Materials Letters 63 (2009) 2672–2675

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Materials Letters

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A simple microemulsion based method for the synthesis of gold nanoparticles

Jitendra Rajput a, Ameeta Ravi Kumar a,b, Smita Zinjarde a,b,⁎a Institute of Bioinformatics and Biotechnology, University of Pune, Pune 411 007, Indiab DST Unit on Nanoscience, Department of Physics, University of Pune, Pune 411 007, India

⁎ Corresponding author. Institute of Bioinformatics anPune, Pune 411 007, India. Tel.: +91 20 25691331; fax:

E-mail address: smita@unipune.ernet.in (S. Zinjarde

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.09.036

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 August 2009Accepted 14 September 2009Available online 22 September 2009

Keywords:Crystal growthNanomaterialsElectron microscopyXRDEmulsifiers

The production of gold nanoparticles and nanoplates by enzymatically-synthesized lauroyl glucose, lauroylfructose and lauroyl ascorbate is described. These emulsifiers formed oil-in-water microemulsions withtoluene and the available reducing groups brought about a rapid reduction of chloroauric acid (HAuCl4). Goldnanoparticles could thus be synthesized without the use of an additional reducing agent. Optical images, UV–visible spectroscopy, scanning electron microscopy, energy dispersive spectra (SEM–EDS) and X-raydiffraction (XRD) analysis revealed the presence of gold nanoparticles, which on further incubationaggregated into nanoplates. This paper thus describes a novel application of the enzymatically-synthesizedesters.

d Biotechnology, University of+91 20 25690087.).

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Nanoparticles have become significant in the recent years andhave created an impact in the areas of electronic, energy, chemical andbiological sciences [1]. In particular, biological synthesis of nanopar-ticles has become a popular alternative [2]. Several organisms produceenzymes such as lipases, proteases and β-glucosidases that have beenused for the production of a variety of surfactants [3]. Amongst these,lipases play a special role in the synthesis of different types ofemulsifiers including sugar fatty acid esters [4]. Such sugar esters aresynthetic amphipathic molecules that have both polar (sugar) andnon-polar (fatty acid) moieties. They exhibit a variety of biologicalactivities [4–7] and display emulsifying, solubilizing as well asfoaming properties [8–10]. Ascorbate fatty acid esters are alsoimportant surfactants that have several applications [11].

The reducing capabilities of glucose, fructose and ascorbate areretained in lauroyl glucose, lauroyl fructose and lauroyl ascorbate asthe esterification generally occurs at 6-O-positions [12–14]. We havehypothesized that these esters could play a dual role in nanoparticlesynthesis (i) by stabilizing oil-in-water emulsions thereby aligningthe reducing groups on the outer side (ii) by reducing HAuCl4 to Au°.Conventionally, water-in-oil emulsions are used to reduce metal saltsin a controlled manner. The process involves the making and mixing

of two types of emulsions (i) containing the metal and (ii) containingthe reducing agent. The present paper describes a simple procedurefor the synthesis of gold nanoparticles using oil-in-water emulsions oflauroyl sugars or ascorbate wherein, the emulsifiers themselves bringabout a reduction of the gold salt thus saving energy as well as rawmaterials.

2. Experiment

2.1. Enzymatic synthesis of emulsifiers

The enzymatic production of lauroyl glucose and its subsequentpurification were carried out as described earlier [10]. Lauroylfructose and ascorbate were also prepared in a similar manner bysubstituting glucose with fructose or ascorbic acid. The esters werepurified and monitored by using the protocols described earlier[10,14,15].

2.2. Synthesis and characterization of gold nanoparticles

The esters equivalent to one unit of emulsification activity [10],HAuCl4 (3 mM gold) and toluene (0.1% final concentration) wereadded to Milli-Q water (reaction mixture volume was 5.0 ml). Thetubes were sealed; vortexed for 1 min, incubated at 70°C for 1 h andthe synthesis of nanoparticles was monitored. Three controls (i)without the esters, (ii) without toluene and (iii) without the vortexingstep were maintained for understanding the significance of each step.Standard protocols and equipment described earlier [16,17] wereused to characterize the nanoparticles. The particle size was analyzed

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by using dynamic light scattering equipment (25°C at a fixed angle of90°) and the Brookhaven 90plus particle sizing software.

3. Results and discussion

In the literature, there are a large number of reports on the use ofwater-in-oil microemulsions for nanoparticle synthesis. Such micro-emulsions are transparent, isotropic liquid media having nanosizedwater droplets (reverse micelles) dispersed in the bulk organicphase that are stabilized by surfactant molecules [18]. These waterpools offer unique microenvironments for nanoparticle synthesis[19]. These nano-reactors continuously collide, a fraction of dropletsform short-lived dimers and they exchange their water contents[20,21]. If two emulsions: one with the metal and the othercontaining a reducing agent are used, the drops collide; an exchangetakes place and the metal gets reduced. Nickel nanoparticles havethus been synthesized by using water-in-oil microemulsions ofwater/cetyltrimethylammonium bromide/n-hexanol. These nano-particles were synthesized by rapidly mixing equal volumes of twowater-in-oil microemulsions (i) with solubilized NiCl2 and (ii) with

Fig. 1. UV–vis spectra of (a) control tubes (i) (ii) and (iii) described in the Experimentsection and (b) spectra with lauroyl glucose (········), lauroyl fructose (- - -) andlauroyl ascorbate (——).

hydrazine (reducing agent) [22]. We describe a unique method ofnanoparticle synthesis based on oil-in-water emulsions wherein, theemulsifiers themselves brought about the reduction of HAuCl4 togold nanoparticles and nanoplates.

Fig. 1(a) shows the UV–visible spectra of the reaction mixtures incontrol tubes. As seen in the figure, there were no characteristic peaksin the range of 520–580 nm. The gold salt was not reduced in any ofthese tubes indicating that all the ingredients as well as the vortexingstep were essential for nanoparticle synthesis. Fig. 1(b) shows thecharacteristic peaks observed when emulsions of lauroyl glucose,lauroyl fructose and lauroyl ascorbate were used in the reactionmixtures. All the esters were thus able to bring about a reduction ofthe chloroauric acid. Gold nanoparticles are known to display vividcolors as they absorb radiation in the visible region due to surfaceplasmon resonance, SPR [16,17]. Nanoparticle synthesis was rapid andwithin 20 min, the pinkish-red color and SPR were observed.

Fig. 2(a) shows SEM images of the spherical nanoparticlesobtained (white arrows) with lauroyl glucose after 20 min. On furtherincubation, the reduced gold aggregated into larger hexagonal ortriangular nanoplates as shown by white arrows [Fig. 2(b)]. Withlauroyl fructose, a variety of forms (spheres, hexagons, triangles,rhomboids and rods) were obtained [Fig. 2(c)]. With lauroyl ascorbate(after 10 min of incubation), high magnification images (60,000×)indicated the presence of nanoparticles that were smaller than100 nm. Their exact size could be estimated by dynamic lightscattering studies as described later. Ascorbic acid is an efficientreducing agent and its ester also brought about a rapid reduction.Hexagonal and triangular nanoplates were formed after 1 h as againstthe 2 h incubation period required for sugar esters [Fig. 2(d), (e) and(f)]. The spot EDS attachment was used to determine the elementalcomposition and confirm the presence of gold in the nanoparticlesand nanoplates. Fig. 2(g) is a representative EDS profile showingpeaks that are characteristic of gold.

The XRD patterns of thin films of the emulsions that had syn-thesized nanoparticles were also obtained [Fig. 2(h)]. A broadening ofthe intense peaks due to (111), (200), (220) and (311) Braggreflection at 2θ=38.36°, 45.55°, 64.65° and 77.73°, respectively, con-firmed the presence of gold nanoparticles.

The size distribution of nanoparticles was estimated by dynamiclight scattering experiments. A representative multimodal sizedistribution graph for the nanoparticles that were obtained withlauroyl glucose after 20 min is shown in Fig. 3(a). The particle sizingsoftware calculated the size of the nanoparticles to be in the range of168 to 226 nm with an average size of 193 nm. These values areconsistent with the observations made by using a SEM [Fig. 2(a)]wherein it can be seen that the average size is around 200 nm. Thesize distribution of the nanoplates (a small proportion) was in therange of 700 nm to 2000 nm after 20 min [indicated by black arrowin Fig. 3(a)]. On further incubation (after 2 h) the nanoplate sizevaried from 2750 nm to 5000 nm as also seen in the SEMobservations [Fig. 2(e) and (f)]. A size range similar to that obtainedwith lauroyl glucose was also observed with lauroyl fructose after20 min. However, the particle size after 2 h could not be estimatedon account of the large size of the nanoplates as also seen in the SEM[Fig. 2(c)].

With lauroyl ascorbate, average size of the nanoparticles wasmuch smaller (39 nm), the minimum andmaximum being 35 nm and48 nm, respectively, after 10 min [Fig. 3(b)]. The figure also showsthat there was a small proportion of particles, in the range of 122 to264 nm and 908 nm to 1970 nm (indicated by black arrows). Onfurther incubation for 1 h, the size varied from 1820 to 5000 nm.

All the three esters were thus able to bring about a reduction of thechloroauric acid. The size and morphology of the nanoparticles thusformedwere varied.With further fine-tuning of the protocol and withthe use of appropriate capping agents such as thiols, chitosan,dodecanethiol or lysine [23–25] their size can be further regulated

Fig. 2. SEM analysis of nanoparticles obtained with lauroyl glucose (a) after 20 min; 18,000×, (b) after 2 h; 10,000×, (c) after 2 h with lauroyl fructose; 2000×, (d) after 1 h withlauroyl ascorbate; 1000×, (e) and (f) magnified images of nanoplates obtained with lauroyl ascorbate after 1 h; 18,000×, (g) representative spot EDS profile, and (h) representativeXRD spectra of nanoparticles obtained with lauroyl glucose.

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and this simple method could be used for custom designingnanoparticles with desired properties.

Fig. 3. Representative size distribution graphs for nanoparticles synthesized with (a)lauroyl glucose and (b) lauroyl ascorbate.

4. Conclusions

The hypothesis that the free reducing groups associated withglucose, fructose or ascorbate could be used for the reductive processduring nanoparticle synthesis has been proven. This article describes ahitherto unreported application of enzymatically-synthesized eco-friendly non-ionic emulsifiers (lauroyl glucose, lauroyl fructose andlauroyl ascorbate). The emulsifiers that were used in the reactionmixtures had two functions (i) to stabilize emulsions of toluenethereby orienting the reducing groups towards the bulk aqueousphase and (ii) to reduce the chloroauric acid using these reducinggroups. This method is simpler than the conventional use of water-in-oil microemulsions and minimizes the steps of dual emulsionformation and the use of an additional reducing agent. There was avariation in the type of nanoparticles being formed when the threetypes of esters were used. The particle size and structure of theresultant nanoparticles produced by these agents have been charac-terized by a variety of techniques such as UV–visible spectroscopy,SEM–EDS diffraction, XRD and particle size analyzer. Such nanopar-ticles are known to have potential applications in the field of optics,electronics, medical diagnostics and treatment, in coating differentmaterials and in making sensors [1,26–29]. Work is underway tocustom design nanoparticles of different shapes and sizes by varying

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the molar ratios of surfactant and the gold salt as well as by using avariety of capping agents.

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