Lanthanide-Complex-Loaded Polymer Nanoparticles for Background-Free Single-Particle and Live-Cell Imaging

Imaging single molecules and nanoparticles in complex biological media is highly challenging notably due to autofluorescence of cells and tissues. Lanthanides and lanthanide complexes, with their particularly long luminescence lifetimes, offer the possibility to perform time-gated imaging and thus to strongly reduce the autofluorescence background. However, their very low brightness and photon flux have limited their use in single-molecule imaging. Here, we encapsulate high amounts of Europium complexes into poly(methyl methacrylate)-based particles of 10, 20, and 30 nm size. The resulting particles contain up to 5000 copies of the complex with a quantum yield of ≥0.2, resulting in a per particle brightness of up to 4 × 107 M–1 cm–1. They can be imaged at the single-particle level using low illumination intensities (0.24 W cm–2) and low acquisition times (300 ms) and internalize well into living cells, where they can be monitored through time-gated imaging at illumination conditions compatible with living specimen. These Eu-complex-loaded nanoparticles can thus be applied for highly sensitive and autofluorescence-free imaging and have the potential to become very performant probes for fast intracellular tracking of single biomolecules.