Controlled Synthesis of NaYF₄:Yb,Er Upconversion Nanocrystals as Potential Probe for Bioimaging: A Focus on Heat Treatment

Abstract

Upconversion nanoparticles are a promising class of materials for bioimaging. Their attractive optical properties are influenced by their crystalline phase, size, and morphology resulting from the preparation method employed. Herein, we synthesized NaYF 4 :Yb,Er nanocrystals by the coprecipitation method followed by a heat treatment step. The influence of the treatment temperature and the type of reactor on the phase transition from the α-cubic to the β-hexagonal phase was investigated. We found that before the phase transition, the size and shape of α-NaYF 4 :Yb,Er nanocrystals evolved due to a coalescence mechanism with microtubular reactors.In contrast to the flask system, we also observed an anisotropic growth of β-NaYF 4 :Yb,Er nanorods in microtubes whose length can be controlled by adjusting the temperature. The diameter of microtubes plays a critical role in the shape evolution of the final products from spheres to nanorods. The type of reactor and thus the resulting temperature gradient could be considered as a promising parameter to control the phase, shape and size of UCNPs without changing the chemical composition.

Low-temperature open-air synthesis of PVP-coated NaYF 4 :Yb,Er,Mn upconversion nanoparticles with strong red emission

“…Further, synthesis requires high-temperature reactions for extended durations, e.g. up to 24 h [35][36][37][38][39]. UCNP synthesis at approximately 200°C typically produces α-phase UCNPs, whereas synthesis at greater than approximately 300°C produces β-phase UCNPs (see electronic supplementary material, figure S1) [36].…”

Lanthanide-Doped Upconversion Luminescent Nanoparticles—Evolving Role in Bioimaging, Biosensing, and Drug Delivery

“…These highly absorbing sensitizer ions provide efficient nonradiative energy transfer to activator ions and result in improved absorption [ 18 , 19 ]; Yb 3+ ions are the most often utilized sensitizers for Er 3+ , Tm 3+ , or Ho 3+ doped UCNPs [ 20 ]. A few examples of nanoparticles with upconversion luminescence include NaYF 4 :Yb:Er [ 21 , 22 ], Y 2 O 3 :Yb:Er [ 23 ], Gd 2 O 2 S:Eu 3+ [ 24 , 25 ], NaPrF 4 :Yb:Tm [ 26 ], and Cs 3 Lu 2 Br 9 :Er 3+ [ 27 ]. Of these, Ln-doped UCNPs, NaYF 4 co-doped with Yb 3+ /Er 3+ or Yb 3+ /Tm 3+ nanoparticles demonstrated the highest UC efficiency with usage in cellular and in vivo animal imaging [ 28 ].…”

From Nanothermometry to Bioimaging: Lanthanide-Activated KY3F10 Nanostructures as Biocompatible Multifunctional Tools for Nanomedicine

“…Among the extensively investigated lanthanides-activated upconverting nanomaterials, the most famous is certainly the ternary host NaYF 4 , which crystallizes in two possible phases, namely, cubic and hexagonal one. 14 Nonetheless, the KY 3 F 10 ternary host has been scarcely investigated, although it has excellent optical properties. In fact, the luminescence of Tm 3+ -, Nd 3+ -, Yb 3+ -, and Er 3+ -doped KY 3 F 10 nanopowders were investigated by Gomes et al, 15 highlighting excellent Ln 3+ emissions upon infrared excitation for the Tm 3+ -, Nd 3+ -, Yb 3+ or Er 3+ -codoped samples.…”

“…Among the extensively investigated lanthanides-activated upconverting nanomaterials, the most famous is certainly the ternary host NaYF 4 , which crystallizes in two possible phases, namely, cubic and hexagonal one . Nonetheless, the KY 3 F 10 ternary host has been scarcely investigated, although it has excellent optical properties.…”