Spontaneous Counterion‐Induced Vesicle Formation: Multivalent Binding to Europium(III) for a Wide‐Range Optical pH Sensor

A counterion-directed molecular design strategy is described for the spontaneous formation of stable vesicles via readily available imidazolium salts with the EDTA counterion in aqueous media. The counteranion is employed to adjust the balance between hydrophobic and hydrophilic parts, which satisfies the requirement of packing parameter for vesicle formation. The counterion-induced vesicles (CIVs) feature spontaneous formation, simple preparation, and easy availability of surfactants. Importantly, the unusual counterion-induced vesicle-like sphere aggregates can further chelate europium(III) ions to enhance the europium-centered emission in aqueous media and make it viable for an optical pH sensor, which exhibits a linear response in a wide range of pH values from 3 to 11. To the best of our knowledge, this constitutes one of the widest ranges of Eu-based pH sensors reported so far. This design concept has offered a new avenue for the preparation of functional vesicles.