Letter pubs.acs.org/NanoLett
Reactive Superhydrophobic Surface and Its Photoinduced Disulfideene and Thiol-ene (Bio)functionalization Junsheng Li,†,‡,§ Linxian Li,†,∥ Xin Du,†,‡ Wenqian Feng,†,∥ Alexander Welle,⊥,# Oliver Trapp,∥ Michael Grunze,‡,⊥ Michael Hirtz,#,∇ and Pavel A. Levkin*,†,‡,∥ †
Institute of Toxicology and Genetics (ITG), Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany Department of Applied Physical Chemistry, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany § School of Chemistry, Chemical Engineering, and Life Sciences, Wuhan University of Technology, 430070 Wuhan, People’s Republic of China ∥ Institut für Organische Chemie, Ruprecht-Karls-Universität Heidelberg, 69120 Heidelberg, Germany ⊥ Institute of Functional Interfaces (IFG), #Karlsruhe Nano Micro Facility (KNMF), and ∇Institute of Nanotechnology (INT), Karlsruhe Institute of Technology, 76021, Karlsruhe, Germany ‡
S Supporting Information *
ABSTRACT: Reactive superhydrophobic surfaces are highly promising for biotechnological, analytical, sensor, or diagnostic applications but are difficult to realize due to their chemical inertness. In this communication, we report on a photoactive, inscribable, nonwettable, and transparent surface (PAINTS), prepared by polycondensation of trichlorovinylsilane to form thin transparent reactive porous nanofilament on a solid substrate. The PAINTS shows superhydrophobicity and can be conveniently functionalized with the photoclick thiol-ene reaction. In addition, we show for the first time that the PAINTS bearing vinyl groups can be easily modified with disulfides under UV irradiation. The effect of superhydrophobicity of PAINTS on the formation of high-resolution surface patterns has been investigated. The developed reactive superhydrophobic coating can find applications for surface biofunctionalization using abundant thiol or disulfide bearing biomolecules, such as peptides, proteins, or antibodies. KEYWORDS: Superhydrophobic surface, photo click chemistry, biofunctionalization, transparent, surface patterning
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and can be directly functionalized under mild conditions with different types of (bio)molecules. The compatibility with different established surface patterning techniques is also critical to make a faster transition from bench to bedside for bioassay, screening of diagnostic applications.13−15 Nevertheless, superhydrophobic surfaces reactive to different biomolecules are very difficult to achieve. The presence of reactive functional groups on a surface often leads to an increase in surface energy and loss of superhydrophobicity.16,17 Transparency of the surface is also important for many biological and diagnostic applications where microscopy readouts are commonly used. To the best of our knowledge, surfaces combining (a) high transparency, (b) photoreactivity compatible with a direct (bio)functionalization, and (c) superhydrophobicity have not been reported yet. In this paper, we describe the first photoreactive, inscribable, nonwettable (superhydrophobic), and transparent surface (abbreviated as PAINTS) based on silicone nanofilaments18−21
uperhydrophobic surfaces are of high importance for different industrial applications ranging from self-cleaning1 or anti-icing2 coatings to membranes for oil−water separations.3,4 For biological or diagnostic applications, a superhydrophobic surface requires further modification to generate functional patterns, such as protein or antibody functionalized microarrays or biochips with water repellant background. However, most of the superhydrophobic surfaces are based on nonreactive low surface energy functional groups, which are either extremely difficult to functionalize (e.g., perfluoroalkyls)5 or their modification requires long treatments often under harsh conditions.6,7 Plasma8 or UV9 treatments are most commonly used for the modification of superhydrophobic surfaces via nonspecific oxidation of surface functional groups,8,9 followed by functionalization of the produced hydroxy and carboxy groups with target molecules. This oxidation-based multistep modification is not convenient for the formation of high-resolution patterns10 and not compatible with many important high-fidelity patterning techniques, such as dip pen nanolithography (DPN)11 or PDMS stamping.12 Thus, there is a clear need for the development of novel transparent superhydrophobic surfaces that are also reactive © XXXX American Chemical Society
Received: October 31, 2014 Revised: December 5, 2014
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Figure 1. (a) A photograph of water droplets on the PAINTS prepared on a glass substrate. (b) UV−vis spectrum of a plain glass substrate, a PAINTS, a superhydrophobic HEMA-EDMA surface,50 and a superhydrophobic BMA-EDMA surface.51 (c) Surface SEM image and TEM image (in the inset) showing the surface topography of the nanofilament coating (left, scale bars: 1 μm and 20 nm in the inset.); cross section (right) SEM micrographs of the PAINTS. Scale bar: 500 nm.
Figure 2. (a) Snapshots of a video showing a water droplet bouncing and sliding from the wall of a PAINTS-coated glass vial. (b) Top: water droplets on an uncoated watch glass. Bottom: a water droplet bouncing from the PAINTS coated watch glass. (c) Antistaining properties of the SLIP-PAINTS coated glass vial (top) and the PAINTS-coated glass vial (bottom). The pictures are snapshots from Supporting Information Video S3.
in various biomolecules. To extend the application of the PAINTS for biological functionalization, we investigated the ability to functionalize the surface vinyl groups available on the PAINTS with disulfides-bearing molecules under UV light. In addition, we show that the reported modification method can be used for the formation of high-precision surface patterns on PAINTS with resolutions ranging from submillimeters to submicrometers by several most common surface patterning techniques. To create the PAINTS, oxygen plasma activated glass substrates were immersed in a coating solution consisting of toluene, 200 ppm of water, and 1000 ppm of trichlorovinylsi-
bearing reactive vinyl groups. Although silicon nanofilaments have been reported for the preparation of superhydrophobic surfaces,18,20,22,23 surface functionalization of these superhydrophobic silicon nanofilaments can be achieved by plasma treatment and subsequent immersion into trichlorosilane solutions in water-free toluene.8 These harsh conditions and multistep procedure limit their application in biofunctionalization. Here, we demonstrate the modification of PAINTS with different thiols through the photo click thiol-ene reaction24 within seconds. While thiols are common in peptides and proteins, only a few natural peptides and proteins such as GSH contain free thiol groups. The disulfide group is more abundant B
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Figure 3. (a) Schematic diagram showing the modification of the PAINTS through the thiol-ene reaction with cysteamine (top) and disulfide-ene reaction with 3,3-dithiodipropionic acid (bottom). (b) ToF-SIMS chemical 2D scans of the 1H,1H,2H,2H-perfluorodecanethiol (top) and 3,3dithiodipropionic acid (bottom) modified PAINTS showing the lateral distribution of CF3− (top) and S−/HS− fragments (bottom), respectively. (c) ToF-SIMS spectra corresponding to CF3(CF2)7CH2CH2S− (top) and S−/HS− (bottom) ions observed on the modified PAINTS.
Slippery lubricant infused porous surfaces (SLIPS)30−34 have received great attention recently due to their excellent liquid repellent, biofilm resistant, anti-icing, and antibiofouling properties. However, until now there are only a few examples for the fabrication of transparent SLIPS on curved surfaces.35−37 We showed that the PAINTS, fluorinated with 1H,1H,2H,2H-perfluorodecanethiol via the thiol-ene reaction, could be transformed into SLIPS by impregnating the fluorinated PAINTS with Krytox 103 lubricant. The SLIPPAINTS could be formed on three-dimensional glass surfaces such as the inner walls of a glass vial. Compared to the uncoated glass vial and PAINTS coated glass vial, the prepared SLIP-PAINTS showed excellent repellency against viscous and sticky materials, such as strawberry jam (Figure 2c, Supporting Information Video S3) and honey (Supporting Information Video S3). Up to now, the most widely studied photoactive superhydrophobic surfaces are the hydrophobized metal oxide surfaces.13,38,39 Dynamic control of the wettability of such surfaces with UV light can be realized due to the generation of surface hydroxy groups and photodegradation of the alkyl chains on these surfaces upon UV irradiation. Therefore, direct immobilization of functionalities, such as biomolecules, on these surfaces is difficult to achieve. The PAINTS reported here represent a new type of photoreactive superhydrophobic surface that can be covalently modified in a direct and facile fashion. The application of trichlorovinylsilane (TCVS) rendered the PAINTS reactive to thiol groups via the thiol-ene photoclick reaction.24,40−42 To demonstrate the functionalization of the PAINTS using the thiol-ene reaction, a glass slide coated with the PAINTS was wetted with an ethanol solution of cysteamine (10 vol %), then covered with a quartz slide and irradiated with 260 nm UV light (∼9 mW/cm2) for 15 s (Supporting Information Figure S2). This transformed the superhydrophobic PAINTS (static WCA 166°) into a highly hydrophilic surface possessing a low static WCA of ∼6°. A hydrophobic 1H,1H,2H,2H-perfluorodecanethiol could be also immobilized to the PAINTS, which was confirmed by time-of-flight
lane (TCVS). Upon immersion, TCVS underwent a polycondensation process20,23,25,26 that led to the formation of a porous silicone nanofilament film bearing photoreactive vinyl groups. Despite the absence of fluoro-containing functionalities, the surface prepared with this method showed superhydrophobicity (Figure 1a). The static water contact angle of the PAINTS was as high as 166° and the water contact angle hysteresis was ∼2°. Transparent superhydrophobic surfaces are usually difficult to obtain due to the fact that multiscale roughness, normally required to achieve robust superhydrophobicity,27 leads to increased light scattering. However, as shown in Figure 1b, the PAINTS possessed very high transparency of about 83% above 400 nm, which was comparable to the reference glass substrate. This high transparency makes requiring PAINTS applicable for biological applications requiring microscopic analysis. The nanofilament morphology of the coating can be clearly seen from the SEM micrographs (Figure 1c). The average diameter of the silicon nanofilament fibers was ∼30−50 nm, which explains the reduced scattering of visible light and high transparency of the PAINTS.28 In addition to the porous bulk nanofilament film, the individual silicon fibers exhibited high surface roughness as seen in the TEM images (Figure 1c, left), which is known to be important for achieving superhydrophobicity. A few silicon fibers with hollow interiors (Supporting Information Figure S1a) were also observed. Some silicon fibers showed a transition from hollow to solid interior (Supporting Information Figure S1b). The hollow structure of the nanofilaments could result from the gaseous byproducts generated during the hydrolysis and condensation of the TCVS.29 Superhydrophobic coatings are usually difficult to achieve on three-dimensional substrates. We demonstrated that the PAINTS could be easily fabricated on glass objects of complex shapes. As an example, we coated on the inner walls of a 20 mL glass vial (Figure 2a, Supporting Information Video S2) and the convex side of a watch glass (Figure 2b, Supporting Information Video S1), thereby rendering the 3D glass surfaces superhydrophobic without compromising their transparency. C
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Figure 4. Patterning on the PAINTS: (a) Schematic representation of different surface patterning methods covering the range of features from submillimeters to submicrometers: (i) UV irradiation through a photomask; (ii) microcontact printing; (iii) microchannel cantilever spotting; and (iv) dip pen nanolithography (DPN). (b) Fluorescence microscope images of (i) arrays of hydrophilic spots prepared on the PAINTS by photolithography and filled with FITC (top) and rhodamine B (bottom) dye solutions using the method of discontinuous dewetting; (ii) rhodamine-SH pattern printed on the PAINTS via microcontact printing and immobilized by UV irradiation; (iii) FITC-SH pattern on the PAINTS created by microchannel cantilever spotting; (iv) patterns of rhodamine-SH and FITC-SH generated using multiplexed DPN on the PAINTS. (c) (i) A microscope image of a superhydrophobic−hydrophilic micropattern immersed in water (the hydrophilic areas are more transparent due to wetting.); (ii) a picture showing an array of water droplets on a superhydrophobic−hydrophilic micropattern; (iii) patterned HEK 293 cells 24 h after seeding on a superhydrophobic−hydrophilic micropattern; (iv) ToF-SIMS chemical map of a superhydrophobic−hydrophilic micropattern prepared by two sequential photo click modifications (blue, overlay S− and HS− signal; green, overlay of F2− and CF3− signal).
We first performed the photopatterning with a photomask (Figure 4a(i)) to create a superhydrophobic−hydrophilic micropattern using the disulfide-ene reaction. Three minutes of UV irradiation of the PAINTS covered with an ethanol solution of 3,3-dithiodipropionic acid solution (10 vol %) through a photomask resulted in the formation of a highly hydrophilic wettable micropattern on the superhydrophobic PAINTS (Figure 4c(i)). The thiol-ene reaction could also be used to create patterns using photolithography. A glass slide coated with the PAINTS was first modified with cysteamine by the thiol-ene reaction through a photomask, transforming the UV exposed regions to highly hydrophilic. The vinyl groups in the nonirradiated region on the PAINTS remained reactive after the first step and could be further modified. A subsequent thiol-ene “backfilling” click modification without a photomask using 1H,1H,2H,2H-perfluorodecanethiol introduced the second functionality on the surface and led to the formation of a well-defined superhydrophobic−hydrophilic pattern (Figure 4c(i), (ii)). Although the second functionalization step was performed without a photomask, there was no crosscontamination between the two areas, as confirmed by ToFSIMS (Figure 4c(iv)). Because of the wettability contrast between the superhydrophobic and highly hydrophilic region, the hydrophilic region can be filled with water solutions without contamination of the superhydrophobic background (Figure 4c(ii)). Figure 4b(i) displays the fluorescence microscope images of rhodamine B (bottom) and FITC (top) arrays formed by the discontinuous dewetting45,46 of the pattern with the corresponding dye solutions. Figure 4c(iii) shows an example of application of the produced superhydrophobic− hydrophilic pattern to form a microarray of live cells. HEK293 cells were cultured on this surface and after 24 h, 94% of the cells were confined to the hydrophilic squares with only 6% of the cells occupying the superhydrophobic barriers.
secondary ion mass spectrometry (ToF-SIMS) (Figure 3b,c (top)). Functionalization of surfaces using disulfide containing molecules is of high importance due to the abundance of biomolecules bearing disulfide groups. Nevertheless, there are no reports on the functionalization of surfaces with disulfides except for the adsorption of disulfides onto metal surfaces.43 Since sulfenyl radicals can be produced from disulfides upon UV irradiation,44 we hypothesized that disulfides can react with alkenes under UV light in a similar way as thiols react with alkenes (photoclick thiol-ene reaction). To verify this hypothesis, a solution of dibutyl disulfide (1 mol/L) and 4pentenoic acid (1 mol/L) in tetrahydrofuran was degassed with nitrogen, followed by UV irradiation at 260 nm for 3 min. Both 4,5-bis(butylthio)pentanoic acid (disubstitued adduct) and 5(butylthio)pentanoic acid (monosubstituted adduct) were observed in the ESI-MS spectra (Supporting Information Figure S5). To investigate the disulfide-ene reaction for the functionalization of the PAINTS, 3,3-dithiodipropionic acid (10 vol % in ethanol) was applied onto the PAINTS, covered with a quartz photomask, and irradiated with UV light (260 nm) for 3 min. The surface was analyzed using ToF-SIMS after rinsing with ethanol and fragments of 3,3-dithiodipropionic acid could be detected in the irradiated regions (Figure 3b,c, bottom). In addition, the superhydrophobic PAINTS was transformed into a highly hydrophilic surface with a static WCA of ∼5.1°. The ability to create macro and micropatterns of different functionalities on nonwettable surfaces is important for different applications. As shown above, despite the superhydrophobic nature, the PAINTS can be conveniently functionalized via both thiol-ene and disulfide-ene reactions. This feature of the PAINTS can be used for the preparation of chemical micropatterns on the surface. D
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inhibition of ink spreading on the superhydrophobic PAINTS, much smaller features (below 3 μm) could be obtained (Figure 4b(iii)). To elucidate the resolution limit for patterning on the PAINTS, we furthermore employed DPN with cantilever arrays (Figure 4a(iv)). Here, a multiplexed deposition of RhodamineSH and FITC-SH into line and dot patterns could be achieved (Figure 4b(iv). Despite the high roughness of the sample, no scratching was observed on the surface. The line pitch in the features of Figure 4b(iv) is 1 μm, yielding an estimate for the line resolution of about 500 nm. High resolution interdigitated micropattern, which is difficult to achieve with other patterning methods, have also been demonstrated on the PAINTS with DPN (Figure 4c(iv) and Supporting Information Figure S5).
Immobilization of biomolecules onto the surface is important in many fields, such as biosensing and tissue engineering. Direct one step functionalization of a superhydrophobic surface with biomolecules has not been shown before. In this paper, conjugation of thiol-containing biomolecules to the superhydrophobic PAINTS was demonstrated using a cysteine containing fluorescently labeled peptide (FITC-β-AlaGGGGC) (Supporting Information Figure S4). To prove the generality of the thiol-ene modification, thiol-modified rhodamine (rhodamine-SH) and thiol-modified fluorescein isothiocyanate (FITC-SH) were used for modification of the PAINTS using a PDMS stamp. The stamp was first impregnated with a solution of rhodamine-SH and then stamped onto the PAINTS, followed by irradiation with UV light (260 nm, ∼9 mW/cm2) for 30 s under air. The surface was washed with ethanol and dried. Patterns of red fluorescence on the modified sample could be clearly observed with a fluorescence microscope (Supporting Information Figure S3a). On the contrary, no fluorescence pattern was observed (Supporting Information Figure S3b) on the PAINTS, which was washed directly after rhodamine-SH stamping without UV irradiation. Another fluorescent dye, FITC-SH was also successfully immobilized onto the PAINTS by the thiol-ene reaction using spotting via microchannel cantilevers47 and subsequent UV irradiation (Figure 4a). Formation of high-resolution chemical patterns on porous surfaces is essential for the creation of high density functional arrays for biological and diagnostic applications. To realize high-resolution patterning on the PAINTS, microcontact printing was used.12,47 A micropattern of Rhodamine-SH is shown in Figure 4b(ii). Spotting with microchannel cantilevers and DPN were used to further increase the patterning resolution on the PAINTS. While spotting with microchannel cantilevers50,51 can deliver small volumes (femtoliter) and leads to features in the range of a few micrometers, DPN11 can potentially reach a resolution well below the micrometer range. The nonwettable feature of the PAINTS could significantly increase the surface patterning resolution by inhibiting the undesired spreading of an “ink” on the substrate if a high surface tension “ink” (e.g., water or glycerol) is used. When a water-based ink is applied on a superhydrophobic surface, the contact area between the ink droplet and the superhydrophobic surface is smaller compared to the contact area between the ink and a hydrophilic surface due to reduced spreading of the ink on the superhydrophobic surface (Figure 5). According to eq
rc =
3
V *3
3 * 1 + cos θ π *(2 + cos θ)* 2 1 − cos θ (1)
To summarize, we demonstrated the formation of transparent, photoreactive and superhydrophobic surfaces (PAINTS) by polycondensation of trichlorovinylsilane from toluene on glass substrates. The PAINTS showed thin highly porous filament of silicon nanofibers bearing reactive vinyl groups with morphology similar to that observed on surfaces based on trichloromethylsilane.20,25 Despite the absence of (fluorinated)alkyl groups on the surface, the PAINTS exhibited both superhydrophobicity and transparency. The photo reactivity of the PAINTS was first demonstrated with the thiol-ene reaction. In addition, we for the first time showed the new disulfide-ene reaction for the immobilization of disulfidebearing molecules. The ubiquity of disulfides in natural biomolecules, such as proteins and peptides, warrants the disulfide-ene reaction a lot of surface functionalization and surface patterning applications. Functionally patterned superhydrophobic surfaces have a potential in various biological, medical, analytical or diagnostic applications due to the combination of spatially ordered functionality with the water and cell repellency of the superhydrophobic areas. Although various techniques have been developed for surface patterning, most of these methods are not applicable for chemical patterning of nonreactive superhydrophobic surfaces due to the inertness of the (fluoro)alkyl groups. Here we showed that the PAINTS could be conveniently modified using several important methods to create patterns with feature sizes ranging from millimeters to submicrometers scale. The developed photoactive, superhydrophobic, and transparent surface can become a useful platform for the immobilization and patterning of biomolecules bearing either thiol or disulfide groups. In addition, PAINTS can be used in numerous applications where multifunctional superhydrophobic surfaces and micropatterns are required.
Figure 5. Scheme showing droplets on the PAINTS and on a hydrophilic surface. The different contact areas between the droplets and the surfaces are indicated with red lines..
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1,48 the contact radius for the PAINTS is lowered by 87% compared to that of a hydrophilic surface with a static water contact angle of 40° (e.g, glass). We previously applied microchannel cantilevers (Figure 4a(iii)) for the spotting of dyes onto hydrophilic HEMA-EDMA polymer films47 and azides onto alkyne modified glass surfaces,49 where feature sizes of about 20 μm (spot diameters) were observed. Due to the
TEM images of the silicone nanofilaments; ToF-SIMS spectra of the as-prepared PAINTS and cysteamine modified PAINTS; fluorescence microscope image of the PAINTS patterned with rhodamine-SH; fluorescence microscope image of the PAINTS patterned with fluorescent peptides bearing a thiol group (FITC-β-Ala-GGGGC); Electrospray ionization mass spectra of the reaction mixture produced by UV irradiation (260 nm, ∼ 9 mW/cm2, 3 min) of a degassed mixture of dibutyl disulfide
ASSOCIATED CONTENT
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and 4-pentenoic acid in tetrahydrofuran; combined channel fluorescence microscope images of interdigitated micropattern on the PAINTS with rhodamine-SH and FITC-SH using multiplexed DPN; and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J.L. and L.L contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the Helmholtz Association’s Initiative and Networking Fund (Grant VH-HG-621) and was partly carried out with the support of the Karlsruhe Nano Micro Facility (KNMF, www.kmf.kit.edu), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, www. kit.edu). We thank Dr. Christian Kübel (KIT, INT) for his help with the TEM and Dr. Udo Geckle (KIT, IMT) for the help with SEM measurements and analysis. We thank Zhengbang Wang and Wei Guo (IFG, KIT) for their help with the plasma treatment of the glass substrates. We thank Erica Ueda for her help with editing this paper and Dr. Cornelia Lee-Thedieck (IFG, KIT) for providing the fluorescently labeled peptide. We acknowledge Dr. Cau from Innopsys for providing the PDMS stamps. J.L., X.D., and W.F. thank the China Scholarship Council for their Ph.D. scholarships. The authors thank Professor Wöll for the access to the infrastructure of IFG.
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