Fluorescent nanocrystals grown in sol–gel thin films for generic stable and sensitive sensors

Through the sol–gel route, we have well-controlled the preparation of fluorescent organic nanocrystals grown in silicate thin films. This process is based on the confined nucleation and growth of dyes in the pores of wet gels. The resulting nanocomposite sol–gel thin films, coated onto low-cost substrates, exhibit coupled properties: transparency, stability, easy shaping of sol–gel thin films and high fluorescence intensity coming from organic nanocrystals. The sensitivity of the fluorescence intensity of nanocrystals to their environments can be exploited for the development of optical sensors. Indeed, Förster Resonance Energy Transfer can inhibit nanocrystal fluorescence when probe molecules are adsorbed or grafted on the nanocrystal surface after their diffusion through the pores of the sol–gel matrix. We investigated by time-resolved fluorescence spectroscopy the effect of nanocrystal size and probe concentration on the fluorescence quenching in presence of Methylene Blue used in this study as molecular probe. As strong fluorescence quenchings can be achieved, even for low probe concentrations, these hybrid organic–inorganic nanocoposites are promising for the development of sensor devices by increasing their fluorescence contrasts under specific chemical or biological environments.     

Effect of iron acetylacetonate on physico-chemical properties of waterglass based aerogels by ambient pressure drying

The effect of iron acetylacetonate on the physico-chemical properties of waterglass based silica aerogels by ambient pressure drying has been investigated. Doping the gels with iron acetylacetonat (FeAA) facilitates in the diminution of the density of the aerogels. The well established silica network provides effective confinement of FeAA nanoparticles which resists the collapse of silica network during ambient pressure drying. Therefore, in the present paper, the effects of FeAA on the physico-chemical properties of the aerogels have been studied by varying the FeAA:Na₂SiO₃ molar ratio from 3 × 10⁻⁴ to 6 × 10⁻⁴. The aerogels were prepared via ambient pressure drying and characterized by the bulk density, thermal conductivity and water contact angle. The aerogel’s surface morphology, elemental analysis and pore structure were characterized by means of EDAX and FTIR, TEM and N₂ adsorption- desorption analyzer. The high temperature hydrophobicity of these aerogels was checked by heating them in temperature controlled furnace. Silica aerogels with low density ~0.050 g/cc have been obtained using the molar ratio of Na₂SiO₃:H₂O:FeAA:Citric acid:TMCS at 1:146.67:3 × 10⁻⁴:0.54:9.46, respectively. EDAX and FTIR studies show that the iron species are entrapped in the mesoporous framework and not took part in the bonding with silica.     

Synthesis,characterization and catalytic activities of bimetallic modified MCM-41 for epoxidation of styrene

Mesoporous molecular sieves MCM-41 modified by bimetal (Zr and Ti) ions with ordered hexagonal arrangement were prepared by direct synthesis under microwave–hydrothermal conditions at 403 K. FT-IR, N₂ adsorption–desorption, scanning electron microscope, high-resolution transmission electron microscope, element mapping, X-ray photoelectron spectroscopy, diffuse Reflectance UV–Visible Spectra etc. were used to characterize as-prepared materials. The results showed that Zr and Ti were introduced into MCM-41 and bonds of Zr–O–Si and Ti–O–Si were formed in framework of the MCM-41. Moreover, their structures with ordered arrangement were still retained. The microwave–hydrothermal method might shorten the synthesis time from days to hours and the as-prepared catalyst had a high catalytic activity and selectivity for the liquid-phase epoxidation of styrene.     

Surface study of the building steps of enzymatic sol–gel biosensors at the micro- and nano-scales

We have studied, by scanning electron and atomic force (AFM) microscopies, how each step involved in the building process of massive carbon-based sol–gel enzymatic biosensors changes and determines the resulting surface morphology and nano-mechanical properties. The biosensor, selected as a model, is developed by the entrapment of glucose oxidase (GOx), a redox mediator and a material conferring conductivity (graphite powder, C) into a polymeric tridimensional network generated by sol–gel technology using tetraethoxysilane (TEOS) as precursor. The smooth TEOS morphology is formed by an irregular nanoporous network, which is very adequate for enzyme encapsulation. Upon addition of carbon powder to the system (TEOS/C), the surface morphology changes but it is still rather irregular since carbon powder micro-grains are found scattered on it. This morphology results in a rather rough surface at the micro- scale whereas at the nano- scale both atomically flat graphitic and nanoporous TEOS domains are found. In contrast, the final biosensing device surface is quite homogeneous and composed by flat platelets separated by deep crevices. On top of most of these platelets there is a soft, as assessed by AFM force indentation experiments, layer of globular structures whose dimensions are compatible with GOx molecules. The final device surface architecture results to be open and accessible both at the micro and nano scales, which turns it as adequate to enhance both the accessibility of the analytes to entrapped proteins and the mass-transfer rates. Finally, in order to show the applicability of the studied biosensor, its response was evaluated towards varying glucose concentrations, displaying a clear electrocatalytic activity.     

Structurally tailored carbon xerogels produced through a sol–gel process in a water–methanol–inorganic salt solution

The impact of solvent composition as well as inorganic salt content and type on carbon xerogel structure was investigated. Carbon xerogels were derived from the sol–gel polycondensation of resorcinol with furfural in a water–methanol–inorganic salt solution. As inorganic salts, NaCl, NH₄ClO₄ and FeCl₃ were used. In order to conduct an accurate examination of the carbon xerogel structures and textures, inorganic salts were removed prior to carbonization. The xerogel structures can be tailored according to the water/methanol ratio and, to a lesser extent, according to the inorganic salt content and type in the starting solution. As a result, a significant amount of salt can be introduced to the gel network of the desired structure. The morphology and physical properties of the organic xerogels, carbon xerogels and their composites were characterized by means of SEM, N₂ sorption and XRD. It was found that samples derived from mixtures with FeCl₃ manifest well developed mesoporosity and depleated microporosity in comparison to samples prepared from mixtures with NaCl and NH₄ClO₄. Iron ions chemically bond to the xerogel matrix and cause its partial graphitization during the carbonization process, resulting in enhanced mesoporosity.     

Effects of diethanolamine on the evolution of silver/titanium dioxide sol–gel process

In order to clarify the effects of diethanolamine (DEA) in the silver (Ag)/titanium dioxide (TiO₂) sol–gel process, sols with and without DEA, and films derived from these sols were prepared. The samples were investigated by X-ray diffraction, transmission electron microscopy, electron diffraction and optical absorption spectra. The results showed that metallic Ag clusters were formed in the sol with DEA and was absent in the sol without DEA. This indicated that DEA worked not only as the stabilizer but also as the reduce agent in Ag/TiO₂ sol–gel process. After annealed, Ag metallic nanoparticles were generated in the films derived from both the sols with and without DEA. The particles in the films derived from the sol with DEA were smaller than those from the sol without DEA. This can be ascribed to the limitation of the growth of Ag cluster formed in the sol with DEA during heat treatment. Mechanisms for the formation of metallic Ag in the Ag/TiO₂ sols and films were discussed. The effects of DEA in the sols and films were studied in detail.     

Characterisation of nickel–zinc ferrite/silica nanocomposites with low ferrite concentration obtained by an improved modified sol–gel method

The paper presents a study regarding the structure, morphology and magnetic behaviour of x% (Ni_0.65Zn_0.35Fe₂O₄)/(100 − x)% SiO₂ ferrimagnetic nanocomposites for low Ni–Zn ferrite concentration (x = 5, 10, 15, 20 and 30 mass percent) obtained by an improved modified sol–gel method. The obtained gels and nanocomposites have been characterized by fast Fourier transform-infrared (FT-IR) spectrometry, X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetic measurements (MM). The addition of a supplementary quantity of diol in the synthesis, corresponding to a molar ratio EG : TEOS = 1:1, and the control of the thermal treatment applied to the precursor xerogels tetraethylortosilicate (TEOS)–metal nitrates (MN)–ethylene glycol (EG) leads to fine (~2–9 nm), almost spherical Ni–Zn ferrite nanoparticles homogenously dispersed inside the amorphous SiO₂ matrix. TEM images reveal the fine nature and the narrow size distribution of the ferrite nanoparticles. Nanoparticles diameter increases with the ferrite concentration and with the annealing temperature. For all concentrations of ferrite in SiO₂ and all annealing temperature, we have obtained Ni_0.65Zn_0.35Fe₂O₄ ferrite as single phase (proven by XRD) in the amorphous silica matrix, only after a pre-treatment of synthesized gels, at 573 K, for 3 h. The magnetic behaviour of ferrite nanoparticles in quasi-static magnetic fields is very particular, depending on the annealing temperature and the ferrite content in silica matrix. We have obtained superparamagnetic behaviour for the nanocomposites, for a concentration of 30% ferrite in SiO₂ at high annealing temperature, of 1,273 K.     

Adsorption of CO<Subscript>2</Subscript>-containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for CO<Subscript>2</Subscript> separation

Adsorption of CO₂, N₂, CH₄ and H₂ on triamine-grafted pore-expanded MCM-41 mesoporous silica (TRI-PE-MCM-41) was investigated at room temperature in a wide range of pressure (up to 25 bar) using gravimetric measurements. The material was found to exhibit high affinity toward CO₂ in comparison to the other species over the whole range of pressure. Column-breakthrough dynamic measurements of CO₂-containing mixtures showed very high selectivity toward CO₂ over N₂, CH₄ and H₂ at CO₂ concentrations within the range of 5 to 50%. These conditions are suitable for effective removal of CO₂ at room temperature from syngas, flue gas and biogas using temperature swing (TS) or temperature-pressure swing (TPS) regeneration mode. Moreover, TRI-PE-MCM-41 was found to be highly stable over hundreds of adsorption-desorption cycles using TPS as regeneration mode.     

Single component and binary diffusion of n-heptane and toluene in SBA-15 materials

In this work, the diffusion properties of single component n-heptane and toluene as well as their binary mixtures in two SBA-15 samples with different structural characteristics were studied by the standard Zero Length Column (ZLC) technique under three different concentration levels. A theoretical ZLC desorption model considering the Generalized Maxwell-Stefan (GMS) formulation was developed. Using the independently measured single component equilibrium and kinetic parameters, the model was able to reasonably predict experimental binary ZLC desorption curve for countercurrent diffusion of toluene in the presence of n-heptane. However, there was a significant deviation between model prediction results and experimental data for countercurrent desorption of n-heptane in the presence of toluene. The diffusion of n-heptane is reduced by the presence of toluene, regardless of the relative content of micropores in the intrawall pores, while that of toluene is virtually unaffected by the counter-diffusion of n-heptane. The observed phenomena cannot be addressed by the simple model considering the cross term diffusion effect. The structural property of material and the molecular characteristics of probe molecules were used to account for the difference in the behavior between n-heptane and toluene.     

On the use of the ‘uncertainty budget’ to detect dominant terms in the evaluation of measurement uncertainty

In the evaluation of measurement uncertainty, the uncertainty budget is usually used to identify dominant terms that contribute to the uncertainty of the output estimate. Although a feature of the GUF method, it is also recommended as a qualitative tool in MCM by using ‘nonlinear’ equivalents of uncertainty contributions and sensitivity coefficients. In this paper, the use of ‘linear’ and ‘nonlinear’ parameters is discussed. It is shown that when and only when the standard uncertainty of the output estimate is nearly equal to the square root of the sum of the squares of the individual uncertainty contributions, will the latter be a reliable tool to detect the degree of contribution of each input quantity to the measurand uncertainty.