HBT-Fl-BnB is a fluorescent probe for the ratiometric detection of ONOO^– in vitro and in vivo. HBT-Fl-BnB consists of an HBT core with Fl groups at the ortho and para positions responding to the zwitterionic excited-state intramolecular proton-transfer (zwitterionic ESIPT) process and a boronic acid pinacol ester with dual roles that block the zwitterionic ESIPT and recognize ONOO^–[1].