In situ neutron reflectivity studies of the adsorption of DNA by charged diblock copolymer monolayers at the air–water interface

Abstract

The absorption of 700 bp DNA by a charged diblock copolymer monolayer at the air–water (D2O) interface was investigated by surface pressure measurements and neutron reflectometry. The amphiphilic diblock copolymer PS-P4VPQ with molecular weights of 19 600 and 12 000, respectively, formed a stable monolayer at the air–water interface, with PS blocks in a highly packed layer at the surface, a dense P4VPQ brush layer adjacent to the PS and a loose P4VPQ brush layer extending into the aqueous subphase. Most of the P4VPQ segments were distributed in the dense brush layer and only 7% of the P4VPQ segments were contained in the loose brush layer. Neutron reflectometry revealed that the distinct two-layer brush structure remained essentially unchanged even upon adsorption of large amounts of DNA (up to a volume fraction of ∼29%). As compared with the DNA adsorption by cationic lipid monolayers, the PS-P4VPQ monolayer has a higher capability to adsorb DNA through its three-dimensionally structured brushes. In the presence of calcium ions in the subphase, the DNA adsorption in both the dense and loose brush layers was significantly enhanced (up to a volume fraction of ∼73%). For a calcium ion concentration of 80 mM, an additional DNA condensation layer was observed below the loose P4VPQ brush layer.