# Spatially Resolved Ion Sensing by Voltammetric Ion Transfer Microscopy

## TL;DR

A new method for rapidly mapping ion concentrations in solution with high spatial resolution using a fluorescence microscope and ion transfer principles.

## Contribution

Introduces a high-frequency, label-free chemical mapping technique for optically silent ionic species with micrometer resolution.

## Key findings

- The method captures concentration maps of millions of pixels in seconds using a conventional fluorescence microscope.
- The technique is demonstrated with tetraethylammonium to map diffusional mixing of solution streams.
- The approach uses a single sensing film to achieve spatially resolved ion sensing.

## Abstract

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.