Reports: DNI750558-DNI7: Self-Assembly of Inorganic Rodlike Colloids

Zvonimir Dogic, PhD, Brandeis University

The long-term goal of the ACS-PRF grant is focused on elucidating the mechanisms by which monodisperse rod-like colloids assemble into 2D colloidal membrane, a novel structure consisting of one rod-length liquid-like monolayer of aligned rods. These methods could potentially be used for robust and easily scalable self-assembly of photovoltaic devices based on semi-conducting colloidal rods. We are pursuing these goals using a combination of experiments and computer simulations. Unlike enclosed lipid bilayers, colloidal membranes are flat two-dimensional disks with exposed edges. This presents a unique opportunity to understand the physical forces which determine the structure and fluctuations of an exposed membrane edge.

During the past year using a combination of experiments and computer simulations we have developed a novel strategy for assembling rod-like molecules into numerous well-defined structures. Our method is based on a demonstration that chirality of the constituent molecules can be used to tune the effective line tension associated with an exposed edge of a two-dimensional colloidal membrane. As a first step, we have demonstrated that in absence of chirality, mebrane’s edges assume a hemi-toroidal structure which minimizes surface tension associated with the rod-polymer interface. As a consequence, rods located at the edges’ vicinity spontaneously tilt, melting from a smectic-A into a nematic phase. Based on the famous analogy established by deGennes we have shown that the twist penetrates into the colloidal membrane in the same way that the magnetic field penetrates a superconductor. Subsequently, we have demonstrated that increasing the chirality of constituent rods reduces the line tension of the exposed edge. For sufficiently large chirality, the effective line tension effectively becomes zero resulting in spontaneous transformation of 2D flat colloidal membranes into 1D supra-molecular ribbons and many other robust structures. These results have recently been published in Nature.

We have also initiated development of a new model system of inorganic rods. Postdoctoral fellow Edward Barry has initiated a collaborative effort with a group at Naval Research Laboratory in order to develop novel methods for synthesizing inorganic rod-like particles by templating cobalt onto monodisperse rod-like viruses. Preliminary manuscript describing these results have recently been published in Journal of Nanobiotechnology

The funds from Petroleum Research Grant have been used to support graduate students and postdoctoral fellows. During the past year Edward Barry, partially supported by the PRF grant, has successfully defended his doctoral thesis. Subsequently he has obtained a highly competitive and prestigious Katz postdoctoral fellowship at Argonne National Laboratory/University of Chicago. He is currently developing novel membranes that could be used for desalination purposes.