Dependence of optical temperature sensing and photo-thermal conversion on particle size and excitation wavelength in β-NaYF4:Yb3+, Er3+ nanoparticles

Hexagonal-phase NaYF4:Yb3+, Er3+ upconversion nanoparticles (UCNPs) have been widely studied as temperature sensors and photothermal nanomaterials in bioscience. In this work, beta-NaYF4:Yb3+, Er3+ UCNPs were synthesized via a thermal decomposition method. Particle size and excitation wavelength dependent temperature sensing properties were studied according to the temperature dependent emission spectra and fluorescence intensity ratio (FIR) technique. It was found that particle sizes (21.6 nm and 64.3 nm) and excitation wavelengths (980 nm and 1550 nm) almost have no effect on the temperature sensing properties of beta-NaYF4:Yb3+, Er3+ UCNPs. In addition, the laser-induced heating effect on the samples with different sizes under different excitation wavelengths were also investigated. It was deduced that the sample temperature was very sensitive to the laser excitation density, excitation wavelength and particle size. The samples reached higher temperature with higher laser excitation density and shorter excitation wavelength. With the same excitation wavelength and excitation density, the larger particle size of beta-NaYF4: Yb3+, Er3+ UCNPs (63.4 nm) generated much more heat than that of the smaller UCNPs (21.6 nm). (C) 2018 Elsevier B.V. All rights reserved.