Subtractive patterning via chemical lift-off lithography.

Conventional soft-lithography methods involving the transfer of molecular "inks" from polymeric stamps to substrates often encounter micrometer-scale resolution limits due to diffusion of the transferred molecules during printing. We report a "subtractive" stamping process in which silicone rubber stamps, activated by oxygen plasma, selectively remove hydroxyl-terminated alkanethiols from self-assembled monolayers (SAMs) on gold surfaces with high pattern fidelity. The covalent interactions formed at the stamp-substrate interface are sufficiently strong to remove not only alkanethiol molecules but also gold atoms from the substrate. A variety of high-resolution patterned features were fabricated, and stamps were cleaned and reused many times without feature deterioration. The remaining SAM acted as a resist for etching exposed gold features. Monolayer backfilling into the lift-off areas enabled patterned protein capture, and 40-nanometer chemical patterns were achieved.

Directing substrate morphology via self-assembly: ligand-mediated scission of gallium-indium microspheres to the nanoscale.

We have developed a facile method for the construction of liquid-phase eutectic gallium-indium (EGaIn) alloy nanoparticles. Particle formation is directed by molecular self-assembly and assisted by sonication. As the bulk liquid alloy is ultrasonically dispersed, fast thiolate self-assembly at the EGaIn interface protects the material against oxidation. The choice of self-assembled monolayer ligand directs the ultimate size reduction in the material; strongly interacting molecules induce surface strain and assist particle cleavage to the nanoscale. Transmission electron microscopy images and diffraction analyses reveal that the nanoscale particles are in an amorphous or liquid phase, with no observed faceting. The particles exhibit strong absorption in the ultraviolet (∼200 nm), consistent with the gallium surface plasmon resonance, but dependent on the nature of the particle ligand shell.

Investigations of the mechanism of gold nanoparticle stability and surface functionalization in capillary electrophoresis.

Covalently functionalized gold nanoparticles influence capillary electrophoresis separations of neurotransmitters in a concentration- and surface-chemistry-dependent manner. Gold nanoparticles with either primarily covalently functionalized carboxylic acid (Au@COOH) or amine (Au@NH(2)) surface groups are characterized using extinction spectroscopy, transmission electron microscopy, and zeta potential measurements. The impact of the presence of nanoparticles and their surface chemistry is investigated, and at least three nanoparticle-specific mechanisms are found to effect separations. First, the degree of nanoparticle-nanoparticle interactions is quantified using a new parameter termed the critical nanoparticle concentration (CNC). CNC is defined as the lowest concentration of nanoparticles that induces predominant nanoparticle aggregation under specific buffer conditions and is determined using dual-wavelength photodiode array detection. Once the CNC has been exceeded, reproducible separations are no longer observed. Second, nanoparticle-analyte interactions are dictated by electrostatic interactions which depend on the pK(a) of the analyte and surface charge of the nanoparticle. Finally, nanoparticle-capillary interactions occur in a surface-chemistry-dependent manner. Run buffer viscosity is influenced by the formation of a nanoparticle steady-state pseudostationary phase along the capillary wall. Despite differences in buffer viscosity leading to changes in neurotransmitter mobilities, no significant changes in electroosmotic flow were observed. As a result of these three nanoparticle-specific interactions, Au@NH(2) nanoparticles increase the mobility of the neurotransmitters while a smaller opposite effect is observed for Au@COOH nanoparticles. Understanding nanoparticle behavior in the presence of an electric field will have significant impacts in separation science where nanoparticles can serve to improve either the mobility or detection sensitivity of target molecules.