Simultaneous Formation of a Self-Wrinkled Surface and SilverNanoparticles on a Functional Photocuring CoatingHongbo Lin,† Yuanlong Wang,‡ Yanchang Gan,† Honghao Hou,† Jie Yin,† and Xuesong Jiang*,†

†School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao TongUniversity, Shanghai 200240, People’s Republic of China‡State Key Laboratory of Dairy Biotechnology, Dairy Research Institute, Bright Dairy & Food Co., Ltd., Shanghai 200436, People’sRepublic of China

*S Supporting Information

ABSTRACT: Bioinspired functional surface with micro/nanostructures are particularly attractive because of thepotential for outstanding characteristics, such as self-cleaning,self-replenishing and antibiosis. Here, we presented a facileapproach to fabricate a functional photocuring coating withboth a self-wrinkling patterned surface and incorporated silvernanoparticles (Ag NPs). Fluorinated polymeric photoinitiator(FPPI) and silver precursor (TFAAg) can self-assembletogether on the air/acrylate interface to form a top layer ofphotocuring liquid resin. Under UV irradiation, a wrinkledpattern was formed as a result of the mismatch in shrinkage caused by photopolymerization between the top layer and the bulklayer. Simultaneously, Ag NPs with sizes of 15 ± 8 nm in diameter were in situ generated in the photocuring coating through thephotoreduction 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) ofthe 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 antibiosiscapabilities as a result of the synergistic effect of the wrinkled surface covered by FPPI and Ag NPs.

■ INTRODUCTION

Multifunctional coatings with a bioinspired patterned surfaceare of great interest because of their versatile properties, e.g.,self-cleaning,1,2 antibiosis,3 or self-replenishing,4,5 as well astheir potential application in functional coatings,6−8 biomedicaldevices,9 or tissue engineering.10 Many of the plant leaves, forexample, remain dirt-free and exhibit hydrophobicity andantibiosis; these features are ascribed to the complexmorphology of their surfaces and the coating of waxy andantimicrobial compounds. Undoubtedly, hierarchical structuresplays an indispensable role in the tunable performance ofmultifunctional coatings. As a result, many techniques, such asnanoimprinting11,12 and lithography,13 have been developedwith the purpose of constructing bioinspired surfaces. Thesetechniques feature a top-down character; however, they aredifferent from the methods of natural ones and are limited bythe disadvantages of requiring a complicated physical process oran elaborate chemical synthesis. Generally, the biologicalsurface patterns are formed spontaneously from the intersystemto the surface in one step. Thus, akin to the biological method,it is of great interest to develop a simple and low-cost methodto fabricate a patterned surface to produce the targetedperformance. As one of the most common methods to generatecomplex patterns in Nature, wrinkling can fabricate a complexpattern on the surface of an organism and usually involves

compressive stress caused by the modulus mismatch betweenthe surface and the bulk of materials involved.Recently, we demonstrated a novel strategy for fabricating

micro- and nanowrinkled patterns on the surface of aphotocuring coating through self-assembly of a fluorinatedadditive and photopolymerization.14 Due to the low surfaceenergy, the fluorinated additive can form the top layer throughself-assembly at the interface of acrylate monomer liquid andair. The mismatch in shrinkage caused by the photo-polymerization between the top and bulk layers can causecompressive stress, resulting in the wrinkled pattern. This one-step approach, which we refer to as “self-wrinkling”, is similar tothe self-organizing method in Nature that used to generate awrinkled surface. Moreover, the combination of the complexwrinkled pattern and the low surface energy of the fluorinatedadditive provides the resulting wrinkled surface with a self-cleaning function.By taking advantage of this self-wrinkling process, we here

demonstrated a multifunctional photocuring coating with thesimultaneous formation of a wrinkled surface and silvernanoparticles. The entire strategy is illustrated in Scheme 1.

Received: June 24, 2015Revised: October 8, 2015Published: October 15, 2015

Article

pubs.acs.org/Langmuir

© 2015 American Chemical Society 11800 DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

Polystyrene grafted with fluorocarbon chains, benzophenoneand amino moieties in the side-chain (FPPI) can be regarded asa photoinitiator and the fluorinated additives that self-assembleat 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 photopolymerizationbetween the top and bulk layers causes compressive stress,resulting in the formation of a wrinkled pattern. Simulta-neously, the Ag+ of TFAAg is reduced to Ag0 by the freeradicals produced from FPS-BPA to form silver nanoparticles(Ag NPs). Due to aggregation of the fluorinated polymerchains and Ag NPs in the top layer, the resulting wrinkledsurface possessed versatile functionalities, such as low surfaceenergy, self-replenishing ability, and antibiosis.

1. RESULTS AND DISCUSSIONFormation of the Self-Wrinkled Pattern. Fluorinated

polymeric photoinitiator (FPPI) is the key component in our

strategy, whose structure is illustrated in Scheme 1. FPPI wassynthesized through free radical copolymerization, and thedetailed synthesis and characterization can be found in theSupporting Information. We considered various physical andchemical factors in designing the PS-based fluorinatedpolymeric photoinitiator. Due to the lower surface energyand higher modulus in comparison to most polyacrylates, PSwas chosen as the backbone to impart excellent mechanical andsurface properties to the resulting coating. Fluorocarbon chainswere introduced to provide a low surface energy for thepolymer chain, which is the key factor for the self-assembly ofthe polymeric photoinitiator in the top layer. Benzophenone(BP) and tertiary amine were chosen as the photoinitiator andthe hydrogen donor, respectively. Due to the high efficiencyand low cost, BP is widely used as a hydrogen-abstractionphotoinitiator in the presence of a co-initiator amine.Furthermore, amino moieties grafted to FPPI can complex

Scheme 1. Strategy to Produce a Self-Wrinkled Pattern and Silver Nanoparticles in a Photocuring Coatinga

a(a) Chemical structure of the components for the photocuring coating; (b) Formulation of the self-assembly strategy to prepare a silver-containedcoating marked with a wrinkled pattern.

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.

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11801

with silver atoms, which promotes the aggregation andphotoreduction of the silver precursor in the top layer.To understand the generation of the wrinkled pattern, the

photocurable mixtures without TFAAg were coated on a glasssubstrate and then irradiated by UV-light under an atmosphereof nitrogen gas. The surface morphology of the resultingphotocuring coating was observed using scanning electronmicroscope (SEM) and atomic force microscope (AFM).Figure 1 presents typical wrinkled patterns, indicating thefeasibility of our strategy. The characteristic wavelength (λ) andamplitude (A) of the wrinkled morphology increased with theincreasing concentration of FPPI (Figure 1 and Figure 2). By

decreasing the air/liquid interface energy, FPPI can migrate tothe surface and self-assemble into the top layer. Because thevolumetric shrinkage of the photocurable resin is proportionalto the content of unsaturated double bonds CC, thephotopolymerization-induced shrinkage of the top layer richin FPPI should be less than that of the bulk layer ofpoly(ethylene glycol) diacrylate (PEGDA). Therefore, themismatch in shrinkage creates a compressive stress that triggersdeformation, leading to the formation of the wrinkled pattern.Because of the high modulus of the PS backbone of FPPI, thetop layer is stiffer than the bulk layer of pure PEGDA afterphotocuring. This system is similar to those typical bilayermodels in which the formation of wrinkles is based on axialcompression of rigid skin resting on a relatively elasticfoundation.15

As shown in Figure 1 and Figure 2, the λ and A of thewrinkled morphology increased with increasing concentrationof FPPI, suggesting that the morphology of the wrinkledpattern can be tunable. For a typical bilayer system, aspredicated by the linear buckling theory,16−18 the λ and Adepend on the thickness of the top hard layer and themechanical properties of the top and bulk (bottom) layers. Theequation for a bilayer system is19

λ πεε

= ̅̅

= −⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟t

EE

A t23

1hh

s

1/3

h0

c

1/2

(1)

where E̅ = E/1 − ν2 is the plane-strain modulus, E is theYoung’s modulus, ν is the Poisson ratio, and t is the thickness.The ε0 is defined as the applied strain and εc as the criticalstrain. The subscripts “h’ and ‘s” refer to the top layer and thesubstrate, respectively.In our system, it is expected that increasing the concentration

of FPPI leads to a greater thickness and modulus of the toplayer, resulting in an increasing wavelength and amplitude.Although the linear theory is applicable to discrete layers andnot to up−down with a gradual change in properties, it didprovide useful insights into our system. According to eq 1, it isexpected that the thickness of the top layer is proportional tothe content of FPPI and can be expressed as follows:

ρ ρρρ

∼ ∼ ∼t hVV

hM M

h C/h 0f

b0

f

f

b

b0

b

fp

(2)

where h0, V, M, ρ, and Cp represent the thickness, volume,mass, density and mass ratio, respectively, of the sample. Thesubscripts “f’ and ‘b” refer to the FPPI layer and the underlyingbulk layer, respectively. Rewriting eq 1 with eq 2 yields

λ πρρ

εε

ρρ

∼ ̅̅

∼

∼ − ∼

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜

⎞⎠⎟

EE

h C k C

A h C k C

23

1

h

s

1/3

0b

fp 1 p

0

c

1/2

0b

fp 2 p

(3)

In eq 3, the parameter k is expected to be a constant,considering the same system. Thus, it is expected that thewrinkling characteristics in our system are linearly linked to themass ratio of FPPI. However, as shown in Figure 2, the

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

Figure 3. Simultaneous formation of the wrinkled pattern and Ag NPs: SEM (top) and AFM (bottom) images of wrinkled surfaces with differentcontents 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.

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11802

experimental data for both wavelength and amplitude do notfollow a strictly linear trend, which could be ascribed to thegradient distribution of FPPI. Surface instability analysis revealsthat a system with gradationally varying mechanical propertiesmay have an amplitude with an exponential trend, which couldbe considered to be a result of the dominant effect of waveamplification with film thickening.20,21 In other words, theobserved nonlinear response agrees with the instability analysis.Simultaneous Formation of the Wrinkled Pattern and

Silver Nanoparticles. Functional nanoparticles are widelyused in constructing materials with different properties, whichcan be introduced through a bottom-up approach of self-assembly.22−24 In our case, silver nanoparticles were introducedto the surface of the photocuring coating through in situphotoreduction and self-assembly of TFAAg with the help ofFPPI. These particles played an important role in disorderingstress 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 withdifferent ratios of TFAAg is revealed in Figure 3, in which thecontent of FPPI is maintained at 6%. We observed that thewrinkles localize into several ridges at a low content of 0.1%TFAAg (Figure S3). We consider this transition to be the resultof the instability induced by the nanoparticles, which leads tofurther compression; thus, ridges appear when the compressionis beyond a third of its initial wrinkle wavelength.25 Furtherconfinement by increasing TFAAg led to prevalent ridges withgrowing amplitude, while neighboring wrinkles decrease inheight, which eventually resulted in much smaller λ and A whencompared 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 adjacentwrinkles are accommodated with short wavelength and smallamplitude.25,26

In fact, what really attracted our attention were the disparatewrinkles, involving the cellular shape (Figure 3a−c) and theregionalization of nested secondary wrinkles (Figure 3d). Theunderlying wrinkling instability may explain this phenomenon,albeit it was mostly elaborated in physical processes.15,27−29 Wepresumed this wrinkling instability might be derived from thegradient distribution of Ag NPs from the surface to the bottomlayer as a consequence of the self-assembly of fluorinatedmolecules. The aggregation of the FPPI and the silver precursorTFAAg in the top layer was confirmed by the XPS spectra(Figure 4a) on both sides of the photocuring coating. Becausethe peak for Ag 3d and F 1s shows a higher intensity on thesurface than on the bottom side, it is reasonable to believe thatmost of the TFAAg complexed with FPPI was self-assembledinto the top layer, resulting in their gradient distribution.Furthermore, SEM with energy dispersive X-ray spectroscopy(SEM-EDX) element mapping of Ag on the top and bottom aswell the etched surface (Figure S5) confirm the gradientdistribution. Upon exposure to UV-light, TFAAg was then insitu photoreduced into Ag NPs by free radicals generated fromFPPI. With increasing content of Ag NPs, the localization ofenhanced stress tends to wrinkle the surfaces with a wrinkle-to-ridge transition, above which ridges grow at the cost ofneighboring wrinkles. This evolution is explicitly presentedwhen the content of TFAAg is lower than 1%; the ridgesadvance to flatten the regions for enclosing an area, and stresssaturation was highlighted during the releasing step.30 Notably,further compression was achieved by adding 1% TFAAg, such

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

Figure 5. Generation of Ag NPs via in situ photoreduction. (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 thenanoparticles. The sample was 100 nm in thickness and prepared by ultramicrotomy under controlled condition.

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11803

that the coating’s surface was partitioned by propagating foldsthat 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 studieshave proved that nanoparticles can be regarded as significantelements to facilitate the formation of ridges, where strainreleases after a stacking process occurs.31−33

The above investigation of the surface morphology indicatedthat the wrinkled pattern is dramatically affected by bothfluorinated polymeric photoinitiators and the silver precursor.By changing the content of FPPI and TFAAg in theformulation of the photocuring coating (2% FPPI and 0.5%TFAAg), a different morphology of the wrinkled surface wasgenerated. An SEM image (Figure S6) revealed the reshapingof wrinkles and the energy instability. To further verify thefeasibility of our strategy, we replaced the PEGDA with twoother photocurable monomers, namely, BPE-100 and poly-(propylene glycol) diacrylate (PPGDA) (Figure S7). Afterbeing irradiated by UV-light, the typical wrinkles were stillrevealed by the SEM images. The differences in the surfacemorphology might be ascribed to the different moduli of theBPE-100 and PPGDA bulk layers. These results confirm thatour strategy is feasible for the fabrication of the self-wrinkledpattern on the surface of the photocuring coating.To fully investigate the in situ reduction of TFFAAg in the

photocuring coating, UV−vis spectra were used to trace theprocess (Figure 5a). The PEGMA-based UV-curable resincontaining 6% FPPI and 0.5% TFAAg was coated on quartzslides and was then irradiated by UV-light under nitrogenatmosphere. There was no sign of the generation of silver

nanoparticles after 10 min of irradiation. However, anabsorption peak at approximately 385 nm was observed after50 min, which was considered as proof of the plasmonabsorbance of the Ag NPs.34 The peak increased in strength toreach a maximum value with slight shifts to 395 nm within 110min of exposure. Presumably, this shift simply reflects thegrowth of the nanoparticles, and the final plasmon absorbancepeak at 395 nm indicates the generation of Ag NPs. A TEMimage (Figure 5b) revealed that the resulting Ag NPs are welldispersed in the matrix of the coating and have a uniform sizewith a diameter of 15 ± 8 nm around the mean value (inset).Such characteristics might be ascribed to the in situphotoreduction, which usually leads to good dispersion ofnanoparticles in a polymer matrix. The formation of Ag NPswas further confirmed by the XPS spectrum of the highresolution narrow scan of the Ag 3d region (Figure 4b). Theobserved peaks located at 368.26 and 374.33 eV are assigned tothe 3d3/2 and 3d5/2 transitions of the Ag0 atom, respectively.35

In comparison to the generation of Ag NPs, the kinetics ofphoto-cross-linking of PEGDA is much faster, as monitored byreal-time Fourier-transform infrared spectroscopy (Figure S8).The characteristic peak at 1642 cm−1 assigned to theunsaturated double bond of PEGDA almost disappearedcompletely after irradiation of 3 min. Based on the kineticresults of photoreduction and photopolymerization, a possiblemechanism for the photo-cross-linking of PEGDA and thegeneration of Ag NPs is proposed in Scheme 2.36 As an efficienthydrogen-abstraction photoinitiator, BP moieties of FPPI canbe excited by irradiation of UV-light. The excited BP abstractshydrogen from DMEA moieties of FPPI to produce active

Scheme 2. Proposed Mechanism for the Photopolymerization and Photoreduction of Ag NPsa

a(a) Generation of free radicals from the photoinitiator system BP/DMEA. (b) Polymerization initiated by free radicals. (c) Photoreduction ofTFAAg by free radicals.

Figure 6. Contact angle and surface energy of the wrinkled surface. (a) Silver-free coatings with varying content of FPPI. (b) Silver-containingcoatings with varying content of TFAAg and a maintained content of 6% FPPI. The surface energy was a result of the calculation of the “Owens andWendt” method.37 The equation used for the calculating surface energy is γL(1 + cos θ) = 2(γS

DγLD)1/2 + 2(γS

PγLP)1/2, 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 thedisperse and polar component, respectively. By measuring the WCA and the DCA, we can obtain γS

D and γSP (γL, γL

D, and γLP are known for water and

diiodomethane), thus providing the surface energy γS = γSD + γS

P.

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11804

amino radicals from DMEA and ketyl radicals from BP(Scheme 2a). Because of steric hindrance and the delocalizationof the unpaired electron, the ketyl radical is usually not reactiveto the vinyl monomer and cannot initiate polymerization. Theresulting amino radicals initiate polymerization of PEGDA toquickly form a cross-linked network because of the high activityof acrylate (Scheme 2b). After almost all the acrylate groups arepolymerized, the active radicals, as well as ketyl radicals, reduceAg ions to produce Ag NPs (Scheme 2c). Therefore, bothphotopolymerization and photoreduction can occur duringphotocuring of the coating, which is the key factor to theformation of the wrinkled pattern and the Ag NPs.Surface Properties: Low Surface Energy, Self-Replen-

ishing Ability, and Antibiosis. The wrinkled surface coveredby the fluorocarbon chains and Ag NPs is expected to possess avariety of useful surface properties.20 Complex micro- andnanopatterned surfaces in Nature, such as the lotus leaf, canresult in unique functions, such as self-cleaning and super-hydrophobicity. The wrinkled surface of our photocuringcoating possesses similar characteristics as the complexpatterned surfaces in Nature. This observation motivated usto investigate the wettability of the wrinkled patterned surfaceby measuring the water contact angle (WCA) and thediiodomethane contact angle (DCA). The data from theWCA measurements and the surface energy that was calculatedfrom the WCA and DCA are shown in Figure 6a. Theincreasing content of FPPI significantly enhanced the WCAvalues and clearly lowered the surface energy. With the contentof FPPI increasing to 6%, the WCA was increased from 67° to112° and the surface energy decreased from 42 to 21 mN/m−1.We consider the increasing content of the fluorocarbon chainto play an important role in the increase of the WCA and thedecrease of the surface energy, while the ongoing roughnessserves a supporting part as well.38 Generally, the WCA isdetermined by the chemical components and the physicalroughness of the surface. The lower surface-energy fluorocar-bon chains and higher roughness can lead to a more

hydrophobic surface and a larger WCA, which might bebeneficial to self-cleaning. In addition, the generation of Ag NPshas no obvious effect on the wettability of the wrinkled surface.As shown in Figure 6b, the WCA and the surface energyexhibited almost no change with an increasing content of AgNPs. This behavior might be explained by the fact that Ag NPsare located in the subsurface covered by fluorocarbon chains ofFPPI, as was confirmed by the following plasma etchingexperiment.Moreover, our coating with wrinkled surface is expected to

possess self-replenishing ability to some extent because of thegradient distribution of the fluorinated polymer and Ag NPs inthe top layer.39 As shown in Figure 7a, the WCA of thephotocured film of PEGDA composed of 6% FPPI and 0.5%TFAAg decreased from 112° to 78° after oxygen plasmaetching for 2 s. The decreased WCA should be ascribed todamage of the fluorocarbon chains covering the surface due toplasma etching, which did not destroy the hierarchicalmicrostructure of the wrinkled morphology. AFM imagesrevealed that the Ag NPs lying in the subsurface were exposedto the surface after plasma etching (Figure 7b). As shown in theXPS spectrum (Figure 7c), the signal assigned to fluorine atomsdisappeared almost completely, while the intensity of the Agatoms was enhanced after plasma etching. When the damagedfilm was heated at 145 °C for 2 h, the WCA increased back to113°, suggesting a self-replenishing property. The increase inthe WCA of the damaged film by heating might be explained asfollows: the temperature is higher than Tg for both the top layerof FPPI and the cross-linking substrate PEGDA, which canenable the subsurface fluorocarbon chains of FPPI to move intothe surface again, consequently restoring the surface hydro-phobicity. This process was confirmed by the XPS analysis.After heating at 145 °C for 2 h, the signal of F 1s became visibleagain in the XPS spectrum of the damaged film, indicating thatthe surface was covered by fluorocarbon chains again. Inaddition, the self-replenishing property was capable of restoringthe original hydrophobicity with a WCA of 113° in the second

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

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11805

cycle of the etching-heating experiment (Figure 7d). Thisbehavior might be ascribed to the synergistic effect of thewrinkled pattern and the fluorocarbon chains, allowing for sucha strong recovery.The aggregation of Ag NPs on the wrinkled surface is

expected to improve antibiosis of the resulting photocuringcoating. A series of trials were conducted for two commonbacteria, Gram-negative Escherichia coli and Gram-positiveStaphylococcus aureus. To fully understand the antibacterialeffect, for each parallel assay, the sample containing both FPPIand TFAAg was investigated, as were the other three sampleswith or without TFAAg as references. Figure 8 shows thestrong antibacterial activity (inhibition zone) of sample 1 inboth cases of the two bacteria as a result of the highconcentration of Ag NPs in the top layer. However, sample 2behaved effectively only in the Gram-negative one, aphenomenon commonly observed in most of the inorganicantibacterial agents.40 Additionally, the inhibition area was notobserved in both samples 3 and 4, suggesting that Ag NPs arethe key factor to antibiosis. The antibacterial performance ofthe silver-coated samples relied on the active silver ions ratherthan other agents. We can also conclude that the wrinkles herehave a minor effect on the antibacterial property, whichindicates that a single topographical pattern is not sufficient toestablish a mechanical defense against microbes, and ahierarchically wrinkled surface might be required.41 Note thatthe antibacterial performance was improved for sample 1 incomparison to sample 2, in the case of either the Gram-positiveor Gram-negative bacteria. The excellent antibacterial perform-ance of sample 1 should be ascribed to the synergistic effect ofAg NPs and FPPI. Due to the self-assembly of the FPPIcomplex with TFAAg at the interface of air/monomer liquid,the enrichment of Ag NPs in the top layer is helpful forsuppressing the bacteria’s growth, resulting in the higherantibacterial activity.

■ CONCLUSIONIn summary, we demonstrated a one-pot bottom-up approachfor the fabrication of a multifunctional photocuring coatingwith a patterned surface and Ag NPs through self-wrinkling andphotoreduction in the presence of FPPI. The induced wrinklesare a consequence of the development of stress, whereas thegeneration of Ag NPs by in situ reduction causes the surface toform elaborate folds. Close investigation of the XPS spectra

further reveals that the wrinkled surface was spontaneouslycovered with FPPI and Ag NPs. In addition to providinghydrophobicity, the fluorocarbon chains also permit the AgNPs embedded in the subsurface to improve the antibacterialperformance, in the case of either E. coli or S. aureus bacteria.Moreover, by taking advantage of the gradient distribution ofFPPI and the wrinkled pattern, the coatings possess a self-replenishing property in terms of hydrophobicity to someextent. It is believed that the feasibility and generality of thisapproach will undoubtedly find practical application in thefabrication of multifunctional coatings.

■ EXPERIMENTAL SECTIONConstruction of Wrinkling Patterned Coating without

TFAAgs via Photochemical Reaction. Glass substrates of about 1cm across were soaked within a 3:1 mixed solution (60 mL 98%H2SO4, 20 mL H2O2, respectively) to expose the hydroxyl groups forfurther modification. After washing by ultrapure water, the slices weredried 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 anddried for the following use.

A tetrahydrofuran solution of PEGDA and FPPIs in differentproportions (0%, 0.5%, 1.0%, 2.0%, 6.0% w/w) was prepared andspread out over modified glass slices to obtain a series of coatings.Here by controlling mass and concentration, the obtained coatings hada thickness around 200 μm, the same as the following TFAAg-complexed coatings. By drying at 60 °C for 1 h to remove the solventand irradiation of 365 nm UV light for 10 min under nitrogenprotection yielded coatings with various morphology.

Construction of Wrinkling Patterned Coating with TFAAgsvia Photochemical Reaction. A tetrahydrofuran solution ofPEGDA, FPPIs (6% w/w) and TFAAgs in different proportions(0%, 0.25%, 0.5%, 0.75%, 1% w/w) was prepared and spread out overmodified glass slices to obtain a series of coatings. By drying at 60 °Cfor 1 h to remove the solvent and irradiation of 365 nm UV light for90 min under nitrogen protection yielded coatings with variousmorphology and also reduced TFAAgs.

Reduction Kinetics of Silver Nanoparticles Traced by UV−visSpectra. A few drops of a solution containing PEGDA, FPPIs (6% w/w) and TFAAgs (0.5% w/w) was deposited on quartz slides, thendried at 60 °C for 1 h to remove the solvent, and finally precured for 2min before testing. After that, UV−vis spectra was taken to follow theabsorption due to generation of silver nanoparticles in a time sequencewith interval time of 10 min. For the precuring process and subsequentreduction, nitrogen was employed to prevent the samples from oxygeninhibition.

Figure 8. Antibacterial trial against (a) Gram-negative E. coli and (b) Gram-positive S. aureus bacteria. The test samples involved four types ofcoatings, 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.

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11806

Curing Kinetics under UV Irradiation Traced by FTIR. Smallamounts of a solution containing 6% (w/w) FPPIs and 0.5% (w/w)TFAAgs were deposited on a silicon wafer, and then thereafter dried at60 °C for 1 h to remove the solvent. After that, FTIR was used tofollow the disappearance of double bonds in a time sequence based ondefault setting (scanning speed: 0.6329 cm/s; resolution: 4 cm−1).Nitrogen was employed to prevent the samples from oxygeninhibition.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 three times. Theformer 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 twocommon bacterial pathogens, Gram-negative strain E. coli and Gram-positive S. aureus, with a reference to the Kirby−Bauer method,42

which is well established and widely used. Before testing, the twostrains were incubated in broth at 37 °C within an overnight periodand transferred on the next day to a tube containing 5 mL of freshmedium with an initial OD value of 0.1 at 600 nm. The incubation wasended after the culture touched an OD value of 0.3 and the strainswere finally diluted to a concentration of OD = 0.65 (1 × 107 CFU/mL) with a 0.9% saline solution. Besides, other samples of differentkinds (2, 3, and 4) were involved and served as a reference. A total of250 μL diluted cells was pipetted into the lawn and the Muller-Hintonagar plate was inoculated by streaking a swab repeatedly overthe entire surface to ensure an even distribution of inoculum. Afterthat, four kinds of samples (1 cm × 1 cm) were inversely loaded ontothe surface with the coated-side contacting with the plate. Thereafterthey were placed in an incubator at 37 °C for 18 h and the inhibitionzones were compared which provides an indication of the effectivenessof antibiotic agents. Notably, for samples without FPPIs and TFAAg,we added BPMS and DMEA with a content referencing the inputradio of synthesis of FPPI.Characterization Method. Nuclear magnetic resonance (1H

NMR) spectra of BPMS and macroinitiator in DMSO (D6) werecollected by a Varian Mercury Plus 400 MHz spectrometer at roomtemperature. SEM was conducted by using a Sirion-200 electronmicroscope (FEI Company, USA) at 5 kV. The samples weresputtered with gold before scanning. AFM was employed to map thesurface in contact mode, by using a scanning probe microscope (E-sweep, SEIKO Company) at room temperature. X-ray photoelectronspectrum (XPS) was performed on an ESCA LAB 250 spectrometer(VG Scientific) with Al Kα radiation (hυ = 1486.6 eV). The slice forTEM observation with a thickness of 100 nm was realized through anUltramicrotomy (UC6-FC6, Leica Company, Germany), equippedwith controllable cutting speed and a temperature control unit. TEMimages of nanoparticles were obtained by transmission electronmicroscope (JEM-2100, JEOL Ltd., Japan) operated at an acceleratingvoltage of 200 kV, and the sample was placed onto a copper gridbefore testing. For kinetics, the reducing process was traced by UV−visspectra carried out with a UV-2550 spectrophotometer (Shimadzu,Japan), and curing was documented by using a PerkinElmer Spectrum100 spectrometer equipped with an outer UV-source of 10 mW/cm2

intensity. The measurement of the contact angles was performed usinga contact angle goniometer (SL200C, USA KINO Industry). Theetching treatment was operated under oxygen plasma (PE 100 fromPlasma Etch, Inc.) with power of 20 W.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.lang-muir.5b03484.

Details on material; synthesis and characterization ofBPMS and FPPI; morphological transition; amplitudeand 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 photocuring kinetics (PDF)

■ AUTHOR INFORMATIONNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors thank the National Basic Research Program(2013CB834506), the National Nature Science Foundation ofChina (21174085, 21274088, 51373098) and the Shanghai KeyLab of Polymer and Electrical Insulation for their financialsupport. X. J. is supported by the NCET-12-3050 Project.

■ REFERENCES(1) Liu, H. L.; Zhang, P. C.; Liu, M. J.; Wang, S. T.; Jiang, L.Organogel-based Thin Films for Self-Cleaning on Various Surfaces.Adv. Mater. 2013, 25, 4477−4481.(2) Kamegawa, T.; Shimizu, Y.; Yamashita, H. SuperhydrophobicSurfaces with Photocatalytic Self-Cleaning Properties by Nano-composite Coating of TiO2 and Polytetrafluoroethylene. Adv. Mater.2012, 24, 3697−3700.(3) Kholmanov, I. N.; Stoller, M. D.; Edgeworth, J.; Lee, W. H.; Li,H. F.; Lee, J.; Barnhart, C.; Potts, J. R.; Piner, R.; Akinwande, D.;Barrick, J. E.; Ruoff, R. S. Nanostructured Hybrid TransparentConductive Films with Antibacterial Properties. ACS Nano 2012, 6,5157−5163.(4) Zhang, M. M.; Xu, D. H.; Yan, X. Z.; Chen, J. Z.; Dong, S. Y.;Zheng, B.; Huang, F. H. Self-Healing Supramolecular Gels Formed byCrown Ether Based Host-Guest Interactions. Angew. Chem. 2012, 124,7117−7121.(5) Zhu, D. D.; Lu, X. M.; Lu, Q. H. Electrically Conductive PEDOTCoating with Self-Healing Superhydrophobicity. Langmuir 2014, 30,4671−4677.(6) Guldin, S.; Kohn, P.; Stefik, M.; Song, J.; Divitini, G.; Ecarla, F.;Ducati, C.; Wiesner, U.; Steiner, U. Self-Cleaning AntireflectiveOptical Coatings. Nano Lett. 2013, 13, 5329−5335.(7) Zhu, J.; Hsu, C.-M.; Yu, Z. F.; Fan, S. H.; Cui, Y. NanodomeSolar Cells with Efficient Light Management and Self-Cleaning. NanoLett. 2010, 10, 1979−1984.(8) Xie, J. X.; Han, X.; Zong, C. Y.; Ji, H. P.; Lu, C. H. Large-AreaPatterning of Polyaniline Film Based on in Situ Self-Wrinkling and ItsReversible Doping/Dedoping Tunability. Macromolecules 2015, 48,663−671.(9) Vasilev, K.; Cook, J.; Griesser, H. Antibacterial surfaces forbiomedical devices. Expert Rev. Med. Devices 2009, 6, 553−567.(10) Gelain, F.; Silva, D.; Caprini, A.; Taraballi, F.; Natalello, A.;Villa, O.; Nam, K. T.; Zuckermann, R. N.; Doglia, S. M.; Vescovi, A.BMHP1-Derived Self-Assembling Peptides: Hierarchically AssembledStructures with Self-Healing Propensity and Potential for TissueEngineering Applications. ACS Nano 2011, 5, 1845−1859.(11) Guo, L. J. Nanoimprint Lithography: Methods and MaterialRequirements. Adv. Mater. 2007, 19, 495−513.(12) Kitagawa, T.; Nakamura, N.; Kawata, H.; Hirai, Y. A noveltemplate-release method for low-defect nanoimprint lithography.Microelectron. Eng. 2014, 123, 65−72.(13) Suh, K.-Y.; Park, M. C.; Kim, P. Capillary Force Lithography: AVersatile Tool for Structured Biomaterials Interface towards Cell andTissue Engineering. Adv. Funct. Mater. 2009, 19, 2699−2712.(14) Gan, Y. C.; Jiang, X. S.; Yin, J. Self-Wrinkling Patterned Surfaceof Photocuring Coating Induced by the Fluorinated POSS ContainingThiol Groups (F-POSS-SH) as the Reactive Nanoadditive. Macro-molecules 2012, 45, 7520−7526.(15) Cerda, E.; Mahadevan, L. Geometry and Physics of Wrinkling.Phys. Rev. Lett. 2003, 90, 074302.

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11807

(16) Yang, C.; Khare, K.; Lin, P. C. Harnessing Surface WrinklePatterns in Soft Matter. Adv. Funct. Mater. 2010, 20, 2550−2564.(17) Lin, P. C.; Yang, S. Spontaneous formation of one-dimensionalripples in transit to highly ordered two-dimensional herringbonestructures through sequential and unequal biaxial mechanicalstretching. Appl. Phys. Lett. 2007, 90, 241903.(18) Bowden, N.; Huck, W. T. S.; Paul, K. E.; Whitesides, G. M. Thecontrolled formation of ordered, sinusoidal structures by plasmaoxidation of an elastomeric polymer. Appl. Phys. Lett. 1999, 75, 2557−2559.(19) Stafford, C. M.; Harrison, C.; Beers, K. L.; Karim, A.; Amis, E. J.;Vanlandingham, M. R.; Kim, H. C.; Volksen, W.; Miller, R. D.;Simonyi, E. E. A buckling-based metrology for measuring the elasticmoduli of polymeric thin films. Nat. Mater. 2004, 3, 545−550.(20) Kim, Y. H.; Lee, Y. M.; Lee, J. Y.; Ko, M. J.; Yoo, P. J.Hierarchical Nanoflake Surface Driven by Spontaneous Wrinkling ofPolyelectrolyte/Metal Complexed Films. ACS Nano 2012, 6, 1082−1093.(21) Lee, D.; Triantafyllidis, N.; Barber, J. R.; Thouless, M. D.Surface Instablity of an Elastic Half Space with Material PropertiesVarying with Depth. J. Mech. Phys. Solids 2008, 56, 858−868.(22) Tsai, H.-J.; Lee, Y.-L. Facile Method to Fabricate Raspberry-likeParticulate Films for Superhydrophobic Surfaces. Langmuir 2007, 23,12687−12692.(23) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. BiomedicalApplications of Layer-by-Layer Assembly: From Biomimetics to TissueEngineering. Adv. Mater. 2006, 18, 3203−3224.(24) Kao, J.; Thorkelsson, K.; Bai, P.; Rancatore, B. J.; Xu, T. Towardfunctional nanocomposites: taking the best of nanoparticles, polymers,and small molecules. Chem. Soc. Rev. 2013, 42, 2654−2678.(25) Pocivavsek, L.; Dellsy, R.; Kern, A.; Johnson, S.; Lin, B. H.; Lee,K. Y. C.; Cerda, E. Stress and Fold Localization in Thin ElasticMembranes. Science 2008, 320, 912−916.(26) Ebata, Y.; Croll, A. B.; Crosby, A. J. Wrinkling and strainlocalizations in polymer thin films. Soft Matter 2012, 8, 9086−9091.(27) Biot, M. A. Folding instability of a layered viscoelastic mediumunder compression. Proc. R. Soc. London, Ser. A 1957, 242, 444−454.(28) Khang, D.-Y.; Rogers, J. A.; Lee, H. H. Mechanical Buckling:Mechanics, Metrology, and Stretchable Electronics. Adv. Funct. Mater.2009, 19, 1526−1536.(29) Kim, P.; Abkarian, M.; Stone, H. A. Hierarchical folding ofelastic membranes under biaxial compressive stress. Nat. Mater. 2011,10, 952−957.(30) Chen, C.-M.; Yang, S. Wrinkling instabilities in polymer filmsand their applications. Polym. Int. 2012, 61, 1041−1047.(31) Jiang, C. Y.; Singamaneni, S.; Merrick, E.; Tsukruk, V. V.Complex Buckling Instability Patterns of Nanomembranes withEncapsulated Gold Nanoparticle Arrays. Nano Lett. 2006, 6, 2254−2259.(32) Leahy, B. D.; Pocivavsek, L.; Meron, M.; Lam, K. L.; Salas, D.;Viccaro, P. J.; Lee, K. Y. C.; Lin, B. H. Geometric Stability and ElasticResponse of a Supported Nanoparticle Film. Phys. Rev. Lett. 2010, 105,058301.(33) Schultz, D. G.; Lin, X. M.; Li, D. X.; Gebhardt, J.; Meron, M.;Viccaro, P. J.; Lin, B. H. Structure, Wrinkling, and Reversibility ofLangmuir Monolayers of Gold Nanoparticles. J. Phys. Chem. B 2006,110, 24522−24529.(34) Henglein, A. Colloidal Silver Nanoparticles: PhotochemicalPreparation and Interaction with O2, CCl4, and Some Metal Ions.Chem. Mater. 1998, 10, 444−450.(35) Kaspar, T. C.; Droubay, T.; Chambers, S. A.; Bagus, P. S.Spectroscopic Evidence for Ag(III) in Highly Oxidized Silver Films byX-ray Photoelectron Spectroscopy. J. Phys. Chem. C 2010, 114,21562−21571.(36) Maretti, L.; Billone, P. S.; Liu, Y.; Scaiano, J. C. FacilePhotochemical Synthesis and Characterization of Highly FluorescentSilver Nanoparticles. J. Am. Chem. Soc. 2009, 131, 13972−13980.(37) Owens, D. K.; Wendt, R. C. Estimation of the Surface FreeEnergy of Polymers. J. Appl. Polym. Sci. 1969, 13, 1741−1747.

(38) Drame, A.; Darmanin, T.; Dieng, S. Y.; Taffin de Givenchy, E.;Guittard, F. Superhydrophobic and oleophobic surfaces containingwrinkles and nanoparticles of PEDOT with two short fluorinatedchains. RSC Adv. 2014, 4, 10935−10943.(39) Dikic,́ T.; Ming, W.; van Benthem, R. A. T. M.; Esteves, A. C.C.; de With, G. Self-Replenishing Surfaces. Adv. Mater. 2012, 24,3701−3704.(40) Lemire, J. A.; Harrison, J. J.; Turner, R. J. Antimicrobial activityof metals: mechanisms, molecular targets and application. Nat. Rev.Microbiol. 2013, 11, 371−384.(41) Efimenko, K.; Rackaitis, M.; Manias, E.; Vaziri, A.; Mahadevan,L.; Genzer, J. Nested Self-Similar Wrinkling Patterns in Skins. Nat.Mater. 2005, 4, 293−297.(42) Bauer, A. W.; Kirby, W. M. M.; Sherris, J. C.; Turck, M.Antibiotic Susceptibility Testing by a Standardized Single DiskMethod. Am. J. Clin. Pathol. 1966, 45, 493−496.

Langmuir Article

DOI: 10.1021/acs.langmuir.5b03484Langmuir 2015, 31, 11800−11808

11808