Electrochemically Imaging the Response of Ion-Selective Membranes with an Ultralow Detection Limit

The development of ion-selective membranes for the selective response of a particular ion has been studied for many years; however, imaging the response of the membrane with a low detection limit is challenging. Here, high spatial-resolution electrochemical imaging of this response down to picomolar is achieved using scanning ion conductive microscopy. The detection strategy relies on the exclusion of a small amount of counter ions from the membrane in the presence of a low concentration of target ions in the solution. These excluded counter ions are adsorbed at the membrane-solution interface, leading to more positive charges at the surface. The resultant elevation of the ionic current in the approach curve behaves as the response for the target ions down to 10^-11 M, which is much more sensitive than that using potentiometric measurement. The constant-current scanning of the membrane exhibits the fluctuation of the apparent surface height that is correlated with the ionic concentration, permitting the imaging of the response at the nanoscale. The achievement of highly sensitive and spatial-resolution imaging for the ionic response enable the collection of spatial response at the ion-selective membrane, which will greatly advance the study of ion-selective electrodes.