A re-evaluation of the role of coumarin C460 as a fluorescent Pd(II) sensor

In 2018, Ashwin and co-workers presented a method for the selective and sensitive detection of Pd²⁺ based on variations in the fluorescence of coumarin (C460). Herein, we re-evaluate the ability of C460 to probe Pd²⁺ concentration via a systematic reproduction of the experiments reported in the original publication and the performance of additional control experiments. Our analysis indicates that C460 is in fact not a fluorescence sensor for Pd²⁺, but rather can act as a pH sensor over a restricted pH range. Furthermore, we are of the opinion that the experimental procedures described in the original paper were inaccurately or incompletely presented, particularly with regard to sample preparation. To support our conclusions, we provide a detailed assessment of the fluorescence properties of C460 in aqueous media as a function of pH. To summarize, our analysis suggests that reductions in fluorescence are not due to molecular interactions with palladium species, but instead result from the inhibition of intramolecular charge transfer (ICT) caused by the protonation of the amine group at low pH values.