Engineered Upconversion Nanoparticles for Resolving Protein Interactions inside Living Cells.

Upconversion nanoparticles (UCNPs) convert near-infrared into visible light at much lower excitation densities than those used in classic two-photon absorption microscopy. Here, we engineered <50 nm UCNPs for application as efficient lanthanide resonance energy transfer (LRET) donors inside living cells. By optimizing the dopant concentrations and the core-shell structure for higher excitation densities, we observed enhanced UCNP emission as well as strongly increased sensitized acceptor fluorescence. For the application of these UCNPs in complex biological environments, we developed a biocompatible surface coating functionalized with a nanobody recognizing green fluorescent protein (GFP). Thus, rapid and specific targeting to GFP-tagged fusion proteins in the mitochondrial outer membrane and detection of protein interactions by LRET in living cells was achieved.