Photonic manipulation with upconverting nanoparticles

Frequency upconverting nanoparticles (UCNPs) utilize long-lived excited states of lanthanides to populate high-energy states of dopant ions (e.g. Tm³⁺ or Er³⁺) by subsequent absorption of near infrared (NIR) photons. UCNPs therefore have the potential to serve as local light sources when targeted to specific sites in biological environments. Here we demonstrate that UCNPs are suited to activate the photoconvertible fluorescence protein Dendra2 with a resolution below the optical diffraction limit. UCNPs were coated with non-fluorescent GFP to target them specifically to plasma membrane proteins tagged with antiGFP nanobodies. We showed that Dendra2 is also activated across the membrane when extracellularly bound UCNPs are excited with an NIR laser. These results identify UCNPs as exciting tools for bioanalytics, such as quantitative investigation of protein interaction kinetics and spatiotemporal organization at the micro- and the nanoscale, as well as the spatially confined, minimally invasive release of caged compounds.