Self-Assembly of DNA Dendrimers in the Bulk and at Interfaces

Conventional wisdom and everyday experience tell us that one cannot place two objects right on top of each other. This is as true for macroscopic objects, such as books and tables, as it is for microscopic ones, such as atoms and molecules. In-between these two extreme length scales, though, lies a mesoscopic scale of the typical macromolecules encountered in soft matter science: polymer chains or tree-like objects called dendrimers, which look like snow-flakes made of carbon. A few years ago, some very robust theoretical predictions have been made, which state that dendrimers could have, under certain conditions, their centers of mass share the same point in space and form large, macroscopic crystals made of such clusters, organized spontaneously. These crystals have been predicted to have elastic and transport properties dramatically different from common solids and to be hybrid materials sharing properties between flowing and rigid matter. In this proposal, we aim at creating these crystals for the first time in the laboratory through a close collaboration between experimentalists and theorists. Our dendrimers will be based on the most versatile and interesting polymer, namely DNA. We will be working hand-in-hand in refining theory and experiment in order to achieve the goal of creating for the first time these unusual materials with a high potential of applications in the laboratory. Additionally, we will be investigating the organization of these molecules on the interface between water and air, to explore the potential of these molecules in nanopatterning of functional surfaces.

More than 20 years ago, theorists around Christos Likos, have predicted that with sufficiently high density certain particles of matter would form a new state of matter that features the properties of both crystalline solids and flowing liquids. Scientists from Forschungszentrum Jülich, the University of Siegen, and the University of Vienna have succeeded in creating this state in a laboratory. Their experimental concept opens up the possibility for further development and could pave the way for further discoveries in the world of complex states of matter. Through their research efforts, the team was able to finally disprove an intuitive assumption that in order for two particles of matter to merge and form larger units (i.e. aggregates or clusters), they must be attracted to each other. As early as the turn of the century, a team of soft matter physicists headed by Christos Likos of the University of Vienna predicted on the basis of theoretical considerations that this does not necessarily have to be the case. They suggested that purely repulsive particles could also form clusters, provided they are fully overlapping and that their repulsion fulfils certain mathematical criteria. It proved difficult to produce particles that had the necessary characteristics for the detection of cluster crystals. However, Emmanuel Stiakakis from Forschungszentrum Jülich and his colleagues have now succeeded in achieving this aim in close collaboration with theoreticians from Vienna led by Christos Likos and polymer chemists from Siegen (Ulrich Jonas). The researchers were able to produce hybrid particles with a pompom-like structure. The core of these particles is comprised of organic polymers to which DNA molecules are attached and which stick out in all directions like the threads of a pompom. This structure enables the molecules to be pushed far inside each other and thus to be sufficiently compressed. At the same time, the combination of an electrostatic repulsion of naturally charged DNA components and a weak interaction of polymers at the centre of the constructs ensures the necessary overall interaction. In this way, the 20-year old search for cluster crystals has reached its goal, discovering thereby experimentally a novel state of matter, which combines the rigidity of solids with the diffusivity of fluids.