Accurate and Rapid Electrochemical Detection of Fentanyl in Hydrophobic Ionic Liquid at Subzero Temperatures

Fentanyl's extreme potency and widespread presence in illicit drug mixtures demand the development of rapid, selective, and field-deployable sensing technologies. In this study, we present an electrochemical impedance-based sensor for fentanyl detection using a polyanilinearginine (PANI-Arg) modified stainless-steel electrode immersed in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyrr-NTf₂). The sensor exhibits nanomolar sensitivity, fast response times, and strong selectivity against common street drug adulterants and over-the-counter medications. Optimization studies revealed that a DC potential of +0.4 V maximizes phase angle shifts (Δφ) without compromising polymer stability. Kinetic analysis demonstrated pseudo-first-order adsorption, with ~90% of signal attained within 5 minutes. Temperature-dependent trials confirmed that Δφ increases with temperature, consistent with a dual thermokinetic model combining Arrhenius-based ion conduction and Langmuir adsorption isotherm behavior. Importantly, the use of BMPyrr-NTf₂ enables stable sensor performance at subzero temperatures, a condition where aqueous systems fail due to freezing. This unique thermal stability makes the platform especially suitable for uncontrolled environments in forensic or field applications. Together, these findings validate the potential of PANI-Arg sensors in ILs as practical, portable tools for real-time fentanyl detection.