Design, Synthesis and Application of a Difunctional Y-shaped Surface

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Design, Synthesis and Application of a Difunctional Y-shaped Surface Tethered Photoinitiator Shuxiang Zhang, Wenying Liu, Yishi Dong, Ting Wei, Zhaoqiang Wu, and Hong Chen Langmuir, Just Accepted Manuscript • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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Design, Synthesis and Application of a Difunctional Yshaped Surface Tethered Photoinitiator Shuxiang Zhang, Wenying Liu, Yishi Dong, Ting Wei, Zhaoqiang Wu* and Hong Chen* State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China. ABSTRACT: Mixed homopolymer brushes have unique interfacial properties that can be exploited for both fundamental studies and applications in technology. Herein, the synthesis of a new catechol-based biomimetic Y-shaped binary photoinitiator (Yphotoinitiator) and its applications for surface modification with polymer brushes through both “grafting to” and “grafting from” strategies are reported. The “leg” of the Y consists of a catechol group as surface anchoring moiety. The arms are photoinitiator moieties which can be “addressed” independently of each other by radiation of different wavelengths. Using ultraviolet (UV) and visible light successively, each arm of the Y-photoinitiator was activated, thereby allowing the synthesis of Y-shaped block copolymer brushes with dissimilar polymer chains. The suitability of the Y-photoinitiator for surface modification was first investigated using N-vinylpyrrolidone and styrene as the model monomers for successive UV-photoiniferter-mediated polymerization and visible-light induced polymerization, respectively. Switching of the wetting properties of the Y-shaped block copolymer brushes poly(N-vinylpyrrolidone)-block-poly(styrene) (PVP-b-PS) grafted surfaces by contact with different solvents was also investigated. To further exploit this novel Y-photoinitiator for the preparation of functional interfaces, Y-shaped block copolymer brushes poly(1-(2-methacryloyloxyhexyl)-3-methylimidazolium bromide)-block-poly(N-vinylpyrrolidone-co-glycidyl methacrylate) (PIL(Br)-b-P(NVP-co-GMA)) were also prepared and subsequently functionalized with the cell-adhesive arginineglycine-aspartic acid (RGD) peptides by reaction with the glycidyl groups (PILPNG-RGD). The PILPNG-RGD grafted surfaces showed excellent cell adhesive, bacteriostatic and bactericidal properties. Thus, it can be concluded that further exploitation of this novel Y-photoinitiator for graft polymerization should allow the preparation of a wide range of functional interfaces with tailored properties. Keywords: Mixed brushes; Catechol; Photoiniferter; RGD; Poly(N-vinylpyrrolidone); Cell adhesion

INTRODUCTION Mixed homopolymer brushes consist of two or more types of polymer chain and can be used to prepare materials with unique surface properties for many applications such as controlling adhesion (including bioadhesion), friction, and ion transport.1-5 It can be obtained by surface-initiated polymerization techniques using mixed monolayers of two different initiators. For example, Vancso and co-workers prepared mixed brushes from mixed self-assembled monolayers (SAMs) of separate photoiniferter and atom transfer radical polymerization (ATRP) initiators.6 However, mixed SAMs may not guarantee a well distributed layer of the two initiators if one initiator is more preferentially adsorbed to the surface than the other. In addition, due to differences in the attachment processes for the different polymers, it is difficult to achieve a uniformly distributed layer whether by grafting to or grafting from.7,8 This may result in the formation of “islands” containing only one of the polymers. One solution for this problem is to use a Y-shaped polymer molecule containing two different polymer chains linked to a single “anchor” group which can attached to a substrate.9,10 Alternatively a binary Y-initiator containing two distinct initiator moieties can be used.2,11,12 Compared with mixed initiators, using a single anchor site via a binary Y-initiator, the possible formation of single brush

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islands is eliminated. According to theoretical predictions, such Y-shaped initiators should lead to uniform mixed polymer brushes.13 Although a number of Y-shaped binary initiators based on trichlorosilane, dimethylchlorosilane, or triethoxysilane anchor moieties have been designed and have allowed the preparation of well-defined mixed homopolymer brush surfaces,14-22 most of these are appropriate for modification of a single material such as silicon wafer or silica particles.1 In addition, the formation of mixed homopolymer brushes based on such binary Y-initiators typically requires different advanced and complex polymerization techniques.2 These limitations have hindered the development of binary Y-initiators for surface modification. The objective of the present study was to design a Y-shaped binary initiator that would overcome these limitations. Motivated by the adhesion characteristics of catechols23,24 and the advantages of photoinitiated polymerization including facile procedure, mild reaction conditions and spatial control,25,26 the design is based on a catechol-type biomimetic Y-shaped photoinitiator (Y-photoinitiator). The leg of the Y consists of a catechol group as surface anchoring moiety. The arms are photoinitiator moieties which can be “addressed” independently of each other by radiation of different wavelengths (Scheme 1). This strategy takes advantage of photoinitiated polymerization and of the adhesion characteristics of catechol-based chemistry. The advantages of this approach are that the two polymers can be grown independently from each arm of the Yphotoinitiator simply by variation of the radiation wavelength, and that surface properties can thus be varied over a wide range.

Scheme 1. Synthesis of the binary Y-photoinitiator. The green and red colors indicate the initiation sites for photoinitiated polymerization using two different wavelengths (λ1 and λ2). The blue color indicates the catechol moiety used to attach the initiator to the solid substrates.

EXPERIMENTAL SECTION Materials All starting chemicals and solvents used in this work were purchased from Sigma-Aldrich unless otherwise stated. Styrene (St, 99%) was distilled from CaH2 under reduced pressure before polymerization. Nvinylpyrrolidone (NVP, 98%) was purified by distillation under reduced pressure to remove inhibitors before use. Glycidyl methacrylate (GMA, 97%) was used after removing the inhibitor by using an inhibitor remover (Sigma-Aldrich). Arginine-glycine-aspartic acid (RGD) peptides were purchased from GL Biochem. Co., Ltd (Shanghai, China). Dimanganese decacarbonyl (Mn2(CO)10, 98%), serinol (98%), N-ethylpiperidine hypophosphite (EPHP, 95%), 2-bromoisobutyryl bromide (98%), diethylamine (98%), carbon disulfide (99%), acrylic acid (99%), N,N’-dicyclohexylcarbodiimide (DCC, 98%), 4-dimethylamino pyridine (DMAP, 99%), 3,4-dihydroxyhydrocinnamic acid (98%) and other reagents were of analytical grade and used as received. S(carboxypropyl)-N,N-diethyldithiocarbamic acid (CNDDA), 2-(2-bromoisobutyrylamido) propane-1,3-diol


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(BPDL) and 1-(2-methacryloyloxyhexyl)-3-methylimidazolium bromide (IL(Br)) were synthesized and purified according to published procedures.27-29 Gold-coated silicon wafers (80 nm of gold deposited on a 10 nm chromium adhesion layer) were cut into 0.5 cm × 0.5 cm pieces. LIVE/DEAD BacLight Bacterial Viability Kits were purchased from Invitrogen (ThermoFisher Scientific, USA). L929 cells were supplied by the China Center of Type Culture Collection (Wuhan, China). Gram-negative E. coli (ATCC-25922) was provided by the China General Microbiological Culture Collection Center (Beijing, China). Instruments and measurements Nuclear magnetic resonance (NMR) spectra were taken using a Varian Mercury-400 spectrometer (Varian, USA). Fourier transform-infrared (FT-IR) spectra were recorded on a Nicolet 6700 FT-IR spectrometer (Thermo Fisher Scientific, USA). Mass spectra (MS) were obtained using a micrOTOF-Q III instrument (Bruker, Germany). The number-average molar masses (Mn) and polydispersity indices (PDI) of the synthesized polymers were determined using a Waters 1515 gel permeation chromatograph (GPC, Waters, USA) with dimethylformamide (DMF) as solvent (flow rate 1.0 mL·min-1) at 30 °C. An ESCALAB MK II Xray photoelectron spectrometer (VG Scientific Ltd, England) was used to measure the surface chemical compositions. A fluorescence microscope (BX51, Olympus, Japan) was used to observe bacteria and cells on the surfaces. Sessile drop water contact angle measurements were made using a SL200C optical contact angle meter (USA Kino Industry Co., Ltd.). Polymer film thicknesses on modified gold surfaces were measured using an α-SE spectroscopic ellipsometer (J.A. Woollam Co., Inc., USA). The topography of the modified gold surfaces was studied with a multimode-8 atomic force microscope (AFM) in tapping mode (Bruker, USA). GY250 devices with 365 nm-centered ultraviolet (UV) light were provided by Beijing Tianmaihenghui Electric Appliance Co., Ltd. (China). Visible light of intensity I420nm = 0.2 mW cm−2 was obtained by filtering the shorter wavelength UV light of a 500 W mercury lamp using JB400 filters (China). Synthesis of the binary Y-photoinitiator The binary Y-photoinitiator was synthesized by sequential ester condensations of S-(carboxypropyl)-N,Ndiethyldithiocarbamic acid (CNDDA) and the commercially available 3,4-dihydroxyhydrocinnamic acid with 2-(2-bromoisobutyrylamido) propane-1,3-diol (BPDL) (Scheme S1). The intermediate compound, 2-(2bromo-2-methylpropanamido)-3-hydroxypropyl

3-((diethyl-

carbamothioyl)thio)propanoate

(BPDL-

CNDDA), was synthesized via an etherification reaction between BPDL and CNDDA. Briefly, BPDL (1.50 g, 6.20 mmol), DCC (1.60 g, 7.50 mmol) and DMAP (0.08 g, 0.66 mmol) were dissolved in 30 mL of dry tetrahydrofuran, and a CNDDA (1.38 g, 6.20 mmol) tetrahydrofuran solution was slowly added to the mixture. The mixture was then stirred at room temperature for 36 h. After filtering and removing the solvent, the product was purified using silica gel column chromatography with ethyl acetate and hexane (1 : 1, v/v) to give 1.10 g (40% yield) of a viscous white liquid. 1H NMR (CDCl3), δ ppm: 7.10 (d, 1H, NH-), 4.29 (d, 2H, CH2-OH), 4.13 (m, 1H, CH-), 4.01 (d, 2H, CH2-O-), 3.71 (m, 4H, (CH2)2-N-), 3.56 (t, 2H, CH2-S-), 2.84 (t, 2H, CH2CO-), 1.94 (s, 6H, (CH3)2-C-), 1.25 (t, 6H, (CH3-CH2-)2) (Figure S1, Supporting Information (ESI†)). The binary Y-photoinitiator was then conveniently prepared by reaction of the intermediate compound with 3,4-dihydroxyhydrocinnamic acid. In brief, BPDL-CNDDA (1.10 g, 2.50 mmol), DCC (0.61 g, 3.00 mmol) and DMAP (0.03 g, 0.25 mmol) were dissolved in 30 mL of dry tetrahydrofuran and placed in an ice bath. After adding 3,4-dihydroxyhydrocinnamic acid (0.45 g, 2.5 mmol) in 10 mL of dry THF dropwise to the mixture, this container was transferred to a glovebox (SG1200/750TS/F, Suzhou Weige Equipment Technology Co., Ltd, China) and the reaction was taken to completion (36 h) at 60 °C. Finally, the mixture was filtered to remove the precipitate, and the solution was concentrated under vacuum, giving the crude product, which was then purified by silica gel column chromatography (ethyl acetate/hexane 1 : 1, v/v). The product was obtained as a white solid (0.55 g, 36% yield). 1H NMR (CDCl3), δ ppm: 7.03 (d, 1H, NH-), 6.79


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(d, 1H, Aryl–H12), 6.71 (s, 1H, Aryl–H13), 6. 64(d, 1H, Aryl–H11), 4.34 (m, 1H, CH-), 4.17(m, 4H, (CH2)2O-), 3.72 (q, 2H, CH2-N-), 4.01 (q, 2H, CH2-N-), 3.56 (t, 2H, CH2-S-), 2.85 (m, 4H, (CH2)2-CO-), 2.61 (t, 2H, CH2-Ph), 1.91 (s, 6H, (CH3)2-C-), 1.07 (t, 6H, (CH3-CH2-)2) (Figure 1a). 13C NMR (CDCl3), δ ppm: 194.39 (C=S), 172.01 (C=O), 143.26 (Aryl–C20,21), 132.46 (Aryl–C16), 120.52 (Aryl–C17), 114.95 (Aryl–C18,19), 62.38 (C-O), 49.43 (C-Br), 48.41 (CH2-N), 46.66 (CH-NH), 35.43 (CH2-S), 33.70 (CH2-C=O), 31.95 (C-CH3), 31.23 (CH2-CH2), 29.90 (Aryl–C16-CH2-CH2) 12.35 (CH3-CH2) (Figure 1b). FT-IR (cm−1): 1740 (s, v(OC=O)), 1640 (s, v(NH-C=O)), 1520 (s, δ(NH)) (Figure S2, ESI†). ESI-MS: calculated for M+: m/z 608.10; found: m/z 609.11 (M+ + 1, M+ + H) (Figure S3, ESI†). Preparation of Y-shaped block copolymer Catechol-(PVP-b–PS) modified surfaces (“grafting to” method) Synthesis of Catechol-PVP homopolymer: Y-photoinitiator (60.81 mg, 0.10 mmol), NVP (2.22 g, 20.0 mmol) and 3.0 mL methanol were placed in a 10 mL round flask, and the solution was deaerated by bubbling with nitrogen gas for 30 min. The mixture was then irradiated with 365 nm UV light at room temperature for 60 min. Afterward, the dithiocarbamate end-group of the resulted polymer was removed using EPHP as a hydrogen atom donor according to published procedures.30 The purified homopolymer Catechol-PVP was obtained after dialysis for 72 h and then vacuum freeze-dried. The 1H NMR spectrum of the prepared catecholPVP confirmed the efficient photopolymerization via Y-photoinitiator. As shown in Figure S4, the proton peaks at 1.3∼3.9 ppm correspond to the expected chemical structure of PVP.31 In addition, the characteristic peaks at 6.4∼6.8 ppm attributed to protons in catechol group. Specifically, GPC traces were unimodal and symmetrical, and Mn and PDI of the prepared Catechol-PVP were 14,200 g/mol and 2.03, respectively (Figure S5, ESI†). Synthesis of Y-shaped block copolymer Catechol-(PVP-b–PS): Briefly, 0.1 g of catechol-PVP, Mn2(CO)10 (8.0 mg, 0.02 mmol), St (0.83 g, 8.0 mmol) and 3.0 mL DMF were placed in a 10 mL round flask, and the solution was deoxygenated by bubbling with argon gas for 30 min. The mixture was then irradiated with visible light at room temperature for 10 min. The purified Y-shaped catechol-(PVP-b–PS) block copolymer was obtained by dialysis for 72 h and then vacuum freeze-dried. The 1H NMR spectrum of the prepared Catechol-(PVP-b– PS) confirmed that photopolymerization via Catechol-PVP was successful. In the 1H NMR spectrum (Figure S6, ESI†), the characteristic peaks for the phenyl protons in PS and the protons in PVP appeared at 7.2∼7.6 and 1.3∼3.9 ppm, respectively. GPC traces were unimodal and symmetrical, and the Mn and PDI of the prepared Catechol-(PVP-b–PS) were 22,700 g/mol and 2.29, respectively (Figure S5, ESI†). Immobilization of Catechol-(PVP-b–PS) on gold surface ((Au-Catechol-(PVP-b–PS)): Gold-coated wafers were cleaned by 30 min ozone plasma treatment, washed with water and pure ethanol, and dried with nitrogen. The cleaned gold surfaces were then immersed in a 30 mg/mL catechol-(PVP-b–PS) DMF solution mixed with 10 μL periodate (1.0 mg/mL) at room temperature,32 in which periodate acted as an oxidant to activate the adhesion. After incubation for 24 h, the gold surfaces were removed from the solution and washed with DMF. Finally, they were dried with nitrogen, to give the Y-shaped catechol-(PVP-b–PS) block copolymermodified gold surfaces. Preparation of surfaces modified with mixed homopolymer brushes Au-g-(PVP-b–PS) (“grafting from” method) Y-photoinitiator immobilization on gold surface (Au-Y-photoinitiator): Briefly, cleaned gold surfaces were immersed in a 0.05 mol/L Y-photoinitiator DMF solution and mixed with 10 μL periodate (1.0 mg/mL) at room temperature. After incubation for 24 h, the gold surfaces were removed and washed with DMF and methanol. Finally, they were dried with nitrogen to afford the required Y-photoinitiator immobilized gold slides (Au-Y-photoinitiator).


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UV photoiniferter-mediated graft NVP polymerization: NVP ((2.22 g, 20.0 mmol) and 3.0 mL of methanol were placed in a 10 mL round flask and the prepared Au-Y-photoinitiator slides were immersed in the mixture. The solution was deoxygenated by bubbling with argon for 30 min and then irradiated with 365 nm UV light at room temperature for 60 min. At the end of the irradiation, the PVP-grafted gold substrates (Au-g-PVP) were treated with EPHP and rinsed with methanol and dried in a nitrogen stream. Successive visible-light induced graft St polymerization: Prepared Au-g-PVP slides were placed in a 10 mL round flask containing Mn2(CO)10 (8.0 mg, 0.02 mmol), styrene (0.83 g, 8.0 mmol) and 3.0 mL DMF. The flask was deoxygenated by bubbling with argon for 30 min and then irradiated with visible light at room temperature for 10 min. Finally, the product gold surfaces modified with mixed homopolymer brushes (Au-g(PVP-b–PS)) were rinsed with DMF and methanol and dried in a nitrogen stream. Water contact angles of Au-Catechol-(PVP-b–PS) and Au-g-(PVP-b–PS) samples To investigate the changes in contact angle of the grafted layers after treatment with different solvents, samples were immersed, respectively, in toluene (good solvent for PS), water (good solvent for PVP) or chloroform (nonselective good solvent for both PS and PVP) for 24 h. Contact angles were measured within 10 min after the solvent treatment. Preparation of a cell adhesive, bacteriostatic and bactericidal coating (“grafting from” method) Preparation of PIL(Br)-grafted surfaces: IL(Br) (2.5 g, 10.0 mmol), 3.0 mL of methanol and the prepared AuY-photoinitiator slides were placed in a 10 mL round flask. The flask was deoxygenated with argon and then irradiated with 365 nm UV light at room temperature for 60 min. At the end of the irradiation, the PIL(Br)grafted gold substrates (Au-g-PIL(Br)) were treated with EPHP and rinsed with methanol and dried with nitrogen. Preparation of PIL(Br)-b-P(NVP-co-GMA) modified surfaces: Prepared Au-g-PIL(Br) slides were placed in a 10 mL round flask containing Mn2(CO)10 (8.0 mg, 0.02 mmol), NVP ((2.22 g, 20.0 mmol), GMA (0.29 g, 2.0 mmol) and 3.0 mL CH3OH/DMF (1:1, v/v). The flask was then deoxygenated by bubbling with argon for 30 min. After irradiation with visible light was allowed at room temperature for 30 min, the slides modified with mixed PIL(Br)-b-P(NVP-co-GMA) brushes (Au-g-PILPNG) were rinsed with DMF and methanol and dried under a nitrogen flow. RGD immobilization: The RGD peptides were immobilized onto the Au-g-PILPNG substrates by nucleophilic ring-opening reaction between amino groups of peptides and epoxy groups. Briefly, Prepared Au-g-PILPNG slides were immersed in a phosphate buffered saline (PBS, pH 7.4) solution containing 2.0 mg/mL RGD at 55 °C for 12 h under gently shaking. The obtained surfaces (Au-g-PILPNG-RGD) were removed from the reaction solution, rinsed with PBS buffer and dried with a stream of nitrogen. Bacterial attachment Gram-negative E. coli was used as a modal bacterium and cultured as previously reported.33 After sterilization with 75% alcohol and washing twice with PBS, the sample surfaces were incubated in 250 μL of an E. coli suspension (1 × 107 CFU/mL) at 37 °C for 2 h to attach the bacteria. Afterwards, the viability of the attached bacteria on sample surfaces was measured using a LIVE/DEAD BacLight Bacterial Viability Kits. Briefly, the staining solution was first prepared by mixing 3 μL of SYTO 9 and 3 μL of propidium iodide in 2 mL of PBS buffer. Then, 20 μL of a staining solution was dropped onto the surfaces and incubated at room temperature in the dark for 15 min. The surfaces were finally gently rinsed with sterile water and dried under a nitrogen stream. The fluorescence images of the stained bacteria on the surfaces were taken in a fluorescence microscope, and Image Pro Plus analysis software was used to determine the bacterial surface density. Cell adhesion L929 cells were cultured in RPMI medium 1640 (Gibco, USA), supplemented with 10% fetal bovine serum,


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2 mmol/L glutamine, 100 U/mL penicillin and 10 mg/mL streptomycin. The cultures were maintained at 37 °C in a humidified atmosphere containing 5% CO2. The cells were seeded on gold substrates in a cell culture plate at a density of 1.0 × 104 cells/cm3 and allowed to attach for 6 h. The cytoplasm of live cells were stained with 4 μmol/L Calcein-AM (Dojindo Inc., Shanghai, China) for 10 min and then washed thrice with PBS buffer for 2 min each. Images of cells were recorded under a fluorescence microscope. The cell number was obtained by analyzing the images of cells using the Image Pro Plus analysis software.

RESULTS AND DISCUSSION Synthesis of the binary Y-photoinitiator The binary Y-photoinitiator is synthesized in two steps (Scheme 1): first, ester condensation of S(carboxypropyl)-N,N-diethyldithiocarbamic acid (CNDDA) with 2-(2-bromoisobutyrylamido) propane-1,3diol (BPDL); second, condensation of 3,4-dihydroxyhydrocinnamic acid with the product of step 1. In Scheme 1 the green color highlights the visible-light initiator moiety,4,34 the red the photoiniferter moiety35,36 and the blue the catechol moiety used to attach the initiator to the solid substrate. The molecular structure of Yphotoinitiator was confirmed using 1H and 13C NMR spectra, MS and FT-IR spectrum. The characteristic chemical shifts at 6.64–6.79, 1.91 and 1.07 ppm (Figure 1a) are assigned to phenyl, CH3 groups of 2bromoisobutyrylamido fragment and CH3 groups of diethylcarbamodithioate fragment, respectively. The 13C NMR spectrum showed chemical shifts at 194.4 and 172.0 ppm assigned to the C=S group in the diethylcarbamodithioate fragment and carbonyl groups, respectively (Figure 1b). In addition, the infrared peaks at 1740 and 1640 cm-1 are attributed to O-C=O, and NH-C=O groups, respectively (Figure S2, ESI†). These data indicate the successful synthesis of the binary Y-photoinitiator.

Figure 1. (a) 1H NMR and (b) 13C NMR spectra of Y-photoinitiator in CDCl3.

Preparation of Au-Catechol-(PVP-b–PS) (“grafting to” method) The suitability of the Y-photoinitiator for surface modification through both “grafting to” and “grafting from” strategies37 was first investigated using N-vinylpyrrolidone (NVP) and styrene (St) as the model monomers for successive UV-photoiniferter-mediated polymerization and visible-light induced polymerization, respectively (Scheme 2). For the “grafting to” strategy, a Y-shaped Catechol-(PVP-b–PS) block copolymer was first prepared. From 1H NMR spectra showing the characteristic chemical shifts for poly(Nvinylpyrrolidone), polystyrene (PS) and catechol units (Figures S4 and S6, ESI†), it may be concluded that the binary Y-photoinitiator is effective for the polymerization of both monomers. Gel permeation chromatography (GPC) data (Figure S5, ESI†), show that the number-average molecular weight (Mn) of the initially obtained Catechol-PVP was 14,200 g/mol, and that of the Catechol-(PVP-b–PS) block copolymer was


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22,700 g/mol, again indicating the successful polymerization of both monomers. The Catechol-(PVP-b–PS) with relatively high PDI values may have resulted from bimolecular recombination between two propagating radicals and disproportional reactions in photo-polymerization process.35 It has been reported that a unimolecular activation mechanism was preferred for the grafting of the second brush component due to the steric hindrance induced by the existing polymer chains.6 In this work, the photoiniferter-mediated polymerization of NVP at 365 nm (λ1) was performed first since visible-light induced polymerization does not proceed in the absence of the catalyst Mn2(CO)10.34 To avoid chain extension via the dithiocarbamate end group in the photoiniferter-mediated system during the subsequent visible-light induced polymerization of styrene, the photoiniferter agent was removed using N-ethylpiperidine hypophosphite (EPHP) as a hydrogen atom donor after the photoiniferter-mediated polymerization step.30 Successful immobilization of the synthesized Catechol-(PVP-b–PS) polymer on gold, used here as a model surface but often used in biosensors,38,39 was confirmed by X-ray photoelectron spectroscopy (XPS) and ellipsometry. XPS spectra (Figure S7, ESI†) showed that the unmodified gold surface was composed mainly of Au, C and O. After immobilization of Catechol-(PVP-b–PS), a substantial decrease in Au from 63% to 29% and increases in C and O from 32 to 47% and 6 to 15% respectively, were observed (Table 1). A nitrogen signal originating from the pyrrolidone ring was also observed. From ellipsometry measurements, the thickness of the immobilized Catechol-(PVP-b–PS) polymer layer was found to be ∼4.4 nm, and using densities of 1.04 g/cm3 for PVP and 1.05 g/cm3 for PS,40,41 the graft density of Catechol-(PVP-b–PS) may be estimated from: δ = hρ𝑁𝐴/𝑀𝑛 as 0.12 chains/nm,2,41 similar to values previously reported for surface-bound Y-shaped amphiphilic brushes.10

Scheme 2. Strategies of “grafting to” and “grafting from” for the preparation of mixed homopolymer brushes through successive photoiniferter-mediated polymerization at 365 nm (λ1) and visible-light induced polymerization at 420 nm (λ2) steps using the binary Y-photoinitiator.

Preparation of Au-g-(PVP-b–PS) (“grafting from” method) For the “grafting from” strategy (Scheme 2) three steps were required: (1) attachment of the binary Yphotoinitiator to the substrate; (2) UV photoiniferter-mediated graft polymerization of NVP monomer; (3) visible-light induced graft polymerization of styrene monomer. The brush thickness and surface elemental composition were determined after each step. As shown in Table 1 and in the XPS spectra (Figure S7, ESI†), N and S signals (5% and 6%, respectively) appeared in the XPS of the Y-photoinitiator layer (Au-Yphotoinitiator) and the Au content, relative to gold, decreased from 63 to 42%. After UV photoinifertermediated graft polymerization of NVP at 365 nm (λ1) for 60 min (Au-g-PVP), a further decrease in Au content


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and a substantial increase in N and O indicated the successful grafting of PVP. Finally, visible-light induced styrene graft polymerization at 420 nm (λ2) (10 min) on the Au-g-PVP surface (to give Au-g-(PVP-b–PS)) was confirmed by an increase in C and decreases in N and O contents. Ellipsometry measurements showed that after photoiniferter-mediated PVP grafting and subsequent visible-light induced PS grafting on gold, the polymer thicknesses were ∼4.5 nm and ∼5.8 nm, respectively, again confirming the successful formation of the mixed homopolymer brush. Table 1. Surface analysis data for polymer-modified gold surfaces XPS atomic concentration (%) Sample [Au]

[C]

[N]

[O]

[S]

Au

62.70

31.70

Au-Catechol-(PVP-b–PS)

28.65

46.89

6.14

14.85

3.47

Au-Y-photoinitiator

42.43

37.70

4.75

9.07

6.05

Au-g-PVP

15.37

54.49

9.52

16.40

4.22

Au-g-(PVP-b–PS)

17.84

59.19

5.48

14.88

2.61

5.60

Stimuli-responsive behavior of Au-Catechol-(PVP-b–PS) and Au-g-(PVP-b–PS) brush samples The effects of solvents on the microstructure and morphologies of Au-Catechol-(PVP-b–PS) and Au-g-(PVPb–PS) layers were also investigated using atomic-force microscopy (AFM). In this regard it has been shown that the properties of conventional mixed brush layers can change significantly under the influence of selective solvents.42 In the dry state in air (a bad “solvent”), both the PVP and PS arms collapsed, forming a relatively homogenous dry layer with root-mean-square (RMS) surface roughness of 0.46 nm and 1.69 nm for AuCatechol-(PVP-b–PS) and Au-g-(PVP-b–PS), respectively(Figures 2a and 2a'). After toluene treatment (good for PS, bad for PVP), the brushes showed heterogeneous segregated surface morphologies (Figures 2b and 2b'), and increased RMS roughness, possibly because the topmost surface layer is composed mainly of low surface energy PS. Compared with the dry state, the contact angle increased by ∼5° after toluene treatment (Table 2), additionally indicating the presence of a small amount of the hydrophobic PS component in the copolymer brushes. In contrast, after water treatment (good for PVP, bad for PS), stratification of the polymer film was observed with PVP in the topmost layer and PS underneath in contact with the gold (Figures 2c and 2c'),43 in agreement with the water contact angles of 39° and 49° for Au-Catechol-(PVP-b–PS) and Au-g(PVP-b–PS) surfaces, respectively (Table 2). In the case of chloroform, a nonselective solvent good for both PVP and PS, the confinement of dissimilar (hydrophobic and hydrophilic) arms caused by covalent attachment to the same grafting point leads to the suppression of microphase separation.10 AFM images of Au-Catechol(PVP-b–PS) and Au-g-(PVP-b–PS) revealed that the grafted layers were of high quality with fairly smooth surfaces (Figures 2d and 2d').


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Figure 2. AFM images (1.0 × 1.0 μm2 overview height scans) of Au-catechol-(PVP-b–PS) (a, b, c, d) and Au-g-(PVP-b–PS) (a', b', c', d') brush samples. Scans are: a, a' in air; b, b' after toluene treatment; c, c' after water treatment; d, d' after chloroform treatment.

Table 2. Water contact angles (degrees) of Au-Catechol-(PVP-b–PS) and Au-g-(PVP-b–PS) brush samples after different solvent treatments After toluene

After water

After chloroform

treatment

treatment

treatment

46 ± 2

51 ± 2

39 ± 2

38 ± 2

55 ± 2

61 ± 2

49 ± 2

46 ± 2

Sample

In air

Au-Catechol-(PVP-b–PS) Au-g-(PVP-b–PS)

Preparation of Au-g-PILPNG-RGD (“grafting from” method) To further exploit this novel Y-photoinitiator for the preparation of functional interfaces, a tri-functional coating that repels contaminating bacteria, kills those that adhere, and promotes cells adhesion was designed. Scheme 3 shows the synthesis of the mixed PIL(Br)-b-P(NVP-co-GMA) brushes with conjugated RGD peptides. PIL(Br) brushes were first prepared through UV photoiniferter-mediated graft polymerization (Aug-PIL(Br)). Due to their strong bactericidal activity,44 this imidazolium-type ionic liquids brushes are expected to kill the bacteria that adhere on the surfaces. Afterwards, P(NVP-co-GMA) brushes were fabricated on the Au-g-PIL(Br) surfaces through visible-light induced graft copolymerization of NVP and GMA (Au-gPILPNG). Each GMA bears an epoxy group which can react with an amino group in RGD, while the PVP provides the repelling property.45 Therefore, this copolymer brushes are expected to repel the adhesion of contaminating bacteria and provide the multiple reactive sites for covalent immobilization of cell-adherent RGD peptides. The brush thickness and static water contact angle were determined after each step. Ellipsometry measurements showed that after photoiniferter-mediated PIL(Br) grafting and subsequent visible-light induced P(NVP-co-GMA) grafting on gold, the dry polymer thicknesses were ∼3.2 nm and ∼11.6 nm, respectively, indicating the successful formation of the mixed polymer brush. In addition, compared to the unmodified gold substrate, the static contact angle decreased by ∼23° after PIL(Br) grafting (Figure S8, ESI†), also confirming the formation of a hydrophilic PIL(Br) layer. In contrast, the water contact angle increased a little after the subsequent P(NVP-co-GMA) grafting and RGD immobilization (Au-g-PILPNG-RGD). However, all conditions rendered contact angle values lower (more hydrophilic) than the unmodified gold substrate. It


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should be mentioned that the nucleophilic ring-opening reaction between –NH2 in RGD peptides and epoxy groups was carried out in PBS solution (pH 7.4) at 55 °C for 12 h. Therefore, the amount of unreacted epoxy groups should be very small and thereby would not affect cell adhesion and other properties significantly.46

Scheme 3. Strategies of “grafting from” for the preparation of mixed PIL(Br)-b-P(NVP-co-GMA) brushes through successive photoiniferter-mediated polymerization at 365 nm (λ1) and visible-light induced polymerization at 420 nm (λ2) steps using the binary Y-photoinitiator, and the covalent immobilization of RGD peptides by reaction with epoxy groups.

Antibacterial properties of the Au-g-PILPNG-RGD surfaces After confirming the successful preparation of the PILPNG-RGD coating, a common clinically relevant gramnegative bacterium E. coli was used as a model bacterium to evaluate the antibacterial efficacy of the Au-gPILPNG-RGD surfaces. As shown in Figure 3a, the attached E. coli bacteria on the unmodified gold surface were stained by a green stain, which means they were alive with an intact membrane. In contrast, most of the attached bacteria (∼86%) to the Au-g-PIL(Br), Au-g-PILPNG and Au-g-PILPNG-RGD surfaces were dead (red stain), indicating that the surfaces containing PIL(Br) brushes have excellent bactericidal properties (Figures 3b, 3c and 3d). Furthermore, due to the low fouling properties of PVP, the adherent bacterial density was drastically reduced on the PILPNG and PILPNG-RGD brushes modified surfaces (Figure 3e). Especially, compared with the unmodified gold surfaces, the Au-g-PILPNG-RGD surfaces reduced the levels of bacterial density by ∼96%. These results indicated that the PILPNG-RGD brushes modified surfaces have excellent bacteriostatic and bactericidal properties. This may attribute to the antifouling character of PVP45 and the bactericidal properties of PIL(Br) brushes,44 which repel bacteria adhesion and kill those that adhere, respectively.

Figure 3. Fluorescent images of attached E.coli bacteria exposed to live/dead stains on Au (a), Au-g-PIL(Br) (b), Au-g-PILPNG (c) and Au-g-PILPNG-RGD (d) after incubation at 37 °C for 2 h. Scale bar represents 100 μm. (e) Density of E. coli on the Au, Au-g-PIL(Br), Au-g-PILPNG and Au-g-PILPNG-RGD surfaces. Error bars represent the standard deviation of the mean (n = 3).


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Cell adhesion on the Au-g-PILPNG-RGD surfaces It is well known that cell adhesion is the very initial interactions of cells to substrates.46 Therefore, the effects of the modified gold surfaces on cell adhesion were investigated by using fluorescence microscopy after 6 h incubation. As shown in Figure 4a, only a few cells were observed on the unmodified gold surface. While on the Au-g-PIL(Br) and Au-g-PILPNG surfaces, two contrast results were observed: a larger number of cells adhered on the Au-g-PIL(Br) surfaces, while a fewer number of cells attached to the Au-g-PILPNG surface (Figures 4b and 4c). This phenomenon can be explained by the antifouling character of PVP and the electrostatic interaction between PIL(Br) brushes and cells,47 which may resist and enhance cell adhesion, respectively. In contrast to the Au-g-PILPNG surface, the cells adhered on the Au-g-PILPNG-RGD surfaces very well (Figure 4d). Compared to those on the Au-g-PILPNG surfaces, the density of adhered cells cultured on the Au-g-PILPNG-RGD surfaces were improved by nearly 3.4 folds (Figure 4e), indicating that covalent immobilization of RGD peptides on Au-g-PILPNG surfaces is an effective way to enhance the cell-adhesive capacity but not bacterial adhesion. This result can be explained by the fact that the RGD motif is known to interact specifically with cell surface integrin receptors, but is not recognized by bacteria.48 Taken these results of cellular and bacterial behavior together, the methods established in this study shed a light for the design of other types of multifunctional interfaces for diverse applications.

Figure 4. Fluorescent Calcein-AM staining images of L929 cells adhered on Au (a), Au-g-PIL(Br) (b), Au-g-PILPNG (c) and Aug-PILPNG-RGD (d) after being cultured for 6 h. Scale bar represents 100 μm. (e) Density of L929 cells on the Au, Au-g-PIL(Br), Au-g-PILPNG and Au-g-PILPNG-RGD surfaces. Error bars represent the standard deviation of the mean (n = 3).

CONCLUSIONS In conclusion, a new catechol-based biomimetic Y-shaped photoinitiator was designed and used for surface modification through both “grafting to” and “grafting from” strategies. For “grafting to”, a Y-shaped Catechol(PVP-b–PS) block copolymer was first prepared via successive UV-photoiniferter-mediated polymerization and visible-light induced polymerization and subsequently immobilized directly on gold surface. For “grafting from”, the Y-shaped photoinitiator was first immobilized on gold substrate; Y-shaped block copolymer brushes PVP-b–PS with dissimilar polymer chains were then obtained by successive surface-initiated photoiniferter-mediated graft polymerization of NVP and visible-light induced graft polymerization of styrene. Switching of the surface wetting properties by treatment with selective solvents was also investigated. Finally, to further exploit this novel Y-photoinitiator for the preparation of functional interfaces, Y-shaped block copolymer brushes PILPNG-RGD grafted surfaces were also prepared through “grafting from” approach and showed excellent cell adhesive, bacteriostatic and bactericidal properties. In principle, the method is feasible for any monomer amenable to photoinitiated polymerization and can be used on different substrates. This general approach should allow the preparation of a wide range of functional surfaces with tailored properties.


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Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. (i) General procedure for the synthesis of the binary Y-photoinitiator, (ii) 1H NMR spectrum of BPDLCNDDA, Catechol-PVP Catechol-(PVP-b-PS), (iii) FT-IR and MS spectrum of Y-photoinitiator, (iv) GPC traces of the Catechol-PVP and Catechol-(PVP-b-PS) block copolymer, (v) XPS survey spectra of modified gold surfaces, (vi) water contact angles (degrees) of modified gold surfaces.

Corresponding Authors *E-mail: [email protected] (Z. Wu), [email protected] (H. Chen)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21774087).

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Design, Synthesis and Application of a Difunctional Y-shaped Surface

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