Contact printing a biomimetic catecholic monolayer on a variety of surfaces and derivation reaction

Biomimic catecholic "ink" is employed in surface patterning by using microcontact printing (μCP) on a variety of surfaces. The surface chemical patterning can be proofed by implementing a derivation reaction, such as specific biological recognition and surface-initiated polymerization for growth of polymer brushes.     

Reactive microCP on ultrathin block copolymer films: investigation of the microCP mechanism and application to sub-microm (bio)molecular patterning

In this paper, the mechanism of the recently introduced soft lithographic patterning approach of reactive microcontact printing on thin substrate-supported polystyrene-block-poly(tert-butyl acrylate) (PS690-b-PtBA1210) films using trifluoroacetic acid (TFA)-inked elastomeric poly(dimethylsiloxane) (PDMS) stamps is investigated in detail. In this approach, solventless deprotection reactions are carried out with very high spatial definition using TFA as a volatile reagent that partitions into the PtBA skin layer. On the basis of a systematic investigation of the process, ink loading was identified as a crucial parameter for obtaining faithful pattern transfer. Using optimized conditions, submicrometer-sized patterns were successfully fabricated. In combination with subsequent wet chemical covalent coupling of various (bio)molecules, reactive microCP is established as an approach to afford positive, as well as negative, images of the features of the stamps used. In addition, the size of the patterned areas was manipulated by exploiting the controlled spreading of the ink; thus, stamps with identical features yield patterns with different sizes, yet identical periodicity, as shown for bovine serum albumin (BSA)-poly(ethylene glycol) patterns. The reactive microCP methodology affords new pathways for submicrometer-scale patterning of bioreactive surfaces.     

Dendrimer-mediated transfer printing of DNA and RNA microarrays

This paper describes a new method to replicate DNA and RNA microarrays. The technique, which facilitates positioning of DNA and RNA with submicron edge resolution by microcontact printing (muCP), is based on the modification of poly(dimethylsiloxane) (PDMS) stamps with dendrimers ("dendri-stamps"). The modification of PDMS stamps with generation 5 poly(propylene imine) dendrimers (G5-PPI) gives a high density of positive charge on the stamp surface that can attract negatively charged oligonucleotides in a "layer-by-layer" arrangement. DNA as well as RNA is transfer printed from the stamp to a target surface. Imine chemistry is applied to immobilize amino-modified DNA and RNA molecules to an aldehyde-terminated substrate. The labile imine bond is reduced to a stable secondary amine bond, forming a robust connection between the polynucleotide strand and the solid support. Microcontact printed oligonucleotides are distributed homogeneously within the patterned area and available for hybridization. By using a robotic spotting system, an array of hundreds of oligonucleotide spots is deposited on the surface of a flat, dendrimer-modified stamp that is subsequently used for repeated replication of the entire microarray by microcontact printing. The printed microarrays are characterized by homogeneous probe density and regular spot morphology.     

Site-selective surface modification using enzymatic soft lithography

Surface modification with functional polymers or molecules offers great promise for the development of smart materials and applications. Here, we describe a versatile and easy-to-use method of site-selective surface modification based on the ease of microcontact printing and the exquisite selectivity of enzymatic degradation. A micropatterned poly-L-lysine (PLL) layer on solid substrates was prepared by enzymatic degradation using trypsin enzyme immobilized on a prestructured poly(dimethlylsiloxane) (PDMS) stamp. After the enzymatic degradation of PLL and the removal of the degradation products, very well defined patterning was revealed over a large scale by fluorescence microscopy and atomic force microscopy (AFM). We investigate the advantage of our method by comparison with traditional microcontact printing and found that lateral diffusion was reduced, yielding a more accurate reproduction of the master. We also demonstrate that the stamp can be reused without reinking. The patterned surface was used for site-selective modification. The strategy was applied to two applications: the first is dedicated to the creation of amino-silane patterned surfaces, and the second illustrates the possibility of patterning polyelectrolyte multilayered thin films.     

Single-feature inking and stamping: a versatile approach to molecular patterning

A technique for micrometer-scale patterning of multiple functional biological molecules on surfaces is demonstrated. The technique is referred to as single-feature inking and stamping (SFINKS). It combines elements of dip-pen nanolithography and microcontact printing. "Inked" atomic force microscopy probes are used to ink individual features of an elastomer stamp. From a single stamp, we printed three different probe ssDNA with <10 mum resolution and showed that they specifically hybridize the complementary DNA labeled with different fluorophores. As a further demonstration of SFINKS' versatility, we patterned a silane onto a silicon wafer consisting of four subpatterns separated by >100 mum and composed of 2 mum lines. We discuss why patterns such as these are impractical with available techniques. Furthermore, we comment on the prospects for multiple stamping after a single inking.     

Microchemical Pen: An Open Microreactor for Region‐Selective Surface Modification

Various micro surface‐modification approaches including photolithography, dip‐pen lithography and ink‐jet systems have been developed and used to extend the functionalities of solid surfaces. While those approaches work in the “open space”, push–pull systems which work in solutions have recently drawn considerable attention. However, the confining flows performed by push–pull systems have realized only the dispense process, while microscale, region‐selective chemical reactions have remained unattainable. This study reports a microchemical pen that enables region‐selective chemical reactions for the micro surface modification/patterning. The chemical pen is based on the principle of microfluidic laminar flows and the resulting mixing of reagents by the mutual diffusion. The tiny diffusion layer performs as the working region. This report represents the first demonstration of an open microreactor in which two different reagents react on a real solid sample. The multifunctional characteristics of the microchemical pen are confirmed by different types of reactions in many research areas, including inorganic chemistry, polymer science, electrochemistry and biological sample treatment.     

Rapid preparation of multifunctional surfaces for orthogonal ligation by microcontact chemistry

Microcontact chemistry has been applied to patterned glass and silicon substrates by successive reaction of unprotected and monoprotected heterobifunctional linkers with alkene-terminated self-assembled monolayers (SAMs) to produce bi-, tri-, and tetrafunctional surfaces. Photochemical microcontact printing of an azide thiol linker followed by immobilization of an acid thiol linker on an undecenyl-terminated SAM results in a well-defined, micropatterned surface with terminal azide, acid, and alkene groups. Biologically relevant molecules (biotin, carbohydrates) have been selectively attached to the surface by means of orthogonal ligation chemistry, and the resulting microarrays display selective binding to fluorescently labeled proteins. An orthogonally addressable, tetrafunctional surface (azide, acid, alkene, and amine) can be prepared by an additional printing step of a tert-butyloxycarbonyl (Boc)-protected alkyne amine linker on the azide structures by using the copper(I)-catalyzed azide-alkyne Huisgen cycloaddition and subsequent removal of the protective group.     

Recent advances in microcontact printing

Microcontact printing is a remarkable surface patterning technique. Developed about 10 years ago, it has triggered enormous interest from the surface science community, as well as from engineers and biologists. The last five years have been rich in improvements to the microcontact printing process itself, as well as in new technical innovations, many designed to suit new applications. In this review, we describe the evolution of microcontact printing over the past five years. The review is categorized into three main sections: the improvements made to the technique, new variations, and new applications.     

Positive microcontact printing with mercaptoalkyloligo(ethylene glycol)s

The soft lithographic replication of patterns with a low filling ratio by microcontact printing (microCP) is problematic due to the poor mechanical stability of common elastomeric stamps. A recently described strategy to avoid this problem employs a modified patterning method, positive microcontact printing ((+)microCP), in which a stamp with a mechanically more stable inverted relief pattern is used. In contrast to conventional negative microCP ((-)microCP), in the contact areas a self-assembled monolayer (SAM) is printed of a "positive ink", which provides only minor etch protection, whereas the noncontacted areas are subsequently covered with a different, etch-resistant SAM, prior to development by chemical etching. With the aim to identify novel, highly versatile positive inks, the patterning of gold by (+)microCP with mercaptoalkyloligo(ethylene glycol)s (MAOEGs), the subsequent adsorption of octadecanethiol (ODT), and the final development by wet chemical etching have now been studied. A polydisperse mixture of mercaptoundecylocta(ethylene glycol) derivatives was found to provide the best patterning results. The surface spreading of the positive ink during stamping, the exchange of printed MAOEGs with ODT, and the choice of the right etching bath were identified as key parameters that influence the achievable pattern resolution and contrast. Due to the modular composition of functionalized alkyloligo(ethylene glycol) derivatives, (+)microCP with these positive inks has the potential for easy adaptation to a variety of materials and development conditions.     

Versatile Stamps in Microcontact Printing: Transferring Inks by Molecular Recognition and from Ink Reservoirs

Microcontact printing is a heavily used surface modification method in materials and life science applications. This concept article focuses on the development of versatile stamps for microcontact printing that can be used to bind and release inks through molecular recognition or through an ink reservoir, the latter being used for the transfer of heavy inks, such as biomolecules and particles. Conceptually, such stamp properties can be introduced at the stamp surface or by changing the bulk stamp material; both lines of research will be reviewed here. Examples include supramolecular stamps with affinity properties, polymer‐layer‐grafted PDMS stamps, and porous multilayer‐grafted PDMS stamps for the first case, and hydrogel stamps and porous stamps made by phase‐separation micromolding for the second. Potential directions for future advancement of this field are also discussed.     

Peroxidase-catalyzed in situ polymerization of surface orientated caffeic acid

Nanoscale surface patterning and polymerization of caffeic acid on 4-aminothiophenol-functionalized gold surfaces has been demonstrated with dip pen nanolithography (DPN). The diphenolic moiety of caffeic acid can be polymerized by biocatalysis with laccase or horseradish peroxidase. In the present study, the DPN patterned features were polymerized in situ through the use of the peroxidase. Using samples prepared by DPN, microcontact printing, and adsorption on macroscopic substrates, the products were characterized by electrostatic force microscopy (EFM), MALDI-TOF, X-ray photoelectron spectroscopy (XPS), UV-vis, and FT-IR. The in situ surface polymerization resulted in the formation of a quinone structure, while the phenyl ester formed in bulk polymerization reactions was not detected. A different coupling site was observed when comparing the polymers obtained from solution (bulk) vs the surface DPN reactions. The structural differences were attributed to surface-induced pre-organization and orientation of the monomers prior to the enzymatic polymerization step. The results of this study expand the application of DPN technology to surface modification and surface chemistry reactions wherein stereo-regularity and regioselectivity can be exploited.     

Reversible covalent patterning of self-assembled monolayers on gold and silicon oxide surfaces

This paper describes the generation of reversible patterns of self-assembled monolayers (SAMs) on gold and silicon oxide surfaces via the formation of reversible covalent bonds. The reactions of (patterned) SAMs of 11-amino-1-undecanethiol (11-AUT) with propanal, pentanal, decanal, or terephthaldialdehyde result in dense imine monolayers. The regeneration of these imine monolayers to the 11-AUT monolayer is obtained by hydrolysis at pH 3. The (patterned) monolayers were characterized by Fourier transform infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, contact angle and electrochemical measurements, and atomic force microscopy. Imines can also be formed by microcontact printing of amines on terephthaldialdehyde-terminated substrates. Lucifer Yellow ethylenediamine was employed as a fluorescent amine-containing marker to visualize the reversible covalent patterning on a terephthaldialdehyde-terminated glass surface by confocal microscopy. These experiments demonstrate that with reversible covalent chemistry it is possible to print and erase chemical patterns on surfaces repeatedly.     

Dip-pen nanolithography of reactive alkoxysilanes on glass

The use of organofunctional silane chemistry is a flexible and general method for immobilizing biomolecules on silicon oxide surfaces, including fabricating DNA, small-molecule, and protein microarrays. The biggest hurdle in employing dip-pen nanolithography (DPN) for extending this general approach to the nanoscopic domain is the tendency of trialkoxy- and trichlorosilanes to rapidly polymerize due to hydrolysis reactions. The control of the local water concentration between the substrate surface and the scanning AFM tip is critical, both to the physical and chemical processes involved in DPN writing and to the ability to form well-defined thin layers of reactive silanes without extensive polymerization induced disorder. We found that we could control the degree of polymerization through careful choice of the alkoxysilane used as the "ink" for DPN and through control of the relative humidity during inking and writing with the coated AFM tip. As a proof-of-principle, we demonstrate that areas patterned with an alkoxysilane on glass with DPN are functional for subsequent immobilization of fluorescently labeled streptavidin via covalent attachment of biotin. This preliminary result sets the stage for the ability to capture proteins in their fully hydrated state from buffered solution, by molecular recognition onto previously written reactive nanoscopic regions on oxidized silicon and glass.     

Covalent microcontact printing of proteins for cell patterning

We describe a straightforward approach to the covalent immobilization of cytophilic proteins by microcontact printing, which can be used to pattern cells on substrates. Cytophilic proteins are printed in micropatterns on reactive self-assembled monolayers by using imine chemistry. An aldehyde-terminated monolayer on glass or on gold was obtained by the reaction between an amino-terminated monolayer and terephthaldialdehyde. The aldehyde monolayer was employed as a substrate for the direct microcontact printing of bioengineered, collagen-like proteins by using an oxidized poly(dimethylsiloxane) (PDMS) stamp. After immobilization of the proteins into adhesive "islands", the remaining areas were blocked with amino-poly(ethylene glycol), which forms a layer that is resistant to cell adhesion. Human malignant carcinoma (HeLa) cells were seeded and incubated onto the patterned substrate. It was found that these cells adhere to and spread selectively on the protein islands, and avoid the poly(ethylene glycol) (PEG) zones. These findings illustrate the importance of microcontact printing as a method for positioning proteins at surfaces and demonstrate the scope of controlled surface chemistry to direct cell adhesion.     

Spatially controlled DNA nanopatterns by "click" chemistry using oligonucleotides with different anchoring sites

DNA patterning on surfaces has broad applications in biotechnology, nanotechnology, and other fields of life science. The common patterns make use of the highly selective base pairing which might not be stable enough for further manipulations. Furthermore, the fabrication of well-defined DNA nanostructures on solid surfaces usually lacks chemical linkages to the surface. Here we report a template-free strategy based on "click" chemistry to fabricate spatially controlled DNA nanopatterns immobilized on surfaces. The self-assembly process utilizes DNA with different anchoring sites. The position of anchoring is of crucial importance for the self-assembly process of DNA and greatly influences the assembly of particular DNA nanopatterns. It is shown that the anchoring site in a central position generates tunable nanonetworks with high regularity, compared to DNAs containing anchoring sites at terminal and other positions. The prepared patterns may find applications in DNA capturing and formation of pores and channels and can serve as templates for the patterning using other molecules.     

Submerged microcontact printing (SmuCP): an unconventional printing technique of thiols using high aspect ratio, elastomeric stamps

A technique for microcontact printing of thiols in liquid media is presented. Elastomeric poly(dimethyl siloxane) stamps are used to pattern gold surfaces with thiol-based self-assembled monolayers. The liquid (water in this case) has been used as an incompressible support and, advantageously, also acts as a medium in which alkylthiol ink molecules are poorly miscible. Consequently, we have been able to produce patterned thiol monolayers using stamps with aspect ratios unsuitable for conventional microcontact printing (i.e., 15:1) and present evidence to suggest that it is possible to use stamps with aspect ratios of up to 100:1.     

Click chemistry by microcontact printing on self-assembled monolayers: a structure-reactivity study by fluorescence microscopy

Microcontact printing (μCP) has developed into a powerful tool to functionalize surfaces with patterned molecular monolayers. μCP can also be used to induce a chemical reaction between a molecular ink and a self-assembled monolayer (SAM) in the nanoscale confinement between stamp and substrate. In this paper, we investigate the Huisgen 1,3-dipolar cycloaddition, the Diels-Alder cycloaddition and the thiol-ene/yne reaction induced by μCP. A range of fluorescent alkyne inks were printed on azide SAMs and fluorescence microscopy was used to monitor the extent of the 1,3-dipolar cycloaddition on a glass substrate. The rate of cycloaddition depends on the reactivity of the alkyne and on the presence of Cu(I). The cycloaddition is accelerated by Cu(I) but it also proceeds readily in the absence of Cu(I). In addition, a range of fluorescent diene inks were printed on alkene SAMs on glass. In this case, fluorescence microscopy was used to monitor the rate of the Diels-Alder cycloaddition as well as its retro-reaction. Finally, fluorescent thiol inks were printed on alkene SAMs on glass, and fluorescent alkenes and alkynes were printed on thiol SAMs. It is shown that reactions by μCP follow structure-reactivity relationships similar to solution reactions. Under optimized conditions all reactions lead to dense microarrays of addition products within minutes of printing time.     

Molecular transfer and transport in noncovalent microcontact printing

Microcontact printing is commonly used to create patterned films of molecules covalently bonded to substrates (e.g., thiols on gold). Here we describe microcontact printing of several types of noncovalently bonding molecules on mica. Due to the weaker interaction of the molecules with the substrate, environmental factors such as temperature and relative humidity play an important role. The vapor pressure of the inks also had a large impact on the fidelity of the stamped patterns. Fingering instabilities were observed for monolayers of octadecanol, docosanol, stearylamine, and stearic acid stamped at moderate relative humidity. The fidelity of the stamped pattern generally increased with the headgroup-surface interaction strength. These stamped monolayer films shed light on molecular transfer and two-dimensional spreading mechanisms.     

Rewritable Polymer Brush Micropatterns Grafted by Triazolinedione Click Chemistry

Triazolinedione (TAD) click reactions were combined with microcontact chemistry to print, erase, and reprint polymer brushes on surfaces. By patterning substrates with a TAD‐tagged atom‐transfer radical polymerization initiator (ATRP‐TAD) and subsequent surface initiated ATRP, it was possible to graft micropatterned polymer brushes from both alkene‐ and indole‐functionalized substrates. As a result of the dynamic nature of the Alder–ene adduct of TAD and indole at elevated temperatures, the polymer pattern could be erased while the regenerated indole substrate could be reused to print new patterns. To demonstrate the robustness of the methodology, the write–erase cycle was repeated four times.     

Controlling the stability and reversibility of micropillar assembly by surface chemistry

For many natural and synthetic self-assembled materials, adaptive behavior is central to their function, yet the design of such systems has mainly focused on the static form rather than the dynamic potential of the final structure. Here we show that, following the initial evaporation-induced assembly of micropillars determined by the balance between capillarity and elasticity, the stability and reversibility of the produced clusters are highly sensitive to the adhesion between the pillars, as determined by their surface chemistry and further regulated by added solvents. When the native surface of the epoxy pillars is masked by a thin gold layer and modified with monolayers terminated with various chemical functional groups, the resulting effect is a graded influence on the stability of cluster formation, ranging from fully disassembled clusters to an entire array of stable clusters. The observed assembly stabilization effect parallels the order of the strengths of the chemical bonds expected to form by the respective monolayer end groups: NH(2) ≈ OH < COOH < SH. For each functional group, the stability of the clusters can be further modified by varying the carbon chain length of the monolayer molecules and by introducing solvents into the clustered samples, allowing even finer tuning as well as temporal control of disassembly. Using these features together with microcontact printing, we demonstrate straightforward patterning of the microstructured surfaces with clusters that can be erased and regenerated at will by the addition of appropriate solvents. Subtle modifications to surface and solvent chemistry provide a simple way to tune the balance between adhesion and elasticity in real time, enabling structures to be designed for dynamic, responsive behavior.