Plasticizer-Free Thin-Film Sodium-Selective Optodes Inkjet-Printed on Transparent Plastic for Sweat Analysis

A novel strategy to functionalize transparent flexible plastic films with an optical ion-sensing layer using an inkjet-printing technology is described. The hydrophobic sensing chemicals that include a sodium ionophore, a lipophilic proton chromoionophore, and a lipophilic ion-exchanger are co-deposited onto substrates such as transparent polyester film sheets in the absence of any plasticizer and/or hydrophobic polymer matrix. The inkjet-printing process enables the formation of optode films with nanoscale thickness/roughness that readily facilitate interfacing with aqueous samples. Using a smartphone detector, the colorimetric response of the optodes is shown to reach 95% of equilibrium values within 100 s in response to different concentrations of sodium ions, which is more rapid than traditional ion-selective optodes based on plasticized PVC films as the sensing layer. The new optodes also exhibit high selectivity to Na⁺ over interfering ions including K⁺, Ca²⁺, and Mg²⁺. Chemical leaching experiments show that the highly hydrophobic optode components remain in place on the plastic substrate surface. Hence, excellent sensor stability and fully reversible optical responses are obtained, which is essential for potential continuous monitoring applications. Further testing of the sensors with undiluted human sweat samples is shown to yield accurate values for sodium concentrations. Therefore, the use of plasticizer-free ion-selective optode nanolayers that enable highly selective ion sensing on a clear plastic support is likely to expand the range of available chemical sensors suited for preparing wearable real-time sweat analysis devices.