Mathieu Etienne

Analytical investigation of the chemical reactivity and stability of aminopropyl-grafted silica in aqueous medium

Various samples of aminopropyl-functionalized silica (APS) have been prepared by grafting an organosilane precursor 3-aminopropyl-triethoxysilane (APTES) onto the surface of silica gel. The amine group content of the materials has been adjusted by varying the amount of APTES in the reaction medium (toluene). The grafted APS solids have been characterized with using several analytical techniques (N(2) adsorption, X-ray photoelectron spectroscopy, infrared spectrometry) to determine their physico-chemical properties. Their reactivity in aqueous solutions was studied by acid-base titration, via protonation of the amine groups, and by way of complexation of these groups by Hg(II) species. APS stability in aqueous medium was investigated at various pH and as a function of time, by the quantitative analysis of soluble Si- or amine-containing species that have been leached in solution upon degradation of APS. The chemical stability was found to increase when decreasing pH below the pK(a) value corresponding to the RNH(3)(+)/RNH(2) couple, but very low pH values were necessary to get long-term stability because of the high local concentration of the amine groups in the APS materials. Adsorption of mercury(II) ions on APS was also performed to confirm the long-term stability of the grafted solid in acidic medium. Relationship between solution pH and APS stability was discussed. For sake of comparison, the stability of APS in ethanol and that of mercaptopropyl-grafted silica (MPS) in water have been briefly considered and discussed with respect to practical applications of silica-based organic-inorganic hybrids, e.g., in separation science or in the field of electrochemical sensors.

Organically-modified mesoporous silica spheres with MCM-41 architecture

New hybrid inorganic–organic materials displaying both an ordered mesoporous structure and spherical morphology have been synthesised by co-condensation of tetraethoxysilane and organoalkoxysilanes in water–ethanol solution containing ammonia in the presence of surfactant as templating agent.

Electrogeneration of highly methylated mesoporous silica thin films with vertically-aligned mesochannels and electrochemical monitoring of mass transport issues

In this work, we describe a versatile approach to generate highly methylated mesoporous silica films exhibiting hexagonally-packed one-dimensional mesochannels oriented uniquely perpendicular to the underlying support. It involves the electro-assisted self-assembly (EASA) of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) with cetyltrimethylammonium bromide (CTAB) under potential control. Transmission electron microscopy and grazing-incidence X-ray diffraction indicate that high level of ordering and vertical alignment of mesopores can be maintained at rather high MTES contents (up to 50–60 mol% with respect to total precursors in the starting sol). The rate of film growth decreased concomitantly to increasing the MTES/TEOS ratio, resulting in thinner films for higher functionalization degrees. This also led to more hydrophobic films (after template extraction), which tended to become less permeable to aqueous phase solutes, as pointed out qualitatively and quantitatively by several electrochemical methods and various redox probes. Effective rate constants for electron transfer were found to be several orders of magnitude higher than for other thin silica films on electrodes, explained by preferential porosity orientation.

Tuning the Sensitivity of Electrodes Modified with an Organic-Inorganic Hybrid by Tailoring the Structure of the Nanocomposite Material

Voltammetric analysis subsequent to open-circuit accumulation was performed with carbon paste electrodes modified with polysiloxane-immobilized mercaptopropyl groups. By using mercury(II) as a model analyte, it was found that the electrode sensitivity was greatly influenced by the structure of the modifier by means of promoted or restricted mass transfer. Synthetic routes leading to open framework structures containing a large amount of accessible binding sites (i.e., surfactant templated organic-inorganic mesoporous materials) gave rise to the highest mass transfer kinetics. These results open the door for further development of sensitive and selective sensors.

Electrochemical approaches for the fabrication and/or characterization of pure and hybrid templated mesoporous oxide thin films: a review

AbstractElectrochemistry can be used for fabrication and characterization of mesoporous oxide films. First, this review provides insight into the methods used to prepare templated mesoporous thin films on an electrode surface, i.e., evaporation-induced self-assembly (EISA) and electrochemically assisted self-assembly (EASA). Electrochemical characterization of mass transport processes in pure and organically functionalized mesoporous oxide films is then discussed. The electrochemical response can be basically restricted by the electron/mass transfer reaction at the electrode–film interface and diffusion through mesopore channels. The contributions of cyclic voltammetry, hydrodynamic voltammetry, electrochemical impedance spectroscopy, and scanning electrochemical microscopy to the characterization of films with distinct mesostructures are finally described, with special emphasis on identification of conditions that can affect the electrochemical response recorded with such modified electrodes. FigurePermeability through mesoporous thin films

Durable cofactor immobilization in sol-gel bio-composite thin films for reagentless biosensors and bioreactors using dehydrogenases.

A new strategy directed to the durable immobilization of NAD(+)/NADH cofactors has been tested, along with a suitable redox mediator (ferrocene), in biocompatible sol-gel matrices encapsulating a bi-enzymatic system (a dehydrogenase and a diaphorase, this latter being useful to the safe regeneration of the cofactor), which were deposited as thin films onto glassy carbon electrode surfaces. It involves the chemical attachment of NAD(+) to the silica matrix using glycidoxypropylsilane in the course of the sol-gel process (in smooth chemical conditions). This approach based on chemical bonding of the cofactor (which was checked by infrared spectroscopy) led to good performances in terms of long-term stability of the electrochemical response. The possibility to integrate all components (proteins, cofactor, mediator) in the sol-gel layer in an active and durable form gave rise to reagentless devices with extended operational stability (i.e. high amperometric response maintained for more than 12h of continuous use under constant potential, whereas the signal completely vanished within the first few minutes of working with non-covalently bonded NAD(+)). To confirm the wide applicability of the proposed approach, the same strategy has been applied to the elaboration of biosensors for D-sorbitol, D-glucose and L-lactate with using D-sorbitol dehydrogenase, D-glucose dehydrogenase and L-lactate dehydrogenase respectively. The analytical characteristics of the glucose sensors are given and compared to previous approaches described in the literature for the elaboration of reagentless biosensors.

Factors affecting copper(II) binding to multiarmed cyclam-grafted mesoporous silica in aqueous solution.

Single- as well as multi-anchored cyclam-functionalized silica samples have been prepared by grafting amorphous silica gel (K60) and mesostructured silica (SBA-15) with silylated cyclam precursors bearing one, two, or four triethoxysilyl groups, respectively ascribed to cyclam-mono, cyclam-di, and cyclam-tetra. Their reactivity toward copper(II) has been thoroughly investigated in aqueous solution and discussed with respect to the number of arms tethering the ligand to the silica surface and the structural ordering of the adsorbent in terms of capacity, long-term stability, and speed of access to the binding sites. Less-than-complete metal ion uptake was always observed, even in excess of cyclam groups with respect to solution-phase Cu(II), suggesting lower stability of immobilized complexes relative to those in solution. Therefore, the number of arms attaching cyclam moieties to the silica walls (one, two, or four) was found to dramatically affect the binding properties of these hybrids toward copper(II), revealing significantly larger capacities when reducing the number of arms (less rigidity constraints in the macrocycle). In parallel, multiarm tethering resulted in better chemical resistance toward degradation as evidenced by UV-visible monitoring of Cu-cyclam complexes in solution (i.e., more ligand leaching from the adsorbent for singly tethered cyclam). On the other hand, electron spin resonance (ESR) experiments did not evidence significant differences between complexes bearing one, two, or four alkyl arms, since all Cu(II)-cyclam surface complexes were found to be hexacoordinated with a strong equatorial ligand field. Comparison of amorphous gels and mesostructured materials indicates that the binding properties of the adsorbents were hardly influenced by their level of ordering, suggesting that accessibility to the binding sites was not the limiting factor. Some advantage belonging to mesostructured adsorbents was however observed with respect to the rate of access to the active centers at pH values close to neutrality (due to faster mass transport), but this was no more the case when operating at lower pH values where the formation of the Cu-cyclam complex became the rate-determining step, as pointed out by electrochemistry.

Multiarm cyclam-grafted mesoporous silica: a strategy to improve the chemical stability of silica materials functionalized with amine ligands

We have explored in this work the stability and the reactivity of multiarm cyclam-grafted mesoporous silica samples in aqueous solution. A series of hybrid materials have been prepared by grafting silylated cyclam molecules bearing one, two, or four silyl groups onto both amorphous silica gel (K60) and ordered mesoporous silica (SBA15). Under these conditions, cyclam moieties are attached to the silica walls via one, two, or four arms. Various physicochemical techniques have been applied to characterize the functionalized solids (elemental analysis, 1H-29Si and 1H-13C CPMAS NMR, and N2 adsorption-desorption isotherms). The interest in two and four arms for improving the chemical stability in solution, by comparison with the system displaying only one arm, has been demonstrated by using a set of complementary experiments involving pH measurements and silicon determination with ICP-AES. Then, the investigation of their protonation and binding properties toward copper(II) has revealed a significant decrease in the reactivity of these hybrids as a consequence of multiarm tethering. A comparison of amorphous and ordered materials has permitted us to point out the influence of mesostructuration on the reactivity of these functionalized solids, especially from a kinetic point of view.

Electrochemically assisted self-assembly of ordered and functionalized mesoporous silica films: impact of the electrode geometry and size on film formation and properties

Surfactant-templated mesoporous silica thin films can be deposited onto solid electrode surfaces by electrochemically assisted self-assembly (EASA). The method involves a cathodically triggered self-assembly of cationic surfactants (cetyltrimethyl ammonium bromide, CTAB) and local pH increase leading to the polycondensation of silica precursors (i.e., tetraethoxysilane, alone or in the presence of (3-mercaptopropyl) trimethoxysilane (MPTMS)) and concomitant growth of the ordered mesoporous silica or organosilica film. The present work shows that the EASA method can be applied to film deposition on electrode supports of various morphologies, geometries and sizes (large and flat discs or non-flat streaked supports, i.e., gold CD-trodes, as well as several kinds of ultramicroelectrodes, including carbon fibers, platinum wires, and platinum microdiscs). Galvanostatic conditions were mainly preferred to potentiostatic conditions to avoid problems related to various overpotentials and surface areas experienced with the various working electrodes used here. The results indicate that film deposition was possible on each electrode support but also that both the film formation and properties were dependent on the experimental conditions for EASA. For example, passing from large electrodes to ultramicroelectrodes required the application of larger current densities to ensure film deposition, which can be due to faster loss of the hydroxyl species in solution in the case of radial or spherical diffusion, in comparison to the linear. Highly porous deposits were obtained after template removal, as ascertained by cyclic voltammetry using Ru(NH3)6(3+) as a redox probe. The advantage of better signal-to-background current ratios for ultramicroelectrodes relative to the macroscopic ones was maintained after film deposition, also resulting in higher sensitivity when used in conditions of preconcentration electroanalysis (using silver(I) or mercury(II) as a probe being accumulated by complexation to MPTMS-based films).

Combined Raman microspectrometer and shearforce regulated SECM for corrosion and self-healing analysis.

Shearforce regulated scanning electrochemical microscopy (SECM) has been associated with Raman microspectrometry in order to perform combined electrochemical and spectrochemical analysis on reactive interfaces. The interest of the method was evaluated by analyzing local corrosion phenomena in damaged Zn(Mg, Al) self-healing coatings deposited on steel. Despite the high aspect ratio of the analyzed sample displaying here more than a 50 μm depth profile, the optimized setup allowed (1) precise electrode positioning with the help of shearforce detection, (2) electrochemical measurement at a constant distance from the sample surface, and (3) local chemical analysis of the solid surface by confocal Raman microspectroscopy performed at a constant focal distance from the sample. All in all, this new setup allows one to approach the detailed reactivity involved in defective metal samples.