Recent advances in fluorescent upconversion nanomaterials: novel strategies for enhancing optical and magnetic properties to biochemical sensing and imaging applications

Upconversion fluorescence is the process of converting optical energy from low absorption energies to high emission energies. The upconversion (UC) materials exhibit significant advantages such as high penetration depth, low unwanted autofluorescence and light scattering from biological species at near-infrared excitation. Nowadays, UC materials are widely applied in optical applications, including solid-state lasers, temperature sensor and chemical sensors. The upconversion materials consist of low phonon host material and sensitizer-activator pair. The sensitizer and activator are often rare earth elements, where energy transfers from the sensitizer to the activator. This review describes the general picture and recent progress of lanthanide upconversion fluorescent materials along with applications in biochemical sensing and imaging. We also provide novel strategies to enhance upconversion intensity and tunable emissions with doping metal ions and core-shell structures, and to integrate magnetic properties to upconversion materials.