Ion exclusion by sub-2-nm carbon nanotube pores

Abstract

Biological pores regulate the cellular traffic of a large variety of solutes, often with high selectivity and fast flow rates. These pores share several common structural features: the inner surface of the pore is frequently lined with hydrophobic residues, and the selectivity filter regions often contain charged functional groups. Hydrophobic, narrow-diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform. Previous studies demonstrated that carbon nanotube pores can support a water flux comparable to natural aquaporin channels. Here, we investigate ion transport through these pores using a sub-2-nm, aligned carbon nanotube membrane nanofluidic platform. To mimic the charged groups at the selectivity region, we introduce negatively charged groups at the opening of the carbon nanotubes by plasma treatment. Pressure-driven filtration experiments, coupled with capillary electrophoresis analysis of the permeate and feed, are used to quantify ion exclusion in these membranes as a function of solution ionic strength, pH, and ion valence. We show that carbon nanotube membranes exhibit significant ion exclusion that can be as high as 98% under certain conditions. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, whereas steric and hydrodynamic effects appear to be less important.

Fig. 1.

CNT/silicon nitride membrane platform for ultrafast nanofiltration of electrolytes. (a) Cross-section schematic of a CNT membrane representing the silicon support chip, the aligned DWNTs, the filling silicon nitride matrix, and the CNT tips functionalized with carboxylic groups. (b) Cross-section SEM image of the CNT/silicon nitride composite showing the gap-free coating of silicon nitride. (c) Photographs of the membrane sides exposed to the feed (Upper) and to the permeate (Lower). (d) Time variation of permeate volume per unit area of freestanding membrane during the filtration of 0.6 mM K₃Fe(CN)₆ solution. The resulting permeation flux, F, is ≈1,000 larger than the calculated value with the Hagen–Poiseuille equation, F_HP. (e) Schematic of the nanofiltration cell showing the column of feed solution (1) pressurized at P = 0.69 bar, the CNT membrane (2), the permeate solution (3), and feed (4) and permeate (5) chambers. (f) Capillary electrophoresis chromatogram for feed (red) and permeate (blue) showing a 91% exclusion of the ferricyanide anion after nanofiltration of a 1.0 mM K₃Fe(CN)₆ solution.

Fig. 2.

Effect of pH on measured rejection for a 0.5 mM Na₄PTS solution. (a) UV spectra for feed (red) and permeate at pH 3.8 (blue) and pH 7.2 (green). (b) Anion (red) and cation (yellow) rejection at pH 3.8 and pH 7.2.

Fig. 3.

Dependence of K₃Fe(CN)₆ (circles) and KCl (diamonds) rejections on solution concentration (a) and Debye length (b). Filled markers correspond to anions, and empty markers correspond to cations. The dashed black vertical line in b marks the average CNT diameter. Dashed green and orange lines show the rejection coefficients calculated using Donnan membrane equilibrium theory (Eq. 1) for a 1:3 and a 1:1 electrolyte, respectively. To illustrate the trends predicted by the Donnan theory, the membrane charge density is set equal to 3.0 mM.

Fig. 4.

Rejection coefficients (bars) measured for six salt solutions that have the same equivalent concentration but different ion valence. Points (filled circles) indicate rejections calculated with the Donnan theory, Eq. 1, with a membrane charge density c_x^m = 2.0 mM [this value was chosen to fit K₃Fe(CN)₆ rejection]. This density corresponds to approximately seven charged groups per nanotube (see SI Text).