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Modulating Zn(OH)2 rods by marine alginate for templates of hybrid tubes with catalytic and antimicrobial properties Lili Lv, Xiaochen Wu, Mingjie Li, Lu Zong, Yijun Chen, Jun You, and Chaoxu Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02192 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 17, 2016
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Modulating Zn(OH)2 rods by marine alginate for templates of hybrid tubes with catalytic and antimicrobial properties
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Lili Lv1,2, Xiaochen Wu1,*, Mingjie Li1, Lu Zong1, Yijun Chen1, Jun You1 and Chaoxu Li1,2,*
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1
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Technology, Chinese Academy of Sciences, Songling Road 189, Qingdao 266101, P. R. China
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CAS Key Laboratory of Bio-based materials, Qingdao Institute of Bioenergy and Bioprocess
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University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, PR China
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Corresponding author: Xiaochen Wu (
[email protected]) and Chaoxu Li (
[email protected]).
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Abstract
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With the help of marine alginate, Zn(OH)2 nanorods were synthesized under mild conditions (e.g.
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room temperature and atmospheric pressure) through a one-step, green and scalable method, in
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which both the morphologies (e.g. nanorods and nanoflakes) and nanoscale sizes of Zn(OH)2 could
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be precisely controlled by alginate. Considering the high reactive activity as well as the adhesion
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property of dopamine, polydopamine nanotubes were produced by the template of Zn(OH)2 rods and
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following a mussel-biomimetic strategy of dopamine coating. The enhanced activities of
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polydopamine surfaces further offered the possibility of embedding Ag nanoparticles to produce Ag
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nanotubes with super catalytic and antibacterial properties. The combination of inexpensive starting
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materials, simple synthesis procedure and exceptional properties makes Zn(OH)2 rods/nanoflakes
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great potential as templates and multiple applications in catalysis, sensing etc..
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Keywords: Zn(OH)2 nanorods, alginate, polydopamine coating, Ag nanoparticles, catalyst,
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antibiosis
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INTRODUCTION
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Because of the large aspect ratios, super structural stability and fascinating physical/chemical
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properties,1, 2 one dimensional (1D) inorganic structures (i.e. tubes, wires, fibers, rods etc.) have
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attracted rapidly-increasing interests in fabricating various novel materials, e.g. Au nanorods in
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sensing and detecting devices,3 and hybrid nanorod arrays of zinc oxide/cadmium sulfur in
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optoelectronic devices.4 Among them, rods of ZnO or Zn(OH)2 were paid particular attention as
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detectors, lighting devices and sensitive antibody microassay,5 on account of their widely-available
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raw materials,6 controllable morphologies,7 good solubility in acid/alkaline solutions and unique
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electrochemical properties.8 In spite of these fascinating newly-discovered properties and emerging
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applications, it is still highly challenged to produce ZnO or Zn(OH)2 rods in large scale along with
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uniform morphologies in environment-friendly approaches.9
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With rapid development of nanotechnology, there have been diverse approaches employed to
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produce ZnO or Zn(OH)2 rods, such as vapor liquid solid growth, metal-organic vapor phase
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epitaxial growth, chemical vapor deposition (CVD), electro-deposition, hydrothermal synthesis and
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wet-chemical method.10 However, these methods show disadvantages either of using active pure Zn
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metal as starting materials,11 specially-designed setups,12 high synthesis temperatures13 or low
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production efficiency. For example, the ZnO nanorods preparing by CVD involved high purity Zn
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metal (99.9999 %) and high temperature (750 ˚C);11 The electrodeposition method could effectively
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prepare ZnO nanorods in large scale, but a complex substrate preparation process were involved
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(i.e.10 nm thick Ti, 30 nm W and porous alumina template were needed before the ZnO nanorod
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arrays deposition).12
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On the other hand, natural evolution has offered various biomacromolecules with powerful
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abilities of greenly synthesizing diverse low-dimensional inorganic materials, such as the
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nanorod-like hydroxyapatite crystals in teeth.14 Followed this inspiration, ZnO nanorods were
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biosynthesized on the surface of cotton fabric, in which the negative charged cellulosic cotton fibers
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and the positive charged zinc ions absorbed together, nucleation and growth;15 flower-like zinc oxide
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nanostructure (whose petal was composed of nanorods) was synthesized with the help of dextran
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((C6H10O5)n), and the excessive hydroxyl groups of dextran assisted the ZnO seeds gradually grow
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into nanorods.16 Furthermore, these organic additives could strongly affect the morphologies of the
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resultant inorganic structures.17-19
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In this work, alginate was chosen as both the reactant and structure-directing agent to synthesize
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Zn(OH)2 nanorods, due to its natural-abundance, excellent physical properties,20 biocompatibility
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and biodegradability.21 Being a natural linear polysaccharide,22 alginate is extracted mainly from
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brown seaweeds and marine algae.23 Among the building unites of β-D-mannuronic acid and
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α-L-guluronic acid, carboxylic acid groups on its chains not only negatively charge the alginate
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molecules, but also enables complexation electrostatically with metals ions. Thus inorganic
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nanomaterials could possibly be synthesized with the help of alginate.24 For example, iron oxide
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nanoparticles were prepared by using sodium alginate as the reducing and stabilizing agent, thus
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avoiding conventional utilization of toxic organic acids and amine;25 hierarchical MnO2 could also
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be synthesized in the presence of alginate for improved supercapacity. 26
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Herein with the help of marine alginate, we prepared Zn(OH)2 nanorods under mild conditions
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(e.g. room temperature and atmospheric pressure) through a one-step, green and scalable method.
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Thanks to the super capabilities of alginate in binding diverse metal ions, both the morphologies (e.g.
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nanorods and nanoflakes) and nanoscale sizes of Zn(OH)2 could be precisely controlled by alginate.
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Followed a mussel-biomimetic strategy of dopamine coating, polydopamine nanotubes were
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produced by the template of Zn(OH)2 rods. The enhanced activities of polydopamine surfaces further
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offered the possibility of embedding Ag nanoparticles to produce Ag nanotubes with super catalytic
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and antibacterial properties. Thus due to this combination of inexpensive starting materials, simple
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synthesis procedure and exceptional properties, these Zn(OH)2 rods/nanoflakes may show great
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potential as templates as well as in applications in catalysis, sensing etc..
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EXPERIMENTAL SECTION
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Materials. Alginic acid was provided by Bright Moon Seaweed Group (Qingdao, China). Dopamine
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was purchased from Shanghai Jinsui Bio-Technology Co., Ltd. Tris-(hydroxymethyl) methyl
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aminomethane (Tris) was bought from Sigma and silver nitrate nitrate (AgNO3) from Solarbio. Other
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reagents were from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as received.
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Synthesis of Zn(OH)2 nanorods (Zn(OH)2-x). Zn(Ac)2 (15 mL, 0.5 M) was added drop-wisely into a
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base solution of NaOH (10 mL, 5 M) under stirring at 15 ˚C to get a clear solution. After adding an
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amount of alginic acid (0.5~3.0 g), the mixture were further stirred for 6 h. The Zn(OH)2 nanorods
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was collected by sequentially centrifuging at 3000 rmp for 5 min, washing three times with deionized
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water and drying at 60 ˚C for 12 h. The product was denoted as Zn(OH)2-x according to the initial
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mass (x) of alginic acid added in the reaction (x=0.5, 1, 2, 2.5, 3). After re-dispersing in Tris buffer
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solution (10 mM, pH 8.5) with a concentration of 2 mg/mL, Zn(OH)2 nanorods could convert into
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ZnO/Zn(OH)2 nanoflakes during thermal incubation at 80 ˚C for 12 h with stirring.
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Polydopamine coating Zn(OH)2 nanorods (Zn(OH)2/PDAy). Zn(OH)2-0.5 nanorods (40 mg) and
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dopamine (4~40 mg) were dispersed in Tris buffer solution (20 mL, 10 mM and pH 8.5) through
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ultrasonication. After incubating at 25 ˚C for 24 h, the mixture was centrifuged at 3000 rmp for 5 min
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to obtain the precipitating product. The precipitate was further washed three times with deionized
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water and re-dispersed in double distilled water. The product was denoted as Zn(OH)2/PDAy
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according to the initial mass ratios (y) of dopamine to Zn(OH)2 (y=0.1, 0.2, 1).
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Hybridization of Zn(OH)2/PDAy with Ag nanoparticles (Zn(OH)2/PDA/Ag). Zn(OH)2/PDA0.2 (3
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mL, 2 mg/mL) was mixed with AgNO3 (1125 µL, 25 mM) at 80 ˚C and pH 8.5 for 12 h under
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stirring. After centrifugation at 3000 rmp for 5 min, the precipitate was washed three times with
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deionized water and then re-dispersed in distilled water.
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Evaluation of catalytic activity. The reduction of 4-nitrophenol was used as a model reaction to test
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the catalytic activity of Zn(OH)2/PDA/Ag. Typically, H2O (520 µL), 4-nitrophenol (100 µL, 20 mM)
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and NaBH4 (330 µL, 3 M) were mixed in a standard quartz cuvette (Volume: 3 mL; Path length: 1
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cm). After quickly adding Zn(OH)2/PDA/Ag (50 µL, 0.5 mg/mL), UV-vis spectrophotometer was
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used to monitor the absorbance variation at 400 nm.
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Evaluation of antimicrobial activity. Antimicrobial activity was evaluated through a spread plate
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method27 and Colibacillus as a model. Typically, the dispersions of Zn(OH)2-0.5, Zn(OH)2/PDA0.2
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and Zn(OH)2/PDA/Ag (2 mL, 2 mg/mL) were added to agar plates (diameter 76 mm) respectively
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and dried at 60 ˚C for 12 h. After inoculating ~105 colony forming units of Colibacillus into the agar
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plate, the incubation was performed at 37 ˚C for 12~48 h. Optical images of these plates were
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documented by a digital camera.
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Characterization. Morphologies of the samples were characterized by field emission scanning
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electron microscopy (FESEM) with a JEOL 7401 instrument (Japan) equipped with X-ray energy
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dispersive spectrometry (EDS). Transmission electron microscopy (TEM) was carried out with
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Hitachi H-7650 instrument (Japan). X-ray diffraction (XRD) measurements were taken on a X-ray
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diffractometer (Bruker D8 ADVANCE) using Cu-Kα (λ = 1.5406 Å) radiation. Thermal gravimetric
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analysis (TGA) was performed on a high-pressure thermogravimetric analyzer (NETZSCH TG 209
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F1 Libra®, Germany) at a ramp rate of 5 ˚C/min from 50 to 800 ˚C under air flow. UV-Vis
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spectrophotometric analysis was performed on a DU800 UV-vis spectrophotometer. FT-IR analysis
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was performed by a Nicolet 6700 FT-IR spectrometer (American) using the KBr pellets method.
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RESULTS AND DISCUSSION
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In contrast to harsh synthesis conditions, utilization of organic/toxic chemicals and complicated
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synthesis procedures reported in the literature10-13, a simple homogenization process could produce
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Zn(OH)2 rods at room temperature in the presence of alginic acid. As shown in Fig. 1a, upon adding
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alginic acid into a mixture solution of Zn(Ac)2 and NaOH, Zn(OH)2 rods were produced gradually.
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The presence of alginic acid not only enabled 1D growth of Zn(OH)2 rods, but also stabilized the rod
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colloids in their dispersions. For example, all the rods seemed to be individually suspended in their
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dispersion and there is no obvious rod aggregation observed in Fig. 1a. X-ray diffraction (XRD)
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patterns in Fig. 1b confirmed the chemical structures of Zn(OH)2 in these rods, which could perfectly
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be indexed to JCPDS card No. 71-2215 of standard Zn(OH)2. Furthermore, these rod morphologies
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maintained even when calcinating at 600 ˚C for 3 h, where ZnO nanoparticles stacked together and
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preserved the rod-like structures in spite of the presence of holes and cracks (Fig. S1).28
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Morphologies of Zn(OH)2 rods depend strongly on the amount of alginic acid added. As shown
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in Fig. 2a-2d, the amount of alginic acid was enhanced from 0.5 to 3 g while keeping a fixed amount
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of Zn(Ac)2 (e.g. 7.5 mmol). The addition of alginic acid tends to increase the average diameters of
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Zn(OH)2 rods while depressed their contour lengths, hereby offering smaller aspect ratios. For
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example (see Fig. S2), when adding 0.5 g alginic acid, the resultant Zn(OH)2-0.5 rods have a mean
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diameter of ~0.22 µm and a mean contour length of ~16.7 µm, with aspect ratio reaches up to 76 (see
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Fig. 2e). On the other hand, when increasing alginic acid up to 3 g, the mean diameter of Zn(OH)2-3
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rods increased up to ~500 nm while the mean contour length decreased lower than ~3 µm, thus
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giving the aspect ratio as low as ~6 (see Fig. S2, Fig. 2e).
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The formation of Zn(OH)2 rods suggested that adding alginic acid may alter the reaction
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pathway of Zn(Ac)2 and NaOH. Without alginic acid, Zn(Ac)2 quickly reacted with NaOH to form
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the precipitate of irregular Zn(OH)2 crystals according to the following chemical equations. Extra
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OH- would further react with Zn(OH)2 into soluble Zn(OH)42-, giving a clear and transparent
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solution.28
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Zn2+ +2 OH- ⇌ Zn(OH)2
(1)
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Zn(OH)2 +2 OH- ⇌ Zn(OH)42-
(2)
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The adding of alginic acid gradually consumed OH- to generate alginate, resulting in the
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conversion of these soluble Zn(OH)42- into insoluble Zn(OH)2 nanocrystals, which could be collected
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by centrifugation (see Fig. 2f). 1D growth of Zn(OH)2 crystals is an indicator that alginate adsorbed
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onto certain crystallographic facets of Zn(OH)2 and prevented their growth along these directions.
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The capping effect of alginate may further be supported by replacement of alginic acid with other
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organic acids such as tartaric acid,28 where rod-like Zn(OH)2 bundle mesocrystals were produced.
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Also, a similar formation mechanism was proposed for the formation of ultralong Cu(OH)2 and CuO
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nanocrystals by the assistant of polyethylene glycol.29 Furthermore, as shown in Fig. S1, after the
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Zn(OH)2 was annealed in air at relatively high temperatures, the remaining alginic acid was
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removed, while the rod-like morphology of Zn(OH)2 still preserved with lots of holes and cracks,
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indicating alginic acid was a major component of the Zn(OH)2 nanorods before annealing. Thus,
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during this reacting process, alginic acid may serve not only as the reactant but also a capping agent.
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When small amounts of alginic acid was added, slower nucleation than growth may be preferred
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according to a typical nucleation-growth mechanism of crystal synthesis, thus producing larger
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aspect ratios of Zn(OH)2 crystals as shown in Fig. 2.
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The presence of alginate also seems essential to stabilize Zn(OH)2 rods. After washing three
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times with distilled water, the synthesized Zn(OH)2 rods were further incubated into a Tris buffer (10
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mM, pH 8.5) at 80 ˚C for 12 h. As shown in Fig. 3a, these rods gradually transformed into
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flower-like nanoflakes. The coexistence of Zn(OH)2 and ZnO was identified by XRD patterns in Fig.
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3b, which could be indexed to Zn(OH)2 (JCPDS card No. 20-1435) and ZnO (JCPDS card No.
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36-1451). Furthermore, Fig. 3c-3f details temporal morphology variations from the Zn(OH)2 rods to
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ZnO/Zn(OH)2 nanoflakes. It is obvious that smooth surfaces of Zn(OH)2 rods first became rough and
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created ridge-like morphologies, which further grow into nanoflakes during the incubation. The
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decreasing cylindrical cores indicate that insoluble Zn(OH)2 may dissolve into Zn(OH)42- ions on the
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rod surfaces in the alkaline environment (pH=8.5) provided by Tris. The concomitant desorption of
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alginate failed to ensure 1D recrystallization of Zn(OH)2 and transformation into ZnO at 80 ˚C.
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On account of the inexpensive starting materials, facile synthesis procedures and good
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dissolution abilities in acid/alkali solutions (Fig. S3), the as-prepared Zn(OH)2 rods could be ideal
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sacrifice templates to synthesize different hybrids and nanotubes. For example, following the
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mussel-inspired catechol redox chemistry, polydopamine (PDA) biomacomolecules could coat these
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Zn(OH)2 rods to produce the Zn(OH)2/PDAy hybrids through self-polymerization of dopamine under
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mild conditions. The polydopamine surfaces have been suggested to be highly active for diverse
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secondary reactions as well as adhesion of various nanomaterials.30 The coating process was
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performed in Tris buffer (10 mM, pH 8.5) under room temperature (~25 ˚C) for 24 h. Dopamine
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molecules were adsorbed on the surface of Zn(OH)2 rods and self-polymerized into PDA layers.
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With a minimum amount of dopamine added, e.g. ~0.1 weight ratio (y) of dopamine/Zn(OH)2,
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isolated PDA nanoparticles decorated on Zn(OH)2 rods (Fig. 4a). Increasing the weight ratio of
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dopamine/Zn(OH)2 up to 0.2 led to a continuous PDA layer of ~50 nm (Fig. 4b and 4e), and also the
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mean diameter of Zn(OH)2 rods increased from 220 nm to 330 nm (Fig. 4f). When the ratio value
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was promoted up to 1, PDA particles reappeared with the size within 200~300 nm (Fig. 4c) on
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continuous PDA layers. Thus the weight ratios of dopamine/Zn(OH)2 ≥0.2 seem to be appropriate to
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produce Zn(OH)2 rods with PDA layers.
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More interestingly, the PDA layers of Zn(OH)2/PDAy kept intact when removing their core of
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Zn(OH)2 rods in acidic conditions (Fig. 4g and 4h). The resultant PDA nanotubes retained high
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aspect ratios and large specific surface area, whose active surfaces renders a super bio-based
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platform for further functionalization serving diverse applications.31
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PDA has the abilities of binding and reducing ions of noble metals,32 with metals ions (e.g. Ag+,
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AuCl4-) reduced to metals (e.g. Ag, Au) and the catechol and hydroxyl groups of PDA oxidized to
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quinone and carbonyl group, respectively. Thus it offers an opportunity of further modifying the
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hybrids
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Zn(OH)2/PDA0.2 in the Ag+ solution at 80 ˚C and pH 8.5, Ag nanoparticles formed and distributed
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evenly on the hybrid surfaces (Fig. 5a). Furthermore, the positively charged PDA could prevent the
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aggregation of Ag nanoparticles by providing reciprocal electrostatic repulsion. The production of
of
Zn(OH)2/PDAy with
metal nanoparticles.33
For example,
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Ag nanoparticles can be confirmed by energy X-ray energy dispersive spectrometry (EDS) in Fig. 5b
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and S4, where the elements of Ag, C, O and Zn distributed uniformly along the rods. Notably, during
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this thermal incubation, Zn(OH)2 rods also partially transformed into ZnO/Zn(OH)2 nanoflakes in
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analogue to Fig. 3a and S5. But Ag nanoparticles were reduced mainly on the PDA layers while less
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on recrystallized nanoflakes (Fig. 5a and 5f).
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FT-IR spectrum of the hybrid of Zn(OH)2/PDA/Ag together with Zn(OH)2-0.5 and
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Zn(OH)2/PDA0.2 was shown in Fig. 5c. The absorptions peaks at 1651 cm-1, 1582 cm-1 and 1491 cm-1
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are ascribed to the stretching vibrations of –C=O–, –C=C– and –C=N functional groups present
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within indoline and/or indole structures of PDA.34 When combining Ag+, the C=O stretching
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vibrations (1651 cm-1) showed a slight blue shift to 1628 cm-1 resulted from bond formation between
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silver and oxygen,35 while the C=C and C=N stretching vibrations decreased. The loading amounts
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of PDA and Ag in the hybrids were evaluated by TGA in Fig. 5d. After scanning at 5 ˚C/min from 50
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to 800 ˚C under air atmosphere, the residue of Zn(OH)2-0.5, Zn(OH)2/PDA0.2 and Zn(OH)2/PDA/Ag
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was 80.1 %, 75.5 % and 84.2 %, respectively. The weight loss between 100~150 ˚C was assigned to
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the decomposition of Zn(OH)2 to ZnO, while the loss between 150~400 ˚C to the decomposition of
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alginate and PDA. Accordingly, a loading amount of 5.7 wt% was obtained for PDA in
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Zn(OH)2/PDA0.2. And the loading amounts of Ag and PDA were 27.3 wt% and 4.2 wt% respectively
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in the hybrid Zn(OH)2/PDA/Ag. Due to Ag nanoparticles forming mainly on the PDA layers, hollow
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hybrids of Ag/PDA could be synthesized after dissolving Zn(OH)2/PDA/Ag in acidic condition (pH
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~2) (Fig. 5e and 5f), in analogue to hollow PDA tubes shown in Fig. 4g and 4h. The Ag nanoparticles
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were found to have a narrow distribution of size within 35~54 nm.
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The uniform size distribution of Ag nanoparticles offers the hybrid of Zn(OH)2/PDA/Ag with
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high catalytic activity. Herein, the reducing of 4-nitrophenol to 4-aminophenol with NaBH4 was used
2
as a model reaction. 4-Aminophenol plays an important role in diverse areas such as analgesic and
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antipyretic drugs, corrosion inhibitor, photographic developer, anticorrosion lubricant, etc.36
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4-nitrophenol alone shows a specific spectral profile with an absorption maximum at 316 nm, 37 and
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a new absorption peak at 400 nm was observed when 4-nitrophenol was mixed with NaBH4 (Fig. 6a)
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due to the conversion of 4-nitrophenol to 4-nitrophenolate ions. Upon the addition of the catalysts
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(e.g. Zn(OH)2/PDA/Ag), the absorption intensity of 4-nitrophenolate ions at 400 nm decreased
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quickly with time, accompanied by the color variation from yellow to colorless, indicating the
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successful conversion of 4-nitrophenol to 4-aminophenol. Thus the decreasing UV-Vis absorption of
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4-nitrophenolate ion at 400 nm could be used to monitor the reaction kinetics. As shown in Fig. 6a,
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the addition of Zn(OH)2/PDA/Ag as low as 0.025 mg/mL accelerated the reaction of 2 mM
12
4-nitrophenol to complete within 9 min, in contrast to almost no reaction in the absence of catalysts
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(Fig. 6b). The accelerated reaction rate depended strongly on the amount of Zn(OH)2/PDA/Ag (Fig.
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6b). For example, an amount of catalyst 0.05 mg/mL could complete the reaction of 2 mM
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4-nitrophenol in
