A Responsive Europium(III) Chelate That Provides a Direct Readout of pH by MRI

A europium(III) DO3A-tris(amide) complex is reported for imaging pH by MRI using ratiometric chemical exchange saturation transfer (CEST) principles. Deprotonation of a single phenolic proton between pH 6 and 7.6 results in an similar to 5 ppm shift in the water exchange CEST peak that is easily detected by MRI. Collection of two CEST images at two slightly different activation frequencies provides a direct readout of solution pH without the need of a concentration marker.