Establishing ¹⁹ F NMR for Understanding Sorbent Interactions with Per- and Polyfluoroalkyl Substances (PFAS): Ranking Affinity and Dissecting Interactions of Nanoscale Carbon Dots and β-Cyclodextrin with PFAS

Developing new technologies for removal of per- and polyfluoroalkyl substances (PFAS) from water is critical for public and environmental safety. In conjunction, suitable analytical techniques are required for understanding interactions between novel sorbent materials and PFAS to facilitate efforts on designing improved materials. In this work, six nanoscale carbon dots (CDs) are synthesized for fundamental PFAS sorption studies, and ¹⁹F NMR is introduced as a primary analytical technique to rank and understand the affinity of the synthesized CDs toward PFAS. The CDs are synthesized from various-amine rich precursors (polyethylenimine or chitosan) to yield cationic CDs and promote electrostatic interactions with anionic PFAS. The affinity of CDs toward perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and a mixture of 24 PFAS was assessed. Three main approaches were used to rank CD affinity for PFAS: K_d determination using 1D NMR and two CPMG-based methods. Based on K_d values, the PEI CDs exhibited higher affinity for PFOA than any of the chitosan-based CDs or β-cyclodextrin, while affinity for PFOS was comparable across all sorbent materials. CMPG-based experiments suggested that PEI CDs have superior affinity for PFOA and operate in a slower exchange regime with PFOA than the chitosan-based CDs. In the PFAS mixture, the PEI-CDs and top-performing chitosan-based CDs interacted strongly with PFAS of varying chain lengths and identity. These results indicate that CDs should continue to be explored for their high PFAS affinity and solidifies the power of ¹⁹F NMR techniques to provide critical molecular-level information about PFAS-sorbent interactions in suspension.