Chemical ion imaging with voltammetric ion transfer microscopy

Mapping ionic species in solution and near surfaces is important for understanding chemical gradients and spatially resolved dynamic processes. Available label-free approaches are either slow or restricted to a few parameters such as pH. We present a novel chemical mapping principle to image optically silent ionic species, 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 solution, electrochemically coupled to electron transfer at the back side of the film. Different solution concentrations shift the ion transfer potential, which is visualized by unquenching a fluorophore when the redox probe in the film is oxidized. The maximum fluorescence change of each pixel is captured by a rapid image burst, mapping excitation peak potentials across the image. We demonstrate this imaging principle with a microfluidic flow junction, resolving the diffusional mixing of tetrae-thylammonium ions, achieving micrometer spatial resolution.