MRI Sensing Based on Displacement of Paramagnetic Ions from Chelated Complexes

A variety of mechanisms have been proposed for detecting inorganic ions using magnetic resonance imaging (MRI) contrast agents. In the most widely explored approach, analyte binding to a paramagnetic metal chelate withdraws ligand(s), increasing the number (q) of inner sphere sites available for water coordination at the paramagnetic center. As a consequence, the longitudinal relaxivity (r1) of protons on exchangeable solvent molecules in the complex is similarly increased.[1] Here we introduce a qualitatively new strategy, in which the analyte completely releases the paramagnetic ion from a polydentate chelating ligand to form the aqua (aq) complex, which serves as an efficient MRI contrast agent with sharply different characteristics from the chelate. In this facile and flexible approach, changes in r1 can be tuned by the choice of the metal chelate complex (Fig. 1a). Measurement of the effect by MRI can then be used to determine the analyte concentration with spatiotemporal resolution.