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Langmuir 2003, 19, 7904-7910
Covalent Surface Modification of Calcium Hydroxyapatite Using n-Alkyl- and n-Fluoroalkylphosphonic Acids Susan C. D’Andrea and Alexander Y. Fadeev* Department of Chemistry and Biochemistry, Seton Hall University, South Orange, New Jersey 07079 Received December 13, 2002. In Final Form: June 22, 2003 This work investigates surface modification of calcium hydroxyapatite (CaHAP) with organophosphonic acids (RP(O)(OH)2). The solution-phase reactions of RP(O)(OH)2 (R ) n-C8H17, n-C18H37, and n-C8F17(CH2)2) were studied with two types of CaHAP substrates: high surface area powder (SN2,BET ) 60 m2/g) and thin films supported on Ti/Si wafers. It has been found that the concentration of the RP(O)(OH)2 solution has a dramatic effect on the structure of modified surfaces. Solutions of low concentration (∼5-10 mM or less) yielded covalently attached monolayers supported on CaHAP. According to Fourier transform infrared spectroscopy, the major products of the reaction were organophosphonic groups bound to the surface via PS-O-P bonds. Grafting densities of alkyl groups in these monolayers were ∼2.4 groups/nm2, which is only ∼0.55 of the maximum values typically observed for closely packed self-assembled monolayers, e.g., alkyltrichlorosilanes on silica, indicating a disordered structure of the surfaces. Loose packing of alkyl groups was also supported by adsorption and wettability studies. Solutions of high concentration (∼10-20 mM or higher) yielded the formation of composites with a high loading of organics believed to be due to the bulk modification of CaHAP. The kinetics of the reactions followed a first-order equation with rate constants ranging from ∼1 L/(mol‚h) using a good solvent (tetrahydrofuran) to ∼1000 L/(mol‚h) using poor solvents (toluene, ethanol). Modified surfaces showed good hydrolytic and thermal stability. No loss of the grafted material was observed after rigorous washing with solvents and ultrasound, which makes this surface modification technique useful for tailoring the CaHAP surfaces and modifying the adsorption and wettability properties of the CaHAP adsorbents, fillers, and biomaterials.
Introduction Covalent modification of surfaces has proven to be an efficient strategy in the development of materials with controllable surface properties. Different selective surface modification techniques have been the focus of research and technology for a variety of applications including sorption and separation, adsorption and adhesion, waterproofing, lubricants, fillers, sensors, and optical and electronic devices.1-4 Most surface modification work has been done on metals and metal oxides (primarily silica), the reactions of which have been extensively characterized and are well understood. In contrast, very little is known about surface modification of non-oxide surfaces, such as that of inorganic phosphates, for example, calcium hydroxyapatite (Ca10(PO4)6(OH)2), the major mineral component of hard tissues (teeth and bones). Surface modification of calcium hydroxyapatite (CaHAP) biological and biomimetic surfaces is of great interest in the development of biomaterials with controllable adhesion and adsorption properties. The use of organosilanes, mostly trialkoxysilanes, for surface modification of CaHAP and calcium phosphate ceramics has been described.5-9 Organosilanes were used (1) Silanes, Surfaces, and Interfaces; Leyden, D. E., Ed.; Gordon and Breach: New York, 1986. (2) Plueddemann, E. P. Silane coupling agents, 2nd ed.; Plenum: New York, 1991. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Adsorption on New and Modified Inorganic Sorbents; Dabrowski, A., Tertykh, V. A., Eds.; Studies in Surface Science and Catalysis 99; Elsevier: New York, 1996. (5) Labella, R.; Braden, M.; Deb, S. Biomaterials 1994, 15, 1197. (6) Nishizawa, K.; Toriyama, M.; Suzuki, T.; Kawamoto, Y.; Yokugawa, Y.; Nagata, F. Chem. Soc. Jpn. 1995, 1, 63. (7) Yoshino, N.; Teranaka, T. J. Biomater. Sci. Polym. Ed. 1997, 8, 623.
to control adhesion in polymer-CaHAP composites and to improve the mechanical properties of dental materials.9 The use of fluorinated silane coupling agents was shown to decrease the adhesion of bacterial plaque on the tooth.7 Reactions with organophosphates (ROP(O)(OH)2) have been described in works of Ishikawa and colleagues.10-12 According to refs 10-12, reaction mechanisms differ with solution concentration of alkyl phosphate: (1) surface modification of CaHAP was observed for reactions of low concentration solutions; (2) bulk modification of CaHAP and the formation of layered alkyl phosphate structures were reported for reactions of high concentration solutions. Recently, the use of isocyanates (RNCO) for surface coupling of polymers on CaHAP has been described.13,14 The present work investigates the reactions of organophosphonic acids (RP(O)(OH)2) with two CaHAP substrates: high surface area powder and thin films supported on Ti/Si wafers. Thermodynamics (adsorption isotherms) and kinetics of the reactions and the structure of the CaHAP-modified surfaces are studied. The use of organophosphonic acids for modification of the adsorption and wettability properties of CaHAP surfaces is demonstrated. (8) Tanaka, H.; Watanabe, T.; Chikazawa, M.; Kandori, K.; Ishikawa, T. J. Colloid Interface Sci. 1998, 206, 205. (9) Santos, C.; Luklinska, Z. B.; Clarke, R. L.; Davy, K. W. M. J. Mater. Sci.: Mater. Med. 2001, 12, 565. (10) Ishikawa, T.; Tanaka, H.; Yasukawa, A.; Kandori, K. J. Mater. Chem. 1995, 5, 1963. (11) Tanaka, H.; Yasukawa, A.; Kandori, K.; Ishikawa, T. Colloids Surf. A: Physicochem. Eng. Asp. 1997, 125, 53. (12) Tanaka, H.; Yasukawa, A.; Kandori, K.; Ishikawa, T. Langmuir 1997, 13, 821. (13) Liu, Q.; de Wijn, J. R.; van Blitterswijk, C. A. J. Biomed. Mater. Res. 1998, 40, 358. (14) Liu, Q.; de Wijn, J. R.; van Blitterswijk, C. A. J. Biomed. Mater. Res. 1998, 40, 257.
10.1021/la027000s CCC: $25.00 © 2003 American Chemical Society Published on Web 08/05/2003
Surface Modification of CaHAP with RP(O)(OH)2
Langmuir, Vol. 19, No. 19, 2003 7905
Table 1. Characteristics of the Samples of CaHAP chem composition
a
sample
Ca
P
CaHAP powder CaHAP film supported on Ti/Si
35.66a
17.83a
O
16.41b
9.23b
51.52b
C
Ca/P (mol)
SBET (m2/g)
1-butanol . tetrahydrofuran. It is noted that the solubility of C18H37P(O)(OH)2 in these solvents ranges in the reverse order. Toluene and ethanol are rather poor solvents for C18H37P(O)(OH)2 with solubility ∼2.5 and
Figure 4. Kinetics of the reaction of C18H37P(O)(OH)2 with CaHAP in different solvents: toluene (squares); 1-butanol (diamonds); tetrahydrofuran (circles). Reaction conditions: 25 °C; concentration of C18H37P(O)(OH)2 9.4 mM. Table 2. Kinetic Parameters for the Reactions of C18H37P(O)(OH)2 with CaHAP solvent toluene ethanol 1-butanol tetrahydrofuran
reaction temp, °C
rate const, L/(mol‚h)
25 80 25 60 25 80 25
1630 7012 1005 5020 170 1191 1.2
60
4.3
EA, kJ/mol 17.3 18.8 22.8 21.9
∼3 g/L, respectively. Solubility in 1-butanol is ∼10 g/L. The highest solubility of C18H37P(O)(OH)2 is observed for THF (>40 g/L). The effect of the solvent can be explained in terms of the solvent-solute and the solute-surface interactions. In a good solvent, strong solvent-solute interactions are favored, diminishing solute adsorption and reaction with the surface. Conversely, in a poor solvent, solute-surface interactions are favored. We note that careful analysis of the interactions in the system should also account for the possible specific interactions between the solvent and the surface, which may include π-bonding, hydrogen bonding, etc. However, since no irreversible adsorption of pure solvents on CaHAP was observed, we assumed that these interactions are not significant. The activation energy of the reaction (EA) was determined from the temperature dependence of the rate constants (Arrhenius plots). For all solvents, the same value, EA ) 20 ( 3 kJ/mol, was obtained. Reaction of Organophosphonic Acids with CaHAP According to FTIR. Since the spectra obtained for both CaHAP powder and CaHAP thin films were similar, only the spectra for CaHAP powder will be discussed. Figure 5 presents the reflectance spectra of CaHAP that reacted with solutions of C18H37P(O)(OH)2 of different concentrations. According to Figure 5, spectra substantially differ depending on the adsorbed amount of octadecylphosphonic acids. Samples with grafting density ∼2.4 groups/nm2 and less (corresponds to the adsorbed amount ∼0.24 mM/g and less, i.e., the first step of the isotherm, Figure 2) show the following characteristic features. First, compared to bare CaHAP there is a decrease in the absorbance at ∼3400-3300 and 1650 cm-1 (stretching and deformation of adsorbed molecular water). Removal of the adsorbed water is explained by the formation of hydrophobic
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Figure 5. IR spectra of CaHAP with different amounts of adsorbed C18H37P(O)(OH)2: 0.2 mM/g (b); 1.2 mM/g (c); 14 mM/g (c). IR spectrum of bare CaHAP (a).
Figure 6. Spectra in the P-O stretching region for CaHAP with different amounts of adsorbed C18H37P(O)(OH)2: 0.2 mM/g (a); 1.2 mM/g (b); 14 mM/g (c). Spectrum of pure C18H37P(O)(OH)2 (d).
surfaces through the reaction of CaHAP with RP(O)(OH)2. Inspection of the high-frequency region of the spectra (OH stretches) reveals a decrease in the absorption at ∼3690 cm-1 (surface PO-H groups23,24), while the sharp peak at ∼3570 cm-1 (OH from the lattice of CaHAP23,24) remains unchanged. Bands at ∼2920, 2850 cm-1 (ν(CH2)), ∼1469 cm-1 (δ(CH2)), and 782 and 716 cm-1 (ν(PC)) are consistent with the appearance on the surface of alkyl groups from the alkylphosphonic acids. Analysis of the P-O stretching region (Figure 6) is complicated due to strong background absorbance of the CaHAP matrix (ν(PO4)). It is noted from Figure 6 that the bands from P-OH (∼1074 and 928 cm-1, νa and νs P(OH)2) and the PdO group (∼1229 cm-1, ν(PdO)), present in the spectra of pure octadecylphosphonic acid, are not seen in the spectra of CaHAP with low grafting density of octadecylphosphonic acid. This indicates a strong interaction between the phosphonic acid and the
surface P-OH groups, probably, through the formation of PS-O-P bonds:
In previous works on the reaction of alkylphosphonic acids with TiO2,26 ZrO2,26 and Al2O3,27 the absence of the (26) Randon, J.; Blanc, P.; Paterson, R. J. Membr. Sci. 1995, 98, 119. (27) Ramser, R. D.; Henriksen, P. N.; Gent, A. N. Surf. Sci. 1988, 203, 72.
Surface Modification of CaHAP with RP(O)(OH)2
PdO band in the spectra has been interpreted as evidence for the formation of phosphonate species (RPO3) attached to the surface via three P-O bonds. We disagree with this explanation and point out that this mechanism would require a change in the oxidation state of phosphorus (from +5 to +4), which is not acknowledged in prior works. We believe that the disappearance of the PdO stretches from the spectra can be explained due to hydrogen bonding (PS-OH‚‚‚OdP). We also offer that the disappearance of this band from the spectra may be due to a decrease in the extinction coefficient, which it is not uncommon in the spectroscopy of adsorbed species.43 Obviously, more work is needed to elucidate the exact mechanism of surface bonding in this system. For samples with high loading of RP(O)(OH)2 the following additional features are present in the spectra. Bands at ∼2340 cm-1 (ν(PO-H)) and ∼1080 and ∼930 cm-1 (ν(P-OH)) appear and grow as the concentration of adsorbed RP(O)(OH)2 exceeds ∼5 groups/nm2 (corresponds to the adsorbed amount of 0.5 mM/g). These bands indicate the presence of noncoupled P-OH groups of alkylphosphonic acid in the samples. The vanishing of the 3570 cm-1 band (lattice OH groups) allows us to propose that the reaction of CaHAP with C18H37P(O)(OH)2 at high concentrations results in changes of the crystalline structure and bulk modification of the CaHAP matrix. The mechanism of this process is yet unclear and is the subject of further investigation.44 Perhaps the most notable feature of Figures 5 and 6 is a series of bands in the ∼1100-1400 cm-1 range that appear only in the spectra of samples with high loadings of C18H37P(O)(OH)2 (effective grafting density greater than ∼5 groups/nm2). These peaks are assigned to the CH2 wagtwist modes, whose appearance indicates the presence of all-trans sequences of the alkyl chains.28,29 The average spacing between these bands (∆ν ) 18 ( 1 cm-1) agrees with the value of 18 cm-1 predicted for fully extended C18 chains,28,29 indicating a high degree of ordering in the system. Further details about the ordering of alkyl chains in the modified CaHAP can be assessed from the position of the CH2 stretching. For a completely disordered structure, the frequency is close to that of a liquid alkane (νa ∼ 2928 and νs ∼ 2856 cm-1). For well-ordered monolayers, e.g., SAMs of alkyl thiols on gold or alkyltrichlorosilanes on (28) Snyder, R. G. J. Mol. Spectrosc. 1960, 4, 411. (29) Snyder, R. G.; Schachtsneider, J. H. Spectrochim. Acta 1963, 19, 85. (30) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (31) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (32) Parikh, A. N.; Leidberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996. (33) Kojio, K.; Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1998, 14, 973. (34) Britt, D. W.; Hlady, V. Langmuir 1999, 15, 1770. (35) Snyder, R. G.; Straus, H. L; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (36) Wasserman, S. R.; Tao, Yu-Tai; Whitesides, G. M. Langmuir 1989, 5, 1074. (37) Bain, C. D.; et al. J. Am. Chem. Soc. 1989, 111, 321. (38) Zisman, W. A. In Advances in Chemistry Series; American Chemical Society: Washington, DC, 1964; Vol. 43, p 1. (39) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (40) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (41) Amati, D.; Kova´ts, E. Sz. Langmuir 1987, 3, 687. (42) Kazakevich, Y. V.; Fadeev, A. Y. Langmuir 2002, 18, 3117. (43) Hair, M. L. Infrared spectroscopy of adsorbed species; Marcel Dekker: New York, 1965. (44) Gelfer, M. Y.; Hsiao, B.; D’Andrea, S. C.; Fadeev, A. Y. Submitted for publication in J. Am. Chem. Soc.
Langmuir, Vol. 19, No. 19, 2003 7909 Table 3. Wettability Data for CaHAP Surfaces Modified with RP(O)(OH)2 water contact angle (adv/rec), °C
hexadecane contact angle (adv/rec), °C
bare CaHAP CaHAP-O3PC18H37 CaHAP-O3PC8H17
