Gopakumar Gopalakrishnan
The central goal of this thesis work is to fabricate novel, functional fluorescent nanostructures in confined systems offered by phospholipid membranes, which are known to have highly ordered, thermotropic and lyotropic structures. In separate approaches, we have used three different lipid systems: multilamellar planar lipid membranes, unilamellar vesicular membranes as well as lipid monolayers for the development of functional fluorescent nano-, micro- and meso-scopic structures. Techniques like fluorescence microscopy, single particle imaging, electron microscopy, electron diffraction were used to achieve fundamental understanding of the resulting structures. Multilayer stacks of phospholipid membranes have been used as an effective template for the growth of high aspect ratio fluorescent nanowires. The room temperature synthesis was achieved in the confined nanometer-sized interlamellar water space of lipid multilayers where supersaturating CdCl2 concentrations were induced by acidification leading to controlled unidirectional growth of nanowires. The possibility to render the nanowires fluorescent by doping with CdS quantum dots (QDs) and the light waveguiding along hundreds of micrometers together with the possibility of lateral manipulation make these nanowires attractive candidates for future optoelectronic applications. Novel organic-inorganic functional nanocontainers have been designed and tested by making use of vesicle forming lipid bilayers in combination with semiconductor QDs. Hydrophobic QDs can be integrated into bilayers of lipid vesicles and such lipid/QD hybrid vesicles are capable to fuse with live cells, thereby stain the cell's plasma membrane selectively with fluorescent QDs and transfer the vesicle's cargo into the cell. Modification of the membrane of such hybrid vesicles on the other hand, made them capable to enter the cytoplasm of live cells. Additionally, these hybrid vesicles were found extremely useful for long-term model membrane imaging studies. The results described in this thesis imply that cell and lipid membranes can integrate any kind of hydrophobic nanoparticle whose size matches the membrane thickness, opening novel possibilities to manipulate them as individuals or in ensemble with wide-ranging applications for nanobiotechnology. In a further step, the ability of phospholipid molecules to exhibit lamellar to non-lamellar transition using external stimuli enabled a directed self-assembly of QDs into mesoscale fluorescent structures. An easy and versatile method for the surface modification of TOPO coated CdSe QDs using 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid (DPPA) have been achieved. DPPA predominantly form non-lamellar phases when dispersed in water, for example, upon addition of Ca2+ or at a pH below 6 are known to form hexagonal II phases. This particular property of DPPA has been exploited to form mesoscale self-assemblies of QD based structures both in solution and in confined systems. Potential applications include the detection or removal of Ca2+ ions in attoliter volumes, the construction of functional devices where QDs are reversibly organized in different forms as well as use of fluorescently labeled DPPA molecules for cellular studies using fluorescence microscopy.
EPFL2006
