Abstract
The ability to synthesize upconversion nanocrystals (UCNCs) with tailored upconversion luminescence and controlled size is of great importance for biophotonic applications. However, until now, limited success has been met to prepare bright, ultrasmall, and monodispersed β-NaYF4:Yb3+/Er3+ UCNCs. In this work, we report on a synthetic method to produce monodisperse hexagonal NaYF4:Yb3+/Er3+ nanocrystals of ultrasmall size (5.4 nm) through a precise control of the reaction temperature and the ratio of Na+/Ln3+/F−. We determined the optimum activator concentration of Er3+ to be 6.5 mol % for these UCNCs, yielding about a 5-fold higher upconversion luminescence (UCL) intensity than the commonly used formula of NaYF4:30% Yb3+/2% Er3+. Moreover, a thin epitaxial shell (thickness, 1.9 nm) of NaLnF4 (Ln = Y, Gd, Lu) was grown onto these ultrasmall NaYF4:Yb3+/Er3+ NCs, enhancing its UCL by about 85-, 70- and 50-fold, respectively. The achieved sub-10-nm core and core–shell hexagonal NaYF4:Yb3+/Er3+ UCNCs with enhanced UCL have strong potential applications in bioapplications such as bioimaging and biosensing.
Highlights
Lanthanide-doped upconversion nanocrystals (UCNCs) are able to produce strong anti-Stokes luminescence, in which two or more low energy photons are stepwise absorbed via real intermediate long-lived electronic states of lanthanides, resulting in an excitation of a higher electronic state that produces an emission of higher energy photon [1]
upconversion luminescence (UCL) intensity than the cubic ones, we are interested in preparing ultrasmall NaYF4 UCNCs of β-phase [23,24]
The nanoparticles were synthesized with a fixed ratio of Na+ /Ln3+ /F− of 2.5:1:4 in the growth solution, but at varied reaction temperature of 220–300 ◦ C
Summary
Lanthanide-doped upconversion nanocrystals (UCNCs) are able to produce strong anti-Stokes luminescence, in which two or more low energy photons are stepwise absorbed via real intermediate long-lived electronic states of lanthanides, resulting in an excitation of a higher electronic state that produces an emission of higher energy photon [1]. They have superior physiochemical characteristics, such as excellent thermal and chemical stability, non-photobleaching, non-blinking, sharp emission bandwidths, low toxicity, and large anti-Stoke shift to entail zero optical imaging background. The uniform β-NaYF4 :Yb3+ /Er3+ nanoparticles >20 nm in size have been
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