Infrared spectroscopy is used to investigate the transformation of carboxyl-terminated alkyl chains immobilized on a surface into succinimidyl ester-terminated chains by reaction with an aqueous solution of N-ethyl-N'-(3-(dimethylamino)propyl)carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The acid chains are covalently grafted at the surface of hydrogenated porous silicon whose large specific surface area allows for assessing the activation yield in a semiquantitative way by infrared (IR) spectroscopy and detecting trace amounts of surface products and/or reaction products of small IR cross section. In this way, we rationalize the different reaction paths and optimize the reaction conditions to obtain as pure as possible succinimidyl ester-terminated surfaces. A diagram mapping the surface composition after activation was constructed by systematically varying the solution composition. Results are accounted for by NHS surface adsorption and a kinetic competition between the various EDC-induced surface reactions.

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The paper reports on a novel surface plasmon resonance (SPR) substrate architecture based on the coating of a gold (Au) or silver (Ag) substrate with 5 nm thin amorphous silicon-carbon alloy films. Ag/a-Si(1-x)C(x):H and Au/a-Si(1-x)C(x):H multilayers are found to provide a significant advantage in terms of sensitivity over both Ag and Au for SPR refractive index sensing. The possibility for the subsequent linking of stable organic monolayers through Si-C bonds is demonstrated. In a proof-of-principle experiment that this structure can be used for real-time biosensing experiments, amine terminated biotin was covalently linked to the acid-terminated SPR surface and the specific streptavidin-biotin interaction recorded.

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For sensitive surface plasmon resonance (SPR) sensing the choice of the metal film and the strategy to bind the receptors to the SPR chip is critical. We have shown recently (Touahir, L.; Niedziolka-Jonsson, J.; Galpin, E.; Boukherroub, R.; Gouget-Laemmel, A. C.; Solomon, I.; Petukhov, M.; Chazalviel, J.-N.; Ozanam, F.; Szunerits, S. Langmuir 2010, 26, 6058) that a 5 nm thick layer of an amorphous silicon−carbon alloy (a-Si1−x C x :H) deposited on a silver-based SPR interface can significantly enhance the sensitivity. In addition, the capping of a surface-plasmon active silver layer with a thin film of hydrogenated amorphous silicon−carbon alloy provides a practical solution for obtaining chemically stable SPR interfaces usable in conditions typical of bioassays with the additional advantage of benefiting from well-controlled processes for a robust covalent immobilization of biological probes to the interface. In this paper we demonstrate that the developed architecture in conjugation with an optimized surface functionalization scheme allows for a highly sensitive analysis of interfacial DNA−DNA binding interactions using surface plasmon-enhanced fluorescence (SPFS) as detection principle. The influence of the density of surface linked DNA probes on the recognition of 50 nM cDNA strands is presented. On an optimized surface (15% of acid-anchoring groups) DNA complementary probes with concentrations of 500 fM could be detected making this approach interesting compared to classical SPR experiments where nanomolar (nM) detection limits are conventionally reached.

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The kinetics of activation reaction of acid-terminated monolayers grafted onto crystalline (111) silicon surfaces with N-ethyl-N′-(dimethylaminopropyl)-carbodiimide in the presence of N-hydroxysuccinimide is investigated using in situ real-time infrared spectroscopy. In the case of fully acid-terminated surfaces, the results show that the reaction rate exhibits a biexponential law, with two characteristic times, a fast one, τ1, and a slow one, τ2. The τ1 and τ2 values depend on temperature and solution composition. Moreover, the reaction pathways related to τ1 are partially suppressed for mixed monolayers, that is, by lowering the density of acid sites. These observations are discussed within the framework of a reaction mechanism.

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