Massimo Gazzano

Ionic substitutions in calcium phosphates synthesized at low temperature.

Ionic substitutions have been proposed as a tool to improve the biological performance of calcium phosphate based materials. This review provides an overview of the recent results achieved on ion-substituted calcium phosphates prepared at low temperature, i.e. by direct synthesis in aqueous medium or through hydrolysis of more soluble calcium phosphates. Particular attention is focused on several ions, including Si, Sr, Mg, Zn and Mn, which are attracting increasing interest for their possible biological role, and on the recent trends and developments in the applications of ion-substituted calcium phosphates in the biomedical field.

High-Contrast Visualization of Graphene Oxide on Dye-Sensitized Glass, Quartz, and Silicon by Fluorescence Quenching

We present a novel approach for detecting and visualizing graphene oxide (GO) with high contrast on different substrates, including glass, quartz, and silicon. Visualization of GO sheets is accomplished through quenching the fluorescence of a thiophene dye, giving high optical contrast without the need to use interference methods. A comparison of fluorescence, AFM, and XRD measurements confirmed that even a single GO sheet can completely quench the fluorescence and thus be quickly visualized.

Magnesium influence on hydroxyapatite crystallization

Abstract X-ray diffraction, infrared absorption, and chemical investigations have been carried out on hydroxyapatite synthesized in the presence of different magnesium concentrations in solution. Magnesium inhibits the crystallization of hydroxyapatite through a reduction of Ca/P molar ratio and crystal sizes of apatite. The reduction of the crystal sizes is also very great for very low magnesium content and increases on increasing magnesium concentration in solution up to 35 Mg atom percent with respect to the total metal ions. The samples are completely amorphous between 35 and 50 Mg atom percent. For higher magnesium concentration different crystalline phases are formed. The results of the x-ray powder pattern fitting indicate that the HA crystal structure at most hosts magnesium amounts of about seven percent. Magnesium substitutes only the calcium atoms which form the channels containing the hydroxy ions. Since magnesium content is much smaller than that found in the solid phase, the greatest amount of magnesium must not be lattice bound. The extent of hydroxyapatite conversion into magnesium substituted β-tricalcium phosphate on heat treatment appears strongly related to magnesium content of the apatitic solid phase. On the basis of these results, the key role of magnesium on the crystallization, crystal growth, and thermal stability of hydroxyapatite has been used to explain the relevant properties of biological apatites.

Inhibiting effect of zinc on hydroxylapatite crystallization

Abstract X-Ray diffraction and spectrophotometric analysis have been used to investigate the role of zinc on hydroxylapatite (HA) crystallization. The presence of zinc in solution strongly inhibits the crystallization of hydroxylapatite, which can be synthesized as a unique crystalline phase only up to zinc concentration of about 25 atom %. This phase exhibits a reduction of Ca/P molar ratio and crystal sizes with increasing zinc concentration. Although the Ca/Zn ratio in the solid phase is almost equivalent to that in solution, the values of the cell parameters of the apatitic phase indicate that zinc cannot appreciably substitute for calcium in HA structure. Therefore, zinc must be assumed to be adsorbed on the surface of apatite crystallites and/or in the amorphous phase. The extent of thermal conversion of HA into s-tricalcium phosphate (s-TCP) increases with increasing zinc concentration in the solid phase, either when it is obtained by means of synthesis in solution or after cyclic pH fluctuation. The decrease of the lattice constants of s-tricalcium phosphate with increasing zinc concentration in the solid phase indicates that zinc partially replaces calcium in this structure. The inhibiting effect of zinc on HA crystallization and its preference for s-TCP structure closely resembles the behavior previously observed for magnesium.

Isomorphous substitutions in β-tricalcium phosphate: The different effects of zinc and strontium

Abstract X-ray diffraction and infrared absorption analyses have been carried out on zinc-substituted and strontium-substituted β-tricalcium phosphate prepared by solid-state reaction. Zinc can substitute calcium up to 20 atom %, inducing a nonlinear variation of the lattice constants and an increase in degeneracy of the PO 4 3− infrared absorption bands. On the other hand, up to 80 atom % of strontium can enter into the crystal structure of β-tricalcium phosphate, causing a linear enlargement of the unit cell, in agreement with its greater ionic radius compared to that of calcium. Furthermore, strontium incorporation provokes the shift of the PO 4 3− absorption bands toward lower frequencies. On the basis of the data previously obtained on magnesium-substituted β-tricalcium phosphate, the different behaviours exhibited by zinc and strontium could be attributed to a different distribution into the cationic sites of the β-tricalcium phosphate structure. The results allow us to relate the effect of bivalent ions on the structure and relative stability of calcium phosphates with their ionic radius, and can be utilized to interpret the role of ionic composition on the properties of biological phosphates.

Interaction of Sr-doped hydroxyapatite nanocrystals with osteoclast and osteoblast-like cells.

This article reports the effect of strontium incorporation into hydroxyapatite nanocrystals on bone cells response. Hydroxyapatite nanocrystals were synthesized at strontium contents of 0, 1, 3, 7 atom %. Strontium incorporation for calcium is confirmed by the linear increase of the unit cell parameters of hydroxyapatite, in agreement with the different ionic radii of the two ions. Moreover, strontium substitution slightly affects hydroxyapatite structural order and the shape of the nanocrystals. Osteoblast-like MG63 cells cultured on the nanocrystals display good proliferation and increased values of the differentiation parameters. In particular, when cultured on samples with Sr concentration in the range 3-7 atom %, osteoblasts display increased values of ALP activity, collagen type I, and osteocalcin production. Moreover, the osteoclast number on all the Sr-doped samples is significantly smaller than on hydroxyapatite, and it decreases on increasing strontium content. The data indicate that strontium stimulates osteoblast activity and exerts its inhibitory effect on osteoclast proliferation even when incorporated into hydroxyapatite.

Oriented Crystallization of Vaterite in Collagenous Matrices

The influence of high super- saturation on kinetic control and the importance of the polypeptide structure in the crystallization of calcium carbo- nate polymorphs were studied in cross- linked gelatin films containing high concentrations of the polypeptides poly-l-aspartate and poly-l-glutamate. Oriented crystallization of vaterite oc- curs in uniaxially deformed gelatin films containing poly-l-aspartate at concenta- tions greater than 100 mg per gram of gelatin. The fact that no orientation of the mineral phase was observed with entrapped poly-l-glutamate at the same concentrations suggests that the orient- ed crystallization is controlled by the b sheet structure assumed by poly-l-as- partate in the presence of calcium ions. These results indicate that local super- saturation in the microenvironment in which nucleation and growth occur plays an important role in controlling the deposition of vaterite in cross-linked gelatin films. However, collagen bundles and the ordered and oriented polypep- tide chains of poly-l-aspartate can con- tribute to the control of polymorphism by inducing the formation of a specific phase by epitaxial crystallization, as suggested by the preferentially oriented deposition of vaterite and aragonite. This is of potential significance in bio- mineralization processes and in materi- als science.

Rietveld structure refinements of calcium hydroxylapatite containing magnesium

The crystal structures of four hydroxylapatite (HA) samples prepared from solutions in the presence of 10, 15, 25 and 30 Mg-atom-% have been investigated by X-ray powder pattern fitting. The total magnesium content of the solid samples, as determined by chemical analysis, was 4.9, 14.1, 20.4 and 30.6 Mg-atom-%, respectively. Rietveld analysis was performed using the computer program PREFIN implemented with routines which allow the refinements of the average crystallite sizes. Different refinement procedures were carried out in order to evaluate the effect of the amorphous and background profiles on the occupancy factor data. For comparison, magnesium-free hydroxylapatite was refined with the same strategies. The results of the different approaches indicate that the degree of magnesium substitution for calcium in the HA structure can be at most ~ 10 atom-%. Magnesium substitutes calcium preferentially at the 6(h) site. The broadening of the diffraction peaks increases on increasing the total magnesium content in the solid phase, which is always significantly higher than the amount incorporated into the HA structure. The excess is probably located in the amorphous phase and/or on the crystallite surface.

Ultrasound Stimulated Nucleation and Growth of a Dye Assembly into Extended Gel Nanostructures

A squaraine dye functionalized with a bulky trialkoxy phenyl moiety through a flexible diamide linkage (GA-SQ) capable of undergoing self-assembly has been synthesized and fully characterized. Rapid cooling of a hot solution of GA-SQ to 0 °C results in self-assembled precipitates consisting of two types of nanostructures, rings and ill-defined short fibers. The application of ultrasound modifies the conditions for the supersaturation-mediated nucleation, generating only one kind of nuclei and prompting the formation of crystalline fibrous structures, inducing gelation of solvent molecules. The unique self-assembling behavior of GA-SQ under ultrasound stimulus has been investigated in detail by using absorption, emission, FT-IR, XRD, SEM, AFM and TEM techniques. These studies reveal a nucleation growth mechanism of the self-assembled material, an aspect rarely scrutinized in the area of sonication-induced gelation. Furthermore, in order to probe the effects of nanoscale substrates on the sonication-induced self-assembly, a minuscule amount of single-walled carbon nanotubes was added, which leads to acceleration of the self-assembly through a heterogeneous nucleation process that ultimately affords a supramolecular gel with nanotape-like morphology. This study demonstrates that self-assembly of functional dyes can be judiciously manipulated by an external stimulus and can be further controlled by the addition of carbon nanotubes.

Microstructural investigation of hydroxyapatite–polyelectrolyte composites

Hydroxyapatite–poly-L-aspartic acid (HA-PASP) and hydroxyapatite–polyacrylic acid (HA-PAA) composite crystals have been prepared by direct synthesis in aqueous solution. The polyelectrolytes are quantitatively incorporated into the crystals up to about 8–9 wt%, as a function of their concentration in solution. The structural and morphological properties of the crystals vary as a function of polyelectrolyte content. TEM images show that the composite crystals display a greater length/width ratio with respect to the control HA crystals. The broadening of the X-ray diffraction reflections on increasing polyelectrolyte content was investigated using three different methods: (i) the Scherrer method, (ii) the Warren–Averbach approach, and (iii) a whole pattern analysis approach. Both polyelectrolytes induce a greater reduction of the mean crystallite size along a direction perpendicular to the c-axis direction, suggesting a preferential interaction of the polymers with the crystal faces parallel to the c-axis. PASP interaction with HA structure provokes a greater increase of strain in comparison to PAA. The data indicate that anionic polyelectrolytes can be usefully applied to modulate the structural and morphological properties of hydroxyapatite crystals.