Novel Microwave-Assisted Solvothermal Synthesis of NaYF4:Yb,Er Upconversion Nanoparticles and Their Application in Cancer Cell Imaging

This work reports the novel microwave-assisted solvothermal synthesis and structural, topographic, spectroscopic characterization of NaYF4:Yb,Er upconversion nanoparticles (UCNPs) as well as their application in the labeling of HeLa cells. The nanoparticles were prepared in ethylene glycol, with rare earth acetates as precursor and NH4F and NaCl as the fluorine and sodium sources. X-ray diffraction, transmission electron microscopy, and luminescence spectroscopy were applied to characterize the nanoparticles. Experimental results showed that the microwave-assisted solvothermal method is an effective approach to create highly crystalline, strongly luminescent UCNPs at a lower temperature (160 degrees C) and within a significantly shortened reaction time (only 1 h) compared to the traditional methods. The effect of fluorine source on the optical properties of UCNPs was investigated by using NH4F, NR4HF2, NaF, and 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF(4)) as different fluorine sources; NH4F proved to be the best one, making the luminescent intensity increase at least 2 orders of magnitude. The UCNPs with four different colors (green, yellow, orange, and cyan) were successfully obtained. After being modified with amino groups and coupled with CEA-8 antibody, the obtained nanoparticles were successfully applied in the specific fluorescent immunolabeling and imaging of HeLa cells to further verify their function as a marker in immunolabeling.