The invention concerns composite nanospheres having a diameter ranging between about 50 and 1000 nm plus or minus 5%, preferably between about 100 and 500 nm plus or minus 5% and advantageously between 100 and 200 nm plus or minus 5%, and comprising an essentially liquid core consisting of an organic phase and inorganic nanoparticles, distributed inside the organic phase, and a skin consisting of at least a hydrophilic polymer derived from the polymerisation of at least one water soluble monomer, in particular N-alkylacrylamide or a N-N-dialkylacrylamide; conjugates derived from said nanospheres; their preparation methods and their uses.