The present invention relates to a nanoparticle comprising a micelle formed by an amphophilic block-copolymer and an agent encapsulated within said micelle. The present invention also relates to a composition comprising such nanoparticle and to the use of such nanoparticle and/or of such composition. More particularly, in one embodiment, the invention describes a new class of polymeric nanoparticles as smart Tl contrast agent for MR imaging for breast cancer early detection. These nanoparticles contrast agents have the capability to remain switched off during circulation and then switch on their imaging capacity upon arrival at the target sites (tissue of interest). These smart nanoparticles contrast agent are self-assembled from pH sensitive amphiphilic polymer, loaded with Gadolinium (Gd3+) complex based Tl agent and then fitted with targeting biomolecules such as antibody, small molecules or DNA to increase its specificity toward the target of interest.
