Synthesis and shape modification of organo-functionalised silica nanoparticles with ordered mesostructured interiors

Organo-functionalised MCM-41 nanoparticles have been prepared by a dilution/neutralisation method involving the surfactant-templated co-condensation of 3-aminopropyltriethoxysilane, allyltriethoxysilane or 3-mercaptopropyltriethoxysilane with tetraethoxysilane under alkaline conditions. The presence of covalently coupled organic groups within the hexagonally ordered silica mesophase was confirmed by solid-state C-13 and Si-29 MAS NMR spectroscopy. TEM studies show that amine- and allyl-functionalised nanoparticles are single-domain oblate ellipsoidal crystals, in which the cylindrical micelles are aligned parallel to the morphological minor axis. In contrast, the thiol-functionalised nanoparticles were synthesised in the form of nanofilaments elongated specifically along the channel direction of the MCM-41 hexagonal mesostructure. A mechanism is proposed in which changes in the nanoparticle morphology are attributed predominantly to an increase in surface charge associated with the anionic mercaptopropyl groups that inhibits the side-on attachment of silica-surfactant micelles to partially ordered primary nanoclusters. In contrast, nanoparticles with neutral side chains, such as amino and allyl moieties, as well as unfunctionalised MCM-41, develop by side-on attachment to radially arranged defect sites of a modulated hexagonal mesophase associated with the oblate ellipsoidal morphology.