The present invention relates to a novel method for the preparation of nanoparticles with a diameter smaller than or equal to 500 nm, comprising bringing a solution (1) comprising nanoparticles of a first polyelectrolyte in the charged state, bearing hydrophobic side groups, together with (2) at least one second polyelectrolyte of opposite polarity to that of the first polyelectrolyte, characterized in that the ratio Z of the number of cationic groups relative to the number of anionic groups in the mixture of the two polyelectrolytes is comprised between 0.1 and 0.75 or between 1.3 and 2； and the total mass concentration C of polyelectrolytes is strictly less than 2 mg/g of the mixture.
