Fabrication and Microscopic and Spectroscopic Characterization of Planar, Bimetallic, Micro- and Nanopatterned Surfaces.

Micropatterns and nanopatterns of gold embedded in silver and titanium embedded in gold have been prepared by combining either photolithography or electron-beam lithography with a glue-free template-stripping procedure. The obtained patterned surfaces have been topographically characterized using atomic force microscopy and scanning electron microscopy, showing a very low root-mean-square roughness (<0.5 nm), high coplanarity between the two metals (maximum height difference ≈ 2 nm), and topographical continuity at the bimetallic interface. Spectroscopic characterization using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), and Auger electron spectroscopy (AES) has shown a sharp chemical contrast between the two metals at the interface for titanium patterns embedded in gold, whereas diffusion of silver into gold was observed for gold patterns embedded in silver. Surface flatness combined with a high chemical contrast makes the obtained surfaces suitable for applications involving functionalization with molecules by orthogonal adsorption chemistries or for instrumental calibration. The latter possibility has been tested by determining the image sharpness and the analyzed area on circular patterns of different sizes for each of the spectroscopic techniques applied for characterization.This is the first study in which the analyzed area has been determined using XPS and AES on a flat surface, and the first example of a method for determining the analyzed area using ToF-SIMS.

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. Atomic force microscopy confirms a roughness of <0.5 nm RMS on both Ti and Au regions, with a topographically indistinguishable gold-titanium interface. Detailed surface-chemical maps of the patterned surfaces have been obtained by means of imaging X-ray photoelectron spectroscopy (i-XPS) as well as time-of-flight secondary-ion mass spectrometry (ToF-SIMS). They confirm the presence of well-separated Ti and Au regions, with a chemical contrast that is sharp (as determined by ToF-SIMS) and complete (as determined by i-XPS) across the Ti-Au interface. Thus, a surface has been fabricated that is physically homogeneous down to the nanoscale incorporating chemically distinct micropatterns consisting of two different metals, with totally contrasting surface chemistries.

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. Additionally, negatively charged proteins (bovine serum albumin and fibrinogen) and positively charged proteins (lysozyme) were adsorbed on the gradients. Negatively charged nanoparticles and proteins adsorb mainly in the positively charged region and vice versa, illustrating that the adsorption behavior is mainly influenced by electrostatic interactions, and showing the potential of the gradient for sorting applications. Despite literature reports to the contrary, no area was found that was completely resistant to protein adsorption.

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. These results are correlated with the chemical composition maps of the surface obtained by X-ray photoelectron spectroscopy. The homogeneity and the ordering of the self-assembled monolayer were investigated by dynamic water contact angle measurements and polarization-modulation infrared reflection-absorption spectroscopy.

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.

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. Optimal conditions for spectral acquisition have been obtained from theoretical calculations. The ability of this methodology to gather high quality infrared spectra of adsorbed monolayers is demonstrated and the analysis of the surface packing and molecular orientation is discussed.

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. The SAMs were characterized by means of dynamic water contact angle (dCA) measurements, variable-angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). A higher degree of order and packing density within the monolayers was found for alkyl phosphates with alkyl chain lengths exceeding 15 carbon atoms. This is reflected in a lower dCA hysteresis, as well as a film thickness measured by VASE and XPS close to the expected values for SAMs with an average alkyl chain tilt angle of 30 degrees to the surface normal. Additionally a shift of the symmetric and antisymmetric C-H stretching modes in the PM-IRRAS spectra to lower wave numbers was observed. These findings imply a higher two-dimensional crystallinity of the films derived from alkyl phosphates with a longer alkyl chain length.

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. Such a gradient, when back-filled with a complementary thiol, having either a hydroxyl or carboxyl end-group, yields two-component gradients that show similar changes in wettability along their lengths. However, while gradients composed of methyl and hydroxyl end-groups show a well-ordered alkyl chain structure over their entire length, methyl-carboxyl gradients exhibit a greater conformational disordering toward the carboxyl-rich end.