Modification of silica by an organic monolayer in aqueous medium using octylphosphonic acid and aluminium species

In the present work we report a "green" method for the grafting of organic monolayers at the surface of silica nanoparticles using water as a solvent. This method is based on the use of water-stable phosphonic acid coupling molecules grafted on an intermediate layer of aluminium species, as the sensitivity of Si-O-P bonds toward hydrolysis precludes a direct anchoring onto silica. Two approaches were explored: anchoring of octylphosphonic acid on aluminated silica and one-pot modification of silica by AlCl3 then by octylphosphonic acid. The modified powders were characterized using elemental analysis, P-31 and Al-27 MAS NMR spectroscopy, FTIR spectroscopy and N-2 physisorption. The one-pot approach appears particularly promising, as it allows the anchoring on silica of phosphonic acid monolayers with controlled densities in a significantly shorter time.