Strong Differential Monovalent Anion Selectivity in Narrow Diameter Carbon Nanotube Porins

Abstract

Inner pores of carbon nanotubes combine extremely fast water transport and ion selectivity that could potentially be useful for high-performance water desalination and separation applications. We used dye-quenching halide assays and stopped-flow spectrometry to determine intrinsic permeability of three small monovalent halide anions (chloride, bromide, iodide) and one pseudohalide anion (thiocyanate) through narrow 0.8 nm diameter carbon nanotube porins (CNTPs). These measurements revealed unexpectedly strong differential ion selectivity with permeabilities of different ions varying by up to 2 orders of magnitude. Removal of the negative charge from the nanotube entrance increased anion permeability by only a relatively small factor, indicating that electrostatic repulsion was not a major determinant of CNTP selectivity. First principle molecular dynamics simulations revealed that the origin of this strong differential ion selectivity is partial dehydration of anions upon entry into the narrow CNTP channels.

Membrane Materials for Selective Ion Separations at the Water–Energy Nexus

“…[ 136 ] Although not as permeable to anions, CNT porins provide selectivity between monovalent anions on the basis of their hydration energy, following the Hofmeister series (SCN − > I − > Br − > Cl − ), with P SCN / P Cl of 412 at neutral pH (Figure 6b). [ 137 ] At lower pH carboxyl groups at the pore mouth protonate, but protonation does not affect the CNT porin selectivity between anions (Figure 6b). [ 137 ] Further increasing the selectivity of CNTs will hinge on the ability to synthesize CNTs with narrower channel‐size distribution, predefined chirality, and chemically tailored rims.…”

“…[ 137 ] At lower pH carboxyl groups at the pore mouth protonate, but protonation does not affect the CNT porin selectivity between anions (Figure 6b). [ 137 ] Further increasing the selectivity of CNTs will hinge on the ability to synthesize CNTs with narrower channel‐size distribution, predefined chirality, and chemically tailored rims. [ 138 ]…”

Putting Together the Puzzle of Ion Transfer in Single-Digit Carbon Nanotubes: Ab Initio Meets Mean-Field

“…Li et al 46 measured chloride permeation rates using stop-flow experiment in vesicles and derive chloride permeability in CNTPs P Cl by fitting the anion permeation rate to a linear dependence on C s . However, the observed trend was clearly non-linear and a much better fit is obtained for C 3/2 s scaling, in agreement with eq.…”

“…27,[35][36][37][38][39][40][41] In parallel, important insights to transport of ions at the molecular level were obtained using molecular dynamics (MD) [42][43][44][45] and ab initio computations. 13,46 The data on ion permeation in CNTPs have mainly come from two types of measurements (a) ion permeation from stop-flow experiments with CNTP-loaded vesicles and (b) conductance or current-voltage (I-V) measurements for CNTPs connecting two solutions across a nanopore. These experiments yielded absolute values of permeabilities to different ions and salts, conductivity and water-salt selectivity for various types of CNTPs and longer tubes.…”

“…These trends are often highly characteristic and may serve as fingerprints of physical mechanisms. The data, combined with simulations, e.g., using classical MD [42][43][44][45] and ab initio computations 13, 46 have significantly advanced understanding of ion transport in CNTPs, yet a full and physically consistent picture unifying different experimental findings is still missing.…”