Lanthanide-doped niobium oxide nanoparticles on nanoarchitectured silica spheres as potential probe for NIR luminescent markers

We have prepared Nb2O5:Eu3+ and Nb2O5:Nd3+ nanostructured materials and SiO2@Nb2O5:Eu3+ core@shell nanoparticles by the sol–gel route to evaluate how the annealing temperature influences their structural and luminescent features, aiming at their potential application as optical markers. An intense red emission under excitation at the blue region was detected from the Nb2O5:Eu3+ solids. Inhomogeneous broadening on the photoluminescence spectra was observed for the sample annealed at 600 °C, while the allocation of the Eu3+ ions on S4 symmetry sites was attributed for higher annealing temperature (1100 °C). A discrete formation of the H-phase (monoclinic) of niobium oxide for the sample annealed at an intermediate temperature of 900 °C was observed, with its photoluminescence spectra revealing the preferential occupation of the Eu3+ ions on sites in this crystalline phase. Photoluminescence decay curves recorded for the 5D0 excited state revealed average lifetime values in the order of milliseconds. The Nb2O5:Nd3+ solids displayed analogous behavior: Nd3+ preferentially occupied the monoclinic phase symmetry sites. In water dispersion and under low-power laser excitation (808 nm), the Nb2O5:Nd3+ systems emitted in biological windows. The sample annealed at 600 °C provided the most promising luminescence intensity and colloidal stability. The surface of the SiO2 nanoparticles was successfully decorated with Nb2O5:Eu3+ nanoparticles in the T-phase. The SiO2@Nb2O5:Eu3+ core@shell nanoparticles presented color tunability due to red light emission from Eu3+ and blue light emission from defect centers in the SiO2 template. The possibility of functionalizing SiO2 nanoparticles with red-emitting Nb2O5:Eu3+ nanoparticles combined with blue excitation and lifetime values in the order of milliseconds renders the prepared SiO2@Nb2O5:Eu3+ core@shell nanoparticles promising biomarkers that can be explored by time-gated luminescence techniques.