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This paper describes the use of several methods of template stripping (TS) to produce ultraflat films of silver, gold, palladium, and platinum on both rigid and polymeric mechanical supports: a composite of glass and ultraviolet (UV)-curable adhesive (optical adhesive, OA), solder, a composite of poly(dimethyl siloxane) (PDMS) and OA, and bare OA. Silicon supporting its native oxide layer (Si/SiO2) serves as a template for both mechanical template stripping (mTS), in which the metal film is mechanically cleaved from the template, and chemical template stripping (cTS), in which the film-template composite is immersed in a solution of thiols, and the formation of the SAM on the metal film causes the film to separate from the template. Films formed on all supports have lower root-mean-square (rms) roughness (as measured by atomic force microscopy, AFM) than films used as-deposited (AS-DEP) by electron-beam evaporation. Monolayers of n-dodecanethiolate formed by the mTS and cTS methods are effectively indistinguishable by scanning tunneling microscopy (STM); molecularly resolved images could be obtained using both types of surfaces. The metal surfaces, before being cleaved, are completely protected from contact with the atmosphere. This protection allows metal surfaces intended to support SAMs to be prepared in large batch lots, stored, and then used as needed. Template stripping thus eliminates the requirement for evaporation of the film immediately before use and is a significant extension and simplification of the technology of SAMs and other areas of materials science requiring clean metal surfaces.

This paper describes the use of micropatterned agarose stamps prepared by molding against PDMS masters to print patterns of bacteria on agar plates. Topographically patterned agarose stamps were inked with suspensions of bacteria; these stamps generated patterns of bacteria with features as small as 200 microm over areas as large as 50 cm2. Stamps with many small features (>200 microm) were used to study patterns of bacteria growing on media containing gradients of small molecules; stamps with larger features (>750 microm) were used to print different strains of bacteria simultaneously. The stamp transfers only a small percentage of cells that are on its surface to the agar at a time; it is thus possible to replica-pattern hundreds of times with a single inking. The use of soft stamps provides other useful functions. Stamps are easily customized to provide a range of patterns. When culture media is included in the agarose stamp, cells divide and thrive on the surface. The resulting "living stamp" regenerates its "ink" and can be used to pattern surfaces repetitively for a month. This method is rapid, reproducible, convenient, and can be used to control the pattern, spacing, and orientation between colonies of different bacteria.

This paper describes the influence of the composition of poly(dimethylsiloxane) (PDMS) on the attachment and growth of several different types of mammalian cells: primary human umbilical artery endothelial cells (HUAECs), transformed 3T3 fibroblasts (3T3s), transformed osteoblast-like MC3T3-E1 cells, and HeLa (transformed epithelial) cells. Cells grew on PDMS having different ratios of base to curing agent: 10:1 (normal PDMS, PDMSN), 10:3 (PDMSCA), and 10:0.5 (PDMSB). They were also grown on "extracted PDMS" (normal PDMS that has reduced quantities of low molecular-weight oligomers, PDMSN,EX) and normal PDMS that had been extracted and then oxidized (PDMSN,EX,OX); all surfaces were exposed to a solution of fibronectin prior to cell attachment. Generally, fibronectin-coated PDMS is a suitable substrate for culturing mammalian cells. Compatibility of cells on some surfaces, however, was dependent on the cell type: PDMSN,EX,OX caused cell detachment of 3T3 fibroblasts and MC3T3-E1 cells, and PDMSCA caused detachment of HUAECs and HeLa cells. Growth of cells on PDMSN, PDMSN,EX, and PDMSB was comparable to growth on tissue culture-treated polystyrene for most of the cell types. All cells grew at similar rates on PDMS substrates regardless of the stiffness of the substrate, for substrates having Young's moduli ranging from E=0.60 +/- 0.04 to 2.6 +/- 0.2 MPa (for PDMSB and PDMSN,EX, respectively).

This work describes the self-assembly of millimeter-sized helical componentsfabricated by replica molding from sections of drill bits and boltsinto 3D aggregates via capillary forces. The components form aggregates of different geometry: components derived from drill bits formed regular helices, and those derived from bolts formed irregular warped sheets. Enantioselective interactions between components derived from drill bits resulted in the formation of aggregates with complex structures, including enantiomerically pure helices, aggregates with multiple helical segments, and chiral aggregates formed from achiral components, i.e., spheres.

This work demonstrates that pressure-driven flow in a microfluidic network can solve mazelike problems by exploring all possible solutions in a parallel fashion. Microfluidic networks can be fabricated easily by soft lithography and rapid prototyping. To find the best path between the inlet and the outlet of these networks, the channels are filled with a fluid, and the path of a second, dyed fluid moving under pressure-driven flow is traced from the inlet to the outlet. Varying the viscosities of these fluids allows the behavior of the system to be tailored. For example, filling the channels with immiscible fluids of different viscosities enhances the resolution of paths of different fluidic resistances.

A mean flow is observed experimentally in a layer of fluid undergoing Marangoni-Benard convection over a heated substrate that presents a pattern of asymmetrical grooves. The direction of the mean flow is a function of the temperature difference across the layer and of the thickness of the layer; the direction can be controlled by changing these parameters. This system acts as a fluidic ratchet: the local structure of the thermally driven convection interacts with the asymmetry of the local topographical pattern and causes a net, global flow in the fluid. This fluidic ratchet may be useful in handling fluids on macroscopic and microscopic scales.

Self-assembled monolayers (SAMs) formed from alkanethiols on thin films of gold were exposed to a beam of metastable argon atoms through a stencil mask. The changes in the organizational structure of the alkyl chains in the SAM that resulted from exposure were characterized using reflection-absorption infrared spectroscopy. All spectroscopic evidence suggested that the SAMs become disordered after exposure to metastable argon atoms and that no apparent oxidation of the alkane chain occurred. The alkanethiolates in the regions of a SAM of dodecanethiolate damaged by the atom beam exchanged readily upon immersion in a solution of 16-mercaptohexadecanoic acid. The exchange reaction was selective for the regions of the SAM exposed to metastable argon atoms with patterns containing critical dimensions of

This paper describes the two-dimensional self-assembly of hexagonal plates at the interface between perfluorodecalin and water. The plates were prepared with five different permutations of hydrophobic and hydrophilic faces. The shapes and amplitudes of the menisci that form on the faces of the plates determine the magnitude of the lateral capillary forces through which they interact. The amplitudes of the menisci also influencethrough their out-of-plane componentsthe position and orientation of the plate relative to the plane of the liquid−liquid interface. In these experiments, the plates were made of poly(dimethylsiloxane) (PDMS) (ρ = 1.05 g/cm3) containing aluminum oxide (ρ = 4.00 g/cm3); this dopant adjusted the density of the plates, the extent to which they sank into the liquid−liquid interface, and thus the structure of their menisci. The plates studied had densities of 1.05 to 1.86 g/cm3. This work complements previous papers (Bowden, N.; Choi I. S.; Grzybowski, B. A.; Whitesides, G. M. J. Am. Che...

This paper presents a hypothesis relating the exclusion of a molecule (solute) from the surface of a protein in aqueous solution with the ability of that molecule to render surfaces “protein-resistant”, that is, resistant to the adsorption of proteins from aqueous buffer. While few current data test this hypothesis, it does suggest that surfaces presenting groups derived from certain osmolytesmolecules synthesized by cells to relieve osmotic stresswill be protein-resistant. These predictions were tested by constructing protein-resistant, self-assembled monolayers (SAMs) based on the osmolytes betaine and taurine. Examination of data from the literature also revealed that most of the known protein-resistant surfaces are based on displays of kosmotropesmolecules that stabilize the native structure of proteins. The connection between protein resistance, kosmotropicity, and biological function as an osmolyte may illuminate all three properties.

This paper describes a technique that uses mixed self-assembled monolayers of two alkanethiolates (-S(CH2)11(OCH2CH2)6OR, R = a hydrophobic group, and -S(CH2)11(OCH2CH2)nOH, n = 3, 6, EGnOH), in combination with surface plasmon resonance spectroscopy, to study the influence of the size and shape of R, and its density at the surface, on the hydrophobic adsorption of proteins at solid−liquid interfaces. Detailed results were obtained for β-galactosidase, carbonic anhydrase, lysozyme, and RNase A using R = C(C6H5)3, CH(C6H5)2, and CH2(C6H5). A hard-sphere model is used to rationalize the adsorption; this model, although very approximate, helps to interpret qualitative trends in the data. Using this model, the extent to which adsorbed proteins undergo conformational rearrangements appears to depend on the density of the hydrophobic groups at the surface and on the concentration of protein in solution. This paper describes the first step toward the development of a system that will allow the study of hydrophob...

Mammalian cells redirect their movement in response to changes in the physical properties of their extracellular matrix (ECM) adhesive scaffolds, including changes in available substrate area, shape, or flexibility. Yet, little is known about the cell's ability to discriminate between different types of spatial signals. Here we utilize a soft-lithography-based, microcontact printing technology in combination with automated computerized image analysis to explore the relationship between ECM geometry and directional motility. When fibroblast cells were cultured on fibronectin-coated adhesive islands with the same area (900 micrometers2) but different geometric forms (square, triangle, pentagon, hexagon, trapezoid, various parallelograms) and aspect ratios, cells preferentially extended new lamellipodia from their corners. In addition, by imposing these simple geometric constraints through ECM, cells were directed to deposit new fibronectin fibrils in these same corner regions. These data indicate that mammalian cells can sense edges within ECM patterns that exhibit a wide range of angularity and that they use these spatial cues to guide where they will deposit ECM and extend new motile processes during the process of directional migration.

This paper describes a synthesis of free-standing, 10−15-nm-thick polymer films of well-defined lateral size and shape. The three key elements of this procedure are (1) formation by microcontact printing (μCP) of a patterned, self-assembled monolayer (SAM) with hydrophobic regions (alkane-terminated) and adsorption-resistant regions (oligo(ethylene oxide)-terminated); (2) initiation of spatially selective growth of films of poly(electrolyte) multilayers by adsorption on the hydrophobic regions of the patterned SAM; and (3) dry transfer of these films to a water-soluble sacrificial backing, from which the films can be released into solution. This technique exploits the hydrophobic effect as an interaction that can be switched off when it is not needed: during the growth of the films in aqueous buffer, the hydrophobic effect anchors the polymers to the surface; once these films have been dried after synthesis, they are bound to the substrate only by van der Waals interactions and can be transferred nondest...

This paper demonstrates the use of microlens projection lithography using gray-scale masks to fabricate arrays of microstructures in photoresist. In microlens projection lithography, an array of microlenses (diameter d = 1−1000 μm) reduces a common, centimeter-scale pattern in an illuminated mask to a corresponding pattern of micrometer-scale images in its image plane. The pattern of intensity projected by the array of microlenses depends on the shape and gray-level distribution of the pattern on the illuminated mask and on the shape and pattern of the lenses. The distribution of intensity in the microimages could be adjusted using gray-scale masks. After the recording of this intensity distribution in layers of photoresist and developing, the developed resist showed arrays of 3D microstructures over areas larger than 10 cm2. We used these arrays of 3D microstructures as masters and cast transparent elastomer onto them to generate complementary replicas. For a specific microlens array and a fixed light so...

The redox behavior of cytochrome c (cyt c) adsorbed to gold electrodes modified with self-assembled monolayers (SAMs) depends on the SAM. This paper examines SAMs generated from alkanethiols terminating in trimethylammonium (1), sulfonate (2), methyl (3), amine (4), and carboxylic acid (5) groups and from an aromatic thiol (6). The redox potentials of cyt c adsorbed on SAMs of 1 and 5 are relatively close to the formal potential of native cyt c measured in solution. The redox potentials of cyt c adsorbed on SAMs of 3, 4, and 6 are significantly shifted from the formal potential, and a reduction peak at about 0.5 V more negative than the formal potential (that is, a value corresponding to a more difficult reduction) was observed in all three cases. These observations suggest that cyt c changes its conformation significantly on adsorption on these surfaces. No redox peaks were observed for cyt c adsorbed on SAMs of 2, although surface plasmon resonance (SPR) studies indicate that the SAMs of 2 irreversibly ...

Composite stamps composed of two layersa stiff layer supported by a flexible layerextend the capabilities of soft lithography to the generation of 50−100-nm features. The preparation of these stamps was adapted from a procedure originally developed by Schmid et al. (Macromolecules 2000, 33, 3042) for microcontact printing. This paper demonstrates how pattern transfer using other soft lithographic techniquesmicromolding in capillaries, microtransfer molding, and phase-shifting lithographycan be improved using two-layer stamps relative to stamps made of Sylgard 184 poly(dimethylsiloxane).

This paper reports a simple and versatile technique for generating structures on the surfaces of poly(dimethylsiloxane) (PDMS), approximately sinusoidal waves with periods between 0.1 and 10 μm, and the use of these structures to study cell contact guidance.1 The features are generated by stretching PDMS slabs mechanically, oxidizing them in an oxygen plasma, and allowing them to relax. These surface features are similar to photolithographically fabricated grooves that have traditionally been used to investigate cell contact guidance, although their edges are rounded rather than angular. Bovine capillary endothelial cells align and elongate on these features. The morphology and cytoskeletal structure of the aligned cells are similar to those of cells described in previous studies of contact guidance on surfaces with other types of topography. These observations and comparisons indicate that sharp edges in the features defining the grooves are not essential in eliciting contact guidance. This technique pro...

This paper examines the hypothesis that surfaces resistant to protein adsorption should also be resistant to the adhesion of bacteria (Staphylococcus aureus, Staphylococcus epidermidis) and the attachment and spreading of mammalian cells (bovine capillary endothelial (BCE) cells). The surfaces tested were those of self-assembled monolayers (SAMs) terminated with derivatives of tri(sarcosine) (Sarc), N-acetylpiperazine, permethylated sorbitol, hexamethylphosphoramide, phosphoryl choline, and an intramolecular zwitterion (−CH2N+(CH3)2CH2CH2CH2SO3-) (ZW); all are known to resist the adsorption of proteins. There seems to be little or no correlation between the adsorption of protein (fibrinogen and lysozyme) and the adhesion of cells. Surfaces terminated with derivatives of Sarc and N-acetylpiperazine resisted the adhesion of S. aureus and S. epidermidis as well as did surfaces terminated with tri(ethylene glycol). A surface that presented Sarc groups was the only one that resisted the adhesion of BCE cells a...

This paper demonstrates the application of projection photolithography, using a standard commercial microscope, for the generation of masters for soft lithography. The procedure is rapid and convenient and produces features smaller in size (as small as 0.6 μm) than those available from other methods of rapid prototyping, albeit over a limited area (∼4 × 104 μm2 per exposure). A transparency photomask (prepared using high-resolution printing) is inserted into the light path of the microscope and projected through the microscope objective onto a photoresist-coated substrate. Features on the order of 1 μm can be produced routinely over the area of sharp focus (a circle of radius r ≅ 100 μm with a 100× objective) by this method without modification or precise calibration of the microscope. The microscope platform also provides two other useful functions, both characteristic of commercial steppers: step-and-repeat exposures and pattern alignment. The developed photoresist is used as a master for the fabricati...

This paper describes the use of surface plasmon resonance (SPR) spectroscopy and self-assembled monolayers (SAMs) to determine the characteristics of functional groups that give surfaces the ability to resist the nonspecific adsorption of proteins from solution. Mixed SAMs presenting different functional groups were prepared for screening using a synthetic protocol based on the reaction of organic amines with a SAM terminated by interchain carboxylic anhydride groups. Surfaces that presented derivatives of oligo(sarcosine), N-acetylpiperazine, and permethylated sorbitol groups were particularly effective in resisting the adsorption of proteins. Incorporation of these groups into single-component SAMs resulted in surfaces that are comparable to (but slightly less good than) single-component SAMs that present oligo(ethylene glycol) in their ability to resist the adsorption of proteins. In the group of surfaces examined, those that resisted the adsorption of proteins had the following properties: they were ...

This paper describes a method for depositing proteins and cells selectively into microwells fabricated in poly(dimethylsiloxane)an elastomeric organic polymer. The arrays of microwells were generated using soft lithography. The surface between the wells was coated with bovine serum albumin, and the surface within the wells was coated with fibronectin (FN)an adhesive extracellular matrix protein. Fluorescence staining of FN using antibodies indicated that the protein had adsorbed to the wells selectively. Exposure of the FN-coated wells to a suspension of bovine capillary endothelial cells resulted in attachment and spreading of the cells only in the wells. This method for patterning cells was demonstrated with wells of several diameters (≤100 μm) and depths (≤50 μm). It should be useful for (i) the development of sensors based on arrays of cells, (ii) screening combinatorial libraries of ligands on cells, and (iii) testing the effect on cells of compounds and samples relevant to problems in environmental ...

Small (100−600 μm in width) hexagonal polymeric plates with faces patterned into hydrophobic and hydrophilic regions, and interacting through lateral capillary forces, were allowed to self-assemble at the perfluorodecalin/water interface. These plates were fabricated from photoresist and patterned by shadow evaporation of gold onto selected faces. The arrays that assembled from the 100 μm objects were similar in structure to those that assembled from millimeter-sized objects with analogous patterns of hydrophobic and hydrophilic faces, but with three important differences. (i) The contribution of buoyancy forces in establishing the level at which the 100 μm objects floated relative to the interface was small compared to the contribution of the vertical capillary forces. (ii) As a result, the designs of hydrophobic edges necessary to generate menisci useful in self-assembly were different for 100 μm objects than for millimeter-sized objects. (iii) The arrays that formed from the 100 μm objects had higher d...

This paper describes the design and preparation of thin polymeric films that resist the adsorption of proteins and the adhesion of bacteria to an extent comparable to, or better than, self-assembled monolayers (SAMs) that present tri(ethylene glycol) groups. These polymeric films were prepared by the reaction of a polyamine, for example, poly(ethylenimine), with a SAM that presented interchain carboxylic anhydride groups, and by the subsequent conversion of the amino groups of the polymer to amido groups on reaction with acyl chlorides. Polyamines functionalized with acetyl chloride produced films that resisted the adsorption of protein and the adhesion of bacteria to a useful extent. Functionalization of the polyamine with acyl chlorides that were derivatives of oligo(ethylene glycol) resulted in films that were 1−10 times more resistant than those obtained by acetylation. The removal of hydrogen bond donor groups from the surface of the polyamines upon acylation seems to be important for the generation ...

This paper describes a simple, versatile method of generating gradients in composition in solution or on surfaces using microfluidic systems. This method is based on controlled diffusive mixing of species in solutions that are flowing laminarly, at low Reynolds number, inside a network of microchannels. We demonstrate the use of this procedure to generate (1) gradients in the compositions of solutions, measured directly by colorimetric assays and (2) gradients in topography of the surfaces produced by generating concentration gradients of etching reagents, and then using these gradients to etch profiles into the substrate. The lateral dimensions of the gradients examined here, which went from 350 to 900 μm, are determined by the width of the microchannels. Gradients of different size, resolution, and shape have been generated using this method. The shape of the gradients can be changed continuously (dynamic gradients) by varying the relative flow velocities of the input streams of fluids. The method is ex...

Several non-photolithographic methods for growing patterned films of parylene-based polymers prepared by chemical vapor deposition are presented. These patterns were generated using transition meta...

We describe the patterning of proteins and cells onto the surfaces of bacteriological Petri dishes, glass, and poly(dimethylsiloxane) (PDMS) with the use of elastomeric lift-off membranesfree-standing polymer films that have circular or square holes with diameters, sides, and height ≥50 μm. Cells are patterned within the physical constraints provided by the holes of the membranes; these constraints can be released to allow the cells to spread onto the rest of the surface or to remain in the pattern by controlling the properties of the surfaces. Careful control of the properties of the surfaces of the substrates are required to cause the cells to adhere to the substrate and not to the membrane, and to avoid damage to the cells on removing the membrane. This strategy of membrane-based patterninggiven the acronym MEMPAT for brevityoffers a more convenient way for patterning cells on surfaces and for studying cell spreading than existing methods.

This paper describes a procedure for preparing mixed self-assembled monolayers (mixed SAMs) on gold that resist the nonspecific adsorption of proteins from solution. This method was tested using α-amino derivatives of ω-hydroxy- and ω-methoxy-oligo(ethylene glycols): H2N(CH2CH2O)nCH3 and H2N(CH2CH2O)nH (n = 3, 6). Mixed SAMs were prepared by allowing these amines to react with a SAM presenting interchain carboxylic anhydride groups. The resistance of the resulting surfaces to adsorption of several proteinscarbonic anhydrase (EC 4.2.1.1), ribonuclease A (EC 3.1.27.5), lysozyme (EC 3.2.1.17), and fibrinogenwas examined using surface plasmon resonance (SPR) spectroscopy. These mixed SAMs resist the nonspecific adsorption of proteins approximately as effectively as single-component SAMs prepared using the conventional method involving chemisorption of oligo(ethylene glycol)-terminated alkanethiols on gold. Characterization of the mixed SAM that presents a 1:1 mixture of −OCNH(CH2CH2O)6CH3 and CO2H/CO2- group...

This paper describes the synthesis and characterization of patterned polymer multilayers on self-assembled monolayers (SAMs) and the use of these structures as etch resists for gold. The procedure used to build polymer multilayers consisted of five steps: (i) A polar thiolHS(CH2)15COOHwas patterned on gold or silver films by microcontact printing (μCP) with a poly(dimethylsiloxane) (PDMS) stamp. (ii) The patterned surface was placed in a solution containing CH3(CH2)15SH to form a nonpolar, methyl-terminated SAM on the remaining bare metal surface. (iii) The regions of the SAM patterned with CO2H groups were activated for further chemical reaction by conversion into interchain anhydride groups. (iv) The activated substrate was allowed to react with poly(ethylene imine) (PEI, branched chain, Mw 750 000). (v) A second polymer layer was attached to the PEI layer by allowing the amine-terminated surface to react with poly(octadecene-alt-maleic anhydride) (POMA, Mw 30 000) or poly(styrene-alt-maleic anhydride)...

Microfiche has been used as the photomask in 1:1 contact photolithography to generate structures of photoresist with features as small as 10 μm. Optical reduction of images that were printed by a high-resolution image setter on transparent polymer sheets generated patterns in microfiche. The photoresist patterns generated using microfiche as the photomask are useful in the fabrication of elastomeric stamps/molds/membranes for soft lithography. Four test structures fabricated using microfiche as photomask and soft lithography are used to evaluate the utility and resolution of this technique.

Elastomeric membranes that contained regular arrays of well-defined holes were formed by spin-coating a prepolymer onto a photolithographically defined master. These membranes were used as dry resists or as masks in dry lift-off to produce simple features as small as 5 μm on both planar and nonplanar surfaces. These procedures were “dry” because the membranes conformed and sealed reversibly to surfaces: no solvent was required either to deposit the membrane or to remove it from the substrate. A variety of materials, some of which would be difficult to pattern using conventional methods, were patterned using this technique. These materials included metals, sol−gels, hydrogels, biological macromolecules, and organometallic molecules. The membranes were used in sequential, dry-lift off steps to produce structures with greater complexity than those generated with a single membrane.

This report describes a method for patterning ligands onto mixed SAMs of alkanethiolates on gold by microcontact printing (μCP). The mixed SAMs were made from thiols presenting terminal tri(ethylene glycol) groups (HS(CH2)11(OCH2CH2)3OH, 1) and terminal hexa(ethylene glycol)−CH2CO2H groups (HS(CH2)11(OCH2CH2)6OCH2CO2H, 2). Ligands were printed using a two-step procedure. The carboxylic acid groups of 2 were first converted to reactive pentafluorophenyl esters. A freshly oxidized PDMS stamp, inked with a ligand derivatized with a primary amine, was then brought into contact with the activated SAM; in the areas of contact, the amine reacted with the activated ester and formed an amide. Two ligands, biotin and benzenesulfonamide, were printed onto these SAMs. The formation of patterned SAMs presenting biotin ligands was detected by fluorescence microscopy of substrates that were incubated with a solution of fluorescently labeled antibiotin antibody. The formation of patterned biotin was also detected using a...

Soft lithography offers a convenient set of methods for the transfer of patterns to planar and nonplanar substrates. Microelectrodeposition can transform thin metal patterns into self-supporting microstructures, weld components together, and strengthen microstructures after deformations. Together, soft lithography and electrochemistry provide synergistic technologies and the basis for a strategy for converting planar patterns into three-dimensional (3D) microstructures with complex topologies. This strategy is illustrated in the formation of folded tetrahedra, square-based pyramids, cylinders with joints, “pop-up” cubes, and linked chains and knots.

This paper describes the patterning of poly(ethylene imine) (PEI) on a surface into structures having submicron edge resolution. This procedure consists of three steps: (1) formation of a reactive self-assembled monolayer (SAM) terminating in interchain carboxylic anhydride groups on gold and silver; (2) patterning of this SAM by microcontact printing (μCP) using a poly(dimethylsiloxane) (PDMS) stamp inked with PEI (this polymer contains primary and secondary amines that are reactive toward the anhydride groups); (3) hydrolysis of the unreacted anhydride groups with base and removal of noncovalently bound PEI. The patterned thin films of PEI are attached covalently to the SAM by amide bonds. The pendant, unreacted primary and secondary amines of the attached PEI can be used as reactive nucleophilic groups in further steps of chemical modification. This type of postmodification has been illustrated by allowing the amine groups of the covalently attached PEI to react with perfluorooctanoyl chloride, palmit...

Microcontact printing (μCP) has been used to pattern octadecanephosphonic acid on the native oxide surface film of aluminum supported on silicon or on silicon nitride-coated silicon wafers. The patterned alkanephosphonic acid protects the Al2O3/Al film from etching in a solution containing phosphoric, acetic, and nitric acids and water in a ratio of 16:1:1:2 and allows the nonpatterned film to be removed selectively. The patterned Al2O3/Al structures resulting from etching are continuous and electrically conductive within each pattern, and separated patterns are electrically isolated. Resistance measurements of the patterned structures are presented. Using μCP, Schottky diodes of aluminum have been prepared on p-type Si(100). The Schottky diodes exhibit rectifying behavior, and the forward-bias current−voltage (I−V) and reverse-bias capacitance−voltage (C−V) characteristics are presented.

This paper demonstrates a new method for the synthesis of mixed self-assembled monolayers (mixed SAMs) by reaction of a reactive intermediatean interchain carboxylic anhydridewith alkylamines. The interchain anhydride was prepared in high yield from a SAM of 16-mercaptohexadecanoic acid (HS(CH2)15COOH) on gold by treatment with trifluoroacetic anhydride. Alkylamines reacted cleanly with the interchain anhydride to generate a mixed SAM, which comprised a mixture of acids and amides in approximately 1:1 ratio on the surface. The SAMs of the interchain anhydride and the mixed SAM were characterized using polarized infrared external reflectance spectroscopy, X-ray photoelectron spectroscopy, contact angle, and ellipsometry. Control of the wettability of the SAMs was demonstrated by allowing the interchain anhydride to react with alkylamines having different alkyl groups; this model system gave wetting data consistent with earlier studies of mixed monolayers and verified the ability of this method to manipulat...

This paper describes the use of patterned self-assembled monolayers (SAMs) in area-selective deposition of conducting polymers (polypyrrole and polyaniline) on insulating, hydroxyl-terminated surfaces such as Si/SiO2 and glass. Patterned SAMs of octadecylsiloxane were generated on the hydroxyl-terminated surfaces using microcontact printing; they defined and directed the deposition of conducting polymers. The rate of deposition on the hydrophobic surface is higher than that on the hydrophilic surface: immersion of a substrate patterned with a methyl-terminated SAM in an appropriate aqueous polymerization bath produced a “positive” pattern of the conducting polymer on the surface. The conducting polymer deposited on the hydrophobic region of a surface completely covered by the polymer could be readily removed by transferring it to adhesive tape to form a “positive” pattern on the tape, leaving a “negative” pattern on the surface of the substrate. The conducting polymer deposited on the hydrophobic surface...

We use electrostatic interactions to direct the patterning of gold disks having ∼10-μm diameters on functionalized surfaces. Planar and curved substrates with patterned surface charge were generated either by microcontact printing or by photolithography. Small charged gold disks were generated by electroplating gold into photoresist molds and derivatizing these disks with charged self-assembled monolayers. When agitated as a suspension in contact with the patterned surfaces, the charged gold disks deposited specifically but as disordered aggregates on regions presenting the opposite charge. Positively-charged disks deposited on phosphonate-, carboxylate-, and SiOH-terminated surfaces but not on trimethylammonium- and dimethylammonium-terminated surfaces, and vice versa for negatively-charged disks. Methyl- and CF3-terminated surfaces resisted deposition of disks of either charge. Selective and dense assembly was achieved in methanol, ethanol, 2-propanol, and dioxane; in water, deposition was nonspecific. ...

This paper describes the use of surface plasmon resonance (SPR) spectroscopy to measure the rates and extents of association of four detergentssodium dodecyl sulfate (SDS), β-octyl glucoside, Triton X-100, and Tween 20to self-assembled monolayers (SAMs) of alkanethiolates on gold. SAMs presenting hexaethylene glycol groups resisted the adsorption of all four detergents. These same detergents associated with hydrophobic SAMs presenting methyl groups; the concentration of detergent molecules on the surface was 120−280 pmol/cm2. The associations of the detergents with the hydrophobic SAM were described well by the Langmuir adsorption isotherm. The dissociation constants Kd (M) for the desorption of the detergents from the surface correlated with the critical micelle concentration (cmc, M) of the detergents in solution, and followed the relationship cmc ≈ 7 (±2)Kd. The efficacy of SDS in removing the protein fibrinogen adsorbed on a hydrophobic SAM depended strongly on the concentration of detergent. SDS at a...

This paper describes a number of approaches that have been employed to reduce the size of features of self-assembled monolayers (SAMs) generated using microcontact printing (μCP). In μCP, an elastomeric stamp is used to print patterned SAMs of alkanethiolates on the surfaces of coinage metals and SAMs of alkylsiloxanes on Si/SiO2. It is a convenient technique for generating patterned microstructures with feature sizes ≥ 500 nm. The capability of this technique could be extended to produce features smaller than 500 nm using the following approaches: (1) μCP with mechanical deformation of the elastomeric stampthat is, with lateral compression or uniaxial stretching in the plane of the stamp and with pressure perpendicular to the plane of the stamp; (2) μCP with physical alternation of the elastomeric stampthat is, with a stamp that has been swelled with a solvent or a stamp whose dimensions have been reduced by extraction of an inert filler; (3) μCP with reduction in the size of features resulting from pro...

This paper describes several methods that generate patterned relief structures for casting the elastomeric stamps required in microcontact printing (μCP). The relief structures include the following: (1) patterns etched in thin films of silver (200 nm thick) with printed SAMs of hexadecanethiolate as resists; (2) patterned polymeric structures assembled on the surface of a thin film of silver (or gold) that has been patterned with SAMs of hexadecanethiolate; and (3) polystyrene microspheres assembled on a flat surface. Commercial diffraction gratings were also useful as masters. Microcontact printing with poly(dimethylsiloxane) stamps cast from these relief structures was used in combination with selective wet etching to generate patterns in thin films of silver and gold. Although some of these techniques used photolithography once in fabricating the initial masters, they could produce patterns more complex than those on the initial masters. These procedures provide a convenient route to moderately compl...

Self-assembled monolayers (SAMs) were formed on gold colloids in 50% aqueous ethanol in the presence of alkanethiols, HS(CH2)nR, where R represents a series of neutral and acidic functional groups. Chemiadsorption of alkanethiols onto the gold colloids significantly changed the rates of flocculation of the gold dispersions; the magnitudes of these pH-dependent changes were a function of chain length, n, and the terminal functionality, R, in a manner consistent with formation of SAMs on the colloid surface. The reduced rate of dissolution of alkanethiol-treated colloids by wet chemical etchants, transmission electron microscopy, and X-ray photoelectron spectroscopy data further support the formation of SAMs.

This paper describes a new method of reducing the size of the metal features made by electroless deposition and fabricating complex-shaped, patterned surfaces. Microcontact printing (μCP) was used to pattern oriented glassy polymers with palladium colloids, stabilized with tetraoctadecylammonium bromide. These colloids are catalysts for the selective electroless deposition of copper. Annealing of the activated polymer at a temperature slightly above its glass transition temperature led to a shrinkage of the substrate. Immersion of the shrunken substrate in the plating bath yielded the metal features. The maximum shrinkage of the feature size achieved was on the order of a factor of ∼4 in one direction of the oriented polymer and of ∼7 in the perpendicular direction. Control of the extent and direction of shrinkage allowed the fabrication of metal features with sizes and shapes different from those on the polydimethylsiloxane stamp used for the patterning of the substrate and from the draw ratios. Free-sta...

This paper describes a new method for forming micron- and submicron-scale patterns of copper on surfaces. This method uses microcontact printing (μCP) to deposit colloids that serve as catalysts for the selective electroless deposition of copper. A patterned elastomeric stamp fabricated from poly(dimethylsiloxane) was used to deliver the catalystpalladium colloids stabilized with tetraalkylammonium bromidesto the substrate surface. The electroless deposition of the copper on the sample occurred only where palladium colloid was transferred to the substrate. Electroless deposition catalyzed by the colloids resulted in the formation of metal structures with features having submicron dimensions, with an edge resolution in the range of 100 nm. This technique of activating substrates for electroless metalization was successfully used to pattern glass, (Si/SiO2), and polymers; both flat and curved substrates were used. Microcontact printing of colloids was also used to fabricate metal structures whose thicknesse...

Model heterogeneous surfaces consisting of alternating and parallel 2.55 μm hydrophobic and 2.45 μm hydrophilic strips were prepared on a gold film by patterning self-assembled monolayers of hexadecanethiol and mercaptohexadecanoic acid using an elastomer stamp. The advancing and receding contact angles were measured for liquid drops (distilled water, buffer solutions with pH = 8.0, 10.0, and 11.0, ethylene glycol, glycerol, and formamide) placed on this specially prepared surface. Contortion of the three-phase contact line is a significant property of these systems. Both contact angles, advancing and receding, were 2−10° lower when measured with the strips normal to the three-phase contact line than those measured with the strips tangential to the three-phase contact line. For most of the systems examined, experimental contact angles, when measured for the liquid drop edge situated along the strips of the model heterogeneous surface (noncontorted three-phase contact line), were in an agreement with theor...

This paper outlines a general synthetic route to alkanethiols useful in forming self-assembled monolayers (SAMs) starting from amines, RNH 2 , and converting them, via α-chloroamides RNHCOCH 2 Cl, to thiols having the structure RNHCOCH 2 SH. The wettabilities (estimated from contact angles of water and hexadecane) and the thicknesses (as measured by ellipsometry and X-ray photoelectron spectroscopy, XPS) of SAMs having an amide moiety β to Au/thiolate were similar to those of SAMs of alkanethiols with similar backbone length on gold. The internal amide group present in SAMs prepared from CF 3 CH 2 -NHCOCH 2 SH increases their stability against desorption or exchange with hexadecanethiol in ethanol relative to SAMs from CF 3 (CH 2 ) 3 SH. The desorption of SAMs from CF 3 (CH 2 ) 3 SH was first order in the alkanethiolate and had a half-life of ∼2 h at 10 -9 Torr ; SAMs from CF 3 CH 2 NHCOCH 2 SH, by contrast, showed no loss after 48 h at ∼ 10 9 Torr. The rate of exchange of a SAM from CF 3 CH 2 NHCOCH 2 SH with hexadecanethiol in ethanol was 10 2 -10 3 times slower than the SAMs from CH 3 (CH 2 ) 3 SH or from CF 3 (CH 2 ) 3 -SH. The susceptibility of a SAM prepared from a short-chain alkanethiol (e.g., CH 3 (CH 2 ) 3 SH or CF 3 -CH 2 NHCOCH 2 SH) to damage by UV was 10 times greater than that observed for a SAM prepared from hexadecanethiol (as measured by XPS). UV damage of SAMs derived from CF 3 CH 2 NHCOCH 2 SH, followed by protection ofthe UV-damaged SAM by replacement with hexadecanethiol and etching, gave gold patterns with minimum feature sizes of 5 μm ; these sizes were limited by the lithographic procedure used and do not reflect the true edge resolution of this photolithographic method.

Electrochemical desorption of a patterned, self-assembled monolayer (SAM) of an organothiol on a gold surface can be used to control the shape of drops of liquid hexadeqanethiol (HDT) on the surface. The contact angle of HDT drops on a thin, transparent film of gold (100 A thick) under an aqueous electrolyte solution could be varied from 128 ± 2° when the gold was held at -1.7 V (vs Ag pseudoreference) to 37 ± 2° when the gold was held at 0.0 V. Switching between these two limiting contact angles could be performed rapidly, reversibly, and reproducibly using this system. These drops could act as lenses and could be used as an optical switch which focuses/defocuses light. An array of 1-nL drops could be formed by differential wetting of a gold surface patterned with methyl- and carboxylic acid-terminated organothiol, and the contact angles of drops in this array could be switched in ca. 50 ms. Alternatively, drops could be confined to an array of gold pads on a conducting glass and switched in a similar fashion.

Departments of Materials Science and Engineering and Chemistry, Uniuersity of lllinois,Urbana-Champaign, (Irbana, Illinois 61801Receiued September 79, 1994eLong-chain alkanehydroxamic acids adsorb on the native oxides of metals and formed oriented self-assembled monolayers (SAMs). This study examined SAMs of hydroxamic acids on the native oxides ofcopper, silver, titanium, aluminum, zirconium, and iron. These SAMs were characterized usingwettability,X-ray photoelectron spectroscopy (XPS), and polarized infrared external reflectance spectroscopy (PIERS).Alkanehydroxamic acids give better monolayers than the corresponding alkanecarboxylic acids on certainbasic metal oxides (especially copper(Il) oxide). On the native oxide of copper (which has an isoelectricpoint greater than the pK" of the hydroxamic acid), the ligand is bound to the surface predominantly asthe hydroxamate. The strength of the interaction between copper oxide and the hydroxamate allowsincorporation of polar tail groups into the monolayer. On acidic or neutral metal oxides (e.g., TiO2), thepredominant species bound to the surface is the hydroxamic acid. Alkanehvdroxamic acids on titaniumdioxide bind relatively weakly but. nonetheless. form SAlts that are more stable than those from carboxylicacids (although not as stable as those from alkanephosphonic acids r.

This paper demonstrates that scanning probe microscopies (SPMs) can image patterned self-assembled monolayers (SAMs) formed by microcontact printing. Lateral force microscopy (LFM) and force modulation microscopy (FMM) showed contrast between regions of patterned SAMs terminated by different chemical functionalities. Normal force microscopy (NFM) typically showed less contrast than LFM or FMM but provided information about the topography of the surface. Chemical functionality (at the interface between the SAM and air) dominated imaging in these experiments. Changes in the morphology of the surface or changes in the humidity of the environment for imaging did not influence significantly the contrast in our experiments. The sharpness of the contrast suggests the use of LFM and SAMs in studying tribology on the submicrometer scale.

This paper establishes conditions necessary for autophobic pinning of solutions of alkanethiols by their reaction with a gold surface. Interfacial tension and the kinetics of organization of the SAMs are important considerations in the design of systems that limit the spread of drops across a surface that is reactive toward those drops. The two systems investigated in this paper-hexadecanethiol (HDT) spreading on gold in air and hydroxyundecanethiol (HHT) in hexadecane spreading on gold under waterprovide a basis for understanding one aspect of the mechanism of microcontact printing (μCP) based on the reaction of thiols with bare gold

This paper describes three types of gold electrodes, each with at least one spatiai dimension as small as 1 prm, fabricated using combinations of micromachining and molecular self-assembly. (i) Large-area (>0.1 mm2) arrays of band-type electrodes were formed by machining micrometer-wide grooves of bare gold into gold frlm covered with a -23-A-thick, electrically insulating, self-assembled monolayer (SAM) of CHs(CHz)rsS. (ii) Micrometer-wide, centimeter-long, 100-nm-thick gold electrodes supported on glass were fabricated by patterning a gold film with monolayers of CHs(CHz)rsS and HO(CHz)zS, and selectively etching the regions of gold covered with HO(CHz)zS using an aqueous solution of CN saturated with Oz. (iii) T\vo microelectrodes, each with electrochemically active areas as small as 100 nm x l rrm and separated from each other by a distance of I !rm, were formed by machining a l-pzm-wide gap into a supported gold wire covered with an electrically insulating SAM of CHs(CHz)rsS; exposed gold on either side of the machined gap formed the electrochemically active surfaces of the microelectrode pair.