There exists a unique class of orthophosphates in which both the phosphorous and metal atoms are tetrahedrally coordinated by oxygen, therefore, to form metal phosphates. Various kinds of metal phosphate particles have many applications of uses in pigments, catalysis, adsorbents, and bioceramics. It is well known that calcium hydroxy apatite(Hap), a kind of calcium phosphate particles, also shows a high affinity for proteins and much attention has been focused on Hap and substituted Hap nanocrystals as sorbents. In this review, I will present (1) the preparation and characterization of monodispersed metal phosphate particles and (2) characteristics of protein adsorption onto various kinds of apatite particles based on our previous investigations.

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The recovery of aqueous phosphate from a model solution consisting of NH₄⁺, NO₂^- and NO₃^-, which are contained in a surplus that phosphate in rivers in Tokushima Prefecture, Japan, using boehmite together with the elution behaviors of phosphate from dephosphorization slag using various eluates were examined. The recovery of aqueous phosphate using boehmite was not influenced by the coexistence of those nitrogen-containing contaminants under various pH conditions. For the elution behaviors of phosphate from dephosphorization slag, it was found that aqueous HNO₃ among various acidic and basic solutions was a suitable solution as an eluate. Present study suggests that the employment of the unused resources such as aqueous phosphate in rivers and phosphate in the dephosphorization slag can be used as a possible candidate as a new raw material for rare phosphate.

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Aqueous ammonium is a threat to the vitality of various fish and the present study examined the feasibility of its removal from both seawater and freshwater using magnesium hydrogen phosphate (MgHPO₄). Regardless of the type of water, aqueous ammonium was favorably removed particularly under basic conditions such as a pH = 10. Even under neutral conditions (pH = 7), some aqueous ammonium can be removed. The results of the present study showed that MgHPO₄ can be used to remove a serious threat to the vitality of fish posed by the dilute concentration of aqueous ammonium in both seawater and freshwater. The resultant formation of magnesium ammonium phosphate (MAP) was detected by X-ray diffraction (XRD), which confirmed the removal of aqueous ammonium. It is common knowledge that MAP is a fertilizer for waterweeds, which are essential for the growth of the fish while simultaneously promoting their nourishment.

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Osteoblasts are susceptible to the surface characteristics of bioceramics and stimulation from outside the cells. The purpose of this study was to evaluate the effects of electrical polarization on surface characteristics and osteoblastic adhesion. The surface characteristics revealed that electrical polarization had no effect on surface roughness, crystallinity, and constituent elements. According to contact angle measurement, electrically polarized HA, which provides two kinds of surfaces, negatively charged HA (N-HA) and positively charged HA (P-HA), was even more hydrophilic than that of normal HA (HA). Morphological observations and quantitative analyses revealed that the typical adhered cells had a round shape on the HA but had a spindle or fan-like spreading configuration on the N-HA and the P-HA 1 h after seeding. After 3 h of cultivation, the rate of the number of spread cells and the size of focal adhesions on the HA increased and approached that of the N-HA and P-HA. However, the cell areas positively stained for actin, which indicates the degree of cell spreading, were distinctly larger on the N-HA and P-HA than that on the HA. The number of focal adhesions per cell was also less than that on the N-HA and P-HA.

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The dispersion of hydroxyapatite, Ca₅(PO₄)₃OH, HAP, in aqueous systems in the presence of variety of polymers of different composition and ionic charge has been investigated. The dispersion data show that polymer effectiveness as HAP dispersant strongly depends upon polymer architecture. It has been found that anionic, non-ionic, amphoteric, and cationic surfactants are ineffective as HAP dispersants. Additionally, it has also been observed that cationic surfactant and cationic polymer exhibit antagonistic effect on the performance of anionic polymeric dispersants.

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Hydroxyapatite (abbreviated to HAp), in general, adsorbs a polypeptide, resulting in a HAp-peptide complex. In the present paper the Hap was of a nanosized particle and peptide was poly-L-arginine (abbreviated to poly(Arg)) or copoly(L-arginine/ L-tryptophan = 4:1) (abbreviated to poly(Arg,Trp)). Physicochemical properties of these complexes were quite different with each other due mainly to the presence or absence of tryptophan residues on the polymer chain. The adsorption amount of poly(Arg,Trp) on HAp was much larger than that of poly(Arg). Zeta potential of the HAp-poly(Arg,Trp) complex was far lower than that of the HAp-poly(Arg) complex. Considering these facts, it was concluded that hydrophobic interaction between Trp residues of poly(Arg,Trp) induced an adsorption bilayer on HAp, while poly(Arg) simply formed an adsorption monolayer through electrostatic interaction. On the other hand, structural flexibility of a copolymer and its translocation through a lipid membrane were lower and slower in poly(Arg,Trp) than in poly(Arg).

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The preparation process was studied to obtain phosphate material with large specific surface area. Tetravalent cerium phosphate was prepared from ammonium cerium nitrate and phosphoric acid. In this system, as a solvent, water and ethanol were used, the solvent volume was limited, propionic acid and n-butylamine were added. These preparation conditions had much influence on the ratio of cerium/phosphorus. By the addition of n-butylamine, specific surface area of cerium phosphate became large. The adsorption ratio of basic stinking gas increased with the addition of propionic acid and n-butylamine in suspension condition.

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Transition metal phosphates are used as inorganic pigments, however these materials had a weak point for acid and base resistance. Because cerium phosphate is insoluble in acidic and basic solution, the addition of cerium was tried to improve the acid and base resistance of nickel phosphate pigment. The cerium doped nickel phosphates were prepared from phosphoric acid, nickel nitrate, and cerium nitrate solution. The additional effects of cerium cation were studied on the chemical composition, particle shape and size distribution, specific surface area, color, acid and base resistance of the precipitates and their thermal products.

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Phosphonylation of 3',4'-dihydroxy-L-phenylalanine (L-DOPA) has been achieved using inorganic diphosphonate (DP) in aqueous solution. The optimum condition for the phosphonylation of L-DOPA with DP is L-DOPA : DP = 1 : 10, pH 6 and 25 ^oC. 3'-Hydroxy-4'-phosphonyl-L-phenylalanine and 3'-phosphonyl-4'-hydroxy-L-phenylalanine were synthesized with the total yield of more than 62%. The phosphonylated products of L-DOPA were stable in neutral and acidic solution. The reaction mechanism of L-DOPA with DP was discussed.

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Li_1+xAl_xGe_2-x(PO₄)₃ (LAGP) solid electrolyte is one of the promising solid electrolytes for lithium ion batteries. The LAGP electrolyte was prepared by a sol-gel method. A gelation powder was calcined at 500 ^oC to obtain precursor powder for LAGP. The obtained powder was pelletized and calcined at 800, 850, and 900 ^oC. The pellets possessed NASICON structure, indicating that the LAGP was successfully prepared. The pellet calcined at 850 ^oC showed the highest lithium ion conductivity, 1.4 × 10^-4 S cm^-1, which is acceptable for all-solid-state lithium battery. Although some modification is still needed to improve the lithium ion conductivity of LAGP prepared by the sol-gel method, this finding provides us a new route for LAGP preparation.

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Hardening of β-tricalcium phosphate (β-TCP) mixed with a 2.5wt% Na₂HPO₄ aqueous solution was investigated by a compressive strength test, XRD, SEM, and calorimetry. The β-TCP starting material was ground using a mortar grinder for 7, 24, and 72 h in an air atmosphere. Compressive strength for β-TCP cement ground for 24 h was stronger than that ground for 7 h and 72 h, and was 42.8±4.1 MPa after soaking in saline for 7 days. It was found that β-TCP requires rapid hydrolysis to harden β-TCP cement. The Na₂HPO₄ aqueous solution facilitates this hydrolysis reaction. The compressive strength of β-TCP cement depends on the length and the number of acicular apatite crystals deposited from β-TCP.

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In order to obtain UV-shielding materials with good comfort, higher safety and effective UV-shielding ability, plate-like cerium phosphate Ce₂(PO₄)₂HPO₄·H₂O (CePHPH)/calcia-doped ceria (Ce_0.8Ca_0.2O_1.8, donated as: CDC) composites were synthesized by a precipitation method. UV-shielding ability and oxidation activity of as-obtained cerium phosphate/CDC composites were characterized. As a result, broad-spectrum UV-shielding composite materials with low oxidation catalytic activity were successfully synthesized. After calcination at 700^oC, the morphology of the composite did not change, while the oxidation catalytic activity decreased.

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Carbonate-containing hydroxyapatite (CHA) particles were synthesized by the hydrothermal treatment of calcium carbonates with phosphate-containing solutions. The influence of synthesis conditions in the hydrothermal synthesis of CHA from calcium carbonate on the morphology and composition has been investigated. Morphological changes in CHA particles were observed depending on the crystal phase and size of starting materials and the pH of solutions. The character of the starting material and pH of the treatment solution were highly important factors to control the morphology and composition of the synthesized CHA.

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The fabrication conditions of porous hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂; HAp) ceramics were investigated using the hollow spherical agglomerates prepared by spray pyrolysis. The starting HAp powder prepared by the spray pyrolysis of 0.50 mol·dm-3 calcium nitrate (Ca(NO₃)₂) and 0.30 mol·dm-3 diammonium hydrogenphosphate ((NH₄)₂HPO₄) solutions at 600°C consisted of hollow spherical agglomerates with diameters ranging from 5 to 15 μm. Prior to firing, the cylindrical compact was fabricated using the HAp powder through the following procedures: (i) the compaction through the "uniaxial pressing" of HAp powder at 20 - 50 MPa, and (ii) the compaction through the "suction filtering" of aqueous slurry containing HAp powder. The resulting HAp compact was fired at a temperature between 1000 and 1300°C for 5 h in air. After firing at 1000°C for 5 h, the maximum porosity of HAp body was 77% (suction filtering technique) › 53% (uniaxial pressing technique). Pore sizes of the porous HAp bodies fabricated using suction filtering were distributed over the range of 1 to 5 μm, and the compressive strength was 1.2 MPa. The releasing behavior of gentamicin sulfate (GEN) loaded on the porous HAp bodies fabricated by firing the suction-filtered compact at 1150°C for 5 h showed that a large amount of GEN was released from the porous HAp body within 1 h in the phosphate buffered saline (i.e., the releasing from the surface of porous HAp body), the reduced releasing in the period of 1 to 7 h, and steady state in the period of 9 h to 14 d (i.e. the releasing of GEN from the insides of HAp body via the intergranular channels).

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Influence of Sr²⁺ on the formation and structure of calcium hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: CaHap) particles prepared by hydrolyzing the phenyl phosphate (C₆H₅PO₄H₂) using alkaline phosphatase (ALP) at 37°C was examined. Increasing the additional amount of Sr²⁺ increased the Sr content in the products and turned the crystal phase of the products as follow: CaHap → solid solution SrCaHap → Sr_1.7Ca_1.3(PO₄)₂ → (3Sr(HPO₄)₂·Sr₃(PO₄)₂)·2H₂O. All the phenyl phosphates were hydrolyzed by ALP at 37°C. The crystallinity and particle size of CaHap decreased with an increase of added Sr²⁺, meaning the inhibition of crystallization and particle growth. These facts suppose that incorporation of Sr²⁺ in animal organism suppresses the formation of hard tissues.

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