NIR-Responsive Silica-Coated NaYbF4:Er/Tm/Ho Upconversion Fluorescent Nanoparticles with Tunable Emission Colors and Their Applications in Immunolabeling and Fluorescent Imaging of Cancer Cells

NaYbF4:RE upconversion (UC) fluorescent nanoparticles (NPs) were synthesized with variable rare-earth dopants (RE = Er3+, Tm3+, or Ho3+, or a combination of these ions) from rare-earth stearate precursors in a water-ethanol-oleic acid system by using a two-phase solvothermal method. The NPs were shown to emit visible light, such as orange, yellow, green, cyan, blue or pink light in response to near-infrared (NIR) irradiation, and their emission colors could be simply tuned by changing either the codopant concentration or the dopant species. The UC NPs were well-dispersed and spherical with an average size of 15-35 nm. They emitted strong UC fluorescence under the 980 nm NIR excitation. The effects of solvothermal reaction time and temperature on nanoparticle size and phase structure as well as UC fluorescence intensity were systematically studied Water dispersibility was achieved by forming a silica coat oil the surface of the UC NPs. After animo functionalization, the silica-coated UC NPs were chemically conjugated with the rabbit anti-CEA8 antibody and then used as fluorescent biolabels for the immunolabeling and imaging of HeLa cells. The NIR-responsive multicolor visible light emission of these UC NPs will enable potential applications in biolabeling and multiplexed analysis because NIR light can penetrate tissue as deep as several inches and is safe to the human body