A new “dual-mode” chromogenic and fluorescent turn-on probe (2) for the selective sensing of biological thiols is reported. In MeOH–H₂O cosolvent at physiological pH 7.40 (MeOH–H₂O = 3:7), biological thiols cleave the 2,4-dinitrobenzenesulfonyl group to release the chromo- and fluorophore merocyanine (3).