Pure near-infrared to near-infrared upconversion of multifunctional Tm3+ and Yb3+ co-doped NaGd(WO4)2 nanoparticles

Abstract

In this work, Tm³⁺ and Yb³⁺ co-doped NaGd(WO₄)₂ nanoparticles were synthesized via surfactant-assisted hydrothermal reactions. The structural analysis indicates that Tm³⁺ and Yb³⁺ ions have been doped into the NaGd(WO₄)₂ host by occupying the Gd³⁺ sites. The as-prepared NaGd(WO₄)₂:Tm³⁺,Yb³⁺ nanomaterials are spherical particles of approximate sizes in the range 50–80 nm. The upconversion spectra results suggest that the NaGd(WO₄)₂:Tm³⁺,Yb³⁺ nanoparticles exhibit pure or virtually pure near-infrared to near-infrared upconversion luminescence when excited by a 975 nm laser. This phenomenon is significant for nanoparticles with applications in bio-imaging, because both excitations and emissions in the near-infrared region reduce the autofluorescence and light scattering from biological tissues. On the basis of the upconversion and Raman spectra, a possible mechanism for the high purity near-infrared upconversion emission is proposed. Moreover, the NaGd(WO₄)₂:Tm³⁺,Yb³⁺ nanoparticles also exhibit paramagnetism, which is promising for bio-separation applications.