huge role in many areas, especially in current global coronavirus outbreak. However, there is a great difficulty in the characterization of piezoelectric and flexoelectric coefficients of the nanoscale DNA film, because the existing experimental methods for hard materials are almost invalid. In addition, the relevant theoretical models for DNA films only consider a single effect without clarifying the difference between the two electromechanical effects on device detection signals. This work aims to present multiscale models for DNA-microcantilever experiments to clarify the competitive mechanism in piezoelectric and flexoelectric effects of DNA films on detection signals. First, a Poisson-Boltzmann (PB) equation is used to predict the potential distribution due to the competition between fixed phosphate groups and mobile salt ions in DNA films. Second, a macroscopic piezoelectric/flexoelectric constitutive equation of the DNA film and a mesoscopic free energy model of the DNA solution are combined to analytically predict the electromechanical coefficients of the DNA film and the relevant microcantilever signals by the deformation equivalent method and Zhang’s two-variable method. Finally, the effects of detection conditions on microscopic interactions, electromechanical coupling coefficients, and deflection signals are studied. Numerical results not only agree well with the experimental observations, but also reveal that the piezoelectric and flexoelectric effects of the DNA film should be equivalently modeled when interpreting microcantilever detection signals. These insights might provide opportunities for the microcantilever biosensor with high sensitivity.