DFT Study of the Adsorption of Aspartic Acid on Pure, N-Doped, and

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DFT Study of the Adsorption of Aspartic Acid on Pure, N-Doped, and Ca-Doped Rutile (110) Surfaces Ya-nan Guo,† Xiong Lu,*,†,‡ Hong-ping Zhang,† Jie Weng,† Fumio Watari,‡ and Yang Leng§ †

Key Lab of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, Sichuan, China ‡ Department of Oral Health Science, Graduate School of Dental Medicine, Hokkaido University, Sapporo 060-8586, Japan § Department of Mechanical Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong, China

bS Supporting Information ABSTRACT:

Understanding the interaction mechanism between titanium oxide surfaces and proteins/peptides/amino acids is crucial to the success of Ti implants. Aspartic acid (abbreviated as Asp or D) is one of the most abundant amino acid in nature. In this study, Dmol3, a quantum mechanics first-principles density functional theory code, was employed to investigate the interaction of Asp with pure, nitrogen-doped, and calcium-doped rutile (R(110)) surfaces. The effect of water on the interaction was also studied. The adsorption energy analysis demonstrated that the strongest adsorption happened when both the amino and carboxyl groups of Asp approached the R(110) surfaces and formed a bidentate coordination to two surface Ti atoms. Hydrogen bonds from the H atoms of Asp and bridging-O atoms on the surface also contributed to the adsorption. Water hindered the Asp adsorption. N-doping and Cadoping were not beneficial to Asp adsorption. The results imply that we may realize selective protein/peptide/amino acid adsorption on materials and determine the adsorption of specific biomolecules by an elaborately designed ion doping process. Our results could have potential impact on the design of effective material surface treatments for biomedical applications.

1. INTRODUCTION Titanium (Ti) and titanium alloys are some of the most popular biomedical materials due to their biocompatibility, good mechanical properties, and excellent corrosion resistance.1,2 The surface of Ti can be spontaneously oxidized into titanium oxide, which is believed to be closely related to the excellent biocompatibility and osteointegration of Ti.3 There are three types of Ti dioxide (TiO2), rutile, anatase, and brookite. Rutile is the most commonly encountered natural form of TiO2, and the (110) crystallographic plane of rutile (hereafter R(110)) is the surface with the lowest free energy of several low-index crystal surfaces and has been widely considered as a model for metal-oxide surfaces.47 When implanted into human bodies, an abundance of proteins adsorb on the surfaces of Ti, which leads to a series of subsequent effects, such as complement activation, platelet activation, coagulation activities, and adherence of cells and bacteria. Thus, understanding the interaction mechanism between titanium oxide surfaces and r 2011 American Chemical Society

proteins/peptides/amino acids is crucial to the success of Ti implants. Aspartic acid (abbreviated as Asp or D) is one of the most abundant amino acids in nature. Its chemical formula is HOOCCH(NH2)CH2COOH. The three-dimensional structure of the Asp molecule is shown in Figure 1. The molecule has three functional groups, referred to as the α-amino group (α-NH2), αcarboxyl group (α-COOH), and β-carboxyl group (β-COOH). Asp is the carbon terminal of tripeptide arginine-glycine-aspartate (RGD), found in many extracellular matrix proteins, and it plays a key role in the protein adsorption process.8 Asp is of great significance in the treatment of bone dysfunction, because it can act as a nucleating agent for apatite and influence bone growth, induce osteoblast differentiation, and increase extracellular Received: January 20, 2011 Revised: July 28, 2011 Published: August 04, 2011 18572

dx.doi.org/10.1021/jp200598t | J. Phys. Chem. C 2011, 115, 18572–18581

The Journal of Physical Chemistry C

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Figure 1. Three-dimensional structure of an aspartic acid molecule.

mineralization.9,10 Asp can be used as a crystal growth catalyst to modulate the process of biomineralization.11 Several studies have been devoted to the adsorption of Asp on TiO2 surfaces, and different adsorption mechanisms have been proposed. Giacomelli et al. studied the adsorption of Asp on anatase in aqueous solution and found that Asp interacts with TiO2 surfaces through an innersphere interaction between the amino group and surface Ti atoms and weak interactions between the carboxyl group and the surface.12 Roddick-Lanzilotta et al. investigated Asp on amorphous TiO2 by in situ attenuated total reflectance infrared (ATR-IR) spectroscopy. They reported that Asp is adsorbed on TiO2 surfaces mainly through the carboxyl groups in a bridging bidentate coordination with two Ti atoms.13 Recently, Jonsson et al. studied Asp on the surface of rutile and found that the Asp configuration is highly dependent on the concentration: at low surface coverage, Asp forms a bridging-bidentate mode binding through both carboxyl groups on the rutile surface (“lying down”); at high surface coverage, Asp stands up on the rutile surface through an outer-sphere linkage or hydrogen bond.14 Although those experimental studies provided valuable results, there is not yet a detailed understanding of Asp interaction with TiO2 surfaces at the molecular level, especially with regard to the electron density distribution and charge transfer during the adsorption process. These properties might not be easily explored by commonly used experimental techniques. Molecular modeling is a useful way to study the interactions between amino acids and material surfaces, because it can provide information about the interaction at the atomic level. Gambino et al.15 used molecular modeling to investigate the interaction of L-lysine in aqueous medium with silanol and methyl sites on quartz substrates, and the results indicate that L-lysine interacts with hydroxylated/methylated surfaces predominantly by electrostatic and hydrogen-bond terms. Rimola et al.16 explored different interactions of amino acids with hydroxylated silica surfaces by ab initio calculation. They reported that the adsorption process is mainly dictated by the hydrogen-bond interactions between both the COOH moiety and the side-chain functionalities of the amino acids and the terminal silanol groups of the surfaces. Irrera et al.17 studied the glycine adsorption on ZnO surfaces and demonstrated that the most favorable conformation is obtained when glycine is parallel to the surface and both the carboxyl and amine groups are interacting with ZnO surfaces. Langel et al.18 employed CarParrinello molecular dynamics (CPMD) to simulate the adsorption of glycine, methionine, serine, and cysteine on partially hydroxylated rutile (100) and (110) surfaces, and their results demonstrated that adhesion is driven by the weak interactions between surface Ti and carboxyl oxygen atoms.

Figure 2. (a) Three-dimensional model of the R(110) surface; (b) top view. Color codes: O, red; Ti, gray.

Ojamae et al.19 studied glycine adsorption on R(110) surfaces and found that stable configurations are obtained when glycine binds to the surface through its carboxylic end in a bridge mode and its amino end forms a hydrogen bond to a surface oxygen atom. Those previous studies indicate that the interaction between amino acids and material surfaces is quite complicated and could involve electrostatic adsorption, covalent bond formation, and hydrogen bonding.20 To the authors’ best knowledge, previous theoretical studies of Asp on TiO2 surfaces are still scant. In this study, Dmol3, a first-principles density functional theory (DFT) code, is used to study Asp interaction with pure, nitrogendoped, and calcium-doped R(110) surfaces. DFT is a quantum mechanical theory that is used in physics and chemistry to investigate the electronic structure and properties of a manybody system. It does not require empirical parameters but relies on the explicit quantum mechanics treatment of the electrons in a model system by solving Schrodinger’s equation indirectly to determine the electronic ground state.21 The purpose of the present study was to investigate the adsorption mechanism of Asp on TiO2 surfaces and to reveal the effect of ion doping on bimolecular adsorption on TiO2 surfaces. First, we calculated the adsorption of Asp on pure R(110) surfaces and studied the effect of different initial configurations; second, we investigated the effects of water on the Asp adsorption; and third, we considered the influence of nitrogen and calcium doping on the Asp adsorption.

2. COMPUTATIONAL DETAILS 2.1. Model Building. Pure R(110) Surfaces. The R(110) surface is the most thermodynamically stable crystallographic surface of stoichiometric rutile.22 It has 5-fold (5f-Ti) and 6-fold (6f-Ti) Ti atoms and two types of surface oxygen atoms, referred to as bridging-O and plane-O atoms (Figure 2). This model contains all of the typical features of the well-understood rutile 18573

dx.doi.org/10.1021/jp200598t |J. Phys. Chem. C 2011, 115, 18572–18581


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Figure 4. Water environment model (WE): the anionic Asp molecule adsorbs on the R(110) surface with eight water molecules surrounding it.

Figure 3. Aspartic acid molecules with four initial configurations on R(110) surfaces. (a) The backbone chain is perpendicular to the surface with the α-NH2 and α-COOH close to the surface. (b) The backbone chain is perpendicular to the surface with the β-COOH close to the surface. (c) The backbone chain is parallel to the surface with the α-NH2 and β-COOH close to the surface. (d) The backbone chain is parallel to the surface with the α-COOH and β-COOH close to the surface.

(110) surface that has been widely used in the literature.23,24 A periodic boundary condition was employed to model this slab, and the area of the surface was 11.836 Å 12.994 Å. Six atom layers were used to simulate the R(110) surface, because the properties of TiO2 surfaces are very closely related to those of atoms of the top six layers.25 A vacuum slab with the thickness of 13 Å was added to separate each slab from its periodic images. TiO2_Asp Systems. Previous results revealed that the intensity of peptide/surface interaction varies widely depending on the initial configuration of peptides.26,27 Four types of initial configurations of Asp were constructed, which considered the functional groups as the active parts and the reaction sites during adsorption, as follows: Case (1) The backbone chain was perpendicular to the surface with the α-NH2 and α-COOH close to the surface (Figure 3a). Case (2) The backbone chain was perpendicular to the surface with the β-COOH close to the surface (Figure 3b). Case (3) The backbone chain was parallel to the surface with the α-NH2 and β-COOH close to the surface (Figure 3c).

Case (4) The backbone chain was parallel to the surface with the α-COOH and β-COOH close to the surface (Figure 3d). Before adsorption, an optimization process was applied to both the Asp molecule and the R(110)surface, and then the Asp molecule was added on the preoptimized surfaces. The atoms in the calculation are labeled in Figure 1 of the Supporting Information (SI). It should be pointed out that there are many other possible initial configurations of Asp on TiO2 surfaces and that the present four models only represent the typical adsorption behavior of Asp considering the functional groups in Asp molecules and possible active sites on TiO2 surfaces. Water Models. The anionic Asp molecule was used in water models, as shown in Figure 2 of the SI. The pKa of the aspartic acid is about 1.9 and 3.4 for the carboxylic groups and around 9.3 for the amino group in the normal pH range of biological systems. Thus, the carboxylic groups are deprotonated and the amino group is protonated in the biological systems. Note that this anionic Asp is not stable without including water molecules. During the building of the water models, several water molecules were added near the functional groups of anionic Asp to form hydrogen bonds.28,29 Two types of water models were built. The first one was the water environment model (WE), in which eight water molecules surrounded the anionic Asp molecule and several of them were near the functional groups of the anionic Asp to interact with the Asp. The initial configuration of the anionic Asp molecule on the R(110) surface was the same as that in Case (1) in a vacuum (Figure 4). The second type of water model was the surface water layer model (WL), in which four water molecules were on the top of 5f-Ti with a OwaterTi distance of about 2.300 Å to form a water layer, as shown in Figure 3 of the SI. The anionic Asp molecule was on the top of water molecules, and four water molecules were added near the functional groups of anionic Asp (Figure 5). 18574



Figure 5. Surface water layer model (WL): the anionic Asp molecule adsorbs on the water layer surface with four water molecules surrounding it.

Figure 6. N-doped R(110) surfaces. (a) Side view; (b) top view. The bridging-O vacancy is marked by the arrow in (a) and the circle in (b), respectively. Color codes: N, blue; O, red; Ti, gray.

N-Doped R(110) Surfaces. The N-doped R(110) model was built by replacing two plane-O atoms with N atoms, accompanied by one bridging-O vacancy adjacent to the substitutional site (marked by the circle in Figure 6). Thus, two nitrogen atoms were substituted for three oxygen atoms in the crystal, to ensure the charge balance of the crystal, and then the substituted surfaces were optimized. At last the Asp molecule was placed on the optimized surfaces to investigate adsorption properties. Here we only considered the initial configuration of Case (1) on N-doped R(110). Figure 7 shows the final configuration of Asp on the N-doped R(110) surface.

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Figure 7. Aspartic acid molecule adsorption on N-doped R(110) surfaces after geometric optimization.

Ca-Doped R(110) Surfaces. The Ca-doped R(110) model was built by substituting two Ca atoms for two 5f-Ti atoms. To guarantee a charge balance, we created two oxygen vacancies by removing the bridging-O adjacent to substituting Ca.30 The Cadoped R(110) model is shown in Figure 8. The rectangular area in Figure 8 marked the lattice deformation that was caused by Ca substitution. We observed that the O atoms were closer to Ti atoms than to Ca atoms. The bond lengths of CaO after substitution were larger than those of TiO before substitution, as listed in Table 1 of the SI. The initial configuration of Asp on Ca-doped R(110) was the same as that of Case (1). The final configuration is shown in Figure 9. 2.2. Simulation Parameters. The simulation was performed by the density functional theory program Dmol3 in Materials Studio (Accelrys, San Diego, CA), in which the physical wave functions are expanded in terms of numerical basis sets.31,32 Dmol3 produces highly accurate results, while keeping the computational cost fairly low. To validate the applicability of Dmol3 code to TiO2 rutile, we calculated a single unit cell of rutile TiO2 and obtained the optimized lattice constants: a = 4.645 Å and c = 2.971 Å. These values were comparable with experimental data (a = 4.593 Å and c = 2.958 Å, ICSD #31322), which validated the applicability of Dmol3 code to the rutile crystal structure. We then used Dmol3 to predict the molecular structure of aspartic acid. The bond lengths and bond angles from Dmol3 and from experiments33,34 were thoroughly compared, because they were important parameters to characterize the structure of amino acids (Figure 4 and Table 2 of the SI). The results revealed that the values from Dmol3 were in good agreement with those from experiments, which validated the applicability of Dmol3 code to Asp molecules. During simulation, the DNP double numerical basis set was used, which was comparable to the 6-31G** basis set. The core 18575



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Table 1. Adsorption Energy of Asp on R(110) Surfaces (unit: eV) EAsp+TiO2

EAsp

ETiO2

Eads

in vacuum Case (1)

207093.630 13930.303 193161.497 1.829

Case (2) Case (3)

207093.078 13930.331 193161.450 1.297 207093.437 13930.293 193161.400 1.744

Case (4)

207093.416 13930.286 193161.489 1.641

in water

WE model 207073.958 13912.288 193160.695 0.975 WL model 215391.043 13912.939 201477.806 0.298

Figure 8. Ca-doped R(110) surfaces. (a) Side view; (b) top view. The rectangular area in (b) indicates the lattice deformation due to Ca substitution. O atoms in the rectangle are closer to the adjacent Ti atoms than Ca atoms. Color codes: Ca, green; O, red; Ti, gray.

N-doping

203928.447 13931.371 189995.761 1.315

Ca-doping

221539.615 13931.367 207606.696 1.552

the PerdewBurkeErnzerhof (PBE) generalized gradient approximation (GGA).36 Special points sampling integration over the Brillouin zone were employed by using the MonkhorstPack schemes with a 2 2 1 k-point mesh.37 A Fermi smearing of 0.005 Ha (1 Ha = 27.211 eV) and a global orbital cutoff of 5.2 Å were employed. The convergence criteria for the geometric optimization and energy calculation were set as follows: (a) a self-consistent field tolerance of 1.0 106 Ha/atom, (b) an energy tolerance of 1.0 105 Ha/atom, (c) a maximum force tolerance of 0.002 Ha /Å, and (d) a maximum displacement tolerance of 0.005 Å. During the optimization process, the aspartic acid molecule and the upper atomic layers of the surface were allowed to relax, while the atoms in the bottom layer were fixed.

3. RESULTS 3.1. Asp on Pure R(110) Surfaces. The adsorption energy (Eads), indicating the intensity of interaction between the Asp and R(110) surface, is derived according to the following equation:38,39

Eads ¼ EAspþTiO2 ðEAsp þ ETiO2 Þ

Figure 9. Aspartic acid molecule adsorption on Ca-doped R(110) surfaces after geometric optimization.

electrons were treated with DFT semicore pseudopotentials.35 The exchange-correlation energy was calculated using

ð1Þ

where EAsp+TiO2, EAsp, and ETiO2 represent the total energy of the system, the energy of the Asp molecule, and the energy of the R(110) surface, respectively. A negative Eads value corresponds to stable adsorption. The more negative the Eads is, the more stable the adsorbed structure is. Eads of the four cases was negative, which indicates that Asp molecule adsorption on R(110) is thermodynamically favored in all four cases (Table 1). The adsorption energy of Case (1) is the most negative, which reveals that the most stable system is gained when both the amino and carboxyl groups approach the R(110) surfaces and form a bidentate coordination to two surface Ti atoms. Hydrogen bonds from the H atoms of Asp and bridging-O atoms on the surface also contribute to the adsorption. The important interactions are marked in Figure 3a: N97Ti38, O110Ti74, H105O44, and H108O20. In the following section we take Case (1) as the adsorption model to analyze the electronic properties of Asp adsorption on R(110) surfaces. The electron density configuration gives us an intuitive physical picture of the Asp adsorption. Figure 10a shows the three-dimensional isosurface plot of the electron density with an isovalue of 0.05 e/Å3. The electron cloud of the α-NH2 and αCOOH overlap parts of the R(110) surface, which suggests that these functional groups play an important role during the adsorption process. There is also electron cloud overlapping between the H atoms of Asp and bridging-O on the surface. 18576



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Figure 10. (a) Electron density isosurface of Asp on pure R(110) surfaces with the isovalue of 0.05 e/Å3 . (b) Electron density difference isosurface with the isovalues of ΔF = +0.005e (red) and 0.005e (blue). Red represents charge accumulation, and blue represents charge depletion.

The electron density difference (ΔF) reveals the change of electron density during adsorption, which is calculated by subtracting the electron density of the isolated Asp (FAsp) and surface (FTiO2) from the total electron density of the system (FAsp+TiO2), as follows: ΔF ¼ FAspþTiO2 FAsp FTiO2

ð2Þ

Figure 10b is the electron density difference map, in which red represents charge accumulation and blue represents charge depletion. Upon adsorption, a significant repolarization of the electron density takes place around the carboxyl group and amino group of the Asp molecule. On the R(110) surface, the repolarization appears on the top of the 5f-Ti atoms and bridging-O atoms. Figure 11 shows the electron density difference slices passing through N in the amino group, in-plane 5f-Ti and 3f-O (N97Ti38O24), H in the Asp molecule and bridging-O (H105O44), H in the amino group and bridging-O (H108O20), and carboxyl-O and in-plane 5f-Ti (O110Ti74). The charge accumulation and charge depletion are represented by red and blue, respectively. There is charge accumulation near the N97, O44, O20, and O110 atoms and charge depletion near the Ti38, Ti74, H108, and H105 atoms. Analysis of the electron density demonstrates that there is charge transfer between these atoms, and chemical bonds are formed between the amino, carboxyl groups, and R(110) surfaces. Analysis of the electron structure on the basis of density of states (DOS) indicates the nature of the bonds between the adsorbed Asp and R(110) surface. Overlap of partial density of states (PDOS) is one piece of evidence of bond formation between atom pairs.4042 The overlap of PDOS in Figure 12 indicates that four pairs of atoms have the ability to form bonds, including N in the amino group (N97) with 5f-Ti (Ti38), N97 with plane 3f-O (O24), amino group H (H108) with bridging-O (O20), H105 with bridging-(O44), and carboxyl group O (O110) with Ti74.

Figure 11. Electron density difference slices of Asp on pure R(110) . The charge accumulation and charge depletion are represented by red and blue, respectively. (a) N in amino group, in-plane 5f-Ti, and 3f-O (N97Ti38O24), (b) H in Asp molecule and bridging-O (H105O44), (c) H in amino group and bridging-O (H108O20), and (d) Carbonyl-O and in-plane 5f-Ti (O110Ti74).

The Asp adsorption induces a surface rearrangement that involves the distortion of the geometry of the atoms, bond lengths, and bond angles during adsorption. Comparison of the configurations of the top atomic layers before and after Asp adsorption helps us to visualize the roles of the surface atoms in the adsorption process. Geometrical displacements in Case (1), as shown in Table 2, are calculated with respect to the geometry before adsorption. The negative displacements indicate inward relaxation, whereas the positive ones indicate outward relaxation. Atoms near Ti38 have large displacements, which reveals that the surface near Ti38 is the main area interacting with Asp. The Asp adsorption also causes variation in the distance between surface bridging-O atoms. Before adsorption, the distance was 2.959 Å (Figure 2a). After adsorption, parts of the distances decreased and other parts increased. For example, the distance of O32O44 increases to 2.961 Å, whereas that of O44O80 decreases to 2.935 Å (Figure 1 of the SI). Geometric rearrangement after adsorbent adsorption was reported when the Ca and Pd atoms interacted with the R(110) surfaces.4,43 To further analyze the effects of surface rearrangement on the adsorption energy, we calculated the Asp adsorption on the unrelaxed R(110) surface. All six layers of R(110) surface atoms were kept frozen at the positions obtained from the surface optimization, and only the configuration of Asp was minimized. The adsorption energy of Asp on the unrelaxed surface was 0.935 eV, which was much smaller than that on the relaxed surface (1.829 eV). This result indicates that the surface rearrangement contributes to the interaction of Asp-TiO2 surface. Table 3 of the SI shows the main structural parameters from relaxed and unrelaxed calculations. The distances between 18577



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Table 2. Geometrical Displacements along the z Axis of Atoms on Pure R(110) Surfaces after Asp Adsorption atom names

Figure 12. Partial density of states of atoms of Asp on pure R(110) surfaces.

Asp atoms and surface atoms in the unrelaxed model were larger than those in the relaxed model, which could be one reason that the adsorption energy was decreased. Sanz et al. studied Pd adsorption on Al2O3 and TiO2 surfaces and found that surface relaxation increased the interaction energy between Pd and metal oxide surfaces.44,45 Our result is in agreement with those previous studies. 3.2. Water Effects. Proteins generally interact with biomaterial surfaces in aqueous solution or at solidliquid interfaces,46,47

Δdz (Å)

5f-Ti38

0.09448

5f-Ti74 6f-Ti28

0.00158 0.02731

6f-Ti40

0.05001

bridging-O32

0.00933

bridging-O44

0.03066

plane-O24

0.13964

plane-O46

0.05758

and thus the effect of the surrounding water environment is important. The behavior of water molecules on TiO2 surfaces is still a controversial issue, although considerable efforts have been devoted to understanding this behavior.4852 Water molecules may fully or partially dissociate into H and OH or exist as water molecules on the R(110) surface.49 Molecular adsorption is preferred on ideal TiO2 surfaces, whereas dissociative adsorption is encountered on defected surfaces.51,52 Our study also demonstrates that OH groups on ideal R(110) surfaces tend to bind with H atoms to form H2O molecules. Thus we adopt the molecular water model in this study, in which only water molecules exist on the surfaces (Figure 3 of the SI). The adsorption energies of Asp on R(110) surfaces in the WE and WL models were lower than that in a vacuum (Table 1), which indicates that water molecules weaken the interaction between the anionic Asp and TiO2 surfaces. The effects of water could be explained from two viewpoints. First, water molecules tend to trap Asp and keep it away from rutile surfaces, preventing further adsorption of Asp. As shown in the WE model (Figure 4), the anionic Asp molecule directly interacted with Ti-sites on the R(110) surface through COO and NH3+ groups. Compared with the distance in a vacuum (2.648 Å; Figure 3a), the distance of O110Ti74 in the WE model (2.613 Å) changed just slightly, indicating that TiCOO was the main interaction. However, the interaction between N97 and Ti38 in the WE model was weak, because the distance of N97Ti38 in the WE model (3.853 Å) was much longer than that in a vacuum (2.395 Å). One of the hydrogen atoms in the NH3+ group interacted with a plane-O atom on the R(110) surface with an interatomic distance of 2.388 Å, which also contributed to the interaction between the Asp and R(110) surface. The surrounding water molecules formed a “cage” to stabilize the configuration of Asp through the intermolecular hydrogen bonds between themselves and also between the water molecules and rutile surfaces. The surrounding water molecules also interacted with anionic Asp through hydrogen bonds, such as the interactions between carboxyl oxygens (Oc) and hydrogens in water molecules (Hw) (dOc-Hw = 1.890, 2.025, and 2.165 Å) and between the amino hydrogen (Ha) and oxygen in a water molecule (Ow) (dHa-Ow = 1.666 Å). Second, water molecules cap the adsorption sites (Ti-sites) and therefore Asp cannot interact with R(110) surfaces directly. In a vacuum condition, Ti38 and Ti74 are the main interaction sites on R(110) surfaces. In the WL model, water molecules occupied the two Ti sites, and the anionic Asp was on the top of those water molecules (Figure 5). Thus, anionic Asp can only interact with the surfaces through the water layer. The wellordered water molecules on R(110) surfaces (Figure 3 of the SI) 18578



Figure 13. (a) Electron density isosurface of Asp on N-doped R(110) surfaces with the isovalue of 0.05 e/Å3 . (b) Electron density difference isosurface with the isovalue of ΔF = +0.005e (red) and 0.005e (blue). Red represents charge accumulation, and blue represents charge depletion.

change their configuration dramatically and form hydrogen bonds with Asp after Asp adsorption (Figure 5). It should be noted that this result is based on the WL model, in which water molecular adsorption on TiO2 is the precondition. However, there is the possibility that water molecules could dissociate into H+ and OH. Thus, further studies are needed to achieve the complete understanding of the interaction of Asp with TiO2 in an aqueous system. In summary, water weakened the Asp adsorption on the R(110) surface in both the WE and WL models. Comparison of the adsorption of the WE and WL models shows that the Asp adsorption process in a water environment is complicated. There is competition between Asp and water molecules during the adsorption process. If Asp occupies the adsorption site first, the interaction between Asp and the R(110) surface is still relatively strong, as in the WE model. However, Asp almost cannot interact with Ti surfaces if water molecules block the interaction sites, as in the WL model. Thus, the adsorption energy of the WL model (0.298 eV) was much smaller than that of the WE model (0.975 eV). These results are consistent with our previous molecular dynamics study on RGD adsorption on rutile surfaces.27 Kang et al.53 also reported that water molecules on a R(110) surface could prevent proteins from moving close to the surface. Zuo et al.54 investigated solvent effects on CO adsorption on a Cu2O(111) surface and found that the adsorption energy decreased in water. Our DFT study further confirms the hindering effect of water on the adsorption of proteins/peptides on rutile surfaces 3.3. Asp on N-Doped R(110) Surfaces. N-doped TiO2 has been investigated by many researchers, most of whom have focused on the photocatalysis properties.5558 In the field of biomaterials, nitrogen is one of the biological friendly elements.59 Nitrogen doping was reported to improve cell adhesion ability,60 hydrophilicity,61 and blood compatibility.62 In this study, we investigated the Asp adsorption on N-doped R(110) surfaces. Compared with pure surfaces, Asp adsorption on N-doped

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Figure 14. (a) Electron density isosurface of Asp on Ca-doped R(110) surfaces with the isovalue of 0.05 e/Å3 . (b) Electron density difference isosurface with the isovalues of ΔF = +0.005e (red) and ΔF = 0.005e (blue). Red represents charge accumulation, and blue represents charge depletion.

surfaces showed lower adsorption energy (Table 1), which demonstrates that N-doping weakened the interaction between the Asp and TiO2. Figure 13a shows the three-dimensional isosurface plot of the electron density of Asp on N-doped R(110) with an isovalue of 0.05 e/ Å3, which is a similar interaction mode to that of Asp on pure R(110) (Figure 10a). Figure 13b reveals the details of the redistribution of electron density after Asp adsorption. Electrons accumulate around amino group N (N97) and bridging oxygen-O (O20, O44) and are depleted around 5f-Ti (Ti74) and H of Asp (H105). PDOS plots of atoms are shown in Figure 5 of the SI. The overlap between PDOS plots indicates bond formation between pairs of atoms, as follows: amino group N and surface 5f-Ti (N97Ti38), carbonyl-O and surface 5f-Ti (O110Ti74), H in Asp and bridging-O (H105O44), and amino group H and bridging-O (H108O20). These results indicate that the interactions between Ti, N, and O in the surfaces and N, O, and H in the Asp molecule affect the adsorption process. The hydrogen bonds from H105O44 and H108O20 also contribute to the stability of adsorption. In summary, our theoretical calculations reveal that N-doping decreases the adsorption energy of Asp on rutile surfaces, which can be ascribed to two causes. First, N-doping causes bridgingO vacancies and therefore weakens the hydrogen-bonding interaction between Asp and bridging-O. Second, the interaction between surface plane-N and Asp is not as strong as that between surface plane-O and Asp. As shown in Figure 6 of the SI, the in-plane 3f-O (O24) on R(110) surfaces and amino group N in Asp (N97) attract each other to some extent before N substitution. After N24 is substituted for O24. they repel each other. 3.4. Asp on Ca-Doped R(110) Surfaces. Ca ions on TiO2 surfaces can improve the apatite-inducing ability of Ti substrates, and Ca deposition is the prerequisite for P deposition on 18579



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Table 3. Distances among Atoms of Different Models (unit: Å) Ti38N97

a

Ti74O110

H105 -O44

H108O20 2.229

pure

2.395

2.648

2.229

N-doping

2.390

2.709

2.146

2.357

Ca-doping

2.625 (N97Ca38)

2.700 (O110Ca74)

a

2.674

No bond formation due to O vacancies.

alkaliheat treated Ti surfaces.63,64 It is interesting to investigate the effect of Ca on the adsorption of proteins, peptides, or amino acids on biomaterial surfaces. Our DFT simulation indicates that Ca doping weakens the adsorption of Asp on TiO2 surfaces. The adsorption energy of Asp on a Ca-doped R(110) surface is less than that of Asp on a pure R(110) surface (Table 1). After adsorption, both NH2 and COOH interact with the doped Ca atoms on the surface, as shown in Figure 9. The isosurface plot of the electron density (Figure 14a) is not very different from that of Asp on the pure (Figure 10a) and N-doped models (Figure 13a). The electron density difference plot indicates that NH2 and COOH have a tendency to react with two Ca atoms individually (Figure 14b). The PDOS plots of NH2, COOH, and Ca have overlap (Figure 7 of the SI), which further proves the interaction of Asp molecules with doped Ca atoms. The interaction of CaN is shown to be weaker than that of TiN if we compare the electron density slices in Figure 8 of the SI with those in Figure 11. However, the interaction of CaO is similar to that of TiO. This also can be proved by comparing the bond lengths. The bond lengths of Ca38N97 and Ca74O110 are 2.625 and 2.700 Å, respectively, which are larger than those of Ti38N97 and Ti74O110 (Table 3).

4. DISCUSSION Characterizing the adsorption mechanism of amino acids on material surfaces could allow the realization of selective adsorption of protein on biomaterial surfaces.20 In this study, we used the DFT method to elucidate the adsorption of Asp on rutile surfaces. Our results reveal that different initial configurations lead to the variation of adsorption energy. Four types of Asp on the R(110) surfaces are modeled, which could be compared to different Asp adsorption modes from experimental investigation in the literature. Adsorption energy analysis indicates that the most stable system is Case (1), in which the α-NH2 and αCOOH groups interact with the R(110) surface in the groove formed by bridging oxygen atoms. In this model, the NH2 and COOH form bonds with the in-plane Ti, and H atoms form hydrogen bonds with bridging-O atoms. Note that the second stable configuration is Case (3), in which the α- NH2 and the βCOOH groups have parallel interaction with the R(110) surface, which further reveals that both the NH2 and COOH could have strong interaction with R(110) surfaces. Our results confirm the reports from Giacomelli et al., in which they proposed that Asp interacts with TiO2 surfaces through an inner-sphere interaction between the amino group and surface Ti atoms and weak interactions between the carboxyl group and the surface.12 The Case (4) model in the present study represents two COOH interactions with R(110) surfaces, which simulates the bridging bidentate coordination to two Ti atoms proposed by Roddick-Lanzilotta et al.13 The |Eads| of Case (4) is relatively large, which indicates that the bridging bidentate coordination configuration could be a stable adsorption configuration.

Case (2) has the smallest |Eads|, which reveals that Asp standing up on rutile surfaces through an outer-sphere or hydrogenbonded linkage to the surface is not stable. However, the standing-up configuration could be possible in cases of high surface coverage.14 In summary, the adsorption through both functional groups (carboxyl and amino) is more favorable than the adsorption through only one functional group. Both the NH2 and COOH are able to coordinate with TiO2 surfaces, and the hydrogen bonds also make a contribution during Asp adsorption on TiO2 surfaces. This conclusion is in line with the results of glycine adsorption on the (0001) surface of ZnO by DFT study.17

5. CONCLUSIONS The adsorption energy analysis demonstrates that the strongest adsorption happens when both the amino and carboxyl groups approach R(110) surfaces. Water hinders the Asp adsorption by competing with Asp for adsorption sites. N-doping and Ca-doping are not beneficial to Asp adsorption, which indicates that the changes in the chemical composition of titanium by ion doping significantly affect the adsorption behavior of Asp on TiO2 surfaces. The results imply that we may realize the selective protein/peptide/amino acid adsorption on materials and determine the adsorption of specific biomolecules by an elaborately designed ion doping process. Our results could have potential impact on the design of effective material surface treatments for biomedical applications. ’ ASSOCIATED CONTENT

bS

Supporting Information. Three additional tables and eight figures that elabrate on experimental results. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +86-28-87634023. Fax: +86-28-87601371. E-mail: [email protected].

’ ACKNOWLEDGMENT This project was financially supported by the NSFC (31070851), NSFC/RGC Joint Research Funding (N_HKUST601/08, 30831160509), Program for New Century Excellent Talents in University (NCET-10-0704), Sichuan Youth Science-Technology Foundation (2011JQ0010), Fundamental Research Funds for the Central Universities, Sichuan, JSPS Postdoctoral Fellowship for Foreign Researchers, and Grants-in-Aid for Scientific Research. ’ REFERENCES (1) Liu, X. Y.; Chub, P. K.; Ding, C. Mater. Sci. Eng., R 2004, 47, 49–121. (2) Lu, X.; Leng, Y.; Zhang, X.; Xu, J.; Qin, L.; Chan, C. Biomaterials 2005, 26, 1793–1801. (3) Lu, X.; Zhao, Z.; Leng, Y. Mater. Sci. Eng., C 2007, 27, 700–708. (4) Lu, X.; Zhang, H. P.; Leng, Y. J. Mater. Sci.: Mater. Med. 2010, 21, 1–10. (5) Ramamoorthy, M.; Vanderbilt, D.; King-Smith, R. D. Phys. Rev. B 1994, 49, 16721–16727. (6) Rohanizadeh, R.; Al-Sadeq, M.; LeGeros, R. Z. Biomed. Mater. 2004, 2, 343–352. 18580


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DFT Study of the Adsorption of Aspartic Acid on Pure, N-Doped, and

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