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Multifunctional BiF3:Ln3+ (Ln = Ho, Er, Tm)/Yb3+ nanoparticles: an investigation on the emission color tuning, thermosensitivity, and bioimaging

Pure cubic phase and uniform BiF3:Ln3+ (Ln = Ho, Er, Tm)/Yb3+ nanoparticles (NPs) were prepared by coprecipitation. The growth mechanism of BiF3:2%Er3+/20%Yb3+ NPs was proposed based on evolution analysis of the time-dependent morphology, in which BiF3:2%Er3+/20%Yb3+ was formed through the growth process of “nucleation to crystallization and Ostwald ripening”. The upconversion luminescence (UCL) properties and mechanism of BiF3:Ln3+ (Ln = Ho, Er, Tm)/Yb3+ under dual-wavelength excitation were also systematically investigated. The emission intensity of BiF3:2%Er3+/20%Yb3+ by dual-wavelength excitation (λ = 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 Ln3+ (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 BiF3:2%Er3+/20%Yb3+ exhibited a downward tendency and its max value was about 0.0036 K−1 at 273 K. Cell toxicity tests showed that the UCNPs in phospholipid aqueous solution presented low cytotoxicity. Also, in vivo imaging and X-ray imaging revealed that the BiF3:2%Er3+/20%Yb3+ NPs had deep penetration and high contrast, which meant it could be used as a potential probe and contrast agent in in vivo optical bioimaging.