Nagaiyanallur V. Venkataraman

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.

Structural Evolution of Self‐Assembled Alkanephosphate Monolayers on TiO2

Alkanephosphates and -phosphonates bind strongly onto the surfaces of a variety of metal oxides, including titania, zirconia, niobia, alumina and tantala, to form self-assembled monolayers (SAMs). The specificity of the phosphate group for the oxide surfaces in the presence of other functional groups, such as hydroxyl or carboxylic acid, allows many useful surface functionalities to be imparted to this important class of materials. In addition to identifying the surface-chemical functional groups, IR spectroscopy has also played an important role in understanding the molecular conformation and orientation of adsorbed monolayers. However, most IR spectroscopic data for alkanephosphate SAMs adsorbed on metal oxides has been restricted to measurements on powders or nanoparticulate samples. Powder spectra do not reveal information concerning molecular orientation, due to the random distribution of the transition dipole moments with respect to the incident IR radiation. Infrared reflection–absorption spectroscopy (IRAS) has been widely used on metallic substrates, and it has also been employed for characterizing monolayers adsorbed on flat, nonmetallic substrates but with high infrared reflections. However, single transmission or reflection methods for organic monolayers adsorbed on flat TiO2 substrates have not yet been reported. Other methods, such as external reflection–absorption using a surface-modified metallic substrate (“buried-metallayer”) or single-reflection germanium-attenuated total reflection (G-ATR) yield reasonable spectra of monolayers. However, since only the spectrum in p-polarization can be obtained, the exact analysis of molecular orientation requires simulation of the complete spectrum in the region of interest for all possible tilt and twist angles and subsequent fitting to the experimental spectrum. Multiple internal reflection (MIR) measurements of thin molecular layers deposited on an ATR crystal surface enable both sand p-polarized spectra—and thereby the dichroic ratios of different vibrational modes—to be measured, thus simplifying the molecular-orientation analysis. However, the need for a specially fabricated ATR crystal restricts its use as a routine analytical method for the study of a series of different metal-oxide interfaces. Here, we report a study of the evolution of the conformational and orientational ordering of the alkyl chains during the self-assembly of an alkanephosphate onto a macroscopically flat TiO2 surface, performed by means of multiple transmission–reflection (MTR) measurements. The high sensitivity afforded by this method enables adsorbed alkanephosphates to be studied even at sub-monolayer coverage on standard metal-oxide-coated, double-sidepolished silicon wafers. Figure 1 presents a simplified schematic depiction of the MTR setup. The measuring device consists of two parallel gold mirrors, the sample being placed between the two mirrors and the distances between the sample and the mirrors being suita-

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.

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.

Chapter 6 – 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.

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.