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A novel fluorescent probe 1 based on acridine orange was developed for the selective detection and bioimaging of biothiols. The probe exhibits higher selectivity and turn-on fluorescence response to cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) than to other amino acids. Importantly, the probe responds to GSH and Cys/Hcy with distinct fluorescence emissions in PBS buffer at pH of 7.4. The Cys/Hcy-triggered tandem S N Ar-rearrangement reaction and GSH-induced S N Ar reaction with the probe led to the corresponding amino-acridinium and thio-acridinium dyes, respectively, which can discriminate GSH from Cys/Hcy through different emission channels. Interestingly, Cys finishes the tandem reaction with the probe and subsequently forms amino-acridinium and Hcy/GSH induces S N Ar reaction with the probe to form thio-acridiniums at weakly acidic conditions (pH 6.0), enabling Cys to be discriminated from Hcy/GSH at different emissions. Finally, we demonstrated that probe 1 can selectively probe GSH over Cys and Hcy or Cys over GSH and Hcy in HeLa cells through multicolor imaging. [ABSTRACT FROM AUTHOR]