Selective Detection of K⁺ by Ion-Selective Optode Nanoparticles on Cellulosic Filter Paper Substrates

This work describes the generation of plasticized poly(vinyl chloride)-based ion-selective optode (ISO) nanoparticles by a piezoelectric inkjet dispensing device. The monodisperse inkjet-generated particles (polydispersity index: 0.177) with an average hydrodynamic diameter of 202 nm are suitable for the highly reproducible deposition onto cellulosic filter paper substrates, resulting in paper-based analytical devices (PADs) for colorimetric cation detection, as demonstrated on the example of potassium-responsive ISOs. In contrast to the deposition of ISO bulk membranes from organic solutions requiring specialized printing equipment, the ISO nanoparticles can be inkjet-deposited from aqueous dispersions by conventional office inkjet printing. The obtained ISO PADs showed highly reproducible (relative standard deviations of the hue mean values in the range of 0.06-0.83% for three independently fabricated batches) and immediate (t_95% ≤ 1 min) response to aqueous K⁺ solutions buffered at pH 7.4, with high selectivity over Li⁺, Na⁺, Ca²⁺, and Mg²⁺ (log KK⁺,M^z+opt ≤ -5.2 (Li⁺); -3.6 (Na⁺); < -5.0 (Ca²⁺); < -5.2 (Mg²⁺)). In addition, causes for the distinctively different response observed for ISO nanoparticles in the liquid phase and on paper substrates were considered. For this purpose, several parameters related to the cation exchange reaction mechanism of ISOs were experimentally evaluated, allowing to semiquantitatively discuss the paper substrate-specific response of ISOs.