We report a Co2-based magnetic resonance (MR) probe that enables the ratiometric quantitation and imaging of pH through chemical exchange saturation transfer (CEST). This approach is illustrated in a series of air- and water-stable CoII2 complexes featuring CEST-active tetra(carboxamide) and/or
hydroxyl-substituted
bisphosphonate ligands. For the complex bearing both
ligands, variable-pH CEST and NMR analyses reveal highly shifted carboxamide and
hydroxyl peaks with intensities that increase and decrease with increasing pH, respectively. The ratios of CEST peak intensities at 104 and 64 ppm are correlated with
solution pH in the physiological range 6.5-7.6 to construct a linear calibration curve of log(CEST104 ppm/CEST64 ppm) versus pH, which exhibits a remarkably high pH sensitivity of 0.99(7) pH unit-1 at 37 °C. In contrast, the analogous CoII2 complex with a CEST-inactive
bisphosphonate ligand exhibits no such pH response, confirming that the pH sensitivity stems from the integration of
amide and
hydroxyl CEST effects that show base- and
acid-catalyzed
proton exchange, respectively. Importantly, the pH calibration curve is independent of the probe concentration and is identical in aqueous
buffer and
fetal bovine serum. Furthermore, phantom images reveal analogous linear pH behavior. The CoII2 probe is stable toward millimolar concentrations of H2PO4-/HPO42-, CO32-, SO42-, CH3COO-, and Ca2+
ions, and more than 50% of
melanoma cells remain viable in the presence of millimolar concentrations of the complex. The stability of the probe in physiological environments suggests that it may be suitable for in vivo studies. Together, these results highlight the ability of dinuclear transition
metal PARACEST probes to provide a concentration-independent measure of pH, and they provide a potential design strategy toward the development of MR probes for ratiometric pH imaging.