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Flexible hydrophobic antifouling coating with oriented nanotopography and non-leaking capsaicin Zhiwei Lu, Zhuo Chen, Yi Guo, Yanyun Ju, Yang Liu, Rui Feng, Chuanxi Xiong, Christopher K. Ober, and Lijie Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19436 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Materials & Interfaces

Flexible hydrophobic antifouling coating with oriented nanotopography and non-leaking capsaicin Zhiwei Lu,a Zhuo Chen,a Yi Guo,a Yanyun Ju,a Yang Liu,a Rui Feng,a Chuanxi Xiong,a Christopher K. Oberb and Lijie Dong*,a a

Center for Smart Materials and Devices, State Key Laboratory of Advanced Technology for

Materials Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, China b

Department of Materials Science and Engineering, Cornell University, New York, 14853, USA

ABSTRACT: Incorporating natural product antifoulants (NPAs) into coatings with controlled surface topography is considered a promising way to suppress marine fouling. However, the rapid leakage of NPAs and the relatively complicated process of constructing well-patterned topography remain unresolved problems for practical applications. In this work, capsaicin bonded

to

CoFe2O4/gelatin

magnetic

nanoparticles

(MNPs)

was

mixed

with

polydimethylsiloxane (PDMS)-based block copolymer. When applied together by a simple spray coating method, these materials formed a film. The leakage of capsaicin was restrained by the chemical bonds with the CoFe2O4/gelatin nanospheres. The primary nanorough structure was constructed by the phase separation of the PDMS-based copolymer. The secondary nanorough structure was formed by the incorporation of capsaicin-loaded CoFe2O4/gelatin nanospheres, which were demonstrated to improve the orientation of the PDMS-based block copolymer

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chains. The combination of oriented nanotopography and non-leaking capsaicin endows the coating with enhanced, long-lasting antifouling ability. KEYWORDS: NPAs, oriented nanotopography, CoFe2O4/gelatin, PDMS, antifouling 1 INTRODUCTION Coatings with specified surface topography, initially inspired by natural marine organisms, have attracted great interest for applications in marine antifouling. Biomimetic structures inspired by sharks, pilot whales, mollusk shells and so on as well as various artificially designed geometries have proven to be effective in deterring fouling.1-4 These studies demonstrated that the topographical features had considerable effects on the antifouling ability. The most widely accepted explanation of the underlying mechanism is based on attachment point theory. Surface textures with a smaller length scale than the fouling organisms were generally unfavorable for biofouling due to the limited number of attachment points.5,6 These concepts have encouraged a variety of research on surface designs based on microtopography and nanotopography. Several approaches, such as electron-beam lithography,7 photolithography8,9 and surface micro-replication,10 have been extensively applied to fabricate coatings with controlled micro/nanotextures. Schumacher and Carman et al.11,12 fabricated the engineered Sharklet AF™ microtopographies with length scales smaller than spores, resulting in significantly less spore settlement density than on the smooth PDMS surface. Although regular artificially textured surfaces have been extensively researched, these methods remain unsuitable for the large-scale application of antifouling coatings due to the limitations of current construction technology. Microphase-separated block copolymer is particularly attractive for the preparation of coatings with antifouling topography due to its spontaneous formation of nanotopography and its


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practicality for large-scale applications.13-15 PDMS-based antifouling coatings have been used commercially due to the low surface energy and elastic modulus and the good chemical stability of PDMS. Fang et al.16 prepared nanostructured coatings by incorporating polyurea segments into the PDMS backbone and found that the PDMS-polyurea coating exhibited higher sporeling removal efficiency than the PDMS coating. The nanotopography formed by the block copolymer was generally disordered. Further effort is required to understand and construct controlled ordered nanotopography for the development of marine antifouling coatings. The surface chemical composition also plays another key role in antifouling. Amphiphilic polymers, as well as enzymes, have displayed universal antibiofouling functions.17-19 Antifouling brushes have been grafted onto biomimetic surfaces to improve their antifouling performance.20,21 In addition, a variety of NPAs have been proposed for incorporation into antifouling coatings. The NPAs possess low toxicity, biodegradability and broad-spectrum antifouling activity.22-24 NPAs that inhibit an organism’s settlement without being biocidal have attracted considerable interest. Capsaicin has been recognized as one of the most promising antifouling NPAs.25 It is the active pungent component of chili peppers and has been proven to effectively prevent biological growth26 and to strongly inhibit multiple fouling species.27-29 The antifouling mechanism was attributed to discouraging the attachment of foulers, rather than killing them by sustained release.30 However, the rapid leakage of capsaicin shortened its service life.31 Therefore, the firm cohesion of capsaicin on the surface must be ensured by physical or chemical interaction. In this work, we have constructed a PDMS-based coating with enhanced and durable antifouling capacity by combining oriented nanorough topography with non-leaking capsaicin. The capsaicin was first bonded to CoFe2O4/gelatin nanospheres, then mixed with polystyrene-


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block-poly(dimethylsiloxane-stat-vinylmethylsiloxane) (PS-b-P(DMS-stat-VMS) or PSDV) solution and made to form a film on a glass substrate. The CoFe2O4/gelatin nanospheres played a critical role in decelerating the leakage of capsaicin and in forming the surface topography. The maximum adsorption of capsaicin on CoFe2O4/gelatin nanospheres was as high as 92 mg/g due to the hydrogen bonding interaction. Characterization of the surface topography revealed that the CoFe2O4/gelatin nanospheres were beneficial for the orientation of PSDV. As a typical fouling organism, Navicula subminuscula was utilized to detect the coating’s antifouling ability. The settlement test of Navicula subminuscula showed that the PDMS-based coating with oriented nanorough topography exhibited excellent antifouling performance. 2 EXPERIMENTAL Materials. Sodium hydroxide (NaOH), diethylene glycol (DEG), sulfuric acid (H2SO4), cobalt (II) chloride (CoCl2·6H2O), ethanol (CH3CH2OH), 3-glycidoxypropyltrimethoxysilane (GPS), acetic acid (CH3COOH), and toluene were all purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Iron (II) chloride (FeCl2·4H2O) was purchased from Damao Chemical Reagent Factory, Tianjin. Gelatin was purchased from Sigma Aldrich, USA. Capsaicin (FW 293.4, 98%) was provided by Energy Chemical, China. Thirty weight percent hydrogen peroxide (H2O2) was purchased from Aladdin, China. Polystyrene-blockpoly(ethylene-ran-butylene)-block-polystyrene (SEBS, MD6945) and SEBS grafted with maleic anhydride (MA-SEBS, D1102) were supplied by Kraton Polymer, USA. PSDV (Mn = 2.01×104, PDI = 1.37, PS: PDMS: PVMS = 146: 343: 100) was synthesized in our laboratory based on a published method.32 All chemicals were used without further purification. Synthesis of CoFe2O4/gelatin nanospheres. The synthesis process began with degassing under a nitrogen atmosphere and stirring at medium speed. FeCl2 ·4H2O (2 mmol), CoCl2 ·6H2O


4

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(2 mmol) and 30 mL of diethylene glycol (DEG) were added to a 100 mL four-necked vacuum flask and heated to 80 ºC within 30 minutes. Nine milliliters of NaOH/DEG solution (2.5 M) pre-heated to 120 ºC and 5 mL of gelatin aqueous solution (10 mg/mL) were then quickly syringed into the mixture. The reactants were further heated to 220 ºC and held there for 3 h. The resulting CoFe2O4/gelatin nanospheres were obtained by magnetic separation and washed 3 times with ethanol and distilled water. Adsorption of capsaicin on CoFe2O4/gelatin nanospheres. The adsorption of capsaicin on CoFe2O4/gelatin was conducted at room temperature. Fifteen milliliters of 0.5 mg/mL capsaicin in ethanol solution was mixed with 5 mL of 4.4 mg/mL CoFe2O4/gelatin nanospheres in aqueous solution and shaken at 100 rpm. Capsaicin bonded to CoFe2O4/gelatin nanospheres was labeled FeCap. Preparation of antifouling coating. The antifouling coating was prepared primarily according to a previous report.13 The glass slides were cleaned in a prepared piranha solution (concentrated sulfuric acid mixed with 30 wt% hydrogen peroxide, 5:3 v/v), rinsed in distilled water until the water reached a neutral pH, and then rinsed in anhydrous ethanol to ensure that no water remained on the surface. A mixture of 200 mL of anhydrous ethanol, 7 mL of 3(glycidoxypropyl)-trimethoxysilane (GPS) and 10 drops of acetic acid was then placed on the glass slides for 12 h, soaked in distilled water, rinsed in anhydrous ethanol and annealed for 12 h at 120 ºC under a vacuum. The silane-functionalized glass slides were coated with solution I (5% (w/v) SEBS, 2% (w/v) MA-SEBS in toluene) in a Cee Model spin coater at 2,500 rpm (acceleration of 500 rpm·s-1) for 30 s, annealed at 120 ºC for 12 h, and labeled the base layer. The base layer was spin-coated 3 times with solution II (12% (w/v) SEBS in toluene) in a Cee Model spin coater at 2500 rpm (acceleration of 500 rpm·s-1) for 30 s, then annealed at 120 ºC for


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12 h and labeled the structure layer. The structure layer was then spray-coated with 10% (w/v) PSDV in toluene mixed with a certain quality of FeCap NPs, which was annealed at 120 ºC for 12 h and then identified as the active layer PSDV/FeCap. The total thickness of the layers was 23±2 μm. Characterization

of

Materials.

The

chemical

structure

and

composition

of

CoFe2O4/gelatin nanospheres were studied by the KBr pellet method on a Fourier transform infrared (FTIR) spectrometer (Nexus Thermo Nicolet) in the range of 500 to 4000 cm-1 and an X-ray diffractometer (XRD, MAX-RB Rigaku) with Cu Kα radiation in the scanning angle range of 10 to 70o. The microtopography of CoFe2O4/gelatin nanospheres was observed by dispersing the nanospheres in ethanol solution and examining them under a high-resolution transmission electron microscope (HRTEM, JEM-2100F Jeol). The components of the coating surfaces were analyzed by an X-ray photoelectron spectroscope (XPS, ESCALAB 250xi). The surface microtopography of the coating was examined by an atomic force microscope (MFP-3DSA Asylum Research) in tapping mode and by a field emission scanning electron microscope (FESEM, Zeiss Ultra Plus) after being coated with gold. The roughness data of each coating surface was analyzed by using the Igor 6 software supplied with the equipment. The underwater bubble water contact angle was measured by a contact angle goniometer (JC2000C Shanghai). The elastic modulus of the PSDV/FeCap layer was determined by a Unitron universal tester (INSTRON5967). Adsorption/desorption of capsaicin on CoFe2O4/gelatin nanospheres. The fluorescence intensity of capsaicin in ethanol solution at various concentrations (0.5 μg/mL, 1 μg/mL, 2 μg/mL, 3 μg/mL, 4 μg/mL, 5 μg/mL) was tested by a Shimadzu RF-5301 PC fluorescence spectrophotometer in order to obtain the quantitative relationship between capsaicin


6

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concentration and fluorescence intensity. After the capsaicin solution was immersed in the CoFe2O4/gelatin solution for a certain time, the upper solution was removed and diluted 100 times. Then, the fluorescence intensity of the upper solution was tested by the fluorescence spectrophotometer. The concentration of capsaicin was then determined by the linear relationship between concentration and fluorescence intensity. The adsorption qe (mg/g) was calculated by the following equation:

qe 

( C 0  C e )V m

(1)

where C0 (mg/mL) and Ce (mg/mL) are the initial and final capsaicin concentrations in the solution, respectively; V (mL) is the volume of the solution; and m (mg) is the dry weight of the CoFe2O4/gelatin nanospheres. The pseudo second-order adsorption model was utilized to reveal the dominant interaction between CoFe2O4/gelatin nanospheres and capsaicin, or more precisely, to reveal whether the adsorption process was governed by a physical or a chemical interaction. Chemical adsorption has been shown to be well fitted by a pseudo second-order adsorption model,33 which is generally given in the form of equation (2):

t 1 1   t qt k 2 qe 2 qe

(2)

where qe (mg/g) is the adsorption uptake at adsorption equilibrium; qt (mg/g) is the adsorption uptake at adsorption time t; and k2 (g/mg·min) is the rate constant of the pseudo second-order adsorption kinetics model. Furthermore, 17.5 g/L NaCl in aqueous solution has been prepared to approximately simulate the salinity of a marine environment. Capsaicin bonded to CoFe2O4/gelatin nanospheres was dispersed in this NaCl aqueous solution and shaken at 100 rpm at room


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temperature. The fluorescence intensity of the upper solution was tested at different times to determine the desorption property of capsaicin on CoFe2O4/gelatin nanospheres. Co release test of the PSDV/FeCap coating. The Co release test was conducted based on the quenching of the fluorescence of quantum dots (CdSe/CdxZn1-xS) upon encountering cobalt ions. The fluorescence of a quantum dot aqueous solution decreases linearly with the addition of Co2+ to the solution. In this case, cobalt chloride was dissolved in deionized water to prepare Co2+ solutions with different concentrations of Co2+. Then, 50 μL of Co2+ solution and 2 mL of CdSe/CdxZn1-xS at a specified concentration were placed in a 5 mL volumetric flask. Then, the fluorescence spectra of the solutions were measured to obtain the curve of quenching (F0/F－1) versus the concentration of Co2+. The PSDV/FeCap coating was immersed in 25 mL of saline water containing 3.5 wt% NaCl. A 50 μL sample of saline water was removed at set intervals and mixed with 2 mL of CdSe/CdxZn1-xS at a specified concentration in a 5 mL volumetric flask. Then, the fluorescence spectra of the solutions were measured to draw the curve of quenching (F0/F) versus time in order to investigate the Co release behavior by obtaining the relation between the Co release amount and the immersion time. Settlement and release of Navicula subminuscula. Cells of Navicula subminuscula were cultured in F/2 medium contained in conical flasks. When the cells were in the log phase of growth, they were ultrasonically dispersed and then diluted to between 1.0×105 and 2.0×105 cells/mL with filtered and sterilized natural seawater. Each sample was immersed in the cell suspension at 25 ºC. After 4 h, the slides were gently washed in natural seawater to remove unattached cells. Five slides were imaged and counted by three-dimensional microscopy. Counts were performed for 10 fields of view (each 0.188 mm2) on each slide, and the average values for each slide are reported. The error bar represents the standard deviation of the 10 counts.


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After imaging, the slides with settled cells were exposed to a shear stress of 10 Pa, 20 Pa, 30 Pa, 40 Pa, or 50 Pa, which corresponded to a moving ship at a flow rate of 0.7, 1.4, 2.1, 2.8, or 3.5 m/s, for 5 min. The remaining biomass after this exposure was determined by the aforementioned method, and the removal rate was calculated. Additionally, the surface morphology of the membrane after exposure to different shear stresses was investigated under a field emission scanning electron microscope (FESEM, Zeiss Ultra Plus). Cell toxicity assessment of the PSDV/FeCap coating. OCCM cells were grown in Dulbecco's Modified Eagle's Medium (Gibco BRL, Gaithersburg, MD, USA) supplemented with 10% fetal calf serum, 100 U/mL penicillin G and 100 μg/mL streptomycin (Gibco BRL). Cultures were kept at 37 ºC in a humidified atmosphere of 5% CO2 and 95% air for 24 hours. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT; Sigma Chemical Co., St. Louis, MO, USA) solution was prepared in 5 mg/mL phosphate-buffered saline. The hydrophobic antifouling coating material was sterilized by high temperature and ultraviolet irradiation. The cells were incubated with or without our material for 24 hours. Briefly, 5×105 cells were seeded on a 96-well plate in 100 μL of medium and left overnight to attach. After treatment, 100 μL of MTT dye was added to each well and incubated for 2 hours at 37 ºC. Then, 100 μL of DMSO was added to each well. The plates were shaken until the crystals were dissolved, and 80 μL of liquid was tested. The optical density was determined by eluting the dye with dimethyl sulfoxide and measuring the absorbance at 570 nm with a spectrophotometer (UV-2401 PC, Shimadzu Co., Tokyo, Japan).

3 RESULTS AND DISCUSSION


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Scheme 1. Preparation of antifouling coating with oriented nanotopography and non-leaking capsaicin Interface adhesion is highly important for an antifouling coating, especially in a marine environment. With the assistance of chemical bonding and physical entanglement, the active PSDV/FeCap layer was firmly adhered to a surface, as illustrated in Scheme 1. As a typical substrate, glass slides were functionalized by hydroxyl via treatment with piranha solution, followed by glycidyl modification via GPS treatment. The glycidyl groups were then reacted with MA-SEBS to covalently attach the SEBS/MA-SEBS base layer.34 A SEBS structure layer with optimized thickness was then coated onto the base layer by controlling the spin coating times because a lower surface elastic modulus allowed the easier release of foulers.35 The active PSDV/FeCap layer was attached to the structure layer by means of the affinity between the PS blocks in PSDV and SEBS. The primary nanorough structure was related to the incompatibility


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between the PS blocks and PDMS-PVMS blocks, while the subsequent orientation of the PSDV was induced by the CoFe2O4/gelatin nanospheres. Furthermore, the abundant amino groups of the CoFe2O4/gelatin nanospheres were exploited to reduce the leakage of capsaicin by hydrogen bonding. 3.1 Surface morphology of the active layer

Figure 1. XPS wide-scan of PSDV layer and PSDV/FeCap layer. The PSDV layer and the PSDV/FeCap layer were prepared by spraying PSDV and PSDV/FeCap solutions onto the SEBS structure layer. The chemical compositions of the two coatings were characterized by XPS. As illustrated in Figure 1, both of the wide-scan spectra contained O1s, C1s, Si2s and Si2p peaks, but they differed in elemental content. The PSDV layer contained 15% O and 14% Si, while the PSDV/FeCap layer contained 26% O and 6.0% Si. This difference in elemental content confirmed the incorporation of FeCap NPs and indicated that the FeCap NPs occupied a large proportion of the surface.


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(a)

(b)

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(c)

(d)

Figure 2. (a) Three-dimensional AFM height image and (b) SEM micrograph of PSDV layer. (c) Three-dimensional AFM height image and (d) SEM micrograph of PSDV/FeCap layer. The morphology of the as-prepared PSDV layer and of the PSDV/FeCap layer was then characterized by AFM and SEM. Figure 2a shows an AFM image of the PSDV layer exhibiting a uniform surface with a root-mean-square (RMS) roughness of 1.5 nm. The SEM micrograph of the PSDV layer displayed in Figure 2b shows microphase separation morphology, which was intimately related to the thermodynamic incompatibility between the PS blocks and the PDMSPVMS blocks. Furthermore, the PS blocks had good affinity with the SEBS underlayer, and the low-surface-energy PDMS-PVMS blocks were enriched in the outer surface. Thus, the outer surface was primarily covered by PDMS-PVMS blocks. After the FeCap NPs were incorporated into the PSDV layer, as shown in Figure 2c, the PSDV/FeCap layer displayed a hierarchical nanorough morphology with a higher RMS roughness of 2.0 nm. Moreover, the maximum height of the PSDV/FeCap layer was less than 15 nm, indicating that most of the FeCap NPs were embedded in the PSDV.


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An SEM micrograph of the PSDV/FeCap layer is presented in Figure 2d. The PSDV/MNP layer was prepared by spraying PSDV/MNP solution onto the SEBS structure layer. Compared with the SEBS (Figure S1) and PSDV layers, the PSDV/MNP layer (Figure S2) and the PSDV/FeCap layer exhibited notably different morphologies. Specifically, the randomly arranged PSDV chains spontaneously formed an oriented structure, which indicated that the magnetic nanoparticles tended to direct the orientation of the PSDV chains. The FeCap NPs likely acted as an anchor to bind the PDMS-PVMS blocks due to the affinity of the polar groups and their common poor solubility in toluene solution. When the toluene evaporated, the PDMSPVMS blocks were inclined to stretch under thermal motion and orient around the FeCap NPs. 3.2 FeCap as a facilitator for oriented nanotopography formation

Figure 3. (a) X-ray diffraction pattern and (b) TEM image of CoFe2O4/gelatin nanospheres To determine the main reason for the anchoring of PSDV on FeCap NPs, the crystal structure and morphology of the CoFe2O4/gelatin nanospheres were characterized by XRD and TEM. Figure 3a shows the XRD spectrum of the CoFe2O4/gelatin nanospheres. The characteristic peaks indexed to (111), (220), (311), (222), (400), (422), (511), and (440) confirmed the formation of CoFe2O4. Since the reaction temperature in the synthesis process was lower than 300 ºC, small amounts of the metallic oxides Fe2O3 and CoO were detected,


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corresponding to the additional peaks at approximately 24º and 45º. These results indicated the high crystallinity of CoFe2O4/gelatin, which ensured that the FeCap was much denser than the PSDV solution. The higher density caused FeCap to deposit first on the SEBS underlayer during the formation of the PSDV/FeCap active layer. The chemical structure of CoFe2O4/gelatin nanospheres was identified by FTIR spectra (Figure S3). The adsorption peaks at 3410 and 1640 cm-1 corresponded to N-H bonds and carbonyls (-C=O) in the abundant amino and carboxyl groups of the gelatin. Figure 3b shows TEM micrographs of the CoFe2O4/gelatin nanospheres. The average size of a single CoFe2O4/gelatin nanosphere was approximately 80 nm, which was consistent with the finding that most of the FeCap NPs were embedded in the PSDV. The CoFe2O4 was encapsulated by gelatin via chelation between the carboxyl groups in the gelatin and the Fe and Co atoms, then assembled in an orderly fashion into larger nanospheres with rough surfaces.36 Considering the surface chemistry, in addition to the polar groups of gelatin, the capsaicin further improved the affinity between the FeCap NPs and PSDV. The PSDV tended to localize around the rough FeCap surface via physical interaction. 3.3 Binding ability of capsaicin in CoFe2O4/gelatin nanospheres for long-term antifouling (a)

(b)


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Figure 4. (a) Adsorption/desorption curve of capsaicin on CoFe2O4/gelatin nanospheres. (b) Adsorption dynamics curve of capsaicin (experimental data points and fit line). The capsaicin was bonded to the CoFe2O4/gelatin nanospheres to achieve long-term antifouling. Capsaicin dissolved in an ethanol solution was mixed with a CoFe2O4/gelatin aqueous solution at room temperature. Determining the amount of capsaicin adsorbed/desorbed on the CoFe2O4/gelatin nanospheres required knowing the quantitative relationship between capsaicin concentration and fluorescence intensity (Figure S4) as well as the fluorescence intensity of the capsaicin solution during adsorption (Figure S5) and desorption (Figure S6). The adsorption process was calculated according to equation (1) and is displayed in Figure 4a. The adsorption proceeded rapidly and reached equilibrium at 10 minutes. Another noteworthy feature was the equilibrium adsorption amount of 92 mg/g, suggesting the high capsaicinadsorption capability of CoFe2O4/gelatin. In practical applications in a marine environment, the salinity may induce the release of capsaicin from CoFe2O4/gelatin nanospheres. Therefore, to discover the impact of salinity on the desorption of capsaicin, the FeCap NPs were placed in 17.5 g/L NaCl aqueous solution at 25 ºC. The amount of capsaicin retained on the CoFe2O4/gelatin nanospheres was obtained by subtracting the released amount of capsaicin and is illustrated in Figure 4a. The retained amount was equal to the adsorption amount after 48 h, indicating that the capsaicin was stably bonded to the CoFe2O4/gelatin nanospheres under saline conditions. To identify the dominant interaction in the adsorption process, the experimental adsorption dynamics curve was obtained according to equation (2) and is illustrated in Figure 4b. The adsorption process fit the pseudo second-order adsorption model well, with R2 = 0.9974. The results suggested that the overall adsorption process was mainly controlled by chemical


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interactions. The abundant amino groups in gelatin formed hydrogen bonds with the phenolic hydroxyl and amide groups in capsaicin. Furthermore, the amino groups in gelatin were present in the interlaced inner channels and outer shell of FeCap, providing sufficient active sites to interact with capsaicin. As shown in Figure S7, the peak corresponding to N-H bonding in free capsaicin was at 3442 cm-1. However, in FeCap, the wavenumber of this peak shifted to 3415 cm-1, indicating the formation of hydrogen bonds between the -NH group in capsaicin and the NH2 group in CoFe2O4/gelatins nanospheres, which weakened the internal atomic interaction of the -NH group.37 The same result was also observed for the peak corresponding to the -OH group. Meanwhile, the peak at 1232 cm-1 was wider than the peak at 1323 cm-1, suggesting the existence of hydrogen bonds between the -OH group in capsaicin and the -NH2 group in CoFe2O4/gelatin nanospheres.38 Consequently, there was a relatively strong interaction between the CoFe2O4/gelatin nanospheres and capsaicin, contributing to the lack of capsaicin leakage from the coating.

3.4 Bubble contact angle analysis

Figure 5. (a) Underwater bubble contact angles of the PSDV layer and the PSDV/FeCap


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layer at different immersion times. (b) Co release amount of the PSDV/FeCap layer immersed in saline water for different times. To understand the influence of the incorporated FeCap NPs on the surface properties, the underwater bubble contact angles of the PSDV layer and the PSDV/FeCap layer were measured, as shown in Figure 5a. The bubble contact angle of the PSDV/FeCap layer was lower than that of the PSDV layer due to the hydrophilic groups of FeCap NPs. The bubble contact angles of both layers increased gradually with time, indicating that the surfaces of both layers became more hydrophobic after immersion in water. This increase in hydrophobicity suggested that the hydrophobic PDMS-PVMS blocks were spaced closer together under water repulsion, which was ascribed to the incompatibility between water and the PSDV chains. Notably, the bubble contact angle of the PSDV/FeCap layer exceeded that of the PSDV layer after approximately 54 h. This phenomenon was further proven by observation of the surface morphology, as shown in Figure S8. The AFM images showed that the RMS roughness increased to 5.6 nm and that the maximum height of the PSDV/FeCap layer was approximately 22 nm, indicating that the layer became bumpy after immersion in water. The explanation can be deduced that FeCap swelled slightly because of H2O penetration from the gelatin channels by the interaction of the hydrophilic -NH2 groups with water. The closer spacing of the PDMS-PVMS chains was expected to affect the antifouling ability of the surface. According to the Wenzel model, the increasing roughness of the hydrophobic surface should further increase the hydrophobicity of the surface. To trace the elemental release of the PSDV/FeCap layer, the Co release test was conducted based on the fluorescence analysis method (Figure S9). As shown in Figure 5b, the release amount of Co was less than 0.006 μmol/L after immersion for 144 h, indicating the strong


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interaction between the CoFe2O4/gelatin NPs and the SEBS layer. This strong interaction contributed to the sustained antifouling ability of the PSDV/FeCap layer. 3.5 Elastic modulus of the PSDV/FeCap layer

Figure 6. Stress-strain curve of the PSDV/FeCap layer. The introduction of the SEBS thermoplastic elastomer into the PSDV/FeCap layer was designed to control the elastic modulus of the PSDV/FeCap layer. Figure 6 shows the stressstrain curve of the PSDV/FeCap layer and its low elastic modulus of 5.0 MPa, which could be attributed mainly to the high elasticity of SEBS. The low elastic modulus enabled the PSDV/FeCap layer to readily release the fouling organisms, as described in the work of Brady.34 3.6 Settlement and release of Navicula subminuscula


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Glass

PSDV

300

PSDV/FeCap

250 2

PSDV/MNPs

SEBS

200

Cells/0.188 mm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 100 50 0

Glass

SEBS

PSDV PSDV/MNPs PSDV/FeCap

Figure 7. Initial images and settlement amounts of Navicula subminuscula on the glass, SEBS, PSDV, PSDV/MNPs, and PSDV/FeCap active layer. Figure 7 shows optical images and the corresponding settlement density of Navicula subminuscula on glass, SEBS, PSDV, PSDV mixed with CoFe2O4/gelatin MNPs and the PSDV/FeCap active layer. The Navicula subminuscula organisms were elliptical-lanceolate with a width and length of several micrometers. Many more cells attached to the smooth glass than to the other four layers with nanorough topography. Significantly fewer cells attached to the PSDV-based layers than to the glass and SEBS structure layers. Upon the incorporation of hydrophilic CoFe2O4/gelatin MNPs, the PSDV/MNPs exhibited lower cell settlement than PSDV due to the rougher surface. The lowest cell settlement appeared on the PSDV/FeCap active layer due to the addition of capsaicin, which played a significant role in deterring fouling by Navicula subminuscula without leaching out from the layer.


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Figure 8. (a) Percentage removal of Navicula subminuscula from the PSDV/FeCap layer exposed to different shear stresses. (b) MTT cell metabolism assay of the blank control group and the experimental group after culturing for 24 h. Figure 8a shows the percentage removal of Navicula subminuscula from the PSDV/FeCap layer. The removal rate was 12.8% under the flow rate of 0.7 m/s and reached 36.2% when the flow rate was 3.5 m/s. The removal of Navicula subminuscula was inversely correlated to the hierarchically oriented nanotopography, which possessed fewer attachment points, significantly weakening the adhesion strength of the cells on the layer and facilitating the release of the cells under water flow. Furthermore, the oriented nanotopography and buried CoFe2O4/gelatin MNPs were unchanged under different shear stresses, indicating that the morphology of the coating was fairly stable (Figure S10). To further assess the effects of the PSDV/FeCap layer on the marine environment and organisms, the cell toxicity of the layer was measured by the MTT assay with 24 h incubation periods, as shown in Figure 8b. The optical density at 570 nm of the control group and the experimental group was 0.31 ± 0.01 and 0.30 ± 0.01, respectively. The cell viability of the PSDV/FeCap layer was appropriately 97% of that of the control group at 24 hours, which


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corresponded to a cytotoxicity grade of 1. Thus, the PSDV/FeCap layer is expected to be highly environmentally friendly, allowing the normal growth of marine life. 4 CONCLUSION In summary, we have presented a practical approach to fabricating oriented nanotopography and meanwhile to preventing the leakage of capsaicin for enhanced and durable antifouling performance. The oriented nanorough structure could be easily constructed by anchoring microphase-separated PSDV chains on FeCap NPs. In addition, chemically binding capsaicin to the CoFe2O4/gelatin nanospheres has been demonstrated to effectively prolong the service life of capsaicin. The PSDV/FeCap active layer efficiently suppressed the settlement of fouling organisms due to the combined factors of capsaicin and hierarchically oriented nanotopography. The proposed approach is suitable for practical applications and might have potential applications in deterring marine biofouling. ASSOCIATED CONTENT Supporting Information SEM images, FTIR spectra, concentration-fluorescence intensity curves and fluorescence intensity curves. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel.: (86) 27-87651775. Fax: (86) 27-87651779 Notes The authors declare no competing financial interests. ACKNOWLEDGEMENTS


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