Apparently, efficient FRET in a one-to-one molecular beacon structure can be achieved only when both the emitter and the quencher are of the same size, e.g., a green CdSe/ZnS QD as a FRET donor and a red QD as an acceptor. This is especially useful in the case of ratiometric sensing platforms where quantitative analysis is based on the intensity ratio between simultaneous green and red emissions. However, permanent background emission from the QD acceptor may be an interfering factor. Instead of a QD-based acceptor, silver or gold plasmonic nanoparticles can be efficiently used to quench the donor emission in the off-state.2 Unfortunately, unlike the optical absorption spectra of semiconductor QDs, those of Ag and Au nanoparticles smaller than 10 nm do not cover the entire spectral range, which is especially problematic for quenching the luminescence of donors emitting in the deep-red and NIR regions of the optical spectrum. In this case, one may use non-emitting core nanocrystals of narrow-gap semiconductors, such as PbS, PbSe, or even CdSe, doped with certain ions (, , etc.) that can completely quench the photoluminescence of CdSe QDs due to the formation of nonradiative recombination traps inside the CdSe core.