Prism coupler and microscopic investigations of DNA films

DNA is a polyelectrolyte capable of forming thin films with interesting optical properties. We investigated refractive indices and optical anisotropy of films of the native, sodium ion-based DNA (Na-DNA) and DNA bearing the cetyltrimethylammonium ion (DNA-CTMA) using a prism coupler technique. The light polarization direction was either parallel (n_TE) or perpendicular to the surface plane of the films (n_TM). The index values and the birefringence of DNA films vary considerably depending on the type of the counter-ion, the film fabrication method and the relative humidity (RH) of the environment. A high negative birefringence in films of Na-DNA, n _TE-n_TM = -0.03 at an RH ∼ 55 %, was measured in solution-cast films, indicating that the optically anisotropic DNA molecules are aligned in the plane parallel to the film surface. Refractive indices of DNA-CTMA thin films were smaller and more isotropic than those for films of Na-DNA polymer. The prism coupler reflectance curves showed a hysteresis of the index values when the RH of a DNA-CTMA film environment varied. Polarization microscopy studies showed liquid-crystalline textures at the edges of Na-DNA and DNA-CTMA films.