Electrochemical Sensing of Perfluorooctanoic Acid via a Rationally Designed Fluorine-Functionalized Cu-MOF and In-Depth Analysis of Sensing Mechanism

Perfluorooctanoic acid (PFOA), a prominent member of the per- and polyfluoroalkyl substance (PFAS) family, has emerged as a new perpetual pollutant posing significant environmental and health risks, necessitating developing highly selective materials for its sensitive detection in water. In this work, we developed an electroactive fluorine-functionalized Cu-MOF (F–Cu–NH₂BDC) through postmodification of the copper-2-amino-terephthalic acid (Cu–NH₂BDC) MOF with 2,3,5,6-tetrafluoroterephthalaldehyde (TFTA). Experimental and computational results suggested that F–F interactions between the decorated tetrafluorobenzaldehyde groups and PFOA, as well as among the PFOA molecules themselves, would induce self-aggregation of PFOA molecules on the surfaces or in the pores of F–Cu–NH₂BDC. This specific aggregation inhibited contact and electron transfer between F–Cu–NH₂BDC and the electrolyte, resulting in a decrease in the inherent electrochemical Cu²⁺/Cu⁺ redox signal from F–Cu–NH₂BDC. Based on this, an F–Cu–NH₂BDC-based label- and probe-free PFOA electrochemical sensor was exploited with an excellent linear range from 5 pM to 500 μM and an extremely low detection limit of 3.54 pM, surpassing most currently reported electrochemical and nonelectrochemical PFAS sensors. This sensor also exhibited good stability, reproducibility, and anti-interference performance, enabling the accurate measurement of PFOA concentrations in actual commercial drinking water. These findings shed light on the design of PFAS sensors utilizing the F–F interaction as the working mechanism.