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Almost all the luminescence upconversion nanoparticles used for Förster resonant energy transfer (FRET) applications are bare cores based on the consideration that the energy transfer efficiency is optimized because the distance between energy donors and acceptors is minimized. On the other hand, it is well proved that core/shell structure is efficient in minimizing the nonradiative loss of excitation energy of the luminescence donors. In this work we use core/shell upconversion NaYF₄:Yb³⁺,Er³⁺@NaYF₄ nanoparticles and rose bengal photosensitizer to construct a FRET conjugate. From the photophysics and singlet oxygen generation we have determined that the core/shell structured nanoparticles are better than bare cores for FRET applications. More importantly, we have found that there exists a critical shell thickness for the best FRET performance. In the model we have established, the critical shell …