Functionalization and Patterning of Self-Assembled Monolayers and

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Functionalization and Patterning of Self-Assembled Monolayers and Polymer Brushes Using Microcontact Chemistry Sebastian Lamping,# Christoph Buten,# and Bart Jan Ravoo*

Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV PARIS-SUD on 04/10/19. For personal use only.

Center for Soft Nanoscience and Organic Chemistry Institute, Westfälische Wilhelms-Universität Münster, Busso-Peus-Strasse 10, 48149 Münster, Germany

CONSPECTUS: Because the surface connects a material to its environment, the functionalization, modification, and patterning of surfaces is key to a wide range of materials applied in microelectronics, displays, sensing, microarrays, photovoltaics, catalysis, and other fields. Self-assembled monolayers (SAMs), which can be deposited on a wide range of inorganic materials, are only a few nanometers thick, yet they can radically change the properties of the resulting interface. Alternatively, thin polymer films composed of polymer brushes grown from the surface provide a more robust molecular modification of inorganic materials. For many applications, patterned SAMs or polymer brushes are desired. Over the past decade, our group has shown that both SAMs as well as polymer brushes can be patterned very efficiently using microcontact printing. In microcontact printing, a molecular “ink” is deposited on a suitable substrate using a microstructured elastomer stamp, which delivers the ink exclusively in the area of contact between stamp and substrate. In contrast to most types of lithography, microcontact printing does not require expensive equipment. Our work has shown that “microcontact chemistry” is a powerful additive surface patterning method, in which molecular inks react with a precursor SAM during printing so that surfaces can be modified with various orthogonal functional groups or molecular recognition sites in microscale patterns. Functional groups include reactive groups for click chemistry or photochemistry and initiators for radical polymerization. Molecular recognition sites include host−guest chemistry as well as biochemical ligands such as carbohydrates and biotin. In this Account, we present an overview of our research in this area including selected examples of work by other groups. In the first part, we review our work on the patterning of SAMs using microcontact chemistry, with a focus on click chemistry and photochemistry. We will show how cycloadditions, thiol−ene reactions, and tetrazole chemistry can be used to obtain versatile surface patterns. In the second part, we demonstrate that microcontact chemistry can be used to pattern polymer brushes. Among others, initiators for surface-induced nitroxidemediated polymerization and atom transfer polymerization were printed and used to grow patterned polymer brushes with molecular recognition groups suitable for responsive surface adhesion. In the third part, we describe how SAMs and polymer brushes can be printed on microparticles instead of flat substrates so that Janus particles with functional patches can be obtained. Finally, we present a brief outlook on further developments expected in this field.

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materials.3 In μCP, an elastomeric stamp, typically poly(dimethylsiloxane) (PDMS), is incubated with a solution of the molecule to be printed, called ink, and dried. Next, the stamp is brought into contact with the desired substrate. If the ink molecules physically adsorb to the surface, this procedure is referred to as μCP, whereas immobilization through a chemical reaction is referred to as reactive μCP or microcontact chemistry (μCC). Both μCP and μCC are attractive additive patterning methods, because they feature advantages such as

MODIFICATION AND PATTERNING OF SELF-ASSEMBLED MONOLAYERS

The term microcontact printing (μCP) was coined in 1994 by George Whitesides for a simple yet powerful method for the fabrication of micrometer sized patterns of self-assembled monolayers (SAMs) on gold and other metals.1,2 μCP belongs to the field of soft lithography, which makes use of elastomer stamps and molds for the spatial functionalization of various materials. Although initially proposed as a tool to pattern alkanethiol SAMs on metals, μCP quickly became a versatile approach to also pattern more complex molecules on a wider variety of surfaces, including glass, silicon, or polymeric © XXXX American Chemical Society

Received: January 21, 2019

A

DOI: 10.1021/acs.accounts.9b00041 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research low cost, low material consumption, and an easy preparation of patterns with resolution in the micrometer range over relatively large areas (∼1 m2).3 Other soft lithographic techniques are micromolding in capillaries (MIMIC),4 replica molding (REM),5 and nanotransfer printing (nTP)6 to name only a few. Although soft lithographic methods, and especially μCP, bear a lot of advantages, there are also some drawbacks and limitations.7 The stamps itself are made by molding the polymeric precursors on patterned silicon masters. Because these masters are typically fabricated by photo or electron beam lithography, soft lithographic methods can never have a better resolution than these techniques. Additionally, forces like gravity, adhesion, and hydrodynamics apply stress to the stamp material, which results in loss of resolution because of deformations or the collapse of the elastomer features. Moreover, typically only one ink can be patterned in each printing step so that applications that require multiplexing (e.g., sensing) remain challenging. Some of these limitations can be circumvented by nanoimprint lithography,8 which uses hard stamps, and scanning probe methods, in which structures are not printed but directly written with scanning probes. Often, atomic force microscopy (AFM) tips are used for this purpose (e.g., dip pen lithography, DPN). To reduce the cost and time barrier between scanning probe techniques and soft lithographic methods, polymer pen lithography (PPL) was invented, which uses arrays of pyramidal elastomeric tips and combines low-cost materials with the high flexibility of scanning probes.9 The deposition of a SAM on a solid support can be the first step toward the fabrication of highly complex functional surfaces. For many applications, the preparation of patterned SAMs is desired. In the past decade, we have shown that μCC is a powerful patterning method, in which molecular inks react with a precursor SAM during printing so that surfaces can be modified with various orthogonal functional groups or molecular recognition sites in microscale patterns. The reaction of molecular inks with SAMs (or any other surface that displays reactive groups) is enhanced by three important factors: (1) the high local concentration of the reacting partners; (2) the preferential orientation of the adsorbate molecules in the SAM (with most terminal groups pointing upward); (3) micropolarity as well as capillary effects in the nanoscale confinement between the stamp and the substrate. Our earliest results were summarized in a feature article published in Langmuir in 2012.10 To illustrate the power and versatility of the method, we would like to open this Account by highlighting a study on four orthogonal reactions induced by μCC on a single surface to yield multifunctional surfaces for the fabrication of biosensors and microarrays (Figure 1).11 Although multifunctional surfaces had been fabricated before,12−16 our work provides the first example of a tetrafunctional surface that can be addressed by four orthogonal functional groups. In a first step, we functionalized a glass surface with an undecenyl-terminated silane SAM. This surface was patterned via μCC with azide and carboxylic acid functionalized thiols using thiol−ene click chemistry in stripe patterns. On the resulting surface, a Bocprotected alkyne−amine linker was printed perpendicularly to the azide termini via Cu(I)-catalyzed alkyne azide cycloaddition (CuAAC). The resulting microarray features azides, acids, amines, and alkenes in a characteristic pattern created by the successive printing of each functional group (two thiols, one alkyne). Each functional group can be addressed one at a

Figure 1. (A) Preparation of a tetrafunctional alkene/azide/acid/ amine-modified substrate surface by microcontact CuAAC of Bocprotected aminoalkyne on an alkene/acid/azide-terminated substrate and successive deprotection. Fluorescence microscopy images of rhodamine streptavidin immobilized on biotin-modified surfaces, prepared by reaction with (B) biotin NHS ester and (C) with biotin alkyne. (D) Binding of rhodamine-labeled PNA to the substrate surface after orthogonal modification with lactose amine. (E) Binding of rhodamine-labeled ConA after attachment of mannose thiol. Copyright Wiley-VCH 2012.11

time by orthogonal reactions with biotin and carbohydrate derivatives (alkynes for azides; thiols for alkenes; amines for acids; NHS esters for amines). Biotin detection was achieved by incubation with fluorescently labeled streptavidin, and carbohydrates were detected by incubation with fluorescently labeled lectins. Although all of these orthogonal reactions are known in surface chemistry, they had never been used in concert on a single substrate before. Pioneering work has been done by printing biotin−alkyneazide on an alkyne-polymer surface,17 depositing biotin−alkynealkene onto thiol-terminated surfaces,18 patterning active-ester functionalized surfaces with amine-terminated synthetic heparin-oligosaccharides,19 and functionalizing poly(allylamine) surfaces with green fluorescent protein (GFP).15 In another example, we prepared bifunctional azide/ benzaldoxime surfaces by μCC of a cyclooctyne−benzaldoxime linker on azide-terminated SAMs by strain-promoted azide−alkyne cycloaddition (SPAAC).20 The functionalities were orthogonally addressed by SPAAC and nitrile oxide− alkene/alkyne cycloadditions (NOAC) (Figure 2). For NOAC, the benzaldoximes are activated with diacetoxyiodobenzene (DIB) to generate nitrile oxide groups on the surface. These reactive 1,3-dipoles can be trapped by cycloadditions with a variety of unsaturated hydrocarbons. We used a β-DB


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Figure 2. Surface modification by metal-free click chemistry. Immobilization of cyclooctyne oxime linker by microcontact SPAAC (A). Surface modifications by nitrile oxide cycloadditions (B). Surface modification by SPAAC (C). Copyright Royal Society of Chemistry 2012.20

galactose norbornene conjugate, a biotin−alkyne derivative, and a cyclooctyne mannose conjugate for this purpose. The remaining azide functionalities can be further functionalized by SPAAC using an α- D -mannose cyclooctyne conjugate. Although cyclooctynes in principle can undergo cycloadditions with both azides and nitrile oxides, both functional groups were addressed orthogonally by a suitable choice of reaction conditions. Whereas high concentrations of cyclooctyne result in a reaction with the azide groups, a selective reaction with the benzaldoximes is obtained by a low concentration of cyclooctyne in the presence of DIB. This approach has proven to be particularly versatile, because it allows biomolecules to be immobilized orthogonally, metal-free, and under extremely mild conditions. All surface functionalizations require a certain trigger, be it temperature, time, or initiation by radicals. Light is a very popular trigger for photochemical surface modification with excellent resolution in space and time. We used light as a trigger for μCP in many different applications. For example, we patterned tetrazole-functionalized surfaces via the lightinduced formation of nitrilimines and subsequent (cyclo-) additions with various functional groups. In detail, tetrazoles eliminate nitrogen upon UV irradiation and form nitrilimines, which are highly reactive 1,3-dipoles. Various functional groups can react with nitrilimines. We used this approach to immobilize alkenes, alkynes, thiols, and carboxylic acids on tetrazole-functionalized surfaces (Figure 3).21,22 We used this procedure to fabricate carbohydrate and biotin chips and immobilized a polymerization initiator for surface-initiated polymerizations. The thiol−ene reaction highlighted in Figure 1 also falls under the heading of photochemistry.11 Having established reliable conditions for the immobilization of molecules via thiol−ene click chemistry, we sought to expand the scope from rather “simple” biomolecules like carbohydrates to more complex ones like proteins. The idea behind this approach was to immobilize this class of biomolecules making use of free

Figure 3. (A) Reaction pathway of tetrazoles upon irradiation with alkenes, alkynes, thiols, and carboxylic acids as dipolarophiles. (B) Analysis of selected dipolarophiles; left: blue fluorescence of the cycloadduct after μCC with maleimide; middle: ToF-SIMS analysis of a surface patterned with mannose acid; right: green fluorescence of FITC−streptavidin after incubation on a surface patterned with biotin−alkynethiol.

thiol groups in the outer part of the quaternary structure (e.g., from cysteines).23 Glucose oxidase (GOx) and lactase (Lac) were immobilized via μCP using a thiol−ene reaction. The immobilized enzymes retained their activity in a cascade reaction. Most immobilization protocols require a bifunctional linker that connects the compound that is to be immobilized with a reactive group that reacts with the complementary functional group at the surface. Especially for the immobilization of carbohydrates, it is common to use linkers with a reactive functional group, because direct O-glycosylations are very sensitive toward the reaction conditions and especially to moisture. We published an example where we used direct Oglycosylation for the patterned immobilization of thioglycosyl donors on hydroxyl-terminated monolayers.24 A solution of thioglycosyl donor and N-iodosuccinimide (NIS) in dry DMF was incubated on hydroxyl-terminated SAMs and dried. Simultaneously, a PDMS stamp was incubated with a trimethylsilyltriflate (TMSOTf) ink, dried, and placed on the surface. In the areas of contact, the TMSOTf reacts with the NIS and produces iodonium ions. These again react with the C


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Figure 4. On surface O-glycosylation by catalytic μCP. A mixture of thioglycosyl donor and NIS is incubated on a hydroxyl-terminated surface. Successive μCP of the promotor TMSOTf yields immobilized carbohydrates exclusively in the areas of contact. Copyright Royal Society of Chemistry 2017.24

Figure 5. Principle of chemical transfer printing and preparation of tetrafluorophenol (TFP)-modified stamps. (A) A TFP stamp is chemically loaded by esterification with a carboxylic acid. (B) The active-ester stamp is brought in conformal contact with (C) an amine-functionalized surface. Because of aminolysis, which takes place in the area of contact, acyl residues are transferred onto the amines of the substrate, and this leads (D) to a patterned surface with newly generated amides. (E) TFP stamps can be prepared from conventional PDMS stamps. In the first step, the PDMS surface is oxidized by ozone and subsequently exposed to 5-hexenyltrichlorosilane to give (F) hexenyl-modified PDMS. In the following step, TFP residues are attached by photochemical microcontact printing of TFP−thiol (1). (G) The obtained TFP stamps have (H) a clear, transparent appearance. (I) The hydrophilic TFP-modified areas can be visualized under an optical microscope by the selective condensation of water droplets on the surface. Copyright ACS 2013.27

functionalized glass surfaces, during which the acyl groups were transferred from the stamps to the surface, retaining the pattern. After printing, the stamps again display free TFP groups, which can again be used for CTP up to three times (Figure 5).

glycosyl donor and cleave the leaving group and leave the carbohydrate as an oxocarbenium ion. The oxocarbenium is attacked by the OH groups of the SAM to form the Oglycosidic bond (Figure 4). The surface of stamps can also be chemically functionalized to alter its properties. For example, polar silanes can be coupled to yield more polar stamps,25 or more recently, diffusion-blocking, low-surface-energy layers of a perfluoropolyether (PFPE) could bind to PDMS stamps to fabricate organic electronic devices.26 We also developed a procedure that makes use of chemically functionalized stamps, coined as chemical transfer printing (CTP). In contrast to conventional μCP where a physically adsorbed ink is transferred from the stamp to a substrate, in CTP, a covalently immobilized monolayer is transferred from one substrate to another.27 To this end, flat PDMS stamps were functionalized with alkene groups via chemical vapor deposition (CVD). These stamps were patterned by thiol−ene click chemistry with a tetrafluorophenol−thiol (TFP−thiol). TFP groups were functionalized with carboxylic acids in the presence of coupling agents via μCP to obtain patterned surface-bound active esters on the stamp. These stamps were used in μCP on amine-

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MODIFICATION AND PATTERNING OF POLYMER BRUSHES The preparation of polymer brushes offers new opportunities and ways of functionalizing surfaces. Polymer brushes are terminally bonded polymer strands with a dense surface coverage.28 In general, a distinction can be made between two different techniques to immobilize polymers on surfaces: the “grafting-to” and the “grafting-from” approaches (Figure 6A,B). In the grafting-to approach, presynthesized polymers are bonded to functional groups on the surface. The advantage of this method is that the polymers can be purified and analyzed by standard techniques, which allows well-defined length and composition. However, the surface density and the height of the polymer layer produced in this way is rather low, and the polymers will form mushrooms rather than brushes.29 In the “grafting-from” approach, the polymerization of the D


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Figure 6. (A) Immobilized polymer obtained by a grafting-to approach. (B) Immobilized polymer obtained by a grafting-from approach.

Figure 7. (A) Scheme of a light triggered polymerization for polymer brushes that are patterned by coverslips. (B) Structured block copolymer brushes are obtained by using different sizes of coverslips. Copyright Wiley-VCH 2018.37

brushes takes place directly on the surface. The surfaces are activated with an initiator as a starting point for so-called surface-initiated (SI) polymerization. Over the years, several techniques including radical, anionic or cationic, ring-opening, and controlled radical polymerizations (CRPs) were introduced.30 CRPs in particular have proven extremely versatile for the production of polymer brushes. Through the uniform growth of the brushes and the possibility to vary the density of the initiator on the surface, it is rather easy to influence other parameters such as architecture, molecular weight, or length of the polymer brushes.31,32 The most widely used techniques of CRP are reversible addition−fragmentation chain transfer polymerization (RAFT), nitroxide-mediated radical polymerization (NMP), ring-opening metathesis polymerization (ROMP), and atom transfer radical polymerization (ATRP), which found broad application in various types of surface functionalization. The fabrication of structured polymer brushes has proven to be a versatile approach for obtaining multifunctional surfaces. Different lithographic techniques, such as scanning probe lithography, photolithography, electron beam lithography, or soft lithography were combined with CRP and found broad application for micro and nanopatterning polymer brushes.33 Recent examples make use of photolithographic approaches, where structured polymer brushes were obtained through photocatalysts or photocleavable groups and photomasks.34−36 In order to circumvent the high effort to avoid oxygen, Hawker et al. developed a method to structure surfaces with coverslips, which shows a very simple possibility to prepare block copolymers but leads to larger structures (Figure 7).37 Soft lithographic approaches have the advantage of directly immobilizing the initiators or grafting polymers in a micropattern on surfaces, without the use of expensive machines or avoidance of oxygen. One of our first examples of structured polymer brushes on surfaces made use of the grafting-to approach. With the help of the RAFT polymerization, three different well-defined polymers (poly(acrylic acid), poly(hydroxyethyl acrylate, and poly(tetraethylene glycol acrylate)) with a molecular weight from 1500 to 6000 g mol−1 were synthesized. Taking advantage of the fact that polymers synthesized by RAFT consist of a thiocarbonylthiol end group, a hetero Diels−Alder reaction could be accomplished to couple the polymer to a cyclopentadiene SAM on surface. This process was further developed by combining it with MIMIC to

obtain patterned polymer brushes. In our example, the selected highly hydrophilic polymers were used to repel peptides at specific parts of the surfaces.38 In principle, with this approach, any RAFT polymer can be patterned on surfaces. For more versatile surface architectures, we started our research in the field of patterned polymer brushes obtained through grafting-from techniques. For the grafting-from approach, the surface has to be functionalized with the initiator of the chosen CRP. This can be used by applying the initiator to the surface in a patterned way. In one of our first approaches to the grafting-from mechanism, a new click reaction was introduced to pattern an ATRP initiator on glass and silicon surfaces.39 A 1,2,4,5-tetrazine functionalized ATRP initiator was synthesized, which reacts as an electron deficient diene with an undecenyl-terminated SAM in an inverse electron demand Diels−Alder reaction. To generate a patterned surface, the initiator was incubated on a PDMS stamp, which was then placed on the undecenyl-terminated SAM. The reaction was carried out in an oven at 60 °C for 55 min. After cleaning, the SI-ATRP polymerization was carried out overnight. With this grafting-from procedure, relatively high poly(methyl acrylate) brushes (40 nm) could be obtained in comparison to the polymer brushes obtained by the graftingto procedure (3 nm).38 A further development of the ATRP of polymer brushes on surfaces could be achieved in collaboration with Du Prez et al. We synthesized an ATRP initiator coupled to triazoline dione (TAD). This type of heterocycle allows ultrafast click reactions to dienes and alkenes in Diels−Alder and Alder−ene reactions.40 Whereas μCP of the tetrazine ATRP initiator took about 1 h, the TAD-ATRP initiator could be printed in seconds (Figure 8). Furthermore, the triazoline dione click shows reversibility via a so-called transclick reaction when it is coupled to indoles. Hence, an indole SAM was fabricated, which reacted via μCP with the TAD-ATRP initiator for polymerization. The obtained patterned polymer brushes were analyzed by AFM. After that, the surfaces were submerged into a hexadien-1-ol solution and heated to 150 °C. The transclick takes place, and the again free TAD moiety reacts with the diene to the nonreversible product while the indole is recovered. The free indole on the surface allows now another reaction with the TAD-ATRP initiator followed by polymerization obtaining different polymer brushes with a E


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Figure 8. μCP of TAD-ATRP initiator on an indole functionalized surface with subsequent ATRP. The transclick reaction is used to erase the surface pattern and an additional functionalization by ATRP is used to write a new surface pattern. Copyright Wiley-VCH 2015.41

Figure 9. Scheme of two poly(ionic liquid) surfaces glued through host−guest interactions between adamantane and ferrocene by Guo et al. Copyright Royal Society of Chemistry 2014.43

groups ensure the compensation of surface roughness for a good contact and lead to multivalent bonding. We note that several groups have reported surface adhesion between multiple weak interactions because of the introduction of polymer brushes on soft materials (e.g., membranes or hydrogels).43−45 An interesting example for a responsible adhesion is the one from Guo et al. using polymers consisting of either ferrocene or cyclodextrin groups.43 Upon oxidation of the ferrocene, the adhesion vanishes because of the dissociation of the host−guest complexes (Figure 9B). One of our adhesives is based on the dynamic covalent bonds between phenylboronic acids and catechols on (hard)

different pattern depending on the stamp. In this way, chemically rewritable substrates for patterned polymer brushes were produced.41 The combination of μCP and CRP enables increasingly complex surface functionalization. Using copolymers consisting of two, three, or more monomers, functional groups can be incorporated into the brushes.42 One option is to develop adhesive systems with weak, noncovalent, or dynamic covalent bonds, which are released upon application of a certain stimulus. To enhance surface adhesion, it was necessary to create an environment where many weak interactions occur at the same time. Polymer brushes with incubated functional F


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Figure 10. (A) Scheme of two surfaces glued by dynamic covalent bonds between catechol and phenylboronic acid brushes and the release by competition reaction when carbohydrates are present. (B) Three systems of “host−guest” adhesives based on polymer brushes. (SysI) Two AAP surfaces with βCD polymer as glue, (SysII) two βCD surfaces with AAP polymer as glue, and (SysIII) two-component glue of one AAP and one βCD surface. All systems showed good adhesion and could be inactivated by UV light. Only SysIII was water resistant and was released by adding a competitive guest.

switch that cannot be complexed by βCD as a cis-isomer, the adhesive could not be released through UV irradiation. To further investigate the host−guest adhesion, we introduced arylazopyrazole (AAP) as a photoswitch with better switchability and higher thermal stability, instead of azobenzene, to the polymer brushes.48 As before, these surfaces could be glued together by a βCD solution with good adhesion (Figure 10B, SysI). Furthermore, we copolymerized a βCD−acrylamide into the polymer brushes showing adhesion between two surfaces functionalized with βCD brushes and an AAP polymer from solution (SysII). To complete this, the βCD brush surface was adhered to an AAP brush surface generating a two-component system with strong adhesion and water resistance (SysIII). Unfortunately, although the AAP surfaces could be inactivated by pre-irradiation with UV light, neither of the adhesive systems showed a photoresponsive release. UV−vis studies showed a fast back isomerization of the AAP brushes from cisto trans-isomer and a poor isomerization from trans- to cisisomer when the AAP was confined in solid state between two surfaces.49 Not only ATRP can be used for the preparation of patterned polymer brushes. Another well-established technique of CRP is the nitroxide-mediated radical polymerization (NMP). In our first example of an NMP-mediated surface-initiated polymerization, an NMP initiator was coupled to a thiol to perform a patterned thiol−ene click via μCP to an undecyl-terminated

glass or silicon surfaces.46 A phenylboronic acid acrylate as well as a catechol acrylamide were synthesized and copolymerized with hydroxyethyl acrylate via ATRP. μCP was utilized to structure the surfaces with an ATRP initiator, which plays an important role for the response of the glue. The adhesion occurs by moistening either the phenylboronic acid functionalized or the catechol functionalized surface and gently pressing one onto the other. After drying, the adhered surfaces show good water resistance and a strong adhesion of 2.38 kg cm−1 in average. Furthermore, this adhesion responds to carbohydrates such as fructose (Figure 9). By placing the glued surfaces in water and adding fructose, the surfaces release, because of the exchange of catechol to fructose, which is available in excess. This effect only appears for patterned surfaces where water is able to penetrate in between. For unpatterned surfaces, no release was observed. Another adhesive based on polymer brushes exploits the host−guest interaction between β-cyclodextrin (βCD) and azobenzene.47 An azobenzene acrylate was copolymerized by ATRP on surfaces. Two of these surfaces were easily glued together when a solution of a βCD polymer in water was placed between them. The βCD polymer interacts with both azobenzene decorated surfaces, holding them together with an average strength of 0.7 kg cm−1. This is reasonable due to the weaker interactions in comparison to the dynamic covalent adhesion. Although azobenzene is well-known as a photoG


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Figure 11. Scheme of an alternating surface-induced polymerization with postfunctionalization by μCP. Copyright Wiley-VCH 2017.52

upper part of the particles to achieve bifunctional Janus particles after an epoxy ring-opening reaction (Figure 12A).

SAM on glass. Afterward, styrene was polymerized via NMP to achieve polystyrene (PS) brushes. Depending on the polymerization time, brushes with a height from 13 to 40 nm were reached. The height of the PS brushes is the decisive factor for whether the polymer brushes are protein repellent or attractive.50 To explore this recognition process, fluorescently labeled proteins were applied to short (13 nm) and long PS brushes (40 nm). As predicted, the short brushes adhere to the proteins, whereas the longer ones tend to repel them.51 In order to bring more functionality to already synthesized polymer brushes, a postmodification procedure was investigated. NMP was used to prepare alternating copolymer brushes consisting of hexafluoroisopropyl acrylate (HFIPA) and 7-octenylvinyl ether (OVE) (Figure 11). Choosing these monomers, two orthogonal functional groups could be introduced into one polymer brush. Althrough the hexafluoroisopropyl ester group could react with amines, the alkenyl group reacts with thiols in a thiol−ene reaction. Thus, both functional groups are still addressable by the respective reaction, which was utilized to pattern the surface in a cross pattern via μCP. By fluorescence microscopy, all areas of the functionalized surfaces could be analyzed: amidation (red in Figure 11), thiol−ene reaction (green), no reaction in areas where the stamp was not in contact to the polymer (black), and both reactions at the junctions (orange).52

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Figure 12. (A) “Sandwich μCP” for Janus particles: A monolayer of particles is picked up by a PDMS stamp with ink number one. A second stamp with ink number two is placed on the particle layer on the first stamp to functionalize the other face of the particles. (B) Janus particles with polymer brushes containing the molecular photoswitch AAP and the formation of host−guest complexes to achieve linear aggregation. Copyright Royal Society of Chemistry 2017.54,57

SURFACE FUNCTIONALIZATION OF PARTICLES USING MICROCONTACT PRINTING The previous sections demonstrate that μCP provides excellent opportunities to functionalize hard and flat surfaces. Whereas photolithographic methods often fail to functionalize curved surfaces in an accurate fashion, μCP is an excellent method to pattern rough or curved surfaces because of the elastic PDMS stamp. Janus particles are anisotropic spheres with two or more faces with distinct physical properties. This fact allows unique chemistry and self-assembly, because each face of the particle will react or interact in specific ways. In general, the production of Janus particles can be divided into two different processes. On the one hand, such particles can be directly synthesized with two or more different faces (e.g., because of phase separation of monomers); on the other hand, an isotropic particle can be postfunctionalized to achieve anisotropy. In 2011, we introduced a method that we called “sandwich” microcontact printing to produce Janus particles.53 In the original paper, monodisperse polymer microparticles (160 μm diameter) are assembled as a monolayer on an inked PDMS stamp. A second stamp with a different ink is placed on the

In further work, we were able to reduce the size of the Janus particles to a diameter of 5 μm. Moreover, two additional types of reactions were introduced, which allows the postfunctionalization of the particles by CuAAC or thiol−ene click reactions. Because of the introduction of click chemistry, the reaction time to achieve bifunctional Janus particles could be remarkably reduced to only a few minutes.54 An orthogonal way to functionalize Janus particles was achieved by combining CuAAC on one side and the thiol−ene click reaction on the other side of one particle. By doing so, a trifunctional Janus bead could be produced, which further showed specific protein adsorption, depending on the functionalization with mannose and biotin on opposite sides.55 Furthermore, through a newly H


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Figure 13. (A) μCP as used by Böker et al. Loading of the stamp with PEI and subsequent printing on silica particles. Release of the particles in ultrasonication bath. (B) Influence of the solvent on patch structure during release. (C) Fabrication of biotinylated and avidin-functionalized patchy particles and their self-assembly. (D) Microscopy image of patchy particles self-assembled into heterodimers. Copyright Royal Society of Chemistry 2018.58

developed synthesis, poly(glycidyl methacrylate-co-divinylbenzene) monodisperse polymer particles could be obtained with a size of 3 μm and a varying amount of functional groups for further reactions. These particles were successfully to prepare bi and trifunctionalized Janus particles by sandwich μCP.56 In a newer example, we combined the “sandwich” μCP with ATRP (Figure 12B). By copolymerizing photoresponsive AAPs into the polymer brushes onto the caps of silica Janus particles, host−guest interactions with CD functionalized magnetite nanoparticles could be enabled, leading to linear aggregates of silica Janus particles that are both light and magnetically responsive.57 For the assembly of these responsive “colloidal molecules”, polymer brushes are elementary for directional aggregation, because without multivalency (i.e., only a monolayer of recognition units printed on the Janus particle), the interaction between the particles is insufficient. Similar to this approach, Böker et al. published a study in which an aqueous solution of polyethylenimine (PEI) was used as ink for sandwich μCP on silica particles via electrostatic interactions.58 They observed that when ethanol was used for the particle release after sandwich μCP, flat 2D patches of the deposited polymer were obtained. When the particles were released in acetone, the morphology changes with increasing PEI concentration in the ink from a flat “coin-like” to a thicker “cap-like” 3D structure (Figure 13B). The amine functionalities were additionally functionalized with biotin and avidin. When mixed, such particles self-assembled into heterodimers because of the strong binding force between avidin and biotin (Figure 13C and D). In contrast to our approach, the deposition of PEI on silica relies solely on electrostatic interactions and not on covalent chemistry. Because of the highly negative charge of silica particles, this method is restricted to positively charged polymers for the patches. Our approach on the other hand needs a more complex sample preparation (e.g., functionalization with ATRP initiator), but in principle, every monomer can be (co)polymerized on the particle surface. Another advantage

of the grafting-to approach by SI-ATRP is the significantly higher polymer density compared to that resulting from grafting-from techniques.

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CONCLUSIONS AND OUTLOOK The combination of μCP and surface chemistry leads to versatile and efficient applications in additive surface functionalization and patterning. Using elastomer stamps, a wide range of molecular “inks” can be deposited on a variety of substrates. Moreover, we have shown that the confinement of reactive groups in the area of contact between the stamp leads to fast surface reactions, including biorthogonal cycloadditions and thiol−ene reactions as well as bioinspired glycosylations and peptide coupling. If suitable initiators are immobilized, patterned polymer brushes can be grown by grafting-from polymerization to bring more functionality and flexibility to surfaces. Because of the higher number of functional groups inside the brushes, multivalent recognition processes including surface adhesionare highly effective. Even postmodification of existing polymer brushes by μCP is possible. Finally, we have shown that μCP can also be a useful method to functionalize microparticles and that complex Janus particles can be obtained in a single printing step. So what will the future bring? First of all, we hope that this Account will inspire more scientists to apply μCP for the functionalization and patterning of surfaces. In contrast to most types of lithography, μCP does not require expensive equipment. Moreover, it is easier than you probably think! We expect that more complex inks, such as nanoparticles and proteins, will be applied to prepare microarrays of functional and hybrid nanomaterials. However, multiplexing (i.e., the printing of multiple inks from a single stamp) remains a fundamental bottleneck of μCP. We also anticipate that, increasingly, 2D patterns will form the basis for 3D structures and that some of the concepts described in this Account will be of use for additive 3D printing. In this respect, we envisage that photochemical reactions will prove extremely useful for surface functionalization in 2D as well as 3D applications.59 Finally, I


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advanced nanoelectronic surface functionalities such as reduced work function and enhanced conductivity may arise when carbenes are patterned on metal surfaces.60

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bart Jan Ravoo: 0000-0003-2202-7485 Author Contributions #

S.L. and C.B. contributed equally to this publication.

Notes

The authors declare no competing financial interest. Biographies Sebastian Lamping received his MSc degree in 2015 and is currently a PhD candidate in chemistry at the WWU Münster. His research focuses on the functionalization of glass and silicon surfaces with various polymer brushes for the development of responsive adhesive surfaces. Christoph Buten received his MSc degree from WWU Münster in 2016. He is currently working on his PhD thesis in the field of surface functionalization with biomolecules and the development of functional surfaces using polymer brush functionalized glass, silicon, and silica substrates. Bart Jan Ravoo received his MSc and PhD degree from the University of Groningen. He was a postdoctoral fellow at University College Dublin and assistant professor at the University of Twente. Currently, he is full professor at the WWU Münster. The “leitmotiv” of his research is self-assembly, and together with his group, he develops responsive soft materials and functional surfaces by bottomup self-assembly of molecular and nanoscale building blocks.

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ACKNOWLEDGMENTS This work was funded by the Deutsche Forschungsgemeinschaft (Ra 1732/9). REFERENCES

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Functionalization and Patterning of Self-Assembled Monolayers and

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