“Seems Bad Turns Good” – traces of precursor in dicationic ionic liquid lead to analytical application

Abstract: We explored a geminal dicationic ionic liquid (DCIL), 1′-(propane-1,3-diyl)bis(4-aminopyridin-1-ium) dibromide, [C₃(Amp)₂][Br]₂, as a fluorescent probe for detection of hydroquinone (HQ). The DCIL, undesirably, was found to incorporate trace precursor (4-aminopyridine). The impurity in DCIL was identified by nuclear magnetic resonance (NMR) spectroscopy and quantified by high-performance liquid chromatography. Surprisingly, trace precursor associated with the sensor actually assisted the quenching process by providing essential in situ mild alkaline environment. The proposed detection method depends on significant quenching of fluorescence of DCIL with increasing concentration of HQ. A plausible quenching mechanism involving photoinduced charge transfer is discussed. The DCIL probe offers a wide detection range of 1–800 µM with detection limit of 0.38 µM for HQ. These results highlight the concept of beneficial intentional doping of impurities in sensing material that does not alter its inherent physicochemical properties but might improve its performance. Graphical abstract: [Figure not available: see fulltext.].