The invention provides a composition comprising core-shell nanoparticles, the nanoparticles comprising (a) cationic core material comprising polymer； and (b) a shell material comprising silica. Preferred core materials comprise diblock copolymer micelles comprising one block of dialkylaminoethyl methacrylate units which are partially or fully quaternised and one block of dialkylaminoethyl methacrylate units that remain non-quaternised . The invention also provides a method for the preparation of the said composition, the method involving (a) preparing a cationic core material comprising polymer； and (b) coating the core material with a shell comprising silica by treating the polymer with a silica precursor under ambient conditions. The invention also envisages a composition comprising core-shell nanoparticles which is adapted to facilitate controlled delivery of at least one active agent into a system in response to controlled changes in the pH of the system.
