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Journal of Nanoscience and Nanotechnology Vol. 6, 1405–1410, 2006

Synthesis of Colloidal Gold Nanoparticles of Different Morphologies in the Presence of Triblock Polymer Micelles Mandeep Singh Bakshi,1
∗ Aman Kaura,1 Poonam Bhandari,1 Gurinder Kaur,2 Kanjiro Torigoe,3 and Kunio Esumi3 1

Department of Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India 2 Department of Physics, Guru Nanak Dev University, Amritsar 143005, Punjab, India 3 Department of Applied Chemistry and Institute of Colloid and Interface Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan

Delivered by Ingenta to: University of Waterloo The Au nanoparticles have been synthesized in the presence of micellar solutions of fixed concenIP poly(ethylene : 129.97.58.73 tration (i.e. 1
4 × 10−3 mol dm−3 ) of each oxide)-poly(propylene oxide)-poly(ethylene Thu,as25P103, MayP84, 2006 14:27:10 oxide), triblock polymers (TBP), such P123, and F127. The nanoparticles have also

Keywords: Gold Nanoparticles, Triblock Polymers, Micelles, Raspberry Morphology, TEM. 1. INTRODUCTION The synthesis and characterization of gold nanoparticles have attracted great attention due to their potential applications in optoelectronics, electronics, catalysis, and other areas.1 Gold nanoparticles are commonly prepared from tetrachloroauric acid (HAuCl4 ) as a precursor using various chemical and photochemical reduction methods. For the preparation of gold nanoparticles with small sizes, narrow size distributions, and stability, the use of various stabilizers such as n-alkanethiols, n-alkylamines, dendrimers, and homopolymers or block copolymers have been extensively studied.2–6 From the solution, the stabilization and separation of the nanoparticles usually achieved by associating them with the micelles. Block copolymers have been known to form micelles. The composition of block copolymers can be tuned to affect the formation of micelles or aggregates of shapes other than spheres.7–10 Block copolymer micelles, which are thermodynamically stable under given set of conditions, have many potential applications. They have been used as templates to produce ∗

Author to whom correspondence should be addressed.

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metal nanoparticles, permanent nanostructures,11 including nanospheres,12–15 shell-cross-linked nanospheres,16–18 star polymers,19 hollow nanospheres,20 21 shaved nanospheres,22 nanofibers, and nanotubes.23–25 A variety of methods for the preparation and stabilization of metal colloids exist, the advantages and disadvantages of which were discussed in detail elsewhere.26 Semiconductor nanoparticles in block copolymer micelles were synthesized and characterized,27 while a preparation of nanoparticles in the segregated blocks of copolymers in solid as well as in thin films were also achieved.28 In most of the cases, the micelle core can be considered as a nanosized reaction vessel for metal colloid formation. The resulting hybrids between block copolymer micelles and metal colloids can be regarded as molecular entities, which can be used for the catalytic activity of metal colloids. The metal colloid size depends on the micelle size and is mainly controlled by the type of reducing agent applied.29 Generally, the stability of the colloids is quite high and is mainly related to the stability of block copolymer micelles. The stability was provided by the location of metal colloids in micelle cores. Since the micelles of block copolymers mainly consist of core and corona, the nanoparticle synthesis has also

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been synthesized in the presence of mixed micellar solutions of binary TBP mixtures such as P103+ P84 and P103 + P123. In the previous case, “raspberry type” Au nanoparticle–TBP aggregates have been observed in which nanoparticles of 2–3 nm have been uniformly distributed throughout the TBP micelle. On the other hand, in the latter case, apart from such aggregates, prominent ordered morphologies of nanoparticles such as rod, sphere, triangle, and hexagonal have also been observed with much larger dimensions. This has been attributed to the nucleation process occurring in the mixed micelles rather than in the micelles of single TBP components.

Colloidal Gold Nanoparticles of Different Morphologies in the Presence of Triblock Polymer Micelles

been achieved in the corona of some copolymer micelles.30 Recent works of Sakai and Alexandridis31–33 have demonstrated the shape and size controlled synthesis of colloidal gold in the presence of various TBPs. They have shown that how the structural modifications of block polymers influence the morphologies of colloidal gold. In view of these interesting applications of block polymers in synthesizing the metallic colloids, we herein have selected some conventional water soluble triblock polymers (TBP) of poly(oxyethylene oxide)-poly(propylene oxide)-poly(oxyethylene oxide) to study the gold (Au) nanoparticle synthesis in their micellar aggregates. Both pure and binary mixtures of different TBPs in their micellar aqueous solutions have been employed to study their influence on the morphology of Au nanoparticles.

2. EXPERIMENTAL DETAILS 2.1. Materials

Bakshi et al.

2.3. Methods UV-visible spectra of solutions before and after the reduction of metal ions were measured by UV spectrophotometer (Perkin Elmer Lambda 25) in the wavelength range of 200–900 nm. The formation of Au nanoparticles was monitored in the visible absorption range of ≈540 nm. The shape and size of gold nanoparticles were characterized by transmission electron microscopy (TEM). The samples were prepared by mounting a drop of a solution on a carbon coated Cu grid and allowed to dry in air. They were observed with a Hitachi H-9000 NAR operating at 200 kV. It is to be mentioned that TEM observations were carried out for each sample after the interval of at least 15 days through a complete reduction was achieved within 30 minutes of the reaction. In the 15 days of time span, there was no color change of the nanoparticle solution and they are considered to be quite stable even for much longer period.32

Delivered by Ingenta to: University of Waterloo Triblock polymers designated by their company names 3. RESULTS AND DISCUSSION IP : 129.97.58.73 such as Pluronic P103, P84, and P123 wereThu, received from 25 May 2006 3.1. 14:27:10 Gold Nanoparticle Synthesis in the Presence of

2.2. Preparation of Gold-Dendrimer Nanoparticles Freshly prepared HAuCl4 in aqueous solution (1 cm3 , 2 mmol dm−3 ) was added to 8.5 cm3 of TBP aqueous micellar solution. The mixture was stirred for 10 minutes at room temperature. Cold freshly prepared solution of NaBH4 (1 cm3 , 20 mmol dm−3 ) was added to the above mixture and the mixture was stirred for 30 minutes. The concentration of TBP micellar solution was kept constant in each case at 14 × 10−3 mol dm−3 . The color of reaction mixture changes from colorless to purple. A slight difference in the color of the solution has also been observed as the nature of TBP changes. In another set of experiments, the equimolar binary mixtures of TBPs were used and the concentration of individual TBP component was again fixed at 14 × 10−3 mol dm−3 , while the total concentration was 28 × 10−3 mol dm−3 . The rest of the conditions were same as mentioned in the previous case.

P103 (EO)17 (PO)60 (EO)17 P123 (EO)20 (PO)70 (EO)20 P84 (EO)19 (PO)43 (EO)19 F127 (EO)97 (PO)69 (EO)97 *Molecular weight ratio.

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Figure 1 shows the UV-vis spectra of Au nanoparticle in the presence of different TBPs (i.e. P123, P103, P84, F127). A clear surface plasmon (SP) band is observed around 540 nm indicating the presence of metal nanoparticles.34 The intensity of the SP bands in the presence of P123, P103, and P84 remains almost close to each other while that in the presence of F127, it is much smaller. It is already known that the magnitude of the intensity depends on the number density of Au nanoparticles either dispersed in the solution or associated with micelles.31–33 It means that the presence of F127 leads to the synthesis of relatively less number of nanoparticles in comparison to other TBPs. This is further supported by TEM studies. TEM micrographs of each of these samples (Figs. 2a–c) show small nanoparticles embedded in a big TBP micelles 2.0

1.5

1.0 P123

Table I. Molecular characteristics of triblock polymers. Triblock polymer

Single TBP Component

Absorbance

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BASF, Germany. Their complete molecular characteristic features have been listed in Table I. Tetrachloroauric acid (HAuCl4 ) and sodium borohydride (NaBH4 ) were obtained from Acros and Aldrich, respectively. Water was used after purification through double distillation.

0.5

(PPO/PEO)*

Molecular weight

2.32 2.31 1.13 0.47

4950 5750 4900 12600

P84 P103

F127 200

300

400

500

600

700

800

Wavelength (nm) Fig. 1. UV-visible spectra of HAuCl4 + TBP (F127, P84, P103, and P123) at [TBP] = 14 × 10−3 mol dm−3 .

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Bakshi et al. (a)

(b)

Colloidal Gold Nanoparticles of Different Morphologies in the Presence of Triblock Polymer Micelles

NaBH4 AuCl4 (c)

TBP micelles

Au + Au (III) +

Au nanoparticle PEO block Fig. 2. (a–c) TEM micrograph of gold nanoparticles in the presence of TBP; (a) P123, (b) P84, and (c) F127, at [TBP] = 14 × 10−3 mol dm−3 . White arrow show the fusion of TBP micelles leading to the nucleation process.

giving rise to a typical “raspberry morphology.”35 36 P123 and P84 samples show a cluster of micelles in a confined area while F127 micelles are singly distributed. Most of the spherical micelles of P123 and P84 are approximately 50 nm in size whereas that of F127 are slightly larger than 100 nm (Fig. 2c) and seem to merge with each J. Nanosci. Nanotechnol. 6, 1405–1410, 2006

PEO block

Fig. 3. Schematic representation of “raspberry type” association between Au nanoparticles and TBP micelle.

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other giving rise to some fused micelles (shown by white arrow). In all cases, the size of gold nanoparticles remains close to 2–3 nm. It is to be noted that the overall number density of nanoparticle unassociated or associated with micelles in the form of “raspberry type” morphologies is much higher in the presence of P123 and P84 (Figs. 2a and 2b, respectively) in comparison to F127 (Fig. 2c), and that could be the reason of low intensity of surface plasmon band in the latter case (Fig. 1). A “raspberry type” morphology of metal colloids has also been observed for palladium colloids in the presence of block polymers.35 36 It has also been reported that the reduction with NaBH4 results in smaller nanoparticles in comparison to any other reducing agent like hydrazine.36 Unlike the traditional kind of micelles of conventional surfactants, the TBP micelles possess large surface area occupied by several small pseudocrown ether cavities,37 38 which provide fast nucleation sites for metallic colloids. We believe that PEO lining these cavities show catalytic effect for the reduction of Delivered by Ingenta to: 0 32 − 39 AuCl to Au . Once Au0 is produced, several Au0 are University of Waterloo 4 available in nanosized cavities and are expected to undergo IP : 129.97.58.73 nucleation process to produce nanoparticles (see Fig. 3). Thu, 25 May 2006 14:27:10 As far as the different TBP structures are concerned (Table I), we do not see much difference in the “raspberry type” morphologies except in the presence of F127, where quite few number of aggregates are present. Our recent studies show that the solution properties of these TBPs significantly depend on the hydrophobic/hydrophilic ratio

Colloidal Gold Nanoparticles of Different Morphologies in the Presence of Triblock Polymer Micelles

(PPO/PEO).40 41 Thus a lowest PPO/PEO = 0.47 value of F127 might be the reason for the presence of low number of aggregates. Some studies31–33 have shown that the particle size mostly increases with the increase in PEO or PPO units at a fixed ratio in the absence of any reducing agent since PEO blocks mainly act as reducing agents and PPO blocks help them in this process. In the present study, when a strong reducing agent (NaBH4 ) is doing this job, a clear dependence of the particle size on the number of either PEO or PPO is not very much expected.

(a) hexagonal

rod

(b)

A contrasting difference is observed in the Au nanoparticle synthesis when carried out in binary TBP mixtures. Figure 4 shows SP bands in the presence of P103 + P84 and P103 + P123 mixtures, though latter is quite significant and shifted towards red while both samples triangle show the presence of large nanoparticles. TEM micro- by Ingenta to: Delivered graphs of Au nanoparticles in the presence of University P103 + P84 of Waterloo equimolar micellar mixture are shown in Figure 5. IPApart : 129.97.58.73 from the spherical TBP micelles decorated with Thu, 25small May 2006 14:27:10 Au nanoparticles of 2–3 nm size in traditional “raspberry morphology,” one can also see clear dark ordered morFig. 5. (a–b) TEM micrographs of gold nanoparticles of equimolar mixphologies (i.e., sphere, rod, triangle, hexagonal) of large tures of P103 + P84. (a) Showing large number of hexagonal aggregates nanoparticles. Figure 5a shows mainly large number of with few rod. (b) Showing triangular shaped Au nanoparticles along with hexagonal aggregates with few rod and sphere shaped, TBP micelles. while Figure 5b shows clear triangular shaped nanoparticles. Both figures also show a contrasting difference small individual Au nanoparticles to ordered morphology. between dark ordered structures and light shaded “raspFigure 6a shows clusters of TBP micelles loaded with berry morphologies.” Practically, there is no difference nanoparticles and at several places, they undergo nuclebetween the reaction conditions applied for single compoation process (shown by white arrows) in order to evaluate nent TBP (mentioned previously) and for their binary mixthe well defined morphologies such as triangle and rod. tures. Thus, the presence of the ordered structures indicates They demonstrate the actual fusion of the TBP micelles the facilitation of the nucleation process in the presence of facilitating the nucleation process. The proposed mechamixed components. nism is shown in Figure 7. During the fusion process the Figure 6 shows similar TEM micrographs of Au surface cavities of TBP micelles containing nanoparticles nanoparticles in the presence of P103 + P123. The microcome so close to each other that the additional AuCl− 4 ions graphs of this mixture show nucleation process from are expected to provide further growth on to the {100} 2.5 2.0

Absorbance

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3.2. Gold Nanoparticle Synthesis in the Presence of Binary Mixtures of TBPs

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1.5

1.0 P103+P123 0.5 P103+P84 200

300

400

500

600

700

800

Wavelength (nm) Fig. 4. UV-visible spectra of HAuCl4 + mixture of TBPs (P103 + P84 and P103 + P123) at [TBP + TBP] = 28 × 10−3 mol dm−3 .

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faces of nanoparticles. Initially, three micelles are thought to fuse (see “b”) and further growth may lead to “c” or “d.” In the case of “c,” more micelles are attached laterally and growth proceeds into a nano-rod. On the other hand, if more micelles fuse three-dimensionally, then the limited space seems to produce only nano-triangle. The former way of TBP micelles fusions looks to be more common since it is quite visible in Figure 6a. The ordered structures are considered to be quite thin and even the structures lying underneath can also be seen (see for example hexagonal and triangle shaped structures in Figs. 5a, 5b, 6b), thus support the fact that they are plate-like and not prisms. Several authors reported such plate-like geometries of Au nanoparticles under different experimental conditions,42–45 and could be the consequence of nanoparticle growth via AuCl− 4 reduction on the {100} faces even in the presence J. Nanosci. Nanotechnol. 6, 1405–1410, 2006

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Colloidal Gold Nanoparticles of Different Morphologies in the Presence of Triblock Polymer Micelles

the ionic micelles due to the presence of charged Stern layer at micelle-solution interface. Thus, increase in the concentration in the case of ionic micelles leads to a well defined structural change in the spherical ionic micelle due to the incorporation of more surfactant monomers in the micelle. However, on the contrary, in the case of TBP, the increase in concentration produces more polydisperse TBP micelles and thus increases their probability of intermicellar fusion. Both TBP and ionic micelles can be used as templates for nanoparticle synthesis, the only difference lies in the fact that the former incorporates hydrophilic while latter accommodates hydrophobic nanoparticles for further growth into organized morphologies. However, why this phenomenon is not working in (b) the presence of single component TBP systems (previously mentioned) needs further attention. Here in the mixed TBP mixtures, it seems that the higher TBP concentration31–33 may generate favorable inter-micellar fusions among Delivered by Ingenta to: unlike TBP micelles and could be the consequence of ordered morphologies. Literature studies47–50 University of Waterloo have also proved that the mixed micelles lead to more IP : 129.97.58.73 ordered morphologies than micelles of single surfactants. Thu, 25 May 2006 14:27:10 Several factors are considered to be responsible for this and are mostly surfactant specific. In the case of ionic surfactants, the selective adsorption of counterions during the crystal growth is more favorable aspect than the nature of Fig. 6. (a–b) TEM micrographs of gold nanoparticles of equimolar mixtemplates. But when the system mainly consists of nontures of P103 + P123. The micrographs of this mixture show nucleation ionic surfactants like mixed TBPs, obviously concentration process of small individual Au nanoparticles (indicated by white arrows) leading to the ordered morphology. effect becomes more prominent.47 (a)

b nucleation site

nano-triangle formation

nano-rod formation

The present study clearly shows the Au nanoparticle synthesis in the presence of TBP micelles leading to the “raspberry” type morphologies. However, clear ordered morphologies (i.e., rod, sphere, triangle, hexagonal) of Au nanoparticles along with TBP micelles have been observed in the presence of binary TBP mixtures. It has been concluded that the mixed micelles of TBP act as templates to trigger the nucleation process of singly distributed nanoparticles. The fusion of several raspberry type morphologies in form of mixed TBP micelles makes it possible.

References and Notes d

growth

4. CONCLUSIONS

growth c

Fig. 7. A schematic representation of proposed mechanism of nucleation process leading to the formation of ordered morphologies in the mixed TBP systems (for details see text).

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Received: 27 October 2005. Revised/Accepted: 3 February 2006.

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