Chaotropic Monovalent Anion-Induced Rectification Inversion at Nanopipettes Modified by Polyimidazolium Brushes

A nonintuitive observation of monovalent anion-induced ion current rectification inversion at polyimidazolium brush (PimB)-modified nanopipettes is presented. The rectification inversion degree is strongly dependent on the concentration and species of monovalent anions. For chaotropic anions (for example, ClO4-), the rectification inversion is easily observed at a low concentration (5mm), while there is no rectification inversion observed for kosmotropic anions (Cl-) even at a high concentration (1m). Moreover, at the specific concentration (for example, 10mm), the variation of rectification ratio on the type of anions is ranged by Hofmeister series (Cl- >= NO3- > BF4- > ClO4- > PF6-> Tf2N-). Estimation of the electrokinetic charge density (sigma(ek)) demonstrates that rectification inversion originates from the charge inversion owing to the over-adsorption of chaotropic monovalent anion. To qualitatively understand this phenomenon, a concentration-dependent adsorption mechanism is proposed.