In this study, a highly fluorescent and receptor-free phenothiazine derivative (PDAB) was developed for detecting picric acid. Steady-state and time-resolved fluorescence studies were conducted to examine the excited-state behavior of PDAB with picric acid in solution. The PDAB probe showed remarkable selectivity and sensitivity toward picric acid with an extremely low detection limit of 9.82 nM. Time-resolved fluorescence quenching provided direct evidence of an inner filter effect-based sensing mechanism. Using the Parker equation, a comprehensive analysis was performed to correct the inner filter effect in picric acid sensing. Overall, these studies offer crucial insights into the sensing mechanism for picric acid detection. © 2023 Elsevier B.V., All rights reserved.