Inkless microcontact printing on self-assembled monolayers of fmoc-protected aminothiols.

Originally developed by Whitesides and co-workers, microcontact printing (μCP) has become the method of choice for microand nanoscale fabrication of surfaces and nanoparticles.1 Despite its many advantages, several limitations remain, specifically (i) relatively high level of defects due to distortion and deformation of the elastomeric stamp,2 (ii) a limited number of substrates and molecular inks, and (iii) practical limits to feature sizes near 100200 nm due to diffusion of molecular inks, both diffusive wetting of surfaces and diffusion of volatile inks through the gas phase.3 Recent efforts to circumvent these limitations include the replacement of liquid inks with solid analogues,4 the use of reactive polymers,5 and the subtraction of inks from flat PDMS stamps in the patterned area followed by printing of the remaining inks on a different substrate.6 In 2003, Reinhoudt et al. obviated the diffusive limitations of molecular inks through the use of functionalized stamps that transfer pattern through covalent modification of a preformed surface.7 Specifically, a SAM surface displaying trimethylsilyl (TMS) ethers was converted to free alcohols during conformal contact with an oxidized PDMS stamp, although the protocol achieved only 30% cleavage. A similar study used plasma-oxidized flat PDMS to promote coupling between amino-terminated SAMs and N-protected amino acids.8 We recently reported a μCP method that transfers pattern through the action of a stamp-immobilized biocatalyst on a monolayer of substrate, a process that does not depend on the transfer of the molecular inks.9 Briefly, an acrylamide stamp bearing immobilized exonuclease I (ExoI) was used to catalyze ablation of singlestranded DNA immobilized on both glass and gold surfaces. This work demonstrated that catalytic μCP can successfully transfer patterns with 14 μm features. Here, we report complete transfer of patterns with sub-micron features using a chemical catalyst bound to a rigid polymeric stamp. The 9-fluorenylmethoxycarbonyl (Fmoc) amino protecting group is selectively cleaved under mildly basic nonhydrolytic conditions using aliphatic amines such as piperidine, morpholine, and 8-diazabicyclo[5.4.0]undec-7-ene (DBU).10 In this embodiment of inkless μCP, a stamp bearing polymerized piperidin-4-ylmethanamine is brought into contact with a gold surface functionalized with the SAM of an Fmoc-protected aminothiol, promoting catalytic cleavage of the Fmoc groups. Fmoc-protected SAMs on gold were formed from (9H-fluoren-9-yl)methyl 11-mercaptoundecylcarbamate (1), prepared in seven steps from 11-aminoundecanoic acid (Supporting Information).11 A significant limitation to the resolution of catalytic μCP reported previously9 was the use of acrylamide stamps: while these materials are easily functionalized, they lack the mechanical rigidity necessary for high fidelity transfer at short length scales. To alleviate this limitation, we utilized a polyurethane acrylate polymer, a material that has previously been used to prepare molds with densely arrayed high aspect ratio nanopatterns with sub-100 nm features for use in replica molding.12 Monomer 2, synthesized from isophorone diisocyanate, polyethylene glycol (av. Mw 400 g/mol), and hydroxypropyl acrylate,13 was diluted by 30% with trimethylolpropane ethoxylate triacrylate (2, av. Mn 912 g/mol) to reduce viscosity. To the mixture were added photoinitiators, and the resulting prepolymeric solution was polymerized between two glass slides under UV light to produce control unfunctionalized flat stamp I (Figure 1). Flat (II) and patterned (III) stamps bearing reactive piperidine functionalities were prepared through Michael addition of 2-aminomethyl piperidine (4) (8 v/v %) to prepolymeric mixtures prior to polymerization. Preceding the printing experiments, stamps were washed with EtOH for at least 1 h, rinsed with EtOH and H2O, and dried with filtered nitrogen. The shape and size of the stamp features were identical to those of the corresponding masters, were unaffected during storage at room temperature, and retained their integrity even after heating to 70 °C. Fmoc-protected SAMs on gold (substrate 1) were formed by immersing clean gold substrates in 1 mM EtOH solution of 1 for at least 2 h at room temperature. Amino-terminated SAMs (substrate 2) were prepared from these surfaces by deprotection with a 1 M piperidine solution in DMSO for 45 min at room temperature (Figure 2).14 Unfunctionalized and reactive featureless stamps were brought into contact with Fmoc-modified SAMs on gold at 50 °C and permitted to react for 3 h (substrates 5 and 3). Following the first application, the reactive stamp was soaked in EtOH for 1 h, Figure 1. Preparation of polyurethane acrylate stamps.