# Tunable Ion-Sensing Using Coulometric-Based Protocols with Permselective Nanomembranes

## TL;DR

This paper presents a tunable ion-sensing method using nanomembranes and electrochemical protocols to detect potassium ions with high accuracy and adaptability for clinical and environmental applications.

## Contribution

A novel coulometric-based ion-sensing approach using permselective nanomembranes with tunable response ranges and adaptability for different ions.

## Key findings

- The sensor demonstrated two distinct potassium ion detection ranges: 3–20 μM in cathodic mode and 200–1000 nM in anodic mode.
- The sensor showed excellent repeatability and reversibility in the cathodic coulometry protocol.
- The method was successfully applied to real samples like human urine and horse serum with linear and tunable responses.

## Abstract

Herein, we investigate all-solid-state ion-selective
electrodes
(ISEs) based on permselective nanomembranes (thickness ∼230
nm) in a coulometric mode. The detection of the potassium ion (K+) has been selected as proof of concept, implementing two
electrochemical protocols based on the anodic and cathodic readouts
of the same ISE. The electrode consists of an ITO glass substrate
with the conducting polymer poly­(3-octylthiophene) (POT) electrodeposited
on it and a potassium-selective nanomembrane spin-coated over the
POT layer. The K+ transfer at the membrane-sample interface
is mediated by the redox activity of POT, which is in excess with
respect to the dopant in the membrane (i.e., the anion part of the
cation exchanger, R–). In the cathodic protocol,
the entry of the K+ into the membrane is promoted by the
POT+ reduction to POT0; while in the anodic
interrogation, first, K+ enters the membrane with a previous
accumulation step, and then it is expelled during the oxidation of
the POT0 to POT+. Both protocols were studied
under linear sweep voltammetry and chronoamperometry, followed by
signal integration to obtain the charge corresponding to K+. It is demonstrated that this charge is directly proportional to
the K+ concentration in the bulk solution. We found two
distinct response ranges: 3–20 μM in the cathodic protocol
and 200–1000 nM in the anodic one. In addition, the cathodic
coulometry strategy revealed excellent repeatability and reversibility
within the linear range of response. The developed analytical approach
demonstrates suitability in the quantification of real samples, i.e.,
human urine, horse serum, canal water, and standard KCl solution,
while providing a linear and tunable coulometric response over a broad
concentration range from the nanomolar to the micromolar level. Moreover,
the sensor can be readily integrated into microfluidic devices, additionally
offering the advantage of small sample volume requirements. The demonstrated
reversibility, along with the ability to customize the ionophore in
the membrane for an analysis of different ions, renders the proposed
concept adaptable and exceptionally suitable for clinical analysis
and environmental monitoring.