HOCl-Mediated Bromothymol Blue Bleaching and Carbon Dot Fluorescence Recovery for Fecal Myeloperoxidase Quantification in Ulcerative Colitis.

Myeloperoxidase (MPO) is a clinically significant enzyme biomarker of neutrophil activation and oxidative inflammation, whose elevated levels in fecal specimens serve as a reliable non-invasive indicator of mucosal damage and disease activity in inflammatory bowel disease, particularly ulcerative colitis, making its accurate and sensitive quantification in stool samples of considerable diagnostic importance. Herein, a fluorometric sensing platform is reported for MPO determination in fecal samples, based on MPO-catalyzed oxidation of chloride ions by hydrogen peroxide to generate hypochlorous acid (HOCl), which selectively bleaches bromothymol blue (BTB) and relieves its quenching effect on carbon dots (CDs), generating a concentration-dependent fluorescence recovery signal proportional to MPO activity. In the absence of MPO, BTB maintains the system in an off-fluorescence state, while increasing MPO activity progressively restores CDs emission, enabling sensitive quantitative detection. The fluorometric mode exhibited linearity over 0-35.0 ng/mL with a limit of detection of 1.12 ng/mL. Applied to spiked stool samples, the platform yielded mean recoveries of 98.3%-99.5%. Application to real stool samples from ulcerative colitis patients revealed markedly elevated MPO levels compared to healthy controls, consistent with active mucosal inflammation, confirming the clinical applicability of the proposed platform as a practical and sensitive tool for non-invasive IBD activity monitoring.