Bifunctional, chemically patterned flat stamps for microcontact printing of polar inks

Different methods to create chemically patterned, flat PDMS stamps with two different chemical functionalities were compared. The best method for making such stamps, functionalized with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFDTS) and 3-(aminopropyl)triethoxysilane (APTS), appeared to be full functionalization of a freshly oxidized flat PDMS stamp with either adsorbate, followed by renewed oxidation through a mask and attachment of the other adsorbate. These stamps were used to transfer polar inks (a thioether-functionalized dendrimer and a fluorescent dye) by microcontact printing. The PFDTS monolayer was used as a barrier against ink transfer, while the APTS SAM areas functioned as an ink reservoir for polar inks. The printing results confirmed the excellent transfer of hydrophilic inks with these stamps to gold and glass substrates, even from aqueous solutions. Attachment of a fluorescent dye on the amino-functionalized regions shows the possibility of the further modification of the chemically patterned stamps for tailoring of the stamps' properties.     

Spreading of 16-mercaptohexadecanoic acid in microcontact printing

Spreading in microcontact printing refers to the process or processes by which the ink molecules end up in the parts of the substrate that are adjacent to the contacted areas but which are not contacted themselves. This has been investigated for different inking concentrations of 16-mercaptohexadecanoic acid (MHDA). Spreading of MHDA takes place with retention of a well-defined demarcation. Feature sizes can be controlled by varying the contact times. Spreading, however, only takes place beyond a certain threshold concentration. For low ink concentrations the edges of stamp features dominate the ink transfer. For these low concentrations the extent of this edge dominance depends strongly on ink concentration rather than on contact time. These observations indicate a dominant role of the stamp surface in the processes of pattern formation and spreading.     

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.     

Hydrophilic elastomers for microcontact printing of polar inks

A moderately hydrophilic, thermoplastic elastomer (poly(ether-ester)) was investigated as a stamp material for microcontact printing of a polar ink: pentaerythritol-tetrakis-(3-mercaptopropionate). Stamps with a relief structure were produced from this polymer by hot embossing, and a comparison was made with conventional poly(dimethylsiloxane) (PDMS) and oxygen-plasma-treated PDMS. It is shown that the hydrophilic stamps can be used for the repetitive printing (without re-inking) of at least 10 consecutive patterns, which preserve their etch resistance, and this in rather sharp contrast to conventional and oxygen plasma-treated PDMS stamps. It is argued that these enhanced printing characteristics of the hydrophilic stamps originate from an improved wetting and solubility of polar inks in the hydrophilic stamp.     

Microcontact printing of monodiamond nanoparticles: an effective route to patterned diamond structure fabrication

By combining microcontact printing with a nanodiamond seeding technique, a precise micrometer-sized chemical vapor deposition (CVD) diamond pattern have been obtained. On the basis of the guidance of basic theoretical calculations, monodisperse detonation nanodiamonds (DNDs) were chosen as an "ink" material and oxidized poly(dimethylsiloxane) (PDMS) was selected to serve as a stamp because it features a higher interaction energy with the DNDs compared to that of the original PDMS. The adsorption kinetics shows an approximately exponential law with a maximum surface DND density of 3.4 × 10(10) cm(-2) after 20 min. To achieve a high transfer ratio of DNDs from the PDMS stamp to a silicon surface, a thin layer of poly(methyl methacrylate) (PMMA) was spin coated onto the substrates. A microwave plasma chemical vapor deposition system was used to synthesize the CVD diamond on the seeded substrate areas. Precise diamond patterns with a low expansion ratio (3.6%) were successfully prepared after 1.5 h of deposition. Further increases in the deposition time typically lead to a high expansion rate (～0.8 μm/h). The general pattern shape, however, did not show any significant change. Compared with conventional diamond pattern deposition methods, the technique described here offers the advantages of being simple, inexpensive, damage-free, and highly compatible, rendering it attractive for a broad variety of industrial applications.     

Surface patterning by microcontact chemistry

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.     

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.     

Heavyweight dendritic inks for positive microcontact printing

Poly(propylene imine) dendrimers with dialkyl sulfide end groups were prepared and developed as inks for positive microcontact printing ((+)muCP) on gold. Long (C10H21-S-C10H20-), medium (C3H7-S-C4H8-), and short (CH3-S-CH2-) dialkyl sulfide end groups were attached to second- and third-generation PPI dendrimers to create a family of dendritic sulfides. The dendritic inks flatten upon adsorption and form monolayers on gold. (+)muCP was performed on gold using commercially available poly(dimethylsiloxane) as stamp material and n-octadecanethiol as etch resist. The gold beneath the dendrimers was selectively etched away with an acidic Fe(NO3)3/thiourea solution to give the positive copy of the original master pattern. The multivalent sulfide attachment and the relatively high molecular mass of these dendrimers ensured minimal lateral ink spreading and thus optimal feature reproducibility. Contact times were varied to analyze the spreading rates of the dendritic inks. The spreading rates of the dendritic inks were found to be much lower than that of pentaerythritol tetrakis(3-mercaptopropionate). (+)muCP with the new inks was extended to submicrometer features. Optical microscopy, scanning electron microscopy, and atomic force microscopy were used to characterize the etched samples. Lines with a width of 100 nm were faithfully replicated with the third-generation dendrimers bearing medium (C3-S-C4-) end groups.     

High-speed microcontact printing

We have demonstrated microcontact printing (muCP) of self-assembled monolayers in the millisecond regime. The contact formation and separation of the stamp and substrate was studied with high-speed video recordings. Using high ink concentrations and contact times as short as 1 ms, we printed monolayers of hexadecanethiol on Au, which served as a selective etch resist. High-speed muCP yields defect-free monolayers that are independent of the dimensions of the printed patterns, have high contrast between printed and unprinted areas, and enable perfect reproducibility of prints.     

Surface modification of elastomeric stamps for microcontact printing of polar inks

Chemical modification of the surface of a stamp used for microcontact printing (microCP) is interesting for controling the surface properties, such as the hydrophilicity. To print polar inks, plasma polymerization of allylamine (PPAA) was employed to render the surface of poly(dimethylsiloxane) (PDMS), polyolefin plastomers (POP), and Kraton elatomeric stamps hydrophilic for long periods of time. A thin PPAA film of about 5 nm was deposited on the stamps, which increased the hydrophilicity, and which remained stable for at least several months. These surface-modified stamps were used to transfer polar inks by microCP. The employed microCP schemes are as follows: (a) a second generation of dendritic ink having eight dialkyl sulfide end groups to fabricate patterns on gold substrates by positive microCP, (b) fluorescent guest molecules on beta-cyclodextrin (beta-CD) printboards on glass employing host-guest recognition, and (c) Lucifer Yellow ethylenediamine resulting in covalent patterning on an aldehyde-terminated glass surface. All experiments resulted in an excellent performance of all three PPAA-coated stamp materials to transfer the polar inks from the stamp surface to gold and glass substrates by microCP, even from aqueous solutions.     

Effect of stamp deformation on the quality of microcontact printing: theory and experiment

Microcontact printing (microCP) is an effective way to generate micrometer- or submicrometer-sized patterns on a variety of substrates. However, the fidelity of the final pattern depends critically on the coupled phenomena of stamp deformation, fluid transfer between surfaces, and the ability of the ink to self-assemble on the substrate. In particular, stamp deformation can produce undesirable effects that limit the practice and precision of microCP. Experimental observations and comparison with theoretical predictions are presented here for three of the most undesirable consequences of stamp deformation: (1) roof collapse of low aspect ratio recesses, (2) buckling of high aspect ratio plates, and (3) lateral sticking of high aspect ratio plates. Stamp behavior was observed visually with an inverted optical microscope while load-displacement data were collected during compression and retraction of stamps. Additionally, a "robotic stamper" was used to deliver ink patterns in precise locations on substrates. These monomolecular ink patterns were then observed in high contrast using the surface potential scanning mode of an atomic force microscope. Theoretical models based on continuum mechanics were used to accurately predict both physical deformation of the stamp and the resultant inking patterns. The close agreement between these models and the experimental data presented clearly demonstrates the essential considerations one must weigh when designing stamp geometry, material, and loading conditions for optimal pattern fidelity.     

微接触印刷法制造聚合物多层次准三维立体微结构

Microstructures of various polymers, such as polystyrene and polymethyl methacrylate, were fabricated with microcontact printing, directly using the corresponding dilute polymeric solutions as “inks”, whose concentrations were about 10 g/L. By repeatedly cross-stamping with the inks, multilayer quasi-three-dimensional polymeric microstructures could be obtained. Both optical photographs and SEM photos showed clear microstructures, which were nearly accurate replication of the original patterns in the PDMS stamps. Microlines of poly-bis-(p-toluene sulfonate)-2,4-hexadiyne-1,6-diol) (PTS) were also fabricated by first processed microcontact printing with solution of the corresponding monomer TS/acetone as ink, then followed with UV polymerization of the monomer micropatterns at solid state. Unlike small molecule processes, the molecules of polymeric inks did not self assembly on the surface of substrates. The formation of polymeric microstructures could be ascribed to the fact that, after volatilization of solvents, polymers tend to stick to the surface of glass substrate which has higher surface free energy (about 72 mN/m), but not to the surface of PDMS stamp which has lower surface free energy (about 20 mN/m). Also the microcontact printing process was studied with optical microscopy, and the main factor--volatilization time of solvent was discussed. The results showed that the volatilization time of solvent is very crucial to the process of polymeric microcontact printing, and with too longer or too shorter volatilization time, the obtained microstructures would become discontinuous or distorted, respectively. For example, with a polystyrene/chloroform solution as ink, the optimal volatilization time was about 15~20 s.     

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.     

CuSO4 Micro-crystal Lattice Produced by Microcontact Printing

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Biocatalytic microcontact printing

Immobilized biocatalytic lithography is presented as an application of soft lithography. In traditional microcontact printing, diffusion limits resolution of pattern transfer. By using an immobilized catalyst, the lateral resolution of microcontact printing would depend only on the length and flexibility of the tether (<2 nm) as opposed to diffusion (>100 nm). In the work, exonuclease reversibly immobilized on a relief-patterned stamp is used to ablate ssDNA monolayers Percent of ablation was determined via confocal fluorescence microscopy to be approximately 70%.     

Growth kinetics and morphology of self-assembled monolayers formed by contact printing 7-octenyltrichlorosilane and octadecyltrichlorosilane on Si(100) wafers

The growth kinetics and morphologies of self-assembled monolayers deposited by contact printing 7-octenyltrichlorosilane (OCT) and octadecyltrichlorosilane (OTS) on Si(100) were studied by ellipsometry and atomic force microscopy. We found that, for both OCT and OTS, full monolayers could be obtained at room temperature after printing times of 120-180 s; the printing-based monolayer assembly processes follow apparent Langmuir adsorption kinetics, with the measured film growth rates increasing both with the ambient humidity and with concentration of the ink used to load the stamp. At a dew point of 10 degrees C and an ink concentration (in toluene) of 50 mM, the observed film growth rate constant is 0.05 s(-)(1). When the printing was carried out at a lower ambient humidity (dew points of 1-3 degrees C), the measured rates of assembly were approximately a factor of 2 slower. Increasing the deposition temperature from 25 to 45 degrees C under these conditions increased the film growth rate only slightly. The morphology of the films depends on the identity of the ink. Uniform, high-coverage films could be obtained readily from the eight-carbon chain length adsorbate OCT, provided that the stamp was not overloaded with the ink; for high concentrations outside of the optimal range, the surface presented significant numbers of adsorbed particles ascribed, in part, to siloxane polymers formed by hydrolysis of the ink on the stamp before printing. In marked contrast, for the 18-carbon adsorbate OTS, the printed films always consisted of a mixture of a uniform monolayer plus adsorbed polysiloxane particles. The different film morphologies seen for OCT and OTS are proposed to result from the different transfer efficiencies of the organotrichlorosilane relative to polysiloxane hydrolysis products formed during the printing process. These transfer efficiencies exhibit sensitivities related to the permeation of the poly(dimethylsiloxane) (PDMS) stamp by the silane reagents. Short-chain inks such as OCT evidently permeate the PDMS stamp more deeply than longer-chain inks such as OTS. This difference, and the different diffusion rates of ink vs oligomeric silane hydrolysis products, determines the film morphology obtained by contact printing. The mass transfer dynamics of the process thus yield surface layers derived from varying quantities of siloxane oligomers, which subsequently transfer to the substrate along with unhydrolyzed silane adsorbate during the printing step. The structural evolution of the contact-printed films so obtained is strikingly different from that of SAMs prepared by immersion.     

Interface interaction controlled transport of CdTe nanoparticles in the microcontact printing process

High-quality CdTe nanoparticles stabilized with thioglycolic acid (TGA) are patterned on SiO2/Si surfaces using microcontact printing (microCP). Due to the weak interaction of the nanoparticles with the stamp surface, tailoring of gas flow rate during the inking process as well as the type and scale of the patterns on the stamp are used to control the distribution of the nanoparticles on the structured stamp surface. This distribution is then transferred the printed regions. Either edge printing or homogeneous printing can be achieved under optimized conditions. In addition, new structures such as nanowires form under certain conditions.     

Microscale features and surface chemical functionality patterned by electron beam lithography: a novel route to poly(dimethylsiloxane) (PDMS) stamp fabrication

Poly(dimethylsiloxane) (PDMS) has become a ubiquitous material for microcontact printing, yet there are few methods available to pattern a completed PDMS stamp in a single step. It is shown here that electron beam lithography (EBL) is effective in writing patterns directly onto cured PDMS stamps, thus overcoming the need for multiple patterning steps. Not only does this method allow the modification of an existing lithographic pattern, but new 3D features such as cones, pits, and channels can also be fabricated. EBL can also be used to fabricate PDMS masks for photolithography whereby 1:1 pattern transfer into a photoresist is achieved. Additionally, direct EBL writing of surface chemical features has been achieved using a PDMS stamp coated with a self-assembled monolayer. An electrostatic mechanism appears to be operative in the EBL patterning process, as supported by calculations, thermogravimetric analysis, time-of-flight secondary ion mass spectroscopy, optical and atomic force microscopy, and chemical functionalization assays.     

Poly(dimethylsiloxane) contamination in microcontact printing and its influence on patterning oligonucleotides

It is well-established that, during microcontact printing (muCP) using poly(dimethylsiloxane) (PDMS)-based stamps, some unexpected siloxane fragments can be transferred from the stamp to the surface of the sample. This so-called contamination effect coexists with the delivery of the molecules constituting the ink and by this way influences the printing process. The real impact of this contamination for the muCP technique is still partially unknown. In this work, we investigate the kinetics of this contamination process through the surface characterization of both the sample and the stamp after imprinting. The way both the curing conditions of the PDMS material and the contact time influence the degree of contamination of the surface is investigated on silicon and glass substrates. We propose a cleaning process of the stamp during several hours which eliminates any trace of contamination during printing. We show that hydrophobicity recovery of PDMS surfaces after hydrophilic treatment using oxygen plasma is considerably slowed down when the PDMS material is cleaned using our procedure. Finally, by comparing cleaned and uncleaned PDMS stamps, we show the influence of contamination on the quality of muCP using fluorescent DNA molecules as an ink. Surprisingly, we observe that the amount of DNA molecules transferred during muCP is higher for the uncleaned stamp, highlighting the positive impact of the presence of low molecular weight siloxane fragments on the muCP process. This result is attributed to the better adsorption of oligonucleotides on the stamp surface in presence of these contaminating molecules.     

Selective depositions on polyelectrolyte multilayers: self-assembled monolayers of m-dPEG acid as molecular template

This paper describes the fabrication of self-assembled monolayer (SAM) patterns of m-d-poly(ethylene glycol) (m-dPEG) acid molecules onto polyelectrolyte multilayers (PEMs). The patterned SAMs on PEMs were created by ionic interactions using microcontact printing (microCP) technique. The created m-dPEG acid monolayer patterns on PEMs act as resistive templates, and thus further depositions of consecutive poly(anion)/poly(cation) pairs of charged particles result in the formation of three-dimensional (3-D) patterned PEM films or selective particle depositions atop the original multilayer thin films. In this study, we illustrate nonlithographic methods of patterning and controlling 3-D PEM architectures and selective particle depositions. We investigated the effect of variables--the choice of solvent, concentration, pH, substrate pretreatment, and stamp contact times--on microcontact printing of m-dPEG acid molecules onto PEM films to determine the optimal conditions for these parameters to achieve efficient transfer of m-dPEG acid patterns onto PEMs. Among the variables, the pH of the m-dPEG acid ink solution played the most important role in the transfer efficiency of the patterns onto the multilayer films. The patterned films were characterized by optical microscopy and atomic force microscopy (AFM).