# Modified Natural Rubber Induced Aqueous to Toluene Phase Transfer of Gold and Platinum Colloids

Authors: Mohamad Abu Bakar; Jamil Ismail; Cheng Hoon Teoh; Wei Leng Tan; Noor Hana Hanif Abu Bakar
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Citrate-stabilized gold and platinum particles were prepared in aqueous phase and transferred to toluene phase by employing 2-propanol as the transfer agent. It was found that the modified natural rubber (ENR) induced the phase transfer and assisted the dispersion of the citrate-free metal particles into the organic phase. The amounts of gold and platinum transferred are 93.4% and 86.1%, respectively. This phase transfer technique produced organosols of smaller particle sizes and narrower size distribution with self-assembly arrangements when compared to those prepared via the previous in situ preparations.  The respective average particle size and standard deviation of gold before and after phase transfer were6.3±1.7 nm and 7.2±1.3 nm, while for platinum they were 4.0±0.7 nm and 4.2±0.8 nm. The slight increase in the average sizes and overall size distributions in both metals after transfer was attributed to multiparticle aggregation in the organic phase.


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## Body

## 1. Introduction

Methods for the preparation of nanosized
noble metals have been widely explored due to their importance in various
applications [1]. Among the applications are sensors [2], catalysts [3], labels
[4], as well as optical [5] and electronic materials [6]. In order to meet the
requirements of these applications, particle synthesis has been tailored to
meet specific features such as dispersity, size, and shapes [7]. These properties are dependent on the
preparation environment and techniques. There are various routes to synthesize
metal particles. Reduction methods such as chemical [8], sonochemical [9],
electromagnetic [10], and electrochemical [11] have been reported. Chemical
reduction is the most favored as a “bottom-up” approach.It is well known that metal particles
can be easily formed in aqueous compared to organic phase due to the ease of
solubility of most metal precursors, chemical reducing agents (e.g.,
borohydrides and citrates), and stabilizers in water. This has afforded a wide
range of metal hydrosols with various particle sizes and morphology as compared
to metal sols prepared in the organic environment. To diversify the range of metal
organosols, phase transfer of metal particles from aqueous to organic phase was
introduced. In general, phase
transfer technique has many advantages due to a number of factors. Among them is
the ease of particles in the organic phase to adopt the size, size distribution
and morphology of the particles prepared in aqueous phase. In addition, the particle
size and morphology can be better manipulated due to the hydrosols that are
already well formed prior to the particles transfer to the organic phase. The
interaction between the metals and the organic solvents with or without
stabilizer often creates particles self-assembly and this is advantageous in
certain applications. Furthermore, aqueous
to organic phase method enables the use of nonwater soluble stabilizers/matrix.Transfer agents and particle
stabilizers are often needed for such particle phase transfer processes. Among the
transfer agents commonly used are tetraoctylammonium bromide (TOAB) [12],
dimethyldioctadecylammonium chloride (DDAC) [13], concentrated hydrochloric
acid [14], and alcohols (e.g., ethanol and 2-propanol) [15]. The common particle
stabilizers are alkylamines (e.g., octadecylamine and dodecylamine) [16],
alkylthiols (1-dodecanethiol) [17], citrates, and surfactants (e.g., mercaptopropionic
acid) [18]. A few of these stabilizers are also known to act as transfer agents
[16, 17]. Few polymers have been employed as stabilizers for particles in the
organic phase. Esumi et al. employed a synthetic poly (amidoamine) dendrimer to
encapsulate gold particles in an organic phase [19]. There are also few reports
on the usage of natural polymers as stabilizers in a similar phase transfer
process. We have previously reported the use of modified natural rubber
(ENR-50) as the stabilizing agent or matrix for the silver nanoparticles in the
organic phase [20]. Our work uses surfactant, tetraoctylammoniumbromide (TOAB),
as the transferring agent. However, a trace presence of TOAB in the organic
phase was observed. Therefore, the matrix comprising a mixture of stabilizer (TAOB) and polymer (ENR) encapsulates
the metal particles. This can affect the properties of the resultant composite
especially in certain applications such as in electrical conductive adhesive
(ECA), thermal interface material (TIM), and polyelectrolyte. For the said
applications, bare metal-polymer composites are much desired. The use of
citrate as aqueous phase particle stabilizer is of particular interest as
compared to alkylamines and alkylthiols. Unlike the latter, citrate has been
reported to be completely detached or replaced by other organic soluble stabilizers
upon particle transfer into the organic phase [16–18]. Thus, in this study, we report the use
of ENR-50 that induces the aqueous to toluene transfer of citrate-stabilized gold
and platinum particles and the ability of the ENR-50 to exert good dispersion of
the citrate-free particles in the toluene layer.


## 2. Experimental

### 2.1. Materials

Hydrogen tetrachloroaurate
trihydrate, HAuCl4·3H2O and platinic chloride, H2PtCl6 (both from Sigma, USA),
sodium borohydride, NaBH4 (Riedel-de Haen, Germany), trisodium citrate, Na3C6H5O4·2H2O
(Ajax Chemicals, Australia), toluene (Fisher Chemicals, UK), and 2-propanol (Merck, Germany) were obtained commercially and used without further
purification. Modified natural rubber with 50% epoxidation (ENR-50) was
supplied by Guthrie
Polymer Sdn. Bhd (Malaysia) and purified according to the previous report [21]. The average molecular
weight of the purified ENR-50 is 400 000.


### 2.2. Preparation and Phase Transfer of Metal Colloids

#### 2.2.1. Metal Colloids

For the preparation of 20 mL aqueous gold
colloids, 1.56 mL2.54×10−6 mol mL−1 of aqueous
HAuCl4·3H2O stock solution was pipette into a reaction
flask and 18.04 mL of distilled water was added. The solution mixture was
stirred with magnetic stirrer. A 0.4 mL of 3.0×10−5 mol mL−1 trisodium citrate solution was then added and stirring was continued for a
further 10 minutes. Subsequently, 0.4 mL of 1.0×10−4 mol mL−1 of freshly prepared, chilled NaBH4 solution was added.Similarly, for the preparation of 20 mL aqueous platinum colloids, 1.44 ml of2.74×10−6 mol mL−1 aqueous H2PtCl6 stock solution and 18.16 mL of distilled
water was mixed and stirred. A 0.4 mL of 3.0×10−5 mol mL−1 solution of trisodium citrate was then added and stirred for another 10
minutes. This was followed by the addition of 2.5 mL of 1.0×10−4 mol mL−1 of chilled NaBH4 solution.


#### 2.2.2. Phase Transfer

A 5 mL of the respective metal colloids
prepared above was added into a flask containing 5 mL of 2-propanol followed by
vigorous stirring. A 5 mL of2.50×10−5 mol mL−1 of ENR-toluene
stock solution was then added. The stirring was stopped after 1 minute and the
mixture was left standing for phase separation.The role of 2-propanol and ENR in the
phase transfer process was also investigated. Thus, experiments without the
2-propanol as phase transfer agent and another in the absence of ENR in the
organic phase were similarly carried out as above.


### 2.3. Characterizations

Prior to any form of
characterization, the organic layer of each preparation was separated and evaporated
off. After that, 5 mL of toluene was added to redisperse the metal colloids and
these were subsequently used for various characterizations as described below.The size and morphology of the metal
particles before (aqueous phase) and after (organic phase) phase transfer was
analyzed using a Philip CM12 transmission electron microscope (TEM) operating
at 80 kV. 5 drops of the respective metal colloids were cast onto a carbon
coated copper grids and left to dry. The average size of the particles,
standard deviations (SD), and size distributions were evaluated using the computer
software “Analysis Docu 2.11” (Soft Imaging System GmbH, Munster, Germany). The
optical property of the metal colloid was measured using a Hitachi U-2000 UV-vis
spectrophotometer. The metal colloid was placed in a 1cm quartz cuvette and
measured within the range of 200–800 nm. X-ray diffraction
analysis was performed using a Siemens D5000 X-ray diffractometer with a monochromatic Cu-Kα radiation filter in the 2θ range of 0–100°. Samples
were prepared as a thin film on a glass slide. The metal contents of the organic phase after phase transfer were determined using the Perkin Elmer 3100
flame atomic absorption spectrometer (AAS). The average molecular weight of purified
ENR was determined using a Perkin Elmer 250 gas permeation chromatography
(GPC). The structure of ENR was analyzed using Perkin Elmer System 2000 FT-infrared
spectrometry.


## 2.1. Materials

Hydrogen tetrachloroaurate
trihydrate, HAuCl4·3H2O and platinic chloride, H2PtCl6 (both from Sigma, USA),
sodium borohydride, NaBH4 (Riedel-de Haen, Germany), trisodium citrate, Na3C6H5O4·2H2O
(Ajax Chemicals, Australia), toluene (Fisher Chemicals, UK), and 2-propanol (Merck, Germany) were obtained commercially and used without further
purification. Modified natural rubber with 50% epoxidation (ENR-50) was
supplied by Guthrie
Polymer Sdn. Bhd (Malaysia) and purified according to the previous report [21]. The average molecular
weight of the purified ENR-50 is 400 000.


## 2.2. Preparation and Phase Transfer of Metal Colloids

### 2.2.1. Metal Colloids

For the preparation of 20 mL aqueous gold
colloids, 1.56 mL2.54×10−6 mol mL−1 of aqueous
HAuCl4·3H2O stock solution was pipette into a reaction
flask and 18.04 mL of distilled water was added. The solution mixture was
stirred with magnetic stirrer. A 0.4 mL of 3.0×10−5 mol mL−1 trisodium citrate solution was then added and stirring was continued for a
further 10 minutes. Subsequently, 0.4 mL of 1.0×10−4 mol mL−1 of freshly prepared, chilled NaBH4 solution was added.Similarly, for the preparation of 20 mL aqueous platinum colloids, 1.44 ml of2.74×10−6 mol mL−1 aqueous H2PtCl6 stock solution and 18.16 mL of distilled
water was mixed and stirred. A 0.4 mL of 3.0×10−5 mol mL−1 solution of trisodium citrate was then added and stirred for another 10
minutes. This was followed by the addition of 2.5 mL of 1.0×10−4 mol mL−1 of chilled NaBH4 solution.


### 2.2.2. Phase Transfer

A 5 mL of the respective metal colloids
prepared above was added into a flask containing 5 mL of 2-propanol followed by
vigorous stirring. A 5 mL of2.50×10−5 mol mL−1 of ENR-toluene
stock solution was then added. The stirring was stopped after 1 minute and the
mixture was left standing for phase separation.The role of 2-propanol and ENR in the
phase transfer process was also investigated. Thus, experiments without the
2-propanol as phase transfer agent and another in the absence of ENR in the
organic phase were similarly carried out as above.


## 2.2.1. Metal Colloids

For the preparation of 20 mL aqueous gold
colloids, 1.56 mL2.54×10−6 mol mL−1 of aqueous
HAuCl4·3H2O stock solution was pipette into a reaction
flask and 18.04 mL of distilled water was added. The solution mixture was
stirred with magnetic stirrer. A 0.4 mL of 3.0×10−5 mol mL−1 trisodium citrate solution was then added and stirring was continued for a
further 10 minutes. Subsequently, 0.4 mL of 1.0×10−4 mol mL−1 of freshly prepared, chilled NaBH4 solution was added.Similarly, for the preparation of 20 mL aqueous platinum colloids, 1.44 ml of2.74×10−6 mol mL−1 aqueous H2PtCl6 stock solution and 18.16 mL of distilled
water was mixed and stirred. A 0.4 mL of 3.0×10−5 mol mL−1 solution of trisodium citrate was then added and stirred for another 10
minutes. This was followed by the addition of 2.5 mL of 1.0×10−4 mol mL−1 of chilled NaBH4 solution.


## 2.2.2. Phase Transfer

A 5 mL of the respective metal colloids
prepared above was added into a flask containing 5 mL of 2-propanol followed by
vigorous stirring. A 5 mL of2.50×10−5 mol mL−1 of ENR-toluene
stock solution was then added. The stirring was stopped after 1 minute and the
mixture was left standing for phase separation.The role of 2-propanol and ENR in the
phase transfer process was also investigated. Thus, experiments without the
2-propanol as phase transfer agent and another in the absence of ENR in the
organic phase were similarly carried out as above.


## 2.3. Characterizations

Prior to any form of
characterization, the organic layer of each preparation was separated and evaporated
off. After that, 5 mL of toluene was added to redisperse the metal colloids and
these were subsequently used for various characterizations as described below.The size and morphology of the metal
particles before (aqueous phase) and after (organic phase) phase transfer was
analyzed using a Philip CM12 transmission electron microscope (TEM) operating
at 80 kV. 5 drops of the respective metal colloids were cast onto a carbon
coated copper grids and left to dry. The average size of the particles,
standard deviations (SD), and size distributions were evaluated using the computer
software “Analysis Docu 2.11” (Soft Imaging System GmbH, Munster, Germany). The
optical property of the metal colloid was measured using a Hitachi U-2000 UV-vis
spectrophotometer. The metal colloid was placed in a 1cm quartz cuvette and
measured within the range of 200–800 nm. X-ray diffraction
analysis was performed using a Siemens D5000 X-ray diffractometer with a monochromatic Cu-Kα radiation filter in the 2θ range of 0–100°. Samples
were prepared as a thin film on a glass slide. The metal contents of the organic phase after phase transfer were determined using the Perkin Elmer 3100
flame atomic absorption spectrometer (AAS). The average molecular weight of purified
ENR was determined using a Perkin Elmer 250 gas permeation chromatography
(GPC). The structure of ENR was analyzed using Perkin Elmer System 2000 FT-infrared
spectrometry.


## 3. Results and Discussion

### 3.1. Synthesis

Aqueous solutions of chloroauric acid and chloroplatinic
acid in the presence of trisodium citrate were yellow and pale yellow,
respectively. Upon addition of sodium borohydride to the aqueous gold solution,
the reduction process causes the color to progressively change from dark blue
to purple then pink and finally wine-red which are the typical colors for
citrate-stabilized metallic gold nanoparticles
[22]. For
the citrate-stabilized
platinum nanoparticles, the color however changed from light brown to dark brown [15]. Equations (1) represent borohydride reductions of the respective metals [23]:(1)NaBH4+HAuCl4+3H2O→Au0+H3BO3+3HCl+NaCl+52H2,NaBH4+H2PtCl6+3H2O→Pt0+H3BO3+5HCl+NaCl+2H2.When the respective
citrate-stabilized metal hydrosol was stirred in the presence of ENR-toluene
solution and 2-propanol, a milky emulsion was formed. The emulsion for gold
was milky pink while that of platinum was milky white as shown in Plates1(a)(ii)
and 1(b)(ii), respectively. After phase separation, the bottom aqueous layer
becomes colorless while the top organic layer adopts the color of the
respective metal hydrosols, (Plates 1(a)(iii) and 1(b)(iii)). However, when
2-propanol was excluded from the preparation, no apparent transfer of the
particles between the phases was observed, as indicated by the color of the
respective phases. They retained their original colors. There was no exchange
of particles between the phases although contact between the aqueous and organic
interface was intentionally increased by stirring. Therefore, the particles phase
transfer was assisted by 2-propanol which acted as the transfer agent. 2-propanol has a mutual solubility in aqueous
and toluene solvents. Alcohol is known to act both as solvent and reducing
agent for noble metals. However, the latter is only possible at elevated
temperatures. The work described uses 2-propanol solely as the phase transfer
agent that acts as the intermediary for both phases thus enabling the transfer
of the metal particles between phases.For these metal organosols, there are
no significant changes observed from the FTIR spectra of ENR before and after
phase transfer. A typical IR spectrum of the metal-ENR-50 organosol is
presented in Figure1. The peaks due to the oxirane at 877 cm−1 [24]
and the C=C at 840 cm−1 remained unchanged. Therefore, it is anticipated
that there are no structural changes of the ENR chain. There was also no FTIR band attributable to the
citrate observed in the organic phase, which meant that citrate was not
transferred into the toluene phase. This implied that citrate stabilizers
remained in the aqueous phase (probably due to the insolubility of citrate in
toluene) after the transfer process of the respective metal particles to the
organic phase.Figure 1
Typical FTIR spectra for the metal-ENR-50 organosol.Plate 1
(a) Gold nanoparticles and (b) platinum nanoparticles at various stages of phase
transfer: (i) before mixing, (ii) during mixing, and (iii) after phase
separation.The optical properties of
the gold sols before (hydrosol) and after (organosol) phase transfer are shown
in Figure2. The citrate-stabilized gold hydrosol showed a typical absorbance at
λmax 525 nm that corresponds to the surface plasmon resonance (SPR)
of gold nanoparticles [25]. After phase
transfer, the λmax is slightly red shifted to 530 nm. This red shift
often indicated the formation of bigger particles but in this work, it may also
be due to the variation in the solvent’s refractive index on going from aqueous
to toluene [16]. Other possible factors that may have caused this red shift are the changes in the
adsorbed molecules on the particle surface (i.e., citrate stabilized to ENR
stabilized) or the aggregation of the particles during the transfer process as previously reported [15]. Platinum
sols do not exhibit SPR within the UV-Vis range studied.The organic layers after phase
transfer were subjected to the atomic absorption spectrophotometry (AAS). The
AAS analyses of the organic aliquots showed that the average metal contents in the organic
phase after the transfer of the metal particles are 93.4% and 86.1% for gold and platinum, respectively.In order to ascertain the role of ENR
as the stabilizer, an experiment was carried out using only citrate-stabilized
metal hydrosols that were
transferred into toluene without ENR. It was found that the particles tend to
aggregate forming several masses of particles at
the interphase as shown in Plate2. This observation indirectly proved that the
transportation of citrate into the organic phase did not occur. This is due to
the low solubility of citrate in the toluene. Thus, without any stabilizer in
the organic phase, the metal particles could not disperse well and therefore tend to agglomerate. Therefore, in those experiments where the ENR is included in the organic
phase, the ENR is responsible for dispersing and stabilizing the transferred metal particles
in the organosols. The ability of ENR to act as a stabilizer arises from its
molecular structure and its chain conformation when in toluene solution. The
degree of epoxidation, distribution of the epoxide group in the polymer chain, molecular
weight and interaction between the chain and toluene molecules determined the chain
conformation. In the case of ENR-50, an isoprene polymer with 50% mol
epoxidation level and the average molecular weight of 400,000, the polymer
chain resumes an
extended coil conformation. This conformation is capable of accommodating the
metal particles, brought to proximity by the carrier 2-propanol, and results in
the metal particles entrapment in the coiled chains. This mode of particle
stabilization is generally termed as trapped within the voids of the polymer
chains [20]. Thus, the particle aggregations are inhibited. Therefore, in the
presence of ENR, the ENR induces
the transfer of metal particles (into the organic phase), assists in the particle dispersion
by providing sites (voids) for the particle, and stabilizes the particles (within the voids) in the
organosols. Although it has been reported that metal particles can interact
with epoxide and the C=C groups of the ENR to form metal-complexes [26, 27], there is no
evidence of such interactions or complexations was observed from the FTIR
spectra as mentioned earlier.Plate 2
Phase transfer in the absence of ENR for (a) gold and (b) platinum.Figure 2
UV-Vis absorbencies of gold sols.


### 3.2. Phase and Morphology of Gold and Platinum Particles

The ENR-gold
organosol cast into film exhibited diffraction peaks at 2θ of 38.2°,
44.4°, 64.6°, and 77.6° that corresponded to the
lattice planes of [111], [200], [220], and [311], respectively, Figure 3(a). Similarly,
the ENR-platinum film also exhibited the diffraction peaks corresponding to the
lattice planes of [111], [200], [220], and [311] at the respective 2θ of 39.7°,
46.2°, 67.4°, and 81.2°, Figure 3(b). These
lattice planes corresponded to the face-centered cubic (fcc) lattice of
metallic gold and platinum, respectively [28, 29]. The very broad peak at 2θ
range of ∼10–30° in both diffractograms was due to the halo-ENR.Figure 3
XRD diffractograms of cast ENR-stabilized metal particles film of (a) gold and (b) platinum.The TEM micrographs and histograms for gold and platinum particles before and after
phase transfer are depicted in Figure4. The gold and platinum
particles were self-assembled. The shapes of the particles in the aqueous phase
are similar as those described by Yang et al. [15]
using citrate as the stabilizer. The shape is basically retained upon the
transfer to the organic phase. However, the platinum particles are less defined
as compared to gold probably due to its smaller size. The average sizes and
standard deviations of the gold particles before and after phase transfer are 6.3±1.7 nm and 7.2±1.3 nm,
respectively. For platinum particles, however, the respective average sizes and
standard deviations before and after phase transfer are 4.0±0.7 nm and
4.2±0.8 nm. Generally, there is a slight increase in particle size and
size distribution after phase transfer. For gold, the majority of the particles
are situated around 6-7 nm range before
the phase transfer. However, the amount of particles in the size range of 7-8 nm increased
after phase transfer. Similarly, for platinum, the increase is from around 3-4 nm to 5-6 nm size range. Especially
for gold, the increases
in particle sizes and size distributions are in accordance with the red shift
of the λmax as observed in the UV-Vis analysis. Generally, the
increase in particle size may be due to the slight aggregation of the particles
upon transfer from the aqueous to the organic phase [15]. As the particles move
into the organic phase, they may come in contact with each other and aggregate
due to the absence of citrate surrounding the particles before the particles are finally surrounded
by ENR and hence inhibiting further growth.TEM micrographs, average
size, standard deviation (SD), and size distribution of ENR-stabilized gold nanoparticles
at (a) before and (b) after phase transfer (scale bar = 50 nm) and platinum nanoparticles at (c) before and (d)
after phase transfer (scale bar = 20 nm).
(a)(b)(c)(d)


### 3.3. Mechanism of Interphase Particles Transfer

In the various phase transfer experiments, it was found that very little transfer of
particles was observed without stirring. Similarly, without 2-propanol there is
also no apparent particle transfer observed, even after prolonged stirring. In
the absence of ENR, the metal particles tend to aggregate and accumulate at the
aqueous/toluene interphase. For those successful phase transfer processes, no citrate was
detected in the organic phase. This inferred that the metal particles underwent
an exchange of stabilizers upon phase transfer, that is, from citrate stabilized
in aqueous phase to ENR stabilized in the toluene phase. Therefore, addition of 2-propanol is required
for phase transfer to occur with 2-propanol acting as the transfer agent while
ENR acts as replacement stabilizer and therefore exerts dispersion of the particles in the toluene
phase. It must be noted that in the several mechanisms previously reported, the
stabilizers are often transferred along with the metal particles from the
aqueous to organic phase, that is, without undergoing any exchange of
stabilizers [30–33].In this work, the probable mechanism
of phase transfer is proposed as follows. Upon formation of the metal particles,
they are initially stabilized by the citrate in the aqueous phase. The microemulsion
was formed when hydrosol and ENR toluene in the presence of 2-propanol were
mixed. The formation of the microemulsion increases the surface area between
the organic and aqueous phase. The addition of 2-propanol will cause the transfer
of the citrate-stabilized particles into the toluene layer due to the mutual
solubility of alcohol in both solvents. Upon particle transfer, the particles
undergo replacement of stabilizer from the citrate to ENR. This is facilitated
by the structure of ENR-50 that is naturally loose and contains many interchain voids [34]. The
citrate disentangled
from the surface of the metal particles and remained in the aqueous layer as it is insoluble in toluene as
previously evidenced
by the FTIR results. Therefore, the presence of the ENR in the organic phase
induces dispersion of the metal particles. The metal particles diffused into
the voids and formed stable ENR-metal organosols as discussed earlier.


## 3.1. Synthesis

Aqueous solutions of chloroauric acid and chloroplatinic
acid in the presence of trisodium citrate were yellow and pale yellow,
respectively. Upon addition of sodium borohydride to the aqueous gold solution,
the reduction process causes the color to progressively change from dark blue
to purple then pink and finally wine-red which are the typical colors for
citrate-stabilized metallic gold nanoparticles
[22]. For
the citrate-stabilized
platinum nanoparticles, the color however changed from light brown to dark brown [15]. Equations (1) represent borohydride reductions of the respective metals [23]:(1)NaBH4+HAuCl4+3H2O→Au0+H3BO3+3HCl+NaCl+52H2,NaBH4+H2PtCl6+3H2O→Pt0+H3BO3+5HCl+NaCl+2H2.When the respective
citrate-stabilized metal hydrosol was stirred in the presence of ENR-toluene
solution and 2-propanol, a milky emulsion was formed. The emulsion for gold
was milky pink while that of platinum was milky white as shown in Plates1(a)(ii)
and 1(b)(ii), respectively. After phase separation, the bottom aqueous layer
becomes colorless while the top organic layer adopts the color of the
respective metal hydrosols, (Plates 1(a)(iii) and 1(b)(iii)). However, when
2-propanol was excluded from the preparation, no apparent transfer of the
particles between the phases was observed, as indicated by the color of the
respective phases. They retained their original colors. There was no exchange
of particles between the phases although contact between the aqueous and organic
interface was intentionally increased by stirring. Therefore, the particles phase
transfer was assisted by 2-propanol which acted as the transfer agent. 2-propanol has a mutual solubility in aqueous
and toluene solvents. Alcohol is known to act both as solvent and reducing
agent for noble metals. However, the latter is only possible at elevated
temperatures. The work described uses 2-propanol solely as the phase transfer
agent that acts as the intermediary for both phases thus enabling the transfer
of the metal particles between phases.For these metal organosols, there are
no significant changes observed from the FTIR spectra of ENR before and after
phase transfer. A typical IR spectrum of the metal-ENR-50 organosol is
presented in Figure1. The peaks due to the oxirane at 877 cm−1 [24]
and the C=C at 840 cm−1 remained unchanged. Therefore, it is anticipated
that there are no structural changes of the ENR chain. There was also no FTIR band attributable to the
citrate observed in the organic phase, which meant that citrate was not
transferred into the toluene phase. This implied that citrate stabilizers
remained in the aqueous phase (probably due to the insolubility of citrate in
toluene) after the transfer process of the respective metal particles to the
organic phase.Figure 1
Typical FTIR spectra for the metal-ENR-50 organosol.Plate 1
(a) Gold nanoparticles and (b) platinum nanoparticles at various stages of phase
transfer: (i) before mixing, (ii) during mixing, and (iii) after phase
separation.The optical properties of
the gold sols before (hydrosol) and after (organosol) phase transfer are shown
in Figure2. The citrate-stabilized gold hydrosol showed a typical absorbance at
λmax 525 nm that corresponds to the surface plasmon resonance (SPR)
of gold nanoparticles [25]. After phase
transfer, the λmax is slightly red shifted to 530 nm. This red shift
often indicated the formation of bigger particles but in this work, it may also
be due to the variation in the solvent’s refractive index on going from aqueous
to toluene [16]. Other possible factors that may have caused this red shift are the changes in the
adsorbed molecules on the particle surface (i.e., citrate stabilized to ENR
stabilized) or the aggregation of the particles during the transfer process as previously reported [15]. Platinum
sols do not exhibit SPR within the UV-Vis range studied.The organic layers after phase
transfer were subjected to the atomic absorption spectrophotometry (AAS). The
AAS analyses of the organic aliquots showed that the average metal contents in the organic
phase after the transfer of the metal particles are 93.4% and 86.1% for gold and platinum, respectively.In order to ascertain the role of ENR
as the stabilizer, an experiment was carried out using only citrate-stabilized
metal hydrosols that were
transferred into toluene without ENR. It was found that the particles tend to
aggregate forming several masses of particles at
the interphase as shown in Plate2. This observation indirectly proved that the
transportation of citrate into the organic phase did not occur. This is due to
the low solubility of citrate in the toluene. Thus, without any stabilizer in
the organic phase, the metal particles could not disperse well and therefore tend to agglomerate. Therefore, in those experiments where the ENR is included in the organic
phase, the ENR is responsible for dispersing and stabilizing the transferred metal particles
in the organosols. The ability of ENR to act as a stabilizer arises from its
molecular structure and its chain conformation when in toluene solution. The
degree of epoxidation, distribution of the epoxide group in the polymer chain, molecular
weight and interaction between the chain and toluene molecules determined the chain
conformation. In the case of ENR-50, an isoprene polymer with 50% mol
epoxidation level and the average molecular weight of 400,000, the polymer
chain resumes an
extended coil conformation. This conformation is capable of accommodating the
metal particles, brought to proximity by the carrier 2-propanol, and results in
the metal particles entrapment in the coiled chains. This mode of particle
stabilization is generally termed as trapped within the voids of the polymer
chains [20]. Thus, the particle aggregations are inhibited. Therefore, in the
presence of ENR, the ENR induces
the transfer of metal particles (into the organic phase), assists in the particle dispersion
by providing sites (voids) for the particle, and stabilizes the particles (within the voids) in the
organosols. Although it has been reported that metal particles can interact
with epoxide and the C=C groups of the ENR to form metal-complexes [26, 27], there is no
evidence of such interactions or complexations was observed from the FTIR
spectra as mentioned earlier.Plate 2
Phase transfer in the absence of ENR for (a) gold and (b) platinum.Figure 2
UV-Vis absorbencies of gold sols.


## 3.2. Phase and Morphology of Gold and Platinum Particles

The ENR-gold
organosol cast into film exhibited diffraction peaks at 2θ of 38.2°,
44.4°, 64.6°, and 77.6° that corresponded to the
lattice planes of [111], [200], [220], and [311], respectively, Figure 3(a). Similarly,
the ENR-platinum film also exhibited the diffraction peaks corresponding to the
lattice planes of [111], [200], [220], and [311] at the respective 2θ of 39.7°,
46.2°, 67.4°, and 81.2°, Figure 3(b). These
lattice planes corresponded to the face-centered cubic (fcc) lattice of
metallic gold and platinum, respectively [28, 29]. The very broad peak at 2θ
range of ∼10–30° in both diffractograms was due to the halo-ENR.Figure 3
XRD diffractograms of cast ENR-stabilized metal particles film of (a) gold and (b) platinum.The TEM micrographs and histograms for gold and platinum particles before and after
phase transfer are depicted in Figure4. The gold and platinum
particles were self-assembled. The shapes of the particles in the aqueous phase
are similar as those described by Yang et al. [15]
using citrate as the stabilizer. The shape is basically retained upon the
transfer to the organic phase. However, the platinum particles are less defined
as compared to gold probably due to its smaller size. The average sizes and
standard deviations of the gold particles before and after phase transfer are 6.3±1.7 nm and 7.2±1.3 nm,
respectively. For platinum particles, however, the respective average sizes and
standard deviations before and after phase transfer are 4.0±0.7 nm and
4.2±0.8 nm. Generally, there is a slight increase in particle size and
size distribution after phase transfer. For gold, the majority of the particles
are situated around 6-7 nm range before
the phase transfer. However, the amount of particles in the size range of 7-8 nm increased
after phase transfer. Similarly, for platinum, the increase is from around 3-4 nm to 5-6 nm size range. Especially
for gold, the increases
in particle sizes and size distributions are in accordance with the red shift
of the λmax as observed in the UV-Vis analysis. Generally, the
increase in particle size may be due to the slight aggregation of the particles
upon transfer from the aqueous to the organic phase [15]. As the particles move
into the organic phase, they may come in contact with each other and aggregate
due to the absence of citrate surrounding the particles before the particles are finally surrounded
by ENR and hence inhibiting further growth.TEM micrographs, average
size, standard deviation (SD), and size distribution of ENR-stabilized gold nanoparticles
at (a) before and (b) after phase transfer (scale bar = 50 nm) and platinum nanoparticles at (c) before and (d)
after phase transfer (scale bar = 20 nm).
(a)(b)(c)(d)


## 3.3. Mechanism of Interphase Particles Transfer

In the various phase transfer experiments, it was found that very little transfer of
particles was observed without stirring. Similarly, without 2-propanol there is
also no apparent particle transfer observed, even after prolonged stirring. In
the absence of ENR, the metal particles tend to aggregate and accumulate at the
aqueous/toluene interphase. For those successful phase transfer processes, no citrate was
detected in the organic phase. This inferred that the metal particles underwent
an exchange of stabilizers upon phase transfer, that is, from citrate stabilized
in aqueous phase to ENR stabilized in the toluene phase. Therefore, addition of 2-propanol is required
for phase transfer to occur with 2-propanol acting as the transfer agent while
ENR acts as replacement stabilizer and therefore exerts dispersion of the particles in the toluene
phase. It must be noted that in the several mechanisms previously reported, the
stabilizers are often transferred along with the metal particles from the
aqueous to organic phase, that is, without undergoing any exchange of
stabilizers [30–33].In this work, the probable mechanism
of phase transfer is proposed as follows. Upon formation of the metal particles,
they are initially stabilized by the citrate in the aqueous phase. The microemulsion
was formed when hydrosol and ENR toluene in the presence of 2-propanol were
mixed. The formation of the microemulsion increases the surface area between
the organic and aqueous phase. The addition of 2-propanol will cause the transfer
of the citrate-stabilized particles into the toluene layer due to the mutual
solubility of alcohol in both solvents. Upon particle transfer, the particles
undergo replacement of stabilizer from the citrate to ENR. This is facilitated
by the structure of ENR-50 that is naturally loose and contains many interchain voids [34]. The
citrate disentangled
from the surface of the metal particles and remained in the aqueous layer as it is insoluble in toluene as
previously evidenced
by the FTIR results. Therefore, the presence of the ENR in the organic phase
induces dispersion of the metal particles. The metal particles diffused into
the voids and formed stable ENR-metal organosols as discussed earlier.


## 4. Conclusion

Citrate-stabilized gold and platinum
particles had been prepared in aqueous phase via sodium borohydride reduction
of their respective salts and subsequently transferred to the organic phase
using 2-propanol as the transfer agent. An absence of ENR in the organic layer
caused the metal particles to accumulate and form aggregates at the interphase.
The presence of ENR in the organic phase induced the metal particles dispersity
with only a minimal growth of the metals particles after phase transfer.
Therefore, ENR can be used as a matrix and/or stabilizer for the transfer of
citrate-stabilized gold and platinum hydrosol to the organosol. The dried metal
colloids can be redispersed using various organic solvents that are compatible
to ENR such as chloroform, tetrahydrofuran (THF), and the likes.



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*Source: 130295-2008-03-09.xml*