Photolithography of self-assembled monolayers: optimization of protecting groups by an electroanalytical method

Abstract

Patterned surfaces presenting a high density of chemically reactive functional groups can be prepared through photolithography of self-assembled monolayers (SAM). In this paper, we report the synthesis and the evaluation of three reagents that can be used in SAM-photolithographic applications. These reagents are made up of a triethoxysilylpropylamine moiety in which the amine is temporarily blocked by photolabile protecting groups: NVOC (o-nitroveratryloxycarbonyl), ONB (o-nitrobenzyl), or DDZ (α,α-dimethyl-3,5-dimethoxybenzyloxycarbonyl). The presence of the triethoxysilyl group allows self-assembled monolayer formation. Release of chemically reactive amino groups is achieved by irradiation of the surface. An electroanalytical method was developed and used to monitor and optimize the three steps of the methodology occurring on surfaces: monolayer formation, photodeprotection, and subsequent functionalization of the released amino groups. Quantitative information on the efficiency of the photodeprotection step was obtained by this method. It was found that the DDZ group is superior to the ONB and NVOC photolabile protecting groups for SAM-photolithographic applications. Percentages of liberated amino groups are generally above 50% with the DDZ group, while they are generally inferior to 25% and 2% for NVOC and ONB, respectively. These differences are attributed to the destruction of some of the released amino groups through a subsequent reaction with the photoproduct, o-nitrosobenzaldehyde for ONB and 3,4-dimethoxy-6-nitrosobenzaldehyde for NVOC, and to partial loss or destruction of the monolayer during prolonged irradiations. Key words: self-assembled monolayers, surface modification, photolithography, photolabile protecting groups, cyclic voltammetry.