Crystals Grown on Templates of Varying Dimensionality - American

Materials Chemistry Division, National Chemical Laboratory, Pune - 411 008, India. Received August 1, 2001; Revised Manuscript Received February 26, 2...
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Morphology of BaSO4 Crystals Grown on Templates of Varying Dimensionality: The Case of Cysteine-Capped Gold Nanoparticles (0-D), DNA (1-D), and Lipid Bilayer Stacks (2-D)

CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 3 197-203

Debabrata Rautaray, Ashavani Kumar, Satyanarayana Reddy, S. R. Sainkar, and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune - 411 008, India Received August 1, 2001;

Revised Manuscript Received February 26, 2002

ABSTRACT: Control over the morphology of ceramic crystals by biomimetic processes is an important goal in contemporary materials engineering. While many parameters have been previously studied in such morphology control, the dimensionality of the crystallization template is one aspect that has not received much attention hitherto. In this paper, we examine the change in morphology of BaSO4 crystals as they are grown on templates such as cysteine-modified colloidal gold particles (0-D), DNA (1-D), and within lipid bilayers stacks (2-D) at two significantly different supersaturation ratios. It is observed that large changes in the morphology of barite crystals occur, and tentative reasons are put forward to explain these changes. Introduction The synthesis of advanced inorganic materials increasingly requires the ability to grow crystals of controllable structure, size, morphology, and indeed, assembly of the crystals into predefined superstructures. In this context, biominerals have served as an inspiration to crystal engineers. This is not surprising given the exquisite control that biological organisms exert over mineral nucleation and growth (both amorphous and crystalline) encompassing the above-mentioned aspects.1,2 It is now established that an important requirement for biomineralization is epitaxy between the crystal nucleating face and underlying bio-organic surface, and, consequently, biomimetic surfaces such as those presented by Langmuir monolayers,3,4 self-assembled monolayers (SAMs),5 and functionalized polymer surfaces6 have been studied in great detail. Attempts have also been made to control the morphology of crystals via addition of suitable crystallization inhibitors7 and carrying out crystal growth in constrained environments such as those afforded by microemulsions.8 An intriguing possibility recently investigated by Tremel and co-workers is the use of nanocurved surfaces as templates in the growth of minerals.9 More specifically, they have shown that the transition from SAMs of mercaptophenols on gold thin films (two-dimensional Euclidean space) to mercaptophenol derviatized gold colloidal particles (curved two-dimensional space) as templates for the crystallization of CaCO3 and SrCO3 resulted in interesting differences in morphology of the minerals.9c In addition to the fact that much larger quantities of the crystals may be obtained by such “heterogeneous nucleation in homogeneous solutions” and that frustrated growth of the crystals consequent to the nanoscale curvature of the template may be used to control the superstructure of the crystalline assembly, * To whom correspondence should be addressed. E-mail: sastry@ ems.ncl.res.in.

Tremel and co-workers also briefly mention the need to investigate the role of the size of nucleation centers in mineral growth and assembly. In this paper, we address the latter issue and investigate the role of dimensionality of the organic template in controlling the morphology of BaSO4 (barite) crystals. We have chosen BaSO4 as a model system since a considerable body of information exists in the literature on barite morphology control,4,7,8,10 possibly due to the importance of this mineral in cosmetics, papermaking, and off-shore oil field applications. BaSO4 crystals have been systematically grown on cysteine-capped gold colloids (zerodimensional surface, particle size 3.5 nm), calf-thymus DNA [CT-DNA, 1000 base-pairs (ca. 350 nm long), onedimensional surface], and within bilayer stacks of stearic acid (two-dimensional surface). The templates together with the Ba2+ counterions are shown in Figures 1-3 (A panels). We would like to mention here that the dimensionality of the templates given above is in relation to the size and nature of the mature BaSO4 crystals grown and has thus been used in a general sense. We have chosen cysteine to modify the surface of the gold colloidal particles for the following reasons: (a) the use of an amino acid would more closely approximate a biological surface; (b) cysteine contains a thiol group that is known to strongly bind to colloidal gold;11 (c) cysteine is water-soluble and thus obviates the need to cap the gold particles separately in an organic phase and, (d) the pI of cysteine is 6.8; therefore, the addition of Ba2+ ions to the colloidal solution prior to crystallization of BaSO4 may be done at relatively mild alkaline conditions (∼ pH 8.5) wherein the carboxylic acid groups of cysteine would be completely charged (important for Ba2+ immobilization), and significant hydroxide formation of the metal ions may be avoided. This is an important point of departure from the use of mercaptophenols for capping colloidal gold which necessitates highly alkaline pH conditions (pH > 12) for dissolution of the colloids in water. The use of thermally evaporated stearic acid bilayer stacks for the

10.1021/cg0155447 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/22/2002

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Figure 1. (A) Cartoon showing a cysteine-capped colloidal gold particle with Ba2+ counterions. (B and C) SEM images of BaSO4 structures formed on cysteine-capped gold colloidal particles at different magnifications at a SSR of ca. 400. (D and E) SEM images of BaSO4 structures formed on cysteinecapped gold colloidal particles at different magnifications at a SSR of ca. 50.

immobilization of cations such as Ba2+ (Figure 3A) prior to formation of barite crystals is interesting since (1) crystal growth occurs in a constrained environment (which is incidentally anisotropic), and (2) the nature of the organic templating layer is somewhat similar to that obtained at the air-water interface using Langmuir monolayers.3,4 The growth of BaSO4 crystals on the three different templates of varying dimensionality mentioned above has been carried out at two different supersaturation ratios and significant differences in the crystal morphology is observed. Control experiments carried out on BaSO4 crystallization without the templates clearly indicate the important role played by the templates. Presented below are details of this study. A preliminary report on the crystallization of SrCO3 in thermally evaporated stearic acid films has been published recently.12 Experimental Details Chemicals. Stearic acid, barium chloride, and sulfuric acid were obtained from Aldrich chemicals and used without purification. 1. Crystallization of BaSO4 in Stearic Acid Bilayers Stacks (2-D). Thin films of stearic acid [StA; CH3(CH2)16COOH] of 500 Å thickness were deposited by thermal evaporation in an Edwards E306A vacuum coating unit at a pressure of better than 1 × 10-7 Torr. The films were deposited on Si (111) wafers (for FTIR, X-ray diffraction, and Scanning electron microscopy measurements) and on a gold-coated 6 MHz quartz crystal for quartz crystal microbalance measurements. The film thickness and deposition rate were monitored

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Figure 2. (A) Cartoon showing CT-DNA with Ba2+ counterions. (B and C) SEM images of BaSO4 crystals formed on CTDNA at different magnifications grown at a SSR of ca. 400. (D and E) SEM images of BaSO4 crystals formed on CT-DNA at different magnifications at a SSR of ca. 50. in situ using an Edwards FTM5 frequency counter. After deposition of the StA films, the StA-coated QCM crystal was immersed in an aqueous BaCl2 solution (1 × 10-2 M in 50 mL of deionized water) at pH ) 5.5, and the frequency change of the quartz crystal was monitored as a function of time of immersion in the electrolyte solution ex-situ, taking care to wash and dry the crystal thoroughly prior to frequency measurement. The frequency of the quartz crystal resonator was measured using an Edwards FTM5 frequency counter that had a frequency resolution and stability of 1 Hz. For the 6 MHz crystal used in this study, this translates to a mass resolution of 12.1 ng/cm2. The frequency changes were converted to mass loading using the standard Sauerbrey formulas.13 The optimum immersion time determined from the QCM kinetics measurements was used to load the StA films on Si (111) substrates that were subjected to FTIR and XRD analysis. This process leads to the electrostatic binding of the Ba2+ ions with the carboxylate groups and formation of barium stearate. On completion of the first cycle of Ba2+ ion incorporation in the fatty acid film, the crystallization of BaSO4 in the barium stearate bilayers stacks was accomplished by immersion of the barium stearate film in an aqueous solution of dil. H2SO4 (6.33 × 10-3 M, pH 6), and the frequency change of the quartz crystal was monitored as a function of time of immersion in the electrolyte solution. This leads to a BaSO4 supersaturation ratio (SSR) of 400 in the lipid film. The optimum immersion time determined from the QCM kinetics measurements after incorporation of SO42- ions, was used to load on the barium stearate films and were subjected to FTIR, XRD, and SEM analysis. The above procedure was also carried out at a significantly lower supersaturation ratio of 50 (50 mL of 1 × 10-3 M BaCl2 and 50 mL of 9.9 × 10-4 M H2SO4 in deionized water). The supersaturation ratios (SSR) were calculated as follows:

SSR ) (IAP/Ksp)1/2

Morphology of BaSO4 Crystals

Figure 3. (A) Cartoon showing a stearic acid bilayer with entrapped Ba2+ counterions. (B and C) SEM images of BaSO4 crystals formed in thermally evaporated stearic acid bilayers (500 Å thick) at different magnifications at a SSR of 400. (D and E) SEM images of BaSO4 crystals formed in thermally evaporated stearic acid bilayers (500 Å thick) at different magnifications at a SSR of ca. 50. where IAP ) ionic activity product and Ksp ) solubility product for BaSO4 (9.9 × 10-11 mol/L). The BaSO4 crystals grown within lipid bilayer stacks were subjected to FTIR measurements, carried out in the diffuse reflectance mode at a resolution of 4 cm-1 on a Shimadzu FTIR - 8201 PC instrument. 2. Crystallization of BaSO4 on Calf-Thymus DNA (1-D). Four milliliters of a solution consisting of equal volumes of barium chloride (2 mL of 1 × 10-2 M) in double distilled water and 10-6 M (2 mL) calf-thymus DNA (CT-DNA) solution was prepared. The CT-DNA used in this study was ca. 1000 base pairs long (∼ 350 nm long). The BaSO4 crystallization was induced by addition of dil. H2SO4 (2 mL of 1.42 × 10-2 M in deionized water) to the solution (pH ) 6), (which results in a supersaturation ratio of 400) under stirring conditions at room temperature. It was observed that precipitation and crystal growth occurred almost immediately (the solution became turbid), and within 1 h, the crystals settled at the bottom of the container resulting in a clear solution. The crystals were separated by filtration, washed with water and placed on Si (111) substrates for further analysis. A similar experiment was carried out with CT-DNA as the template at a supersaturation ratio of 50 (2 mL of 1 × 10-6 M CT-DNA in deionized water, 2 mL of 1 × 10-3 M BaCl2 solution, and 2 mL of 2.25 × 10-3 M H2SO4 in deionized water). 3. Crystallization of BaSO4 on Cysteine-Capped Gold Nanoparticles (0-D). Gold colloidal particles were synthesized by borohydride reduction of HAuCl4 as described elsewhere.14 This procedure yields a clear ruby-red colored solution at a pH ) 9 containing gold particles of size 35 ( 7 Å.14 The gold particles were thereafter capped with the amino acid L-cysteine (Sigma Chemicals) by mixing a carefully weighed quantity of the amino acid to yield an overall cysteine

Crystal Growth & Design, Vol. 2, No. 3, 2002 199 concentration of 10-4 M in solution. The pI of cysteine is ca. 6.8, and therefore, the colloidal gold particles are expected to acquire a net negative charge at pH ) 9. To the cysteinecapped gold colloidal solution, 50 mL of aqueous BaCl2 solution (1 × 10-2 M) was added. Thereafter, 20 mL of dil. H2SO4 (2.28 × 10-2 M in deionized water) was added to the colloidal solution under stirring conditions at room temperature (supersaturation ratio ) 400). It was observed that precipitation and crystal growth occurred almost immediately (the solution became turbid) and within 1 h, the crystals settled at the bottom of the container resulting in a clear solution. The crystals were separated by filtration, washed with water, and placed on Si (111) substrates for further analysis. A similar experiment using gold nanoparticles was also carried out at a supersaturation ratio of 50 (50 mL of 1 × 10-3 M BaCl2 and 20 mL of 3.56 × 10-3 M H2SO4 in deionized water). Preliminary thermogravimetric analysis (TGA) of the cysteinecapped gold nanoparticle powder indicated a ca. 8% weight loss at 180 °C TGA and is attributed to desorption of surface bound amino acid molecules at this temperature. This weight loss is consistent with almost 100% surface coverage of the gold nanoparticles with cysteine. 4. Crystallization of BaSO4 in Solution. In control experiments, the crystallization of BaSO4 was accomplished directly in solution by a mixture of aqueous solutions of BaCl2 and dil. H2SO4 at two different supersaturation ratios of 400 and 50. The crystals were formed predominantly in the bulk of the solution and slowly settled down within 1 h at the bottom of the container. The crystals were separated by filtration, washed with water, and separately placed on Si (111) substrates for further analysis. The BaSO4 crystals grown on the different templates were subjected to X-ray diffraction (XRD) analysis carried out on a Philips PW 1830 instrument operating in the transmission mode at 40 kV voltage and a current of 30 mA with CuKR radiation. The morphology and chemical composition of the crystals were studied by scanning electron microscopy (SEM) on a Leica Stereoscan-440 microscope equipped with a Phoenix EDAX attachment.

Results and Discussion Figure 1B,C shows scanning electron microscopy (SEM) images obtained from BaSO4 crystals grown on cysteine-capped colloidal gold particles (0-D templates, Figure 1A) at an SSR of 400. At lower magnification, a number of star-shaped structures of very uniform size can be seen which are evenly distributed over the surface of the substrate (Figure 1B). At higher magnification, the finer details of the structure can clearly be resolved (Figure 1C). The structures have 4-fold symmetry and appear to be composed of assemblies of crystallites, possibly due to secondary nucleation on crystallites organized along the two perpendicular axes. In gross detail, the BaSO4 structures shown in Figure 1B,C are rather similar in morphology to barite crystals obtained by Bromley et al.7a They observed that at certain concentrations of additives containing (iminodimethylene) diphosphonate motifs, crystal growth along the [010] direction was more severely inhibited than along the [100] direction, which taken together with rounding along the [100] direction resulted in rounded, star-shaped single crystals of BaSO4. The XRD pattern from these crystals yielded sharp Bragg reflections characteristic of highly ordered crystalline barite (Figure 6B, curve 1).15 Spot profile energy dispersive analysis of X-rays (EDAX) measurements well within one of the BaSO4 stars yielded a Ba:S:O in excellent agreement with expected composition. A strong gold signal was also recorded (Au:Ba ratio of 1:10) clearly

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Figure 4. (A) SEM image of BaSO4 crystals grown in solution by mixture of aqueous solutions of BaCl2 and dil. H2SO4 at a supersaturation ratio of 400. (B) SEM image of BaSO4 crystals grown in solution by mixture of aqueous solutions of BaCl2 and Na2SO4 at a supersaturation ratio of 50.

showing the presence of cysteine-capped gold nanoparticles in the structures. It is pertinent to point out that even though the cysteine-capped gold nanoparticles did act as nucleation centers for BaSO4 growth, we did not obtain a spherical assembly of crystallites around a core of nanoparticle/nanoparticles as observed by Kunther et al. in their studies on the formation of CaCO3/SrCO3 on mercaptophenol-capped gold nanoparticles.9c The reason for this difference is not clear at present. Figure 1D,E shows SEM images obtained from crystals grown on the gold nanoparticles at a SSR of ca. 50. A number of crystals with a flat plate like morphology of fairly uniform size can be seen (Figure 1D). Closer observation of this image shows the occurrence of secondary nucleation on the surface of the crystals, the secondary nucleation being rather similar to that observed by Wong et al. in their study of BaSO4 crystallization in solution as a function of ion excess.10 Such surface reactions leading to secondary nucleation is known to occur at lower SSRs.10 The point of differ-

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ence between the barite crystals of this study and those of Wong et al. is the shape of the crystals obtained. Figure 1E shows one of the BaSO4 crystals wherein it can be seen that the crystals are rectangular in morphology as opposed to a more irregular morphology observed by Wong et al. The XRD pattern from the crystals grown at the lower SSR yielded sharp Bragg reflections characteristic of highly ordered crystalline barite (Figure 7, curve 1). An important concern in the crystallization of barite on cysteine-modified gold nanoparticles is the extent of surface coverage of the particles by the amino acid. As mentioned in the experimental section, preliminary TGA analysis of the cysteine-gold nanoparticle conjugates indicated a nearly 100% surface coverage of the amino acid. It is clear that the degree of coverage will impact the surface charge on the nanoparticles, the Ba2+ concentration at the surface, and, consequently, the morphology (and indeed the supersaturation in solution) of the crystals formed. We would like to stress that this is a preliminary report and that future reports will focus on addressing such issues. Figure 2B,C shows SEM images recorded from the BaSO4 crystals grown on CT-DNA as the template at a supersaturation ratio of 400. In this experiment, the DNA to Ba2+ ion molar ratio is 1:10000. Accounting for the charge on the DNA in terms of the number of base pairs, this leads to a Ba2+ ion excess over the phosphate charges on the DNA by a factor of 5. At lower magnification, some flat, star-shaped crystals can be observed. However, the density of more intricately structured crystals was much higher, some of which can be seen in the center of Figure 2B. A higher magnification SEM image of one of the BaSO4 crystals shows the structure in greater detail (Figure 2C). The BaSO4 crystals have

Figure 5. (A) QCM mass uptake recorded ex-situ during Ba2+ (cycle-1) and SO42- (cycle-2) ion incorporation in a 500 Å thick thermally evaporated StA film. The different cycles of ion exchange are indicated. (B) The FTIR spectra recorded from a 500 Å thick stearic acid film on a Si (111) substrate (curve 1), the stearic acid film after incorporation of Ba2+ ions (curve 2), and the barium stearate film after incorporation of SO42- ions. (curve 3).

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Figure 6. (A) XRD pattern recorded from a 500 Å thick stearic acid film (curve 1), and XRD pattern recorded from a 500 Å thick stearic acid film after entrapment of Ba2+ ions (curve 2). The (00l) Bragg reflections are identified in the figure. (B) XRD patterns recorded from BaSO4 crystals grown on gold nanoparticles (curve 1), on CT-DNA (curve 2) and in thermally evaporated stearic acid bilayers (curve 3) at a supersaturation ratio of 400.

Figure 7. XRD patterns recorded from BaSO4 crystals grown on gold nanoparticles (curve 1), on CT-DNA (curve 2) and in thermally evaporated stearic acid bilayers (curve 3) at a supersaturation ratio of 50.

a needle-shaped morphology, and as in the case of gold nanoparticle templates, growth of secondary crystals appears to occur along two mutually perpendicular directions. While the more open and tenuous nature of the structures suggests greater imperfection in the crystals relative to those grown on the gold nanoparticles (Figure 1B,C), the XRD pattern from these crystals yielded sharp Bragg reflections characteristic of highly ordered crystalline barite (Figure 6B, curve 2). Spot profile EDAX analysis of one of the crystallites yielded a composition consistent with barite. In the case of BaSO4 crystals grown on the CT-DNA templates at a lower SSR of ca. 50, the crystal morphology is quite different (Figure 2D,E). At lower magnification, some flat sheet like crystals with flowerlike morphology can be observed in some regions (Figure 2D). A higher magnification SEM image of one of the flower-shaped BaSO4 crystals shows the structure in greater detail (Figure 2E). The XRD pattern from these crystals yielded sharp Bragg reflections characteristic of highly ordered crystalline barite (Figure 7, curve 2). We would like to mention here that the morphologies of BaSO4

crystals obtained in this study with gold nanoparticles and DNA as templates have not been observed in the numerous earlier studies on barite crystallization on Langmuir monolayers3,4 and in solution with crystallization inhibitors.7 It has been shown in this laboratory that thermally evaporated fatty acid films when immersed in electrolyte solutions such as PbCl2 and CdCl2 resulted in the electrostatic entrapment of the metal cations and the spontaneous ordering of the lipid films into a lamellar c-axis oriented structure.16 Figure 5A shows the QCM mass uptake data recorded from a 500 Å thick StA film during Ba2+ ion incorporation and subsequent incorporation of SO42- ions into the barium stearate film. It is seen that there is a fairly large mass increase and this is attributed to electrostatically controlled diffusion (and entrapment) of the Ba2+ ions in the fatty acid film. At pH 5.5, the carboxylate ions of the fatty acid matrix are expected to be fully charged leading to maximum electrostatic interaction to the metal cations. From the equilibrium mass uptake of Ba2+ ions (ca. 40000 ng cm-2) and the mass of the stearic acid film, a Ba2+: stearic acid molar ratio of 15:1 was calculated. This result indicates considerable overcompensation of the negative charge in the acid matrix by the Ba2+ ions. Such charge overcompensation is known to occur in layer-by-layer electrostatically assembled systems.17 From the mass uptake of ca. 60 300 ng cm-2 recorded during entrapment of sulfate ions, a Ba2+:SO42- molar ratio of 1.3:1 was calculated, which is in excellent agreement with the expected stoichiometry of barite crystals. Figure 5B shows FTIR spectra recorded from the asdeposited 500 Å thick stearic acid film on a Si (111) substrate (curve 1), the stearic acid film after immersion in BaCl2 solution for 40 min (curve 2), and the barium stearate film after reaction with H2SO4 for 36 h (curve 3). A prominent absorption is seen at 1700 cm-1 in the case of the as-deposited stearic acid film (curve 1) as well as with the fatty acid film after immersion in BaCl2 solution (curve 2). This band is due to excitation of

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carbonyl stretch vibrations in the carboxylic acid groups of the fatty lipid film. In addition to this band, the barium stearate film shows an additional absorption at ca. 1515 cm-1 (curve 2). This band is assigned to the carbonyl stretch of the carboxylate groups of the salt of stearic acid. The presence of the 1700 cm-1 band in the barium stearate film indicates that complete salt formation had not occurred. The FTIR spectrum recorded from the barium stearate film after reaction with H2SO4 (curve 3) is essentially featureless. The formation of lamellar barium stearate was ascertained by the presence of characteristic odd-even intensity oscillations in the (00l) Bragg reflections in the XRD pattern of the film (Figure 6A, curve 2) which is not observed in the XRD pattern recorded from the as-deposited stearic acid film on Si (111) substrate (Figure 6A, curve 1). Similar XRD results were also obtained using glass substrates (data not shown). The above results clearly indicate that the nature of the substrate, viz. crystalline or amorphous, does not play a crucial role in ordering the fatty acid film prior to entrapment of the Ba2+ ions. Therefore, epitaxial effects contributing to the templating action observed for the stearic acid bilayer stacks may be ruled out. However, the lamellar c-axis oriented structure inferred from the presence of strong (00l) reflections in the barium stearate film16 indicates some degree of in-plane order which may serve to assemble the Ba2+ ions in a periodic manner as illustrated in Figure 3A. SEM pictures recorded from a 500 Å thick stearic acid film after formation of BaSO4 (by maintaining the SSR at ca. 400) are shown in Figure 3B,C. The lower magnification image (Figure 3B) shows a number of well-formed barite crystals of fairly uniform size and with faceted surfaces. Sharp Bragg reflections characteristic of barite were obtained for crystals grown in the lipid bilayers (Figure 6B, curve 3). The mature crystals exhibit an elongated hexagonal morphology similar to that obtained for barite crystals grown in solution along with block copolymers.7b The two SEM images show fortuitously a large crystal viewed edge-on (center of Figure 3B) and parallel to the lipid bilayer templating surface (Figure 3C). It is observed that secondary nucleation occurs around a central crystal leading to the formation of a flowerlike structure with four petals. What is interesting is that the secondary crystals occur in pairs that can be clearly seen in Figure 3B and to some extent in Figure 3C. Such a flowerlike structure consisting of pairs of barite crystals in each petal have been observed by Qi et al.7b In this study, flowers consisting of up to 10 petals were formed during the growth of barite in the presence of sulfonated derivative of poly(ethyleneglycol)-block-poly(ehtyleneimine)-poly(sulfonic acid) (PED-b-PEIPSA). While the exact mechanism leading to the formation of such flowerlike barite structures in the stearic acid bilayers is not fully understood and may be a consequence of inhibition of growth along certain crystallographic directions by the acid molecules in the bilayer, another factor that could contribute is the kinetics of crystal growth and the builtin anisotropy of the bilayer geometry. Unlike in the case of BaSO4 growth on cysteine-capped gold nanoparticles and DNA where the reaction could proceed rapidly and isotropically, the diffusion of the sulfate anions into the

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barium stearate bilayers (Figure 3A) during immersion in the acidic medium would control the rate of growth of the crystals. Yet another factor that could lead to the interesting pair-coordinated petals in the flowerlike barite structures could be hydrophobic interactions between individual crystallites covered by a monolayer of stearic acid. During the initial stages of growth of the crystals, the bilayers would distort to accommodate the large structures and eventually rupture to form a monolayer around the crystals. Indeed, contact angle measurements carried out with a sessile water drop on the BaSO4 film indicated that the films were hydrophobic (contact angle > 90° at all points on the film surface). The possibility of such hydrophobic interaction driven hierarchical assembly of crystals grown within fatty acid bilayers has been discussed by us in our study of SrCO3 growth in thermally evaporated stearic acid films.12 As in the previous experiments involving gold nanoparticles and DNA as templates, growth of BaSO4 crystals within the lipid bilayers at a lower SSR of ca. 50 resulted in a morphology different from that obtained at higher SSR values (Figure 3D,E). At lower magnification, a number of very small crystallites of rectangular shape can be seen (Figure 3D). A higher magnification SEM image of one of the BaSO4 crystals shows that the structures possess 4-fold symmetry and that the growth occurs uniformly along all the four directions directed away from a central nucleus (Figure 3E). Bragg reflections characteristic of barite were obtained for crystals grown in the lipid bilayers at lower SSRs as well (Figure 7, curve 3). It is clear from the SEM studies that the barite crystal morphology is a strong function of the nature of the template used (Figures 1-3) and that the crystal morphology is unique to a particular template and SSR. However, an important aspect to be verified is the variation in the BaSO4 crystal morphology during growth in solution in the absence of any additives at different supersaturation ratios. In control experiments, BaSO4 crystals were grown without additives at SSR of 400 and 50 as explained in the experimental section, and the SEM images recorded from these crystallite assemblies are shown in Figure 4, panels A and B, respectively. At the higher SSR, the crystallites are exhibit a very open structure, with a great degree of branching. This morphology is clearly distinct from the morphology of barite crystals grown on any of the templates at this SSR (Figures 1-3) and indeed, from the lower SSRs as well. At a lower SSR () 50), the crystals are flat and have a predominantly rectangular morphology, although occasionally, square-shaped crystals are also observed (Figure 4B). At this SSR, the morphology of the control crystals approximates those grown within lipid bilayer stacks (Figure 3E) and on gold nanoparticles (Figure 1E). The control experiments clearly show the templating effect of varying dimensionality plays an important role in determining the morphology of the barite crystals formed especially at large supersaturation ratios. In conclusion, the growth of BaSO4 crystals on surfaces of varying dimensionality with two significantly different super saturation ratios has been studied. Interesting changes in the morphology of the crystals is observed on going from 0-D (gold nanoparticle tem-

Morphology of BaSO4 Crystals

plates) to 1-D (DNA templates) to 2-D surfaces (lipid bilayers), these differences being predominant for crystals grown at large supersaturation ratios. Such strategies based on simple dimensionality considerations would prove useful in crystal engineering and could also throw light on the fundamental process of nucleation and growth of crystals near charged surfaces. Acknowledgment. D.R., S.R., and A.K. thank the Department of Science and Technology (DST), the University Grants Commission and the Council of Scientific and Industrial Research (CSIR), Government of India, respectively, for financial assistance. This work was partially funded by grants to M.S. from the DST and the Indo-French Centre for the Promotion of Advanced Scientific Research (IFCPAR, New Delhi) and is gratefully acknowledged. The authors thank Dr. K. N. Ganesh for a gift of the calf thymus DNA. References (1) (a) Biomineralization, Chemical and Biochemical Perspectives; Mann, S., Webb, J., Williams, R. J. P., Eds.; VCH Weinheim, 1989; (b) Mann, S. In Inorganic Materials; Bruce, D. W., O’Hare, D., Eds.; John Wiley & Sons: New York, 1996; p 255; (c) Mann, S. J. Chem. Soc., Dalton Trans. 1993, 1. (2) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 153. (3) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9, and references therein. (4) BaSO4: (a) Heywood, B. R.; Mann, S. Langmuir 1992, 8, 1492; (b) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681; CaCO3: (c) Litvin, A. L.; Valiyaveettil, S.; Kaplan, D. L.; Mann, S. Adv. Mater. 1997, 9, 124; (d) Buijnsters, P.

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