In Situ Monitoring of Water–Sodium Chloride Diffusion via a Fiber-Integrated Plasmonic Microsensor

Water-sodium chloride diffusion, accompanied by ion movement, is a representative and important liquid-liquid diffusion system in the fields of physics, chemistry, and biology. Here, we have in situ monitored the water-sodium chloride solution diffusion process by a fiber-integrated plasmonic microsensor with high space-time resolution. The sodium chloride concentration space-time evolution shows a non-Fickian law in which the diffusion coefficient is constant but the diffusion layer center moves to the water side during the diffusion process, which is explained by the equivalent surface tension effect. Moreover, for the system with a higher concentration of sodium chloride solution, the diffusion coefficient is smaller and the amount of movement is greater. The observation of the non-Fickian water-sodium chloride diffusion behavior enriches the liquid-liquid diffusion theory and demonstrates the practicability of the microsensor for studying the ion diffusion processes in liquids.