Lanthanide-doped Sr ₂ YF ₇ nanoparticles: controlled synthesis, optical spectroscopy and biodetection

Sr2YF7, as an important matrix for trivalent lanthanide (Ln(3+)) ions to fabricate upconversion (UC) or downshifting (DS) phosphors, has been rarely reported. Herein, monodisperse and size-controllable tetragonal-phase Ln(3+)-doped Sr2YF7 nanoparticles (NPs) were synthesized via a facile thermal decomposition method. Upon excitation at 980 nm, UC luminescence properties of Sr2YF7:Ln(3+)/Yb(3+) (Ln = Tm, Er) NPs were systematically surveyed. Particularly, after coating an inert Sr2YF7 shell, the UC luminescence intensities of Sr2YF7:Tm(3+)/Yb(3+) and Sr2YF7:Er(3+)/Yb(3+) NPs were enhanced by ∼22 and 4 times, respectively. Furthermore, intense multicolor DS luminescence was also achieved in Ce(3+)/Tb(3+) or Eu(3+) doped Sr2YF7 NPs, with absolute quantum yields of 55.1% (Tb(3+)) and 11.2% (Eu(3+)). The luminescence lifetimes of (5)D4 (Tb(3+)) and (5)D0 (Eu(3+)) were determined to be 3.7 and 8.1 ms, respectively. By utilizing the long-lived luminescence of Ln(3+) in these Sr2YF7 NPs, we demonstrated their application as sensitive heterogeneous time-resolved photoluminescence bioprobes to detect the protein of avidin and the tumor marker of the carcinoembryonic antigen (CEA) with their limits of detection down to 40.6 and 94.9 pM, and thus reveal the great potential of these Sr2YF7:Ln(3+) nanoprobes in cancer diagnosis.