Tailoring up-conversion luminescence for single band located in first biological windows and optical thermometry of Yb3+/Ln3+ (Ln = Er, Tm) doped oxyfluoride ceramics via Cr3+ doping

Highlights: • In this work, for the first time, Cr³⁺ and Ln³⁺ (Er³⁺, Tm³⁺, Yb³⁺) ions were doped into oxyfluoride ceramics, simultaneously. • Tailoring up-conversion luminescence of Er³⁺ and Tm³⁺ under 980 nm laser excitation for single red band by Cr³⁺ doping was observed. • β-PbF₂:Yb³⁺/Ln³⁺/Cr³⁺ (Ln = Er, Tm) oxyfluoride ceramics as thermometer operating in the BW-I was evaluated on the basis of FIR. -- Abstract: Successful up-conversion luminescence (UCL) tuning from multi-bands to a single band located in first biological windows (BW-1) was realized in oxyfluoride ceramics doped with Yb³⁺/Ln³⁺ (Ln = Er, Tm) via Cr³⁺ doping. Based on UCL spectra, decay curves and diffuse reflectance spectra (DRS) of β-PbF₂:Yb³⁺/Er³⁺/Cr³⁺ oxyfluoride ceramics with different Cr³⁺ contents, strong energy transfer between Cr³⁺ and Er³⁺ ions was confirmed to explain the UCL tuning mechanism. In β-PbF₂:Yb³⁺/Tm³⁺/Cr³⁺ system, besides energy transfer between Cr³⁺ and Tm³⁺ ions, that Cr³⁺ ions doping into β-PbF₂ nanocrystals absorbing the blue and other higher-energy emissions of Tm³⁺ ions directly was the leading role of β-PbF₂:Yb³⁺/Tm³⁺/Cr³⁺ exhibiting an intense single red emission under 980 nm laser excitation. Additionally, the potentiality of β-PbF₂:Yb³⁺/Ln³⁺/Cr³⁺ (Ln = Er, Tm) oxyfluoride ceramics as optical thermometry operating in the first biological window (BW-I) was evaluated on the basis of a thermo-responsive fluorescence intensity ratio (FIR) of the Stark sublevels of ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ion and deep-red (700 nm) to NIR emission (800 nm) from completely separated ³F₃/³H₄→³H₆ transitions of Tm³⁺ ion. These results offer a promising strategy to manipulate single band UCL located in BW-1 region for the deep tissue imaging. And temperature dependent UCL properties of Ln³⁺ (Ln = Er, Tm) doped fluoride nanocrystals has great potential applications as biological thermometry.