Silica covered porphyrin microstructures obtained in sol–gel processes

In this article, we report on the formation of well-defined highly emissive silica-covered porphyrin microstructures in base-catalyzed sol–gel processes. The microstructures were obtained by self-assembly of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (mTHPP) at room temperature. Tetraethoxysilane (TEOS) was used as a silica precursor. The hybrid mTHPP- silica particles were characterized by means of reflectance UV–Vis and microscopy techniques including atomic force microscopy, scanning electron microscopy (SEM) and confocal fluorescence microscopy (CM). The SEM and TEM observations revealed that depending on the porphyrin concentration used in the synthesis, the shape of the hybrid mTHPP-silica particles has changed from ribbon-like (c _mTHPP = 2.09 mM) to rhombus-like structures (c _mTHPP = 4.35 mM). The ribbons were straight-edged, uniform in width (1.2–1.8 μm) and height (350–400 nm), and variable in length (40–100 μm). The rhombs were 1–3.7 μm in height, 7–25 μm in length, and 3.5–15 μm in width, and the ratio of length to width was uniform and equal to ca. 1.8–2. UV–Vis absorption spectra indicated that the J-aggregates and H-aggregates formed in the systems with lower and higher porphyrin content, respectively. Formation of different type of porphyrin aggregates in both systems resulted in different emission spectra, as it was shown with CM.     

Improved biocompatibility of protein encapsulation in sol–gel materials

By using the fluorescent dye 6-propionyl-2-(N,N-dimethylamino) naphthalene (PRODAN) to monitor methanol generated during tetramethyl orthosilicate polymerization we have optimised the encapsulation of protein in silica sol–gel monoliths with respect to completion of hydrolysis and distillation in order to remove methanol such that protein can be added without denaturation. A minimum of 24 h at +4 °C was found to be required before hydrolysis is complete and 3–5 min of vacuum distillation at 50 °C and 300 mbar needed to remove methanol before the gel is formed. The biocompatibility of a tetramethyl orthosilicate sol–gel monolith was demonstrated by preserving the trimer protein allophycocyanin (APC) in its native form for up to 500 h. This obviates the previously essential requirement of covalently binding the trimer together in order to prevent dissociation into monomers and has enabled observation of native APC trimer in a sol–gel pore for the first time down to the single molecule level using combined fluorescence spectroscopy and confocal microscopy. The higher stability afforded by the protocol we describe could impact on the application of sol–gel materials to single-molecule studies of wider bearing such as protein folding and aggregation.     

Role of precursors and coating polymers in sol–gel chemistry toward enhanced selectivity and efficiency in solid phase microextraction

To evaluate the selectivity and efficiency of solid phase microextraction (SPME) fiber coatings, synthesized by sol–gel technology, roles of precursors and coating polymers were extensively investigated. An on-line combination of capillary microextraction (CME) technique and high performance liquid chromatography (HPLC) was set up to perform the investigation. Ten different fiber coatings were synthesized in which five of them contained only the precursor and the rests were prepared using both the precursor and coating polymer. All the coatings were chemically bonded to the inner surface of copper tubes, intended to be used as the CME device and already functionalized by self-assembly monolayers of 3-(mercaptopropyl)trimethoxysilane (3MPTMOS). The selected precursors included tetramethoxysilane (TMOS), 3-(trimethoxysilyl)propylmethacrylate (TMSPMA), 3-(triethoxysilyl)-propylamine (TMSPA), 3MPTMOS, [3-(2,3-epoxypropoxy)-propyl]-trimethoxysilane (EPPTMOS) while poly(ethyleneglycol) (PEG) was chosen as the coating polymer. The effects of different precursors on the extraction efficiency and selectivity, was studied by selecting a list of compounds ranging from non-polar to polar ones, i.e. polycyclic aromatic hydrocarbon, herbicides, estrogens and triazines. The results from CME–HPLC analysis revealed that there is no significant difference between precursors, except TMOS, in which has the lowest extraction efficiency. Most of the selected precursors have rather similar interactions toward the selected analytes which include Van der Walls, dipole–dipole and hydrogen bond while TMOS has only dipole–dipole interaction and therefore the least efficiency. TMOS is silica but the other sorbents are organically modified silica (ORMOSIL). Our investigation revealed that it is rather impossible to prepare a selective coating using conventional sol–gel methodologies. The comparison study performed among the fiber coatings contained only a precursor and those synthesized by a precursor along with coating polymer proved that the extraction efficiency obtained for all coatings are the same. This is an indication that by selecting the appropriate precursor there is no need to use any coating polymer. In overall, a fiber coating in sol–gel process could be synthesize with no coating polymer which leads to faster, easier, cheaper and more controllable synthesis.     

Silica sol–gel chemistry: creating materials and architectures for energy generation and storage

There is widespread recognition that the use of energy in the twenty-first century must be sustainable. Because of its extraordinary flexibility, silica sol–gel chemistry offers the opportunity to create the novel materials and architectures which can lead to significant advances in renewable energy and energy storage technologies. In this paper, we review some of the significant contributions of silica sol–gel chemistry to these fields with particular emphasis on electrolytes and separators where sol–gel approaches to functionalization and encapsulation have been of central importance. Examples are presented in the areas of dye-sensitized solar cells, biofuel cells, proton exchange membrane fuel cells, redox flow batteries and electrochemical energy storage. Original work is also included for the sol–gel encapsulation of a room temperature ionic liquid to create a solid state electrolyte for electrochemical capacitors. In view of the critical importance of energy and the versatility of the sol–gel process, we expect the sol–gel field to play an increasingly important role in the development of sustainable energy generation and storage technologies.     

Effect of sol–gel hydrophobicity on the distribution and structure of different proteins in organically modified sol–gel thin films

Although the use of silica sol–gels for protein entrapment has been studied extensively our understanding of the interactions between the immobilization matrix and the entrapped biomolecules is still relatively poor. Non-invasive in situ spectroscopic characterization is a promising approach to gain a better understanding of the fundamentals governing sol–gel immobilization of biomolecules. This work describes the application of Fourier transform infrared (FTIR) microscopy to determine the influence of modifying the sol–gel hydrophobicity, by varying the content of the organically modified precursor propyltrimethoxysilane (PTMS), on the distribution and structure of three model proteins (lysozyme [EC 3.2.1.17], lipase [EC 3.1.1.3] and bovine serum albumin (BSA)) in silica sol–gel thin films. FTIR analysis of the overall immobilized protein positional distribution showed a Gaussian type distribution. FTIR microscopic mapping however, revealed that the spatial distribution of proteins was heterogeneous in the sol–gel thin films. When this positional information provided by FTIR microscopy was taken into account, areas of high protein concentration (clusters) were found and were not found to be homogeneously distributed. The shape of these clusters was found to depend on the type of protein entrapped, and in some cases on the composition of the sol–gel. Positional analysis of the distribution of the organically modified precursor PTMS in relation to the protein distribution was also conducted. The localized concentration of PTMS was found to positively correlate with the protein concentration in the case of lipase and negatively correlate in the case of lysozyme and BSA. These results indicate that lysozyme and BSA concentration was higher in areas of low hydrophobicity, while lipase concentration was higher in areas of high hydrophobicity within the sol–gel. Additionally, as determined by peak shape analysis of the amide I peak a higher PTMS content appeared to conserve protein structure in high concentration clusters for lipase. In contrast, lysozyme and BSA, appeared to retain their structure in high concentration clusters better at lower PTMS contents. A hypothesis speculating on the nature of the hydrophobic/hydrophilic interactions between the proteins and the sol–gel domains as the reason for these differences is presented.     

High water solubility and sol–gel transition behavior of titania nanoparticles obtained by an in situ functionalization sol–gel process

Herein, addition reaction occurred between glycidol and partially hydrolyzed Ti⁴⁺ complexes provides a opportunity to obtain dry anatase nanopowder with high redispersity in water. This property is considered to be originated from the two OH groups located in the two ends of glycidol resulted chlorinated propandiol molecules. In aqueous solution, the two OH groups are respectively connected with particle surface and external free water by the formation of hydrogen bonds, resulting in high water redispersity of nanoparticles. Due to the much less amount of chlorinated propandiol molecules than adsorbed molecule water on particle, the wide space between organic molecules facilitates the mutual physical surface touch of individual particles to form hydrogen bond between them. A novel property is then obtained for surface modified titania nanoparticles, which is the gelation of redispered nanoparticles in aqueous solution.     

Electrochemical oxidation of aniline in a silica sol–gel matrix

Electrochemical oxidation of aniline encapsulated in a silica solid electrolyte prepared by a sol–gel process yielded products that were dependent on the pore size. An acid-catalyzed process that used tetramethyl orthosilicate as the precursor and aniline as a dopant yielded the silica. When the aging time was limited to one day so that a mesoporous solid was obtained, the potentiodynamic oxidation of aniline at a carbon fiber electrode resulted in the formation of polyaniline. With aging times of 3–5 days, microporous silica was obtained. In this electrolyte, the formation of dimers and other oligomers was observed by cyclic voltammetry. Evidence for these products was the presence of a quasi-reversible redox couple at 0.2 V vs Ag/AgCl that was previously related to oligomeric aniline by Raman spectroscopy. The results supported the hypothesis that the pore structure of sol–gel electrolytes can influence the pathways of electrode reactions therein.     

Heat and moisture transfer in sol–gel treated cotton fabrics

Cotton fabrics have been treated by sol–gel processes in order to produce an inorganic coating on fibres, able to modify their thermal conductivity under an irradiating flow. To this aim, fabric specimens with different silica content have been tested following the ISO 6942 standard in order to establish the effect of the coating on coupled heat and moisture transfer through the cotton fibres. The collected results have been compared with those obtained by an optimized method using a cone calorimeter as the heating source. By this latter approach, it has been demonstrated that sol–gel cotton treated fabrics with high moisture uptake possess a significantly lower thermal conductivity with respect to cotton alone.     

Lactic acid based sol–gel process of Ag nanoparticles and crystalline phase control of Ni particles in aqueous sol–gel process

Nickel and silver particles were prepared by using sol–gel auto-combustion method under N₂ atmosphere where lactic acid was applied as chelating agent. The synthesis of nickel particles was carried out at various pH conditions (2–7), resulting in the face-centered-cubic or hexagonal-close-packed crystalline nickel phase. The morphology and structure of synthesized nickel particles and silver nanoparticles were characterized by X-ray diffraction, transmission electron microscope, energy dispersive X-Ray spectroscopy and differential scanning calorimetry-thermogravimetric analysis. The results show that the spherical Ag nanoparticles with diameters in the range of 18–27 nm and narrow size distribution can be obtained by this sol–gel process.     

Induced neodymium luminescence in sol–gel derived layered double hydroxides

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Fluorescence studies of the sol–gel transition in deoxygenated conditions

The results of steady-state and time-resolved fluorescence studies of the sol-gel transition initiated by acidic hydrolysis of silane sols of pH 1.8–5.9 are presented. The gelation process was carried out in an oxygen-free atmosphere using pyrene as a fluorescence probe at concentrations of 1×10 ^-5–1×10 ^-2 M. The silica gels were prepared from sols consisting of tetraethylorthosilicate, ethanol and water in a molar ratio of 1:6:6. The steady-state spectra recorded during gelation allowed the polarity to be determined in the microenvironment of the probe for different pH and pyrene concentration. The investigation of the kinetics of fluorescence decay permitted us to discuss the formation of pyrene aggregates due to variations occurring in the gel net.     

Room temperature ferromagnetism in sol–gel deposited un-doped ZnO films

Dilute magnetic semiconductors are fast emerging spintronic materials where advantage of magnetic properties of semiconductor materials (usually doped with small quantities of magnetic ions) is being explored. Sol–gel technique, being low-cost simple and application oriented method, has been used in the present case. ZnO films of <150 nm thickness have been deposited by spin coating onto single crystal p-type Si substrates. The optimized sol is of paramagnetic nature, whereas, mixed para- dia-magnetic phase is observed for the as-prepared films. A complete ferromagnetic phase transition has been observed after heating the films in vacuum at a temperature of 300 °C. These sol–gel prepared films exhibit hexagonal wurtzite structure as observed by X-ray diffraction. After the magnetic field annealing in vacuum the films showed strengthened magnetic as well as structural properties. This work presents a clear evidence of ferromagnetic behavior of the un-doped ZnO films deposited by sol–gel at room temperature. It is also pointed out that Zn vacancies rather than oxygen deficiency are responsible for ferromagnetism in these sol–gel deposited ZnO thin films, whereas, the experimental evidence has been substantiated with the theoretical calculations using density functional theory.     

Fluorescent organic nanocrystal confined in sol–gel matrix for bio-imaging

The sol–gel chemistry combined to a spray-drying process allowed us to control the formation of original hybrid core–shell nanoparticles constituted by molecular nanocrystals of rubrene embedded in biocompatible silicate spheres. With a good management of all the physical (gas flows, temperatures) and chemical (dye, solvent and alkoxide natures, concentrations, and hydrolysis and condensation conditions) parameters, we optimized a one-step and self-assembly process allowing to obtain nanoparticles exhibiting promising optical properties such as highly fluorescent labels (two-photon excitation) for medical imaging. Moreover, the presence of Si–OH functions on the silicate shell surface make easy to functionalize these fluorescent nanoparticles by grafting biomolecules for targeting properties. The confined nucleation and growth of rubrene nanocrystals in sol–gel silicate spheres during their drying in the air laminar flows, prevents any phase segregation or particle coalescence and stabilizes mechanically and chemically the organic cores. The first particle sizes obtained in these first experiments are ranging between 80 and 600 nm, but lower diameters will be easily prepared by increasing the solvent amount. Transmission electron microscopy was used to characterize the rubrene organic cores. The electron diffraction patterns performed at 100 K, under low-dose illumination to avoid amorphization of the samples during electron irradiation, have shown the good crystallinity of the NP rubrene cores that seem to be constituted by single rubrene nanocrystals. Finally, optical confocal microscopy, used in reflection and fluorescence modes, showed that all the core–shell particles are strongly fluorescent. This high fluorescence intensity arises from the high molecule numbers of rubrene nanocrystals, which enhance the absorption and emission cross sections.     

Materials doping through sol–gel chemistry: a little something can make a big difference

Several examples of sol–gel preparation of doped materials are taken to illustrate the various situations where the doping elements are responsible for the main function of the material or govern its structure. Other examples are used to illustrate that sometimes unexpected effects can be observed like structural modification and the appearance of new properties. Rare earth doped scintillators demonstrate higher homogeneity for materials prepared via sol–gel chemistry when compared with classical solid state reaction. The XRD study of rare earth doped orthoborates shows that doping can affect the vaterite to calcite phase transition observed in these compounds. A Raman spectroscopic study has been performed on doped silica xerogels and it has been shown that doping ions can modify greatly the densification process in these amorphous materials. Finally, it has been evidenced that sol–gel chemistry allows the preparation of bioactive ceramics with enhanced properties. In particular Zn-doped HAP with anti inflammatory properties has been prepared and Sr-doped bioactive glasses have demonstrated superior in-vitro bioactivity as evidenced by PIXE-RBS study.     

Glass formation and confined melting in sol–gel derived nano-ZnO aggregates

Sol–gel processing of ZnO nanocolloids has been revisited to prepare various alkaline metal acetate (MAc: K, Na, Li-acetates) containing ZnO nanopowders. Using differential scanning calorimetry and X-ray diffraction investigations, several new relationships between the morphology and the thermodynamic behavior in the aforementioned particulate ZnO/MAc nanocomposites were noticed: (1) large MAc melting depression shifts ?ΔT_m ~ 80 °C due to salt confinement within porous ZnO aggregates, (2) corresponding MAc crystallization and re-crystallization temperature depression shifts, (3) presence of multimodal pore size distributions within nanoaggregates and (4) glass formation at temperatures T_g between ?40 and 130 °C. We also note that the T_g value drops with rising alkaline metal size, increasing water content and decreasing ZnO particle size. In contrast, T_g rises in the case of co-doped Fe–LiAc/ZnO compositions.     

Photochromic hybrid sol–gel films containing naphthopyrans

Abstract  

Several photochromic benzo- and naphthopyrans were embedded in hybrid organic–inorganic ureasilicate based films obtained by a sol–gel process. The resulting films, containing 0.1% (w/w) of the photochromic compound, were transparent and flexible and for naphthopyrans they were colourless or presented just a slight colouration. Under UV continuous irradiation the films developed a significant absorption in the visible region that fades in the dark with variable kinetics depending on the naphthopyran structure. 2,2-Diphenyl-3H-naphtho[2,1-b]pyran films showed a very fast colouration under UV light giving rise to coloured open forms, containing mainly the TC isomer (98%) along with a very small amount of the more stable TT isomer (2%), that practically returned to the initial absorbance in 1 min.     

Proteins conjugation with ZnO sol–gel nanopowders

The conjugation between probe biomolecules and inorganic nanoparticles has been studied. Three different and biologically relevant proteins, bovine serum albumin (BSA), lysozyme (LSZ) and Ribonuclease A (RNAseA), have been selected as model systems because of their difference in size and isoelectric point. Zinc oxide nanoparticles, synthesized via sol–gel, have been thoroughly characterized by X-ray Photoelectron Spectroscopy, Scanning Electron Microscopy and X-ray Diffraction, and subsequently used as platforms for immobilization of the biomolecules. The interaction of the three proteins with the ZnO surface was performed in phosphate buffer solutions at pH 7.2 in order to mimic physiological fluids and was investigated through fluorescence experiments. The obtained results indicate that conjugation of BSA, LZS and RNAseA on the oxide nanoparticles was mostly dictated by the overall charge of the different proteins. Electrostatic bonds dominate the formation of the protein/ZnO conjugates, whereas the size of the proteins seems to play a negligible role under the adopted experimental conditions.     

Synthesis and sol–gel assembly of nanophosphors

Some recent works made in our group on inorganic nanophosphors are briefly reviewed in this paper. We first present the synthesis of highly concentrated semiconductor quantum dot colloids allowing the extension of the well-known oxide sol–gel process to chalcogenide compounds. Secondly, we show the synthesis and the chemical functionalization of lanthanide-doped insulator nanoparticles. In particular, the annealing process of these particles at high temperature leads to highly bright nanocrystals, which can be used as biological luminescent labels or for integration in transparent luminescent coatings. Finally, we consider luminescent transition metal clusters, which combine the inorganic structure of nanoparticles with the monodispersity and the easy functionalization of the organic molecules. Emphasis is put on the original thermochromic luminescence properties of copper iodide clusters trapped in siloxane-based films.