Ratiometric Imaging Detection of Amyloid-β Fibrils by a Dual-Emissive Tris-Heteroleptic Ruthenium Complex

Two dual fluorescent/phosphorescent tris-heteroleptic mononuclear Ru(ΙΙ) complexes (2 and 3) were designed and applied in amyloid-β (Aβ) sensing. These complexes have a general formula of [Ru(phen)(dppz)(L)](PF₆)₂, where L is (2-pyrazinyl)(2-pyridyl)(methyl)amine (H-L) with different substituents (-OMe for 2, -H for 3), phen is 1,10-phenanthroline, and dppz is dipyridophenazine, respectively. Compared with the previously reported ratiometric probe 1 with a di(pyrid-2-yl)(methyl)amine ligand, complex 2 can be employed for not only ratiometric emissive detection of Aβ aggregation but also ratiometric imaging detection of Aβ fibrils. In ratiometric emissive detection, as the incubation time of the Aβ sample (Aβ₄₀ and Aβ₄₂) was prolonged, a new phosphorescence emission band appeared with gradual enhancement of the emission intensity, while the fluorescence emission was basically unchanged, which could be treated as an intrinsic internal reference signal. In comparison, a larger ratiometric photoluminescence enhancement (I₆₄₀/I₄₄₀) was observed for Aβ₄₀ aggregation with respect to Aβ₄₂. In ratiometric imaging detection, the imaging signals obtained from the phosphorescence emission are much brighter than the fluorescence emission in both Aβ₄₀ and Aβ₄₂ fibrils. As indicated by molecular docking results, stronger interactions were found between complex 2 with Aβ₄₀ fibrils, which included π/π, π/C-H, and π/H interactions between bidentate ligands dppz and phen with amino acid residues. Moreover, computational calculations were carried out to assist the interpretation of these experimental findings.