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Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 61–65

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

ynthesis of gold nanorods using concentrated aerosol OT in hexane and itspplication as catalyst for the reduction of eosin

hruti Srivastavaa, Surender Kumar Sharmab, Rakesh Kumar Sharmaa,∗

Department of Chemistry, University of Delhi, Delhi 110007, IndiaMSIP, Janak Puri, New Delhi, India

r t i c l e i n f o

rticle history:eceived 28 June 2010eceived in revised form 13 October 2010ccepted 14 October 2010vailable online 21 October 2010

eywords:anorodsurfactant aggregates

a b s t r a c t

The microemulsion method for the preparation of nanoparticles is well known. We have used the aqueouscore of highly concentrated aerosol OT in hexane solution to synthesize gold nanorod by utilizing theaqueous core of surfactant aggregates as host nanoreactor. The shape and size of the aqueous core aswell as the particles formed inside the core can be controlled by changing the parameter W0 (water tosurfactant ratio), concentration of gold salt and the concentration of surfactant. When the concentrationof the surfactant is very high the shape of the aqueous droplet does not remain spherical but take theshape of prolate. In our study we have made gold nanorods by the reduction of gold chloride with sodiumborohydride in the aqueous core of 1 M AOT hexane at a W0 of 10. The rods are highly monodispersed with

icroemulsionsosin

a diameter of about 20 nm and a length of 200 nm with an aspect ratio of 10. The absorption spectra of thegold nanorods show two different peaks one at 535 nm and the other at 965 nm. The particles were usedas a catalyst for the reduction of eosin with sodium borohydride. The rate constant comes out be very largein comparison with that of uncatalysed reaction. The reaction was carried out at various temperaturesbetween 20 and 60 ◦C and the activation energy of the reaction was calculated using Arrhenius plotbetween–ln k and 1/T. The activation energy of the gold nanorods catalysed reaction comes out to be

ompa

more than two times as c

. Introduction

The progressive decrease in size of metal and semiconduc-or nanoparticles has been observed to be significant from bothundamental and application points of view. There is widespreadnterest in understanding the shape and size-dependent proper-ies of nanomaterials for the development of various applications.

any characteristic properties of the nanoscale materials includingptical, electronic and catalytic activity are shape and size depen-ant [1–3]. The catalytic activity of nanoparticulate materials isecause of the enormous number of active sites available for theeactants [4–6]. Of course, the efficacy of nanoparticle catalystsepends on various other factors [7–10] such as morphology, aver-ge size distribution, porosity, and phase composition. Thereforehe technique of fabrication of nanomaterials with controlled sizend morphology is required so as to tailor their properties. In recent

ears, a great deal of progress has been made in the synthesis ofarious nanostructures with controllable morphology and prop-rties, including one dimensional structure such as nanorods andanowires [11–14]. These are important class of the nanomaterials

∗ Corresponding author. Tel.: +91 9310050453.E-mail address: sharmark101@yahoo.com (R.K. Sharma).

927-7757/$ – see front matter © 2010 Published by Elsevier B.V.oi:10.1016/j.colsurfa.2010.10.022

red to uncatalysed reaction.© 2010 Published by Elsevier B.V.

due to their promising applications in both fundamental researchand nanodevice fabrication [15–17] and have compelled scientiststo devise direct and simple methods to accomplish synthesis withhigh yield [18–21]. Growth of nanostructures having different mor-phologies can result in the modification of its physical propertiesand improvement in performance characteristics. By tuning theaspect ratio of nanorods one can follow the transition from zeroto one dimensional structure such as nanorods and nanowires.Those structures can also be used as building blocks for anisotropicnanostructured materials and devices like high density data stor-age devices [22]. Nanoparticles with specific shapes display moreparticular and valuable characters in optical properties than spher-ical ones [23,24]. There is an increase in research activities on goldnanorods, to a large extent, is a result of the unique nature of thesenanostructures. Gold nanorods are particularly suitable for pho-tonic, optoelectronic, and biotechnological applications in the NIRregion [25]. Among many other nonspherical nanostructures, goldnanorods are more directed to advanced uses such as anticanceragent, near-infrared (IR) filter, and storage media, since nanorods

have a longitudinal plasmon band at near-infrared region, [26–28].

In the present study we have synthesized gold nanorods inhighly concentrated AOT solution in hexane by the reduction ofchloroauric acid with sodium borohydride. The particles were pre-pared at a high concentration of surfactant and the transmission

6 A: Physicochem. Eng. Aspects 373 (2011) 61–65

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lectron microscopy confirmed the formation of nanorods. Cat-lytic study of the gold nanorods-catalyzed reaction was followedy monitoring the absorbance of eosin in the visible spectrum ofosin. Reactions were also followed at different temperatures toeasure the activation energy of the above reaction.

. Materials and experimental procedure

.1. Materials

All the chemicals used were of AR grade and were used with-ut further purification. The aqueous solutions were prepared inouble distilled water. The Chloroauric acid (HAuCl4) and the dyeosin was purchased from Thomas Baker, Sodium borohydride wasurchased from s d fine-chem limited. AOT (i.e., sodium bis(2-thylhexyl)sulphosuccinate) was procured from ACROS organics.he solvent hexane and ethanol was procured from Spectrochemnd Merck Ltd respectively.

.2. Synthesis of gold Nanorods

Gold nanorods (GNRs) was prepared by the reduction of aurichloride by sodium borohydride in the aqueous core of concen-rated solution of AOT in hexane. To 10 ml of 1 M AOT in hexane,00 �l of 2% (w/v) HAuCl4 and 1700 �l of water was added to main-ain a W0 of 10 (W0 is the molar ratio of water to AOT). The solutionas stirred till the clear solution ‘A’ is obtained. In another 10 ml ofM AOT in hexane solution, 100 �l of 2% sodium borohydride and700 �l of water was added to maintain a W0 of 10. The solutionas stirred to get a clear solution ‘B’. When both the solutions A andbecome clear, the solution B was added to solution A very slowly

t 20 ◦C. After the complete addition of solution B to A the mix-ure was kept on stirring for another 2 h. To extract the nanorodsml of distilled ethanol was added to the mixture and kept in coldvernight and decanting off the supernatant solution. The nanorodsere washed 4–5 times with ethanol and water to remove AOT

ompletely. The nanorods were then dispersed in distilled water byonication for 10 min for characterization and further applications.

.3. Characterization of Gold nanorods

Au nanorods prepared in H2O/AOT/hexane solution was charac-erized by TEM and UV–Vis spectophotometrically. Transmissionlectron microscopic (TEM) pictures were taken with a Technai2 30 U-Twin, Technai 300 kv ultra twin microscope. The goldanorods after extraction were redispersed in ethanol by sonica-ion for 15 min. A drop of the dilute ethanolic solution was put onhe polymer coated grid and dried. This step was repeated for aouple of times after complete drying of the grid, a TEM picture ofhe particles was taken.

The UV–visible spectra of the particles was taken after dispers-ng the particles in water by sonication and was recorded in the

ave length range of 300–1100 nm using Shimadzu-1601 UV–Vispectrophotometer.

.4. Kinetic studies of the reduction of eosin

To 2 ml of distilled water taken in a 3 ml cuvette, 40 �l of 10−2 Mosin, 60 �l of 0.2%(w/v) aqueous dispersion was added. The reac-ion was started by adding 400 �l of 0.5 M sodium borohydrideolution to the reaction mixture. The reaction was studied spec-

rophotometerically by measuring the decrease in absorbance ofosin at 535 nm with time in the temperature range of 20–60 ◦C. Theemperature of the reaction mixture was kept constant by circulat-ng water of desired temperature around the sample holder from

thermostatted water bath. For the uncatalyzed reaction 60 �l

Fig. 1. TEM picture of gold nanorods having a length of around 200 nm and a diam-eter of about 20 nm, prepared using concentrated AOT in hexane solution.

of 0.2% (w/v) gold nanorods was replaced by an equal volume ofdouble-distilled water.

3. Results and discussion

Various methods [29–36] have been reported in the litera-ture for the preparation of nanorods. We have used the aqueouscore of water/aerosol OT/hexane solution as a host nanoreactorto make gold nanorods. It is already known that water, AOT andoil mixed together at different proportion represented by a trian-gular phase diagram [37], which generally include both solutionphases (isotropic phase) and liquid crystalline phases. In a threeor four component system there are usually two isotropic solu-tion phases, one rich in water generally termed as L1 and otherin oil continuous phase termed as L2. At a higher concentration ofAOT where the solution crosses the L2 phase, the spherical dropletstake the shape of prolate. It was further observed that at a particularpoint in the triangular phase diagram the aspect ratio of rod shapedgold nanoparticles remains constant even on repeated synthesis.We used the same principle in our synthesis procedure i.e. thereduction of gold chloride to metallic gold takes the shape of non-spherical rod types inside the prolate shaped droplets. The shapeand size of the nanorods can be controlled by varying the shapeand size of the aqueous droplets of the ternary system. We haveprepared gold nanorods at 1 M concentration of AOT and at a W0 of10. Maintaining these parameters of the ternary water/AOT/hexanesystem it was observed that the aspect ratio of the nanorods remainconstant at 10 which has been confirmed from TEM pictures. Fig. 1shows the transmission electron microscopic picture of the goldnanorods prepared using 1 M AOT in hexane solution and at a W0of 10. The nanorods are reasonably monodispersed and are around200 nm long and about 20 nm in diameter and having an aspectratio of 10.

Fig. 2 shows the UV–Vis spectra of gold nanorods. Au nanorodspossess two characteristic absorption bands [38], a transverse band,similar to that of gold nanospheres, in the visible region corre-sponding to electron oscillations along the short axis (at 530 nm)and another longitudinal band in the near-infrared region corre-sponding to electron oscillation along the long axis (at 967 nm)which undergoes a shift with increasing aspect ratio. The latter is

particularly sensitive toward the size and the organization of theNRS.

The gold nanorod catalyzed reduction of eosin by NaBH4involves the reduction of double bond in the eosin molecule tosingle bond and was studied spectrophotometrically. In the gold

S. Srivastava et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 373 (2011) 61–65 63

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ig. 2. UV–visible spectra of the gold nanorods showing two peaks at 530 nm andt 967 nm. Inset shows dispersion of gold nanorods in water.

anorod catalysed reduction of eosin, because of the adsorptionf eosin onto the gold nanorods, eosin adsorption spectra suf-ered a red shift in the band peak position which otherwise is atlower wavelength in aqueous solution, So the reduction of eosin

s monitored at 535 nm spectrophotometrically [39], On additionf sodium borohydride, commencement of reduction of eosin wasbserved. The absorbance value of eosin absorption band decreasedapidly indicating the progress of the reduction of the dye. The falln concentration of eosin both in the presence and absence of goldanorods was followed at 535 nm and is shown in Fig. 3. Since theold nanorods also absorbs at 530 nm (in the vicinity of �max ofosin (535 nm)) so same amount of gold nanorods was also takenn reference cuvette during the reaction to deduct the absorbanceue to gold nanorods. So the decrease in absorbance with time

s the absorbance of the eosin and not the gold nanorods. In thebsence of nanorods there is small decrease in absorbance of the

ye indicating that the reaction is slower. On the other hand there

s a sharp decrease in the absorbance of eosin in the presence ofold nanorod which shows the reaction is strongly catalysed byold nanorods. As the temperature of the reaction increases theoncentration of eosin decreases faster shown as in Fig. 4. A plot of

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ig. 3. Decrease in concentration of eosin with time measured at 535 nm in theresence and absence of GNRs.

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Fig. 4. Decrease in the concentration of eosin with time at different temperature inthe range 20–60 ◦C.

–ln A535 against time gives almost a straight line (Fig. 5) indicatinga first order kinetics and the rate constant can be obtained fromthe slope of the straight line. It is noteworthy that the rate constantobtained from the straight line portion of the curve at 27 ◦C for GNRcatalysed reaction comes out to be 1.49 × 10−2 s−1 which is morethan two times in comparison to rate constant of 6.49 × 10−3 s−1

for uncatalysed reaction at the same temperature. The value of rateconstant increases with temperature and is shown in Fig. 6.

Fig. 7 shows the Arrhenius plots of natural logarithm of rate con-stant against 1/T for the reaction catalysed by gold nanorods in thetemperature range of 20–60 ◦C. The activation energy was obtainedfrom the slope of linear portion of the Arrhenius plot. The activa-tion energy of the catalysed reaction is comes out to be 32.84 kJ/molwhile that for unanalyzed reaction is 41.83 kJ/mol. The lower acti-

vation energy of the catalysed reaction indicates the strong activityof gold nanorods. It has been presumed that the mechanism of thereaction involves adsorption of eosin molecules on the surface ofthe catalyst and as a result it provides an alternate path with lower

Fig. 5. First order regression of the reaction between eosin and sodium borohydridemeasured at different temperatures in the presence of GNRs.

64 S. Srivastava et al. / Colloids and Surfaces A: Phy

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nergy for the reaction to take place. The mechanism of the catal-sed reaction is different from that of uncatalysed reaction. In theater case the mechanism may involves multisteps reaction with alow rate determining step.

. Conclusion

The concentrated ternary solution of water/AOT/hexane haseen used to prepare gold nanorods of length 200 nm and about0 nm wide. The nanorods are reasonably monodispersed. Auanorods possess two characteristic absorption bands, a transverseand, similar to that of gold nanospheres, in the visible region cor-esponding to electron oscillations along the short axis (at 530 nm)nd another longitudinal band in the near-infrared region corre-ponding to electron oscillation along the long axis (at 967 nm).he gold nanorods were used as a catalyst for the reduction of eosin.he rate of reduction of eosin in the presence of gold nanorods is

igher as compared to uncatalysed reaction. The reaction rate con-tant obtained at different temperatures was used to find out thectivation energy using Arrhenius equation. The activation energyf the reaction in presence of gold nanorods comes out to be small31 kJ/mol) as compared to uncatalysed reaction (41 kJ/mol)

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sicochem. Eng. Aspects 373 (2011) 61–65

Acknowledgements

We are grateful to Prof. Amarnath Maitra, Department of Chem-istry, University of Delhi, India for his valuable comments andfruitful discussion. We are highly thankful to University Grant Com-mission, India, for financial assistance in the form of a researchproject.

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