Isotropic and Patchy Colloids with Engineered Surface Functionality

Abstract

The formation of structures from a disordered starting point is a common feature in biological systems. Examples include formation of virus capsids from proteins, folding of proteins and formation of micro-tubules. Capturing the physics behind these self-assembly processes is not a trivial task due to two fundamental reasons. Firstly, the individual building blocks are of molecular dimensions and therefore not easily visualized. The second difficulty arises from the fact that the interactions between the building blocks are complex and compose of various contributions i.e. electro-statics, hydrophobic interactions, steric repulsion and hydrogen bonding. The importance of each individual contribution is generally difficult to extract from experimental results, since independent variation of contributions is usually not possible. In this context, the use of colloidal particles as analogues for the molecular building blocks is an attractive option. Colloids are objects with at least one dimension in the order of 1 nm – 1 μm, that are dispersed in a continuous medium. Thermal fluctuations result in constant uncorrelated movements of the colloids, similar to the motion of molecules and atoms. However, due to the relatively large dimensions of colloids compared to atoms and molecules, the dynamics are much slower. The large size and related slow dynamics facilitate direct visualization and therefore studying structure formation upon assembly of colloidal building blocks. An additional advantage of using colloidal systems is that inter-particle interactions and particle shape are tunable to a large extent. Unlike molecular systems, colloidal interactions are tunable in magnitude and range by physical/chemical alternations. This allows for the rational design of particles to either mimic molecular self-assembly or the formation of new assemblies with tailored properties. Systematic variation of either shape or interactions can in principle reveal the minimal complexity required to form a desired superstructure. The focus of this thesis can be divided into two main topics. In this first part we explore synthesis procedures for the preparation of large quantities of anisotropic, dumbbell-shaped colloids. These particles consist of two lobes of which one bears chemically modifiable moieties. In principle this allows for introducing directional interactions between the particles. Therefore, these particles are promising candidates as building blocks in self-assembly studies. In the second part we exploit chemical surface modifications of colloids to tune the inter-particles interactions. We mainly focus on Atom Transfer Radical Polymerization as the modification tool. With this technique we are able tether well-defined polymers on the colloidal surface. The physical properties of the immobilized polymers are used to steer the behavior of the resulting colloidal system. This enables us to prepare colloidal systems that showed thermo-reversible aggregation and the first colloidal system that shows out-of-equilibrium/dissipative assembly. An alternative modification strategy relies on controlling the overall charge of colloidal particles by performing a surface reaction that introduces positive moieties on a negative charged particle. The extend of the reaction determines the ratio between positive and negative groups and therefore the overall charge of the particles. These particles are subsequently used in electrostatic driven assembly to form well-defined and finite-sized colloidal aggregates.