Lanthanide-based luminescence represents an attractive alternative to conventional organic chromophores used for optical imaging applications. The unique electronic configuration of trivalent lanthanides results in narrow emission bands independent of the coordinative environment of the metal between 490-700 nm. Due to the Laporte-forbidden nature of f-f transitions, the excitation from aromatic chromophores at 250-350 nm is required to access the metal ion-based excited states. The need for this short-wave excitation renders luminescent lanthanides not suitable for in vivo applications due to the limited permeability of light through biological media below 500 nm.
We developed a system that employs Cherenkov radiation energy transfer (CRET) to excite luminescent lanthanide complexes in situ. Cherenkov radiation (CR) is emission of short wavelength light generated by charged particles emitted during the decay of radioisotopes in dielectric media. CR exhibits high relative intensity at wavelengths required to efficiently excite antennae. To demonstrate the feasibility of this approach, we doped aqueous lanthanide(III) complexes with radioisotope (¹⁸F, ⁸⁹Zr) and recorded the lanthanide-based emission using a small animal optical imager. With only 10 μCi of ¹⁸F, quantities as low as 5 nmol of discrete terbium complex can be detected. In addition to our proof-of-concept studies, we evaluated the relationship between lanthanide complex quantum yield (QY) and CRET by imaging a library of Tb(III) complexes (QY 0.01 – 0.65). A direct correlation allows for a systematic, high-throughput relative quantum yield determination of lanthanide complexes, which also aided in discovery of the importance of the excited triplet state on lanthanide quantum yield. We then investigated the direct modulation of the triplet state for a robust lanthanide turn-on probe. Utilizing the CR-imaging method, we observe a 500-fold increase in radiance upon reduction of a nitro-antenna Tb-complex to an amine-antenna Tb-complex in the presence of biologically relevant analyte H₂S. These cumulative approaches advance lanthanide-based probes for optical imaging. Efforts to develop intramolecular CRET probes to minimize CR quenching is on-going.

Patrick Nawrocki

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