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Nucleation and Growth of Glycine Crystals on Self-Assembled Monolayers on Gold Jung F. Kang,†,‡ Julien Zaccaro,† Abraham Ulman,*,†,‡ and Allan Myerson*,† Department of Chemical Engineering, Chemistry and Material Science, Polytechnic University, Six Metrotech Center, Brooklyn, New York 11201, and The NSF MRSEC for Polymers at Engineered Interfaces Received October 26, 1999. In Final Form: December 6, 1999 Control of crystal morphology is critical in many pharmaceutical and food applications. Here we show that SAMs and mixed SAMs of rigid thiols on gold can serve as nucleation planes and modify the morphology of glycine crystals. Self-assembled monolayers (SAMs) and mixed SAMs of 4′-hydroxy-4-mercaptobiphenyl, 4-(4-mercaptophenyl)pyridine, and their mixed SAMs with 4′-methyl-4-mercaptobiphenyl were prepared on gold (111) surfaces and used as templates for the nucleation and growth of glycine crystals. Glycine nucleates in the R-glycine structure independent of hydroxy or pyridine surface concentration. The crystallographic planes corresponding to the nucleation surfaces, for the different SAM surfaces under study, were determined by interfacial angle measurements. For nucleation on 100% OH surfaces, the glycine crystallographic plane corresponding to the nucleation is {011}, whereas for the 0 and 50% OH surfaces, the crystallographic plane corresponding to the nucleation surface is a {h0l} face, probably {101}. For 25%, 75%, and 100% surface pyridine concentrations, the crystallographic planes corresponding to the nucleation are {010}, {121}, and {1105}, respectively. These differences are attributed to differences in H-bonding between glycine molecules in the nucleating layer and the SAM surface. As interfacial H-bonding increases, the dipoles of glycine molecules within the crystal become more perpendicular to the SAM surface. The direction of dipoles of glycine molecules that nucleated on a pyridine surface are not as close to the surface normal as those of molecules that nucleated on hydroxyl surface. This implies that the overall H-bonding interactions between the CO2- and NH3+ groups of the glycine and the hydroxyl groups of the SAMs surface are stronger than those between the NH3+ and the pyridine group.
Organic monolayer films1 have been used as an interface2 across which the geometric matching3 and interactions such as van der Waals forces and hydrogen bonding4 can transfer order and symmetry from the monolayer surface to a growing crystal. The nucleation and growth of organic crystals, the nucleation rates, polymorphic selectivity,5 patterning of crystal, crystal morphology, and orientation with respect to the surface can be modified through site-directed nucleation,6 using supramolecular assemblies7 of organic molecules such as chemically and spatially specific surfaces. Compressed at the plane of water/air interface, Langmuir monolayers8 are mobilized by and commensurate9 with the adsorption of aggregates during crystallization. *To whom correspondence may be addressed. Polytechnic University. ‡ The NSF MRSEC for Polymers at Engineered Interfaces. †
(1) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692. (2) (a) Chen, B. D.; Cilliers, J. J.; Davey, R. J.; Garside, J.; Woodburn, E. T. J. Am. Chem. Soc. 1998, 120, 1625. (b) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261, 3. (c) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819. (3) (a) Landau, E. M.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Nature 1985, 318, 353. (b) Weissbuch, I.; Berfeld, M.; Bouwman, W.; Kjaer, K.; Als, J.; Lahav, M.; Leiserowitz, L. J. Am. Chem. Soc. 1997, 119, 933. (4) Weissbuch, I.; Popvitz, R.; Lahav, M.; Leiserowitz, L. Acta Crystallogr. 1995, B51, 115. (5) Carter, P. W.; Ward, M. D. J. Am. Chem. Soc. 1994,116, 769. (6) Ahn, D. J.; Berman, A.; Charych, D. J. Am. Chem. Soc. 1998,120, 243. (7) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; Mcvay, G. L. Science 1994, 264, 48. (8) (a) Frostman, L. M.; Ward, M. D. Langmuir 1997, 13, 330. (b) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681. (9) Landau, E. M.; Wolf, S. G.; Levanon, M.; Lahav, M.; Sagiv, J. J. Phys. Chem. 1996, 100, 12455.
Self-Assembled monolayers (SAMs) and mixed SAMs10 lack the mobility of molecules at an air-water interface and hence the possibility to adjust lateral positions to match a face of a nucleating crystal. This is especially true for SAMs of rigid thiols where even conformational adjustment is not possible. Recently we have shown that SAMs of 4-mercaptobiphenyls are superior to those of alkanethiolate and provide stable model surfaces11 and the ability to engineer surface dipole moments.12 Taken together with the ability to engineer surface functionalities at the molecular level, SAMs and mixed SAMs of rigid thiol may offer unique surfaces for nucleation and growth of inorganic and organic crystals. Silane SAMs have been used to promote heterogeneous nucleation and growth of iron hydroxide crystals13 and to study the effect of surface chemistry on calcite nucleation, attachment, and growth.14 The crystallization of CaCO3 was investigated on surfaces of alkanethiolate SAMs on gold,15 and recently it was reported that the oriented growth of calcite can be controlled by SAMs of functionalized alkanethiols.16 The heterogeneous nucleation and growth of malonic acid (HOOCCH2COOH) was investi(10) For review on SAMs of thiols on gold see: (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (11) (a) Jung F. Kang, Ulman, A.; Liao, S.; Jordan, R. J. Am. Chem. Soc. 1998, 120, 9662. (b) Kang, J. F.; Jordan, R.; Ulman, A. Langmuir 1998, 14, 3983. (12) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R. Langmuir 1999, 15, 2095. (13) Tarasevich, B. J.; Rieke, P. C.; Liu, J. Chem. Mater. 1996, 8, 292. (14) Archibald, D. D.; Qadri, S. B.; Gaber, B. P. Langmuir 1996, 12, 538. (15) Ku˜ther, J.; Tremel, W. Chem. Commun. 1997, 2029. (16) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500.
10.1021/la9914054 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/21/2000
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Figure 1. Rigid 4-mercaptobiphenyls.
gated using alkanethiolate SAMs on gold that terminated with carboxylic acid and with methyl groups.17 In this paper we report that SAMs of the rigid 4′-hydroxyl-4-mercaptobiphenyl (I), 4-(4-mercaptophenyl)pyridine (II), and their mixed SAMs with 4′-methyl-4mercaptobiphenyl (III) (Figure 1) on gold18 can be used as templates for the nucleation and growth of glycine crystals. The motivation for these studies comes from the fact that while surface OH groups can be both H-bond donors and acceptors, the pyridine electron pairs at the surface can only serve as H-bond acceptors. A comparison between these two surfaces may provide understanding of the underlying surface chemistry that controls twodimensional nucleation and growth. We observed that glycine nucleates in the R-glycine structure, independent of OH or pyridine surface concentration. However, the morphology of the glycine crystals is very sensitive to the OH and pyridine site densities, and the direction of the dipoles of glycine molecule within the crystal is tilted further away from the nucleation plane as the surface H-bonding strength and concentration increase. Gold substrates were prepared on mica sheets in a procedure similar to one previously published.11,19 Optical constants for gold substrates prepared in this manner are Ns ) 0.186 ( 0.01 and Ks ) 3.400 ( 0.05. Atomic force microscopy (AFM) studies revealed terraces of Au(111) with typical crystalline sizes of 0.5-1 µm2. These gold substrates when modified with SAMs were used as templates in glycine crystallization. For the preparation of SAMs and mixed SAMs, 0.5 mM stock solutions of thiols I, II, and III were prepared in ethanol. Solutions with 0%, 25%, 50%, 75%, and 100% I and II mixed with III were prepared from these stock solutions, with total concentration of 10 µM. The gold substrates were kept in the thiol solution for 16 h under nitrogen, then rinsed with ethanol, and blown dry by a jet of nitrogen. The thickness of all SAMs and mixed SAMs, as established by ellipsometry, was 13 ( 1 Å. The SAMs of biphenylthiolate on gold are stable in water for 6 months without any apparent change in thickness or surface wetting properties. For glycine crystallization experiments, 28.125 g of glycine was dissolved in 100 mL of Millipore water to provide a 25% supersaturated solution at 25 °C. The (17) Frostman, L. M.; Bader, M. M.; Ward, M. D. Langmuir 1994, 10, 576. (18) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R. Submitted for publication in Langmuir. (19) (a) Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, 243. (b) Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M.; Sokolov, J. J. Am. Chem. Soc. 1999, 121, 1016.
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Figure 2. The νs(E2g and Ag) IR band for mixed SAMs II/III made in ethanol.
complete dissolution was obtained after heating the solution up to 60 °C in an ultrasonic bath. The solution was cooled to room temperature (∼2 h) and then slowly poured into the vile (1 oz) used as the crystallization reactor. The SAMs were carefully introduced and were placed close to a vertical position to avoid crystals forming in the bulk solution from attaching to the SAM surface. The vial was sealed and placed on a vibration isolation table at room temperature. Usually, very few mature macrocrystals (millimeters in size) of glycine were nucleated at the surface and near to the edge of SAM-covered substrate. The latter were discarded, and only crystals that have visible SAM area around them were considered. Being still attached to the substrate, the crystals were removed from the glycine solution and stored in cyclohexane for later analysis. In all cases force was needed to remove the crystal from the SAM surface. Frequently, the crystal face that was attached to the SAM surface had gold marks, indicating gold peeling due to strong adhesion of the crystal to the nucleating SAM surface. The faces closing the tips of the crystal will be referred as the “dome faces” and the parallel faces creating its stick shape will be called the “side faces”. Single crystals of a few millimeters in size were placed on a two-circle optical goniometer, and a He-Ne laser with almost parallel beam was used as the light source. The interfacial angles were obtained from the change in the crystal orientation necessary to place consecutively two faces in the same reflection position. This position was defined by directing the reflected beam on a mark on a distant screen. The small divergence of the laser beam and the high crystal-screen distance (∼3 m) enabled us to orient the crystal within 5′ of arc. The uncertainty is mainly due to the diffusion associated with the roughness of the crystal faces. This allowed us to determine the morphology of the crystals, and that the “dome faces” are {011} faces, and the “side faces” are {110} or {120} faces. The best matches for the nucleated planes were found with the crystallographic planes {010}, {121}, and {1105}. Detailed studies of mixed SAMs of I and III were published elsewhere.11 Figure 2 shows external reflection Fourier transform infrared (ERFTIR) spectra of mixed SAMs of II and III. As the concentration of II in its mixed SAMs II/III decreases, the intensity of the ν(E2g and Ag) bands from I decreases monotonically and is shifted to lower frequencies. On the basis of the absorbance of this band, the quantitative composition analysis of the mixed SAMs has been accomplished. The integrals of the areas under the curves are plotted in Figure 3 as the function of the molar fraction of II in solution. Notice that there
Glycine Crystals on Gold
Figure 3. Integrated area under the νs(E2g and Ag) band, in mixed SAMs II/III vs the molar fraction of II in solution, for adsorption from ethanol.
Figure 4. Advancing and receding water contact angles on mixed SAMs II/III.
is a closely linear relationship between the surface and solution composition of II. This is different from what we observed for mixed SAMs of 4′-hydroxyl and 4′-methyl4-mercaptobiphenyls on gold, where deviation from linearity was significant,11b and suggests that I is more stabilized in ethanol than II. Figure 4 shows linear relationships between cos θ of advancing and receding water contact angles and surface composition in mixed SAMs II/III. This is in agreement with the Cassie equation predicting that the contact angle of heterogeneous surfaces is an area average of the contact angles of the two homogeneous surfaces. Thus, no largescale phase separation, resulting in domains of II and III, exists. Many factors such as the temperature, the solvent, the concentration, the impurities, and hydrodynamics (vibrations) affect the morphology of a growing crystal. In the studies reported here, all these factors were maintained constant, and the only variable was the surface composition, allowing for the systematic change of H-bonding, and the investigation of how it affects crystallization. Two SAMs with OH surfaces were studied, a 100% OH surface and a 50% OH (1:1 I:III) surface. For comparison a 0% OH (100% III) surface was also tested. Figure 5 shows crystals that grew on SAMs with 100% and 50% OH surfaces.
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Figure 5. Glycine crystals grown on 100% OH (a) and on 50% OH (b) SAM surfaces.
Figure 6. X-ray powder-diffraction patterns for glycine nucleated on 100% OH (top), and 0% OH (100% III) (middle) SAM surfaces, compared with the R-glycine pattern (bottom). The patterns have artificially translated along the intensity axis for clarity.
Figure 7. Glycine crystals grown in aqueous solution and on a 100% OH SAM surface.
We have measured X-ray powder-diffraction patterns for all crystals that nucleated on SAM surfaces to determine which of the three stable glycine polymorphs nucleate (R, β, or γ). The powders were obtained by grinding a piece of the crystal that nucleated on the surfaces, and the diffracted intensities were measured using a diffractometer. The structure of all crystals studied match with that of the R-glycine as can be seen in Figure 6, where only the patterns corresponding to glycine crystals that nucleated on 100% OH and 0% OH surfaces and to known R-glycine that grew in a bulk solution are presented for clarity. X-ray studies confirmed that for all SAM
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Figure 8. The nucleation faces in glycine crystals grown on 100% OH (right) and on 0 and 50% OH (left) SAM surfaces.
Figure 9. A Wulff plot using Eatt{hkl} as distance to the center.
Figure 11. Crystallographic image (a), sketched structure (b), and proposed unit cell (c) of glycine crystal nucleated on 25% pyridine surface.
Figure 10. Proposed molecular structure of the nucleation planes for 100% OH SAM (top) and for 0 and 50% OH surfaces (bottom).
surfaces we studied, glycine nucleates as the R-polymorph independently of surface OH or pyridine concentration. To investigate possible changes in the morphology of the crystals, the natural growth faces occurring for the different surfaces have been determined by interfacial angle measurement. The crystal form of R-glycine grown in aqueous solution is bipyramidal and composed of large {011} and {110} faces and smaller {010} faces (Figure 7).4 The millimeter-size crystals obtained by nucleation on the 100% OH surface SAM present a different habit, {110} and {011} faces are always present but no {010} face has been observed, whatever the covering ratio of
hydroxide group. On the other hand, for 0% OH surfaces, this morphology is enriched by {120} faces (Figure 8). Several extrinsic factors can influence the crystal morphology: temperature, solvent nature, impurity content, and supersaturation. In the present case, the first four parameters were kept constant for all the SAM compositions studied; only the crystal quality may have change from one experiment to another. Indeed, the tension at the nucleation interface can induce dislocations in the growing crystal in a way that is difficult to control or reproduce. Hence, the presence of {120} faces for the same SAM composition and for different crystals seems to indicate that it is rather independent of the dislocation density in the crystal, thus indicating that the face is growing by two-dimensional surface nucleation. One of the possible explanations of the observed habit modification could be a change in the surface. Indeed, the growth rate of each face is proportional to its attachment
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Figure 13. Crystallographic image (a), sketched structure (b), and proposed unit cell (c) of glycine crystal nucleated on 100% pyridine. Figure 12. Crystallographic image (a), sketched structure (b), and proposed unit cell (c) of glycine crystal nucleated on 50% pyridine.
energy,20 Rhkl ∝ Eatt{hkl}. To determine the magnitude of the energy modification necessary to make the {120} face appear, we have calculated the attachment energies of the {110}, {120}, and {020} faces with the Cerius2 software package. Force field parameters from Scheraga et al.21 were used and charges were calculated by the Gasteiger method.22 The resulting attachment energies are Eatt{020} ) -0.2551 kcal/mol2, Eatt{110} ) -0.5672 kcal/mol2, and Eatt{120} ) -0.5545 kcal/mol2. The corresponding intrinsic morphology is then obtained by drawing a Wulff plot using Eatt{hkl} as distance to the center (Figure 9). As can be seen in Figure 6, the appearance of large {120} faces is a direct consequence of a large increase of the relative (20) Hartman, P.; Bennema, P. J. Cryst. Growth 1980, 49, 145. (21) Nemethy, G.; Pottle, M. S.; Scheraga, H. A. J. Phys. Chem. 1983, 87, 1883. (22) Gasteiger, J.; Marsili, M. Tetrahedron 1980, 36, 3219.
growth rate R020/R110 (approximately doubled), while R110/ R120 remaining approximately constant. Nevertheless, it is not possible to determine whether it is R120 that is increased, R110 and R120 that are reduced, or both simultaneously. Considering the crystal structure of R-glycine, it can be proposed that in crystallization on 0 and 50% OH surfaces the glycine molecules located at the crystal-surface interface are oriented in such a way that their permanent dipole moment is set parallel to the surface (Figure 10). This is because the molecular orientation at the surface is the result of H-bonding as well as of van der Waals interactions between the glycine methylene groups and the surface methyl groups. On the other hand, for crystallization on a 100% OH surface, the molecules at the interface present CO2- and NH3+ groups, well oriented to form hydrogen bonds with the hydroxide groups of the organic monolayer (Figure 10). As a result, the nucleation plane is no longer a {h0l} face, but becomes almost parallel to a {011} face.
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Three SAMs with pyridine surfaces were studies, 100% pyridine (100% II), 50% pyridine (1:1 II:III), and a 25% pyridine surfaces (1:3 II:III). Figures 11-13 present the results. Considering the repetition of the growth faces of the crystal, 2-fold axis of symmetry is present indicating R- or β-glycine structure. As for the OH surfaces, interfacial angle measurement and X-ray powder diffraction show that, for all SAM compositions studied here, the structure is R-glycine. Figures 11, 12, and 13 present pictures (a) of crystals on the SAMs surfaces onto which they nucleated, 25%, 75%, and 100% surface pyridine concentration, respectively. For better clarity, sketches of the crystal morphology (b) are also presented. The dashed lines show the hidden edges, and the dotted lines help visualize the complementary part cut off by substrate. The interfacial angle measurements also enabled us to determine precisely the orientation of the nucleation surfaces, which are found to match the {010}, {121}, and {1105} crystallographic plans for 25%, 75%, and 100% surface pyridine, respectively. The representation of these planes (dark areas) in the R-glycine structure in Figures 11-13 allows us to observe the evolution of the orientation of the glycine molecules at the nucleation surface. As seen in Figure 11, crystallization of glycine on mixed SAMs with 25% pyridine occurs in a fashion similar to that on 100% CH3, with the permanent molecular dipole moment almost parallel to the nucleated surface. The increase in pyridine concentration to 50% (Figure 12) results in tilting of the dipole moment away from the nucleation surface; this is even more pronounced for 100% pyridine surface (Figure 13), but to less extent than for glycine grown on 100% OH SAM surfaces. Across the interface between the nucleated plane and mixed SAMs, weak van der Waals interactions are gradually transformed into hydrogen bonding as observed from wetting experiments (in Figure 4). Unlike the case where glycine molecules nucleated with their permanent dipole moment almost perpendicular to OH surfaces, the glycine dipoles are not oriented closely to the surface normal of pyridine surface even though the wetting behavior of water on these two monolayer systems is the same. This means that the detailed chemical nature of the SAM surfaces and not their surface energy dominates the nucleation and growth process. There is an apparent
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difference between glycine molecules H-bonded to a phenol surface and to a pyridine surface. Modeling studies are underway to investigate this difference. Finally, we have observed that the actual area of contact between a glycine crystal and a SAM seems to be a function of the surface concentration of pyridine moieties. As this concentration increases, the area a crystal occupies decreases. This may suggest that when H-bonds between glycine molecules and a SAM surface are the dominating interactions, fewer molecules are required to form a viable nucleation plane. Conclusions Self-assembled monolayers (SAMs) and mixed monolayers of 4-(4-mercaptophenyl)pyridine and 4′-methyl-4mercaptobiphenyl were prepared on planar gold (111) surfaces and used as templates for the nucleation and growth of glycine crystals. Glycine nucleates in the R-glycine structure independent of pyridine surface concentration. The crystallographic planes corresponding to the nucleation surfaces, for the different surface pyridine concentration studied, are determined by interfacial angle measurements. They are found to be {010}, {121}, and {1105} for 25%, 75%, and 100% surface pyridine, respectively. This change has been attributed to an evolution of interfacial interactions between the glycine molecules and the SAM surfaces. Indeed, the direction of the dipoles of glycine molecules within the crystal is tilted further away from the nucleation plane as the surface pyridine concentration increases. Additionally, the direction of dipoles of glycine molecules that nucleated on a pyridine surface is not as close to the surface normal as that of the molecules that nucleated on hydroxyl surface. It implies that the overall H-bonding interactions between the CO2- and NH3+ groups of the glycine and the hydroxyl groups of the SAMs surface are stronger than those between the NH3+ and the pyridine group. Acknowledgment. A.U. gratefully acknowledges support by the NSF through the MRSEC for Polymers at Engineered Interfaces. A.M. thanks NSF and NASA (Grant NAG8-1455) for financial support. LA9914054
