Electrochemical Control of Ion Transport through a Mesoporous Carbon Membrane Sumedh P. Surwade, † Song-Hai Chai, † Jai-Pil Choi, ‡ Xiqing Wang, † Je Seung Lee, § Ivan V. Vlassiouk, ⊥ Shannon M. Mahurin,* ,† and Sheng Dai* ,†,∥ † Chemical Sciences Division, Oak Ridge National Lab, Oak Ridge, Tennessee 37831, United States ‡ Department of Chemistry, California State University, Fresno, California 93740, United States § Department of Chemistry, Kyung Hee University, Seoul, Republic of Korea ∥ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ⊥ Energy and Transportation Science, Oak Ridge National Lab, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: We report a carbon-based, three-dimensional nano- fluidic transport membrane that enables gated, or on/off, control of the transport of organic molecular species and metal ions using an applied electrical potential. In the absence of an applied potential, both cationic and anionic molecules freely diffuse across the membrane via a concentration gradient. However, when an electrochemical potential is applied, the transport of ions through the membrane is inhibited. 1. INTRODUCTION The transport of fluids through nanometer scale channels typically on the order of 1−100 nm often exhibit unique properties compared to the bulk fluid. 1,2 These phenomena occur because the channel dimensions and molecular size become comparable to the range of several important forces including electrostatic and van der Waals forces. Small changes in properties such as the electric double layer or surface charge can significantly affect molecular transport through the channels. Based on these emerging properties, a variety of nanofluidic devices such as nanofluidic transistors, nanofluidic diodes, or lab-on-a-chip devices have been developed 3−7 with a diverse range of applications including water purification, biomolecular sensing, DNA separation, and rectified ion transport. 8−13 Nanofluidic devices are typically fabricated using expensive lithography techniques or sacrificial tem- plates. 14−20 Here we report a carbon-based, three-dimensional nanofluidic transport membrane that enables gated, or on/off, control of the transport of organic molecular species and metal ions using an applied electrical potential. In the classic model, the interface between a charged surface and an ionic solution consists of an electrical double layer (EDL) where ions are electrostatically attracted to the charged surface while co-ions are repelled, creating a region in which the potential decays exponentially with a characteristic length known as the Debye length. 1,2 In microchannels, the Debye length is usually much smaller than the channel dimensions, and therefore direct electrostatic manipulation of ions across the microchannel is not possible. However, as the channel dimension becomes comparable to the Debye length, direct manipulation of ions through the nanochannel using either a surface charge or electric field becomes feasible. Moreover, when the nanochannel is small enough that the electric double layer overlaps, fluid transport in the channels becomes strongly affected leading to a rich diversity of new properties. For an overlapped EDL, the magnitude of the effect on transport properties is connected to both the channel size and the ion concentration since the Debye length is inversely proportional to the square root of ionic concentration, λ D ∝1/ √ρ s , where ρ s is the ion concentration. To obtain measurable effects from the double layer, particularly for an overlapped double layer, nanochannels with sizes on the order of 10−100 nm are required for an ion concentration higher than 0.1 mM. The nanochannel size must be reduced even further to maintain the effect at higher concentrations. A number of methods to fabricate such nanofluidic channels including photolithography, nanowires as sacrificial template, and surfactant-templated mesoporous thin films have been reported. 14−20 The direct use of carbon nanotubes as nanofluidic channels has also been described. 21,22 Additionally, fabrication of two-dimensional (2D) and three-dimensional (3D) channels using methods such as laser writing, electron beam induced etching, and etching and bonding for nano- fluidics applications have been explored. 14,15,17−20,23,24 How- ever, all of these methods are expensive, time-consuming, and applicable mainly for fundamental studies of ion transport through nanochannels. In contrast, a three-dimensional nano- fluidic membrane where the transport of ions can be controlled Received: December 4, 2013 Revised: March 6, 2014 Published: March 21, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 3606 dx.doi.org/10.1021/la404669m | Langmuir 2014, 30, 3606−3611