Abstract
Yb and Er codoped NaT(XO4)2 (T = Y, La, Gd, Lu and X = Mo, W) disordered oxides show a green (Er3+ related) up-conversion (UC) efficiency comparable to that of Yb:Er:β-NaYF4 compound and unless 3 times larger UC ratiometric thermal sensitivity. The similar UC efficiency of Yb:Er doped NaT(XO4)2 and β-NaYF4 compounds allowed testing equal subcutaneous depths of ex-vivo chicken tissue in both cases. This extraordinary behavior for NaT(XO4)2 oxides with large cutoff phonon energy (ħω≈ 920 cm-1) is ascribed to 4F9/2 electron population recycling to higher energy 4G11/2 level by a phonon assisted transition. Crystalline nanoparticles of Yb:Er:NaLu(MoO4)2 have been synthesized by sol-gel with sizes most commonly in the 50–80 nm range, showing a relatively small reduction of the UC efficiency with regards to bulk materials. Fluorescence lifetime and multiphoton imaging microscopies show that these nanoparticles can be efficiently distributed to all body organs of a perfused mouse.
Highlights
The development of nanoscience and nanotechnology has opened new challenges for the measurement of physical properties at the nanometer scale and for actuation on nano- or micrometric systems
The phase and crystalline quality of the synthesized products was assessed by room temperature (RT) powder X-ray diffraction. θ-2θ scans were performed on a Bruker AXS D8
Individual quasi-spherical nanoparticles with diameters in the 50–80 nm range and good crystalline quality have been obtained by the sol-gel method when the calcination temperature and time is limited to 600 ̊C and 12 h, respectively, to prevent particle clustering and sintering observed at higher temperatures (700–800 ̊C) and times (> 12 h)
Summary
The development of nanoscience and nanotechnology has opened new challenges for the measurement of physical properties at the nanometer scale and for actuation on nano- or micrometric systems. Our purpose in the present work is to present a significant evaluation of the green UC efficiency for eight different DMo/DW hosts in direct comparison to the UC efficiency of Yb: Er:β-NaYF4 with the same Yb and Er ion densities, as well as to determine the Yb and Er composition that optimizes the green UC efficiency in these hosts We address this point by measuring the intensity of the output green photon flux at constant solid angle as well as by simulation of subcutaneous testing using ex-vivo breast chicken tissue and both types of compounds (DMo/DW and β-NaYF4). Since the reduction of the UC efficiency with particle size is a hot issue for applications, we evaluate the UC efficiency of sub-100 nm UCNPs and we explore their suitability as probes for biomedical applications For the latter purpose, quasispherical sol-gel synthesized Yb:Er:NaLu(MoO4) NPs with diameter ranging 50–80 nm have been perfused in a mouse and their body distribution monitored with lifetime and multiphoton optical microscopies. A thermal assisted energy transfer excitation mechanism is proposed as responsible for the extraordinary UC properties of DMo/DW compounds
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