Carbon Nanotube Nanocomposites with Highly Enhanced Strength

Jun 24, 2015 - Simple and cost-effective method of highly conductive and elastic ... Kim , Joong Hoon Lee , Jae-Won Seo , Ueon Sang Shin , Sang-Hoon L...
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Carbon Nanotube Nanocomposites with Highly Enhanced Strength and Conductivity for Flexible Electric Circuits Ji-Young Hwang,† Han-Sem Kim,§,∥ Jeong Hun Kim,†,‡ Ueon Sang Shin,*,§,∥ and Sang-Hoon Lee*,†,‡ †

Department of Biomedical Engineering and ‡Korea University & Korea Institute of Science and Technology (KU-KIST) Graduate School of Converging of Sciences & Technologies, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea § Department of Nanobiomedical Science & BK21 Plus NBM Global Research Center for Regenerative Medicine and ∥Institute of Tissue Regeneration Engineering (ITREN), Dankook University, Anseo-dong, Dongnam-gu, Cheonan 330-714, Republic of Korea S Supporting Information *

ABSTRACT: Carbon nanotubes (CNTs) have an important role in nanotechnology due to their unique properties, retaining the inherent material flexibility, superior strength, and electrical conductivity, unless the bottleneck of CNTs persists and the aggregated structure is overcome. Here, we report on the highly enhanced mechanical and electrical properties of the CNT− chitosan nanocomposites through homogeneous dispersion of CNTs into chitosan solution using a high-pressure homogenizer. The optimal condition is a 50% (w/w) chitosan−CNT film, providing about 7 nm thickness of homogeneous chitosan layer on CNTs, a good tensile strength of 51 MPa, high electrical conductivity under 16 Ω/sq, and a stable bending and folding performance. This CNT−chitosan nanocomposite with highly enhanced properties is an amenable material to fabricate structures of various shapes such as films, sensors, and circuits and also enables a simple and cost-effective approach to improve the performance of a device that presents the first flexible and soft electric circuits yet reported using only CNT−chitosan as the conductor.

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INTRODUCTION

useful for the design of composite materials, electrochemical biosensors, and nanometer-scale electronic devices.6,8−11 In recent decades, nanocomposite materials consisting of CNTs and chitosan have been employed in a variety of applications, including biosensors, biofuel cells, drug delivery systems, and scaffold fabrication.12−14 The chitosan polymer is a natural cationic polysaccharide consisting of 2-amino-2deoxy-β-D-glucose units and has been broadly used in drug delivery, tissue engineering, and regenerative medicine due to its excellent biocompatibility and biodegradability, low immunogenicity, high antibacterial effects, drug/gene delivery, and amenability to chemical modification through its amino and hydroxyl functional groups.15−19 Although numerous studies have focused on improving the physical, mechanical, and electrical properties of the CNT− chitosan nanocomposite using a variety of techniques, a wide range of applications of the material have continually faced

Development of flexible electronics integrated with the commercial electronic components on a soft substrate is critical technology in the biomedical engineering and electronic industries. Achieving this technology using the conventional carbon paste or metal line patterning methods faces several challenges, and new paradigms in the materials and processes must be developed. Recent progress in the synthesis of nanocomposites containing nanomaterials and polymers has highlighted several new possible routes to overcoming these conventional limits.1 Carbon nanotubes (CNTs) present a candidate nanomaterial that has attracted significant attention due to its unique chemical, physical, mechanical, and electrical properties, which provide a platform for a variety of modified approaches, while retaining the inherent material flexibility, superior strength, and electrical conductivity.2,3 CNTs have accordingly enabled a variety of applications in basic and applied research.4−7 The sp2-hybridized carbon structures of CNTs promote electron transfer reactions magnificently, thereby rendering the CNTs © XXXX American Chemical Society

Received: April 6, 2015 Revised: May 28, 2015

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using 4 mg samples heated at a rate of 10 °C/min at temperatures ranging from 25 to 900 °C under air. The FTIR spectra were recorded in solid conditions over a spectral range of 400−4000 cm−1. Images of the sample morphologies were obtained using high-resolution transmission electron microscopy (HR-TEM; JEM 3010, JEOL, Japan) and field emission scanning electron microscopy (FE-SEM; MIRA II LMH microscope, Tescan, Czech Republic). The SEM studies were conducted using the samples that had been sputter-coated with a 10 nm gold layer prior to analysis. Characterization of Surface on the CNT−Chitosan Films. A comparative analysis of the different substrates was performed using Raman spectrometry (Horiba LabRam Aramis IR2, Japan) and X-ray photoelectron spectroscopy (XPS; AES-XPS ESCA 2000, Thermo Fisher Scientific, United States) for chemical quantification, contact angle analysis (Phoenix 300, Surface Electro Optics, Republic of Korea) for the wettability analysis, contact resistance measurements (Tonghui, TH2683, China) for the electrical conductivity analysis, and SEM images (JEOL 2000, Japan) for the detailed morphologic analysis. For measurement of the degree of swelling, 35 mm diameter circles were cut from the prepared chitosan and CNT−chitosan sheets with 0.50 mm thickness and soaked in 5 mL of distilled water at room temperature for 24 h. After the films were dried in the fume hood for 1 h to remove extra water, the weight of each sample was measured (before and after swelling), and then the swelling ratio was calculated using the following formula: swelling ratio (fold) = mass of swollen film/mass of dried film. The mechanical properties of the composite films were measured using a tensile testing machine (Instron 5966, Instron, United States). Five samples for the mechanical characterization were prepared, and the thicknesses of the samples were measured by a Vernier caliper with a resolution of 0.02 mm. The dimensions of the samples were 10 mm length, 3 mm width, and 0.50 ± 0.07 mm thickness. Fabrication of the CNT−Chitosan Electronic Circuits. We fabricated a master device template on a silicon wafer using SU-8 (SU8 100, Microchem Corp., United States) and photolithography techniques. The master pattern was 500 μm thick and 1 mm wide. Poly(dimethylsiloxane) (PMDS; Sylgard 184, Dow Corning Corp., United States) was poured over the master pattern for replica molding. The negative channels in the PDMS were filled with CNT−chitosan nanocomposite hydrogels. Electric devices outfitted with a lightemitting diode (LED)-based seven-segment decoder driver and a seven-segment display (SN74LS47N, Texas Instruments, United States) were inserted into the PDMS, and the physical connections between the CNT−chitosan lines and the electric devices were reinforced using a CNT−PDMS composite.10,11 We then electrically connected the CNT−chitosan lines and the electric devices. All LEDs of the seven-segment decoder driver were turned on using a power supply (UP-100D, Unicorn, Republic of Korea). With the LEDs turned on, the PDMS circuit was submitted to several repeated bending tests.

significant challenges. The previously reported CNT−chitosan nanocomposites, which have been made using relatively weak energy such as that of a mechanical agitator (about 695 W and 1400 rpm) and ultrasonic frequencies (about 40 kHz) and have no homogeneous distribution of the two components (no same (or similar) margin between a CNT molecule and the next on the nanoscale throughout the nanocomposite), have not been sufficiently strong or flexible for practical applications, and their electrical conductivity must still be significantly improved for future uses in fields such as conductive ink, electronic devices, and biomedical applications. The high value of a homogeneous mixture of the two components might be realized by formation of a CNT-core and chitosan-shell structure, because the same margins between CNT molecules on the nanoscale could be achieved only in a three-dimensional arrangement of the CNTcore and chitosan-shell structured fibers. This paper presents (i) an advanced and novel method for preparing CNT−chitosan nanocomposite material using a highpressure homogenizer (Nano DeBEE, maximum pressure of 60 000 psi) that resulted in close-packing CNT-core and chitosanshell structured nanofibers, (ii) optimization and validation of the highly enhanced electrical and mechanical properties of the CNT−chitosan nanocomposite consisting of CNT nanofibers wrapped with a chitosan layer, and (iii) practical applications for development of working flexible electric circuits that integrate commercial electronic digital components. The proposed method is simple and cost-effective, and could be used to synthesize nanocomposites consisting of other polymers and nanomaterials. The devices described here represent the first working digital electric circuit yet reported using the CNT-core and chitosan-shell structured nanocomposites.

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EXPERIMENTAL SECTION

Materials. Multiwalled carbon nanotubes (>95%, 20−30 nm outer diameter, 10−30 μm in length) were obtained from EMP (EM-Power Co., Republic of Korea). Low molecular weight chitosan (MW = 50000−190000, 75%−85% deacetylated) was purchased from SigmaAldrich (United States). All chemicals, including glacial acetic acid, KOH, and organic solvents, were purchased from Sigma−Aldrich, were of analytical grade, and were used without further purification. Preparation of the CNT−Chitosan Nanohybrids. Pristine CNTs were refluxed in a 5 N HCl solution for 1 day, and all possible impurities were washed out prior to use. Chitosan (200 mg) was dissolved in 50 mL of a mixture (25/25, pH < 2) of 5 N HCl solution and glacial acetic acid for 24 h, and then 200 mg of CNTs was homogenized in the chitosan solution with the assistance of a highpressure homogenizer (Nano DeBEE 45-3, BEE International, South Easton MA). A high level of homogenization could be achieved via a constant and high-velocity jet stream through the homogenizing cell. The maximum pressure was 60 000 psi. The acidic CNT−chitosan solution was slowly neutralized by adding 2 N NaOH solution for 3 h, which was then followed by the use of dialysis membrane tubing with a molecular weight cutoff of 12000−14000 (Spectrum Laboratories, Savannah, GA) against distilled water for 3 days to remove any small molecules, including inorganic side products. Prior to use, both the chitosan and CNT− chitosan solutions were sonicated for 30 min to homogeneously disperse the CNTs in the aqueous solution. Next, these solutions were transferred to appropriately sized containers and were left to stand at room temperature for 2 days in a fume hood. Characterization of the CNT−Chitosan Nanohybrids. Each chitosan and CNT−chitosan sample was characterized quantitatively using thermogravimetric analysis (TGA; Seiko Exstar 6000 TG/ DTA6100, Japan) and Fourier transform infrared spectroscopy (FTIR; JASCO 470 PLUS, Japan). The TGA measurements were conducted

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RESULTS AND DISCUSSION Fabrication of CNT−Chitosan Nanocomposites. To date, three methods have been used in attempts to modify the surface of CNTs with chitosan for good dispersion: (i) physical coating of chitosan on the surfaces of pristine CNTs using a mechanical force, such as sonication or milling techniques,14 (ii) ionic coating of the positively charged chitosan bearing primary amino groups onto the negatively charged carboxylic acid-functionalized CNTs through electrostatic interactions,20 and (iii) covalent immobilization of chitosan molecules on the surfaces of the carboxylic acid-functionalized CNTs using chemical reactions.14,21−23 The ionic or covalent grafting of any biomolecule requires that the CNTs be chemically functionalized prior to use. Highly reactive chemicals, such as alkali metals and Brønsted superacids, have been employed for this purpose,24−26 often leading to charged CNT derivatives that are quite unstable under ambient conditions. Given the significant B

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Figure 1. Fabrication of the CNT−chitosan nanocomposites: (A) schematic representation of the CNTs functionalized with chitosan for the fabrication of a CNT−chitosan film, (B) HR-TEM and (C) FE-SEM images of the prepared CNT−chitosan under high magnification, (D) photograph of the fabricated CNT−chitosan film.

Figure 2. Homogeneous dispersion of CNTs into chitosan solution using a high-pressure homogenizer: (A) homogeneous dispersion of the CNT− chitosan nanocomposites (I, sonication with neutralization; II, homogenization without neutralization; III, homogenization with neutralization), (B, C) schematic illustrations of (B) the nonhomogeneous solution due to partial dispersion and (C) the homogeneous solution produced by the highpressure homogenizer.

dispersed in the chitosan solution using a NanoDeBee highpressure homogenizer. This is the most important process to make the CNT-core and chitosan-shell structure nearly perfect. The third step involved a neutralization reaction via the slow addition of a KOH solution. During this step, the protonated forms of the chitosan molecules were neutralized by the acid− base reaction, and the neutralized chitosan molecules were selfassembled by hydrogen bonding onto the outer surfaces of the individual CNT molecules in a core−shell structure (Figure 1A).27,28 The attraction between the chitosan shell and the surface of the CNT core could be caused by a possible van der Waals interaction. The three TEM images of the three types of samples, which were prepared with different concentrations of chitosan (25, 50, and 75 wt %), are compared in Figure S2 in the Supporting Information. The TEM images clearly show increasing thickness of the chitosan layer (respectively, about 19.1 ± 0.4, 23.1 ± 1.3, and 27.8 ± 0.7 nm) on the surface of CNTs according to the increasing concentration of chitosan, and clearly indicate the formation of the CNT-core and chitosan-

damage introduced during these harsh processes, the pristine nanotubes tend to lose their primary electrical and mechanical properties and often become sensitive to air. These serious consequences may be avoided by physically attaching the chitosan onto the CNT surfaces. The noncovalent attachment of chitosan onto the CNT surfaces would preserve the sp2 structure of the CNTs to facilitate electron flow and simultaneously minimize the interfacial tension between the CNT surface and water. Especially the realization of a perfect CNT−chitosan core−shell structure by a physicochemical method would be the best way to achieve a complete homogeneous nanohybrid and to maximize the electrical and mechanical properties (Figure 1). The first step toward physicochemicalyl coating the chitosan onto the CNTs involved transforming the amino functional groups of the chitosan molecules (150 mg) into a water-soluble form, −NH3+Cl−, in 5 N HCl solution to enable homogeneous dispersion of chitosan molecules in water. As the second step, a stable and homogeneous CNT−chitosan hybrid solution was obtained after the CNT molecules (150 mg) were completely C

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Figure 3. Analysis of pristine CNTs, chitosan, and CNT−chitosan powder by (A) thermal gravimetric analysis and (B) Fourier transform infrared spectroscopy.

The neutralization process also appeared to significantly improve the electrical performances of the CNT−chitosan sample, as shown in Figure 2A. Through neutralization (or deprotonation) of the acidic CNT−chitosan nanocomposite solution, chitosan molecules will be suddenly self-assembled over the individual carbonaceous fibers, resulting in the formation of a CNT-core and chitosan-shell structure. The resulting same margins between CNT molecules within the CNT-core and chitosan-shell structured nanocomposite may indicate an extremely high level of homogeneous dispersion of CNTs into chitosan molecules and will promise a significantly increased electrical conductivity. Chemical Properties of CNT−Chitosan Nanocomposites. Relative chitosan amounts deposited on the surface of CNTs according to the different ratios of CNT and chitosan concentrations were quantitatively measured by TGA and compared to the pure samples of CNTs and chitosan (Figure 3A). The results demonstrated that a major mass loss of the pristine CNTs occurred over the temperature range of 600− 700 °C, whereas the thermal decomposition of pure chitosan took place in two steps involving depolymerization and decomposition of glucosamine units at about 300 °C, followed by an oxidative decomposition at 400−600 °C. The TGA results obtained from the 50 wt % CNT−chitosan nanohybrids showed that the material decomposed clearly in two steps. Weight loss corresponding to the loss of the chitosan occurred at 200−300 °C, and weight loss corresponding to the CNT part occurred at 500−600 °C.35 The thermal decomposition temperatures of both materials were shifted to lower values compared to those of the pure material cases. These shifts may have been due to the reduced concentration (wt %) (by half) of each component within the 50 wt % CNT−chitosan sample. The FTIR spectrum of the chitosan-functionalized CNT nanocomposites was compared with the corresponding spectra of the pristine CNTs and chitosan over the range of 500−4000 cm−1 (Figure 3B). The pristine CNTs did not display any characteristic peaks because the surfaces were not functionalized. By contrast, the 50 wt % CNT−chitosan sample displayed a chitosan-like peak pattern, with peaks at 3120− 3385 cm−1 (−OH and −NH stretching of chitosan), 2925 and 2856 cm−1 (−CH stretching of chitosan), 1654 cm−1 (CC stretching of pristine CNTs), and 1632 cm−1 (CO stretching of chitosan). These IR spectra indicated that the CNTs were well-functionalized with chitosan.

shell structured nanocomposites and an even distribution of chitosan over the outer walls of the multiwalled CNTs (Figure 1B). The SEM images shown in Figure 1C reveal that the individual CNTs of the CNT−chitosan composite were fully covered by the chitosan molecules, which formed a network of intermolecular hydrogen bonds. These images of the chitosanfunctionalized CNTs revealed that the defined surfaces of the samples were smoother and thicker (28.7 ± 5.4 nm of diameter) than those of the pristine CNTs (24.7 ± 4.3 nm), indicating that the CNTs were wrapped with hydrophilic chitosan molecules. The chitosan shell thickness can be controlled by adjusting the ratio of the CNTs and the chitosan concentration. The prepared CNT−chitosan nanocomposites could be cast using a simple drying method, resulting in a dense close-packing of the CNT−chitosan fiber units via intermolecular hydrogen bonding to form a CNT−chitosan film (complete and homogeneous dispersion) and elevation of their electrical and mechanical properties. Figure 1D shows the images of the CNT−chitosan film fabricated after drying. CNTs have not escaped from the dried CNT−chitosan film due to the perfect encapsulation of CNT cores within chitosan shells and the van der Waals force between the CNT cores and chitosan shells. Homogeneous Dispersion of CNTs into Chitosan. Figure 2A presents data supporting the excellent electrical conductivity (about 16.5 ± 0.6 Ω/sq) of the CNT−chitosan nanohybrid prepared with a 50% (w/w) solution using a highpressure homogenizer, NanoDeBEE, followed by neutralization. Sonication techniques have been used in most previously published reports.29−34 The electrical conductivities of the resultant CNT−chitosan hybrids tended to be considerably lower compared with that of the hybrid prepared by a highpressure homogenizer. Indeed, the electric conductivities increased significantly after homogenization, compared to those of the samples obtained using sonication. Conventional techniques, including stirring, ball milling, and sonication, can promote chitosan−chitosan agglomeration and nonhomogeneous wrapping of chitosan onto the CNT surfaces, thereby introducing irregularities into the CNT−chitosan packing structures and decreasing the electron flows or increasing the electrical resistivity (Figure 2B). The significantly increased conductivity resulting from the high-pressure homogenization process, therefore, could be considered to be a consequence of a highly homogeneous chitosan−CNT dispersion in water and a homogeneous chitosan-wrapping on the CNTs (Figure 2C). D

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Figure 4. Analysis of characteristics of the CNT−chitosan films: (A) Raman spectroscopy and (B) contact angle measurements and swelling test (n = 6).

Figure 5. XPS analysis of the surface elements on chitosan and on the CNT−chitosan sheets: (A) deconvoluted high-resolution XPS C 1s spectral intensity relative to the carbon atoms, (B) N 1s spectrum in the range of 395−410 eV, (C) O 1s spectrum in the range of 525−540 eV. The gray solid line corresponds to chitosan, and the green bold line corresponds to CNT−chitosan.

Figure 6. Characteristics of the CNT−chitosan films as a function of the percentag eof CNT relative to the percentage of chitosan: (A) thickness of the chitosan layer and sheet resistance, (B) tensile strength and elastic modulus versus the percentage of chitosan in the chitosan−CNT film. The error bars represent standard deviations (n = 3−8). The dashed lines indicate the curve fits to the data.

Surface Characteristics of the CNT−Chitosan Films. After fabrication of the chitosan and CNT−chitosan films, the Raman spectra display the retention of the carbon nanotube symmetry structure (Figure 4A). This Raman method offers a useful technique for quantitatively characterizing the carbon nanocomponents. The D-band peak at 1350 cm−1 is attributed to the presence of sp3-hybridized carbon atoms in the nanotubes, and also to a disordered graphite structure that is proportional to the amount of amorphous carbons in the dispersion. The high-frequency G-band peak at 1580 cm−1 corresponds to the structural intensity of the sp2-hybridized carbon atoms according to the vibrational modes of the CNTs. The sharp shapes of the G and G′ peaks show possible association of our nanotubes with metallic-type conductivity.

Figure 5 shows an XPS analysis of chitosan and the CNT− chitosan film as a method of quantitatively analyzing the biomaterial surfaces and certifying the modifications to the CNT−chitosan film. A wide-scan XPS analysis of the surfaces of chitosan and the CNT−chitosan hybrid revealed three distinct peaks at 284.60 eV (C 1s), 399.63−400.16 eV (N 1s), and 532.36−533.17 eV (O 1s). The C 1s, N 1s, and O 1s of XPS spectra obtained from the functionalized CNTs revealed bonding energies at 280−295, 395−410, and 525−540 eV, respectively. The C 1s spectra of the chitosan-modified CNTs indicated the presence of a higher quantity of carbon atoms due to the increased number of sp2 carbon atoms in the CNT molecules strongly attached to the chitosan molecules. The deconvoluted high-resolution C 1s spectrum revealed the E

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Langmuir carbon atoms loaded onto the CNT−chitosan with higher C− O peak at 286 eV.36 These XPS spectra clearly suggested that the CNT surfaces were functionalized with chitosan. The water contact angle of the CNT−chitosan composite film was 76.5 ± 3.9°, whereas the contact angle of the neat chitosan film was 51.1 ± 5.5°.37 It should be noted that the slightly increased contact angles of the nanohybrid films could be attributed to the increased hydrophobicity of the film surfaces and the porous nanomorphology created by selfassembly of CNT−chitosan units (Figure 4B). On the other hand, the effects of the CNT components on the volumetric swelling of the CNT−chitosan composite demonstrated that the CNT−chitosan film exhibited a significant swelling ratio with a volumetric change of 57-fold. By contrast, the neat chitosan film showed an increase of 8-fold. This increase in the swelling response was attributed to the existence of the nanosized spaces between the CNT−chitosan fibers and the large surface area of the chitosan-shell phase. These results indicated that the CNT surfaces were successfully functionalized by the chitosan biopolymer chains. Strong hydrogen bonding between the chitosan-wrapped tubes and this matrix could enhance the mechanical and electrical properties of the CNT−chitosan nanocomposite materials. Electrical and Mechanical Properties of the CNT− Chitosan Films. Figure 6 shows the electrical and mechanical properties of the CNT−chitosan films prepared with different ratios of CNTs and chitosan (10, 25, 50, 75, and 85 wt %). The electrical resistance values of the sheet samples having 10, 25, or 50 wt % CNT content decreased exponentially, from 85.93 ± 2.03 to 16.51 ± 0.62 and 8.48 ± 0.38 Ω/sq, respectively. The samples that contained more than 50 wt % CNT, however, displayed saturated electrical resistance (Figure 6A). The thickness of the chitosan layer on the CNTs decreased in proportion to the percentage of CNTs in the solution and highly affected the sheet resistance (Figure S2, Supporting Information). Less chitosan contents show more electrical conductivity. Figure 6B shows the dramatic impact of the chitosan content on the tensile strength and the elastic modulus of the hybrid material. The CNT−chitosan sheet samples prepared with CNT contents of 50−75 wt % displayed the best mechanical properties: a 51.04 MPa tensile strength and a 22.75 N/mm elastic modulus, indicating a 3.5−4.1-fold increase relative to the properties of the pure chitosan sample. The test results of mechanical and electrical properties revealed that the 50 wt % nanohybrid film was optimal for practical applications. All data related to the mechanical and electrical properties of the CNT−chitosan films are summarized in Table S1 in the Supporting Information. Consequently, the effective homogenization by the Nano DeBEE, the completion of the CNTcore/chitosan-shell structure through neutralization, and the regular spaces between adjacent CNT−chitosan fibers in the film might contribute to the excellent enhancement in the mechanical and electrical properties. Application of the CNT−Chitosan Films for Flexible Electrical Circuits. Figure 7 demonstrates the high conductivity and flexibility of the CNT−chitosan nanocomposite material as an electrical conductor. We designed an electrical circuit containing a seven-segment decoder and a sevensegment display. An electrical circuit (negative relief) was patterned onto the PDMS substrate using conventional soft lithography technologies. Through holes were punched to form connections between the seven-segment display and its driving chip, power supply, and input signal. Figure 7A illustrates the

Figure 7. Application of the CNT−chitosan electrode in electrical circuits: (A) operation of a functional LED in a plate circuit and (B) bending test to determine the flexibility of the CNT−chitosan circuit.

electric circuit that integrated conventional electric digital components. The system operated well, even though metal lines were not employed at all. Variations in the input signals yielded variations in the readout. No disconnections were observed during operation. Bending of the electric circuit did not disrupt the operation of the LEDs mounted on the sevensegment display, and the LEDs remained luminescent without displaying a decrease in the light intensity (Figure 7B). These results indicated that the CNT−chitosan nanocomposites could serve as an economical and sustainable alternative for use in electrical applications.

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CONCLUSION The present study describes a novel approach to preparing chitosan-functionalized CNT composites, which is capable of dispersing and modifying CNTs in an aqueous solution. This dispersion method could provide a simple, low-cost, and effective route to improve the performance of the advanced hybrid nanocomposites for use in flexible electric circuits. Highresolution electron microscopic images demonstrated that chitosan was homogeneously wrapped around the outer walls of the individual nanotubes. The CNT−chitosan nanocomposites possessed two advantages. Chitosan conveyed excellent hydrophilicity and chemical diversity, whereas the CNTs provided extraordinary electrical, mechanical, and structural properties. The hydrogen-bonding network stabilized the CNT−chitosan film, which displayed the properties of a durable and strong sheet of paper. The sheets displayed a high conductivity, excellent mechanical properties, a high structural stability, and long and durable properties. These characteristics enable a variety of useful applications, including those in the biomedical field. In conclusion, the proposed physical coating of chitosan on the surfaces of CNTs using a high-pressure homogenizer enabled a thorough dispersion of both materials, and allowed the construction of diverse biomedical systems with highly enhanced mechanical and electrical properties. We suggested that a 50 wt % chitosan−CNT provides optimal conditions for the fabrication of sensitive biosensing and flexible circuits, which has about 7 nm thickness of the chitosan layer on CNTs, a good tensile strength of about 51 MPa, and high electrical conductivities under about 16 Ω/sq, exhibiting a stable performance regardless of bending and folding. This method for CNT−chitosan nanocomposites could be used for a homogeneous dispersion of other types of nanomaterials and polymers such as single-walled carbon nanotubes, graphene, nanoparticles and PDMS, alginate, or cellulose to enhance the mechanical and electrical properties in numerous applications such as conductive printed inks, functional aerogel sponges, wearable electronic textiles, flexible transparent electronics, and so on. F

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(9) Saleh Ahammad, A. J.; Lee, J. J.; Rahman, M. A. Electrochemical sensors based on carbon nanotubes. Sensors (Basel) 2009, 9 (4), 2289−319. (10) Jung, H. C.; Moon, J. H.; Baek, D. H.; Lee, J. H.; Choi, Y. Y.; Hong, J. S.; Lee, S. H. CNT/PDMS composite flexible dry electrodes for long-term ECG monitoring. IEEE Trans. Biomed. Eng. 2012, 59 (5), 1472−9. (11) Lee, J. H.; Lee, S. M.; Byeon, H. J.; Hong, J. S.; Park, K. S.; Lee, S. H. CNT/PDMS-based canal-typed ear electrodes for inconspicuous EEG recording. J. Neural Eng. 2014, 11 (4), 046014. (12) Aryaei, A.; Jayatissa, A. H.; Jayasuriya, A. C. Mechanical and biological properties of chitosan/carbon nanotube nanocomposite films. J. Biomed. Mater. Res., A 2013, 102 (8), 2704−2712. (13) Kumar, A.; Jena, P. K.; Behera, S.; Lockey, R. F.; Mohapatra, S. Efficient DNA and peptide delivery by functionalized chitosan-coated single-wall carbon nanotubes. J. Biomed. Nanotechnol. 2005, 1 (4), 392−396. (14) Zhang, M.; Smith, A.; Gorski, W. Carbon nanotube-chitosan system for electrochemical sensing based on dehydrogenase enzymes. Anal. Chem. 2004, 76 (17), 5045−50. (15) Keong, L. C.; Halim, A. S. In vitro models in biocompatibility assessment for biomedical-grade chitosan derivatives in wound management. Int. J. Mol. Sci. 2009, 10 (3), 1300−13. (16) Aoki, T.; Iskandar, S.; Yoshida, T.; Takahashi, K.; Hattori, M. Reduced immunogenicity of beta-lactoglobulin by conjugating with chitosan. Biosci., Biotechnol., Biochem. 2006, 70 (10), 2349−56. (17) Chung, Y. C.; Wang, H. L.; Chen, Y. M.; Li, S. L. Effect of abiotic factors on the antibacterial activity of chitosan against waterborne pathogens. Bioresour. Technol. 2003, 88 (3), 179−84. (18) Hagiwara, K.; Anastasia, R.; Nakata, M.; Sato, T. Physicochemical properties of pDNA/chitosan complexes as gene delivery systems. Curr. Drug Discovery Technol. 2011, 8 (4), 329−39. (19) Lee, K. H.; Shin, S. J.; Kim, C. B.; Kim, J. K.; Cho, Y. W.; Chung, B. G.; Lee, S. H. Microfluidic synthesis of pure chitosan microfibers for bio-artificial liver chip. Lab Chip 2010, 10 (10), 1328− 1334. (20) Baek, S. H.; Kim, B.; Suh, K. D. Chitosan particle/multiwall carbon nanotube composites by electrostatic interactions. Colloids Surf., A 2008, 316 (1−3), 292−296. (21) Jiang, L.; Liu, C.; Jiang, L.; Peng, Z.; Lu, G. A chitosan-multiwall carbon nanotube modified electrode for simultaneous detection of dopamine and ascorbic acid. Anal. Sci. 2004, 20 (7), 1055−9. (22) Ke, G.; Guan, W.; Tang, C.; Guan, W.; Zeng, D.; Deng, F. Covalent functionalization of multiwalled carbon nanotubes with a low molecular weight chitosan. Biomacromolecules 2007, 8 (2), 322−6. (23) Pan, Y.; Bao, H.; Li, L. Noncovalently functionalized multiwalled carbon nanotubes by chitosan-grafted reduced graphene oxide and their synergistic reinforcing effects in chitosan films. ACS Appl. Mater. Interfaces 2011, 3 (12), 4819−30. (24) Rao, A. M.; Eklund, P. C.; Bandow, S.; Thess, A.; Smalley, R. E. Evidence for charge transfer in doped carbon nanotube bundles from Raman scattering. Nature 1997, 388 (6639), 257−259. (25) Penicaud, A.; Poulin, P.; Derre, A.; Anglaret, E.; Petit, P. Spontaneous dissolution of a single-wall carbon nanotube salt. J. Am. Chem. Soc. 2005, 127 (1), 8−9. (26) Liu, C. M.; Cao, H. B.; Li, Y. P.; Xu, H. B.; Zhang, Y. The effect of electrolytic oxidation on the electrochemical properties of multiwalled carbon nanotubes. Carbon 2006, 44 (14), 2919−2924. (27) Nishi, N.; Nishimura, S.; Ebina, A.; Tsutsumi, A.; Tokura, S. Preparation and characterization of water-soluble chitin phosphate. Int. J. Biol. Macromol. 1984, 6 (1), 53−64. (28) Kumar, M. N. V. R. A review of chitin and chitosan applications. React. Funct. Polym. 2000, 46 (1), 1−27. (29) Liu, Y.; Qu, X.; Guo, H.; Chen, H.; Liu, B.; Dong, S. Facile preparation of amperometric laccase biosensor with multifunction based on the matrix of carbon nanotubes-chitosan composite. Biosens. Bioelectron. 2006, 21 (12), 2195−201.

ASSOCIATED CONTENT

S Supporting Information *

Details of XPS analysis of the surface elements on chitosan and on the CNT−chitosan sheets, TEM images for analysis of various chitosan layer thicknesses on the CNT surfaces versus pristine CNTs, stress−strain curve for mechanical properties, and more detailed schematic illustrations for dispersion of CNTs into chitosan solution. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00845.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +82-2-3290-5654. Fax: +82-2-921-6818. E-mail: [email protected]. *Phone: +82-41-550-3889. Fax: +82-41-550-3081. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This research was supported by the Public Welfare & Safety Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant Number 2014039349). This research was also supported by the Basic Science Research Program through the NRF funded by the Ministry of Education, Science and Technology (Grant Number 2009-0093829).

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ABBREVIATIONS CNT, carbon nanotube; PMDS, poly(dimethylsiloxane); TGA, thermogravimetric analysis; FTIR, Fourier transform infrared spectroscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy; XPS, X-ray photoelectron spectroscopy

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DOI: 10.1021/acs.langmuir.5b00845 Langmuir XXXX, XXX, XXX−XXX