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Biomimetic robust superhydrophobic stainless-steel surfaces with antimicrobial activity and molecular dynamics simulation
Chemical Engineering Journal 372 (2019) 852–861
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Biomimetic robust superhydrophobic stainless-steel surfaces with antimicrobial activity and molecular dynamics simulation
T
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Shuyi Lia, Yan Liua, , Zaihang Zhenga,b, Xin Liub, Honglan Huangc, Zhiwu Hana, Luquan Rena a
Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, China School of Chemical Engineering, Changchun University of Technology, Changchun 130012, China c College of Basic Medical Science, Jilin University, Changchun 130022, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
stainless-steel surfaces • Biomimetic were prepared by laser etching and modification.
simulations suggested the PDA@ • MD ODA compounds how to adsorb on the substrate.
results of MD simulations were • The consistent with experimental data. surfaces presented super• The hydrophobicity, antibacterial property and stability.
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomimetic Superhydrophobic Molecular dynamics simulation Antimicrobial Stainless-steel substrate
Wettability is an effective strategy to reduce the adhesion force between bacteria and a solid surface, also is favor to effectively overcome the limitation of the substrate materials and expand their practical applications. Herein, inspired by the special micro/nano structures of the creatures and the bioadhesion of marine mussels, a green and simple two-step method, the laser interference patterning and in-situ polymerization, was used to fabricate the superhydrophobic surface with antibacterial property on the stainless-steel (SS) substrate. As well, this patterned surface also has successfully realized the wettability transition from superhydrophilic to superhydrophobic. Meanwhile, the results of MD simulations directly suggested the PDA@ODA compounds how to adsorb on the SS substrate and interact with water molecules, which was consistent with the experimental. In addition, according to the agar plate assays and fluorescent microscope tests, the obtained specimens presented a certain antimicrobial activity. Moreover, the samples had good robustness in the heat resistance, physical performance and durability, which will expand the application fields and working life.
1. Introduction Currently, biocontamination or biofouling is not an uncommon problem, which is the main source of health associated infections,
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including water purification systems, surgical equipment, food packaging and food storage, marine and industrial equipment [1–6]. Through evolution and optimization over millions of years, lots of creatures in the nature, such as lotus leaves [7], geckos [8], shark skin [9], and
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.cej.2019.04.200 Received 26 December 2018; Received in revised form 25 April 2019; Accepted 28 April 2019 Available online 29 April 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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simulations are not common. Herein, inspired by the special micro/nano structures of the creatures and the bioadhesion of marine mussels, we reported a two-step method to fabricate the superhydrophobic surface with antibacterial property, which anchored the chemical compositions onto the patterned stainless-steel substrate. Firstly, the Laser Interference Patterning (LIP) is applied to prepare micro/nano structures with advantages such as low pollution, efficient, and selective area processing. Moreover, the in-situ polymerization reaction about polydopamine and octadecylamine also is mild, simple, inexpensive, quick and green. Polydopamine (PDA), the adhesive proteins secreted by mussels, can be regarded as a versatile platform for secondary surface-mediated reactions, leading ultimately grafting polymer coatings to metal under mild wet conditions [38–41]. Meanwhile, combined with molecular dynamic simulation, no matter the formation process of superhydrophobic coatings on the stainless steel or the surface adsorption process of water molecules were systematically verified in theory. In addition, we also investigated the surface properties such as antimicrobial activity, wettability and robustness. Comparatively speaking, this method has the advantages of mild, green, and can be applied to the large-scale preparation of a variety of materials.
insect wings such as cicadae [10], dragonflies and butterflies [11] etc., have been formed a diverse range of micro/nano-structures and special properties, which will provide a potentially rich blueprint for the development of artificial antifouling surfaces for applications in medicine and industry. Meanwhile, lots of research results show that the way to inhibit the bacterial infection is not only limited to antibiotics or the material property, but also can change the biological adhesion activity of materials by varying the surface wettability of the substrate materials [12–15]. Superhydrophobic surface is known to possess a high degree of water repellency, which enables dirt particles to be carried away while the liquid droplet rolls off, famous as “lotus effect” [6,16,17]. Nowadays, developing biofouling material based on superhydrophobicity is an effective strategy to reduce the adhesion force between bacteria and a solid surface and has yet to be studied thoroughly and systematically, due to the weak bacterial attachment for the low protein adsorption and easy protein detachment [18–22]. In addition, the surface microstructure can induce the mechanical disruption of cell membranes to kill bacteria, which is extremely effective in achieving antimicrobial properties [8,10,13,23]. At the same time, the surface microstructure is also critical for the construction of superhydrophobic surface. The preparation process of these surfaces usually accompanied either by covalently attaching molecules with low surface energies onto a roughened surface or by roughening the surface of a hydrophobic material, including surface modification using nanoparticles, lithography, etching, deposition, and sol-gel etc. [24–27]. Such surfaces are of interest for the metal substrates in a diverse area of applications. The main reason is that, it not only doesn't change the material characteristics, but also overcomes the limitation of the substrate materials and expands the practical applications. However, methods for the fabrication super-hydrophobic and antimicrobial surfaces for metal are still relatively limited. Via anodizing and postetching processes, Choi et al. [14] designed and fabricated various wettability types of nanostructured surfaces on aluminum substrates, showed a significant reduction in CFU adhesion for S. aureus and E. coli while more pronounced for the hydrophobic surface condition. Popat et al. [28] have fabricated superhydrophobic and superhydrophilic titania nanotube arrays by anodizing and chemically etching titanium and then modifying the surface chemistry through silanization, which showed fewer bacteria adhered to the superhydrophobic surfaces than any other surface. Based on nanosecond laser processing, Boinovich et al. [29] designed superhydrophobic and superhydrophilic copper alloy substrates, and analyzed the evolution of the bactericidal activity of the copper substrates with different wettability. Among that, laser processing has been widely used to prepare micro/nano microstructures on metals with advantages such as low pollution, efficient, and selective area processing [29–31]. Analyzing wettability mechanism requires a thorough understanding of the interaction between the liquid and the solid surface, which is also related to molecular conformation. Recently, several researchers have tried to study the interactions between droplets and solid surfaces via molecular dynamics (MD) simulation, and also suggested that MD simulations constitute a powerful method for understanding interactions and conformational dynamics in wetting [32–35]. Gao et al. [36] characterized the condensation processes on various nanopillar surfaces, including the nucleation, growth and coalescence of nanodroplets by MD simulations. Similarly, combining MD simulations, Zhu et al. [37] showed that the transition between the Cassie and Wenzel states can be controlled via precisely designed trapezoidal nanostructures on a surface. Suganuma et al. [32] used (MD) simulations to investigate the wettability of Al2O3 (0 0 0 1) by organic molecules, which also can aid in understanding the consequences of molecular structure for the wettability of Al2O3 (0 0 0 1) surfaces and thereby help to control the wettability. However, the adsorption between organic molecules and metals and their oxides remains poorly understood. Yet the interaction process between water droplets and metal surfaces via
2. Experimental section 2.1. Materials and fabrication processing Anhydrous ethanol, acetone, hydrochloric acid (HCl, 36%–38%), dopamine hydrochloride, octadecylamine (ODA), tris(hydroxymethyl) aminomethane (Tris base) were all of analytical grade and used without further purification. After the sandpaper polished, stainless-steel 316L sheets (SS 316L sheets, 20 mm × 20 mm × 1.5 mm) were cleaned with acetone, ethanol and deionized water sequentially in an ultrasonic cleaner before using. Specimens were prepared by a two-step synthetic process, presented in the Scheme 1. Firstly, the SS sheets with different morphology were irradiated by fiber laser, the parameters employed of 50 W average power, 20 kHz repetition rate, 200 ns pulse duration and 500 mm/s scanning speed. In addition, the diameter and interval of irradiated patterns about the circle were about 100 μm and 75 μm respectively, which was based on the related parameters on the gecko back structures [8]. Each sample was processed five times under the above parameters. Afterwards, cleaning the as-prepared specimens for use. Secondly, 2 mg/mL dopamine hydrochloride aqueous solution (Tris-HCl buffer, 50 mM, pH = 8.5) and 50 mM ODA ethanol solution (1:1 v/v) were mixed by magnetic stirring of 5 min. Then SS sheets were immersed in the resulting dispersions under 37 °C for 6 h with a water-bathing constant temperature vibrator. After the reaction process, these specimens were ultrasonically cleaned in ethanol for 10 min, dried in air, and waited for the next tests. 2.2. Characterization The surface morphologies of specimens were characterized by scanning electron microscope (SEM, EVO18, ZEISS) and laser confocal scanning microscope (LCSM, OLS3000). The chemical components were recorded on Fourier-transform infrared spectrophotometer (FTIR, JACSCO, Japan) and X-ray photoelectron spectroscopy (XPS, SPECS XR50) at room temperature. The surface wettability was characterized by the water contact angles (WCA) and sliding angles (SA) measurement via a contact angle meter (Kruss DSA25S, Germany) at ambient temperature. Water droplets of 3 μL were cautiously dropped via a micro syringe onto the specimens, and the value record of contact angle was done after 30 s with a steady droplet state. The measurements were performed for at least three trials at different position of the sample surface. 853
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Scheme 1. Illustration of specimens’ fabrication on SS substrate, including the laser etching process (a) and in-situ polymerization (b).
2.3. Molecular dynamics (MD) simulation section
3. Results and discussions
The MD simulation process was conducted with the Materials Studio v5.0 package using the COMPASS force field, NVT ensemble, the temperature under 310 K, 1 fs time step and the Andersen method as thermal bath, which records the trajectories and forces of all atoms in the system during the simulation for data analysis. The COMPASS force field is a very effective force field to support the simulation of condensed metal atoms, which can accurately determine the structure, conformation, vibration, and thermophysical properties under conditions of wide temperature and pressure for the simulation and calculation [42]. And the other simulation details are in the Supplement materials.
3.1. Structural analysis To investigate the morphological information of the stainless-steel surface with or without the laser etching/coating treatment, the SEM images were analyzed as following. As shown in Fig. 1a and d, without laser etching, the surfaces clearly exhibited several scratches which resulted from the sandpaper using, and the intrinsic contact angle of the stainless-steel substrate was about 67.3° ± 1°. After laser etching, as shown in Fig. 1b, an orderly matrix of periodic round humps can be obviously observed on the substrates, as well as the gaps irradiated by laser beam in low magnification. While the laser beam came into contacting with the substrate, ablation proceeded along with the interference process, making the molten metal surface periodic, and finally made the samples form certain regular surface structures. Meanwhile, the sputtering phenomenon appeared on the substrate, which resulted in the formation of other microstructures, such as secondary textures and clusters structures, as shown in Fig. 1e. In this situation, the water contact angle was as low as 0°, which belonged to the category of superhydrophilic state. Compared with the unmodified surface, while with the functionalization treatment of PDA@ODA in Fig. 1c, there appeared lots of small floccules, adhering on the substrate on the surface in the magnification images (Fig. 1f), making the surface seem rougher, which also could be proved by the measurement of laser scanning confocal microscope in Fig. 2. However, for the etching substrate, the roughness value was slightly decreased after the modification, which may be resulted from the formation of nanolayer (PDA@ ODA compounds). In addition, the contact angle value reached 157.3° ± 1° in the inset of Fig. 1c, exhibiting a superhydrophobic state. Based on the experimental phenomenon, it can be concluded that the cooperative effect of the micro/nano hierarchical structure and low surface energy materials provided basic conditions for superhydrophobicity.
2.4. Antibacterial tests The Staphylococcus aureus (S. aureus, ATCC 6538) and Escherichia coli (E. coli, CMCC 44568) were purchased from the China General Microbiological Culture Collection Center (CGMCC). Sodium chloride (NaCl), tryptone, peptone, yeast extract and agar were all of analytical grade and used without further purification. A single isolated bacteria colony of E. coli (GN) and S. aureus (GP) bacteria was inoculated in 50 mL LB broth and Nutrient Agar overnight at 37 °C, respectively. Afterwards, 500 μL culture was taking out to transfer into a fresh sterilized broth, culturing another 3 h at 37 °C. Subsequently, the optical density (OD) of the bacterial suspension was measured by the UV–Vis spectrophotometer, and diluted to get an OD of 0.5 at 600 nm. Next, removed 200 μL bacterial suspension into fresh medium, and added different content of PDA@ODA powders (0, 0.1, 1.0, or 2.0 mg/mL), incubated for 24 h in an incubator-shaker at 37 °C under the 50 rpm. In addition, the PDA@ODA powders came from the reaction solution of polymerization process with centrifugation and drying. After incubation, 100 μL culture medium was taken out and gradually diluted (10−5), used for the solidified agar plates and placed into the incubator 24 h at 37 °C. Finally, the number of bacterial colonies on the plates was counted and plotted against the concentration of PDA@ODA powders, and the antibacterial efficiency was also estimated. The experiment was repeated thrice, and all of the readings were taken in triplicate. In addition, while incubated into the well containing a volume of 100 μL of bacterial culture at 37 °C for 2 h, taking the samples from bacterial culture, then washed with PBS and deionized water thrice (to remove the detached or un-adhered bacteria), and stained with 100 μL of DNA-binding dye 4′,6-diamidino-2phenylindole (DAPI) dye solution (10 μg/mL) for 20 min. Afterwards, the samples, with a washing step with PBS thrice, were imaged using Olympus IX73 Fluorescent Microscope.
3.2. Chemical characterization In order to study the chemical composition of the SS surface, the superhydrophilic and superhydrophobic specimens were probed by XPS and FTIR spectra and presented in Fig. 3. As for the XPS spectra analysis, in comparison with the superhydrophilic sample, survey scan results showed that there were only C, N and O elements on the superhydrophobic specimen after laser etching. As shown in Fig. 3a, the Fe element disappeared after the treatment, which indicated that the substrate surface was successfully covered by the PDA@ODA coatings. In Fig. 3b, via laser etching, Fe 2p spectrum of the superhydrophilic specimen showed the typical Fe 2p3/2 (710.6 and 712.6 eV), Fe 2p1/2 (724.3 and 726.7 eV) and satellite (718.8 and 732.4 eV) peaks for iron 854
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Fig. 1. SEM images of the (a) bare SS substrate, (b) laser-etched SS substrate, (c) etched SS with the PDA@ODA modification, (d, e, f) are the corresponding high magnification, respectively. The insets are the corresponding water contact angles.
vibrations of methylene. Hence, the PDA@ODA components with alkyl chain have been functionalized on the substrates successfully. Based on the above results, a possible mechanism for the immobilization of octadecylamine molecules was proposed in the Scheme 1. Firstly, as a supramolecular aggregate, DA usually gathers together via the noncovalent interactions, i.e. PDA. In this reaction process, while the solution pH is about 8.5, belonging to alkaline condition, lots of dopamine was oxidized to o-quinone, furthermore linked the dopamine, and then anchored on the ODA molecules and substrates. Meanwhile, part of o-quinone might directly react with eNH2 group of the anchored dopamine and ODA molecules via intermolecular Schiff base reaction and/or Michael addition [45,46]. In addition, there also existed bidentate groups among the PDA, ODA molecules and stainlesssteel substrate, such as H-bonding or metal coordination. The detail reaction mechanism of the immobilization process may not be so simple, which still need further exploration.
oxides, which also can be supported by the O 1s in the Supplement information (Fig. S2a). For the O 1s of superhydrophilic specimen, there were two peaks at the binding energy of 529.8 eV and 531.1 eV, corresponding to the iron oxide (Fe3O4) and CeO respectively. Meanwhile, the characteristic nitrogen N 1s signal at 400.5 eV provided an evidence for the successful incorporation of PDA@ODA, which was absent in the superhydrophilic sample. In the N 1s spectrum, the characteristic nitrogen N 1s peak could be deconvoluted into four components at 397.8, 398.8, 399.9 and 401.2 eV in Fig. 3c, which were attributed to the N atoms of C]N, eNHe, eNH2 and hydrogen bonded amine, respectively. Especially, the presence of C]N and eNHe species suggested the reactions between eNH2 and o-quinone through Schiff base reaction and/or Michael addition [43]. As shown in Fig. 3d, the vibration peak appeared at 1647 cm−1 after coating modification, which was assigned to the C]N stretching vibrations of Schiff base reaction product between PDA and ODA [44]. Meanwhile, the peaks appeared at 2920 and 2850 cm−1, which was attributed to C–H stretching
Fig. 2. Laser confocal microscopy images of (a) bare SS substrate, (b) modified SS substrate, (c) laser-etched SS substrate, and (d) etched SS with the PDA@ODA modification for 6 h. Also, the roughness values (Ra) of the surface have been listed in the figures. 855
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Fig. 3. XPS spectra of the superhydrophilic and superhydrophobic specimens, (a) Survey scan, (b) Fe 2p, (c) N 1s; and (d) FTIR spectra.
further testified the surface wetting behavior, which were attributed to the presence of the air cushion on the superhydrophobic surface, inducing the total light reflectance. Meanwhile, this surface kept completely dry after removed from the water. Thus, this property as well as the self-cleaning ability enables the substrate to effectively reduce surface contaminations [19,22,25]. Moreover, the solid-gas interface is difficult to be completely replaced by the solid-liquid interface, so as to effectively prevent the solution from infiltrating. Also, due to the existence of air cushion, a capillary system was also formed on the superhydrophobic surface. Based on the Eq. (3.3) [49,50], the value of pressure difference (ΔP) was negative while the WCA value is larger than 150°, resulting in the height of liquid level raising in the capillary. This phenomenon will promote the liquid expelling instead of passing through the void into the surface of the material. The reason was that, for a concave curvature (ΔP > 0), the capillary provides energy for transport in the direction of narrowing, whereas a convex curvature (ΔP < 0) leads to retardation of flow [50].
3.3. Surface properties 3.3.1. Surface wettability Surface wettability was characterized by static water contact angles and sliding angles. As shown in the inset of Fig. 1a, for the bare SS substrate, the WCA value was only about 67.3 ± 1°. According to the Wenzel model [41], the WCA will be lower with the increase of roughness if the surface is originally hydrophilic. Thus, after the processing of laser etching, the hydrophilic SS substrate turned into a superhydrophilic state with a WCA nearly 0°, and the droplets spread quickly and formed a water film on the surface, as described by Fig. 4a, which can be explained by the magnification effect [47,48]. Subsequently, the modification treatment made the structured surface exhibit a WCA of 157.3 ± 1°, successfully realizing the state transition from superhydrophilicity to superhydrophobicity, which fully confirmed the significance of chemical components in the construction of superhydrophobic surfaces. Moreover, as shown in Fig. 4c, the water droplets were easy to roll down from the superhydrophobic surface while the angle of inclination was only 5°, implying a lower contact angle hysteresis and a Cassie-Baxter wetting state. In addition, the quartz sands had no residue on the sample after water washing in Fig. 4d, further confirming the excellent self-cleaning ability. In addition, as described in Fig. 4b, the micro-nano hierarchical structures on the substrate could capture a lot of air, forming an “air cushion” as the solution immersing, which also can be confirmed by the phenomenon of light reflection in Fig. 4e. While immersed in the water, the optical image of bare SS and superhydrophilic surfaces almost exhibited complete wetting. However, as for the superhydrophobic specimen, there appeared a mirror-like phenomenon at a specific angle,
ΔP = 2γ cosθ / r
(3.3)
where θ denotes contact angle, γ denotes the surface tension between liquid and air, r denotes the principal radii of curvature. 3.3.2. Molecular dynamics (MD) simulation for wettability analysis In order to further investigate the superhydrophobic mechanism, we used MD simulations to study the adsorption process of water nanodroplets on the SS surface or which treated with PDA@ODA compounds. As for the substrate, combined with the above composition analysis, it is assumed that the main oxide component on the SS surface is Fe3O4, and the (1 1 1) crystallographic plane is chosen to study the 856
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Fig. 4. Schematic illustrations of (a) Wenzel model and (b) Cassie-Baxter model, (c) the dynamic behavior of water droplet on the superhydrophobic surface, (d) optical image about the wettability state, (e) the optical images of the SS (1), superhydrophilic (2) and superhydrophobic (3, 4) specimens in sequence.
−0.437e (N atom in the secondary amine at Num 12), and others (C atom and H atom). Hence, similarly, the alkyl chains at the tail deviate from the surface. While introducing the ethanol-water mixed solution, there is no significant difference compared with the adsorption state in the absence of solvent. In conclusion, both A and B molecules can form strong interaction with Fe3O4 (1 1 1) surface in solvent environment, and the alkyl chains are all far from the surface to remain an approximately vertical state, as we have inferred to form a hydrophobic surface. Meanwhile, observing the adsorption state in Fig. 6a, it could be found that, as time progresses, the water molecules gradually adsorb on the Fe3O4 (1 1 1) surface and eventually spread completely. And then, the snapshot almost no change under the dynamic balance. The main reason is that, while coming into contacting with a nano-cluster of water molecules or even a larger-scale water droplet, Fe3O4 (1 1 1) surface, as a hydrophilic surface, will promote the water cluster or droplet to spread rapidly and present a hydrophilic state. While for the surface with the PDA@ODA modification, after establishing the adsorption model, the hydrophobic molecular membrane was first relaxed by 300 ps to immobilize all the molecules in the water molecule cluster. Then, the snapshots of the final moments of relaxation recorded as b1300 ps, which is equivalent to b2-0 ps. Afterwards, it can be seen from the snapshots at different MD simulation moments in Fig. 6b, the adsorption state of water molecules on hydrophobic surface always maintains a relatively regular spherical shape as the simulation time extending, which is different from the Fe3O4 (1 1 1) surface. Therefore, via MD simulations, as a result of the presence of product A and B, the surface can effectively prevent water clusters adsorbing and spreading, and exhibit a superhydrophobic state which is consistent with the
interaction with the PDA@ODA compounds and water droplets [42,51,52]. Due to the in-situ polymerization grafting process occurring on the surface is too complicated to analyze the reaction process in detail, we assumed the surface mainly occurred the Schiff base reaction and the Michael addition reaction based on the previous composition analysis, so the final products are assumed to be the following A (Schiff base reaction) and B (Michael addition process). As shown in Fig. 5a, for the product A, under the environment without solvent, the adsorption of A molecule on the surface is substantially vertical, mainly by the interaction of the amino group and the carbonyl group. Among that, the alkyl chain is slightly curved because of the flexibility, but the overall tendency is away from the surface. If the adsorption process is in an ethanol-water mixed solvent environment, i.e. Fig. 5b, the ring-shaped head, connecting the carbonyl group and the amino group, has completely adsorbed on the surface of Fe3O4 (1 1 1) surface. Meanwhile, the alkyl chain bends and detaches from the surface due to its hydrophobic nature and the flexibility of the chain. And for the product B, unlike A, the head containing the hydroxyl and the amino group is completely flatly adsorbed on the surface of Fe3O4 (1 1 1), presenting a stronger interaction between the composite and surface. This phenomenon can be ascribed for the charge distribution law of the quantum chemistry calculation, which is consistent with the data in the Supplement of Tables S4.1 and S4.2. The charge values in product A is by a turn of −0.481e (N atom in the amino group at Num 9), −0.357e (O atom in the carbonyl), −0.263e (C atom in benzene ring at Num 6), −0.237e (N atom in the secondary amine at Num 11), others (C atom and H atom). However, the charge values in product B is as followed, −0.521e and −0.447e (O atom in the two phenolic hydroxyl groups), −0.482e (N atom in the amino group at Num 9), 857
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To verify the antibacterial property of PDA@ODA compound, the growth of typical GN and GP bacteria, i.e. S. aureus and E. coli, was observed under different concentrations. As can be seen from the agar plate assay in Fig. 7a, with the increase in the concentration of the complex, no matter what kind of bacteria, its growth was greatly inhibited, and the number of colonies showed a gradual decreasing tendency. Via bacterial counting, the corresponding relationship between compound concentrations and Colony-Forming Units (CFU) is shown in Fig. 7b. In addition, antibacterial efficiency, given as the percentage of bacterial reduction by calculating the ratio between the number of survival bacteria colonies before and after, is listed in Table 1, reflecting the effects of compound concentrations directly. Compared with the control experiment, for S. aureus, the number of colonies has decreased by nearly two orders of magnitude at the concentration of 2.0 mg/mL, presenting a decrease of almost 99.6% reduction. But for the E. coli, antibacterial effect had been greatly improved with 90.3% under the same concentration, maybe resulting from the difference of cell walls. Up to now, the interaction between the compounds and bacteria is still complicated, especially the complex formation on the surface. As for the PDA@ODA compounds, the antibacterial mechanism may be partly attributed to the structural changes on the surface of bacterial membrane, by affecting the function of the cell membrane to inhibit bacterial growth [53]. Another reason is due to the binding of its long lipophilic side chain -C18H37 and lipid bilayer of the outer membrane, causing changes in cell membrane permeability and leakage of cytoplasmic compounds, indicating the effects on the membrane [54]. In the binding process, the hydrocarbon tail of the substance became intercalated into the interior hydrophobic zone of the microbial membrane. In addition, longer hydrophobic chains (C18) for the formed compound have a greater hydrophobic effect, which may affect antimicrobial potency and can be confirmed by the results of fluorescence microscope evaluated by the density of bright dots in Fig. 8. By comparison, after surface treatment, the antibacterial properties of the SS substrate have a certain improvement. Unlike the bare substrate in Fig. 8a, the blue dots in the yellow circle were sporadic distribution on the coated surface (Fig. 8b and c). Always, the superhydrophobic surface possessed a relatively more effective inhibition for the bacterial adhesion, which can be ascribed to the results of multifactor synthesis, such as biocide agents of the PDA@ODA compounds, micro/nano hierarchical surface structures and the reduction of bioadhesion by the presence of air cushion. Meanwhile, in the red circle, the adhesion of large amount dye mainly is because of the dye deposition, and maybe a little attributed to the bacterial adhesion.
Fig. 5. Snapshot of the MD simulation when the interaction between the A product molecule (a, b) or B product molecule (c, d) and Fe3O4 (1 1 1) surface reaches equilibrium in the presence/absence of ethanol-water mixed solvent environment.
previous experimental data. 3.3.3. Antimicrobial activity For the material property of stainless-steel, the antimicrobial activity is also regarded as one of the most important material properties.
Fig. 6. MD simulations of nano-water clusters adsorbed on the (a) Fe3O4 (1 1 1) surface and (b) surface modified with PDA@ODA membrane at different times. 858
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Fig. 7. (a) Agar plate assays and (b) Plots of the colony forming units (CFU) vs different concentrations (mg/mL) on E. coli and S. aureus.
contact angle still was as high as 153.5 ± 1°, in a superhydrophobic state. As well, the film layer didn’t appear cracks and peeling phenomenon, confirming the good stability in the heat resistance. Moreover, we immersed the superhydrophobic specimen into ultrasonic cleaner containing 50 mL ethanol and operated for 2 h. Meanwhile, the CA value only had a slight decline, was about 155.0 ± 1° and still exhibited superhydrophobicity. In conclusion, the as-prepared samples had good stability in the heat resistance, physical performance and durability, which will enhance the materials utilization in the application fields and working life.
Table 1 The antibacterial efficiency of E. coli and S. aureus under different compound concentrations. Strain
E. coli S. aureus
Concentration (mg/mL) 0.1
1.0
2.0
21.3% 40.1%
88.5% 90.8%
90.3% 99.6%
3.3.4. Robustness Due to the importance of surface stability for metal substrate, corresponding tests were carried out to assess the physical stability, thermostability, long-term stability of the prepared surfaces. As shown in Fig. 9a, we tested the physical stability of the as-prepared samples with a sandpaper wear method, so as to evaluate the conventional physical damage for the durable superhydrophobic ability [55]. As the wear distance increasing, the values of WCA had a slight decrease, and the SA gradually increased. Even though wear distance reached 2000 mm, the WCA value still maintain 136.4°, which exhibit a hydrophobic state. The wear resistance mechanism of as-prepared surface mainly can be ascribed by special microstructures on the SS substrate, which was not completely in contact with the sandpaper during the wear process. The processed SS substrates had a rough three-dimensional structure and were covered by nanolayers. The abrasion progress will only damage the outermost structure, and the internal structure will be protected. Furthermore, the long-term stability of the superhydrophobic surface for storage in air for six months was also examined by investigating changes in static WCAs with time evolution in Fig. 9b. A maximum contact angle of 154.5 ± 1° was observed on the specimen after exposed 6 months in the air. As presented in the Fig. 9c, it can be clearly seen that the surface wettability did not change significantly under 200 °C with the heating time extended to 2 h, the value of surface
4. Conclusion Inspired by the special micro/nano structures of the creatures and the bioadhesion of marine mussels, a two-step method can be used to fabricate the superhydrophobic surface with antibacterial property, including the laser interference patterning and in-situ polymerization, which anchored the chemical compositions onto the patterned stainless-steel substrate. Combined with the analysis of the structural and chemical characterization, the cooperation effect of the micro/nano hierarchical structure and low surface energy materials provided basic conditions for superhydrophobicity. Meanwhile, the surface also realized the state transition from superhydrophilic to superhydrophobic successfully. According to the results of MD simulations, the alkyl chains of PDA@ODA compounds were far from the surface to remain an approximately vertical state, promoting the formation of superhydrophobic surfaces. Also, the adsorption process of water clusters on superhydrophobic surfaces was well shown by the MD simulations, still maintaining spherical finally, which was consistent with the experimental data. Also, the PDA@ODA compounds (∼2.0 mg/mL) effectively inhibited the growth and multiplication of bacterial colonies of E. coli (about 90.3%) and S. aureus (up to 99.6%) via agar plate assays. In addition, the superhydrophobic specimen exhibited good antimicrobial
Fig. 8. Fluorescence microscopy for S. aureus of (a–c) bare SS substrate, laser-etched superhydrophilic surface and superhydrophobic surface. 859
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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.04.200. References [1] P. Wang, D. Zhang, S.M. Sun, T.P. Li, Y. Sun, Fabrication of slippery lubricantinfused porous surface with high underwater transparency for the control of marine biofouling, ACS Appl. Mater. Interfaces 9 (2017) 972–982. [2] Y.C. Fu, J.X. Jiang, Q.H. Zhang, X.L. Zhan, F.Q. Chen, Robust liquid-repellent coatings based on polymer nanoparticles with excellent self-cleaning and antibacterial performances, J. Mater. Chem. A 5 (2017) 275–284. [3] Z.Y. Liu, Y.X. Hu, C.F. Liu, Z.Y. Zhou, Surface-independent one-pot chelation of copper ions onto filtration membranes to provide antibacterial properties, Chem. Commun. 52 (2016) 12245–12248. [4] Z.Y. Liu, Y.X. Hu, Sustainable antibiofouling properties of thin film composite forward osmosis membrane with rechargeable silver nanoparticles loading, ACS Appl. Mater. Interfaces 8 (2016) 21666–21673. 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Choi, Nanoengineered superhydrophobic surfaces of aluminum with extremely low bacterial adhesivity, ACS Appl. Mater. Interfaces 9 (2017) 12118–12129. [15] E.C. Cho, C.W. Chang-Jian, H.C. Chen, K.S. Chuang, J.H. Zheng, Y.S. Hsiao, K.C. Lee, J.H. Huang, Robust multifunctional superhydrophobic coatings with enhanced water/oil separation, self-cleaning, anti-corrosion, and anti-biological adhesion, Chem. Eng. J. 314 (2017) 347–357. [16] B. Bhushan, Biomimetics: lessons from nature-an overview, Philos. Trans. R. Soc. A 367 (2009) 1445–1486. [17] Z.W. Han, Z.Z. Mu, W. Yin, S.C. Niu, J.Q. Zhang, L.Q. Ren, Biomimetic multifunctional surfaces inspired from animals, Adv. Colloid Interface 234 (2016) 27–50. [18] C. Thamaraiselvan, Y. Carmiel, G. Eliad, C.N. Sukenik, R. Semiat, C.G. Dosoretz, Modification of a polypropylene feed spacer with metal oxide-thin film by chemical bath deposition for biofouling control in membrane filtration, J. Membr. Sci. 573 (2019) 511–519. [19] E.J. Falde, S.T. Yohe, Y.L. Colson, M.W. Grinstaff, Superhydrophobic materials for biomedical applications, Biomaterials 104 (2016) 87–103. [20] B.B. Zhang, J.R. Li, X. Zhao, X.H. Hu, L.H. Yang, N. Wang, Y.T. Li, B.R. Hou, Biomimetic one step fabrication of manganese stearate superhydrophobic surface as an efficient barrier against marine corrosion and Chlorella vulgaris-induced biofouling, Chem. Eng. J. 306 (2016) 441–451. [21] G. Wen, X.Y. Gao, P. Tian, L.S. Zhong, Z.L.L. Wang, Z.G. Guo, Modifier-free fabrication of durable and multifunctional superhydrophobic paper with thermostability and anti-microbial property, Chem. Eng. J. 346 (2018) 94–103. [22] X.Q. Dou, D. Zhang, C.L. Feng, L. Jiang, Bioinspired hierarchical surface structures with tunable wettability for regulating bacteria adhesion, ACS Nano 9 (2015) 10664–10672. [23] A. Tripathy, A. Kumar, S. Sreedharan, G. Muralidharan, A. Pramanik, D. Nandi, P. Sen, Fabrication of low-cost flexible superhydrophobic antibacterial surface with dual-scale roughness, ACS Biomater. Sci. Eng. 4 (2018) 90–97. [24] Z. Li, B.L. Nguyen, Y.C. Cheng, J.M. Xue, G. MacLarencd, C.H. Yap, Durable, flexible, superhydrophobic and blood-repelling surfaces for use in medical blood pumps, J. Mater. Chem. B 6 (2018) 6225–6233. [25] T. Wei, Z.C. Tang, Q. Yu, H. Chen, Smart antibacterial surfaces with switchable bacteria-killing and bacteria-releasing capabilities, ACS Appl. Mater. Interfaces 9 (2017) 37511–37523. [26] A. Jaggessar, H. Shahali, A. Mathew, P.K.D.V. Yarlagadda, Bio-mimicking nano and micro-structured surface fabrication for antibacterial properties in medical
Fig. 9. The surface stability of the superhydrophobic specimen. Change of WCAs and SAs of the as-prepared superhydrophobic surface as a function of (a) abrasion distances, and the inset shows a schematic of the abrasion test; (b) exposed times in air; and (c) heating temperatures or ultrasonication.
property, thermostability, physical stability and long-term stability, which will expand the application fields and working life.
Acknowledgements The authors thank the Key Program for International S&T Cooperation Projects of China (2016YFE0132900), National Natural Science Foundation of China (No. 51775231), National Postdoctoral Program for Innovative Talents (BX20180123), China Postdoctoral Science Foundation (2018M641782), and JLU Science and Technology Innovative Research Team (No. 2017TD-04). 860
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Biomimetic robust superhydrophobic stainless-steel surfaces with antimicrobial activity and molecular dynamics simulation
T
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Shuyi Lia, Yan Liua, , Zaihang Zhenga,b, Xin Liub, Honglan Huangc, Zhiwu Hana, Luquan Rena a
Key Laboratory of Bionic Engineering (Ministry of Education), Jilin University, Changchun 130022, China School of Chemical Engineering, Changchun University of Technology, Changchun 130012, China c College of Basic Medical Science, Jilin University, Changchun 130022, China b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
stainless-steel surfaces • Biomimetic were prepared by laser etching and modification.
simulations suggested the PDA@ • MD ODA compounds how to adsorb on the substrate.
results of MD simulations were • The consistent with experimental data. surfaces presented super• The hydrophobicity, antibacterial property and stability.
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomimetic Superhydrophobic Molecular dynamics simulation Antimicrobial Stainless-steel substrate
Wettability is an effective strategy to reduce the adhesion force between bacteria and a solid surface, also is favor to effectively overcome the limitation of the substrate materials and expand their practical applications. Herein, inspired by the special micro/nano structures of the creatures and the bioadhesion of marine mussels, a green and simple two-step method, the laser interference patterning and in-situ polymerization, was used to fabricate the superhydrophobic surface with antibacterial property on the stainless-steel (SS) substrate. As well, this patterned surface also has successfully realized the wettability transition from superhydrophilic to superhydrophobic. Meanwhile, the results of MD simulations directly suggested the PDA@ODA compounds how to adsorb on the SS substrate and interact with water molecules, which was consistent with the experimental. In addition, according to the agar plate assays and fluorescent microscope tests, the obtained specimens presented a certain antimicrobial activity. Moreover, the samples had good robustness in the heat resistance, physical performance and durability, which will expand the application fields and working life.
1. Introduction Currently, biocontamination or biofouling is not an uncommon problem, which is the main source of health associated infections,
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including water purification systems, surgical equipment, food packaging and food storage, marine and industrial equipment [1–6]. Through evolution and optimization over millions of years, lots of creatures in the nature, such as lotus leaves [7], geckos [8], shark skin [9], and
Corresponding author. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.cej.2019.04.200 Received 26 December 2018; Received in revised form 25 April 2019; Accepted 28 April 2019 Available online 29 April 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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simulations are not common. Herein, inspired by the special micro/nano structures of the creatures and the bioadhesion of marine mussels, we reported a two-step method to fabricate the superhydrophobic surface with antibacterial property, which anchored the chemical compositions onto the patterned stainless-steel substrate. Firstly, the Laser Interference Patterning (LIP) is applied to prepare micro/nano structures with advantages such as low pollution, efficient, and selective area processing. Moreover, the in-situ polymerization reaction about polydopamine and octadecylamine also is mild, simple, inexpensive, quick and green. Polydopamine (PDA), the adhesive proteins secreted by mussels, can be regarded as a versatile platform for secondary surface-mediated reactions, leading ultimately grafting polymer coatings to metal under mild wet conditions [38–41]. Meanwhile, combined with molecular dynamic simulation, no matter the formation process of superhydrophobic coatings on the stainless steel or the surface adsorption process of water molecules were systematically verified in theory. In addition, we also investigated the surface properties such as antimicrobial activity, wettability and robustness. Comparatively speaking, this method has the advantages of mild, green, and can be applied to the large-scale preparation of a variety of materials.
insect wings such as cicadae [10], dragonflies and butterflies [11] etc., have been formed a diverse range of micro/nano-structures and special properties, which will provide a potentially rich blueprint for the development of artificial antifouling surfaces for applications in medicine and industry. Meanwhile, lots of research results show that the way to inhibit the bacterial infection is not only limited to antibiotics or the material property, but also can change the biological adhesion activity of materials by varying the surface wettability of the substrate materials [12–15]. Superhydrophobic surface is known to possess a high degree of water repellency, which enables dirt particles to be carried away while the liquid droplet rolls off, famous as “lotus effect” [6,16,17]. Nowadays, developing biofouling material based on superhydrophobicity is an effective strategy to reduce the adhesion force between bacteria and a solid surface and has yet to be studied thoroughly and systematically, due to the weak bacterial attachment for the low protein adsorption and easy protein detachment [18–22]. In addition, the surface microstructure can induce the mechanical disruption of cell membranes to kill bacteria, which is extremely effective in achieving antimicrobial properties [8,10,13,23]. At the same time, the surface microstructure is also critical for the construction of superhydrophobic surface. The preparation process of these surfaces usually accompanied either by covalently attaching molecules with low surface energies onto a roughened surface or by roughening the surface of a hydrophobic material, including surface modification using nanoparticles, lithography, etching, deposition, and sol-gel etc. [24–27]. Such surfaces are of interest for the metal substrates in a diverse area of applications. The main reason is that, it not only doesn't change the material characteristics, but also overcomes the limitation of the substrate materials and expands the practical applications. However, methods for the fabrication super-hydrophobic and antimicrobial surfaces for metal are still relatively limited. Via anodizing and postetching processes, Choi et al. [14] designed and fabricated various wettability types of nanostructured surfaces on aluminum substrates, showed a significant reduction in CFU adhesion for S. aureus and E. coli while more pronounced for the hydrophobic surface condition. Popat et al. [28] have fabricated superhydrophobic and superhydrophilic titania nanotube arrays by anodizing and chemically etching titanium and then modifying the surface chemistry through silanization, which showed fewer bacteria adhered to the superhydrophobic surfaces than any other surface. Based on nanosecond laser processing, Boinovich et al. [29] designed superhydrophobic and superhydrophilic copper alloy substrates, and analyzed the evolution of the bactericidal activity of the copper substrates with different wettability. Among that, laser processing has been widely used to prepare micro/nano microstructures on metals with advantages such as low pollution, efficient, and selective area processing [29–31]. Analyzing wettability mechanism requires a thorough understanding of the interaction between the liquid and the solid surface, which is also related to molecular conformation. Recently, several researchers have tried to study the interactions between droplets and solid surfaces via molecular dynamics (MD) simulation, and also suggested that MD simulations constitute a powerful method for understanding interactions and conformational dynamics in wetting [32–35]. Gao et al. [36] characterized the condensation processes on various nanopillar surfaces, including the nucleation, growth and coalescence of nanodroplets by MD simulations. Similarly, combining MD simulations, Zhu et al. [37] showed that the transition between the Cassie and Wenzel states can be controlled via precisely designed trapezoidal nanostructures on a surface. Suganuma et al. [32] used (MD) simulations to investigate the wettability of Al2O3 (0 0 0 1) by organic molecules, which also can aid in understanding the consequences of molecular structure for the wettability of Al2O3 (0 0 0 1) surfaces and thereby help to control the wettability. However, the adsorption between organic molecules and metals and their oxides remains poorly understood. Yet the interaction process between water droplets and metal surfaces via
2. Experimental section 2.1. Materials and fabrication processing Anhydrous ethanol, acetone, hydrochloric acid (HCl, 36%–38%), dopamine hydrochloride, octadecylamine (ODA), tris(hydroxymethyl) aminomethane (Tris base) were all of analytical grade and used without further purification. After the sandpaper polished, stainless-steel 316L sheets (SS 316L sheets, 20 mm × 20 mm × 1.5 mm) were cleaned with acetone, ethanol and deionized water sequentially in an ultrasonic cleaner before using. Specimens were prepared by a two-step synthetic process, presented in the Scheme 1. Firstly, the SS sheets with different morphology were irradiated by fiber laser, the parameters employed of 50 W average power, 20 kHz repetition rate, 200 ns pulse duration and 500 mm/s scanning speed. In addition, the diameter and interval of irradiated patterns about the circle were about 100 μm and 75 μm respectively, which was based on the related parameters on the gecko back structures [8]. Each sample was processed five times under the above parameters. Afterwards, cleaning the as-prepared specimens for use. Secondly, 2 mg/mL dopamine hydrochloride aqueous solution (Tris-HCl buffer, 50 mM, pH = 8.5) and 50 mM ODA ethanol solution (1:1 v/v) were mixed by magnetic stirring of 5 min. Then SS sheets were immersed in the resulting dispersions under 37 °C for 6 h with a water-bathing constant temperature vibrator. After the reaction process, these specimens were ultrasonically cleaned in ethanol for 10 min, dried in air, and waited for the next tests. 2.2. Characterization The surface morphologies of specimens were characterized by scanning electron microscope (SEM, EVO18, ZEISS) and laser confocal scanning microscope (LCSM, OLS3000). The chemical components were recorded on Fourier-transform infrared spectrophotometer (FTIR, JACSCO, Japan) and X-ray photoelectron spectroscopy (XPS, SPECS XR50) at room temperature. The surface wettability was characterized by the water contact angles (WCA) and sliding angles (SA) measurement via a contact angle meter (Kruss DSA25S, Germany) at ambient temperature. Water droplets of 3 μL were cautiously dropped via a micro syringe onto the specimens, and the value record of contact angle was done after 30 s with a steady droplet state. The measurements were performed for at least three trials at different position of the sample surface. 853
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Scheme 1. Illustration of specimens’ fabrication on SS substrate, including the laser etching process (a) and in-situ polymerization (b).
2.3. Molecular dynamics (MD) simulation section
3. Results and discussions
The MD simulation process was conducted with the Materials Studio v5.0 package using the COMPASS force field, NVT ensemble, the temperature under 310 K, 1 fs time step and the Andersen method as thermal bath, which records the trajectories and forces of all atoms in the system during the simulation for data analysis. The COMPASS force field is a very effective force field to support the simulation of condensed metal atoms, which can accurately determine the structure, conformation, vibration, and thermophysical properties under conditions of wide temperature and pressure for the simulation and calculation [42]. And the other simulation details are in the Supplement materials.
3.1. Structural analysis To investigate the morphological information of the stainless-steel surface with or without the laser etching/coating treatment, the SEM images were analyzed as following. As shown in Fig. 1a and d, without laser etching, the surfaces clearly exhibited several scratches which resulted from the sandpaper using, and the intrinsic contact angle of the stainless-steel substrate was about 67.3° ± 1°. After laser etching, as shown in Fig. 1b, an orderly matrix of periodic round humps can be obviously observed on the substrates, as well as the gaps irradiated by laser beam in low magnification. While the laser beam came into contacting with the substrate, ablation proceeded along with the interference process, making the molten metal surface periodic, and finally made the samples form certain regular surface structures. Meanwhile, the sputtering phenomenon appeared on the substrate, which resulted in the formation of other microstructures, such as secondary textures and clusters structures, as shown in Fig. 1e. In this situation, the water contact angle was as low as 0°, which belonged to the category of superhydrophilic state. Compared with the unmodified surface, while with the functionalization treatment of PDA@ODA in Fig. 1c, there appeared lots of small floccules, adhering on the substrate on the surface in the magnification images (Fig. 1f), making the surface seem rougher, which also could be proved by the measurement of laser scanning confocal microscope in Fig. 2. However, for the etching substrate, the roughness value was slightly decreased after the modification, which may be resulted from the formation of nanolayer (PDA@ ODA compounds). In addition, the contact angle value reached 157.3° ± 1° in the inset of Fig. 1c, exhibiting a superhydrophobic state. Based on the experimental phenomenon, it can be concluded that the cooperative effect of the micro/nano hierarchical structure and low surface energy materials provided basic conditions for superhydrophobicity.
2.4. Antibacterial tests The Staphylococcus aureus (S. aureus, ATCC 6538) and Escherichia coli (E. coli, CMCC 44568) were purchased from the China General Microbiological Culture Collection Center (CGMCC). Sodium chloride (NaCl), tryptone, peptone, yeast extract and agar were all of analytical grade and used without further purification. A single isolated bacteria colony of E. coli (GN) and S. aureus (GP) bacteria was inoculated in 50 mL LB broth and Nutrient Agar overnight at 37 °C, respectively. Afterwards, 500 μL culture was taking out to transfer into a fresh sterilized broth, culturing another 3 h at 37 °C. Subsequently, the optical density (OD) of the bacterial suspension was measured by the UV–Vis spectrophotometer, and diluted to get an OD of 0.5 at 600 nm. Next, removed 200 μL bacterial suspension into fresh medium, and added different content of PDA@ODA powders (0, 0.1, 1.0, or 2.0 mg/mL), incubated for 24 h in an incubator-shaker at 37 °C under the 50 rpm. In addition, the PDA@ODA powders came from the reaction solution of polymerization process with centrifugation and drying. After incubation, 100 μL culture medium was taken out and gradually diluted (10−5), used for the solidified agar plates and placed into the incubator 24 h at 37 °C. Finally, the number of bacterial colonies on the plates was counted and plotted against the concentration of PDA@ODA powders, and the antibacterial efficiency was also estimated. The experiment was repeated thrice, and all of the readings were taken in triplicate. In addition, while incubated into the well containing a volume of 100 μL of bacterial culture at 37 °C for 2 h, taking the samples from bacterial culture, then washed with PBS and deionized water thrice (to remove the detached or un-adhered bacteria), and stained with 100 μL of DNA-binding dye 4′,6-diamidino-2phenylindole (DAPI) dye solution (10 μg/mL) for 20 min. Afterwards, the samples, with a washing step with PBS thrice, were imaged using Olympus IX73 Fluorescent Microscope.
3.2. Chemical characterization In order to study the chemical composition of the SS surface, the superhydrophilic and superhydrophobic specimens were probed by XPS and FTIR spectra and presented in Fig. 3. As for the XPS spectra analysis, in comparison with the superhydrophilic sample, survey scan results showed that there were only C, N and O elements on the superhydrophobic specimen after laser etching. As shown in Fig. 3a, the Fe element disappeared after the treatment, which indicated that the substrate surface was successfully covered by the PDA@ODA coatings. In Fig. 3b, via laser etching, Fe 2p spectrum of the superhydrophilic specimen showed the typical Fe 2p3/2 (710.6 and 712.6 eV), Fe 2p1/2 (724.3 and 726.7 eV) and satellite (718.8 and 732.4 eV) peaks for iron 854
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Fig. 1. SEM images of the (a) bare SS substrate, (b) laser-etched SS substrate, (c) etched SS with the PDA@ODA modification, (d, e, f) are the corresponding high magnification, respectively. The insets are the corresponding water contact angles.
vibrations of methylene. Hence, the PDA@ODA components with alkyl chain have been functionalized on the substrates successfully. Based on the above results, a possible mechanism for the immobilization of octadecylamine molecules was proposed in the Scheme 1. Firstly, as a supramolecular aggregate, DA usually gathers together via the noncovalent interactions, i.e. PDA. In this reaction process, while the solution pH is about 8.5, belonging to alkaline condition, lots of dopamine was oxidized to o-quinone, furthermore linked the dopamine, and then anchored on the ODA molecules and substrates. Meanwhile, part of o-quinone might directly react with eNH2 group of the anchored dopamine and ODA molecules via intermolecular Schiff base reaction and/or Michael addition [45,46]. In addition, there also existed bidentate groups among the PDA, ODA molecules and stainlesssteel substrate, such as H-bonding or metal coordination. The detail reaction mechanism of the immobilization process may not be so simple, which still need further exploration.
oxides, which also can be supported by the O 1s in the Supplement information (Fig. S2a). For the O 1s of superhydrophilic specimen, there were two peaks at the binding energy of 529.8 eV and 531.1 eV, corresponding to the iron oxide (Fe3O4) and CeO respectively. Meanwhile, the characteristic nitrogen N 1s signal at 400.5 eV provided an evidence for the successful incorporation of PDA@ODA, which was absent in the superhydrophilic sample. In the N 1s spectrum, the characteristic nitrogen N 1s peak could be deconvoluted into four components at 397.8, 398.8, 399.9 and 401.2 eV in Fig. 3c, which were attributed to the N atoms of C]N, eNHe, eNH2 and hydrogen bonded amine, respectively. Especially, the presence of C]N and eNHe species suggested the reactions between eNH2 and o-quinone through Schiff base reaction and/or Michael addition [43]. As shown in Fig. 3d, the vibration peak appeared at 1647 cm−1 after coating modification, which was assigned to the C]N stretching vibrations of Schiff base reaction product between PDA and ODA [44]. Meanwhile, the peaks appeared at 2920 and 2850 cm−1, which was attributed to C–H stretching
Fig. 2. Laser confocal microscopy images of (a) bare SS substrate, (b) modified SS substrate, (c) laser-etched SS substrate, and (d) etched SS with the PDA@ODA modification for 6 h. Also, the roughness values (Ra) of the surface have been listed in the figures. 855
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Fig. 3. XPS spectra of the superhydrophilic and superhydrophobic specimens, (a) Survey scan, (b) Fe 2p, (c) N 1s; and (d) FTIR spectra.
further testified the surface wetting behavior, which were attributed to the presence of the air cushion on the superhydrophobic surface, inducing the total light reflectance. Meanwhile, this surface kept completely dry after removed from the water. Thus, this property as well as the self-cleaning ability enables the substrate to effectively reduce surface contaminations [19,22,25]. Moreover, the solid-gas interface is difficult to be completely replaced by the solid-liquid interface, so as to effectively prevent the solution from infiltrating. Also, due to the existence of air cushion, a capillary system was also formed on the superhydrophobic surface. Based on the Eq. (3.3) [49,50], the value of pressure difference (ΔP) was negative while the WCA value is larger than 150°, resulting in the height of liquid level raising in the capillary. This phenomenon will promote the liquid expelling instead of passing through the void into the surface of the material. The reason was that, for a concave curvature (ΔP > 0), the capillary provides energy for transport in the direction of narrowing, whereas a convex curvature (ΔP < 0) leads to retardation of flow [50].
3.3. Surface properties 3.3.1. Surface wettability Surface wettability was characterized by static water contact angles and sliding angles. As shown in the inset of Fig. 1a, for the bare SS substrate, the WCA value was only about 67.3 ± 1°. According to the Wenzel model [41], the WCA will be lower with the increase of roughness if the surface is originally hydrophilic. Thus, after the processing of laser etching, the hydrophilic SS substrate turned into a superhydrophilic state with a WCA nearly 0°, and the droplets spread quickly and formed a water film on the surface, as described by Fig. 4a, which can be explained by the magnification effect [47,48]. Subsequently, the modification treatment made the structured surface exhibit a WCA of 157.3 ± 1°, successfully realizing the state transition from superhydrophilicity to superhydrophobicity, which fully confirmed the significance of chemical components in the construction of superhydrophobic surfaces. Moreover, as shown in Fig. 4c, the water droplets were easy to roll down from the superhydrophobic surface while the angle of inclination was only 5°, implying a lower contact angle hysteresis and a Cassie-Baxter wetting state. In addition, the quartz sands had no residue on the sample after water washing in Fig. 4d, further confirming the excellent self-cleaning ability. In addition, as described in Fig. 4b, the micro-nano hierarchical structures on the substrate could capture a lot of air, forming an “air cushion” as the solution immersing, which also can be confirmed by the phenomenon of light reflection in Fig. 4e. While immersed in the water, the optical image of bare SS and superhydrophilic surfaces almost exhibited complete wetting. However, as for the superhydrophobic specimen, there appeared a mirror-like phenomenon at a specific angle,
ΔP = 2γ cosθ / r
(3.3)
where θ denotes contact angle, γ denotes the surface tension between liquid and air, r denotes the principal radii of curvature. 3.3.2. Molecular dynamics (MD) simulation for wettability analysis In order to further investigate the superhydrophobic mechanism, we used MD simulations to study the adsorption process of water nanodroplets on the SS surface or which treated with PDA@ODA compounds. As for the substrate, combined with the above composition analysis, it is assumed that the main oxide component on the SS surface is Fe3O4, and the (1 1 1) crystallographic plane is chosen to study the 856
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Fig. 4. Schematic illustrations of (a) Wenzel model and (b) Cassie-Baxter model, (c) the dynamic behavior of water droplet on the superhydrophobic surface, (d) optical image about the wettability state, (e) the optical images of the SS (1), superhydrophilic (2) and superhydrophobic (3, 4) specimens in sequence.
−0.437e (N atom in the secondary amine at Num 12), and others (C atom and H atom). Hence, similarly, the alkyl chains at the tail deviate from the surface. While introducing the ethanol-water mixed solution, there is no significant difference compared with the adsorption state in the absence of solvent. In conclusion, both A and B molecules can form strong interaction with Fe3O4 (1 1 1) surface in solvent environment, and the alkyl chains are all far from the surface to remain an approximately vertical state, as we have inferred to form a hydrophobic surface. Meanwhile, observing the adsorption state in Fig. 6a, it could be found that, as time progresses, the water molecules gradually adsorb on the Fe3O4 (1 1 1) surface and eventually spread completely. And then, the snapshot almost no change under the dynamic balance. The main reason is that, while coming into contacting with a nano-cluster of water molecules or even a larger-scale water droplet, Fe3O4 (1 1 1) surface, as a hydrophilic surface, will promote the water cluster or droplet to spread rapidly and present a hydrophilic state. While for the surface with the PDA@ODA modification, after establishing the adsorption model, the hydrophobic molecular membrane was first relaxed by 300 ps to immobilize all the molecules in the water molecule cluster. Then, the snapshots of the final moments of relaxation recorded as b1300 ps, which is equivalent to b2-0 ps. Afterwards, it can be seen from the snapshots at different MD simulation moments in Fig. 6b, the adsorption state of water molecules on hydrophobic surface always maintains a relatively regular spherical shape as the simulation time extending, which is different from the Fe3O4 (1 1 1) surface. Therefore, via MD simulations, as a result of the presence of product A and B, the surface can effectively prevent water clusters adsorbing and spreading, and exhibit a superhydrophobic state which is consistent with the
interaction with the PDA@ODA compounds and water droplets [42,51,52]. Due to the in-situ polymerization grafting process occurring on the surface is too complicated to analyze the reaction process in detail, we assumed the surface mainly occurred the Schiff base reaction and the Michael addition reaction based on the previous composition analysis, so the final products are assumed to be the following A (Schiff base reaction) and B (Michael addition process). As shown in Fig. 5a, for the product A, under the environment without solvent, the adsorption of A molecule on the surface is substantially vertical, mainly by the interaction of the amino group and the carbonyl group. Among that, the alkyl chain is slightly curved because of the flexibility, but the overall tendency is away from the surface. If the adsorption process is in an ethanol-water mixed solvent environment, i.e. Fig. 5b, the ring-shaped head, connecting the carbonyl group and the amino group, has completely adsorbed on the surface of Fe3O4 (1 1 1) surface. Meanwhile, the alkyl chain bends and detaches from the surface due to its hydrophobic nature and the flexibility of the chain. And for the product B, unlike A, the head containing the hydroxyl and the amino group is completely flatly adsorbed on the surface of Fe3O4 (1 1 1), presenting a stronger interaction between the composite and surface. This phenomenon can be ascribed for the charge distribution law of the quantum chemistry calculation, which is consistent with the data in the Supplement of Tables S4.1 and S4.2. The charge values in product A is by a turn of −0.481e (N atom in the amino group at Num 9), −0.357e (O atom in the carbonyl), −0.263e (C atom in benzene ring at Num 6), −0.237e (N atom in the secondary amine at Num 11), others (C atom and H atom). However, the charge values in product B is as followed, −0.521e and −0.447e (O atom in the two phenolic hydroxyl groups), −0.482e (N atom in the amino group at Num 9), 857
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To verify the antibacterial property of PDA@ODA compound, the growth of typical GN and GP bacteria, i.e. S. aureus and E. coli, was observed under different concentrations. As can be seen from the agar plate assay in Fig. 7a, with the increase in the concentration of the complex, no matter what kind of bacteria, its growth was greatly inhibited, and the number of colonies showed a gradual decreasing tendency. Via bacterial counting, the corresponding relationship between compound concentrations and Colony-Forming Units (CFU) is shown in Fig. 7b. In addition, antibacterial efficiency, given as the percentage of bacterial reduction by calculating the ratio between the number of survival bacteria colonies before and after, is listed in Table 1, reflecting the effects of compound concentrations directly. Compared with the control experiment, for S. aureus, the number of colonies has decreased by nearly two orders of magnitude at the concentration of 2.0 mg/mL, presenting a decrease of almost 99.6% reduction. But for the E. coli, antibacterial effect had been greatly improved with 90.3% under the same concentration, maybe resulting from the difference of cell walls. Up to now, the interaction between the compounds and bacteria is still complicated, especially the complex formation on the surface. As for the PDA@ODA compounds, the antibacterial mechanism may be partly attributed to the structural changes on the surface of bacterial membrane, by affecting the function of the cell membrane to inhibit bacterial growth [53]. Another reason is due to the binding of its long lipophilic side chain -C18H37 and lipid bilayer of the outer membrane, causing changes in cell membrane permeability and leakage of cytoplasmic compounds, indicating the effects on the membrane [54]. In the binding process, the hydrocarbon tail of the substance became intercalated into the interior hydrophobic zone of the microbial membrane. In addition, longer hydrophobic chains (C18) for the formed compound have a greater hydrophobic effect, which may affect antimicrobial potency and can be confirmed by the results of fluorescence microscope evaluated by the density of bright dots in Fig. 8. By comparison, after surface treatment, the antibacterial properties of the SS substrate have a certain improvement. Unlike the bare substrate in Fig. 8a, the blue dots in the yellow circle were sporadic distribution on the coated surface (Fig. 8b and c). Always, the superhydrophobic surface possessed a relatively more effective inhibition for the bacterial adhesion, which can be ascribed to the results of multifactor synthesis, such as biocide agents of the PDA@ODA compounds, micro/nano hierarchical surface structures and the reduction of bioadhesion by the presence of air cushion. Meanwhile, in the red circle, the adhesion of large amount dye mainly is because of the dye deposition, and maybe a little attributed to the bacterial adhesion.
Fig. 5. Snapshot of the MD simulation when the interaction between the A product molecule (a, b) or B product molecule (c, d) and Fe3O4 (1 1 1) surface reaches equilibrium in the presence/absence of ethanol-water mixed solvent environment.
previous experimental data. 3.3.3. Antimicrobial activity For the material property of stainless-steel, the antimicrobial activity is also regarded as one of the most important material properties.
Fig. 6. MD simulations of nano-water clusters adsorbed on the (a) Fe3O4 (1 1 1) surface and (b) surface modified with PDA@ODA membrane at different times. 858
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Fig. 7. (a) Agar plate assays and (b) Plots of the colony forming units (CFU) vs different concentrations (mg/mL) on E. coli and S. aureus.
contact angle still was as high as 153.5 ± 1°, in a superhydrophobic state. As well, the film layer didn’t appear cracks and peeling phenomenon, confirming the good stability in the heat resistance. Moreover, we immersed the superhydrophobic specimen into ultrasonic cleaner containing 50 mL ethanol and operated for 2 h. Meanwhile, the CA value only had a slight decline, was about 155.0 ± 1° and still exhibited superhydrophobicity. In conclusion, the as-prepared samples had good stability in the heat resistance, physical performance and durability, which will enhance the materials utilization in the application fields and working life.
Table 1 The antibacterial efficiency of E. coli and S. aureus under different compound concentrations. Strain
E. coli S. aureus
Concentration (mg/mL) 0.1
1.0
2.0
21.3% 40.1%
88.5% 90.8%
90.3% 99.6%
3.3.4. Robustness Due to the importance of surface stability for metal substrate, corresponding tests were carried out to assess the physical stability, thermostability, long-term stability of the prepared surfaces. As shown in Fig. 9a, we tested the physical stability of the as-prepared samples with a sandpaper wear method, so as to evaluate the conventional physical damage for the durable superhydrophobic ability [55]. As the wear distance increasing, the values of WCA had a slight decrease, and the SA gradually increased. Even though wear distance reached 2000 mm, the WCA value still maintain 136.4°, which exhibit a hydrophobic state. The wear resistance mechanism of as-prepared surface mainly can be ascribed by special microstructures on the SS substrate, which was not completely in contact with the sandpaper during the wear process. The processed SS substrates had a rough three-dimensional structure and were covered by nanolayers. The abrasion progress will only damage the outermost structure, and the internal structure will be protected. Furthermore, the long-term stability of the superhydrophobic surface for storage in air for six months was also examined by investigating changes in static WCAs with time evolution in Fig. 9b. A maximum contact angle of 154.5 ± 1° was observed on the specimen after exposed 6 months in the air. As presented in the Fig. 9c, it can be clearly seen that the surface wettability did not change significantly under 200 °C with the heating time extended to 2 h, the value of surface
4. Conclusion Inspired by the special micro/nano structures of the creatures and the bioadhesion of marine mussels, a two-step method can be used to fabricate the superhydrophobic surface with antibacterial property, including the laser interference patterning and in-situ polymerization, which anchored the chemical compositions onto the patterned stainless-steel substrate. Combined with the analysis of the structural and chemical characterization, the cooperation effect of the micro/nano hierarchical structure and low surface energy materials provided basic conditions for superhydrophobicity. Meanwhile, the surface also realized the state transition from superhydrophilic to superhydrophobic successfully. According to the results of MD simulations, the alkyl chains of PDA@ODA compounds were far from the surface to remain an approximately vertical state, promoting the formation of superhydrophobic surfaces. Also, the adsorption process of water clusters on superhydrophobic surfaces was well shown by the MD simulations, still maintaining spherical finally, which was consistent with the experimental data. Also, the PDA@ODA compounds (∼2.0 mg/mL) effectively inhibited the growth and multiplication of bacterial colonies of E. coli (about 90.3%) and S. aureus (up to 99.6%) via agar plate assays. In addition, the superhydrophobic specimen exhibited good antimicrobial
Fig. 8. Fluorescence microscopy for S. aureus of (a–c) bare SS substrate, laser-etched superhydrophilic surface and superhydrophobic surface. 859
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Fig. 9. The surface stability of the superhydrophobic specimen. Change of WCAs and SAs of the as-prepared superhydrophobic surface as a function of (a) abrasion distances, and the inset shows a schematic of the abrasion test; (b) exposed times in air; and (c) heating temperatures or ultrasonication.
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Acknowledgements The authors thank the Key Program for International S&T Cooperation Projects of China (2016YFE0132900), National Natural Science Foundation of China (No. 51775231), National Postdoctoral Program for Innovative Talents (BX20180123), China Postdoctoral Science Foundation (2018M641782), and JLU Science and Technology Innovative Research Team (No. 2017TD-04). 860
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