Prototropic Exchange Governs T1 and T2 Relaxivities of a Potential MRI Contrast Agent Nanozeolite Gd−LTL with a High pH Responsiveness

Abstract

Aqueous suspensions of lanthanide exchanged zeolite LTL have been investigated by 1H, 17O NMR, and EPR relaxivity studies. Both the longitudinal and the transverse relaxivity of these Gd3+ loaded materials are strongly pH dependent and therefore, they have great potential as pH responsive contrast agents. For example, LTL-nanocrystals loaded with 3.5 wt % Gd show a dramatic decrease in the longitudinal relaxivity from 32 to 7 s–1 mM–1 (7.5 T and 25 °C) when going from pH 4 to 9. 1H and 17O NMR show that this phenomenon can be rationalized by a decrease in proton mobility between the zeolite interior and the exterior due to a change from a fast prototropic exchange to a 3 orders of magnitude slower water exchange mechanism. The same material also has a high transverse relaxivity (98 s–1 mM–1 at 7.5 T, 25 °C, and pH 5 as measured with the CPMG pulse sequence), which is governed by proton exchange too, while water diffusion plays a minor role. The high relaxivities and pH dependence render Gd-loaded LTL mate...