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], where i = 1, 2, or 3 and Ln = Gd or 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 dimethylformamide 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 hydration numbers 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 nuclear magnetic resonance 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 water molecules coordinated to Gd(III) and the scarce accessibility of Gd(III) sites to water were highlighted as the main causes of limited relaxivity.