Tailoring Na⁺ and Cl^--Selective Colorimetric Optode Arrays for Wearable Sweat Analysis: Composition Optimization and...

Sweat testing is central to cystic fibrosis diagnosis, but conventional analysis depends on clinical instrumentation, creating a need for portable point-of-care alternatives. As Part 1 of this two-part study, we systematically optimized Na⁺- and Cl^--selective colorimetric optodes and their measurement protocols for potential integration into a wearable device for in situ sweat analysis. Fifteen chromoionophore-based sensor compositions were screened over the physiologically relevant range of 5-100 mmol/L. Candidate optodes were selected based on stability in NaCl solutions and artificial sweat, hysteresis below 0.1 log units, and equilibration times under 15 min. Their analytical performance was evaluated by spectrophotometry and digital color analysis using smartphones and research-grade cameras, with a robustness parameter used to quantify signal reliability under different imaging conditions. A simple smartphone setup provided more robust performance than the tested laboratory imaging configurations. Incorporating light-scattering TiO₂ particles into the PVC matrix produced opaque films that significantly reduced interference from colored samples without compromising sensitivity or response kinetics. Validation in artificial sweat yielded recoveries above 93% across pH 5.5-8.0. These results establish optimized sensor compositions and measurement conditions for colorimetric Na⁺ and Cl^- determination in sweat and provide the analytical basis for wearable-device development in Part 2.