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Commercial interest is growing in biomimetic methods that employ self assembled mono-layers (SAMs) to produce biocompatible HA coatings on Ti-based orthopedic implants. Recently, separate studies have considered HA formation for various SAM surface functional groups. However, these have often neglected to verify crystallinity of the HA coating, which is essential for optimal bioactivity. Furthermore, differing experimental and analytical methods make performance comparisons difficult. This article investigates and evaluates HA formation for four of the most promising surface functional groups: --OH, --SO(3)H, --PO(4)H(2) and --COOH. All of them successfully formed a HA coating at Ca/P ratios between 1.49 and 1.62. However, only the --SO(3)H and --COOH end groups produced a predominantly crystalline HA. Furthermore, the --COOH end group yielded the thickest layer and possessed crystalline characteristics very similar to that of the human bone. The --COOH end group appears to provide the optimal SAM surface interface for nucleation and growth of biomimetic crystalline HA. Intriguingly, this finding may lend support to explanations elsewhere of why human bone sialoprotein is such a potent nucleator of HA and is attributed to the protein's glutamic acid-rich sequences.

We report a simple and facile hydrothermal pseudomorphic replacement route to synthesize three-dimensional (3D) ordered arrays of zeolite nanocrystals with uniform size and crystallographic orientation. We demonstrate this route by synthesizing analcime monoliths as an example using leucite crystals as precursors. The leucite crystals contain an inherent 3D ordered network of nanometer-sized lamellar twins. Such highly ordered 3D patterns were precisely preserved during hydrothermal pseudomorphic replacement reactions in pH buffered NaCl solutions, resulting in 3D ordered arrays of analcime nanocrystals. Moreover, these analcime nanocrystals have a uniform size and crystallographic orientation due to epitaxial nucleation and growth facilitated by the similarity of crystal lattice between leucite and analcime. The morphology of the nanocrystals is tunable by simply changing solution pH values. Mild acidic to mild alkaline conditions tend to produce cuboid-shaped nanocrystals, while strong alkaline conditions favor the formation of cylindrical-shaped nanocrystals. The replacement follows the coupled dissolution−reprecipitation mechanism that the rate of leucite dissolution equals the rate of analcime precipitation. This pseudomorphic replacement route has the potential to synthesize other ordered arrays of functional nanocrystals with controlled shape, size, and crystallographic orientation.

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A novel route for the synthesis of complex metal sulfides using hydrothermal coupled dissolutionreprecipitation reactions is reported. Two thiospinels, (Ni,Fe) 3 S 4 (violarite) and Co 3 S 4 (linnaeite), were synthesized using (Fe,Ni) 9 S 8 (pentlandite) and Co 9 S 8 (cobaltpentlandite) as precursors. The Fe/Ni ratio of (Ni,Fe) 3 S 4 can be adjusted by varying the reaction conditions, for example, temperature (125-145 °C), pH (2.90, 3.90, 5.00), and precursor stoichiometry ((Fe x Ni 1-x ) 9 S 8 , x ) 0.4, 0.5, 0.55, 0.6). Pure (Ni,Fe) 3 S 4 can be synthesized by utilizing a flow-through hydrothermal cell rather than a static hydrothermal cell as the fluid flow improves mass transfer and flushes away the Fe 2 O 3 byproduct from the reaction front. Synthesis times range from 10 to 20 days, compared to the traditional dry synthesis route for (Ni,Fe) 3 S 4 that requires 3 months annealing to obtain a product of only 72 ( 3 wt % purity. This synthesis route is ideal for preparing compounds with low thermal stabilities (<500 °C).

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