Multifunctional BiF:Ln (Ln = Ho, Er, Tm)/Yb nanoparticles: an investigation on the emission color tuning, thermosensitivity, and bioimaging.

Pure cubic phase and uniform BiF:Ln (Ln = Ho, Er, Tm)/Yb nanoparticles (NPs) were prepared by coprecipitation. The growth mechanism of BiF:2%Er/20%Yb NPs was proposed based on evolution analysis of the time-dependent morphology, in which BiF:2%Er/20%Yb was formed through the growth process of "nucleation to crystallization and Ostwald ripening". The upconversion luminescence (UCL) properties and mechanism of BiF:Ln (Ln = Ho, Er, Tm)/Yb under dual-wavelength excitation were also systematically investigated. The emission intensity of BiF:2%Er/20%Yb by dual-wavelength excitation ( = 980 nm + 1550 nm) was 1.49 times more than that excited by 1550 nm or 980 nm individually. Furthermore, the properties of the bright white and multicolor UCL showed that yellow, purple, green, or pinkish light could be observed by controlling the doping concentration of Ln (Ln = Yb, Er, Tm, and Ho), indicating that they had potential applications in backlight sources of color displays and security labeling. The temperature sensitivity of BiF:2%Er/20%Yb exhibited a downward tendency and its max value was about 0.0036 K at 273 K. Cell toxicity tests showed that the UCNPs in phospholipid aqueous solution presented low cytotoxicity. Also, imaging and X-ray imaging revealed that the BiF:2%Er/20%Yb NPs had deep penetration and high contrast, which meant it could be used as a potential probe and contrast agent in optical bioimaging.