HKOCl-4 (BXY2142) is a rhodol-based yellow fluorescent probe known for its exceptional sensitivity and selectivity in detecting hypochlorous acid. With an absorption wavelength longer than fluorescein-based probes, it offers improved pH stability. Notably, HKOCl-4 exhibits excitation at 530 nm and emission at 557 nm.

HKOCl-4 (10 μM) demonstrates a proportional increase in fluorescence intensity with rising concentrations of hypochlorous acid[1]. It exhibits an ultra-fast response to 1 equivalent of HOCl, achieving maximum fluorescence within 2 minutes, and displays a more than 20-fold increase in fluorescence intensity compared to its reaction with other analytes, including 1 equivalent of ?OH and ONOO? as well as 10 equivalents of O2, NO, 1O2, ROO?, TBHP, and H2O2[2]. Initial toxicity tests on HKOCl-4 have confirmed its non-toxicity in living cells, specifically in RAW264.7 macrophages, even at concentrations as high as 50 μM[2].