The use of microelectrodes to study ion recognition by a squaramide-based ion pair receptor consisting of a ferrocene reporter

Here, we report a novel electrochemical ion-pair sensor R1, consisting of crown ether and ferrocene units linked to the squaramide group. The presence of the electron-withdrawing phenyl substituent, through which 1-aza-18-crown ether is attached in the R1 structure, allows for stronger interaction with anions and ion pairs. For comparison, an anion sensor R2 lacking a crown ether unit was also obtained and tested. UV–vis and 1H NMR binding studies revealed that all the anions tested were bound more strongly to R1 in the presence of sodium cations, while the homotopic anion sensor R2 did not have this property. Next, the ability of receptor R1 to act as an electrochemical ion-pair sensor was investigated. The most pronounced potential shifts induced by the addition of ions were obtained using a microelectrode in a solution containing no supporting electrolyte. Obtained results are in good agreement with the data collected from UV–vis and NMR spectroscopies. In addition, the data obtained from electrochemical experiments were used to predict conformation changes of the receptor caused by the binding of a cation and/or anion.