Preparation of Highly Concentrated Silver Nanoparticles in Reverse Micelles of Sucrose Fatty Acid Esters through Solid-Liquid Extraction Method

doi:10.4236/aces.2011.14041

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Advances in Chemical Engi neering and Science , 2011, 1, 299-304

doi:10.4236/aces.2011.14041 Published Online October 2011 (http://www.SciRP.org/journal/aces)

Preparation of Highly Concentrated Silver Nanoparticles

in Reverse Micelles of Sucrose Fatty Acid Esters through

Solid-Liquid Extraction Method

Hidetaka Noritomi*, Yoshihiro Umezawa, Saori Miyagawa, Satoru Kato

Department of Applied Chemistry, Tokyo Metropolitan University, Tokyo, Japan

E-mail: *[email protected]

Recieved August 12, 2011; revised September 16, 2011; accepted September 20, 2011

Abstract

Silver nanoparticles were synthesized in reverse micelles consisting of sucrose fatty acid esters by dissolving

reactant powder in the water pool of reverse micelles through the solid-liquid extraction. Silver nanoparticles

having various sizes and shapes were obtained at high concentration. The size of silver nanoparticles was

controlled by reaction temperature. Moreover, the size of silver nanoparticles was dependent upon the aver-

age esterification degree of sucrose fatty acid esters forming reverse micelles. The wavelength in the peaks,

which corresponded upon the localized surface plasmon resonance of resultant silver nanoparticles, was cor-

related with their sizes.

Keywords: Reverse Micelle, Silver Nanoparticle, Size Control, Solid-Liquid Extraction, Sucrose Fatty Acid

Ester

1. Introduction

In recent years, the production and applications of metal

nanoparticles have rapidly been increased in the field of

nanotechnology, since their novel physical and chemical

properties are not only different from those of bulk sub-

stances due to their extremely small size and large spe-

cific surface area, but also metal nanoparticles exhibit

specific colors due to localized surface plasmon reso-

nance (LSPR) corresponding to the wavelength of light

that induces the largest electromagnetic field on the

nanopartices, when metal nanoparticles are impinged by

a beam of light [1]. Especially, the applications of silver

nanoparticles have widely been investigated, since they

exhibit some profitable properties such as catalysis [2],

antibacterial agent [3], nanoparticle colorant [4], nano-

paste for electrical circuits and electrodes [5], and sub-

strates for surface-enhanced Raman scattering [6]. The

colors of silver nanoparticles result from changes of

LSPR induced by their size and shape.

In order to produce metal nanoparticles having suit-

able sizes and/or shapes, many physical or chemical

techniques such as coprecipitation, gas-evaporation, sol-

gel method, and sputtering have been developed so far

[1]. Additionally, more attention has been paid on the

preparation of metal nanoparticles in reverse micelles [7].

Reverse micelles are thermodynamically stable nanome-

ter-sized aggregates of surfactant molecules dispersed in

a hydrophobic organic phase like octane, and can form

w/o type microemulsions containing a small amount of

water in their centers. The water phase in w/o type mi-

croemulsions is called water pool. Metal nanoparticles

are synthesized in the water pool, in which reactants are

dissolved. As the preparation of metal nanoparticles us-

ing reverse micelles does not require a special apparatus

and extreme conditions of temperature and pressure, it is

comparatively easy to expand the scale of reverse micel-

lar reaction system. However, the productivity of metal

nanoparticles per volume of reverse micellar system is

limited due to low overall concentration of reactants in

the reverse micellar system, since the volumetric ratio of

water phase playing a role as the dissolution of reactants

and the reaction field to the bulk organic phase is too

small [1].

In order to solubilize reactants in reverse micelles,

three solubilization methods are utilized: injection me-

thod; liquid-liquid extraction method; solid-liquid ex-

traction method [8]. First, the injection method is carried

out by injecting a few microliters of the concentrated

stock solution of reactants into the hydrocarbon solution

300 H. NORITOMI ET AL.

of surfactants. This method is most commonly used to

prepare metal nanoparticles in reverse micelles. Second,

the liquid-liquid extraction method is carried out by

transferring solutes dissolved in an aqueous phase from

the aqueous phase into the hydrocarbon phase of surface-

tants. This method is relatively slow, and the concentra-

tion of reactants in the hydrocarbon phase is limited due

to the distribution between aqueous and reverse micellar

organic phases in thermodynamic equ ilibrium. Third, the

solid-liquid extraction method is carried out by mixing

solid reactants with the reverse micellar solution con-

taining already a certain amount of water. This method

can dissolve reactants until their dissolution reaches

saturation in water pool. Moreover, when using the

solid-liquid extraction instead of the conventional inject-

tion method to solubilize reactants into reverse micelles,

the following benefits are expected. First, the productive-

ity of metal nanoparticles is markedly promoted, since

reactants consumed for the formation of metal nanoparti-

cles are successively supplied from solid reactants al-

ready added in large excess, compared to the solubility

of the water pool of reverse micelles. Second, only a

small amount of water is needed for solid-liquid extrac-

tion. Consequently, the aggregation of resultant nanopar-

ticles is inhibited, since the lower water content is kept in

reverse micellar system, the more stably nanoparticles

are dispersed [9]. However, there have not been any re-

ports about the preparation of nanoparticles in reverse

micelles using the solid -liquid extraction method.

In our previous work, we have reported that silver

nanoparticles are prepared in reverse micelles of sucrose

fatty acid esters by using the conventional injection

method [10]. Sucrose fatty acid esters are commercial

food grade nonionic surfactants, and are biodegradable

and nonhazardous to the environment [11]. In the present

work, we examined the preparation of silver nanoparti-

cles in reverse micelles of sucrose fatty acid esters by

using solid-liquid extraction method to address how the

preparation conditions such as the reaction temperature

and the composition of surfactants affect the size and

shape of silver nanoparticles.

2. Experimental

Silver nitrate and sodium borohydride were the guaran-

teed reagents of Kanto Chemicals (Tokyo, Japan). DK-

SS (sucrose fatty acids of 99 wt% monoesters and 1 wt%

di- and triesters, fatty acid constituent consisting of 60

wt% stearic acid and 40 wt% palmitic acid) and DK-F-

20W (sucrose fatty acids of 11wt% monoesters, 36wt%

di- and triesters, and 53 wt% more than tetraesters, fatty

acid constituent con sisting of 70 wt% stearic acid and 30

wt% palmitic acid) were supplied from Dai-Ichi Kogyo

Seiyaku (Kyoto, Japan). The surfactant was used without

further purification. Isooctane and n-butanol were from

Kanto Chemicals (Tokyo, Japan), and were of analytical

grade.

The reverse micellar solutions were prepared by add-

ing the required amounts of sucrose fatty acid esters and

a small amount of water into the solution of n-butanol/

isooctane (3:7 (v/v)), and then were used for the prepa-

ration of nanoparticles within a few minutes.

The preparation of nanoparticles was carried out by

adding a certain amounts of AgNO3 powder and sodium

borohydride powder into the reverse micellar solution.

Since the preliminary study indicated that the synthe-

sis of particles finished at 25˚C after 3 h, and no residual

solid reactants existed in the reaction medium, the sam-

ples for various analyses were taken after 3 h. In the pre-

sent work, the concentrations of sucrose fatty acid esters

and water based on the overall volume of reverse micel-

lar solution were fixed at 50 g/L and 60 mM, respect-

tively. As a typical condition, the overall concentrations

of AgNO3 and sodium borohydride were 0.1 and 1.0 M,

respectively, and the temperature was fixed at 25˚C.

The TEM micrograph was obtained using a JEOL

JEM-2000FX electron microscope operating at 200 kV.

The sample for TEM was prepared by dilutin g a resultan t

colloidal solution one hundred-fold with n-butanol and

placing a drop of colloidal solution onto the standard

carbon-coated copper grids and drying it under vacuum.

The UV-vis spectra of the reverse micellar solutions

containing nanoparticles were measured by UV/vis spec-

trophotometer (Ubest-55, Japan Spectroscopic Co. Ltd.)

with a 10 mm quartz cell.

3. Results and Discussion

3.1. Productivity of Silvar Nanoparticles through

Solid-Liquid Extraction Method

In order to successively supply reactants consumed for

the formation of silver nanoparticles to the water poo ls of

reverse micelles, which are reaction fields, and produce

silver nanoparticles at high concentration, we have em-

ployed solid-liquid extraction method instead of the

conventional injection method [8]. The colloidal solution

of silver nanoparticles prepared by solid-liqu id ex traction

method was much concentrated, compared to that pre-

pared by the injection method examined in our previous

method [10]. The concentration of silver nanoparticles

obtained by the present system was about one hundred

and thirty times larger than that obtained by the injection

method. After diluting the resultant colloidal solution

one hundred-fold with n-butanol, the measurement of

UV-visible absorption spectrum was carried out. The

H. NORITOMI ET AL.

301

color of the solution after dilution was yello w, similar to

the case of injection method, and, as shown in Figure 1,

its UV-visible absorption spectrum exhibited the peak

around 403 nm, which corresponds upon the LSPR of

silver nanoparticles [12]. Figure 2 shows the relationship

between surfactant concentration and the absorbance at

403 nm of colloidal solutions of silver na noparticles pre-

liminarily diluted one handred-fold with n-butanol. Any

absorbance at 403 nm was not observed without surfac-

tant. On the other hand, the absorbance at 403 nm in-

creased above 20 g/L of surfactant concentration. We

have reported that the apparent critical micelle concen-

tration in sucrose fatty acid/n-butanol/isooctane system

was observed around 10 g/L [13]. Consequently, it is

suggested that the preparation of silver nanoparticles

proceeds as follows. First, empty reverse micelles con-

taining a small amount of water approach solid reactants

added in large excess, compared to the solubility of the

water pool of reverse micelles. When the water pools of

reverse micelles contact solid reactants, reactants are

solubilized into the water pools. Then, silver nanoparti-

cles are synthesized from reactants solubilized in reverse

micelles. Resultant silver nanoparticles are coated by

surfactants, and are dispersed in organic solvents. Emp-

Figure 1. Absorption spectrum of silver nanoparticles pre-

pared by solid-liquid extraction method.

Figure 2. Effect of surfactant concentration on absorbance

at 403 nm of colloidal solution dispersing silver nanoparti-

cles prepared by solid-liquid extraction method: [AgNO3]ov

= 0.1 M; [NaBH4]ov = 1.0 M; n-butanol/isooctane containing

50 g/L DK-SS and 60 mM H2O; reaction time = 3 h; reac-

tion temperature = 25˚C.

tied reverse micelles approach solid reactants again.

Thus, the production of silver nanoparticles is promoted

by repeating the process composed of the extraction of

reactants, the synthesis of silver nanoparticles, and the

dispersion of resultant silver nanoparticles.

3.2. Dependence of Reaction Temperature upon

Size of Silver Nanopartices

Figure 3 shows the typical transmission electron micro-

graphs and size distributions of silver nanoparticles when

silver nanoparticles were prepared in the reverse micellar

system of DK-SS for 3 h at different reaction tempera-

tures. The TEM images showed that the obtained silver

nanoparticles displayed a wide variety of shapes. The

size of resultant silver nanoparticles was almost similar

to that prepared in DK-SS or AOT (sodium bis(2-ethyl-

hexyl) sulfosuccinate) reverse micelles by the conven-

tional injection method [10,14,15]. The size of silver na-

noparticles was strongly dependent upon reaction tem-

perature. Figure 4 shows the plots of the mean diameter

of silver nanoparticles against reaction temperature. As

seen in the figure, the mean diameter of silver nanoparti-

cles gradually increased with an increase in reaction

temperature. The line in the figure represents the fitting

curve with the correlation coefficient of 0.99 as

0.079 6.2dt



 (1)

where d is the mean diameter of silver nanoparticles, and

t is the reaction temperature. This tendency was similar

to the case using the conventional injection method [10].

The solubilization of reactants, the exchange of reactants

between reverse micelles, the reduction reaction, and the

growth of nanoparticles increase with increasing the re-

action temperature [16-19].

Figure 5 shows the plots of the mean diameter of sil-

ver nanoparticles obtained at different reaction tempera-

ture against the wavelength in the peak of UV-visible

absorption spectrum due to the LSPR of silver nanopar-

ticles. The standard deviation concerning the size distri-

bution of silver nanoparticles depended upon reaction

temperature. The wavelength in the peaks increases with

an increase in the mean diameter of silver nanoparticles.

The line in the figure represents th e fitting curve with the

correlation coefficient of 0.99 as

λ = 8.2d + 340 (2)

where λ is the wavelength of peaks, and d is the mean

diameter of silver nanoparticles. The position of the

plasmon adsorption peak depends upon the particle size

and shape, and especially tends to be red-shifted with

ncreasing the particle size [14,16,20,21]. i

H. NORITOMI ET AL.

302

Figure 3. Transmission electron micrographs and particle size distributions of silver nanoparticles synthesized in DK-SS re-

verse micelles: reaction temperature = (a) 5˚C, (b) 15˚C, (c) 25˚C, and (d) 40˚C.

3.3. Dependence of Average Esterification

Degree of Surfactants upon Size of Silver

Nanoparticles

The formation and stability of micelles are due to the

structure and/or HLB of surfactants [22-24]. The struc-

ture and HLB of sucrose fatty acid esters alter with the

esterification degree [11]. In order to elucidate the effect

of those factors on the formation of silver nanoparticles,

we have examined the synthesis of silver nanoparticles in

the reverse micelles of sucrose fatty acid esters by mix-

ing DK-SS (average esterification degree = 1.01, HLB =

19) with DK-F-20W (average esterification degree = 3.1,

HLB = 2). The more weight fraction of DK-F-20W is,

the larger the average esterification degree is. Figure 6

shows the typical transmission electron micrographs and

size distributions of silver nanoparticles when silver

nanoparticles were prepared in the reverse micellar sys-

tem of sucrose fatty acid esters at different average es-

terification degrees for 3 h. The silver nanoparticles in

various shapes were obtained in those reverse micellar

systems. The standard deviation concerning the size dis-

tribution of silver nanoparticles was strongly dependent

upon the average esterification degree of surfactants.

When the average esterification degree was 1.85, the

minimal standard deviation was obtained. This result

indicated that at that average esterification degree, mono-

disperse silver nanoparticles were formed. As shown in

Figure 7, the mean diameter of silver nanoparticles was

dependent upon the average sterification degree, similar

Figure 4. Effect of reaction temperature on the mean di-

ameters of silver nanoparticles in reverse micelles of DK-SS:

[AgNO3]ov = 0.1 M; [NaBH4]ov = 1.0 M; n-butanol/ isooctane

containing 50 g/L DK-SS and 60 mM H2O; reaction time =

3 h.

Figure 5. Relationship of the mean diameter of silver nano-

particles obtained at different reaction temperature with

the wavelength in the peak of UV-visible absorption spec-

trum due to the LSPR of silver nanoparticles. e

H. NORITOMI ET AL.

303

Figure 6. Transmission electron micrographs and particle size distributions of silver nanoparticles synthesized in reverse

micelles of DK-SS and DK-F-20W: average esterification degree = (a) 1.01, (b) 1.43, (c) 1.85, (d) 2.68, and (e) 3.10.

Figure 7. Effect of average esterification degree of surfac-

tants on the size of silver nanoparticles synthesized in re-

verse micelles of DK-SS and DK-F-20W: [AgNO3]ov= 0.1 M;

[NaBH4]ov = 1.0 M; n-butanol/isooctane containing 50 g/L

DK-SS/DK-F-20W and 60 mM H2O; reaction temperature

= 25 ˚C; reaction time = 3 h.

to the case of injection method [10,25]. The size of silver

nanoparticles gradually increased with an increase in the

average esterification degree. Thus, when the bulkiness

of surfactants increased or the HLB value decreased, the

size of silver nanoparticles tended to increase.

4. Conclusions

We have demonstrated that the synthesis of silver

nanoparticles is drastically promoted in the sucrose fatty

acid ester/n-butanol/isooctane system by supplying reac-

tants to the water pool through solid-liquid extraction.

The size of silver nanoparticles decreased with a de-

crease in reaction temperature. The wavelength in the

peaks corresponding upon the LSPR was linearly cor-

related with the size of silver nanoparticles prepared by

the present method. The size of silver nanoparticles

tended to depend upon the average esterification degree

of sucrose fatty acid esters.

5. Acknowledgements

Authors thank Mr. Misaki for taking TEM micrographs,

and Dai-Ichi Kogyo Seiyaku Co., Ltd. for supplying su-

crose fatty acid esters.

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