Selective Binding and Light-Driven Release of Fluorous PF6– and Radioactive 99TcO4– Anions for All-to-Nothing Liquid–Liquid Extraction

Abstract

The purity of fresh water supplies fit for human, agriculture, and industrial sectors is evolving into a leading global priority. Separation of ionic analytes from water using liquid-liquid extraction is one strategy for high selectivity and large throughput volumes. However, it remains a challenge to selectively extract target ions, like the radioactive pertechnetate (99TcO4–) anion, at low concentrations from complex mixtures that can be present in accidental environmental release. Furthermore, studies on the triggered release of the extracted anion to help minimize generation of secondary waste are still in their infancy. Herein, we describe a strategy for selective extraction of 99TcO4– anions from aqueous and salt solutions using cyanostar macrocycles that form strong 2:1 anion complexes. We used PF6– and ReO4– anions as surrogates together with NMR and UV-vis spectroscopy to establish methods of extraction. These methods were translated to the selective extraction of radioactive 99TcO4– with an efficiency of ~85% at 400 ppb (2.5 µM). Use of clamshell receptors composed of two linked cyanostars lowers the extraction limit of 99TcO4– ions to 4 ppb (25 nM) with ~80 % efficiency. We also harnessed cyanostar’s ability to photoisomerize under UV light leading to a single-shot release of the bound anions into an aqueous receiving phase. These findings highlight the potential of receptor-mediated selective extraction of environmentally hazardous anions and their corresponding release that avoids production of secondary ionic waste.