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.