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Direct Visualization of Perm-Selective Ion Transportation

Perm-selective ion transportation in a nanoscale structure has been extensively studied with aids of nanofabrication technology for a decade. While theoretical and experimental advances pushed the phenomenon to seminal innovative applications, its basic observation has relied only on an indirect analysis such as current-voltage relation or fluorescent imaging adjacent to the nanostructures. Here we experimentally, for the first time, demonstrated a direct visualization of perm-selective ion transportation through the nanostructures using an ionic plasma generation. A micro/nanofluidic device was employed for a micro bubble formation, plasma negation and penetration of the plasma through the nanojunction. The direct observation provided a keen evidence of perm-selectivity, i.e. allowing cationic species and rejecting anionic species. Furthermore, we can capture the plasma of Li+, which has lower mobility than Na+ in aqueous state, passed the nanojunction faster than Na+ due to the absence of hydrated shells around Li+. This simple, but essential visualization technique would be effective means not only for advancing the fundamental nanoscale electrokinetic study but also for providing the insight of new innovative engineering applications.