Mechanical properties of Langmuir-Blodgett films of a discogen-DNA complex by atomic force microscopy

We have studied the mechanical properties of films of a novel ionic discogen, pyridinium tethered with hexaalkoxytriphenylene (PyTp) and its complex with DNA (PyTp-DNA) using atomic force microscope (AFM). The PyTp and PyTp-DNA complex monolayer films were first formed at air-water interface and then transferred onto silicon substrates by Langmuir-Blodgett (LB) technique. For the mechanical properties, particularly to obtain elastic modulus, we have carried out nanoindentation measurements on the LB films of PyTp and also PyTp-DNA complex. The load versus indentation curves from the nanoindentation measurements were analyzed quantitatively using Hertz model. Our analysis yields Young's modulus values of 54 and 160 MPa for the PyTp and PyTp-DNA complex films, respectively. In addition, the LB films were imaged in the tapping mode AFM to obtain topography and phase images simultaneously. The energy dissipation maps were constructed from the phase images to determine qualitatively the variation in stiffness on the film surfaces. We find that the complex film exhibits a nonuniform surface with varying stiffness while the pure film exhibits a uniform surface.