Towards a Tetravalent Chemistry of Colloids

We propose coating spherical particles or droplets with anisotropic nano-sized objects to allow micron-scale colloids to link or functionalize with a fourfold valence, similar to the sp3 hybridized chemical bonds associated with, e.g., carbon, silicon and germanium. Candidates for such coatings include triblock copolymers, gemini lipids, metallic or semiconducting nanorods and conventional liquid crystal compounds. We estimate the size of the relevant nematic Frank constants, discuss how to obtain other valences and analyze the thermal distortions of ground state configurations of defects on the sphere. Self-assembly or functionalization of micron-sized particles has many potential uses, including particle-based bioassays [1], catalysis [2] and photonic band gap materials [3]. However, attaching a small, predetermined number of chemical groups, DNA strands or kinesin molecules to considerably larger colloidal spheres can be an inefficient process, since the number of ligands per sphere is highly variable. If the ligand concentration in solution is kept very low, many spheres will have no ligands at all. Higher ligand concentrations require tedious sorting methods such as gel electrophoresis to select the spheres with the correct number of attached objects [4]. Controlled fabrication of tetravalent colloids would be of particular interest. Dense glassy and crystalline colloidal arrays (possibly involving DNA linker elements [5]) usually display