Nanoparticle delivery systems for use in targeting biologically active agents to the central nervous system comprise a composition comprising (a) a nanoparticle comprising: (i) a core comprising a metal and/or a semiconductor and (ii) a corona comprising a plurality of ligands covalently linked to the core, wherein said ligands comprise a carbohydrate, insulin and/or a glutathione and (b) the at least one agent to be delivered to the CNS. Methods of treatment and diagnosing CNS disorders utilising the nanoparticle delivery systems and related screening methods are also disclosed.
