The introduction of a thick shell structure has been widely used to enhance the emission intensity of upconversion nanoparticles (UCNPs). However, a thick shell could increase the distance between UCNPs and photosensitizers, which is not favourable to the generation of singlet oxygen (O-1(2)) in photodynamic therapy (PDT) due to the low fluorescence resonance energy transfer (FRET) efficiency. In this study, we used a facile method to prepare UCNPs that the emission intensity could increase with the shell thickness decreasing, which facilitated the efficient FRET between UCNPs and photosensitizers. In detail, the Nd3+ doped UCNPs with different dopant concentration of Yb3+ were prepared and characterized firstly. The irjg (intensity of red luminescence to green luminescence) was tuned to increase largely by precisely controlling Yb3+ concentration in core-shell, which could make UCNPs effectively activate methylene blue (MB). Then, a unique procedure was used to prepare NaYF4:Yb/Er/Nd@NaYF4:Nd (Yb3+:30%) core shell nanoparticles with different shell thickness by tuning the amount of the core. The upconversion luminescence (UCL) intensity of those UCNPs enhanced dramatically with the shell thickness decreasing. Furthermore, UCNPs and MB were encapsulated into SiO2 nanoparticles. FRET efficiency between UCNPs and MB largely increased with the shell thickness of UCNPs decreasing. Correspondingly, the efficiency of O-1(2) generation obviously increased. We provided a new method to optimize the UCL intensity and FRET efficiency at the same time to produce O-1(2) efficiently. (C) 2016 Elsevier B.V. All rights reserved.