Lanthanide doped Bi2O3 upconversion luminescence nanospheres for temperature sensing and optical imaging

Abstract

Water-soluble lanthanide (Ln³⁺) doped Bi₂O₃ nanospheres have been successfully prepared through a solid-state-chemistry thermal decomposition process. The nanospheres exhibit intense upconversion luminescence (UCL) by doping the Ln³⁺ (Ln = Yb, Er/Ho/Tm) ions into the Bi₂O₃ host matrix under 980 nm excitation. The ratio of red/green emission of Bi₂O₃:Yb³⁺/Er³⁺ nanospheres exhibits a significant change as the calcination temperature increases and the value could reach 105.6. Moreover, the UCL of Bi₂O₃:Yb³⁺/Tm³⁺ nanospheres are temperature-sensitive, where the intensity ratios of 799 and 808 nm emissions increase monotonously with temperature. The MTT assay reveals that Bi₂O₃:Yb³⁺/Tm³⁺ nanospheres exhibit good biocompatibility by grafting citric acid molecules on the surface. The application possibility of Bi₂O₃:Yb³⁺/Tm³⁺ nanospheres as bioprobes for optical imaging in vivo is also confirmed by the high-contrast photoluminescence images between the background and the UCL imaging area.