Ionophore-Based Voltammetric Ion Transfer Microscopy: Imaging Enzyme-Generated Ammonium in Flow

Spatially resolved measurements of ion concentration gradients provide valuable insight into chemical, biological, and environmental processes. Recently, Voltammetric Ion Transfer Microscopy (VITM) was introduced as a novel opto-electrochemical imaging technique capable of rapidly mapping ionic species across a large (>mm²) surface at high spatial resolution. Its working principle is based on electrochemically driven ion transfer across a thin membrane coupled to the modulation of a fluorescent signal. Local ion concentration changes at the membrane are visualized by a shift in the potential (and therefore acquisition time) at which the fluorescence change occurs. Although this technique allows for the acquisition of millions of chemical data points within seconds, its analytical applicability has so far been limited to lipophilic species in the Hofmeister selectivity sequence. Here, we report the first ionophore-based implementation of VITM, demonstrating a general strategy to overcome this limitation by the incorporation of an ionophore into the sensing membrane. This introduces selective chemical recognition while preserving the fundamental sensing mechanism of VITM. Ammonium was selected as a model ion by using a recently introduced tripodal ionophore containing triazole groups as hydrogen bond-forming recognition elements. Simultaneous electrochemical and optical selectivity measurements demonstrate that the optical response faithfully reproduces the electrochemical selectivity, giving a two-orders-of-magnitude preference for ammonium over potassium ions. As a proof of concept, the methodology was applied to visualize ammonium generated in situ by hydrolysis of urea catalyzed by jack bean urease at a microfluidic flow junction. The resulting concentration maps reveal the formation of an ammonium concentration gradient that shows excellent agreement with simulations realized for the same experimental conditions. The proposed strategy should be adaptable to other target ions by the incorporation of appropriate selective ionophores, thereby extending the analytical applicability of VITM.