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Journal of Nanoscience and Nanotechnology Vol. 7, 916–924, 2007

Lamellar Phase Supported Synthesis of Colloidal Gold Nanoparticles, Nanoclusters, and Nanowires Mandeep Singh Bakshi1
∗ , Poonam Sharma2 , Tarlok Singh Banipal2 , Gurinder Kaur3 , KanjiroTorigoe4 , Nils O. Petersen5 , and Fred Possmayer1 1

RESEARCH ARTICLE

Department of Ob/Gyn and Biochemistry, University of Western Ontario, London, ON, Canada N6A 5A5 2 Department of Applied Chemistry, Guru Nanak Dev University, Amritsar 143005, Punjab, India 3 Department of Chemistry, University of Western Ontario, London, ON, Canada 4 Department of Industrial Chemistry and Institute of Colloid and Interface Science, Tokyo University of Science, 2461 Yamazaki, Noda 278-8510, Japan 5 National Institute for Nanotechnology (National Research Council and University of Alberta), Edmonton, Alberta, Canada

Delivered by Ingenta to: University of Waterloo Gold nanoparticles (Au NP) have been synthesized in aqueous phase under ambient conditions in the presence of a series of various cationic double chain as well as dimeric (gemini) surfactants. IP : 129.97.58.73 The spacer chain and twin tail length has been systematically varied to see Mon,of05these Feb surfactants 2007 15:17:39 the effect of hydrophobicity on their capping ability. It has been observed that the increase in the length of spacer chain (from 12-2-12 to 12-6-12) and twin tails (from 10-2-10 to 14-2-14) significantly increases the lamellar phase formation and which in return acts as a wonderful template to accommodate the NP in the form of nanoclusters and nanowires. The lamellar phase practically facilitates the nucleation of Au and produces large NP (15 ± 2 nm). All reactions have also been carried out in the presence of
-cyclodextrin (CYC) which has strong ability to complex with surfactant tail. The presence of CYC induces a tendency to form nanowire and it is more prominent in the case of surfactants with longer spacer group. Keywords: Gold Nanoparticles, Gemini Cationic Surfactants, Lamellar Phase, Cyclodextrin, Transmission Electron Microscopy.

1. INTRODUCTION Among the cationic surfactants, cetyltrimethylammonium bromide (CTAB) is frequently used as capping agent in the synthesis of gold (Au) nanoparticles (NP)1–5 . Short range electrostatic interactions mainly lead to the adsorption of CTAB molecules on the surface of Au NP thus preventing their aggregation. The concentration of CTAB significantly influences the morphology of NP since the spherical as well as rod like micelles act as templates for such structures.6 The rod like micelles appear due to structure transitions in the spherical micelles beyond second critical micelle concentration (cmc). A variation in the concentration of CTAB ranging from pre- to post-micellar regions influences the synthesis of Au NP and leads to anisotropic geometries.6 The synthesis of NP with anisotropic morphologies is the basic requirement to explore their applications in nanotechnology. Quaternary ammonium gemini surfactants are basically dimeric homologues of monomeric cationic surfactants ∗

Author to whom correspondence should be addressed.

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such as CTAB7–9 . They are much more hydrophobic than their monomeric homologues. The greater hydrophobicity in the case of gemini surfactants comes from the twin tail effect and from the non-polar spacer group contribution.10–13 As far as the micellar templates are concerned, the micelles of gemini surfactants undergo several structure transitions (i.e., micelles → rod shaped or worm like micelles → vesicles → multilayer lamellar structures) in a low concentration range as compared to their homologues.14–16 It has been observed that the hydrophobicity of spacer group and the length of the twin tails govern the structures of micellar aggregates.14–16 The strong hydrophobicity generated by these two factors is mainly responsible for the formation of worm like micelles or bilayers which further lead to the multilayer structures known as lamellar phase or commonly called L
phase. A freshly prepared solution of a gemini surfactant with concentration much greater than its cmc produces L
phase which gets dehydrated in the presence of salt and leads to a much stable phase known as L/
. All micellar structures are expected to act as templates in the synthesis of organized morphologies of NP. There are few reports17 18 1533-4880/2007/7/916/009

doi:10.1166/jnn.2007.200

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Lamellar Phase Supported Synthesis of Colloidal Gold Nanoparticles, Nanoclusters, and Nanowires

2. EXPERIMENTAL DETAILS 2.1. Materials

Table I. Values of gemini surfactant concentrations used in preand post-micellar regions, literature cmc, cmc in the presence of CYC (cmcCYC ) (all concentration values × 10−4 mol dm−3 ), and [CYC]/[HAuCl4 ] ratio. Surfactant 12-0-8 12-0-10 12-0-12 12-2-12 12-3-12 12-6-12 10-2-10 14-2-14 ∗

cmc∗

cmcCYC

[CYC]/[HAuCl4 ]

10.88 5.4, 5.88 2.08 8.4, 8.944 45 10.1, 9.621 11.0, 12.021 6546 1.6, 0.6413 47

7.2 4.1 1.4 6.8 6.7 6.9 42 1.4

2.8 1.5 0.5 2.3 2.6 2.9 17 0.4

Literature values.

in each case (Table I). Similar reactions have also been carried out in the presence of constant amount of CYC (i.e., CYC = surfactant = 1/2 cmc, in each case of gemini surfactant).

Ingenta to: University of 2.3. Waterloo Methods IP : 129.97.58.73 Mon, 05 Feb 2007 15:17:39 UV-visible spectra of as prepared Au NP solutions

Chloroauric acid (HAuCl4 ), sodium borohydride (NaBH4 ), and -cyclodextrin (CYC) were obtained from Aldrich. A series of gemini surfactants such as dodecyldimethyloctylammonium bromide(12-0-8), decyldodecyldimethylammonium bromide(12-0-10), didodecyldimethylammonium bromide(12-0-12), dimethylene bis (decyldimethylammonium bromide)(10-2-10), dimethylene bis (dodecyldimethylammonium bromide)(12-2-12), dimethylene bis (tetradecyldimethylammonium bromide) (14-2-14), trimethylene-1,3-bis (dodecyldimethylammonium bromide)(12-3-12), and hexamethylene-1,6-bis (dodecyldimethylammonium bromide)(12-6-12) were synthesized as reported in literature.8 21 22 All surfactants were used after repeated crystallization from ethanol. The cmc values of all surfactants at 25  C have been taken from our previous studies as well as from literature, and have been listed in Table I. Pure water was used after purification through double distillation.

were measured by UV spectrophotometer (Perkin Elmer Lambda 25) in the wavelength range of 200–900 nm. The formation of NP was monitored in the visible absorption range of ≈540 nm. The inclusion complex formation between gemini surfactant and CYC has been studied with the help of pyrene fluorescence using Hitachi fluorescence spectrophotometer F2500. The ratio of the intensities of I1 to I3 vibronic bands of pyrene emission spectrum gives the polarity of the medium in which it is solubilized. The emission spectra were recorded employing an excitation wavelength of 334 nm, and the intensities I1 and I3 were measured at the wavelengths corresponding to the first and third vibronic bands located at ca. 373 and 384 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

2.2. Preparation of Au NP In a typical synthesis, freshly prepared HAuCl4 in aqueous solution (1 ml, 2 mmol dm−3 ) was added to 8.5 ml of aqueous surfactant solution. The mixture was stirred for 10 minutes at room temperature. Then cold freshly prepared solution of NaBH4 (1 ml, 20 mmol dm−3 ) was added to the above mixture and the mixture was stirred for 30 minutes. The color of reaction mixture changes from colorless to wine red (see few sample tubes in Fig. 1) indicating the formation of Au NP in each case. The reactions were carried out at two surfactants concentrations i.e., below the cmc = 1/2 cmc and above the cmc = 5×cmc J. Nanosci. Nanotechnol. 7, 916–924, 2007

12-2-12 Post

12-2-12 Pre

14-2-14 Post

14-2-14 Pre

Fig. 1. Sample tubes showing the ruby red color of gold nanoparticle aqueous solutions in the presence of 12-2-12 and 14-2-14 in the preas well as in the post-micellar regions. Note the greater intensity of the color in the pre-micellar region of both surfactants.

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related to the metal nanoparticle synthesis in the presence of gemini surfactants. These studies have suggested that gemini surfactants act as excellent capping agent and which lead to the synthesis of a narrow size distribution of metal nanoparticles. The present study is the first report suggesting the Au NP synthesis supported by the gemini lamellar phase which leads to the formation of large aggregates of Au NP. The synthesis has been carried out both in the pre- as well as in the post-micellar regions of a series of gemini surfactants by varying the length of twin tails and spacer group. The purpose of this study is also to broaden the applicability of various gemini surfactants as capping agents in order to produce organized morphologies of Au NP. Apart from this, the Au NP synthesis has also been carried out in the presence of -cyclodextrin (CYC), which has strong ability to interact with the surfactant tail of cationic surfactants through the hydrophobic interactions.19 20 The purpose of choosing CYC is to simply study its additive effect on the morphology of Au NP. by Delivered

Lamellar Phase Supported Synthesis of Colloidal Gold Nanoparticles, Nanoclusters, and Nanowires

out for each sample after the interval of at least 15 days though a complete reduction was achieved within 30 minutes of the reaction. In 15 days of time span, there was no color change of the NP solution and they were quite stable even for several months.

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Fig. 2. UV-visible spectra of aqueous gold nanoparticle solutions in the presence of 12-2-12 and 14-2-14 at surfactant concentrations 1/2 cmc and 5 × cmc.

12-0-12

Delivered by Ingenta to: 12-2-12 0.8 12-3-12 University of Waterloo 12-6-12 IP : 129.97.58.73 0.6 Spacer Effect Mon, 05 Feb 2007 15:17:39

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RESEARCH ARTICLE

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[14-2-14] = 1/2 cmc [12-2-12] = 1/2 cmc

3. RESULTS The synthesis of Au NP in aqueous gemini surfactant solutions was confirmed by measuring the UV visible spectrum at 540 nm. Each surfactant solution containing NP gave clear absorbance at 540 nm due to the characteristic surface plasmon resonance. Figure 2 shows such spectra for NP solutions in the presence of 12-2-12 and 14-2-14 at both in the pre- as well as in the post-micellar concentration ranges.23 24 The absorbance bands in both cases are

Bakshi et al.

0.6 0.4 0.2

450

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650

700

0 400

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Fig. 3. UV-visible spectra of aqueous gold nanoparticle solutions in the absence (a) and presence of -cyclodextrin (b) for 12-0-12, 12-2-12, 12-3-12, and 12-6-12, showing the effect of spacer. Similarly, in the absence (c) and presence of -cyclodextrin (d) for 10-2-10, 12-2-12, and 14-2-14, showing the effect of twin tail; and in the absence (e) and presence of -cyclodextrin (f) for 12-0-8, 12-0-10, and 12-0-12, showing the effect of single tail.

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(a) higher in the pre-micellar region than in the post-micellar region. For present gemini surfactants, this trend was observed in most of the cases. The UV plots of Au NP in the presence of gemini = 1/2 cmc as well as gemini = 5 × cmc indicate (not shown) that absorbance maximum is quite weak for 12-0-8 and 10-2-10 among all gemini surfactants, however both these surfactants along with other gemini surfactants show strong absorbance bands in the post-micellar region. It means that the premicellar regions of 12-0-8 and 10-2-10 do not possess sufficient amount of surfactant required for the nucleation of Au NP. The (b) absorbance maximum is also having higher intensity value for the surfactant with lower cmc, and this trend remains even in the presence of CYC. Apart from this, there is no significant dependence of the absorbance wavelength on the cmc of surfactants which indicates that mainly spherical Au NP are present. These results suggest that the present series of gemini surfactants can be divided into two categories i.e., with lower cmc values (includes majority Delivered by Ingenta to: of the present surfactants) which produce greater number University of Waterloo of Au NP or larger NP; and with relatively higher cmc IP : 129.97.58.73 value (i.e., 10-2-10) which produces less number of Au (c) Mon, 05 Feb 2007 15:17:39 NP or smaller NP. While it is not clear, why the presence of 12-0-8 causes low UV absorbance despite low cmc, a large unequal difference among the twin tails length might reduce the capping ability. Figure 3 compares the effect of variation of spacer and hydrophobic tails in the absence and presence of CYC. Figure 3(a) shows that increase in the spacer length from 12-2-12 to 12-6-12 reduces the UV absorbance systematically and significantly though their cmc values are quite close to each other (Table I). In the presence of CYC (Fig. 3(b)), although the same trend still exists, there is about 10 nm red shift in the absorbance maxi(d) mum for 12-3-12 and 12-6-12. Such a shift is generally 25 26 On the attributed to nanorod or nanowire formation. contrary, increase in the length of twin tails i.e., from 102-10 to 14-2-14 (Fig. 3(c)) shows a drastic increase in the absorbance from 10-2-10 to 12-2-12, however further increase to 14-2-14 does not lead to any appreciable rise. Addition of CYC (Fig. 3(d)) essentially keeps everything same to what has been observed in Figure 3(c). Increase in the length of one of the twin tails (i.e., single tail) has dramatic influence on the Au NP synthesis (Fig. 3(e)). The absorbance increases from 12-0-8 to 12-0-10, but decreases with further increase to 12-0-12. The addition of CYC (Fig. 3(f)) still keeps everything the same except it Fig. 4. TEM micrographs of gold nanoparticles in the presence of 12reduces the absorbance for 12-0-10 significantly. Collec2-12 in pre-micellar (a) and post-micellar range (b); and in the presence tively, the results of Figure 3 suggest that the increase in of 14-2-14 in pre-micellar (c) and post-micellar range (d). All scale bars are equal to 100 nm. the spacer and twin tails lengths seems mainly reduces the UV absorbance. Decrease in the UV absorbance can be Some of the TEM micrographs of 12-2-12 and 14-2-14 attributed to the dampening of plasmon resonance due to below and above the cmc have been shown in Figure 4. some kind of aggregation (which will be explained from Fig. 4(a) and 4(b) show the TEM images of 12-2-12 the TEM images in the next section). below and above the cmc, respectively. One can see the The results have been further evaluated by determincontrasting difference between the two images. In the ing the shape and size of the NP with the help of TEM.

Lamellar Phase Supported Synthesis of Colloidal Gold Nanoparticles, Nanoclusters, and Nanowires (a)

RESEARCH ARTICLE

(a1)

(b)

(c)

(a2)

Bakshi et al.

below cmc (Fig. 4(c)), NP (8 ± 1 nm) still can be seen closely associated without any clear NW like order. But clearly show the presence of nanocluster (NC) (Fig. 4(d)) in the post-micellar region, where large NP (17 ± 2 nm) are arranged in spherical fashion. Thus, the presence of larger NP (Fig. 4(b)) and NC formation (Fig. 4(d)) is considered to be mainly responsible for a decrease in UV absorbance of Au NP in the post-micellar region in comparison to that in pre-micellar region (Fig. 2) for both surfactants. However, increase in the length of spacer group as in the case of 12-3-12 and 12-6-12, leads to the formation of predominantly lamellar phase in the post-micellar region (Fig. 5) which is clearly visible as dark area in contrast to the light background. It acts as template for the nucleation of Au NP and thus, brings various NP in confined areas. This results in a significant decrease in UV absorbance, and that is why it is lowest for 12-3-12 and 12-6-12 in comparison to all other surfactants in the post-micellar Delivered by Ingenta to: region. The lamellar phase of 12-3-12 occupies large University of Waterloogroups of NP (10 ± 2 nm) in confined areas (Fig. 5(a1) and (a2)), while that of 12-6-12 completely IP : 129.97.58.73 occupied with large NP (15 ± 4 nm), and even nanotubes Mon, 05 Feb 2007 15:17:39 can also be seen bearing NP (Fig. 5(c)). The increase in the spacer length induces significant hydrophobicity to the micellar phase due to its folding and penetration into the palisade layer of the micelle. This facilitates the structure transitions from spherical micellar to thread like micelles which ultimately arrange themselves in a bilayer arrangement (L
phase).14–16 One can see a significant difference between the Au NP arrangement in the presence of 14-2-14 (Fig. 4(d)) and 12-6-12 (Figs. 5(b), 5(c)). The former drives the spherical NC formation whereas the latter accommodates the NP on the lamellar phase. It suggests that the increase in the length of twin tails facilitates the vesicle formation which might bring NP together in a spherical arrangement (NC), while increase in the spacer 1.75 12-0-12 12-2-12 12-3-12 12-6-12

1.7 1.65 cmc

I1/I3

1.6 1.55 1.5

Fig. 5. TEM micrographs of gold nanoparticles in the presence of 123-12 in post-micellar range (a). Insets (a1) and (a2) are the magnified images; and in the presence of 12-6-12 in post-micellar range (b), while (c) shows the nanoparticles on nanotube. Dark area bearing nanoparticles is the lamellar phase (see the contrast with the light background). All scale bars are equal to 100 nm.

pre-micellar range, the Au NP (5 ± 1 nm) are arranged in small nanowires (NW), whereas in the post-micellar region the Au NP nucleate to form large NP (ca. 10 ± 2 nm) apart from showing some NW. In the case of 14-2-14 920

cmc cmc

1.45 1.4 1.35 0

2

4

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14

[Surfactant] /10–4 mol dm–3 Fig. 6. Plot of pyrene micropolarity (I1 /I3  versus the gemini surfactant concentration for 12-0-12, 12-2-12, 12-3-12, and 12-6-12 at constant amount of -cyclodextrin ([-cyclodextrin] = 1/2 cmc in each case). Dark arrows show the 1 : 1 complex between surfactant and -cyclodextrin, while vertical dotted lines show the cmc.

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length arrange NP on L
phase. Apart from this, it is also evident that increase in the twin tails and spacer length produces large NP. In order to further evaluate this behavior, we carried out the Au NP synthesis in the presence of CYC. CYC is an oligosaccharide, which has a strong ability to entrap surfactant hydrophobic tail in its hydrophobic cavity.27–30 All the reactions were carried out in the presence of gemini = 1/2 cmc = CYC gemini + CYC = 2 × 1/2

cmc) i.e., in the pre-micellar region of each surfactant so to achieve at least possible 1 : 1 complexation of inclusion complex between the surfactant and CYC cavity. Figure 6 demonstrates a change in the I1 /I3 intensity ratio with the increase in [gemini] at constant [CYC]. Each plot shows a gradual decrease in I1 /I3 value with the increase in [gemini]. But at a certain [gemini], it shows a break in the curve (shown by dark arrows) and thereafter, it falls with a steep slope before tending to a constant value at cmc.

(d) (a) RESEARCH ARTICLE

Delivered by Ingenta to: University of Waterloo IP : 129.97.58.73 Mon, 05 Feb 2007 15:17:39 500 nm

(e)

(b)

500 nm

(c)

(f)

500 nm Fig. 7. TEM micrographs of gold nanoparticles in the presence of -cyclodextrin for 12-0-10 (a), 12-4-12 (b), 12-3-12 (c), and 12-6-12 (d). Figure (e) shows the close up of “pearl necklace” type nanowire, while inset demonstrates the presence of small nanoparticles on 12-6-12 + -cyclodextrin beads. Fig (f) shows the wire like arrangement of 12-6-12 + -cyclodextrin crystalline complexes while block arrows indicate the presence of nanoparticles at some specific locations. All scale bars are equal to 100 nm or otherwise specified.

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of NW like arrangement of such crystalline complexes The dark arrows in each case represent the 1 : 1 complex. with scattered Au NP (shown by arrows). These complexes The cmc values thus computed in each case have been have also been reported by some recent studies31–33 while compared with those in the absence of CYC in Table I. -CYC derivative has been found to show maximum tenOne would see that the cmc values in the presence of dency to form rod shaped aggregates with nonionic gemini CYC are much smaller than in its absence. It suggests that surfactants.19 20 gemini-CYC complex is taking part in the micelle forma19 20 tion. Recently, Alami et al. have reported the complexation of heterogemini nonionic surfactant with
, , and 4. DISCUSSION -CYC. They have found that it is possible to include only one tail in the -CYC cavity and the complex thus forms The greater hydrophobicity of the gemini surfactants than still capable of undergoing micellization. This is contrary their corresponding monomeric homologues produces preto what has been observed for single tail conventional surmicellar aggregates in the pre-micellar range, while vesicfactants where micellization is in fact delayed in the presular and lamellar aggregates in the post-micellar region at relatively much lower concentrations.9 These structures ence of CYC in comparison to that in its absence.27–29 act as templates and accommodate large number of NP Therefore, gemini-CYC complex seems to undergo some in confined areas which may otherwise disperse in the sort of pre-micellar aggregates which in return act as tembulk phase, and thus result in overall decrease in the UV plates for the nucleation of NP. Figure 7(a) shows the absorbance. However, the capping mechanism is expected TEM micrograph of Au NP synthesized in the presence to be the to: same in CTAB5 and gemini surfactants where the of CYC for 12-0-10. Here the NP are mainly arranged Delivered by Ingenta the surface of Au NP in somewhat NW like arrangement and probably due to of surfactant University Waterloomonomer interacts with 17 18 34 through electrostatic interactions. This arises due to this arrangement there is a significant reduction inIP the: UV 129.97.58.73 − the electrostatic adsorption of Br counterions at the surabsorbance (compare Fig. 3(e) and 3(f)). This behavior is Mon, 05 Feb 2007 15:17:39 face of NP which in return interacts with electropositive not very clear in the case of 12-0-8/12-0-12 + CYC (not cationic head groups (Fig. 8). But since the gemini surfacshown) due to the unknown reasons. Now if we move on tants are much more hydrophobic (due to their twin tails) to a gemini surfactant with spacer group and longer twin than their homologous monomeric surfactants, therefore, tails as in the case of 14-2-14 + CYC (Fig. 7(b)), a systhe stronger hydrophobicity acts as driving force for the tematic arrangement of NP (10 ± 3 nm) in linear fashion aggregation of NP in small aggregates (such as NW and is observed which seems to be more ordered in comparNC) as has been observed especially in case of 12-2-12, ison to that in the absence of CYC (Fig. 4(c)). On the 14-2-14 (Fig. 4), and 12-3-12, 12-6-12 (Fig. 5). Figure 9 other hand, increase in the spacer length as in the case explains the schematic representation of small aggregate of 12-3-12 + CYC (Fig. 7(c)), still retains the NC forformation due to strong hydrophobic interactions and that mation with large size distribution of NP (19 ± 4 nm). is also expected even in the pre-micellar range. The stabiThis size of NP is almost double than that observed in lization of such aggregates is generally explained in terms the absence of CYC (not shown) and even larger than that obtained in the post-micellar range of 12-3-12 without CYC i.e., 10 ± 2 nm (Fig. 5(a)). Further increase in (A) (B) the spacer length as in the case of 12-6-12 + CYC leads to a fine NW network of fused NC just like a typical “pearl necklace” model (Fig. 7(d)). A close inspection of these NW (Fig. 7(e)) indicates that the average width of each NW is close to 100 nm and that is presumed to be the diameter of each NC which consists of fine NP with average size of 3 ± 2 nm (see inset of Fig. 7(e)). Thus a comparison with Figure 7(c) suggests that an increase in (C) the spacer length from 12-3-12 to 12-6-12 leads to a self arrangement of NC in NW in the presence of CYC though the NC formed in the presence of 12-3-12 (Fig. 7(c)) are of much larger size than 100 nm. The NC and NW formation for 12-3-12 and 12-6-12 is also evident from their UV spectra (Fig. 3(a) and 3(b)), where the presence of CYC shows a clear red shift of 10 nm in UV absorbance. It is also evident that 12-6-12 + CYC complexes mainly crystallize in the form of aggregates which arrange themFig. 8. Schematic representation of gold nanoparticles (in red spheres) selves in NW. These NW act as templates for the growth capped with CTAB monomers (a), gemini surfactant with smaller spacer of Au NP on them. Figure 7(f) clearly shows the presence group (b), and with longer spacer group monomers (c). 922

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normally small NP (≈5 nm) obtain at corresponding CTAB concentration.6 This concludes that micellar arrangement of cationic double tail surfactants is indeed a better template than that of CTAB even at comparable concentration range. Hydrophobic interactions However, the additive effect of CYC requires more explanation. Recent studies41 42 have shown that when CYC is used in the absence of any surfactant, it actually leads to a significant decrease in the NP size and no NPCYC inclusion complex formation has been observed. The increase in [CYC] even leads to a further decrease in the size.41 On the other hand, [CYC]/[HAuCl4 ] ratio is also expected to influence the NP size significantly.43 A higher ratio in fact produces NP of small dimensions whereas low ratio leads to a NW type arrangement.43 Looking at Table I, one would find that in the presence of all gemini surfactants except 10-2-10, this ratio is quite low and close to unity. Also not all the gemini + CYC systems lead to the NW formation, though most of them have the Delivered by Ingenta to: tendency to do so. The most prominent and clear NW University of formation Waterloois demonstrated by 12-6-12 + CYC system and IP : 129.97.58.73 obviously this is not only due to the low CYC/HAuCl4 Mon, 05 Feb 2007 15:17:39 ratio. Hence, the formation of large NC and NW by gemini surfactants with longer twin tails and spacer groups, Fig. 9. Schematic representation of aggregates of gold nanoparticles respectively, in the presence of CYC can only be attributed (in red spheres) capped with gemini surfactant monomers showing the to strong hydrophobic interactions among gemini surfachydrophobic interactions which drive the aggregation. tant monomers as shown in Figure 9. The existence of 1 : 1 complex (Fig. 6) allows only one tail out of two of of charge (due to DLVO theory35 ) and steric stabilization. gemini surfactant to complex with CYC cavity19 20 and The charge stabilization is achieved when Br− counterions the other still remains active for hydrophobic interactions. adsorb at the NP surface and induce coulombic repulsions The strong hydrophobic interactions thus generated may due to the presence of electric double layer around the reduce the solubility of gemini-CYC complex in the aqueNP.36–38 Steric stabilization on the other hand plays an ous phase and consequently leads to the formation of crysimportant role when the NP is covered by long tail surtalline complexes31–33 (Fig. 7(f)). The latter should be more factant molecules which provide a protective layer around favorable in the case of 12-6-12 + CYC rather than 12-3the NP.36 39 Such protective layers from the adjoining 12+CYC, because longer spacer would contribute towards NP when intermingle with each other, they induce congreater freedom for the uncomplexed hydrophobic tail to figurational constrains, and keep the NP separate from undergo hydrophobic interactions with other similar tails. each other. It prevents coagulation and enhances further Since larger NP are obtained in the presence of CYC than nucleation. Therefore, in conclusion both electrostatic and its absence, therefore, such gemini + CYC complexes even steric stabilizations are required to keep the NP stable further facilitates the nucleation process. in solution.38 40 Both properties are easily contributed by the ionic surfactants to stabilize NP in solution. In the 5. CONCLUSIONS case of present gemini surfactants, apart from charge and steric stabilizations, the third most significant factor i.e., The NC and NW formation of Au NP was achieved by the strong hydrophobic interactions, is mainly responsible using various cationic gemini surfactants and their combito bring several gemini capped NP in well defined mornation with CYC. Cationic double tail surfactants act as phologies such as NC and NW. Here, the NP have already wonderful capping agent just like that of CTAB, however, been prevented from coagulation by screening their short due to their stronger hydrophobic interactions, they bring range electrostatic interactions due to the presence of interNP in NC or NW formation and hence act as better temvening hydrophobic domains (Fig. 9). Therefore, these NC plates than CTAB. With the increase in the length of twin and NW located in confined areas further act as nucleation tails (from 10-2-10 to 14-2-14), mainly cluster formation sites to generate large NP. This has been seen in both pre is achieved while increase in the spacer group (from 12-2as well as post-micellar regions of present gemini surfac12 to 12-6-12) accommodates NP on lamellar phase. Both tants (Figs. 4 and 5), where NP of much larger dimensions kinds of arrangements result in the facilitation of nucle(i.e., more than 10 nm) are observed in comparison to ation process leading to the formation of large NP.

Lamellar Phase Supported Synthesis of Colloidal Gold Nanoparticles, Nanoclusters, and Nanowires

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The presence of CYC along with the gemini surfactants further facilitates the nucleation process and produces NP with larger dimensions. Though, a tendency of NW formation exists in most of the cases, the clear NW formation is demonstrated by 12-6-12 + CYC system. It has been observed that the wire like arrangement of crystalline complexes of 12-6-12+CYC acts as template for the formation of NW. Thus, present study is step forward in achieving goals to accommodate NP in confined areas in an ordered manner so that they can be used for various applications in nanotechnology such as the fabrication of supramolecular machines and functional materials.

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Received: 18 July 2006. Revised/Accepted: 5 September 2006.

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