Morena Nocchetti

New Synthetic Routes to Hydrotalcite-Like Compounds − Characterisation and Properties of the Obtained Materials

Hydrotalcite-like anionic clays of general formula [M(II)1–xM(III)x(OH)2]x+[CO32–x/2]x– · m H2O with M(III) = Al and M(II) = Mg, Ni, Zn have been prepared by precipitation of the hydroxycarbonates from the “homogeneous” solution after the thermally induced hydrolysis of urea. The effect of the temperature of precipitation, of the total metal cations concentration, of the molar fraction M(III)/M(III) + M(II) and of the molar fraction urea/M(II) + M(III) in solution on the composition and on the crystallinity degree of the samples has been investigated. The optimum conditions are reported to obtain micro-crystalline powders with a narrow distribution of particle size in a short time with a simple procedure. The compounds obtained have been characterised for chemical composition, thermal behaviour, particle-size distribution and BET-surface area. In addition, the crystal structure of Mg0.67Al0.33(OH)2 (CO3)0.165 · 0.4 H2O has been refined by X-ray diffraction powder methods. The carbonate form has been converted into the chloride form by letting gaseous HCl flow over the hydrotalcite-like compounds, heated at 150 °C. The exchange of Cl– anions with some alkoxide anions in the presence of the respective alkanols has been investigated. The exchange reaction was driven by the segregation of NaCl crystals poorly soluble in alkanols and led to the co-intercalation of the alkoxide ions together with the alkanol with the formation of a bi-layer of extended alkyl chain in the interlayer region of the Mg-Al hydrotalcite. The intercalation compound, washed with water, produces a hydrotalcite with Cl– and OH– as balancing anions.

Recent progress in the synthesis and application of organically modified hydrotalcites

Abstract This review is focused on the preparation and potential applications of hydrotalcite like compounds organically modified by ion-exchange procedure and the data reviewed have been supplemented with unpublished results. It is divided in two Parts. Part I deals with intercalation of biologically active species such as amino-acids, anti-inflammatory and antibiotic drugs, UV-absorbers to produce nano-hybrids with versatile application as biomolecule reservoir and in the pharmaceutical and personal care fields. Part II deals with the intercalation of several anions with either hydrophobic or hydrophilic properties in order to make the inorganic sheets compatible with different polymers. Moreover, if the guest is an active molecular anion such as antimicrobial, antioxidant, antibiotic or anti-inflammatory, the polymer can acquire the peculiar properties of the guest opening novel interesting application fields.

Preparation and deprotection of 1,1-diacetates (acylals) using zirconium sulfophenyl phosphonate as catalyst

Layered zirconium sulfophenyl phosphonate was found to be an efficient heterogeneous catalyst for the preparation and deprotection of 1,1-diacetates.

Preparation and photo-physical characterisation of nanocomposites obtained by intercalation and co-intercalation of organic chromophores into hydrotalcite-like compounds

Several chromophores with donor–acceptor properties and containing a carboxylic or sulfonic group [coumarin-3-carboxylic acid (3-CCA), 9-anthracenecarboxylic acid (9-ACA), 4-benzoylbenzoic acid (4-BBA) and 2-naphthalenesulfonic acid (2-NSA)] have been intercalated into Mg–Al hydrotalcite-like compounds, both via anion exchange procedures and by using the “memory effect” of the hydrotalcites. The obtained intercalation compounds have been characterised by X-ray diffractometry, thermal analysis and chemical composition. Data obtained indicate that the guest species are accommodated in the interlayer region as a monofilm of interdigitaded anions with the C–COO− or C–SO3− bond almost perpendicular to the layer plane. The pairs of chromophores investigated, 3-CCA–9-ACA and 4-BBA–2-NSA, can give rise to energy transfer processes because of the characteristics of their excited states. Co-intercalation of the above-mentioned pairs of chromophores has been achieved with different synthetic procedures. The mono-intercalated and co-intercalated systems were investigated by absorption and emission spectroscopy and by laser flash photolysis. The absorption spectrum of MgAl-9-ACA shows an unexpected band around 490 nm that was attributed to an aggregate. The fluorescence characteristics of the various co-intercalated samples depend on the excitation wavelength and on the preparation methods, which control the relative amounts of the two fluorophores in the interlayer region. In all the samples containing 9-ACA excitation around 500 nm generates an emission attributed to the anthracene aggregate. In mono-intercalated and co-intercalated samples, the fluorescence decays satisfactorily fit bi-exponential and tri-exponential functions, respectively. Laser flash photolysis experiments showed that excitation of the intercalated chromophores produces transients which can be attributed to the guest triplets and, in the case of co-intercalated 4-BBA–2-NSA composite, the probable occurrence of an energy transfer process.

Ag/AgCl nanoparticle decorated layered double hydroxides: synthesis, characterization and antimicrobial properties

A layered double hydroxide (LDH) surface was employed as a substrate for growing silver nanoparticles (NPs). An efficient method to produce stable silver/silver chloride nanoparticles supported on the ZnAl-LDH surface was developed. NPs of AgCl were grown on the ZnAl-LDH surface by using AgNO3 as the silver source. The ZnAl-LDH in chloride form acts as a nucleating agent, and depending on the pH of the LDH dispersion, AgClNPs with different dimensions were obtained. In particular AgClNPs with a diameter of 60 nm were formed at pH 5. The AgClNPs supported on LDH sheets were partially reduced by different reducing agents (NaBH4 and formaldehyde) resulting in a Ag/AgCl-LDH nanocomposite. The silver chloride and silver NP dimensions were evaluated by X-ray powder diffraction, field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). UV-Vis spectra of the samples upon reduction showed a band centred at 415 nm due to the surface plasmon resonance of silver nanoparticles with a diameter of about 10 nm, in agreement with the TEM analysis. The AgCl-LDH and Ag/AgCl-LDH nanocomposites, subjected to antimicrobial tests, exhibited good antimicrobial activity against both Gram-negative (Pseudomonas aeruginosa) and Gram-positive (Staphylococcus epidermidis and S. aureus) bacteria and yeast (Candida albicans). The nanocomposites were also studied for their ability to release silver by obtaining release curves, under conditions of antibacterial assays. Finally, the nanocomposites antibacterial behavior, as a function of time, was investigated by performing time-kill experiments using S. aureus and Candida albicans.

Preparation and characterisation of hydrotalcite/carboxyadamantane intercalation compounds as fillers of polymeric nanocomposites

Mono- and di-carboxyadamantane derivatives were intercalated via ion-exchange procedure onto hydrotalcites with formula [Zn0.65Al0.35(OH)2](A)0.35·nH2O where A is Cl– or ClO4–. Furthermore, an intercalation compound of [Mg0.65Al0.35(OH)2](CO3)0.175·0.5H2O containing the mono-carboxyadamantane has been obtained by using the “memory effect” of the hydrotalcite. The obtained materials were characterised for their composition, thermal behaviour, X-ray diffraction patterns, FT-IR absorption spectra and BET specific surface areas. Structural and chemical considerations indicated that mono-carboxylic guests are accommodated in the interlayer region as an ordered bifilm of anions while the di-carboxylic guests give rise to a bifilm of partially interdigitated anions. The presence of hydrophobic anions makes the interlayer region of the hydrotalcites compatible with hydrophobic polymers, such as medium density polyethylene. As an example, the preparation and characterisation of a nanocomposite of polyethylene filled with MgAl hydrotalcite/carboxyadamantane are reported. The nanocomposite, containing 5% w/w of inorganic residue, was prepared by melt compounding and was characterised with X-ray powder diffraction and thermogravimetric techniques. The obtained results showed that the presence of the organically modified hydrotalcite lamellae shields in part the polymer from thermal oxidation, increasing the degradation temperature by about 160 °C and producing a char residue greater than pure polyethylene.

On the Intercalation of the Iodine−Iodide Couple on Layered Double Hydroxides with Different Particle Sizes

Molecular iodine was intercalated from nonaqueous solution into microsized ZnAl-layered double hydroxide (LDH) in the iodide form, generating the I(3)(-)/I(-) redox couple into the interlayer region. Chloroform, ethanol, acetonitrile, or diethyl ether were used as solvents to dissolve the molecular iodine. The intercalation compounds were characterized by thermogravimetric analysis, X-ray powder diffraction, UV-vis spectroscopy, and scanning and transmission electron microscopy. The stability of iodine-solvent adducts and the iodine concentration affected the LDH iodine loading, and samples with I(2)/I(-) molar ratio ranging from 0.14 to 0.82 were prepared. Nanosized, well dispersible LDH, synthesized by the urea method in water-ethylene glycol media, were also prepared and successfully functionalized with the I(3)(-)/I(-) redox couple applying the conditions optimized for the micrometric systems.

Unexpected chromogenic properties of 1,3,3-trimethylspiro(indoline-2,3′-[3H]naphtho [2,1-b][1,4]oxazine) in the solid phase: photochromism, piezochromism and acidichromism

New chromogenic properties of a spirooxazine {1,3,3-trimethylspiro(indoline-2,3′-[3H]naphtho[2,1-b][1,4]oxazine), SO} are presented in this work. This molecule was found to be photochromic and piezochromic in the pure microcrystalline solid phase and also exhibited acidichromism when kept in contact with a hydrogen-donating solid phase. Photochromism was observed under continuous UV irradiation. Biexponential thermal bleaching followed the interruption of irradiation. Piezochromism was detected by applying a mechanical force to the solid powder, using either a hydraulic press or manual crushing. After discontinuing pressure, biexponential bleaching kinetics were observed. The biexponential nature of the fading processes is interpreted in terms of merocyanine (M) micro-spot formation inside the crystalline phase. The third chromogenic property of the solid spirooxazine, acidichromism, is due to the extraction of protons from an acidic solid matrix, alpha zirconium phosphate [α-Zr(HPO4)2·H2O]. Proton transfer to SO in the solid phase causes the appearance of a purple colour. The spectral evolution is qualitatively similar to that observed in CH3CN solution when HClO4 is added at room temperature and in the dark. Temporal evolution of the colour bands, in both solid and liquid phases, is interpreted in terms of an initial protonation, inducing ring-opening, and a second protonation, leading to a colourless product.

Effect of gliclazide immobilization into layered double hydroxide on drug release.

This paper deals with a new hydrotalcite-like compound used as a matrix to improve dissolution rate of the poorly water-soluble drug gliclazide and to administer at the same time micro- and oligo-elements useful to improve insulin performance. Gliclazide is a second-generation sulfonylurea compound used in the treatment of type II diabetes mellitus. As a consequence of the poor water solubility, its absorption is limited. Thus, a new hydrotalcite-like compound containing Zinc and Chromium, micronutrients directly involved in the physiology of insulin and in the carbohydrate, lipid and protein metabolism, was synthesized. The gliclazide-hydrotalcite-like clay nanohybrid was prepared via ion-exchange in its nitrate form and was characterized by inductively coupled plasma-optical emission spectrometry and thermogravimetric analysis. The drug loading resulted in 42.9% (w/w). As a consequence of the intercalation, the interlayer distance of the host increased from 0.89 nm (interlayer distance of nitrate form) to 1.5 nm. The intercalation product was submitted to solubility measurements and in vitro dissolution test. A remarkable improvement of the apparent solubility and dissolution rate in comparison to the crystalline drug was observed in acid fluids (pH 1.2 and 3). The presence of chromium and zinc cations was also found in the medium. These results showed that the hybrid nanostructure could represent a promising system to improve drug dissolution rate and to release cations involved in the performance of insulin.

Space-resolved fluorescence properties of phenolphthalein-hydrotalcite nanocomposites

Phenolphthalein has been included into Zn–Al hydrotalcite-like compounds by taking advantage of the “memory effect” of hydrotalcites. In particular [Zn0.65Al0.35(OH)2] (CO3)0.1750.5H2O was calcined at 500 °C and the obtained mixture of Zn and Al oxides was dispersed in an aqueous alcohol solution containing 7 × 10−2 mol dm−3 phenolphthalein. The reconstruction of the layered hydrotalcite structure in the presence of the dye yielded formation of the composites in which phenolphthalein is intercalated in the interlayer region and/or adsorbed on the layer surface. Two samples were studied, the first, obtained at 25 °C, contained 0.17 mmol g−1 of phenolphthalein, mainly adsorbed on the surface, the second, obtained at 60 °C, contained 0.3 mmol g−1, in part intercalated. The samples were characterized by their X-ray diffraction patterns, specific surface areas and scanning electron micrographs. The photophysical characterisation of the bulk samples was based on the determination of their reflectance absorption and fluorescence spectra, the fluorescence lifetimes and fluorescence anisotropy. The results indicated different properties of the lowest singlet excited state of the adsorbed and intercalated dye. The space-resolved fluorescence images and fluorescence spectra obtained by confocal fluorescence spectroscopy of the two samples gave valuable information on the dye distribution and on the nature of the interactions between the dye and the inorganic matrix.