Effects of Ion Concentration on Ion Current Rectification Using a Glass Nanocapillary

When a glass capillary with a nanopore at its tip is filled with a solution of low ion concentration, the ion concentration at the tip is higher than expected. This is because the ions form an electrical double layer inside the wall. In this case, the ion currents conducted via the nanopore differ dramatically depending on the polarity of the applied potential. This phenomenon is known as ion current rectification (ICR). ICR shows great potential for applications in bioassay methodology, as it is heavily influenced by the amount of surface charges on the inside wall of the nanopore, allowing it to be seamlessly applied in biosensing procedures. However, ICR can only be induced under low ion concentrations, preventing it from being applied for biosensing under physiological conditions. To address this issue, the effects of the ion concentration of phosphate buffer solutions (PBS) in a glass nanocapillary and bulk outside of the capillary on ICR were investigated in this study. Three ion concentrations were introduced into the capillary and the bulk, and ion current via the nanopore was measured. The results show that ICR was successfully monitored even though PBS of a physiological condition was used in either the glass nanocapillary or bulk. In addition, cell spheroids were monitored inside the capillary as a preliminary study for biosensing. This finding can be utilized for ICR-based biosensing applications under a physiological condition.