Solubilization of Single-Walled Carbon Nanotubes Using a Peptide

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Solubilization of Single-Walled Carbon Nanotubes Using a Peptide Aptamer in Water below the Critical Micelle Concentration Zha Li,† Tomoshi Kameda,‡,§ Takashi Isoshima,† Eiry Kobatake,∥ Takeshi Tanaka,⊥ Yoshihiro Ito,*,†,§ and Masuki Kawamoto*,†,§,# †

Nano Medical Engineering Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Computational Biology Research Center, National Institute of Advanced Industrial Science and Technology, Koto, Tokyo 135-0064, Japan § Emergent Bioengineering Materials Research Team, RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ∥ Department of Environmental Chemistry and Engineering, Graduate School of Interdisciplinary Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8501, Japan ⊥ Carbon Nanomaterials Research Group, Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8562, Japan # Photocatalysis International Research Center, Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ‡

S Supporting Information *

ABSTRACT: The solubilizing ability of single-walled carbon nanotubes (SWCNTs) in water with several dispersants was investigated. Among the dispersants, including low-molecular-weight surfactants, peptides, DNA, and a water-soluble polymer, the peptide aptamer, A2 (IFRLSWGTYFS), exhibited the highest dispersion capability below the critical micelle concentration at a concentration of 0.02 w/v%. The dispersion of supernatant aqueous solution of SWCNTs containing aptamer A2 was essentially unchanged for several months after high-speed ultracentrifugation and gave rise to an efficient and stable dispersion of the SWCNTs in water. From the results of isothermal titration calorimetry and molecular dynamics simulations, the effective binding capability of A2 was due to π−π interaction between aromatic groups in the peptide aptamer and the side walls of SWCNTs. Interestingly, the peptide aptamer showed the possibility of diameter separation of semiconducting SWCNTs using a uniform density gradient ultracentrifuge. These phenomena are encouraging results toward an effective approach to the dispersion and separation of SWCNTs.

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demonstrated that surfactants,4,5 polymers,6 DNA,7−10 proteins,11 and peptides12−14 exhibit dispersants of SWCNTs in water. Peptides are regarded as promising noncovalent dispersants for SWCNTs in water because the peptide backbone forms a folded helical conformation stabilized by hydrogen bonds. The amphiphilic helices of the peptides spontaneously assemble to minimize the exposure of a hydrophobic region and to present a hydrophilic region to the aqueous interface. An interesting approach to the computational design of peptides, such as Hex (AEGESALEYGQQALEKGQLALQAGRQALKA), is to use self-organizing dispersants that wrap SWCNTs.14 Both molecular recognition and suitable molecular packing interactions of Hex gave rise to virus-like assemblies on the surfaces of SWCNTs. Biological screening using phage-displayed peptide libraries has been used to evaluate the interaction

INTRODUCTION Single-walled carbon nanotubes (SWCNTs) have received a great deal of attention in the past two decades; their extreme mechanical strength and thermal, kinetic, and electrical properties are of great interest for carbon-based devices.1 These materials are expected to find application in the fields of energy, environment, medicine, and textiles. However, SWCNTs form bundles owing to van der Waals interactions and result in poor solubility; biological applications in water are thus very limited. To overcome this difficulty, promising approaches have been developed through chemical modification of the surface of SWCNTs or by the noncovalent attachment of dispersants. The surface functionalization of SWCNTs is effective for dispersion in an aqueous solution,2 though the intrinsic electronic and mechanical properties of the nanotubes are adversely affected.3 On the other hand, noncovalent methods have an advantage of preserving π-conjugated structures and electrical properties of SWCNTs. The solubilization of SWCNTs using noncovalent chemical interactions have © 2015 American Chemical Society

Received: December 9, 2014 Revised: February 25, 2015 Published: March 6, 2015 3482

DOI: 10.1021/la504777b Langmuir 2015, 31, 3482−3488

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Langmuir Table 1. Lists of Abbreviations of the Dispersants abbreviation peptide

surfactant

DNA

polymer

A2 P1 Hex SDBS SC SDS GT20 T30 salmon DNA PVP

our selected peptide aptamer (IFRLSWGTYFS)16 selected by a phage display (HWSAWWIRSNQS)12 computer-designed peptide (AEGESALEYGQQALEKGQLALQAGRQALKA)14 sodium dodecyl benzenesulfonate sodium cholate sodium dodecyl sulfate (GT)2017 T308 salmon genomic DNA polyvinylpyrrolidone6

between the peptides and SWCNTs.15,16 Previously, we successfully demonstrated that a ribosome display from a diverse random library is applied for selecting peptide aptamers with a higher binding affinity to SWCNTs.16 In this study, the dispersion ability of SWCNTs in water using different types of dispersants including a polymer, DNA, surfactants, and peptides was explored below the critical micelle concentration (CMC) of each dispersant. We found that peptide aptamer A2 shows the highest dispersion capability below the CMC at a low concentration of 0.02 w/v%. Binding enthalpies between SWCNTs and dispersants were determined with isothermal titration calorimetry (ITC) in water. We found that the binding enthalpy of A2 in an aqueous solution is 3 and 5 times larger than those of the conventional low-molecularweight dispersant and the computer-designed peptide, respectively. Furthermore, molecular dynamics (MD) simulations revealed that the high binding strength of A2 is attributed to the π−π interaction between an aromatic unit of amino acid and a side wall of SWCNT. We also prepared a peptide-assisted dispersion of SWCNTs in water using a uniform density gradient ultracentrifuge (U-DGU) and demonstrated the possibility of a chiral separation of mixtures of metallic and semiconducting SWCNTs.

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Figure 1. Schematic illustrations of the preparation of solubilized SWCNTs using a conventional ultracentrifuge (a) and a uniform density gradient ultracentrifuge (U-DGU) (b). (i) Ultrasonication, (ii) ultracentrifugation, (iii) fraction of supernatant collected, (iv) addition of iodixanol to the supernatant, and (v) ultracentrifugation. Original solution before U-DGU treatment (1) and upper (2) and lower (3) layers of the supernatant after U-DGU treatment. for 15 min after sonication. The supernatant was homogeneous and gray (Figure 1a). When iodixanol is added to mixtures of surfactants and water, a spontaneous density gradient is observed after ultracentrifugation.18 In this method, SWCNTs moved toward their isopycnic points due to the difference in the density of metallic and semiconducting SWCNTs. Iodixanol (2 mL) was slowly added to the supernatant of surfactants and SWCNTs in water (4 mL, Figure 1a(iv)), and the resultant solution (Solution 1) became homogeneous (Figure 1b). Five milliliters of the original solution was centrifuged using a uniform density gradient ultracentrifuge (U-DGU) at 417 000g for 30 min, and the colored solution was collected from the upper (solution 2) and lower (solution 3) layers of the supernatant. Spectroscopic and Microscopic Characterization. Absorption spectra of dispersed SWCNTs were measured using a Jasco V670DS spectrometer with 10 mm path length disposable cells. Transmission electron microscopy (TEM) measurements for mixtures of SWCNTs and dispersions in aqueous solutions were conducted on a JEOL JEM2100F/SP. The samples were prepared by casting the solution onto a carbon-coated TEM grid (JEOL Cu 200 mesh) and then drying in air. Isothermal Titration Calorimetry (ITC). ITC is a convenient method for finding the binding interactions of biological materials.19−22 When free ligands are titrated into biological macromolecules, a calorimetric response occurs after the binding process. Binding enthalpies between SWCNTs and dispersants were determined with an ITC (GE MicroCal iTC200 System) at 25 °C. The volumes of the sample cell and injection syringe were 200 and 40 μL, respectively. Each titration consisted of 20 continuous injections of a dispersant of 2 μL, except for the first injection of 0.2 μL. During titration, the solution was stirred at 1000 rpm. Titration intervals were 60 s for A2 and 120 s for SDBS and Hex. The differential power of titrations was recorded as negative (μcal/s) when heat was released in an exothermic process and as positive when heat was absorbed in an endothermic process. The concentrations of A2, Hex, and SDBS solutions in the injection syringe were 0.1, 0.5, and 1.2 w/v%, respectively. Preparation of SWCNT/Dispersant Mixtures in the Cell for ITC Measurements. SWCNT/A2 Solution. Raw HiPCO SWCNTs (0.015 w/v%) were predispersed in water (1.5 mL) containing peptide aptamer A2 at a concentration of 0.004 w/v%. The solution was sonicated for 10 min using a tip-type ultrasonic homogenizer in a

EXPERIMENTAL SECTION

Materials. Raw HiPCO SWCNTs (12.3 wt %), synthetic peptides, and salmon genomic DNA salt were purchased from Nano Integris (Evanston, IL, USA), Pi Proteomics LLC (Huntsville, AL, USA), and Sigma-Aldrich (St.Louis, MO, USA), respectively. T30 and GT20 were obtained from Operon. Co. Ltd (Tokyo, Japan). All of the surfactants and PVP were purchased from Wako Pure Chemical Industries, Ltd (Osaka, Japan). Table 1 shows abbreviations of the dispersants used in this study. Critical Micelle Concentration (CMC) Measurements. Pyrene was used as a fluorescence probe to detect the CMC of aqueous solutions. The dispersants were prepared at concentrations ranging from 20 to 500 μM, and then pyrene was dissolved in methanol as a stock solution of 60 μM. The final concentration of the probe was 0.5 μM, and the excitation wavelength was 334 nm. The intensity ratio (I1/I3) of the first (I1) and third (I3) vibronic bands of the emission spectra of pyrene, at 373 and 384 nm, respectively, was used to detect the formation of microdomains of micelles. Dispersion of Single-Walled Carbon Nanotubes (SWCNTs). A typical experimental procedure is described below (Figure 1). Ten milligrams of raw HiPCO SWCNTs was predispersed in water (4 mL) containing dispersants at a concentration of 0.02 w/v%. The solution was sonicated for 15 min using a tip-type ultrasonic homogenizer in a water bath at 25 °C. During sonication, the aqueous solution changed from colorless to black, indicating the dispersion of SWCNTs. To remove big bundles, impurities of catalysts, and amorphous carbon, the resultant solution was centrifuged via ultracentrifugation at 417 000g 3483

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Langmuir water bath at room temperature. The resultant colloidal mixture was centrifuged at 1000g for 5 min; the SWCNT/A2 solution was collected from the upper layer of the supernatant. SWCNT/Hex Solution. The solution was prepared using a method similar to that described for the SWCNT/A2 solution except 0.07 w/v % of SWCNTs was dispersed in water (1.5 mL) containing Hex at a concentration of 0.1 w/v%. SWCNT/SDBS Solution. SWCNTs (0.15 w/v%) were predispersed in water (2 mL) containing SDBS at a concentration of 0.2 w/v%. The solution was treated by sonication using the tip-type ultrasonic homogenizer in a water bath at room temperature for 10 min. The resultant colloidal mixture was centrifuged at 3000g for 20 min; the solution was collected from the upper layer of the supernatant. Molecular Dynamics (MD) Simulations. The microscopic states of interaction between an aptamer A2 and a SWCNT were studied in a water box using MD simulations. The 100 ns calculations were performed three times. The system contained SWCNT, A2, one chloride ion, and 14 678 water molecules. The structure of a (7,6) SWCNT with a length of 46 Å was generated by Tubegen 3.4.23 The SWCNT was described using the general AMBER force field (GAFF).24 The charges of the carbon atoms of the SWCNT were set to 0.0. The polypeptide of A2 was described using the AMBER force field (param99SB).25 Water molecules were described using the TIP3P model.26 The initial conformation for MD simulations was prepared in the following steps. First, the energy minimization of the extended form of A2 was carried out under vacuum, which lead to the peptide in a condensed form. The conformation was also placed 10 Å from the surface of the SWCNT. Finally, the A2 and SWCNT system was solvated by water and ion molecules. The three 100 ns simulations were conducted with an NPT ensemble (300 K, 1 bar) in an orthorhombic dodecahedral box of approximately 86 Å per side. The temperature was controlled using a Langevin thermostat with a viscosity of 1 p/s. The pressure was controlled by a Parrinello−Rahman barostat27 with a relaxation time of 2.0 ps. The electrostatics was handled using the particle mesh Ewald (PME) method with a 10 Å cutoff distance. The van der Waals interactions were expressed by the twin range cutoff method with cutoff distances of 10.0 and 12.0 Å. The covalent bonds for the hydrogen atoms in A2 were constrained using the linear constraint solver (LINCS).28 The covalent bonds in water were constrained using the SETTLE algorithm.29 The integration time step was 2 fs. The simulations were conducted using the GROMACS 4.6.4 simulator.30

However, the addition of surfactant-dispersed SWCNTs and their absorption spectra were obtained. The multiple peaks in the absorption spectra can be attributed to the existence of multiple SWCNTs with different chiralities.31 To estimate the number of dispersed SWCNTs, we integrated the absorbance between 400 and 940 nm, corresponding to the first optical transition of metallic SWCNTs (M11, 400−620 nm) and the second absorption band of the semiconducting SWCNTs (S22, 620−940 nm), respectively (Figure 2).32

Figure 2. Integration of absorbance between 400 and 940 nm, corresponding to the first optical transition of metallic SWCNTs and the second absorption band of the semiconducting SWCNTs, respectively, at various dispersants below the critical micelle concentration.

The order of dispersing capability was A2 > Hex > SDBS > P1 > SC > GT20 > T30 > SDS > salmon DNA > PVP. The dispersion ability of low-molecular-weight surfactants depended on the CMC. The synthetic polymer and DNAs (GT20, T30, and salmon DNA) showed low dispersion capabilities compared with surfactants. Because they have higher CMCs, they had less dispersion ability. The amount of dispersed SWCNTs by A2 was larger than that by SDBS,33 which is generally used to prepare dispersions of SWCNTs, at the same concentration of 0.02 w/v%. Considering the low molar CMC of A2 (0.15 mM) compared to that of SDBS (1.2 mM) (Table 2), the dispersion ability of A2 was observed not only because of their micelle formation but also because of their affinity toward SWCNTs. Among the peptides, A2 had the highest dispersion ability. Figure 3c shows the TEM image of insoluble raw SWCNTs, which forms large bundles. In contrast, well-dispersed SWCNTs are clearly observed using peptide aptamer A2 even below the CMC (Figure 3d).

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RESULTS AND DISCUSSION Table 2 lists CMCs of the dispersants. The CMCs of peptides are lower than those of DNAs, PVP, and low-molecular-weight surfactants. To investigate the dispersion abilities of dispersants, the visible−near-infrared absorption spectra of the supernatant solutions were evaluated below the CMC. In the absence of the dispersants, no absorption was obtained from SWCNTs. Table 2. Critical Micelle Concentration of the Dispersants dispersant

CMC (mM)

CMC (w/v%)

A2 P1 Hex SDBS SC SDS GT20 T30 salmon DNA PVP

0.15 0.2 0.08 1.2 9−14 8 0.16 0.16

0.02 0.03 0.02 0.04 0.4−0.6 0.23 0.2 0.15 0.12 0.5

0.12

Figure 3. SWCNTs in water before (a) and after dispersion (b) using the peptide aptamer A2 below the CMC. TEM images of raw SWCNTs (c) and dispersed SWCNTs (d). 3484

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Figure 4. Titration behavior of aqueous solutions of dispersants in SWCNTs/dispersant mixtures in water at 25 °C. (a) A2, (b) SDBS, and (c) Hex. (i) The blank titration, (ii) the raw ITC response, and (iii) the integrated enthalpy of the binding interaction. Red lines in (iii) indicate nonlinear least-squares fitting of experimental data.

Both dispersants exhibited large values of ΔH in Figure 4b,c. We found that the ΔH value of A2 is 3 and 5 times larger than those of SDBS and Hex, respectively. The large ΔH value of A2 is mainly due to π−π interactions between SWCNTs and the peptide aptamer. A2 (IFRLSWGTYFS) is composed of 11 amino acids containing four aromatic units of phenylalanine (F), tyrosine (Y), and tryptophan (W). In contrast, Hex (AEGESALEYGQQALEKGQLALQAGRQALKA) has 30 amino acids with only one aromatic unit of Y. The ITC results suggest that specific interactions between the aromatic units of amino acids and the side walls of SWCNTs play an important role in imparting a high binding strength. We also found that the introduction of hydrophobic units in the dispersants enhances the dispersion properties of SWCNTs. SDBS is an amphiphilic aromatic compound with hydrophobic units consisting of long alkyl chains. Hex also possesses 12 hydrophobic units that contain alanine (A) and leucine (L). Because of the hydrophobicity of SWCNT side walls, hydrophobic units in the dispersants enhance their binding affinity. The large binding strength of A2 is an encouraging result for MD simulations. Three MD simulations on a time scale of 100 ns from the initial conformation were carried out. The initial distance between A2 and SWCNT was 10 Å. Figure S1 shows a time series for the solvent-accessible surface area (SASA) of the SWCNTs and the aromatic amino acids (F2, W6, Y9, and F10) in A2 (IFRLSWGTYFS) and the total potential energy. The SASA values and the total potential energy decayed in the time period of 0−10 ns, which confirmed that the A2 peptide bound the SWCNTs in the early stages of the simulations. Furthermore, no dissociation occurred during the trajectory after A2 was attached to SWCNT once. Figure 5 shows a representative conformation of A2 on SWCNT; almost all of the amino acid side chains in A2 are bound to the side wall of the SWCNT. In particular, the binary composite gave rise to specific interactions between the guanidinium group of arginine (R) and aromatic groups of F and W in A2 with the sidewalls of SWCNT (Figure 5). These results are consistent with the view that R units in polyarginine37 and lysozyme38 are bound to SWCNTs. As we mentioned above, aptamer A2 contains four aromatic units and one guanidinium unit in the side chains. MD simulations also revealed that the high binding strength of A2 is

To evaluate the effective dispersion capability of aptamer A2, we used ITC to estimate a binding affinity. We titrated aqueous solutions of dispersants into SWCNT/dispersant mixtures in water (Figure 4). The blank titration of A2 at a concentration of 0.1 w/v% into pure water exhibited an exothermic response corresponding to the effect of dilution (Figure 4a (i)). The raw ITC response for A2 in the SWCNT/A2 mixture showed an exothermic response due to the binding behavior (Figure 4a (ii)). After subtracting the information from the blank titration, the integrated change in heat, which was the change in enthalpy during the binding action, was plotted as a function of the injected volume of peptide aptamer A2 (Figure 4a (iii)).19 Although the concentration of A2 in the mixture was 5 times lower than the CMC (0.02 w/v%), the sigmoidal character indicates single-site homogeneous binding behavior in the welldefined ITC standard curve.22 The value of the binding enthalpy (ΔH) was estimated to be −2.2 kcal/mol according to an extrapolation of the fitting curve to the y intercept. We found that the ΔH value of A2 is significantly higher than those of nucleobases (−0.13 to −0.32 kcal/mol)34 and DMF/NMP (−0.002 to −0.004 kcal/mol).35 It is suggested that aptamer A2 shows only the binding action without aggregation and dissociation of SWCNTs (Figure 4a). We also evaluated the binding behaviors of conventional, low-molecular-weight dispersant SDBS33 and computer-designed peptide Hex14 using a similar manner to A2 (Figure 4b,c). The blank titration of SDBS was an endothermic reaction due to the π−π stacking of phenyl groups in water (Figure 4b (i)).36 A clear sigmoidal curve was obtained for the SWCNT/ SDBS mixture, which evidently results from substantial binding behavior during the raw ITC response (Figure 4b (iii)). A ΔH value of −0.63 kcal/mol was achieved by extrapolation of the fitting curve. Conversely, the blank titration of Hex exhibited an endothermic reaction in the initial state (Figure 4c (i)). Hex has been designed to show hierarchical control of helix−helix interactions, which stabilize the secondary structure of the peptide without SWCNTs.14 The endothermic events originate from self-assembled architecture during the blank titration. From the fitting curve, the value of ΔH for the binding interaction between SWCNTs and Hex is −0.39 kcal/mol (Figure 4c (iii)). In Figure 2, SDBS and Hex shows good dispersion capabilities such that high binding strengths were expected. 3485

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Figure 5. Representative conformation of A2 on a SWCNT. (a) Side view and (b) front view.

attributable to π−π interactions with SWCNT. In contrast, Hex has only one aromatic unit of Y and exhibits good dispersion capabilities of SWCNTs because of the hydrophobicity of amino acids in the side chains. These results suggest that the noncovalent binding behavior of SWCNTs is sensitive to the functional groups of dispersants. SWCNTs can exhibit two types of electric behavior, metallic and semiconducting in nature, depending on their chirality. Because electric and optical properties of SWCNTs originate from the chirality, the mixture of metallic and semiconducting SWCNTs hinders intrinsic electrical properties.39,40 Though many efforts have been made to realize the selective separation of metallic and semiconducting SWCNTs, promising techniques should be developed not only with the fabrication of highperformance nanoscaled molecular devices but also with a further understanding of the fundamental properties of SWCNTs. We evaluated the separation of metallic and semiconducting SWCNTs at a low dispersant concentration using a U-DGU method. U-DGU is one of the most effective techniques for the selection of a single chirality of SWCNTs.41 Figure 6 shows

Figure 7. Absorption spectra of the supernatant of A2 (a) and P1 (b) before and after U-DGU treatment. Solution 1: the original solution before U-DGU treatment. Solutions 2 and 3: upper (2) and lower (3) layers of the supernatant after U-DGU treatment. Normalization at 940 nm.

(7,6), and (8,6) chiralities (diameters of 8.29, 8.40, 8.95, and 9.66 Å) of semiconducting SWCNTs in a near-infrared (NIR) region.9 This means that the relatively large-diameter semiconducting SWCNTs existed in solution 1 before U-DGU treatment. The S22 band (620−940 nm) region based on the semiconducting SWCNTs in the lower layer (solution 3) treated with A2 shifted to a longer-wavelength region than that in the upper layer (solution 2). This red shift of absorption spectra in the NIR region is owing to electron delocalization and/or fast nonradiative recombination, resulting from both aggregation and reduced coverage of the dispersant for largediameter semiconducting SWCNTs. Calvaresi et al. reported that large-diameter SWCNTs tend to exist in the bottom layer, probably owing to their high density.11 In our case, the diameter of SWCNTs in solution 3 was larger than that in solution 2 after U-DGU treatment using aptamer A2. We roughly estimated that solution 3 in the bottom part contains SWCNTs in the diameter range of 8−10 Å. No significant shifts of absorption peaks at appropriate wavelengths in the dispersion of peptide aptamer P1 were observed before or after U-DGU treatment in Solutions 1 and 3 (Figure 7b). Furthermore, the low separation ability of P1 gave rise to the weak absorbance of SWCNTs in the upper layer corresponding to solution 2. These results suggest that P1 has no obvious separation ability at a concentration of 0.1 w/v%. The origin of dispersant behavior of the peptides is not fully understood in the present stage of research; however, there is a specific interaction of the chemical structure of peptides with SWCNTs that plays an important role in the separation ability and dispersion stability of SWCNTs over the property of selfassembly indicted by the CMC. As we mentioned above, the aromatic amino acids exhibit significant π−π stacking interactions between the SWCNTs and the peptides.42 The U-DGU method is a highly scalable and automatable process.43 When peptides that exhibit high dispersion and separation capabilities of SWCNTs below the CMC are developed, the dispersant enables control of not only separation but also purification of the sorted SWCNTs in an elegant way.

Figure 6. Absorption spectra of the supernatant with various dispersants at concentrations of 0.02 w/v% after U-DGU treatment.

absorption spectra of the supernatant after U-DGU treatment. According to the signal of electric structures of SWCNTs, only peptide aptamer A2 exhibited the dispersion ability at a low concentration in the supernatant. The dispersants, except A2, did not show any characteristic signs of SWCNTs in the aqueous solutions using U-DGU, even though SDBS, SC, and SDS typically act as dispersants to induce the dispersion capability of SWCNTs. This indicates that peptide aptamer A2 possesses high solubilizing stability at the low concentration of 0.02 w/v%. Next, we compared dispersant aqueous solutions of A2 and P1 at a higher concentration of 0.1 w/v% using a U-DGU method (Figure 7). A2 showed characteristic absorption peaks at 1040, 1134, and 1199 nm, corresponding to (7,5), (8,4), 3486

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CONCLUSIONS The solubilizing ability of SWCNTs in water with various dispersants was investigated. We found for the first time that SWCNTs can be dispersed in water using peptide aptamer A2 below the CMC. The resultant solution remained unchanged after high-speed ultracentrifugation, indicating that the peptide gave rise to the efficient and stable dispersions of SWCNTs with a low concentration. To evaluate the effective dispersion capability of A2, ITC was used to estimate binding enthalpies in water. The ΔH value of A2 was 3 and 5 times higher than those of SDBS and Hex, respectively. MD simulations also showed that π−π interactions between aromatic units of amino acids and the side walls of SWCNTs play an important role in imparting a high binding strength. It is believed that the introduction of aromatic units into the side chains of amino acids is an effective approach to realizing peptide aptamers with dispersion abilities. New peptide aptamers with good performance are expected to be developed using this line of molecular design. Furthermore, the diameter and metallic and semiconducting separations of SWCNTs were evaluated using a UDGU method. At present, U-DGU of the supernatant with a low peptide concentration is not enough to bring about separation and purification of SWCNTs. However, these results are expected to open a new methodology of manipulating SWCNTs in a simple and effective way. Because the hybridized materials composed of the peptides and SWCNTs can be useful in developing biosensing, bioimaging, and bioanalytical applications, a further detailed investigation of this novel and interesting phenomenon is now in progress.

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ASSOCIATED CONTENT

S Supporting Information *

Time series of the solvent-accessible surface area for the SWCNTs and aromatic amino acids in the peptide aptamer and the total potential energy using MD simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to members of the Materials Characterization Support Unit of RIKEN Center for Emergent Matter Science (CEMS) for TEM measurements. We also thank Dr. Keisuke Tajima of the Emergent Functional Polymers Research Team of RIKEN CEMS for measurements of absorption spectra and his valuable comments.

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REFERENCES

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DOI: 10.1021/la504777b Langmuir 2015, 31, 3482−3488