Adsorption behavior of DNA onto a cationic surfactant monolayer at the air–water interface

This communication reports the adsorption of DNA to the preformed Langmuir monolayer of cationic surfactant Octadecylamine (ODA) at the air–water interface and thereby formation of ODA/DNA complex monolayer at the interface. Effect of concentration of DNA in the subphase as well as subphase pH on the adsorption of DNA onto ODA monolayer assemblies have been studied by monitoring the change in surface pressure of ODA/DNA complex monolayer as a function of time. The complex monolayer was also transferred onto solid substrate to prepare ODA/DNA Langmuir–Blodgett films which were analyzed by UV–vis absorption, ATR–FTIR spectroscopic techniques. The most significant observations is that the extent of interactions between ODA and DNA at the air–water interface increases with increasing concentration of DNA in the subphase and also subphase pH. At higher pH, hydrophobic interaction dominates over electrostatic interaction between DNA and ODA in the aqueous subphase. DNA immobilized in the backbone of ODA lies almost flat or extended onto solid substrate at neutral pH whereas, they lie aggregated and compacted coil rather than flat when adsorbed from high pH namely, 11.5 of the subphase. This was confirmed by atomic force microscopy of these complex LB films onto solid substrate.