Rare-earth-doped luminescent nanothermometers are not reliable as their emission spectra can be affected by numerous environmental and experimental factors.

We propose two effective approaches to enhance the Förster resonance energy transfer (FRET) efficiency from nearinfrared (NIR) excited upconverting nanoparticles (UCNPs, namely LiYF4:Yb 3+ ,Tm 3+ ) to CuInS2 quantum dots (QDs) upon engineering of the donor's architecture. The study of the particles' interaction highlighted a radiative nature of the energy transfer (ET) among the moieties under investigation when in solution. However, analyses performed on dry powders allowed to observe clear evidence of a FRET mechanism. In particular, photoluminescence lifetime measurements showed that FRET efficiency could be effectively increased by, both, reducing the size of the UCNPs and directly controlling the distribution of the active ions throughout the donor's volume, i.e. doping them only in the outer shell of a core/shell system. Both strategies resulted at least in a more than doubled FRET efficiency compared to larger core-only UCNPs. Obtained experimental values were compatible with those predicted from geometrical considerations on the active ions' distribution over the UCNP volume. These results provide a concrete proof of the potential of UCNP-QD FRET pair when the system is properly designed, hence setting a solid base for the development of robust and efficient all-inorganic probes for FRET-based assays.

An approach to unequivocally determine the three-dimensional orientation of optically manipulated studies. Based on the strong polarization dependent upconverted luminescence of UCNRs it is possible to unequivocally determine, in real time, their three-dimensional orientation when optically trapped. In single-beam traps, polarized single particle spectroscopy has concluded that UCNRs orientate parallel to the propagation axis of the trapping beam. On the other hand, when multiple-beam optical tweezers are used, single particle polarization spectroscopy demonstrated how full spatial control over UCNR orientation can be achieved by changing the trap-to-trap distance as well as the relative orientation between optical traps. All these results show the possibility of real time three-dimensional manipulation and tracking of anisotropic nanoparticles with wide potential application in modern nanobiophotonics.