Upconverting Nanophosphors for bioimaging: Preparation Strategies for hydrophilic colloidal Stable Particles

Currently, there is a widespread interest to use neodymium containing upconverting nanophosphors (UCNPs) for bioimaging applications. The infatuation of these nanoparticles derives from their capacity for excitation in the biologically transparent window (700-1000 nm), exceptional ability to convert near infrared radiation into visible light (upconversion), which prevents autofluorescence and over-heating effect of biological tissues and capability for deep tissue, high contrast imaging related to minimum autofluorescence in this spectral range [1]. Despite the fast progress in lanthanide-doped upconversion nanoparticles, the preparation of ultrasmall, monodisperse and hydrophilic UCNPs that display intense luminescence is still a challenging issue. Only a few examples of ultrasmall and hydrophilic UCNPs have been reported [2-3]. Here, we report the elaboration, synthesis and surface modification of sub-10 nm UCNPs with an excitation wavelength of 795 nm, which are composed of a host lattice of crystalline NaYF4 doped with Nd3+ and Yb3+ as sensitizers, and Er3+,Ho3+ or Tm3+ as emitter ions. Established strategies for synthesizing UCNPs yield mainly hydrophobic particles[1]. We report the conversion of these into hydrophilic, colloidally-stable, and biocompatible by using mainly ligand exchange strategies, and the influence of the coating on the UCNPs’ photophysical properties. This study will also allow establishing information about biodistribution, pharmacokinetics and formation of protein corona for ultrasmall UCNPs.