Enhanced upconversion emission of Er³⁺-Yb³⁺ co-doped Ba₅(PO₄)₃OH powder phosphor for application in photodynamic therapy

Er³⁺-Yb³⁺ co-doped Ba₅(PO₄)₃OH nanoparticle powder phosphors were successfully synthesized by urea combustion method. The resulting powder phosphors were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), high resolution scanning electron microscopy (HRSEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). XRD data confirmed crystallization of pure hexagonal phase of Ba₅(PO₄)₃OH and HRSEM images showed formation of ellipsoidal particles. XPS data combined with EDS analysis confirmed the materials composition that corresponds with identification of all the chemical elements constituting the materials. The in vitro dark cytotoxicity of the particles confirmed lack of cytocidal behaviour in the absence of light, but considerable photodynamic therapy (PDT) activity was observed upon illumination. Upon excitation using a 980 nm laser, multiple emission peaks in the green and red regions corresponding to the optical transitions of Er³⁺ ion were observed. Upon co-doping with Yb³⁺, upconverted red emission was detected and this was attributable to non-radiative energy transfer from Yb³⁺ to Er³⁺. The proposed mechanism of upconversion photoluminescence is discussed.