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Showing papers by "George M. Whitesides published in 1996"


Journal ArticleDOI
TL;DR: The convenience and broad application offered by SAMs and microcontact printing make this combination of techniques useful for studying a variety of fundamental phenomena in biointerfacial science.
Abstract: Self-assembled monolayers (SAMs) formed on the adsorption of long-chain alkanethiols to the surface of gold or alkylsilanes to hydroxylated surfaces are well-ordered organic surfaces that permit control over the properties of the interface at the molecular scale. The ability to present molecules, peptides, and proteins at the interface make SAMs especially useful for fundamental studies of protein adsorption and cell adhesion. Microcontact printing is a simple technique that can pattern the formation of SAMs in the plane of the monolayer with dimensions on the micron scale. The convenience and broad application offered by SAMs and microcontact printing make this combination of techniques useful for studying a variety of fundamental phenomena in biointerfacial science.

910 citations


Journal ArticleDOI
19 Jul 1996-Science
TL;DR: The versatility of this procedure for fabricating surfaces with complex, micrometer- and submicrometers-scale patterns was demonstrated by the production of diffraction gratings with periods smaller than the original grating.
Abstract: Complex, optically functional surfaces in organic polymers can be fabricated by replicating relief structures present on the surface of an elastomeric master with an ultraviolet or thermally curable organic polymer, while the master is deformed by compression, bending, or stretching. The versatility of this procedure for fabricating surfaces with complex, micrometer- and submicrometer-scale patterns was demonstrated by the production of (i) diffraction gratings with periods smaller than the original grating; (ii) chirped, blazed diffraction gratings (where the period of a chirped grating changes continuously with position) on planar and curved surfaces; (iii) patterned microfeatures on the surfaces of approximately hemispherical objects (for example, an optical surface similar to a fly's eye); and (iv) arrays of rhombic microlenses. These topologically complex, micropatterned surfaces are difficult to fabricate with other techniques.

708 citations


Journal ArticleDOI
TL;DR: Surface plasmon resonance studies showed that His-tagged proteins adsorbed on the NTA-SAM retained a greater ability to participate in binding interactions with proteins in solution than protein immobilized in a thin dextran gel layer by covalent coupling.
Abstract: This paper reports the generation of a self-assembled monolayer (SAM) that selectively binds proteins whose primary sequence terminates with a His-tag: a stretch of six histidines commonly incorporated in recombinant proteins to simplify purification. The SAM was prepared by the adsorption onto a gold surface of a mixture of two alkanethiols: one thiol that terminated with a nitrilotriacetic acid (NTA) group, a group that forms a tetravalent chelate with Ni(II), and a second thiol that terminated with a tri(ethylene glycol) group, a group that resists protein adsorption. His-tagged proteins bound to the SAM by interaction of the histidines with the two vacant sites on Ni(II) ions chelated to the surface NTA groups. Studies with model proteins showed the binding was specific for His-tagged proteins and required the presence of Ni(II) on the surface. Immobilized His-tagged proteins were kinetically stable in buffered saline at pH 7.2 but could be desorbed by treatment with 200 mM imidazole. Surface plasmo...

597 citations


Journal ArticleDOI
07 Aug 1996-Langmuir
TL;DR: In this article, 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.
Abstract: 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.

538 citations


Journal ArticleDOI
TL;DR: In this paper, an elastomeric stamp made of poly(dimethylsiloxane) and having relief features in its surface was placed on a substrate; contact between the elastomers and the substrate formed a network of interconnected channels.
Abstract: Procedures based on micromolding in capillaries (MIMIC) were used to pattern a surface of a substrate with micrometer- and submicrometer-scale structures. An elastomeric stamp made of poly(dimethylsiloxane) and having relief features in its surface was placed on a substrate; contact between the elastomeric stamp and the substrate formed a network of interconnected channels. A fluida precursor to a polymer, a solution, or a suspension of the material to be patternedfilled these channels by capillary action. After the material in the fluid had cross-linked, crystallized, cured, adhered, or deposited onto the surface of the substrate, the elastomeric component was removed. The microstructures remained on the surface in the pattern complementary to that present in the mold. MIMIC was used to fabricate microstructures of organic polymers, inorganic and organic salts, ceramics, metals, and crystalline microparticles.

483 citations


Journal ArticleDOI
TL;DR: The method combines optically transparent contoured surfaces with self-assembled monolayers of alkanethiolates on gold to control interfacial characteristics; these tailored interfaces, in turn, control the adsorption of proteins and the attachment of cells.
Abstract: This paper describes a method based on experimentally simple techniques--microcontact printing and micromolding in capillaries--to prepare tissue culture substrates in which both the topology and molecular structure of the interface can be controlled. The method combines optically transparent contoured surfaces with self-assembled monolayers (SAMs) of alkanethiolates on gold to control interfacial characteristics; these tailored interfaces, in turn, control the adsorption of proteins and the attachment of cells. The technique uses replica molding in poly(dimethylsiloxane) molds having micrometer-scale relief patterns on their surfaces to form a contoured film of polyurethane supported on a glass slide. Evaporation of a thin (< 12 nm) film of gold on this surface-contoured polyurethane provides an optically transparent substrate, on which SAMs of terminally functionalized alkanethiolates can be formed. In one procedure, a flat poly(dimethylsiloxane) stamp was used to form a SAM of hexadecanethiolate on the raised plateaus of the contoured surface by contact printing hexadecanethiol [HS(CH2)15CH3]; a SAM terminated in tri(ethylene glycol) groups was subsequently formed on the bare gold remaining in the grooves by immersing the substrate in a solution of a second alkanethiol [HS(CH2)11(OCH2CH2)3OH]. Then this patterned substrate was immersed in a solution of fibronectin, the protein adsorbed only on the methyl-terminated plateau regions of the substrate [the tri(ethylene glycol)-terminated regions resisted the adsorption of protein]; bovine capillary endothelial cells attached only on the regions that adsorbed fibronectin. A complementary procedure confined protein adsorption and cell attachment to the grooves in this substrate.

452 citations


Patent
01 Mar 1996
TL;DR: In this article, an elastomeric stamp is deformed during and/or prior to using the stamp to print a self-assembled molecular monolayer on a surface.
Abstract: Improved methods of forming a patterned self-assembled monolayer on a surface and derivative articles are provided. According to one method, an elastomeric stamp is deformed during and/or prior to using the stamp to print a self-assembled molecular monolayer on a surface. According to another method, during monolayer printing the surface is contacted with a liquid that is immiscible with the molecular monolayer-forming species to effect controlled reactive spreading of the monolayer on the surface. Methods of printing self-assembled molecular monolayers on nonplanar surfaces and derivative articles are provided, as are methods of etching surfaces patterned with self-assembled monolayers, including methods of etching silicon. Optical elements including flexible diffraction gratings, mirrors, and lenses are provided, as are methods for forming optical devices and other articles using lithographic molding. A method for controlling the shape of a liquid on the surface of an article is provided, involving applying the liquid to a self-assembled monolayer on the surface, and controlling the electrical potential of the surface.

382 citations



Journal ArticleDOI
TL;DR: An ELISA assay is described for measuring the binding of influenza virus A-X31 to α-sialoside groups that are linked to biotin-labeled polyacrylamides, and the role of steric stabilization in the mechanism is shown to be directly related to the binding affinity of the polymers for the viral surface.
Abstract: An ELISA assay is described for measuring the binding of influenza virus A-X31 to α-sialoside groups that are linked to biotin-labeled polyacrylamides. The efficacy of these polymers in inhibiting the adhesion of influenza virus to erythrocytes (as measured by a hemagglutination assay) was shown to be directly related to the binding affinity of the polymers for the viral surface: the differences in inhibitory efficacy among the polymeric inhibitors and monomeric α-methyl sialoside, among fractions of a polymeric, polyvalent inhibitor with narrow molecular weight ranges, and among polymeric inhibitors prepared by copolymerization or modification of a preformed polymer chain, all correlated with differences in the affinity of the inhibitors for the surface of the virus. The polymeric inhibitors studied had affinities for the viral surface that ranged between 103 and >106 greater than α-methyl sialoside, on the basis of total sialic acid groups in solution. The role of steric stabilization in the mechanism ...

333 citations


Journal ArticleDOI
06 Mar 1996-Langmuir
TL;DR: In this paper, a patterned elastomeric stamp fabricated from poly(dimethylsiloxane) was used to deliver the catalystpalladium colloids stabilized with tetraalkylammonium bromidesto the substrate surface.
Abstract: 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...

276 citations


Patent
16 Aug 1996
TL;DR: In this article, a self-assembled monolayer pattern is formed on a surface and a material on the surface in a pattern complementary to the self-assembling pattern is created.
Abstract: A technique for creating patterns of material deposited on a surface involves forming a self-assembled monolayer in a pattern on the surface and depositing, via chemical vapor deposition or via sol-gel processing, a material on the surface in a pattern complementary to the self-assembled monolayer pattern. The material can be a metal, metal oxide, or the like. The surface can be contoured, including trenches or holes, the trenches or holes remaining free of self-assembled monolayer while the remainder of the surface is coated. When exposed to deposition conditions, metal or metal oxide is deposited in the trenches or holes, and remaining portions of the article surface remain free of deposition. The technique finds particular use in creation of conductive metal pathways selectively within holes passing from one side of a substrate to another.

Journal ArticleDOI
03 Apr 1996-Langmuir
TL;DR: In this article, a model heterogeneous surfaces consisting of alternating and parallel 255 μm hydrophobic and 245 μm hyrophilic strips were prepared on a gold film by patterning self-assembled monolayers of hexadecanethiol and mercaptohexadecanoic acid using an elastomer stamp.
Abstract: Model heterogeneous surfaces consisting of alternating and parallel 255 μm hydrophobic and 245 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 = 80, 100, and 110, 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

Journal ArticleDOI
TL;DR: In this paper, the authors consider that registered names, trademarks, etc. are not to be considered unprotected by law, even without soecific indications thereof, and they propose an approach to protect them.
Abstract: Registered names, trademarks, etc. used in this journal, even without soecific. indications thereof. are not to be considered unprotected by law. Printed in the Federal Republic of Germany

Journal ArticleDOI
TL;DR: In this article, the authors describe applications in microfabrication using patterned self-assembled monolayers (SAMs) formed by microcontact printing, which is a flexible new technique that forms patterned SAMs with regions terminated by different chemical functionalities (and thus different physical and chemical properties), in patterns with dimensions.
Abstract: This paper describes applications in microfabrication using patterned self-assembled monolayers (SAMs) formed by microcontact printing. Microcontact printing is a flexible new technique that forms patterned SAMs with regions terminated by different chemical functionalities (and thus different physical and chemical properties), in patterns with dimensions. Patterns of SAM are formed using an alkanethiol as an `ink', and printing the alkanethiol on a metal support with elastomeric `stamp'. We fabricate the stamp by moulding a silicone elastomer using a master prepared by optical or x-ray microlithography or by other techniques. SAMs of long-chain alkanethiolates on gold and other metals can act as nanometer resists by protecting the supporting metal from corrosion by appropriately formulated etchants: the fabrication of microstructures of gold and silicon demonstrates the utility of patterned SAMs (formed by ) as nm resists. Patterned SAMs formed by can also control the wettability of a surface on the scale. The organization of liquids in patterned arrays with dimensions, and the patterned deposition of microcrystals and microcrystal arrays illustrate the use of controlled wettability for microfabrication.

Patent
06 Jun 1996
TL;DR: In this article, a plate defining a surface and a plurality of cytophilic islands that adhere cells which are isolated by cytophobic regions to which cells do not adhere and further is contiguous with the cytophobic islands.
Abstract: The disclosed invention is a device for adhering cells in a specific and predetermined position. The device comprises a plate defining a surface and a plurality of cytophilic islands that adhere cells which are isolated by cytophobic regions to which cells do not adhere and further is contiguous with the cytophilic islands. The islands or the regions or both may be formed of a self-assembled monolayer (SAM). Further, the cytophobic regions are wide enough such that less than 10 percent of the cells adhered to the cytophilic islands are allowed to form bridges across the cytophobic regions and contact each other. The device is used in a method for culturing cells on a surface or in a medium and also for performing cytometry. Furthermore, the device is used in immobilization of cells at a surface and for controlling the shape of a cell.


Journal ArticleDOI
TL;DR: In this paper, a review of the preparation, structure and properties of self-assembled monolayers (SAMs), techniques for patterning SAMs, including microcontact printing (mCP), UV-photolithography, and e-beam writing, is presented.

Journal ArticleDOI
TL;DR: In this article, the use of micromolding in capillaries (MIMIC) was described to produce complex polymeric microstructures supported on different substrates and the applications of these micro-structures in microfabrication.
Abstract: This paper describes the use of micromolding in capillaries (MIMIC) to produce complex polymeric microstructures supported on different substrates and the applications of these microstructures in microfabrication. Patterned microstructures of several organic polymerspolyurethane, polyacrylate, and epoxywere formed by molding in enclosed, continuous channels formed by conformal contact between a solid support and an elastomeric mold whose surface had been patterned with a relief structure having micrometer-scale dimensions. A liquid prepolymer filled these channels by capillary action and was allowed to cure photochemically or thermally. The mold was then removed. Polymeric microstructures formed on films of Saran Wrap could be folded into different shapes, while these microstructures retained their forms; they could also be stretched uniaxially to generate microstructures having distorted forms. The patterned polymeric microstructures formed on SiO2, glass, and metals (Au, Ag, and Cr) could be used direct...

Journal ArticleDOI
TL;DR: In this article, a scanning tunneling microscope (STM) imaging mechanism was proposed to describe the STM imaging mechanism for these insulating, physisorbed films in which th
Abstract: Monolayer films of several primary substituted hydrocarbons CH3(CH2)nCH2X (X = CH3, OH, NH2, SH, Cl, Br, I; n = 16−30) have been imaged on graphite at the solution−substrate interface using a scanning tunneling microscope (STM) The straight chain hydrocarbons form well-ordered 2-D films on graphite and physisorb with their molecular axes parallel to the surface The NH2, SH, Br, and I end groups are observed as bright spots in the STM image corresponding to an enhancement in the tunnel current in the vicinity of the functional group, relative to the remainder of the carbon chain On the other hand, the OH and Cl substituents were not distinguishable from the alkyl chain in the STM images Comparison of the relative “brightness” of the functional groups with respect to the carbon chain reveals an empirical relationship between increasing relative brightness and increasing molecular polarizability A model is proposed to describe the STM imaging mechanism for these insulating, physisorbed films in which th

Journal ArticleDOI
TL;DR: In this article, microcontact printing was used to generate patterned self-assembled monolayers of alkanethiolates on the surfaces of evaporated films of copper, which could be directly used as ultrathin resists that protected the underlying copper from etching in aqueous solutions of FeCl3/HCl or FeCl 3/NH4Cl.
Abstract: Microcontact printing was used to generate patterned self-assembled monolayers of alkanethiolates on the surfaces of evaporated films of copper. These patterned SAMs could be directly used as ultrathin resists that protected the underlying copper from etching in aqueous solutions of FeCl3/HCl or FeCl3/NH4Cl. Arrays of junctions of Ag/Ag2O/Cu were fabricated using a multistep procedure that combines metal evaporation, microcontact printing, and selective etching.

Journal ArticleDOI
07 Aug 1996-Langmuir
TL;DR: In this paper, the authors describe several methods that generate patterned relief structures for casting the elastomeric stamps required in microcontact printing (μCP), including patterns etched in thin films of silver (200 nm thick) with printed SAMs of hexadecanethiolate as resists, patterned polymeric structures assembled on the surface of a thin film of silver or gold that has been patterned with SAMs, and polystyrene microspheres assembled on a flat surface.
Abstract: 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...

Journal ArticleDOI
TL;DR: In this article, an extension of microcontact printing with self-assembled monolayers (SAMs) of hexadecanethiolates on gold and silver has been proposed to fabricate patterned microstructures of these metals with dimensions of > 0.2 μm.
Abstract: Microcontact printing (μCP)1 is a convenient technique for generating patterned self-assembled monolayers (SAMs)2 of alkanethiolates on gold1 and silver3 and of alkylsiloxanes on silicon dioxide and glass.4,5 Patterned SAMs of hexadecanethiolate on gold and silver can be used directly as ultrathin resists in selective wet etching to fabricate patterned microstructures of these metals with dimensions of >0.2 μm.1,3,6-9 The present study is a natural extension of this technique, since alkanethiols also form organized monolayers on evaporated films of copper.10 The chemical reactivity of coinage metals increases in the order of Au < Ag < Cu; it is possible to find an etchant that dissolves only Cu but not Ag and Au or only Ag but not Au. For example, aqueous FeCl3 solution, an etchant that is widely used in semiconductor industry to produce printed circuits of copper,11 etches silver and gold very slowly or not at all.3 This ability to carry out “orthogonal etching” offers an opportunity to generate junctions of Cu-Ag, Cu-Au, and Ag-Au by using a multistep procedure that includes metal evaporation, microcontact printing with hexadecanethiol, and selective wet etching. Figure 1 shows scanning electron micrographs (SEMs) of copper samples (50 nm thick) that had been patterned with SAMs of hexadecanethiolate and etched in an aqueous FeCl3 solution (0.012 M) for 2-3 s.12 The microparticles (∼0.5 μm in size) on the bare regions of copper are CuCl (shown by energy-dispersed spectroscopy, EDS). They were formed in situ during the dissolution of bare copper:

Patent
22 Jul 1996
TL;DR: In this paper, a chemically active agent such as a catalyst is applied from an applicator in a pattern to an exterior surface of an article, and metal is deposited according to the pattern and optionally, removed from the substrate.
Abstract: Techniques for fabrication of small-scale metallic structures such as microinductors, microtransformers and stents are described. A chemically active agent such as a catalyst is applied from an applicator in a pattern to an exterior surface of an article, metal is deposited according to the pattern and optionally, removed from the substrate. Where the substrate is cylindrical, the pattern can serve as a stent. Alternatively, a pattern of a self-assembled monolayer can be printed on a surface, which pattern can dictate metal plating or etching resulting in a patterned metal structure that can be cylindrical. In another embodiment, a structure is patterned on a surface that serves as a phase-modulating pattern or amplitude-modulating pattern. The article subsequently is exposed to radiation that can induce a change in refractive index within the article, and the phase-modulating or amplitude-modulating pattern results in different indices of refraction being created in different portions of the article. By this technique, a grating can be written into a core of an optical fiber.

Journal ArticleDOI
TL;DR: The ability of ESI-MS to estimate simultaneously the relative binding affinities of a protein to soluble ligands in a library, if general, should be useful in drug development.
Abstract: This paper describes the use of electrospray ionization-mass spectrometry (ESI-MS) to screen two libraries of soluble compounds to search for tight binding inhibitors for carbonic anhydrase II (EC 4.2.1.1). The two libraries, H2NO2SC6H4C(O)NH-AA1-AA2-C(O)NHCH2CH2CO2H (1), where AA1 and AA2 are l-amino acids (library size: 289 compounds) or d-amino acids (256 compounds), were constructed by attaching tripeptides to the carboxyl group of 4-carboxybenzenesulfonamide. Screening of both libraries yielded, as the tightest binding inhibitor, compound 1 (AA1 = AA2 = l-Leu; binding constant Kb = 1.4 × 108 M-1). The ability of ESI-MS to estimate simultaneously the relative binding affinities of a protein to soluble ligands in a library, if general, should be useful in drug development.

Journal ArticleDOI
TL;DR: In this paper, a self-assembled monolayers (SAMs) of alkanethiolates on gold that present tri(propylene sulfoxide) groups prevent the nonspecific adsorption of protein and subsequent attachment of cells.
Abstract: This Communication demonstrates that self-assembled monolayers (SAMs) of alkanethiolates on gold that present tri(propylene sulfoxide) groups prevent the nonspecific adsorption of protein and subsequent attachment of cells. A common strategy used to passivate surfaces against protein adsorption is to coat them with poly(ethylene glycol) (PEG).1 It is not clear whether PEG is unique in its ability to confer resistance to adsorption, or if there are other materials with comparable (or superior) properties. SAMs of alkanethiolates on gold are a class of model organic surfaces that are well-suited to study the relationships between the structure of a surface and the adsorption of protein on the surface.2 We have previously demonstrated that SAMs presenting short oligomers of the ethylene glycol group ([-CH2CH2O-]n, n ) 2-7) effectively resist the nonspecific adsorption of protein.3,4 The goal of the present work was to design a new material that resists the adsorption of protein, but that has no counterpart in available biomaterials. In designing a new “inert” material, we sought to preserve three characteristics of the oligo(ethylene glycol) chains: (i) a hydrophilic repeating unit; (ii) a unit that can hydrogen bond with water, and that is well-solvated; (iii) an oligomer that is conformationally flexible. We chose oligomers of the propylene sulfoxide group [-CH2CH2CH2S(O)-] as candidates that shared these properties with the oligo(ethylene glycol) chains. Both materials are hydrogen bond acceptors, but not donors. SAMs presenting tri(propylene sulfoxide) groups5 are more hydrophilic than those presenting hexa(ethylene glycol) groups, as measured by advancing contact angles of water of 27° and 38°, respectively. A propylene linker was chosen in place of the ethylene linker because elimination reactions of the latter limit its stability. The biocompatibility of the propylene sulfoxide group is not known, but the parent functional groupsdimethyl sulfoxidesis more biocompatible than is ethylene glycol.6 We used surface plasmon resonance (SPR) spectroscopy4,7 to measure the adsorption of the proteins RNAse A and fibrinogen to mixed SAMs prepared from 1 and undecanethiol.8 In the SPR experiment, p-polarized light is incident on the backside of a glass slide coated with a thin layer of gold that is modified with a SAM.4,9,10 The angle of reflected light that shows a minimum in intensity (the resonance angle, θm) is related linearly to the amount of protein in the interfacial region. In the experiments described here, a buffer containing phosphate (10 mM) and sodium chloride (150 mM) (pH 7.4) was allowed to pass through the flow cell for 4 min, replaced with a solution of protein in the same buffer (1 mg/mL) for 6 min, and returned to buffer for 20 min. Both RNAse A and fibrinogen adsorbed quickly and irreversibly to hydrophobic SAMs presenting only methyl groups (o1 ) 0); a SAM prepared from only 1 (o1 ) 1.0) entirely resisted the adsorption of these two proteins (Figure 1). Even mixed SAMs containing as much as 60% undecanethiolate resisted the adsorption of both proteins; the amount of protein that adsorbed to SAMs having values of o1 < 0.4 increased with the mole fraction of undecanethiolate (Figure 1). In all cases, greater than 90% of the protein remained adsorbed irreversibly during the final wash with buffer. Figure 2 compares the adsorption of RNAse A and fibrinogen on mixed SAMs prepared from undecanethiol and either 1 or a hexa(ethylene glycol)-terminated alkanethiolate (-S(CH2)11(OCH2CH2)6OH). SAMs presenting only tri(propylene sulfoxide) groups or hexa(ethylene glycol) groups are indistinguishable in their behavior: they entirely resist the in situ adsorption of proteinseven “sticky” proteins such as fibrinogen.4 Comparing the effectiveness of these groups when mixed with methyl-terminated alkanethiolates in the SAM is complicated for four reasons: (i) the lengths of the monomeric polymethylene units are different (propyl and ethyl); (ii) the number of oligomeric units differs for the two alkanethiolates; (iii) there is an error of ∼5% in determining the value of o in the SAM; (iv) the structures of mixed SAMs comprising alkanethiolates terminated in methyl groups, and these two (1) Gombotz, W. R.; Guanhui, W.; Horbett, T. A.; Hoffman, A. S. J. Biomed. Mater. Res. 1991, 15, 1547-1562. Harris, J. M., Ed. Poly(ethylene glycol) chemistry: Biotechnical and biochemical applications; Plenum: New York, 1992. (2) Whitesides, G. M.; Gorman, C. B. In Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC Press: Boca Raton, FL, 1995; pp 713-732. Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437-463. (3) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164-1167. Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 1071410721. (4) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 4383-4385. (5) The synthesis of alkanethiol 1 is given in the supporting information. (6) Yu, Z. W.; Quinn, P. J. Biosci. Rep. 1994, 14 (6), 259-281. Chen, C. Y.; Chang, K. K.; Chow, N. H.; Leow, T. C.; Chou, T. C.; Lin, X. Z. Dig. Dis. Sci. 1995, 40 (2), 419-426. Igimi, H.; Asakawa, S.; Tamura, R.; Yamamoto, F.; Shimura, H. Hepato-Gastroenterology 1994, 4 (1), 65-69. (7) Raether, H. In Physics of Thin Films; Hass, G., Francombe, M., and Hoffman, R., Eds.; Academic Press: New York, 1977; pp 145-261. Liedberg, B.; Lundstrom, I; Stenberg, E. Sens. Actuators, B 1993, 11, 6372. (8) SAMs were prepared by immersing gold-coated glass slides in solutions of mixtures of the two alkanethiols in chloroform (2 mM total thiol) for 1 h. Experimental details are given in the supporting information. (9) Mrksich, M.; Grunwell, J. R.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 12009-12010. (10) Sigal, G. B.; Bamdad, C.; Barberis, A.; Strominger, J.; Whitesides, G. M. Anal. Chem. 1996, 68, 490-497. Figure 1. Data for the adsorption of RNase A to mixed SAMs comprising 1 and undecanethiolate. The relatiVe change in the resonance angle (θm) is plotted versus time for each mixed SAM. The region of time during which protein was present in the buffer is indicated above the plot; the rapid increase (and decrease) in signal is due to dissolved protein in the bulk solution. The curves are offset vertically for clarity. The mole fraction of 1 in each SAM (o1) was determined by XPS and is indicated on the right side of the plot. 5136 J. Am. Chem. Soc. 1996, 118, 5136-5137

Journal ArticleDOI
TL;DR: Reaction alpha- and beta-xanthates 2 and 3 of sialic acid with glycosyl acceptors 5-8 in the presence of phenylsulfenyl triflate (PST) as a promotor in a 2:1 mixture of CH(3)CN/CH(2)Cl( 2) at low temperature affords alpha-sialosides in good yield and stereoselectivity.
Abstract: Reaction alpha- and beta-xanthates 2 and 3 of sialic acid with glycosyl acceptors 5-8 in the presence phenylsulfenyl triflate (PST) as a promotor in a 2:1 mixture of CH(3)CN/CH(2)Cl(2) at low temperature affords alpha-sialosides in good yield and stereoselectivity. PST is prepared in situ by reacting benzenesulfenyl chloride with silver triflate. Less reactive acceptors 5 and 6 give a higher alpha/beta ratio than more reactive allylic alcohol 7 and primary alcohol 8; alpha-stereoselectivity is increased in a dilute solution. A possible mechanism of the reaction that involves intermediate alpha- and beta-nitrilium cations 16 and 17 is discussed.

Journal ArticleDOI
Abstract: Six derivatives of 2-benzimidazolone, disubstituted in the 4 and 5 positions, have been synthesized, and their structures have been determined in the solid state. Four of these compounds crystallize as molecular tapes. Compounds 1-(CH3)2, 1-Cl2, and 1-Br2 form tapes that pack with their long axes parallel; compounds 1-H2 and 2-H2 form tapes that pack with their long axes at an angle to one another. Compounds 1-F2 and 1-I2 crystallize with a three-dimensional network of hydrogen bonds. The packing arrangement of molecular tapes is rationalized on the basis of closest packing and electrostatic interactions between aromatic rings. The occurrence of the network motif rather than the tape motif for 1-F2 and 1-I2 is rationalized on the basis of secondary interactions involving the halogen atoms.


Journal ArticleDOI
TL;DR: In this paper, the use of microcontact printing (μCP) and capillary filling (CF) was demonstrated to pattern the deposition of iron oxides on a surface with feature sizes of microns.
Abstract: This paper demonstrates the use of microcontact printing (μCP) and capillary filling (CF) to pattern the deposition of iron oxides on a surface with feature sizes of microns. Selective wetting of both self-assembled monolayers (SAMs) of alkanethiolates on gold and alkylsiloxanes on Si/SiO2 formed by microcontact printing limited the deposition of the iron oxides to the hydrophilic areas on the surfaces; thereby, the chemical functionality of the hydrophilic SAM had only a minor influence on the wetting behavior and the deposition. The iron oxides were deposited either as magnetite particles from colloidal solution, by precipitation of the oxide from previously deposited drops of water containing an iron(III) salt, or by ferrite plating. The size of the metal oxide patterns was limited to the size of the areas that could be patterned using μCP. Capillary filling using a colloidal solution of magnetite could also be used to fabricate continuous, interconnected structures of magnetite. The magnetic properties of the deposited iron oxides were characterized by magnetic force measurement (MFM) and by measurement of the magnetization. The magnetite particles deposited in these experiments showed superparamagnetic behavior; they were too small individually to support a permanent magnetization.

Journal ArticleDOI
26 Jun 1996-Langmuir
TL;DR: In this paper, the surface density of self-assembled monolayers (SAMs) of hexadecanethiolates on gold that lead to etching of the gold when the system is etched with aqueous ferricyanide solution is measured.
Abstract: This paper describes a technique to measure the surface density of the defects in self-assembled monolayers (SAMs) of hexadecanethiolates on gold that lead to etching of the gold when the system is etched with aqueous ferricyanide solution. This technique uses two steps of amplification through chemical reaction to convert pinhole defects in SAMs into easily imaged, micron-scale pits in an underlying Si support. With this technique, it is possible to evaluate the density of defects in SAMs and in the structures prepared by using SAMs as resists under conditions that may be encountered in lithographic processing. At present, the lowest density of pits that we have measured for SAMs of hexadecanethiolates on 50 nm thick gold is ∼5 pits/mm2. This density is an upper limit for the density of defects that expose bare gold to the etching solution or that comprise regions of SAM sufficiently thin that the SAM is not able to block access of the etchant to the gold.