Sr6Y(PO4)5: Nd3+ a novel whitlockite-type phosphor for optical temperature sensing applications: Synthesis and luminescence properties

Abstract

Active research in science and technology is improving the development of materials that emit infrared light, with the aim of better optical thermometric sensors. In this paper we report the synthesis of a new NIR-emitting phosphate phosphor, Sr6Y(PO4)5:Nd3+ (SYP: Nd3+), at differing concentrations of Nd3+ using a conventional solid-state reaction route. X-ray diffraction analysis showed that the prepared phosphors exhibit a singular phase with a monoclinic whitlockite structure (space group I2/a). SEM images show micrometer-sized particle. FTIR and Raman spectroscopies confirm the presence of PO43− groups in the examined phosphates. Luminescence features, comprising emission spectra, concentration quenching, and fluorescence lifetime, were investigated for the prepared SYP: Nd3+ samples at various Nd3+ doping concentrations. Excited at 808 nm, the phosphors emitted near-infrared light with four distinct bands corresponding to specific Nd3+ transitions from 4F3/2 to (4I9/2,4I11/2 and 4I13/2) and 4F5/2 to 4I11/2, falling within the first and second biological windows (NIR-I and NIR-II). Importantly, Nd3+ emission characteristics are affected by concentration and temperature. The powders' effectiveness as optical temperature sensors using fluorescence intensity ratio (FIR) between coupled and non-coupled levels was researched across 303–473 K, resulting in two distinctive FIR-based temperature sensor strategies employing emissions at 874.8 nm (4F3/2 → 4I9/2) and 956 nm (4F5/2 → 4I11/2) transitions, all within the first biological window for excitation and emission. Using this method, SYP: Nd3+ revealed a notably elevated sensitivity of approximately 1.16 %K−1 at 303 K, surpassing other Nd3+-doped materials when stimulated within the infrared range. These results strongly suggest the prospective applicability of the analyzed material in monitoring the temperature of biological systems.