Preparation of Spherical and Balloonlike Calcium Phosphate Particles

Mar 19, 2010 - The effects of addition of HCl on the formation of monodispersed spherical calcium hydroxyapatite (Hap) particles from the forced hydro...
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Preparation of Spherical and Balloonlike Calcium Phosphate Particles from Forced Hydrolysis of Ca(OH)2-Triphosphate Solution and Their Adsorption Selectivity of Water Kazuhiko Kandori,*,† Yuka Noguchi,† Masao Fukusumi,‡ and Yoshiaki Morisada‡ School of Chemistry, Osaka UniVersity of Education, Asahigaoka 4-698-1, Kashiwara-shi Osaka 582-8582, Japan and Department of Processing Technology, Osaka Municipal Technical Research Institute, 1-6-50 Morinomiya, Joto-ku, Osaka 536-8553, Japan ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: March 04, 2010

The effects of addition of HCl on the formation of monodispersed spherical calcium hydroxyapatite (Hap) particles from the forced hydrolysis reaction of Ca(OH)2-triphosphate (tripolyphosphate, tpp: P3O105-) mixed solution were investigated. The fairly uniform spherical particles were formed at concentration of HCl (abbreviated as [HCl]) of 8-12 mM, but the particles with various shapes such as very small rodlike and/or large thin platelike were produced at high [HCl] region. These fairly uniform spherical particles produced only at [HCl] ) 8-12 mM where the solution exhibited almost neutral pH of 6.89-7.24 after aging. The XRD patterns of the spherical particles as prepared were amorphous but they crystallized to β-Ca2P2O7 after the calcining the samples above 600 °C in air. The large amounts of H2O molecules were adsorbed on all the particles after evacuating the samples at 25 °C though they decreased drastically after evacuating the samples above 100 °C, except for the particles precipitated with a high concentration of HCl ([HCl] g 16 mM). The high selective adsorption of H2O was achieved for those amorphous spherical particles and was explained by rehydration of the particles by adsorbed H2O. The cavities formed near the cations by evacuation serve for H2O adsorption but not for N2. The time-resolved TEM experiment revealed that polydispersed spherical particles ranging from 100-300 nm in diameter are formed after aging the solutions for 3-5 h by aggregation of the primary nanometer-sized particles, produced at the first step of the reaction within 2 h, with incorporating calcium and pyrophosphate ions. The calcium and pyrophosphate ions were further deposited on these polydispersed spherical particles and grew to fairly uniform spherical particles with ca. 600-700 nm in diameter. The transparent balloonlike hollow spheres were also formed at 120-140 °C together with solid spheres. This temperature region for producing balloonlike hollow spheres is much wider than those for the system without HCl. This fact was explained by the high acidity of the present system with HCl to dissolve the inside of the solid spheres. Since the inner substances of hollow spheres to penetrate H2O molecules were dissolved, the balloonlike hollow spheres exhibited low adsorption selectivity of H2O. 1. Introduction Various kinds of metal phosphate have many applications of uses in pigments, catalysis, adsorbents, and bioceramics.1,2 To develop the materials into high-quality products, the studies on the preparation and characterization of uniform spherical metal phosphate particles have been extensively done.3-7 In the past decade, the authors’ group has investigated the inner structure of these uniform particles and found that cobalt phosphate particles possess a thermally unstable layer structure that exhibits a high selective adsorption of H2O by their molecular sieve effects,8 while nickel,9 aluminum,10-12 and ferric phosphate13 particles are agglomerates of the fine primary particles that exhibit a high mesoporosity. Recently, calcium phosphate particles such as spherical calcium hydroxyapatite (Ca10(PO4)6(OH)2, Hap) has been desired, because Hap is a major inorganic component of biological hard tissues14-17 and used as a row materials of artificial bone and an adsorbent for liquid chromatography. However, controlling the particle shape and size of Hap is very difficult because of its low-solubility product. In our previous study, we investigated the formation * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-729-78-3395. † Osaka University of Education. ‡ Osaka Municipal Technical Research Institute.

of calcium phosphate particles by a forced hydrolysis reaction of CaCl2-triphosphate (tripolyphosphate, tpp: P3O105-) mixed solution,18 because phosphate (orthophosphate: PO43-) ions can be expected to produce slowly with heating the solution by utilizing its endothermic reaction. In the previous study, however, a single phase of monodispersed spherical Hap particles could not be produced, the amorphous spherical particles were precipitated together with a lot of small rodlike particles, but the authors incidentally produced transparent balloonlike hollow spheres by raising the temperature for producing these nearly spherical particles. In addition, from the chemical analysis chemical composition of these spherical particles was expressed as β-Ca2P2O7, indicating that the hydrolysis of tpp was not sufficient to supply orthophosphate ions but to produce diphosphate (pyrophosphate, pp: P2O74-). More recently, we also examined Ca(OH)2-tpp system (Ca(OH)2 was used instead of CaCl2) to prepare Hap because Ca(OH)2 can be expected to increase solution pH that is suitable for Hap formation.19 However, no spherical Hap particle was formed in the previous paper. Since it is known that the hydrolysis reaction of condensed phosphate ions is accelerated with acids such as hydrochloric acid (HCl),20-23 the addition of HCl in the solution is more suitable to produce Hap. Therefore, the authors investigated in the present study the effects of the

10.1021/jp100829f  2010 American Chemical Society Published on Web 03/19/2010

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Figure 1. TEM pictures of typical samples produced at various concentrations of HCl. Numbers in the bracket represent the Ca/P atomic ratio for each particle.

addition of HCl on the formation of monodispersed spherical Hap particles by using Ca(OH)2-tpp mixed solution employed in the previous study.19 The present study focused on the formation of spherical Hap particles, their adsorption selectivity of H2O, and their formation mechanism. The formation of a transparent balloonlike hollow sphere was also attempted. 2. Experimental Section 2.1. Materials. Following the method reported by the authors in the previous paper,18 we produced the calcium phosphate particles by aging a solution of 3.16 mM Ca(OH)2 and 10.0 mM sodium tpp with various concentration of HCl (abbreviated as [HCl]) ranging from 0 to 20 mM in a 20 cm3 Teflon-lined screw-capped Pyrex test tube. The mixed solution without HCl attained fairly monodispersed spherical particles as reported in a previous study. To facilitate the aging, the test tubes were allowed to stand for 18 h at various temperatures (100-150 °C) in an air turbulent oven. The standard aging temperature examined was 100 °C. The resulting particles were thoroughly washed by filtration with deionized and distilled water and finally dried in air-oven at 70 °C for 18 h. The solution pH before and after aging was monitored at room temperature. Guaranteed reagent grade chemicals from Wako Pure Chemical Co. Inc. (Osaka, Japan) were used without further purification. 2.2. Characterization. The resulting particles were characterized by transmission and scanning electron microscope (TEM, JEOL JEM-2100; SEM, JEOL JCM-5700), thermogravimetry and differential thermal analysis (TG-DTA, SII-220), Fourier transform infrared spectrophotometery embedded in KBr pellets (0.2 wt %) (FTIR, Nicolet Prote´ge´ 460), powder X-ray diffraction measurement (XRD, Rigaku Rad-RC), adsorption experiments of N2 and H2O.8-13 Adsorption isotherms of N2 were measured at liquid N2 temperature with an ordinary house built automatic volumetric flow method using a Baratron diaphragmtype manometer attaching a mass flow meter. Adsorption isotherms of H2O were also determined by using a gravimetric apparatus built in-house at 25 °C. The degrees of confidence on specific surface area measured by these apparatuses were within 2%. Prior to these N2 and H2O adsorption measurements, the samples were evacuated at 25-300 °C for 2 h. The chemical composition of the precipitated particles (concentration of calcium and phosphorus) was determined by dissolving a few milligrams in concentrated nitric acid. The solutions were diluted and the calcium and phosphorus concentrations were determined by an inductively coupled plasma atomic emission spectroscopy (ICP-AES, SII SPS3520UV-2).

Figure 2. The pH of the solutions before and after aging at various concentrations of HCl.

3. Results and Discussion 3.1. Particle Morphology and Crystal Form. Figure 1 displays the electron micrographs of the particles precipitated after aging the solution for 18 h at 100 °C with various [HCl] from 0-20 mM. The numbers in brackets in each micrograph represent the Ca/P atomic ratio of each particle assayed by the ICP-AES measurement. The fairly spherical particles with ca. 200-300 nm in diameter are precipitated after aging the solution in the absence of HCl. The precipitated particles change their shape to rodlike at [HCl] ) 2-4 mM. The fairly uniform spherical particles are formed at [HCl] ) 8-12 mM, but the particles with various shapes such as very small rodlike and/or large thin platelike are produced above [HCl] ) 16 mM. The solution pH before and after aging measured at room temperature were shown in Figure 2. It is reasonable that pH of the solution before aging (full circle symbols) are reduced from 12.20 to 5.8 by increase in [HCl]. The pH of the solution after aging (open circle symbols) are also reduce by increased at [HCl] e 14, though the solution pH after the reaction at [HCl] g 16 mM was slightly increased by aging at 100 °C. It is interesting that the particles with no spherical but various morphologies such as rodlike and platelike were produced at this high concentration region of HCl. The solution pH of 7.10-12.20 was dropped 0.4-2.4 unit by the reaction at 0 e [HCl] e 14 mM. It has reported that the hydrolysis of tpp ions is expressed as the following two reactions, 1 and 224

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Figure 3. XRD patterns of samples produced at [HCl] ) 10 mM.

P3O105- + 2H2O f 3PO43- + 4H+

(1)

P3O105- + H2O f P2O74- + PO43- + 2H+

(2)

Reaction 1 represents that tpp ions are completely hydrolyzed to orthophosphate ions, though tpp ions are not completely hydrolyzed and still remain pp ions in reaction 2. Since the solutions are almost neutral to alkaline after the reaction (6.43 e pH e 9.80), the protonated states of orthophosphate (HPO42-) and pyrophosphate (HP2O72-) ions were ignored in reactions 1 and 2. Both the reactions generate protons in any event. Therefore, the pH drop watched in this experiment at 0 e [HCl] e 14 mM proves that tpp ions were hydrolyzed. It should be emphasized that fairly uniform spherical particles produced only at [HCl] ) 8-12 mM were exhibiting almost neutral pH of 6.89-7.24 after aging. Figure 3 shows the XRD patterns of the typical particles precipitated with 10 mM HCl. The spherical particles as prepared are amorphous; unfortunately these are not Hap, but they crystallized to β-Ca2P2O7 after calcining the samples above 600 °C in air. All other samples produced at various HCl concentrations showed the same XRD pattern of Figure 3. The fact that Ca/P atomic ratios of the particles shown in Figure 1 exhibit almost unity strongly supports the formation of β-Ca2P2O7. Figure 3 also shows clearly the presence of a residual amorphous phase even after heating at 600 and 800 °C together with β-Ca2P2O7 phase because there are broad lines that can be recognized at 20-30 °C. However, the reason for this fact is obscure. From above discussion, the formation of β-Ca2P2O7 can be expressed as eq 3.

2P3O105- + 6Ca2+ + H2O f 3β-Ca2P2O7 + 2H+ (3) Hence we can obtained the yield of the reaction by using eq 3. The obtained yield values are plotted as a function of [HCl] in Figure 4. Clearly, the yield values rise and become constant ca. 90% up to [HCl] ) 8 mM, supporting that the hydrolysis reaction is accelerated by HCl. However, the tpp ions are still not hydrolyzed to PO43-ones even though the present system contains HCl. Figure 5 shows TG-DTA curves of the precipitates. The small exothermic peak can be recognized at 540-570 °C in the DTA curves on the samples produced at [HCl] ) 0-12 mM. These peaks can be also naturally identified as a crystallization to β-Ca2P2O7. In addition, a large endothermic peak appears at ca. 25-140 °C together with a large weight loss (ca. 8 wt.) in the TG curves. This endothermic peak and weight loss are due to dehydration of water molecules remained in poorly ordered amorphous particles and structural water. On the samples produced at [HCl ]g 16 mM, no large endothermic peak at 25-140 °C and no sharp exothermic one at 540-570 °C in the

Figure 4. Change of the yield of the reaction as a function of [HCl].

DTA curves can be seen, indicating that these particles have less amounts of structural water and crystallinity of the rodlike and/or platelike particles are slightly higher than spherical and large rodlike particles. Figure 6 shows FTIR spectra of the particles. The strong peaks at 567, 928, and 1142 cm-1 due to the pyrophosphate ions in amorphous particles can be recognized, especially for the spherical particles produced at [HCl] ) 10 and 12 mM, supporting the low crystallinity of these spherical particles. The small absorption peak due to the pyrophosphate ions at 728 cm-1 can be also recognized. On the other hand, the absorption peak at 1142 cm-1 for the particles produced at 16 and 20 mM HCl shifted to 1190 cm-1 due to the crystallized pyrophosphate groups, suggesting again their higher crystallinity rather than spherical ones. 3.1.1. AdsorptiWe Properties. We found in previous works that the AlPO411 and Ni3(PO4)29 particles exhibit a high selective adsorption of H2O though they adsorb small amounts of N2. Since the calcium phosphate particles prepared in this work contain a large amount of hydrated H2O molecules as discussed before, it can be expected that similar characteristic adsorptive property will emerge. To reveal this point, we measured N2 and H2O adsorption isotherms for the samples prepared at varied [HCl] as shown in Figure 1. All the N2 adsorption isotherms for the particles outgassed at 25 °C were of Type II in the BDDT classification25 and they did not change significantly by changing the evacuation temperature from 25 to 300 °C (data not shown). The specific surface area (Sn) of the samples, calculated by applying the BET equation to these N2 adsorption isotherms using a cross-sectional area of N2 molecule of 0.162 nm2. The Sn values were decreased from 30 to 8 m2/g with increase in the [HCl] up to 12 mM, but they steeply increased to ca. 140 m2/g above 16 mM HCl where produce small rodlike and/or platelike particles. The small Sn values at 8-12 mM HCl supports the large particles with spherical shape. All H2O adsorption isotherms for these particles were also Type II in the BDDT classification as well as N2 adsorption (data not shown). The large amounts of H2O molecules adsorbed on all the particles precipitated either with and without HCl after evacuated at 25 °C though they decreased drastically after evacuated above 100 °C, except for the particles precipitated with a high concentration of HCl ([HCl] g 16 mM.) To clarify the adsorption selectivity of H2O, the monolayer adsorption capacity of H2O molecules per unit surface area (nw) of the samples was calculated and plotted as a function of aging temperature in Figure 7. The nw value of the particles produced at 0 mM HCl is 67.0 which is much larger than 9.3 molecules/ nm2 of the theoretical nw estimated using the 0.108 nm2 as the

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Figure 5. TG and DTA curves of samples produced at various concentrations of HCl.

Figure 8. Time-resolved TEM pictures of precipitates for producing spherical particles at [HCl] ) 10 mM. Numbers in the bracket represent the Ca/P atomic ratio for each particle. Figure 6. FTIR spectrum of samples produced at various concentrations of HCl.

Figure 7. Change of nw as a function of [HCl].

cross-sectional area of a H2O molecule. The nw is increased to 156 by an increase in [HCl] up to 8 mM, where it attains the spherical particles, but they are decreased steeply to 12.6 at [HCl] g 14 mM where it gave small rodlike and/or thin platelike particles, indicating that the high selective adsorption of H2O is achieved for those amorphous spherical particles. The similar high adsorption selectivity of H2O was observed on amorphous AlPO4 particles11,12 that was explained by rehydration of the particles by adsorbed H2O. A similar fact was reported in the case of chromium gel prepared from chromium nitrate solution utilizing the slow hydrolysis of urea.26 The hydrogels, freshly formed from highly hydrated Ca2+ ions in the present study, are usually poorly ordered and retain considerable amounts of hydrated H2O. These hydrated H2O molecules are removed by

outgassing without an appreciable structural change or collapse of the solid framework. The cavities formed near the cations by outgassing serve for H2O adsorption but not for N2. Thus, the rehydration of Ca2+ ions by H2O penetrated in the cavities may take place due to the coordination of adsorbed H2O to Ca2+ ions. Indeed, the adsorption selectivity of H2O reduced by crystallization of particles after elevating the evacuation temperature above 100 °C. Hence, it is reasonable that the spherical particles exhibit high adsorption selectivity of H2O because these spherical particles possess small Sn values. This high selective adsorption of H2O suggests that the amorphous calcium phosphate particles can be applied for adsorbents. 3.2. Formation Mechanism of Spherical Particles. To disclose the formation mechanism of spherical particles, the time-resolved TEM experiment was done at [HCl] ) 10 mM. The time-resolved TEM pictures are displayed in Figure 8 together with their Ca/P atomic ratios assayed. Clearly, very small nanometer sized particles are precipitated after aging for 2 h, though they suddenly changed to polydispersed spherical particles ranging from 100-300 nm in diameter after being aged for 4-5 h. After further aging for 10 h, the highly uniform spherical particles are observed. Figure 9 displays the change of calcium, [Ca], and phosphorus, [P], concentrations in the aging solution for producing uniform spherical particles as is shown in Figure 8. Clearly, it can be seen three steps in their curves, that is, (1) 0-3, (2) 3-5, and (3) over 6 h. Compared to TEM pictures in Figure 8, it can be discussed as follows. At step (1), very small nanometer-sized primary particles were produced at the first step of the reaction. Following step (1), polydispersed spherical particles ranging from 100-300 nm in diameter are formed by aggregation of the primary nanometersize particles by incorporating calcium and pyrophosphate ions at step (2). Finally, the calcium and pyrophosphate ions were deposited on these polydispersed spherical particles and grew to fairly uniform spherical particles with ca. 600-700 nm in

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Figure 9. Changes of concentrations of phosphorus and calcium ions in the aging solution for producing spherical particles at [HCl] ) 10 mM.

Figure 11. The change of nw of the samples in Figure 10 as a function of their aging temperatures.

diameter. Indeed, the Ca/P atomic ratio of these particles assayed as shown in Figure 7 are closed to unity, indicating the formation of β-Ca2P2O7. However, the reason for the formation of fairly uniform spherical particles is still obscure at the present. 3.3. Effects of Forced Hydrolysis Temperature. In their previous paper,18 the authors attempted to raise the aging temperature from 100 to 150 °C to increase the degree of hydrolysis of tpp ions under the condition for producing fairly uniform spherical particles. The authors incidentally found that the aging temperature of 125 °C supplies transparent balloonlike hollow spheres. Therefore, we investigated again in the present study under the condition for producing solid spherical particles at [HCl] ) 10 mM. Figure 10 shows the TEM pictures of the particles formed at various aging temperatures. Clearly, the solid spherical particles are produced up to 110 °C. However, transparent balloonlike hollow spheres can be recognized at 120-140 °C together with solid spheres. The wall thickness of the balloonlike hollow spheres is approximately 20 nm (measured from TEM micrographs). However, no balloonlike hollow sphere but aggregated needlelike particles is recognized after aged at 150 °C. As the authors already reported in a previous paper, a balloonlike hollow sphere is formed by dissolving the inside of the solid spherical particles. Since tpp and pp ions have very strong adsorption ability to the Hap surface as the authors already revealed before,27 the unhydrolyzed tpp and also pp ions of hydrolysis product of tpp ions were adsorbed on the solid

spherical particles initially produced. The adsorption of tpp and pp ions may protect the dissolution of the particles’ shell and turns out the balloonlike structure. This mechanism was further identified by the SEM measurement. The partly hollow spheres and hollow spheres with a part of the surfaces disappeared are observed together with solid undissolved spherical ones on the SEM pictures of the particles produced at 130 °C as displays in Figure 10, proving that the balloonlike hollow spheres were produced by dissolving inside of the particles. It should be noted here that the temperature region for producing balloonlike hollow spheres are much wider than those for the system without HCl as reported before (125-127 °C). This fact is due to the high acidity of the present system with HCl to dissolve the inside of the solid spheres. The specific surface area of the hollowlike particles produced at 140 °C measured after evacuated the particles at 25 °C for 2 h by N2 adsorption measurement was 56 m2/g, fairly coinciding with a calculated specific surface area of ca. 63.3 m2/g in the hollow spheres with an outer diameter (φ1) of 647 nm and an inside diameter (φ2) of 627 nm (the theoritical density F of Hap is 3.16 g/cm3, Sn ) [6(φ21 + φ22)]/ (φ31 - φ32)).28 This coincidence of Sn value strongly supports that balloonlike particles have hollow structure. Slightly small experimental Sn value is attributed to the coexisting solid spheres. Figure 11 plots the change of nw value of the samples as was shown in Figure 10 as a function of their aging temperatures. The high nw value(119) exhibits for solid particles formed at 100 °C suggesting high adsorption selectivity of H2O

Figure 10. TEM pictures of precipitates for producing spherical solid particles at various aging temperatures. [HCl] ) 10 mM. The bottom picture is a SEM micrograph of samples produced at 130 °C.

Spherical and Balloonlike Calcium Phosphate Particles as reported in a previous section, but they are depressed by raising the aging temperature by formation of the balloonlike hollow spheres and finally no selective adsorption property can be observed at 150 °C. This is naturally attributed to the lack of inner substances of hollow spheres to penetrate H2O molecules. 4. Conclusions (1) The fairly uniform spherical and amorphous particles were produced from the forced hydrolysis reaction of Ca(OH)2-tpp mixed solution in the presence of HCl. The fairly uniform spherical particles are formed at [HCl]) ) 8-12 mM, but the particles with various shapes such as very small rodlike and/or large thin platelike are produced at a high [HCl] region. (2) These fairly uniform spherical and amorphous particles produced only at neutral pH of 6.89-7.24 after aging. (3) The XRD patterns of the spherical and amorphous particles were crystallized to β-Ca2P2O7 after calcined the samples above 600 °C in air. (4) The high adsorption selectivity of H2O was observed for those amorphous spherical particles by rehydration of the particles. (5) The spherical particles were formed by aggregation of the primary nanometer sized particles, produced at the first step of the reaction, by incorporating calcium and pyrophosphate ions and grew to fairly uniform spherical particles with ca. 600-700 nm in diameter. 6) The transparent balloonlike hollow spheres were formed at 120-140 °C together with solid spheres. The balloonlike hollow spheres exhibited low adsorption selectivity of H2O. References and Notes (1) Susa, E. J. Polym. Sci. Part A: Polym. Chem. 1963, 399. (2) Klein, T.; Driessen, A. A.; de Groot, K.; van der Hoff, A. J. Biomed. Mater. Res., Part A 1983, 17, 769. (3) Katsanis, E.; Matijevic, E. Colloids Surf. 1982, 5, 43.

J. Phys. Chem. C, Vol. 114, No. 14, 2010 6445 (4) Wilhelmy, R. B.; Patel, R. C.; Matijevic, E. Inorg. Chem. 1985, 24, 3290. (5) Ishikawa, T.; Matijevic, E. J. Colloid Interface Sci. 1988, 123, 122. (6) Wilhelmy, R. B.; Matijevic, E. Colloids Surf. 1987, 22, 111. (7) Springsteen, I. I.; Matijevic, E. Colloids Polym. Sci. 1989, 267, 1007. (8) Kandori, K.; Toshioka, M.; Nakashima, H.; Ishikawa, T. Langmuir 1993, 9, 1031. (9) Kandori, K.; Nakashima, H.; Ishikawa, T. J. Colloid Interface Sci. 1993, 160, 499. (10) Kandori, K.; Imazato, T.; Yasukawa, A.; Ishikawa, T. Colloid Polym. Sci. 1996, 274, 290. (11) Kandori, K.; Ikeguchi, N.; Yasukawa, A.; Ishikawa, T. J. Colloid Interface Sci. 1996, 181, 425. (12) Kandori, K.; Ikeguchi, N.; Yasukawa, A.; Ishikawa, T. J. Colloid Interface Sci. 1998, 202, 369. (13) Kandori, K.; Kuwae, T.; Ishikawa, T. J. Colloid Interface Sci. 2006, 300, 225. (14) Kawasaki, T.; Takahashi, S.; Ikeda, K. Eur. J. Biochem. 1985, 152, 361. (15) Kawasaki, T.; Niikura, M.; Takahashi, S.; Kobayashi, K. Biochem. Int. 1986, 13, 969. (16) Kawasaki, T.; Ikeda, K.; Takahashi, S.; Kuboki, Y. Eur. J. Biochem. 1986, 155, 249. ¨ Arch. Biochem. Biophys. 1956, (17) Tiselius, A.; Hjerte´n, S.; Levin, O 65, 132. (18) Kandori, K.; Tada, K.; Fukusumi, M.; Morisada, Y. Bull. Chem. Soc. Jpn. 2008, 81, 1567. (19) Kandori, K.; Takeguchi, K.; Fukusumi, M.; Morisada, Y. Polyhedron 2009, 28, 3036. (20) Kura, G.; Tsukuda, T. Phos. Res. Bull. 1991, 1, 107. (21) Kura, G. Polyhedron 1991, 10, 697. (22) McGilvery, J. D.; Crowther, J. P. Can. J. Chem. 1954, 32, 174. (23) Mitra, R. P.; Thukral, B. R. Indian J. Chem. 1970, 8, 350. (24) Osterheld, R. K. In Topics in Phosphorus Chemistry; Barman, H., Ed.; John Wiley & Sons Inc: New York, 1972; Vol. 7, p 103. (25) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemienniewska, T. Pure Appl. Chem. 1985, 57, 603. (26) Balker, F. S.; Sing, K. S. W.; Stryker, L. J. Chem. Ind. 1970, 30, 718. (27) Kandori, K.; Oda, S.; Tsuyama, S. J. Phys. Chem. B 2008, 112, 2542. (28) He, T.; Chen, D.; Jias, X.; Xu, Y.; Gu, Y. Langmuir 2004, 20, 8404.

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