Luc Vellutini

PM-IRRAS Investigation of Self-Assembled Monolayers Grafted onto SiO2/Au Substrates

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to characterize self-assembled monolayers (SAMs). Novel ester-terminated organosilicon coupling agents possessing a trialkoxysilyl headgroup and a urea group in the linear alkyl chains (4) were synthesized and grafted onto SiO(2)/Au substrates (SiO(2) film of 200 Å thickness deposited on gold mirror). This composite substrate allowed the anchoring of SAMs and preserved the high reflectivity for infrared radiation. PM-IRRAS spectra with very high signal-to-noise ratios have been obtained in the mid-infrared spectral range allowing monitoring of the grafted SAMs. Quantitative analysis of the measured signal is described to compare PM-IRRAS and conventional IRRAS spectra. This quantitative analysis has been validated since the band intensities in the corrected PM-IRRAS and conventional IRRAS spectra are identical. Orientation information on the different functional groups has been obtained comparing the corrected PM-IRRAS spectrum with the one calculated using isotropic optical constants of ester-terminated organosilicon coupling agents 4. The carbonyls of the urea groups are preferentially parallel to the substrate surface favoring intermolecular hydrogen bonding and consequently a close packing of the molecules attached to the surface. By contrast, the alkyl chains present gauche defects and are poorly oriented.

Functionalized hydrogen-bonding self-assembled monolayers grafted onto SiO2 substrates.

A novel urea coupling agent possessing a vinyl-terminal group and trimethoxysilyl anchoring group was synthesized and grafted onto SiO(2)/Au substrates. This ureido coupling agent exhibits a good capacity to directly yield homogeneous SAMs with a surface smoothing. Polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) was used to monitor these SAMs. Indeed, the different functional groups (alkyl chain, urea, and vinyl) of this coupling agent were clearly observed in the PM-IRRAS spectra. Chemical modifications of the terminal function for the covalent immobilization of biomolecules were monitored by PM-IRRAS for the first time. We have demonstrated the successful reactions of the conversion of the vinyl-terminated SAMs successively into SAM-COOH and SAM-NHS without any degradation of the monolayer. The reactivity of activated esters was successfully investigated in order to immobilize the protein A.

Route to Smooth Silica-Based Surfaces Decorated with Novel Self-Assembled Monolayers (SAMs) Containing Glycidyl-Terminated Very Long Hydrocarbon Chains†

Novel glycidyl-terminated organosilicon coupling agents possessing a trialkoxysilyl head group and a very long hydrocarbon chain (C22) were synthesized. Their ability to afford densely packed self-assembled monolayers (SAMs) grafted on silica-based surfaces was investigated. Transmission FT-IR spectra showed that the most regular films were obtained by using trichloracetic acid as the catalyst (10 M%). Atomic force microscopy (AFM) and optical ellipsometry were consistent with well ordered monolayers exhibiting a marked decrease of the surface roughness. Epifluorescence microscopy revealed that these SAMs possessed a better surface reactivity than monolayers obtained with the commercially available (3-glycidoxypropyl) trimethoxysilane (GPTS) upon grafting of a fluorescent probe (dansylcadaverin). Moreover, direct attachment of fluorescent antibodies (RAG-TRITC) through covalent binding led to higher mean fluorescence intensities, showing that these new SAMs possess high potential for the immobilization of biological molecules.

New Approach for the Organisation and the Shaping of Organo-Bridged Silicas: An Overview

A new and general route to synthesize shape-controlled bridged silsesquioxanes has been developed by the hydrolysis-condensation of molecularly designed precursors bearing urea groups. The auto-association of the bridging organic units, owing to the hydrogen bonds developing between the urea groups, has been exploited to create new multifunctional organo-bridged silicas with peculiar shapes from the nano- to the micro-scale. Helical fibers with controlled handedness, hollow tubes and spheres, and lamellar plates have been produced according to the main organic substructure and also depending on the reaction conditions (catalyst, solvent and temperature). Spectroscopic techniques (solid state 13C and 29Si NMR, FTIR and X-ray diffraction) and electronic microscopic measurements (SEM and TEM) allowed the characterisation of these hybrid materials.

Chirality Transcription of Molecular Precursors to Hybrid Silicas

Silylated racemic, chiral (R,R)- and (S,S)-diureidocyclohexane derivatives have been synthesized in quantitative yields by the reaction of respectively trans racemic, (R,R)- or (S,S)-1,2-diaminocyclohexanes with 3-isocyanatopropyltriethoxysilane. The enantiomeric compounds have the ability to auto-associate through inter-molecular hydrogen bonding owing to the presence of the urea groups. Consequently they formed organic gels in non-polar solvents such as cyclohexane and mesitylene at very low concentration (ca < 10 mg per ml). Such gelating properties were not observed in the case of the racemic mixture. Under acidic aqueous media the controlled hydrolysis-condensation of the (R,R)- and (S,S)-enantiomers led to the hybrid silicates with fibrous structures. Interestingly right- or left-handed helices were respectively observed by Scanning Electronic Microscopy (SEM) images. Conversely, a featureless structure was obtained from the racemic mixture under the same conditions. A significantly reduced wavelength distance (Δ ν = 60 cm−1) between the C=O stretching mode and the NH bending mode was observed by FTIR spectroscopy, respectively at 1636 and 1576 cm−1. These results confirm the presence of the H-bonded urea groups in the solid materials. The solid state 13C NMR spectra exhibit in all cases a signal at 160 ppm attributed to the C=O, and a series of chemical shifts at 55, 43, 34, 25 and 10 ppm corresponding to the different sp3 carbon atoms. The preservation of the C–Si bonds was confirmed by the solid state 29Si CP-MAS spectra in which only T2 (−58 ppm) and T3 (−67 ppm) structures, respectively assigned to C—Si(OH)(OSi)2 and C—Si(Si)3, were observed. The formation of the right- and left-handed helical morphologies demonstrates the transcription of chirality from the enantiomers to the hybrid solids.

Cumulative effects of electrode and dielectric surface modifications on pentacene-based transistors

Surface modifications of the dielectric and the metal of pentacene-based field effect transistors using self-assembled monolayer (SAM) were studied. First, a low interfacial trap density and pentacene 2D-growth were favored by the nonpolar and low surface energy of octadecyltrichlorosilane-based SAM. This treatment leaded to increased mobility up to 0.4 cm2 V−1 s−1 and no observable hysteresis on transfer curves. Second, reduced hole injection barrier and contact resistance were achieved by fluorinated thiols deposited on gold contacts resulting in an increased mobility up to 0.6 cm2 V−1 s−1. Finally, a high mobility of 2.6 cm2 V−1 s−1 was achieved by cumulative effects of both treatments.

Self-assembled monolayer for AFM measurements of Tobacco Mosaic Virus (TMV) at the atomic level

Biosensors are based on the conversion of a biological response to an electrical signal. One of the major challenges is to ascertain that the receptor is not denatured when immobilised (covalently or not) on the device. In this work, a protein receptor (virus) was immobilised on two different surfaces, mica and self-assembled monolayer (SAM), and its height was determined by atomic force microscopy measurements at the atomic level. Results clearly showed that expected dimensions of Tobacco Mosaic Virus (TMV) are obtained after immobilisation onto a soft organic SAM.

Resolving the Chemical Nature of Nanodesigned Silica Surface Obtained via a Bottom-up Approach

The covalent grafting on silica surfaces of a functional dendritic organosilane coupling agent inserted, in a long alkyl chain monolayer, is described. In this paper, we show that depending on experimental parameters, particularly the solvent, it is possible to obtain a nanodesigned surface via a bottom-up approach. Thus, we succeed in the formation of both homogeneous dense monolayer and a heterogeneous dense monolayer, the latter being characterized by a nanosized volcano-type pattern (4-6 nm of height, 100 nm of width, and around 3 volcanos/μm(2)) randomly distributed over the surface. The dendritic attribute of the grafted silylated coupling agent affords enough anchoring sites to immobilize covalently functional gold nanoparticles (GNPs), coated with amino PEG polymer to resolve the chemical nature of the surfaces and especially the volcano type nanopattern structures of the heterogeneous monolayer. Thus, the versatile surface chemistry developed herein is particularly challenging as the nanodesign is straightforward achieved in a bottom-up approach without any specific lithography device.

Epoxy-Terminated Self-Assembled Monolayers Containing Internal Urea or Amide Groups

We report the synthesis of new coupling agents with internal amide or urea groups possessing an epoxy-terminal group and trimethoxysilyl-anchoring group. The structural characterizations of the corresponding self-assembled monolayers (SAMs) were performed by polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS). The molecular assembly is mainly based on the intermolecular hydrogen-bonding between adjacent amide or urea groups in the monolayers. Because of the steric hindrance of amide or urea groups, the distance between the alkyl chains is too large to establish van der Waals interactions, inducing their disorder. The reactivity of the epoxy-terminal groups was successfully investigated through reaction with a fluorescent probe. We show that SAMs containing internal urea or amide groups exhibited a higher density of accessible epoxide groups than the corresponding long-chain (C22) glycidyl-terminated SAM.

Dendritic maleimide functionalization of core–shell (γ-Fe2O3/polymer) nanoparticles for efficient bio-immobilization

A new route for the preparation of stable and water-dispersible core–shell γ-Fe2O3/polymer MNPs has been developed in order to ensure a selective and covalent immobilization of biomolecules using maleimide–thiol coupling chemistry. A high maleimide functionalization was achieved by the grafting of dendritic coupling agent via a convergent approach.