Gold cellular networks inside water-solublesodium alginate sol

Jishi Wei, Jianmao Yang, and Shuyan Gao

Abstract: The present work demonstrates the fabrication of unique gold cellular networks inside water-soluble sodium algi-nate sol. In conjunction with control experiments, UVvis spectra and TEM images structurally give a piece of robust evi-dence that Au(III) and sodium alginate combined with preparation parameters synergically favor the formation of goldcellular networks. From the viewpoint of green chemistry, the cheap and safe reactant sodium alginate and the friendly aque-ous medium make the present method possible for applications in large-scale production of metallic cellular networks. Thus,coupled with facile and green creation, the gold cellular networks open new opportunities for noble metal nanostructures,with different morphologies, as biological labels, energy transfer pairs, and other nanoscale electronics.

Key words: biopolymer, cellular network, gold.

Rsum : Dans ce travail, on prsente une mthode de prparation de rseaux cellulaires uniques dor lintrieur dun soldalginate de sodium soluble dans leau. En relation avec des expriences de contrle, les spectres UVvis et les images ob-tenues par microscopie lectronique transmission (MET) donnent un appui de taille au fait que le Au(III) et lalginate desodium se combinent avec des paramtres de prparation qui favorisent par synergie la formation des rseaux cellulairesdor. Du point de vue de la chimie verte, l'alginate de sodium peu dispendieux et facilement accessible et le milieu ractionaqueux cologiquement correct font que la prsente mthode pourrait s'appliquer sans problme la production grandechelle de rseaux cellulaires mtalliques. Les rseaux cellulaires d'or, qui peuvent tre obtenus facilement et d'une faoncologiquement correcte, ouvrent donc de nouvelles possibilits pour la production de nanostructures de mtaux noblescomportant des morphologies diffrentes qui pourraient tre utiliss comme marqueurs biologiques, paires de transfertd'nergie et d'autres combinaisons lectroniques l'chelle nano.

Motscls : biopolymre, rseau cellulaire, or.

[Traduit par la Rdaction]

Introduction

Nanoparticles are nanometer-size materials with uniquephysical and chemical properties and have been widely usedfor many years.1 Meanwhile, it has been well-demonstratedthat their physical and chemical properties are closely relatedto their size and shape.2,3 Therefore, there has been an in-creasing interest in developing new methods for fabricatingshape-controlled nanoparticles. For example, Ag and Auwith different shapes such as nanorods,3,4 nanoplates,512nanodisks,1316 sponges,17 three-dimensional (3D) thornynanostructures,18 and hollow microspirals19 have been syn-thesized. However, the procedure for porous metallic nano-structures is still a great challenge for materials scientists,although nanomaterials with very abundant pores have be-come increasingly attractive in the area of materials science,20which is fueled by their use in catalysis, gas storage, and sep-aration.21,22Biopolymers belong to a class of materials that can swell

largely in water other than organic solvents and maintaintheir 3D network structure on a nanometer length scale in

the swollen state. This makes them have many potential ap-plications including high-surface-area supports for catalysts,separation media, and templates for the synthesis of othernanoscopic materials, which undoubtedly motivates the prep-aration of nanostructural materials inside biopolymers.23,24 Infact, a sol-gel-derived 3D network of nanoscale particles(aerogel/ambigel),25 and a 3D (hierarchically) ordered macro-porous (3DOM) solid with an inverted opal structure have al-ready been created by a combination of sol-gel chemistry andtemplating.26,27 Among biopolymers that are widely used isSodium alginate is a biopolymer that is widely in use. It is apopular additive and naturally derived linear anionic copoly-mer of 1,4-linked b-D-mannuronic acid (M block) and a-L-guluronic acid (G block) residues arranged in a nonregularblockwise pattern of varying proportionsof GG, MG, andMM blocks.2831 The chemical structure of sodium alginateis shown in Scheme 1. The extensive number of carboxylgroups present in sodium alginate can make cross-linked hy-drogels form in the presence of cations via an ionic interac-tion between the acid groups on the G blocks and thechelating ions, generally, Ca2+.32,33 As a result, it is plausible

Received 13 February 2012. Accepted 13 April 2012. Published at www.nrcresearchpress.com/cjc on 6 June 2012.

J. Wei and S. Gao. College of Chemistry and Environmental Science, Henan Normal University, 46 Jianshe street, Xinxiang, 453007,Henan, P.R. China.J. Yang. Research Center for Analysis and Measurement, Donghua University, Shanghai, 201620, P.R. China.

Corresponding author: Shuyan Gao (e-mail: shuyangao@htu.cn).

551

Can. J. Chem. 90: 551556 (2012) doi:10.1139/V2012-031 Published by NRC Research Press

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that Au(III) ions can play a significant role in guiding thesupramolecular organization among sodium alginate sol,which results in the nanogold formation upon the reduction.That is to say, sodium alginate sol can act as nanoreactor totemplate and stabilize nanoparticles in the cross-linked sys-tem. However, little attention has been paid to the applicationof their cross-linked systems as nanoreactors.In conjunction with the biocompatible nature and unique

optical properties of gold nanoparticles, we report herein onthe generation of gold cellular networks through the in situreduction of the gold precursor existing inside the 3D net-work structure in sodium alginate sol. This bulky natural so-dium alginate sol allows the stabilization of the porousnanostructues. UVvis spectra and TEM images structurallywitness the formation of gold cellular networks. Control ex-periments give a piece of robust evidence that Au(III) andthe cheap and safe sodium alginate reactant combined withpreparation parameters synerically favor the formation ofgold cellular networks. Thus, coupled with facile and greencreation, the gold cellular networks open new opportunitiesfor noble metal nanostructures with different morphologies

as biological labels, energy transfer pairs, and other buildingblocks in nanoscale electronics.

Materials and methodsSodium alginate was purchased from Sigma-Aldrich. All

other reagents are of analytic grade and used as receivedwithout further purification. All water used in this investiga-tion was deionized by a Nanopure filtration system to a resis-tivity of 18 M cm. The preparation of gold cellularnetworks is quite straightforward. In a typical preparation, a0.5 mL aliquot of a 0.0254 mol/L aqueous solution ofHAuCl4 was added to 10 mL of a 0.055% (wt) aqueous solu-tion of soluble sodium alginate. The mixture was heated to90 C and was maintained at this temperature for 30 min.After the subsequent dropwise introduction of 0.2 mL offreshly prepared 0.1 mol/L aqueous sodium borohydride, thesolution turned red, indicating the formation of gold nanopar-ticles. They were structurally investigated by UVvis spectra,TEM images, selected area electron diffraction (SAED), andX-ray diffraction (XRD) to obtain detailed information ontheir nanometer-scale morphology. The UVvis spectra werecollected on a UVvis spectrophotometer (Beijing PurkinjeGeneral Instrument Co., Ltd.). TEM and SAED measure-ments were made on a JEOL 2010 transmission electron mi-croscope operated at an accelerating voltage of 200 kV. TheXRD pattern was collected on a Rigaku-D/Max 2500 V/PCX-ray diffractometer using high-intensity Cu Ka1 radiation(l = 1.54056 ). Samples for TEM and XRD characteriza-tion were prepared by placing the gold colloidal sample oncarbon-coated copper grid and glass substrates, respectively,and dried at room temperature.

Results and discussion

UVvis absorption observationsThe synthesis of gold cellular networks is carried out by

the reduction of Au(III) to Au(0) in heated sodium alginate/HAuCl4 aqueous solution. Gold nanoparticles exhibit strongplasmon resonance absorption that is dependent on particlesize and shape. For roughly spherical gold nanoparticles, thefingerprint surface plasmon absorbance band generally falls

Scheme 1. Chemical structure units of sodium alginate (M =mannuronic acid and G = guluronic acid).

Fig. 1. UVvis absorption spectrum of the product.

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between 520 and 530 nm.27,3436 The formation of goldcolloid in our system was confirmed by the UVvis spec-trum, displaying a plasmon band maximum at 538 nm(2.31 eV) (shown in Fig. 1). The slight red shift to longerwaves was observed for the sample, presumably stemmingfrom the collective network behavior. It is worthwhile men-tioning that the colloidal sample is stable for a month or lon-ger without any observable aggregation, indicating thatsodium alginate can serve as a very effective protective agentfor the formation of gold colloid. There was quick colorchange from light yellow to purple, accompanied by the ob-servable aggregation, for the control experiment with no so-dium alignate in the solution performed. This simultaneouslyproved the protective agent role of sodium alginate in the for-mation of gold colloid.

TEM measurementsFigure 2 shows the typical TEM images of the obtained

gold cellular networks. The lower magnification image(Fig. 2A) indicates that the product consists of porous cellu-lar networks. The higher magnification image (Fig. 2B)shows that the cellular networks are composed of discretegold nanoparticles with average diameters of 3 nm. Figure2D shows the size distributions. In general, the particles areisotropic (i.e., low aspect ratio) in shape. These results illus-trate the synthesis of Au(0) nanoparticles through the reduc-tion of Au(III) inside the cross-linked nanoscopic sodiumalginate templates. The massive carboxyl groups act as stabi-lization and passivation contacts for the stabilization of thenanoparticles formed inside these templates. The relatedSAED pattern (Fig. 2C) obtained by focusing the electronbeam on porous cellular networks on the TEM grid gives apiece of powerful evidence of the gold component. TheSAED pattern reveals a ringlike pattern, suggesting that thegold nanoparticles are multicrystalline.

XRD measurementWhereas SEM and TEM often sample only a small portion

of the products, XRD can be used to assess the overall qual-ity and purity of the product.37 The XRD pattern recorded onthe product is compiled in Fig. 3. The observed five Braggreflections clearly correspond to the face-centered cubicstructure of metallic gold with space group Fm3m (JPCDSfile, card No. 04-0784). The observation of all the expecteddiffraction peaks substantiates the formation of crystallinegold.38 The nanometer size of the gold crystallites is evi-denced by the broad X-ray reflection peaks. From the well-known Scherrer formula, the average crystallite size (L) is

L KlBmcos q

where l is the X-ray wavelength, q is the diffraction angle,Bm is the line width of the pure diffraction profile resultingfrom small crystallite size, and K is a constant approximately

Fig. 2. Low magnification TEM Image (A), high magnification TEM image (B), SAED (C) of the gold cellular networks, and the size dis-tribution (D).

Fig. 3. XRD pattern of the gold cellular networks. The vertical linesrepresent the data of JPCDS file, card No. 04-0784.

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equal to unity and related to crystallite shape. According tothe Scherrer formula, the average crystallite size of goldcellular networks is 5 nm, which is consistent with that cal-culated from the TEM images.

Control experimentsTo clarify the effect of the reaction temperatures on the

special morphology of the product, we conducted experi-ments by changing the reaction temperature to room temper-ature and 50 C under otherwise identical conditions. TheTEM image (Fig. 4) indicates that the particles invariablyshow 2D arrays consisting of close-packed and well-separatedspherical particles, with an average particle size of 3.7 nm(Fig. 4A) and 4 nm (Fig. 4B), respectively. This indicatesthat the gold nanoparticles are capped by sodium alginate,which prevents physical contact between the particles andalso provides sufficient hydrophobicity to the gold nanopar-ticles to enable their dispersion in water. More importantly,it suggests that the reaction temperature plays a vital role inthe formation of cellular networks. This is presumably at-tributed to its well-stretched state resulting from heatingtreatment. The stretched state offers enough space for metalions to approach carboxyl, thus facilitating the cross-linkedsystem and the nanoparticles more stable. By this token,

heat treatment in the present case is employed not only toincrease the reaction velocity and narrow the particle-sizedistributions,39 but also for the formation of the cross-linkedsystem. This is a distinct feature superior to conventionalstabilizers.To make clear the role of the reactant concentrations in the

formation of the porous networks, we also investigated theeffect of the molar ratios of sodium alginate to Au on themorphology of the product, whereby we conducted the ex-periment by changing the molar ratio of sodium alginate /HAuCl4 to 10:1 and 1:1 under otherwise identical conditions.The TEM image (Fig. 5) indicates that the particles also in-variably show 2D arrays consisting of close-packed andwell-separated spherical particles, with the average particlesize of 4 nm (Fig. 5A) and 6.4 nm (Fig. 5B), respectively.This likewise indicates that the gold nanoparticles are cappedby sodium alginate, which prevents physical contact betweenthe particles and also provides sufficient hydrophobicity tothe gold nanoparticles to enable their dispersion in water.The disappearance of cellular networks under the presentconditions suggests that the molar ratios of sodium alginateto Au have evident effect on the formation of cellular net-works. This is presumably attributed to the fact that the bio-polymers swell enough to form the cross-linked system

Fig. 4. Representative TEM images when the reaction temperature is changed to room temperature (A) and to 50 C (B) under the otherwiseidentical conditions and the corresponding size distributions: C from A and D from B.

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through metal ions bridging only at an appropriate concentra-tion.

Conclusion

In conclusion, the results from UV-vis absorption spectra,TEM images,and XRD are in agreement, and provide robustevidence to support the successful fabrication of gold cellularnetworks. The cheap and safe of sodium alginate reactantcombined with easy control of preparing parameters makesthe present method possible for applications in large-scaleproduction of metallic cellular networks. Control experimentsshow that both the reaction temperatures and reactant concentra-tions play a vital role in the formation of the cellular networks.Intense work aimed at supporting this sol-template-controlledsynthesis strategy for other porous systems, particularly fornanomagnets is in progress.A key issue in nanotechnology is the development of con-

ceptually simple construction techniques for the mass fabrica-tion of identical nanoscale structures. The present approachrelies on the exploitation of specific cross-linking interactionsbetween biopolymer and metal ions and is one of the keybuilding principles of all living organisms. It is thus obvious

to search for more biological structures that can be used astemplates for directing and perfecting the self-assembly.40

AcknowledgementThis work was supported by the National Natural Science

Foundation of China (Grant No. 21071047), the Program forNew Century Excellent Talents in University (Grant No.NCET-110944), the Project Sponsored by the Scientific Re-search Foundation for the Returned Overseas Chinese Schol-ars, State Education Ministry of China, the Excellent YouthFoundation of Henan Scientific Committee, and the Programfor Science & Technology Innovation Talents in Universitiesof Henan Province (Grant No. 2011HASTIT010).

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