Controlled Silanization using Functional Silatrane for Thin and

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Interface-Rich Materials and Assemblies

Controlled Silanization using Functional Silatrane for Thin and Homogeneous Antifouling Coatings Chun-Jen Huang, and Ya-Yun Zheng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01981 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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Controlled Silanization using Functional Silatrane for Thin and Homogeneous Antifouling Coatings Chun-Jen Huang, ,#,* and Ya-Yun Zheng#

Department of Biomedical Sciences and Engineering, # Department of Chemical and Materials

Engineering, National Central University, Jhong-Li, Taoyuan 320, Taiwan

*Corresponding author: Email: [email protected] (C.-J.H.)

ACS Paragon Plus Environment

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ABSTRACT Organosilicons for surface modification are gaining prominence because of their easy and rapid preparation, high availability, and effective modification for varying interfacial properties. However, their implementation has been humbled by poor control on the packing density, thickness and molecular structures due to the uncontrollable hydrolysis and condensation. This study reports for the first time new functional silatrane chemistry for precision deposition of a thin and homogeneous zwitterionic coating. Sulfobetaine silatrane (SBSiT) carries tricyclic caged structure and transannular N → Si dative bond, which shows excellent chemical stability in presence of water, and acid-modulated hydrolysis characteristic. Results from X-ray photoelectron spectroscopy indicate progressive deposition of SBSiT on silicon surface. Characterizations using atomic force microscopy and ellipsometry show the uniform and thin SBSiT films on silicon surfaces. The superior antifouling properties of SBSiT coatings were demonstrated by resisting bacterial and protein adsorption. More importantly, the stable and complete formation of the SBSiT coatings allows accurate interpretation of the interfacial phenomena for sensing and nanomaterial applications. Keywords: Zwitterionic material, self-assembling material, organosilicon, silatrane, antifouling property.


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INTRODUCTION A wide variety of surface modification methods for silica-based materials have been explored within industrial and biomedical realms. Among which, organosilanes are considered as the most attractive agents for effective and robust modification. Since the first introduction by Sagiv and co-workers in the 1980s,1, 2, 3, 4 the studies of self-assembled monolayers (SAMs) on SiO2 with organosilanes have grown rapidly. The spontaneous formation of silane-based coatings, or referred to salinization, relies on alkoxy hydrolysis and condensation to afford siloxane bonds between the silane molecules and hydroxyl groups on surface. Therefore, the modification with organosilanes on oxide surfaces provides advantageous features, such as stable covalent conjugation, post-modification without deterioration of the coatings, high compatibility with silicon technology and optical techniques.5 However, the highly ordered two-dimensional SAMs were constructed mainly from organosilanes with long hydrocarbon chains, such as octadecyltrichlorosilane, to allow assembly through the van der Waals interaction. Well control over the structure and formation of functional organosilane coatings appears of fundamental importance and challenge for various sensor, energy, and nanomaterial applications.6,

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For

example, the sensitivity of semiconductor field-effect transistor biosensors is strongly associated with the Debye screening length that is about 1 nm in a 0.1 M aqueous NaCl solution at 25°C.8 Moreover, the Förster resonance energy transfer (FRET) efficiency depends on the donor-toacceptor separation distance with an inverse sixth power law.9 The cases point out the critical demand for the precision control of the functional adlayers. The formation process and condition of the silane-derived layers are critical to the success of the tuning of the surface properties as well as a desired follow-up reactivity or functionality. Generally, the surface is activated by strong acids or an oxygen plasma to clean the surface and


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generate silanol groups at surfaces.10 The treatment renders the surface hydrophilic and prone to the formation of a thin water layer, which is essential for the formation of well-packed silane film. Besides, several studies indicate that the deposition process highly depends on solvent,11 deposition time,12, 13 temperature,14 water content, and solution age.15 In addition, the storage of organosilanes typically subjects the formation of polysiloxane aggregates in solutions, which is fast at high water contents and lower temperatures.16 The aggregates loss the reactivity to covalently conjugate on surfaces but physically deposited on the surfaces, giving rise to an unstable and inhomogeneous film. The complex deposition process involves reactions with silanol/silanolate groups, hydrogen bonding, electrostatic attractions, and siloxane bonds.17, 18 The complexity leads to low silane grafting density and weak attachment. Therefore, organosilanes simultaneously and spontaneously react in a solution with themselves and the silanol groups on surfaces upon the kinetics of hydrolysis and condensation. The controlled reaction of organosilanes is rarely to be found for well orientation, high packing, uniformity and thinness of functional films. Several advanced modification approaches have been developed for homogeneous and wellordered organic adlayers on silicon substrates. Functional 1-alkenes were grafted by thermally induced or visible light-induced hydrosilylation on a Si-H substrate.19, 20, 21, 22 The process relies on the mechanism in which the nucleophilic attack of positively charged surface Si sites by alkenes leads to the formation of a stable Si-C bond. The unique feature of this technology allows temporal and spatial controlled-modification on silicon via UV irradiation.23 However, pre-treatment to the silicon substrate is needed to remove the oxide layer for exposure of Si-H. Recently, heterocyclic silanes containing Si-N or Si-S in the ring were developed for the modification of surface silanols via a ring-opening click reaction.24 The heterocyclic silanes


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permit rapid modification of silicon porous nanostructures and functionalization. Moreover, amino- or thiol-terminated silatranes have been applied for DNA conjugation and nanoparticle immobilization.25,

26, 27, 28, 29

The five-membered tricyclic silatranes were synthesized by

treatment of triethanolamine with an alkoxysilanes. In the silatrane structures, the N atom is forced to approach towards the Si, resulting in the formation of strong transannular bonds (NSi) and high stability toward hydrolysis.30 However, the ordered and controlled formation of silatrane thin films has not been found due to limited understanding of modification mechanism. Zwitterionic materials are recently emerged antifouling materials as an alternative to hydrophilic oligo(ethylene glycol) (OEG)-based materials.31,

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The superhydrophilic and

relatively temperature-stable zwitterionic materials enable to effectively resist micro- and macroorganisms in complex media, such as human blood and seawater.34 Zwitterionic silane-based materials were developed for self-assembled modification of SiOx, metal oxides, and oxidized silicone for long-term fouling repellence, antifog property, oil-water separation, and colloidal stability.35, 36, 37 However, the superhydrophilic zwitterioinc moieties in the silanes are prone to strongly interact with water molecules, leading to the fast hydrolysis, cross-linking and aggregation of alkoxy groups in a solution as happening to the other silane molecules. It makes poor control over the stability and homogeneity of organosilane adlayers on substrates18. Moreover, thick coatings and clogging of micro- or mesopores of zwitterionic silanes can occur, resulting in restricted applications in biosensing and nanomaterials.24, 38 In this work, a new zwitterionic sulfobetaine silatrane (SBSiT) is established with controlled reactive characteristic for smooth, thin and robust modification on oxide surfaces for antifouling properties (Scheme 1). The stable chemical structure of SBSiT under an ambient environment and in an aqueous solution will be demonstrated by using 1H nuclear magnetic


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resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopies. For the first time, the acid-modulated deposition of SBSiT molecules on surfaces for fast and uniform modification was conducted. The plausible grafting chemistry is associated with rapid detachment of triethanolamine from silatranyl group to present reactive silanol groups. The complete and effective coating of SBSiT on silicon wafer was confirmed by X-ray photoelectron spectroscopy (XPS). The roughness and thickness of the SBSiT films were verified by atomic force microscopy (AFM) and ellipsometry. In addition, the antifouling properties of the zwitterionic films were challenged by Gram-positive and –negative bacteria and observed using fluorescence microscope. More importantly, quartz crystal microbalance with dissipation (QCM-D) was applied to quantitatively evaluate the repellence against protein and the stability of the coatings. Consequently, the work not only presents the new zwitterionic assembly for the biocompatible and antifouling modification, but also a breakthrough for organosilicon chemistry to afford a smooth and thin coating in a controlled manner. The silatrane chemistry in this study offers a cornerstone for the development of silicon-based sensors and nanomaterials for a wide spectrum of applications.


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Scheme 1. a) Synthesis of SBSiT. b) The formation of SB-based antifouling coating on SiO2 substrate. EXPERIMENTAL SECTION Materials.

(N,N-Dimethylaminopropyl)

trimethoxysilane

(DMASi)

and

1,3-

propanesultone were purchased from Gelest Inc. and Alfa Aesar, respectively. Triethanolamine (TEOA), n-pentane, acetone, ethanol, acetic acid, toluene, dimethyl sulfoxide and bovine serum albumin (BSA) were obtained from Sigma-Aldrich. Phospoate buffered saline (PBS), toluene, npentane and Luria-Bertani broth (LB broth) were acquired from Acros Organics. Dulbecco's Modified Eagle's Medium (DMEM) and fetal bovine serum (FBS) were from Gibco. Synthesis of SBSiT and SBSi. A flask containing 22.8 mmol of DMASi and 24.03 mmol of TEOA in 33.69 mL of toluene was heated at 110 °C and stirred under nitrogen for 30 h. Afterward, the flask was kept for 1 h at room temperature, and then copious cooled n-pentane


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was added. The reaction solution was evaporated in vacuo to obtain white precipitate. The white precipitate was washed with cooled n-pentane for three times and collected by centrifugation at 9,000 rpm for 5 min. The white product was analyzed as (N,N-Dimethylaminopropyl) silatrane (DMASiT) with a yield of 66%. Afterward, 3.84 mmol of DMASiT and 3.84 mmol of 1,3propanesultone were dissolved in 4 mL of anhydrous acetone and the solution was stirred under nitrogen at rt for 6 h. The white product appears after the reaction. The product was washed with copious anhydrous acetone for 3 times and collected by centrifugation at 9,000 rpm for 5 min. The white product was dried in vacuo to afford SBSiT with a yield of 83 %. The schematic illustration of SBSiT synthesis is presented in Scheme 1a. SBSi was synthesized as described in the previous work.39, 40 The nucleophile (Nu), such as tertiary amine in DMASi and DMASiT, has a reactive pair of electrons that is capable of participating in nucleophilic substitution, e.g., SN2 type, ring opening of a sultone ring to afford the sulfobetaine group. Therefore, 24 mmol of DMASi and 25 mmol of 1,3-propanesultone were dissolved in 25 ml anhydrous acetone and stirred for 6 h under nitrogen protection at rt. The white powder was produced. The solution was filtered with G3 glass Gooch filter. The white solid product on the filter was washed with acetone, followed by drying in vacuo. SBSi was obtained with a yield of 65 %. Hydrolysis Tests. The chemical structure and stability of SBSi and SBSiT in the presence of water and acid were characterized using 500 MHz 1H NMR (Cryomagnet Oxford). FTIR (JASCO Corp.) analysis was performed for the structural characterization of SBSi and SBSiT before and after storage under an ambient condition for 24 h. Turbidity of SBSi and SBSiT solutions with and without addition of acetic acid was tested using UV-vis spectroscopy (Model


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V-600, JASCO) at a wavelength of 550 nm. In addition, the size of aggregates of SBSi and SBSiT molecules in solutions was followed by dynamic light scattering (DLS, Nano-S, Malvern). Preparation of Zwitterionic Organosilicon Coatings. The silicon wafer or glass substrates were cleaned sequentially in a sonication bath of 0.1% SDS, acetone, and ethanol for 10 min of each, followed by drying in a stream of nitrogen. The substrates were transferred to a plasma cleaner (PDC-001, Harrick Plasma, NY) to expose O2 plasma twice with a power of 10.5 W for 10 min to remove trace amounts of contaminants from the surfaces. The clean substrates were immediately immersed into a 5 mM SBSi or SBSiT solution containing 10 v/v % H2O. In order to accelerate the hydrolysis process, 2 v/v % acetic acid was additionally added. The coating solutions were heated at 60 °C for 4 h. The modified substrates were removed and cleaned in a sonication bath of ethanol, followed by drying in a stream of nitrogen. The substrates were baked in an oven at 70 °C for 1 h for the formation of Si-O-Si bonds by condensation in the organosilicon films. Contact Angle Measurements. Static water contact angle measurements for organosilicon coatings were estimated by using a contact angle goniometer (Phoenix mini, Surface Electro Optics, Seoul). The 5 µL water droplets from a microsyringe were dropped on the substrates and after the equilibrium was established for 10 s, the shape of the water droplet was recorded for determination of the contact angle. The contact angles were measured at least three times at random positions. XPS Measurements. The chemical elemental spectra were detected by an XPS system with a microfocused and monochromatic Al Kα x-ray source (1486.6 eV, 400 µm; Sigma Probe, Thermo Scientific). The takeoff angle of the photoelectron source was set at 45°. The pressure of the system was below 10-10 Pa using an oil-less ultrahigh vacuum pumping system. A dual


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beam charge neutralizer was employed to compensate for charging effects.

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Spectra were

collected with a pass energy set to 58.8 eV, and the energy resolution was 0.2 eV. The typical data acquisition time was around 30 min. The spectra were analyzed using Multipak software package. Ellipsometric Measurements. Ellipsometric measurements for the coatings on SiO2 were performed with a ellipsometer equipped with a He-Ne laser (λ = 632.8 nm) at an incidence angle of 70° (Stokes Ellipsometer LSE-Traveler, Gaertner Scientific Corp). The bare substrates were measured to ﬁnd the Ns (2.5), Ks (-3.2), and refractive index (n = 1.00) of the ambient. The refractive index of the organic thin films on the substrates was ﬁxed to n = 1.446. The measurements were performed at least five times at random locations on each sample. AFM Measurements. The surface morphology and roughness of substrates were explored by using AFM (NW3 NanoOptics AFM, JPK Instruments AG). The AFM was operated in a tapping mode with a scanning rate of 0.1 Hz. A cantilever with a resonance frequency of 160 kHz and force constant of 7.4 N/m (NanoWorld AG) was used. MTT Assay. The cytotoxicity of chemicals of 1,3-propanesultone, SBSi and SBSiT was assessed by NIH-3T3 fibroblast cells in MTT assay. Initially, NIH-3T3 fibroblasts were cultured in DMEM medium containing 10% FBS in 96-well plate with a cell number of 6×103 per well. The culture plates were placed in an incubator in 5% CO2 content of the atmosphere at 37 °C for 16 h. After culturing, the medium was replaced by serum-free DMEM medium containing the chemicals at a concentration ranging from 0.2 mM to 25 mM, and afterwards the plates were incubated for 24 h. MTT solution in PBS was added to the cultures and, then, cells were incubated for additional 3 h. Then the medium was removed, and 20 µL of DMSO was added to each well to dissolve purple crystals of formazan. The absorbance was measured in a


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spectrophotometer (Synergy 2, BioTek) at a wavelength of 540 nm. The reported value is the mean from triplicates and is expressed as percentages with respect to the control values. Bacterial Fouling Tests. A single bacteria colony of Gram negative E. coli or Gram positive S. epidermidis was picked from the LB agar plate to inoculate 25 mL liquid LB growth media. After 16 h inoculation at 37 °C shaking at 200 rpm, 1 µL bacteria containing media from the first culture was used for a secondary culture in LB in a conical flask. The bacteria were then washed with sterile PBS for three times. After the final wash, the bacterial samples in PBS were diluted to OD670 of 0.1, corresponding to ~8 × 107 cells/mL, to be tested for antifouling properties of substrates. The substrates were dipped into the bacterial solution at 37 °C for 3 h, followed by washing with sterile PBS and shaking at 100 rpm for 10 min for three times. The adsorbed bacteria were stained with 50 µL of LIVE/DEAD BacLight for 15 min. Afterward, the substrates were observed under fluorescence microscopy (ZEISS Microscope Axio Obserber A1, Germany) equipped with a CCD camera (Roper Scientific). The microscopy was operated with a magnification of 400 × at an excitation wavelength of 488 nm. The measurements were performed at five random locations on each sample, and the bacteria numbers were analyzed using an ImageJ software package (developed at National Institutes of Health, MA).

QCM-D for Protein Fouling Tests. The SiO2 covered QCM crystal chips (AT-cut quartz crystals, f0 = 5 MHz) (Q-Sense AB, Gothenburg, Sweden) were cleaned with the protocol used in a previous publication41, and the SBSi and SBSiT coatings were prepared as the same as abovementioned process. Before the measurement, the chamber was rinsed with phosphate buffered saline (PBS) and the temperature was stabilized at 25 °C. After the equilibrium was established, a 1 mg/mL BSA solution in PBS was brought into the chamber at a flow rate of 0.3


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mL/min for 10 min, followed by rinsing with PBS. All measurements were recorded at the third overtone (15 MHz), and the data shown here were normalized to the fundamental frequency (5 MHz) by dividing by the overtone number. The increased mass on the chip is related to changes in frequency of the oscillating crystal through the Sauerbrey relationship: ‫ܥ‬ொ஼ெ ∙ ∆݂ ∆m = ݊ where ∆m represents the mass adsorbed on the quartz sensor, ∆f is resonance frequency, CQCM is the mass-sensitivity constant (=17.7 ng cm-2 Hz-1 at f = 5 MHz), and n is the overtone number (= 1, 3, 5 and 7).42, 43, 44


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RESULTS and DISCUSSION Synthesis and Characterization of SBSiT. In this work, zwitterionic SBSiT was synthesized for robust antifouling, smooth, stable and thin coating. Firstly, DMASi reacted with TEOA in toluene at 110 °C for 30 h to obtain (N,N-Dimethylaminopropyl) trimethoxysilatrane (DMASiT) with tricyclic caged structure and transannular N → Si dative bond. Secondly, 1,3propanesultone underwent ring-opening reaction with DMASiT in anhydrous acetone at room temperature for 6 h. Thirdly, the white solid product, SBSiT, was collected and purified (Scheme 1a, and 1H NMR spectrum in Fig. S1, ESI). The total yield after two synthesis steps is 55 %. In addition, sulfobetaine silane (SBSi) was synthesized as described in literature35,

37

as a

counterpart for comparison in terms of chemical stability, thin film formation, and antifouling properties. On the basis of the silatrane chemistry, the hydrolysis of SBSi and SBSiT in water was verified using time course 1H NMR in Fig. 1. The existence of methoxy (chemical shift δ= 3.6 ppm) and silatranyl groups ( δ =3.0 and 3.8 ppm) for SBSi and SBSiT (Fig. 1a and b), respectively, was monitored to follow the degree of hydrolysis. As indicated, the resonance shift for the methoxy group disappears after the incubation of SBSi in D2O for 240 min. On the contrary, the silatranyl ring of SBSiT nearly remains intact in the time frame of the test. We quantified the degree of hydrolysis according to the signal strengths of the corresponding peaks (Fig. 1c). The silatranyl ring of SBSiT can avoid the hydrolysis, whereas the exponential decrease in the number of the methoxy for SBSi was observed, showing the rapid hydrolysis in agreement with previous works. 37


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Figure 1.

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H NMR spectra for SBSi (a) and SBSiT (b) in D2O at different time points. The

quantitative analysis for the hydrolysis percentages of SBSi and SBSiT in D2O (c). Moreover, to pursue the long term usability, the dried solids of SBSi and SBSiT were left in a laboratory environment (RH = 79 % and temperature = 24 °C) for 24 h. FTIR was applied to examine their chemical stability (Fig. 2a). The signals centered at 1050 and 1612 cm-1 corresponding to Vas(SO3-) and Vb(N(CH3)+), respectively, indicate the presence of the sulfobetaine moiety in all samples. In order to compare the integrity of the methoxy and silatranyl groups, signals from Vs(CH2) (2884 cm-1) and Vas(CH2) (2948 cm-1) for SBSiT, and


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those from Vs(CH3) (2841 cm-1) and Vas(CH2) (2959 cm-1) for SBSi were followed. As the results, the spectra for SBSiT before and after 24-h storage was almost identical, whereas the absorption intensities for Vs(CH3) and Vas(CH2) for SBSi considerably decrease after storage (Fig. 2a). The photographs of the SBSiT and SBSi solids were shown in Fig. 2b. The deliquescence obviously occurs to SBSi, showing its hygroscopic character. Contrarily, SBSiT remains dry powder after exposure to the humid environment, demonstrating insensitiveness to water.

Figure 2.

FTIR analysis for SBSi and SBSiT before and after storage under an ambient

condition for 24 h (a). The photographs of SBSi and SBSiT samples before and after the storage in the regular laboratory environment (b).

Acetic acid was chosen to increase the hydrolysis rate of silatrane molecule.6 1H NMR was employed to follow the hydrolysis of silatrane in a MeOD solution containing 2 % v/v acetic acid


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in Fig. 3. The signal intensities for the silatranyl group (positions: I and J) decreased with the incubation time in contrast to that in D2O only solution (Fig. 1b). It is found that the hydrolysis rapidly proceeds with a half-life (t1/2) of 180 min in the solution, exhibiting better susceptibility of SBSiT to acid. Accordingly, the addition of acid in the solution enables the quick detachment of TEOA from the silatranyl ring to expose silanol groups, which likely faciliates the chemical conjugation of SBSiT on silicon oxides.

Figure 3. 1H NMR spectra for hydrolysis of SBSiT in MeOD solution containing 2 % v/v acetic acid at different time points.

Formation of zwitterionic thin films. Formation of thin films on solid substrates from silatranes is strongly associated with the experimental conditions. Herein, we carefully evaluated the effects of solvent (methanol and ethanol), temperature (25 and 60°C), time and acid on the deposition of sulfobetaine films (Figs. S2 and 4). From the time-course contact angle measurements (Fig. 4), the SBSiT solution in ethanol containing 2% acetic acid to react with cleaned glass slides at 60 °C enables formation of a superhydrophilic coating (contact angle < 5°) after 3-h reaction. However, the surface coatings from the SBSiT solution without addition of acetic acid needs 4 h to reach superhydrophilicity. It should be noted that SBSi allows fast


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deposition on glass to obtain superhydrophilic coating in 1 h due to its rapid hydrolysis of the silane group in ethanol. Therefore, the hydrolysis rate determines the formation of organosilicon adlayers with SBSi and SBSiT on surfaces and, nevertheless, it may come along with the formation of polysiloxane aggregates in solutions.

45 40 Contact angle ( degree)

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35 30 25

bare glass SBSi w/ acid SBSiT w/ acid SBSi w/o acid SBSiT w/o acid

20 15 10 5 0 0

50

100

150 200 250 Time (min)

300

350

400

Figure 4. Time-course contact angle measurements for the formation of hydrophilic coatings using SBSi and SBSiT under different deposition conditions.

We further used UV-Vis spectrometer and DLS to compare the aggregation behaviors of SBSi and SBSiT in solutions in Fig. 5. From the transmittance measurements, the solution containing SBSi without acid quickly turns to turbid and then gradually becomes clear (Fig. 5a). The increased transmittance was attributed to the precipitation of large particles on the bottom of the cuvette. For the SBSi solution containing 2% acetic acid in ethanol, the transmittance continuously drops in the time course. The photograph of the sample after incubation for 6 h in a cuvette is shown in Fig. 5a. In the SBSiT solution without acid, the solution remains totally transparent until 250 min. For the SBSiT solution with acid, after 180 min, the solution starts


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becoming turbid. Besides, DLS measurements support the observation in turbidity tests. As found in Fig. 5b, the increased hydrodynamic diameters of the SBSi and SBSiT in solutions indicate that SBSi subject fast aggregation, especially in solution without acid forming big particles with an increased diameter >800 nm after incubation for 1 h. This reflects the observation in the increased transmittance and precipitated particles in Fig. 5a. As a result, the kinetics of SBSi assembly and aggregation is largely associated with the pH value.45,

46

At

neutral pH, the condensation is faster, but the hydrolysis is slower.46 At low pH, fast hydrolysis can lead to fast aggregation and good colloidal stability. In the acidic solution, electrophilic attack of the proton on an alkoxide oxygen atom leads to the development of a positive charge on it. The electrophilic attack makes the bond between the silicon center and the attacked oxygen more polarized, facilitating its breakage in the departure of the alcohol leaving group.47 Moreover, in the absence of acid, fast gelation can occur, leading to fast precipitation.45 Herein, SBSiT displays chemical stability and slow hydrolysis to avoid the aggregation and condensation.


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Figure 5. The UV-vis spectrometer and DLS measurements for SBSi and SBSiT in the solutions with and without acetic acid in ethanol at 60 °C.

To examine the formation of zwitterionic SBSiT adlayers, XPS was applied to analyze elemental compositions and chemical ratios. The SBSiT was deposited in ethanol solutions with and without 2 % v/v acetic acid at 60 °C. In order to confirm the complete formation of SBSiT organosilicon adlayers, N1S spectra were followed at deposition times of 0.5 and 3 h (Fig. 6). Since SBSiT exhibits the quaternary (-N+(CH3)3) and tertiary (-N(CH3)2) ammonium in the sulfobetaine and silatranyl groups, respectively, the detachment of the tertiary ammonium (TEOA) from the surface provides a clear indication of the intact reaction with silanol groups on


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the surface. In the results, the signal intensity for the tertiary ammonium decreased with the deposition time, and the detachment of TEOA in the presence of acetic acid was faster than that without acid. The data are in agreement with previous results from turbidity and aggregation measurements from UV-vis spectrometer and DLS (Fig. 5).

Figure 6. XPS analysis for N1s spectra of SBSiT deposition in ethanol solutions without acetic acid for 0.5 (a) and 6 h (b), and that with acetic acid for 0.5 (c) and 6 h (d).

Moreover, the XPS characterization was conducted for the silicon oxide samples treated in the SBSiT solution containing 2 %v/v acetic acid at 60 °C for 4.5 h. In Fig. 7, the N1s spectrum reveals complete detachment of TEOA from the silatranyl ring, and the S2p spectrum shows the appearance of sulfonate -SO3- (doublet BE = 167.0 and 168.2 eV). The N/S elemental ratio was estimated as 0.95, which is approximately close to the stoichiometric value of N/S = 1.36, 40, 48 The O1s spectrum presents siloxane –Si-O-Si- (BE = 532.4 eV) and -SO3- (BE = 531.7 eV); the


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C1s spectrum reveals presence of –C-N/-C-S (BE = 285.7 eV) and –C-C-/-C-Si- (BE = 284.6 eV).35 Consequently, the XPS spectra clearly demonstrate the complete formation of the sulfobetaine organosilicon adlayer on oxide surfaces through the acid-modulated hydrolysis.

Figure 7. XPS spectra for N1s (a), S2p (b), O1s (c), and C1s (d) of zwitterionic organosilicon adlayer by the deposition of SBSiT in the ethanol solution containing 2 %v/v acetic acid at 60 °C for 4.5h.

In order to examine the uniformity of the organosilicon adlayers from SBSi and SBSiT, AFM and ellipsometry were employed to access surface morphology and thickness, respectively. Samples were prepared in solutions with and without acetic acid for a treatment time of 4.5 h. In Fig. 8a and b, the surface features of bare and SBSiT-modified wafer are generally flat and almost identical as indicating by the comparable root-mean-square (RMS) roughness (Rq) of 4.4 and 5.4 Հ, respectively. The data for the first time show a smooth organosilicon coating by


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deposition of the silatrane molecule. The slow hydrolysis of silatranyl groups enables the order grafting of SBSiT molecules onto surfaces and subsequent formation of Si-O-Si bonds. Contrarily, abundant micro-sized granules appear on SBSi-coated surfaces, resulting in high Rq values of 66.5 and 112.6 Հ for samples prepared in solutions with and without acetic acid in Fig. 8c and d, respectively. Compared with previous evidences in Fig. 5, the large roughness should arise from the aggregation and deposition of SBSi polycyclosilane and polysiloxane due to the fast hydrolysis and condensation in the solution. In addition, the film thicknesses for SBSi and SBSiT films were measured using ellipsometry (Fig. 9). Overall, the thicknesses of the organosilicon adlayers increase with the deposition time. It appears that the SBSi films prepared from the acidic solution is considerable thicker than other samples. After the deposition for 4.5 h, the thickness is 72.1 ± 2.9 nm. On the contrary, the SBSiT films from the acidic solutions exhibit thicknesses as small as 6.5 ± 1.1 nm, which is much smaller than the SBSi coatings. More importantly, the thicknesses of SBSiT films do not obviously increase with the deposition time, implying that the progressive reaction of silatrane with silanol groups on the surface avoids the formation of crosslinking polycyclosilane. The SBSiT film developed in the ethanol solution without acid can be controlled to a thin layer with a thickness of 3.4 ± 0.4 nm after deposition for 4.5 h. Accordingly, the slow hydrolysis of SBSiT facilitates the controlled deposition of functional organosilicon adlayers with great improved smoothness and thickness.


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Figure 8. AFM images for bare silicon wafer (a), SBSiT (b) and SBSi (c) coatings deposited in acidic solutions, and SBSi coating deposited in non-acidic solution (d).

t = 0.5 h t = 4.5 h

80

Thickness (nm)

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

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60 40 20 0 SBSi

SBSi + Acid

SBSiT

SBSiT + Acid

Figure 9. Ellipsometry measurements for the film thicknesses of SBSi and SBSiT coatings with and without addition of acid in the deposition solutions.


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Antifouling properties of SBSiT. The cytotoxicity of SBSiT was evaluated before the biological tests. SBSi, SBSiT and 1,3propanesultone were dissolved in the culture medium at concentrations ranging from 0.2 to 25 mM. After incubation with NIH-3T3 fibroblasts for 24 h, the cellular viability was determined by MTT assay. As shown in Fig. 10, 1,3-propanesultone exhibits high cytotoxicity, which is classified as a toxic and carcinogenic agent. Contrarily, the SBSi and SBSiT have comparable cellular viability. At a high concentration of 25 mM, both can maintain the viability higher than 80%. Thus, the zwitterionic coating possesses good biocompatibility for potential uses in medical applications.

1,3-propane sultone SBSi SBSiT

100

Cell viability (%)

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

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80 60 40 20 0 0 mM

0.2 mM 1 mM 5 mM Concentration

25 mM

Figure 10. MTT assays for NIH 3T3 fibroblasts in the presence of SBSi, SBSiT and 1,3propanesultone at various concentrations.

The antifouling properties of the SBSiT films were characterized to verify fully exploitation of the zwitterionic function. Gram negative, E. coli, and Gram positive, S. epidermidis, were brought into contact with coatings prepared from SBSi and SBSiT solutions with and without


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addition of acetic acid in ethanol for 4.5 h. The adhered bacteria on surfaces were stained with Live/Dead fluorescence dye (Fig. 11a). The bacterial fouling levels were examined by ImageJ, showing in Fig. 11b. Obviously, the fouling levels for all SBSi coatings are all lower than that on bare glass by >99.8 %, revealing the excellent antifouling properties of SBSi in agreement with previous literature.35 For SBSiT films, the samples prepared from the acidic solution enables resisting the bacterial fouling to a level similar to that found with SBSi, which is about three orders lower than bare samples. However, the amount of bacteria attached onto the SBSiT coatings prepared without the addition of acid remains larger compared with other zwitterionic coatings. The data can conclude that: 1) the coating thickness and structure of SBSi do not affect the functionality of zwitterionic moieties to repel the bacterial adhesion; 2) the undetached TEOA caged structure in SBSiT can lead to non-specific adsorption of bacteria, likely due to unbalanced surface charge; 3) the complete formation of SB organosilicon coatings from SBSiT ensures the effective resistance of bacterial adhesion.


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Figure 11. Bacterial adsorption tests for E. coli and S. epidermidis on wafer, coatings of SBSi, SBSi with addition of acid, SBSiT, and SBSiT with addition of acid. The fluorescence images were shown for all samples after staining with fluorescence dye (a). The quantitative analysis for the adsorbed bacteria on samples was presented (b).

QCM-D was applied to simultaneously evaluate the adsorbed mass and viscoelastic property of the films.49, 50, 51 Since the Sauerbrey equation is to calculate the surface mass of the films with low viscoelasticity.49 In this study, the zwitterionic SAMs from SBSi or SBSiT and BSA were relatively rigid (low viscoelasticity) and, therefore, the Sauerbrey equation is applicable to determine the surface mass. BSA protein solution prepared in PBS at a concentration of 1 mg/mL was flowed over organosilicon surfaces, and the changes in frequency and dissipation were recorded in Fig. 12a and b. After PBS washing, the net adsorption levels (Γ) were


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quantitatively calculated using Sauerbrey equation in Fig. 12c.49 Protein adsorption levels on all modified surfaces demonstrate much lower than that on bare silicon. However, the incomplete formation of SBSiT coating prepared from the solution without addition of acetic acid show the slightly higher fouling level of Γ= 18.1 ng/cm2 compared with that prepared in the acidic solution ( Γ = 3.2 ng/cm2). It should be noted that Δ D measures properties related to the viscoelastic properties of the adlayer.49 In Fig. 12b, after washing with PBS, ΔD of the SBSiT coating with acid treatment returned to zero, indicating no change in the viscoelastic property of the film. However, ΔD values of SBSi coatings after washing exhibit negative, showing the viscoelastic structure of the coatings changes from hydrated and soft to compact and rigid. This should be ascribed to the replacement and detachment of swollen thick SBSi films by BSA, which should be similar to the Vroman effect, in which the loosely bound SBSi aggregates were replaced by rigid BSA proteins on surfaces. Nevertheless, the frequency measurements hardly detect structural changes, reflecting the false estimation of actual adsorption levels. Therefore, the complete formation of the SBSiT coating can be advantageous in applications such as biosensors and coatings for nanomaterials, where depletion of the modifiers in contact with complex environment should be restricted.


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Figure 12. QCM-D studies for the adsorbed mass and viscoelastic property of the films. The samples include bare silicon wafer, coatings prepared from SBSi and SBSiT with and without addition of acid.


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CONCLUSIONS In summary, we established a new zwitterionic organosilicon molecule, SBSiT, with tricyclic caged silatranyl ring and transannular N → Si dative bond for stable, homogeneous and robust antifouling coating. Because of the slow hydrolysis, SBSiT enables long-term storage in ambient environment and aqueous solution to avoid intermolecular crosslinking and aggregations. Hydrolysis and condensation of SBSiT for surface coating can be activated by addition of acid into the deposition solution, which permits modification in a controlled way. SBSiT undergoes the progressive deposition of silatrane with silanol groups on the surface to form a thin and smooth adlayer. The excellent antifouling properties of SBSiT coatings were demonstrated by the adsorption of bacteria and protein. In addition, from the QCM-D measurements, the thin and stable SBSiT film prepared from the acid-triggered reaction on silicon has been verified in protein fouling resistance. For the regular SBSi films, we observed that the loosely bound organosilicon aggregates were replaced by proteins. Consequently, SBSiT offers great promise as a suitable organosilicon material for applications in biosensors and nanomaterials.

ACKNOWLEDGMENT The authors acknowledge the Ministry of Science and Technology (MOST 105-2628-E-008 -007 -MY3; 106-2119-M-194 -002) and Environmental Protection Administration of Taiwan for financial support of this project.

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