Investigation of eosin Y as a colorimetric sensor for the detection of fentanyl hydrochloride and fentanyl freebase.

The increasing prevalence of fentanyl within the illegal global drug market underscores the need for rapid, low-cost, and straightforward detection methods. Eosin Y demonstrates a measurable color change in the presence of fentanyl, making it a promising candidate for colorimetric sensing and preliminary identification of fentanyl hydrochloride in seized drug samples. This study investigated the interactions of fentanyl freebase and fentanyl hydrochloride with eosin Y in both solution and solid states. Quinine sulfate was included in the solid-state analysis due to its visually similar colorimetric response. CIE-Lab colorimetric evaluation revealed that the peach-to-pink color shift produced by solid fentanyl hydrochloride closely matched that of quinine sulfate, indicating that quinine sulfate may serve as a safe and practical simulant for fentanyl hydrochloride in eosin Y-based assays. In contrast, fentanyl freebase produced no observable color change. Solution-state studies, assessed visually and through UV-Vis and fluorescence spectroscopy, showed that fentanyl hydrochloride generated the strongest absorbance increase at 560 nm, while fluorescence emission at 542 nm increased for both the freebase and hydrochloride forms. These findings demonstrate that the protonation state of fentanyl is critical for eliciting eosin Y's colorimetric response, while the presence of hydrochloric acid alone does not account for the observed changes. Overall, the results supported the potential of eosin Y as a practical colorimetric indicator for protonated fentanyl species and highlighted quinine sulfate as a viable simulant for safer laboratory testing.