Equipment-Free Detection of K ⁺ on Microfluidic Paper-Based Analytical Devices Based on Exhaustive Replacement with Ionic Dye in Ion-selective Capillary Sensors

A distance-based analysis of potassium ion (K ⁺ ) is introduced that is performed on a microfluidic paper-based analytical device (μPAD) coupled to an ion-selective capillary sensor. The concept is based on two sequential steps, the selective replacement of analyte ion with an ionic dye, and the detection of this dye in a distance-based readout on paper. To achieve the first step, the capillary sensor holds a poly(vinyl chloride) (PVC) membrane film layer plasticized by dioctyl sebacate (DOS) that contains the potassium ionophore valinomycin, a lipophilic cation-exchanger and the ionic indicator Thioflavin T (ThT) on its inner wall. Upon introduction of the sample, K ⁺ in the aqueous sample solution is quantitatively extracted into the film membrane and replaced with ThT. To convert the ion exchange signal into a distance-based analysis, this solution was dropped onto the inlet area of a μPAD to flow the ThT along a channel defined by wax printing, resulting in the electrostatic binding of ThT to the cellulose carboxylic groups. The initial amount of K ⁺ determines the amount of ThT in the aqueous solution after ion-exchange, and consequently the distance of ThT-colored area reflects the sample K ⁺ concentration. The ion exchange reaction was operated in a so-called "exhaustive sensing mode" and gave a distinct response in a narrow range of K ⁺ concentration (1-6 mM) that cannot be achieved by the classical optode sensing mode. The absence of hydrogen ions from the equilibrium competition of the capillary sensor contributed to a complete pH-independence, unlike conventional optodes that contain a pH sensitive indicator. A very high selectivity for K ⁺ over Na ⁺ and Ca ²⁺ has been confirmed in separate solutions and mixed solutions tests. K ⁺ measurements in pooled serum samples at concentrations between 2 and 6 mM are successfully demonstrated on a temperature controlled support.