Nd(3+)-Doped TiO(2) Nanoparticles as Nanothermometer: High Sensitivity in Temperature Evaluation inside Biological Windows

TiO(2) nanoparticles doped with different amounts of Nd(3+) (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 Nd(3+). 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 Nd(3+) in TiO(2) was obtained by EDXS, and the oxidation state of titanium and neodymium was investigated via XPS and NEXAFS, respectively. Nd(3+) was present in all the samples, unlike titanium, where besides Ti(4+), a significantly amount of Ti(3+) was observed; the relative concentration of Ti(3+) increased as the amount of Nd(3+) in the TiO(2) 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 TiO(2), increased when the concentration of Nd(3+) was increased. Using 600 nm for excitation, the (4)F(7/2)→(4)I(9/2), (4)F(5/2)→(4)I(9/2) and (4)F(3/2)→(4)I(9/2) transitions of Nd(3+) 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 (4)F(7/2)→(4)I(9/2) and (4)F(5/2)→(4)I(9/2) transitions increased significantly, in contrast to the (4)F(3/2)→(4)I(9/2) transition, in which the intensity emission decreased. The fluorescence intensity ratio between the transitions [Formula: see text] and [Formula: see text] were used to calculate the relative sensitivity of the sensors. The relative sensitivity was near 3% K(−1) for [Formula: see text] and near 1% K(−1) for [Formula: see text].