A new turn-off fluorescent substrate for horseradish peroxidase is introduced, termed clics, which enhances the sensitivity of commercial enzyme-linked immunosorbant assays (ELISA) by 32-fold and detects chlamydial infections in patients. Clics are cost-effective, stable, and can be easily incorporated in commercial ELISA kits, and therefore should have a great impact in the fields of medicine and biology.