Self-assembly of DNA-coated colloidal particles

Nature itself is an illustration of the fact that highly complex, functional systems can self-assemble from smaller, suitably pre-fabricated units. The grand challenge in nano-material design is to apply a similar approach in the design of complex, functional materials. To achieve this goal, two key problems have to be faced: one is to design suitable building blocks, defined---for instance---by their physical or chemical properties and their shapes. The other is to create the right physical conditions that cause these entities to come together so that they can self- assemble into the desired structures. The aim of the present project is to explore the concept of selective self-assembly for a particular system, namely DNA-coated colloidal particles. In particular, we aim to develop and use theoretical-numerical tools that will allow us to predict whether such particles can assemble to form complex three-dimensional target structures. The choice for this particular system is motivated by two factors: first and foremost, the building blocks for such systems can at present be produced relatively easily in experiments. In this process, several double-stranded DNA fibres with a short, single-stranded DNA sequence ("sticky end") protruding from it, are chemically grafted onto the surface of colloidal particles. The DNA linkages give unique selectivity to the design process. Secondly, it has been shown that such units can then assemble in rather simple crystal arrangements which are stabilised by the hybridisation between the sticky ends of DNA linkers on neighbouring colloidal particles. Having said that, our theoretical understanding of the static and dynamic properties of such DNA-coated colloid systems is still very limited. In this project, we first plan to study the properties of spherical DNA-coated colloidal particles. In particular, we aim to investigate the effects of characteristic system parameters, such as the number of DNA linkers, their length, and their flexibility on the behaviour of the system. To this end, we intend to employ computer simulations that we will suitably extend to guarantee an accurate evaluation of the free energy. As a first step, the interaction between two such colloidal particles as a function of the distance will be computed. In the subsequent step we will evaluate the equilibrium phase diagram of the systems at hand. Inspired by the fact that colloidal particles can nowadays be synthesised in a rich variety of shapes, we are going to extend the investigations outlined above to non-spherical particles (e.g., rods, plates, and polyhedra). In particular, we will address the question whether the use of non- spherical building blocks allows the system to form anisotropic crystals, or even novel kinds of ordered structures. Moreover, we plan to investigate whether the additional use of stiff spacers between the colloidal particles might support the growth of rather open crystals instead of close-packed particle arrangements. A further topic of interest that we plan to investigate towards the end of the proposed project is the kinetic behaviour of such systems. Both the host research group as well as the University of Cambridge in general offer the ideal setting for the realisation of this project. Not only am I going to profit from Prof. Daan Frenkel`s substantial expertise in the field of theoretical and numerical physics and chemistry, but also from the vicinity of world-leading experimental groups that work on closely related topics.