Electrochemical Control of Ion Transport through a Mesoporous

Mar 21, 2014 - Department of Chemistry, California State University, Fresno, California 93740, United States ... Department of Chemistry, University o...
0 downloads 12 Views 4MB Size
Download PDF
Article pubs.acs.org/Langmuir

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 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 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. 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, λD ∝1/ √ρs, where ρs 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 nanofluidics applications have been explored.14,15,17−20,23,24 However, all of these methods are expensive, time-consuming, and applicable mainly for fundamental studies of ion transport through nanochannels. In contrast, a three-dimensional nanofluidic membrane where the transport of ions can be controlled

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 developed3−7 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 templates.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 © 2014 American Chemical Society

Received: December 4, 2013 Revised: March 6, 2014 Published: March 21, 2014 3606

dx.doi.org/10.1021/la404669m | Langmuir 2014, 30, 3606−3611

Langmuir

Article

mesoporous carbon membrane. The mesoporous carbon membrane was mounted between two pieces of adhesive-backed aluminum foil with a hole of 0.5 cm diameter in the center exposing the membrane. One side of the Al foil/membrane was further covered with plastic with a hole in between exposing the membrane to prevent any possibility of electrochemical reactions occurring on the source side of the U-shaped tube. Alligator clips were used to establish electrical connections. The carbon membrane served as the working electrode, and an electrochemical potential (EAPP) was applied to the membrane with either a CHI 604B or a CHI 660C (CH Instruments, Inc.) potentiostat. A silver/silver chloride (Ag/AgCl) reference electrode and platinum counter electrode were used for all the measurements and were placed in the receiver solution. The aqueous solutions of methylene blue (MB+; Cl− salt) and methyl orange (MO−; Na+ salt) and KCl were used to test the ion transport measurements. The UV− vis absorption experiments were carried out with a Cary 5000 UV−vis spectrometer (Varian, Inc.) or a UV-1800 spectrophotometer (Shimadzu, Inc.). The absorption spectra were measured within a certain time interval, using 3 mL of the solution taken from the receiver compartment (∼5 mL) of a cell. After measurement, the solution was brought back to the receiver compartment.

could have enormous potential in a variety of applications such as drug delivery, desalination, catalysis, and selective chemical separation. Porous carbon materials with high surface area and controlled nanostructure have been used for diverse applications including electrode materials for energy storage, catalyst supports, and separation membranes.25−28 Recent developments in carbon synthesis methods have made it possible to design a variety of morphologies ranging from monolithic fibers, wires, and spheres to more complex shapes with high mechanical strength and tailored surface properties.29−34 The high chemical and thermal stability, controlled morphology, and tailorable surface properties of the porous carbon make it a favorable candidate for ion separation.26 These properties along with characteristics such as a high density of nanochannels, mechanical stability, and the conductivity of the carbon, which enables direct control of the surface charge, motivated us to examine porous carbon membranes as a prototype 3D nanofluidic device to control the passage of large molecules and ions. Here, we report gated control of ion transport through a mesoporous carbon membrane using an applied electric potential.

3. RESULTS AND DISCUSSION Figure 1 is a representation of the mesoporous carbon membrane used to control the transport of ions. As illustrated

2. MATERIALS AND METHODS 2.1. Membrane Preparation. Free-standing mesoporous (pore size between 2 and 50 nm) carbon−carbon black (MC−CB) nanocomposite membranes were synthesized by carbonization of polymeric composite films fabricated by casting an ethanolic solution of phenolic resin−CB block copolymer (F127, PEO106-PPO70-PEO106) composite on a hydrophilic Mylar sheet via a tape casting technique. The gel-like composite was obtained by self-assembly of in situ formed phloroglucinol−formaldehyde resin, CB, and F127 under acidic conditions (more details given in the Supporting Information). After removal of the air bubbles by centrifugation, the solution was cast onto a hydrophilic Mylar sheet using a doctor blade, dried overnight at room temperature, and cured at 80 °C for 24 h, thus generating a freestanding polymeric film (∼100 μm in thickness). The polymeric film was then punched into small circular disks with a diameter of 20 mm, detached from the substrate, and carbonized after loading between two pieces of carbon paper and sandwiched by quartz strips to maintain the flat-sheet morphology. The carbonization was conducted in flowing Ar (100 mL/min) at an initial temperature of 450 °C for 2 h followed by heating at 850 °C for an additional 2 h with a heating rate of 5 °C/ min. 2.2. Surface Etching and Characterization. The surface of the as-prepared membrane is usually nonporous or microporous.34 Therefore, the thin surface layer was etched by exposing each side of the carbon membrane to oxygen plasma (Diener Electronic, Femto) for 45 min. In order to characterize the pore volume and pore size, N2sorption analysis was carried out with a Micromeritics Gemini 2375 analyzer at −196 °C (77 K). Before measurement, the etched membrane was degassed in vacuum at 120 °C overnight. The specific surface area was estimated from the N2 adsorption data in the relative pressure range (P/P0 = 0.06−0.3) using the Brunauer−Emmett− Teller (BET) method. The total pore volume was determined from the N2 uptake at P/P0 = 0.98. The micropore volume was calculated from the intercept of the V−t plot where the t values were calculated as a function of the relative pressure using the de Bore equation t/Å = [13.99/(log(P0/P) + 0.0340)]1/2. The pore size distribution (PSD) plot was derived from the adsorption branch of the isotherm based on the Kruk−Jaroniec−Sayari (KJS) model. For surface images, a scanning electron microscope (JEOL operated at 15 kV) or a scanning transmission electron microscope (Hitachi HD-2000 operated at 200 kV) was used. 2.3. Ion Permeation Measurement. The ion permeation experiments were conducted using a U-shaped cell. The U-shaped cell consisted of the source and the receiver solutions, separated by a

Figure 1. (a) Illustration of the porous carbon membrane on an aluminum support showing no dye transport with an applied voltage (V). Though not shown, in the absence of the voltage, dye molecules diffuse through the membrane (see Supporting Information). The SEM image shows the pore structure of the surface of the membrane.

in the figure, in the presence of an applied potential, ion transport through the membrane is inhibited. The mesoporous carbon−carbon black (MC−CB) nanocomposite membranes were created by incorporating carbon black into a typical softtemplating synthesis method (see the Materials and Methods section).35 Membranes with carbon black initially varying from 25% to 75% were synthesized to primarily control the pore size and electrical conductivity of the material.35 An added advantage of incorporating CB into the composite was the improved stability of the membrane since the MC−CB composite membrane was less fragile and more mechanically robust than the pristine mesoporous carbon membrane. Figure 2 shows SEM images of the carbon membrane demonstrating the small pores in the carbon. Based on the lowmagnification image in Figure 2a, the thickness of the carbon membrane is approximately 30 μm. The higher-magnification images (Figures 2b and 2c) show that the pore structure is present throughout the membrane, thus confirming the 3D nature of the porosity. Figure 3 shows the N2-physisorption isotherms and pore size distributions of the MC−CB nanocomposite membranes. The adsorption isotherms (type IV) and hysteresis loops (H-1) for all MC−CB nanocomposite membranes are typical for materials with large mesopores 3607

dx.doi.org/10.1021/la404669m | Langmuir 2014, 30, 3606−3611

Langmuir

Article

Figure 2. SEM images of the carbon membrane showing (a) the edge with a thickness of 30 μm, (b) the edge taken at an angle of approximately 30°, and (c) zoomed in on the edge showing the pore structure.

Figure 3. (a) Nitrogen adsorption isotherms and (b) pore size distributions for MC−CB composite membranes.

(Figure 3a). As the CB content increased from 0 to 75 wt %, the pore size distribution (Figure 3b) changed from a relatively narrow distribution centered at 7.3 nm to a broader distribution centered at 11.3 nm while with the BET surface area decreased from 493 to 324 m2 g−1 (Table 1). Thus, by controlling the amount of carbon black, we were able to control the pore size of the carbon. Because the MC−CB composite membrane with 25% carbon black had the smallest pore size of all the composites, hence the highest potential for electrochemical control, we focus our discussion on the MC−CB-25 membrane. The ion permeability through the MC−CB-25 membrane was first tested without an electrochemical potential. Ion transport for two different dye solutions, i.e., methyl orange as the anionic dye and methylene blue as the cationic dye, were initially examined as a visual demonstration of the transport properties as a function of applied potential. Two separate source solutions were prepared:

Table 1. Textural Properties of MC−CB Composite Membranes sample

MC−CB-0

MC−CB-25

MC−CB-50

MC−CB-75

SBETa (m2 g−1) Smib (m2 g−1) VSPc (cm3 g−1) Vmid (cm3 g−1) we (nm)

493 369 0.61 0.06 7.3

406 311 0.58 0.04 7.8

352 270 0.65 0.04 9.7

324 245 0.68 0.04 11.3

a

Specific surface area calculated using the BET equation in the relative pressure range of 0.05−0.20. bMicropore surface area calculated in the αS plot range of 0.80−1.00. cSingle point pore volume from adsorption isotherms at P/P0 ∼ 0.98. dMicropore volume calculated in the αS plot range of 0.80−1.00. ePore width calculated according to the improved KJS method (see Supporting Information) using statistical film thickness for nonporous reference carbon material (see Supporting Information).

3608

dx.doi.org/10.1021/la404669m | Langmuir 2014, 30, 3606−3611

Langmuir

Article

and the ionic solution which resulted in no ion transport through the membrane. More specifically, when an electrochemical potential of −0.5 V was applied to the membrane, as shown in Figure 5, no spectral evidence of the anionic dye

(1) the anionic source solution was 0.5 mM methylene blue (MB+) and 1 mM KCl, and (2) the cationic source solution was 0.5 mM methyl orange (MO−) and 1 mM KCl in deionized (DI) water. As shown in Figure 4a, the source solution was

Figure 5. Images of the experimental setup with −0.5 V applied to the membrane at (a) 0 h and (b) after 24 h. There is no change in the color of the DI water in the receiver side of the U-shaped tube after 24 h, indicating no dye diffusion through the membrane. (c) UV−vis absorbance spectra of methyl orange at 0 h and after 24 h at −0.5 V.

Figure 4. Images of the experimental setup at (a) 0 h and (b) after 24 h with no applied voltage. The DI water in the receiver side of the Ushaped tube turns orange after 24 h, indicating diffusion of dye molecules through the membrane. (c) UV−vis absorbance spectra of methyl orange at 0 h and after 24 h with no applied potential. The inset shows similar UV−vis spectra for methylene blue.

MO− was observed in the receiver solution even after 24 h. Similarly, no MO− transport was measured when a positive potential was applied either, though the magnitude of the potential was limited to less than +0.3 V due to oxidation. Comparable results were obtained for the cationic dye MB+ where application of −0.5 and +0.3 V also prevented dye transport (see Figure S3). Unlike ion-selective membranes, the polarity of the potential did not matter, and the diffusion of dye ions through the membrane was prohibited when a potential was applied regardless of polarity. The mechanism that we propose for this gating effect observed in the dye solutions is that the EDL in the pores of the carbon membrane was larger than the pore radius, resulting in an overlapped EDL within the pores. The pore diameter for the MC−CB-25 membrane is ∼7.8 nm, while the Debye length of the ion solution was estimated to be ∼9 nm using the equation

placed on the right side of the U-shaped tube while the receiver side contained 1 mM KCl. The transport of the MO− and MB+ solutions was measured separately to avoid any possible precipitation. For each solution, the transport of dye through the membrane was monitored by measuring the UV−vis spectrum of the receiver solution at regular time intervals. After a retention time of approximately 4 h, both the MB+ and MO− species could be detected at the receiver side of the U-shaped cell by UV−vis absorption spectroscopy. The ion concentration on the receiver side of the membrane gradually increased with retention time for both MO− and MB+ (Figure 4b and Figure S2). Figure 4c shows the UV−vis absorption spectra of MB+ and MO− measured at the receiver side after a retention time of 24 h where the transport of the ions could be easily seen with the eye. Thus, both cations (MB+) and anions (MO−) were transported by diffusion driven by a concentration gradient across the membrane. These results were quite reproducible and similar to our previous results obtained with anilinium and Rhodamine B.36 In the absence of an applied potential, both cations and anions diffused through the membrane with no gating or separation selectivity. In contrast, the results were remarkably different with the application of an electrical potential between the membrane

λD =

ε0εrRT 2F 2C0

where ε0 is permittivity of free space, εr is the dielectric constant of the solution, R is the gas constant, T is the temperature, F is the Faraday constant, and C0 is the electrolyte concentration. Thus, the electrostatic field readily extended throughout the channel, enabling control over the transport of ions using the electrochemical potential. When the applied voltage was in the range of 0 to −0.4 V, the electrostatic field 3609

dx.doi.org/10.1021/la404669m | Langmuir 2014, 30, 3606−3611

Langmuir

Article

Figure 6. (a) Conductivity of the solution on the receiver side at different applied voltages showing the diffusion of salt through the membrane. (b) Diffusion rate at various applied voltages.

to 0 V was slightly lower than the initial flow rate at 0 V. The reason for this is not clear and is under evaluation. The unique aspect of the 3D carbon membrane is the ability to gate the transport of ions rather than act as an ion selective membrane. The use of nanopores as ion-transport selective membranes is well-known dating back to the work of Nishizawa, who showed that cylindrical metal nanotubes could selectively transport cations or anions.37 The gating ability of the carbon can result from the overlap of the EDL as described previously. In addition, a number of reports have shown that the transport of both cation and anion species can be prevented in nanopores, and this effect can be modulated by the double layer and the presence of functional groups.38,39 In addition, since the two reservoirs are not connected by the external circuitry in our experiment, the semipermeable membrane which separates two reservoirs with different salt concentrations forces the development of a Donnan potential between these reservoirs. The electric field gradient developed across the membrane counteracts the diffusion gradient making the net flux through semipermeable membrane of all ionic species zero. Alternatively, Boo et al. showed that as the EDL becomes larger than the pore size of a mesoporous material due to changes in the electrolyte concentration, the shape of the EDL changes.40 At low concentration and large EDL, the equipotential lines of the EDL do not extend into the pores, leading to a potential difference along the length of the pore. This ultimately results in a potential difference across the pore connecting the two reservoirs that serves to inhibit ion transport through the membrane.

was not strong enough to shield the entire cross section of the channel, allowing the dye molecules to diffuse through the membrane and be detected at the receiver end. However, the stronger electric field at −0.5 V prevented the passage of dye ions through the membrane, regardless of the charge of the ion. To explore this further, we decreased the Debye length of the dye solution by increasing the salt concentration from 1 mM KCl to 0.25 M KCl and repeated the dye transport measurements using a 0.5 mM dye solution. The Debye length of the 0.25 M KCl solution was ∼0.6 nm, much smaller than the pore radius of the carbon membrane. It should be noted that the electrolyte concentration that produces an EDL equal to the radius of the pores is ∼6 mM. Even with an electric potential applied to the membrane, transport of dye through the membrane was observed on the receiver side further suggesting that this is indeed an EDL effect. On the basis of the ability to gate the large dye ions and to evaluate the potential of the carbon membranes to control the transport of smaller salt ions for application in desalination, we repeated the measurements using KCl rather than the molecular (dye) ions. Briefly, 1 mM KCl was used as the source solution, and the receiver side of the U-shaped glass tube was filled with DI water. The conductivity of the DI water in the receiver side was continuously monitored over time. As shown in Figure 6a, in the absence of an electrical potential on the membrane, the conductivity of the receiver solution continuously increased, indicating ion diffusion through the membrane. However, when an electrical potential of −0.9 V was applied to the membrane, ion transport through the membrane was drastically reduced. There was no significant effect on the diffusion rate when the applied potential was less than −0.8 V. When the potential was increased to −0.9 V, however, ion transport through the membrane was inhibited. Because the K+ and Cl− ions are much smaller than the organic dye molecules, a larger applied potential was necessary to prevent ion transport through the membrane. To further confirm that this effect was not due to an electrochemical reaction at the electrode surface causing pore blockage, we sequentially applied and then removed the potential applied to the membrane. As shown in Figure 6b, when no potential was applied to the membrane, the ions freely flowed through the membrane; however, when a potential was applied, the rate at which ions passed through the membrane decreased (indicated by the change in slope). When the potential was removed, the rate at which ions flow through the membrane increased. It is to be noted that the rate of ion flow after changing back from −0.9

4. CONCLUSION In conclusion, we report a mesoporous carbon-based bulk nanofluidic transport membrane that can be used to regulate the flow of large molecular dye molecules as well as smaller ions using a small electrochemical potential. The transport of ions does not depend on the polarity of the potential applied to the membrane, and thus the membrane directly acts as an on/off switch allowing gated control of the transport of ionic dye molecules and smaller salt ions. This unique property of the carbon-based, bulk nanofluidic transport membranes can open up tremendous opportunities in controlled drug release, desalination, and chemical separations. 3610

dx.doi.org/10.1021/la404669m | Langmuir 2014, 30, 3606−3611

Langmuir

■

Article

(20) Duan, C.; Majumdar, A. Anomalous ion transport in 2-nm hydrophilic nanochannels. Nat. Nanotechnol. 2010, 5, 848−852. (21) Liu, H.; He, J.; Tang, J.; Liu, H.; Pang, P.; Cao, D.; Krstic, P.; Joseph, S.; Lindsay, S.; Nuckolls, C. Translocation of single-stranded DNA through single-walled carbon nanotubes. Science 2010, 327, 64− 67. (22) Mattia, D.; Gogotsi, Y. Review: Static and dynamic behavior of liquids inside carbon nanotubes. Microfluid. Nanofluid. 2008, 5, 289− 305. (23) Li, J.; Stein, D.; McMullan, C.; Branton, D.; Aziz, M. J.; Golovchenko, J. A. Ion-beam sculpting at nanometer length scales. Nature 2001, 412, 166−169. (24) Stavis Samuel, M.; Strychalski Elizabeth, A.; Gaitan, M. Nanofluidic structures with complex three-dimensional surfaces. Nanotechnology 2009, 20, 165302. (25) Chang, H.; Joo, S. H.; Pak, C. Synthesis and characterization of mesoporous carbon for fuel cell applications. J. Mater. Chem. 2007, 17, 3078−3088. (26) Liang, C.; Li, Z.; Dai, S. Mesoporous carbon materials: Synthesis and modification. Angew. Chem., Int. Ed. 2008, 47, 3696−3717. (27) Liu, R.; Duay, J.; Lee, S. B. Heterogeneous nanostructured electrode materials for electrochemical energy storage. Chem. Commun. 2011, 47, 1384−1404. (28) Ndamanisha, J. C.; Guo, L.-p. Ordered mesoporous carbon for electrochemical sensing: A review. Anal. Chim. Acta 2012, 747, 19−28. (29) Huang, Y.; Cai, H.; Feng, D.; Gu, D.; Deng, Y.; Tu, B.; Wang, H.; Webley, P. A.; Zhao, D. One-step hydrothermal synthesis of ordered mesostructured carbonaceous monoliths with hierarchical porosities. Chem. Commun. 2008, 2641−2643. (30) Jin, J.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Vapor phase synthesis of ultrathin carbon films with a mesoporous monolayer by a soft-templating method. Chem. Commun. 2009, 1371−1373. (31) Steinhart, M.; Liang, C.; Lynn, G. W.; Goesele, U.; Dai, S. Direct synthesis of mesoporous carbon microwires and nanowires. Chem. Mater. 2007, 19, 2383−2385. (32) Yan, Y.; Zhang, F.; Meng, Y.; Tu, B.; Zhao, D. One-step synthesis of ordered mesoporous carbonaceous spheres by an aerosolassisted self-assembly. Chem. Commun. 2007, 2867−2869. (33) Zhang, F.; Meng, Y.; Gu, D.; Yan, Y.; Yu, C.; Tu, B.; Zhao, D. A facile aqueous route to synthesize highly ordered mesoporous polymers and carbon frameworks with ia3d bicontinuous cubic ̅ structure. J. Am. Chem. Soc. 2005, 127, 13508−13509. (34) Wang, X. Q.; Zhu, Q.; Mahurin, S. M.; Liang, C. D.; Dai, S. Preparation of free-standing high quality mesoporous carbon membranes. Carbon 2010, 48, 557−560. (35) Fulvio, P. F.; Mayes, R. T.; Wang, X.; Mahurin, S. M.; Bauer, J. C.; Presser, V.; McDonough, J.; Gogotsi, Y.; Dai, S. ″Brick-andmortar” self-assembly approach to graphitic mesoporous carbon nanocomposites. Adv. Funct. Mater. 2011, 21, 2208−2215. (36) Hou, C.-H.; Wang, X.; Liang, C.; Yiacoumi, S.; Tsouris, C.; Dai, S. Molecular-sieving capabilities of mesoporous carbon membranes. J. Phys. Chem. B 2008, 112, 8563−8570. (37) Nishizawa, M.; Menon, V. P.; Martin, C. R. Metal nanotubule membranes with electrochemically switchable ion-transport selectivity. Science 1995, 268, 700−702. (38) Calvo, A.; Yameen, B.; Williams, F. J.; Soler-Illia, G.; Azzaroni, O. Mesoporous films and polymer brushes helping each other to modulate ionic transport in nanoconfined environments. An interesting example of synergism in functional hybrid assemblies. J. Am. Chem. Soc. 2009, 131, 10866−10868. (39) Park, S.; Kim, H. C.; Chung, T. D. Electrochemical analysis based on nanoporous structures. Analyst 2012, 137, 3891−3903. (40) Boo, H.; Park, S.; Ku, B. Y.; Kim, Y.; Park, J. H.; Kim, H. C.; Chung, T. D. Ionic strength-controlled virtual area of mesoporous platinum electrode. J. Am. Chem. Soc. 2004, 126, 4524−4525.

ASSOCIATED CONTENT

S Supporting Information *

Complete synthesis and fabrication of mesoporous carbon membrane and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.M.M.). *E-mail [email protected] (S.D.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy.

■

REFERENCES

(1) Bocquet, L.; Charlaix, E. Nanofluidics, from bulk to interfaces. Chem. Soc. Rev. 2010, 39, 1073−1095. (2) Daiguji, H. Ion transport in nanofluidic channels. Chem. Soc. Rev. 2010, 39, 901−911. (3) Daiguji, H.; Oka, Y.; Shirono, K. Nanofluidic diode and bipolar transistor. Nano Lett. 2005, 5, 2274−2280. (4) Kalman, E. B.; Vlassiouk, I.; Siwy, Z. S. Nanofluidic bipolar transistors. Adv. Mater. 2008, 20, 293−297. (5) Karnik, R.; Fan, R.; Yue, M.; Li, D.; Yang, P.; Majumdar, A. Electrostatic control of ions and molecules in nanofluidic transistors. Nano Lett. 2005, 5, 943−948. (6) Kovarik, M. L.; Jacobson, S. C. Nanofluidics in lab-on-a-chip devices. Anal. Chem. 2009, 81, 7133−7140. (7) Vlassiouk, I.; Siwy, Z. S. Nanofluidic diode. Nano Lett. 2007, 7, 552−556. (8) Hou, X.; Yang, F.; Li, L.; Song, Y.; Jiang, L.; Zhu, D. A biomimetic asymmetric responsive single nanochannel. J. Am. Chem. Soc. 2010, 132, 11736−11742. (9) Levy, S. L.; Craighead, H. G. DNA manipulation, sorting, and mapping in nanofluidic systems. Chem. Soc. Rev. 2010, 39, 1133−1152. (10) Piruska, A.; Gong, M.; Sweedler, J. V.; Bohn, P. W. Nanofluidics in chemical analysis. Chem. Soc. Rev. 2010, 39, 1060−1072. (11) Saleh, O. A.; Sohn, L. L. An artificial nanopore for molecular sensing. Nano Lett. 2003, 3, 37−38. (12) Stein, D.; Van Den Heuvel, M.; Dekker, C. Transport of ions, DNA polymers, and microtubules in the nanofluidic regime. Nanofluidics 2009, 1−30. (13) Vlassiouk, I.; Kozel, T. R.; Siwy, Z. S. Biosensing with nanofluidic diodes. J. Am. Chem. Soc. 2009, 131, 8211−8220. (14) Cao, H.; Yu, Z.; Wang, J.; Tegenfeldt, J. O.; Austin, R. H.; Chen, E.; Wu, W.; Chou, S. Y. Fabrication of 10 nm enclosed nanofluidic channels. Appl. Phys. Lett. 2002, 81, 174−176. (15) Duan, C.; Wang, W.; Xie, Q. Review article: Fabrication of nanofluidic devices. Biomicrofluidics 2013, 7, 026501. (16) Goldberger, J.; Fan, R.; Yang, P. Inorganic nanotubes: A novel platform for nanofluidics. Acc. Chem. Res. 2006, 39, 239−248. (17) Guo, L. J.; Cheng, X.; Chou, C.-F. Fabrication of sizecontrollable nanofluidic channels by nanoimprinting and its application for DNA stretching. Nano Lett. 2004, 4, 69−73. (18) Han, A.; de Rooij, N. F.; Staufer, U. Design and fabrication of nanofluidic devices by surface micromachining. Nanotechnology 2006, 17, 2498−2503. (19) Mao, P.; Han, J. Fabrication and characterization of 20 nm planar nanofluidic channels by glass-glass and glass-silicon bonding. Lab Chip 2005, 5, 837−844. 3611

dx.doi.org/10.1021/la404669m | Langmuir 2014, 30, 3606−3611