Exploring the adsorption and sensing properties of multiscale COFs toward PFAS: Effects of pore size, chain length,...

Poly- and perfluoroalkyl substances (PFAS) are a class of persistent environmental pollutants that poses significant risks to ecosystems and human health. Covalent organic frameworks (COFs) have emerged as promising candidates for removing and detecting PFAS. However, their adsorption and sensing mechanisms are not well understood at the molecular level. The objective of this study is to reveal how COFs pore size, as well as PFAS chain length and functional groups affect adsorption characteristics and sensing properties from a theoretical perspective. To this end, four representative PFAS (PFOA, PFOS, PFBA and PFBS) adsorbed on COFs with distinct pore sizes are carefully studied and discussed. For PFAS with the same functional group, long-chain PFOS and PFOA exhibit more negative adsorption energies than their short-chain PFBS and PFBA. For compounds with the same chain length, those containing the -SO₃H group (PFOS and PFBS) show more negative adsorption energies compared to those with the -COOH group (PFOA and PFBA). Wavefunction analysis attributed this trend to the elevated electrostatic potential (ESP) values of -SO₃H group and more adsorption sites of long-chain compounds. Quantum theory of atoms in molecules (QTAIM) analysis confirms that the adsorption process is primarily driven by the partial covalent interactions. Natural bond orbital (NBO) shows that PFOS and PFBS gain more electrons than PFOA and PFBA. Furthermore, larger pore sizes in COFs are more beneficial for improving sensitivity towards PFAS, owing to the considerable LUMO shift during the adsorption process. These results deepen the understanding of the structure-property relationship in COF/PFAS systems, providing important guidance and insights for optimizing COFs as sensing materials and sorbents for specific PFAS.