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Biomimetic superhydrophobic surfaces by combining mussel-inspired adhesion with lotusinspired coating
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 335602 (http://iopscience.iop.org/0957-4484/26/33/335602) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 26 (2015) 335602 (9pp)
doi:10.1088/0957-4484/26/33/335602
Biomimetic superhydrophobic surfaces by combining mussel-inspired adhesion with lotus-inspired coating Chao-Hua Xue1,2, Xue-Qing Ji1, Jing Zhang1, Jian-Zhong Ma1,2 and Shun-Tian Jia1 1
College of Resource and Environment, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China 2 Shaanxi Research Institute of Agricultural Products Processing Technology, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China E-mail: [email protected] Received 14 April 2015, revised 10 June 2015 Accepted for publication 1 July 2015 Published 29 July 2015 Abstract
Superhydrophobic surfaces on PET textiles were fabricated by combined bioinspiration from the strong adhesion of marine mussels and the two-scale structure of lotus leaves under mild conditions. Dopamine can spontaneously polymerize in alkaline aqueous solution to form a thin adhesive layer of polydopamine (PDA) wrapping on the micro-scale fibers. The as-formed thin PDA layer worked as a reactive template to generate PDA nanoparticles decorated on the fiber surfaces, imparting the textiles with excellent UV-shielding properties as well as a hierarchical structure similar to the morphology of the lotus leaf. After further modification with perfluorodecyl trichlorosilane, the textiles turned superhydrophobic with a water contact angle higher than 150°. Due to the strong adhesion of PDA to a wide range of materials, the present strategy may be extendable to fabrication of superhydrophobic surfaces on a variety of other substrates. Keywords: superhydrophobic surfaces, polydopamine, mussel-inspiration, lotus-inspiration, UVshielding property (Some figures may appear in colour only in the online journal) 1. Introduction
most of the existing methods are either complicated in operation or involving solvent exposure, hindering practical applications. The challenge remains to obtain superhydrophobic surfaces by using environmentally friendly and low cost chemicals as well as simple and mild conditions. In addition, enhancing the interfacial interaction between the nanostructured roughening layer and the substrate is also important for surface science and nanotechnology. The bioadhesion of marine mussels has drawn great interest in the last decades. Mussels can adhere to virtually all types of substrate with high binding strength in wet environments by secreting an adhesive protein [22]. Multifunctional polymer coatings could be formed through simple dip-coating of materials in aqueous solutions of dopamine on a wide range of inorganic and organic substrates, including
Surface modification plays an important role in combining the functionality of different materials or tuning the properties of a single surface, and bioinspired modification of surfaces has attracted considerable interest [1–7]. One typical example is biomimetic fabrication of artificial lotus leaves with superhydrophobic properties, which requires the combination of a specific surface topography based on dual-sized roughness and low surface energy of the substrates. In order to construct suitable roughening structures on substrates, various methods have been put forward, including physical or chemical etching [8–10], chemical vapor deposition [11, 12], electrospinning [13–15], template processing [16], sol–gel coating [17, 18], nanoparticle-based finishing [19–21], etc. However, 0957-4484/15/335602+09$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Scheme 1. Procedure for preparation of biomimetic superhydrophobic surfaces by combining mussel-inspired adhesion and lotus-inspired
coating.
originates from covalent binding as well as noncovalent interfacial interactions between substrates and catecholic derivatives of either small molecules or macromolecules in the protein [1, 23–26]. Studies have shown that polydopamine can easily form particles and aggregates on substrates after deposition [27, 28]. In this regard, the strong adhesion and aggregation of mussel protein may offer an alternative biomimetic strategy for in situ growth of nanoparticles or nanostructures on solid surfaces under mild wet conditions, which may help to roughen substrates to meet the topological requirements of superhydrophobic surfaces. Additionally, it
noble metals, oxides, polymers, semiconductors, and ceramics. Importantly, self-polymerization of dopamine on substrates can induce active groups on materials, which favor secondary reactions for construction of a variety of ad-layers, including self-assembled monolayers with long-chain molecules, metal films, and bio-based coatings of macromolecules. The strong interfacial interaction between the ad-layers and the original substrates has paved a broad way for chemical and physical modification of materials, including classically surface inert and adhesion-resistant materials such as poly (tetrafluoroethylene). It was revealed that the strong adhesion 2
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has been reported that polydopamine is a major pigment of naturally occurring melanin which possesses true absorption in the UV and visible light range [1, 26]. This property might endow outdoor materials with excellent photoprotection properties. In this work, the two attractive biomimetic subjects were incorporated together to develop a novel route to fabricate superhydrophobic surfaces, combining the remarkable adhesive ability of dopamine with the dual-scale structure of lotus leaves. Firstly, microscale fibers of textiles were immersed in an aqueous solution of dopamine at pH 8.5 for coating the adhesive film and in situ generation of nanostructures of polydopamine on the fiber surfaces. Then the polydopaminemodified textiles were hydrophobized using 1% perfluorodecyl trichlorosilane in ethanol to obtain superhydrophobic surfaces. The whole procedure was carried out in a mild environment, and no special instruments were needed. A typical procedure for preparing the superhydrophobic surfaces is shown in scheme 1.
2.4. Hydrophobic modification of fibers
The pristine or polydopamine-modified textiles were immersed in an ethanol solution of 1% perfluorodecyl trichlorosilane (PFDTS) for a given time, dried at 80 °C and baked for 5 min at 170 °C. 2.5. Characterization
To observe the surface morphology of the blended fabrics before and after modification, SEM measurements were carried out on a field emission scanning electron microscope (Hitachi S-4800, Japan). Textile samples were fixed on SEM holders using adhesive conductive tape. Prior to observation using SEM, all samples were metalized using a thin layer of gold. The wettability of the coated samples was evaluated at room temperature by means of the sessile-drop technique of a video optical contact angle system (OCA 20, Dataphysics, Germany). The reported values arise from the average contact angle (CA) measurements on five different spots of each sample. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific K-Alpha XPS spectrometer. The thermal decomposition of the textile samples was evaluated on a Simultaneous Thermal Analyzer (STA 409PC, NETZSCH, Germany). The reflectance and transmittance of the textiles were measured on an Agilent Cary 5000 UV–vis-NIR spectrophotometer in the range of 200 to 800 nm.
2. Experimental section 2.1. Materials
Dopamine hydrochloride (BR 99%) was purchased from Xiya Reagent Research Center, China. Tris(hydroxymethyl aminomethane) (Tris) was obtained from Aladdin Industrial Inc., Shanghai, China. All other chemicals were of analytical reagent grade and were used without further purification. Deionized water was used throughout all the experiments. In this study, the textiles used as substrates were cut into pieces of 6 × 20 cm2, and cleaned with anhydrous ethanol and deionized water, then dried at 80 °C before use.
3. Results and discussion 3.1. Mussel-inspired coating of polydopamine on fibers
Surface coating is an important strategy to modify inert substrates for further functionalization. As shown in figure 1(a), conventional PET fibers usually have smooth and clear surfaces with few functional chemical groups, showing little affinity for functional chemicals under aqueous conditions. Inspired by the bioadhesion of marine mussels, which can adhere to virtually all types of substrate with high binding strength in wet environments [29], dopamine deposition was considered as a technique to modify PET fibers in chemistry. After treatment with 2 mg mL−1 DA for 20 h in Tris buffer, the smooth fibers were coated with numerous nanostructured papillae, indicating successful modification of PET with polydopamine from oxidization of dopamine. In order to evaluate the roughness contribution of the polydopamine coating, the original and DA-treated textiles were modified using 1% PFDTS in ethanol solution. It was found that the original textile became hydrophobic with a CA of 137.36° ± 2.21°, while the DA-treated textiles showed higher hydrophobicity. Treatment with DA of 2 mg mL−1 increased the CA to 151.74° ± 1.27°, while concentrations of DA higher than 2 mg mL−1 contributed little to the hydrophobicity of the textiles. This might be because much higher concentration did not increase the roughness of the fibers, although aggregation depends on the concentration of dopamine [30, 31]. Previous studies demonstrated that when the
2.2. Surface modification of fibers with dopamine in Tris buffer
The fabric was immersed in 150 mL dopamine solution at various concentrations (1.0, 2.0, 3.0, 4.0, 5.0 g L−1) in Tris buffer (10 mM, pH 8.5) for different times (5, 10, 15, 20, 25 and 30 h) in a sealed container. During the reaction, the container was continuously rotated to keep the fabric evenly coated at 35 °C. After that, the sample was washed sequentially with anhydrous ethanol and deionized water until the solution was clear without apparent suspension. 2.3. Surface modification of fibers with dopamine in phosphate and Tris buffer
Firstly, the textiles were immersed in freshly prepared dopamine solution at a given concentration (1.0, 2.0, 3.0, 4.0, 5.0 g L−1) in phosphate buffer (pH 8.5) for 4 h at 40 °C. Secondly, the textile was taken out and put into another 150 mL dopamine solution in Tris buffer (10 mM, pH 8.5, 40 °C) for another 5 h. During the whole procedure, the container was continuously rotated. After that, the sample was washed sequentially using anhydrous ethanol and deionized water until the solution was clear without apparent suspension. 3
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Figure 1. SEM images of PET: (a) original; (b) modified with DA of 2 mg mL
for 20 h.
Figure 2. CA changes of PDA-treated PET textiles after hydrophobization. (a) Variation of CA with concentration of DA over 20 h; (b)
variation of CA with modification time in DA.
dopamine concentration is high enough, its oxidation polymerization happens not only at the surface of the substrate, but also in solution. However, dopamine molecules are more easily trapped within the solution, accelerating aggregation rather than depositing on the substrate, and there is a saturation in the deposition kinetics, thus resulting in a decrease in the surface roughness [31, 32]. The effect of polymerization time on the CA of the textiles was also investigated, and the results are shown in figure 2(b). It was found that prolonging the treatment time of DA increased the CA, while a longer time than 20 h did not result in enhancement in the hydrophobicity of the textiles.
however, it only increased the CA to just over 150°. A previous study showed that polydopamine particles are larger in phosphate solution and remarkably small in Tris buffer [30]. In this work, combination treatment of PET textiles in phosphate and Tris buffers was applied in order to improve the roughness by increasing the size of the polydopamine aggregates on the fibers. Figure 3 shows SEM images of PET fibers treated with various concentrations of DA in phosphate followed by Tris buffers. It was found that combination treatment led to a denser coating of polydopamine with nanostructures on the fibers, thus making the textiles superhydrophobic after hydrophobization, as shown in figure 6. Similarly, higher concentration of DA did not lead to an enhancement in the roughness of the fiber surfaces; this might be because higher concentration would cause partial shedding of polydopamine films from the fiber surfaces, as indicated by the arrows in figures 3(d), (e). It was found that in this procedure treatment with DA of 2 mg mL−1 for 20 h led to higher roughness, making the textile have a CA as high as 160.26° ± 1.12°.
3.2. Lotus-inspired roughening of fibers by polydopamine aggregates
With regards to the stable superhydrophobicity of materials, suitable roughness of surfaces is very important. The treatment of PET textiles with DA in Tris buffer as described roughened the fiber surface with a polydopamine coating, 4
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Figure 3. SEM images of PET fibers treated with various concentrations of DA: (a) 1 g L ; (b) 2 g L ; (c) 3 g L ; (d) 4 g L ; (e) 5 g L ;
(f) CA changes of treated PET textiles with concentration of DA for 20 h after hydrophobization; insets are higher magnification images of the corresponding samples.
substrate [1, 22, 23, 25]. In this work, besides noncovalent interactions, covalent bonds of PDA on PET may be formed by the interactions of the hydroxyl groups on PDA and PET, and the interactions between the hydroxyl groups on PDA and the carboxyl groups on PET, which may be similar to the
3.3. Chemical analysis of the modified textiles
It was confirmed in previous work that dopamine through self-polymerization can form strong covalent and noncovalent interactions with virtually all types of inorganic and organic 5
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Figure 4. (a) XPS survey spectra of the PET fabric before and after modification. High-resolution XPS spectra of C 1 s: (b) original PET
textiles; (c) PDA-treated PET textiles; (d) superhydrophobic PDA-treated PET textiles.
(figure 4(a)) and the formation of new peaks for –CF2 (291.8 eV) and –CF3 (293.2 eV) groups (figure 4(d)).
covalent surface modification of substrates by catechol [33]. The chemical composition of the PET- and PDA-coated PET was measured by XPS, which is commonly used to confirm successful modification of substrates with PDA [31, 34]. As shown in figure 4(a), the XPS survey spectrum indicates that the original PET textile was composed of the elements carbon and oxygen. The presence of an N1s peak, F1s peak and –CFx peak (–CF2 and –CF3) together with the attenuation of –CH, C–O and O–C–O peaks, confirmed that PDA particles and PFDTS had been successfully coated and self-assembled on the PET surface, respectively. Figure 4(b) shows the high resolution C 1 s peak of the original PET textile, which exhibited four distinct sub-peaks at 285.0 eV for –CHX (C–C and C–H) bonds, 286.8 eV for C–O bonds, 290.3 eV for C=O bonds and 291.7 eV for C=C. The weak N1s signal for the PET-PDA textile is consistent with the nature of the polydopamine deposition on the PET textile surface, and a new curve-fitted peak component with binding energy at about 287 eV appeared, attributable to C–N. The obvious indications of the presence of a PFDTS molecular layer were the strong increase in the intensity of the F 1 s peak at 689.0 eV
3.4. Thermogravimetric analysis of the modified textiles
The thermal decomposition of the pristine PDA, original PET textiles, PDA-coated textiles, and superhydrophobic PDAtreated PET textiles in nitrogen atmosphere is shown in figure 5. It can be seen from the curves that the original PET textiles went through two weight losses, and the sharp weight loss between 380 and 450 °C corresponds to the decomposition of the PET backbone. As the temperature reached 800 °C, the carbonized PET remaining was about 9.4%. After treatment with PDA, the residue amount increased to 16.5% at 800 °C indicating that the PDA-treated PET textile shows higher stability than PET. This is attributed to the good thermal stability of PDA, as seen from the weight loss curve of PDA in figure 5 which is in agreement with a previous report [35]. However, the thermal stability of the superhydrophobic PDA-treated PET textiles was close to that of PET. This may be because the modification of PDA-treated 6
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colourless to yellow-brown and the textiles became hydrophilic. In a strong alkaline environment, catechol groups are deprotonated and the hydrogen bonding between PDA molecules is destroyed [36], hence most of the coating layer of PDA fell off the fiber surface into the solution, eliminating the roughening structures. Further improvement of the stability of the roughening structures could be achieved by utilizing the strong adhesion of PDA and constructing covalent bonding between the fibers and PDA. 3.6. Optical properties of the superhydrophobic textiles
Polydopamine is a major pigment of naturally occurring melanin (eumelanin) [1, 37]. The colour of the textiles changed from white to brown after the coating of polydopamine. There is no specific reflectance in the range of visible light, as shown in figure 7(a). To examine the colour change effect on its optical properties, we also checked the transmittance of the textiles in the range of 200 to 800 nm. In figure 7(b), the untreated textile has a low transmittance at 200–300 nm, but a higher one at 400–800 nm. After treatment with PDA, the transmittance of the textiles was lowered to near zero from the ultraviolet to the visible region. This is consistent with a previous study [26] which showed that the optical density spectrum of PDA-coated substrates across all wavelengths can be interpreted as true absorption. As there is no distinct chromophoric band, it is believed that polydopamine is a disordered organic semiconductor. The appearance of absorption in the UV region of the light spectrum was ascribed to the oxidation of dopamine to dopachrome and dopaindole, and the pronounced absorption in the visible region is considered to be a result of the subsequent self-polymerization process [26, 38]. The strong absorbance of the superhydrophobic textiles in the ultraviolet wavelengths may find great potential applications in photoprotection. This strategy using PDA to impart UV-shielding properties to materials should be superior to those using UVabsorbing nanomaterials such as ZnO, TiO2, and others, because these inorganic nanomaterials have photodegradation issues to be solved [39, 40].
Figure 5. Thermogravimetric analysis results for (a) pristine PDA;
(b) original PET textiles; (c) PDA-treated PET textiles; (d) superhydrophobic PDA-treated PET textiles.
PET textiles using PFDTS interrupted the effective carbonization of PDA on PET, but this needs further investigation. 3.5. Durability of the superhydrophobic textiles
It has been demonstrated that polydopamine can adhere to and roughen fiber surfaces with nanostructures through the asdescribed bioadhesion technique, and hydrophobization of the polydopamine-modified fibers can impart the textiles with superhydrophobicity. It was considered that the strong adhesion of polydopamine structures might make the hydrophobized textiles have durable superhydrophobicity. The abrasion resistance of the textiles was evaluated by using a modified procedure according to our previous report based on the AATCC Test Method 8-2001 [9, 10]. All of the textiles used in the test were modified with perfluorodecyl trichlorosilane for 15 min. The CAs were almost unchanged over 100 cycles of abrasion, as shown in figure 6(a), showing the good durability of superhydrophobicity of the textiles to some degree. Further abrasion led to a decrease in CA, making the textiles lose their superhydrophobicity. When the abrasion cycles proceeded 1000 times, the CA showed a little increase which may be due to the improved roughness from the mechanically-induced fibers protruding [9]. It is obvious that the mechanical durability of the superhydrophobic properties of the textiles is not as strong as that reported in our previous work [9, 10]. The washing durability was also evaluated in 2 mol L−1 HCl solution and deionized water through sonication at 100 W. Figure 6(b) shows that under high power ultrasonic treatment, the hydrophobicity of the textiles decreased, indicating that the polydopamine coated on the fibers may be removed by ultrasound under neutral and acidic conditions, weakening the roughness. Meanwhile, ultrasonic treatment of fibers in NaOH solution changed the solution colour from
4. Conclusions Superhydrophobic surfaces were fabricated on PET textiles by combined bioinspiration from the strong adhesion of marine mussels and the two-scale structures of lotus leaves. A PDA coating with nanostructures roughened the fibers, favoring the superhydrophobicity of the textiles. The whole procedure was carried out in a mild environment, and no special instruments were needed. Utilizing the excellent UV absorption properties, PDA-treated textiles with superhydrophobicity might find great potential applications in photoprotection for outdoor products. Considering the high adhesion of catecholic derivatives to almost any substance,
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Nanotechnology 26 (2015) 335602
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Figure 6. (a) Changes of CAs of superhydrophobic textiles with abrasion cycles; (b) changes of CAs of superhydrophobic textiles with
ultrasonication time.
Figure 7. (a) UV-visible reflectance of PET, PDA-treated PET and superhydrophobic PDA-treated PET textiles; (b) UV-visible transmittance of PET, PDA-treated PET and superhydrophobic PDA-treated PET textiles.
References
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Acknowledgments This work was supported by the National Natural Science Foundation of China (51372146), the Program for New Century Excellent Talents in University (NCET-12-1042), the Research Fund for the Doctoral Program of Higher Education of China (20116125110002, 20136125110003), the Major Program of Science Foundation of Shaanxi Province (2011ZKC05-7), the Key Scientific Research Group of Shaanxi province (2013KCT-08), and the Scientific Research Group of Shaanxi University of Science and Technology (TD12-03).
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Biomimetic superhydrophobic surfaces by combining mussel-inspired adhesion with lotusinspired coating
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 335602 (http://iopscience.iop.org/0957-4484/26/33/335602) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 131.111.164.128 This content was downloaded on 30/07/2015 at 04:49
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 335602 (9pp)
doi:10.1088/0957-4484/26/33/335602
Biomimetic superhydrophobic surfaces by combining mussel-inspired adhesion with lotus-inspired coating Chao-Hua Xue1,2, Xue-Qing Ji1, Jing Zhang1, Jian-Zhong Ma1,2 and Shun-Tian Jia1 1
College of Resource and Environment, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China 2 Shaanxi Research Institute of Agricultural Products Processing Technology, Shaanxi University of Science and Technology, Xi’an 710021, People’s Republic of China E-mail: [email protected] Received 14 April 2015, revised 10 June 2015 Accepted for publication 1 July 2015 Published 29 July 2015 Abstract
Superhydrophobic surfaces on PET textiles were fabricated by combined bioinspiration from the strong adhesion of marine mussels and the two-scale structure of lotus leaves under mild conditions. Dopamine can spontaneously polymerize in alkaline aqueous solution to form a thin adhesive layer of polydopamine (PDA) wrapping on the micro-scale fibers. The as-formed thin PDA layer worked as a reactive template to generate PDA nanoparticles decorated on the fiber surfaces, imparting the textiles with excellent UV-shielding properties as well as a hierarchical structure similar to the morphology of the lotus leaf. After further modification with perfluorodecyl trichlorosilane, the textiles turned superhydrophobic with a water contact angle higher than 150°. Due to the strong adhesion of PDA to a wide range of materials, the present strategy may be extendable to fabrication of superhydrophobic surfaces on a variety of other substrates. Keywords: superhydrophobic surfaces, polydopamine, mussel-inspiration, lotus-inspiration, UVshielding property (Some figures may appear in colour only in the online journal) 1. Introduction
most of the existing methods are either complicated in operation or involving solvent exposure, hindering practical applications. The challenge remains to obtain superhydrophobic surfaces by using environmentally friendly and low cost chemicals as well as simple and mild conditions. In addition, enhancing the interfacial interaction between the nanostructured roughening layer and the substrate is also important for surface science and nanotechnology. The bioadhesion of marine mussels has drawn great interest in the last decades. Mussels can adhere to virtually all types of substrate with high binding strength in wet environments by secreting an adhesive protein [22]. Multifunctional polymer coatings could be formed through simple dip-coating of materials in aqueous solutions of dopamine on a wide range of inorganic and organic substrates, including
Surface modification plays an important role in combining the functionality of different materials or tuning the properties of a single surface, and bioinspired modification of surfaces has attracted considerable interest [1–7]. One typical example is biomimetic fabrication of artificial lotus leaves with superhydrophobic properties, which requires the combination of a specific surface topography based on dual-sized roughness and low surface energy of the substrates. In order to construct suitable roughening structures on substrates, various methods have been put forward, including physical or chemical etching [8–10], chemical vapor deposition [11, 12], electrospinning [13–15], template processing [16], sol–gel coating [17, 18], nanoparticle-based finishing [19–21], etc. However, 0957-4484/15/335602+09$33.00
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Scheme 1. Procedure for preparation of biomimetic superhydrophobic surfaces by combining mussel-inspired adhesion and lotus-inspired
coating.
originates from covalent binding as well as noncovalent interfacial interactions between substrates and catecholic derivatives of either small molecules or macromolecules in the protein [1, 23–26]. Studies have shown that polydopamine can easily form particles and aggregates on substrates after deposition [27, 28]. In this regard, the strong adhesion and aggregation of mussel protein may offer an alternative biomimetic strategy for in situ growth of nanoparticles or nanostructures on solid surfaces under mild wet conditions, which may help to roughen substrates to meet the topological requirements of superhydrophobic surfaces. Additionally, it
noble metals, oxides, polymers, semiconductors, and ceramics. Importantly, self-polymerization of dopamine on substrates can induce active groups on materials, which favor secondary reactions for construction of a variety of ad-layers, including self-assembled monolayers with long-chain molecules, metal films, and bio-based coatings of macromolecules. The strong interfacial interaction between the ad-layers and the original substrates has paved a broad way for chemical and physical modification of materials, including classically surface inert and adhesion-resistant materials such as poly (tetrafluoroethylene). It was revealed that the strong adhesion 2
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has been reported that polydopamine is a major pigment of naturally occurring melanin which possesses true absorption in the UV and visible light range [1, 26]. This property might endow outdoor materials with excellent photoprotection properties. In this work, the two attractive biomimetic subjects were incorporated together to develop a novel route to fabricate superhydrophobic surfaces, combining the remarkable adhesive ability of dopamine with the dual-scale structure of lotus leaves. Firstly, microscale fibers of textiles were immersed in an aqueous solution of dopamine at pH 8.5 for coating the adhesive film and in situ generation of nanostructures of polydopamine on the fiber surfaces. Then the polydopaminemodified textiles were hydrophobized using 1% perfluorodecyl trichlorosilane in ethanol to obtain superhydrophobic surfaces. The whole procedure was carried out in a mild environment, and no special instruments were needed. A typical procedure for preparing the superhydrophobic surfaces is shown in scheme 1.
2.4. Hydrophobic modification of fibers
The pristine or polydopamine-modified textiles were immersed in an ethanol solution of 1% perfluorodecyl trichlorosilane (PFDTS) for a given time, dried at 80 °C and baked for 5 min at 170 °C. 2.5. Characterization
To observe the surface morphology of the blended fabrics before and after modification, SEM measurements were carried out on a field emission scanning electron microscope (Hitachi S-4800, Japan). Textile samples were fixed on SEM holders using adhesive conductive tape. Prior to observation using SEM, all samples were metalized using a thin layer of gold. The wettability of the coated samples was evaluated at room temperature by means of the sessile-drop technique of a video optical contact angle system (OCA 20, Dataphysics, Germany). The reported values arise from the average contact angle (CA) measurements on five different spots of each sample. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific K-Alpha XPS spectrometer. The thermal decomposition of the textile samples was evaluated on a Simultaneous Thermal Analyzer (STA 409PC, NETZSCH, Germany). The reflectance and transmittance of the textiles were measured on an Agilent Cary 5000 UV–vis-NIR spectrophotometer in the range of 200 to 800 nm.
2. Experimental section 2.1. Materials
Dopamine hydrochloride (BR 99%) was purchased from Xiya Reagent Research Center, China. Tris(hydroxymethyl aminomethane) (Tris) was obtained from Aladdin Industrial Inc., Shanghai, China. All other chemicals were of analytical reagent grade and were used without further purification. Deionized water was used throughout all the experiments. In this study, the textiles used as substrates were cut into pieces of 6 × 20 cm2, and cleaned with anhydrous ethanol and deionized water, then dried at 80 °C before use.
3. Results and discussion 3.1. Mussel-inspired coating of polydopamine on fibers
Surface coating is an important strategy to modify inert substrates for further functionalization. As shown in figure 1(a), conventional PET fibers usually have smooth and clear surfaces with few functional chemical groups, showing little affinity for functional chemicals under aqueous conditions. Inspired by the bioadhesion of marine mussels, which can adhere to virtually all types of substrate with high binding strength in wet environments [29], dopamine deposition was considered as a technique to modify PET fibers in chemistry. After treatment with 2 mg mL−1 DA for 20 h in Tris buffer, the smooth fibers were coated with numerous nanostructured papillae, indicating successful modification of PET with polydopamine from oxidization of dopamine. In order to evaluate the roughness contribution of the polydopamine coating, the original and DA-treated textiles were modified using 1% PFDTS in ethanol solution. It was found that the original textile became hydrophobic with a CA of 137.36° ± 2.21°, while the DA-treated textiles showed higher hydrophobicity. Treatment with DA of 2 mg mL−1 increased the CA to 151.74° ± 1.27°, while concentrations of DA higher than 2 mg mL−1 contributed little to the hydrophobicity of the textiles. This might be because much higher concentration did not increase the roughness of the fibers, although aggregation depends on the concentration of dopamine [30, 31]. Previous studies demonstrated that when the
2.2. Surface modification of fibers with dopamine in Tris buffer
The fabric was immersed in 150 mL dopamine solution at various concentrations (1.0, 2.0, 3.0, 4.0, 5.0 g L−1) in Tris buffer (10 mM, pH 8.5) for different times (5, 10, 15, 20, 25 and 30 h) in a sealed container. During the reaction, the container was continuously rotated to keep the fabric evenly coated at 35 °C. After that, the sample was washed sequentially with anhydrous ethanol and deionized water until the solution was clear without apparent suspension. 2.3. Surface modification of fibers with dopamine in phosphate and Tris buffer
Firstly, the textiles were immersed in freshly prepared dopamine solution at a given concentration (1.0, 2.0, 3.0, 4.0, 5.0 g L−1) in phosphate buffer (pH 8.5) for 4 h at 40 °C. Secondly, the textile was taken out and put into another 150 mL dopamine solution in Tris buffer (10 mM, pH 8.5, 40 °C) for another 5 h. During the whole procedure, the container was continuously rotated. After that, the sample was washed sequentially using anhydrous ethanol and deionized water until the solution was clear without apparent suspension. 3
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Figure 1. SEM images of PET: (a) original; (b) modified with DA of 2 mg mL
for 20 h.
Figure 2. CA changes of PDA-treated PET textiles after hydrophobization. (a) Variation of CA with concentration of DA over 20 h; (b)
variation of CA with modification time in DA.
dopamine concentration is high enough, its oxidation polymerization happens not only at the surface of the substrate, but also in solution. However, dopamine molecules are more easily trapped within the solution, accelerating aggregation rather than depositing on the substrate, and there is a saturation in the deposition kinetics, thus resulting in a decrease in the surface roughness [31, 32]. The effect of polymerization time on the CA of the textiles was also investigated, and the results are shown in figure 2(b). It was found that prolonging the treatment time of DA increased the CA, while a longer time than 20 h did not result in enhancement in the hydrophobicity of the textiles.
however, it only increased the CA to just over 150°. A previous study showed that polydopamine particles are larger in phosphate solution and remarkably small in Tris buffer [30]. In this work, combination treatment of PET textiles in phosphate and Tris buffers was applied in order to improve the roughness by increasing the size of the polydopamine aggregates on the fibers. Figure 3 shows SEM images of PET fibers treated with various concentrations of DA in phosphate followed by Tris buffers. It was found that combination treatment led to a denser coating of polydopamine with nanostructures on the fibers, thus making the textiles superhydrophobic after hydrophobization, as shown in figure 6. Similarly, higher concentration of DA did not lead to an enhancement in the roughness of the fiber surfaces; this might be because higher concentration would cause partial shedding of polydopamine films from the fiber surfaces, as indicated by the arrows in figures 3(d), (e). It was found that in this procedure treatment with DA of 2 mg mL−1 for 20 h led to higher roughness, making the textile have a CA as high as 160.26° ± 1.12°.
3.2. Lotus-inspired roughening of fibers by polydopamine aggregates
With regards to the stable superhydrophobicity of materials, suitable roughness of surfaces is very important. The treatment of PET textiles with DA in Tris buffer as described roughened the fiber surface with a polydopamine coating, 4
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Figure 3. SEM images of PET fibers treated with various concentrations of DA: (a) 1 g L ; (b) 2 g L ; (c) 3 g L ; (d) 4 g L ; (e) 5 g L ;
(f) CA changes of treated PET textiles with concentration of DA for 20 h after hydrophobization; insets are higher magnification images of the corresponding samples.
substrate [1, 22, 23, 25]. In this work, besides noncovalent interactions, covalent bonds of PDA on PET may be formed by the interactions of the hydroxyl groups on PDA and PET, and the interactions between the hydroxyl groups on PDA and the carboxyl groups on PET, which may be similar to the
3.3. Chemical analysis of the modified textiles
It was confirmed in previous work that dopamine through self-polymerization can form strong covalent and noncovalent interactions with virtually all types of inorganic and organic 5
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Figure 4. (a) XPS survey spectra of the PET fabric before and after modification. High-resolution XPS spectra of C 1 s: (b) original PET
textiles; (c) PDA-treated PET textiles; (d) superhydrophobic PDA-treated PET textiles.
(figure 4(a)) and the formation of new peaks for –CF2 (291.8 eV) and –CF3 (293.2 eV) groups (figure 4(d)).
covalent surface modification of substrates by catechol [33]. The chemical composition of the PET- and PDA-coated PET was measured by XPS, which is commonly used to confirm successful modification of substrates with PDA [31, 34]. As shown in figure 4(a), the XPS survey spectrum indicates that the original PET textile was composed of the elements carbon and oxygen. The presence of an N1s peak, F1s peak and –CFx peak (–CF2 and –CF3) together with the attenuation of –CH, C–O and O–C–O peaks, confirmed that PDA particles and PFDTS had been successfully coated and self-assembled on the PET surface, respectively. Figure 4(b) shows the high resolution C 1 s peak of the original PET textile, which exhibited four distinct sub-peaks at 285.0 eV for –CHX (C–C and C–H) bonds, 286.8 eV for C–O bonds, 290.3 eV for C=O bonds and 291.7 eV for C=C. The weak N1s signal for the PET-PDA textile is consistent with the nature of the polydopamine deposition on the PET textile surface, and a new curve-fitted peak component with binding energy at about 287 eV appeared, attributable to C–N. The obvious indications of the presence of a PFDTS molecular layer were the strong increase in the intensity of the F 1 s peak at 689.0 eV
3.4. Thermogravimetric analysis of the modified textiles
The thermal decomposition of the pristine PDA, original PET textiles, PDA-coated textiles, and superhydrophobic PDAtreated PET textiles in nitrogen atmosphere is shown in figure 5. It can be seen from the curves that the original PET textiles went through two weight losses, and the sharp weight loss between 380 and 450 °C corresponds to the decomposition of the PET backbone. As the temperature reached 800 °C, the carbonized PET remaining was about 9.4%. After treatment with PDA, the residue amount increased to 16.5% at 800 °C indicating that the PDA-treated PET textile shows higher stability than PET. This is attributed to the good thermal stability of PDA, as seen from the weight loss curve of PDA in figure 5 which is in agreement with a previous report [35]. However, the thermal stability of the superhydrophobic PDA-treated PET textiles was close to that of PET. This may be because the modification of PDA-treated 6
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colourless to yellow-brown and the textiles became hydrophilic. In a strong alkaline environment, catechol groups are deprotonated and the hydrogen bonding between PDA molecules is destroyed [36], hence most of the coating layer of PDA fell off the fiber surface into the solution, eliminating the roughening structures. Further improvement of the stability of the roughening structures could be achieved by utilizing the strong adhesion of PDA and constructing covalent bonding between the fibers and PDA. 3.6. Optical properties of the superhydrophobic textiles
Polydopamine is a major pigment of naturally occurring melanin (eumelanin) [1, 37]. The colour of the textiles changed from white to brown after the coating of polydopamine. There is no specific reflectance in the range of visible light, as shown in figure 7(a). To examine the colour change effect on its optical properties, we also checked the transmittance of the textiles in the range of 200 to 800 nm. In figure 7(b), the untreated textile has a low transmittance at 200–300 nm, but a higher one at 400–800 nm. After treatment with PDA, the transmittance of the textiles was lowered to near zero from the ultraviolet to the visible region. This is consistent with a previous study [26] which showed that the optical density spectrum of PDA-coated substrates across all wavelengths can be interpreted as true absorption. As there is no distinct chromophoric band, it is believed that polydopamine is a disordered organic semiconductor. The appearance of absorption in the UV region of the light spectrum was ascribed to the oxidation of dopamine to dopachrome and dopaindole, and the pronounced absorption in the visible region is considered to be a result of the subsequent self-polymerization process [26, 38]. The strong absorbance of the superhydrophobic textiles in the ultraviolet wavelengths may find great potential applications in photoprotection. This strategy using PDA to impart UV-shielding properties to materials should be superior to those using UVabsorbing nanomaterials such as ZnO, TiO2, and others, because these inorganic nanomaterials have photodegradation issues to be solved [39, 40].
Figure 5. Thermogravimetric analysis results for (a) pristine PDA;
(b) original PET textiles; (c) PDA-treated PET textiles; (d) superhydrophobic PDA-treated PET textiles.
PET textiles using PFDTS interrupted the effective carbonization of PDA on PET, but this needs further investigation. 3.5. Durability of the superhydrophobic textiles
It has been demonstrated that polydopamine can adhere to and roughen fiber surfaces with nanostructures through the asdescribed bioadhesion technique, and hydrophobization of the polydopamine-modified fibers can impart the textiles with superhydrophobicity. It was considered that the strong adhesion of polydopamine structures might make the hydrophobized textiles have durable superhydrophobicity. The abrasion resistance of the textiles was evaluated by using a modified procedure according to our previous report based on the AATCC Test Method 8-2001 [9, 10]. All of the textiles used in the test were modified with perfluorodecyl trichlorosilane for 15 min. The CAs were almost unchanged over 100 cycles of abrasion, as shown in figure 6(a), showing the good durability of superhydrophobicity of the textiles to some degree. Further abrasion led to a decrease in CA, making the textiles lose their superhydrophobicity. When the abrasion cycles proceeded 1000 times, the CA showed a little increase which may be due to the improved roughness from the mechanically-induced fibers protruding [9]. It is obvious that the mechanical durability of the superhydrophobic properties of the textiles is not as strong as that reported in our previous work [9, 10]. The washing durability was also evaluated in 2 mol L−1 HCl solution and deionized water through sonication at 100 W. Figure 6(b) shows that under high power ultrasonic treatment, the hydrophobicity of the textiles decreased, indicating that the polydopamine coated on the fibers may be removed by ultrasound under neutral and acidic conditions, weakening the roughness. Meanwhile, ultrasonic treatment of fibers in NaOH solution changed the solution colour from
4. Conclusions Superhydrophobic surfaces were fabricated on PET textiles by combined bioinspiration from the strong adhesion of marine mussels and the two-scale structures of lotus leaves. A PDA coating with nanostructures roughened the fibers, favoring the superhydrophobicity of the textiles. The whole procedure was carried out in a mild environment, and no special instruments were needed. Utilizing the excellent UV absorption properties, PDA-treated textiles with superhydrophobicity might find great potential applications in photoprotection for outdoor products. Considering the high adhesion of catecholic derivatives to almost any substance,
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Figure 6. (a) Changes of CAs of superhydrophobic textiles with abrasion cycles; (b) changes of CAs of superhydrophobic textiles with
ultrasonication time.
Figure 7. (a) UV-visible reflectance of PET, PDA-treated PET and superhydrophobic PDA-treated PET textiles; (b) UV-visible transmittance of PET, PDA-treated PET and superhydrophobic PDA-treated PET textiles.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (51372146), the Program for New Century Excellent Talents in University (NCET-12-1042), the Research Fund for the Doctoral Program of Higher Education of China (20116125110002, 20136125110003), the Major Program of Science Foundation of Shaanxi Province (2011ZKC05-7), the Key Scientific Research Group of Shaanxi province (2013KCT-08), and the Scientific Research Group of Shaanxi University of Science and Technology (TD12-03).
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