A molecular dynamics study on ionic current rectification of ultra-narrow conical nanopore

Understanding the ionic current rectification (ICR) is crucial for elucidating the physical mechanisms of ions transport in various processes and for advancing the development of nanodevices. Using molecular dynamics simulations, we explore the properties of ionic motion in negatively charged conical nanopores with a tip radius fixed at 1.51 nm. The effects of ion transport behavior on ICR under different electric fields, angles, and cation species were studied. The rectification ratio increases linearly with an increase in angle (0-15 degrees), slows down and then eventually stabilizes with an increase in electric fields, and depends on the cation species being used. Our results indicate that the ion current is mainly contributed by the flow of cations in ultra-narrow nanopores. Further analysis of the ion concentration distribution reveals that the inverse ICR phenomenon is mainly caused by the cation concentration polarization at the tip under negative electric field, making cations are difficult to enter the nanopore from the tip, resulting in a decrease in ionic current. Additionally, cations entering the nanopore from the tip become trapped in the potential well of the tip at a negative electric field, leading to lower ion mobility and ionic current. Finally, it is found the difference in ICR ratio mainly results from the migration rate of cations with different nanopore angles and different ionic types. Our study provides valuable insights into the behavior of ions in ultra-narrow conical nanopores and the mechanisms behind ICR, which could guide the design and development of nanodevices that rely on ionic transport, such as biosensors and energy storage systems.