Means and methods for producing surface-activated semiconductor nanoparticles suitable for in vitro and in vivo applications that can fluoresce in response to light excitation. Semiconductor nanostructures can be produced by generating a porous layer in semiconductor substrate comprising a network of nanostructures. Prior or subsequent to cleavage from the substrate, the nanostructures can be activated by an activation means such as exposing their surfaces to a plasma, oxidation or ion implantation. In some embodiments, the surface activation renders the nanostructures more hydrophilic, thereby facilitating functionalization of the nanoparticles for either in vitro or in vivo use.