Dissolution of the Calcite (104) Face under Specific Calcite–Aspartic

Article pubs.acs.org/crystal

Dissolution of the Calcite (104) Face under Specific Calcite−Aspartic Acid Interaction As Revealed by in Situ Atomic Force Microscopy Congmeng Wu,† Kang Zhao,† Xiaoqiang Wang,† Meiwen Cao,† Hai Xu,*,† and Jian R. Lu*,‡ †

Centre for Bioengineering and Biotechnology, China University of Petroleum (East China), 66 Changjiang West Road, Qingdao Economic Development Zone Qingdao 266555, China ‡ Biological Physics Group, School of Physics and Astronomy, University of Manchester, Schuster Building, Manchester M13 9PL, United Kingdom S Supporting Information *

ABSTRACT: In the presence of aspartic acid (Asp), the calcite (104) face shows distinct dissolution pit morphology, presumably resulting from the surface reaction between calcite and Asp. However, the specific nature of this interaction and the influence of solution hydrodynamics remain unclear. To this end, we have followed the calcite (104) surface dissolution using in situ fluid cell atomic force microscopy (AFM). The results showed that at pH 4.5 and in 100 mM Asp the surface reactions were controlled by diffusion under static conditions and that trapezoidal etch pits were formed. In contrast, elliptical etch pits were rapidly developed upon flowing due to the increased transfer of Asp to the [010] step edge and the dissolution of Asp-surface complexes away from the step edge. The occurrence of the [010], [461̅], and [4̅11] steps of trapezoidal etch pits was attributed to the stabilization of the (001), (11̅ 2), and (011̅ ) faces by Asp through bridging between the two carboxyl groups and two adjacent Ca atoms, with the α-NH3+ group forming a hydrogen bond with the oxygen of the H2O from the bulk solution and the surface CO3 groups from the (1̅12) and (01̅1) faces. The mirror images of the etch pits formed in D-Asp and L-Asp solutions resulted from the enantio-specific interaction, supporting the tripodal contact of Asp with the crystal surface. Thus, the etch pit morphology is affected by Asp concentration, mass transfer, and specific surface reaction.

1. INTRODUCTION Calcium minerals constitute nearly half of the known biogenic minerals.1−3 Dissolution of calcium minerals is of great interest in many fields, such as weathering or decay of rocks and building stone, removing of scale inside industrial pipes, and biomineralization. Because of its ubiquity and abundance in the biosphere, calcite has long been regarded as a model system for geochemical and biological research. Dissolution of the calcite (104) surface in the presence of foreign ions or biomolecules has been the focus of a number of recent investigations.4−9 Metal ions such as Mg2+ tend to incorporate into the calcite bulk structure while the behavior of organic additives is dominated by their interactions with the mineral surfaces.10−13 AFM investigations reveal that these solid/fluid interfacial reactions are often direction specific and highly dependent on molecular geometry and chirality of organic additives.12,13 Since both the kink environments and molecular properties of additives have delicate and significant effects on surface interaction, the microenvironment adopted in AFM experiments must be carefully chosen. Important factors include the state of supersaturation of calcium carbonate, solution pH, fluid flow rate, and the purity of additives. While calcite dissolution in the presence of inorganic ions is the main concern of geochemical studies, biomolecules are the main focus for biomimetic understanding. It is noteworthy that © 2012 American Chemical Society

many of the macromolecules that are involved in the biomineralization of calcium crystals are aspartic acid-rich (Asp-rich) proteins.14−19 Hence, understanding the calcite− aspartate interaction is of fundamental importance. A previous study showed that enantiomer-specific binding of amino acids to the step edges on the calcite (104) surface results in chiral morphology in crystal growth and this also holds in crystal dissolution.13 Thus, investigating the dissolution of calcite in the presence of foreign additives may provide an alternative for elucidating the solid/fluid interaction mechanism. Although the dissolution process of the calcite (104) face in aspartate solution has been studied earlier,7 only the stabilization of new facets and steps was discussed and there was no discussion of the specific calcite−Asp interaction, the binding geometry of Asp molecules on the (104) surface and the effect of hydrodynamics. Because the biomineralization microenvironment is often complex and many organisms are most likely to take advantage of local hydrodynamic environments to control pattern formation of biominerals, this work focuses on the comparison of the structural dissolution under static and dynamic flow. We used in situ fluid-cell AFM to follow the Received: February 8, 2012 Revised: March 25, 2012 Published: April 6, 2012 2594

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Figure 1. AFM deflection images of the calcite (104) surface, (a) freshly cleaved in air, (b) in deionized water under flow at pH 4.5, (c) in deionized water under static conditions at pH 4.5, and (d) cross section of an etch pit on the (104) face.

Figure 2. AFM deflection images of the calcite (104) surface in D-Asp solutions under static conditions with a concentration of (a) 1, (b) 10, (c) 50, and (d) 100 mM, respectively, and (e) with a concentration of 100 mM under flow.

and [481̅] step edges which are crystallographically equivalent, as shown in Figure 1b. Note that all the AFM images presented in the present study have the same orientation as in Figure 1b. The acute steps are on the upper part of the rhombus forming an acute angle of 78° with the (104) face and the obtuse steps are on the lower part forming an obtuse angle of 102° with the (104) face (Figure 1d). The anisotropic dissolution features on the (104) face originate from structural differences in the surface kinks. In light of the geometrical difference between kink sites of [4̅41]+/+ and [4̅41]−/− (shown in Figure 1), the [4̅41]+/+ kink site is less sterically constrained with respect to the approaching/departing of ions and possesses a more open environment than the [4̅41]−/− kink site. The obtuse steps therefore retreat faster than the acute steps in pure water. Under static conditions etch pit morphology retains a typical rhombohedral shape in the initial tens of minutes, after which the obtuse−obtuse corner appears to have become round. With time the rounding becomes more marked. Ultimately, the equilibrium morphology is an asymmetric rhombus with rounding of the obtuse−obtuse corner (Figure 1c) arising from an impurity effect.20 2.2. The Concentration Effect of Asp on Calcite Dissolution. The effect of D-Asp was tested in four concentrations of 1, 10, 50, and 100 mM under static conditions. At 1 mM, the etch pits retained a regular rhombus shape and no discernible changes were observed (Figure 2a). As the concentration was increased to 10 mM, the pit shape

evolution of dissolution features with the introduction of Aspbearing solution with a view to provide a better understanding of the interfacial processes and the underlying mechanisms of calcite−Asp interaction in aqueous solution. Changes in morphology were followed under both static and flow conditions. Our study has revealed significant alteration in etch pit shape from static to flow, suggesting that the solution hydrodynamics, which is often neglected in the investigation of crystal-additive interactions, must be taken into account when mimicking the mineral formation/dissolution in the biosphere.

2. RESULTS The freshly cleaved calcite (104) surface is characterized by atomically flat terraces separated by wedge-shaped steps. These cleaved steps are one molecule high, that is, 3.0 Å, and the step edges indicate the cleavage direction, as shown in Figure 1a. 2.1. The Influence of Mass Transfer in Deionized Water. Upon the introduction of deionized water with a flow rate of 0.5 mL/min, the calcite (104) surface underwent a dissolution process characterized by the retreat of the original cleavage steps and the formation of etch pits, followed by coalescence and deepening of the pits. The depth of these pits is either monolayer or multilayer. Because of the difference in surface defects, dissolution pits on calcite (104) surface can be divided into two categories, the nucleation pits (the black arrow in Figure 1b) and the inverted pyramid pits (the yellow arrow in Figure 1b). The etch pits reflect the inherent symmetry of the {104} faces with the shape of a rhombus, bounded by [4̅41] 2595

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Figure 3. AFM deflection images of the morphological changes from static to flowing conditions on the calcite (104) surface in the presence of 100 mM D-Asp at pH 4.5. (a) Under static conditions. (b) 2 min of flow. (c) 3 min of flow. (d) 3.8 min of flow. (e) 4.5 min of flow. (f) 6.5 min of flow.

[421̅] direction. Further exposure to the flowing solution did not cause any change in pit shape. The equilibrium morphology therefore exhibited a network of interconnected elliptical etch pits. Despite the curvature of the [421̅] step, the step edges were smooth and showed no sign of roughness. The reverse process, that is, the pit shape evolution from flowing to static conditions, is shown in Figure S1 where it can be seen that when flow was stopped the [010] step began to emerge on the acute side while the [421̅] step diminished. Meanwhile, the [461̅] steps appeared on both the acute side and the obtuse side, with the former showing a slower pace. As a result, the etch pits changed into trapezoidal shapes eventually. 2.4. The Influence of Molecular Chirality. Our AFM results showed that the etch pit morphology developed in Asp solution also depends on the chirality of Asp, broadly consistent with previous studies.13 In L-Asp solution, the newly formed etch pits are also trapezoidal under static conditions, but are comprised of two [4̅21], one [010], and one [451̅] step edges, as shown in Figure 4a. This pit shape well mirrors the one in DAsp solution, where the trapezoidal etch pits are comprised of two [461̅], one [010], and one [4̅11] step edges (Figure 4b). As shown in Figures 5 and 7, the [451]̅ and [41̅ 1] steps are symmetrical with regard to the c-glide, as are the [4̅21] and [461̅] steps. Under static conditions, the anisotropic appearance of the [010] step is the same for D- and L-Asp. Under flow, the overall shape of the etch pits is elliptical with the expression of curved [421]̅ steps, and the etch pits developed in D-Asp and L-Asp solutions are symmetrical with regard to the c-glide (Figure 4c,d). Hence, whether the fluid is under flow or static, the etch pit shapes generated in D-Asp and L-Asp solutions are always the mirror image of each other.

was elongated and no longer rhombic (Figure 2b). However, the four step edges remained straight and no other steps appeared. When the D-Asp concentration was increased to 50 mM, the equilibrium morphology became trapezoidal and three new step edge directions emerged (Figure 2c). A further increase in D-Asp concentration did not affect etch pit shape, as revealed by the fact that the pit shape developed in the presence of 100 mM D-Asp was the same as that in 50 mM solution (Figure 2d). Note that under flow, the effect of Asp exhibited the same trend with regard to concentration; that is, the elliptical pit morphology developed in 50 mM D-Asp did not change when the concentration increased to 100 mM (Figure 2e). 2.3. The Influence of Mass Transfer in 100 mM D-Asp Solution. We chose the concentration of 100 mM for the subsequent study in order to have an etch pit shape independent of additive concentration and compare it with the previous work.7,21 As indicated above, the etch pits evolved into trapezoidal shape under static conditions after tens of minutes and then became stable (Figure 2d). The overall shape was not symmetrical with regard to the c-glide plane. It is evident that the acute−acute corner disappears and the obtuse−obtuse corner becomes narrower. The corresponding crystallographic directions of the four step edges are [010], [461]̅ , and [41̅ 1], at which the (001), (11̅ 2), and (011̅ ) planes intersect the (104) face, respectively. Two of the step edges are parallel to each other, so as to have the same step edge vector of [461̅]. Note that the modifying effect of D-Asp on pit morphology was much more severe on the acute side than that on the obtuse side, and the occurrence of the [010] step was always located at the acute side of etch pits. Surprisingly, the equilibrium pit shape developed in 100 mM D-Asp solution under flow was significantly different from that under static conditions. Under flow-through conditions, etch pits took on an elliptical or pseudoquadrilateral morphology (Figure 2e). The [010] step present under static conditions disappeared while the [421̅] step emerged, though it was not perfectly straight. The dynamic process of pit shape evolution from static to flowing conditions was followed and is shown in Figure 3. Once the solution started to flow, the acute side of trapezoidal etch pits was first affected, giving rise to the elongation along the [421]̅ direction. At the beginning, the [010] step still existed and the obtuse side was not affected. With time, the [010] step gradually diminished, and the [421̅] step edge lengthened. Finally, the pit shape became oval with some bending along the

3. DISCUSSION In pure water, the step edges of the rhombus are [481̅] and [4̅41], demonstrating that these two step edge directions are most stable. One prevailing view is that the Ca atoms and the CO3 groups are alternately arranged along the [481̅] and [4̅41] directions, making these two steps much more stable than any others (Figure 5). In the presence of Asp at pH 4.5, the etch pit shape on the calcite (104) surface depends on both additive concentration and hydrodynamics. The influence exerted over pit morphology became more apparent when the concentration of D-Asp increased. The shape of etch pits became stable when D-Asp 2596

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Under static conditions, Asp prefers to interact with the step edge at the [010] direction through the functional carboxyl group,21 and the surface complexes hinder the dissolution due to diffusion limitation, thus making the [010] step stable and expressed.24 Because the fluid flowing in the AFM fluid-cell is laminar,25 it is incapable of changing the adsorption state of Asp molecules on the crystal surface. The switch of pit shape from trapezoid to ellipse when the solution starts to flow should then be ascribed to the effect of mass transfer. Specifically, under flow, the disappearance of the [010] step indicates that the [010] step retreat is extremely fast and this therefore leads to the elongation of etch pits along the [421̅] direction. The accelerated dissolution rate of the [010] step can be attributed to the increased transfer of reactant (Asp) to the step edge and the products (the Asp-surface complexes) away from the step edge. In practice, the biomineralization microenvironment is very complex and many organisms are most likely to take advantage of local hydrodynamic environments to control the pattern formation of biominerals. Hence, this significant alteration in etch pit shape from static to flow suggests that the solution hydrodynamics, which is often neglected in the investigation of crystal-additive interactions, must be taken into account when mimicking the mineral formation/dissolution in the biosphere. On the other hand, from static to flow, Figure 3 shows that the [010] step retreats faster than the [461]̅ and [41̅ 1] steps and this can be interpreted in terms of surface interaction strength. The [010], [461̅], and [4̅11] steps possess different crystal structures and therefore create different atomic environments for surface binding. Comparing the [010] with the [461̅] and [4̅11] steps, the Ca atoms at the [010] step edge are more closely aligned, separated by 4.99 Å, in contrast to 15.72 Å at the [461̅] step and 9.51 Å at the [4̅11] step (Figure 5). Thus for a certain length of step edge, the Asp molecules can bind more Ca2+ from the [010] step than from the [461]̅ and [41̅ 1] steps, and this greatly accelerates the retreat rate of the [010] step when the surface product is continuously removed under flow. Figure 4 shows the mirror images generated by the two enantiomers of Asp. This enantio-specific interaction suggests that there ought to be tripodal contact of Asp with the crystal surface.26 Our previous results indicated that the α-amine group (α-NH3+) of amino acids is also active in surface binding besides the carboxyl groups, and therefore all three functional groups on Asp are probably involved in surface binding.27 In comparison with Asp, succinic acid does not have the α-NH3+ in molecular structure, and the common point in their effects on calcite dissolution is that the [010] step is expressed at pH 4.5.12 This implies that the expression of the [010] step in the solution is controlled by the two carboxyl groups (−COO−), consistent with the previous result that carboxyl groups have stronger adsorption on the calcite (104) face than −NH3+ groups.28 In addition, the etch pits generated in succinic acid are symmetrical with respect to the c-glide plane. In contrast, this symmetry is broken in Asp solution, thus signifying the important role of α-NH 3 + in Asp in enantio-specific interactions. Since electrostatic interaction is not directionspecific while covalent bonding and hydrogen bonding are,29 the interaction between the α-NH3+ group and the (104) surface probably involves hydrogen bonding. As the (001), (1̅12), and (01̅1) planes intersect the (104) face at the [010], [461̅], and [4̅11] directions, respectively, we attribute the expression of the [010], [461̅], and [4̅11] steps to

Figure 4. AFM deflection images of calcite (104) face observed in aqueous solutions at pH 4.5 in the presence of (a) 100 mM L-Asp under static conditions, (b) 100 mM D-Asp under static conditions, (c) 100 mM L-Asp under flow, and (d) 100 mM D-Asp under flow.

Figure 5. Atomic structure of the calcite (104) face. Dark green, Ca in plane; white, C in plane; yellow, O in plane; light red, O below the plane; red, O above the plane. Ca−Ca distances along the crystallographic directions are shown; green circles stand for binding sites of the two carboxylates of Asp, and red circles stand for possible binding sites of the ammonium group of Asp.22

concentration reached 50 mM or higher, indicating a saturated adsorption of D-Asp on the (104) surface. The newly formed etch pits took on an elliptical shape under flow but evolved into a trapezoidal shape when flow was stopped and vice versa (Figures 3 and S1). Fundamentally, the etch pit shape is determined by the relative retreat rate of the step edges. The steps that retreat slowly will be expressed while the ones that dissolve fast will disappear. The dissolution rate of steps is limited by the combination of the surface reaction and the mass transfer of reactants and products through the fluid boundary layer.10,23,24 2597

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the stabilization of the (001), (11̅ 2), and (011̅ ) faces by D-Asp. The (001) plane consists of either Ca atoms or CO3 groups. Given that −COO− groups interact more strongly with the Caterminated surface than the CO3-terminated surface,28 the expressed (001) face could be composed of pure Ca atoms rather than CO3 groups. Also, the stabilized [010] step edge ought to be terminated with Ca atoms. The surface structure of the (001) plane in Figure 6 is composed of single Ca atoms

which are separated by 4.99 Å in the three directions and every three neighboring Ca atoms make up an equilateral triangle. Unlike the (1̅12) and (01̅1) faces, there is no oxygen on the (001) surface, making it impossible for Asp to form hydrogen bonding with surface atoms. The binding of Asp to the (001) face is probably dominated by the −COO− group, and the αNH3+ is only able to form hydrogen bonds with water molecules from bulk solution. This also explains why the expression of the [010] step is not affected by the α-NH3+ group on Asp.12 As the Ca−Ca distances along the [010], [100], and [110] directions are all the same at 4.99 Å, this structural characteristic enables geometrical matching between the two carboxyl groups and the adjacent Ca atoms. The expression of the (001) face presumably resulted from the combination of electrostatic interaction and geometrical matching. Figure 7a shows the atomic structure of the (1̅12) face. Both Ca and oxygen atoms are available on the surface and the Ca− Ca distance along the [110] direction is 4.99 Å. It is clear that only the [110] direction matches the two carboxyl groups of Asp.27 A possible binding geometry for D-Asp is therefore as shown in Figure 7a, with the two carboxyl groups of Asp bridging the two adjacent Ca atoms in the [110] direction and the α-NH3+ group forming a hydrogen bond with the surface oxygen. This type of binding was probably responsible for the

Figure 6. Atomic structure of the (001) plane. Ca−Ca distances along different crystallographic directions are indicated.

Figure 7. (a−d) Atomic structure of the (1̅12), (01̅2), (01̅1), and (1̅11) planes. Ca−Ca distances along different crystallographic directions are shown; the blue triangles represent the binding of Asp to the crystal surface and the three apexes of one triangle indicate possible binding sites of an Asp molecule on the surface. Details about the conformation of Asp can be found in refs 30 and 31. 2598

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stabilization of the [461]̅ step. Figure 7c shows the surface structure of the (01̅1) face, together with the proposed binding geometry for D-Asp. In resemblance to the [461̅] step, the expression of the [4̅11] step could also arise from multiple Asp binding. The (1̅11) and (01̅2) planes formed in the presence of L-Asp intersect the (104) face at the [451̅] and [4̅21] step edges, respectively. The c-glide plane bisects the (1̅12) and the (01̅2) as well as the (1̅11) and (01̅1) planes. The (1̅12) plane possesses the same surface structure as the (01̅2) plane and the (1̅11) plane possesses the same surface structure as the (01̅1) plane. Evidently, Asp binding on the (1̅12) and (01̅1) planes involves hydrogen bonding while there is no hydrogen bonding on the (001) plane. Since hydrogen bonding is directionspecific, this leads to the chiral formation of the [461̅] and [4̅21], and the [4̅11] and [451̅] steps in D-Asp and L-Asp solutions. The expression of the [010] step is the same in both D-Asp and L-Asp solutions, implying that there is no hydrogen or covalent bonding between Asp and the (001) plane (Figure 6). The expression of the [010] step edge was always initiated from the acute sides of the pits, indicating that the effect on the acute side is stronger than on the obtuse side. The anisotropic occurrence of the [010] step can be best understood from the kink surroundings and the surface hydrogen bonding environment. It has nothing to do with molecular chirality since the [010] is always expressed anisotropically in both D-Asp and LAsp solutions. Figure 8a shows the four kinks composed of ⟨4̅41⟩ steps on the calcite (104) face. It is clear that the three carbonates located in the [481̅]−/[4̅41]− kink are coplanar in (1̅02) and form an isosceles triangle. The edge lengths of the isosceles triangle are 4.99, 4.05, and 4.05 Å, respectively. In the [481̅]+/[4̅41]+ kink site, the three carbonates are coplanar in (001) and make up an equilateral triangle with the edge length of 4.99 Å. Moreover, the step risers on the upper part of the pit form acute angles of 78° with the (104) face, while the step risers on the lower part form obtuse angles of 102° with the (104) face. Figure 8b is the profile view of a (104) plane viewed from the [010] direction. The Ca atom on the [010] step edge at the obtuse side has three dangling bonds, one of which is in plane and two of which are pointing upward. The Ca atom on the [010] step edge at the acute side also has three dangling bonds, but they point in different directions from the obtuse one. One is in the (104) plane, one is pointing upward and the remaining one is pointing downward. At the same time, the angle on the acute side is smaller than that on the obtuse side and the two Ca atoms at the acute corner are spaced more closely. All these factors suggest that the more restricted geometry of the [481̅]−/[4̅41]− kink favors preferential interaction with small ions, whereas the [481̅]+/[4̅41]+ kink site favors interaction with larger groups.32 The molecular length of L-Asp in solution is about 5 Å which is much larger than any cations. Arguments applicable to small cations do not hold for Asp, indicating the importance of some degree of site specific adsorption. A previous study also documented that Asp modified calcite crystal shape through adsorption rather than incorporation.13 The adsorbed amount of Asp on the crystal surface depends on the balance between adsorption and desorption. The [481]̅ −/[44̅ 1]− kink is more sterically constrained and thus is less accessible to attachment or detachment of Asp. On the contrary, the [481̅]+/[4̅41]+ kink is more open and more accessible to attachment and detachment. Provided that the

Figure 8. The kink environment for obtuse and acute steps: (a) top view of the calcite (104) face; (b) side view of the calcite (104) face, viewed from the [010] direction.12,32

adsorption of Asp on the (104) surface is limited by detachment under the experimental conditions, adsorption at the [481̅]−/[4̅41]− kink will be favored and Asp will preferentially interact with the acute side of the [010] step. This type of kink dependent adsorption rationalizes the direction-specific interaction in the calcite-Asp system. Another factor that accounts for the site-specific interaction of Asp with the [010] step edge is the environment for hydrogen bonding on the (104) surface. As shown in Figure 5, the two green circles represent the binding sites of the two carboxyl groups of Asp, while the red circles represent the binding sites of the α-NH3+ group. When the carboxyl group binds to the [010] step edge by bridging two adjacent Ca atoms, there are two possible binding sites for the α-NH3+ group, on the upper part or the lower part. The protruding oxygen on the upper part is closer to the two Ca atoms than the lower oxygen, and the α-NH3+ may therefore gain an easier access to form a hydrogen bond with the upper oxygen. Given that the step edge of the [010] is terminated with pure Ca atoms rather than CO3 groups, the emergence of the [010] step only occurs on the acute side.

4. CONCLUSIONS The dissolution behavior of the calcite (104) face in Asp solution is quite different from that in deionized water. At pH 4.5, the etch pit shape developed in Asp depends on concentration, hydrodynamics, and molecular chirality. The 2599

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and a resolution of 256 × 256 or 512 × 512, while minimizing the tip− surface force to avoid any artifacts on crystal morphology and step retreat. All of the images were captured in Contact Mode at ambient temperature. Before the dissolution experiment, deionized water (resistivity 18.2 MΩ·cm, Milli-Q) was allowed to flow over the calcite cleavage face so that the orientation of the calcite dissolution surface was established correctly.

pit shape becomes independent of concentration above 50 mM, and then calcite dissolution is controlled by the combination of surface reaction and mass transfer at pH 4.5. Asp prefers to interact with the step edge at the [010] direction. Under flow, the dynamic processes of the approach of Asp to the [010] step edge and the departure of Asp-surface complexes from the step edge are enhanced, thus accelerating the dissolution rate of the step, leading to the formation of elliptical etch pits. Under static conditions, surface reaction is apparently diffusion limited and the equilibrium shape of the pits becomes trapezoidal with the [010], [461̅], and [4̅11] steps being expressed. The occurrence of the [010], [461̅], and [4̅11] steps is ascribed to the stabilization of the (001), (11̅ 2), and (011̅ ) faces by Asp, for which bridging is established between the two carboxyl groups of Asp and two adjacent Ca atoms, while the αNH3+ group can form a hydrogen bond with the oxygen. Chiral molecules can break the symmetry of the etch pit with regard to the c-glide plane, and the mirror images generated by the D-Asp and L-Asp probably result from enantio-specific adsorption of the two enantiomers. The anisotropic emergence of the [010] step is attributed to the differences in the kink surrounding and hydrogen bonding environment.

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ASSOCIATED CONTENT

S Supporting Information *

AFM deflection images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-532-86981569 (H.X), 44-161-2003926 (J.R.L); Email: [email protected] (H.X), [email protected] (J.R.L). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. R. K. Thomas, University of Oxford, for critical reading of the manuscript and providing valuable comments. This work was supported by the National Natural Science Foundation of China (Grant No. 21071151), the Natural Science Funds of Shandong Province of China for Distinguished Young Scholar (Grant No. JQ201105), and UK Engineering and Physical Sciences Research Council (EPSRC).

5. MATERIALS AND METHODS Sample and Solution Preparation. A large crystal of opticalquality Iceland spar (3 × 3 × 2 cm3) of Mexican origin was purchased from Ward’s Scientific. It was washed consecutively using ethanol, 1% hydrochloric acid, and Milli-Q water. The washing procedure was done twice and the crystal was then completely dried by N2. The cleaned crystal of Iceland spar was then placed on a clean substrate and cleaved by a scalpel scoring along the cleavage plane. Since the cleavage plane (104) requires minimal energy for initiating a cleavage, it is easy to generate a rhombohedral fragment which comprises six crystallographically equivalent {104} facets. The cleaved fragments (2 × 2 × 1 mm3) were handled with tweezers to avoid any contamination and a jet of N2 was applied to remove small particles from the cleaved surfaces. The freshly cleaved fragment was then mounted on a steel puck using cyanoacrylate. L- and D-Aspartic acid (L-Asp and D-Asp) with purity ≥99.5% were purchased from GL Biochem (Shanghai). The Asp molecule comprises two carboxyl groups (α-carboxylate and β-carboxylate) and one α-amine group. The pKa values for α-carboxylate, βcarboxylate, and α-amine are 2.0, 3.9, and 9.8 respectively, giving a pI of 2.95 for Asp.30 The point of zero charge (pHpzc) for the calcite (104) surface is 9.5.33 The Asp-doped solutions used in dissolution experiments had a concentration of 0.1 M, and the pH was adjusted to 4.5 by adding reagent grade NaOH or HCl solutions. Therefore, under the experimental conditions, Asp molecules were predominantly in the zwitterionic form and were negatively charged, and the calcite (104) surface was positively charged. In-Situ AFM Imaging. Fluid cell AFM experiments were carried out on a Nanoscope IVa scanning probe microscope (Digital Instruments, Santa Barbara) equipped with a J-type scanner (maximum scan area 125 × 125 μm2) and gold-coated Si3N4 tips with a nominal spring constant of 0.06 N/m. A freshly cleaved calcite sample (2 × 2 × 1 mm3) was glued onto a steel puck, and then the Oring in the liquid cell was placed on top of the calcite (104) face and aqueous solutions continuously flowed into the cell by sucking solution from the outlet of the fluid cell with a peristaltic pump. In this way, leakage of liquid from the fluid cell was avoided. In flow through experiments, a flow rate of 0.5 mL/min was chosen so that the surface kinetics was not affected by diffusion. In batch experiments, flow was stopped once the solution had filled the fluid cell. Before each AFM run, the piezoelectric scanner was calibrated in the x, y, and z directions to ensure the correctness of surface morphology and depth of etch pits. AFM images were collected using scan rates of 5−25 Hz

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REFERENCES

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