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A study of ion transport in aqueous-filled silica channels as thin as 70 nm reveals a remarkable degree of conduction at low salt concentrations that departs strongly from bulk behavior: In the dilute limit, the electrical conductances of channels saturate at a value that is independent of both the salt concentration and the channel height. Our data are well described by an electrokinetic model parametrized only by the surface-charge density. Using chemical surface modifications, we further demonstrate that at low salt concentrations, ion transport in nanochannels is governed by the surface charge. DOI: 10.1103/PhysRevLett.93.035901 PACS numbers: 66.10.-x, 82.65.+r Nanoscale fluidic channels represent a new regime in the study of ion transport. Recent advances in the fabrication of ultraconfined fluidic systems such as nanoscale ''lab-on-a-chip'' type devices [1][2][3] and synthetic nanopores [4 -6] raise fundamental questions about the influence of surfaces on ion transport. In particular, surface charges induce electrostatic ion (Debye) screening and electrokinetic effects such as electro-osmosis, streaming potentials, and streaming currents [7][8][9] that may have large effects on conductance in nanochannels, as suggested by studies of colloidal suspensions [10] and biological protein channels [11]. Here we present experiments that directly probe ion transport in the nanoscale regime, and reveal the role of surface charge in governing conductance at low salt concentrations.Nanofluidic channels [ Fig. 1(a)] were fabricated following a silicate bonding procedure similar to that of Wang et al. [12]. Briefly, channels 50 m wide and 4:5 mm long were patterned between 1:5 mm 1:5 mm reservoirs using electron beam lithography on fused silica substrate. A reactive CHF 3 =O 2 plasma then etched into the fused silica at a rate of 30 nm=min, and was timed to stop when the desired submicron channel depth was attained. The depth of the resulting channel, h, was measured using an -stepper profilometer. The channels were sealed by first spinning a 20 nm layer of sodium silicate from 2% solution onto a flat fused silica chip, then pressing the silicate-coated surface to the patterned channel surface, and finally curing the device at 90 o C for 2 h. The channels were filled by introducing distilled, deionized (18 M cm) water into the large fluidic reservoirs, from which point capillary forces were sufficient to draw the water across the channels. The electrical voltage source and IV converter were connected to the fluidic channel with negligible resistive loss via silver wires inserted into the reservoirs [ Fig. 1(b)]. The channels were cleaned of ionic contaminants using electrophoretic pumping: The ionic current was observed to decay while 10 V were applied across the channels to drive out ionic impurities. The reservoirs were periodically flushed with fresh solution until the current equilibrated to a minimum, which typically took 20 min. This procedure was also followed to replace different dilutions of 1 M potassium ...

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Simulations and measurements have established that water moves through carbon nanotubes with exceptionally high rates due to nearly frictionless interfaces1–4. These observations have stimulated interest in nanotube-based membranes for applications that range from desalination to nano-filtration and energy harvesting5–10, yet the exact water transport mechanisms inside the nanotubes and at the water-carbon interface continue to be controversially discussed11,12 because existing theories fail to provide a satisfying explanation for the limited number of experimental results available to date13. This is because even though controlled and systematic studies have explored transport through individual nanotubes8,9,14–7, none has met the considerable technical challenge of unambiguously measuring the permeability of a single nanotube11. Here we show that the pressure-driven flow rate across individual nanotubes can be determined with unprecedented sensitivity and without dyes from the hydrodynamics of water jets as they emerge from single nanotubes into a surrounding fluid. Our measurements reveal unexpectedly large and radius-dependent surface slippage in carbon nanotubes (CNT), and no slippage in boron-nitride nanotubes (BNNT) that are crystallographically similar to CNTs but differ electronically. This pronounced contrast between the two systems must originate from subtle differences in atomic-scale details of their solid-liquid interfaces, strikingly illustrating that nanofluidics is the frontier where the continuum picture of fluid mechanics confronts the atomic nature of matter.