Surface Patterning by Microcontact Chemistry - Langmuir (ACS

Invited Feature Article pubs.acs.org/Langmuir

Surface Patterning by Microcontact Chemistry Christian Wendeln and Bart Jan Ravoo* Organic Chemistry Institute and Center for Nanotechnology (CeNTech), Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany ABSTRACT: In this Feature Article we describe recent progress in covalent surface patterning by microcontact chemistry. Microcontact chemistry is a variation of microcontact printing based on the transfer of reactive “ink” molecules from a microstructured, elastomeric stamp onto surfaces modified with complementary reactive groups, leading to a chemical reaction in the area of contact. In comparison with other lithographic methods, microcontact chemistry has a number of advantageous properties including very short patterning times, low consumption of ink molecules, high resolution and large area patterning. During the past 5 years we and many others have investigated a set of different reactions that allow the modification of flat and also spherical surfaces in an effective way. Especially click-type reactions were found to be versatile for substrate patterning by microcontact chemistry and were applied for chemical modification of reactive self-assembled monolayers and polymer surfaces. Microcontact chemistry has already found broad application for the production of functional surfaces and was also used for the preparation of DNA, RNA, and carbohydrate microarrays, for the immobilization of proteins and cells and for the development of sensors.

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INTRODUCTION Ever since the development of the printing press in the middle of the 15th century by the goldsmith Johannes Gutenberg, printing has developed into a standard method for the patterning of a variety of materials and substrates. Whereas the first letterpress only allowed patterning in the range of several millimeters, modern approaches can guarantee the localized transfer of material with micrometer and even submicrometer resolution. Nowadays, especially microcontact printing (μCP) is a commonly used method for the modification of surfaces. μCP was invented in the early 1990s by Kumar and Whitesides, who used elastomeric stamps made of poly(dimethylsiloxane) (PDMS) with micrometer-size relief structures for the patterned transfer of alkanethiols on Aucoated silicon surfaces.1 The elastomeric stamps were exposed to dilute thiol solutions for inking and then carefully placed on the Au covered substrates. Since thiols form strong metal− sulfur bonds with Au, printing results in self-assembled monolayers (SAMs) of alkanethiols on the metal exclusively in the area of conformal contact. After seconds of printing, a dense monolayer is formed, and the stamps are removed from the substrates. Due to the high chemical stability of alkanethiol monolayers,2 the patterns can be used as etch masks for the removal of the unprotected metal, resulting in very accurate Au microstructures. Although the authors most probably envisaged applications in the area of microelectronics when they invented the technique, μCP has meanwhile found extremely widespread applications in physics, chemistry, and biology. In addition to the patterning of thiols on metal surfaces, μCP has meanwhile been used for the patterned deposition of various substances including DNA, proteins, synthetic polymers and dendrimers, © 2012 American Chemical Society

nanoparticles, metal films, and carbon nanotubes on a range of inorganic and organic substrates.3 PDMS is the most common material to make stamps for μCP. Suitable stamps can easily be prepared with micrometersize features by curing commercially available PDMS prepolymer on a patterned master. Moreover, PDMS is cheap, transparent, nontoxic, and chemically very resistant. Also, it has the advantage that its wettability can be changed from hydrophobic to hydrophilic by oxidation with UV/ozone or oxygen plasma under the formation of silanol groups, allowing the printing of polar inks as well.4 The oxidized stamps can be further modified by reaction with silanes such as polyethylene glycol silanes,5 perfluorosilanes,6 or aminopropyl triethoxysilane (APTES)7 and subsequent chemical derivatizations.8 Also layer-by-layer (LbL) assembly of polymers,9 surface-initiated polymerizations,10 and plasma polymerization of allylamine11 were used to tailor the surface properties of PDMS. Although PDMS remains the material of choice to make stamps, the ever broader spectrum of useful “inks” triggered the development of alternative polymer stamps that allow a controlled loading with ink and yet at the same time still deliver the ink during contact with the substrate surface. Various polymers such as poly(styrene-block-butadiene-blockstyrene) (SBS, Kraton D1102), photocurable thiol−ene polymers based on poly[(3-mercaptopropyl)-methylsiloxane] (PMMS), and hydrogels have been applied as stamp material in Received: November 30, 2011 Revised: January 17, 2012 Published: January 21, 2012 5527

dx.doi.org/10.1021/la204721x | Langmuir 2012, 28, 5527−5538

Langmuir

Invited Feature Article

Figure 1. Different approaches for patterned surface modification by μCC. (A) A reaction is induced by printing a functionalized reactive ink on a chemically modified surface, leading to the covalent immobilization of the ink (X reacts with Y). (B) In some cases, the addition of a catalyst or the use of catalyst modified stamps may be required to induce the reaction of X and Y. (C) Chemical reactions on surfaces are also possible by printing with catalyst-modified stamps, which converts Y to Z without the addition of any ink. (D) Chemical reactions on the surface can also be induced by printing a sacrificial reagent that converts Y to Z.

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μCP. An overview of stamp materials and modifications is provided in two recent reviews.4,12 In the majority of μCP experiments, the transfer of ink from the stamp onto the substrate is not only driven by the concentration gradient but also relies on the fact that the substrate surface shows a higher affinity for the ink than the stamp. Electrostatic interactions, hydrogen bonding and nonpolar interactions, or a combination of these effects play a key role in efficient ink transfer. Also, chemisorption and host−guest interactions can be exploited as the driving force. In this Feature Article we mainly focus on μCP approaches that are based on the immobilization of inks by covalent reactions with chemically modified substrates. In this case, both ink and substrate have functional groups that are reactive to each other. We use the term microcontact chemistry (μCC) for this variation of μCP,3 since conformal contact of ink loaded stamp and the reactive substrate surface leads to the covalent attachment of the ink to the surface, allowing the synthesis of chemically well-defined patterned monolayers and multilayers. The covalent immobilization of a monolayer of ink makes further multistep chemical surface modifications very convenient as such surfaces tolerate the use of almost all types of organic solvents and extensive washing procedures. In comparison to other methods that can be used for the chemical patterning of layers such as photolithography, electron beam lithography, ion beam lithography, and dip pen nanolithography, μCC has the advantage that a broad range of reactions can be induced, no expensive technical equipment is required, and no restrictions concerning the substrate material apply. Moreover, μCC allows the patterning of relatively large areas in a short period of time and is still very economic regarding the consumption of ink. The patterning of SAMs and polymer films is a key step for the preparation of biologically active surfaces, microarrays, redox-responsible surfaces, and sensors. In this respect, we believe that the investigation and development of stamp induced chemical surface modifications is of high interest for a broad range of applications in different fields of science and technology.

PRINCIPLES OF MICROCONTACT CHEMISTRY There is an increasing number of reports in the literature describing chemical surface patterning by μCP. The most frequently used method is the covalent attachment of molecules to chemically functionalized substrates (ligation). In the simplest case, a microstructured stamp is inked with a molecule or particle R that has a functional group X which is reactive toward a group Y on the substrate (figure 1A). The reaction is induced in the area of contact by printing with the stamp and leads to the formation of one or more new chemical bonds. It is also conceivable that the ink solution does not only contain the substance RX, but also other reagents that participate in the chemical reaction during μCP. In some cases, the reaction must be induced by an external stimulus such as heat or irradiation. In most cases, the surface group Y remains intact within the noncontacted areas, allowing further printing on the same substrate. Using this type of μCC, amines have been printed on anhydrides, aldehydes, and epoxides, alkynes have been printed on azides, thiols have been printed on alkenes and so forth. One category of additives that might be required in order to induce a ligation reaction are catalysts (typically metal salts, metals or metal complexes). Catalysts can simply be added to the ink in order to allow the chemical patterning by using a structured stamp (Figure 1B). In other cases, the structured stamp itself is coated with a layer of catalyst before inking and contact between the inked stamp and surface induces the reaction. For example, Cu (I)-catalyzed cycloadditions have been performed by modification of the relief stamp surfaces with copper metal films. If an enzyme is immobilized on a stamp, enzyme catalyzed surface reactions are also possible. Catalyst coatings can also be carried out in a patterned way on flat stamps (Figure 1B). In this case, the covalent attachment of the ink to the substrate only occurs in areas with both ink and catalyst on the stamp surface. The preparation of such prepatterned catalyst-modified flat stamps is generally possible by applying any other lithographic method or μCP itself for the local attachment of the catalyst onto the stamp surface. The main advantage of the use of catalyst-patterned flat stamps is that, principally, resolutions down to several nanometers can be achieved, since flat stamps do not undergo deformation effects 5528


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such as pairing, buckling, or roof collapse,13 which can take place during μCP with structured stamps. Moreover, ink diffusion on the substrate surface is irrelevant because the catalyst is immobilized on the stamp and in most cases does not diffuse. Using this type of μCC, Pd-catalyzed Heck reactions and hydrosilylation reactions have been successfully carried out. Another (rarely used) way for μCC with catalyst-coated flat or structured stamps is to contact the modified stamp with an inkcoated or ink solution-coated reactive surface (not described in Figure 1). In many cases, printing with catalyst-modified stamps can also be carried out without the addition of any ink. In this case, the catalyst itself induces a surface reaction in the area of contact under conversion of a layer Y into a different layer Z (Figure 1C). The induced reactions are usually dissociative reactions, in particular deprotection reactions. Finally, μCC can be used for the chemical modification of surfaces by deposition of sacrificial reagents in the area of contact with the stamp. In this approach the stamp is inked with the reagent and applied to the surface. The surface reaction leads to the conversion of a layer Y into a layer Z (Figure 1D) and consumption of the reagent that was printed. These reactions can, for example, be etching reactions or functional group transformations such as oxidations or reductions. Although different stamp materials have been applied in μCC, PDMS has a unique position and is commonly used due to its advantageous properties. Besides the fact that its surface can easily be modified with different coatings, PDMS is chemically very inert and tolerates a variety of functional groups including amines, aldehydes, thiols, alcohols, unsaturated hydrocarbons, azides, and even strong acids. Moreover, it is thermally extremely robust and allows μCP at high temperatures. The use of PDMS for μCP at temperatures up to 150 °C has been reported.14 Its optical transparency down to a wavelength of about 300 nm with considerable transmission even under 300 nm also makes PDMS attractive as stamp material for a broad range of photochemical surface reactions. Nevertheless, chemical surface patterning by μCC can be challenging for several reasons. First of all, the inking procedure, stamp modifications and printing conditions must be optimized for every reaction and sometimes even for every ink molecule in order to on the one hand achieve high yields yet on the other hand generate high-resolution patterns. Especially the diffusion of low molecular weight ink molecules and reagents from the stamp into noncontacted substrate areas can be problematic since ink spreading reduces the edge resolution and leads to the formation of broadened patterns. Moreover, stamp deformations that might be caused by high temperatures during printing or by swelling due to the extensive exposure to nonpolar inks and organic solvents must be avoided. In spite of these limitations, μCC benefits from its general applicability, simplicity, and rapidness to pattern surfaces and has developed into a standard method to achieve patterned, chemical surface modifications.

reactive substrates were subsequently patterned by printing with n-hexadecylamine inked PDMS stamps for 1 min. In the contact area with the stamp, the anhydride SAMs were converted into a mixed SAM of N-alkylamides and carboxylic acids in a 1:1 ratio. Remaining areas with unreacted anhydride were treated with solutions of CF3(CF2)6CH2NH2 in order to produce a patterned SAM of N-hexadecylamides and fluorinated N-alkylamides. The substrates were analyzed by scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). In later work, the group used the same anhydride SAMs and immobilized poly(ethylene imine) (PEI) by contact printing.16 It was found that the printing results were significantly improved when the stamps were oxidized prior to inking with a solution of PEI in isopropanol. The authors assumed that the ink solution wetted the hydrophilic PDMS surface and subsequent drying led to the formation of a homogeneous thin layer of PEI on the stamp surface. Following up on the first reports by Whitesides and co-workers, the patterned immobilization of various amines (simple amines, polymers including DNA, dendrimers, bioconjugates, fluorophores, and proteins) has been carried out by stamp-induced aminolysis on different monolayers and polymer films. Besides carboxylic anhydride surfaces,15,16 patterning of amines was carried out on surfaces with N-hydroxy succinimide esters,17 pentafluorophenol esters,18 and acid fluorides.19 Moreover, amines have been printed on imidazolide monolayers20 and on monolayers with terminal alcohol groups that were reacted with succinimidyl carbonate in order to generate amine reactive surfaces.21 Furthermore, pyrylium SAMs22 have successfully been pattered by reaction with amines using μCP. In this case, the immobilization could directly be verified by fluorescence microscopy. The inverse patterning method, i.e., the printing of active esters on amine modified surfaces has also been described recently.23,24 Fluorescent dyes, L-penicillamine and the tripeptide arginine-glycine-aspartic acid (RGD) were printed on amine-functionalized zeolite L monolayers. L-penicillamine and RGD were activated prior to printing by 1-ethyl-3-(3dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide. The RGD-modified zeolite monolayers are useful substrates for the growth and adhesion of various cells. Besides the covalent and irreversible attachment of (macro)molecules to surfaces, reversible attachment is of interest for several applications. The imine formation between amines and aldehydes is highly attractive for the synthesis of such “erasable” surfaces and was applied for the reversible patterning on aldehyde-terminated substrates.25 Aldehyde surfaces on Au were prepared by chemisorption of 11-amino-1-undecanethiol and subsequent reaction with terephthaldialdehyde. The surfaces were successfully patterned by printing n-octadecylamine using poly(styrene-b-(ethylene-cobutylene)-b-styrene) (SEBS) stamps within 1 min of contact time (Figure 2A). Inking was carried out by immersing the stamps for 1 min in an ethanolic (1 mM) solution of the amine and drying the stamp in a stream of argon. The reaction was confirmed by atomic force microscopy (AFM) in height and friction mode. Figure 2 shows the analysis of aldehyde-modified Au surfaces before (Figure 2B,C) and after printing n-octadecylamine (Figure 2D,E). The immobilization of the amine was verified by the generation of patterns, visible in height (Figure 2D), but especially in friction (Figure 2E). Reversibility of the immobilization reaction was demonstrated by acid-catalyzed hydrolysis of the imine in aqueous acetic solution (pH = 3)

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MICROCONTACT CHEMISTRY WITH AMINES Seminal work for the chemical pattering of SAMs by μCP was done by Whitesides and co-workers,15 who coined the term “reactive μCP” for this variation of μCP. They prepared monolayers of 16-mercaptohexadecanoic acid on Au and Ag and chemically activated them by treatment with trifluoroacetic anhydride in order to generate interchain anhydrides. The 5529


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of RGD by cell surface receptors of the integrin family. The successful immobilization of the protein on aldehydeterminated Au surfaces by μCC was verified by AFM. Figure 3A shows an AFM height image of the Au surface after printing

Figure 3. Patterning of proteins by imine formation using μCC and binding to HeLa cells.27 (A) Printing of col3a1 protein stripes on aldehyde-terminated SAMs on Au surfaces and detection by AFM. (B) Labeling of the col3a1 protein dot pattern by exposure to lissamine rhodamine B and visualization by fluorescence microscopy. (C) Optical microscopy image of attached HeLa cells to dot patterns of col3a1.

with a patterned stamp. The visible protein pattern with an average height of 1.3 nm indicates successful attachment to the surface via reaction of protein amines with the aldehyde surface. Furthermore, protein attachment to glass surfaces could be visualized by labeling with lissamine rhodamine B and detection using fluorescence microcopy (Figure 3B). Before labeling, remaining aldehyde groups were deactivated by reaction with methoxy poly(ethylene glycol) amine. The same surfaces were also applied for cell binding studies. Exposure of the protein patterned substrates to HeLa cells resulted in selective cell attachment to the protein patterns, clearly visible under the optical microscope (Figure 3C). The patterning of proteins by microcontact imine chemistry has also applied by Zin et al., who printed the Au binding protein GBP-1 on aldehydeterminated glass surfaces.28 The protein patterns could be used for the selective self-assembly of Au nanoparticles. Similarly, the method was recently used for the immobilization of urease and subsequent mineralization of ZnO nanoparticles on the top of the catalytically active enzymes.29 Moreover, microcontact imine chemistry was successfully applied for the preparation of laminin patters, which were used as substrates to grow and differentiate human embryotic stem cells into pancreatic endoderm-like cells.30 In addition to μCC with simple amines and proteins, aminefunctionalized DNA and RNA was also successfully immobilized on aldehyde-functionalized substrates.31 In order to achieve an effective inking of the stamp with nucleic acids and to allow optimal transfer of the molecules onto the reactive aldehyde surfaces, the usage of dendrimer-coated stamps (“dendri-stamps”) was essential. The stamps were prepared by immersing oxidized PDMS stamps for 30 s in ethanolic solutions of fifth-generation PPI dendrimers (1 μM) and subsequently dried in a stream of nitrogen. Loading with nucleic acids was carried out by subsequent exposure to an oligonucleotide solution in buffer for 20 min. Stamp modification with dendrimers produces a high density of positive charges and induces the attachment of the negatively charged oligonucleotides in a LbL arrangement. Immobilization on aldehyde surfaces by microcontact imine formation and removal of the transferred dendrimer layer by washing with basic ethanol results in patterned oligonucleotide surfaces (Figure 4A). Irreversible ligation was achieved by subsequent

Figure 2. (A) Schematic description of imine formation by μCC.25 (B−G) AFM analysis of octadecylamine printed on an aldehydeterminated SAM on Au in height (B,D,F) and friction (C,E,G). (B,C) Before printing no pattern is visible. (D,E) Imine patterns due to successful immobilization by μCP. (F,G) Removal of the pattern by acid-catalyzed hydrolysis.

within 1 h at room temperature. After hydrolysis, the pattern completely vanished (Figure 2F,G). In another experiment, Lucifer yellow ethylenediamine was printed on aldehydemodified glass substrates. The immobilization was confirmed by fluorescence microscopy. Also in this case the fluorescent pattern could be removed by acid catalyzed hydrolysis. Stable and irreversible attachment of amines to surfaces could generally be achieved by reduction of imine patterns with sodium borohydrate. In another paper, the reversibility of imine formation was additionally demonstrated by the directional movement of rhodamine-labeled poly(propylene imine) (PPI) dendrimers on glass substrates with gradients in aldehyde surface concentration.26 The dendrimers were immobilized in a patterned fashion by μCP with PDMS stamps and allowed to diffuse by hydrolysis and reformation of amine bonds. To this end, the surfaces were immersed in water overnight and then investigated by fluorescence microscopy. Detailed analysis showed increasing broadening of the patterns with rising aldehyde concentrations, proving the reversibility on imine formation on surfaces. In order to demonstrate the applicability of μCC by imine formation for the preparation of surfaces with biological relevance, cytophilic proteins such as col3a1 were printed on aldehyde-terminated Au and glass substrates using microstructured, oxidized PDMS stamps.27 Col3a1 is a gelatin-like extracellular matrix protein (ECM) that contains the RGD sequence and can mediate cell attachment through recognition 5530


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exposed to a solution of complementary Cy5-labeled oligonucleotides. Using fluorescence microscopy, it was found that specific attachment of the complementary strand was exclusively observed in areas of the printed fluorescein-labeled DNA (Figure 4D,E). In contrast to this, hybridization with cDNA with one mismatching oligonucleotide in the middle of the sequence only showed a marginal fluorescence intensity, proving the suitability of the concept for the preparation of printed DNA microarrays. Not only are aminolysis and imine formation valuable reactions for the immobilization of amines on substrates by μCC, but the nucleophilic opening of epoxides has also been applied for covalent immobilization. Thierry et al. used flat, oxidized PDMS stamps in order to attach the protein lysozyme homogenously on epoxy-functionalized silicon surfaces.32 The surfaces were prepared by pulsed plasma polymerization of allyl glycidyl ether. Our group used the ring-opening reaction of epoxides for the patterned immobilization of low molecular weight amines on flat epoxide-coated substrates and also on spherical epoxide-containing polymer beads.33 To investigate the printing conditions in a first step, amine functionalized dyes and carbohydrates were printed on (3-glycidyloxypropyl)trimethoxysilane modified substrates using oxidized PDMS stamps (Figure 5A). It was found that μCC proceeds at room

Figure 5. (A) Schematic description μCC of amines on epoxy substrates.33 (B) Fluorescence microscopy image of lissamine rhodamine B ethylenediamine, immobilized on epoxy-functionalized glass surfaces. (C) Fluorescence microscopy image of a mannose amine conjugate printed on epoxy-functionalized glass and subsequent binding of fluorescein-labeled Con A.

temperature, but is slow and not very efficient under these conditions. Better printing results with higher immobilization yields were achieved if μCC was carried out at elevated temperatures. Figure 5B shows a fluorescence microscopy image of lissamine rhodamine B ethylenediamine printed in dots within 4 h at 120 °C on epoxy-functionalized glass. The reaction conditions guaranteed an effective immobilization of the dye under maintenance of good pattern fidelity. In another experiment, the amine-functionalized carbohydrate conjugate 5aminopentyl-α-D-mannopyranoside was printed using the same conditions on the reactive glass surface. The surfaces were then exposed to a bovine serum albumin (BSA)-containing solution in order to passivate remaining epoxide areas. Subsequent exposure to the fluorescein-labeled lectin concanavalin A (Con A) and fluorescence microscopy analysis of the surface demonstrated the accessibility of the mannose to bind the labeled protein (Figure 5C). The high fluorescence intensity indicates multivalent binding and verifies the presence of a high density carbohydrate layer within the modified areas.

Figure 4. Patterning of oligonucleotides by μCC using dendrimercoated PDMS stamps.31 (A) Preparation of dendrimer-coated PDMS stamps and transfer printing of oligonucleotides on aldehyde modified glass. Fluorescence microscopy images of fluorescein-labeled DNA printed with (B) dendri-stamps and (C) APTES-modified stamps. (D) Immobilization of fluorescein-labeled DNA. (E) Hybridization with Cy5-labeled complementary oligonucleotides.

reduction of the imines to amines with sodium borohydride. Printing of fluorescein-labeled DNA and analysis by fluorescence microscopy revealed that the approach leads to very efficient attachment of the target molecules, and high resolution can be achieved (Figure 4B). Interestingly, simple modification of the oxidized PDMS stamps with 3-aminopropyltriethoxysilane was not effective for immobilization of the fluoresceinlabled DNA, which is probably due to the higher surface density of positive charges in the case of the dendrimer modified stamps (Figure 4C). For investigation of the availability of the printed DNA for hybridization, fluoresceinlabeled DNA was immobilized with a dendri-stamp and 5531


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fluorescein-labeled Con A. Fluorescence microscopy analysis showed attachment of PNA on one side of the beads (Figure 6E) and of Con A on the opposing side (figure 6F). This result is due to the selectivity of PNA for β-D-galactose and to the selectivity of Con A for α-D-mannose, yielding bifunctional protein-decorated polymer particles (Figure 6G).

For the patterning of spherical epoxide-containing polymer beads, prepared by polymerization of glycidyl methacrylate and ethylene glycol dimethacrylate, another approach was followed. The polymer beads with an average size of approximately 160 μm were assembled into a monolayer on a flat PDMS stamp, which was loaded with an amine-containing ink (Figure 6A). A

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MICROCONTACT CHEMISTRY USING AZIDE−ALKYNE CYCLOADDITION Nowadays the cycloaddition of azides and alkynes is one of the most applied reactions in material sciences due to its outstanding properties. The fact that the educts are easily available and very stable makes the use of this reaction convenient. Moreover, the cycloaddition is biocompatible and orthogonal to many functional groups, which led to its intensive application also in different fields of biochemistry and biology. The reaction of nonactivated alkynes with azides are usually carried out in the presence of a Cu (I) catalyst. This allows the reaction to proceed even at room temperature with near quantitative yields. Besides the Cu catalyzed azide−alkyne cycloaddition (CuAAC), the reaction can also be carried out at elevated temperatures without the need of any catalyst by means of thermal azide−alkyne cycloaddition (TAAC). In recent years, azide−alkyne cycloaddition has also extensively been applied for surface patterning by μCP. In a first report by Rozkiewicz et al., the use of PDMS stamps for the immobilization of n-octadecyne and of a lissamine rhodamine alkyne conjugate on azide-terminated monolayers is described.34 Printing was carried out at room temperature without the addition of a catalyst, and the modified surfaces were characterized by different analytical methods. Although Cu-free click chemistry is attractive, more recent papers do not refrain from the addition of Cu (I) in the case of nonactivated alkynes, as the reaction without a catalyst usually results in limited conversion, even after prolonged printing times.35 Microcontact CuAAC was first reported by Lahann and coworkers, who used the method for the immobilization of a biotin-azide conjugate on an alkyne-modified polymer film, prepared by chemical vapor deposition polymerization.36 To this end, a thin film of the azide and sodium ascorbate was spread on the polymer surface and contacted with a patterned PDMS stamp, which was previously inked with a CuSO4 solution. Cu (I) was exclusively generated in the area of contact with the stamp and initiated the immobilization reaction. The biotin patterns were readily visualized by the binding of streptavidin. Our group used a slightly different approach for the printing of different alkyne-modified carbohydrates (see Figure 7A) on azide-terminated monolayers.37 A small amount of aqueous Cu (I) was added to a solution of carbohydrate conjugates in ethanol, and the ink was exposed to the surfaces of oxidized PDMS stamps. After drying, the stamps were placed on the azide-functionalized substrates in order to induce the reaction. The immobilization was investigated by X-ray photoelectron spectroscopy (XPS). Whereas the C(1s) signal in the XPS spectra of the undecylazide-SAM consists only of a single peak at 285 eV due to the saturated hydrocarbon chains (Figure 7B), β-glucoside decorated surfaces show a shoulder at about 287 eV (Figure 7C), caused by the carbohydrate-conjugate specific carbons in a C−O environment. The carbohydrate patterned surfaces could be used for the selective immobilization of carbohydrate-binding proteins (lectins). μCP of a α-mannoside-alkyne conjugate and subsequent substrate exposure to a

Figure 6. Sandwich μCC by epoxide ring-opening with amines.33 (A) Schematic illustration of the method (see text). (B) Fluorescence microscopy image of lissamine rhodamine B ethylenediamine immobilized on one side of the beads and of (C) dansylcadaverine immobilized on the opposing side. (D) Overlay of the previous two images. Fluorescence microscopy analysis of (E) rhodamine-labeled PNA and of (F) fluorescein-labeled Con A attached to opposing sides of bifunctionalized carbohydrate-decorated beads and (G) overlay of both images.

second flat PDMS stamp, inked with another amine was applied on the top of the beads, and the sandwich-like arrangement was heated in an oven at 120 °C for 4 h in order to induce the chemical reaction. After removal of the stamps and intensive washing of the particles, bifunctional Janus polymer beads were obtained. Different amines have been used for bead modification by μCC. In a first experiment, lissamine rhodamine B ethylene diamine and dansylcadaverine were printed on the epoxide beads. Fluorescence microscopy analysis showed red light emission on one side of the beads under green light excitation (Figure 6B) and blue to green light emission on the opposing side under UV light excitation (Figure 6C). The overlay of the two pictures visualize the Janus-type functionalization of the particles and prove successful immobilization of both dyes on almost every bead, demonstrating the effectiveness of the method (figure 6D). In another experiment, 5-aminopentyl-α-D-mannopyranoside and its β-D-galactose analogue were printed on the polymer particles by using the sandwich μCP technique. The beads were incubated with a mixture of rhodamine-labeled peanut agglutinin (PNA) and 5532


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out from solution, (ii) for reactions carried out by using PDMS stamps that were inked with an alkyne and a Cu (I) salt, and (iii) for reactions that were induced by an alkyne-inked, Cu film-coated PDMS stamp. The catalytic activity of the Cucoated stamp was attributed to the formation of Cu2O on the metal surface. In addition to XPS, IR, and fluorescence microscopy, the reaction and its kinetics were especially investigated by cyclic voltammetry. It was found that printing of a ferrocene-alkyne conjugate in the presence of Cu (I) salts is the fastest immobilization method, and complete surface conversion can be observed after approximately 30 min of reaction time. Solution-based immobilization of the ferrocene derivate proceeded at very high rates initially, but for the production of a fully modified surface, 180 min of reaction time were required. Both methods have pseudo-first-order kinetics, whereas in the case of the Cu-coated stamps, a zero-order linear growth was observed. In this case, full conversion of the azides on the surface was observed after 60 min of printing, if a stamp was used that was allowed to age for 24 h prior to printing. The kinetics of the CuAAC and TAAC reactions in μCC were also analyzed in a recent paper using fluorescently labeled alkyne inks printed on azide substrates.40 Complete conversion was observed in less than 15 min. It was found that the TAAC reaction displays pseudo-first-order kinetics with increasing reaction rates for the most electron poor alkynes. The CuAAC reaction is even faster than the fastest TAAC reaction. Besides microcontact CuAAC, the printing of azides on activated, electron deficient alkynes has also been used for metal-free surface patterning. Alkyne-modified polymer surfaces consisting of propiolate groups were prepared by chemical vapor deposition polymerization and successfully patterned by μCP of a biotin-azide conjugate.41 Detection of the biotin residues was carried out by fluorescence microscopy after exposure to rhodamine-labeled streptavidin. If the same experiment was carried out on a similar polymer film with nonactivated alkynes, no binding of the streptavidin was observed. Cu-free microcontact azide−alkyne cycloaddition with nonactivated alkynes has been realized by thermal immobilization at elevated temperatures.14 Reactive bifunctional azide and alkyne containing polymers have been prepared by the copolymerization of a mixture of styrene, trimethyl silyl-protected propargyloxy-styrene, and vinylbenzyl azide with subsequent deprotection of the alkyne by tetrabutylammonium fluoride. Spin coating and thermal crosslinking by TAAC resulted in azide and alkyne functionalized polymer films. These films were successfully modified by μCP of an azido-coumarin dye and an alkynyl-perylene derivative at 90 °C for 30 min. Analysis of the patterned surfaces was carried out by fluorescence microscopy (Figure 8).

Figure 7. (A) Carbohydrate immobilization by microcontact CuAAC.37 (B) C(1s) region in the XPS spectrum of an undecylazide SAM on a silicon wafer. (C) C(1s) region in the XPS spectrum of a glucose-decorated surface obtained by μCC with a flat stamp. (D) Fluorescence microscopy image of fluorescein-labeled Con A bound to patterns of α-D-mannose. (E) AFM friction image of Con A immobilized on patterns of β-D-maltose.

buffered solution of fluorescein-labeled Con A resulted in the selective binding of the protein to the mannose residues, readily visible under a fluorescence microscope (Figure 7D). Additionally, the binding of the protein to β-maltose patterns was verified by AFM (Figure 7E). In the same way, Bertozzi and coworkers prepared carbohydrate-modified surfaces by printing of dual end-functionalized mucin-lice glycopolymers38 on azidemodified substrates. Also the immobilized glycopolymers maintained their ability to bind lectins. Microcontact CuAAC has recently also been applied for the generation of patterned alkyne/cyclodextrin surfaces.39 To this end, azide-modified β-cyclodextrin was printed on a monolayer of alkyne-terminated coumarins. The fluorogenic click reaction of the azide and the coumarin unambiguously demonstrates the rapid surface immobilization by μCC. The patterned substrates could successfully be used for the orthogonal covalent or noncovalent immobilization: Alkynes were attached to the surface by CuAAC and adamantane-modified molecules by host−guest chemistry. Also these subsequent reactions could be carried out either from solution or by μCP. The cycloaddition proceeded with and without Cu(I). In addition to different applications of the microcontact CuAAC reaction, the first investigation of its reaction kinetics was reported by Spruell et al. in 2008.35 The authors compared time-dependent surface reaction yields for (i) reactions carried

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MICROCONTACT CHEMISTRY USING DIELS−ALDER REACTIONS Diels−Alder cycloadditions are very attractive reactions in the area of bioconjugation and also for the immobilization of biomolecules since they are compatible with a variety of functional groups and additionally proceed in aqueous environment under mild conditions. Moreover, they can be very fast and quantitative, depending on the combination of diene and dienophile. Due to the advantageous properties of Diels−Alder reactions, we investigated the suitability of two cycloadditions, namely the reaction between furans and maleimides and also between cyclopentadienes and maleimides for the immobilization of biomolecules, in particular of 5533


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Figure 8. Surface patterning by μCC using TAAC on a polymer film with reactive azide and alkyne groups.14 Fluorescence microscopy image of (A) an azido-coumarin and (B) an alkynyl-perylene patterned surface.

Figure 9. (A) μCP of carbohydrate−cyclopentadienyl conjugates on maleimide-terminated monolayers.42 C(1s) region in the XPS spectrum of (B) a maleimide-terminated SAM on a silicon wafer, (C) carbohydrate decorated surfaces prepared by μCP of β-Dgalactose-cyclopentadiene conjugate, and (D) β-D-maltose-cyclopentadiene conjugate.

carbohydrates by μCC.42 Maleimide-terminated surfaces on glass and silicon were prepared in three steps: (i) self-assembly of undecyltrichlorosilane monolayers; (ii) nucleophilic substitution of the bromine by 3,6-endoxo-Δ4-tetrahydropthalimide (furan protected maleimide); and (iii) removal of the protecting group by a thermally induced retro-Diels−Alder reaction to give the unprotected maleimide-terminated surface. Using oxidized PDMS stamps, the surfaces were patterned in a preliminary experiment by μCP of a furan−galactose conjugate. It was found that the reaction required relatively harsh conditions for effective patterning, as the reaction was carried out at 80 °C for 1 h. In contrast to that, cyclopentadiene− carbohydrate conjugates (Figure 9A) could be printed within minutes at room temperature, due to the much higher reactivity of cyclopentadienes to maleimide groups. In view of this result, further investigations were carried out using cyclopentadiene conjugates as reactive ink molecules. The successful immobilization by μCP was verified by XPS. In comparison to the C(1s) signal of the maleimide-terminated surface (Figure 9B), the C(1s) signals of β-D-galactose-functionalized substrates (Figure 9C) and more significantly β-D-maltose-functionalized substrates (Figure 9D) show the presence of many carbons in a C−O environment. This finding is in accordance with the chemical structure of the ink molecules and proves the effectiveness of μCC. The resolution of the structures, generated by the microcontact cyclopentadiene−maleimide chemistry was investigated by secondary ion time-of-flight mass spectrometry (ToF-SIMS). Analysis of a silicon substrate, patterned by printing of the β-D-galactose conjugate in 5 μm broad lines that are spaced by 20 μm, indicated the formation of high-resolution patterns in several anion images (Figure 10A). The fact that a number of oxygen-rich fragments such as C3H3O2−, C2H3O2−, and C2H2O2− were found is again in accordance with the chemical structure of the carbohydrate conjugates. To demonstrate the suitability of such carbohydrate modified surfaces for protein binding, the corresponding β-Dlactoside was printed in stripes on the maleimide surface, and

Figure 10. (A) ToF-SIMS analysis of maleinimide-terminated SAMs on silicon wafers patterned by μCC of β-D-galactose-cyclopentadiene conjugate.42 (B) Fluorescence microscopy image of rhodamine-labeled PNA selectively attached to a carbohydrate modified glass surface, prepared by μCP of β-D-lactose-cyclopentediene conjugate. (C) Fluorescence microscopy image of selective binding of fluoresceinlabeled Con A to α-D-mannose and β-D-maltose residues, but not to βD-glucose residues.

remaining dienophiles were deactivated by reaction with cysteine. Fluorescence microscopy analysis after exposure to rhodamine-labeled PNA showed selective binding of the 5534


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protein to the lactose residues (Figure 10B). In addition to the simple preparation of substrates with a single carbohydrate, arrays with two and three sugars were successfully prepared by sequential cross-printing of the carbohydrate ink molecules. For example, first printing α-D-mannose-cyclopentadienyl conjugate in horizontal lines and then β-D-maltoside in perpendicular direction led to a surface with two carbohydrates. The surface was further modified by reacting all remaining maleimide groups in the noncontacted areas by printing the corresponding β-D-glucoside with a flat stamp. Exposure to fluorescein-labeled Con A indicated the selective binding to α-D-mannose and β-Dmaltose residues (lattice structure), but not to β-D-glucose units (rectangular interspaces), which is in agreement with the recognition properties of this lectin (Figure 10C). Protein binding was to some extent accompanied by unspecific protein adsorption, which could successfully be reduced by the implementation of an additional BSA blocking step (data not shown here).42

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MICROCONTACT CHEMISTRY USING PHOTOCHEMICAL REACTIONS The use of polymer stamps to chemically pattern surfaces is not limited to catalyzed and noncatalyzed reactions. Due to the transparancy of several polymers, they also allow photochemical surface modifications. Especially PDMS with its high transparency down to 300 nm and below is very suitable in this context. For instance, Suh et al. demonstrated the use of structured PDMS stamps to pattern poly(ethylene glycol) dimethacrylate (PEG-DMA) films on substrate surfaces by capillary force lithography.43 During contact of the polymer film with the PDMS stamp, capillary forces the polymer into the void space of the stamp, leading to a negative replica of the stamp. Curing of the polymer was achieved by UV light irradiation through the PDMS. The direct use of PDMS stamps for the patterned μCC of photoreactive inks on chemically modified substrates with induction of the immobilization reaction by irradiation has been demonstrated by Campos et al. and by our group independently in 2010.44,45 In both cases, the radical thiol−ene reaction was applied for the addition of thiols on alkene-terminated monolayers. This click-type reaction has achieved intensive interest during past years due to its compatibility to various functional groups and its effectiveness. In the communication of Campos et al., a nonoxidized PDMS stamp was inked with a concentrated solution of thioglycolic acid and placed on an alkene-terminated monolayer on oxygen-free silicon. Irradiation with UV-light induced the addition of the thiol to the alkene-terminated SAM in the area of contact. In our approach, we used oxidized PDMS stamps and dilute ethanolic thiol solutions in order to reduce the quantity of thiol necessary for inking. A range of thiols such as N-acetyl cysteine, β-mercaptopropiolic acid, and dithiothreitol were successfully attached to undecenyl-modified substrates. Also complex thiols, such as an acetylated and a nonacetylated galactose-thiol conjugate (see Figure 11A) could be immobilized. The use of a high-power UV-LED that was placed directly over the stamps resulted in substantial surface coating in less than 1 min of reaction time. A detailed analysis of the printing of the acetylated galactose-thiol conjugate was carried out by using ToF-SIMS. The observation of heteroatom-containing fragments such as CN−, HS− and of oxygen-rich anions such as CHO2− and C2H3O2− in the negative ion mode verify the immobilization of the carbohydrate (Figure 11B). The formation of CH3− fragments can be ascribed to the presence

Figure 11. (A) Photochemical μCP using the thiol−ene radical addition reaction.45 (B) ToF-SIMS analysis in the negative ion mode of acetylated galactose-thiol conjugate printed in lines on an undecenyl-SAM. (C) Fluorescence microscopy image of rhodaminelabeled PNA bond to β-D-galactose residues, immobilized by photochemical μCP of unprotected galatose−thiol conjugate. (D) Fluorescence microscopy surface of a β-D-galactose patterned surface after exposure to fluorescein-labeled Con A.

of acetyl protective groups on the surface. Detailed investigation of the patterns shows that photochemical μCP is possible with high-resolution and reproducibility. We also demonstrated the suitability for the binding and immobilization of proteins. To this end, unprotected galactose-thiol conjugate was printed in stripes, and remaining alkenes were subsequently decorated by attachment of a triethylene glycol thiol. After exposure to a solution of rhodamine-labeled PNA, fluorescence microscopy verified the selective attachment of the lectin to the carbohydrate pattern (Figure 11C). Fluorescein-labeled Con A, which does not bind to β-D-galactose residues, did in not show any interaction (Figure 11D). Furthermore, we also investigated the printing of thiols on alkyne-terminated monolayers. In contrast to alkenes, alkynes can react twice with thiols to produce a 1,2-double-addition product, leading potentially to a more dense surface coating. μCP of galactose−thiol conjugates on alkyne-terminated SAMs was verified by contact angle measurements, ToF-SIMS, and by the selective binding of PNA. Light-induced immobilization using the μCP technique has also been applied for the reaction of 1-alkynes with hydrogenterminated silicon.46 In a report by ter Maat et al., PDMS stamps were inked with 1-octadecyne or with 2,2,2-trifluoroethyl undec-10-ynoate and placed on hydrogen-terminated 5535


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Figure 12. Surface patterning by catalytic stamp lithography. A flat PDMS stamp patterned with an array of Pd-nanoparticles is inked with a solution of alkene or alkyne and brought into conformal contact with the surface of H-terminated silicon in order to induce a hydrosilylation reaction.54

modified with catalytically active Pd nanoparticles. The stamps were successfully used to attach functionalized molecules covalently to different substrate surfaces by “catalytic stamp lithography”. In a first report, these catalyst modified stamps were applied to attach alkenes and alkynes covalently to a surface of hydrogen-terminated silicon via hydrosilylation reaction in areas with immobilized nanoparticles (Figure 12).54 The method could be applied for sub-100 nm patterning since stamp deformation (flat stamps) or ink diffusion (immobilized catalyst) is excluded. Synthesis of the stamps was carried out in two steps: Preparation of patterned pseudohexagonal close-packed Pd nanoparticle arrays on silicon substrates from self-assembled block-copolymer templates and transfer of the catalyst onto flat PDMS surfaces using a peelingoff technique. In further reports, the same authors extended the concept to the printing of aldehydes on hydrogen-terminated silicon55 and the coupling of aryliodides to alkene-terminated SAMs by using the Heck reaction.56 Another reaction that could be induced by printing with Pd-nanoparticle-modified PDMS stamps was the reduction of azides to amines, resulting in patterned amine/azide surfaces.56 In contrast to the previously described catalyzed reactions, in the last approach, the stamp was not inked with a molecule prior to printing but was contacted with an azide-modified substrate under a saturated solution of hydrogen in 2-propanol. Surface patterning with catalyst-modified stamps was also frequently applied to induce cleavage or deprotection reactions on surfaces. In its simplest case, PDMS stamps with a relief structure were inked with trifluoroacetic acid and brought in contact with a tert-butylester containing polymer surface for selective deprotection. The resulting patterns of free carboxylic acid groups could readily be modified by peptide synthesis.57 In another approach, Huskens and co-workers used oxidized, structured PDMS stamps for the selective deprotection of alcohols with trimethylsilyl and tert-butyl dimethylsilyl protective groups on SAMs on Au.58 This inkless printing technique avoided diffusion processes, as the acetic silanol groups are covalently attached to the PDMS stamp. In a similar way, piperidine-modified polyurethane acrylate stamps have been prepared by photoinduced polymerization and were used for the inkless deprotection of Fmoc-protected SAMs in the area of contact.59 Furthermore, sulfonic acid modified polyurethane acrylate stamps were developed and applied for the deprotection of SAMs with tert-butyl dimethylsilyl-protected alcohols60 or Boc-protected amines.60,61 Additionally, the

silicon under an inert atmosphere. Irradiation for 3 h with 658 nm visible light lead to the formation of dense alkenyl monolayers by hydrosilylation of the alkynes on the silicon surface. Backfilling with the other alkyne resulted in patterned, two-component substrates that were successfully characterized by SEM and AFM.

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OTHER REACTIONS INDUCED BY MICROCONTACT PRINTING In addition to the reactions discussed above, many other reactions have been applied for covalent surface modifications by μCC. For example, alcohols have been printed on imidazolide monolayers20 and amines were immobilized on surfaces with thermally generated acyl ketenes.14 One common way for the preparation of patterned SAMs on silicon dioxide is the direct printing of alkoxysilanes47 or chlorosilanes.48 These molecules condense with surface silanol groups on the silicon oxide surface form a monolayer in the area of contact. Not only have flat surfaces been patterned by the printing of silanes, but silica particles were also successfully modified.49 In an inverse approach, chlorine-terminated silicon surfaces have been patterned by μCP of dodecanol.50 The surfaces were characterized by AFM. Another powerful method for covalent surface modification is the printing of diazonium salts.51,52 Under acetic conditions, the salts release dinitrogen and form aryl radicals that can react with different kinds of substrates and surface coatings. Depending on the combination of substrate and diazonium salt, the reaction leads to different organic films, ranging from relatively irregular multilayers to smooth monolayer coatings. μCC of an Al(III) complex on carboxy-modified polymer films through ligand exchange reaction has also been demonstrated recently.53 The complexes bind a Pd/Sn colloidal catalyst from solution that afterward could successfully be used to initiate the deposition of Cu in an electroless plating solution. This method allowed the efficient fabrication of patterned Cu films on both rigid and flexible polymer films.

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MICROCONTACT CHEMISTRY WITH CATALYST-MODIFIED STAMPS Catalytic μCC is a particularly effective method for patterned surface modification by covalent reactions. Similarly to the structured Cu-coated stamps used to induce CuAAC reactions (see above), Buriak and co-workers designed flat PDMS stamps 5536


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stamps were used for the hydrolysis of N-hydroxysuccinimide functionalized monolayers62 in the area of contact. Stamps with catalytic activity have also successfully been prepared by modification with enzymes. Snyder et al. prepared polyacrylamide-based stamps with nitrilotriacetic acid groups for the reversible surface attachment of hexahistidine-tagged enzymes.63 The applicability of the approach was demonstrated by the degradation of surface-bound DNA with exonuclease Imodified stamps. Recently, trypsin-modified PDMS stamps were used for the removal of poly-L-lysine (PLL) layers in the contact area between stamp and substrate.64 The surface patterns could be visualized by fluorescence microscopy and AFM.

parallel immobilization of thousands of different molecules is a central element, the potential of surface patterning by μCP is limited. In an attempt to overcome this limitation, μCP has, for example, been combined with microspotting. To this end, DNA was in a first step spotted on a flat stamp, and afterward μCC with this stamp was used to subsequently pattern a number of reactive surfaces without reinking.31 This approach combines the advantages of both methods: easy patterning of a large number of substances by microspotting and high speed ligation to a reactive substrate by μCC. It can be assumed that the combination of μCC with other patterning techniques will also be developed and applied for the synthesis of multifunctional surfaces or for multiprotein immobilization. A further challenge in the area of μCP is to develop patterning methods that allow submicrometer resolution, which is usually not applicable to conventional relief-structured stamps. As discussed above, the immobilization of catalysts on stamps circumvents the loss of resolution to reagent diffusion into noncontact areas. Another sophisticated approach is the use of flat stamps that are chemically patterned with perfluorinated silanes by nanoimprint lithography. The perfluorinated areas act as ink diffusion barriers and allow the local transfer of ink from the stamp onto the substrate with high resolution.68 The preparation of such chemically patterned stamps, suitable for submicrometer patterning, has also been achieved by dip-pen nanolithography,69 so that virtually any complex nanoscale pattern can be reproduced faithfully by μCP. Similarly, PDMS stamps modified with metallic nanostencil masks that act as a diffusion barrier could successfully be applied for nanocontact printing.70 The design of new stamps and stamp coatings represents a key to induce chemical reactions on surfaces with nanometer resolution and will certainly further extend the applicability of μCC. In conclusion, we envisage that in the future not only will μCC be extended toward an even wider scope of reactions, but also the development of printing technique and stamp design will show new opportunities toward nanopatterned, multicomponent surfaces, bioactive surfaces, and responsive coatings.

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MICROCONTACT CHEMISTRY BY REAGENT PRINTING In addition to printing approaches where the ink is either attached to the surface via a reactive group (ligation) or acts as a catalyst, the ink can also be a simple sacrificial reagent that induces a surface reaction and is consumed during printing. One example is use of NaBH4-loaded PDMS stamps for the reduction of quinone-presenting SAMs to hydroquinones in the area of contact.65 The hydroquinones were designed in such a way that a cyclization reaction followed upon generation of the hydroquinones, leading to the release of free amine groups. The amine patterns could successfully be further modified by reaction from solution. Another example for reagent-induced surface patterning is the use of hydrogel stamps, soaked with appropriate chemical etchants for the selective removal of material from different kind of substrate surfaces. This so-called “reactive wetstamping” approach was among others used for the etching of silicon dioxide with HF or of Cu with HCl/FeCl3.66 Furthermore, the method could be applied for the oxidation of PDMS with potassium dichromate or of polystyrene with H2O2 in the area of contact.67

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CONCLUSION AND OUTLOOK

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The use of contact printing for the chemical patterning of a wide range of monolayers, polymers, and other substrate materials demonstrates the power and versatility of μCC. Not only have different stamp materials and coatings been developed during the past years in order to optimize the printing process itself, many reactions have also been successfully applied to create a wide variety of functional surfaces, ranging from simple patterned monolayers to complex (bio)sensors. Printing with elastomeric stamps enables a nanoscale contact of reagents and surfaces and therefore represents a very elegant and extremely versatile method to induce area specific surface reactions. Moreover, μCC is usually very fast, easy to perform, and economic toward the consumption of ink, which renders the method attractive not only for chemists but also for other scientists in various fields of surface engineering. Nevertheless, one of the main disadvantages of μCP in comparison to other patterning methods such as robotic microspotting is the fact it is very hard to print more than one ink in a single printing step, so that the synthesis of multicomponent surfaces and complex microarrays remains difficult. In this respect, μCP is very suitable for large area patterning of surfaces with only a few components, but becomes increasingly challenging with a rising number of printing steps. Especially in microarray fabrication where the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Deutsche Forschungsgemeinschaft DFG for financial support of this work (Grant Ra 1732/ 2-1).

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REFERENCES

(1) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (2) Kumar, A.; Biebuyck, A.; Abott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188. (3) Ravoo, B. J. J. Mater. Chem. 2009, 18, 8902. (4) Kaufmann, T.; Ravoo, B. J. Polym. Chem. 2010, 1, 371. (5) Delamanche, E.; Donzel, C.; Kamounah, F. S.; Wolf, H.; Geissler, M.; Stutz, R.; Schmidt-Winkel, P.; Michel, B.; Mathieu, H. J.; Schaumburg, K. Langmuir 2003, 19, 8749. (6) Sharpe, R. B. A.; Burdinski, D.; Huskens, J.; Zandvliet, H. J. W.; Reinhoudt, D. N.; Poelsema, B. J. Am. Chem. Soc. 2005, 127, 10344. (7) Lee, N. Y.; Chung, B. H. Langmuir 2009, 25, 3861. 5537


Langmuir


(42) Wendeln, C.; Heile, A.; Arlinghaus, H. F.; Ravoo, B. J. Langmuir 2010, 26, 4933. (43) Suh, K. Y.; Seong, J.; Khademhosseini, A.; Laibinis, P. E.; Langner, R. Biomaterials 2004, 25, 557. (44) Campos, M. A. C.; Paulusse, J. M. J.; Zuilhof, H. Chem. Commun. 2010, 46, 5512. (45) Wendeln, C.; Rinnen, S.; Schulz, C.; Arlinghaus, H. F.; Ravoo, B. J. Langmuir 2010, 26, 15966. (46) Ter Maat, J.; Yang, M.; Scheres, L.; Kuypers, S.; Zuilhof, H. Chem. Commun. 2010, 46, 8005. (47) Li, H.; Zhang, J.; Zhou, X.; Lu, G.; Yin, Z.; Li, G.; Wu, T.; Boey, F.; Venkatraman, S. S.; Zhang, H. Langmuir 2010, 26, 5603. (48) Cristiano, A.; Lim, C. W.; Rozkiewicz, D. I.; Reinhoudt, D. N.; Ravoo, B. J. Langmuir 2007, 23, 8944. (49) Jiang, S.; Granick, S. Langmuir 2009, 25, 8915. (50) Jun, Y.; Le, D.; Zho, X.-Y. Langmuir 2002, 18, 3415. (51) Garrett, D. J.; Lehr, J.; Miskelly, G. M.; Downard, A. J. J. Am. Chem. Soc. 2007, 129, 15456. (52) Lehr, J.; Garrett, D. J.; Paulik, M. G.; Flavel, B. S.; Brooksby, P. A.; Williamson, B. E.; Downard, A. J. Anal. Chem. 2010, 82, 7027. (53) Miller, M. S.; Filatrault, H. L.; Davidson, G. J. E.; Luo, M.; Carmichael, T. B. J. Am. Chem. Soc. 2010, 132, 765. (54) Mizuno, H.; Buriak, J. M. J. Am. Chem. Soc. 2008, 130, 17657. (55) Mizuno, H.; Buriak, J. M. ACS Appl. Mater. Interfaces 2009, 1, 2711. (56) Mizuno, H.; Buriak, J. M. ACS Appl. Mater. Interfaces 2010, 2, 2301. (57) Feng, C. L.; Embrechts, A.; Bredebusch, I.; Schnekenburger, J.; Domschke, W.; Vancso, G. J.; Schönherr, H. Adv. Mater. 2007, 19, 286. (58) Li, X.-M.; Péter, M.; Huskens, J.; Reinhoudt, D. N. Nano Lett. 2003, 3, 1449. (59) Shestopalov, A. A.; Clark, R. L.; Toone, E. J. J. Am. Chem. Soc. 2007, 129, 13818. (60) Shestopalov, A. A.; Clark, R. L.; Toone, E. J. Nano Lett. 2010, 10, 43. (61) Shestopalov, A. A.; Clark, R. L.; Toone, E. J. Langmuir 2010, 26, 1449. (62) Shestopalov, A. A.; Morris, C. J.; Vogen, B. N.; Hoertz, A.; Clark, R. L.; Toone, E. J. Langmuir 2011, 27, 6478. (63) Snyder, P. W.; Johannes, M. S.; Vogen, B. N.; Clark, R. L.; Toone, E. J. J. Org. Chem. 2007, 72, 7459. (64) Guyomard-Lack, A.; Delorme, N.; Moreau, C.; Bardeau, J.-F.; Cathala, B. Langmuir 2011, 27, 7629. (65) Seo, H.; Choi, I.; Lee, J.; Kim, S.; Kim, D.-E.; Kim, S. K.; Yeo, W.-S. Chem.Eur. J. 2011, 17, 5804. (66) Grzybowski, B. A.; Bishop, K. J. M. Small 2009, 5, 22. (67) Campbell, C. J.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A Langmuir 2005, 21, 2637. (68) Duan, X.; Zhao, Y.; Perl, A.; Berenschot, E.; Reinhoudt, D. N.; Huskens, J. Adv. Mater. 2009, 21, 2798. (69) Zheng, Z.; Jang, J.-W.; Zheng, G.; Mirkin, C. A. Angew. Chem., Int. Ed. 2008, 47, 9951. (70) Lee, H.; Lin, J. Y.; Odom, T. W. Angew. Chem., Int. Ed. 2010, 49, 3057.

(8) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamanche, E. Adv. Mater. 2001, 13, 1164. (9) Xu, H.; Gomez-Casado, A.; Liu, Z.; Reinhoudt, D. N.; Lammertink, R. G. H.; Huskens, J. Langmuir 2009, 25, 13972. (10) Wu, Y.; Huang, Y.; Ma, H. J. Am. Chem. Soc. 2007, 129, 7226. (11) Sadhu, V. B.; Perl, A.; Péter, M.; Rozkiewicz, D. I.; Engbers, G.; Ravoo, B. J.; Reinhoudt, D. N.; Huskens, J. Langmuir 2007, 23, 6850. (12) Xu, H.; Huskens, J. Chem.Eur. J. 2010, 16, 2342. (13) Hui, C. Y.; Jagota, A.; Lin, Y. Y.; Kramer, E. J. Langmuir 2002, 18, 1394. (14) Spruell, J. M.; Wolffs, M.; Leibfarth, F. A.; Stahl, B. C.; Heo, J.; Connal, L. A.; Hu, J.; Hawker, C. J. J. Am. Chem. Soc. 2011, 133, 16698. (15) Yan, L.; Zhao, X.-M.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 6179. (16) Yan, L.; Huck, W. T. S.; Zhao, X.-M.; Whitesides, G. M. Langmuir 1999, 15, 1208. (17) Feng, C. L.; Vansco, G. J.; Schönherr, H. Adv. Funct. Mater. 2006, 16, 1306. (18) Lahari, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055. (19) Scheres, L.; ter Maat, J.; Giesbers, M; Zuilhof, H. Small 2010, 6, 642. (20) Hsu, S.-H.; Reinhoudt, D. N.; Huskens, J.; Velders, A. H. J. Mater. Chem. 2008, 18, 4959. (21) Lee, B. S.; Chi, Y. S.; Lee, K.-B.; Kim, Y.-G.; Choi, I. S. Biomacromolecules 2007, 8, 3922. (22) Scaramuzzo, F. A.; González-Campo, A.; Wu, C.-C.; Velders, A. H.; Subramaniam, V.; Doddi, G.; Mencarelli, P.; Barteri, M.; Jonkheijm, P; Huskens, J. Chem. Commun. 2010, 46, 4193. (23) Kehr, N. S.; Schäfer, A.; Ravoo, B. J.; De Cola, L. Nanoscale 2010, 2, 601. (24) Kehr, N. S.; Riehemann, K.; El-Gindi, J.; Schäfer, A.; Fuchs, H.; Galla, H.-J.; De Cola, L. Adv. Funct. Mater. 2010, 20, 2248. (25) Rozkiewicz, D. I.; Ravoo, B. J.; Reinhoudt, D. N. Langmuir 2005, 21, 6337. (26) Chang, T.; Rozkiewicz, D. I.; Ravoo, B. J.; Meijer, E. W.; Reinhoudt, D. N. Nano Lett. 2007, 7, 978. (27) Rozkiewicz, D. I.; Kraan, Y.; Werten, M. W. T.; de Wolf, F. A.; Subramaniam, V.; Ravoo, B. J.; Reinhoudt, D. N. Chem.Eur. J. 2006, 12, 6290. (28) Zin, M. T.; Ma, H.; Sarikaya, M.; Jen, A. K. Y. N Small 2005, 1, 698. (29) Fabijanic, K. I.; Perez-Castillejos, R.; Matsui, H. J. Mater. Chem. 2011, 21, 16877. (30) Van Hoof, D.; Mendelsohn, A. D.; Seerke, R.; Desai, T. A.; German, M. S. Stem Cell Res. 2011, 6, 276. (31) Rozkiewicz, D. I.; Brugman, W.; Kerkhoven, R. M.; Ravoo, B. J; Reinhoudt, D. N. J. Am. Chem. Soc. 2007, 129, 11593. (32) Thierry, B.; Jasieniak, M.; de Smet, L. C. M.; Vasilev, K.; Griesser, H. J. Langmuir 2008, 24, 10187. (33) Kaufmann, T.; Gokmen, M. T.; Wendeln, C.; Schneiders, M.; Rinnen, S.; Arlinghaus, H. F.; Bon, S. A. F.; Du Prez, F. E.; Ravoo, B. J. Adv. Mater. 2011, 23, 79. (34) Rozkiewicz, D. I.; Jańczewski, D.; Verboom, W.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2006, 118, 5418. (35) Spruell, J. M.; Sheriff, B. A.; Rozkiewicz, D. I.; Dichtel, W. R.; Rhode, R. D.; Reinhoudt, D. N.; Stoddart, J. F.; Heath, J. R. Angew. Chem., Int Ed. 2008, 47, 9927. (36) Nandivada, H.; Chen, H.-Y.; Bondarenko, L.; Lahann, J. Angew. Chem., Int. Ed. 2006, 45, 3360. (37) Michel, O.; Ravoo, B. J. Langmuir 2008, 24, 12116. (38) Godula, K.; Rabuka, D.; Nam, K. T.; Bertozzi, C. R. Angew. Chem., Int. Ed. 2009, 48, 4973. (39) González-Campo, A.; Hsu, S.-H.; Puig, L.; Huskens, J.; Reinhoudt, D. N.; Velders, A. H. J. Am. Chem. Soc. 2010, 132, 11434. (40) Mehlich, J.; Ravoo, B. J. Org. Biomol. Chem. 2011, 9, 4108. (41) Deng, X.; Friedmann, C.; Lahann, J. Angew. Chem., Int. Ed. 2011, 50, 6522. 5538


Surface Patterning by Microcontact Chemistry - Langmuir (ACS

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