Energy transfer between optically trapped single ligand-free upconversion nanoparticle and dye.

The quenching in luminescence emission of an optically trapped ligand-free hydrophilic NaYF4:Yb, Er upconversion nanoparticle as a function of rose Bengal dye molecule is investigated here. The removal of oleate capping of the as-prepared upconversion nanoparticles was achieved via acid treatment and characterized via FTIR and Raman spectroscopic techniques. Further, the capping removed hydrophilic single upconversion nanoparticle is optically trapped and the emission studies were carried out as a function of excitation laser power. Compared to the studies using the bulk solution, the single upconversion nanoparticle luminescence spectrum exhibited additional spectral lines. The excitation laser power-dependent studies using the bulk solution yield a slope value between 1 and 2 for Blue, Green1, Green 2, and Red emission and thus indicate that upconversion is a two-photon upconversion process. On the other hand, in the case of laser power-dependent studies on an optically trapped single-particle study, Blue and Green 2 yield a slope value of less than 1 whereas Green 1 and Red emission gave a slope value between 1 and 2. The energy transfer studies between an optically trapped ligand-free single upconversion nanoparticle (UCNP) and the rose Bengal dye show a concentration-dependent quenching in the emission of Green emissions and illustrate the potential of developing sensor platforms.