Nd³⁺-Doped TiO₂ Nanoparticles as Nanothermometer: High Sensitivity in Temperature Evaluation inside Biological Windows

TiO₂ nanoparticles doped with different amounts of Nd³⁺ (0.5, 1, and 3 wt.%) were synthetized by the sol-gel method, and evaluated as potential temperature nanoprobes using the fluorescence intensity ratio between thermal-sensitive radiative transitions of the Nd³⁺. XRD characterization identified the anatase phase in all the doped samples. The morphology of the nanoparticles was observed with SEM, TEM and HRTEM microscopies. The relative amount of Nd³⁺ in TiO₂ was obtained by EDXS, and the oxidation state of titanium and neodymium was investigated via XPS and NEXAFS, respectively. Nd³⁺ was present in all the samples, unlike titanium, where besides Ti⁴⁺, a significantly amount of Ti³⁺ was observed; the relative concentration of Ti³⁺ increased as the amount of Nd³⁺ in the TiO₂ nanoparticles increased. The photoluminescence of the synthetized nanoparticles was investigated, with excitation wavelengths of 350, 514 and 600 nm. The emission intensity of the broad band that was associated with the presence of defects in the TiO₂, increased when the concentration of Nd³⁺ was increased. Using 600 nm for excitation, the ⁴F_7/2→⁴I_9/2, ⁴F_5/2→⁴I_9/2 and ⁴F_3/2→⁴I_9/2 transitions of Nd³⁺ ions, centered at 760 nm, 821 nm, and 880 nm, respectively, were observed. Finally, the effect of temperature in the photoluminescence intensity of the synthetized nanoparticles was investigated, with an excitation wavelength of 600 nm. The spectra were collected in the 288-348 K range. For increasing temperatures, the emission intensity of the ⁴F_7/2→⁴I_9/2 and ⁴F_5/2→⁴I_9/2 transitions increased significantly, in contrast to the ⁴F_3/2→⁴I_9/2 transition, in which the intensity emission decreased. The fluorescence intensity ratio between the transitions I821I880=F5/24I49/2F43/2I49/2 and I760I880=F47/2I49/2F43/2I49/2 were used to calculate the relative sensitivity of the sensors. The relative sensitivity was near 3% K^-1 for I760I880 and near 1% K^-1 for I821I880.