Surface-Charge-Governed Ion Transport in Nanofluidic Channels

Abstract

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 ...

Single-layer MoS2 nanopores as nanopower generators

“…The conductance also depends on the salt concentration (Fig. 2b) and shows saturation at low salt concentrations-a signature of the presence of surface charge on the nanopore 17,18 . The predicted pore conductance (G), taking into account the contribution of the surface charge (Σ), is given by 19 :…”

Self-assembled two-dimensional nanofluidic proton channels with high thermal stability

“…This lengthscale is similar to the size of solvated ions as well as the Debye length of electrolytes at up to physiological concentrations. Many works have used similarly sized nanofluidic channels, fabricated by lithography or using a sacrificial template, to demonstrate that ions behave very differently in confined environments than they do in bulk [13][14][15][16][17] . For example, when the channel dimension becomes comparable to Debye length, ions carrying the opposite charge to the channel wall become enriched, and ions carrying the same charge as the channel wall are excluded.…”

Sub-10 nm transparent all-around-gated ambipolar ionic field effect transistor

“…However, in a high concentration range, i.e. c 0 > 10 −1 M, the experiments deviated from the theoretical calculation, while previous literature followed the theoretical calculation over 1 M. 10,33,34 The discrepancy could be as a result of highly confined microchannels. Our microchannels had the thickness of 1.5 µm, while previous studies usually provided the demonstration with an open reservoir.…”

“…Consequently, electrokinetic fields such as concentration distributions and flow fields can be inverted by the applied gate voltage. order of magnitude because those concentrations were in surface-charge-governed regime 10 where ion transport through the nanochannel is affected only by the surface charge density of nanochannel rather than bulk property. Meanwhile, deviations between measured and numerical results occurred in the high concentration range (10 −1 M-1 M).…”

“…10,11 Those platforms employed passive methods which were unable to change the behavior of charged species on-demand, once the devices were fabricated. In contrast, IFET can provide an active method which enables it to enhance, diminish or even reverse the behavior of charged species in situ by introducing gate potential.…”