Strong Red-Emitting near-Infrared-to-Visible Upconversion Fluorescent Nanoparticles

Near infrared (NIR)-to-red upconversion fluorescent nanoparticles of yttrium oxyfluoride, ytterbium, erbium/yttrium oxyfluoride (YOF:Yb,Er/YOF) with a core/shell structure have been prepared for the first time. Under 980 nm NIR excitation, they emitted strong red upconversion fluorescence with a sharp emission band topping at ∼669 nm. Compared with the most efficient green emitting hexagonal phase NaYF4:Yb,Er upconversion fluorescent nanoparticles, our red-emitting fluorescent nanoparticles exhibited even stronger fluorescence. The YOF shell played a dual-role in the upconversion: it greatly enhanced the red emission at ∼669 nm (∼18 times) and suppressed the green emission of erbium at ∼530 nm. These nanoparticles were rendered hydrophilic by using several strategies and were further conjugated to antibodies for cancer cell labeling and imaging. As both excitation and emission were in the long wavelength range (>650 nm), which are transparent to tissues, along with their strong and narrow emission, these red emission upconversion fluorescent nanoparticles offer excellent opportunities for biosensing and in vivo bioimaging applications.