Thin-Layer Sample-Ion-Selective Nanomembrane Architecture for Coulometric Cation Sensing

Herein, we investigate a coulometric nanomembrane-based ISE integrated into a microfluidic cell pursuing the absolute analysis of potassium ions (K+), selected as a model example. The sensor consisted of a glass substrate coated with the conducting polymer poly(3-octylthiophene) (POT) and a nanometer-sized, potassium-selective membrane casted on top. In the microfluidic cell, the sample is confined to a thin-layer domain being sandwiched between the working electrode and the counter/reference electrode. Two electrochemical protocols are developed toward exhaustive ion transfer at the membrane-sample interface: (i) an anodic mode, where K+ is first accumulated in the membrane and then released to the sample, and (ii) a cathodic mode, where ion release from the membrane is followed by a complete uptake. Tuned POT oxidation and reduction are respectively utilized as the driving force for the ultimate ion transfer to be analytically exploited, with linear sweep voltammetry (LSV) and chronoamperometry being explored as charge readout strategies. The calibration-free operation is validated for the anodic LSV protocol within a range of 1-30 μM; while other routes exhibited deviations from the 100% coulometric efficiency that may arise from several factors: (i) variations in the sample volume due to the spacer material, (ii) lateral diffusion in the microfluidic channel, and (iii) interferences. A fundamental asymmetry was observed between ion uptake and release at the membrane interface, meaning that calibration-free operation via Faradaic quantification relies heavily on the specific electrochemical protocol applied. The developed sensor was tested for the quantification of K+ in diverse matrices, including urine, blood serum, environmental water, and standard solutions, showing overall good agreement with ion chromatography when appropriate protocols were employed.