The present invention relates to particles, useful for drug release detection, comprising: at least one luminescent metal complex of formula [Ln].[L], wherein: Ln is a lanthanide ion, L is an organic ligand comprising a hydrophilic unit with at least 6 chelating groups chosen from the group consisting of hydroxyl, carboxyl, optionally in the carboxylate form, carbonyl, ether, phosphonate, phosphinate, thiolate, amino and imino functions, and at least one drug comprising a conjugated aromatic system, wherein said drug is not covalently bonded to the metal complex, said particle having a size comprised from 3 nm to 2000 nm, and wherein the distribution of the metal complex and the drug within the particle is such that the mean distance between a drug molecule and a metal complex is less than 15 nm.