Tetrathiatriarylmethyl radical with a single aromatic hydrogen as a highly sensitive and specific superoxide probe

► The radical probe CT02-H specifically reacts with O2
- − forming a quinone methide. ► O2
- − rapidly reacts with CT02-H with decreased EPR signal, enabling O2
- − detection. ► HPLC with EC detection can detect <0.2 pmol of the quinone methide product. ► CT02-H can measure O2
- − generation as low as 0.34 nM/min in an enzyme or cell system. ► With EPR spectroscopy, CT02-H can simultaneously measure O2
- − and O2.