The invention relates to composite particles comprising a hydrophobic polymer core and inorganic nanoparticles. Said hydrophobic polymer forms a polymer matrix inside which the inorganic nanoparticles are stabilized and distributed in a relatively homogenous manner. Said particles are at least partially surrounded by an amphophilic copolymer comprising a hydrophobic part and a hydrophilic part, said hydrophobic part being at least partially immobilized in the polymer matrix. The inventor also relates to the preparation method thereof, the conjugates comprising such particles and a ligand and the use of same in reactants and therapeutic compositions.