Halogen-bond mediated charge transfer for visual competitive colorimetric detection of fentanyl

Colorimetric assays are inexpensive and attractive tools for the detection of fentanyl (FTN), yet further enhancing their sensitivity remains a major challenge. Herein, we develop a halogen-bond mediated visual competitive colorimetric assay for FTN using Erythrosine B (EB) as a probe. The addition of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide (F-127) induces the EB aggregation, strongly suppressing the background signal. Upon the introduction of FTN, the subsequent competitive reaction displaces F-127 to generate EB-FTN charge-transfer complexes via N^…I halogen bonds between electron-rich amine groups and electron-deficient iodine sites, accompanying a significant absorbance wavelength shift and pink-to-purple color change. The limits of detection of this approach for FTN are 2 mg·L⁻¹ by the naked eye and 0.19 mg·L⁻¹ by UV–vis spectroscopy, which are approximately 3.7-fold to 4 orders of magnitude more sensitive than the reported colorimetric assays. Meanwhile, the present method is well applied for FTN-spiked domestic sewage samples, and an easy-to-use smartphone-based digital image colorimetry is also fabricated. It is expected that such an assay can play a key role in alleviating the worldwide opioid overdose crisis.