Cryst. Res. Technol. 43, No. 7, 740 – 744 (2008) / DOI 10.1002/crat.200711105

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Simultaneous synthesis of different structures of calcium oxalate

by living bi-template

Sheng Li1, Dong-Mei Sun

1, Qing-Sheng Wu*

1, and Ya-Ping Ding

2

1 Department of Chemistry, Tongji University, Shanghai 200092, P. R. China 2 Department of Chemistry, Shanghai University, Shanghai 200436, P. R. China

Received 4 October 2007, accepted 16 December 2007

Published online 29 February 2008

Key words biocrystallization, nanomaterials, calcium oxalate dehydrate, crystallization, living bi-template.

PACS 61.82.Rx, 79.60.Jv

Biomimetic living bi-templates, mung bean sprouts (MBS), were employed to control the crystallization of

calcium oxalate dehydrate (COD). Two kinds of crystals in different shapes were simultaneously grown on

the outer surface and the inner stem wall of MBS respectively. The whole process is in the living system that

material flow and energy exchange ceaselessly. The products were respectively characterized by SEM, XRD.

A presumable mechanism is proposed.

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Nature uses organic molecules to control and direct the crystallization of biominerals with special orientation, texture and morphology under ambient conditions [1]. Furthermore, biominerals are organized from the nano- to macroscopic scales to form structures with hierarchical long-range order. Inspired by the formation of biominerals, many studies are carried out to control the polymorph and morphology of minerals in lab.

In the field of biomineralization, the study of the crystallization of calcium oxalate arouses much interest, since calcium oxalate crystal has been known as a possible source of urinary and kidney stones [2,3]. In addition, mineral formation in plants is a widespread phenomenon. There has been considerable interest in the crystallization of calcium oxalate [4], which is the most commonly formed biomineral in higher plants [5]. However calcium oxalate dehydrate is less commonly found in plants. Generally the crystals adopt raphides or styloids shapes. Some studies have focused on developing and understanding interactions between calcium oxalate crystals, organic templates and additives [6,7]. For example, Langmuir monolayer [8-10], membrane vesicles [11], and phospholipids micelles [12] have been used for the direct crystallization of calcium oxalate.

However, the control of calcium oxalate crystal has been achieved to a very limited extent so far compared with the unusual morphologies adopted by plant calcium oxalate crystals [6]. People can not control the process of crystallization according to their own will. Bionic research is still considered to be different from natural mineralization for, at least, the following two reasons: firstly, there is no living cell which can directly control the complex process in the laboratory bionic research; secondly, laboratory bionic process is not developed in flowing system in which material and energy exchange ceaselessly.

Based on the above considerations, a new method was proposed to control the crystallization by using living biological template. The whole reaction process is a state of the material flow and energy exchange. Mung bean sprout, the sprout form of mung bean, a traditional vegetable in Asia, is available in supermarket, rather inexpensive and without season limitation, which can remain stable at temperature between 0°C to 35°C and at pH values from 4 to 10. Besides, MBS consists of large amount of celluloses, proteins and more than seventeen kinds of amino acids, such as aspartic acid, glutamic acid, valine, etc. In principle, these various ____________________

* Corresponding author: e-mail: qswu@mail.tongji.edu.cn

Cryst. Res. Technol. 43, No. 7 (2008) 741

www.crt-journal.org © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

functionalities such as hydroxyl, primary amide, carboxylate, and amine groups should offer a wide variety of nucleation sites for surface-controlled inorganic deposition [13]. The epidermal surface and the inner stem of MBS have different structures, which have been proved by biology. Figure 1b shows the outside surface and figure 1c shows the inside stem patterns of MBS. From figure 1b, it can be seen that the epidermis is totally not smooth but has well-distributed grooves, with the widths around 80~120 µm and diameters around 10~20 µm. Figure 1c is the cross section of the inner stem of MBS. Stems are composed of many canaliculi with uniform size, whose apertures are at about 100 µm. In addition, there are a lot of pleats on the inner stem wall. It is considered that this study can provide insights on the cooperation between bionic research and living vegetal cells.

Fig 1 (a) Photograph of living MBS; (b) SEM images of the cross section of the interior stem SEM images of the

epidermal surface of mung bean sprout (outside); (c) the cross section of the interior stem of mung bean sprout (inside).

2 Experimental

Synthesis Mung bean sprouts were purchased from supermarket. Prior to experiment, the plants were washed by distilled water several times to ensure no impurities attached on them. 1 The MBS (except laminae) were immersed in a solution of 0.05 mol/L CaCl2 for 8 h. 2 Taken out of the CaCl2 solution and washed by distilled water, the MBS were divided into tow parts. One

was immersed in a solution of 0.05 mol/L Na2C2O4, and the other was immersed in the same solution but 0.01 mol/L sodium dodecyl sulfate (SDS) was added. Then the MBS were kept to immerse in the solution for 8 h until reaction completed.

3 After being washed by distilled water and absolute ethanol repeatedly, the different products, accreted inside and outside of the MBS, were successively collected into two beakers.

4 Before the precipitates were characterized, all precipitates were carefully washed repeatedly with distilled water and absolute ethanol followed by centrifuged to ensure no residual MBS attached on the products. Characterization Powder X-ray diffraction (XRD) patterns were recorded on a Philips PW1700 X-ray

diffractometer (XRD) with Cu Kα radiation (λ=0.154 18 nm) (Holand). Scanning electron microscopy (SEM) measurements were performed with a field-emission environmental scanning electron microscope FEI/ Phillps XL30 ESEM-FEG (SEM, Holand). All measurements were carried out at room temperature.

3 Results and discussion

In the control experiments without any crystal modifiers, the morphology image of the as-grown COD (outside and inside of the MBS) was obtained using SEM, as shown in figure 2a. The outside products exhibit some dendritic hierarchical architecture. The inside products, as shown in figure 2b, exhibit bouquet-like. Further observation reveals that all the bouquet-like productions were assembled by substructure just like nano-rods.

Our former experimental studies have been devoted to the polymorphic formation of calcium carbonate [14]. Crystal modifier is considered one of the most important factors in controlling the formation of the crystalline phase and morphology. In this paper, we also observed the effect of crystal modifier on the growth process. The morphology images of the COD prepared by MBS in presence of SDS as crystal modifier were

742 Sheng Li et al.: Synthesis of different structures of calcium oxalate

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

obtained using SEM, the morphology of outside products is rod-like (as shown in Fig. 3a), and the inside products is sphere (as shown in Fig. 3b).

Fig. 2 SEM images of as-prepared COD: (a) outside sample; (b) inside sample.

Fig. 3 SEM images of as-prepared COD after SDS is added: (a) outside sample; (b) inside sample.

The result of powder X-ray diffraction depicts that all the above products prepared by MBS have the same structure. The peaks are corresponding to <hkl> values of (200), (211), (411), and so on, which are in good agreement with reference (JCPDS File No. 17-0541). The unit cell of the structure is body-centered tetragonal, with references of the cell unit a=b=12.35 Å and c=7.363 Å. Figure 4 is the XRD pattern. Figure 4a is the pattern of outside sample and figure 4b is inside.

4 Mechanism exploration

It can be seen from the above experimental results that MBS themselves play an important role as template in the process of COD crystallization. MBS arrange inside product dendritic. After modifier is added, the order is changed, so a new morphology is obtained.

Cryst. Res. Technol. 43, No. 7 (2008) 743

www.crt-journal.org © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A probable mechanism of the morphology of COD is proposed: first of all, the mineralization experiment is carried out with the existence of living plant cells. The reaction process is full of material and energy exchanging. Biologically, the MBS method has a template effect for its spatial structure and various organic groups’ arrangements. And the main groups include hydroxyl, primary amide, carboxylate, and some phospholipid groups, etc. According to the theory of soft-hard acid-base, Ca2+ ion belongs to hard acid, which will preferentially combine with hard alkaline groups, such as oxygen in the hydroxyl and carboxylate groups. Therefore, Ca2+ ion was firstly combined on the wall of the MBS containing many hydroxyl and carboxylate groups mainly by electrostatic attraction. It means that the nucleation of COD will be affected by the MBS. After the C2O4

2- ion entered the MBS, COD molecules were formed, in the presence of the organic phase of MBS, the COD were continuously produced on the interface, later the substructure came to being. Then, induced by the bio-template, substructure was assembled to the last morphology. This step is also supported by our earlier findings that the superstructures do not really grow from a supersaturated ion solution but by transformation of the primary clusters formed [15].

Fig. 4 XRD patterns of as-prepared COD sample.

While the modifier was added, the process was considered to be under the control of the cooperation template effect of MBS with the crystal modifier. MBS is a living bio-template, which played an important role in the whole formation process. In the process of nucleation, templates are dominant in the nucleation positions through special confinement. During further growth period, the external and internal surface template groups preferentially adsorbed in some crystal faces and inhibited these faces from further growth. Meanwhile, the special function of organic groups induced substructure to directionally assemble and oriented grow. In the end, the outside and inside dendritic patterns and other morphologies were formed, respectively. When the modifier was added, the interaction between MBS and modifier was changed, so the morphologies were changed too. Detailed mechanism needs to be further investigated in the future work.

4 Conclusion

In this paper, an easy and environment-friendly route, biomimetic living bi-templates, mung bean sprouts (MBS), is reported for effective control of the crystallization of calcium oxalate dehydrate. The two kinds of crystals in different shapes were simultaneously grown on the outer surface and the inner stem wall of MBS respectively. Modifier is an important factor for the morphology control. The detailed formation mechanism may be very complicated and further investigation is to be continued. Finally, it is interesting to consider our results in light of studies of calcium carbonate biomineralization.

Acknowledgements The authors are grateful to the financial support of the National Natural Science Foundation of China

(Nos. 50772074, 20471042) and the Nano-Foundation of Shanghai in China (No. 0652nm007).

References

[1] S. Mann, J. Webb, and R. J. P. Williams, “Biomineralization Chemical and Biochemical Perspectives”, VCH

Publishers, New York, p. 135.

744 Sheng Li et al.: Synthesis of different structures of calcium oxalate

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

[2] N. Bouropoulos, S. Weiner, and L. Addadi, Chem. Eur. J. 7, 1881 (2001).

[3] S. Guo, M. D. Ward, and J. A. Wesson, Langmuir 18, 4284 (2002).

[4] M. J. Lochhead, L. Touryan, and V. Vogel, J. Phys. Chem. B 103, 3411 (1999).

[5] S. V. Pennisi, D. B. McConnell, L. B. Gower, M. E. Kane, and T. Lucansky, New Phytol. 150, 110 (2001).

[6] D. B. Zhang, L. M. Qi, J. M. Ma, and H. M. Cheng, J. Mater. Chem. 12, 3677 (2002).

[7] J. J. De Yoreo, S. R. Qiu, and J. R. Hoyer, Am. J. Physiol. Renal. Physiol. 291, 1123 (2006).

[8] M. J. Lochhead, S. R. Letellier, and V. Vogel, J. Phys. Chem. B 101, 10821 (1997).

[9] S. R. Letellier, M. J. Lochhead, A. A. Campbell, and V. Vogel, Biochim. Biophys. Acta-Gen. Subj. 1380, 31

(1998).

[10] R. Backov, C. M. Lee, S. R. Khan, C. Mingotaud, G. E. Fanucci, and D. R. Talham, Langmuir 16, 6013 (2000).

[11] J. M. Ouyang, H. Zheng, and S. P. Deng, J. Cryst. Growth 293, 118 (2006).

[12] C. M. Brown, F. Novin, and D. L.Purich, J. Cryst. Growth 135, 523 (1994).

[13] W. Shenton, S. A. Davis, and S. Mann, Adn. Mater. 11, 449 (1999).

[14] D. M. Sun, Q. S. Wu, and Y. P. Ding, J. Appl. Cryst. 39, 544 (2006).

[15] Y. Yan, Q. S. Wu, L. Li, and Y. P. Ding, Cryst. Growth Des. 6, 769 (2006).