Phase and luminescent intensity control of hydrophilic rare-earth up-converting nanophosphors prepared by one-pot solvothermal synthesis

Rare-earth up-converting nano-phosphors (RUNPs) have wide applications, and most of these applications require hydrophilic RUNPs with high up-converting luminescence efficiency. In this work, we report a simultaneous control of the phase and luminescent intensity of hydrophilic Gd3+ doped NaYF4:Yb/Er nanoparticles with diameters of 40–100 nm, which were prepared by a facile one-pot solvothermal synthesis with ethylene glycol as the solvent and poly(vinylpyrrolidone) as the ligands at 220 °C for different time. When reaction time is 3 h, the increase of Gd3+ dopant concentration from 0 to 30 mol% results in the transformation from cubic to hexagonal phase, and pure hexagonal phase NaYF4:Yb/Er nanoparticles can be obtained with Gd3+ dopant concentration up to 30 mol%. Gd3+ dopant concentration at 15 mol% leads to a maximal luminescent intensity in a wide dopant range of 0–80 mol%. Furthermore, the increase of reaction time from 3 to 24 h favors the formation of hexagonal phase samples and therefore improves greatly luminescence intensity. 15 mol% Gd3+ doped NaYF4:Yb/Er nanoparticles prepared for 24 h exhibit the highest upconverting luminescence intensity which is almost 11 times as strong as that of ones prepared for 3 h and almost 28 times as strong as that of hexagonal phase NaGdF4:Yb/Er (namely NaYF4:Yb/Er sample with 80 mol% Gd3+ prepared for 3 h). Due to its small size, high hydrophilicity and excellent up-converting luminescence, this 15 mol% doped NaYF4:Yb/Er sample has great superiority for biological applications.

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► Hydrophilic Gd3+ doped NaYF4:Yb,Er nanoparticles have been prepared by a facile one-pot solvothermal synthesis.
► The control of their phase and upconverting luminescent intensity can be realized by tuning Gd3+ dopant concentration and solvothermal time.
► Due to its small size, high hydrophilicity and excellent up-converting luminescence, the 15 mol% doped NaYF4:Yb/Er sample has great superiority for biological applications.