J O U R N A L O F C H E M I S T R Y Materials Oriented growth of hydroxyapatite on (0001) textured titanium with functionalized self-assembled silane monolayer as template Chuanbin Mao,* Hengde Li, Fuzhai Cui, Qinglin Feng, Hao Wang and Chunlai Ma Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 18th February 1998, Accepted 19th August 1998 A highly (0001) textured hydroxyapatite [Ca10(PO4)6(OH)2, HA] coating on polycrystalline titanium plate is successfully synthesized by a biomimetic process mimicking biomineralization.To simulate the first stage of biomineralization, that is, supramolecular preorganization, a template surface with highly organized arrangement of carboxyl (KCOOH) and alcoholic hydroxyl (KOH) groups is prepared through self-assembly of vinyltriethoxysilane [(C2H5O)3SiCHLCH2, VTS] on hydroxylated titanium with strong (0001) texture, followed by oxidation of the vinyl groups (KCHLCH2) with dilute KMnO4 solution into alcoholic hydroxyl and then into carboxyl groups.The functionalized substrate can induce oriented nucleation and growth of HA with (0001) planes parallel to the substrate surface from supersaturated HA solution through interfacial molecular recognition.The mechanisms of molecular recognition are also discussed. is to highly texture the substrate before functionalization 1 Introduction through self-assembly, resulting in a large quantity of domains Biomineralization has been regarded as an excellent archetype with nearly the same arrangement of surface atoms.Then a for inorganic materials synthesis by materials chemists.1 Four surface with a large quantity of domains with nearly the same sequential stages are involved in the biomineralization includ- arrangement of functional groups may be prepared after ing supramolecular preorganization, interfacial molecular rec- chemisorption and self-assembly of functional molecules. That ognition, vectorial regulation and cellular processing.1 In is, an organized SAM can be fixed on a highly textured recent years, with these four stages as archetypes, a new substrate through a self-assembly process.synthetic strategy, termed biomimetic synthesis or template In this paper, we hope to demonstrate the above synthesis, has been developed to synthesize a variety of inor- consideration through biomimetic synthesis of an HA coating ganic materials such as nanoparticles,2 thin films,3 coatings,3 on polycrystalline titanium. The titanium is textured with porous materials4 and materials with complex forms.5 (0001) preferably parallel to the surface through rolling defor- Currently, biomimetic synthesis of inorganic materials through mation since the (0001) texture is a typical kind of rolling a process mimicking biomineralization has been a very deformation induced texture for hexagonal metals.8 The funcpromising approach to prepare materials of low cost.5 tionalized surface is prepared through initial hydroxylation To perform a successful biomimetic synthesis, it is most with H2O2 and subsequent self-assembly of vinyltriethoxysilane important to mimic the first stage of biomineralization (i.e., (VTS), followed by oxidation of vinyl groups with dilute supramolecular preorganization) where a highly organized KMnO4 solution into alcoholic hydroxyl groups and then into reaction template is built through self-assembly of an organic carboxyl groups.The principle of such a biomimetic process matrix.For synthesis of films or coatings on inorganic sub- is illustrated in Fig. 1. strates, a surface with organized functional groups is often prepared to act as reaction template through chemisorption 2 Experimental and self-assembly of functional organic molecules on the substrates.3,6,7 The functional molecules bond with the surface 2.1 Biomimetic synthesis atoms which are usually coordinatively unsaturated. Therefore, A supersaturated solution of HA ([Ca2+]=4mM) was prepared the arrangement of the functional organic molecules is often according to the procedure in ref. 9. A 2 mm thick titanium determined by the surface lattice. For a single crystal substrate, plate was prepared by rolling deformation to introduce the the surface to be functionalized is always a single plane (i.e., (0001) texture and was cut into small square pieces no grain boundaries and little variation in atomic arrangement).For a polycrystalline substrate, the arrangement of atoms usually diVers significantly between two exposed crystal grains unless the substrate surface is highly textured with one plane preferably parallel to the surface, therefore the arrangement of chemisorbed and self-assembled functional molecules is usually not uniform and organized across the whole surface.Hence, the successful biomimetic growth of inorganic films or coatings is usually realized on self-assembled monolayer (SAM) covered single crystals.6,7 However, practical films or coatings are usually on polycrystalline substrates, for example, the hydroxyapatite (HA) coating on titanium as biomedical bone implant.Therefore, it is necessary to find a way to obtain a highly organized functionalized surface on polycrystalline substrates. We consider that one eYcient way to increase the degree of Fig. 1 Principle of biomimetic synthesis of HA coating on titanium through a process mimicking biomineralization. organization of the arrangement of the functional molecules J.Mater. Chem., 1998, 8, 2795–2801 2795(1 cm×1 cm). The plates were metallographically polished with SiC emery paper to remove the oxide surface layer. The final polishing was performed with No. 800 paper and the final thickness of the polished plates was about 1 mm. The plates were ultrasonically washed in acetone for ca. 10 min and rinsed in deionized water for 1 min, and then placed in a 10% (by volume) solution of VTS in benzene for 2 weeks.After being rinsed in deionized water for several seconds, they were further aged in 5 mass% aqueous alkaline KMnO4 solution first at 0 °C for one week and then in 5 mass% aqueous KMnO4 solution containing sodium periodate (NaIO4) at room temperature for one week. The resulting plates are denoted as VTS-Ti.One VTS-Ti plate was put into 50 ml HA solution and aged for up to 2 months. For comparison, a 1 cm×1 cm×2 mm control titanium plate with poor (0001) texture was prepared by a diVerent rolling procedure,8 and subjected to the subsequent experiment as above. 2.2 Characterization Fig. 3 Time dependences of pH in VTS-Ti solution and control The changes of pH during aging were measured in situ with a solution.numeric acidity meter. The phase composition and orientation of the substrate and coating were determined by X-ray diVracnaked eye upon aging both titanium plates in HA solutions. tion (XRD) on a Rigaku D/max RD diVractometer with a In HA solution containing VTS-Ti (VTS-Ti solution), crys- Cu target.To observe the morphologies of the coating by tallization only occurs on the surface of VTS-Ti to form a scanning electron microscopy (SEM), a knife was used to cut continuous white layer, indicating that heterogeneous between the coating and the substrate, and then the coatings nucleation and growth of calcium phosphates is favored on were fractured to expose a fresh fractured cross section, VTS-Ti.However, in the HA solution containing control VTS- followed by coating with gold by ion beam sputtering. SEM Ti (control solution), crystallization occurs not only on the images were observed by a Hitachi S-450 scanning electron surface of control VTS-Ti to form several deposits of island- microscope coupled with an energy dispersive X-ray analysis like white precipitates, and at the air/solution and solution/ (EDAX) system.Functional groups were detected by X-ray beaker interfaces to form island-like white precipitates, but photoelectron spectroscopy (XPS) on an ESCALAB 220i-XL also in the bulk solution to form floating white precipitates. system with Mg-Ka source. In VTS-Ti solution, the white coating can be observed by the naked eye after about 2 days and grows thicker with aging for 3 Results about one week.Crystallization elsewhere was not observed for up to 2 months. In the control solution, the island-like 3.1 Coating formation precipitates did not form a continuous layer for up to 2 The XRD results in Fig. 2 indicate that the degree of (0001) months, which is consistent with the results of Hanawa and texture of VTS-Ti is much higher than that of control VTS- Ota.10 Ti through the comparison of the relative intensity ratio Crystallization of HA can be written in terms of eqn.(1) between (0001) and (10191) diVraction peaks. Much diVerent 10Ca2++6PO43-+2OH-=Ca10(PO4)6(OH)2 (1) crystallization of calcium phosphate was observed by the and reduction of pH can be an eVective indicator of HA crystallization on titanium or elsewhere in the bulk solution.Fig. 3 depicts the time dependences of pH values of the VTSTi solution and control solution. The pH of the VTS-Ti solution decreases after an incubation period due to the crystallization on VTS-Ti and levels oV at about 6.6 after about one week indicative of a typical nucleation and growth process. For the control solution, the pH begins to decrease a little later and then decreases more rapidly.It also levels oV at about 6.6 after ca. 3 days. 3.2 Coating characterization The XRD spectrum for VTS-Ti aged for 2 days as shown in Fig. 2(c) suggests that the HA crystals have nucleated and grown with (0001) preferably parallel to the substrate. After VTS-Ti has been aged for 7 days, i.e., when the coating on VTS-Ti no longer grows as judged from a constant solution pH (Fig. 3) the coating is composed of highly (0001) textured HA as shown in Fig. 2(d). Since only several small deposits of island-like precipitates occur on the control VTS-Ti, they can not be characterized by XRD and thus SEM-EDAX is adopted. Fig. 4 shows the SEM morphologies of the fractured cross-sections of the coating on VTS-Ti at diVerent stages.After being aged for 2 days [Fig. 4(a)], the coating on VTSTi is ca. 10 mm thick. Previous studies have shown that HA Fig. 2 XRD patterns of (a) control VTS-Ti; (b) VTS-Ti; (c) coating on VTS-Ti after 2 days and (d) coating on VTS-Ti after 7 days. grains always tend to grow along the [0001] direction, which 2796 J. Mater. Chem., 1998, 8, 2795–2801of plate-like HA grains which are ca. 10 mm wide. Since it is diYcult to make a fractured cross-section of one island, the length of the HA grains can not be determined. However, analogous to the grains on VTS-Ti, the length of HA grains may be ca. 40 mm. Most of the ball-like islands are much thicker than the coating on VTS-Ti. This is because there are several HA grains along the normal of the substrate.From the SEM images in Fig. 5(b), the plate-like HA grains have grown toward the solution preferably with [0001] parallel to the normal of the substrate. These results demonstrate our considerations in section 1, that is, a functionalized surface on the textured substrate can be an eVective template for biomimetic growth. 4 Discussion 4.1 Formation of an organized and hydroxylated surface After the old oxide surface layer is removed through polishing, a fresh oxide layer will quickly form on the plate. Fig. 6 shows the XPS Ti 2p spectrum of a titanium surface before treatment with H2O2. Curve-fitting revealed that the spectrum can be resolved into two peaks centered at 463.89 and 458.27 eV, corresponding to Ti 2p1/2 and Ti 2p3/2 in TiO2, respectively.12,13 It is thus evident that the surface oxide layer of titanium is Fig. 4 SEM morphologies of the fractured cross sections of coatings TiO2. Many researchers also found that TiO2 is the natural on VTS-Ti, after 2 (a), 4 (b) and 7 days (c). A top-view SEM oxide layer on the titanium at room temperature.13–15 Since morphology of the coating on VTS-Ti after 7 days is shown in (d).Regions A in (a) denote the crystallization area that is not on the thermodynamically stable form of TiO2 at low temperature SAM-(110)TiO2 domains. Note that the incident electron beam was is anatase (tetragonal, a=0.3783 nm, c=0.951 nm, JCPDS 4- tilted by 45° in (a), (b) and (c). 477),16 the surface oxide layer can be regarded as the anatase form.17 Recently, Azoulay et al.18 made a systematic investiresults in a plate habit, and the long axes of the plate-like gation on the interactions of oxygen with polycrystalline grains are parallel to the crystallographic [0001] direction.11 titanium surfaces at a very low pressure of O2.They found Thus, one can judge the type of preferred orientation by the that the initial accumulation of oxygen follows an island fashion of the grain arrangements shown in the SEM images.formation model, resulting in a patch-like pattern of oxide From Fig. 4(a), one can observe that the HA crystals have surface layer containing diVerent titanium valence states, and been oriented with [0001] perpendicular to the substrate, the oxide with better lattice match with underlying parent consistent with the XRD result in Fig. 2(c). After 4 days titanium structure will be favored to form first. Ti2O (hexag- [Fig. 4(b)], the coating is ca. 30 mm thick and the highly onal, a=0.29593 nm, c=0.4845 nm, JCPDS 11-218) shows (0001) textured state is very obvious in the SEM figure. After the best lattice match with titanium (a=0.2950 nm, c= 7 days [Fig. 4(c)], the coating is ca. 40 mm thick and consists 0.4686 nm, JCPDS 5-682) among all the titanium oxides and of highly textured plate-like HA grains, consistent with the can be directly transformed from titanium through the ordering XRD result in Fig. 2(d). From the top-view SEM morphology of the dissolved oxygen atoms (cf. notes on JCPDS 11-218). of the coating on VTS-Ti [Fig. 4(d)], one can see that the Electron microscopy also revealed that Ti2O can grow out of coating is nearly uniform across the whole surface.The above titanium through coherent transformation.19 In addition, Ti2O results suggest that HA crystals nucleate on VTS-Ti with has the lowest content of oxygen among all the titanium (0001) preferably parallel to the substrate and grow toward oxides.20 Consequently, the fresh surface oxide layer, which is the solution, giving rise to a textured coating with the thickness too thin to be detected by XRD, may be formed from Ti equal to the length of one HA crystal along the [0001] directly to TiO2, or through intermediate oxides with low direction. Fig. 5 shows the top-view SEM morphologies of the oxygen content, i.e., low chemical valence of titanium, such island-like precipitates on the control VTS-Ti after 7 days.as Ti2O, and finally to TiO2. The intermediate Ti2O will be This reveals that the precipitates consist of islands, which are also (0001) textured due to coherent transformation from significantly diVerent from the coating on VTS-Ti as shown (0001) textured titanium. Since there is a good lattice matching in Figs. 4(c) and (d).The islands are ball-like in shape and their size ranges between 50 and 150 mm. They are composed Fig. 5 Top-view SEM morphologies of island-like precipitates on Fig. 6 Curve-fitting of the Ti 2p XPS spectrum for titanium before control substrate after aging for one week: (a) whole morphology and (b) one island at higher magnification. H2O2-treatment. J. Mater. Chem., 1998, 8, 2795–2801 2797Fig. 7 Schematic illustration of the lattice match between (0001)Ti and Fig. 8 Schematic illustration of formation and functionalization of a (110)anatase. The dashed and bold line frames show the outlines of the self-assembled silane monolayer. unit cells of Ti along [0001] and TiO2 along [110], respectively. To demonstrate the formation of functionalized SAM, VTS- between (0001)Ti or (0001)Ti2O and (110)anatase (for structure Ti was subjected to XPS characterizations.Fig. 9 shows the O of anatase see ref. 16) as illustrated in Fig. 7, the transformed 1s XPS spectrum of VTS-Ti. Curve-fitting revealed three peaks TiO2 will be (110) textured due to the (0001) texture of with binding energies of 533.04, 531.32 and 529.65 eV, corre- underlying titanium and may be why the texture of substrate sponding to O 1s electrons in KOKSi(O)KOK before SAM-functionalization plays an important role in biom- Si(O)KOK( like SiO2), KCOOH and KOH, and surface oxide imetic mineralization of HA.It should be stated that the lattice, respectively.12 It is worthwhile to point out that due process of surface oxidation on titanium in air has scarcely to adsorption of some other phases containing oxygen such been studied perhaps because the oxidation from Ti to TiO2 as H2O and KMnO4, their contribution in O 1s photoemission is very quick and TiO2 is always the main detected oxide.13–15 peaks may be overlapped and cannot be resolved from the During subsequent aging in H2O2 solution, a surface interabove species.For example, the O 1s binding energy in KMnO4 action will occur between the fresh oxide layer formed on the is very similar to that in KCOOH.12 Therefore, the C 1s XPS titanium and H2O2 to form active hydroxyl groups.21,22 The spectrum (Fig. 10) may be more suitable to characterize the peroxide ion (O22-) can be regarded as an active state of O2.functionalized SAM. Curve-fitting showed three peaks with Its electron configuration is sls2, s*ls2, s2s2, s*2s2, s2px2, binding energies of 288.24, 286.02 and 284.63 eV, which corre- p2py2, p2pz2, p*2py2, p*2pz2, s*2px0 where s and p refer to spond to carbon in KCOOH, KCH(OH)CH2OH, KCHLCH2 bond-forming orbitals, and s* and p* to antibonding orbitals, respectively and there is an empty s*2px orbital in O22-.21,22 The coordinatively unsaturated surface (c.u.s.) Ti4+ ions have unbonded valence electrons and will interact with O22- in H2O2.The coordinatively saturated bulk Ti4+ ions show an outermost electron configuration of 3d04s0. Some c.u.s. titanium sites may occur as Ti3+ (3d14s0).23,24 During the surface reaction, 3d electrons will be favored to enter empty s*2px orbitals, resulting in cleavage of the OKO bond in O22- and thereby the formation of hydroxyl groups (KOH).21,22 The fresh KOH will be thermodynamically favored to be chemisorbed on the surface oxide layer through bonding with c.u.s.Ti4+ ions.17,25 In addition, chemisorbed hydroxyl groups can also be formed through dissociation of H2O on titanium. However, the concentration of such chemisorbed hydroxyl groups is very low because the major part of H2O is physisorbed. 17,25 Hence, a highly hydroxylated surface will be Fig. 9 Curve-fitting of the O 1s XPS spectrum for VTS-Ti. realized after H2O2 treatment. The chemisorbed hydroxyl groups on the c.u.s. Ti4+ ions of the fresh oxide layer can be denoted as TiKOH. Moreover, the texturing of the substrate before chemisorption of hydroxyl groups makes a majority of TiO2 crystals with (110) planes exposed, that is, a majority of domains with same c.u.s. Ti4+ arrangement on surface and thus a majority of domains with the same arrangement of TiKOH.Consequently, the OH groups on the textured substrate are more organized than a nontextured substrate, and may be more suitable to act as a template for biomimetic growth of coatings after further functionalization. 4.2 Formation of a self-assembled monolayer (SAM) According to known organic processes,26 vinyl groups will be oxidized by dilute KMnO4 solution first to give alcoholic hydroxyl groups at low temperature and then into carboxyl groups at room temperature and the formation of VTS-SAM Fig. 10 Curve-fitting of the C 1s XPS spectrum for VTS-Ti. and further functionalization of SAM is illustrated in Fig. 8.27 2798 J. Mater. Chem., 1998, 8, 2795–2801and contaminated carbon inherent to the XPS spectrometer, respectively.12,28 These results suggest that the oxidation of KCHLCH2 into KCH(OH)CH2OH and further into KCOOH is not complete, which is characteristic for this organic reaction. Therefore, the SAM on VTS-Ti is a mixture of OH-terminated and COOH-terminated silane moleculars fixed on the titanium plate, i.e., a mixture of states as shown in Fig. 8(b)–(d).Since we are dealing with a self-assembly process and the hydroxylated surface on highly (0001) textured titanium is highly organized, the mixture may be uniform, i.e., the arrangement of KCHLCH2, OH and COOH groups in the functionalized SAM on highly textured titanium is highly organized.Among these groups, OH and COOH can induce biomimetic mineralization6,7 and may be the reason why control VTS-Ti can not induce coating formation; this will be discussed later. 4.3 Mechanism of oriented coating formation In biomineralization, oriented nucleation and growth result from the mediation by preorganized supramolecules through interfacial molecular recognition including the complementarities of lattice geometry, electrostatic potential, polarity, stereochemistry, space symmetry and topography.1 Similarly, in this biomimetic synthesis, the oriented nucleation and growth of HA originate from the regulation by OH- and COOHfunctionalized SAM fixed on textured titanium through interfacial molecular recognition.The interfacial molecular recognition involves at least four aspects: (1) Crystal lattice matching.The arrangement of OH and COOH groups in SAM fixed on the (110) oriented TiO2 layer along [001]anatase shows excellent one-dimensional (1-D) coherent matching with the (0001) plane of HA along [01190]HA as illustrated in Fig. 11.29 (2) Hydrogen bonding interaction. OH and COOH on SAM can form hydrogen bonds not only with OPO33- on (0001) planes of HA, i.e., OH,OPO3 and Fig. 11 Schematic illustration of the one-dimensional lattice match relation between (0001) planes of HA and SAM treated TiO2. COOH,OPO3,29,30 but also with OH- on the (0001) planes (a) Idealized crystal structure of HA[30] (note that the atomic arrange- of HA, i.e., OH,OKH and COOH,OKH, respectively. ment of PO4 and OH is not resolved for clarity), (b) (0001) plane of (3) Electrostatic potential interaction.The negatively charged HA at z=3/4 and (c) lattice overlapping between the functionalized COO- on SAM can attract the positively charged Ca2+ in (110) plane of TiO2 and (0001) plane of HA. The dashed and bold HA solution to substrate surface. (4) Stereochemistry match. line frames show the outlines of the unit cells of HA along [0001] and The directions of the valence bonds of OH and OH in COOH TiO2 along [110], respectively. tend to be parallel to that of OKH on (0001) planes of HA (parallel to the c-axis of HA).29 The recognition elements (1) and (4) explain why (0001) Thus finally a continuous textured coating can be formed on VTS-Ti [Fig. 12(d)]. However, on control VTS-Ti, few isolated textured HA is favored to form.Now one can understand why VTS-Ti can induce formation of (0001) textured HA domains are exposed with (110) planes of TiO2, and then the eVective molecular recognition, especially the 1-D lattice coating while control VTS-Ti can only induce formation of island-like precipitates, and moreover, both the coating on matching, can only occur on such few isolated domains.Consequently, for control VTS-Ti, after HA crystals nucleate VTS-Ti and the island-like precipitates on the control substrate exhibit order on a length scale larger than the expected size and grow on these few SAM-(110)TiO2 domains as the case on VTS-Ti [Fig. 12(a¾) and (b¾)], the degree of supersaturation is of SAM-(110)TiO2 domains as shown in Fig. 4 and 5. This can be explained by the formation process of HA on the substrate relatively higher for HA to nucleate in head-to-head fashion, through a 2-D lattice match between (0001) planes of the new as illustrated in Fig. 12. On VTS-Ti, many domains are exposed with (110) planes of TiO2 and functionalized with OH or HA nuclei and the underlying HA crystals31 [Fig. 12(b¾) and (c¾)] and also in other places such as the beaker/solution COOH groups and thus there will be many domains in which all the above molecular recognition processes are active and and air/solution interface.The distance between two SAM-(110)TiO2 domains is much longer for the control sub- induce oriented nucleation and growth of HA with (0001) planes parallel to VTS-Ti. Therefore, HA crystals nucleate strate than for VTS-Ti.For control VTS-Ti, before the crystallization can cover the exposed area between preferably on many SAM-(110)TiO2 domains with (0001) preferably parallel to the substrate on such domains [Fig. 12(a)]. SAM-(110)TiO2 domains, the solution has been saturated. Therefore, there will be exposed areas where no HA crystallizes As these crystals are growing toward solution along the [0001] direction, HA crystals will also begin to nucleate and grow on and island-like precipitates exist on the substrate [Fig. 12(d¾)].One can thereby understand why the height of islands on other SAM domains in the nearby pores between SAM-(110)TiO2 domains close to the growing crystals in a side- control substrate is much larger than that of the coating on VTS-Ti, and both the coating and the island-like precipitates by-side fashion, through the above recognition processes except lattice matching between SAM and nuclei and 2-D show order on a length scale larger than the expected size of SAM-(110)TiO2 domains.It is possible that the final coating lattice matching along substrate normal to the growing HA crystals and the new nuclei31 [Fig. 12(b) and (c)].The crystals on VTS-Ti will tend to be uniform as shown in Fig. 4 because there will be a larger driving force for growth for smaller grown according to this fashion are labelled A in Fig. 4(a). J. Mater. Chem., 1998, 8, 2795–2801 2799HA will begin earlier on VTS-Ti than on control VTS-Ti, and the crystallization always occurs on VTS-Ti before elsewhere in the VTS-Ti solution while relatively few crystals crystallize on control VTS-Ti before crystallizing elsewhere in the control solution.Thus the pH decreases at a earlier stage in the VTSTi solution than in the control solution, i.e., the incubation period is shorter in the VTS-Ti solution than in the control solution. After the incubation period, the crystallization on VTS-Ti is controlled by the reaction template through the above recognition in VTS-Ti solution.However, in the control solution, crystallization occurs elsewhere such as at the air/ solution and beaker/solution interface besides the control substrate due to the existence of a much smaller area of the eVective reaction template. Thus the pH value decreases more rapidly and levels oV more quickly in the control solution than in the VTS-Ti solution, i.e., the nucleation and growth period is longer in the VTS-Ti solution than in the control one. 4.4 Importance of texturing HA coatings So far, there have been no reports concerning the successful synthesis of highly (0001) oriented HA coatings on titanium or other substrates by biomimetic or other processes.3,32–36 In natural bone, HA is highly oriented with (0001) perpendicular to the collagen fibrils.37 Therefore, as a bone implant material, it may be necessary for HA coating to be (0001) oriented to help growth of collagen fibrils in surrounding bone tissue into the implant to enhance biointegration.35 Fig. 12 Schematic illustration of the biomimetic mineralization process on (A) VTS-Ti and (B) control VTS-Ti.Note that the thicker and 5 Conclusion thinner lines correspond to the SAM domains fixed on (110)TiO2 and those on TiO2 crystal surfaces except (110). A highly oriented HA coating is successfully prepared by a biomimetic process on a textured titanium substrate. The oriented growth of HA results from the mediation by a crystals (i.e., HA crystals which nucleate later on SAM functionalized self-assembled monolayer with organized domains fixed on TiO2 surfaces except (110)].It seems that arrangement of OH and COOH groups fixed on the textured the geometry matching between the arrangement of functional titanium plate through interfacial molecular recognition such groups and 1-D or 2-D crystal lattice of inorganic nuclei plays as crystal lattice matching, hydrogen bonding interaction, an important role in biomimetic mineralization.Further inves- electrostatic potential interaction and stereochemistry match. tigation of this point is now under way in our laboratory. Another interesting result to be explained is the diVerent Acknowledgements time dependences of the pH values between VTS-Ti solution and control VTS-Ti solution.From reaction (1), it can be This project was granted financial support from China deduced that the more rapidly the HA crystals nucleate and Postdoctoral Science Foundation. grow, the more quickly the solution pH decreases. Therefore, the diVerent time dependences of pH values reflect the diVerent rates of HA crystallization. The pH-time curves in Fig. 3 can References be resolved into three stages.The initial stage is the incubation 1 S. Mann, J. Mater. Chem., 1995, 5, 935 and references therein. period during which the pH value remains unchanged and no 2 F. J. Fendler and F. C. Meldrum, Adv. Mater., 1995, 7, 607. HA crystals are formed. The subsequent stage is the nucleation 3 B. C. Bunker, P. C. Rieke, B. J. Tarasevich, A. H.Campbell, and growth period during which the pH decreases because G. E. Fryxell, G. L. Graft, L. 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