Biomimetic Selective Ion Transport through Graphene Oxide

Subscriber access provided by SELCUK UNIV

Article

Biomimetic Selective Ion Transport through Graphene Oxide Membranes Functionalized with Ion Recognizing Peptides Sunho Kim, Jeasun Nham, Yo Sub Jeong, Chang Sun Lee, Sung Hoon Ha, Ho Bum Park, and Yun Jung Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504212j • Publication Date (Web): 22 Jan 2015 Downloaded from http://pubs.acs.org on January 30, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Biomimetic Selective Ion Transport through Graphene Oxide Membranes Functionalized with Ion Recognizing Peptides ∥

∥

Sunho Kim,†, Jeasun Nham,‡, Yo Sub Jeong,‡ Chang Sun Lee,‡ Sung Hoon Ha,‡ Ho Bum Park,‡ and Yun Jung Lee*,‡ †Department of Material Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea ‡Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea ∥

These authors contributed equally to this work.

ABSTRACT: Membranes that differentiate ions are being actively developed to meet the needs in separation, sensing, biomedical, and water treatment technologies. Biomimetic approaches that combine bio-inspired functional molecules with solid state supports offer great potential for imitating the functions and principles of biological ion channels. Here we report the design and fabrication of biomimetic graphene oxide (GO) based membranes functionalized with a peptide motif that has the capabilities for selective recognition and transport. The peptide, which has ion binding affinity to Co2+ ions, was adopted to enable the ion selective filtration capability, and was then anchored on a GO surface. The resulting GO-based membranes show remarkable ion selectivity toward the specific ion of interest, for the transport across the membranes as in the biological ion channels. Ion recognition capability of this peptide motif successfully translates into ion specificity for selective transport. This study provides a new avenue for developing artificial ion channels via a synergistic combination of biomimetic recognition chemistry, with a novel nanoplatform such as GO.

■ INTRODUCTION Membranes that precisely recognize and selectively transport specific molecular species are of great interest in various technical fields.1 Especially, the ability that differentiates ions is the basis for the ion filtration/separation,2, 3 biosensing,4, 5 nanomedicine,6-8 and water treatment.9-11 Among many approaches addressing selective ionic transport, considerable efforts have been devoted to simulate the ion transport of naturally occurring biological channels through bio-inspired approaches by mimicking principles or functions of biological membranes.12-16 Biological membranes are made of lipid bilayers and the embedded proteins responsible for sensing outside and controlling the exchange of substances between the cell and its environment. Ion channels are self-assembled proteins embedded for the facilitated and highly selective transport of ions through the cell membranes. Central to the ion channel is the selectivity filter composed of peptides, responsible for the discrimination between ions.17 Performance in biological nanochannels is sophisticated and shows huge difference in qualities compared to that of the artificial counterpart. Biological ion channels offer precise control over ion selectivity, rectification and gating as well as outstanding permeability.17 Molecular recognition is a hallmark of biological interactions enabling these functions, especially the ion selectivity.

Several types of artificial nanochannels have been studied to transfer properties of naturally occurring nanochannels. However, mimicking these nanochannels has several problems to overcome. Supporting a lipid bilayer on a solid state membrane18, 19 is considered the most reliable way to reproduce functional attributes of biological nanochannels since many embedded biochannels often lose their activity upon leaving the biological environment. Though supported, however, flexible lipid bilayers are still fragile and not perfectly compatible with the practical applications. Similar approaches were the use of polymeric bilayer-like membranes incorporating biological channels such as aquaporin water channels20, 21 and Gramicidin-A ion channels.22 Although synthetic bilayerlike membrane supports are more stable than the purely biological one, their mechanical strength is not sufficient for practical application. Synthetic solid state membranes offer several advantages over bilayer-based approaches, such as mechanical stability, controls over pore dimension and shape, and modifiable surfaces for desired functions.14, 16, 23, 24 Tracketched membranes of polyethylene terephthalate (PET),14 polyimide (PI),25 or polycarbonate (PC)13 with cylindrical nanopores are mostly used as the solid support. Loading biological channels into the solid state nanopores has been recently investigated,13, 26 and showed a partial success. Purely artificial solid state ion channels have been explored with modification of the surface for functionality. The surface modification was generally accomplished by

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

immobilization of the functional ligands, and the immobilization has been performed by covalent attachment,12, 15, 27-29 hydrophobic interaction,15 and electrostatic selfassembly 4, 30, 31 of the ligands with the active chemical sites on the pristine nanochannel surface. Therefore, the beginning of the development for biomimetic ion channels is the fabrication of novel nanochannel scaffolds with modifiable surface. Graphene oxide (GO) has been recently suggested as a material for solid state membranes.32-38 Stacked GO films showed promising filtration properties with remarkable selectivity towards gases32, 39 and liquids.33, 35-37 Selective ionic transport through GO membranes has been also reported.34, 40 The holes on the basal plane and the interlayer spacing between stacked GO flakes construct the transport path in laminated GO membranes. In filtration applications, GO membranes have utilized its inherent properties so far and surface modified GO membranes with desired functionalities have not been actively explored. Since the surface oxygen-containing sites such as hydroxyl, epoxy, and carboxyl groups offer modifiable surface chemistry, GO can be a suitable scaffold for equipping functional ligands. 41, 42 The chemical functionalities on the channel surface determine the selectivity. The selection mechanism depends on the interaction between ligands on the channel wall and the molecules passing through. Most artificial ion channels studied have relied on electrostatic 12, 29, 43, 44 or size sieving mechanism.15 Molecular recognition has been used mostly as a method of channel surface modification for biosensing.4, 5, 45-50 As a selection mechanism for selective transport, molecular recognition chemistry has been applied to the selective transport of molecules such as enantiomeric drugs27 and complementary DNA,51 however, selective ion transport based on recognition interaction has not been explored yet. Here we adopted GO films as solid state membranes for the study of selective ion transport. GO has been functionalized by a synthetic peptide with ion recognizable capability as a selective filter. Peptides, the biomolecules composed of amino acids, have inherent recognition properties52-54 thus offering possibility for selective transport through the channels on which they are anchored. Peptides with specific affinity toward targets have been extensively identified through so-called bio-panning techniques.55-58 The Co2+ ion binding octapeptide of sequence EPGADAVP (Glu-Pro-Gly-Ala-Asp-Ala-Val-Pro), named Copep here, has been screened previously by a bio-panning method.59 To evaluate selective Co2+ ion transport facilitated by Copep, we have immobilized this peptide on GO surface as a solid state support. The resulting GO membranes functionalized with ion recognizable peptides showed transport of Co2+ ions remarkably selective over other transition metal ions acting as an ionselective filter.

■ EXPERIMENTAL SECTION

Page 2 of 9

GO based membrane Synthesis: Graphene oxide (GO) was prepared by a modified Hummers method as described elsewhere.60 The ester, hydroxyl, and epoxide groups were converted go carboxylic groups by treating as-prepared GO with ClCH2COONa and HCl subsequently.41 Briefly, 5 g of NaOH and 5 g of ClCH2COONa were added to 100 mL of aqueous GO suspension of 1 mg ml-1 concentration, followed by bath sonication for 2 h. The resulting product was neutralized with 1 M of hydrochloric acid to produce GO-COOH and purified by repeated rinsing and centrifugation. The sediment was welldispersed in 100 ml phosphate buffer (pH 7.4). For preparation of GO-Copep, Co2+ ion binding peptide (EPGHDAVP: Copep) was attached to the carboxylated GO-COOH by carbodiimide protocol.41 To activate COOH groups on GO-COOH, 50 ml of 1 mg ml-1 mg/ml GO-COOH in phosphate buffer (pH 7.4) was stirred with 20 mM EDC and 5 mM NHS in 50 ml of phosphate buffer (pH 7.4) for 10 min and bath sonicated for an hour and stirred for an additional 10 min. The solution was centrifuged at 14000 rpm for 10 min, and supernatant was discarded. The solid product was washed for 3 times to remove the excess EDC and NHS. 1 mM peptide dissolved in 50 ml phosphate buffer (pH 8) was mixed with activated GO-COOH and sonicated for an hour followed by overnight stirring. The resulting GO-peptide conjugate (GOCopep) was washed with deionized water three times by centrifuging at 14000 rpm for 10 min to remove unbounded peptides and remaining ions. Subsequently, GO-Copep was dispersed in deionized water and stored at 4 ℃. The prepared GO, GO-COOH, and GO-Copep solutions were diluted to 0.1 mg ml-1 and vacuum-filtered through mixed cellulose ester (MCE) membranes with pore size of 0.2 μm. The loading amount was ~ 0.5 mg cm-2. Characterization: The GO based membranes were characterized by X-Ray Diffraction (XRD, Rigaku D/Max-2500 powder diffractometer with Cu Kα radiation), and Scanning Electron Microscopy (Hitachi S-4800). Chemical attributes analysis by X-ray Photoelectron Spectroscopy (XPS) was performed on Thermo VG K-alpha. Silica nanotube channels were observed by Transmission Electron Microscopy (JEOL JEM-2100F). Concentration of transported ions through membranes was done by UV-Vis (JASCO, V-650) absorption measurement. Conductivity (Mettler Toledo SevenMulti S47-k) measurement of the filtrate was done to complement UV-Vis measurement. Ion transport test: Ion transport test was performed in a homemade diffusion cell. Two identical compartments with a hole of 1.5 cm in diameter in one side were used. The membrane coupons were mounted with two rubber O-rings between the two compartments facing the hole in each cell (Figure S1). The two compartments were filled with 40 ml of given aqueous solutions and pure water, respectively. The aqueous solutions investigated here were 0.1 M CoCl2, NiCl2, and CuCl2. The two compartments were continuously stirred during the experiments to ensure the concentration uniformity in each compartment. The ion permeability passing through the


Page 3 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic of the GO-Copep membrane fabrication process and scanning electron microscopic (SEM) analysis of GObased membranes. (a) Synthesis step of the GO-Copep membranes and a photograph of a GO-Copep membrane on a MCE support filter. Surface of GO membranes. (b), (c) SEM images of pristine GO and GO-Copep membranes. (b) Surface (left) and cross-section (right) of GO membranes and (c) surface (left) and cross-section (right) of GO-Copep membranes.

membranes was monitored by measuring ion concentrations of pure water side with UV-Vis spectroscopy. The conductivity of the filtrate was also observed during the transport experiments.

■ RESULT AND DISCUSSION Fabrication and characterization of GO-Copep membranes: The schematic of the GO-Copep membrane fabrication process is presented in Figure 1a. The Co2+ ion binding peptide of a sequence EPGADAVP, Copep was covalently attached to the carboxylic groups on GO surface.41 To increase efficiencies of conjugation, the surface oxygen-containing functional groups such as hydroxyl, ester, and epoxide were converted to carboxylic groups by treating aqueous GO solutions with chloroacetic acid under a strong basic condition and subsequently neutralize with HCl, generating GO-COOH. Copep was then conjugated with GO surface by a carbodiimide coupling (EDC/NHS) method. GO, GO-COOH, and GO-Copep were vacuum-filtrated onto the mixed cellulous ester (MCE) membranes with 0.2 μm pores (photograph in Figure 1a). The loading amount was ~ 0.5 mg cm-2. The surface morphology and thickness of the GO based membranes were evaluated by a Scanning Electron Microscopy (SEM). In Figures 1b and 1c, the GO and GOCopep film present the typical wrinkled lamellar of graphene-based films. The thickness of two films is ~ 2.5 μm. The gross morphologies of GO and GO-Copep were simi-

lar in Figure 1. The detailed morphological profile of GOCopep was characterized by atomic force microscopy (AFM). The thickness of GO sheets in Figure S2a was 1~2 nm corresponding to single or double-layered sheets of GO. After peptide conjugation, the GO-Copep showed a topological height of 2~3 nm due to the incorporation of octapeptide on the GO surface. The lamellar structure of GO and GO-Copep was further examined using X-ray Diffraction (XRD, shown in Figure S3). In a completely dry state, the diffraction peak was found at 2θ =11 and 10 degree for GO and GO-Copep. The corresponding interlayer spacing is 8 Å and 9 Å, respectively. Presence of octapeptide on the basal plane slightly increases the interlayer spacing in a dry state. For wet films immersed in water, hydration of GO increases the interlayer spacing and the interlayer spacing was reported as ~ 13 ±1 Å giving ~ 1 nm free space between the sheets.33 Chemical attributes of GO and GO-Copep were examined by X-ray Photoelectron Spectroscopy (XPS). GO has many oxygenated surface functional groups such as hydroxyl (C-OH), epoxide (C-O-C), and carboxylic (COOH) groups. The high resolution C1s spectra of GO in Figure 2a shows the hydroxyl and epoxide as dominant oxygen functionalities. After functionalization with Copep, peaks of hydroxyl and epoxide were significantly reduced while those of carbonyl (C=O) and carboxylic groups were increased. Amide bonds and carboxylic groups in Copep might contribute to the peaks in C1s of GO-Copep. The



a

b

c

O=C-NH (amide) and possibly N=C-NH

C-C/C=C sp2

Intensity [A.U.]

C-OH

C-O-C

C=O

C-O-C C=O

O-C=O

292

290

Intensity [A.U.]

C-C/C=C sp2

Intensity [A.U.]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 9

C-OH

GO-Copep GO

+

NH 3

N=C-NH

O-C=O

288

286

284

Binding Energy [eV]

282

280

292

290

288

286

284

282

280

Binding Energy [eV]

406

404

402

400

398

396

394

Binding Energy [eV]

Figure 2. High resolution XPS spectra of GO and GO-Copep membranes. (a) C1s spectra of GO membranes, (b) C1s spectra of GO-Copep, and (c) N1s spectrum of GO and GO-Copep membranes.

presence of nitrogen in GO-Copep (Figure 2c), which is absent in GO, also confirmed the successful functionalization with peptides. The nitrogen content was 5.6 at.%. The successful peptide conjugation was further verified by Fourier Transform Infrared Spectroscopy (FT-IR). In Figure S4, GO showed distinctive absorption bands 1055, 1240, 1580, 1700, and 3400 cm-1 from the C-O-C, C-OH, C=C, C=O, and –OH functional groups of GO, respectively. Enhanced absorption at 1630 cm-1 in GO-COOH indicated the increase of carboxylic groups (COO-) by the chloroacetic acid treatment. In GO-Copep, the absorption band at 1650 and 1240 cm-1 can be assigned the stretching vibration of C-N and –CO-NH- respectively and the small absorption around 2900 cm-1 is to that of -CH2. The strong absorption at 1650 cm-1 and the appearance of the small bands around 2900 cm-1 confirmed the successful functionalization of GO with peptides. Ion transport through GO based membranes: Transport of transition metal cations through GO based membranes were evaluated by ion permeation tests in a homemade apparatus (for details, see Supporting Information). Briefly, the two compartments of the diffusion cell were separated by a membrane under investigation, and each compartment was filled with 40 ml of aqueous solution (0.1 M CoCl2, NiCl2, and CuCl2) and pure water, respectively (Figure S2). The concentration of transition metal cations in pure water side was monitored with UV-Vis spectroscopy for 2 h. The ion permeability through the membranes was calculated from the slope of a concentration-time curve in Figure S5, following the formulation in the previous reports.13, 33 (For details, see Figure S6) Here, the ion permeability P is defined as P=D∙K where D is the diffusivity and K is the partition coefficient (or solubility coefficient). Permeability of Co2+, Ni2+, and Cu2+ ions through GO, GOCOOH, and GO-Copep membranes was on the order of 10-2 cm2 s-1 (Figure S6a), that is more than three-orders of magnitude faster than the bulk diffusivities (Figure S6b). To enumerate the degree of ion transport enhancement, we defined the relative permeability of a specific ion as the ratio of permeability through membranes over the bulk diffusivity in Figure S4b. This ultrafast transport through GO membranes has been recently reported and

attributed to the inherent property of GO membranes.33 Here our focus is on the ion selection by the functional molecules. Co2+ ion selectivity of membranes over the other ions was obtained from the ratio of relative permeability of Co2+ ion to that of the ion under comparison. In Figure 3a, GO and GO-COOH showed no noticeable selectivity for Co2+ over Ni2+ and Cu2+ ions. After functionalized with Copep, the Co2+ ion selective peptide, GOCopep demonstrated a marked increase in selectivity for Co2+ over Cu2+ ions. The selectivity of ~5 was attained for Co2+ over Cu2+ ions. On the other hand, Copep did not show clear selectivity for Co2+ over Ni2+ ions. The artificial charge selective ion channels, which select cations over anions or vice versa, have presented selectivity of several hundreds61 to thousands62 in the literature. However, selectivity between cations of similar properties was about 2~3 for artificial ion channels.13, 15, 61 The selectivity achieved in GO membranes with Copep is thus significantly higher than those reported previously from artificial solid state ion channels.13, 15, 61 We assumed that the ion selection by Copep is the origin of this superior selectivity. As negative controls, GO was functionalized with two other octapeptides, named RF8 and random peptide. RF8 has a sequence of RFRFRFRF (Arg-Phe-Arg-Phe-ArgPhe-Arg-Phe), a peptide mimicking the water selective filter in aquaporin.63 The random peptide with a sequence of GLDGSAVD (Gly-Leu-Asp-Gly-Ser-Ala-Val-Asp) is adopted from a previous report64 in which the random motif was selected randomly from a library. As shown in Figure 3b, there was no selectivity between Co2+, Ni2+ and Cu2+ ions in the transport across GO-RF8 and GO-random membranes, which confirmed the hypothesis that the remarkable selectivity for Co2+ ion in the GO-Copep membrane is rooted in the functional Copep molecules. To complement the concentration measurement by UV -Vis, conductivity measurement was performed on the pure water side for 2 h. The Co+2 ion selectivity estimated by conductivity measurement differs in numeric values from the ones by UV-Vis concentration measurement; however, the trends are similar in both evaluation methods (Figure S7). GO-Copep showed apparent Co2+ selectivity of 2.81 over Cu2+ and 1.19 for Ni2+. Apparent ion selectivity by the conductivity measurement is evaluated


Co2+ selectivity over Ni2+ and Cu2+

3 2 1 0

Ni2+

Cu2+

b Co2+ selectivity over Ni2+

6 rel

5

rel

2+

Cu2+

4 3 2 1 0

GO-Copep

GO-RF8

GO-random 2+

2+

2+

Figure 3. Permeation test of Co , Ni and Cu ions through 2+ GO-based membranes. (a) Selectivity of membranes for Co 2+ 2+ over Ni and Cu ions at each stage of functionalization. Selectivity was calculated from the ratio of relative permea2+ bility of Co ions to that of the ions compared ty=

ೝ೐೗ ௉಴೚ మశ ೝ೐೗ ௉೔೚೙

). (b) Comparison of ion selectivity of GO-Copep, 2+

2+

2+

GO-RF8 and GO-random membranes for Co , Ni , and Cu . RF8 represents a octapeptide with a sequence of RFRFRFRF and random indicates a random octapeptide of sequence 64 GLDGSAVD selected from literature.

rel

rel

Bulk MCE GO GO-COOH GO-Copep

6 5

2+

4

+2

rel

Selectivity (PCo /Pion)

5

ing Information Methods). After grafting Copep onto the nanochannel surface, Co2+ ion selectivity over Cu2+ was evaluated as 2.86 for membranes with 3.5 nm-size nanochannels in Figure 4. Nanochannels with a 7 nm diameter did not show Co2+ ion selectivity. Ion selectivity clearly increases with decreasing channel size. The function is maximized when the functional molecules fully cover the channel since there can be no area not affected by the molecules anchored on the wall. If the pore width of channel is much larger than the dimension of functional molecules attached on the wall, the central region of channel is not a region of selectivity. In a fully extended state the length of octapeptide is ~ 2.8 nm, however, it would assume a different conformation in water. The dynamic range covered by Copep would be much smaller. The dimension covered by Copep is apparently smaller than 1.7 nm in Figure 4. As in GO-Copep membranes, no Co2+ selectivity over Ni2+ was found in membranes with larger nanochannels. Figure 4 emphasizes the significance of fabricating platforms with a desirable feature size in truly mimicking the biological attributes. Through the synergistic combination of the nano-platform by GO with bio-inspired ion filters, remarkable selectivity has been achieved. We did not totally exclude the influence of membrane materials (GO vs. silica and alumina) on the selectivity at this stage, but the effect of different support might be ignored since pristine GO, GO-COOH, and GOs functionalized with negative controls did not show selectivity, supporting negligible effect of GO support on Co2+ selectivity.

rel

a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 5 of 9

Co2+ selectivity over Ni2+

Cu2+

4

smaller than by the UV-Vis method, since other ions such as counter anions and protons that exist in aqueous salt solutions also contribute to conductivity while only specific cation of interest is traced in UV-Vis spectroscopy. Especially, fast proton conduction through GO membrane has been noticed recently and would increase the conductivity of the pure water side more than expected from the cation concentration by UV-Vis. The pH of 0.1 M CoCl2, NiCl2, and CuCl2 aqueous solutions are 5.9, 6.7, and 3.9, respectively, thus the contribution from proton would be higher for CuCl2 solution lowering the apparent selectivity over Cu2+.

7 nm 3.5 nm 1.0 nm CTAB-SiNC P123-SiNC GO Figure 4. Effect of nanochannel diameter on selectivity. Ion 2+ selectivity of Copep-functionalized nanochannels for Co 2+ 2+ over Ni and Cu ions depending on the size of nanochannels. SiNC: nanochannels in silica hierarchically formed in the pores of AAO membranes.

Figure 4 presents the variation of Co2+ selectivity of Copep depending on the channel size. Copep has been functionalized onto the surface of nanochannels in different membrane platforms. The pore width of nanochannel formed in GO membrane is assumed 1 nm. The nanochannels with diameters of 3.5 nm and 7 nm were fabricated using mesoporous silica. Silica rods with cylindrical or helical-shaped pores were filled in pores of anodic alumina membrane (AAO) with nominal pore size of 0.2 μm. Depending on the size of structure guiding agent or surfactant, pores of 3.5 nm and 7 nm were hierarchically formed in silica rods in Figure S8 (for details, see Support-

Mechanism of Copep ion filter selecting Co2+ over other ions: Copep has been identified to have affinity toward Co2+ ions in a bio-panning method. It indicates that this peptide motif has a favorable binding affinity to Co2+ ions among the competing peptide sequences, but its affinity for other transition metal ions are not known. We postulated that the recognition of targets such as Co2+ ions would facilitate the transport of corresponding ions through the channels where the recognition elements sit when the dissociation constant between peptides and the ions is not too small. If the binding with the targets are too strong, the functional molecules will capture the tar-

3 2 1 0



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

gets blocking the transport through the channel. Previously, strong binding of Cu2+ ions to the carboxylic groups on GO membrane restricted the passage of Cu2+ ions due to the strong adsorption.34 To check if the same mechanism works for the Co2+ ion selectivity in GO-Copep membranes, salt adsorption has been examined. In Figure 5a, salt intake to GO-Copep membranes was measured after 12 h of immersion to the corresponding aqueous salt solution. To exclude salt adsorption by the MCE support, free-standing GO-Copep films were used for the adsorption test. Mass increases were ~ 5 wt.% for all the salt tested (CoCl2, NiCl2, and CuCl2) and no difference between salts was observed. Surface images by SEM after penetration experiments in Figures 5b~d reveal a few nanoparticles precipitated on the GO-Copep surfaces, possibly due to the salt adsorption onto the GO-Copep. However, the surfaces were relatively clean on the whole indicating that no significant adsorption happened during the penetration test. More importantly, no noticeable difference was found between different transition metal cations. The presence of element Co, Ni, or Cu on the surface of GO-Copep after the permeation test was identified in energy dispersive spectroscopy (EDS) analysis in Figure S9. Quantitative EDS analysis also revealed no difference in the amount of element Co, Ni, an Cu on the GO-Copep surface. The atomic composition of Co, Ni, and Cu was ~ 1 at % which is consistent with the elemental analysis by XPS in Figure S9b. The adsorption test in Figure 5 thus signifies that the Co2+ ion selectivity of GO-Copep might not originate from the different absorption efficiency of GO-Copep toward different ions. We believe that the preferential affinity of Copep toward Co2+ compared to Cu 2+ significantly contributes to the

Figure 5. Salt adsorption of GO-Copep membranes. (a) Mass increase by salt adsorption. GO-Copep membranes were immersed in the corresponding aqueous salt solutions for 12 h. After washing with pure water 3 times, the membranes were completely dried and mass increase was examined. (b)~(d) SEM images of GO-Copep membranes after 2 h of 2+ 2+ 2+ permeation test for (b) Co , (c) Ni ,and (d) Cu .

Page 6 of 9

selectivity of GO-Copep. To be noted is that, if Copep has no or little affinity for Cu2+, there must be a difference in binding with Copep between Cu2+ and Co2+, but the difference is not manifested by a macroscopic salt adsorption here. The binding of Co2+ with Copep might be not too strong to be captured but favorable enough to induce preferential interaction for selective transport. In biology, ion transporters contribute to the trafficking of transition metals such as Fe, Zn, Mn, Cu, Ni, and Co. The ion specificity of the transporter is endowed by specific metal binding proteins or peptide motifs. The coordination chemistry of metal ion-peptides is fundamental to this biological discrimination. Many metal binding motifs have been identified and studied. According to hard-soft acid-base theory, Cu2+ and Zn2+ ions prefer nitrogen and sulfur ligands,65 hence they mainly bind to histidine (His, H), cysteine (Cys, C) and methionine (Met, M) amino acid ligands.66 Co2+ and Ni2+ ions form stable binding with carboxylate and nitrogen,65 thus showing preference to glutamate (Glu, E), aspartate (Asp, D) and histidine (His, H) residues.67 Copep with EPGHDAVP sequence has histidine (H), and acidic residue (glutamate E and aspartate D) as in most Co2+ ion binding motif. It also shows similar amino acid residues with a VXLHVLGXAL (Val-Xaa-Leu-His-Val-Leu-Gly-Xaa-AlaLeu) sequence found in cobalt transporters of COT1 and NhlF.68 Along with this ligand preference, coordination geometry might be the origin of Co2+ ion selectivity since not only chemical but also geometric components constitute the recognition. The coordination geometry is closely related to the d-block electronic configuration, and it has been theoretically calculated and experimentally observed in previous reports.67, 69, 70 The Co2+ and Ni2+ ions showed preference for octahedral,67, 69 Zn2+ ions for tetrahedral,69 and Cu2+ ions for square planar coordination geometry.69, 70 In Zn2+ and Mn2+ transporter systems, Mn2+ binding proteins have 5 coordination ligands while Zn2+ show 4 coordination numbers, which is assumed a primary criteria for ion selectivity in these systems.71 The exact mechanism and coordination geometry how Copep recognizes Co2+ ions are not totally clear and beyond the scope of this study. However, GO-Copep membranes showed clear selectivity for Co2+ over Cu2+ while there was no noticeable preference over Ni2+, probably due to the chemical similarity between Co2+ and Ni2+. In the discussion above, cobalt and nickel ions share similar taste on the ligand preference and coordination geometry. In biological metal transporter systems, Co2+ and Ni2+ ions are grouped together and NiCoT (nickel-cobalt transporter) families transit Co2+ ions as well as Ni2+ ions.72 On the other hand, Zn2+ and Mn2+ ions belong to the same transporter systems. In Figure S7, GO-Copep membranes showed transport preference on the order of Co2+ ≥Ni2+∼ Mn2+>Zn2+ >>Cu2+ in a conductivity measurement method. It may reflect the chemical similarity on the ligand preference and coordination geometry between ions.

■ CONCLUSION


Page 7 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We have explored the feasibility of producing ionselective membranes by integrating GO sheets and bioinspired ion selective filters. When peptides with cobalt ion recognition capability were applied as ion selective filters anchored on the surface of GO sheets, the resulting GO-Copep membranes showed Co2+ ion selectivity over other transition metal cations. Notably, the selectivity over Cu2+ ion is ~5 folds, which is more than twice of the previously reported values for selectivity between cations. 13, 15, 61 The same peptides functionalized on the larger nanochannels have lower selectivity and nanochannels larger than 6 nm lost selectivity completely. It suggests that the novel platform with desirable dimensions is the prerequisite for truly mimicking biological attributes. This study thus demonstrates the first remarkably encouraging results for using biomimetic recognition chemistry for selective ion transport and synergistic combination of such chemistry with novel nanomaterials like GO.

ASSOCIATED CONTENT Supporting Information. A photograph of diffusion cell. AFM, XRD of GO and GO-Copep films FT-IR of GO, GOCOOH, and GO-Copep. Time evolution of concentration for ions under investigation. Ion permeability and relative permeability through membranes. Conductivity change of solutions. TEM images of silica nanochannels. SEM-EDS spectra of GO-Copep membranes after ion permeation tests. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail addresses: [email protected] (Y.J.L.)

Notes The authors declare no completing financial interest.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program (grant no. NRF-2012R1A1A1009029 and NRF2012R1A1A2021678) and Pioneer Research Center Program (grant no. NRF-2012-0009577) through the National Research Foundation of Korea (NRF).

REFERENCES (1) Baker, L. A.; Bird, S. P. Nat. Nanotechnol.2008, 3, 73-74. (2) Nishizawa, M.; Menon, V. P.; Martin, C. R. Science 1995, 268, 700-702. (3) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M. S.; Lee, S. B. Adv. Mater. 2001, 13, 1351-1362. (4) Ali, M.; Yameen, B.; Neumann, R.; Ensinger, W.; Knoll, W.; Azzaroni, O. J. Am. Chem. Soc. 2008, 130, 16351-16357. (5) Vlassiouk, I.; Kozel, T. R.; Siwy, Z. S. J. Am. Chem. Soc. 2009, 131, 8211-8220. (6) Spohr, R.; Reber, N.; Wolf, A.; Alder, G. M.; Ang, V.; Bashford, C. L.; Pasternak, C. A.; Omichi, H.; Yoshida, M. J. Control. Release 1998, 50, 1-11. (7) Sinha, P. M.; Valco, G.; Sharma, S.; Liu, X. W.; Ferrari, M. Nanotechnology 2004, 15, S585-S589.

(8) Majd, S.; Yusko, E. C.; Billeh, Y. N.; Macrae, M. X.; Yang, J.; Mayer, M. Curr. Opin. Biotechnol. 2010, 21, 439-476. (9) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Marinas, B. J.; Mayes, A. M. Nature 2008, 452, 301-310. (10) Logan, B. E.; Elimelech, M. Nature 2012, 488, 313-319. (11) Xu, T. W. J. Membr. Sci. 2005, 263, 1-29. (12) Ali, M.; Ramirez, P.; Mafe, S.; Neumann, R.; Ensinger, W. Acs Nano 2009, 3, 603-608. (13) Balme, S.; Janot, J. M.; Berardo, L.; Henn, F.; Bonhenry, D.; Kraszewski, S.; Picaud, F.; Ramseyer, C. Nano Lett. 2011, 11, 712716. (14) Ali, M.; Nasir, S.; Nguyen, Q. H.; Sahoo, J. K.; Tahir, M. N.; Tremel, W.; Ensinger, W. J. Am. Chem. Soc. 2011, 133, 1730717314. (15) Cazacu, A.; Legrand, Y. M.; Pasc, A.; Nasr, G.; Van der Lee, A.; Mahon, E.; Barboiu, M. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 8117-8122. (16) Dekker, C. Nat. Nanotechnol. 2007, 2, 209-215. (17) Hille, B., Ion Channels of Excitable Membranes. 3rd ed.; Sinauer Associates, Sunderland, MA,: 2001. (18) Ariga, K. Chem. Rec. 2004, 3, 297-307. (19) Fyles, T. M. Chem. Soc. Rev. 2007, 36, 335-347. (20) Meier, W.; Nardin, C.; Winterhalter, M. Angew. Chem., Int.. Ed. 2000, 39, 4599-4602. (21) Kumar, M.; Grzelakowski, M.; Zilles, J.; Clark, M.; Meier, W. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 20719-20724. (22) Gonzalez-Perez, A.; Stibius, K. B.; Vissing, T.; Nielsen, C. H.; Mouritsen, O. G. Langmuir 2009, 25, 10447-10450. (23) Hou, X.; Guo, W.; Jiang, L. Chem. Soc. Rev. 2011, 40, 23852401. (24) Healy, K.; Schiedt, B.; Morrison, A. P. Nanomedicine 2007, 2, 875-897. (25) Guo, W.; Cao, L. X.; Xia, J. C.; Nie, F. Q.; Ma, W.; Xue, J. M.; Song, Y. L.; Zhu, D. B.; Wang, Y. G.; Jiang, L. Adv. Funct. Mater. 2010, 20, 1339-1344. (26) Hall, A. R.; Scott, A.; Rotem, D.; Mehta, K. K.; Bayley, H.; Dekker, C. Nat. Nanotechnol. 2010, 5, 874-877. (27) Lee, S. B.; Mitchell, D. T.; Trofin, L.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. Science 2002, 296, 2198-2200. (28) Cao, L. X.; Guo, W.; Ma, W.; Wang, L.; Xia, F.; Wang, S. T.; Wang, Y. G.; Jiang, L.; Zhu, D. B. Energy Environ. Sci. 2011, 4, 2259-2266. (29) Vlassiouk, I.; Apel, P. Y.; Dmitriev, S. N.; Healy, K.; Siwy, Z. S. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 21039-21044. (30) Vilozny, B.; Actis, P.; Seger, R. A.; Vallmajo-Martin, Q.; Pourmand, N. Anal. Chem.2011, 83, 6121-6126. (31) Ali, M.; Yameen, B.; Cervera, J.; Ramirez, P.; Neumann, R.; Ensinger, W.; Knoll, W.; Azzaroni, O. J. Am. Chem. Soc. 2010, 132, 8338-8348. (32) Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K. Science 2012, 335, 442-444. (33) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Science 2014, 343, 752-754. (34) Sun, P.; Zhu, M.; Wang, K.; Zhong, M.; Wei, J.; Wu, D.; Xu, Z.; Zhu, H. Acs Nano 2013, 7, 428-437. (35) Han, Y.; Xu, Z.; Gao, C. Adv. Funct. Mater. 2013, 23, 36933700. (36) Hu, M.; Mi, B. X. Environ. Sci. Technol. 2013, 47, 37153723. (37) Huang, H. B.; Song, Z. G.; Wei, N.; Shi, L.; Mao, Y. Y.; Ying, Y. L.; Sun, L. W.; Xu, Z. P.; Peng, X. S. Nat. Commun. 2013, 4. (38) Raidongia, K.; Huang, J. J. Am. Chem. Soc. 2012, 134, 16528-16531.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(39) Kim, H. W.; Yoon, H. W.; Yoon, S. M.; Yoo, B. M.; Ahn, B. K.; Cho, Y. H.; Shin, H. J.; Yang, H.; Paik, U.; Kwon, S.; Choi, J. Y.; Park, H. B. Science 2013, 342, 91-95. (40) Sun, P. Z.; Zheng, F.; Zhu, M.; Song, Z. G.; Wang, K. L.; Zhong, M. L.; Wu, D. H.; Little, R. B.; Xu, Z. P.; Zhu, H. W. Acs Nano 2014, 8, 850-859. (41) Wang, H. B.; Zhang, Q.; Chu, X.; Chen, T. T.; Ge, J.; Yu, R. Q. Angew. Chem., Int.. Ed. 2011, 50, 7065-7069. (42) Zhang, L. M.; Xia, J. G.; Zhao, Q. H.; Liu, L. W.; Zhang, Z. J. Small 2010, 6, 537-544. (43) Calvo, A.; Yameen, B.; Williams, F. J.; Soler-Illia, G.; Azzaroni, O. J. Am. Chem. Soc. 2009, 131, 10866-10868. (44) Ali, M.; Schiedt, B.; Healy, K.; Neumann, R.; Ensinger, A. Nanotechnology 2008, 19, 085713. (45) Han, C. P.; Hou, X.; Zhang, H. C.; Guo, W.; Li, H. B.; Jiang, L. J. Am. Chem. Soc. 2011, 133, 7644-7647. (46) Ali, M.; Neumann, R.; Ensinger, W. Acs Nano 2010, 4, 7267-7274. (47) Siwy, Z.; Trofin, L.; Kohli, P.; Baker, L. A.; Trautmann, C.; Martin, C. R. J. Am. Chem. Soc. 2005, 127, 5000-5001. (48) Ali, M.; Ramirez, P.; Tahir, M. N.; Mafe, S.; Siwy, Z.; Neumann, R.; Tremel, W.; Ensinger, W. Nanoscale 2011, 3, 18941903. (49) Jagerszki, G.; Gyurcsanyi, R. E.; Hofler, L.; Pretsch, E. Nano Lett. 2007, 7, 1609-1612. (50) Sexton, L. T.; Horne, L. P.; Sherrill, S. A.; Bishop, G. W.; Baker, L. A.; Martin, C. R. J. Am. Chem. Soc. 2007, 129, 1314413152. (51) Kohli, P.; Harrell, C. C.; Cao, Z. H.; Gasparac, R.; Tan, W. H.; Martin, C. R. Science 2004, 305, 984-986. (52) Sarikaya, M.; Tamerler, C.; Jen, A. K. Y.; Schulten, K.; Baneyx, F. Nat. Mater.2003, 2, 577-585. (53) Hersel, U.; Dahmen, C.; Kessler, H. Biomaterials 2003, 24, 4385-4415. (54) Shin, H.; Jo, S.; Mikos, A. G. Biomaterials 2003, 24, 43534364. (55) Chen, C. L.; Rosi, N. L. Angew. Chem., Int.. Ed. 2010, 49, 1924-1942. (56) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665-668. (57) Lee, Y. J.; Yi, H.; Kim, W. J.; Kang, K.; Yun, D. S.; Strano, M. S.; Ceder, G.; Belcher, A. M. Science 2009, 324, 1051-1055. (58) Oh, D. Y.; Dang, X. N.; Yi, H. J.; Allen, M. A.; Xu, K.; Lee, Y. J.; Belcher, A. M. Small 2012, 8, 1006-1011.

Page 8 of 9

(59) Lee, S. K.; Yun, D. S.; Belcher, A. M. Biomacromolecules 2006, 7, 14-17. (60) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339-1339. (61) Barboiu, M.; Le Duc, Y.; Gilles, A.; Cazade, P. A.; Michau, M.; Legrand, Y. M.; van der Lee, A.; Coasne, B.; Parvizi, P.; Post, J.; Fyles, T. Nat. Commun. 2014, 5, 4142. (62) Zhou, X. B.; Liu, G. D.; Yamato, K.; Shen, Y.; Cheng, R. X.; Wei, X. X.; Bai, W. L.; Gao, Y.; Li, H.; Liu, Y.; Liu, F. T.; Czajkowsky, D. M.; Wang, J. F.; Dabney, M. J.; Cai, Z. H.; Hu, J.; Bright, F. V.; He, L.; Zeng, X. C.; Shao, Z. F.; Gong, B. Nat. Commun. 2012, 3, 949. (63) Hub, J. S.; De Groot, B. L. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 1198-1203. (64) Huang, Y.; Chiang, C. Y.; Lee, S. K.; Gao, Y.; Hu, E. L.; De Yoreo, J.; Belcher, A. M. Nano Lett. 2005, 5, 1429-1434. (65) Roat-Malone, R. M., Bioinorganic Chemistry: A short course John Wiley & Sons: 2003. (66) Sun, X. S.; Xiao, C. L.; Ge, R. G.; Yin, X. F.; Li, H.; Li, N.; Yang, X. Y.; Zhu, Y.; He, X.; He, Q. Y. Proteomics 2011, 11, 32883298. (67) Okamoto, S.; Eltis, L. D. Metallomics 2011, 3, 963-970. (68) Komeda, H.; Kobayashi, M.; Shimizu, S. Proc. Natl. Acad. Sci. U. S. A. 1997, 94, 36-41. (69) Rulisek, L.; Havlas, Z. J. Am. Chem. Soc. 2000, 122, 1042810439. (70) Viles, J. H.; Cohen, F. E.; Prusiner, S. B.; Goodin, D. B.; Wright, P. E.; Dyson, H. J. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 2042-2047. (71) Ma, Z.; Jacobsen, F. E.; Giedroc, D. P. Chem. Revi.2009, 109, 4644-4681. (72) Degen, O.; Kobayashi, M.; Shimizu, S.; Eitinger, T. Arch. Microbiol. 1999, 171, 139-145.


Page 9 of 9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Insert Table of Contents artwork here


9

Biomimetic Selective Ion Transport through Graphene Oxide

Recommend Documents