  DNA plays a special role in polymer science not just because of the highly
selective recognition of complementary single DNA strands but also because
bacteria can express DNA chains that are very long yet perfectly monodisperse.
The latter reason makes long DNA molecules widely used as model systems in
polymer science. Here, we report the unusual self-assembly that takes place in
systems of colloids coated with very long double-stranded DNA. In particular,
we find that colloids coated with such long DNA can assemble into unique
floating crystalline monolayers. Floating colloidal structures have potentially
interesting applications as such ordered structures can be assembled in one
location and then deposited somewhere else. This would open the way to the
assembly of multi-component, layered colloidal crystals.
