Capsaicin-Inspired ThiolâEne Terpolymer Networks Designed for

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Capsaicin-Inspired Thiol-Ene Terpolymer Networks Designed for Antibiofouling Coatings Haiye Wang, Joshua Jasensky, Nathan Ulrich, Junjie Chen, Hao Huang, Zhan Chen, and Chunju He Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03098 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017

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Capsaicin-Inspired Thiol−Ene Terpolymer Networks Designed for Antibiofouling Coatings Haiye Wang,1,2 Joshua Jasensky,2 Nathan W. Ulrich, 2 Junjie Cheng, 2 Hao Huang, 2 Zhan Chen, 2 Chunju He1 1

College of Materials Science and Engineering, Donghua University, Shanghai, 201620, P. R. China 2

Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States

ABSTRACT: Novel photocurable ternary polymer networks were prepared by incorporating N-(4-hydroxy-3-methoxybenzyl)-acrylamide (HMBA) into a cross-linked thiol−ene network based on poly(ethylene glycol) diacrylate (PEGDA) and (mercaptopropyl) methylsiloxane homopolymers (MSHP). The ternary network materials displayed bactericidal activity against Escherichia coli, Staphylococcus aureus, and reduced the attachment of marine organism Phaeodactylum tricornutum. Extensive soaking of the polymer networks in aqueous solution indicated that no active antibacterial component leached out from the materials, and thus the ternary thiol-ene coating killed the bacteria by surface contact. The surface structures of the polymer networks with varied content ratios were studied by sum frequency generation (SFG) vibrational spectroscopy. The results demonstrated that the PDMS Si-CH3 groups and mimic-capsaicine groups are predominantly present at the polymer-air interface of the coatings. Surface reorganization was apparent after polymers were placed in contact with D2O: the hydrophobic PDMS Si-CH3 groups left the surface and returned to the bulk of the polymer networks, while the hydrophilic PEG chains cover the polymer surfaces in D2O. The capasaicine methoxy groups are able to segregate to the surface in an aqueous environment, depending upon the ratio of HMBA/PEGDA. SFG measurements in situ showed that the antibacterial HMBA chains, rather than the “non-fouling” PEG, played a dominant role in 1

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mediating the antibiofouling performance in this particular polymer system. KEYWORDS: thiol-ene network; capsaicin-mimic, antibiofouling; surface reorganization, SFG vibrational spectroscopy Introduction Over the past decades, the development of antifouling coatings for marine ships has been of great interest due to adhesion of marine organisms onto ship surfaces causing increased hydrodynamic drag, greater fuel consumption, as well as an increase in ship maintenance and environment compliance costs.1-5 Biocide-containing antifouling coatings have been extensively used in order to resist the settlement of marine organisms.6-8 The most widely used antifouling compounds are mainly based on heavy metals such as copper, lead, tin, and arsenic.9-11 Even though they have been shown to be effective antifouling materials, their toxicity has limited their use in recent years.12-14 The restriction on the use of such toxic compounds has promoted the research on natural and eco-friendly synthetic biocides.6, 15-16 Since adhesion of marine organisms to a surface is known to be highly related to the surface chemical composition or structure,17-18 great effort has been made to design surfaces with non-fouling performance (with or without biocides), with a focus on varying the surface chemical composition.19-22 In particular, a marine biofouling process could start from the adsorption of protein-containing glues secreted by marine organisms to a surface. Thus, protein-resistant materials have been employed to prevent adhesion of marine organisms. Poly(ethylene glycol) (PEG) and zwitterionic polymers are the most popular nonfouling materials used for protein resistance.23-28 The strong surface hydration of PEG is known to play an important role in its mechanism of protein resistance. Such a strong hydration was formed at the PEG surface through extensive hydrogen bonding between water molecules and the ether oxygen atoms.29-30 The tightly bound hydration layer acts as a physical barrier, where the removal of water by biomolecules poses as a significant energy cost. This leads to 2


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the excellent non-fouling property of the material.31 However, incorporation of the “non-fouling” PEG chains to the surface can only effectively resist protein adsorption, which may not be able to reduce bacterial colonization.32-33 Indeed, bacterial colonization on a surface is another crucial factor to induce the marine biofouling process.1 The fouling process is highly dynamic and the adhered bacteria incubate rapidly to form a biofilm, which contribute to the microfouling community.34 Thus, effective strategies to prevent biofouling may comprise (1) preparation of “non-fouling” surfaces that can resist initial protein adsorption, (2) fabrication of antibacterial surfaces that can kill bacteria when they are deposited. We believe that an effective way to design an antifouling coating is to combine both protein-resistant and antibacterial properties into the surface.35 In order to ensure a long-term antibacterial effectiveness for the coating, the leaching of biocides from the coating should be avoided as much as possible. To do so it is necessary to introduce the polymer with antimicrobial activity by binding the antibacterial agent covalently to the polymer.33, 36-37 Capsaicin, the main active component of chili peppers, has strong deterrence toward microorganisms. Previous research has shown that its derivatives and analogs could possess much more attractive properties than the current toxic antifoulants,32 and have consequently been used as an additive for anti-fouling coating on ships and water craft in ocean transportation.38-40 As one of the capsaicin-mimic materials, a functional monomer N-(4-hydroxy-3-methoxybenzyl)-acrylamide (HMBA) was derived from acrylamide by reaction with guaiacol through the Friedel-Crafts reaction. Also, the carbon–carbon double bond can be used for polymerization with other monomers to produce polymer network antifouling coatings.41-43 This study aimed to develop a new concept of using hot and spicy surfaces for antibiofouling. In the long run, the materials need to be optimized with better performance. In 3


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this work, a simple and versatile method to prepare bio-inspired ternary polymer networks was developed. This method was based on the photo-curing of polysiloxane macromonomer (PSMM), capsaicin-mimic HMBA and poly(ethylene glycol) diacrylate (PEGDA). The above polymer network compositions were chosen due to their photo-curing behavior and extensively characterized physicochemical properties. The protein resistance and antibacterial property of the prepared polymer network films were investigated and found to be dependent on the ratios of the different components in the films. Also, a study on the antibiofouling activity of the prepared polymer network films against the diatom Phaeodactylum tricornutum, taken as a commonly accepted model organism, was performed. In addition, the surface structures of the polymer network films in different chemical environments were studied by sum frequency generation (SFG) vibrational spectroscopy in situ; the effect of the polymer bulk composition on the surface structure of the polymer network was elucidated. The surface structural results can be well correlated to the antibiofouling performance of various polymer samples, providing an in-depth understanding of structural – function relationships of the prepared materials as well as the antibiofouling mechanisms of such materials. Experimental Section Materials. All chemical reagents and solvents were obtained at the highest purity available from Aldrich Chemical Co. and used without further purification unless otherwise specified. 2,2-dimethoxy-2-phenylacetophenone (DMPA), dimethyl formamide (DMF), ethanol, poly(ethylene

glycol)

diacrylate,

N-hydroxymethylacrylamide,

and

guaiacol

(2-methoxyphenol) were purchased from Aladdin. (Mercaptopropyl) methylsiloxane homopolymer (MSHP) (Mn=4000-7000, average numbers of thiols were 40) was obtained from Gelest and used as received. Bovine serum albumin (BSA) and lysozyme were 4


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purchased from Sinopharm Chemical Reagent Co. Phaeodactylum tricornutum was supplied by the Ocean University of China. Synthesis of the HMBA by Friedel-Craft Reaction. The synthesis procedure of HMBA was similar to that presented in the previous report with some modifications.33 10 g of N-hydroxymethylacrylamide and 12.5 g of guaiacol were dissolved in 50 ml of absolute ethanol. The mixture was cooled with an ice-bath, and 5 mL of H2SO4 was added dropwise under stirring over a period of 1 h. The reaction mixture was kept stirring for 7 days at room temperature. Then the product was washed with water, and concentrated under vacuum to give a yellow liquid, which was poured into diethyl ether and white precipitate of HMBA was formed. The crude product was recrystallized from ethanol. The results of the 1H NMR and FT-IR measurements of the synthesized HMBA are presented in the supplementary material. Synthesis of Ternary Thiol-Ene Polymer Networks. A series of ternary thiol-ene polymer networks containing different molar ratios of PEGDA and HMBA were synthesized from resins maintaining a 1:1 thiol/total alkene functional group stoichiometry (The bulk concentration of MSHP in the coating was 37 wt%). HMBA and poly(ethylene glycol) diacrylate were successively added to the (Mercaptopropyl) methylsiloxane homopolymer to prepare the three-component mixtures. For all cases, 1 wt % photoinitiator DMPA was first dissolved in MSHP, and subsequently HMBA and PEGDA were added with the proper amounts (see Table 1). Once completely dissolved, the mixture was transferred onto a glass slide and then cured by exposure to UV radiation under N2 purge.

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Scheme 1. Schematic showing the photo-cured ternary thiol−ene/capsaicin polymer networks. Table 1 Reaction conditions and the obtained ternary thiol-ene polymer networks. Each sample contains a 1:1 molar ratio of siloxane vs. the sum of HBMA and PEGDA, but with a different HBMA/PEGDA molar ratio. Molar ratio of

Coating composition

HMBA

HMBA (wt %)

Light intensity Sample

Curing time

color

16 min 14 min 12 min 10 min 8 min

colorless colorless colorless colorless yellow

and distance /PEGDA MHP (1:9) MHP (3:7) MHP (5:5) MHP (7:3) MHP (9:1)

1:9 3:7 5:5 7:3 9:1

5 15 26.9 39.3 53

80%, 12 cm 80%, 12 cm 80%, 12 cm 80%, 12 cm 80%, 12 cm

Characterization Attenuated Total Reflectance FTIR (ATR-FTIR). A Nicolet 7600 FTIR spectrometer was used to collect ATR-FTIR spectra in this research. The spectra were collected in the wavenumber range between 400 and 5000 cm-1 with a resolution of 4 cm-1. Contact Angle Measurements. A contact angle goniometer (Dataphysics OCA40, Germany) was used to measure static water contact angles on sample surfaces using the sessile drop method. The measurements were performed at room temperature. For each measurement, a 6


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syringe was used to drop 3 µl of deionized water onto the sample surface. The contact angle data presented in this research for each sample were the average of five contact angles measured on different regions of the sample. SFG Vibrational Spectroscopy. As a second order nonlinear optical spectroscopic method, SFG has a submonolayer surface specificity, which comes from its selection rule.44-54 Previous papers have extensively reported the details of the SFG instrumentation, date collection geometry, and SFG experimental procedures,55-57 which will not be repeated here. The polymer films used in this SFG study were prepared by spin coating polymer solutions onto fused silica substrates. A spin coater from Specialty Coating Systems (Indianapolis, IN) was used to coat thin films of MHP-3/7, MHP-5/5, or MHP-7/3 with their 2 wt % solutions in DMF. The samples were then cured by exposure to UV radiation under N2 purge. The thickness of the resulting films was ~55 nm. In SFG experiments, the two input beams, the visible (30 µJ pulse energy) and the frequency tunable infrared (IR, 100 µJ pulse energy) beams, overlapped spatially and temporally on the sample surface in air or in water after penetrating through a right angle prism. The incident angles of the beams were 60° (visible) and 54° (IR) with respect to the surface normal, respectively. A monochromator along with a photomultiplier tube was used to collect the reflected SFG signal from the sample surface. In this research, SFG ssp (s-polarized sum frequency output, s-polarized visible input, and p-polarized IR input) signals were collected. Such signals were normalized by using the input visible and IR beam intensities. Optical Properties. A Shimadzu UV-1800 UV/Vis spectrophotometer was used for the optical absorption measurements of 400-700 nm. The quantitative transmittance reported below was 7


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measured at 550 nm. Fluorescence Microscopy. BSA with FITC-label was prepared according to a published procedure.58 Each polymer coating sample with a size of 1×1 cm2 was incubated with 4 ml FITC labeled BSA solution in a vial. After samples were left to shake for 12 at 4 °C in the dark, the polymer sample was thoroughly washed using PBS so that any loosely adsorbed proteins on the surface could be removed. An Olympus BX-51 inverted microscope equipped with a 20× objective, a mercury arc lamp, and an Olympus DP52 digital camera was used for the fluorescence imaging experiments. The same experimental conditions such as collection time, CCD gain, and optics set (U-MWB2 filter cube with excitation of 450-480 nm and emission of 510-520 nm) were used in all the experiments. Therefore it is reasonable to compare images from frame-to-frame. Protein Adsorption Tests. Protein adsorption was determined using an ELISA method according to a previous procedure59 (see Supporting Information). Biofouling Assays Phaeodactylum tricornutum Settlement and Adhesion Assays. Phaeodactylum tricornutum was used to test the antifouling behaviors of various materials. Supplied by the Ocean University of China, Phaeodactylum tricornutum has been grown in f/2 medium for several days at 15°C. In this study, the diatoms were cultivated in a climate chamber at 20°C with a light intensity of 2000 lux. Then diatom suspension (5 ml, 104 cells/ml) was added to the surface of each polymer sample (coated on glass slide) in an individual dish. After 7 days, all the samples were gently dipped into a beaker with 3% salinity, 0.22 µm FSW to wash off any unattached cells. This washing step was repeated three times. Then a hemocytometer was 8


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used to determine the density of cells on each polymer sample surface and digital images of diatom growth were recorded. Antimicrobial Test. Antimicrobial tests were performed according to standard antimicrobial test protocols described in detail elsewhere.35 E. coli and S. aureus were chosen as representative Gram-negative and Gram-positive bacteria, respectively. Antimicrobial

Activity

of

Polymer

Leached

from

Ternary

Polymer

Networks.

Polymer-coated prisms were incubated with 5 mL of PBS (pH 7.4, without calcium and magnesium) in a sterile 15 mL Falcon tube at 37°C with orbital rotation at 200 rpm for 1 hour. An untreated prism was used as a control. The PBS solution (4.5 mL) was removed from each Falcon tube and added to another tube containing E. coli fresh culture (0.5mL, OD = ~0.05), then incubated at 37 °C for 2 hour. The density of cells was determined using a cell density meter. Some other test methods of physical and chemical properties, such as hardness, adhesion force, chemical resistance and swelling, are presented in supporting information. Results and Discussion Photo-Curing of Ternary Thiol−Ene/Capsaicin Polymer Networks. In order to investigate the compositional effect on the polymer surface structure, as well as the physicochemical and biological properties of the resulting ternary networks, thiol-ene polymer networks containing different ratios of HMBA and PEGDA were synthesized. HMBA and di-functional poly(ethylene glycol) diacrylate (PEGDA) were used in the polymer network sample along with a linear siloxane polymer (mercaptopropyl) methylsiloxane homopolymers (MSHP) to study the structure-property relationships of the thiol-ene networks. Reaction conditions on the ternary thiol-ene polymer networks and different samples prepared are shown in Table 1.

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The coating MHP(9:1) containing more than 53 wt% HMBA appeared rough and light yellow. Although all coatings were clear and smooth, they were difficult to cure and could not solidify completely when the HMBA content was less than 5 %. Perhaps this is due to the fact that the PEGDA content is too high, which prevents the complete curing. For these reasons, only optically transparent and flexible coatings having HMBA between 5-53 wt% were studied in detail. Stability of the Ternary Thiol-Ene Polymer Networks. Although there exist many classes of marine antifouling coatings ranging from resins to hydrogels, the ideal characteristics of highly active and long lasting materials include low water-uptake and high mechanical stability.60 After immersion in water for 48 hrs, thiol-ene cross-linked networks with 15-39.3 wt% HMBA were found to maintain visual integrity on the substrates, exhibiting excellent stability in water, as shown in Fig.1(a). We also studied the stability for a week, and no change was observed. All the coatings were still transparent (88.4%~89.3% transmittance measured at 550 nm), only a slight decrease was observed upon the increased addition of HMBA (Fig. 1(b)).

Fig. 1 (a) Stability of the ternary thiol-ene polymer network coatings against water immersion. (b) Transmission spectra of the control and corresponding coatings. (The samples are transparent and colorless. The papers underneath the samples are green). Physicochemical Characters of the Ternary Thiol-Ene Polymer Networks. Cross-hatch 10


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adhesion, hardness, seawater resistance, and chemical resistance were determined to evaluate the physicochemical performance of these network samples. As shown in Table 2, all three coatings yielded similar results for the seawater and 20% HCl resistance, where the coatings with different compositions kept their intact morphology. Adhesion tests were used to measure the adhesive properties of the ternary polymer networks to glass. With an increased HMBA content, the adhesive property of the MHP (7:3) coating improved to 0B, where absolutely no film was removed from the surface of the substrate. This originates from the special structure of HMBA, which was very similar to a widely studied strong bioadhesive 3,4-dihydroxyphenylalanine (DOPA). In hardness tests, the rigidity of the ternary polymer networks also increased, which was ascribed to the stiffness of the benzene ring from the HMBA. Despite the slight deficiency in alkali and acetone resistance, other measured properties all demonstrated outstanding physicochemical performance for these samples. Table 2 Results of thiol-ene polymer network coatings from the measurements on cross-hatch adhesion, hardness, seawater resistance, chemical resistance, water-uptake, and transmittance at 550 nm.

Sample

MHP(3:7) MHP(5:5) MHP(7:3)

cross-hatch adhesion 3B 1B 0B

Hard ness HB HB 1H

Seawater resistance （24h） E E E

20% NaOH resistance (24h) R C F

20% HCl resistance (24h)

Aceton resistan ce

Water uptake (%)

Transmit tance (%)

E E E

R R C

1.12 0.76 0.28

89.35 88.74 88.49

Remark: E, F, C, R, P, corresponding to the intact, bubble, wrinkle, fracture, peeling phenomenon, respectively. Static Water Contact Angle. It has been shown that wettability is a critical parameter for an amphiphilic system to resist protein adsorption.61 The amphiphilic balance of the ternary thiol-ene polymer networks can be controlled by altering the ratio of the HMBA and PEGDA monomers, which should lead to the variation of surface wettability. As shown in Fig. 2, the static water contact angles depended on the ratio of the HMBA and PEGDA monomers in the 11


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ternary network (e.g. the contact angle increased from 85.2° for MHP(3:7) to 93.6° for MHP(5:5) and further to 106.4° for MHP(7:3)). This indicated that the surface became more hydrophobic with an increasing content of HMBA, leading to an enhanced bacteria killing capability by surface contact.37,38 In contrast, the PEGDA segment played a significant role in increasing the surface hydrophilicity, which was favorable to reduce the nonspecific adhesion of proteins.

Fig. 2 Static water contact angle of thiol-ene polymer network coatings.

Nonspecific Protein Adsorption Resistance

12


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Fig. 3 (a) Fluorescence microscopic images showing BSA and lysozyme adsorption results on the thiol-ene polymer networks. (b) The quantitative results of protein adsorption on polymer networks. The adsorption of protein on a material surface was used as one of the most important criteria to evaluate the antifouling performance of the material, e.g., for using as a marine coating.62 In this study, both BSA and lysozyme adsorptions onto polymer networks were tested (Fig. 3). The Polydimethylsiloxane elastomer (PDMSe: Silastic T2) coating (as a control surface) exhibited a high protein adsorption on the surface. This is reasonable because it is hydrophobic which likes to adsorb more protein molecules. The protein adsorption amounts on amphiphilic polymer networks were measured to be less, because their surface PEG functionality could resist BSA and lysozyme adsorption, especially when compared to the control PDMSe sample. It was also observed that with an increase in PEG content within the coating, the protein adsorption amount was reduced. For the amphiphilic coating with a high PEG content, barely any BSA or lysozyme protein was found to be adsorbed on the surface. Interestingly, on each coating surface, the adsorption amounts for BSA and lysozyme were slightly different. This likely was due to that fact that BSA and lysozyme had different net charges in a PBS solution (BSA was negative while lysozyme was positive). The electrostatic interaction effect between the protein and a coating surface would also contribute to protein adsorption. That is why all the coatings always have a slightly higher lysozyeme adsorption. Nevertheless, the improved protein adsorption resisting property will have benefits to enhance the antibiofouling performance of a material.

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Fig. 4 Results of bactericidal tests against E. coli and S. aureus.

Table 3 Antibacterial efficiency for the ternary thiol-ene polymer networks. E. coli Sample Blank MHP-3/7 MHP-7/3

No. of bacteria colonies 97 49 6

S. aureus average Eb (%)a 0 49.4 93.9

No. of bacteria colonies 124 40 5

average Eb (%)a 0 67.7 96.0

Antibacterial Activity of MHP Coating. Escherichia coli and Staphylococcus aureus were chosen as representative gram-negative and gram-positive bacteria to evaluate the antimicrobial activities of MHP coatings. With the pristine glass serving as a control, the adherent bacteria colonies within 24 h incubation were shown in Fig. 4. There were considerable reductions in E. coli and S. aureus colonies after 24 h incubation with the MHP coatings, indicating that the presence of HMBA in the coating significantly inhibited bacteria growth. With an increase of the HMBA content in MHP coating, the antibacterial effect against both S. aureus and E. coli became stronger. The antibacterial efficiencies (Eb) of the different coatings are presented in Table 3. For the MHP coatings, the average antibacterial efficiency of the MHP-7/3 sample surface for E. coli and S. aureus reached 93.9% and 96.0%, respectively, which had a greater killing effect than the MHP-3/7 coating. As the 14


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Gram-negative E. coli possesses an additional lipopolysaccharide-containing outermost membrane as compared to Gram-positive S. aureus, the polymer coatings had a slightly lower killing efficiency against E. coli as compared to S. aureus.63 The high bacteria killing efficiencies further confirmed that the MHP cross-linked coating should be especially useful for marine antibiofouling. To determine whether any bacteria were killed by the antimicrobial contents leached out of the coatings in solution, each of the prisms with coated polymer films was placed in PBS buffer without bacteria and then the buffer solution was added to an E. coli suspension. If the antimicrobial polymer components were leached out from a coating film into the buffer solution, then the biocidal activity of the buffer could be detected.64 As shown in Table 4, the PBS buffer solution incubated with the coatings of MHP-3/7, MHP-5/5, MHP-7/3 exhibited no antimicrobial activity. This suggests that no antimicrobial polymer contents leached to the solution. Therefore the biocidal activity of MHP coatings must come from the contact between the bacteria and the coating surface. Table 4 Antibacterial effects of possible leachables from the ternary thiol-ene polymer networks. Sample solution Pretreatment (a) Control (b) MHP-3/7 MHP-5/5 MHP-7/3

OD of the E. coli. cells 0.05±0.01 0.44±0.02 0.43±0.03 0.42±0.02 0.43±0.03

a) The PBS solution with E.coli.before incubation. b) The PBS solution was incubated with a blank (control) and polymer coated prisms after two hours. Phaeodactylum tricornutum Adhesion Assays. Two ternary thiol-ene polymer networks with 15


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different contents of HMBA as well as two controls (glass and PDMSe) were chosen for testing their antifouling performance against Phaeodactylum tricornutum. The purpose of this research was to understand whether the “non-fouling” PEG component or the antibacterial HMBA chains played a dominating role in the determination of the antibiofouling performance of a polymer network material. As shown in Fig. 5A, the numbers of Phaeodactylum tricornutum diatoms attached on the surfaces of MHP-3/7 and MHP-7/3 coatings are 195 × 103 and 85 × 103/cm2 respectively, which are much lower than those on the hydrophilic glass control (290 × 103 /cm2), and the hydrophobic commercially available PDMS coating (370 × 103 /cm2) surfaces. The results indicated that both the polymer network amphiphilic coatings exhibited significantly better resistance to Phaeodactylum tricornutum settlement. More importantly, the chemical composition of the polymer films markedly affected their antifouling properties. The sample containing a higher HBMA exhibited a better antifouling effect, indicating that here in this type of polymer network polymers the anti-bacterial component is primarily responsible for the improved anti-biofouling performance.

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Fig. 5 (A) The average numbers of Phaeodactylum tricornutum cells attached on various sample surfaces. (B) Example optical microscopic images of the settlement of Phaeodactylum tricornutum on the surfaces after 7 days. The bars are 10 µm.

Fig. 6. SFG measurement of the polymer networks on a right angle silica prism in contact with diluted water. Molecular Surface Structures of Thiol-Ene Polymer Networks in Air and diluted Water. To understand the varied antifouling performance of the polymer networks with different components, molecular surface structures of these materials were probed using sum frequency generation (SFG) vibrational spectroscopy systematically in air and in an aqueous environment (Fig. 6). Fig. 7 (a) shows SFG ssp spectra detected in the frequency region between 2800 and 3050 cm−1 observed from the surfaces of various coating samples in air. The SFG spectra collected from the surfaces of the three samples are similar, dominated by peaks centered at 2835, 2880, 2910, 2940, and 2985 cm-1, which can be assigned to the C-H symmetric stretching modes of the methoxy group (OCH3), the regular methyl (C-CH3) group, and the Si−CH3 group, the Fermi resonance modes of the regular methyl and/or the Si-CH3 groups, and the asymmetric of the methoxy group, respectively. Therefore the peaks at 2835 cm-1 and 2985 cm-1 are believed to originate from the HMBA component, and the 2880, 2910, and 2940 cm-1 peaks are from the PDMS. This shows that both HMBA and PDMS components cover the polymer network surfaces in air. The presence of a strong 2910 cm-1 17


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peak indicates that the PDMS chains are dominating the polymer-air interface. The relative intensities of the methoxy signals increased as the HMBA/PEG ratio in the sample increased from 3:7 to 7:3. This suggests that the surface coverage of HMBA increased as a function of bulk content, as expected. It is worth mentioning that the above peaks are overlapped with each other (Fig. 7), thus we cannot exclude the possible presence of the signals around 2850-2860 cm-1 range, where the signals from the PEGDA methylene groups should be located. Therefore, we could not exclude the possibility that PEGDA also was present on the sample surface in air.

Fig. 7. SFG ssp spectra of the polymer surfaces in air. Since the antifouling polymers need to be used in an aqueous environment, we then studied the surface restructuring behavior of the polymer networks in water. Previously we have demonstrated that SFG is a powerful tool to elucidate the surface restructuring behavior of polymer materials in water in situ in real time.

57, 65-70

To exclude spectral interference from

the water molecules, D2O was used. The SFG spectrum collected from the 7/3 polymer/water interface was markedly different from that collected from the surface in air (Fig. 8a), showing that the polymer surface structure changed in water. In the SFG spectrum collected from the polymer/D2O interface (Fig. 8b), the strong peak at 2910 cm-1 disappeared, showing that the 18


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hydrophobic PDMS backbone component returned to the bulk in water. On the other hand, the spectrum from the MHP-7/3 surface in water exhibited a distinct peak around 2850 cm−1, indicative of the good ordering of the PEG component at the polymer/water interface, showing that PEG component was clearly present on the surface in water. This is reasonable because PEG is very hydrophilic; its presence on the surface reduced the free energy of the entire system. The detection of the 2880 cm-1 signal indicates that the end groups of the PDMS still can exist on the polymer surface in water. This surface restructuring behavior is reversible upon drying of the surface. The SFG signal recovered to the initial feature after the sample was removed from water and exposed to air again. Moreover, the polymer surface structure can recover to the initial state after repeating the process of placing the sample in water and then drying multiple times. We believe that this result also supports the results obtained from the previous leaching experiment, showing that no materials leached or removed from the sample in water, otherwise the surface structure should gradually altered as a function of time during many repeat washings.

Fig. 8. SFG spectra collected from the MHP-7/3 coating surface in (a) air, (b) in contact with D2O, (C) in air, after removal from the D2O. To understand the surface restructuring behaviors of all the polymer networks in water, SFG 19


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spectra were also collected from the other two polymer networks with the 3/7 and 5/5 ratios of HMBA and PEG in D2O. All the three SFG spectra collected from the polymer/D2O interface are shown in Fig. 9. As presented above, the peak at 2850 cm-1 is assigned to the CH2,s of the PEGDA chains. The peak observed at 2880 cm-1 is ascribed to the CH3,s of the terminal methyl group of the PDMS side chains. The spectral features of the MHP coating surfaces in water are similar, dominated by the PEG and PDMS end group. However, the methoxy C-H symmetric stretching signal at 2835 cm-1 varied. At the MHP-3/7 interface, no SFG signal at ~2835 cm-1 was detected in water, indicating that the lack of methoxy group at the polymer/D2O interface. However, on the MHP-5/5 and 7/3 surfaces in D2O, the peak centered at ~2835 cm−1 was clearly detected, indicating that the HMBA component migrated to the interface, because of the increased overall content in the sample. The different surface structures of the 3/7 and 7/3 samples showed that the different bulk contents led to the varied surface molecular structures in water: More HMBA contents in the bulk resulted in a higher surface coverage in water. We believe that the better antifouling performance against Phaeodactylum tricornutum by the 7/3 sample is due to the high surface coverage of HMBA, which presents a higher bacteria killing capability.

Fig. 9. SFG spectra in the C−H stretching frequency region collected from the interfaces 20


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between D2O and (a) MHP-3/7, (b) MHP-5/5, and (c) MHP-7/3. Conclusions A simple method was developed to prepare novel bio-inspired thiol-ene polymer networks for antibiofouling application. More specifically, ternary amphiphilic polymer materials were synthesized via photo-polymerization of a polysiloxane macromonomer MSHP carrying mercaptopropyl

side

groups

with

poly(ethylene

glycol)

diacrylate

PEGDA and

capsaicin-mimic N-(4-hydroxy-3-methoxybenzyl)-acrylamide HMBA monomer, without using the conventional heavy-metal catalyzed crosslinking reactions. The measured results on cross-hatch adhesion, hardness, and stability indicated that such materials exhibit desired physicochemical properties. It was found that the varied compositions of the two components in the bulk led to different surface structures and surface properties. These materials were found to be able to resist protein adsorption, likely due to the PEG component. The incorporation of the capsaicin-mimic HMBA enables the polymer network materials to demonstrate good antibacterial activity against E. coli and S. aureus. The molecular structural information obtained from the SFG data provides direct experimental evidence to interpret the mechanism of the antibiofouling performance of the thiol-ene polymer network materials. The better antibacterial activity of MHP-7/3 was due to the surface presence of more HMBA content. The diatom adhesion assay results indicated that the antibacterial component HMBA played a dominating role in the improvement of antifouling performance of a polymer network material against P. tricornutum. This research demonstrates that substantial presence of the antibacterial component in a polymer material on the surface is needed to ensure excellent antibiofouling performance of a polymer material. 21


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors *E-mail [email protected] *E-mail [email protected] ORCID Zhan Chen: 734-615-4189 Chunju He: +86-21-67792842 ACKNOWLEDGMENTS This work was financially supported by Fundamental Research Funds for the Central Universities (CUSF-DH-D-2016037) and US Office of Naval Research (N00014-16-1-3115). REFERENCES (1) Callow, J. A.; Callow, M. E., Trends in the Development of Environmentally Friendly Fouling-Resistant Marine Coatings. Nature Communications 2011, 2. (2) Kim, S.; Gim, T.; Kang, S. M., Versatile, Tannic Acid-Mediated Surface Pegylation for Marine Antifouling Applications. ACS Appl Mater Interfaces 2015, 7, 6412-6. (3) Shivapooja, P.; Wang, Q.; Szott, L. M.; Orihuela, B.; Rittschof, D.; Zhao, X.; Lopez, G. P., Dynamic Surface Deformation of Silicone Elastomers for Management of Marine Biofouling: Laboratory and Field Studies Using Pneumatic Actuation. Biofouling 2015, 31, 265-74. (4)

Pollack,

K.

A.;

Imbesi,

P.

M.;

Raymond,

J.

E.;

Wooley,

K.

L.,

Hyperbranched 22


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Fluoropolymer-Polydimethylsiloxane-Poly(Ethylene

Glycol)

Cross-Linked

Terpolymer

Networks

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Pollack,

K.

A.;

Imbesi,

P.

M.;

Raymond, J.

E.;

Wooley,

K.

L.,

Hyperbranched 29


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Fluoropolymer-Polydimethylsiloxane-Poly(Ethylene

Glycol)

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Cross-Linked

Terpolymer

Networks

Designed for Marine and Biomedical Applications: Heterogeneous Nontoxic Antibiofouling Surfaces. Acs Applied Materials & Interfaces 2014, 6, 19265-19274. (63) Voo, Z. X.; Khan, M.; Xu, Q.; Narayanan, K.; Ng, B. W. J.; Bte Ahmad, R.; Hedrick, J. L.; Yang, Y. Y., Antimicrobial Coatings against Biofilm Formation: The Unexpected Balance between Antifouling and Bactericidal Behavior. Polym. Chem. 2016, 7, 656-668. (64) Han, H.; Wu, J.; Avery, C. W.; Mizutani, M.; Jiang, X.; Kamigaito, M.; Chen, Z.; Xi, C.; Kuroda, K., Immobilization of Amphiphilic Polycations by Catechol Functionality for Antimicrobial Coatings. Langmuir 2011, 27, 4010-9. (65) Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z., Measuring Polymer Surface Ordering Differences in Air and Water by Sum Frequency Generation Vibrational Spectroscopy. Journal of the American Chemical Society 2002, 124, 7016-7023. (66) Hankett, J. M.; Lu, X. L.; Liu, Y. W.; Seeley, E.; Chen, Z., Interfacial Molecular Restructuring of Plasticized Polymers in Water. Physical Chemistry Chemical Physics 2014, 16, 20097-20106. (67) Hankett, J. M.; Liu, Y. W.; Zhang, X. X.; Zhang, C.; Chen, Z., Molecular Level Studies of Polymer Behaviors at the Water Interface Using Sum Frequency Generation Vibrational Spectroscopy. Journal of Polymer Science Part B-Polymer Physics 2013, 51, 311-328. (68) Lu, X. L.; Zhang, C.; Ulrich, N.; Xiao, M. Y.; Ma, Y. H.; Chen, Z., Studying Polymer Surfaces and Interfaces with Sum Frequency Generation Vibrational Spectroscopy. Analytical Chemistry 2017, 89, 466-489. (69) Leng, C.; Sun, S. W.; Zhang, K. X.; Jiang, S. Y.; Chen, Z., Molecular Level Studies on Interfacial Hydration of Zwitterionic and Other Antifouling Polymers in Situ. Acta Biomaterialia 2016, 40, 6-15. 30


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(70) Leng, C.; Buss, H. G.; Segalman, R. A.; Chen, Z., Surface Structure and Hydration of Sequence-Specific Amphiphilic Polypeptoids for Antifouling/Fouling Release Applications. Langmuir 2015, 31, 9306-9311.

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Capsaicin-Inspired ThiolâEne Terpolymer Networks Designed for

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