Evolution of Size and Optical Properties of Upconverting Nanoparticles during High-Temperature Synthesis

Abstract

We investigated the growth of β-phase NaYF4:Yb3+,Er3+ upconversion nanoparticles synthesized by the thermal decomposition method using a combination of in situ and offline analytical methods for determining the application-relevant optical properties, size, crystal phase, and chemical composition. This included in situ steady state luminescence in combination with offline time-resolved luminescence spectroscopy as well as small-angle X-ray scattering (SAXS) transmission electron microscopy (TEM), X-ray diffraction analysis (XRD), and inductively coupled plasma optical emission spectrometry (ICP-OES). For assessing the suitability of our optical monitoring approach, the in situ-collected spectroscopic data, which reveal the luminescence evolution during nanocrystal synthesis, were compared to measurements done after cooling of the reaction mixture of the as-synthesized particles. The excellent correlation of the in situ and time-resolved upconversion luminescence with the nanoparticle sizes determined during the course of the reaction provides important insights into the various stages of nanoparticle growth. This study highlights the capability of in situ luminescence monitoring to control the efficiency of UCNP synthesis, particularly the reaction times at elevated temperatures and the particle quality in terms of size, shape, and crystal structure, as well as luminescence lifetime and upconversion quantum yield.

Simple Self-Referenced Luminescent pH Sensors Based on Upconversion Nanocrystals and pH-Sensitive Fluorescent BODIPY Dyes

“…216 Their findings support a thermodynamically driven dissolution/recrystallization process. They even proposed a four stage process for the growth of β-phase UCNPs, which is supported by our results.. 159,217 As the most efficient UCNPs are mainly synthesized in organic solvents, the applicability is limited as the particles are stabilized by hydrophobic ligands. However, much effort was made to overcome solubility issues.…”

“…Representative molecular structures of BODIPYs used as building blocks for artificial photosynthetic models. 46,47 159 Copyright 2018 American Chemical Society. b) Energy scheme of Yb 3+ and Tm 3+ in NaYF 4 host lattice showing possible ETU pathways.…”

“…Figure 15: Reprinted with permission from S. Radunz et al 159. Copyright 2018 American Chemical Society.…”

Assessing the protective effects of different surface coatings on NaYF4:Yb3+, Er3+ upconverting nanoparticles in buffer and DMEM

“…NaYF 4 :20%Yb 3+ ,2%Er 3+ UCNP were synthesized according to a previously reported procedure 26 – 28 with some modifications 25 , 44 . Surface modifications and silica coating experiments followed published procedures 45 – 49 .…”

Detecting Variable Resistance by Fluorescence Intensity Ratio Technology

“…772042) mixed phase NaYF 4 [35,36], as shown in Figure 2b. The up-conversion (UC) photoluminescence spectra of the sensor was measured with varying current under a fixed pump power (160.8 mW/mm 2 ), among which, three emission bands were respectively ascribed to 2 H 11/2 → 4 I 15/2 (524 nm), 4 S 3/2 → 4 I 15/2 (543 nm), and 4 F 9/2 → 4 I 15/2 (658 nm) transitions of Er 3+ ions [37,38,39], as depicted in Figure 2c. The luminescence intensity of three UC emission bands decreased with the increasing current, as described in Figure 2d.…”