Journal of Colloid and Interface Science 335 (2009) 62–69

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier .com/locate / jc is

Preparation and stabilization of monodisperse colloidalgold by reduction with aminodextran

Benjamin J. Morrow, Egon Matijevic, Dan V. Goia *

Center for Advanced Materials Processing, Clarkson University, Box 5814, Potsdam, NY 13699, USA

a r t i c l e i n f o

Article history:Received 30 January 2009Accepted 26 February 2009Available online 5 April 2009

Keywords:Gold solAminodextranMonodisperse gold

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.02.053

* Corresponding author. Fax: +1 315 269 2139.E-mail address: goiadanv@clarkson.edu (D.V. Goia

a b s t r a c t

Stable concentrated aqueous dispersions of monodisperse gold nanoparticles were prepared using dieth-ylaminoethyl-dextran as reductant and stabilizer. The effectiveness of dextran as reducing agent wasstrongly affected by the pH. In alkaline solutions, the Au(III) species were rapidly and completely reduced,yielding stable dispersions of spherical uniform gold nanoparticles. Their modal diameter could beadjusted from 18 to 40 nm by varying the pH, temperature, and the Au3+/dextran ratio. In contrast, underacidic conditions (pH � 4.0) the reduction was very slow, favoring the formation of large gold crystals ofother shapes. A general mechanism explaining the reducing and stabilizing actions of polysaccharides ingeneral, and of diethylaminoethyl-dextran in particular, is proposed.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction compounds not only simplify the precipitation systems but also of-

The unique properties of finely dispersed gold particles havebeen exploited since antiquity as proven by the Lycurgus Cup dis-played at the British Museum in London [1]. However, it was notuntil Michael Faraday’s pioneering work in the mid 19th century[2] that the science behind the formation and properties of suchsols was described. Recently, gold dispersions have been widelyused in numerous applications, including electronics [3,4], nonlin-ear optics [5,6], catalysis [7–9], SERS [10,11], pigments [12,13],sensors [14,15], biosensors [16–19], and drug delivery [20]. Amongthe many available processes for the preparation of such particles,the reduction in homogeneous solutions remains the simplest andmost versatile method. Various approaches differ mainly in theway the electrons needed for the reduction are provided. In somecases they are generated ‘‘in situ” by exposing a tetrachloroau-ric(III) acid solution to visible [21] or ultraviolet light [22,23],gamma rays [24], pulse radiolysis, or laser beams [25]. In mostcases the source of electrons is a dedicated reducing agent, suchas phosphorous [2,26], citric or salicylic acids [26–28], oleylamine[29], polyols [30–32], hydrazine [33], borohydrides [33–35],hydrogen peroxide [36], and alcohols [37–41].

As a rule, stable gold sols can be obtained without difficulties indilute systems due to a very effective electrostatic stabilizationmechanism. In contrast, at high reactant concentrations the repul-sive forces are greatly weakened due to the high ionic strength, andadditives (usually polymers or surfactants) are necessary to pro-vide stable dispersions. Of significant scientific and practical inter-est are stabilizers which can also act as reductants. Such

ll rights reserved.

).

fer opportunities for developing environmentally friendly pro-cesses [42–51]. The most likely candidates for this purpose arelarge organic molecules containing functional groups with reduc-ing properties. Particularly well suited are high molecular weightpolysaccharides, which consist of reducing mono- and di-saccha-rides, joined in extended chains. This work is an illustration of thisstrategy in the preparation of metal colloids. More specifically, asimple and reproducible method is described that yields concen-trated stable gold dispersions by using diethylaminoethyl-dextran(aminodextran) as reductant and stabilizer. Setting this work apartfrom other research conducted with dextrans and gold [16] are thereaction conditions involving both high pH and moderately ele-vated temperature. Under such conditions, complete reduction ofthe Au(III) solute species is achieved in reasonable times, whileexceptionally stable gold sols are obtained at concentrations atleast one order of magnitude higher than by other reportedmethods.

2. Experimental

2.1. Materials

A stock solution of tetrachloroauric(III) acid (HAuCl4) (23 wt%Au, Umicore, South Plainfield, NJ) served as the source of Au(III)ions. The diethylaminoethyl-dextran hydrochloride (aminodex-tran, MW �500,000, Sigma–Aldrich) was used as received.

2.2. Preparation of gold sols

In a typical experiment, 8.57 g HAuCl4 stock solution (contain-ing 1.97 g or �0.01 mol Au) was added to 500 cm3 of deionized

B.J. Morrow et al. / Journal of Colloid and Interface Science 335 (2009) 62–69 63

water maintained at 50 �C in a 1 dm3 jacketed glass beaker. The pHof the gold solution was subsequently adjusted to 12.2 ± 0.1 with a10 mol dm�3 NaOH solution. Next, 2.30 g of aminodextran previ-ously dissolved in 50 cm3 deionized water was poured rapidly intothe reaction vessel under stirring. The mixture was continuously

Fig. 1. UV/Vis spectrum of a diluted gold sol obtained in the reference experiment(dilution factor 40�).

Fig. 2. (a) FESEM image and (b) histogram of the particles prepared in the referenceexperiment.

agitated at 50 �C for 7.5 h, during which time the Au(III) specieswere completely reduced, forming a stable gold sol. The progressof the reduction was assessed by monitoring the absorption inten-sity at 318 nm, which is characteristic for [AuCl4]� square planarcomplex ions [49]. Once all gold was reduced, the stabilizingaminodextran was decomposed by heating the dispersion atP84 �C for 4 h. After the decomposition of the stabilizer, the goldparticles settled (due to the intrinsic high ionic strength of the dis-persion) and the supernatant solution was decanted. The resultingsolids were washed several times with water and ethanol, and fi-nally freeze-dried. Under otherwise the same conditions, the effectof other experimental parameters on the properties of the resultinggold particles was evaluated, including the pH, [Au3+]/aminodex-tran ratio, temperature, and HAuCl4 concentration.

2.3. Characterization

The optical properties of intermediate species and gold disper-sions were investigated by UV/Vis spectrophotometry using aPerkinElmer Lambda 35 instrument. Field emission scanning elec-tron microscopy (FESEM, JEOL JSM-7400F) and scanning transmis-sion electron microscopy (STEM, JEOL JEM-2010) were used toassess the particle size, shape, and crystal structure of the particles.The latter was also evaluated by X-ray powder diffraction (XRD)with a Bruker D8 diffractometer. The amount of residual dispersant(aminodextran) was determined by thermogravimetric analysis(TGA) with a PerkinElmer Pyris 1 instrument. The size distributionof the particles was obtained from FESEM images by measuring thediameter of at least 250 particles, as well as by dynamic light scat-

Fig. 3. (a) XRD pattern and (b) high-resolution TEM micrograph of a single goldparticle shown in Fig. 2.

Fig. 5. FESEM of gold particles obtained at pH 4.0.

64 B.J. Morrow et al. / Journal of Colloid and Interface Science 335 (2009) 62–69

tering with a ZetaPlus (Brookhaven Instruments Corporation)analyzer.

3. Results

In the reference experiment, the yellow-orange color of thealkaline reacting solution changed to dark red-violet immediatelyafter the addition of aminodextran, due to the formation of goldnanoparticles. The turbidity of the dispersion increased duringthe remaining 7.5 h while the color changed progressively to deepred, characteristic of gold sols containing particles below �50 nmin diameter. The UV/Vis spectrum of the final dispersion displayeda sharp peak at �519 nm (Fig. 1), indicating the uniformity andexcellent dispersion of gold nanoparticles.

The field emission electron micrograph of the isolated particles(Fig. 2a) and the corresponding histogram based on the SEM image(Fig. 2b) confirmed their narrow distribution with a modal diame-ter of �20 nm.

The X-ray diffractogram (Fig. 3a) of the dried particles was typ-ical for metallic gold. The crystallite size calculated using theScherrer equation (�12 nm) was very close to their observed size,suggesting that they are single crystals. The high-resolution trans-mission electron microscopy confirmed their highly crystallinenature, showing an orderly arrangement of atoms in well-definedextended planes (Fig. 3b).

The weight loss determined by the TGA indicated that the driedparticles contained only a small amount (less than 1% wt) of resid-ual aminodextran.

3.1. Effect of the pH

To study the effect of the pH, the reference experiment was alsocarried out with gold solutions adjusted to pH 10.0 and 4.0, respec-

Fig. 4. (a) FESEM of gold particles obtained at pH 10.0 and (b) UV/Vis spectra of goldsols obtained at pH 12.2 and 10.0, respectively.

tively, before adding aminodextran. At pH 10.0 the reduction wassignificantly slower than at pH 12.2 as it took several minutes forthe color of the dispersion to change to violet-red and �90 h to

Fig. 6. (a) Gold nanoparticles obtained at 25 �C and (b) UV/Vis spectra for solsobtained at 25 �C and at 50 �C.

B.J. Morrow et al. / Journal of Colloid and Interface Science 335 (2009) 62–69 65

completely reduce the gold. The resulting particles were still quiteuniform, but their average diameter increased to �40 nm (Fig. 4a)and the plasmon band of the diluted dispersion shifted to 530 nm(Fig. 4b).

At pH 4, the nucleation of gold occurred only after several hours,and the complete reduction of gold took �240 h. The resultingunstable gold dispersions were light brown and did not display aplasmon band, and the majority of the particles settled as soonas the agitation was stopped. Electron microscopy revealed a mix-ture of spheroidal polydispersed aggregates interspersed withlarge (1to10 lm) trigonal and hexagonal particles (Fig. 5).

3.2. Effect of the reaction temperature

To investigate the temperature effect, experiments were carriedout at 25 and 80 �C. At the lower temperature, the reduction wasconsiderably slower than in the reference experiment (50 �C). Ini-tial color changes were noted only after several minutes and thered gold sol formed after several hours. Furthermore, 16 days werenecessary to completely reduce the gold species. The resultingnanoparticles were uniform and slightly larger than at 50 �C, asindicated by the electron micrographs in Fig. 6a and in the red-shifted plasmon band (Fig. 6b).

In contrast, the reduction at 80 �C yielded large aggregates ofgold particles, which settled rapidly.

3.3. Effect of the gold concentration

For practical and theoretical reasons it is often important to as-sess the upper concentration limit of reactants that still generatesstable dispersions of uniform particles. In the studied system, it

Fig. 7. UV/Vis absorption spectra for the sol prepared under the referenceconditions and at 2.5 times higher gold concentration.

Table 1Experiments carried out with different [aminodextran]/[Au3+] ratios.

Sample HAu(OH)4 solution Aminodextran solution

Volume(cm3)

Concentration(mol dm�3)

Volume(cm3)

Concentration(mol dm�3)

1 540 1.85 � 10�2 50 10.00 � 10�5

2a 540 1.85 � 10�2 50 5.00 � 10�5

3 540 1.85 � 10�2 50 2.50 � 10�5

4 540 1.85 � 10�2 50 1.25 � 10�5

a Reference experiment described in Section 2.2.

was possible to obtain stable gold sols even when the HAuCl4 con-centration was increased 2.5 times to 4.2 � 10�2 mol dm�3. Thenanoparticles formed were slightly larger than in the referenceexperiment, which caused a small shift in the position of the plas-mon band from 519.5 to 521.7 nm (Fig. 7). The dispersion re-mained stable, as confirmed by the relatively constant ratiobetween the absorption intensities at the plasmon peak and at600 nm (Fig. 7 inset) and by the lack of particle settling even afterseveral months.

These results are of significant practical importance consideringthat stable gold dispersions are typically obtained in systems withgold concentrations 1to2 orders of magnitude lower.

3.4. Effect of aminodextran concentration

To test the effect of the aminodextran amount, experimentswith different amounts of the polymer were carried out as summa-rized in Table 1. Doubling the aminodextran concentration to10.0 � 10�5 from 5.0 � 10�5 mol dm�3 (sample 1) had no effecton the size or the stability of the resulting particles (Fig. 8a andb), or on the UV/Vis spectrum (Fig. 9). At half of the amount ofaminodextran (sample 3) the particle size increased from �18 to�28 nm (Fig. 8c), with the corresponding shift of the plasmon bandfrom �520 to 523 nm. The stability of the sol also decreased,resulting in the settling of a considerable fraction of particles afterseveral days. Reducing further the amount of the polysaccharide(sample 4) did not affect the size of the nanoparticles (Fig. 8d)any further, but yielded a less stable sol, as testified by the emer-gence of a broad absorption band at �670 nm (Fig. 9) and the com-plete particle settling after only 2 days.

4. Discussion

The essential component in the described procedure for the for-mation of gold nanoparticles is the aminodextran, which actedsimultaneously as the reducing and stabilizing agent. Dependingon the pH and temperature, this additive controlled the rate of for-mation, size, shape, uniformity, and the stability of the finely dis-persed metal. Based on the experimental findings presented, amechanism of the reduction and stabilization of gold by aminodex-tran can be proposed.

In general, dextrans are single-branch homopolysaccharideswith a backbone consisting of C1–C6 connected glucose units withperiodically attached side chains of C1–C4 and/or C1–C3 bondedglucose molecules [52]. The side chains consist of only 2–3 glucosemolecules [53] and contain exposed functional groups [54,55] withreducing properties similar to those of individual sugar monomers.It has been previously shown that the reducing power of monosac-charides increases dramatically with pH, making possible a rapidreduction of silver salts [56]. It was also demonstrated that, at suf-ficiently high alkalinity, the C–C bonds in sugar molecules cancleave according to the well-known Malaprade-type oxidationmechanism [57] until the entire molecule has been converted to

[Aminodextran]:[Au3+] molarratio

Gold sol concentration (mol dm�3)

5.41 � 10�3:1 1.67 � 10�2

2.71 � 10�3:1 1.67 � 10�2

1.35 � 10�3:1 1.67 � 10�2

0.67 � 10�3:1 1.67 � 10�2

Fig. 8. (a–d) FESEM micrographs of samples 1–4 (Table 1).

Fig. 9. UV/Vis spectra of gold sols obtained at different [aminodextran]/[Au3+]ratios (Table 1).

Fig. 10. UV/Vis spectra taken at different times during the reference experiment.

66 B.J. Morrow et al. / Journal of Colloid and Interface Science 335 (2009) 62–69

CO2 and H2O. Having strong reducing properties, the side chainsare capable of reducing rapidly the [Au(OH)4]� species under theconditions used in the reference experiment. The emergence ofthe red-violet color immediately after the addition of the amino-dextran into the gold solution confirms this fast reduction. Oncethe side chains are exhausted, the reaction slows considerably

and further reduction of Au(III) is possible only as a result of thebreakdown of the C1–C6 bonds in the polymer backbone. Since thisprocess is slow, the complete reduction of gold requires severaladditional hours. Changes in the pH and the temperature stronglyaffect the cleaving of the C–C bonds in both the side chains and thebackbone. Thus, the time scales of nucleation and particle growth

B.J. Morrow et al. / Journal of Colloid and Interface Science 335 (2009) 62–69 67

stages depend on these two parameters, as demonstrated by theresults presented in this study.

The UV/Vis spectra of the gold sol generated in the referenceexperiment at different times confirm the ‘‘two-stage” dynamicsof the reduction process suggested above (Fig. 10). Despite a fastreduction in the first few seconds that leads to the rapid formationof gold nanoparticles (and the emergence of the red-violet color),most of the gold is reduced slowly over several hours. Indeed, at1.5 h the absorption intensity of the plasmon band was only�10% of the value at the end of the reaction.

The slow growth of the gold nanoparticles is also captured inthe TEM images in Fig. 11, which show that between 1.5 h andthe end of the reduction (7.5 h) the size of the particles increasedby a factor of �1.8. This implies that �6 times more gold wasdeposited on the particles during the last 6 h of the process, whichis consistent with the approximate increase in the absorptionintensity of the plasmon band.

Fig. 11. TEM of gold nanoparticles removed from the refer

The evolution of the UV/Vis spectra during the reference exper-iment invites for a separate discussion. The sequence of colors ob-served (violet ? blue ? red) is normally associated with aprogressive decrease in the size of dispersed gold, which contra-dicts the electron microscopy evidence (Fig. 11a–d). Since solidparticles cannot cleave, they must be aggregated in the early stagesand, as the reaction progresses, they slowly peptize. Thus, the pur-ple color observed during the first 1.5 h indicates the aggregationof the primary particles formed soon after the nucleation step. Inthe remaining time, the same particles grow slowly as more goldis deposited by diffusion, while freeing themselves from theagglomerates. As a result, the final particles absorb at a lowerwavelength (519 nm) despite the fact that the TEM shows thembeing almost twice as large as those at 1.5 h. This phenomenonwas previously observed and amply discussed by Chow and Zuko-ski [58] in the case of the reduction of tetrachloroauric acid withsodium citrate. They showed that the coagulating effect was in-

ence experiment at (a) 1.5, (b) 3, (c) 4.5, and (d) 7.5 h.

68 B.J. Morrow et al. / Journal of Colloid and Interface Science 335 (2009) 62–69

duced by the excess [AuCl4]� ions, which decreased the surfacecharge and reduced the repulsive forces between particles. Despitea much higher ionic strength in the system described here, themagnitude of the red shift is less pronounced than in the citratemethod simply because the aminodextran prevents a severe coag-ulation of the early formed primary particles.

An important finding of this study was the effect of the pH onthe rate of gold reduction. The stepwise substitution of the Cl� li-gands with OH� in the [AuCl4]� complex ion results in the forma-tion of progressively more stable complexes, as indicated by thefollowing equation:

AuCl5�nðOHÞ�n�1 þ OH� ! AuCl4�nðOHÞ�n þ Cl� ð1Þ

and the graph in Fig. 12. Thus, one would expect that the reductionof gold to be more facile in acidic conditions, where the [AuCl4]�

species is preponderant, than at pH > 12.0, where the more stable[Au(OH)4]� ions are present [49].

The opposite behavior observed in this system can be explainedby an even stronger effect of the pH on the reducing capabilities ofthe aminodextran. It appears that the ability of this polymer to re-lease electrons increases sharply with the pH and overcomes thestabilization effect of the OH� ligands. As a result, the standard po-tential of the reduction reaction (DE�) is larger under alkaline con-ditions favoring the rapid and complete reduction of Au(III)species. In contrast, at lower pH, aminodextran is a weak reductantand, despite the lower stability of [AuCl4]�, the reduction of gold ismuch slower. A slower nucleation leads to larger particles and fa-vors preferential directional growth, which results in the formationof large anisometric gold entities (trigonal and hexagonal plate-lets). The observed effect of pH on the reducing ability of amino-dextran is quite similar with that previously reported in thereduction of silver with glucose [56].

The oxidative degradation of the aminodextran is essential inproviding the electrons needed for the reduction of gold. By con-trolling its kinetics through pH and temperature adjustments, itis possible to provide not only a rapid and complete reduction ofAu(III) species, even in concentrated systems, but also to tailorthe size and shape of gold particles. Specific experimental condi-tions must be imposed, however, to ensure that dextran also suc-cessfully fulfills its second role as a dispersing agent. First, it isessential that the fragments of dextran at the end of the reductionprocess are large enough to prevent the aggregation of the goldnanoparticles, especially at the high ionic strength employed inthe experiments. This condition implies that both a large molecularweight polymer and a sufficiently high dextran-to-metal ratio are

Fig. 12. Speciation diagram for Au(III) complexes formed on addition of NaOH to aconcentrated solution of tertachloroauric acid [49].

necessary. As shown, the dispersion became unstable when theamount of dextran was reduced too much from the referenceexperiment (sample 4, Table 1). Secondly, one must make sure thatthe oxidative degradation of dextran is caused predominantly bythe reaction with Au(III) species. The inability to obtain stable goldsols at elevated temperature (>80 �C) and high pH is a good illus-tration of the importance of this aspect. Under these conditions,the dextran backbone is hydrolyzed by the OH� ions as well and,as a result, the final polymeric fragments are too small to providean effective stabilization of the nanoparticles.

The described reaction failed to produce stable dispersions ofgold nanoparticles when using either anionic or nonionic dextrans.The unique behavior of the aminodextran may be explained by factthat the fragments resulting from the hydrolysis of the backbonemaintain a branched structure. Such branched fragments are capa-ble of a more effective steric stabilization than linear chains ofcomparable size. Furthermore, the amino group in the side DEAEchains may also serve as an anchor to the surface of the gold, whichcould further enhance their stabilizing action.

5. Summary

A precipitation method capable of yielding very stable disper-sions of uniform gold nanoparticles was developed. The experi-mental approach described relies on the combined abilities ofaminodextran to reduce Au(III) species in aqueous solutions andto stabilize the gold particles formed. It was found that the precip-itation process must be conducted at elevated pH and tempera-tures between 50 and 70 �C in order to ensure a reasonably fastand complete reduction of gold. At the same time, to obtain goldsols with long-lasting stability, it is necessary to work with highmolecular weight aminodextrans (>500,000) and provide a highdispersant/metal ratio. Under these conditions, it is possible to re-duce, quantitatively and rapidly, concentrated solutions of Au(III)and obtain, on a large scale, exceptionally stable dispersions of uni-form gold nanoparticles.

Acknowledgment

This paper was supported in part by NSF Grant DMR-0509104.

References

[1] D.J. Barber, I.C. Freestone, Archaeometry 32 (1990) 33.[2] M. Faraday, Philos. Trans. Roy. Soc. 147 (1857) 145.[3] M.D. Musick, C.D. Keating, M.H. Keefe, M.J. Natan, Chem. Mater. 9 (1997) 1499.[4] D. Huang, F. Liao, S. Molesa, D. Redinger, V. Subramanian, J. Electrochem. Soc.

150 (2003) G412.[5] I. Tanahashi, Y. Manabe, T. Tohda, J. Appl. Phys. 79 (1996) 1244.[6] J. Sasai, K. Hirao, J. Appl. Phys. 89 (2001) 4548.[7] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647.[8] M. Haruta, M. Daté, Appl. Catal. A 222 (2001) 427.[9] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293.

[10] L.G. Olson, Y.S. Lo, T.P. Beebe Jr., J.M. Harris, Anal. Chem. 73 (2001) 4268.[11] G.R. Souza, C.S. Levin, A. Hajitou, R. Pasqualini, W. Arap, J.H. Miller, Anal. Chem.

78 (2006) 6232.[12] J. Carbert, Gold Bull. 13 (1980) 144.[13] P. Mulvaney, MRS Bull. 26 (2001) 1009.[14] Y. Jin, X. Kang, Y. Song, B. Zhang, G. Cheng, S. Dong, Anal. Chem. 73 (2001)

2843.[15] A. Sugunan, C. Thanachayanont, J. Dutta, J.G. Hilborn, Sci. Technol. Adv. Mater.

6 (2005) 335.[16] Y. Ma, N. Li, C. Yang, X. Yang, Anal. Bioanal. Chem. 382 (2005) 1044.[17] P. Yáñez-Sedeño, J.M. Pingarrón, Anal. Bioanal. Chem. 382 (2005) 884.[18] P.K. Jain, I.H. El-Sayed, M.A. El-Sayed, Nano Today 2 (2007) 18.[19] S.E. Skrabalak, J. Chen, L. Au, X. Lu, X. Li, Y. Xia, Adv. Mater. 19 (2007) 3177.[20] G.F. Paciotti, L. Myer, D. Weinreich, D. Goia, N. Pavel, R.E. McLaughlin, L.

Tamarkin, Drug Deliv. 11 (2004) 169.[21] N. Toshima, J. Macromol. Sci. A 27 (1990) 1225.[22] K. Esumi, K. Matsuhisa, K. Torigoe, Langmuir 11 (1995) 3285.[23] Y. Zhou, C.Y. Wang, Y.R. Zhu, Z.Y. Chen, Chem. Mater. 11 (1999) 2310.[24] H. Fujita, M. Izawa, H. Yamazaki, Nature 196 (1962) 666.

B.J. Morrow et al. / Journal of Colloid and Interface Science 335 (2009) 62–69 69

[25] K. Kurihara, J. Kizling, P. Stenius, J.H. Fendler, J. Am. Chem. Soc. 105 (1983)2574.

[26] J. Turkevich, P.C. Stevenson, J. Hillier, Discuss. Faraday Soc. 11 (1951) 55.[27] G. Frens, Kolloid-Z. Z. Polym. 250 (1972) 736.[28] N. Malikova, I. Pastoriza-Santos, M. Schierhorn, N.A. Kotov, L.M. Liz-Marzán,

Langmuir 18 (2002) 3694.[29] M. Aslam, L. Fu, M. Su, K. Vijayamohanan, V.P. Dravid, J. Mater. Chem. 14

(2004) 1795.[30] P.Y. Silvert, K. Tekaia-Elhsissen, Solid State Ionics 82 (1995) 53.[31] M. Tsuji, M. Hashimoto, Y. Nishizawa, T. Tsuji, Mater. Lett. 58 (2004) 2326.[32] C. Li, W. Cai, B. Cao, F. Sun, Y. Li, C. Kan, L. Zhang, Adv. Funct. Mater. 16 (2006)

83.[33] J.P. Wilcoxon, R.L. Williamson, R. Baughman, J. Chem. Phys. 98 (1993) 9933.[34] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc., Chem.

Commun. (1994) 801.[35] M. Brust, J. Fink, D. Bethell, D.J. Schiffrin, C. Kiely, J. Chem. Soc., Chem.

Commun. (1995) 1655.[36] D.B. Zilversmit, G.A. Boyd, M. Brucer, J. Lab. Clin. Med. 40 (1952) 261.[37] M. Gola, K. Lutz, Ger. Offen., GP 2 330413, 1975..[38] Y. Nagata, Y. Mizukoshi, K. Okitsu, Y. Maeda, Radiat. Res. 146 (1996) 333.[39] S. Ayyappan, R.S. Gopalan, G.N. Subbanna, C.N.R. Rao, J. Mater. Res. 12 (1997)

398.[40] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Y. Yoo, Bull. Chem. Soc.

Jpn. 75 (2002) 2289.[41] R.A. Caruso, M. Ashokkumar, F. Grieser, Langmuir 18 (2002) 7831.

[42] S. Mandal, P.R. Selvakannan, S. Phadtare, R. Pasricha, M. Sastry, Proc. IndianAcad. Sci. (Chem. Sci.) 114 (2002) 513.

[43] P.R. Selvakannan, S. Mandal, S. Phadtare, A. Gole, R. Pasricha, S.D. Adyanthaya,M. Sastry, J. Colloid Interface Sci. 269 (2004) 97.

[44] Y. Shao, Y. Jin, S. Dong, Chem. Commun. (2004) 1104.[45] J. Huang, Q. Li, D. Sun, Y. Lu, Y. Su, X. Yang, H. Wang, Y. Wang, W. Shao, N. He, J.

Hong, C. Chen, Nanotechnology 18 (2007) 105104.[46] P. Mukherjee, A. Ahmad, D. Mandal, S. Senapati, S.R. Sainkar, M.I. Khan, R.

Ramani, R. Parischa, P.V. Ajayakumar, M. Alam, M. Sastry, R. Kumar, Angew.Chem., Int. Ed. 40 (2001) 3585.

[47] P. Mukherjee, S. Senapati, D. Mandal, A. Ahmad, M.I. Khan, R. Kumar, M. Sastry,ChemBioChem 3 (2002) 461.

[48] H. Huang, X. Yang, Carbohydr. Res. 339 (2004) 2627.[49] D.V. Goia, E. Matijevic, Colloids Surf. A 146 (1999) 139.[50] D. Andreescu, T.K. Sau, D.V. Goia, J. Colloid Interface Sci. 298 (2006) 742.[51] S. Panigrahi, S. Kundu, S.K. Ghosh, S. Nath, T. Pal, J. Nanopart. Res. 6 (2004) 411.[52] M. Glicksman (Ed.), Food Hydrocolloids, vol. 1, CRC Press, Boca Raton, FL, 1989.[53] O. Larm, B. Lindberg, S. Svenson, Carbohydr. Res. 20 (1971) 39.[54] J.J. Marshall, Adv. Carbohydr. Chem. Biochem. 30 (1974) 257.[55] R.L. Sidebotham, Adv. Carbohydr. Chem. Biochem. 30 (1974) 371.[56] D. Andreescu, C. Eastman, K. Balantrapu, D.V. Goia, J. Mater. Res. 22 (2007)

2488.[57] C.A. Bunton, in: K.B. Wiberg (Ed.), Oxidation in Organic Chemistry, Academic

Press, New York, 1965, pp. 367–369.[58] M.K. Chow, C.F. Zukoski, J. Colloid Interface Sci. 165 (1994) 97.