Self-assembled lipid tubules: Structures, mechanical properties, and applications.

摘要

Self-assembled lipid tubules are interesting supramolecular structures for both basic research and technological applications. In this thesis work, we have synthesized lipid tubules of 1,2-bis(tricosa-10,12-dinoyl)- sn-glycero-3-phosphocholine (DC8,9PC) by self-assembly and polymerization in solutions. The structures of DC8,9PC lipid tubules are characterized by transmission electron microscope (TEM), atomic force microscope (AFM), and optical microscope. TEM images confirm that lipid tubules are hollow cylinders with open ends, indicating their high surface-area to volume ratio. The external diameters of DC8,9PC tubules are 0.5 mum with little variation, but their length varies from 5 mum to 100 mum, and their wall thickness varies from a single bilayer to a stack of 14 lipid bilayers. The self-assembly mechanism of DC8,9PC lipid molecules is studied by imaging molecular ordering in lipid tubules with liquid crystals as an optical amplification-probe. This work demonstrates for the first time that both uniform and modulated molecular tilt orderings exist in the tubule walls, which have been predicted by current theories.;Axial and radial mechanical properties of DC8,9PC lipid tubules are studied with different loading methods. We find that the interface tension of the shrinking liquid droplets exerts compression force on the ends of the trapped lipid tubules, and causes them to buckle. This provides an efficient method to measure their mechanical properties. The bending rigidity and axial Young's modulus is calculated to be ∼ 2.6x10-18 Nm 2 and ∼ 1.07 GPa, respectively. As the strain energy of the buckled tubules build up, they poke through the interface of shrinking liquid droplets and then adhere onto glass substrates to form looplike shapes. The persistence length of DC8,9PC lipid tubules is measured accordingly to be ∼ 41 mum by applying worm-like chain model. Radial deformation of the DC 8,9PC lipid tubules is studied using AFM tips as nanoindenters. From measured force-distance curves, a reversible linear region that persists up to indentations of 15% of the tubule diameter is observed. We find that the elastic responses of DC8,9PC lipid tubules is sensitive to the thickness of tubule walls and the position along the long axis of DC 8,9PC tubules. Finite element models have been established to model the lipid tubule---AFM tip system and to simulate the indentation process. Comparing simulating results with experimental force-displacement curves, the radial Young's modulus is estimated to be 705 MPa for DC8,9PC lipid tubules. The difference in axial and radial Young's moduli suggests an anisotropy of DC8,9PC lipid tubules in terms of their mechanical properties. Other interesting mechanical behaviors such as recovery and surface stiffening are observed and their mechanisms are discussed.;Due to the high aspect ratio of lipid tubules, the hierarchical assembly of lipid tubules into ordered arrays and desired architectures, which is critical in developing some of their applications, remains to be challenging. Two efficient methods for fabricating ordered arrays of lipid tubules on solid substrates have been developed. In the first method, DC8,9PC lipid tubules are positioned and aligned by combining surface patterning and dipcoating. The moving contact line of air-liquid interface during the dipping process is able to align lipid tubules on patterned Au substrates formed by microcontact printing. The density, position, and orientation of lipid tubules on patterned Au substrates are controlled by adsorption time, surface pattern, withdrawal direction and rates. In the second method, DC8,9PC lipid tubules are aligned and confined in the recessed channels of a thin poly(dimethylsiloxane) (PDMS) stamp with capillary action. We find that the aligned lipid tubules can serve as an "ink" for microcontact printing. 2-D ordered arrays and 3-D cross-bar junctions are constructed on planar Au-coated mica, patterned Au electrodes, and curved glass tubules. It is believed that these two methods will open up simple ways to integrate lipid tubules with current fabrication technology and devices.;The hollow cylindrical shape and molecular order of bilayer walls, coupled with moderate stiffness, make DC8,9PC lipid tubules attractive as templates for controlled deposition of inorganic nanomaterials. Hybrid silica-lipid tubes are synthesized by sol-gel condensation of tetraethoxysilane (TEOS) on DC8,9PC lipid tubules. Meanwhile, by filling nematic liquid crystal into parallel aligned DC8,9PC tubules, an ordered array of optical anisotropic fibers are formed. These hybrid materials have great potentials in heterogeneous chiral catalysis and separation, sensors, photonic devices, and optical communications.