F. Ozanam

The titration of carboxyl-terminated monolayers revisited: in situ calibrated fourier transform infrared study of well-defined monolayers on silicon.

The acid-base equilibrium at the surface of well-defined mixed carboxyl-terminated/methyl-terminated monolayers grafted on silicon (111) has been investigated using in situ calibrated infrared spectroscopy (attenuated total reflectance (ATR)) in the range of 900-4000 cm (-1). Spectra of surfaces in contact with electrolytes of various pH provide a direct observation of the COOH <--> COO (-) conversion process. Quantitative analysis of the spectra shows that ionization of the carboxyl groups starts around pH 6 and extends over more than 6 pH units: approximately 85% ionization is measured at pH 11 (at higher pH, the layers become damaged). Observations are consistently accounted for by a single acid-base equilibrium and discussed in terms of change in ion solvation at the surface and electrostatic interactions between surface charges. The latter effect, which appears to be the main limitation, is qualitatively accounted for by a simple model taking into account the change in the Helmholtz potential associated with the surface charge. Furthermore, comparison of calculated curves with experimental titration curves of mixed monolayers suggests that acid and alkyl chains are segregated in the monolayer.

Development of a metal-chelated plasmonic interface for the linking of His-peptides with a droplet-based surface plasmon resonance read-off scheme.

Monolayers of metal complexes were covalently attached to the surface of lamellar SPR interfaces (Ti/Ag/a-Si(0.63)C(0.37)) for binding histidine-tagged peptides with a controlled molecular orientation. The method is based on the activation of surface acid groups with N-hydroxysuccinimide (NHS), followed by an amidation reaction with (S)-N-(5-amino-1-carboxypentyl)iminodiacetic acid (NTA). FTIR and X-ray photoelectron spectroscopy (XPS) were used to characterize each surface modification step. The NTA modified SPR interface effectively chelated Cu(2+) ions. Once loaded with metal ions, the modified SPR interface was able to bind specifically to histidine-tagged peptides. The binding process was followed by surface plasmon resonance (SPR) in a droplet based configuration. The Cu(2+)-NTA modified interface showed protein loading comparable to commercially available NTA chips based on dextran chemistry and can thus be regarded as an interesting alternative. The sensor interface can be reused several times due to the easy regeneration step using ethylenediaminetetraacetic acid (EDTA) treatment.

Thermal decomposition of alkyl monolayers covalently grafted on (111) silicon

Alkyl monolayers have been covalently grafted onto atomically flat (111) hydrogenated silicon surfaces through hydrosilylation of 1-alkenes. The thermal stability of the layers under oxidizing or reducing atmospheres has been investigated by quantitative in situ infrared spectroscopy, using a specially designed variable-temperature cell. The layers are thermally stable up to 250°C. In the range of 250–300°C, the main reaction is alkene desorption accompanied with silicon oxidation. The characteristic desorption temperature is not significantly affected by changing the atmosphere, the initial packing density of the layers, or the chain length from C18 to C6, but very short chains appear more stable.

Active Acetylcholinesterase Immobilization on a Functionalized Silicon Surface

In this work, we studied the attachment of active acetylcholinesterase (AChE) enzyme on a silicon substrate as a potential biomarker for the detection of organophosphorous (OP) pesticides. A multistep functionalization strategy was developed on a crystalline silicon surface: a carboxylic acid-terminated monolayer was grafted onto a hydrogen-terminated silicon surface by photochemical hydrosilylation, and then AChE was covalently attached through amide bonds using an activation EDC/NHS process. Each step of the modification was quantitatively characterized by ex-situ Fourier transform infrared spectroscopy in attenuated-total-reflection geometry (ATR-FTIR) and atomic force microscopy (AFM). The kinetics of enzyme immobilization was investigated using in situ real-time infrared spectroscopy. The enzymatic activity of immobilized acetylcholinesterase enzymes was determined with a colorimetric test. The surface concentration of active AChE was estimated to be Γ = 1.72 × 10(10) cm(-2).

Quantitative Infrared Investigation of the Acido-Basic Equilibrium at a Carboxy-Terminated Silicon Surface

Silicon surfaces grafted with mixed alkyl/carboxylic acid monolayers have been investigated by in-situ quantitative infrared spectroscopy at the contact with electrolytes of various pH. Ionization of the carboxy groups is found to start around pH 6 and to extend over a wide pH range. Ionization is not complete even at pH 11. These observations point to the importance of the interactions between surface charges. This effect may be accounted for by a simple model taking into account the change in the Helmholtz potential associated with the surface charge. However, the agreement is only qualitative, which indicates that the surface charge is screened more efficiently than expected, possibly due to ion penetration between the carboxy groups.