Real-Time-Monitoring of the Synthesis of β-NaYF₄:17% Yb,3% Er Nanocrystals Using NIR-to-Visible Upconversion Luminescence

Abstract

In situ real-time monitoring of upconversion emission is applied to study the reaction mechanism for the synthesis of β-NaYF4:17% Yb,3% Er nanoparticles in oleic acid and octadecene via the heat-up method. Transmission electron microscopy is used to correlate the spectroscopic signature of the reaction mixture with its composition. The power of real-time spectroscopic monitoring to precisely time the duration of the various stages of the reaction, and to accurately identify the transitions between those stages, including the completion of the reaction, is demonstrated. During the heat-up stage, the initial precipitate present is transformed into small α-phase nanoparticles. In the second and longest stage of the reaction, a period of relative stasis is maintained, during which there is at most a slight change in the size distribution of the α-phase nanoparticles formed during heat-up. In the third stage, a relatively rapid conversion of small α-phase nanoparticles to the larger β-phase product nanoparticles is observed. The size distribution of the β product remains relatively constant during the phase-transition stage, indicating that, once the β particles begin to form, they grow very rapidly to the final product size. A significant variation in the time required to complete the reaction is observed, which is due almost entirely to the variation in the duration of the second reaction stage. The time required to complete the reaction does not appear to affect the size or uniformity of the product β-NaYF4:17% Yb,3% Er nanoparticles.

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

“…188,[212][213][214][215] Later, Suter et al were the first who exploited the intrinsic UCL properties of UCNPs which enables real-time monitoring of the particle growth. 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.…”

Analysing the effect of the crystal structure on upconversion luminescence in Yb3+,Er3+-co-doped NaYF4 nanomaterials

“…TRANES could also be a great tool for monitoring the phase composition of UCNP in situ during synthesis. 84,85…”

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

“…Our results strongly suggested that the -phase nanocrystals may also grow by the coalescence process beside the Ostwald ripening phenomenon that was reported in the literature. 26,30,32 Surprisingly, despite the high temperature (310 °C) and homogeneous heat transfer of the microtube 879 , one could not observe any hexagonal β-phase with HRTEM before 90 min of heating. This was probably related to the slow change in shape and size of the α-phase nanocrystals.…”

“…23,24 Significant efforts have also been invested in the characterization of their growth and phase transition mechanism. [25][26][27][28][29][30][31][32] Suter et al have investigated the growth of nanocrystals and the transition from cubic α-phase to hexagonal β-phase in real-time by monitoring their NIR-to-visible upconversion emission. 30 They have defined four stages related to the evolution of NaYF 4 nanocrystals during the heat treatment step, in which the longest period of time is between the initial formation of the small cubic α-phase nanocrystals and the start of the phase transition that was called relative stasis phase.…”

“…[25][26][27][28][29][30][31][32] Suter et al have investigated the growth of nanocrystals and the transition from cubic α-phase to hexagonal β-phase in real-time by monitoring their NIR-to-visible upconversion emission. 30 They have defined four stages related to the evolution of NaYF 4 nanocrystals during the heat treatment step, in which the longest period of time is between the initial formation of the small cubic α-phase nanocrystals and the start of the phase transition that was called relative stasis phase. May et al have studied the mechanism of crystals growth by a mathematical model, 32 while Radunz et al have investigated the evolution of size and optical properties of NaYF 4 nanocrystals with different techniques such as small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-OES).…”