Nd3+-doped LiBaPO4 phosphors for optical temperature sensing within the first biological window: A new strategy to increase the sensitivity

In this work, powders of LiBaPO4:Nd3+ were synthesized by the sol–gel method using PVA as a chelating agent. X-ray diffraction indicates a tridymite structure with a hexagonal unit cell and space group P63. SEM images show that the samples have particles of different sizes with the majority varying between 5 and 20 μm. Under 532 nm excitation, four infrared emission bands were observed, with maxima at 749.9, 802.4, 873.2 and 953.4 nm assigned to the transitions from 4F7/2, 4F5/2, 4F3/2 to 4I9/2 and from 4F5/2 to 4I11/2. The performance of this material aiming optical thermometry based on the fluorescence intensity ratio methodology (FIR) was investigated within the range from 294 to 373 K, suitable for applications in biological systems. Under visible excitation, sensor performance is comparable to previously investigated Nd schemes. However, the sensing performance under infrared excitation was strongly enhanced by exploiting the FIR associated to the 4F5/2 → 4I11/2 and 4F3/2 → 4I9/2 transitions. The obtained relative sensitivity with this approach is one order of magnitude higher when compared with other Nd-doped materials excited in the infrared. These results suggest that the analyzed material can be used for application in temperature sensing of biological systems exploring the first biological window.