Layered Double Hydroxide Matrix Research Articles

DFT-Based Simulation and Experimental Validation of the Topotactic Transformation of MgAl Layered Double Hydroxides.

The thermal topotactic transformation mechanism of MgAl layered double hydroxides (LDHs) is investigated by a combined theoretical and experimental study. Thermogravimetric differential thermal analysis (TG-DTA) results reveal that the LDH phase undergoes four key endothermic events at 230, 330, 450, and 800 °C. DFT calculations show that the LDH decomposes into CO2 and residual O atoms via a monodentate intermediate at 330 °C. At 450 °C, the metal cations almost maintain their original distribution within the LDH(001) facet during the thermal dehydration process, but migrate substantially along the c-axis direction perpendicular to the (001) facet; this indicates that the metal arrangement/dispersion in the LDH matrix is maintained two-dimensionally. A complete collapse of the layered structure occurs at 800 °C, which results in a totally disordered cation distribution and many holes in the final product. The structures of the simulated intermediates are highly consistent with the observed in situ powder XRD data for the MgAl LDH sample calcined at the corresponding temperatures. Understanding the structural topotactic transformation process of LDHs would provide helpful information for the design and preparation of metal/metal oxides functional materials derived from LDH precursors.

Photostability enhancement of azoic dyes adsorbed and intercalated into Mg–Al-layered double hydroxide

Abstract Two azoic dyes 4-aminoazobenzene-4-sulfonic (AS) and ethyl orange (EO) were adsorbed on or intercalated into Mg–Al–CO 3 layered double hydroxide (LDH) for photostability enhancement. Fluorescence analysis results showed that the photostability of two dyes could be greatly improved after being adsorbed on the surface of Mg–Al–CO 3 -LDH matrix. Furthermore, photostability of adsorbed dyes was superior to that of intercalated dyes. It was suggested that AS or EO was adsorbed on LDHs surface through a strong chemisorption interaction, resulting in the enhancement of photostability. After the UV irradiation under N 2 atmosphere, the absorbed dyes not only show great increase of fluorescence intensity but also exhibited high stability against UV irradiation. This work provides a feasible approach to enhance the photostability of azoic dye confined in an inorganic two-dimensional (2D) matrix via changing the microenvironment, which may be considered to be a promising method of improving photostability of solid fluorescent materials.

Sorption of 60Co from aqueous solutions onto layered double hydroxides of various compositions

Newly obtained and published data on 60Co sorption from aqueous solutions onto layered double hydroxides (LDHs) of various compositions are summarized. The influence of double-(Mg2+, Zn2+, Ni2+) and triple-charged (Al3+, Fe3+, Nd3+) cations incorporated in the LDH matrix and of anions (CO 3 2− , SO 4 2− , NO 3 − , OH−, Cl−, C2O 4 2− , H2EDTA2−) intercalated in the LDH structure on the physicochemical properties of LDHs, including their ability to take up 60Co, is analyzed.

A pH-responsive layered double hydroxide (LDH)-phthalocyanine nanohybrid for efficient photodynamic therapy.

A pH-responsive nanohybrid (LDH-ZnPcPS4 ), in which a highly hydrophilic zinc(II) phthalocyanine tetra-α-substituted with 4-sulfonatophenoxy groups (ZnPcPS4 ) is incorporated with a cationic layered double hydroxide (LDH) based on electrostatic interaction, has been specially designed and prepared through a facile co-precipitation approach. ZnPcPS4 is an excellent singlet-oxygen generator with strong absorption at the near-infrared region (692 nm) in cellular culture media, whereas the photoactivities of ZnPcPS4 were remarkably inhibited after incorporation with the LDH. The nanohybrid is essentially stable in aqueous media at pH 7.4; nevertheless, in slightly acidic media of pH 6.5 or 5.0, ZnPcPS4 can be efficiently released from the LDH matrix, thus leading to restoration of the photoactivities. The nanohybrid shows a high photocytotoxicity against HepG2 cells as a result of much more efficient cellular uptake and preferential accumulation in lysosomes, whereby the acidic environment leads to the release of ZnPcPS4 . The IC50 value of LDH-ZnPcPS4 is as low as 0.053 μM, which is 24-fold lower than that of ZnPcPS4 . This work provides a facile approach for the fabrication of photosensitizers with high photocytotoxicity, potential tumor selectivity, and rapid clearance character.

Sorption of strontium and yttrium radionuclides from aqueous solutions onto layered double hydroxides of various compositions

New experimental results and published data on sorption of strontium and yttrium radionuclides from aqueous solutions onto layered double hydroxides (LDHs) of various compositions are summarized. The effect of double-charged (Mg2+, Ba2+, Sr2+, Cu2+, Zn2+, Ni2+) and triple-charged (Al3+, Fe3+, Nd3+) cations incorporated in the LDH matrix and of anions incorporated in LDH (CO32−, SO42−, NO3−, OH−, Cl−, C2O42−, H2EDTA2−) on the physicochemical properties of this class of compounds, including the ability to sorb strontium and yttrium radionuclides, is examined.

A chiroptical switch based on DNA/layered double hydroxide ultrathin films.

A highly oriented film was fabricated by layer-by-layer self-assembly of DNA and MgAl-layered double hydroxide nanosheets, and its application in chiroptical switch was demonstrated via intercalation and deintercalation of an achiral molecule into the DNA cavity. DNA molecules are prone to forming an ordered and dispersive state in the interlayer region of rigid layered double hydroxide (LDH) nanosheets as confirmed by scanning electron microscopy and atomic force microscopy. The induced chiroptical ultrathin film (UTF) is achieved via the intercalation of an achiral chromophore [5,10,15,20-tetrakis(4-N-methylpyridyl)porphine tetra(p-toluenesulfonate) (TMPyP)] into the spiral cavity of DNA stabilized in the LDH matrix [denoted as TMPyP-(DNA/LDH)20]. Fluorescence and circular dichroism spectroscopy are utilized to testify the intercalation of TMPyP into (DNA/LDH)20 UTF that involves two steps: the electrostatic binding of TMPyP onto the surface of (DNA/LDH)20 followed by intercalation into base pairs of DNA. In addition, the TMPyP-(DNA/LDH)20 UTF exhibits good reversibility and repeatability in induced optical chirality, based on the intercalation and deintercalation of TMPyP by alternate exposure to HCl and NH3/H2O vapor, which can be potentially used as a chiroptical switch in data storage.

Understanding the thermal motion of the luminescent dyes in the dye–surfactant cointercalated ZnAl-layered double hydroxides: a molecular dynamics study

Previous work has demonstrated that cointercalation of luminescent dyes and surfactants into layered double hydroxides (LDHs) is an efficient approach to inhibit the aggregation of dye and therefore enhance its photoluminescence behavior. In this work, molecular dynamics simulations are performed on different ZnAl-LDHs cointercalated with dye (fluorescein or 1-anilinonaphthalene-8-sulfonate) and alkylsulfonate with different alkyl chain length (CnH2n+1SO3, n = 5, 6, 7, 10 and 12, respectively), together with dye–alkylsulfonate solutions for comparison. The structure, binding energy and the thermal motion characterized by the diffusion coefficient of each dye are analyzed. In the dye–alkylsulfonate/LDHs, the diffusion coefficient and the binding energy of the dye show a minimum when the dye is cointercalated with heptanesulfonate (HPS, n = 7), whose size is the closest to that of the dye. While in the case of dye–alkylsulfonate solutions, the diffusion coefficient and the binding energy vary monotonously with the increasing alkylsulfonate size. Furthermore, it is found that the increase of Al3+ content in LDH matrix in dye–HPS/LDHs is favorable for the restriction of the dye motion. These results indicate that the dye–alkylsulfonate/LDH system is more effective in restraining both the thermal motion and the aggregation of the dye than that of dye–alkylsulfonate solutions due to the confined microenvironment provided by the LDH matrix. Therefore, it is possible to inhibit the aggregation of the dye in dye/LDHs by two aspects: choosing a surfactant with a size close to that of the dye as the cointercalant and increasing the content of trivalent cations in the LDH matrix.

Enhancement of thermal properties of polyacrylonitrile by reinforcement of Mg-Al layered double hydroxide

The in situ polymerization technique was used for synthesis of polyacrylonitrile nanocomposites (PAN/LDH) by reinforcement of different concentrations of Mg-Al layered double hydroxide (LDH). Molecular interaction between Mg/Al LDH and PAN matrix was studied by Fourier transform infrared spectroscopy. The morphology of PAN/LDH nanocomposite was investigated by field emission scanning electron microscopy. Energy dispersive spectroscopy of nanocomposites were noticed to establish the elemental composition of the composite. High resolution transmission electron microscopy was used to study the nanostructural distribution of LDH with PAN matrix, whereas crystalline patterns were explained by selected area electron diffraction studies. The thermal properties of nanocomposites were studied by thermo-gravimetric analysis. It was found that, the thermal property of PAN was substantially enhanced due to strong interfacial adhesion of Mg-Al-LDH with PAN matrix. Further, it was noticed that the thermal stability of PAN/LDH nanocomposites were increased with increasing concentration of LDH. POLYM. COMPOS., 2014. © 2014 Society of Plastics Engineers

A Supermolecular Photosensitizer with Excellent Anticancer Performance in Photodynamic Therapy

A supermolecular photosensitizer with excellent anticancer behavior when used for photodynamic therapy (PDT) is fabricated by the incorporation of zinc phthalocyanines (ZnPc) into the gallery of a layered double hydroxide (LDH). The composite material possesses uniform particle size (hydrodynamic diameter ∼120 nm), and the host–guest and guest–guest interactions result in a high dispersion of ZnPc in a monomeric state in the interlayer region of the LDH matrix, with high singlet oxygen production efficiency. In vitro tests performed with HepG2 cells reveal a satisfactory PDT effectiveness of the ZnPc(1.5%)/LDH composite photosensitizer: a cellular damage as high as 85.7% is achieved with a rather low dosage of ZnPc (10 μg/mL). An extraordinarily high specific efficacy is demonstrated (31.59 μg−1 (J/cm2)−1), which is over 185.5% enhancement compared with the previously reported photosensitizers under similar test conditions. Furthermore, an in vivo study of the ZnPc(1.5%)/LDH demonstrates excellent PDT performance with an ultra‐low dose (0.3 mg/kg) and a low optical fluence rate (54 J/cm2). In addition, the ZnPc/LDH photosensitizer displays high stability, good biocompatibility, and low cytotoxicity, which would guarantee its practical application. Therefore, this work provides a facile approach for design and fabrication of inorganic–organic supermolecular materials with greatly enhanced anticancer behavior.

Nanohybrids from NiCoAl-LDH coupled with carbon for pseudocapacitors: understanding the role of nano-structured carbon

Transition metal layered double hydroxides (LDHs) are one of the great potential electrode materials for pseudocapacitors. However, the aggregation and low conductivity of these metal compounds will constrain electrolyte ion and electron transfer and further affect their electrochemical performances. The nano-structured carbon coupled with the LDH matrix can act as an active component or conducting scaffold to enhance or improve the rate capacity and cycle life. Here, various nano-structured carbon species, including zero-dimensional carbon black (CB), one-dimensional carbon nanotubes (CNTs), two-dimensional reduced graphene oxide (RGO), and CNT/RGO composites were used to couple with the NiCoAl-LDHs to construct LDH-carbon nanohybrid electrodes for pseudocapacitors, and the role of the nanostructured carbon was investigated and discussed in terms of the pore structure of nanohybrids and electrical conductivity. The results show that all of the carbons can be well incorporated into the LDH nanosheets to form homogeneous nanohybrid materials. The pore structure properties and electrical conductivity of nanohybrids have statistically significant effects on the electrochemical performances of the LDH-carbon nanohybrids. Of the electrodes adopted, the nanohybrid electrode consisting of NiCoAl-LDHs, CNTs, and RGO exhibits excellent electrochemical performance with a specific capacitance as high as 1188 F g(-1) at a current density of 1 A g(-1) due to the synergistic effect of NiCoAl-LDHs, RGO, and CNTs, in which the RGO nanosheets are favorable for high specific surface area while the CNT has a fast electron transport path for enhancing the electrical conductivity of nanohybrids. This will shed a new light on the effect of nano-structured carbon within the electrode matrix on the electrochemical activity and open a new way for the carbon-related electrode configuration/design for supercapacitors, and other energy storage and conversion devices.

Mg–Al and Zn–Fe layered double hydroxides used for organic species storage and controlled release

Layered double hydroxides (LDH) containing (Mg and Al) or (Zn and Fe) were prepared by coprecipitation at constant pH, using NaOH and urea as precipitation agents. The most pure LDH phase in the Zn/Fe system was obtained with urea and in Mg/Al system when using NaOH. The incorporation of phenyl-alanine (Phe) anions in the interlayer of the LDH was performed by direct coprecipitation, ionic exchange and structure reconstruction of the mixed oxide obtained by the calcination of the coprecipitated product at 400°C. The reconstruction method and the direct coprecipitation in a medium containing Phe in the initial mixture were less successful in terms of high yields of organic–mineral composite than the ionic exchange method. A spectacular change in sample morphology and yield in exchanged solid was noticed for the Zn3Fe sample obtained by ionic exchange for 6h with Phe solution. A delivery test in PBS of pH=7.4 showed the release of the Phe in several steps up to 25h indicating different host–guest interactions between the Phe and the LDH matrix. This behavior makes the preparation useful to obtain late delivery drugs, by the incorporation of the anion inside the LDH layer.

Universal Chemiluminescence Flow-Through Device Based on Directed Self-Assembly of Solid-State Organic Chromophores on Layered Double Hydroxide Matrix

In this work, a universal chemiluminescence (CL) flow-through device suitable for various CL resonance energy transfer (CRET) systems has been successfully fabricated. Highly efficient CRET in solid-state photoactive organic molecules can be achieved by assembling them on the surface of layered double hydroxides (LDHs). We attribute these observations to the suppression of the intermolecular π-π stacking interactions among aromatic rings and the improvement of molecular orientation and planarity in the LDH matrix, enabling a remarkable increase in fluorescence lifetime and quantum yield of organic molecules. Under optimal conditions, using peroxynitrous acid-fluorescein dianion (FLUD) as a model CRET system, trace FLUD (10 μM) was assembled on the surface of LDHs. Peroxynitrous acid/nitrite could be assayed in the range of 1.0-500 μM, and the detection limit for peroxynitrous acid/nitrite (S/N = 3) was 0.6 μM. This CL flow-through device exhibited operational stability, high reproducibility, and long lifetime. While LDHs were immobilized in a flow-through device in the absence of FLUD, the detection limit for peroxynitrous acid/nitrite was 100 μM. On the other hand, FLUD at the same concentration can not enhance the CL intensity of peroxynitrous acid system. This fabricated CL flow-through column has been successfully applied to determine nitrite in sausage samples with recoveries of 98-102%. These satisfactory results demonstrated that our studies pave a novel way toward flow-through column-based CRET using solid-state organic molecules as acceptors for signal amplification.

Preparation of pH-Sensitive Polymers/Layered Double Hydroxide Hybrid Beads for Controlled Release of Insulin

The present work introduces new hybrid material based on the combination of layered double hydroxides (LDH) and a pH-sensitive polymer (PSP), alginate, a polysaccharide widely applied for encapsulating drugs to produce a LDH–polymer nanocomposite, able to act as an effective drug delivery system (DDS) in comparison to the Zn–Al LDH or the polymer alone. Insulin has been chosen as a model drug, being loaded in a Zn–Al LDH matrix. In vitro release profiles were established separately in both the presence and absence of glucose phosphate in pH 7. Drug release studies showed that the in the presence of glucose phosphate release rate increases.

Two dimensional restriction-induced luminescence of tetraphenyl ethylene within the layered double hydroxide ultrathin films and its fluorescence resonance energy transfer

Neutral poly(vinyl carbazole) (PVK) and tetraphenylethylene (TPE) molecules were assembled within the interlayers of layered double hydroxide (LDH) nanosheets to form (TPE@PVK/LDH)n ultrathin films (UTFs) by the hydrogen-bonding layer by layer assembly method. These UTFs exhibit well defined blue luminescence of TPE, which implies that the intercalation anchoring of aggregation induced emission (AIE)-type chromophores to the rigid LDH matrix can serve as an alternative mechanism for restricting the rotation of the phenyl rings of TPE. The concentration–fluorescence relationship study implied that the AIE effect of intercalated TPE can be regarded as the 2D restriction-induced photoemission (RIE) effect within the interlayers. Moreover, the 2D Förster/fluorescence resonance energy transfer (FRET) process between PVK and TPE was realized within these UTFs, by which the UTFs exhibit reversible two-state switching of fluorescence emission (violet vs. blue) with the free/restriction states of TPE triggered by the external stimuli (e.g. volatile organic compound (VOC) vapors). Therefore, this 2D assembly of UTFs worked not only as the 2D RIE environment for TPE molecules, but also as a FRET 2D protocol for fluorescence sensing applications.

CdTe Quantum Dots/Layered Double Hydroxide Ultrathin Films with Multicolor Light Emission via Layer‐by‐Layer Assembly

AbstractQuantum dots (QDs) luminescent films have broad applications in optoelectronics, solid‐state light‐emitting diodes (LEDs), and optical devices. This work reports the fabrication of multicolor‐light‐emitting ultrathin films (UTFs) with 2D architecture based on CdTe QDs and MgAl layered double hydroxide (LDH) nanosheets via the layer‐by‐layer deposition technique. The hybrid UTFs possess periodic layered structure, which is verified by X‐ray diffraction. Tunable light emission in the red‐green region is obtained by changing the particle size of QDs (CdTe‐535 QDs and CdTe‐635 QDs with green and red emision respectively), assembly cycle number, and sequence. Moreover, energy transfer between CdTe‐535 QDs and CdTe‐635 QDs occurs based on the fluorescence resonance energy transfer (FRET), which greatly enhances the fluorescence efficiency of CdTe‐635 QDs. In addition, a theoretical study based on the Förster theory and molecular dynamics (MD) simulations demonstrates that CdTe QDs/LDH UTFs exhibit superior capability of energy transfer owing to the ordered dispersion of QDs in the 2D LDH matrix, which agrees well with the experimental results. Therefore, this provides a facile approach for the design and fabrication of inorganic‐inorganic luminescent UTFs with largely enhanced luminescence efficiency as well as stability, which can be potentially applied in multicolor optical and optoelectronic devices.

Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement

Layered double hydroxides (LDHs) and derived mixed oxides with different Mg/Al/Fe contents were investigated. Two super-saturation precipitation methods were used for the synthesis of LDHs with general formula [Mg1−xM(III)x(OH)2](CO3)x/2⋅mH2O where M(III) presents Al and/or Fe. The content of trivalent ions x=M(III)/[M(II)+M(III)], was varied between 0.15<x<0.7. Such a wide range of trivalent ions was chosen with the aim to induce the formation of different multiphase mixed oxides. Iron was introduced as constituent metal in order to obtain redox properties. LDHs and their derived mixed oxides were characterized with respect to their crystalline structure (XRD), thermal stability (TG/DTA), textural (N2 adsorption), redox (H2 TPR) and acid properties (NH3 TPD) as well as the nature of the iron species (Mössbauer spectroscopy). Catalytic behavior was studied in two test reactions: N2O decomposition and reduction with NH3. It has been demonstrated that extended M(III) substitution influences the structure and surface properties of Mg–Al–Fe LDHs and derived mixed oxides, weakens Mg–Al–Fe–O interactions and improves catalytic behavior correlated with the presence of Fe–O–Fe–O–Fe entities providing possibility for facilitated extraction of oxygen with simultaneous redox Fe3+Fe2+ conversion. The catalytic behavior is mainly determined by redox properties, nature of iron species in mixed oxides and by structural properties of initial LDHs. The best catalytic results were obtained when the amount of M(III) was near the limit for the incorporation into LDH matrix.

Mg/Al layered double hydroxide-Pt nanoparticle composite by delamination-restacking route

Platinum nanoparticle intercalated magnesium–aluminum layered double hydroxide (LDH) composite was prepared by restacking the colloidal dispersion of organically modified LDH in the presence of preformed oleylamine capped platinum nanoparticles in n-butanol. The resultant nanocomposite was characterized using powder X-ray diffraction (PXRD), infrared (IR) spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction and energy dispersive X-ray spectroscopy (EDX). The PXRD pattern of the composite indicates an expanded interlayer due to the intercalation of the Pt-nanoparticles into the LDH interlayer region. PXRD patterns along with the IR spectra confirm the formation of the precursors and the composite. TEM image of Pt-nanoparticles shows spherical particles with an average particle size of ~3 nm and that of the composite shows dispersed Pt-nanoparticles on the LDH matrix. The presence of Pt, Mg and Al in the composite is indicated in the EDX data.

Mechanochemical synthesis of Fe3O4@(Mg-Al-OH LDH) magnetic composite

A magnetic composite of a meixnerite (Mg-Al-OH) type layered double hydroxide (LDH) and magnetite (Fe3O4), abbreviated as Fe3O4@(Mg-Al-OH LDH), was successfully synthesized by a mechanochemical route with a two-step milling operation. In the first step, the mixture of Mg(OH)2, Al(OH)3 and Fe3O4 as the starting materials was milled without water for 1h, and then with addition water for another 5h in the second step. The so-synthesized composite has the structure of Fe3O4 cores homogeneously dispersed in the LDH matrix and demonstrates super-paramagnetic behavior. This mechanochemical process provides a facile and environmentally safe method for synthesizing magnetic composites based on LDHs.

Photoactive Self-Standing Films Made of Layered Double Hydroxides with Arranged Porphyrin Molecules

Transparent films of layered double hydroxides (LDH) have been prepared by the deposition of their chloroform dispersions. The advantage of this method is the potential to obtain large and transparent self-standing films, composed of highly ordered LDH platelets, with durability that allows further modifications by anion exchange. The LDH interlayer can be filled with anionic porphyrins that impose interesting photophysical and photochemical properties on the films. Intercalated porphyrins, 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and its palladium complex (PdTPPS), exhibit luminescence emission. The triplet state dynamics in the absence of oxygen are not affected by the LDH matrix, whereas the interaction of the triplet states with oxygen to produce singlet oxygen, O2(1Δg), is less effective in the film than in solution. High local concentrations of O2(1Δg) and of the triplet states within the films induce singlet oxygen-sensitized delayed fluorescence (SODF). This work is the first report on LDH-based materials capable of emitting SODF. The detailed kinetic analysis of SODF behavior indicates a short singlet oxygen lifetime of ∼0.3 μs, probably due to the close proximity of hydroxide groups at the LDH layers.

An optical sensor based on H-acid/layered double hydroxide composite film for the selective detection of mercury ion

A novel optical chemosensor was fabricated based on 1-amino-8-naphthol-3,6-disulfonic acid sodium (H-acid) intercalated layered double hydroxide (LDH) film via the electrophoretic deposition (EPD) method. The film of H-acid/LDH with the thickness of 1 μm possesses a well c-orientation of the LDH microcrystals confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The fluorescence detection for Hg(II) in aqueous solution was performed by using the H-acid/LDH film sensor at pH 7.0, with a linear response range in 1.0 × 10 −7 to 1.0 × 10 −5 mol L −1 and a detection limit of 6.3 × 10 −8 mol L −1. Furthermore, it exhibits excellent selectivity for Hg(II) over a large number of competitive cations including alkali, alkaline earth, heavy metal and transitional metals. The specific fluorescence response of the optical sensor is attributed to the coordination between Hg(II) and sulfonic group in the H-acid immobilized in the LDH matrix, which was verified by NMR spectroscopy and UV–vis spectra. In addition, density functional theory (DFT) calculation further confirms that the coordination occurs between one Hg 2+ and two O atoms in the sulfonic group, which is responsible for the significant fluorescence quenching of the H-acid/LDH film. The results indicate that the H-acid/LDH composite film can be potentially used as a chemosensor for the detection of Hg 2+ in the environmental and biomedical field.