Ana Isabel Becerro

Synthesis and properties of multifunctional tetragonal Eu:GdPO4 nanocubes for optical and magnetic resonance imaging applications.

A simple and fast (7 min) procedure for synthesis of gadolinium phosphate nanocubes (edge = 75 nm) based on the microwave-assisted heating at 120 °C of gadolinium acetylacetonate and phosphoric acid solutions in buthylene glycol is reported. These nanocubes were highly crystalline and crystallized into a tetragonal structure, which has not been ever reported for pure gadolinium phosphate. Determination of such crystal structure has been carried out here for the first time in the literature by means of powder X-ray diffraction. The developed synthesis procedure was also successful for preparation of multifunctional europium(III)-doped the gadolinium phosphate nanocubes, which were nontoxic for cells and exhibited strong red luminescence under UV illumination and high transverse relaxivity (r(2)) values. These properties confer them potential applications as biolabels for in vitro optical imaging and as negative contrast agent for magnetic resonance imaging.

Pore structure analysis of the mesoporous titanosilicate molecular sieve MCM-41 by 1H NMR and N2 sorption

The pore structure of the titanosilicate mesoporous molecular sieve [Si, Ti]-MCM-41 has been examined by N2 sorption and 1H NMR. Pore sizes determined by both methods are in good agreement. Titanosilicate MCM-41 has a broader pore size distribution than the purely siliceous material, and the diameter of its pores is larger. The thickness of the pore walls, calculated by subtracting the pore diameter from the unit cell parameter, increases with the incorporation of titanium into the framework.

Hydrothermal reactivity of Lu-saturated smectites: Part I. A long-range order study

Abstract Changes produced after hydrothermal treatments at 400 °C and at different pressures in a set of Lu-saturated smectites have been compared with those occurring in a mixture of 2SiO2:1Lu2O3. The effect of the mineralogical composition of the smectite on the reactivity has been analyzed. The growth of a new crystalline phase, Lu2Si2O7 (detected by means of X-ray diffraction), at a different pressure for each smectite, has allowed us to establish a reactivity order among the samples. All the samples were much more reactive than the mixture of oxides. The reactivity of the smectites was modulated by their different mineralogical compositions. First, smectites having Al in the tetrahedral sheet were the most reactive. This finding is interpreted as due to a strong electrostatic interaction created by the Al that activates neighboring Si for the reaction. Second, a higher reactivity was observed for the smectites with total occupancy of the octahedral sheet, due to the simultaneous disruption of the tetrahedral-octahedral shared oxygen layer and the diffusion of octahedral cations into the interlayer space. Third, the layer charge of the smectite does not induce any variation in the reactivity for the set of samples analyzed.

The transition from short-range to long-range ordering of oxygen vacancies in CaFexTi1−xO3−x/2 perovskites

The transition from isolated and finite oxygen vacancy clusters in disordered cubic perovskites to structures containing infinite chains of oxygen vacancies in an ordered arrangement was investigated by a combined Mossbauer spectroscopy, X-ray and electron diffraction and high-resolution transmission electron microscopic study. With increasing defect concentration (equivalent to an increase in the CaFeO2.5 component concentration) and/or decreasing annealing temperature, a disorder–order phase transition occurs leading to infinite chains of vacancies in the [101]cubic direction, arranged in sheets containing essentially tetrahedral iron. In the early stages of the ordering process and at low iron concentrations, these sheets exhibit orientational disorder with two different directions of vacancy chains in addition to stacking disorder with variable sequences of tetrahedral and octahedral layers. At higher vacancy concentrations, ordered layer sequences with proportions defined by stoichiometry are formed. The ordering of vacancies into infinite chains and sheets leads to a large and discontinuous decrease in ionic conductivity.

Structure-directing effect of phyllosilicates on the synthesis of y-Y2Si2O7. Phase transitions in Y2Si2O7

This paper describes the low temperature hydrothermal synthesis of y-Y2Si2O7 from a layered silicate (2 ∶ 1 clay), a precursor which acts as a structure-templating agent. Both the transformations undergone by the clay during the hydrothermal process and the new products formed at different reaction stages have been characterised at both long and short range orders. Secondly, the influence of the aluminium content of the clay on the stability of y-Y2Si2O7 has been investigated. For this purpose, another layered clay, containing nominally no Al, has been submitted to the same hydrothermal treatment as the previous one. The results indicate that the amount of aluminium present in the starting material assists the stabilisation of y-Y2Si2O7. The behaviour of y-Y2Si2O7 with increasing temperature has been studied and the sequence of polymorphs and transition temperatures are reported and compared with those found in the literature.

Ligand-Free Synthesis of Tunable Size Ln:BaGdF5 (Ln = Eu3+ and Nd3+) Nanoparticles: Luminescence, Magnetic Properties, and Biocompatibility

Bifunctional and highly uniform Ln:BaGdF5 (Ln = Eu(3+) and Nd(3+)) nanoparticles have been successfully synthesized using a solvothermal method consisting of the aging at 120 °C of a glycerol solution containing the corresponding Lanthanide acetylacetonates and butylmethylimidazolium tetrafluoroborate. The absence of any surfactant in the synthesis process rendered hydrophilic nanospheres (with tunable diameter from 45 nm 85 nm, depending on the cations concentration of the starting solution) which are suitable for bioapplications. The particles are bifunctional because they showed both optical and magnetic properties due to the presence of the optically active lanthanides (Eu(3+) in the visible and Nd(3+) in the NIR regions of the electromagnetic spectrum) and the paramagnetic gadolinium ion, respectively. The luminescence decay curves of the nanospheres doped with different amounts of Eu(3+) and Nd(3+) have been recorded in order to determine the optimum dopant concentration in each case, which turned out to be 5% Eu(3+) and 0.5% Nd(3+). Likewise, proton relaxation times were measured at 1.5 T in water suspensions of the optimum particles found in the luminescence study. The values obtained suggested that both kinds of particles could be used as positive contrast agents for MRI. Finally, it was demonstrated that both the 5% Eu(3+) and 0.5% Nd(3+)-doped BaGdF5 nanospheres showed negligible cytotoxicity for VERO cells for concentrations up to 0.25 mg mL(-1).

Application of 29Si and 27Al MAS NMR spectroscopy to the study of the reaction mechanism of kaolinite to illite/muscovite.

Understanding the mechanisms for illitization of clay minerals has important applications in reconstructing geologic histories and determining the origins of physical and chemical characteristics of buried sediments. While many studies have been carried out on this topic, few have focused on the mechanism of illite formation from kaolinite. The purpose of this study was to investigate more deeply the illitization of kaolinite in KOH solution at a high solid/liquid ratio (1000 mg/mL). X-ray diffraction (XRD) and infrared spectroscopy were used to follow the formation of new crystalline phases and the composition of the octahedral sheet, while the transformation of the Si and Al local environments was analyzed by 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). The results revealed that the first reaction stage consists of the diffusion of Al from the octahedral to the tetrahedral sheet of the kaolinite TO layers, giving rise to the precursors of the illite/muscovite nuclei. Combination of XRD with 27Al MAS NMR measurements indicated that a minimum amount of tetrahedral Al is required in the original TO layer before condensation of a second tetrahedral sheet occurs to complete the formation of the illite/muscovite TOT layers.

High-resolution 1H MAS NMR spectra of 2∶1 phyllosilicates

Solid-state 1H MAS NMR at high magnetic fields and high spinning speeds provides a powerful means of identifying the different proton sites in smectites and affects information on the octahedral nature, the octahedral layer charge deficit and the charges on the cations residing in the interlayer space of the silicate.

The hydrothermal conversion of kaolinite to kalsilite: Influence of time, temperature, and pH

Abstract Kalsilite (the low-temperature form of KAlSiO4) is used as the precursor of leucite, an important component in porcelain-fused-to-metal and ceramic-restoration systems, and it has also been proposed as a high-thermal expansion ceramic for bonding to metals. The present study reports the hydrothermal synthesis and characterization of pure kalsilite from kaolinite in subcritical conditions, as well as the characterization of the intermediate products by means of XRD, 29Si and 27Al MAS NMR, IR, SEM, and TEM. Effects of time, temperature, and pH on the reaction products are analyzed. The experimental data indicate that pure kalsilite is obtained after hydrothermal treatment of kaolinite at 300°C for 12 h in 0.5 M KOH solution. Longer reaction times increase the crystallinity of the structure, whereas lower reaction times give rise to the metastable ABW-type KAlSiO4 polymorph. Lower temperatures are not sufficient to produce kalsilite, but zeolite W is obtained instead as the unique reaction product. Finally, the pH of the aqueous solution in contact with kaolinite is an important parameter for the synthesis of kalsilite, which must be ≥13.70

Rare earth based nanostructured materials: synthesis, functionalization, properties and bioimaging and biosensing applications

Abstract Rare earth based nanostructures constitute a type of functional materials widely used and studied in the recent literature. The purpose of this review is to provide a general and comprehensive overview of the current state of the art, with special focus on the commonly employed synthesis methods and functionalization strategies of rare earth based nanoparticles and on their different bioimaging and biosensing applications. The luminescent (including downconversion, upconversion and permanent luminescence) and magnetic properties of rare earth based nanoparticles, as well as their ability to absorb X-rays, will also be explained and connected with their luminescent, magnetic resonance and X-ray computed tomography bioimaging applications, respectively. This review is not only restricted to nanoparticles, and recent advances reported for in other nanostructures containing rare earths, such as metal organic frameworks and lanthanide complexes conjugated with biological structures, will also be commented on.