Enhanced adhesion of carbon nanotubes by dopamine modification

Subscriber access provided by ECU Libraries

Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Enhanced adhesion of carbon nanotubes by dopamine modification weijun li, Yang Li, Mao Sheng, Shitong Cui, Zhihang Wang, Xiaojie Zhang, Chen Yang, Zhiyi Yu, Yilin Zhang, Shouceng Tian, Zhendong Dai, and Quan Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00192 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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 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 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.

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 30 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

Langmuir

Enhanced Adhesion of Carbon Nanotubes by Dopamine Modification Weijun Li,†,= Yang Li,‖,= Mao Sheng, † Shitong Cui,† Zhihang Wang,† Xiaojie Zhang,† Chen Yang,† Zhiyi Yu,† Yilin Zhang,‡ Shouceng Tian,† Zhendong Dai,‖,* Quan Xu†,* † State

Key Laboratory of Petroleum Resources and Prospecting, China University of

Petroleum-Beijing, 102249, China ‖ Institute

of Bio-inspired Structure and Surface Engineering, College of Astronautics,

Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China ‡C.

Eugene Bennett Department of Chemistry, West Virginia University, Morgantown,

West Virginia 26506-6045, United States

=These

authors contributed equally to this work.

*Corresponding

authors: [email protected], [email protected]

ACS Paragon Plus Environment

Langmuir 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

Abstract According to the fact that gecko-inspired vertically aligned carbon nanotubes (VACNTs) exhibit ultra-strong adhesion, dopamine is utilized to make a modification to this traditional biomimetic material. The composite material is tested for adhesion performance under different environmental conditions by an atomic force microscope (AFM). The adhesion force of the modified VA-CNTs doesn’t show obvious fluctuation during the gradual heating process, however, the material gains improved adhesion when increasing the ambient humidity. And the modified CNTs show a stronger adhesion force than the original CNTs in their performance tests. The dopamine polymer has a good combination with CNTs, which is responsible for the aforementioned excellent performance. Overall, this modification method is simple, convenient, efficient and environmentally friendly, which all indicates a promising future in its application. The modified CNTs are expected to be used for super-adhesion in harsh environments, as well as in the field of microelectronics. Keywords: carbon nanotubes, dopamine, gecko, mussel protein, adhesion


Page 2 of 30

Page 3 of 30 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

Langmuir

1. Introduction Materials that imitate the adhesion of gecko's feet have achieved great success after decades of development. The focus of adhesion research is on the peculiar dry reversible adhesion,1 as well as the dry self-cleaning performance of the gecko's seta.2-3 In this study, we focus on the dry reversible adhesion characteristics of geckos. This ability has attracted lots of attention to explore and discover the mystery in it. In the past decade, a series of theoretical studies have been conducted and several biomimetic materials based on hierarchical fibrillar structure of seta have been developed.4-5 In theory, reversible adhesion arises due to the featured geometry of contact components,6 and the van der Waals forces between the spatula and the substrate.7-8 Many research groups have attempted to fabricate the bionic surfaces with different aspect ratio, various directions of pillar bending, as well as changing the tip contact pattern of each pillar (point contact or shovel-like contact).4 Three outstanding materials are widely used to produce excellent bionic surfaces, including carbon nanotubes, polymers, and photosensitive materials.4, 9 In 2003, Sitti et. al10 used polyimide to fabricate a cluster forest-like nano-pillars, which was placed perpendicularly to the surface. A lot of research work was conducted on changing the angle of inclination of the nano-pillars and the shape of the stud to maximize the adhesion between the materials and the contact surface in the past decade.11-12 However, the adhesion force of the polymer materials was still limited. The fact that carbon nanotubes can reach 10 times of adhesion than the gecko's feet was first discovered by Qu et. al13 in 2008, and since that, a series of studies based on the CNTs have been published.14-16 However, most of


Langmuir 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

these studies only involved minor change of the CNTs, such as the arrangement or surface morphology, lack of significant modification.17 In terms of adhesive materials applicable in wet environment, scientists have made great efforts to address the challenges existed in such environment. A few appropriate gecko-adhesive materials have been developed.18-21 Many studies have demonstrated that gecko-inspired materials are able to show greatly enhanced wet adhesion when the surfaces modified by dihydroxyphenylalanine (DOPA) quasi polymers.22-24 DOPA found in mussel adhesive proteins has been discovered to be the most crucial component for the strong adhesive ability of mussels onto wet substrates25-27 and it is believed that the catechol group in the polymer is responsible for the powerful wet adhesion of materials and substrate.28-29 Dopamine (DA), which has a similar molecular structure to the DOPA, can be easily polymerized to form polydopamine under alkaline conditions. The obtained polydopamine (PDA) contains an indole scaffold which can be found in many materials and natural products.30-31 The PDA is found to be able to adhere onto almost all types of substrates.30-34 Although PDA-inspired materials have shown strong adhesion in wet conditions, there remains great challenges to combine PDA with gecko-mimic materials efficiently so that great adhesion can be achieved under tough conditions in a controllable manner. Many composite adhesives are prepared by combining mixtures in a simple way and do not possess good adhesion strength.35-36 And it remains relatively impossible to greatly enhance the adhesion properties of those materials. In addition, some methods for preparing composite polymer materials are complicated, costly, and require rigorous experimental


Page 4 of 30

Page 5 of 30 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

Langmuir

conditions, which all adds the difficulty of the preparation process.9, 37 Herein, we report a simple method to modify the hierarchically structured VACNT arrays. The modification is based on the mussel adhesion chemistry and the modified CNTs, named polydopamine modified carbon nanotubes (PDA@CNTs), successfully integrated the wet adhesion property with the hierarchically structured VA-CNTs. Meanwhile, the PDA@CNTs exhibited enhanced adhesion ability than the original CNTs, which was confirmed by atomic force microscope (AFM) test. Moreover, the adhesion property was found to relate with the humidity of environment and more humidity contributes to a higher strength of adhesion. The adhesive material prepared in this study has shown a strong binding on both the shear and normal direction while maintaining excellent performance under various environmental conditions.

2. Experimental Section 2.1 Materials Dopamine hydrochloride was purchased from Shanghai Energy Chemical Co., Ltd.. Tris-HCl buffer solution (pH=9) was purchased from Beijing Solarbio Science ﹠ Technology co., ltd., and deionized water was prepared on site using a NABAICHUAN deionized water machine. All chemicals were used directly without further purification. 2.2 Preparation of VA-CNTs Electron beam catalyst deposition (E-beam 500, Xingnan Investment Co., Ltd.) and thermal chemical vapor deposition (CVD) were used to fabricate vertically aligned multi-walled carbon nanotubes (VA-MWCNTs). In the first step, a 20 nm thick Al2O3 film and a 2 nm thick Fe layer were successively deposited on a 4×4 mm2 Si/SiO2


Langmuir 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

substrate using an electron beam system. VA-MWCNTs were grown by thermal CVD in a tube furnace (OTF-1200X-80, Kejing) with 1200 sccm of Ar as carrier gas. At 650 °C, 450 sccm of H2 was injected into the furnace to pretreat the catalyst. When 250 sccm of C2H2 was introduced into the reactor at 720 °C, CNTs started to upspring on substrate. The reaction time was 3-30 minutes and the CNTs grew up to 100-800 microns. The C2H2 and H2 flows were stopped when the growth completed, but the Ar flow was discontinued until the reactor temperature reached 25 °C. After 45 min of deposition, VA-CNTs with excellent adhesion performance were obtained. 2.3 Polydopamine VA-CNTs composite Firstly, a 0.001 M dopamine solution was prepared by dissolving 0.002 g dopamine hydrochloride in 10 mL deionized water. 10 μl of Tris•HCl was added to the solution to adjust the pH. The solution was ultrasonicated for 15 minutes to generate polydopamine. In the next step, the polydopamine solution was added dropwise to the side of the container in which VA-CNTs were placed. The container was sealed and placed in an oven at 45 °C for 6 hours to produce the dopamine-supported VA-CNTs composite. In this work, we configured the concentration of dopamine solution as the regulatory variables. Composite materials in different dopamine concentrations were obtained by the above method. 2.4 Morphology and performance characterizations The top and cross-sectional morphologies of materials were visualized by scanning electron microscope (SEM, Hitachi SU 8220). Chemical structure was characterized by attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR)


Page 6 of 30

Page 7 of 30 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

Langmuir

(Nicolet iS10). X-ray photoelectron spectroscopy (XPS) tests were performed using ESCALAB 250 spectrometer with Al Ka excitation (1486.6 eV). Morphologies of CNTs were observed by field emission transmission electron microscope (HRTEM) (FEI Tecnai G2 F20 S-TWIN) with an accelerating voltage of 200 kV. The contact angles of water were determined by Dataphysics DCAT21. 2.5 Determination of adhesion under different environmental conditions Initially, a single PS ball (hydrophobic microballoon) was pasted to the bottom of the tipless AFM cantilever (ACTA-TL-FM-50, AppNano) using a slow curing epoxy as shown in Figure 6b. The cantilever is operated by the AFM (Dimension Icon AFM, Bruker Co., Inc..). Each probe was examined before and after the AFM test using a scanning electron microscope (SEM, FEI Inc.) to eliminate premature failure due to poor adhesion and/or excessive glue. The hydrophilic microparticle (Al2O3) was also pasted to cantilever in order to compare with the PS micro-ball. Herein, adhesions at different conditions (including different temperature and humidity) were examined under nitrogen. A sealed electrochemical cell (Bruker ECAFM) was purged by 99.999% dry nitrogen for 30 minutes at -10 to 50 °C before testing. The relationship between interaction force and movement time of the AFM z-piezoelectric scanner was investigated during each approaching-dragging-retracting cycle. The thermal tuning method was utilized to determine the normal spring constants of the probes and the mean value was ~3.9 N m-1. The adhesion of the probe and the substrate was measured at least 5 times per group. The back of the modified CNTs was glued to a self-made hook and the entire device adhered to different glass surfaces. Different weights can be


Langmuir 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

hung at the lower end of the hook. In this work, 500 g counterweight was used and the adhesion forces in four directions (0°, 30°, 60°, 90°) were tested.

3. Results and discussion Dopamine is an analogue of DOPA, which is a major amino acid residue of mussel adhesive proteins. It shows strong impact on the adhesion of mussels underwater. As one of the main components of the adhesion protein, DOPA has good biological affinity such as tissue and cell compatibility. For these merits, DA has been widely applied in various fields. In biomimetic gels, the adhesion of normal hydrogels has been greatly improved with the addition of dopamine. The prepared gel was rendered moist adhesion, even in aqueous environment. In addition, the injectable glue38 which has underwater adherence can be obtained by integrating dopamine or analogs. Due to the excellent adhesion performance, DA was used in our work to further improve the performance of CNTs. Generally, the VA-CNTs prepared by the catalyst growth method have a diameter of about 10 nm with a regular arrangement. This microstructure has similar pattern observed on the gecko foot, which endows the material with excellent adhesion. Direct surface modification of CNTs with aqueous solution is infeasible due to its superhydrophobic property. Herein, we have developed a relatively simple and rapid chemical vapor deposition method to effectively modify the CNTs. The schematic diagram of CNTs modification with dopamine is shown in Figure 1. As can be seen from the figure, the prepared polydopamine solution was added to a container in which CNTs sample was placed. The polydopamine solution was heated to modify the surface of the CNTs and produce the composite CNTs with enhanced adhesion property. The


Page 8 of 30

Page 9 of 30 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

Langmuir

picture in the left red circle represents the morphology of the adult gecko and the spatulae microstructure of the gecko's foot. As can be clearly seen, the spatulae of the gecko's setae has very neatly arranged micro-nano structures. Therefore, carbon nanotubes can be the best alternative materials for reversible adhesion materials. The schematic diagram of mussel byssus adhering to the surface of the object was depicted in the red circle on the right. Mussel can permanently stick to wet substrates ranging from soft surfaces to hard rocks by proteinaceous holdfast known as the byssus, which radiated from the ventral groove of the foot. The green ring in the middle shows the chemical structure of dopamine. Dopamine has quite similar structure to the PODA and can be converted to PDA in alkaline solutions in the presence of oxygen. The two hydroxyl groups on the ortho-position of the benzene ring are critical to the adhesion property, however, they could be easily oxidized, causing the complete loss of adhesion. Herein, the two aforementioned adhesive materials are combined in a facile way. The polymerized dopamine was used to modify the CNTs by generating a composite micro-nano structure (Figure 2). The schematic description of surface functionalization of CNTs and dopamine via the self-polymerization to produce PDA in substrate surface were showed in Figure 2. The SEM was used to characterize surface microstructure of the obtained PDA@CNTs sample. As shown in Figure 3a, the top of the original CNTs shows many layered nanotubes having a diameter of about 8-20 nm. These cluster-like microscopic nanotubes play a key role in the adhesion of the CNTs.39 These materials can be adhered directly to various surfaces with external force just like the gecko's foot and can also


Langmuir 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

support a certain weight.40 The newly produced CNTs bundles had small tip size but did not contain relatively weak entangled nanotubes and a π–π conjugated carbon structure. The intimate contact between the target surface and the electron-rich π–π conjugated nanotubes could ensure sufficient adhesion force of the CNTs which was tested in this experiment. The microstructure of the PDA@CNTs was characterized to compare with the original carbon nanotubes. It can be seen from Figure 3b that there is no significant difference in the microstructures of these CNTs. Several tiny particles are observed in the PDA@CNTs indicating that the CNTs are successfully modified by PDA. Figure 3c and d show the side microstructure of the original CNTs. As seen in Figure 3c, the prepared nanotubes aligned nearly perpendicular to the silicon surface and have a fairly ordered tubular length of approximately 600 μm. The partial crosssectional view at a higher magnification reveals that these CNTs were very neat and formed packed “bundles” (Figure 3d). It can be seen that the growth structure of the CNTs is closely arranged, which ensures their good adhesion performance. The wettability of the CNTs and PDA@CNTs were comprehensively evaluated by contact angle measurements. In the air, when a water droplet touches the CNTs surface, the contact angle of 157.4±1.0° is observed (Figure S1a). However, the PDA@CNTs shows a contact angle of 132.1±1.0° (Figure S1b). Thus, the PDA@CNTs shows lower hydrophobicity than the raw CNTs due to the presence of PDA. In order to further confirm the successful modification of CNTs by PDA, X-ray photoelectron spectroscopy (XPS) analysis was performed to compare the CNTs before and after the modification (Figure 4a). Four peaks are found in C1s of CNTs (Figure


Page 10 of 30

Page 11 of 30 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

Langmuir

4b), including C-C bond at 284.31 eV, C-O bond at 285.23 eV, O=C-O bond at 285.93 eV, and π-π* bond at 287.75 eV. For PDA@CNTs, four peaks are shown in C1s (Figure 4c), which are assigned to C-C bonds (284.32), C-O groups (285.64), O=C-N groups (286.44) and π-π* transitions (292.91). As shown in Figure S2a, the O1s XPS of CNTs indicates four functional groups, including C=O bond at 530.98 eV, C-O bond at 532.93 eV and O=C-O bond at 531.76 eV. Three peaks were found in the O1s XPS of PDA@CNTs (Figure S2b), which are attributed to C=O bonds (532.45), C-O groups (533.78) and O=C-O groups (533.21). Typically, the peaks at 398.92, 398.17 and 399.7 eV in N1s (PDA@CNTs) are assigned to the C-N, N-H and N+ bonds, respectively (Figure S2c). A Raman spectrum of the modified sample recorded at 633 nm (Figure 4d) clearly shows two characteristic peaks for both CNTs and PDA@CNTs. As seen in Figure 4d, the D-band at 1350 cm-1 represents adsorbed amorphous carbon, carbon nanoparticles, and other micro-nano crystals on the tube walls. The G-band at 1580cm-1 demonstrates the ordered graphitization of the nanotubes. The high-intensity ratio of G-band to Dband (∼1.52) indicates a graphitic structure exists in the original CNTs. However, the ratio became 0.93 after the modification with PDA because active polydopamine particles were generated. In FT-IR spectrum of PDA@CNTs (Figure 4e), an absorption band at 1632 cm−1 is observed which corresponds to the indole group of PDA.41-42 Absorption band at 1632 cm−1 is attributed to carbonyl (C=O) stretching vibration of an amide (CON(H)) group, which indicates that a polydopamine has been successfully deposited on the surface of


Langmuir 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

CNTs. Meanwhile, the absorption bands at ~2910 cm−1 and ~2950 cm−1 suggest the existence of C-H bond and C=O bond. In summary, through a series of characterization methods, we have demonstrated that the carbon nanotubes have been successfully modified by polydopamine, which is the prerequisite for the subsequent studies. A transmission electron microscope (TEM) image of scattered nanotubes dispersed from an ethanol solution shows that the CNTs are almost free of amorphous carbon (Figure 4f). As mentioned above, we have successfully obtained polydopamine-modified CNTs. In the next step, AFM measurement of the adhesion forces between spherical probes and PDA@CNTs surfaces was conducted, which is the most significant part of our study. The Polystyrene (PS) attached AFM probes were placed in contact with target substrates in an ‘approach-contact-pull’ manner to simulate the motion of a live gecko. The obtained force-distance curves in AFM-based force-spectroscopy experiments are shown in Figure 5a. The PS probe was approached and repeatedly retracted by the piezo scanner in AFM experiments. In addition, the probe was allowed to reach multiple areas of the surface so that the most representative result could be obtained (Figure 6a). The optimized parameters of the consecutive studies are shown in Table 1. Figure 5a shows the representative force-distance curve (retraction) and the force between the PS probes and the surfaces of PDA@CNTs versus time. It reveals the adhesion stage (i.e., direct separation after contact) between PS normal approaching and pull-off. It can be seen that the PS probe is close to the surface of the object before


Page 12 of 30

Page 13 of 30 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

Langmuir

750 ms, and reaches the lowest point on the sample that pressed by the probe at 980 ms. After that, the probe begins to slowly retract. During this process, the tip of the PS probe has some weak attraction to the surface of the sample, mainly caused by some faint static electricity, and is much weaker than adhesion, so we can ignore this influence. Figure 5b indicates that the max adhesion force (619 nN) could be obtained at the dopamine concentration of 0.3 wt%. In the following experiments, we used this optimum concentration to modify the original carbon nanotubes. Since the PDA@CNTs have relatively strong adhesion force, their adhesion performance has been further evaluated under different environmental conditions. Firstly, the microscopic adhesion of the modified CNTs was examined under different temperatures. Variation of pull-off force under different temperature (Figure 5c) and relative humidity (Figure 5d) in the AFM experiment was studied. As shown in Figure 5c, the adhesion force of raw CNTs has only slight fluctuation as the rise of temperature, and the adhesion of PDA@CNTs is constantly greater than the original CNTs. The adhesion strength of the two CNTs shows significant difference (60-90 nN) and each of them fluctuates within small range (5-20 nN). Overall, temperature has not shown great influence on CNTs adhesion. Representative adhesion force curves of CNTs and PDA@CNTs are shown in Figure S3. Typical curves of CNTs and PDA@CNTs are shown in Figure S3a and Figure S3b respectively. The representative curves of PDA@CNTs adhesion force against temperatures and relative humidity are shown in Figure S3c and S3d respectively. Moreover, as shown in Figure 5d, the adhesion force of PDA@CNTs is


Langmuir 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

consistently showing a greater strength than the unmodified CNTs under environments of varying relative humidity. For PDA@CNTs, the adhesion force gradually increases with the rising humidity and slightly declines at extremely high relative humidity (>80%). Conversely, for the unmodified CNTs, the adhesion force exhibits a decreasing trend despite the increase of environmental humidity. This difference is believed to be caused by the polydopamine which enhances adhesion effectively. This unique feature remarkably extends the application of CNTs to different environmental conditions. In general, this study proved that the PDA modified CNTs are capable of showing excellent adhesion ability under various environmental conditions. Ambient temperature is not expected to have a significant impact on the adhesion of the materials because the temperature variation is very minimal at ambient environment and the CNTs could withstand a wide temperature range. Correspondingly, no obvious change in the adhesion performance of the material was observed in the varying experimental temperature. However, changes in relative humidity was known to have great impact on the adhesion of the material. As the humidity increases, water droplets would be repelled to accumulate on the surface of the unmodified CNTs due to the hydrophobic nature of the CNTs. However, the polydopamine modified CNTs was shown to have smaller water contact angles (Figure S1a and b), implying a lower hydrophobicity. In the AFM experiments, when using the hydrophilic probe, a mutual attraction could be established between the probe and water droplets. As a result, the probe and the surface of the nanotubes containing water droplets develops a stronger attraction


Page 14 of 30

Page 15 of 30 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

Langmuir

during the pull-off process. The adhesion of PDA@CNTs continues to increase as the environmental humidity grows (Figure S4), most likely due to the formation of stronger hydrogen bonding between the hydroxyl groups of dopamine and water molecules (Figure 6a and b). On the other hand, the hydrophobic probe has similar adhesion force as the hydrophilic probe, but the curve against humidity is different. As the probe approaches the water droplets, repulsion is initially predominant between them. However, attraction becomes dominant after the probe is pressed down in the pull-off process, also resulting in stronger adhesion force of this material (Figure 5 and Figure 6). In conclusion, the polydopamine modified carbon nanotubes have demonstrated the high level of pull-off force during the AFM test. In order to evaluate the super adhesion performance of PDA@CNTs, this material was applied in a weight-bearing experiment to test its endurance. As shown in Figure 7a, the normal adhesion of PDA@CNTs can bear a weight up to 500 g (Figure S5) and shows a “linear” adherence to the glass base. The surface area of PDA@CNTs is about 0.6*0.55 cm2, and the corresponding pressure is approximately 151.52 MPa. As shown in Figure 7b, the shear adhesion of CNTs increased dramatically from 10 to 56 MPa with the nanotube length increased from 0.2 to 0.7 mm (supplemental data is shown in the Figure S6 a-f). However, the corresponding normal adhesion force only increased slightly from 8 to 23 MPa. The shear adhesion observed is about four times stronger than the normal adhesion. The high shear adhesion ensures a strong bonding of the material to the target substrate to suspend the weight in the shear direction, while a weak normal adhesion allows the materials to be easily detached in the normal direction.


Langmuir 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

In order to investigate the angular effect on adhesion, the pull-out forces in different directions were measured. The tensile force shown in Figure 7c decreases as the stretch angle increases, indicating that the shear adhesion is much stronger than the normal adhesion. Based on the experimental results above, we further studied the mechanism of dry adhesion. SEM images of the adhesive surface after detachment was used to elucidate this mechanism (Figure S7). The top view image of the PDA@CNTs shows an even distribution of reticular domains with a center distance of 1-2 μm, which is entirely different from that of the arrays on top of the unmodified CNTs. This also revealed the greater adhesion performance of the PDA@CNTs than raw CNTs. Thus, the PDA@CNTs displayed excellent adhesion properties in the load-bearing experiment, which illustrates the material will have innumerous potential application in many fields such as mechanical engineering and electronics.

4. Conclusion In summary, the effect of dopamine on adhesion performance of CNTs was investigated. The modified carbon nanotubes reveal greater adhesion force than the original CNTs under different environmental conditions. The environment study discloses that, under various conditions, the modified material is able to demonstrate more than expected adhesion and excellent environmental adaptability and stability. Furthermore, the modified carbon nanotubes displayed superior load-bearing performance, which greatly expands its application to different areas such as surface engineering, mechanical engineering and electronics.

Supporting Information


Page 16 of 30

Page 17 of 30 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

Langmuir

The supplementary information contains the auxiliary characterization of the materials (including Figure S1, S2 and S7), the typical data of adhesion test (including Figure S3, S4 and S6) and other regarding information (Figure S5).

Acknowledgements The project was supported by The National Science Foundation of China (51875577), The Beijing Nove Program (No. Z171100001117058), The Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF16A06), The Science Foundation of China University of Petroleum-Beijing (No. 2462018BJC004). PetroChina Innovation Foundation(No.2018D-5007-0308.

Notes The authors declare no competing financial interest.

Reference (1) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive Force of a Single Gecko Foot-Hair. Nature 2000, 405 (6787), 681. (2) Hansen, W. R.; Autumn, K. Evidence for Self-Cleaning in Gecko Setae. PNAS 2005, 102 (2), 385-389. (3) Li, M.; Xu, Q.; Wu, X.; Li, W.; Lan, W.; Heng, L.; Street, J.; Xia, Z. Tough Reversible Adhesion Properties of a Dry Self-Cleaning Biomimetic Surface. ACS Appl. Mater. Interfaces 2018, 10 (31), 26787-26794. (4) Hu, S.; Xia, Z. Rational Design and Nanofabrication of Gecko‐Inspired Fibrillar Adhesives. Small 2012, 8 (16), 2464-2468. (5) Brodoceanu, D.; Bauer, C.; Kroner, E.; Arzt, E.; Kraus, T. Hierarchical Bioinspired Adhesive Surfaces—a Review. Bioinspiration Biomimetics 2016, 11 (5), 051001. (6) Varenberg, M.; Pugno, N. M.; Gorb, S. N. Spatulate Structures in Biological Fibrillar Adhesion. Soft Matter 2010, 6 (14), 3269-3272.


Langmuir 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

(7) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. Evidence for Van Der Waals Adhesion in Gecko Setae. PNAS 2002, 99 (19), 12252-12256. (8) Sun, W.; Neuzil, P.; Kustandi, T. S.; Oh, S.; Samper, V. D. The Nature of the Gecko Lizard Adhesive Force. Biophys. J. 2005, 89 (2), L14-L17. (9) Ma, Y.; Ma, S.; Wu, Y.; Pei, X.; Gorb, S. N.; Wang, Z.; Liu, W.; Zhou, F. Remote Control over Underwater Dynamic Attachment/Detachment and Locomotion. Adv. Mater. 2018, 1801595. (10) Sitti, M.; Fearing, R. S. Synthetic Gecko Foot-Hair Micro/Nano-Structures as Dry Adhesives. J. Adhes. Sci. Technol. 2003, 17 (8), 1055-1073. (11) Murphy, M. P.; Aksak, B.; Sitti, M. Gecko‐Inspired Directional and Controllable Adhesion. Small 2009, 5 (2), 170-175. (12) Mengüç, Y.; Yang, S. Y.; Kim, S.; Rogers, J. A.; Sitti, M. Gecko ‐ Inspired Controllable Adhesive Structures Applied to Micromanipulation. Adv. Funct. Mater. 2012, 22 (6), 1246-1254. (13) Qu, L.; Dai, L.; Stone, M.; Xia, Z.; Wang, Z. L. Carbon Nanotube Arrays with Strong Shear Binding-on and Easy Normal Lifting-off. Science 2008, 322 (5899), 238-242. (14) Hu, S.; Xia, Z.; Gao, X. Strong Adhesion and Friction Coupling in Hierarchical Carbon Nanotube Arrays for Dry Adhesive Applications. ACS Appl. Mater. Interfaces 2012, 4 (4), 1972-1980. (15) Chen, B.; Zhong, G.; Goldberg Oppenheimer, P.; Zhang, C.; Tornatzky, H.; Esconjauregui, S.; Hofmann, S.; Robertson, J. Influence of Packing Density and Surface Roughness of Vertically-Aligned Carbon Nanotubes On Adhesive Properties of Gecko-Inspired Mimetics. ACS Appl. Mater. Interfaces 2015, 7 (6), 3626-3632. (16) Xu, M.; Du, F.; Ganguli, S.; Roy, A.; Dai, L. Carbon Nanotube Dry Adhesives with Temperature-Enhanced Adhesion over a Large Temperature Range. Nat. Commun. 2016, 7, 13450. (17) Lu, M.; He, Q.; Li, Y.; Guo, F.; Dai, Z. The Effects of Radio-Frequency CF4


Page 18 of 30

Page 19 of 30 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

Langmuir

Plasma on Adhesion Property of Vertical Aligned Carbon Nanotube Arrays. Carbon 2018. (18) Stark, A. Y.; Badge, I.; Wucinich, N. A.; Sullivan, T. W.; Niewiarowski, P. H.; Dhinojwala, A. Surface Wettability Plays a Significant Role in Gecko Adhesion Underwater. PNAS 2013, 201219317. (19) Ji, K.; Zhang, J.; Chen, J.; Meng, G.; Ding, Y.; Dai, Z. Centrifugation-Assisted Fog-Collecting Abilities of Metal-Foam Structures with Different Surface Wettabilities. ACS Appl. Mater. Interfaces 2016, 8 (15), 10005-10013. (20) Soltannia, B.; Sameoto, D. Strong, Reversible Underwater Adhesion via GeckoInspired Hydrophobic Fibers. ACS Appl. Mater. Interfaces 2014, 6 (24), 2199522003. (21) Shahsavan, H.; Salili, S. M.; Jákli, A.; Zhao, B. Thermally Active Liquid Crystal Network Gripper Mimicking the Self‐Peeling of Gecko Toe Pads. Adv. Mater. 2017, 29 (3), 1604021. (22) Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448 (7151), 338. (23) Glass, P.; Chung, H.; Washburn, N. R.; Sitti, M. Enhanced Wet Adhesion and Shear of Elastomeric Micro-Fiber Arrays with Mushroom Tip Geometry and a Photopolymerized p(DMA-co-MEA) Tip Coating. Langmuir 2010, 26 (22), 17357-17362. (24) Xue, L.; Kovalev, A.; Eichler-Volf, A.; Steinhart, M.; Gorb, S. N. HumidityEnhanced Wet Adhesion on Insect-Inspired Fibrillar Adhesive Pads. Nat. Commun. 2015, 6, 6621. (25) Lee, B. P.; Messersmith, P. B.; Israelachvili, J. N.; Waite, J. H. Mussel-Inspired Adhesives and Coatings. Annu. Rev. Mater. Res. 2011, 41, 99-132. (26) Yang, H. C.; Waldman, R. Z.; Wu, M. B.; Hou, J.; Chen, L.; Darling, S. B.; Xu, Z. K. Dopamine: Just the Right Medicine for Membranes. Adv. Funct. Mater. 2018, 28 (8), 1705327. (27) Waite, J. H. The Formation of Mussel Byssus: Anatomy of a Natural Manufacturing Process. In Structure, cellular synthesis and assembly of


Langmuir 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

biopolymers; Springer: 1992; pp 27-54. (28) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-Molecule Mechanics of Mussel Adhesion. PNAS 2006, 103 (35), 12999-13003. (29) Yu, J.; Kan, Y.; Rapp, M.; Danner, E.; Wei, W.; Das, S.; Miller, D. R.; Chen, Y.; Waite, J. H.; Israelachvili, J. N. Adaptive Hydrophobic and Hydrophilic Interactions of Mussel Foot Proteins with Organic Thin Films. PNAS 2013, 201315015. (30) Ryu, J. H.; Messersmith, P. B.; Lee, H. Polydopamine Surface Chemistry: A Decade of Discovery. ACS Appl. Mater. Interfaces 2018, 10 (9), 7523-7540. (31) Qiu, W.-Z.; Yang, H.-C.; Xu, Z.-K. Dopamine-Assisted co-Deposition: An Emerging and Promising Strategy for Surface Modification. Adv. Colloid Interface Sci. 2018. (32) Yang, X.; Yan, L.; Ran, F.; Pal, A.; Long, J.; Shao, L. Interface-Confined Surface Engineering Constructing Water-Unidirectional Janus Membrane. J. Membr. Sci. 2019. (33) Sun, H.; Yang, X.; Zhang, Y.; Cheng, X.; Xu, Y.; Bai, Y.; Shao, L. SegregationInduced in Situ Hydrophilic Modification of Poly (Vinylidene Fluoride) Ultrafiltration Membranes via Sticky Poly (Ethylene Glycol) Blending. J. Membr. Sci. 2018, 563, 22-30. (34) Yang, X.; Sun, H.; Pal, A.; Bai, Y.; Shao, L. Biomimetic Silicification on Membrane Surface for Highly Efficient Treatments of Both Oil-in-Water Emulsion and Protein Wastewater. ACS Appl. Mater. Interfaces 2018, 10(35), 29982-29991. (35) Murphy, M. P.; Kim, S.; Sitti, M. Enhanced Adhesion by Gecko-Inspired Hierarchical Fibrillar Adhesives. ACS Appl. Mater. Interfaces 2009, 1 (4), 849855. (36) Jia, X.; Sheng, W.-b.; Li, W.; Tong, Y.-b.; Liu, Z.-y.; Zhou, F. Adhesive Polydopamine Coated Avermectin Microcapsules for Prolonging Foliar Pesticide Retention. ACS Appl. Mater. Interfaces 2014, 6 (22), 19552-19558. (37) Guo, D.-J.; Liu, R.; Cheng, Y.; Zhang, H.; Zhou, L.-M.; Fang, S.-M.; Elliott, W.


Page 20 of 30

Page 21 of 30 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

Langmuir

H.; Tan, W. Reverse Adhesion of a Gecko-Inspired Synthetic Adhesive Switched by an Ion-Exchange Polymer–Metal Composite Actuator. ACS Appl. Mater. Interfaces 2015, 7 (9), 5480-5487. (38) Li, L.; Yan, B.; Yang, J.; Chen, L.; Zeng, H. Novel Mussel‐Inspired Injectable Self‐Healing Hydrogel with Anti‐Biofouling Property. Adv. Mater. 2015, 27 (7), 1294-1299. (39) Chen, B.; Zhong, G.; Goldberg, O. P.; Zhang, C.; Tornatzky, H.; Esconjauregui, S.; Hofmann, S.; Robertson, J. Influence of Packing Density and Surface Roughness of Vertically-Aligned Carbon Nanotubes on Adhesive Properties of Gecko-Inspired Mimetics. Acs Appl Mater Interfaces 2015, 7 (6), 3626-3632. (40) Murphy, M. P.; Kim, S.; Sitti, M. Enhanced Adhesion by Gecko-Inspired Hierarchical Fibrillar Adhesives. ACS Appl Mater Interfaces 2009, 1 (4), 849-855. (41) Lee, H. D.; Kim, H. W.; Cho, Y. H.; Park, H. B. Experimental Evidence of Rapid Water Transport through Carbon Nanotubes Embedded in Polymeric Desalination Membranes. Small 2014, 10 (13), 2653-2660. (42) Yang, H.-C.; Liao, K.-J.; Huang, H.; Wu, Q.-Y.; Wan, L.-S.; Xu, Z.-K. MusselInspired Modification of a Polymer Membrane for Ultra-High Water Permeability and Oil-in-Water Emulsion Separation. J. Mater. Chem. A 2014, 2 (26), 1022510230.


Langmuir 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 diagram of carbon nanotubes modification with polydopamine. The red circle on the left shows the structure of gecko seta and the image of gecko. The right red ring reveals the adhesion of mussel byssus. The green circle at the middle shows the structural formula of dopamine.


Page 22 of 30

Page 23 of 30 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

Langmuir

Figure 2. Schematic description of surface functionalization of carbon nanotubes and the structure of PDA.


Langmuir 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 3. The scanning electron microscope (SEM) of original CNTs (a) and polydopamine modified CNTs (b). The side SEM of original CNTs (c) and polydopamine modified CNTs (d).


Page 24 of 30

Page 25 of 30 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

Langmuir

Figure 4. (a) The XPS spectrum of original CNTs and PDA@CNTs. The highresolution XPS spectra of C1s of CNTs (b) and PDA@CNTs (c). The high-resolution XPS spectra of N1s of PDA@CNTs (d). (e) Raman spectra. (f) FT-IR spectrum.


Langmuir 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 5. (a) The typical force-time (distance)curve. The curve of adhesion force against Dopamine concentration (b); Temperature (c) and Relative humidity (d).


Page 26 of 30

Page 27 of 30 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

Langmuir

Figure 6. (a) Schematic diagram of the AFM experiment. (b) Schematic diagram of the contacting between probe and PDA@CNTs at high relative humidity.


Langmuir 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 7. (a) Schematic diagram of the material bearing external force in shear direction. (b) Adhesion force in normal and shear direction against the length of dependent carbon tubes. (c) Representative diagram of the forces applied in different directions and angles. All the values in the figure are the average of 5 measurements in the same direction and the same force value.


Page 28 of 30

Page 29 of 30 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

Langmuir

Table 1. The preloading forces, retraction velocity and contact time in AFM tests. Preloading Force/μN Retraction Velocity/μm s-1 CNTs CNTs PDA@CNTs PDA@CNTs

1.0 0.1 1.0 0.1

1.0 0.1 1.0 0.1


Contact Time/s 0.1 1.0 0.1 1.0

Langmuir 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

Table of contents

Dopamine modification carbon nanotubes with enhanced adhesion property


Page 30 of 30

Enhanced adhesion of carbon nanotubes by dopamine modification

Recommend Documents