NIR FRET Luminescence in Rhenium Complex and Dye Co‐Loaded Polymer Nanoparticles

Abstract

Photoactive transition‐metal complexes are luminophores combining high photostability and long luminescence lifetimes. However, reduced optical performance in aqueous solutions has limited their use in biological systems. Herein, the physicochemical and photophysical properties and bioimaging compatibility of Re diimine complexes and near‐infrared (NIR) emitting Cy5 dyes coencapsulated in polymer nanoparticles (NPs) are investigated. By varying the polymers, NPs with sizes from 20 to 70 nm and encapsulating ≤ 40 wt.% of Re complexes, i.e., ≈11 000 Re complexes per NP, are obtained. The photoluminescence (PL) quantum yields of the Re complexes increase eightfold to ≈50% upon encapsulation (vs 6–7% in acetonitrile), resulting in PL brightness up to 108 m−1 cm−1 and PL lifetimes of 3–4 µs. Coencapsulation of Cy5 yields very bright NIR emission upon Re complex excitation. Very close Re‐to‐Cy5 donor–acceptor distances down to ≤2 nm and FRET efficiencies over 90% are confirmed by PL lifetime measurements. The Re‐Cy5 NPs enter mammalian cells for high‐contrast PL imaging in both visible and NIR. This detailed characterization provides a better understanding of the photophysical properties of the transition‐metal‐dye FRET NPs and presents a vital step toward the efficient design of a new class of bright luminescent NP probes.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

“…Hildebrandt, Charbonniere, Reisch, and co-workers have coencapsulated a phosphorescent rhenium(I) complex (156) and an NIR fluorescent cyanine derivative 1,1′dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine (DiD) as a FRET pair into PNPs to prepare NIR luminescent PNPs for cellular imaging with high constrast. 326 Choudhury, Alemany, Jana, Laskar, and co-workers have utilized a biodegradable block copolymer PEG−PLA (PLA = poly(lactic acid)) to encapsulate an AIE-active monocyclometalated iridium(III) complex (157) (Chart 71) for cellular imaging. 327 The amphiphilic block copolymer self-assembles into micelles in aqueous solution, with the hydrophobic PLA block located inside and hydrophilic PEG block outside.…”

“…Cellular imaging shows that the PNPs are internalized into live KB cells via an energy-dependent pathway and enriched in the cytoplasmic region. Hildebrandt, Charbonnière, Reisch, and co-workers have coencapsulated a phosphorescent rhenium(I) complex ( 156 ) and an NIR fluorescent cyanine derivative 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine (DiD) as a FRET pair into PNPs to prepare NIR luminescent PNPs for cellular imaging with high constrast …”

Control of the fluorescence lifetime in dye based nanoparticles

“…, for time gating or using energy transfer to other emitting species). 12–15 Compared to lanthanide-based emitters, organic fluorophores with ns lifetimes allow for a higher photon flux (photons per time). The low abundance of organic dye based NPs for lifetime imaging is likely due to their unpredictable fluorescence lifetimes.…”