Biomimetic synthesis of mesoporous zinc phosphate nanoparticles

Journal of Alloys and Compounds 477 (2009) 657–660

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Biomimetic synthesis of mesoporous zinc phosphate nanoparticles Wen He a,b,∗ , Shunpu Yan b , Yingjun Wang a,∗ , Xudong Zhang b , Weijia Zhou b , Xiuying Tian b , Xianan Sun b , Xiuxiu Han b a b

Biomaterials Research Center, South China University of Technology, Guangzhou 510640, PR China Department of Materials Science and Engineering, Shandong Institute of Light Industry, Jinan, 250353, PR China

a r t i c l e

i n f o

Article history: Received 23 August 2008 Received in revised form 6 October 2008 Accepted 22 October 2008 Available online 10 December 2008 Keywords: Chemical synthesis Precipitation Nanostructures X-ray diffraction

a b s t r a c t Self-assembled biomolecular structures on microbial cells are particularly attractive, due to their versatile chemistry, molecular recognition properties, and biocompatibility. Their ability to work as templates for the fabrication of mesoporous nanocomposite has already been demonstrated. Here, we report on the preparation of zinc phosphate crystals with mesoporous structure by chemical precipitation using yeasts as a template. The technique, which combines biomineralization technology and crystals assembly on biomacromolecules, can be applied in a wide variety of fields. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

1. Introduction Much attention has been attracted to porous materials since the discovery of the ordered mesoporous silica M41S in 1992 [1]. Due to their high surface area, pore volumes and controllable pore sizes, these properties make them potential candidates for catalysts [2–4], molecular sieves [5,6], photovoltaic devices [7], chemical and biological sensors [8], and functional materials in general. A number of mesostructured materials have been synthesized in the past few years by various molecular templates. Zinc phosphate, as a new type of non-toxic, ecological anticorrosive pigment with excellent properties, has been applied in a wide variety of fields, from coating industry to tissue engineering. Since the discovery of zeolite-like zinc phosphates by Gier and Stucky in 1991 [9], open-framework zinc phosphates have been extensively studied. Though there are a number of processing routes for preparing porous materials, more and more people focus on the template method. Since the word “template” was firstly used by Birchall who worked in ICI in 1985, much research has been done on the synthesis, properties and applications of nanomaterials with the template approach. In biological systems, a large variety of organisms form organic/inorganic composites with ordered structures

∗ Corresponding authors at: Biomaterials Research Center, South China University of Technology, Guangzhou, 510640, PR China. Tel.: +862087114645; fax: +862022-23608. E-mail addresses: [email protected] (W. He), [email protected] (YJ. Wang).

by the use of biopolymers such as proteins, which have defined monomer sequences and controlled three-dimensional structures [10–19]. Yeast cell is one kind of the easiest microbes to cultivate. The rough surface of the cells possesses of many biomacromolecules. The biomacromolecules could induce crystals formation on them with special structures. Comparing with other surfactants, yeast cell is non-toxic, easy to degrade and no pollution which is consistent to environmentally friendly chemistry. The present work is to provide information on synthesis of nanosized zinc phosphate powder with mesoporous structure by chemical precipitation method with yeast cells which are non-toxic and no pollution as templates at room temperature. The explanation about the effect of the yeast cells on the synthesis of zinc phosphate is attempted. 2. Sample preparation and experimental procedure 2.1. Materials All reagents such as ZnSO4 ·7H2 O (99%, Tianjin Bodi Chemical and Industry Ltd.), Na3 PO4 ·12H2 O (99%, Tianjin Bodi Chemical and Industry Ltd.) are used at analytical grade as received and yeasts (Angel Yeast Co., Ltd.) without further purification. Distilled water was used throughout all the experiments. 2.2. Sample preparation First of all, 2.00 g of yeast cells were incubated in an aqueous solution of glucose (2 wt.%, 150 ml) at a temperature of 36 ◦ C for 30 min. Then 4.31 g of ZnSO4 ·7H2 O was put into the mixture under the condition of stirring. After stirring vigorously at 36 ◦ C for 2 h, an aqueous solution of 0.1 M (100 ml) trisodium phosphate was

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gradually added to the mixture drop by drop under stirring condition. An aqueous solution of NaOH (0.05 M) was employed to adjust the pH of the reaction solution to 8–10 which is the range of the presence of zinc phosphate precipitation. Continue to stir the mixture for another 2 h and a white gel was formed, which was aged at 25 ◦ C for 48 h. The resulting deposition was recovered by centrifugation at a rotation speed of 4500 rpm, washed five times with distilled water until the conductivity of the filtrate was less than 2 ms/m and washed one time with ethanol. The resultant product was dried at 80 ◦ C for 24 h. The dried sample was calcined at 500 and 750 ◦ C for 2 h to remove the surfactant. 2.3. Characterization techniques The as-prepared samples were examined by X-ray diffraction (XRD) technique, on a PANalytical X’Pert PRO X-ray diffractometer (Netherlands) with Cu K␣ radiation ( = 0.15418 nm). The diffraction patterns were collected at room temperature over the diffraction angle 2 in the range of 1.1–10◦ , 10–70◦ , with an acquisition time of 12.0 s at 0.02◦ step size. The BET (Brunauer–Emmett–Teller) surface area and pore size distribution were determined by N2 adsorption–desorption at 77 K using a computer controlled sorption analyzer (Micromeritics, Gemini V2.0, USA) operating in the continuous mode. Before measurement, the samples were degassed in vacuum at 150 ◦ C. Transmission electron microscopy (JEM-100X, Japan) operating at 100 kV was employed to characterize the morphologies of the as-prepared samples. Fourier transform infrared spectrograph (FTIR) was performed using a Nicolet Nexus spectrometer (NEXUS 470, Nicolet, USA) by using a KBr wafer technique in order to study the composition of the samples. Infrared spectra were recorded in the region 4000–400 cm−1 , with a resolution of 4.00 cm−1 .

3. Results and discussion The XRD patterns of the representative prepared samples using YCs (yeast cells) as templates are shown in Fig. 1. The XRD peaks of all the as-prepared samples are in good agreement with standard Zn3 (PO4 )2 ·4H2 O(PDF no. 39-0079). No characteristic peaks of impurities could be found in Fig. 1. So pure Zn3 (PO4 )2 ·4H2 O was obtained under the present experimental conditions. The Scherrer line width analysis of the [3 1 1] reflection gives an estimate of the

Fig. 1. Wide-angle XRD and low-angle XRD (inset) patterns of the sample.

primary crystallite size in the range of 11 nm. In the XRD patterns, no peak was observed in the low-angle range (2 = 1–10◦ , Fig. 1 inset), that suggests the absence of ordered mesoporous network structure in the sample. The TEM micrographs of Zn3 (PO4 )2 powders are shown in Fig. 2. In Fig. 2a, a spherical yeast cell can be observed with many channels on it. Fig. 2b shows the magnification of the framed region in Fig. 2a. If yeasts are not added during synthesis then bulk precipitation of much larger hydrated Zn3 (PO4 )2 particles occurs. When yeasts are added during processing, a spontaneous nucleation of

Fig. 2. (a) Typical TEM micrographs of the sample. (b) An amplificatory image of the framed region in (a). (c) A high magnification. (d) The sample calcined at 750 ◦ C for 2 h.

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Fig. 4. FTIR spectra of various samples studied in this work (a) YC, (b) Zn3 (PO4 )2 /YCs and curves of heat-treated at (c) 500 ◦ C and (d) 750 ◦ C for 2 h. Fig. 3. N2 gas adsorption–desorption isotherms and Barret–Joyner–Halenda (BJH) pore size distribution plot (inset) of the sample.

large numbers of very tiny Zn3 (PO4 )2 particles occur (Fig. 2c). Fig. 2c shows clearly the presence of some isolated tiny particles of size 10 nm forming soft agglomerates of varied shapes. Also, the presence of large numbers of pores with size 10 nm is observed. The calculated size of particles approximately 10 nm using Scherrer equation agrees well with that measured from TEM image. The stable three-dimensional connectivity/network is developed to form spongy scaffold by linking of nanoparticles one by one via reaction limited aggregation (RLA) process [20]. The pore network structure collapsed completely at 750 ◦ C (Fig. 2d). The specific surface area as high as 146 m2 /g was obtained and the average pore size of the as-prepared material was 10 nm. The typical type IV-isotherm curve can be seen in N2 gas adsorption–desorption isotherms as shown in Fig. 3. The pore diameter estimated from the equation d (nm) = 6000/S [where, density,  (gm/cm3 ) and surface area, S (m2 /g)] is found to be in the range of 10–11 nm. The pore distribution obtained from isotherms is given in Fig. 3 inset that shows the presence of pores having size 10 nm. The average crystallite size evaluated from the line width of XRD peak agrees well with the TEM data.

FTIR spectras shown in Fig. 4 were carried out in order to study the as-prepared particles. The IR bands for native yeast cells are attributed to characteristic chemical functional groups in their structure. Even though the samples were dried in a vacuum oven at 100 ◦ C, a broad peak appears at 3400 and 1609 cm−1 in the YC–Zn3 (PO4 )2 IR spectra which is attributed to water absorption. That suggests that zinc phosphate binds water. The existence of CH2 peaks (1438, 2852 and 2925 cm−1 ) and NH2 peaks (608 and 1629 cm−1 ) show that there are some residual yeasts or biomacromolecules after the extraction by water and ethanol mixtures. The bands still retain at 500 ◦ C heat-treated samples indicating its presence which helps to retain the porous nature of aggregates. However, the intensity of these bands vanished completely at 750 ◦ C heat-treated material. Thus, IR study confirmed that some organic groups bond on the aged Zn3 (PO4 )2 particles. The absorption bands at 561, 960, and 1033 cm−1 are the characteristic peaks of PO4 3− . It was noted that the peaks at 960 and 1033 cm−1 were located on the shoulders of the 1060 cm−1 peak instead of two sharp bands. This shape seemed to indicate the presence of a poorly crystalline Zn3 (PO4 )2 , which is in accordance with the XRD results. Biomineralization involves initial nucleation and subsequent growth of crystals from aqueous solutions. Surface functional

Fig. 5. Schematic diagrams of various interaction processes (a) Zn3 (PO4 )2 crystals grow around the trapped ions and (b) on phosphate radicals.

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groups of substrate materials play a decisive role in both nucleation and growth. Much research has been conducted to elucidate mechanisms of inorganic matter formation on an outer surface and to determine its substrate dependence [21–24]. As is known to all, the pericellular membrane is made up of glycoprotein, membrane lipid and membrane protein which have many carboxyl groups [25], amino groups [23] and hydroxyl groups [26]. Yeast cells incubating in liquid are negatively charged [25,26] (have been tested) that could induce crystals formation. To take a kind of glycoprotein as an example, the responses may be interpreted by the following mechanisms (shown in Fig. 5). It is believed that the nonionic hydroxyl groups, carboxyl groups, amino groups and other negatively charged groups may firstly bind the zinc ions through ionic–dipolar interaction as nucleating points. Nuclei of zinc phosphate crystals emerge and will grow around these trapped ions when phosphate radicals are added. The process is schematically displayed in Fig. 5a. However, a different process still exists for the yeast cells. For one thing, as shown in Fig. 5b, esterification takes place between hydroxyl groups on the glycoprotein and phosphate radicals who are bond on the cells’ surface through strong covalent bonds. Then zinc ions were trapped on the phosphate radical groups, forming zinc phosphate complexes that act as nuclei of Zn3 (PO4 )2 ·4H2 O crystals and the crystals grow by further complexation with zinc ions in the aqueous solution as shown in Fig. 5. Note that the bonding for the former is via ionic–dipolar interaction while the later is via strong ionic bonds. Thanks to the multiple performance of glycoprotein in the process, crystals will grow around biomacromolecules on the coarse surface of cells or around groups on the template. Particles congregated around the floppy template which plays as a scaffold. Groups on the template also prevented the aggregation of zinc phosphate particles. When the template was removed by centrifugation, piled pores of zinc phosphate particles could be obtained. 4. Conclusion We have demonstrated a simple chemical precipitation route in aqueous media resulting in the formation of very fine crystalline zinc phosphate particles after aging at room temperatures. These particles stuck together one by one to form mesoporous aggregates with high surface area. Clearly, the use of yeast cells may not be the ideal route for inorganic nanomaterial synthesis, but it does provide a novel method that might be worth considering in the bottom-up fabrication of materials with mesoporous structure.

The technique can be expanded to many material systems to synthesize mesoporous materials like mesoporous metal-oxide and metal phosphate, and it provides a general, simple, convenient, and innovative strategy for the synthesis of nanoparticles of metallic phosphates which may have special configuration. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 50572029 and 50732003), 973 Project Grant of China (Grant No. 2005CB623902) and Natural Science Foundation Cooperative Project Grant of Guangdong (Grant No. 04205786). The authors wish to thank Xinguo Gao and Jingyun Ma for their assistance in characterization. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. [2] M.D. Wei, M. Okabe, H. Arakawa, Y. Teraoka, React. Kinet. Catal. Lett. 77 (2002) 381–387. [3] I. Yuranov, P. Moeckli, E. Suvorova, P. Buffat, L. Kiwi-Minsker, A. Renken, J. Mol. Catal. A: Chem. 192 (2003) 239–251. [4] V.I. Parvulescu, S. Boghosian, V. Parvulescu, S.M. Jung, P. Grange, J. Catal. 217 (2003) 172–185. [5] K.Y. Ho, G. McKay, K.L. Yeung, Langmuir 19 (2003) 3019–3204. [6] A. Stein, Adv. Mater. 15 (2003) 763–775. [7] D.H. Wang, W.L. Zhou, B.F. McCaughy, J.E. Hampsey, et al., Adv. Mater. 15 (2003) 130–133. [8] W. Yantasee, Y.H. Lin, G.E. Fryxell, Z.M. Wang, Electroanalysis 16 (2004) 870–877. [9] T.E. Gier, G.D. Stucky, J. Nat. 349 (1991) 508–510. [10] P. Calvert, S. Mann, J. Mater. Sci. 23 (1988) 3801–3815. [11] L. Addadi, S. Weiner, Angew. Chem. Int. Ed. Engl. 31 (1992) 153–169. [12] S. Weiner, L. Addadi, J. Mater. Chem. 7 (1997) 689–702. [13] S. Mann, J. Nat. 332 (1988) 119–124. [14] S. Mann, G.A. Ozin, J. Nat. 382 (1996) 313–318. [15] C.S. Sikes, A.P. Wheeler, Chemtech 18 (1988) 620–626. [16] N. Watabe, J. Ultrastruct. Res. 12 (1965) 351–370. [17] G.A. Ozin, Chem. Commun. 6 (2000) 419–432. [18] G. Krampitz, G. Graser, Angew. Chem. Int. Ed. Engl. 27 (1988) 1145–1156. [19] S. Weiner, L. Addadi, Trends Biochem. Sci. 16 (1991) 252–256. [20] B.C. Bunker, P.C. Rieke, B.J. Tarasevich, A.A. Campbell, G.L. Graff, Science 264 (1994) 48–55. [21] M. Tanahashi, T. Matsuda, J. Biomed. Mater. Res. 34 (1997) 305–315. [22] E.G. Derek, J.K. Amanda, Science 259 (1993) 1439–1442. [23] H.M. Kim, F.F. Miyaji, T. Kokubo, T. Nakamura, J. Biomed. Mater. Res. 38 (1997) 409–417. [24] M. Tanahashi, T. Yao, T. Kokubo, M. Minoda, T. Miyamoto, T. Nakamura, T. Yamamuro, Mater. Res. 29 (1995) 349–358. [25] A. Tachibana, S. Kaneko, T. Tanabe, K. Yamauchi, Biomaterials 26 (2005) 297–302. [26] L. Addadi, S. Weiner, Proc. Natl. Acad. Sci. U.S.A. 85 (1985) 4110–4114.