BaWO₄:Ln³⁺ Nanocrystals: Controllable Synthesis, Theoretical Investigation on the Substitution Site, and Bright Upconversion Luminescence as a Sensor for Glucose Detection

Simultaneous morphology and upconversion luminescence tuning of BaWO4 nanocrystals excited at 980 nm has been achieved by the successful double doping of different Ln3+ ions. Especially, the substitution sites of Ln3+ into BaWO4 have been investigated by the combination of experimental and density functional theory (DFT) calculations for the first time. The Ba site is the most likely location for BaWO4:Yb3+, BaWO4:Er3+, BaWO4:Tm3+, and BaWO4:Ho3+. However, when codoping Yb3+ with other Ln3+ species into the unit cell of BaWO4, the DFT calculations indicated that it is more likely that Yb3+ engages the W site of BaWO4, while the doped Ln3+ remains at its location at the Ba site, with Yb3+ in the near W layer in the c axis. In addition, a low-cost and sensitive BaWO4:Yb3+/Er3+@Au for glucose detection is designed through a luminescence resonance energy-transfer process and characterized by fluorescence techniques. The limit of detection of BaWO4:Yb3+/Er3+@Au was 3.1 nM. BaWO4:Yb3+/Er3+@Au can be conveniently extended to detect many analytes involving hydrogen peroxide.