Spatially Resolved Ion Sensing by Voltammetric Ion Transfer Microscopy.

The visualization and mapping of ionic species in solution and near surfaces are important to understand chemical gradients and spatially resolved dynamic processes in various fields. Available label-free approaches are either slow or restricted to a few parameters, such as pH. We introduce here a novel chemical mapping principle for the spatially resolved sensing of optically silent ionic species at high frequency, acquiring a concentration map of millions of pixels in seconds using a conventional fluorescence microscope. The principle relies on ion transfer from a thin polymeric film into a solution phase, electrochemically coupled to electron transfer at the back side of the film. Different solution concentrations change the potential at which ion transfer is observed, which is visualized by unquenching a fluorophore when the redox probe in the film is electrochemically oxidized. The moment of maximum fluorescence change for each pixel is captured by a rapid image burst to simultaneously find the excitation peak potentials for all pixels. This produces a concentration map, turning a single sensing film into a chemical imaging platform that provides millions of concentration points. The imaging principle is demonstrated with a flowing junction to map diffusional mixing of two solution streams with different ion concentrations, using tetraethylammonium as an initial model ion, to achieve micrometer spatial resolution.