The invention relates to an assay based on resonance energy transfer (RET), comprising a first molecule grope A, which is marked with at least one energy donor, and at least one second molecule group B which is marked with at least one energy acceptor, the donor comprising a molecule or particle which can be energetically excited by an external radiation source and which is fluorescence enabled and the acceptor comprising a molecule or particle which can be excited by energy transfer via the donor with partial or complete quenching of the donor fluorescence, and the donor and/or acceptor comprise luminescing inorganic dope nanoparticles having an expansion of ≦50 nanometers, emitting electromagnetic radiation with stokes or anti-stokes scattering after energetic excitation.