Patterning gradients.

Surface-chemical and -morphological gradients can be extremely useful in cell-biological research as high-throughput screening tools-for example, exposing a given set of cells to many different surface conditions at once, under identical ambient conditions, in order to monitor cell behavior such as proliferation or specific gene expression. They can also be used to investigate the effects of gradients themselves on cell behavior, such as migration. A number of simple, reliable techniques for both chemical- and morphological-gradient fabrication have been developed in our laboratories and are described in detail in the following.

Template-stripped, ultraflat gold surfaces with coplanar, embedded titanium micropatterns.

Ultraflat gold surfaces with coplanar, embedded titanium micropatterns, exhibiting extremely low roughness over the entire surface, have been obtained by a modified template-stripping procedure. Titanium is deposited onto photolithographically predefined regions of a silicon template. Following photoresist lift-off, the entire surface is backfilled with gold, template stripping is conducted, and an ultraflat micropatterned surface is revealed.

Protein and nanoparticle adsorption on orthogonal, charge-density-versus-net-charge surface-chemical gradients.

An orthogonal, charge-density-versus-net-charge, surface-chemical gradient, composed of ternary mixed self-assembled monolayers, has been prepared from three hydrophilic components: positively chargeable amine-terminated, negatively chargeable carboxylic-acid-terminated, and hydroxyl-terminated alkanethiols, with the latter bearing a slight negative charge in electrolytes. The chemical composition and its distribution have been monitored by X-ray photoelectron spectroscopy. The adsorption behavior of negatively charged SiO(2) nanoparticles and positively charged amine-modified SiO(2) nanoparticles has been studied.

Influence of salt on the aqueous lubrication properties of end-grafted, ethylene glycol-based self-assembled monolayers.

We have investigated the influence of a high-concentration salt solution (1 M NaCl) on the aqueous lubrication properties of ethylene glycol-based molecules, namely, alpha-methoxy-omega-mercaptopoly(ethylene glycol) (MW 5000 Da) and alpha-methoxy-omega-mercaptoheptakis(ethylene glycol) (MW 356 Da), which have been end-grafted onto polycrystalline gold surfaces at high surface density. Macroscopic-scale, yet nondestructive, pin-on-disk tribometry experiments revealed that a high concentration of sodium chloride is deleterious to the aqueous lubricating properties of both films under low-sliding-speed conditions. This behavior was observed to be closely associated with the more collapsed conformation of surface-grafted poly(ethylene glycol) polymer chains in concentrated salt solutions, as confirmed by quartz-crystal microbalance measurements.

Orthogonal, three-component, alkanethiol-based surface-chemical gradients on gold.

An orthogonal surface-chemical gradient composed of self-assembled monolayers on gold has been prepared by successive, controlled immersions in orthogonal directions into dilute solutions of dodecanethiol and perfluorododecanethiol. The resulting two-component orthogonal gradient in surface coverage was backfilled with 11-mercaptoundecanol, leading to a two-directional, three-component surface-chemical gradient. Water and hexadecane show distinctly different wetting behaviors on the gradient surface because of the differences in the hydrophobic and oleophobic natures of the three different constituents.

Multiple transmission-reflection infrared spectroscopy for high-sensitivity measurement of molecular monolayers on silicon surfaces.

A new infrared spectroscopic measurement involving multiple transmissions and reflections for molecular monolayers adsorbed on silicon surfaces has been established. Compared to the well-known multiple internal reflection (MIR) method, the distinctive advantage of multiple transmission-reflection infrared spectroscopy (MTR-IR) is the convenient measurement using standard silicon wafers as samples, while in the MIR setup special fabrication of geometric shapes such as 45 degrees bevel cuts on an attenuated total reflection silicon crystal is needed. Both p- and s-polarized spectra can be obtained reproducibly with the same order of sensitivity as by the MIR spectroscopy.

Influence of alkyl chain length on phosphate self-assembled monolayers.

A series of alkyl phosphates with alkyl chain lengths ranging from C10 to C18 have been synthesized. Self-assembled monolayers (SAMs) of these molecules were prepared on titanium oxide surfaces by immersion of the substrates in alkyl phosphate solutions of 0.5 mM concentration in n-heptane/isopropanol.

Order and composition of methyl-carboxyl and methyl-hydroxyl surface-chemical gradients.

A detailed infrared and XPS characterization of surface-chemical gradients of dodecanethiol with 11-mercaptoundecanol or 11-mercaptoundecanoic acid self-assembled on gold, is reported. Gradients were prepared using a simple, two-step process previously reported from our laboratory, which involves a controlled immersion of a polycrystalline gold substrate in a dilute (5 microM) solution of one component and a subsequent back-filling with the other. FTIR measurements show that a single-component gradient of dodecanethiol is composed of disordered, liquidlike alkyl chain conformations.