Self-assembly in nanoscale and mesoscale regimes: Biomolecular complexes and photonic crystals

In this work, we have investigated two self-assembled systems in the nanoscale (<100 nm) and mesoscale (between 0.2 and 2 μm) regimes. The nanoscale self-assembly involved biomolecular complexes made from the cationic lipid, didodecyldimethyl ammonium bromide (DDAB), the neutral lipid, dilauroyl sn-glycero phosphocholine (DLPC) and the anionic polypeptide, poly(glutamic acid) (PGA). Small angle X-ray scattering data are consistent with a condensed multilamellar structure with the PGA sandwiched between the bilayers of the lipids. Lipid dilution experiments at the isoelectric point of the complex resulted in large increase in the interlamellar “d” spacing from 39 Å for the pure DDAB membrane to 60 Å at very high dilutions. The data are consistent with a model of a “pinched lamellar” phase of the complexes where the PGA molecules and DDAB associate to form localized pinched regions. Between these “pinches” the system behaves as a nearly pure DLPC membrane with a “d” spacing of 60 Å.; The mesoscale self-assembly involved colloidal crystals that were used as templates in the fabrication of ordered macroporous materials. We present a new way of making macroporous materials called colloidal templating that employs aqueous colloidal suspensions of nanoparticles as the “precursor”. These nanoparticles when mixed with monodisperse emulsions droplets or latex suspension, form a particulate gel around the emulsion droplets or latexes upon slow evaporation of water. Removal of the template results in a macroporous material. Emulsion templates were used to fabricate macroporous bulk gels and thin films of silica. Relatively crack-free bulk gels with volume greater than 0.5 cm3 could be easily produced because of low shrinkage associated with particulate gels. Use of latex templates produced highly ordered macroporous materials such as titania, silica, alumina, and silicon with potential applications as photonic crystals. Optical experiments on macroporous titania revealed the presence of photonic pseudogaps. Again the low shrinkage enabled us to produce larger samples with dimensions of several millimeters, a significant improvement over the sol-gel process that produces powders.