Lanthanide-doped nanoparticles (LnNPs)h ave becomea ni mportant class of fluorophores for advanced biosensing and bioimaging. LnNPs that are photosensitized by surface-attached antenna ligands can possess exceptional brightness. However,their functional bioconjugation remains an important challenge fortheir translation into bioanalytical applications.T os olvet his problem,w ed esigned al igand that can be simultaneously applieda se fficientl ight harvesting antenna for Tb surface ions and strong linker of biomolecules to the LnNPs surfaces. To demonstrate generic applicability of the photosensitized TbNP-bioconjugates, we applied them in two prototypical applications for biosensing and bioimaging. First, in-solution biorecognition was shown by time-resolved Fçrster resonance energy transfer (FRET)b e-tweens treptavidin-functionalized TbNPs to biotinylated dyes (ATTO 610). Second,i ns itu detection of ligand-receptor binding on cells wasa ccomplished with TbNP-antibody (Matuzumab)c onjugates that could specifically bind to transmembrane epidermalg rowth factor receptors (EGFR). High specificity and sensitivity were demonstrated by time-gated imaging of EGFR on both strongly (A431)a nd weakly (HeLa and Cos7) EGFR-expressing cell lines, whereas non-expressing cell lines (NIH3T3) and EGFR-passivated A431 cells did not show any signals. Despitet he relatively large size of TbNP-antibody conjugates,t hey could be internalized by A431 cells upon binding to extracellular EGFR,w hich showed their potential as bright and stable luminescence markersfor intracellular signaling.
A new tripodal ligand, based on a central nitrogen atom tris‐functionalized with 6‐methylene‐2‐pyridyl phosphonic acid was synthesized and characterized, in particular by its X‐ray crystal structure. The coordination behaviour of the tripod with lanthanide cations in aqueous solutions was studied by means of UV/Vis electronic absorption spectroscopy and steady‐state and time‐resolved luminescence spectroscopy, revealing the formation of a [LnL] complex followed by polynuclear species with 2:1 and 3:1 metal/ligand stoichiometries. The [LnL] complexes (Ln = Eu, Tb and Yb) were isolated and characterized and the solid‐state structure of the Eu complex was determined by X‐ray diffraction analysis on monocrystals, revealing the observation of dimeric species, in which the Ln3+ cations are firmly held in the cavity formed by the three pyridylphosphonate arms, the coordination of the cations being completed by a water molecule and a phosphonate function of the second complex, allowing for the formation of the dimers, which are further stabilized by π–π stacking interactions between one pyridyl unit of each adjacent monomer. The spectroscopic properties of the complexes in aqueous solutions were studied, showing an impressive 16 µs excited state lifetime for the Yb complex in D2O, despite the presence of a water molecule in the first coordination sphere.
The heptadentate ligand L was shown to form an extremely stable Gd complex at neutral pH with ap Gd value of 18.4 at pH 7.4. The X-ray crystal structures of the complexes formed withG da nd Tb displayed two very different coordination behaviors being, respectively,o cta-and nonacoordinated. The relaxometric properties of the Gd complex were studied by field-dependent relaxivity measurements at various temperatures and by 17 ONMR spectroscopy.T he pHdependence of the longitudinal relaxivity profile indicated large changes around neutralp Hl eading to av ery large value of 10.1 mm À1 ·s À1 (60 MHz, 298 K) at pH 4.7. The changes were attributed to an increaseo ft he hydration number from one water molecule in basic conditions to two at acidic pH. As imilart rend was observedf or the luminescence of the Eu complex, confirming the change in hydration state. DOSY experiments were performed on the Lu analogue,p ointing to the absence of dimers in solutioni nt he considered pH range. Ab reathing mode of the complex was postulated, which was further supported by 1 Ha nd 31 PNMR spectroscopy of the Yb complex at varying pH and was finally modeled by DFT calculations. Figure 1. UV absorption spectra of a1:1 Gd 3 + /L mixture( 4.8 10 À5 m,0.15 m NaCl, 25 8C) recorded at different pH values from 1.9 (black line,12.5 %f ree Gd 3 + )t o0.8 (red line, 96 %o ffree Gd 3 + ).
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