Surfactant–DNA interactions at the liquid crystal–aqueous interface

Abstract

The presence of single-stranded (ssDNA) vs. double-stranded (dsDNA) DNA at a surfactant-laden aqueous–nematic liquid crystal (LC) interface results in distinctly different orientations of the LC molecular axis; this is of practical interest as a method to detect DNA hybridization. Results presented here provide new insights into the molecular-level mechanisms of these phenomena. The adsorption of ssDNA to a cationic surfactant-laden aqueous–LC interface caused LC reorientation, leading to coexistence between homeotropic and planar (birefringent) oriented regions. Fluorescence microscopy revealed that ssDNA preferentially partitioned into the birefingent regions, presumably causing a decreased surface coverage of surfactant and the resultant planar LC orientation. Both electrostatic and hydrophobic effects were found to be critical to inducing LC reorientation. In particular, insufficient ssDNA adsorption occurred in the absence of a cationic surfactant (e.g. with no surfactant or with a non-ionic surfactant), demonstrating the importance of electrostatic interactions with the polyanionic ssDNA. Even in the presence of a cationic surfactant, however, polyanions without hydrophobic side-group moieties (poly[acrylic acid] and dsDNA) caused no LC reorientation, while polyanions with hydrophobic side groups (polystyrene sulfonate and ssDNA) initiated the desired LC reorientation. These observations are consistent with the fact that interfacial hybridization of adsorbed probe ssDNA to complementary target ssDNA caused a reorientation from planar back to homeotropic. We propose that ssDNA forms an electrostatic interfacial complex with cationic surfactant where the hydrophobic nucleobases associate directly with the LC phase, effectively competing with surfactant molecules for interfacial sites. Upon hybridization, the hydrophobic character of the ssDNA is lost and the nucleobases no longer associate directly with the LC phase, allowing the surfactant molecules to pack more closely at the interface.