Design and hierarchical synthesis of branched

INSTITUTE OF PHYSICS PUBLISHING

SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 15 (2006) N46–N50

doi:10.1088/0964-1726/15/2/N04

TECHNICAL NOTE

Design and hierarchical synthesis of branched heteromicrostructures Tierui Zhang1 , Wenjun Dong1 , Jay Kasbohm1, Vijay K Varadan2 and Z Ryan Tian1 1 2

Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA Department of Electrical Engineering, University of Arkansas, Fayetteville, AR 72701, USA

E-mail: [email protected]

Received 9 January 2006, in final form 2 February 2006 Published 16 March 2006 Online at stacks.iop.org/SMS/15/N46 Abstract Smart heteromicrostructures of a new kind, composed of SiO2 microspheres and ZnO branches, have been prepared hierarchically for the first time. Such heteromicrostructures have been characterized by means of x-ray diffraction and field emission scanning electron microscopy. Suggesting a possible formation process has been tackled. This type of smart branched heteromaterial is expected to have applications in fields of nanomedicine such as clearing clogged arteries, breaking aggregated amyloids and delivering drugs.

1. Introduction Nanotechnology is widely believed to have unusual potential for greatly advancing the existing technologies in the new fields of nanomedicine involving diagnosis, drug delivery and tissue regeneration, to name but a few. The advances would largely rely on the development of smart nanomaterials and nanodevices including nanoparticles and nanofibers (e.g. nanotubes, nanowires, nanobelts, nanoribbons) that can function as biomaterials, sensors etc [1]. In contrast to other normal nanomaterials, branched structures could have two different structures and functions rationally integrated to give unique application potential [2–4]. Great efforts are currently devoted to the controlled solution synthesis and fabrication of branched nanostructures of different sorts. Alivisatos et al have reported a process for controlled synthesis of branched (such as tetrapods and inorganic dendrimer heterostructures) and hyperbranched (the shapes range from ‘thorny balls’, to tree-like ramified structures, to delicate ‘spiderweb’-like particles) nanocrystals of semiconducting CdTe and CdSe [5–7]. The lengths and widths of the branching arms, and the magnitude of branching could be controlled by varying the amount and kind of organic surfactant. CdS and MnS tetrapods were made by Cheon using a monosurfactant system under atmospheric conditions [8, 9]. Most recently, Liu et al used a method of 0964-1726/06/020046+05$30.00

sequential nucleation and growth to systematically assemble large arrays of hierarchical architecture ZnO nanocrystals. Oriented secondary ZnO and CdS nanobranches have also been grown on facets of primary ZnO nanorods [10]. Magnetic nanoparticles have been extensively studied in biomedical fields for magnetic resonance imaging, protein and enzyme immobilization, immunoassay, RNA and DNA purification, tissue-specific release of therapeutic agents, labeling and sorting of cells, separation of biochemical products and targeting drugs [11–18]. If a ‘spiky’ heteromicrostructure contains magnetic components, its movement in the three-dimensional (3D) space could be manipulated by a magnet or an external magnetic field. Such a controllable movement could be applied to the destruction of, for instance, plaques in biological systems, which is the driving force of this study. On the basis of our previous experience, we think that ZnO nanorods or microrods could be good candidate spikes. Further, the surface of the ZnO branches would be modified with various coupling agents such as fluorescent organic dyes and inorganic nanoparticles [19] for nanomedicine applications such as clearing clogged arteries, breaking aggregated amyloids and delivering drugs. ZnO is a wide band-gap ( E g = 3.37 eV) semiconductor with a large exciton binding energy (60 meV), exhibiting near-ultraviolet emission, transparent conductivity and piezo-

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O Zn

NH4OH

Zn(NO3)2 + HMT SiO2

SiO2

90 oC

TEOS + C2H5OH + H2O

2. Experimental section 2.1. Preparation of SiO2 microspheres covered with ZnO microbranches (SiO2 /ZnO) The SiO2 microspheres were prepared according to the method of Wang’s paper [34]. 200 µg of thus-prepared SiO2 microspheres were dispersed in 54 ml of deionized water. The suspension was then heated to 90 ◦ C in a 120 ml bottle sealed with an autoclavable screw cap. After 10 min, 0.3 ml of 0.2 M zinc nitrate (Zn(NO3 )2 ·6H2 O, Aldrich) was added into the hot suspension. This system was then heated for another 10 min, and this was followed by the addition of 0.3 ml of 0.2 M hexamethylenetetramine (HMT, Aldrich). The whole system was thereafter heated for 9 h at 90 ◦ C. The resulting powders were collected after a centrifugation, washed repeatedly with deionized water and ethanol, and then dried overnight in vacuum at room temperature. A schematic illustration of the fabrication process is shown in figure 1.

30

40

50

60

(200) (112) (201)

(103)

(102)

(002)

(110)

(101)

Intensity (CPS)

electricity [20, 21]. In addition, ZnO is commonly used in sunscreens and dental filler, reflecting a certain biocompatibility for commercial powdery ZnO [22–24]. Via simple and inexpensive wet chemistry routes, rod-, wire-, tube-, tower-, rotor-, doughnut-and flower-like ZnO nanostructures [25–28] together with oriented helical ZnO nanorod arrays [29] have been fabricated recently. Biocompatible SiO2 , on the other hand, could be a good core candidate, because much of our previous work on making oriented ZnO nanorods was done on silica glass. Amorphous silica has been widely used as an adjuvant in pharmaceutical technology due to its very good thermal and chemical stability, and nontoxic and biocompatible characteristics [30]. The surface of silica is often terminated by a wealth of silanol groups that can easily bind metal ions and react with various agents or active functional groups for new structures to grow on [31]. Thus, SiO2 would be an ideal coating on a magnetic core for forming the Fe3 O4 /SiO2 core–shell structure that can prevent anisotropic magnetic dipolar attraction in the absence of an external magnetic field, can enhance the wear and corrosion resistance of the magnetic particles, and can finetune magnetic properties with temperature [32, 33]. In this work, SiO2 microspheres with the surface modified by ZnO branches have been fabricated for the first time. This synthetic advance will enable us to further explore similar synthesis but using a magnetic core (Fe3 O4 ) covered with an SiO2 shell. These unusual heteromicrostructures could be useful in the above-mentioned important nanomedicine applications.

(100)

Figure 1. Schematic illustration of the fabrication of the SiO2 /ZnO heteromicrostructures.

70

2θ (degree)

Figure 2. XRD pattern of the SiO2 /ZnO heteromicrostructures.

2.2. Characterizations The phase composition and structure of SiO2 /ZnO were characterized by means of x-ray diffraction using a Philips ˚ scanning X’pert x-ray Cu Kα diffractometer (λ = 1.5418 A) from 10◦ to 80◦ (2θ ) at a speed of 1◦ min−1 . A drop of the aqueous powder suspension was introduced onto a 1 cm × 1 cm glass substrate and then dried in air for the x-ray diffraction (XRD) studies. The morphology of the resulting powders was mainly examined under a field emission scanning electron microscope (FESEM; Philips ESEM XL30) operated at 10 kV.

3. Results and discussion Figure 2 shows an XRD pattern of the SiO2 /ZnO heteromicrostructures. Nine diffraction peaks, (100), (002), (101), (102), (110), (103), (200), (112) and (201), could be indexed to the wurtzite type of ZnO with lattice constants (a = ˚ , c = 5.200 A) ˚ close to the reported data (JCPDS 793.248 A ˚ , c = 5.207 A). ˚ Since no other XRD peaks 2205, a = 3.250 A were detected, the SiO2 is amorphous and no other crystalline phases coexisted in the samples. The morphologies of the SiO2 microspheres and SiO2 /ZnO heteromicrostructures were examined by means of the FESEM. As shown in figure 3(a), these SiO2 microspheres are nearly monodisperse and about 475 nm in diameter. For the SiO2 /ZnO composites, many branched structures can be observed in figure 3(b). The detailed morphological variations are illustrated in figures 3(c) and (d). It can be clearly seen N47

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Figure 3. SEM images of the SiO2 microspheres and SiO2 /ZnO heteromicrostructures: (a) SiO2 microspheres; (b) a low magnification SEM image of the heteromicrostructures; ((c), (d)) high magnification SEM images of the heteromicrostructures.

in figure 3(c) that the SiO2 microsphere is in the center, with several ZnO branches grown around it in a nearly radial

fashion. The ZnO branches are about 1.6 µm in length and 520 nm in diameter. Figure 3(d) further shows that the SiO2 microsphere has been grown into a form fully covered by ZnO branches, thus making the microsphere center difficult to see. Although no ZnO hexagon [001] facets could be clearly seen for this sample, each of the ZnO branches would still be wurtzite ZnO [35] according to our XRD studies. The possible mechanism for the variation in the number of branches on the SiO2 microspheres will be discussed in the next section. The formation of the SiO2 /ZnO heteromicrostructures could be understood as follows. The surface of the silica microsphere could be largely covered by either Si–OH or Si– O–Si groups [31], each of which could be coordinating with the Zn2+ cations in the solution. In this system, the HMT would act as a pH buffer and react with water to produce ammonia (NH3 ) at an elevated temperature, providing the system with a slow and constant supply of OH− anions [36]. The OH− anions would then react with the Zn2+ at the elevated temperature, resulting in wurtzite-type ZnO rather than zinc hydroxide or hydrated zinc oxide. We observed that different synthesis conditions lead to different numbers of ZnO branches. Due possibly to the various features of the complexity of ZnO heterogeneous nucleation on SiO2 surface sites (e.g. the ratio of the Si–OH to the Si–O–Si), ZnO branches could grow more densely on certain SiO2 surface sites than on the others, producing an uneven distribution of ZnO microbranches on the SiO2 microspheres. When the HMT was replaced by NaOH during the synthesis, no branched composites could be found except the irregularly shaped ZnO particles plus the bare SiO2 microspheres. Hence, the HMT certainly plays an important role in forming the ZnO branches on the SiO2 microsphere surface. In this sense, the slow supply of a low content of OH− anions could be a main driving force for the formation of ZnO/SiO2 heteromicrostructures. Nonetheless, the synthesis of such heteromicrostructures would enable us to further incorporate magnetic nanostructures into the heteromicrostructures to meet challenges in the development of new nanomedicine technologies. Many technological applications require magnetic nanoparticles to be embedded in a nonmagnetic matrix. Encapsulating magnetic nanoparticles in biocompatible silica could help prevent nanoparticle aggregation in solution and improve their chemical stability [37]. Likewise, the biocompatible secondary ZnO rods could be readily grown on a magnetic iron oxide nanoparticle covered by a biocompatible SiO2 shell (see figure 4), resulting in an unusual heteromicrostructures with integrated properties for the above-mentioned nanomedicine

O Zn SiO2

SiO2

NH4OH

Na2SiO3 Fe3O4

Fe2+ + Fe3+

80 oC

Zn(NO3)2 + HMT Fe3O4

Fe3O4

90 oC

Figure 4. A schematic illustration of fabricating new Fe3 O4 /SiO2 /ZnO heteromicrostructures.

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applications. With such inspiration, further synthesis explorations are being actively undertaken in our group.

4. Conclusions This study presents for the first time a rational hierarchical synthesis of smart branched heteromicrostructures based on coating SiO2 microspheres with highly oriented ZnO microrod branches. XRD and SEM characterizations reveal the crystal nature of the ZnO rods and that of the SiO2 /ZnO heteromicrostructures. Both the surface properties of SiO2 microspheres and the HMT concentration in the solution play critical roles in the formation of the oriented ZnO microbranches. This method could be further extended to fabrications of magnetic Fe3 O4 core–SiO2 shell–ZnO branch kinds of new heteromicrostructures for potential nanomedicine applications.

Acknowledgments This work was supported by a UA start-up grant and a grant from the ABI. The authors would like to thank Mr A Toland and Dr J Shultz of Arkansas Analytical Lab (AAL) for their help with the FESEM and XRD measurements.

References [1] Roco M C 2003 Nanotechnology: convergence with modern biology and medicine Curr. Opin. Biotechnol. 14 337–46 [2] Gao P X and Wang Z L 2002 Self-assembled nanowire–nanoribbon junction arrays of ZnO J. Phys. Chem. B 106 12653–7 [3] Yan H Q, He R R, Johnson J, Law M, Saykally R J and Yang P D 2003 Dendritic nanowire ultraviolet laser array J. Am. Chem. Soc. 125 4728–9 [4] Hao E C, Bailey R C, Schatz G C, Hupp J T and Li S Y 2004 Synthesis and optical properties of ‘branched’ gold nanocrystals Nano Lett. 4 327–30 [5] Manna L, Milliron D J, Meisel A, Scher E C and Alivisatos A P 2003 Controlled growth of tetrapod-branched inorganic nanocrystals Nature Mater. 2 382–5 [6] Milliron D J, Hughes S M, Cui Y, Manna L, Li J B, Wang L W and Alivisatos A P 2004 Colloidal nanocrystal heterostructures with linear and branched topology Nature 430 190–5 [7] Kanaras A G, Sonnichsen C, Liu H T and Alivisatos A P 2005 Controlled synthesis of hyperbranched inorganic nanocrystals with rich three-dimensional structures Nano Lett. 5 2164–7 [8] Jun Y W, Lee S M, Kang N J and Cheon J 2001 Controlled synthesis of multi-armed CdS nanorod architectures using monosurfactant System J. Am. Chem. Soc. 123 5150–1 [9] Jun Y W, Jung Y Y and Cheon J 2002 Architectural control of magnetic semiconductor nanocrystals J. Am. Chem. Soc. 124 615–9 [10] Sounart T L, Liu J, Voigt J A, Hsu J W P, Spoerke E D, Tian Z R and Jiang Y B 2006 Sequential nucleation and growth of complex nanostructured films Adv. Funct. Mater. 16 335–44 [11] Babes L, Denizot B, Tanguy G, Le Jeune J J and Jallet P 1999 Synthesis of iron oxide nanoparticles used as MRI contrast agents: a parametric study J. Colloid Interface Sci. 212 474–82

[12] Kondo A and Fukuda H 1997 Preparation of thermo-sensitive magnetic hydrogel microspheres and application to enzyme immobilization J. Ferment. Bioeng. 84 337–41 [13] Khng H P, Cunliffe D, Davies S, Turner N A and Vulfson E N 1998 The synthesis of sub-micron magnetic particles and their use for preparative purification of proteins Biotechnol. Bioeng. 60 419–24 [14] Gupta P K and Hung C T 1989 Magnetically controlled targeted micro-carrier systems Life Sci. 44 175–86 [15] Chemla Y R, Crossman H L, Poon Y, McDermott R, Stevens R, Alper M D and Clarke J 2000 Ultrasensitive magnetic biosensor for homogeneous immunoassay Proc. Natl Acad. Sci. USA 97 14268–72 [16] Sonti S V and Bose A 1995 Cell-separation using protein-a-coated magnetic nanoclusters J. Colloid Interface Sci. 170 575–85 [17] Dyal A, Loos K, Noto M, Chang S W, Spagnoli C, Shafi K V P M, Ulman A, Cowman M and Gross R A 2003 Activity of Candida rugosa lipase immobilized on gamma-Fe2 O3 magnetic nanoparticles J. Am. Chem. Soc. 125 1684–5 [18] Shinkai M, Suzuki M, Iijima S and Kobayashi T 1995 Antibody-conjugated magnetoliposomes for targeting cancer-cells and their application in hyperthermia Biotechnol. Appl. Biochem. 21 125–37 [19] Kim J Y and Osterloh F E 2005 ZnO–CdSe nanoparticle clusters as directional photoemitters with tunable wavelength J. Am. Chem. Soc. 127 10152–3 [20] Wang Z L 2004 Nanostructures of zinc oxide Mater. Today 7 26–33 [21] Von Preissig F J, Zeng H and Kim E S 1998 Measurement of piezoelectric strength of ZnO thin films for MEMS applications Smart Mater. Struct. 7 396–403 [22] Salvador A, Pascual-Marti M C, Adell J R, Requeni A and March J G 2000 Analytical methodologies for atomic spectrometric determination of metallic oxides in UV sunscreen creams J. Pharm. Biomed. 22 301–6 [23] Schwarz V A, Klein S D, Hornung R, Knochenmuss R, Wyss P, Fink D, Haller U and Walt H 2001 Skin protection for photosensitized patients Laser Surg. Med. 29 252–9 [24] Boisnic S, Branchet-Gumila M C, Merial-Kieny C and Nocera T 2005 Skin Pharmacol. Physiol. 18 201–8 [25] Vayssieres L 2003 Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions Adv. Mater. 15 464–6 [26] Wang Z, Qian X F, Yin J and Zhu Z K 2004 Large-scale fabrication of tower-like, flower-like, and tube-like ZnO arrays by a simple chemical solution route Langmuir 20 3441–8 [27] Gao X P, Zheng Z F, Zhu H Y, Pan G L, Bao J L, Wu F and Song D Y 2004 Rotor-like ZnO by epitaxial growth under hydrothermal conditions Chem. Commun. 12 1428–9 [28] Liang J B, Liu J W, Xie Q, Bai S, Yu W C and Qian Y T 2005 Hydrothermal growth and optical properties of doughnut-shaped ZnO microparticles J. Phys. Chem. B 109 9463–7 [29] Tian Z R, Voigt J A, Liu J, McKenzie B and McDermott M J 2002 Biomimetic arrays of oriented helical ZnO nanorods and columns J. Am. Chem. Soc. 124 12954–5 [30] Naim J O, van Oss C J, Wu W, Giese R F and Nickerson P A 1997 Mechanisms of adjuvancy. 1. Metal oxides as adjuvants Vaccine 15 1183–93 [31] Srinivasan S, Datye A K, Smith M H and Peden C H F 1994 Interaction of titanium isopropoxide with surface hydroxyls on silica J. Catal. 145 565–73 [32] Tartaj P and Serna C J 2002 Microemulsion-assisted synthesis of tunable superparamagnetic composites Chem. Mater. 14 4396–402 [33] Tartaj P and Serna C J 2003 Synthesis of monodisperse superparamagnetic Fe/silica nanospherical composites J. Am. Chem. Soc. 125 15754–5 [34] Wang W, Gu B H, Liang L Y and Hamilton W A 2003 Fabrication of near-infrared photonic crystals using

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highly-monodispersed submicrometer SiO2 spheres J. Phys. Chem. B 107 12113–7 [35] Umetsu M, Mizuta M, Tsumoto K, Ohara S, Takami S, Watanabe H, Kumagai I and Adschiri T 2005 Bioassisted room-temperature immobilization and mineralization of zinc oxide—The structural ordering of ZnO nanoparticles into a flower-type morphology Adv. Mater. 17 2571–5

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[36] Govender K, Boyle D S, Kenway P B and O’Brien P 2004 Understanding the factors that govern the deposition and morphology of thin films of ZnO from aqueous solution J. Mater. Chem. 14 2575–91 [37] Lu Y, Yin Y D, Mayers B T and Xia Y N 2002 Modifying the surface properties of superparamagnetic iron oxide nanoparticles through a sol–gel approach Nano Lett. 2 183–6