Simultaneous Formation of a Self-Wrinkled Surface and Silver

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Simultaneous Formation of a Self-Wrinkled Surface and Silver Nanoparticles on a Functional Photo-Curing Coating Hongbo Lin, Yuanlong Wang, Yanchang Gan, Honghao Hou, Jie Yin, and Xuesong Jiang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03484 • Publication Date (Web): 15 Oct 2015 Downloaded from http://pubs.acs.org on October 18, 2015

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Simultaneous Formation of a Self-Wrinkled Surface and Silver Nanoparticles on a Functional Photo-Curing Coating Hongbo Lin1, Yuanlong Wang2, Yanchang Gan1, Honghao Hou1, Jie Yin1, Xuesong Jiang1* 1. School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China. 2. State Key Laboratory of Dairy Biotechnology, Dairy Research Institute, Bright Dairy & Food Co., Ltd., Shanghai 200436, People’s Republic of China.

Abstract Bio-inspired functional surface with micro/nanostructures are particularly attractive because of the potential for outstanding characteristics, such as self-cleaning, self-replenishing and antibiosis. Here, we presented a facile approach to fabricate a functional photo-curing coating with both a self-wrinkling patterned surface and incorporated silver nanoparticles (Ag NPs). Fluorinated polymeric photoinitiator (FPPI) and silver precursor (TFAAg) can self-assemble together on the air/acrylate interface to form a top layer of photo-curing liquid resin. Under UV irradiation, a wrinkled pattern was formed as a result of the mismatch in shrinkage caused by photopolymerization between the top layer and the bulk layer. Simultaneously, Ag NPs with sizes of 15±8 nm in diameter were in-situ generated in the photo-curing coating through the photo-reduction of TFAAg. Their number density is higher in the top layer than in the bulk. Scanning electron microscope (SEM) and atomic force microscope (AFM) measurements revealed that the characteristic wavelength (λ) and amplitude (A) of the wrinkled morphology increased with growing concentration of FPPI, and that the generation of Ag NPs led to the wrinkle-to-fold transition. Furthermore, the obtained functional coatings possess a low surface energy and self-replenishing and antibiosis capabilities as a result of the synergistic effect of the wrinkled surface covered by FPPI and Ag NPs.

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Keywords Self-wrinkled surface, nanoparticles, hydrophobicity, antibiosis, self-replenishing

Introduction Multi-functional coatings with a bio-inspired patterned surface are of great interest because of their versatile properties, e.g., self-cleaning,[1,2] antibiosis,[3] or self-replenishing,[4,5] as well as their potential application in functional coatings,[6,7,8] biomedical devices,[9] or tissue engineering.[10] Many of the plant leaves, for example, remain dirt-free and exhibit hydrophobicity and antibiosis; these features are ascribed to the complex morphology of their surfaces and the coating of waxy and antimicrobial compounds. Undoubtedly, hierarchical structures plays an indispensable role in the tunable performance of multi-functional coatings. As a result, many techniques, such as nanoimprinting[11,12] and lithography[13], have been developed with the purpose of constructing bio-inspired surfaces. These techniques feature a top-down character; however, they are different from the methods of natural ones and are limited by the disadvantages of requiring a complicated physical process or an elaborate chemical synthesis. Generally, the biological surface patterns are formed spontaneously from the intersystem to the surface in one step. Thus, akin to the biological method, it is of great interest to develop a simple and low-cost method to fabricate a patterned surface to produce the targeted performance. As one of the most common methods to generate complex patterns in Nature, wrinkling can fabricate a complex pattern on the surface of an organism and usually involves compressive stress caused by the modulus mismatch between the surface and the bulk of materials involved. Recently, we demonstrated a novel strategy for fabricating micro- and nano-wrinkled patterns on the surface of a photo-curing coating through self-assembly of a fluorinated additive and photopolymerization[14]. Due to the low surface energy, the fluorinated additive can form the top layer through self-assembly at the interface


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of acrylate monomer liquid and air. The mismatch in shrinkage caused by the photopolymerization between the top and bulk layers can cause compressive stress, resulting in the wrinkled pattern. This one-step approach, which we refer to as “self-wrinkling”, is similar to the self-organizing method in Nature that used to generate a wrinkled surface. Moreover, the combination of the complex wrinkled pattern and the low surface energy of the fluorinated additive provides the resulting wrinkled surface with a self-cleaning function. By taking advantage of this self-wrinkling process, we here demonstrated a multi-functional photo-curing coating with the simultaneous formation of a wrinkled surface and silver nanoparticles. The entire strategy is illustrated in Scheme 1. Polystyrene grafted with fluorocarbon chains, benzophenone and amino moieties in the side-chain (FPPI) can be regarded as a photoinitiator and the fluorinated additives that self-assemble at the interface of air/acrylate monomer with silver precursor (TFAAg) to form the top layer. Upon irradiation with UV light, the difference in shrinkage induced by photopolymerization between the top and bulk layers causes compressive stress, resulting in the formation of a wrinkled pattern. Simultaneously, the Ag+ of TFAAg is reduced to Ag0 by the free radicals produced from FPS-BPA to form silver nanoparticles (Ag NPs). Due to aggregation of the fluorinated polymer chains and Ag NPs in the top layer, the resulting wrinkled surface possessed versatile functionalities, such as low surface energy, self-replenishing ability and antibiosis.


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Scheme 1: The strategy to produce a self-wrinkled pattern and silver nanoparticles in a photo-curing coating. (a) Chemical structure of the components for the photo-curing coating; (b) Formulation of the self-assembly strategy to prepare a silver-contained coating marked with a wrinkled pattern.

1. Results and Discussion Formation of the self-wrinkled pattern Fluorinated polymeric photoinitiator (FPPI) is the key component in our strategy, whose structure is illustrated in Scheme 1. FPPI was synthesized through free radical copolymerization, and the detailed synthesis and characterization can be found in the supporting information. We considered various physical and chemical factors in designing the PS-based fluorinated polymeric photoinitiator. Due to the lower surface energy and higher modulus in comparison to most polyacrylates, PS was chosen as the backbone to impart excellent mechanical and surface properties to the resulting coating. Fluorocarbon chains were introduced to provide a low surface energy for the polymer chain, which is the key factor for the self-assembly of the polymeric photoinitiator in the top layer. Benzophenone (BP) and tertiary amine were chosen as the photoinitiator and the hydrogen donor, respectively. Due to the high efficiency and low cost, BP is widely used as a hydrogen-abstraction photoinitiator in the presence of a co-initiator amine. Furthermore, amino moieties grafted to FPPI can


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complex with silver atoms, which promotes the aggregation and photo-reduction of the silver precursor in the top layer.

Figure 1. Formation of the wrinkled pattern with different contents of FPPI. SEM (top) and AFM (bottom) images of the wrinkled surfaces (a-0.5%, b-1%, c-2.0%, d-6.0%). Scale bars in a-d correspond to 25 µm.

To understand the generation of the wrinkled pattern, the photo-curable mixtures without TFAAg were coated on a glass substrate and then irradiated by UV-light under an atmosphere of nitrogen gas. The surface morphology of the resulting photo-curing coating was observed using SEM and AFM. Figure 1 presents typical wrinkled patterns, indicating the feasibility of our strategy. The characteristic wavelength (λ) and amplitude (A) of the wrinkled morphology increased with the increasing concentration of FPPI (Figure 1 and Figure 2). By decreasing the air/liquid interface energy, FPPI can migrate to the surface and self-assemble into the top layer. Because the volumetric shrinkage of the photo-curable resin is proportional to the content of unsaturated double bonds C=C, the photopolymerization-induced shrinkage of the top layer rich in FPPI should be less than that of the bulk layer of PEGDA. Therefore, the mismatch in shrinkage creates a compressive stress that triggers deformation, leading to the formation of the wrinkled pattern. Because of the high modulus of the PS backbone of FPPI, the top layer is stiffer than the bulk layer of pure PEGDA after photo-curing. This system is similar to those typical bilayer models in which the formation of wrinkles is based on axial compression of rigid skin resting


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on a relatively elastic foundation.[15] As shown in Figure 1 and Figure 2, the λ and A of the wrinkled morphology increased with increasing concentration of FPPI, suggesting that the morphology of the wrinkled pattern can be tunable. For a typical bilayer system, as predicated by the linear buckling theory[16-18], the λ and A depend on the thickness of the top hard layer and the mechanical properties of the top and bulk (bottom) layers. The equation for a bilayer system is [19] λ = 2πth

1 h 3 E s 3E

A = th 0 -1 ε

1 2

(1)

εc

where = E⁄1 − ν is the plane-strain modulus, E is the Young’s modulus, ν is the Poisson ratio, and t is the thickness. The ε0 is defined as the applied strain and εc as the critical strain. The subscripts ‘h’ and ‘s’ refer to the top layer and the substrate, respectively. In our system, it is expected that increasing the concentration of FPPI leads to a greater thickness and modulus of the top layer, resulting in an increasing wavelength and amplitude. Although the linear theory is applicable to discrete layers and not to up-down with a gradual change in properties, it did provide useful insights into our system. According to eq. 1, it is expected that the thickness of the top layer is proportional to the content of FPPI and can be expressed as follows: th ~h0

Vf Vb

~h0

ρ M f Mb / ~h0 b Cp ρf ρb ρf

(2)

where h0, V, M, ρ and Cp represent the thickness, volume, mass, density and mass ratio, respectively, of the sample. The subscripts ‘f’ and ‘b’ refer to the FPPI layer and the underlying bulk layer, respectively. Rewriting eq. 1 with eq. 2 yields E

λ ~2π 3Eh s

1 3

h0

ρb ρf

Cp ~k1Cp

A ~ 0 -1 ε

εc

1 2

h0

ρb ρf

Cp ~k2 Cp

(3)

In eq. 3, the parameter k is expected to be a constant, considering the same system. Thus, it is expected that the wrinkling characteristics in our system are linearly linked to the mass ratio of FPPI. However, as shown in Figure 2, the experimental data for both wavelength and amplitude do not follow a strictly linear trend, which could be


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ascribed to the gradient distribution of FPPI. Surface instability analysis reveals that a system with gradationally varying mechanical properties may have an amplitude with an exponential trend, which could be considered to be a result of the dominant effect of wave amplification with film thickening.

[20,21]

In other words, the observed

nonlinear response agrees with the instability analysis. 16 Amplitude Wavelength

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0 0.5

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2.0

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Figure 2. Amplitude and wavelength of the wrinkled surfaces with different content of FPPI.

Simultaneous formation of the wrinkled pattern and silver nanoparticles Functional nanoparticles are widely used in constructing materials with different properties,

which

can

be

introduced

through

a

bottom-up

approach

of

self-assembly.[22-24] In our case, silver nanoparticles were introduced to the surface of the photo-curing coating through in-situ photo-reduction and self-assembly of TFAAg with the help of FPPI. These particles played an important role in disordering stress field, which resulted in countless focal points of stress, thus leading to the generation of Ag NP-dependent wrinkles. The morphology of the resulting patterns of samples with different ratios of TFAAg is revealed in Figure 3, in which the content of FPPI is maintained at 6%. We observed that the wrinkles localize into several ridges at a low content of 0.1% TFAAg (Figure S3). We consider this transition to be the result of the instability induced by the nanoparticles, which leads to further compression; thus, ridges appear when the compression is beyond a third of its initial wrinkle wavelength.[25] Further confinement by increasing TFAAg led to prevalent ridges with growing amplitude, while neighboring wrinkles decrease in height, which eventually


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resulted in much smaller λ and A when compared with the silver-free sample (Figure

3 and Figure S4). This behavior might be ascribed to localized ridges, where stress confinement lowers the total energy and adjacent wrinkles are accommodated with short wavelength and small amplitude.[25,26]

Figure 3. Simultaneous formation of the wrinkled pattern and Ag NPs: SEM (top) and AFM (bottom) images of wrinkled surfaces with different contents of TFAAg (a-0.25%, b-0.50%, c-0.75%, d-1.00%). The concentration of FPPI is 6%, and the scale bars in a-d correspond to 25 µm.

In fact, what really attracted our attention were the disparate wrinkles, involving the cellular shape (Figure 3a-c) and the regionalization of nested secondary wrinkles (Figure 3d). The underlying wrinkling instability may explain this phenomenon, albeit it was mostly elaborated in physical processes.[15,

27-29]

We presumed this

wrinkling instability might be derived from the gradient distribution of Ag NPs from the surface to the bottom layer as a consequence of the self-assembly of fluorinated molecules. The aggregation of the FPPI and the silver precursor TFAAg in the top layer was confirmed by the XPS spectra (Figure 4a) on both sides of the photo-curing coating. Because the peak for Ag 3d and F 1s shows a higher intensity on the surface than on the bottom side, it is reasonable to believe that most of the TFAAg complexed with FPPI was self-assembled into the top layer, resulting in their gradient distribution. Furthermore, SEM-EDX element mapping of Ag on the top and bottom as well the etched surface (Figure S5) confirm the gradient distribution. Upon exposure to


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UV-light, TFAAg was then in-situ photo-reduced into Ag NPs by free radicals generated from FPPI. With increasing content of Ag NPs, the localization of enhanced stress tends to wrinkle the surfaces with a wrinkle-to-ridge transition, above which ridges grow at the cost of neighboring wrinkles. This evolution is explicitly presented when the content of TFAAg is lower than 1%; the ridges advance to flatten the regions for enclosing an area, and stress saturation was highlighted during the releasing step.[30] Notably, further compression was achieved by adding 1% TFAAg, such that the coating’s surface was partitioned by propagating folds that outline the border of each domain (Figure 3d). The in-dwelled wrinkles nucleate perpendicular to the existing ridges, which represents a portrayal of stress growth.[29] Previous studies have proved that nanoparticles can be regarded as significant elements to facilitate the formation of ridges, where strain releases after a stacking process occurs. [31-33]

(b) C 1s

O 1s

Top Side Bottom Side

Ag 3d5/2 368.26 eV

1200

1000

800

N 1s Ag 3d

Intensity

Ag 3d3/2

F KLL

O KLL

F 1s

(a)

Intensity

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600

400

200

0

Binding Energy (eV)

374.33 eV

380

375

370

365

Binding Energy (eV)

Figure 4. XPS spectra of the top and bottom sides of the photo-curing coating with 0.5% TFAAg and 6% FPPI (a); high solution Ag 3d region (b).

The above investigation of the surface morphology indicated that the wrinkled pattern is dramatically affected by both fluorinated polymeric photoinitiators and the silver precursor. By changing the content of FPPI and TFAAg in the formulation of the photo-curing coating (2% FPPI and 0.5% TFAAg), a different morphology of the wrinkled surface was generated. An SEM image (Figure S6) revealed the reshaping of wrinkles and the energy instability. To further verify the feasibility of our strategy, we replaced the PEGDA with two other photo-curable monomers, namely, BPE-100


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and PPGDA (Figure S7). After being irradiated by UV-light, the typical wrinkles were still revealed by the SEM images. The differences in the surface morphology might be ascribed to the different moduli of the BPE-100 and PPGDA bulk layers. These results confirm that our strategy is feasible for the fabrication of the self-wrinkled pattern on the surface of the photo-curing coating. To fully investigate the in-situ reduction of TFFAAg in the photo-curing coating, UV-vis spectra were used to trace the process (Figure 5a). The PEGMA-based UV-curable resin containing 6% FPPI and 0.5% TFAAg was coated on quartz slides and was then irradiated by UV-light under nitrogen atmosphere. There was no sign of the generation of silver nanoparticles after 10 min of irradiation. However, an absorption peak at approximately 385 nm was observed after 50 min, which was considered as a proof of the plasmon absorbance of the Ag NPs.[34] The peak increased in strength to reach a maximum value with slight shifts to 395 nm within 110 min of exposure. Presumably this shift simply reflects the growth of the nanoparticles, and the final plasmon absorbance peak at 395 nm indicates the generation of Ag NPs. A TEM image (Figure 5b) revealed that the resulting Ag NPs are well dispersed in the matrix of the coating and have a uniform size with a diameter of 15±8 nm around the mean value (inset). Such characteristics might be ascribed to the in-situ photo-reduction, which usually leads to good dispersion of nanoparticles in a polymer matrix. The formation of Ag NPs was further confirmed by the XPS spectrum of the high resolution narrow scan of the Ag 3d region (Figure 4b). The observed peaks located at 368.26 eV and 374.33 eV are assigned to the 3d3/2 and 3d5/2 transitions of the Ag0 atom, respectively.[35]


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Figure 5. Generation of Ag NPs via in-situ photo-reduction. (a) UV-vis spectra following irradiation of a coating containing 6% FPPI and 0.5% TFAAg. (b) TEM image of the cross-sectional view of a sample containing 6% FPPI and 1% TFAAg. The inset shows the size distribution of the nanoparticles. The sample was 100 nm in thickness and prepared by ultramicrotomy under controlled condition.

In comparison to the generation of Ag NPs, the kinetics of photo-crosslinking of PEGDA is much faster, as monitored by real-time Fourier-transform infrared spectroscopy (Figure S8). The characteristic peak at 1642 cm-1 assigned to the unsaturated double bond of PEGDA almost disappeared completely after irradiation of 3 min. Based on the kinetic results of photo-reduction and photopolymerization, a possible mechanism for the photo-crosslinking of PEGDA and the generation of Ag NPs is proposed in Scheme 2.[36] As an efficient hydrogen-abstraction photoinitiator, BP moieties of FPPI can be excited by irradiation of UV-light. The excited BP abstracts hydrogen from DMEA moieties of FPPI to produce active amino radicals from DMEA and ketyl radicals from BP (Scheme 2a). Because of steric hindrance and the delocalization of the unpaired electron, the ketyl radical is usually not reactive to the vinyl monomer and cannot initiate polymerization. The resulting amino radicals initiate polymerization of PEGDA to quickly form a cross-linked network because of the high activity of acrylate (Scheme 2b). After almost all the acrylate groups are polymerized, the active radicals, as well as ketyl radicals, reduce Ag ions to produce Ag NPs (Scheme 2c). Therefore, both photopolymerization and photo-reduction can


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occur during photo-curing of the coating, which is the key factor to the formation of the wrinkled pattern and the Ag NPs.

Scheme 2. The proposed mechanism for the photopolymerization and photo-reduction of Ag NPs. (a) Generation of free radicals from the photoinitiator system BP/DMEA. (b) Polymerization initiated by free radicals. (c) Photo-reduction of TFAAg by free radicals.

Surface Properties: low surface energy, self-replenishing ability, and antibiosis The wrinkled surface covered by the fluorocarbon chains and Ag NPs is expected to possess a variety of useful surface properties.[20] Complex micro- and nano-patterned surfaces in Nature, such as the lotus leaf, can result in unique functions, such as self-cleaning and superhydrophobicity. The wrinkled surface of our photo-curing coating possesses similar characteristics as the complex patterned surfaces in Nature. This observation motivated us to investigate the wettability of the wrinkled patterned surface by measuring the water contact angle (WCA) and the diiodomethane contact angle (DCA). The data from the WCA measurements and the surface energy that was calculated from the WCA and DCA are shown in Figure 6a. The increasing content of FPPI significantly enhanced the WCA values and clearly lowered the surface energy. With the content of FPPI increasing to 6%, the WCA was increased from 67o to 112o and the surface energy decreased from 42 to 21 mN/m-1. We consider the increasing content of the fluorocarbon chain to play an important role in the increase of the WCA and the decrease of the surface energy, while the ongoing roughness serves a supporting part as well.[38] Generally, the WCA is determined by the chemical components and the physical roughness of the surface. The lower surface-energy fluorocarbon chains and higher roughness can lead to a more


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hydrophobic surface and a larger WCA, which might be beneficial to self-cleaning. In addition, the generation of Ag NPs has no obvious effect on the wettability of the wrinkled surface. As shown in Figure 6b, the WCA and the surface energy exhibited almost no change with an increasing content of Ag NPs. This behavior might be explained by the fact that Ag NPs are located in the subsurface covered by fluorocarbon chains of FPPI, as was confirmed by the following plasma etching experiment. (b) 150

SE

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30

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Contact Angle (degree)

(a) 125 Contact Angle (degree)

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0 0.00

0.25

0.50

0.75

1.00

Content of TFAAg (%)

Content of FPPI (%)

Figure 6. Contact angle and surface energy of the wrinkled surface. (a) Silver-free coatings with varying content of FPPI. (b) Silver-containing coatings with varying content of TFAAg and a maintained content of 6% FPPI. The surface energy was a result of the calculation of the “Owens and Wendt” method.[37] The equation used for the calculating surface energy is: 1 2

γL1+ cos θ=2γD γD S L

1 2

+2γPS γPL

, where γ is the surface tension, and θ is the solid-liquid

contact angle. The subscripts ‘S’ and ‘L’ refer to solid and liquid, respectively. The superscripts ‘D’ and ‘P’ represent the disperse and polar component, respectively. By measuring the WCA and the DCA, we can obtain γD and γPS (γL , γD and γPL are known for water and diiodomethane), thus S L providing the surface energy γS =γD +γPS . S

Moreover, our coating with wrinkled surface is expected to possess self-replenishing ability to some extent because of the gradient distribution of the fluorinated polymer and Ag NPs in the top layer.[39] As shown in Figure 7a, the WCA of the photo-cured film of PEGDA composed of 6% FPPI and 0.5% TFAAg decreased from 112o to 78o after oxygen plasma etching for 2 seconds. The decreased WCA should be ascribed to damage of the fluorocarbon chains covering the surface


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due to plasma etching, which did not destroy the hierarchical microstructure of the wrinkled morphology. AFM images revealed that the Ag NPs lying in the subsurface were exposed to the surface after plasma etching (Figure 7b). As shown in the XPS spectrum (Figure 7c), the signal assigned to fluorine atoms disappeared almost completely, while the intensity of the Ag atoms was enhanced after plasma etching. When the damaged film was heated at 145 °C for 2 h, the WCA increased back to 113o, suggesting a self-replenishing property. The increase in the WCA of the damaged film by heating might be explained as follows: the temperature is higher than Tg for both the top layer of FPPI and the cross-linking substrate PEGDA, which can enable the subsurface fluorocarbon chains of FPPI to move into the surface again, consequently restoring the surface hydrophobicity. This process was confirmed by the XPS analysis. After heating at 145 °C for 2 h, the signal of F 1s became visible again in the XPS spectrum of the damaged film, indicating that the surface was covered by fluorocarbon chains again. In addition, the self-replenishing property was capable of restoring the original hydrophobicity with a WCA of 113° in the second cycle of the etching-heating experiment (Figure 7d). This behavior might be ascribed to the synergistic effect of the wrinkled pattern and the fluorocarbon chains, allowing for such a strong recovery.

Figure 7. Investigation of the self-replenishing ability. The sample here contains 6% FPPI and 0.5% TFAAg. (a) Schematic demonstration of the self-replenishing test and the corresponding


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images of the contact angle. (b) AFM images of the wrinkled surface after the 1st etching. The magnified image shows the hierarchical surface, where the silver nanostructures prevalently rest on the wrinkles. (c) XPS data of the 1st cycle with regard to the changes of fluorine and silver elements. (d) Test cycle of the self-replenishing property, illustrating the reversible hydrophobicity of the wrinkled surface. The results demonstrate the contact angles of the testing area where the sample sequentially underwent plasma treatment and the heating process over 3 times; in addition, the results show robust recovery of hydrophobicity after the 2nd heating.

The aggregation of Ag NPs on the wrinkled surface is expected to improve antibiosis of the resulting photo-curing coating. A series of trials were conducted for two common bacteria, Gram-negative E. coli and Gram-positive S. aureus. To fully understand the antibacterial effect, for each parallel assay, the sample containing both FPPI and TFAAg was investigated, as were the other three samples with or without TFAAg as references. Figure 8 shows the strong antibacterial activity (inhibition zone) of sample 1 in both cases of the two bacteria as a result of the high concentration of Ag NPs in the top layer. However, sample 2 behaved effectively only in the Gram-negative one, a phenomenon commonly observed in most of the inorganic antibacterial agents.[40] Additionally, the inhibition area was not observed in both samples 3 and 4, suggesting that Ag NPs are the key factor to antibiosis. The antibacterial performance of the silver-coated samples relied on the active silver ions rather than other agents. We can also conclude that the wrinkles here have a minor effect on the antibacterial property, which indicates that a single topographical pattern is not sufficient to establish a mechanical defense against microbes and a hierarchically wrinkled surface might be required.[41] Note that the antibacterial performance was improved for sample 1 in comparison to sample 2, in the case of either the Gram-positive or Gram-negative bacteria. The excellent antibacterial performance of sample 1 should be ascribed to the synergistic effect of Ag NPs and FPPI. Due to the self-assembly of the FPPI complex with TFAAg at the interface of air/monomer liquid, the enrichment of Ag NPs in the top layer is helpful for suppressing the bacteria’s growth, resulting in the higher antibacterial activity.


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Figure 8. Antibacterial trial against (a) Gram-negative E. coli and (b) Gram-positive S. aureus bacteria. The test samples involved four types of coatings, which are marked as follows: (1) monomers with 0.5% TFAAg and 6% FPPI, (2) monomers with 0.5% TFAAg, (3) monomers with 6% FPPI, and (4) monomers without TFAAg and FPPI.

Conclusion In summary, we demonstrated a one-pot bottom-up approach for the fabrication of a multifunctional photo-curing coating with a patterned surface and Ag NPs through self-wrinkling and photo-reduction in the presence of fluorinated polymeric photoinitiator (FPPI). The induced wrinkles are a consequence of the development of stress, whereas the generation of Ag NPs by in-situ reduction causes the surface to form elaborate folds. Close investigation of the XPS spectra further reveals that the wrinkled surface was spontaneously covered with FPPI and Ag NPs. In addition to providing hydrophobicity, the fluorocarbon chains also permit the Ag NPs embedded in the subsurface to improve the antibacterial performance, in the case of either Escherichia coli or Staphylococcus aureus bacteria. Moreover, by taking advantage of the gradient distribution of FPPI and the wrinkled pattern, the coatings possess a self-replenishing property in terms of hydrophobicity to some extent. It is believed that the feasibility and generality of this approach will undoubtedly find practical application in the fabrication of multifunctional coatings.


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Experimental Section Construction

of

Wrinkling

Patterned

Coating

without

TFAAgs

via

Photochemical Reaction: Glass substrates of about 1 cm across were soaked within a 3:1 mixed solution (60 ml 98% H2SO4, 20 ml H2O2, respectively) to expose the hydroxyl groups for further modification. After washing by ultrapure water, the slices were dried and immersed in toluene solution of 1% 3-(trimethoxysilyl) propyl methacrylate for 24 h to improve its compatibility with the UV-curing resins. Finally, the slices were cleaned with acetone solution and dried for the following use. A tetrahydrofuran solution of PEGDA and FPPIs in different proportions (0%, 0.5%, 1.0%, 2.0%, 6.0% w/w) was prepared and spread out over modified glass slices to obtain a series of coatings. Here by controlling mass and concentration, the obtained coatings had a thickness around 200 µm, the same as the following TFAAg-complexed coatings. By drying at 60 °C for 1 h to remove the solvent and irradiation of 365 nm UV light for 10 min under nitrogen protection yielded coatings with various morphology.

Construction of Wrinkling Patterned Coating with TFAAgs via Photochemical Reaction: A tetrahydrofuran solution of PEGDA, FPPIs (6% w/w) and TFAAgs in different proportions (0%, 0.25%, 0.5%, 0.75%, 1% w/w) was prepared and spread out over modified glass slices to obtain a series of coatings. By drying at 60 °C for 1 h to remove the solvent and irradiation of 365 nm UV light for 90 min under nitrogen protection yielded coatings with various morphology and also reduced TFAAgs.

Reduction Kinetics of Silver Nanoparticles Traced by UV-vis Spectra: A few drops of a solution containing PEGDA, FPPIs (6% w/w) and TFAAgs (0.5% w/w) was deposited on quartz slides, then dried at 60 °C for 1h to remove the solvent, and finally pre-cured for 2 min before testing. After that, UV-vis spectra was taken to follow the absorption due to generation of silver nanoparticles in a time sequence with interval time of 10 min. For the pre-curing process and subsequent reduction, nitrogen was employed to prevent the samples from oxygen inhibition.

Curing Kinetics under UV Irradiation Traced by FTIR: Small amounts of a


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solution containing 6% (w/w) FPPIs and 0.5% (w/w) TFAAgs were deposited on a silicon wafer, and then thereafter dried at 60 °C for 1 h to remove the solvent. After that, FTIR was used to follow the disappearance of double bonds in a time sequence based on default setting (scanning speed: 0.6329 cm/s; resolution: 4 cm-1). Nitrogen was employed to prevent the samples from oxygen inhibition.

Self-replenishing Test: In this test, the coating containing 6 % (w/w) FPPIs and 0.5% TFAAgs was chosen for the etching process and the subsequent heating treatment was repeated for three times. The former was operated for 2 s with controlled etching speed (5 nm/s) and the latter was done at 145 °C for 5 h.

Antibacterial Trials: The investigation of antibacterial activity was performed in Muller-Hinton medium by exposing samples 1 to two common bacterial pathogens, Gram-negative strain E. coli and Gram-positive S. aureus, with a reference to Kirby-Bauer method [42] which is well established and widely used. Before testing, the two strains were incubated in broth at 37 °C within an overnight period and transferred on the next day to a tube containing 5 mL of fresh medium with an initial OD value of 0.1 at 600 nm. The incubation was ended after the culture touched an OD value of 0.3 and the strains were finally diluted to a concentration of OD = 0.65 (1×107 CFU/mL) with a 0.9% saline solution. Besides, other samples of different kinds (2, 3 and 4) were involved and served as a reference. A total of 250 µL diluted cells was pipetted into the lawn and the Muller-Hintonagar plate was inoculated by streaking a swab repeatedly over the entire surface to ensure an even distribution of inoculum. After that, four kinds of samples (1 cm × 1 cm) were inversely loaded onto the surface with the coated-side contacting with the plate. Thereafter they were placed in an incubator at 37 °C for 18 hours and the inhibition zones were compared which provides an indication of the effectiveness of antibiotic agents. Notably, for samples without FPPIs and TFAAg, we added BPMS and DMEA with a content referencing to input radio of synthesis of FPPI.

Characterization Method: Nuclear magnetic resonance (1H-NMR) spectra of BPMS and macroinitiator in DMSO (D6) were collected by a Varian Mercury Plus 400 MHz spectrometer at room temperature. Scanning electron microscopy (SEM) was


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conducted by using a Sirion-200 electron microscope (FEI Company, USA) at 5 kV. The samples were sputtered with gold before scanning. Atomic force microscope (AFM) was employed to map the surface in contact mode, by using a scanning probe microscope (E-sweep, SEIKO Company) at room temperature. X-ray photoelectron spectrum (XPS) was performed on an ESCA LAB 250 spectrometer (VG Scientific) with Al Kα radiation (hυ = 1486.6 eV). The slice for TEM observation with a thickness of 100 nm was realized through an Ultramicrotomy (UC6-FC6, Leica Company, Germany), equipped with controllable cutting speed and a temperature control unit. TEM images of nanoparticles were obtained by transmission electron microscope (JEM-2100, JEOL Ltd., Japan) operated at an accelerating voltage of 200 kV and the sample was placed onto a copper grid before testing. For kinetics, the reducing process was traced by UV-vis spectra carried out with a UV-2550 spectrophotometer (Shimadzu, Japan) and curing was documented by using a Perkin-Elmer Spectrum 100 spectrometer equipped with an outer UV-source of 10 mW/cm2 intensity. The measurement of the contact angles was performed using a contact angle goniometer (SL200C, USA KINO Industry). The etching treatment was operated under oxygen plasma (PE 100 from Plasma Etch, Inc.) with power of 20 watt.

Acknowledgements The authors thank the National Basic Research Program (2013CB834506), the National Nature Science Foundation of China (21174085, 21274088, 51373098) and the Shanghai Key Lab of Polymer and Electrical Insulation for their financial support. X. J. is supported by the NCET-12-3050 Project.

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Supporting Information Details on material; Synthesis and characterization of BPMS and FPPI; Morphological transition; Amplitude and wavelength of wrinkled surfaces with varying TFAAg; SEM-EDS scanning of Ag on different surface; SEM image of samples with 2% FPPI and 0.5% TFAAg; SEM images of samples with alternative monomers; experiments of photo-curing kinetics. This material is available free of charge via the Internet at http://pubs.acs.org.

Table of content


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254x98mm (150 x 150 DPI)


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Simultaneous Formation of a Self-Wrinkled Surface and Silver

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