We present a novel approach for preparation of nanometric protein arrays, based on binding of avidin molecules to nanotemplates generated by conductive AFM lithography on robust oligo(ethylene glycol)-terminated monolayers on silicon (111) surfaces that are protein-resistant. We showed that only biotinated-BSA but not the native BSA bind to the avidin arrays and that the resulting arrays of biotinated BSA could bind avidin to form protein dots with a feature size of approximately 30 nm. This result demonstrates that the avidin array may serve as templates for preparation of nanoarrays of a wide variety of biotin-tagged proteins for studying their interactions with other protein molecules at nanoscale.

We report the formation and characterization of self-assembled monolayers (SAMs) derived from the adsorption of 4-mercaptophenylboronic acid (MPBA) on gold. For comparison, SAMs derived from the adsorption of thiophenol (TP), 4-mercaptophenol (MP), and 4-mercaptobenzoic acid (MBA) were also examined. The structure and properties of the SAMs were evaluated by ellipsometry, contact-angle goniometry, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Specifically, ellipsometry was used to assess the formation of monolayer films, and contact angle measurements were used to determine the surface hydrophilicity and homogeneity. Separately, PM-IRRAS was used to evaluate the molecular composition and orientation as well as the intermolecular hydrogen bonding within the SAMs. Finally, XPS was used to evaluate the film composition and surface coverage (i.e., packing density), which was observed to increase in the following order: TP < MP < MPBA < MBA. A rationalization for the observed packing differences is presented. The XPS data indicate further that ultrahigh vacuum conditions induce the partial dehydration of MPBA SAMs with the concomitant formation of surface boronic anhydride species. Overall, the analytical data collectively show that the MPBA moieties in the SAMs exist in the acid form rather than the anhydride form under ambient laboratory conditions. Furthermore, stability studies find that MPBA SAMs are surprisingly labile in basic solution, where the terminal B-C bonds are cleaved by the attack of hydroxide ion and strongly basic amine nucleophiles. The unanticipated lability observed here should be considered by those wishing to use MPBA moieties in carbohydrate-sensing applications.

Atomically flat, homogeneous, and protein-resistant monolayers can be readily prepared on H-Si(111) surfaces by photo-induced hydrosilylation of alpha-oligo(ethylene glycol)-omega-alkenes.

Multidentate thiolate films were prepared by solution phase deposition of dendritic oligothiols on gold surfaces. The dendritic molecules contain three (G0) and nine (G1) thiol groups at the periphery and a functional group (bromophenyl) at the corn. The films were characterized by contact-angle goniometry, ellipsometry, polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Results of the XPS measurement indicate the presence of ∼20% of unbound S in the G0 films while ∼28% in the G1 films. Some of the unreacted thiol groups may be located at the upper part of the films and some of the bromophenyl groups at the lower part of the films. Three G0 molecules (containing three SH groups) cover about the same area on the substrate as one G1 molecule (containing nine SH groups), and both of the films exhibit similar thicknesses, indicating the flattening of the dendrons to maximize the bonding of SH groups to the gold surface. The G1 films exhibit higher stability than the G0 films, and both of them are much more stable than octadecanethiolate monolayers on gold in hot solvents, attributed to the formation of multiple S-Au bonds per dendron unit. The high stability of the films allows for the palladium-catalyzed Heck reaction on thiolate films. Treatment of both the G0 and G1 films with 4-fluorostyrene under Heck reaction conditions led to the disappearance of about 76 and 68% of the Br atoms among which about 60-70% were replaced by the 4-fluorostyryl groups, as shown by XPS. Prolonging the reaction time did not consume the remaining Br atoms, probably due to unfavorable orientation of the bromophenyl groups and/or the presence of adjacent thiol groups that may deactivate the catalyst in the film assembly.

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