Vera R. L. Constantino

Layered niobate nanosheets: building blocks for advanced materials assembly

Layered materials with intracrystalline reactivity undergo intercalation and pillaring reactions to produce materials with useful properties for catalysis, electrodes for Li batteries and adsorbents. New possibilities for the use of layered inorganic solids came out from the layered structures capable of delamination. The exfoliated particles are considered a new class of nanomaterial based on single crystal nanosheets. Due to their unique morphological features and properties, these nanosheets can be used as building blocks for nanomaterials with innovative properties. In this feature article we describe the aspects related to layered niobate exfoliation and the new possibilities that arises from the use of niobate nanosheets in the manufacturing of thin films, layer-by-layer (LbL) assemblies, hybrid structures, sensors and other materials.

Luminescence properties of the layered niobate KCa2Nb3O10 doped with Eu3+ and La3+ ions

Abstract In the present work the layered perovskite phase KCa 2 Nb 3 O 10 was doped with rare earth ions through a solid-state reaction, producing compounds of general formula K 1− x Ln x Ca 2− x Nb 3 O 10 (where Ln=La 3+ or Eu 3+ and x =0.02). The photoluminescence properties of the layered oxide doped with rare earth ions in the perovskite-type layers were investigated based on the luminescence data. The KCa 2 Nb 3 O 10 phase does not exhibit emission bands when excited in the UV region. On the other hand the matrices doped with Ln 3+ ions show a blue emission ( λ exc =270 nm) at 298 and 77 K, indicating that the electronic properties of the layered materials are modified by the lanthanide insertion. In the case of K 0.98 Eu 0.02 Ca 1.98 Nb 3 O 10 the characteristic red emission of the trivalent europium ion is also observed under excitation at 270 and 394 nm. The luminescence spectra of the Eu 3+ -compound show the 5 D 0 → 7 F 0 transition split into two peaks, indicating that the Eu 3+ ions are located in at least two distinct site symmetries. The results also suggest the occurrence of an energy transfer process between the niobate matrix and the Eu 3+ ion. The luminescence decays exhibit biexponential curves corroborating the presence of Eu 3+ ions in more than one chemical environment.

Photoluminescence study of layered niobates intercalated with Eu3+ ions

Abstract In the present work the lamellar phases of composition K 4 Nb 6 O 17 , KNb 3 O 8 and KCa 2 Nb 3 O 10 were used as precursors to prepare new compounds through the potassium exchange reaction with Eu 3+ ion via a soft chemistry route. The precursors show absorption bands in the UV range and KNb 3 O 8 exhibits blue emission at room and liquid nitrogen temperature, while K 4 Nb 6 O 17 presents emission spectra only at low temperature. The aim of this work is to investigate the photoluminescence properties of Eu 3+ -exchanged layered oxides through the analysis of their excitation and emission spectra. The compounds show the general formulae Eu x /3 K 4− x Nb 6 O 17 · z H 2 O (1), Eu x /3 K 1− x Nb 3 O 8 · z H 2 O (2) and Eu x /3 K 1− x Ca 2 Nb 3 O 10 . z H 2 O (3) and were characterized by X-ray diffraction and europium and potassium analyses. The emission spectra of the samples recorded at 298 and 77 K temperatures showed transitions between the 5 D 0 and 7 F J (J=0–4) levels that indicate the presence of Eu 3+ ions in C nv site symmetry. It was observed for systems (2) and (3) that the 5 D 0 → 7 F 0 transition does not split, which indicates that the Eu 3+ ion is found only in one site symmetry. On the other hand, the system (1) spectra indicated the presence of the rare earth ions in two different site symmetries. Since the precursor K 4 Nb 6 O 17 has two crystallographically distinct interlayer regions, we suggest that the K + ion is replaced by the Eu 3+ ion in the two interlamellar regions of the material (1). In the case of the system (2), two types of emission spectra were observed at room temperature: blue luminescence due to matrix excitation and red emission due to the direct excitation of the Eu 3+ ion. The values of the Ω λ (λ=2, 4) experimental intensity parameters for these three lamellar compounds suggest that the short distance effects are not dominant.

Porphyrin intercalation into a layered niobate derived from K4Nb6O17

The incorporation of guest species into two-dimensional inorganic structures can lead to materials with interesting chemical, catalytic, electronic, optical or mechanical properties. Concerning porphyrins and metalloporphyrins intercalation compounds, nanostructured materials have been obtained and evaluated in studies about photoprocess and catalytic reactions in confined media. The intercalation of bulky species such porphyrins into layered niobates is not easy to perform due to their high layer charge densities when compared to other layered materials. In this work we describe a method for TMPyP [5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H, 23H-porphyrin] intercalation into a layered niobate derived from K4Nb6O17. The potassium precursor was converted into the acidic-exchanged form and then intercalated with n-butylamine to produce an expanded material that was later used in the production of a dispersion containing exfoliated niobate sheets. The niobate dispersion was dropped into a porphyrin solution originating an organic-inorganic hybrid composite of formula TMPyP0.35H0.6K2Nb6O17·3H2O. XRD data suggest a tilted arrangement of the TMPyP ring with respect to the layers. Spectroscopic data (uv-visible absorption, fluorescence and resonance Raman) showed that TMPyP is intercalated in a non protonated form and that the interaction with the niobate layers surface is weak, corroborating with the proposed tilted orientation in the interlayer region.

Hidróxidos duplos lamelares: nanopartículas inorgânicas para armazenamento e liberação de espécies de interesse biológico e terapêutico

Studies about the inorganic nanoparticles applying for non-viral release of biological and therapeutic species have been intensified nowadays. This work reviews the preparation strategies and application of layered double hydroxides (LDH) as carriers for storing, carrying and control delivery of intercalated species as drugs and DNA for gene therapy. LDH show low toxicity, biocompatibility, high anion exchange capacity, surface sites for functionalization, and a suitable equilibrium between chemical stability and biodegradability. LDH can increase the intercalated species stability and promote its sub-cellular uptake for biomedical purposes. Concerning the healthy field, LDH have been evaluated for clinical diagnosis as a biosensor component.

Preparation and Characterization of Cu(II) Phthalocyanine Tetrasulfonate Intercalated and Supported on Layered Double Hydroxides

The aim of the present work was to synthesize and characterize layered doublehydroxides (LDHs), in the magnesium/aluminum form, intercalated with copper(II)phthalocyanine tetrasulfonate (CuPcTs). The metal complex was immobilized intothe LDH gallery region through the reconstitution method and this material wascharacterized by X-ray diffraction (XRD), surface area and porosity measurements,elementary analysis, thermogravimetry (TGA), vibrational (IR) and electronic(UV-visible) spectroscopies, and electronic paramagnetic resonance (EPR). Thecatalytic performance of CuPcTs intercalated and supported on the LDH wasevaluated by carrying out the hydrogen peroxide dismutation. The CuPcTs wassuccessfully intercalated into the LDH layers according to XRD data (the basalspacing of the carbonate precursors increases by approximately 15Å inthe intercalated samples). The surface area and porosity analysis suggested thatthe CuPcTs intercalated materials are not microporous solids. Samples containingthe metal complex confined between the LDH layers have an appreciable thermalstability: decomposition is not observed at least up to 400 °C. TGA experiments also show that the weight-loss curves of the CuPcTs supported samples superimpose those recorded for the CuPcTs complex and the LDH-carbonate while the curves for theintercalated materials are unique. CuPcTs intercalated or supported on LDHs is notactive in the hydrogen peroxide dismutation although the free form shows activity at pH above 8.

Spectroscopic Studies on the Interaction of Tetramethylpyridylporphyrins and Cationic Clays

In the present work, the interaction between5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine (TMPyP) and its metallated form(CoTMPyP) with three cationic clays was investigatedby X-ray diffraction (XRD), UV-VIS and resonance Ramanspectroscopies. Sodium montmorillonites K10 and KSFand a synthetic fluorohectorite (FHT) containingdifferent macrocycle loadings, were prepared by an ionexchange reaction. In nonsaturated KSF and FHT, theCoTMPyP molecule assumes a flat orientation, relativeto the host layers, giving rise to at least twoabsorption bands in the Soret region (ca. 445 and 465 nm)assigned to adsorbed and intercalated CoTMPyP,respectively. For the delaminated K10 sample, a broadband centered around 456 nm, indicates a majorcontribution from the metalloporphyrin on the clayexternal surfaces. The electronic spectra of FHTsamples containing increasing amounts of CoTMPyPshow bands red shifted even when a small amount ofporphyrin is used, suggesting that the electroniclevels of the macrocycle are more affected by theinteraction with the clay than by the metalloporphyrindistortion inside the galleries. The resonance Ramanspectra obtained for all CoTMPyP samples presentedonly minor shifts in peak positions and band width,with the exception of the FHT saturated sample, wherethe bands are clearly broader when compared to otherloadings, suggesting that porphyrin aggregation isoccurring. In the case of TMPyP, the bands at ca. 430and 468 nm were assigned to nonprotonated andprotonated molecules, respectively. This assignment issupported by resonance Raman spectroscopy, which alsoshowed the ν2 mode (ca. 1550 cm-1) to bethe most sensitive peak to protonation.

Iron oxyhydroxide nanostructured in montmorillonite clays: Preparation and characterization.

Akaganéite is a very rare iron oxyhydroxide in nature. It can be obtained by many synthetic routes, but thermohydrolysis is the most common method reported in the literature. In this work, akaganéite-like materials were prepared through the thermohydrolysis of FeCl(3).6H(2)O in water and suspensions containing clay minerals. X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) data show that the clays determine the crystal phase and size of the iron oxyhydroxide crystals. According to XRD and FTIR data, beta-FeO(OH) (akaganéite) is the main metal oxyhydroxide phase. Considering the small basal spacing (d(001)) displacement observed when comparing the XRD patterns of pristine clays with the composites containing beta-FeO(OH), the iron oxyhydroxide should be mostly located on the basal and edge surfaces of the clay minerals. UV-Vis electronic absorption spectra indicate that the preferred phase of the iron oxyhydroxide is determined by the nature of the clay minerals.

Biopolymer-clay nanocomposites: cassava starch and synthetic clay cast films

Polymer-clay nanocomposites (PCN) based on cassava starch, synthetic hectorite clay and inverted sugar cane syrup (plasticizer) were prepared by solvent-assisted (casting) process producing transparent and homogeneous films. Small amounts of clay (5-15 wt.%) resulted mainly in exfoliated nanocomposites while large amounts (30 wt.%) promote the intercalated nanocomposites formation. FT-Raman bands sensitive to hydrogen bonding in starch granules are progressively shifted to lower wavenumbers as the clay content is raised. Nanocomposites show a similar thermal behavior up to 320 oC while the biomolecule decomposition at about 500 oC is dependent on the clay content. CO2 release at about 300 oC (non-oxidative decomposition of polymeric chains) decreases if compared to the gas delivery at ca. 500 oC, as the clay content is increased. Films with clay content higher than 10 wt.% show no substantial benefit for either elongation or resistance properties.

Mefenamic Acid Anti-Inflammatory Drug: Probing Its Polymorphs by Vibrational (IR and Raman) and Solid-State NMR Spectroscopies

This work deals with the spectroscopic (supported by quantum chemistry calculations), structural, and morphological characterization of mefenamic acid (2-[(2,3-(dimethylphenyl)amino] benzoic acid) polymorphs, known as forms I and II. Polymorph I was obtained by recrystallization in ethanol, while form II was reached by heating form I up to 175 °C, to promote the solid phase transition. Experimental and theoretical vibrational band assignments were performed considering the presence of centrosymmetric dimers. Besides band shifts in the 3345-3310 cm(-1) range, important vibrational modes to distinguish the polymorphs are related to out-of-phase and in-phase N-H bending at 1582 (Raman)/1577 (IR) cm(-1) and 1575 (Raman)/1568 (IR) cm(-1) for forms I and II, respectively. In IR spectra, bands assigned to N-H bending out of plane are observed at 626 and 575 cm(-1) for polymorphs I and II, respectively. Solid-state (13)C NMR spectra pointed out distinct chemical shifts for the dimethylphenyl group: 135.8 to 127.6 ppm (carbon bonded to N) and 139.4 to 143.3 ppm (carbon bonded to methyl group) for forms I and II, respectively.