Abstract

Trivalent rare earth (RE)-doped upconversion nanoparticles (UCNPs) can be considered as a dilute guest-host system, where trivalent lanthanide ions are dispersed as guests in an appropriate inorganic host lattice with a dimension of less than 100 nm. They are able to convert near-infrared (NIR) light into visible or ultraviolet luminescence through a set of coupled linear processes in a system of real energy levels of RE ions incorporated at the Bravais lattice points of the host material (Auzel 1990, 2004; Chen et al. 2014a; Haase and Schäfer 2011; Wang and Liu 2009). An important advantage offered by coupled linear excitation is the ability to generate nonlinear UC luminescence (UCL) with an excitation power density as low as ~10⁻¹ W/cm², which can be easily provided by low cost continuous-wave laser diodes or incoherent light sources (light emitting diodes and incandescent light bulbs). Along with the low cytotoxicity entailed by the inorganic host lattice, the frequency converting capability of UCNPs imparts a number of advantages for them such as absence of autofluorescence, deep penetration of light in biological tissues, and minimum photodamage to living organisms (Achatz et al. 2011; Wang et al. 2010a). These merits engage them for a plethora of applications in bioimaging and therapy. As a rule of thumb, the luminescence efficiency of UCNPs is of particular importance for their embodiment in biophotonic applications, which is, however, severely limited by nanosize-induced surface-related quenching effects and detrimental interactions between the doped RE ions. The reported maximum upconversion quantum yield (UCQY) of UCNPs, which are devoid of any shell, is typically less than 1% even under a high laser irradiance of ~10² W/cm².