Structural characterization of nanoparticles from thermoresponsive poly(N-isopropylacrylamide)-DNA conjugate.

Poly(N-isopropylacrylamide) (PNIPAAm) grafted with single-stranded (ss) DNA conjugate (PNIPAAm-g-DNA) self-assembles above its lower critical solution temperature to form colloidal particles. When the ssDNA within the particle hybridizes with its complementary DNA, the particles aggregate above a certain threshold of salt concentration with drastically increased turbidity in solution. Detailed structural information of the particle was obtained mainly by small-angle X-ray scattering. The influence of copolymer composition on the morphology of particle and non-crosslinking aggregation was examined. The particle consists of hydrophobic PNIPAAm core surrounded by hydrophilic DNA strands. The increase in DNA fraction brought about a significant decrease in core size, whereas the shell thickness little changed and corresponded to the length of DNA. A structural model with a sticky potential was applied to the analysis of particle aggregate. This analysis provided that the particles aggregate while the coronal layers interpenetrate each other. The interaction between the particles was quantified in terms of the sticky potential and showed a trend to be influenced by the particle size rather than the graft density of DNA strands on the particle.