A solvothermal method for the synthesis of dual paramagnetic and upconverting luminescent β-NaYF4:Yb3+,Ln3+@NaGdF4 (Ln3+=Er3+ or Tm3+) core@shell nanoparticles

Abstract

The interest in the applications of upconverting luminescent nanomaterials has expanded significantly, particularly in the biomedical field. An opportunity for innovation in this realm lies in the development of multimodal contrast agents that exhibit both luminescent and magnetic responses. This study presents an easily reproducible strategy for the solvothermal synthesis of NaYF 4 :Yb 3+ ,Ln 3+ @NaGdF 4 (Ln = Er 3+ or Tm 3+ ) core@shell nanoparticles. The shell incorporation process initially involved dissolving the outer layer of NaYF 4 :Yb 3+ ,Ln 3+ cores, followed by the radial growth of NaGdF 4 around them. This shell provides intense paramagnetic properties without affecting the luminescent response of the cores. The optimized process yielded individual core@shell nanoparticles, with average diameters of 20 ± 5 nm for NaYF 4 :Yb 3+ ,Er 3+ @NaGdF 4 and 19 ± 3 nm for NaYF 4 :Yb 3+ ,Tm 3+ @NaGdF 4 UCNPs. The presence of NaGdF 4 shells was further confirmed by Raman spectroscopy, which revealed a change in polarizability within the structure due to the presence of Gd 3+ instead of Y 3+ . Based on the hysteresis loops, the magnetic response of NaYF 4 :Yb 3+ ,Ln 3+ was negligible, whereas NaYF 4 :Yb 3+ ,Ln 3+ @NaGdF 4 exhibited saturation at 1.04 emu/g at 20 kOe. Compared to common thermal decomposition approaches, this method offers several advantages: it provides nanoparticles that are colloidally stable in aqueous solutions, a higher reaction yield, eliminates the need for an inert atmosphere, reduces the excessive use of solvents for separation and washing, and requires less experimental manipulation. Therefore, this method enables the synthesis of materials with applications in the development of sensors, contrast agents, or theranostic tools with dual magnetic-luminescent responses.

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