Ion sensing is essential for applications ranging from clinical diagnostics and environmental monitoring to the study of interfacial chemical processes. Ion-selective electrodes (ISEs) are particularly attractive sensing platforms due to their tunable membrane composition, operational simplicity, and compatibility with different electrochemical interrogation methods. Recent advances in membrane chemistry, solid-contact transduction, and dynamic electrochemical techniques have broadened the analytical capabilities of these sensors. At the same time, optical approaches and scanning electrochemical probe techniques have enabled spatially resolved measurements of ionic gradients. However, challenges remain in achieving high temporal resolution, simultaneous detection of multiple ions, and integrated bioanalytical functionality. This thesis explores new principles for ion-selective chemical sensing based on electrochemical control of ion transfer and redox-mediated processes at polymeric membranes. First, a new concept for electrochemical enzyme immunoassays is introduced using a biofunctionalized ion-selective membrane interrogated by dynamic electrochemistry. In this system, an enzymatic substrate is released on demand from the membrane by a galvanostatic pulse, enabling localized activation of a choline-oxidase-based sandwich immunoassay for the detection of human lysozyme in saliva. Surface functionalization of the membrane with capture antibodies is achieved through click chemistry and NHS ester coupling, while enzymatic turnover is monitored potentiometrically. Second, a lipophilic TEMPO derivative is investigated as a redox mediator for controlling ion transfer across immiscible phases. The mediator enables electrochemical modulation of both anion and cation transfer in ion-exchanger membranes, producing voltammetric responses consistent with theoretical predictions combining redox potentials and phase-boundary equilibria. Building on this concept, thin-layer membranes containing lipophilic TEMPO and ionophores are developed for multianalyte sensing. Selective redox-driven transfer of Li⁺ and K⁺ generates distinct ion-transfer waves with near-reversible voltammetric behavior and independent Nernstian shifts with ion activity. This system enables simultaneous determination of lithium and potassium in undiluted human serum with high accuracy. Finally, a new imaging principle is introduced for high-speed spatial mapping of optically silent ions using fluorescence microscopy. Electrochemically induced ion transfer in a thin polymeric film modulates fluorescence, allowing each pixel to report the local ion-transfer potential and thus ion concentration. This approach enables micrometer-scale imaging of ionic gradients with high temporal resolution. Together, these results demonstrate new strategies for electrochemically controlled ion sensing, enabling advances in immunoassays, multianalyte detection, and high-resolution ion imaging.