The present invention relates to nanoparticles having a mean diameter of < 500nm and comprising, at their surface, a selected material. The nanoparticles are taken up by cells under physiological conditions and can be used to isolate interaction partners of the selected material within the cells. The present invention provides important advantages in that it opens up new ways of identifying cellular components and of delivering a substance of interest specifically to a selected cell compartment. The nanoparticles are also useful as a tool of diagnosis and for the constitution of chemical libraries.