Multicolor Core/Shell-Structured Upconversion Fluorescent Nanoparticles

Near-infrared (NIR)-to-Visible upconversion fluorescent nanoparticles emit visible light upon NIR-light excitation, and are well suited for bioimaging, compared to the commonly used downconversionfluorescentmaterials.Thesenanoparticleshave advantages such as minimum photodamage to living organisms, weak background fluorescence, high detection sensitivity, and high light-penetration depth in tissues. However, development of upconversion fluorescent nanoparticles is still in its infancy, and such materials have yet to be used for bioimaging applications due to their unsuitable surface properties, poor dispersibility inwater, and limited colors. In this work, facile and user-friendly methods are developed to synthesize uniform NaYF4 nanospheres with strong upconversion fluorescence and core–shell silica/NaYF4 nanospheres with uniform silica coating on the surface. Multicolor upconversion nanospheres are produced by encapsulating organic dyes or quantum dots (QDs) in the silica shell, and upconversion fluorescence is generated based on fluorescence resonance energy transfer (FRET) from the NaYF4 nanospheres to these organic dyes or QDs.Useof theupconversionnanospheres for imagingof cells is also demonstrated, the first report in the field using such nanoparticles for bioimaging. Fluorescence imaging is a very important technique for biological studies andclinical applications. It hasbeenused for in vivo imaging due to its high temporal and spatial resolutions. Conventional fluorescence imaging is based on single-photon excitation, emitting low-energy fluorescence when excited by a high-energy light. It has some limitations, such as causing DNA damage and cell death due to long-term irradiation with UV or short-wavelength light, significant auto-fluorescence from