Ionic transport through carbon nanotube porins

Abstract

Author(s): Yao, Yun-Chiao | Advisor(s): Noy, Aleksandr | Abstract: Nanofluidic channels confine water and ions down to length scales that are comparable to the sizes of individual molecules. This strong confinement creates unique transport phenomena, such as enhanced water flow, unusual selectivity patterns, and strong electroosmotic coupling. To probe transport behavior and the underlying physics under extreme confinement, our group developed a model system - carbon nanotube porins (CNTPs), which are synthetic analogs of aquaporins made of carbon nanotubes. CNTPs have well-defined, nanometer-sized diameters and ultrashort lengths of ca. 10 nm. In addition, CNTPs have the ability to self-insert into lipid bilayers, forming artificial membrane channels. Using ionic transport measurement setups adapted from protein channels, I was able to obtain ionic conductance scaling and selectivity values through CNTPs with average diameters of 0.8 nm and 1.5 nm. While the 0.8 nm-diameter CNTPs showed exclusive-cation selectivity, the 1.5 nm-diameter CNTPs demonstrated strong ion-water coupling during transport. Furthermore, I demonstrated that built-in charges (end-groups), environmental changes (such as altering pH), or external forces (such as applying a gate voltage) could alter the ionic distribution and selectivity of the CNTP channels. Our CNTPs represent a versatile nanofluidic model system that we can utilize to advance our understanding and control of ion and water behavior at the nanoscale, which could further benefit desalination membrane design and lab-on-a-chip sensing.