One-step synthesis of amino-functionalized up-converting NaYF₄:Yb,Er nanoparticles for in vitro cell imaging

The emerging up-conversion nanoparticles (UCNPs) offer a wide range of biotechnology applications, from biomarkers and deep tissue imaging, to single molecule tracking and drug delivery. Their successful conjugation to biocompatible agents is crucial for specific molecules recognition and usually requires multiple steps which may lead to low reproducibility. Here, we report a simple and rapid one-step procedure for in situ synthesis of biocompatible amino-functionalized NaYF₄:Yb,Er UCNPs that could be used for NIR-driven fluorescence cell labeling. X-ray diffraction showed that UCNPs synthesized through chitosan-assisted solvothermal processing are monophasic and crystallize in a cubic α phase. Scanning and transmission electron microscopy revealed that the obtained crystals are spherical in shape with a mean diameter of 120 nm. Photoluminescence spectra indicated weaker green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) and stronger red emission (⁴F_9/2 → ⁴I_15/2), as a result of enhanced non-radiative ⁴I_11/2 → ⁴I_13/2 Er³⁺ relaxation. The presence of chitosan groups at the surface of UCNPs was confirmed by Fourier transform infrared spectroscopy, thermogravimetry and X-ray photoelectron spectroscopy. This provides their enhanced internalization in cells, at low concentration of 10 μg ml^-1, without suppression of cell viability after 24 h of exposure. Furthermore, upon 980 nm laser irradiation, the amino-functionalized NaYF₄:Yb,Er UCNPs were successfully used in vitro for labeling of two human cell types, normal gingival and oral squamous cell carcinoma.