Yves Chevalier

Surface hydroxylation and silane grafting on fumed and thermal silica.

The optimization of the surface functionalization of flat thermal silicon oxide by silanes was investigated. The difficulties are the low density of silanols at the surface of thermal silica, the lack of precise knowledge of the actual surface chemistry of thermal silica and of its hydroxylation, and the limited number of possible chemical analyses at flat surfaces of small area. This steered our study toward a comparative investigation of the hydroxylation and silane grafting of thermal silica and the well-known fumed silica. The silane grafting density for fumed silica that had undergone thermal treatments of dehydroxylation was related to the surface density of silanols. The surface density of silane on the flat thermal silica as measured by FTIR-ATR spectroscopy was 1.4 micromol/m2, similar to that of fumed silica dehydroxylated at 1000 degrees C. This moderate value was related to the low silanol density present on such silica surfaces. Several rehydroxylation treatments that proved their efficiency on dehydroxylated fumed silica did not lead to any noticeable improvement on thermal silicon dioxide.

New surfactants: new chemical functions and molecular architectures

New properties at interfaces can be brought about by new surfactant molecules. Suitable chemical functions (polymerizable groups, electroactive functions) can be attached to surfactant molecules in order to create a specific property. In a different way, new molecular architectures have appeared at the borderlines between classical surfactants and hydrotropes and towards copolymers. Gemini (dimeric) surfactants include various headgroups and incorporate hydrophilic spacers. The route to oligomeric and comb-like polymers has been laid out. The control of intermolecular hydrogen bonding allows supramolecular association by headgroups. Hybrid surfactants and unsymmetrical bolaform amphiphiles introduce additional interactions to be played with.

Surface Sensitization Techniques and Recognition Receptors Immobilization on Biosensors and Microarrays

The quality of a biosensing system relies on the interfacial properties where bioactive species are immobilized. The design of the surface includes both the immobilization of the bioreceptor itself and the overall chemical preparation of the transducer surface. Hence, the sensitivity and specificity of such devices are directly related to the accessibility and activity of the immobilized molecules. The inertness of the surface that limits the nonspecific adsorption sets the background noise of the sensor. The specifications of the biosensor (signal-to-noise ratio) depend largely on the surface chemistry and preparation process of the biointerface. Lastly, a robust interface improves the stability and the reliability of biosensors. This chapter reports in detail the main surface coupling strategies spanning from random immobilization of native biospecies to uniform and oriented immobilization of site-specific modified biomolecules. The immobilization of receptors on various shapes of solid support is then introduced. Detection systems sensitive to surface phenomena require immobilization as very thin layers (two-dimensional biofunctionalization), whereas other detection systems accept thicker layers (three-dimensional biofunctionalization) such as porous materials of high specific area that lead to large increase of signal detection. This didactical overview introduces each step of the biofunctionalization with respect to the diversity of biological molecules, their accessibility and resistance to nonspecific adsorption at interfaces.

Grafting of phosphonate groups on the silica surface for the elaboration of ion-sensitive field-effect transistors

Ion-sensitive field-effect transistors (ISFETs) sensitive to Ca(2+) ions could be elaborated by means of a new grafting process of the phosphonate group at the surface of the silica gate of FETs. A grafting process involving only one chemical reaction step at the surface afforded a significant improvement of the ISFET properties. The sensitivity of the ISFET towards Ca(2+) ions at pH 10 was quasi-linear in the concentration range from 10(-1) to 10(-3) M, and the slope was 10 mV pCa(-1). The site-binding model works well in predicting the experimental data, giving the complexation constant of 10(2.7) and a low value of the grafting density. The origin of the poor response of ISFETs sensitized by means of a multistep grafting process was investigated on silica powders of high specific area: the cleavage of the organic grafts at the SiOSi bonds occurring at each step could be disclosed by means of elemental analyses, infrared, and cross-polarization and magic angle spinning nuclear magnetic resonance of the grafts.

Adsorption of poly(isobutenylsuccinimide) dispersants at a solid-hydrocarbon interface

The adsorption at the solid—xylene interface of poly(isobutenyl-succinimides) (PIBSI) has been studied on carbon black by means of adsorption isotherms and small-angle neutron scattering. Simple diblock PIBSI having various chemical structures and poly(PIBSI) with a comblike structure were compared. The adsorption is due to the hydrophilic polyamine part. It was related to the chemical structure of the dispersants (length of the polyamine part, simple diblock structure versus comblike). The adsorption phenomenon was irreversible at low concentrations; the adsorbed macromolecules are fully stretched and form a monolayer of 30-A thickness. The consequences for the colloidal stability of carbon black dispersions in xylene were analyzed by means of quasielastic light scattering and rheology measurements.

The structure of porous silica–polysiloxane hybrid materials

Porous organic–inorganic hybrid materials are prepared by means of a sol–gel process where tetraethoxysilane (TEOS) is hydrolyzed in a water–tetrahydrofuran (THF) mixture containing the reactive polar polysiloxane PS124. A porous material consisting of silica and PS124 domains is obtained after evaporation of the solvents. Such materials can be deposited as thin layers at the surface of electrodes and various transducers for the design of chemical sensors. A very important property of the materials is the accessibility to the organic domains where the active species are immobilized. Thus, the key parameters of the structure have been investigated by means of different techniques, solid state NMR, electron microscopy, nitrogen adsorption and small angle scattering. Solid state 29Si NMR shows the high condensation of the silica and the reaction of the PS124 during the sol–gel process. The porous volume mainly consists of macropores in interstices between large aggregated particles (0.3–1 μm). The size distribution of the pores displays only few micropores and no mesopores. The values of the specific areas of the three interfaces separating silica and the external medium, the PS124 polymer and the external medium and silica and PS124 have been determined by means of an original method combining nitrogen adsorption, X-ray and neutron small angle scattering measurements. The silica–exterior interfacial area is vanishingly small and the PS124–exterior interfacial area is much larger than that of the silica–PS124 interface. This means complete wetting of the silica by PS124 and a rough outer surface of the PS124 domains.

Small angle neutron scattering studies and kinetic analysis of laponite-enzyme hydrogels in view of biosensors construction

Clay hydrogels were designed as an immobilization matrix for electroactive enzymes at the surface of electrodes. Such materials are prepared by drying an aqueous colloidal suspension of laponite clay and enzyme on the surface of the electrodes. The properties of such materials have been investigated with regards to their structure and permeability, which determine the entrapment efficiency and the accessibility of the analyses to the enzymatic sites. The structure of such materials containing similar amounts of globular protein (bovine serum albumin (BSA)) and laponite have been investigated both in the dry and swollen states by means of small angle neutron scattering (SANS). The macroscopic orientation parallel to the faces of the dry films and the short-range ordering in the direction perpendicular to the faces was observed. This structure corresponds to that of the clay template swollen by the proteins. These structural features resist against moderate swelling by a concentrated electrolytic solution. The macroscopic orientation and the short-range order progressively disappear upon swelling with aqueous solutions of decreasing ionic strength. Since the structure turns from a quasi-impermeable stack of clay sheets to an open isotropic structure with enhanced accessibility below 100 mM of phosphate buffer, the sensitivity of an enzymatic clay-modified electrode (CME) working with the polyphenol oxidase (PPO) increases when the ionic strength of the swelling medium decreases. However, the gain of sensitivity remains moderate although the structural changes are huge. The rate determining mechanism seems to be a diffusion of the reacting species through defects (microchannels) inside the film as was previously proposed based on electrochemical and kinetics studies on the electrodes.

The synthesis of colloidal particles of calcium thiophosphates in a microemulsion: influence of the parameters of the process

Abstract The synthesis of colloidal particles of calcium thiophosphate from calcium oxide and phosphorus pentasulfide catalyzed by water in a microemulsion medium involves a reaction process which is controlled by out-of-equilibrium steps. The reaction product, the yield and the colloidal stability then strongly depend on the process. In particular, the rate of addition of the water in the reaction medium is of importance. Its influence was investigated by means of a kinetic study where both the products of the chemical reaction and the structure of the suspension were studied as a function of the conversion. A progressive addition of water leads to stable suspensions of calcium thiophosphate particles of small size with high yields. On the contrary, a one-shot addition of water leads to a suspension with a bimodal size distribution and with lower yields. The transient formation of calcium hydroxide colloidal particles is proposed as the origin of the formation of larger particles found in the one-shot process.

Chemically grafted field-effect transistors for the recognition of ionic species in aqueous solutions

The direct chemical grafting of complexing agents onto the silica grid of field-effect transistors (FETs) was realized, allowing the selective complexation of ions at the surface of the FETs. The chemical recognition of ions at the surface provides sensitivity to the presence of such ions. Electrochemical devices known as ion-sensitive field-effect transistors (ISFETs) for the quantitative analysis of ions were elaborated and their electrochemical behaviour was studied. Thus, the grafting of the crown ether benzo-15-crown-5 onto the silica grid brought about sensitivity to potassium ions that the bare silica had not. In the same way, the grafting of phosphonate groups led to ISFET devices sensitive to calcium ions. The electrical response was related to the surface potential at the silica-water interface and could be rationalized by means of a modified site-binding model.

The emulsification of inorganic colloidal particles in an organic medium

Colloidal suspensions of inorganic particles in xylene could be prepared by means of reactions between two solid powders. In all cases, the reagents were transported to the reaction sites by means of a microemulsion of the reverse type containing a surfactant, water and tetrahydrofuran as a cosurfactant. This process, where colloidal particles are formed and emulsified from a chemical reaction site at the surface of a solid, is illustrated with examples of the synthesis of particles containing calcium thiophosphate, calcium hydroxide or sodium phosphate. The role of the cosurfactant was to speed up the transport rate of the reagents, allowing a high yield of colloidal particles and the control of side reactions in the case of calcium thiophosphate synthesis. The mechanism by which the inorganic particles were emulsified is still an open question and this is discussed. The particle sizes did not depend on the amount of surfactant as in the case of classical emulsions. The type of inorganic product formed inside the reverse micelles and the spontaneous curvature of the surfactant film appear as the most relevant parameters which control the particle size.