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 (O 2 ) levels. Using the deuterated analogue, dMTP 5, which has an optimal p K a as well as high sensitivity to bioreduction and O 2 , we reasonably evaluated pH effects on efflux of reducing agents from HepG2 cells and cellular O 2 consumption.