Readily Visualized Perfluorooctanoic Acid Detection Using a Small Molecule Chemosensor

Mounting concerns regarding per‐/poly‐fluoroalkyl substances (PFAS) on human health are focusing attention on trace level PFAS detection in aqueous environments. Here we report a readily prepared small molecule, 2,6‐bis(3,5‐diethyl‐1H‐pyrrol‐2‐yl)pyridine (receptor 1), that displays high binding affinities (logKa = 4.9‐6.2) and produces a strong “turn‐on” emission response when exposed to representative PFAS in hexanes. The hydrophobic nature of 1, and its strong affinity for various PFAS, allowed hexanes solutions of 1 to be used as "turn‐on" emission sensors for dilute aqueous solutions of long‐chain (≥C8) PFAS under acidic conditions (pH = 2) by liquid‐phase extraction (LPE). In the case of perfluorooctanoic acid (PFOA), the response was rapid (under 10 min) and sensitive. Limits of detection (LOD) as low as 250 ppt were readily achievable by direct naked‐eye observation. LOD as low as 40 ppt and 100 ppt, respectively, could be reached for deionized and tap water solutions of PFOA using a smartphone color‐scanning application. Little change in the sensitivity was seen in the presence of a range of inorganic and organic species that could act as potential interferants. Support for the present findings came from UV‐vis absorbance, fluorescence, 1H/19F NMR spectroscopic analyses, DFT calculations, and single‐crystal X‐ray diffraction analyses.