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The association of a mycotoxin-ochratoxin A (OTA)-with a high-affinity DNA aptamer (anti-OTA) immobilized on a functionalized surface has been investigated at the molecular level. Anti-OTA aptamers are coupled by aminolysis in several steps on an acid-terminated alkyl monolayer grafted on a silicon substrate, and Fourier transform infrared spectroscopy in attenuated total reflection geometry is used to assess the immobilization of anti-OTA (in its unfolded single-strand form) and determine its areal density (ca. 1.4/nm 2 ). IR spectra further demonstrate that the OTA/anti-OTA association is efficient and selective and that several association/dissociation cycles may be conducted on the same surface. The areal density of OTA measured after association on the surface (IR spectroscopy) and after dissociation from the surface (UV-vis spectroscopy) falls in the range 0.16-0.3/nm 2 which is close to the areal density of a closed-packed monolayer of anti-OTA aptamers folded to form their G-quadruplex structure. The interactions between OTA and its aptamer at the surface are discussed with the help of density functional theory calculations-to identify the complex IR vibrational modes of OTA in solution-and UV-vis spectroscopy-to determine the protonation state of the adsorbing species (i.e., OTA dissolved in the buffer solution).

The structure of mixed acid/decyl monolayers (MLs) grafted on oxide-free Si(111) surfaces by photochemical hydrosilylation in a mixture of neat undecylenic acid and 1-decene is studied in detail. After appropriate surface cleaning of the as-grafted surfaces, atomic force microscopy (AFM) (topography and phase imaging) and calibrated FTIR analysis demonstrate that a mixed monolayer is formed, free of residue. When the acid-molecule fraction (Γ SOL ) is >0.1, mixed MLs are homogeneous on the scale of observations and they are only slightly enriched in acid chains with respect to the solution. Conversely, when Γ SOL < 0.1, the acid chain fraction within the ML becomes quasi-independent of the solution composition and may become much larger than Γ SOL . In addition, dark domains are observed in AFM phase images. Correlations between the characteristic parameters of νCO IR bands and AFM phase images suggest a strong phase separation of acid and alkyl chains at various length scales. A model involving a structuration of the grafting solution is proposed to explain observations.

Various poly(ethylene glycol) monomethyl ether moieties were grafted onto hydrogenated silicon surfaces in order to investigate the influence of the molecular design on the antifouling performance of such coatings. The grafted chains were either oligo(ethylene oxide) chains (EG)(n)OMe bound to silicon via Si-O-C covalent bonds, or hybrid alkyl/oligo(ethylene oxide) chains C(p)(EG)(n)OMe bound via Si-C covalent bonds (from home-synthesized precursors). Quantitative IR spectroscopy gave the molecular coverage of the grafted layers, and AFM imaging demonstrated that a proper surfactinated rinse yields C(p)(EG)(n)OMe layers free of unwanted residues. The protein-repellent character of these grafted layers (here, toward BSA) was studied by IR and AFM imaging. C(p)(EG)(n)OMe layers exhibit a lower surface concentration than (EG)(n)OMe layers, because of the presence of a solvent in the grafting solution; they however demonstrate high resistance against BSA adsorption for high values of the n/p ratio and a higher stability than (EG)(n)OMe. This behavior is consistently explained by the poor ordering capability of the alkyl part of the layer, contrary to what is observed for similar layers on Au, and the key role of an entangled arrangement of the ethylene oxide chains which forms when these chains are long enough.

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.

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.

This work demonstrates that well-defined mixed carboxyl-terminated/methyl-terminated alkyl monolayers can be prepared in one step on H-terminated Si(111) via direct photochemical hydrosilylation of undecylenic acid and 1-decene mixtures. As evidenced by AFM imaging and IR spectroscopy, a final rinse in hot acetic acid leaves the functionalized surface atomically smooth and perfectly free of physisorbed contaminants while unwanted material remains atop the monolayer with most other common solvents. The compositional surface chemistry was determined from a truly quantitative IR (ATR geometry) study in the range of 900-4000 cm(-)(1). Results prove that neither surface oxidation nor grafting through the carboxyl end groups occurs. Monolayers are fairly dense for such bulky end groups, with a total molecular surface density of approximately 2.7 10(14) cm(-)(2) corresponding to a surface coverage of 0.35 (maximum theoretical value approximately 0.5). Careful analysis of the CH- and COOH-related IR bands reveals that the composition of the grafted layers is richer in acid chains than the starting grafting mixture. A simple model is presented that shows that the grafting kinetics is about twice as fast for undecylenic acid as for 1-decene. Complementary electrochemical impedance measurements indicate the excellent electronic properties of the interface with a very low density of gap states. They also show that the acid terminal groups promote the penetration of water in the outer part of the organic film.

Covalent grafting of alkyl chains on silicon can be obtained by thermal treatment in Grignard reagents. Alkyl halide present in the Grignard solution as an impurity appears to play a key role in the grafting process. Grafting efficiency is improved when the alkyl halide concentration is increased. It is also enhanced on n-type substrates as compared to p-type substrates and when alkyl bromides are present in solution rather than alkyl chlorides. The grafting reaction involves a zero-current electrochemical step. A reaction model in which simultaneous Grignard oxidation and alkyl halide reduction take place at the silicon surface accounts for all these observations. Alkyl halide reduction is the rate-determining step. Negative charging of the silicon surface lowers the energetic barrier for this reaction, allowing for efficient grafting on n-Si., (Copyright 2004 American Chemical Society)