• cyclodextrins nanosponges
• europium
• fluorescence microscopy
• protein bioconjugation

Luminescent lanthanide complexes, particularly those involving Europium (III) with TTA (thenoyltrifluoroacetone) ligands, have garnered significant interest for their applications in materials science and biomedicine. This study explores their use in fluorescence labeling of proteins and the delivery of nucleic acids, facilitated by cyclodextrin sponge nanoparticles. TTA derivatives coordinate effectively with Eu3+ ions, producing emissions in the visible spectrum at wavelengths over 600 nm. The longer excited state lifetimes of Eu3+ ions compared to organic fluorescent molecules like protein residues and coenzymes allow for time-gated fluorescence microscopy, reducing cell autofluorescence and enhancing sensitivity.
For protein labeling, the free amino groups of proteins (such as terminal -NH2 and lysine residues) were reacted with a chlorosulfonic derivative of TTA. Adding an excess of Eu3+ resulted in pink-red fluorescence under UV light (365 nm).
In cyclodextrin nanosponges, the chlorosulfonic derivative of TTA was reacted with 1-adamantanamine to form a sulfonamide derivative, which then formed fluorescent complexes with Eu3+ in a 1:1 ratio. These derivatives were incorporated into β-cyclodextrin and corresponding nanosponges. To create nucleic acid transport systems, the cyclodextrin nanosponges were loaded with 1-adamantanamine and a small amount of the sulfonamide-Eu3+ complex. At physiological pH, the free amine groups of the included amine become protonated, forming a polycationic system capable of transporting nucleic acids (polyanionic systems) across lipid bilayers. The Eu3+ complex's fluorescent labeling enables observation of DNA entry into cells via time-gated fluorescence microscopy.