Magnetic resonance imaging (19F MRI) is an accurate, noninvasive imaging modality used for human diagnostic testing. 1H MRI is traditionally used in a clinical setting, providing fundamental biological images with high penetration depth. 19F MRI maintains the advantages of 1H MRI with decreased background signal. Current 19F MRI probes contain C-F bonds (19F signal roughly -70 ppm) with indistinguishable chemical shifts in an MR setting, preventing the use of multiple imaging probes simultaneously. By introducing heteroatom fluorine sources, specifically S-F bonds (19F signal between +60 and +100 ppm), distinguishable chemical shifts would be present compared to traditional C-F bonds, potentially allowing for the simultaneous use of this pH-sensing compound with others that sense alternative analytes containing C-F bonds instead. CoNO2ASF5 as a 19F MRI probe functions via coordination change when introduced to different pH environments. The cobalt metal center has been chosen for the probes due to its favorable PCS properties and the core of the complex consists of five donors coming from a macrocyclic TACN (1,4,7-triazacyclononane) and two carboxylate donors that help stabilize the Co(II) center. The sixth ligand donor comes in the form of an anilide with our SF5 tag. Crystal structure characterization shows the anilide has varied coordination depending on pH due to the ability of the anilide NH to deprotonate in basic environments while the protonated complex has the oxygen coordinating with the Co(II). Studies show that the complex gives a 19F MR signal at +66 ppm in acidic conditions and +99 in basic conditions, affording a chemical shift difference greater than 30 ppm. Ongoing work with multicolor 19F MRI probes includes increasing the fluorine density per probe while maintaining aqueous solubility and synthesizing additional heteroatom probes to enhance simultaneous imaging.

Synthesis and characterization of cobalt-based multicolor 19F MRI pH sensing probes

ACS Fall 2021

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