Fig. 2
From: Label-free upconversion nanosensor for water safety monitoring of permanganate and dichromate ions
TEM image of the (a) core-shell-shell-shell NaYF4:0.2Eu@NaYbF4:0.39Gd/0.01Tm@NaYbF4@NaLuF4 UCNPs, (b) randomly chosen core-shell-shell-shell UCNPs for elemental analysis, (c–h) elemental mapping and (i) EDS spectrum of the core-shell-shell-shell UCNPs; (j) HRTEM image of individual core-shell-shell-shell nanoparticles (d-spacing = 0.520 nm); (k) XRD pattern of the core-shell-shell-shell UCNPs; (l) UCL spectra of NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 in comparison with NaYF4:Eu@NaYbF4:Gd/Tm, NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4 and NaYF4:Eu@NaYbF4:Gd/Tm@NaLuF4 nanoparticles (in cyclohexane) measured under 980 nm laser excitation, where the right inset shows the photographs of the as-synthesized NaYF4:Eu@NaYbF4:Gd/Tm@NaYbF4@NaLuF4 UCNPs (denoted as CSSS) and NaYF4:Eu@NaYbF4:Gd/Tm@NaLuF4 (denoted as CSS(Lu)) upon 980 nm irradiation