Recent advances in the optimization and functionalization of upconversion nanomaterials for in vivo bioapplications

Abstract

Upconversion nanophosphors are considered to be important components in advanced materials designed for in vivo bioapplications and benefit from their unique anti-Stokes upconversion luminescence properties. The near-infrared light excitation allows for a large penetration depth in biotissues during in vivo applications. This review presents recent advances in the optimization and functionalization of upconversion nanomaterials for bioimaging and in vivo therapy. The most effective protocols are introduced in detail. Current limitations and future prospects are also included to provide a general summary and suggest future research directions. Upconversion nanophosphors have unique ability to generate anti-Stokes luminescence. The rapid developments of nanotechnology in recent years have demonstrated the successful synthesis and optimization of such upconversion nanophosphors. This review summarizes the recent advances using upconversion nanophosphors as bioprobe for in vivo applications. Upconversion nanoparticles emit photons at shorter wavelengths than the wavelengths of the photons they absorb under excitation, and thus convert low-energy light into higher-energy light. With obvious potential for solar cells, these nanoparticles also show promise in bioimaging and therapeutic applications: they can be excited by near-infrared radiation and emit shorter near-infrared or visible light, leading to minimal scattering by biological tissues and good penetration depth. Fuyou Li and colleagues from Fudan University, China, review recent progress toward optimizing and functionalizing these nanoparticles for in vivo applications such as multi-modality imaging and near-infrared light-driven therapy. Challenges include insufficient luminescent efficiencies, which can be improved through core-shell structures, as well as the alleviation of overheating effects—achieved, for example, through incorporating additional ions that act as antennas and result in more efficient wavelength conversion. Additionally, the potential toxicity of these inorganic particles—including their distribution and effects in the body—needs to be investigated further.