At high loads, the pressure exerted by an atomic force microsope (AFM) probe on a molecular film is sufficient to break covalent bonds. Such processes have been used to carry out nanolithography, but they lack specificity. We describe a very different process, in which an AFM probe under a modest load is used to selectively remove a photocleavable protecting group. Films formed by the adsorption of (methoxyheptaethylene glycol)nitrophenylethoxycarbonyl-protected aminopropyltriethoxysilane on silica are highly resistant to the adsorption of proteins, but may be patterned by exposure to UV light, causing removal of the protein-resistant nitrophenyl protecting group and its protein-resistant oligoethylene glycol adduct and exposing the amine. The same result can be achieved using an AFM probe at a load of ca. 100 nN. A full width at half maximum height (FWHM) of ca. 20 nm is obtained. The FWHM increases with load, reaching ca. 90 nm at a load of 10 mN. At larger loads an abrupt transition occurs to a regime dominated by mechanical abrasion, yielding broader features. We hypothesize that at low loads the AFM probe causes selective cleavage of the same C-N bond in the carbamate group that is cleaved during UV irradiation. The selectivity has been confirmed by derivatizing patterned surfaces with nitrilotriacetic acid (NTA) functional groups, and coupling His-tagged green fluorescent protein (GFP) to the surface. Confocal fluorescence microscopy confirms that the GFP attaches to nanolines formed in the lithographic process, but is released when the samples are treated with imidazole, which disrupts the interaction between NTA and the His tag on the protein. Density functional calculations suggest an explanation for the surprising selectivity of this process. The main energetic barrier in the photodeprotection scheme is the initial S0 to S1 transition. The molecular conformation was then constrained in an effort to model the likely deformations occurring in the lithographic process. In the compressed configuration the energy barrier to the S0 to S1 transition was found to be much reduced, so that the deprotection may occur at near IR wavelengths. Hence application of the AFM probe facilitates deprotection by exposure to low energy photons, while UV photons are required in the absence of a mechanical deformation. These findings indicate that with careful design of the resist system, a high degree of specificity is possible in tribochemical surface modification.

Tribochemical nanolithography

255th National Meeting (2018)

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