Photocatalytic Nanolithography of Self-Assembled Monolayers and Proteins

Self-assembled monolayers of alkylthiolates on gold and alkylsilanes on silicon dioxide have been patterned photocatalytically on sub-100 nm length-scales using both apertured near-field and apertureless methods. Apertured lithography was carried out by means of an argon ion laser (364 nm) coupled to cantilever-type near-field probes with a thin film of titania deposited over the aperture. Apertureless lithography was carried out with a helium–cadmium laser (325 nm) to excite titanium-coated, contact-mode atomic force microscope (AFM) probes. This latter approach is readily implementable on any commercial AFM system. Photodegradation occurred in both cases through the localized photocatalytic degradation of the monolayer. For alkanethiols, degradation of one thiol exposed the bare substrate, enabling refunctionalization of the bare gold by a second, contrasting thiol. For alkylsilanes, degradation of the adsorbate molecule provided a facile means for protein patterning. Lines were written in a protein-resistant film formed by the adsorption of oligo(ethylene glycol)-functionalized trichlorosilanes on glass, leading to the formation of sub-100 nm adhesive, aldehyde-functionalized regions. These were derivatized with aminobutylnitrilotriacetic acid, and complexed with Ni2+, enabling the binding of histidine-labeled green fluorescent protein, which yielded bright fluorescence from 70-nm-wide lines that could be imaged clearly in a confocal microscope

Odd Even Effects in the Structure and Stability of Azobenzene Substituted Alkanethiolates on Au 111 and Ag 111 Substrates

Structural properties and stability of the self-assembled monolayers (SAMs) of two prototypical azobenzene-based alkanethiols (C₆H₅–NN–C₆H₄–(CH₂)_<i>n</i>–SH) on Au(111) and Ag(111) substrates were studied in detail using a combination of complementary experimental techniques. The azobenzene moiety in these films was linked to the thiol headgroup via short aliphatic spacers of variable length, i.e., (CH₂)₃ or (CH₂)₄, corresponding to a different parity of <i>n</i>. For both Au(111) and Ag(111) substrates, a pronounced dependence of the packing density and molecular inclination on the parity of <i>n</i> was observed, with a higher packing density (by ∼14%) and smaller inclination (by ∼17°) of the azobenzene moieties for <i>n</i> = odd as compared to <i>n</i> = even on Au(111) and reversed, but somewhat reduced, behavior on Ag(111). This dependence was related to the well-known odd–even effects in molecular assembly on noble metal substrates, reported previously for a variety of oligophenyl-substituted alkanethiolate SAMs and observed now for the azobenzene-substituted monolayers as well, underlining their generality. The structural odd–even behavior was accompanied by odd–even effects in the stability of the substrate–S bond, with the latter effects being directly correlated to the respective structure variation. The above results are of general importance for the design of functional monomolecular films and of a particular significance as a basis for dedicated photoisomerization experiments