Relaxivity Enhancement of Ditopic Bishydrated Gd(III) Complexes Conjugated to Mesoporous Silica Nanoparticles

Binuclear Gd3+-complexes based on thermodynamically stable and bis-hydrated GdAAZTA chelates were covalently linked to the surface of mesoporous silica nanoparticles followed by their decoration with PEG5000 molecules. Surface and textural properties of the samples were analysed. A detailed 1H NMR relaxometric characterisation as a function of the applied magnetic field strength was carried out in aqueous solution in order to evaluate the efficacy of the conjugated systems as potential MRI diagnostic probes. The samples showed enhanced longitudinal relaxivity values (per ion: 37.5 mM-1 s-1 at 0.7 T and 310 K) as compared to the free complex (+270%) and related nanosystems based on monohydrated Gd3+-chelates (+42%). The relaxivity gain is the result of the rational design of the ditopic complex that ensures an increased hydration state (q = 2), improvement of the rotational dynamics and a fast rate of water exchange between the inner coordination sphere and the bulk. These favourable properties are obtained without compromising the high thermodynamic stability and kinetic inertness of the chelating units.