Investigation of zone migration in a current rectifying nanofluidic/microfluidic analyte concentrator.

A simple microfluidic device that uses a nanocapillary membrane (NCM) to connect a microfluidic channel and solution reservoir is capable of rectifying ionic current and enrichment of ionic species. Application of a potential induces concentration polarization (CP), which creates ion-depleted and ion-enriched zones on opposite sides of the permselective NCM. A force balanced (FB) enriched zone forms at the interface of the bulk buffer solution and depleted CP zone in the off state or the low-current case. After polarity reversal, the migration of a FB enriched zone of anionic tracer is imaged. By decreasing the solution volume at the microchannel and NCM interface, the response time of the current rectifier is decreased and elution of the zone of anionic tracer is achieved. The decrease in response time is most dramatic for the on to off state transition. For this transition, the response time decreases from approximately 50 to approximately 1 s. The decrease in response time for the off to on state is not as dramatic and is characterized by the time from polarity reversal to current peak, which decreased from 84 to 21 s. The features in the I-t plots can be accounted for with schematics of the zone migration that show the migration of depleted CP and enriched CP zones. Together, the fluorescent images and the I-t plots provide the foundation for schematics that describe the zone elution following polarity reversal. These results provide an improved understanding of the zone migration and current rectification in nanofluidic-microfluidic interfaces with symmetric nanochannels.