In solution, only collisions between molecules of sufficient energy to overcome the activation barrier lead to the formation of reaction products. The rate of the reaction is influenced by the temperature, concentration of the reactants, solvent, orientation of the reactants, and structure of the molecules. Catalysts or enzymes lower the activation energy by making available alternative reaction pathways and preorganize the reactant molecules in close proximity, which leads to an increase in the effective concentration of the reactants.[1] Herein, we report the first example in which a reaction (amide-bond formation) is made possible because the reactant molecules are physically pushed together. An elastomeric stamp (as used in microcontact printing [2] ) comes into conformal (van der Waals) contact with a functionalized surface, [3] thus forcing "ink" molecules very close to the surface. We speculate that the nanoscale confinement of the ink at the interface between the stamp and the selfassembled monolayer (SAM), in combination with the preorganization of the reactants in the monolayer, facilitates the formation of covalent bonds.The on-chip synthesis of singlestrand DNA molecules has revolutionized genotyping research. Fodor and co-workers [4,5] developed photolithographic techniques in combination with wet surface chemistry to attain > 99 % yield in coupling steps for the preparation of libraries of peptides or oligonucleotides. Usually, a range of catalysts and activated substrates are required for efficient covalent-bond formation on surfaces. In our research on patterned self-assembled monolayers, we were intrigued by the rapid and efficient formation of SAMs on gold or Si/SiO 2 through the use of microcontact printing (mCP).[2] When an elastomeric stamp inked with trichloroalkyl silanes is placed on a clean Si/SiO 2 surface, the formation of the polysiloxane network is essentially complete within minutes, whereas this process could take hours in solution. Microcontact printing has previously been used to couple molecules to a reactive surface, but in all cases a catalyst or activated substrate was used to induce covalentbond formation.[6-9] Herein we report the formation of new bonds solely as a result of the nanoscale confinement of molecules between stamp and surface.Amide-bond formation is a suitably challenging test case, as such reactions require catalysts (4-(dimethylamino)pyridine (DMAP), N-hydroxybenzotriazole (HOBT), or dicyclohexylcarbodiimide (DCC)) or elevated temperatures and long reaction times when performed in solution. In a typical reaction (Figure 1), we prepared a clean amine-terminated SAM surface on gold. To ensure all amines were deprotonated, we washed the surface with a saturated solution of K 2 CO 3 . A flat hydrophilic stamp (treated for 30 s with an oxygen plasma and stored under millipore water) was inked with a solution of an appropriate Boc-protected amino acid (1 mm) in ethanol, dried under a stream of nitrogen, and placed on this surface. If necessary the sample was heated ...

Dedicated to Professor David N. Reinhoudt on the occasion of his 65th birthday Polydimethylsiloxane (PDMS) elastomers are widely used in biomedical devices, such as medical implants, catheters, and contact lenses, because of their biocompatibility and physical properties. [1][2][3] PDMS elastomers can be easily molded into (sub)micrometer features and are optically transparent and relatively chemically inert, which are ideal properties for applications in soft-lithographic techniques. In many softlithographic applications, the low-energy surface of PDMS can be used advantageously, for example, in microcontact printing of nonpolar species [4] and the transfer-printing technique pioneered by Rogers and co-workers. [5,6] However, the unmodified PDMS surface is hydrophobic, and surface modification is required to print polar inks or proteins, [7][8][9] to facilitate cell growth, [10] to improve compatibility, [11,12] and for use in microfluidics.[13] A range of methods have been developed to alter the surface chemistry of PDMS; most involve UV/ozone treatment [14] or an oxygen plasma, [15] and subsequent functionalization by forming alkylsilane monolayers. Yet, there is to date no single-step procedure that allows the introduction of a wide range of functional groups on the PDMS surface in a spatially controlled way. Herein, we demonstrate a novel way to selectively modify the surface of a silicone elastomer through minimization of interfacial free energy and the self-assembly of functional molecules at the surface by mirroring the distribution of surface energies on a template. Chaudhury and co-workers [16] have exploited surface energy to drive allyl-functionalized perfluorinated polyethers to the surface of PDMS. Our approach generalizes the principle of surface free energy minimization to drive both hydrophilic and hydrophobic molecules to a surface, thus replicating the pattern on the template surface.This surface-induced self-assembly-based method of chemically micropatterning of PDMS is shown schematically in Figure 1. A micropatterned alkylthiolate self-assembled monolayer (SAM) on gold is used as a topographically flat, chemical master. Subsequently, a mixture of Sylgard 184 PDMS and a small amount (< 5 wt %) of vinyl-terminated small molecules with different head groups is cured against this master. The small molecules preferentially accumulate near the surface areas with complementary surface energy to minimize the unfavorable interactions of the PDMS polymers with the SAMs. During the hydrosilylation (curing) reaction, the silicon hydride groups present in the cross-linking agent of the Sylgard 184 react with the vinyl groups of both the crosslinking dimethylsiloxane oligomers as well as the added functional molecules, thereby "freezing" the chemical pattern into the PDMS elastomer (Figure 1 e). After careful removal of the PDMS, facilitated by water to weaken the interfacial bonds on the hydrophilic areas, flat PMDS surfaces patterned with sub-micrometer features of different chemical functionalities are...

No abstract

In solution, only collisions between molecules of sufficient energy to overcome the activation barrier lead to the formation of reaction products. The rate of the reaction is influenced by the temperature, concentration of the reactants, solvent, orientation of the reactants, and structure of the molecules. Catalysts or enzymes lower the activation energy by making available alternative reaction pathways and preorganize the reactant molecules in close proximity, which leads to an increase in the effective concentration of the reactants. [1] Herein, we report the first example in which a reaction (amide-bond formation) is made possible because the reactant molecules are physically pushed together. An elastomeric stamp (as used in microcontact printing [2] ) comes into conformal (van der Waals) contact with a functionalized surface, [3] thus forcing "ink" molecules very close to the surface. We speculate that the nanoscale confinement of the ink at the interface between the stamp and the selfassembled monolayer (SAM), in combination with the preorganization of the reactants in the monolayer, facilitates the formation of covalent bonds.The on-chip synthesis of singlestrand DNA molecules has revolutionized genotyping research. Fodor and co-workers [4,5] developed photolithographic techniques in combination with wet surface chemistry to attain > 99 % yield in coupling steps for the preparation of libraries of peptides or oligonucleotides. Usually, a range of catalysts and activated substrates are required for efficient covalent-bond formation on surfaces. In our research on patterned self-assembled monolayers, we were intrigued by the rapid and efficient formation of SAMs on gold or Si/SiO 2 through the use of microcontact printing (mCP). [2] When an elastomeric stamp inked with trichloroalkyl silanes is placed on a clean Si/SiO 2 surface, the formation of the polysiloxane network is essentially complete within minutes, whereas this process could take hours in solution. Microcontact printing has previously been used to couple molecules to a reactive surface, but in all cases a catalyst or activated substrate was used to induce covalentbond formation. [6][7][8][9] Herein we report the formation of new bonds solely as a result of the nanoscale confinement of molecules between stamp and surface.Amide-bond formation is a suitably challenging test case, as such reactions require catalysts (4-(dimethylamino)pyridine (DMAP), N-hydroxybenzotriazole (HOBT), or dicyclohexylcarbodiimide (DCC)) or elevated temperatures and long reaction times when performed in solution. In a typical reaction (Figure 1), we prepared a clean amine-terminated SAM surface on gold. To ensure all amines were deprotonated, we washed the surface with a saturated solution of K 2 CO 3 . A flat hydrophilic stamp (treated for 30 s with an oxygen plasma and stored under millipore water) was inked with a solution of an appropriate Boc-protected amino acid (1 mm) in ethanol, dried under a stream of nitrogen, and placed on this surface. If necessary the sample w...

Dedicated to Professor David N. Reinhoudt on the occasion of his 65th birthday Polydimethylsiloxane (PDMS) elastomers are widely used in biomedical devices, such as medical implants, catheters, and contact lenses, because of their biocompatibility and physical properties. [1][2][3] PDMS elastomers can be easily molded into (sub)micrometer features and are optically transparent and relatively chemically inert, which are ideal properties for applications in soft-lithographic techniques. In many softlithographic applications, the low-energy surface of PDMS can be used advantageously, for example, in microcontact printing of nonpolar species [4] and the transfer-printing technique pioneered by Rogers and co-workers. [5,6] However, the unmodified PDMS surface is hydrophobic, and surface modification is required to print polar inks or proteins, [7][8][9] to facilitate cell growth, [10] to improve compatibility, [11,12] and for use in microfluidics.[13] A range of methods have been developed to alter the surface chemistry of PDMS; most involve UV/ozone treatment [14] or an oxygen plasma, [15] and subsequent functionalization by forming alkylsilane monolayers. Yet, there is to date no single-step procedure that allows the introduction of a wide range of functional groups on the PDMS surface in a spatially controlled way. Herein, we demonstrate a novel way to selectively modify the surface of a silicone elastomer through minimization of interfacial free energy and the self-assembly of functional molecules at the surface by mirroring the distribution of surface energies on a template. Chaudhury and co-workers [16] have exploited surface energy to drive allyl-functionalized perfluorinated polyethers to the surface of PDMS. Our approach generalizes the principle of surface free energy minimization to drive both hydrophilic and hydrophobic molecules to a surface, thus replicating the pattern on the template surface.This surface-induced self-assembly-based method of chemically micropatterning of PDMS is shown schematically in Figure 1. A micropatterned alkylthiolate self-assembled monolayer (SAM) on gold is used as a topographically flat, chemical master. Subsequently, a mixture of Sylgard 184 PDMS and a small amount (< 5 wt %) of vinyl-terminated small molecules with different head groups is cured against this master. The small molecules preferentially accumulate near the surface areas with complementary surface energy to minimize the unfavorable interactions of the PDMS polymers with the SAMs. During the hydrosilylation (curing) reaction, the silicon hydride groups present in the cross-linking agent of the Sylgard 184 react with the vinyl groups of both the crosslinking dimethylsiloxane oligomers as well as the added functional molecules, thereby "freezing" the chemical pattern into the PDMS elastomer (Figure 1 e). After careful removal of the PDMS, facilitated by water to weaken the interfacial bonds on the hydrophilic areas, flat PMDS surfaces patterned with sub-micrometer features of different chemical functionalities are...