BaTiO₃ nanoparticles were successfully synthesized by a new coconut water-based sol–gel method using Ba(CH₃COO)₂ and TiCl₃ as the starting salts. The influence of the amount of coconut water and calcination conditions on the barium titanate crystallization was investigated. The resulting nanoparticles were characterized by thermogravimetric and differential thermal analysis, X-ray diffraction, scanning electron microscopy, and Raman and Fourier transform infrared (FTIR) spectroscopies. The ferroelectric tetragonal single phase of BaTiO₃ was obtained in samples prepared with a ratio of coconut water volume (mL)/BaTiO₃ mass (g) of 25 : 2 and 30 : 2, calcined at 1100°C, which was confirmed by XRD measurements. Crystallites with an average size of about 31 nm for both samples were obtained, and microscopy images revealed the presence of particles in the range of 40 to 60 nm. Raman and FTIR spectra confirmed the dominant tetragonal phase of BaTiO₃, meanwhile traces of BaCO₃ were identified in FTIR spectra.

Luminescent thermometers based on the luminescence intensity ratio between two thermally coupled levels have a strong appeal in the biomedical area due to the possibility of monitoring the temperature of deep tissues. In such procedures, it is necessary that the excitation and emission wavelengths are within the biological windows. Probes based on neodymium luminescence, with excitation and emission around 800 and 880 nm, are frequently proposed but have low relative sensitivity (0.2%.K⁻¹) due to the small energy separation between the explored Stark sublevels. By changing the excitation wavelength to around 750 nm, it is possible to explore the thermal coupling between the ⁴F_5/2 and ⁴F_3/2 levels. However, lasers in this wavelength range are not common. An alternative is to use LEDs as an excitation source. As a proof of concept, we investigated the thermometric performance of three distinct Nd-doped luminescent probes under 730 nm LED excitation and 532 nm laser excitation: nanocrystalline Y₂O₃, LiBaPO₄ microcrystals, and lithium-boron-aluminum (LBA) glass. The results indicated that the use of LEDs as an excitation source can be applied in nano-, micro- and macro-structured probes, as it does not compromise the thermometric performance of the systems, which exhibited relative sensitivities of approximately 2%.K⁻¹.