Tuning DNA-amphiphile condensate architecture with strongly binding counterions

A. V. Radhakrishnan, S. K. Ghosh, G. Pabst, V. A. Raghunathan, A. K. Sood
Proceedings of the National Academy of Sciences. 2012-04-09; 109(17): 6394-6398
DOI: 10.1073/pnas.1115541109

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Electrostatic self-assembly of colloidal and nanoparticles has attracted a lot of
attention in recent years, since it offers the possibility of producing novel
crystalline structures that have the potential to be used as advanced materials
for photonic and other applications. The stoichiometry of these crystals is not
constrained by charge neutrality of the two types of particles due to the
presence of counterions, and hence a variety of three-dimensional structures have
been observed depending on the relative sizes of the particles and their charge.
Here we report structural polymorphism of two-dimensional crystals of oppositely
charged linear macroions, namely DNA and self-assembled cylindrical micelles of
cationic amphiphiles. Our system differs from those studied earlier in terms of
the presence of a strongly binding counterion that competes with DNA to bind to
the micelle. The presence of these counterions leads to novel structures of these
crystals, such as a square lattice and a √3 x √3 superlattice of an underlying
hexagonal lattice, determined from a detailed analysis of the small-angle
diffraction data. These lower-dimensional equilibrium systems can play an
important role in developing a deeper theoretical understanding of the stability
of crystals of oppositely charged particles. Further, it should be possible to
use the same design principles to fabricate structures on a longer length-scale
by an appropriate choice of the two macroions.


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