Colorimetric detection of fentanyl using a supramolecular displacement assay

Fentanyl is a potent synthetic opioid with an alarmingly low lethal dosage of 2 mg. The equipment necessary to detect fentanyl in field settings (e.g., handheld spectrometers) is restricted to highly trained, well-funded, and specialized personnel. Established point-of-need technologies, such as lateral flow immunochromatographic strips, are available; however, they often involve multiple contact-based steps (e.g., collection, mixing) that pose a higher risk to users handling unknown substances. Herein, we developed a colorimetric displacement assay capable of contactless detection of fentanyl as liquid or solid samples. The basis of our assay relies on the presence of fentanyl to displace a redox mediator, ferrocene carboxylic acid, inclusively bound in the cavity of a supramolecular host, CB[7]. The displacement is only possible in the presence of high affinity binding guests, like fentanyl (KA ~ 106 M-1). The liberated redox guest can then react with indicator reagents that are free in solution, producing either: (i) a distinct blue color to indicate the presence of fentanyl or (ii) remain a pale blue tint in the absence of fentanyl. We demonstrate rapid and specific detection of fentanyl free base and fentanyl derivatives (e.g., acetyl fentanyl, furanyl fentanyl) against a panel of 9 other common drugs of abuse (e.g., morphine, cocaine, heroin). Furthermore, we highlight the intended use of this assay by testing grains of fentanyl derivatives on a surface with a drop (i.e., 25 µL) of assay reagent. We anticipate this approach can be applied broadly to identify the presence of fentanyl at the point of need.