Dysprosium complexes and their micelles as potential bimodal agents for magnetic resonance and optical imaging

Six diethylene triamine pentaacetic acid (DTPA) bisamide derivatives functionalized with p-toluidine (DTPA-BTolA), 6-aminocoumarin (DTPA-BCoumA), 1-naphthalene methylamine (DTPA-BNaphA), 4-ethynylaniline (DTPA-BEthA), p-dodecylaniline (DTPA-BC12PheA) and p-tetradecyl-aniline (DTPA-BC14PheA) were coordinated to dysprosium(III) and the magnetic and optical properties of the complexes were examined in detail. The complexes consisting of amphiphilic ligands (DTPA-BC12PheA and DTPA-BC14PheA) were further assembled into mixed micelles. Upon excitation into the ligand levels, the complexes display characteristic Dy(III) emission with quantum yields of 0.3-0.5% despite the presence of one water molecule in the first coordination sphere. A deeper insight into the energy-transfer processes has been obtained by studying the photophysical properties of the corresponding Gd(III) complexes. Since the luminescence quenching effect is decreased by the intervention of non-ionic surfactant, quantum yields up to 1% are obtained for the micelles. The transverse relaxivity r2 per Dy(III) ion at 500 MHz and 310 K reaches a maximum value of 27.4 s(-1) mM(-1) for Dy-DTPA-BEthA and 36.0 s(-1) mM(-1) for the Dy-DTPA-BC12PheA assemblies compared with a value of 0.8 s(-1) mM(-1) for Dy-DTPA. The efficient T2 relaxation, especially at high magnetic field strengths, is sustained by the high magnetic moment of the dysprosium ion, the coordination of water molecules with slow water exchange kinetics and long rotational correlation times. These findings open the way to the further development of bimodal optical and magnetic resonance imaging probes starting from single lanthanide compounds.