Synthesis and Self-Assembly of Nanoparticles Mediated by Phospholipid

Abstract

Nanotechnology based on nano-sized materials is believed to play a large part of future technology in the 21st century together with biotechnology and information technology. Key to the successful application of nanotechnology on an industrial scale is the ability to manipulate these nano-objects into a spatially ordered pattern. In general, the low throughput and high capital investment of physical template, various organic molecules have been used to drive the self-assembly of metal nanoparticle. In spite of solution-based chemical methods have a long history dating back to the pioneering work of faraday on the synthesis of aqueous Au colloids, recently, increasing pressure to develop green chemistry, eco-friendly methods for synthesis and self-assembly of metal nanoparticles have resulted in researchers turning to biological organisms for inspiration. However, to generate the 2-dimensional ordered nanoparticle array extending to millimeter or centimeter scale or more complicates nanoparticle superstructure, lithographic and electron beam writing technique is still a prerequisite. To overcome the limitation, it was attempted to use a phospholipid as a template to synthesize and self-assemble the metal nanoparticle. Phospholipid typically consists of one hydrophilic head group and two hydrophobic tail groups. The head group contains the phosphate group, providing the hydrophilicity while the fatty acids in the tail group generate the hydrophobicity. Such amphiphilic property enables generation of multitude of microscopic lipid structures: micelle, vesicle, bilayer, microtubules and nanotubes and would also render the phospholipid an ideal material to drive the assembly of metal nanoparticles. In this research, a monolayer of noble metal nanoparticle, such as Ag and Au nanoparticle (~ 6 nm in size), as well as metal core-metal oxide shell nanoparticles, such as Sn-SnO2, In-In2O3, and Bi-Bi2O3 nanoparticles (~10 nm in size), were produced by direct deposition onto a solid-supported liquid-crystalline phospholipid membrane and the formation mechanism of nanoparticle chemically interacting with lipid membrane was studied in detail. I also demonstrated that the structure and morphology of the resulting nanoparticles was correlated to the chemical and physical properties of the deposited metal. Furthermore, the lipid membrane can be dissolved in a solvent in order to reclaim the nanoparticles for further processing such as formation of 2 - or 3 - dimensional ordered superlattice nanoparticle array or a honeycomb-like superstructure. It is also possible that functionalization of the surface of nanoparticles with others bio-materials for using bio-medical applications. The shape of Au nanoparticles encapsulated with lipid molecules can be modulated by heat-treatment at low temperature. Because the lipid membrane, being in a liquid crystalline state, is flexible, and chemically bound to the Au nanoparticle, the mobility of the encapsulating lipid molecules induces the diffusion of Au atoms to change spherical shaped Au nanoparticle into triangular or hexagonal shaped nanoparticles during heat-treatment at 80oC. These biocompatible Au nanoparticles array can be led to many biomedical applications apart from applications in catalysis and sensors. Phospholipid molecules also enable long-range ordering, in particular allowed self-assembling into a 2-dimensional superlattice over an area extending 2 cm x 2 cm using spin-coating by encapsulating Fe3O4 nanoparticles (~10 nm in size). It is indicated that the polydispersity of the nanoparticles can be improved through lipid-mediated mass distribution. The proposed method can be easily extended to other metals or metal oxide whose properties can be engineered as desired through the controlling the mobility of lipid molecules chemically bound to the particle surface. It is also expected that the dispersion process is better controllable and easier to scale up to an industrial process.