Amidation of monolayers on silicon in physiological buffers: A quantitative IR study

Mixed carboxyl-terminated monolayers grafted onto monocrystalline (111) silicon are prepared by photochemical hydrosilylation of undecylenic acid/1-decene mixtures on hydrogenated surfaces. The attachment of a simple primary amine (hexylamine) to the mixed-acid-terminated monolayers is achieved in a physiological buffer by a two-step process using the water-soluble coupling agents N-ethyl-N'-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) to activate the carboxyl terminations. The chemical compositions of the acid-terminated, activated, and amidated monolayers are studied carefully as a function of the morphology of the initial hydrogenated surface (rough or flat) by quantitative FTIR spectroscopy (ATR geometry) coupled with AFM imaging. For the amidation reaction, in situ IR experiments are performed in hexylamine solutions at three different concentrations. A simple model is proposed to explain the sublinear variation of the reaction rate as a function of the amine concentration.