Ratiometric pH Imaging with a Co^II₂ MRI Probe via CEST Effects of Opposing pH Dependences

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 Co^II₂ 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(CEST_104 ppm/CEST_64 ppm) versus pH, which exhibits a remarkably high pH sensitivity of 0.99(7) pH unit^-1 at 37 °C. In contrast, the analogous Co^II₂ 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 Co^II₂ probe is stable toward millimolar concentrations of H₂PO₄^-/HPO₄^2-, CO₃^2-, SO₄^2-, 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.