In accordance with the present invention there is provided a nanoparticle, comprising—a core, —an outer layer, and —a buffer layer located between said core and said outer layer, wherein the buffer layer prevents energy transfer between the core and the outer layer. In another aspect of the invention there is provided a nanoparticle as described herein for use in therapy, a nanoparticle according as described herein for use in treatment of a tumor, and a nanoparticle as used herein for use in in vivo imaging According to another aspect of the invention there is provided a method for the production of a nanoparticle as described herein, comprising the steps of—heating of a solution comprising core precursors, followed by—injection of a dispersion comprising buffer layer precursors, followed by—injection of a dispersion comprising outer layer precursors. Another aspect of the invention is a method for in vivo imaging or treatment or both of a human or animal, comprising the steps of: —administering nanoparticles as described herein to a patient, —irradiating at least part of the patient's body with one or more types of radiation. According to yet another aspect of the invention there is provided the use of a nanoparticle as described herein for therapy, in vivo imaging, or both.