Helical Silica - Lipid Mesostructures
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Helical Silica – Lipid Mesostructures Annela M. Seddon and Stephen Mann School of Chemistry, University of Bristol, Cantocks Close, Bristol, BS8 1TS, United Kingdom ABSTRACT In this work, diacetylenic-containing chiral lipid, 1,2-bis(10,12- tricosadiyonyl)-sn-glycero3-phosphatidylcholine (DC8,9PC) was used simultaneously as a structure-directing template, “morphogen” and photoactive monomer to synthesize high aspect ratio helical tubules and ribbons consisting of polymerizable periodic mesostructured silica-lipid walls with a twisted lamellar architecture. Intercalation of silica in the ordered hybrid lamellar mesostructure significantly increased the photochemical reactivity of the diacetylenic moieties, such that eneyne conjugation became facile compared with non-mineralized tubules or tubules coated in a surface film of silica. INTRODUCTION The use of amphiphilic surfactants and polymers in the template-directed synthesis of periodically ordered hybrid mesophases has produced a diverse range of new inorganic-based materials with controlled mesoporosity and functionality [1]. A key challenge for the future lies with the reproducible processing of mesostructured materials, and specifically with the control of higher-order properties such as structural hierarchy and macroscopic shape. Although mesostructured nanoparticles [2], colloids [3], tubes [4], fibres [5], hollow shells [6], gyroids and helicoids [7] have been prepared, these rely on the subtle interplay of extrinsic factors associated with the growth mechanism, which are often difficult to control. An alternative strategy is to use molecular structure to direct the expression of form so that increased levels of morphological control can be achieved via the transcription of internal information. In this respect, chiral organic molecules known to be efficient gelators of organic solvents have been used recently to prepare helical tapes of silica by in situ coating of gel filaments [8]. However, although these molecules are capable of self-assembly and directing the deposition of silica, they show no propensity to co-assemble with the inorganic phase, and are therefore not mesostructurally ordered. In this work, however, in situ synthesis of silica by acid hydrolysis and condensation of TEOS in the presence of DC8,9PC lipid molecules results in the formation of a lamellar hybrid mesostructure containing polymerizable diacetylinic groups, which can be subsequently twisted into tubules and ribbons with helical architecture. EXPERIMENTAL DETAILS DC8,9PC was obtained from Avanti Polar Lipids and used without further purification. TEOS was obtained from Aldrich and used as received. Mineralized lipid tubules were prepared in situ as follows. Typically, DC8,9PC (40mg) was dissolved in ethanol (2mL) by sonication (lipid concentration, 0.044 mM), and TEOS (82.4 µL) added to give a TEOS : lipid molar ratio of 8.3 : 1. A solution (0.38g) prepared from 0.8g H2O and 3.0g of 48 wt% aqueous HBr was added to the lipid / TEOS mixture (final pH = 0.5) and a white precipitate fo
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