Trityl Quinodimethane Derivatives as Unimolecular Triple-Function Extracellular EPR Probes for Redox, pH, and Oxygen.

Electron paramagnetic resonance (EPR) spectroscopy and imaging coupled with the use of suitable probes is a promising tool for assessment of the tumor microenvironment (TME). Measurement of multiple TME parameters by EPR is very desirable but challenging. Herein, we designed and synthesized a class of negative-charged trityl quinodimethane MTPs as unimolecular triple-function extracellular probes for redox, pH, and oxygen (O2) levels. Using the deuterated analogue, dMTP5, which has an optimal pKa as well as high sensitivity to bioreduction and O2, we reasonably evaluated pH effects on efflux of reducing agents from HepG2 cells and cellular O2 consumption.