Modification of porous silicon surfaces with activated ester monolayers

A series of activated esters of undecylenic acid with activated ester end groups (3-nitrophenyl, N-succinimidyl, S-ethyl, and 2,2,2-trifluoroethyl) have been synthesized and covalently linked to a porous silicon (PSi) surface via a thermal hydrosilylation reaction. Diffuse reflectance infrared Fourier transform (DRIFT) or transmission FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the monolayers and to follow the course of reactions with primary amines in aqueous solution. It was found that the reaction of the 3-nitrophenyl undecylenate with the PSi surface resulted in significant oxidation of the surface. A control reaction with nitrobenzene confirmed that the arylnitro group was responsible for the surface oxidation and grafting of the nitroaromatic on the surface via a Si-O-N linkage. The N-succinimidyl (NHS) and thioethyl activated ester-modified surfaces formed monolayers with no apparent oxidation of the PSi surface. However, we find that the hydrophobic end groups prevent wetting of the porous structure and further reaction is not possible. Reactions of these groups could be achieved by the addition of 25% (v/v) of ethanol to the aqueous solution of amine, electrochemical partial oxidation of the surface, or preparation of mixed monolayers consisting of mixtures of undecylenic acid and the activated undecylenate ester in a ratio of 5:1 or higher. Under these conditions the NHS group was displaced by ethanolamine or n-butylamine. As expected, exposure of primary amines to the activated S-ethyl thioester did not result in any reaction. However, amide bond formation was observed upon reaction of the methyl ester of cysteine illustrating specificity of this activating group toward cysteine-terminated proteins.