Eight-Coordinate, Stable Fe(II) Complex as a Dual ¹⁹F and CEST Contrast Agent for Ratiometric pH Imaging

Accurate mapping of small changes in pH is essential to the diagnosis of diseases such as cancer. The difficulty in mapping pH accurately in vivo resides in the need for the probe to have a ratiometric response so as to be able to independently determine the concentration of the probe in the body independently from its response to pH. The complex Fe^II-DOTAm-F12 behaves as an MRI contrast agent with dual ¹⁹F and CEST modality. The magnitude of its CEST response is dependent both on the concentration of the complex and on the pH, with a significant increase in saturation transfer between pH 6.9 and 7.4, a pH range that is relevant to cancer diagnosis. The signal-to-noise ratio of the ¹⁹F signal of the probe, on the other hand, depends only on the concentration of the contrast agent and is independent of pH. As a result, the complex can ratiometrically map pH and accurately distinguish between pH 6.9 and 7.4. Moreover, the iron­(II) complex is stable in air at room temperature and adopts a rare 8-coordinate geometry.