Polyanion and anionic surface monitoring in aqueous medium enabled by an ionic host-guest complex

Abstract

Detection of polyanions and anionic surfaces/interfaces in aqueous medium remains a challenge for years. Herein, we describe a highly selective sensing strategy for the critical issue via utilization of a new host-guest complex formed from a recently developed fluorescent cationic Cage and the tetrasodium salt of 3, 4, 9, 10-perylene tetracarboxylic acid (PTCA-Na). Binding behavior studies revealed that the cage is a very efficient host for PTCA-Na to form a 1:1 host-guest inclusion complex (PTCA-Na⊂Cage). Surprisingly, the fluorescence of both the host and the guest quenches with the formation of the complex as a result of charge transfer from the guest to the cage. With further increasing the ratio of the guest to the host, the PTCA-Na⊂Cage can bind another PTCA-Na on the outer positive-charged surface of Cage via electrostatic interaction to give a charge-balanced 1:2 host-guest ternary complex. Interestingly, the PTCA-Na⊂Cage is highly sensitive to the presence of polyanions, such as inorganic polyanion salts, anionic surfactants-based micelles, and even bacteria, as a result of competitive binding to the cationic cage or PTCA-Na. We believe our finding not only realizes a universal method for polyanions and anionic surfaces/interfaces monitoring, but also provides an example for exploring the sensing applications of supramolecular systems in different ways.