Fabrication of a Photocontrolled Surface with Switchable Wettability

Publication Date (Web): July 22, 2014 ... The results demonstrate that the surface modified with PEG possesses good ... Cyclodextrin - ferrocene host ...
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Fabrication of a Photocontrolled Surface with Switchable Wettability Based on Host−Guest Inclusion Complexation and Protein Resistance Qiongxia Shen, Lichao Liu, and Weian Zhang* Shanghai Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China S Supporting Information *

ABSTRACT: A novel surface-modification strategy has been developed for the construction of a photocontrolled silicon wafer surface with switchable wettability based on host−guest inclusion complexation. The silicon wafer was first modified by guest molecule azobenzene (Azo) via a silanization reaction. Subsequently, a series of polymers with different polarities were attached to host molecule β-cyclodextrin (β-CD) to prepare β-CD-containing hemitelechelic polymers via click chemistry. Finally, a photocontrolled silicon wafer surface modified with polymers was fabricated by inclusion complexation between β-CD and Azo, and the surface properties of the substrate are dependent on the polymers we used. The elemental composition, surface morphology, and hydrophilic/ hydrophobic property of the modified surfaces were characterized by Xray photoelectron spectroscopy (XPS), atomic force microscope, and contact angle measurements, respectively. The antifouling property of the PEG-functionalized surface was evaluated by a protein adsorption assay using bovine serum albumin, which was also characterized by XPS. The results demonstrate that the surface modified with PEG possesses good protein-resistant properties.

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INTRODUCTION The fabrication of functional surfaces has aroused great interest because of their potential applications in protective coatings, biomedicine, sensors, biochips, and so on.1−4 A variety of strategies have been developed to improve the surface properties of the substrate in the past several decades such as self-assembly, grafting, and coating.5−7 Among them, it is common and effective to attach polymers onto solid surfaces via grafting methods including grafting-to and grafting-from techniques8 since a wide range of characteristic surface features such as wettability and protein resistance could be well tuned via changing the polymer composition and polarity. Recently, living polymerization techniques have been developed to prepare well-defined polymers with a rich range of properties in terms of varying the chemical nature of the components and chemical structures. In the grafting-from technique, living polymerization could be directly initiated from initiator-functionalized surfaces, thus the well-defined polymers are covalently tethered to surfaces.9,10 The grafting-to strategy involves the attachment of prefabricated welldefined polymers via noncovalent or covalent bonds.11−13 For example, Kang and coworkers constructed the antifouling and antimicrobial surfaces using functional polymer brushes via click reactions in both grafting-from and grafting-to processes.14 In overviews of previous studies of surfaces functionalized with polymers including well-defined polymers, we find that the polymer chains are often tethered to surfaces via covalent bonds or strong interactions such as hydrogen bonds and ion pairs. © 2014 American Chemical Society

Thus, these modified surfaces are irreversible and nonrenewable due to those strong interactions between the surfaces and polymers. In the past few years, considerable attention has been focused on cyclodextrin-based host−guest inclusion complexation since cyclodextrins (CDs) possess a hydrophobic cavity which can accommodate a series of guest molecules such as adamantane, ferrocene, and azobenzene (Azo).15−17 Harada et al. designed a series of supramolecular polymers, rotaxanes and hydrogels via CD-based host−guest chemistry.18−20 The host−guest inclusion complexations based on CDs have also been exploited to construct functional surfaces.21−26 Reinhoudt and Huskens have investigated multivalent interactions and surface patterns via inclusion complexation between CD self-assembled monolayers (SAMs) and guest-functionalized proteins, fluorescent molecular, quantum dots, and so on.27−29 More recently, this responsive host−guest chemistry has attracted significant interest and has been applied in the modification of material surfaces.30−34 Among these responsive inclusion complexes, the host−guest interaction between β-CD and Azo has been widely investigated, since Azo exhibits a photocontrolled reversible transition between the trans and cis forms by alternating UV/ visible light irradiation.35 In the host−guest inclusion complexReceived: March 9, 2014 Revised: June 6, 2014 Published: July 22, 2014 9361

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ation between β-CD and Azo, trans-Azo tends to bind in the hydrophobic cavity of β-CD easier than does cis-Azo driven by hydrophobic interactions and the complementary character of size and shape between their structures.36 When trans-Azo is transformed into cis-Azo upon irradiation with UV light, β-CD can no longer include the bulky cis-Azo because of the mismatch between host and guest.37 Zhang et al. have fabricated a biosurface based on inclusion complexation between β-CD grafted with poly(acrylic acid) and Azo, which has been applied in the reversible immobilization of cytochrome c.38,39 To the best of our knowledge, there are few reports on the wettability of silicon wafers which can be tuned by the photocontrolled inclusion complexation between β-CD-containing polymers and guest moieties, especially for the surface properties of modified surfaces tuned using the attached polymers with different polarities.40−44 Herein, we develop a new strategy for the preparation of a functional silicon wafer surface with photocontrolled switchable wettability based on host−guest chemistry between β-CD and the azobenzene moiety, and the surface properties could be tuned by changing the polarity of β-CD-containing polymers (Scheme 1). The silicon wafer was first modified with the guest

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EXPERIMENTAL SECTION

Materials. Silicon wafers were purchased from Beijing XXBR Technology Co. Ltd (China). β-Cyclodextrin (β-CD) (Shanghai Seebio Biotechnology, Inc., China) was recrystallized three times from deionized water. 2,2,3,4,4,4-Hexafluorobutyl methacrylate (HFBMA) (Aladdin, China) and methyl methacrylate (MMA) (Aladdin, China) were passed through an alumina column to remove inhibitor and were refrigerated before use. Copper(I) bromide (CuBr, Aladdin Reagents, China) was purified by stirring in acetic acid three times and then washing with ethanol and drying under vacuum. Dimethylformamide (DMF) was dried over calcium hydride. Tetrahydrofuran (THF) and toluene were refluxed to remove water over sodium. Sodium azide (NaN3), sodium iodide (NaI), N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA), (3-chloropropyl)triethoxysilane (Cl-PTES), methoxy poly(ethylene glycol) (PEG, Mn = 2000), propargyl bromide, and bovine serum albumin were all purchased from Aladdin Reagents of China and used directly as received. Characterization. Nuclear Magnetic Resonance Spectroscopy (NMR). 1H NMR spectra were recorded at 400 MHz using a Bruker AV400 spectrophotometer in CDCl3 or DMSO-d6 with tetramethylsilane (TMS) as an internal reference. Fourier Transform Infrared (FT-IR) Spectra. FT-IR spectra were recorded on a Paragon 1000 instrument using a KBr sample holder method. Gel Permeation Chromatography (GPC). The number-average weight (Mn) and polydispersity index (PDI) were determined by gel permeation chromatography (GPC, Waters 1515) at 35 °C. A series of monodisperse polystyrene standards were used for calibration, and THF was used as the eluent at a flow rate of 1 mL min−1. UV/Vis Absorption Spectra. Absorption spectra were recorded with a Shimadzu UV-2550 UV/visible spectrophotometer using a quartz cuvette with a 1 cm beam path length. Contact Angle Measurements. The contact angle was measured via a sessile drop on a silicon wafer surface with a contact angle meter (Powereach, Shanghai, China). In each measurement, a droplet (4.5 μL) of ultrapure water was placed carefully onto the substrate, and images were recorded after 60 s at room temperature. The contact angle value was obtained by averaging the measurements at three different positions on the silicon wafer specimen. Atomic Force Microscope (AFM). AFM observation was carried out using tapping mode on a Digital Instruments Nanoscope IV equipped with a 125 μm silicon cantilever and an E-type vertically engaged piezoelectric scanner. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed on a Thermo Escalab 250 spectrometer. Nonmonochromatic Al Kα radiation was used as the primary excitation. The binding energies (BE) were calibrated with the C 1s level of adventitious carbon (284.6 eV) as the internal standard reference. The samples for XPS analysis were dried at 50 °C for 24 h under high vacuum. Synthesis of Azobenzene-Containing Propyltriethoxysilane (AzoPTES). Azo-PTES was synthesized via copper-catalyzed azide−alkyne click chemistry45 between the propargyl azobenzene derivative and (3azidopropyl) triethoxysilane (Scheme S1), and the detailed preparation procedure is described in the Supporting Information. Synthesis of β-CD-Containing Polymers (β-CD-PEG, β-CD-PMMA, and β-CD-P(MMA-co-HFBMA)). These β-CD-containing polymers were synthesized via click chemistry between alkynyl-terminated polymers and azide-modified β-CD (β-CD-N3). Alkynyl-terminated hydrophilic PEG was directly prepared by the reaction between PEG− OH and propargyl bromide (Scheme S2). Alkynyl-terminated PMMA and P(MMA-co-HFBMA) were respectively prepared by atom-transfer radical polymerization (ATRP) using an alkynyl ATRP initiator (Schemes S3 and S4). The detailed preparation procedures are described in the Supporting Information Pretreatment of Silicon Wafer Substrates. At first, silicon wafer substrates covered with an approximately 1.5-nm-thick native oxide layer with dimensions of 0.5 cm × 0.5 cm were washed with acetone, dichloromethane, ethanol, and ultrapure water in turn. Then, the clean silicon substrate was immersed in piranha solution [H2SO4 (98%)/ H2O2 (30%) = 7:3 (v/v)] for 2 h at 90 °C to get the silanol-terminated

Scheme 1. Schematic Presentation of the Formation of Si-Azo Surfaces, the Photocontrolled Inclusion Complexation between β-CD Polymers and Si-Azo Surfaces, and the Protein Adsorption Assay of the PEG-Functionalized Surface

molecule Azo to form an Azo-terminated silicon wafer (Si-Azo) substrate via a silanization reaction; subsequently, a series of polymers with different polarities including poly(ethylene glycol) (PEG), poly(methyl methacrylate) (PMMA), and poly(methyl methacrylate-co-hexafluorobutyl methacrylate) (P(MMA-co-HFBMA)) were attached to the β-CD host molecule to prepare β-CD-containing hemitelechelic polymers via click chemistry. The photocontrolled silicon wafer surfaces with polymers were finally constructed via inclusion complexation between β-CD and Azo, and the wettability of the substrate is dependent on the polymers we used. The elemental composition, surface morphology, and hydrophilic/hydrophobic property of the modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), and contact angle measurements, respectively. The antifouling properties of the poly(ethylene glycol) (PEG)functionalized surface were evaluated by a protein adsorption assay using bovine serum albumin (BSA). 9362

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surface. (Caution! Piranha solution is caustic and reacts violently with organic materials, so it needs to be handled with extreme care.) Finally, the silanol-terminated substrate was rinsed thoroughly with ultrapure water and dried under nitrogen. Immobilization of Azobenzene (Azo) on Silicon Wafers. The Azoterminated silicon wafer (Si-Azo) was formed by immersing a fresh silanol-terminated silicon wafer in 10 mmol L−1 Azo-PTES in dry toluene at 45 °C under UV irradiation. After 5 h, the modified silicon wafer was taken out of the original solution and sonicated for 1 min in pure toluene to remove physically absorbed molecules, then washed with ethanol and ultrapure water, and finally dried under nitrogen. Photocontrolled Inclusion Complexation between β-CD-Terminated Hydrophilic Poly(ethylene glycol) (β-CD-PEG) and Si-Azo Surfaces. The Azo-terminated silicon wafer was first immersed in a 15 mg mL−1 aqueous solution of β-CD-PEG at room temperature overnight. The silicon wafer substrate was scrupulously rinsed with ultrapure water four times and dried under nitrogen. Afterward, the SiAzo/β-CD-PEG surface was immersed in ultrapure water and exposed to UV light (365 nm) for 60 min to release β-CD-PEG. UV light (365 nm) and visible light (450 nm) were used from a 500 W Xe lamp equipped with 365 and 450 nm cutoff filters (E = 2000 mW cm−2, CELHXF300/CEL-HXUV300, China), respectively, and the distance between the UV lamp and the sample was about 20 cm. Photocontrolled Inclusion Complexation between β-CD-Terminated Hydrophobic Polymers (β-CD-PMMA, β-CD-P(MMA-coHFBMA)) and Si-Azo Surfaces. The Azo-terminated silicon wafer was immersed in a 15 mg mL−1 DMF solution of hydrophobic polymers including β-CD-PMMA and β-CD-P(MMA-co-HFBMA) at room temperature overnight, respectively. Then the solution was decanted, and the treated silicon wafer substrates were placed in ethanol and rinsed scrupulously to remove DMF solution. Finally, silicon wafer substrates were rinsed with ultrapure water and ultrasonically rinsed to precipitate and remove the free β-CD-containing polymers. The detachment process is similar to that of the Si-Azo/β-CD-PEG surface. Protein Adsorption Studies. Bovine serum albumin (BSA) was dissolved in 0.01 M phosphate buffer saline (pH 7.4) to a concentration of 1 mg mL−1. Samples were rinsed initially with phosphate-buffered saline (PBS) solution and then placed in the prepared BSA solution at 37 °C for 1 h. Then the substrates were gently rinsed three times with PBS solution and rinsed twice with ultrapure water to remove the PBS salt. After being dried in a stream of nitrogen, these substrates were characterized by XPS.

elemental composition, and surface topography of Si-Azo surfaces were investigated by sessile drop water contact angle measurement, XPS, and AFM, respectively. The contact angle of the fresh silicon wafer treated with H2SO4/H2O2 is 16 ± 2° (Figure 1a). The contact angle increases to 79 ± 1° after

Figure 1. Images of water drops on silicon wafer substrates of (a) Si− OH, (b) Si-Azo, (c) Si−OH treated with toluene, and (d) Si-Azo after UV irradiation.

treatment of the silanol-terminated silicon wafers with Azo-PTES solution (Figure 1b). Additionally, the silanization reaction was carried out in toluene, so we also performed a control experiment by treating a silicon wafer with pure toluene without Azo-PTES. The contact angle was about 30° (Figure 1c), which further confirms that the increase in the contact angle on the silicon wafer surface treated with Azo-PTES solution resulted from the modification of Azo-PTES. The isomerization process of Azo molecules is effective in solution but is often ineffective after attachment to planar solid substrates since tightly assembled Azo molecules lose their free volume, which is needed for isomerization to take place.47,49 We wonder whether the photoisomerization of the Azo moiety on the silicon wafer surface occurs through the variation of the contact angle. After the Si-Azo surface is exposed to UV light for 30 min, the contact angle decreases to 75 ± 1°, as shown in Figure 1d, due to the change in the dipole moment of the Azo molecules upon trans to cis photoisomerization via UV irradiation.32,50 Interestingly, the contact angle increases to 79 ± 1° again upon visible irradiation for 15 min. This phenomenon indicates that Azo molecules on silicon wafers can undergo the reversible configuration transformation between the cis and trans forms. XPS was used to analyze the surface chemical composition. An XPS wide-scan spectrum and C 1s core-level spectrum of Si-Azo surfaces are shown in Figure 2. The C 1s core-level spectrum (Figure 2b) can be curve-fitted into three peak components with binding energies (BEs) of 284.6, 285.6, and 286.4 eV, which are attributable to the C−C, C−N, and C−O species, respectively.14,44 The [C−C]/[C−O]/[C−N] peak component area ratio of about 3.6:1.1:1.0 is roughly consistent with the theoretical value of 3.5:1.0:1.0 calculated from the structural formula of Azo-PTES. However, the [C]/[O] ratio (determined from the corrected C 1s and O 1s core-level spectral peak area ratio) is 0.67, suggesting that the oxidized layer of the substrate is detected since the Azo layer is thinner than the sampling depth of the XPS instrument (7.5 nm).51 The results of both contact angle measurements and XPS suggest that Azo-PTES has been attached to the silicon wafer surface successfully.

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RESULTS AND DISCUSSION Preparation of Azo-Terminated Silicon Wafer via the Silanization Reaction. The silanization reaction is a common approach to modifying silicon substrates,46 which could be effectively performed between the silanol groups of silicon surfaces and triethoxysilane or trimethoxysilane derivatives. The 1 H NMR spectrum of Azo-PTES is shown in Figure S1. Besides the characteristic signals from the Azo and PTES moieties, the signal for the proton peak of the 1,2,3-triazole ring clearly appears at δ = 7.56 ppm, indicating the occurrence of the 1,3-dipolar cycloaddition reaction. Figure S2 shows the UV/vis absorption spectra of Azo-PTES in toluene. Upon irradiation with UV light, the absorption band at around 350 nm decreased remarkably, and the absorption band at 455 nm slightly increased. The strong absorption band at 350 nm is due to the π−π* electronic transition of the trans form of Azo, and the weaker absorption band at 455 nm is due to the n−π* transition of the cis form of Azo. The result indicates that the photoisomerization of the trans to cis form of Azo occurred.47,48 The inset shows that Azo underwent cis to trans isomerization upon irradiation with visible light (455 nm). Azo-PTES was used to construct the Si-Azo surface by the immobilization of Azo-PTES onto the silanol-terminated surfaces through the silanization reaction. The surface properties, 9363

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Figure 2. XPS wide-scan spectrum (a) and C 1s core-level spectra (b) of the Si-Azo surface.

Fabrication of Photocontrolled Surfaces with Switchable Wettability Based on Inclusion Complexation between β-CD Polymers and Si-Azo Surfaces. To construct the responsive surfaces with switchable wettability, we prepared a series of β-CD-polymers with different polarities to complex with the Azo group on silicon wafers. These β-CD-polymers were directly synthesized via click chemistry between alkynylterminated polymers and azide-modified β-CD (β-CD-N3). βCD-N3 was synthesized according to the literature,52,53 The FTIR spectrum of β-CD-N3 as shown in Figure S3 clearly exhibits a sharp peak at 2107 cm−1 which is ascribed to the stretching vibration of the azide group. Alkynyl-terminated hydrophilic PEG was prepared by the reaction between PEG−OH and propargyl bromide. A typical 1H NMR spectrum of alkynylterminated PEG is shown in Figure S4. The signals at δ = 4.21, 3.45, and 2.47 ppm are respectively assigned to the resonance of the methylene protons (−OCH 2CCH), methyl protons (−COCH 3 ), and methine proton (−OCH 2 CCH). The alkynyl-terminated PEG was further used to construct β-CDcontaining PEG (β-CD-PEG) via click cycloaddition with β-CDN3. To complete the coupling reaction, the initial molar ratio of β-CD-N3 to the alkynyl group was 1.5/1. The 1H NMR of β-CDPEG is shown in Figure S5. Except for the proton signals from the CD ring and PEG, the signal at δ = 8.04 ppm, is assigned to the proton peak of the 1,2,3-triazole ring, indicating the occurrence of the 1,3-dipolar cycloaddition reaction. Also, the sharp peak (2107 cm−1) disappeared (Figure S3), which also suggests that the final product of β-CD-PEG was successfully prepared. To further fabricate photocontrolled surfaces with a range of wettability, we also prepared alkynyl-terminated hydrophobic polymers of PMMA and P(MMA-co-HFBMA) using ATRP. Alkynyl-terminated PMMA and P(MMA-co-HFBMA) were synthesized via ATRP using an alkynyl-functionalized ATRP initiator, propargyl 2-bromoisobutyrate (PBiB), in the presence of Cu(I)/PMDETA as the catalyst system. The 1H NMR spectra of alkynyl-terminated PMMA and P(MMA-co-HFBMA) are separately shown in Figures S6 and S7. The similar click cycloaddition between these alkynyl-terminated hydrophobic polymers and β-CD-N3 was also performed to produce β-CDterminated hydrophobic polymers, and the click reaction was confirmed by FT-IR and 1H NMR (Figures S3, S8, and S9). To study the surface properties of the photocontrolled surface, contact angle measurement is a conventional and efficient tool for revealing the switchable wettability. The water contact angle became 46 ± 1.5° (Figure 3a) when the Si-Azo surface was immersed in an aqueous solution of β-CD-PEG overnight, which means that the hydrophobic surface became hydrophilic. After UV light (365 nm) irradiation, the contact angle increased to 70

Figure 3. Images of water drops on the surfaces of Si-Azo/β-CD-PEG (a) and Si-Azo/β-CD-PEG after UV irradiation (b) and the switchable wettability transition of the Si-Azo/β-CD-PEG surface with UV and vis irradiation (c).

± 1°, since β-CD-PEG was released from the surface because of a mismatch between the host and the guest (Figure 3b). From the results, we found that the contact angle was slightly lower than the initial 75° for the Si-Azo surface after UV irradiation, perhaps because there is some trace of β-CD-PEG left on the surface. The photocontrolled property of the surface was also studied by alternating UV/vis irradiation, and the changes in the contact angle were switchable for many cycles (Figure 3c). These results clearly indicate that the wettability of surfaces modified with βCD-PEG is photocontrolled. After immersing the Azo-terminated silicon wafer in the solution of β-CD-PMMA to form the Si-Azo/β-CD-PMMA surface, we found that the contact angle did not increase obviously compared to that of the Si-Azo surface. However, with further decreases in the polarity of the β-CD-polymer, the contact angle of the functional surface containing β-CDP(MMA- co-HFBMA) became 87 ± 1° as shown in Figure 4. Furthermore, the contact angle could decrease back to 80° after the surface was exposed to UV light. This indicates that the surface constructed by the hydrophobic polymers has a photocontrolled property that is similar to that of hydrophilic β-CD-PEG. On the basis of the above results, surfaces with photocontrolled wettability have been successfully prepared via host−guest chemistry. However, the contact angle of the Si-Azo 9364

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between β-CD and Azo is lower than that in the aqueous medium. X-ray photoelectron spectroscopy (XPS) was used to evaluate the variation in the elemental composition of the Si-Azo/β-CDPEG surface before and after UV irradiation. The XPS C 1s corelevel spectrum of the Si-Azo/β-CD-PEG surface can be curvefitted into four peak components (Figure 7a) located at about 284.6, 285.6, 286.4, and 287.8 eV, which are attributable to the C−H/C−C, C−N, C−O, O−C−O species, respectively.56 Compared to the C 1s core-level spectrum of Si-Azo (Figure 2b), the intensity of the C−O peak component significantly increases, and a new peak appears at 287.8 eV, which is attributable to the O−C−O species, indicating that β-CD-PEG had been successfully attached to the Si-Azo surface. The XPS C 1s core-level spectrum of the Si-Azo/β-CD-PEG surface after UV light (365 nm) irradiation is shown in Figure 7b. The O−C−O peak component almost disappears; moreover, the intensity of the C−O peak component is much weaker than that of the C− H/C−C peak component. This result further confirms that most β-CD-PEG chains detach from the Si-Azo surface. Additionally, the [C−C]/[C−O] peak component area ratio in Figure 7b is slightly higher than that in Figure 2b, indicating that β-CD-PEG does not completely detach from the Si-Azo surface. The reason for this phenomenon may be that not all of the tran-Azo groups were converted to the cis isomer under UV irradiation. XPS analysis was also used to evaluate the variation of element composition of both Si-Azo/β-CD-PMMA and Si-Azo/β-CDP(MMA-co-HFBMA) surfaces. XPS wide-scan spectra and C 1s core-level spectra of both Si-Azo/β-CD-PMMA and Si-Azo/βCD-P(MMA-co-HFBMA) surfaces are shown in Figures S10 and S11, respectively, and the elemental composition is summarized in Table S1. Compared to the wide-scan spectrum of Si-Azo, SiAzo/β-CD-P(MMA-co-HFBMA) presents an F 1s peak at 687.8 eV, indicating that the Si-Azo surface has been modified with P(MMA-co-HFBMA). As we know, the sampling depth of the XPS technique is about 7.5 nm, and the atomic concentration of silicon should decrease with the increase in polymer thickness on the silicon wafer. As shown in Table S1, after the Si-Azo surface was immersed in β-CD-PMMA and β-CD-P(MMA-co-HFBMA) solution, the atomic concentration of silicon decreases from 38.68 to 31.72 and 32.58%, respectively. However, the atomic concentration of silicon on the Si-Azo/β-CD-PEG surface drops to 23.29%. Those phenomena are in agreement with the AFM results of both Si-Azo/β-CD-PMMA and Si-Azo/β-CD-P(MMA-co-HFBMA) surfaces, indicating that hydrophobic polymers possess a low grafting density. In previous studies, the substrates modified with PEG chains have exhibited good protein resistance, and they have been widely used in biomedical fields.57−59 These conventional PEG-

Figure 4. Surface hydrophilic/hydrophobic performance revealed by contact angle analysis.

surface before and after attachment with hydrophobic polymers does not apparently change compared to that of the hydrophilic PEG. We can attribute this phenomenon to the following two reasons: first, the hydrophobicity of β-CD-PMMA- or β-CDP(MMA-co-HFBMA)-modified surfaces is weakened due to the presence of multiple hydroxyl groups on the β-CD ring;44 second, the density of hydrophobic polymers on silicon surfaces is less than that of PEG. AFM was used to directly observe the surface morphology and density of polymers on the surface. The 3D AFM image showed that the Si-Azo surface was quite smooth in Figure 5a. After attachment to β-CD-PEG, the Si-Azo/β-CD-PEG surface presents globular-like morphology (Figure 5b) resulting from the heterogeneity of the PEG layer. Similar morphology has also been observed by other groups.54,55 Under UV irradiation, the globular-like morphology disappears, and the surface morphology returns almost to its original smooth surface (Figure 5c). The result is in agreement with that of contact angle measurements. For the surfaces fabricated by β-CD-PMMA and β-CD-P(MMAco-HFBMA), we also find that the surfaces are not uniform, compared to the original Si-Azo surfaces, which demonstrates that polymers are attached to the surface (Figure 6a,c). Moreover, after UV irradiation, the hydrophobic polymers can detach from the surfaces to form smooth morphologies (Figure 6b,d), suggesting that these surfaces have good reproducibility. But the density of polymers on these two surfaces is obviously lower than that of the surface constructed from β-CD-PEG. This is because the PEG-modified surface is fabricated in water where inclusion complexation can easily occur. Nevertheless, for β-CDPMMA and β-CD-P(MMA-co-HFBMA), the inclusion complexations are performed in DMF, where the binding strength

Figure 5. AFM images of (a) Si-Azo, (b) Si-Azo/β-CD-PEG, and (c) Si-Azo/β-CD-PEG surfaces after UV irradiation. 9365

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Figure 6. AFM images of (a) the Si-Azo/β-CD-PMMA surface, (b) the Si-Azo/β-CD-PMMA surface after UV irradiation, (c) the Si-Azo/β-CDP(MMA-co-HFBMA) surface, and (d) the Si-Azo/β-CD-P(MMA-co-HFBMA) surface after UV irradiation.

Figure 7. XPS C 1s core-level spectra of (a) Si-Azo/β-CD-PEG and (b) Si-Azo/β-CD-PEG surfaces after UV irradiation.

Figure 8. (A) XPS wide scan of the surfaces of (a) Si-Azo, (b) Si-Azo+BSA, (c) Si-Azo/β-CD-PEG, and (d) Si-Azo/β-CD-PEG+BSA. (B) Evaluation of protein adsorption on the Si-Azo and Si-Azo/β-CD-PEG surfaces before and after incubation in bovine serum albumin (BSA), expressed as XPS-derived surface [N]/[Si] ratios.

modified surfaces have been mostly fabricated via covalent bonds or irreversible strong interactions. In this work, the Si-Azo/β-

CD-PEG surface was constructed by host−guest chemistry, and its protein resistance was also studied. We have measured the 9366

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (nos. 21074035 and 51173044), the Research Innovation Program of SMEC (no. 14ZZ065), and a project sponsored by SRF for ROCS, SEM. W.Z. also acknowledges support from Fundamental Research Funds for the Central Universities.

contact angle of the Si-Azo surface, which is supposed to be prone to protein adsorption. The contact angle actually decreases from 79 to 62° after BSA immersion. However, the contact angle of the Si-Azo/β-CD-PEG surface after immersion in a BSA solution stays at 47°, which is close to that of the Si-Azo/β-CDPEG surface without BSA adsorption. The Si-Azo/β-CD-PEG surface after immersion in a BSA solution was subjected to UV irradiation, and the contact angle returns to 70.5°. Moreover, the changes in the contact angle could be switchable for many circles (Figure S12). The results suggest that the Si-Azo/β-CD-PEG surface is resistant to BSA protein adsorption. XPS was also used to analyze the variation of the surface chemical composition in the above process. The wide-scan spectra of the Si-Azo/β-CDPEG surface before and after immersion in BSA solution are shown in Figure 8A, respectively. After the Si-Azo surface was immersed in BSA solution, the atomic concentration of silicon significantly decreased from 38.68 to 23.6%, which suggests that a mass of BSA has been adsorbed on the Si-Azo surfaces. The N 1s signal intensity at a binding energy of about 399.8 eV is stronger than that of the original Si-Azo surfaces in Figure 8A. Moreover, the [N]/[Si] ratio increases from 0.085 prior to exposure to BSA to about 0.255 after 1 h of protein adsorption (Figure 8B), indicating that the Si-Azo surface has a strong affinity for proteins. However, for the Si-Azo/β-CD-PEG surface, the atomic concentration of silicon varies from 23.29 to 20.55% after exposure to the BSA solution, and there is also little variation in the strength of the N 1s signal before and after protein adsorption (Figure 8A). Additionally, the [N]/[Si] ratio also increases slightly after exposure to the BSA solution. All of these results indicate that the photocontrolled surface modified with β-CD-PEG via host−guest chemistry exhibits good resistance to protein adsorption.

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CONCLUSIONS A novel surface-modification strategy has been developed for the preparation of a photocontrolled silicon wafer surface with switchable wettability based on host−guest inclusion complexation between β-CD and Azo. The wettability of the silicon substrates could be tuned by varying the polymer polarities from hydrophilic β-CD-PEG to hydrophobic β-CD-P(MMA-coHFBMA), and the photocontrolled property of these surfaces could be determined via alternating UV/vis irradiation. The antifouling properties of PEG-functionalized surfaces are evaluated by the protein adsorption assay of bovine serum albumin, and the result shows that the surface modified with PEG possesses good protein resistance. ASSOCIATED CONTENT

S Supporting Information *

Synthesis schemes of Azo-PTES, β-CD-PEG, β-CD-PMMA, and β-CD-P(MMA-co-HFBMA). 1H NMR, UV/vis, FT-IR, and XPS spectra of various CDs. Elemental composition of polymerfunctionalized surfaces determined by XPS. Photocontrolled cyclic wettability of the Si-Azo/β-CD-PEG+BSA surface before and after UV irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 9367

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