Lyotropic Liquid Crystal Templating Method Assembly Of Noble Metal Nano-structured Materials

Synthesis of noble metal nanostructures has been an active research area for many decades, because of the importance of these materials for catalysis, photography, electronics, photonics, information storage, optoelectronics, biological labeling, imaging and sensing. A lot of efforts have been focused on controlling of the shape, size, crystallinity and structures. Particular emphasis has recently been placed on the control of shape, because in many cases it allows one to finely tune the properties with a greater versatility than can be achieved otherwise. In this dissertation lyotropic liquid crystals are used as templates to obtain noble metal nanostructures with controllable shape and size. Three main experimental researches are included.In the first part, anisotropic gold and silver nanostructures are prepared from LLC hexagonal phase templates made of PEO-PPO-PEO block copolymer and water. Single-crystalline gold nano- and microplates. with triangular or hexagonal shapes. are successfully synthesized for the first time with a large-scale amount from LLC templates. And long silver nanofibers are also obtained. POM (polarized optical microscope). SAXS (small angle X ray scattering). TEM (transmission electron microscope) and UV-vis spectrum measurements are used to investigate effect of different conditions on the product, including reduction time, concentration of capping agents (cetyltrimethylammonium bromide (CTAB), tetrabutylammonium bromide (TBAB) or sodium di-2-ethylhexylsulfosuccinate(AOT)) and metal salts.In HAuCl₄-containing hexagonal phase, the long range ordered structure of LLC directs the reagents and the naissant crystal nuclei which are distributed in water domain separated by hydrophobic cylinders, indicating a long-range ordered nuclei arrangement. With more AuCl₄^- being reduced, such arranged nuclei would orientationally attach and coalescence with adjacent ones. This is very important for the following single-crystal growth when the selective adsorption of CTAB or TBAB on {111} crystallographic facets plays the key role. The binding between cappingagents and Au nuclei inhibits particles going randomly and favors single-crystal growth with {111} facets extending and results in large plate-like products (edge size even longer than 10 um).In AgNO3-containing LLC, the reduction rate of Ag+ is much lower than that of AuCU" Therefore the templating effect of LLC plays the most important role in Ag product formation. It is favorable for the small Ag particles aggregating in long-range ordered water domain of LLC and at last the long nanofibers (-60 um) are formed with diameter of 1-4 um. In this process, added AOT molecules reduce the lattice spacings, and their binding with Ag+ through negtive headgroups is helpful to immoblize Ag+ to prevent the Ag particle from growing larger.Obtained results suggest a novel, simple and effective way to produce particles with controllable shape and size, as well as a clue to understand the mechanism for crystal growth.In the second part of dissertation, LLC lamellar phase made of lecithin is formed to mimic biomembrane, where salt (NaCl) and biocompatible molecules including polyethylene glycol (PEG) macromolecules, nonionic surfactants (Ci2EO4, P123, F127), protein (bovine scrum albumin, BSA), amino acid (L-cysteine, Cys) and oleic acid (OA) as model drugs or drug carriers are introduced, with the aim to systematically study effects of these molecules on the biomembrane and preparation new "soft" nanomaterials. Then gold and silver nanoparticles are synthesized by templating of such lamellar phase containing nonionic block copolymers as reducers.In the neutral lecithin/H2O lamellar phase, different doped molecules will interact with lecithin bilayers by dissolving in water domain, adsorbing on lecithin headgroups or inserting themselves into hydrophobic parts of lecithin bilyers. NaCl can bind with lecithin headgroups and result in dehydration. Therefore, reduced repulsion between bilayer makes the lamellar phase more ordered as well as the lattice spacing (d) decreased. PEG, as the water-soluble macromolecule, swells the water layer and destroys the original ordered phase, inducing a phase seperation with an isotropic phase (rich PEG) and a anisotropic one (poor PEG). When doped in lecithin LLC, the hydrophobic tails of nonionic surfactant CnEO4 are inserted intothe hydrophobic part of bilayers, breaking its compact state. The EO groups will lie in the hydrophilic domain of bilayers and introduce electrostatic repulsion into the neutral system. Such arrangement of C12EO4 (and OA) decreases the phase order and increases d spacing. After doped into LLC, PPO blocks of PI23 will be inserted in bilayer through hydrophobic interaction, with PEO blocks extending in water. Due to the large volume of PPO blocks and the steric repulsion induced by PEO swelling in water, two lamellar phases are formed with poor PI23 or rich PI23 respectively. In the latter, the long PEO blocks can act as bridges between two bilayers and reduce the d spacing. F127 molecules in LLC act also in the same way. When BSA is introduced to lecithin LLC, it may be embedded in the bilayer, just like the manner of membrane proteins in native biomembranes. Its large volume induces stacking stress in bilayers, which brings about transition from lamellar to hexagonal phase at higher BSA concentration. Cys can adsorb on headgroups of lecithin and introduce electrostatic repulsion between neutral bilayers to decrease the long-range order of LLC.Templated by lecithin lamellar phases containing C12EO4, PI23 and F127. gold and silver nanostructures are prepared. The selective adsorption of lecithin headgroups on metal surfaces plays an important role in plate-like nanoparticle formation. However, such effect is weakened by the larger volume of headgroups and only a few plates can be observed. As EO blocks become longer, the reduction rate increases and the product sizes are smaller. The same effect is observed for increased HAuCL; concentration. Adding salts to LLC may change the product morphology through disturbing the headgroup adsorption on metal. For AgNO3 system, no regular silver plates can be obtained. This may be due to the slower reaction rate, which makes the templating effect more important and products smaller. The formation of irregular silver plates also suggests the headgroup adsorption on silver.Our effort for using biological LLC as template to synthesize anisotropic noble metal nanoparticles will be helpful to understand the formation mechanism of nanoparticles with controllable shape and size in biomimic system.In the last part, we successfully fabricate novel LLC made of nonionic blockcopolymer PI23 and ionic liquid (ILs) [Bmim]PF6(l-n-butyl-3-methyl imidazolium hexafluorophosphate), whose formation mechanism is investigated in detail. Moreover, the LLC phase containing of a small amount of ionic liquids ([Bmim]PF6, [0mim]PF6 (l-n-Octyl-3-methyl imidazolium hexafluorophosphate), [Ci6mim]Cl (l-n-Cetyl-3-methyl imidazolium chloride)) are used for templating synthesis of anisotropic gold nanostructures.POM and SAXS measurements showing hexagonal and lamellar phases can be obtained with increasing PI23 concentration. The various interactions between PEO-[Bmim]PF6 (hydrogen bonding) and PP0-[Bmim]PF6 (hydrophobic interaction) play the key role in LLC formation. Additionally, the cation [Bmim]+ with a hydrophobic butyl group tends to act as a cosurfactant and cooperate with the block copolymer in forming interfaces, thus enhancing the structural order. And also, as a melting salt, the ionic liquid (IL) may have a salting-out effect on the block copolymer system, which is helpful for the formation of self-assembled structures. The good stability of such system is due to strong interactions mentioned above. The structures of hexagonal and lamellar phases are all similar to those in wate systems.In PI23/H2O hexagonal phases doped with ILs, we also obtained a large amount of planar gold single-crystals with {111} facets. Here the ILs act as capping agents like CTAB, whose selective adsorption on certain crystallographic facets plays the crucial role in forming plates. It should be noted that the interactions of ILs with AuCLf, PEO, and Au nuclei are much stronger than those of CTAB, so that the reduction rates is decreased much, resulting in larger gold singe-crystals (edge size even longer than 12 um).These results will be helpful to explore applications of such self-assembled aggregates with ILs as "green" template, and understand the weak interactions (such as hydrogen bonding, coordination bond) between molecules.Thank the supports from the National Natural Science Foundation of China (20073025,20373035).