Electrophoretically induced aqueous flow through single-walled carbon nanotube membranes

Abstract

Electrophoresis, the motion of charged species through liquids and pores under the influence of an external electric field, has been the principle source of chemical pumping for numerous micro- and nanofluidic device platforms. Recent measurements of ion currents through single or few carbon nanotube channels have yielded values of ion mobility that range from close to the bulk mobility to values that are two to seven orders of magnitude higher than the bulk mobility. However, these experiments cannot directly measure ion flux. Experiments on membranes that contain a large number of nanotube pores allow the ion current and ion flux to be measured independently. Here, we report that the mobilities of ions within such membranes are approximately three times higher than the bulk mobility. Moreover, the induced electro-osmotic velocities are four orders of magnitude faster than those measured in conventional porous materials. We also show that a nanotube membrane can function as a rectifying diode due to ionic steric effects within the nanotubes.

Figure 1

Characterization of single wall carbon nanotubes. a, TEM image of SWCNT with a scale bar of 10nm. b, Raman spectra of SWCNT powder sample showing diameters calculated from peak location. c, SEM image of the Cross-sectional view of CNT membrane with a scale bar of 5μm, showing carbon nanotubes are uniformly dispersed in epoxy resin matrix as no bundles of nanotubes can be observed. d, Histogram of SWCNT inner core diameters.

Figure 2

Enhanced electrophoretic mobilities in SWCNTs generated by highly efficient electroosmotic flow and effect of charged carboxyl groups on the electrophoretic flow. a, Ionic currents (0.6V) and K⁺ EMs vs. KCl concentrations; Membrane pore area is 5.0×10⁻¹² m². Shown are mobilities calculated with Ap (equation 1) using bulk diffusion (K_(λ)= 1) or hindered diffusion (K_(λ)= 0.48) assumption.b, Ionic current as a function of pH in 10 mM KCl. HCl is used to change pH values, applied bias is 0.6V and Ag/AgCl electrodes are used.

Figure 3

Effects of ionic concentrations and species on the electrophoretic flow through SWCNTs and SWCNT membrane functioning as a rectifying diode, confirming that the ionic currents are through the SWCNT cores.a, Current vs. Voltage of KCl and Ru(bpy)₃Cl₂ through SWCNT-COO^- membrane. There is no apparent threshold voltage to pump K⁺ through SWCNT and the electrophoretic mobility of Ru(bpy)₃Cl₂ through SWCNT is dramatically reduced at a high ionic strength. b, Ionic currents through SWCNT membranes rectification is seen when one side is filled with 25 mM K₃(Fe(CN)₆), and another side is filled with 50 mM Ru(bpy)₃Cl₂, and the control experiment with KCl. c, current shown on log scale. d, Schematic with space filling molecular models of ionic transport in SWCNT (10, 10) under electric field under opposite bias conditions (Dark green, K⁺; light green Cl⁻ grey, C; blue, N; dark brown, Ru²⁺; light brown, Fe³⁺; white, H). Pore area: 5.0×10⁻¹² m².

Figure 4

SWCNT membrane functions as a rectifying diode using other ionic species. a, Ionic currents through SWCNT membranes obtained using 50 mM Ru(bpy)₃Cl₂ and 25 mM Na₇(SO₃)₇CD; after working (WE) and reference electrodes (RE) positions are exchanged, the IV cure was reversed (2). b, molecular structure of Na₇(SO₃)₇CD. Pore area is 5.0×10⁻¹² m².