Covalent immobilization of amino acids on the porous silicon surface

In this work, we report on a multi-step route fully compatible with the mild conditions required for anchoring amino acids onto the porous silicon (PSi) surface. A dense monolayer of acid chains is first grafted via thermal hydrosilylation of undecylenic acid at the surface of hydrogenated PSi. The reaction takes place at the terminal C=C double bond and yields an organic monolayer covalently attached to the surface through Si-C bonds. Next, the acid terminal groups are transformed to succinimidyl ester terminations. This reaction, called activation, is achieved using N-hydroxysuccinimide (NHS) in the presence of the water-soluble N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide (EDC). The surface, now bearing an 'activated ester' termination, is subsequently made to react with the amino end of the amino acid, allowing for its covalent attachment through the formation of an amide bond. The reaction efficiency at each stage of functionalization was confirmed using Fourier transform infrared (FT-IR) measurements. SIMS depth profiling showed a significant level of carbon incorporation throughout the PSi layer. Electrochemical behavior of the amino-acid-modified PSi in the presence of copper ions was studied by means of cyclic voltammetry measurements.