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.     

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%.     

Single-colloidal-particle microcontact printing

We have utilized colloidal probe atomic force microscopy (AFM) in conjunction with microinterferometry to study the forces acting in the microcontact printing process. The procedure we have developed yields well defined asymmetric imprints on colloidal probes, which could find further application in interaction force measurements between Janus-type particles and surfaces.     

Positive microcontact printing

Microcontact printing alkanethiols from an inked, microstructured stamp onto Au or Cu results in the formation of a self-assembled monolayer, which can locally protect the substrate from wet etching. This lithographic technique resembles a negative-type of lithography but can be inverted to the positive process. This is done by printing a type of oligothiol that does not protect the substrate from etching but prevents the adsorption of a protective monolayer from solution.     

Tunable micropatterned substrates based on poly(dopamine) deposition via microcontact printing

A simple technique was developed to fabricate tunable micropatterned substrates based on mussel-inspired surface modification. Polydopamine (PDA) was developed on polydimethylsiloxane (PDMS) stamps and was easily imprinted to several substrates such as glass, silicon, gold, polystyrene, and poly(ethylene glycol) via microcontact printing. The imprinted PDA retained its unique reactivity and could modulate the chemical properties of micropatterns via secondary reactions, which was illustrated in this study. PDA patterns imprinted onto a cytophobic and nonfouling substrates were used to form patterns of cells or proteins. PDA imprints reacted with nucleophilic amines or thiols to conjugate molecules such as poly(ethylene glycol) for creating nonfouling area. Gold nanoparticles were immobilized onto PDA-stamped area. The reductive ability of PDA transformed silver ions to elemental metals as an electroless process of metallization. This facile and economic technique provides a powerful tool for development of a functional patterned substrate for various applications.     

Peptides on GaAs surfaces: comparison between features generated by microcontact printing and dip-pen nanolithography

Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS) were employed to understand the size, composition, and conformation of lithographic patterns composed of peptide molecules. GaAs surfaces were patterned by microcontact printing (microCP) and dip-pen nanolithography (DPN) using a peptide sequence composed of 15 amino acids. The detailed surface evaluation showed that the patterns have similar chemical compositions but differ in the bonding among the molecules anchored on the GaAs substrate. Both types of patterns were crystalline-like in nature. The features created by DPN exhibited interchain hydrogen bonding, while the ones generated by microCP displayed non-hydrogen bonding. The differences in the lithographic structures can be utilized in future biorecognition experiments that take advantage of the electronic properties of the GaAs substrate and the tunable behavior of the covalently anchored biomolecules on the surface.     

Characterization of supported membranes on topographically patterned polymeric elastomers and their applications to microcontact printing

This article describes the fluorescence microscopy and imaging ellipsometry-based characterization of supported phospholipid bilayer formation on elastomeric substrates and its application in microcontact printing of spatially patterned phospholipid bilayers. Elastomeric stamps, displaying a uniformly spaced array of square wells (20, 50, and 100 mum linear dimensions), are prepared using poly(dimethyl)siloxane from photolithographically derived silicon masters. Exposing elastomeric stamps, following UV/ozone-induced oxidation, to a solution of small unilamellar phospholipid vesicles results in the formation of a 2D contiguous, fluid phospholipid bilayers. The bilayer covers both the elevated and depressed regions of the stamp and exhibits a lateral connectivity allowing molecular transport across the topographic boundaries. Applications of these bilayer-coated elastomeric stamps in microcontact printing of lipid bilayers reveal a fluid-tearing process wherein the bilayer in contact regions selectively transfers with 75-90% efficiency, leaving behind unperturbed patches in the depressed regions of the stamp. Next, using cholera-toxin binding fluid POPC bilayers that have been asymmetrically doped with ganglioside Gm1 ligand in the outer leaflets, we examine whether the microcontact transfer of bilayers results in the inversion of the lipid leaflets. Our results suggest a complex transfer process involving at least partial bilayer reorganization and molecular re-equilibration during (or upon) substrate transfer. Taken together, the study sheds light on the structuring of lipid inks on PDMS elastomers and provides clues regarding the mechanism of bilayer transfer. It further highlights some important differences in stamping fluid bilayers from the more routine applications of stamping in the creation of patterned self-assembled monolayers.     

Fabrication of dipolar colloid particles by microcontact printing

A novel technique for preparation of dipolar colloid particles has been developed which is based on microcontact printing of films of water-insoluble ionic surfactants onto monolayers of colloid particles of opposite surface charge.     

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.     

Multiprotein microcontact printing with micrometer resolution

Depositing multiple proteins on the same substrate in positions similar to the natural cellular environment is essential to tissue engineering and regenerative medicine. In this study, the development and verification of a multiprotein microcontact printing (μCP) technique is described. It is shown that patterns of multiple proteins can be created by the sequential printing of proteins with micrometer precision in registration using an inverted microscope. Soft polymeric stamps were fabricated and mounted on a microscope stage while the substrate to be stamped was placed on a microscope objective and kept at its focal distance. This geometry allowed for visualization of patterns during the multiple stamping events and facilitated the alignment of multiple stamped patterns. Astrocytes were cultured over stamped lane patterns and were seen to interact and align with the underlying protein patterns.     

Intact transfer of layered, bionanocomposite arrays by microcontact printing

A novel approach is presented that allows high-quality, 3D patterned bionanocomposite layered films to be constructed on substrates whose surface properties are incompatible with existing self-assembly methods.     

Size-tuneable and micro-patterned iron nanoparticles derived from biomolecules via microcontact printing SAM-modified substrates and controlled-potential electrolyses

Site-selected and size-controlled iron nanoparticles were prepared on coplanar surfaces via microcontact printing of SAM-modified Au/mica electrodes and controlled-potential electrolytic reactions using ferritin biomolecules. Ferritin molecules packed like a full monolayer on 6-amino-1-hexanethiol (AHT)- and 11-amino-1-undecanethiol (AUT)-modified Au/mica surface via electrostatic interactions, which did not depend on the chain length of the amino terminal alkane thiols. After heat-treatment at 400 degrees C for 60 min, iron oxide nanoparticles (ca. 5 nm in diameter) derived from ferritin cores were observed at the Au/mica surface by atomic force microscopy (AFM). On the study on the electrochemistry of ferritin immobilized onto AHT- and AUT-modified Au/mica electrodes, the redox response of the ferritin immobilized AHT-modified electrode was clearly observed. On the other hand, no redox peak for ferritin was obtained at the AUT-modified electrode. The electron transfer between ferritin and the electrode through the AUT membrane could not take place. The difference in the electrochemical response of ferritin immobilized onto AHT- and AUT-modified Au/mica was caused by the chain length of the amino terminal alkane thiols. Uniform patterns of AHT and AUT on the Au/mica electrode surface were performed by use of a poly(dimethylsiloxane) (PDMS) stamp. After the immobilization of ferritin onto both AHT- and AUT-modified electrode surfaces, the modified electrode was applied to a -0.5 V potential for 30 min in a phosphate buffer solution. After this procedure, the PDMS stamp patterning image appeared by scanning electron microscopy (SEM) image. The SEM results induced by the size change of the ferritin core consisting of iron(III) by electrolysis.     

Chemically patterned flat stamps for microcontact printing

Locally oxidized patterns on flat poly(dimethylsiloxane) stamps for microcontact printing were used as a platform for the transfer of a hydrophilic fluorescent ink to a glass substrate. The contrast was found to be limited. These locally oxidized patterns were conversely used as barriers for the transfer of hydrophobic n-octadecanethiol. In this case a good contrast was obtained, but the pattern was found to be susceptible to defects (cracks) in the barrier layer. Local stamp surface oxidation and subsequent modification with 1H,1H,2H,2H-perfluorodecyltrichlorosilane, for use as a barrier in the transfer of n-octadecanethiol, 16-mercaptohexadecanoic acid, and octanethiol, resulted in remarkably good contrast and stable patterns. The improved ink transfer control is ascribed to the reduction of undesired surface spreading and a superior mechanical stability of the stamp pattern. This new approach substantially expands the applicability of microcontact printing and provides a tool for the faithful reproduction of even extremely low filling ratio patterns.     

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.     

Supramolecular microcontact printing and dip-pen nanolithography on molecular printboards

The transfer of functional molecules onto self-assembled monolayers (SAMs) by means of soft and scanning-probe lithographic techniques-microcontact printing (muCP) and dip-pen nanolithography (DPN), respectively-and the stability of the molecular patterns during competitive rinsing conditions were examined. A series of guests with different valencies were transferred onto beta-cyclodextrin- (beta-CD-) terminated SAMs and onto reference hydroxy-terminated SAMs. Although physical contact was sufficient to generate patterns on both types of SAMs, only molecular patterns of multivalent guests transferred onto the beta-CD SAMs were stable under the rinsing conditions that caused the removal of the same guests from the reference SAMs. The formation of kinetically stable molecular patterns by supramolecular DPN with a lateral resolution of 60 nm exemplifies the use of beta-CD-terminated SAMs as molecular printboards for the selective immobilization of printboard-compatible guests on the nanometer scale through the use of specific, multivalent supramolecular interactions. Electroless deposition of copper on the printboard was shown to occur selectively on the areas patterned with dendrimer-stabilized gold nanoparticles.     

Langmuir monolayers of Co nanoparticles and their patterning by microcontact printing

In this paper, we describe an easy and reliable method for the production of patterned monolayers of Co nanoparticles. A two-dimensional monolayer of Co nanoparticles is fabricated by spreading a nanoparticle solution over an air-water interface and then transferring it to a hydrophobic substrate by using the Langmuir-Blodgett (LB) method. Transmission electron microscopy (TEM) was used to show that, with increasing surface pressure, the Co nanoparticles become well-organized into a Langmuir monolayer with a hexagonal close-packed structure. By controlling the pH of the subphase, it was found that a monolayer of Co nanoparticles with long-range order could be obtained. Further, by transferring the Langmuir monolayer onto a poly(dimethoxysilane) (PDMS) mold, the selective micropatterning of the Co nanoparticles could be achieved on a patterned electronic circuit. The electronic transport properties of the Co nanoparticles showed the ohmic I-V curve.     

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.