Molecular monolayers on silicon as substrates for biosensors.

(111) silicon surfaces can be controlled down to atomic level and offer a remarkable starting point for elaborating nanostructures. Hydrogenated surfaces are obtained by oxide dissolution in hydrofluoric acid or ammonium fluoride solution. Organic species are grafted onto the hydrogenated surface by a hydrosilylation reaction, providing a robust covalent Si-C bonding. Finally, probe molecules can be anchored to the organic end group, paving the way to the elaboration of sensors. Fluorescence detection is hampered by the high refractive index of silicon. However, improved sensitivity is obtained by replacing the bulk silicon substrate by a thin layer of amorphous silicon deposited on a reflector. The development of a novel hybrid SPR interface by the deposition of an amorphous silicon-carbon alloy is also presented. Such an interface allows the subsequent linking of stable organic monolayers through Si-C bonds for a plasmonic detection. On the other hand, the semiconducting properties of silicon can be used to implement field-effect label-free detection. However, the electrostatic interaction between adsorbed species may lead to a spreading of the adsorption isotherms, which should not be overlooked in practical operating conditions of the sensor. Atomically flat silicon surfaces may allow for measuring recognition interactions with local-probe microscopy.

Semiquantitative study of the EDC/NHS activation of acid terminal groups at modified porous silicon surfaces.

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.

Highly sensitive and reusable fluorescence microarrays based on hydrogenated amorphous silicon-carbon alloys.

We have designed a new architecture of fluorescent microarrays based on a thin layer of hydrogenated amorphous silicon-carbon alloy (a-Si(0.85)C(0.15):H) deposited on an aluminium-on-glass back reflector. These substrates are modified with an organic monolayer anchored through Si-C bonds and terminated with carboxyl groups, allowing for the covalent immobilization of biological probes. The fluorescence yield is maximized by optimization of the a-Si(0.85)C(0.15):H layer thickness. This approach is assessed for DNA recognition, demonstrating an increase in sensitivity by over one order of magnitude as compared to commercial slides, and the possibility of following in situ the molecular recognition event (hybridization). The immobilization chemistry provides these substrates with a superior chemical stability toward ageing or long-term exposure to physiological buffers, which allows for many successive hybridization/dehybridization cycles without measurable changes in performance.