Synthesis of dl-Alanine Mesocrystals with a Hollow Morphology

Sep 4, 2008 - Masakatsu Tsubota , Katsuhiko Inagaki , Toru Matsuura , and Satoshi Tanda. Crystal Growth & Design 2011 11 (11), 4789-4793. Abstract | F...
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Synthesis of

DL-Alanine

Mesocrystals with a Hollow Morphology

Dana D. Medina and Yitzhak Mastai* Department of Chemistry, Nanomaterials Research Center, Institute of Nanotechnology and AdVanced Materials, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3646–3651

ReceiVed March 3, 2008; ReVised Manuscript ReceiVed June 2, 2008

ABSTRACT: The crystallization of DL-alanine from water/alcohol supersaturated solutions results in the precipitation of a racemic compound displaying a needlelike hollow morphology, in contrast to the needlelike crystals grown from pure aqueous solutions. This unusual morphology is explained by assuming that the supersaturated solutions crystallized rapidly to create DL-alanine nanocrystals that assemble to give mesoscale crystals. The resulting DL-alanine crystals are characterized by a range of techniques such as X-ray diffraction, scanning electron microscopy, micro-Raman, and differential scanning calorimetry. The kinetics was studied by light scattering and ion-solution conductivity. These measurements revealed that these superstructures of DL-alanine mesocrystals consist of three-dimensional DL-alanine nanocrystals. Finally, a mechanism for the formation of DL-alanine mesocrystals with a hollow morphology is proposed. Introduction The organization of nanostructures across extended lengths is a key challenge in modern colloid and materials chemistry for the design of integrated materials with advanced functions.1 In this field, much can be learned from biomineralization processes that yield well-defined, organic/inorganic hybrid materials with superior materials properties, complex morphologies, and hierarchical order. Overall, self-assembly is one of the key mechanisms by which nature builds products from biological molecules, such as proteins, to larger structures such as cells and extracellular matrices. The spatial arrangement of atoms is determined in large part by information built into the assembling units. For example, in biomineralization, the control and ordering of mesoscopic materials is generally achieved through self-organization and transformation.2 Recently, it was found that many crystallization processes can propagate via nonclassical pathways, employing colloidal intermediates,precursorparticles,andmesoscaletransformation.3-5 The notation “mesocrystal” has been introduced as a new term to describe self-assembled nanostructured crystals with welldeveloped external faces consisting of almost perfectly aligned nonspherical crystalline nanoparticle building units, which scatter like a single crystal.6 Mesocrystal assembly was reported for a number of inorganic systems, including iron oxide,7 cerium oxide,8 copper oxide,9 and copper oxalates.4 The structure of monodispersed spherical nano- or microparticles to colloidal crystals has also been investigated extensively.10-16 In a series of articles, Colfen et al. described doublehydrophilic block copolymers (DHBCs)17-20 as directing agents for the synthesis of inorganic materials with complex higherorder hierarchical structures. Both inorganic and organic mesocrystals, such as calcium carbonate and amino acids, were recently synthesized using DHBCs.21,22 They also showed that DL-alanine mesocrystals can be formed in the absence of any additives and that these mesostructures can crystallize via the simple control of pH and temperature.23 In spite of the large number of examples for the crystallization of mesocrystals, the mechanism for the formation of mesocrystals is as yet unknown. In this paper, we put forward, for the first time, that the formation of organic mesocrystals can be achieved based on * To whom correspondence should be addressed. Tel: ++972-3-5317682. Fax: + +972-3-5351250. E-mail: [email protected].

simple crystallization from water-alcohol mixtures. We will show that the crystallization of DL-alanine from water-alcohol mixtures can be used as a new method to induce the formation of DL-alanine nanoparticles and their self-assembly to mesocrystals with a hollow rodlike morphology. Overall, in our work, the construction of mesocrystals is achieved by kinetic control of the crystallization process using a water-alcohol mixture system with a different water-alcohol volume ratio. Furthermore, the present study is also devoted to a better understanding of the crystallization process and the formation mechanism of mesocrystals. Results and Discussion The crystallization of DL-alanine from aqueous solutions under a variety of crystallization conditions is well-documented in the literature.24-26 The effect of solvents such as alcohols and polymeric additives on the crystallization kinetics and crystal morphology of DL-alanine was also studied.27,28 For example, Lahav et al.29 studied the morphology changes of DL-alanine crystals in the presence of small amounts of organic additives, namely, optically active R-amino acids. It was shown that DLalanine crystallized as a twine crystal with a propeller-like morphology. Lahav et al.30 also examined the rule of solvent mixtures, such as water/methanol on the crystallization process and crystal morphology. It has been shown that in aqueous solutions, the polar crystals of DL-alanine grow more quickly at the (001j) crystal face than at the opposite crystal face. Another example of the effect of additives in crystallization was reported by Colfen and co-workers, who demonstrated that chiral block copolymers can selectively adsorb onto the (001) crystal face of DL-alanine, resulting in the formation of DL-mesocrystals.31,32 The formation of DL-alanine mesocrystals without the presence of organic additives was recently illustrated by Colfen et al.,23 who showed that the simple control of crystallization pH and temperature can lead to the formation of DL-alanine mesocrystals with a partly hollow needlelike morphology. The formation of mesocrystals by this method was confirmed by employing small angle neutron scattering.33 In this work, we present a new approach for the formation of mesocrystal crystals, and we demonstrated this approach in the crystallization of DL-alanine. Generally, our approach is based on the alteration of crystallization kinetics using alcohols

10.1021/cg800239g CCC: $40.75  2008 American Chemical Society Published on Web 09/04/2008

DL-Alanine

Mesocrystals with a Hollow Morphology

Figure 1. Time-dependent conductivities of DL-alanine crystallization from (A) water and 40% IPA and (B) water and 25% IPA; solutions at room temperature.

as a cosolvent that leads to rapid crystallization. We will demonstrate that alcohol as a cosolvent can affect the kinetics of the crystallization process and result in the formation of mesocrystals. The crystallization experiments on DL-alanine were performed from water/isopropyl alcohol (IPA) solutions with different volume ratios of IPA to water, ranging from pure water up to 35% IPA. In general, the crystallization experiments were performed as follows. First, supersaturated solutions of DLalanine were prepared by dissolving DL-alanine in deionized double-distilled (DD) water (0.8 g in 5 mL) to attain a saturation level, after which the solutions were heated to T ) 70 °C until complete dissolution was achieved, then, they were cooled to 40 °C and filtered with 0.2 µm filters. IPA was then added to the solution to initiate rapid crystallization, and the solution was allowed to cool to room temperature (T ) 25 °C). At the end of the crystallization process (ca. crystal yield of 90%), the crystals were collected from the solution by filtration onto filter paper and kept at room temperature for further measurements. Generally, the crystallization of DL-alanine from water/IPA solutions shows various effects on the crystallization kinetics and crystal morphology, specifically the formation of mesocrystals, which will be discussed separately. The crystallization kinetics of DL-alanine from pure water is relatively slow, for example, half of the material crystallizes after 18 h. The crystallization kinetics of DL-alanine from water/ IPA is relatively rapid. To investigate the crystallization kinetics from water/IPA solutions, we measured the solution’s electrical conductance during crystallization. In view of the fact that the solution conductivity is proportional to the dissolved DL-alanine concentration, the drop in the solution conductivity is related to the formation of DL-alanine crystals. The results of the kinetic measurements of crystallization experiments at different water/ IPA ratios are shown in Figure 1. In all of the experiments, the conductivity of the DL-alanine solution decreased quickly during the first 30 min. For example, in the crystallization of DL-alanine from water and 25% IPA (Figure 1B), the solution conductivity dropped from 180 to 100 µS/cm after ca. 15 min, indicating a strong decrease in the dissolved DL-alanine. Furthermore, there is a slow second moderate decrease of conductivity after 20 min, to a constant value of 25 µS/cm. These results can be explained by simple two crystallization mechanism stages. In the first stage, the rapid drop in the conductivity is due to the nucleation and formation of DL-alanine nanoparticles, while the second stage is characterized by a slow decrease in solution conductivity, which is attributed to crystal growth and the

Crystal Growth & Design, Vol. 8, No. 10, 2008 3647

formation of mesocrystals. Light scattering experiments confirmed this mechanism by showing the formation of nanozise crystals (typically 30-40 nm) during the first phase of the crystallization after ca. 15 min. The same behavior is observed at other water/IPA ratios. In general, as the amount of alcohol increases, the kinetics of the nucleation and formation of mesocrystals rise (see Figure 1A). These results underline the key issue in the formation of mesocrystals, that is, the need to stimulate and increase nucleation and crystallization kinetics in orders of magnitude comparable to the bulk crystallization. The morphology of DL-alanine crystals crystallized from water as well-facetted prismatic crystals with a relatively uniform wall thickness (see the Supporting Information, Figure S1). The crystals’ diameter is about 250 µm, while they can have a length of up to few millimeters. It should be mentioned that DL-alanine crystallized as a polar crystal and, as such, is elongated along the c-direction of the orthorhombic unit cell (as proven by X-ray diffraction results, see below). To examine the morphology changes of the DL-alanine crystals crystallized from solutions with different water/IPA volume ratios, a scanning electron micrograph (SEM) was employed. Overall, the effect of the IPA on the crystals’ morphology depended on the water/IPA volume ratio. In up to 20% of IPA in water, no effect on the crystals’ morphology was observed by SEM or X-ray diffraction measurements. In a water/IPA volume ratio of 25-35%, a strong effect of on the crystals’ morphology is observed. Figure 2a shows a SEM image of DLalanine crystals that were crystallized from a water/IPA volume ratio of 25%. As can be seen, the crystals exhibit a unique tubular hollow morphology with a length of more than 100 µm and a diameter of 10 µm. The high magnification image of the DL-alanine tube shown in Figure 2b suggests that crystals have a well-defined hollow morphology with a relatively uniform wall thickness of 2.5-4 µm and quadratic cross-sections. It is difficult to determine if these structures are hollow tubes of single crystals. However, on the basis of the prismatic morphology of the DL-alanine particles, we can assume that the tubes are probably single crystalline grown along the c-direction and exhibit the [210] planes on their sides. Figure 2c,d presents the crystal morphology of DL-alanine grown at a water/IPA volume ratio of 35%. At this ratio, the DL-alanine crystals kept their bulk morphology as needlelike crystals. However, the crystals’ end faces displayed a high degree of roughness and porosity, unlike the DL-alanine crystallized from water. The X-ray diffraction pattern of DL-alanine crystallized from pure water and from solutions of 25% and 35% IPA showed almost identical X-ray patterns. Nevertheless, the X-ray diffraction of DL-alanine at a very high IPA volume (50%) revealed differences in the diffraction pattern, as compared to the bulk case. This has to be clarified. In other words, the X-ray diffraction of DL-alanine at a high volume ratio of IPA (50%) gives us information on the overall role of IPA on the crystal morphology, which we assume is also meaningful for lower volume ratios. Figure 3 displays the powder X-ray diffraction of DL-alanine crystals crystallized from pure water and for DLalanine crystals crystallized from a water/IPA volume ratio of 50%. The X-ray powder diffraction of pure DL-alanine matched the data reported in the literature,34 with a orthorhombic space group (Pna21) and unit cell parameters (in Å) a ) 12.0263, b ) 6.0321, and c ) 5.829 (Figure 3a). For pure DL-alanine, the very rich spectrum with main diffraction peaks at 2θ ) 20.63, 30.85, and 34.47°, corresponding to the (210), (400), and (112) intense crystal diffraction planes. This X-ray diffraction pattern,

3648 Crystal Growth & Design, Vol. 8, No. 10, 2008

Medina and Mastai

Figure 2. SEM images of DL-alanine crystals crystallized from various water/IPA ratio volumes. (A and B) IPA 25% volume ratio. (C and D) DL-Alanine crystallized from a water/IPA 35% volume ratio.

Figure 3. Powder X-ray diffraction of volume ratio.

DL-alanine.

(A)

DL-Alanine

crystallized from water and (B)

particularly the intensity of the (210) crystal plane at 2θ ) 20.63°, proves directional growth of the crystals along the c-axis, as reported in the literature,34 and therefore leading to the needlelike morphology. The X-ray diffraction of DL-alanine crystallized from a water/IPA volume at a ratio of 50% displays only four X-ray diffraction peaks at (110), (210), (400), and (112) (Figure 3b). Furthermore, the X-ray peaks are very broad, and on the basis of the peak widths at half-height, crystals with typical sizes of 100 nm are calculated by the Debye-Scherrer equation.35 The formation of DL-alanine mesocrystals can also be reflected by different thermal behavior, and we assume that the phase transition temperature of DL-alanine crystals crystallized from pure water and from water-IPA solutions may be different. Accordingly, we employed a differential scanning calorimetry

DL-Alanine

DL-alanine

crystallized from a water/

crystallized from a water/IPA 50%

(DSC) method to measure the phase transition temperatures of pure crystals and of mesocrystals. The results of the DSC scan are shown in Figure 4. Pure DL-alanine has a sharp melting peak at 275 °C, as reported in the literature36 (Figure 4a). The DSC scan of DL-alanine crystals crystallized from a water/IPA 25% volume ratio solution shows two phase transition peaks: one sharp peak at 275 °C, corresponding to bulk DL-alanine, and a second broad peak at 215 °C, assumed to correspond to DLalanine mesocrystals (Figure 4b). From the DSC results, we can conclude that we have two types of crystals, the bulk DLalanine and the scan type with a lower melting peak that corresponds to mesocrystals that are less stable due to their hollow morphology. Moreover, on the basis of the relative area under these DSC peaks, we calculated that approximately 19% of the overall crystals are in the form of mesocrystals. The same

DL-Alanine

Mesocrystals with a Hollow Morphology

Figure 4. DSC for DL-alanine crystals. (A) DL-Alanine crystallized from water and (B) DL-alanine crystallized from a water/IPA volume ratio of 25%.

Figure 5. Micro-Raman of a DL-alanine single crystal. (a) DL-Alanine outer walls, corresponding to (210) faces and (b) DL-alanine crystal end faces, corresponding to (011), (201), and (001j) crystals faces.

phenomenon was also observed at different water/IPA ratios. It should be mentioned that the heating rate had a minimal effect on the position of the melting peaks. High-resolution SEM showed that DL-alanine crystallized from mixed solvents exhibits a different morphology. The outer walls of a single DL-alanine crystal that correspond to (210) faces exhibit a well-facetted crystalline morphology, while the end faces of the same crystal that correspond to (011), (201), and (001j) show a high degree of roughness and porosity (see the Supporting Information, Figure S2). To explore the degree of crystallinity of the different crystal faces, micro-Raman spectroscopy was used. Figure 5a presents the Raman scattering patterns of the DL-alanine (210) crystal face. The micro-Raman spectra taken from the (210) crystal face show the vibrational modes of DL-alanine crystal, as reported at the literature,37 and also fit the micro-Raman of DL-alanine crystals crystallized from water in our control crystallization. The main vibrational peaks, 546, 648, and 851 cm-1, correspond to the deformation of CdO and CO-O-O, CO-O-- wagging38 and C-C skeletal stretching/C-CH3 stretching, respectively. This Raman measurement shows that the (210) face exhibits a high degree of order and, therefore, is crystalline. Figure 5b presents the Raman scattering pattern of the DL-alanine end faces, namely, the (011), (201),

Crystal Growth & Design, Vol. 8, No. 10, 2008 3649

and (001j) faces. The scattering factors of these crystals faces are very low due to very poor crystallinity. This lack of crystallinity of crystal edges may be due to the formation of nanocrystals having a low Ramam scattering factor. It may also be that these crystal faces are amorphous. The above results strongly support our proposed mechanism for the formation of mesocrystals, as will be explained below. Finally, it should be noted that similar effects were observed in the crystallization of DL-alanine in the presence of other alcohols such as methanol and butanol. For example, in Figure 6a, the SEM images of crystals formed in a water-methanol 25% volume ratio solution are shown. In this case, the crystals also have a high degree of porosity and a rough surface at the end faces of the crystals. The crystals preserve their general elongated needle morphology, corresponding to the crystals crystallized from water, but in some cases, the crystals exhibit a strong change in morphology at the (201) and (011) crystals faces. To understand the rule of the IPA on the crystal morphology, we used molecular dynamic calculation DL-alanine morphology in the presence of IPA.39 For more details, see the Supporting Information. In summary, our results indicate the formation of DL-alanine mesocrystals. On the basis of these results, it is possible to develop a molecular mechanism for understanding the formation of DL-alanine mesocrystals during the crystallization process. However, before discussing our proposed mechanism for the formation of DL-alanine mesocrystals, we will briefly describe other suggested mechanisms for the formation of mesostructures. The formation of DL-alanine mesostructures was reported by Colfen and co-workers in a few papers.23,31-33 For example, the formation of DL-alanine mesocrystals in the presence of a chiral DHBC is interpreted by polymer adsorption onto the highly polar face of the DL-alanine crystals. The process leads to the formation of plateletlike crystals with a strong dipole moment, normal to the crystals’ growth direction. This induces the crystal-oriented attachment in the structure of the layers, resulting in the formation of mesocrystals. A similar mechanism for the formation of DL-alanine mesostructures is also proposed in the pH control crystallization of DL-alanine. In other words, the formation of the mesostructures is based on the alignment and assembly of nanoparticles.23 Overall, the formation of mesocrystals is understood based on two general rules, the formation of nanocrystals with a specific crystallographic orientation, followed by the self-assembly of the nanocrystals into mesostructures at well-defined directions. On the basis of our own results, we would present a somewhat different mechanism for the formation of DLalanine mesocrystals. From the kinetic measurements of crystallization based on solution conductivity and dynamic light scattering, we conclude that the first step in the formation DL-alanine mesostructures is the rapid nucleation of a high quantity of nanocrystals of DL-alanine. It is possible that a large amount of the DL-alanine participates as amorphous material, due to the rapid crystallization conditions, as a result of IPA effect on the supersaturation level of DL-alanine. It should be mentioned that at this stage we do not observe any particular crystallographic orientation of the nanocrystals different from the bulk crystallization. In the second stage, we assume the aggregation of the nanocrystals and amorphous intermediates into a well-defined DLalanine mesostructure. The mesocrystals clearly contain larger amounts of amorphous materials, as seen by the porous and rough surfaces of the as-deposited mesostructures. At this

3650 Crystal Growth & Design, Vol. 8, No. 10, 2008

Figure 6. (a and b) SEM images of

DL-alanine

Medina and Mastai

crystallized from water and 25% volume ratio methanol.

Figure 7. Proposed growth mechanisms of hollow DL-alanine mesocrystals. (a) Rapid precipitation of amorphous nanoparticles. (b) Self-assembly of DL-alanine nanoparticles. (c) Recrystallization and formation of hollow DL-alanine mesocrystals.

phase, the DL-alanine has a rodlike morphology, and the hollow structure is not observed. Finally, the last stage involves the phase transition of the amorphous DL-alanine in the mesostructure into crystalline DL-alanine. On the basis of the micro-Raman results, the SEM images, and the DSC measurements of the crystals, we can conclude that this phase transition occurs mostly from the core of the mesostructure to the edges, resulting in the formation of a hollow structure. The overall proposed scenario for the formation of the DLalanine mesocrystals is schematically shown in Figure 7. To sum up, our suggested mechanism is based on (i) the rapid nucleation of nanocrystals and amorphous material, (ii) the aggregation of these intermediates into a high order structure, and finally a phase transition step that involves a dissolution-recrystallization process of the amorphous material into a crystalline state. As opposed to the previous proposed mechanisms for the formation of mesostructures, our model does not include the prealignment of nanoparticles to mesostructures as a result of the presence of chemical additives (such as block copolymers31,32) in the crystallization solution. By varying the volume of IPA as a cosolvent in the crystallization process, we were able to demonstrate the formation of DL-alanine crystals with major morphology changes. It should be mentioned that in recent years a most interesting development in the understanding of biomineralization process is the realization that transient amorphous precursors of crystals are a widely used mechanism by organisms for producing crystals.40-42 Our proposed mechanism for the formation of DL-

alanine mesostructures is in line with the general approach of the biomineralization process. Conclusion Understanding the crystallization process and the aim of rational design crystals morphology, namely, crystal engineering, is the subject of both fundamental and practical interest. New discoveries related to our understanding of crystallization process recently emerged and have fundamentally changed our views of some processes involved in crystallization and morphology control of crystals. In this paper, the crystallization of DL-alanine from water/alcohol mixtures and the formation of well-defined hollow DL-alanine crystals are demonstrated. DL-Alanine crystallized from water/alcohol mixtures forms mesocrystals composed of three-dimensional, rodlike nanocrystals and amorphous material. At a certain range, alcohol acts as a cosolvent and mesocrystals with pores and rough surfaces are formed and are stable at a time scale of several hours, as demonstrated by SEM and DSC analysis. Micro-Raman measurements show a significant difference in crystallinity between the outer walls of the DL-alanine crystals and the crystals’ edges, indicating an amorphous or poor crystalline material. Time-resolved solution conductivity, which examines the kinetics of the formation of DL-alanine crystals, indicates the rapid precipitation of alanine to a metastable structure, presumably a defined assembly of nanocrystalline material. Finally, on the basis of our results, a formation mechanism is proposed for the DL-alanine hollow crystals. This work demonstrates a new principle for the

DL-Alanine

Mesocrystals with a Hollow Morphology

organization of nanocrystals to a high order structure, leading to the formation of mesocrystals. Experimental Section Analytical-grade amino acids were purchased from Aldrich-Sigma and used with no further purification. Analytical grade methanol, ethanol, and IPA were used as cosolvents. The crystallization experiments of DL-alanine were performed from various water/alcohol volume ratio solutions. In this section, a water/IPA solution mixture was chosen to represent the crystallization experiments from a water/alcohol solution. A 0.8 g amount of DL-alanine was suspended in 5 mL of double distillated water. The suspension was stirred and heated to 70 °C for 1 h until complete dissolution of the suspended powder. The solution was cooled to 40 °C and then filtered with a Teflon filter (0.2 µm mash). A 2.5 mL amount of IPA was added to the solution while stirring. The solution was then allowed to cool to room temperature. The crystallization process started after ca. 5 min when a suspended powder appeared in the solution. The first precipitation accrued after ca. 20 min as a white powder at the bottom of the glass flask. The crystals were collected at the end of the crystallization process and were separated from the solution by filtration under vacuum and left to dry on filter paper for a further half an hour under air. The crystals were kept dry for further characterization in a desiccator at room temperature. Using the same procedure, DL-alanine was crystallized from various water/alcohol volume ratio mixtures. Time-resolved conductivity measurements were performed using a conductivity meter (model CDM230 Radiometer Analytical SAS). X-ray diffraction analysis was performed using a Bruker AXS D8 Advance Diffractometer (using Cu KR λ )1.5418 Å radiation) operating at 40 kv/40 mA, with a graphitereflected beam monochromatic and variable divergence slits for powder X-ray diffraction. Average crystals sizes were calculated employing Debye-Scherrer equation35 with correction for instrumental broadening (∆θ ) 0.2 degree as measured for gold single crystal). SEM images were acquired on a JEOL 840 instrument at an accelerating voltage of 20 kV. Raman measurements were done with a Jobin Yvon Micro Raman (model HR800), with λ ) 632.817 nm at 20 mW, slit ) 100 µm, and hole ) 1000 µm. DSC was carried out with a Mettler Toledo DSC-822 equipped with a liquid nitrogen cooling system (scanning rate, 2 °C/min under nitrogen).

Acknowledgment. We thank Adi Kol for the graphical animation of Figure 7. D.D.M. thanks the Bar-Ilan President’s Ph.D. Scholarship Foundation and the Bar-Ilan Nanocenter Ph.D. Scholarship. Supporting Information Available: Light microscope images of crystallized from water, high-resolution SEM images, and molecular mechanics and dynamics calculations for DL-alanine crystal morphology. This material is available free of charge via the Internet at http://pubs.acs.org. DL-alanine

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