Paramagnetic Relaxation Enhancement in Hydrophilic Colloids Based on Gd(III) Complexes with Tetrathia- and Calix[4]arenes

Hydrophilic colloids (PSS-[Ln2(TCAi)2] and PSS-[LnCAi], with i = 1, 2, 3 and Ln=Gd, Tb) were prepared by precipitation of Gd(III) or Tb(III) complexes with tetrathiacalix[4]arenes (TCAi) and calix[4]arenes bearing two 1,3-diketone groups (CAi) from dimethyl formamide to an aqueous solution of poly(sodium 4-styrenesulfonate) (PSS). Dynamic Light Scattering and Transmission Electron Microscopy demonstrated the formation of nanoparticles coated by the polymer. Luminescence decay measurements on Tb(III)-based colloids allowed a hydration number of 2 and 4 per metal ion to be determined for PSS-[Ln2(TCAi)2] and PSS-[LnCAi] samples, respectively. Longitudinal and transverse water proton relaxivity values measured at 20.8 MHz were remarkably high for the PSS-[GdCAi] colloids, but unexpectedly low for the PSS-[Gd2(TCAi)2] ones. 1H Fast Field Cycling NMR relaxometry was applied to shed light on the origin of the different relaxation enhancement in the investigated systems. Extremely slow exchange with the bulk of ...