A general and versatile method for the functionalization and subsequent modification of single dispersed magnetite nanoparticles by direct reaction of alkoxysilanes on the particle surface is reported. Our data supports the formation of a dense monolayer that is controlled by sterical needs. By selecting the functionality of alkoxysilanes, the surface properties of the particles can be tailored. Depending on the surface functionality, the modified particles can be used as macrocomonomers or macrocrosslinkers, or as macroinitiators for surface-initiated polymerization and thus enable the covalent attachment of polymers.