Dipole field guided orientated attachment of nanocrystals to twin-brush ZnO mesocrystals.

Mesocrystals of ZnO were synthesized hydrothermally by using gum arabic as a structure-directing agent. Their hierarchical structure has a unique twin-brush form consisting of vertically aligned nanorods in a single-crystal-like porous form. The formation mechanism of the twin-brush ZnO was investigated by quenching a series of samples at different times and examining them by TEM, SEM, and XRD. The alignment of ZnO crystal units can be modulated by adding simple salts such as KCl to change the units from nanorods to nanoplates. This can be explained by screening the dipolar force of the polar crystal. Local cathodoluminescence of twin-brush ZnO was used to follow the local structure changes.

Biomimetic ZnO plate twin-crystals periodical arrays.

We report a periodically patterned organization of hierarchical ZnO twin-crystals on a planar substrate in a biomimetic solution process. The deposition of the porous complex ZnO structure on specific sites of the substrate is achieved by introduction of the substrate surface modification and gelatin in a hydrothermal crystallization of ZnO.

Solid-state P-31 NMR study of octacalcium phosphate incorporated with succinate.

Octacalcium phosphate (OCP) is an important model compound in the study of biomineralization. The octacalcium phosphate-succinate (OCPS) compound is prepared and characterized by (31)P solid-state NMR spectroscopy. Taking advantage of the fact that the crystal structures of OCP and OCPS are very similar, an NMR strategy based on the (31)P homonuclear double-quantum spectroscopy is developed to assign all the peaks observed in the (31)P magic-angle spinning spectrum of OCPS. On the basis of our experimental data, the molecular formula of OCPS is determined to be Ca(7.81)(HPO(4))(1.82)(PO(4))(3.61)(succinate)(0.56).zH(2)O, where z

Adsorption of a statherin peptide fragment on the surface of nanocrystallites of hydroxyapatite.

Statherin is an active inhibitor of calcium phosphate precipitation in the oral cavity. For many studies of the interaction between statherin and hydroxyapatite (HAp), the samples are prepared by a direct mixing of statherin or its fragment with well-crystalline HAp crystals. In this work, the HAp sample is precipitated in the presence of peptide fragment derived from the N-terminal 15 amino acids of statherin (SN-15). The in situ prepared HAp crystallites are nanosized, leading to a significant increase of the peptide amount adsorbed on the HAp surface. The enhancement in NMR sensitivity allows, for the first time, the measurement of a two-dimensional 13C-13C correlation spectrum for a 13C uniformly labeled peptide sample adsorbed on mineral surface. The measurement time is about 18.5 h at a field strength of 7.05 T. Preliminary results suggest that there may exist two different mechanisms for the interaction between SN-15 and HAp. In addition to the one which will cause a conformational change near the N-terminal, SN-15 may also be absorbed on the HAp surface by simple electrostatic interaction, without any significant conformational changes of the peptides.

Solid-state P-31 NMR study of the formation of hydroxyapatite in the presence of glutaric acid.

We demonstrate that glutaric acid can be used to prepare nanorods of hydroxyapatite under hydrothermal condition at 100 degrees C with a Ca(2+):glutaric acid molar ratio of 1:4. Frequency-switched Lee-Goldburg irradiation is employed to obtain high-resolution (31)P{(1)H} correlation spectra of the reaction mixture at two different reaction periods, from which it is shown that octacalcium phosphate is the precursor phase of the final hydroxyapatite product. In addition, the spectra show that a substantial amount of water molecules is trapped between the glutaric acid and the hydroxyapatite surface, indicating that water molecules may play a prominent role in the noncovalent interaction of the glutaric acid and the HAp surface.

Double-quantum filtered heteronuclear correlation spectroscopy under magic angle spinning.

We present a simple experimental method to extract the van Vleck second moment of a multiple-spin system under high-resolution condition. The idea is to incorporate a double-quantum (DQ) filter into the pulse sequence of heteronuclear correlation spectroscopy so that a DQ excitation profile can be obtained by measuring a series of 2D spectra. The effects of spinning frequency and proton decoupling are demonstrated on the measurements of two model compounds, viz. hydroxyapatite and brushite. Based on the results obtained for the model compounds, the P-31 homonuclear second moment of the apatite component in rat dentin is characterized. The method is generally suited for the study of bone, enamel and dentin.

Solid-state NMR study of the transformation of octacalcium phosphate to hydroxyapatite: a mechanistic model for central dark line formation.

For many years, octacalcium phosphate (OCP) has been postulated as the precursor phase of biological apatite in bones and teeth. In this work, we study the molecular mechanism of OCP to hydroxyapatite (HAp) transformation in vitro by several physical techniques, with particular emphasis on solid-state (31)P homonuclear double-quantum (DQ) NMR spectroscopy. The in vitro system is prepared by mixing urea, sodium phosphate monobasic dehydrate, and calcium nitrate tetrahydrate at 100 degrees C. The images obtained by scanning electron microscopy and transmission electron microscopy show that the bladelike OCP crystals will transform into hexagonal rod-shaped HAp crystals as the pH of the reaction mixture increases slowly from 4.35 to 6.69 in 12 h. Powder X-ray diffraction patterns indicate that a trace amount of monetite was also precipitated when the pH was around 5. Together with computer-assisted lattice matching, our DQ NMR data reveal that OCP crystals transform to HAp topotaxially, where the [000](HAp) and [20](HAp) axes are along the same directions as the [001](OCP) and [010](OCP) axes, respectively. On the basis of our in vitro results, the formation of the central dark line commonly found in biological hard tissues could be explained by the inherent lattice mismatch between OCP and HAp. Furthermore, the data of the (31)P{(1)H} cross-polarization NMR suggest that water molecules enter the hydration layers of OCP crystals via the hydrolysis reaction HPO(4)(2)(-) + OH(-) = PO(4)(3)(-) + H(2)O, which also accounts for the deprotonation of the HPO(4)(2)(-) ions during the transformation.

Spectral editing based on selective excitation and Lee-Goldburg cross-polarization under magic angle spinning.

We show that a Gaussian-shaped pulse can be used to excite selected 1H signals in hydroxyapatite, monetite and H-Y zeolite loaded with trimethylphosphine oxide (TMPO). This selective excitation method can be incorporated into Lee-Goldburg (LG) cross-polarization to obtain useful spectral editing opportunity. This new strategy has been applied to identify the Brønsted and the Lewis acid sites in H-Y zeolite using TMPO as the probe molecule.

Double-quantum NMR spectroscopy based on finite pulse RFDR.

We demonstrate that the finite pulse RFDR sequence (J. Chem. Phys. 114 (2001) 8473) can be used effectively for 31P double-quantum NMR spectroscopy at a spinning frequency of 10 kHz. The 31P NMR data measured for hydroxyapatite and octacalcium phosphate show that sizable double-quantum excitation efficiency can be obtained with the ratio of the recoupling field to spinning frequency set equal to 1.67.

High resolution 31P NMR study of octacalcium phosphate.

We have assigned the (31)P high-resolution spectrum of octacalcium phosphate by (31)P double quantum and HETCOR spectroscopy. The (31)P peaks at -0.2, 2.0, 3.3 and 3.7 ppm are assigned to P5/P6, P3, P2/P4 and P1, respectively. Our data reveal that substantial amount of the PO(4)(3-) groups at the P2 and P4 sites have been transformed to HPO(4)(2-) in our octacalcium phosphate sample.