Spectral engineering of lanthanide-doped upconversion nanoparticles and their biosensing applications

Upconversion nanoparticles (UCNPs), which can convert low-energy excitation to high-energy emission, have been extensively investigated owning to their advantageous optical properties. However, despite significant achievements, UCNP-based nanomaterials still suffer from several intrinsic issues such as narrow absorption cross-sections, weak upconversion luminescence efficiency, and fixed emission wavelength. In this review, we summarize the recent progress in the spectral engineering of UCNPs, focusing on their emission intensity enhancement and emission color tuning. The various bioapplications of UCNPs in the detection of intracellular ions and biomolecules and sensing the physiological environment are highlighted. Finally, challenges and potential solutions in this area are provided. We hope that this review will offer guidance for the design and fabrication of new UCNP-based nanophosphors and further broaden their applications.