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Dispersion of Thiol Stabilized Gold Nanoparticles in Lyotropic Liquid Crystalline Systems P. Suresh Kumar, Santanu Kumar Pal, Sandeep Kumar, and V. Lakshminarayanan* Raman Research Institute, C.V. Raman AVenue, SadashiVanagar, Bangalore-560080, India ReceiVed NoVember 13, 2006. In Final Form: December 21, 2006 A new method of forming stable dispersions of alkanethiol and aromatic thiol stabilized gold nanoparticles in two different lyotropic liquid crystalline mediums, namely, a columnar hexagonal phase made up of a Triton X-100/water system and an inverse columnar hexagonal phase made up of pure AOT, are presented. The dispersions have been characterized using small-angle X-ray scattering (SAXS) and polarizing optical microscopy. Our studies show that the gold nanoparticles are distributed outside the columns formed by both the surfactants. Such dispersions can find applications in the study of nanoparticles as well as in the development of devices based on some unique properties of metal nanoparticles.
Introduction Nanoparticles have several interesting characteristics including size-tunable chemical reactivity and physical properties, which find diverse applications in devices based on ordered arrays in surface films, molecular electronic devices, analytical measurements, and as new types of polyfunctional molecules. The last mentioned application has found widespread interest after the advent of thiol passivated metal nanoclusters.1-6 The synthetic advance made by Brust et al.,7a in their toluene phase separation of alkanethiolate-monolayer-coated gold nanoparticles (GNPs) of 1-3 nm, has opened up a whole new field in material science. This method, extensively followed by several research groups, has allowed the facile synthesis of stable GNPs with a variety of chemical functionalities.8 These alkanethiolate-monolayerprotected gold nanoclusters provide versatile precursors for the fabrication of nanoscale systems.9 In addition to alkanethiols, there are several other functional groups that have been used for stabilizing the nanoparticles. For example, Turkevich et al. have used citrate as a reducing agent for the synthesis of gold nanoparticles, which can also act as a stabilizer.10-12 Herrera et al. prepared the nanoparticles using surfactants in a reverse micelle,13 while Jana et al. studied the * Corresponding author. E-mail:
[email protected]. Tel.: 91-8023610122, ext. 366. Fax: 91-80-23610492. (1) (a) Fendler, J. H.; Meldrum, F. C. AdV. Mater. 1995, 7, 607. (b) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (c) Pileni, M. P.; Tanori, J.; Filankembo, A.; Dedieu, J. C.; Gulik-Krzywicki, T. Langmuir 1998, 14, 7359. (2) (a) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (b) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (3) Collier, C. P.; Saykally, R. J.; Shiang, J. J; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (4) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (b) Storhoff, J. J.; Mirkin, C. A. Chem. ReV. 1999, 99, 1849. (5) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27, and references therein. (6) Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface Sci. 1997, 2, 42. (7) (a) Brust, M.; Walker, M.; Bethel, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 801. (b) Song, Y.; Huang, T.; Murray, R. W. J. Am. Chem. Soc. 2003, 125, 11694. (8) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (9) Liu, J.; Xu, R.; Kaifer, A. E. Langmuir 1998, 14, 7337. (10) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (11) Lee, S.; Perez-Luna, V. H. Anal. Chem. 2005, 77, 7204. (12) Dahanayaka, D. H.; Wang, J. X.; Hossain, S.; Bumm, L. A. J. Am. Chem. Soc. 2006, 128, 6052.
gold nanoparticles covered by CTAB.14 Nanoparticles using Triton X-100 as a stabilizer have also been reported.15 Recently, Longo et al. have studied the synthesis and stabilization of nanoparticles by AOT and its incorporation in a reverse micelle.16,17 The GNPs can also be prepared in the polymer matrix by trapping the nanoparticles in the polymer18 or polyelectrolyte19 systems. Nanoparticles stabilized by hydrocarbons,20 streptavidin,21 PVP,22 and silica23 have also been reported in the literature. There are also several studies on the deposition of nanoparticles on carbon nanotubes and other nanomaterials that act as templates for the nanoparticle deposition.24-28 As the physical properties of nanosized materials are quite different from the bulk material, a number of potential applications have been envisaged.8,29 On the other hand, liquid crystals exhibit unique properties such as long-range orientational order, co-operative effects, and anisotropic optical and electronic properties due to their selforganizing nature in a certain temperature range with fluidity.30 This self-organizing property of anisotropic liquid crystals, both thermotropic as well as lyotropic, could be easily exploited in (13) Herrera, A. P.; Resto, O.; Briano, J. G.; Rinaldi, C. Nanotechnology 2005, 16, S618. (14) Jana, N. R.; Gearheart, L. A.; O’Bare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909. (15) Ghosh, S. K.; Sharma, N.; Mandal, M.; Kundu, S.; Esumi, K.; Pal, T. Curr. Sci. 2003, 84 (6), 791. (16) Longo, A.; Calandra, P.; Casaletto, M. P.; Giordano, C.; Venezia, A. M.; Turco Liveri, V. Mater. Chem. Phys. 2006, 96, 66. (17) Calandra, P.; Giordano, C.; Longo, A.; Turco Liveri, V. Mater. Chem. Phys. 2006, 98, 494. (18) Gascon, I.; Marty, J. D.; Gharsa, T.; Mingotaud, C. Chem. Mater. 2005, 17, 5228. (19) Angelatos, A. S.; Radt, B.; Caruso, F. J. Phys. Chem. B 2005, 109, 3071. (20) Mirkhalaf, F.; Paprotny, J; Schiffrin, D. J. J. Am. Chem. Soc. 2006, 128, 7400. (21) Smorodin, T.; Beierlein, U.; Kotthaus, J. P. Nanotechnology 2005, 16, 1123. (22) Correa-Duarte, M. A.; Liz-Marzan, L. M. J. Mater. Chem. 2006, 16, 22. (23) Tsunoyama, H.; Sakurai, H.; Ichikuni, N.; Negishi, Y.; Tsukuda, T. Langmuir 2004, 20, 11293. (24) Jiang, K.; Eitan, A.; Schadler, L. S.; Ajayan, P. M.; Siegel, R. W.; Grobert, N.; Mayne, M.; Reyes-Reyes, M.; Terrones, H.; Terrones, M. Nano Lett. 2003, 3, 275. (25) Ma, X. C.; Lun, N.; Wen, S. L. Chinese Chem. Lett. 2005, 16, 265. (26) Qu, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 10806. (27) Quinn, B. M.; Decker, C.; Lemay, S. G. J. Am. Chem. Soc. 2005, 127, 6146. (28) Zanella, R.; Basiuk, E. V.; Santiago, P.; Basiuk, V. A.; Mireles, E.; PuenteLee, I.; Saniger, J. M. J. Phys. Chem. B 2005, 109, 16290. (29) Schwerdtfeger, P. Angew. Chem., Int. Ed. 2003, 42, 1892. (30) Imrie, C. T., Luckhurst, G. R. In Handbook of Liquid Crystals; Demus, D., Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 2B, p 801.
10.1021/la063318z CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007
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liquid crystalline systems. The hexagonal columnar phase was made either by a Triton X-100/water system or by a pure AOT system. While a Triton X-100/water system has a normal hexagonal columnar phase (H1 phase), AOT exhibits a reverse hexagonal columnar liquid crystalline phase (H2 phase). The dispersions are characterized using polarizing optical microscopy (POM) and small-angle X-ray scattering (SAXS) studies. Experimental Procedures
Figure 1. (a) Structure of Triton X-100 molecule, n ) 9.4. (b) Structure of AOT molecule.
the study of metal nanoparticles.31 In recent times, lyotropic liquid crystals formed by surfactant Triton X-100 (Figure 1a) with water have been intensively and systematically studied.32 The formation of a highly ordered columnar hexagonal phase (H1 phase) and the non-ionic nature of this system makes it interesting for several physicochemical studies. Recently, we have reported the preparation of a high surface area nickel deposit using this medium as a template for the application of supercapacitors.33a We have also shown that this medium can be used for the formation of self-assembled monolayers (SAMs) of alkanethiols on gold, which provides a highly hydrophobic environment to solubilize alkanethiols and to facilitate their delivery to the gold surface.33b Weiss et al. have recently reported a method for the preparation and characterization of carbon nanotube dispersion in a Triton X-100/water lyotropic liquid crystalline medium.34 Sodium (2-ethylhexyl) sulfosuccinate, which also has the trade name Aerosol-OT or AOT (Figure 1b), is of considerable interest in liquid crystal research since it can form a reverse hexagonal phase (H2 phase). The phase diagram of AOT has been wellstudied and has been shown to form a lamellar phase and a reverse hexagonal phase as a function of water concentration.35 AOT microemulsion is shown to be a potential candidate for oral drug delivery systems.36 AOT has also been used as a medium for the preparation of gold nanoparticles as well as a stabilizer.16,17 Hybridization of these two systems, namely, GNPs and the lyotropic liquid crystalline phase, may lead to novel materials with interesting properties, which can find many device applications. The gold nanoparticles in an aqueous micellar medium lend themselves to potential applications in both drug delivery and immunoassay studies. But until now, there have been no reports in the literature on incorporating thiol stabilized gold nanoparticles in a lyotropic liquid crystalline matrix. In the present work, we demonstrate for the first time the integration of welldispersed hexanethiol and cyanobiphenylthiol (10CB-thiol)protected GNPs in highly ordered lyotropic hexagonal columnar (31) Lee, M. H.; Oh, S. G.; Suh, K. D.; Kim, D. G.; Sohn, D. Colloids Surf., A 2002, 210, 49. (32) (a) Beyer, K. J. Colloid Interface Sci. 1982, 86, 73. (b) Ahir, S. V.; Petrov, P. G.; Terentjev, E. M. Langmuir 2002, 18, 9140. (c) Galatanu, A. N.; Chronakis, I. S.; Anghel, D. F. A.; Khan, A. Langmuir 2000, 16, 4922. (d) Maza, A. D. L.; Parra, J. L. Biochem. J. 1994, 303, 907. (33) (a) Ganesh, V.; Lakshminarayanan, V. Electrochim. Acta 2004, 49, 3561. (b) Ganesh, V.; Lakshminarayanan, V. Langmuir 2006, 22, 1561. (34) Weiss, V.; Thiruvengadathan, R.; Regev, O. Langmuir 2006, 22, 854. (35) (a) Rogers, J.; Winsor, P. A. Nature 1967, 216, 477. (b) Winsor, P. A. Chem. ReV. 1968, 68, 1. (c) Fontell, K. J. Colloid Interface Sci. 1973, 44, 318. (d) Coppola, L.; Muzzalupo, R.; Ranieri, G. A.; Terenzi, M. Langmuir 1995, 11, 1116. (e) Petrov, P. G.; Ahir, S. V.; Terentjev, E. M. Langmuir 2002, 18, 9133. (36) El-Laithy, H. M. Pharm. Sci. Technol. 2003, 4 (1), 11.
Chemicals. Triton X-100 (Spectrochem), HAuClO4·3H2O (Aldrich), NaBH4 (Aldrich), hexanethiol (Aldrich), tetraoctylammonium bromide (Aldrich), AOT (sodium (2-ethylhexyl) sulfosuccinate) (SD fine Chem. Ltd), and toluene (Aldrich) were used as received. 10CBThiol (4′-[10-sulfanyldecyl)oxy][1,1′-biphenyl]-4-carbonitrile) was synthesized as reported earlier.37 Millipore water of resistivity 18 MΩ cm was used for the preparation of all the samples. GNP Preparation. GNPs covered with a hexanethiolate monolayer were prepared exactly following the literature method.7 Thus, a solution of tetraoctylammonium bromide (1.1 g) in toluene (65 mL) was added with stirring to a solution of 158 mg of HAuCl4· 3H2O. This solution was stirred for 20 min and mixed with n-hexanethiol (142 mg) with further stirring for 10 min. To this mixture, a solution of 450 mg of NaBH4 dissolved in 5 mL of water was added. The reaction mixture was stirred at room temperature for 24 h. The organic phase was separated, evaporated to about 2-3 mL in a rotary evaporator under vacuum at room temperature, mixed with 50 mL of ethanol, and centrifuged at 5000 rpm for 1 h. The supernatant liquid was removed, and the resulting hexanethiolprotected gold nanoparticles were dissolved in about 1 mL of dichloromethane and precipitated with ethanol. The centrifugation and re-dispersal process was repeated several times to ensure the complete removal of noncovalently bound organic material. Removal of the solvent afforded 60 mg of hexanethiol-capped gold nanoparticles (C6-GNP). It has been demonstrated that this procedure furnished GNPs with an average composition of Au140[S(CH2)5CH3]537. The 10CB-thiol stabilized gold nanoparticles (10CB-GNP) were prepared by following the same procedure except using 10CBthiol instead of hexanethiol. Preparation of GNP Dispersions. The lyotropic hexagonal columnar liquid crystalline phase (H1 phase) has been prepared with a composition of 42 wt % Triton X-100 and 58 wt % water as reported earlier.33 A total of 2 mL of this mixture was heated to about 35 °C to the isotropic phase, and 12 mg of the hexanethiol functionalized GNPs were dispersed in this medium. This was followed by ultrasonication for 10 min and later was allowed to cool down to 25 °C. Two more samples with compositions of 2.4 mg of GNP/2 mL of H1 phase and 1 mg of GNP/2 mL of H1 phase were prepared following the same process. For 10CB-thiolated GNP, two samples of compositions of 1 mg of GNP/2 mL of H1 phase and 0.5 mg of GNP/2 mL of H1 phase were prepared by the same procedure. The samples showed a homogeneous phase, indicating that the GNPs are well-dispersed in the H1 phase and found to be stable for several days (Figure 2a). For the preparation of GNP dispersions in AOT, 1 mg of the nanoparticles was mixed with 2 g of pure AOT dissolved in diethyl ether. This solution was mixed well, and the solvent was evaporated to obtain the GNP dispersion in AOT (Figure (2b). Pure samples of Triton X-100/water and AOT systems were also prepared following the same procedure without adding the gold nanoparticles. Polarizing Optical Micrographic Studies. The polarizing optical micrographic studies of the samples were performed using an Olympus POM instrument. The textures have been taken by sandwiching the sample between glass slides and cover slips. In the Triton X-100/water systems, the samples were heated to the isotropic temperature, and the textures were obtained while cooling. For the AOT samples, the textures were taken at room temperature. Small-Angle X-ray Scattering (SAXS). The small-angle X-ray studies have been carried out using an X-ray diffractometer (Rigaku, (37) Kumar, S.; Pal, S. K. Liq. Cryst. 2005, 32 (5), 659.
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Figure 2. (a) Hexanethiolated GNPs dispersed in the lyotropic columnar hexagonal phase of Triton X-100/water system. (b) Hexanethiolated GNPs dispersed in the pure AOT system. UltraX 18) operating at 50 kV and 80 mA using Cu KR radiation having a wavelength of 1.54 Å. Samples were prepared by filling the capillary with the liquid crystal or the GNP dispersion and sealed. All the scattering studies were carried out at room temperature.
Results and Discussion Dispersion of GNPs in Triton X-100/Water System. From the STM studies, the size of a hexanethiol stabilized nanoparticle was measured to be about 3.6 nm. The measured size includes that of the hexanethiol molecules around the core, which is about 2 nm. The liquid crystal dispersions were characterized in terms of the stability and dimensions of nanoparticle-dispersed liquid crystalline phases using POM and SAXS. Figure 3a shows the texture of the Triton X-100/water system, while Figure 3b,c shows the textures of the dispersions, 1 mg of GNP/2 mL of Triton X-100/water system for hexanethiolated and 10CBthiolated GNPs, respectively. In all the cases, the classical texture of the columnar mesophase appeared upon cooling from the isotropic liquid, which remained stable down to room temperature. The textures for the GNP/Triton X-100/water systems are identical to the texture for H1 phase shown by the virgin Triton X-100/ water system. This clearly demonstrates that even after the GNPs incorporation, the H1 phase was maintained. The temperature of the phase transition from the isotropic to the columnar phase was found to be dependent on the concentration of GNPs. The pure Triton X-100/water system underwent a phase transition at 29 °C. At the lowest concentration of hexanethiolated GNPs we have used (1 mg of GNP/2 mL of H1 phase), the transition temperature for the isotropic phase to the H1 phase shifted from 29 to 33.1 °C. This increase in the transition temperature clearly shows that the nanoparticles stabilize the H1 phase of the Triton X-100/water system. But at higher concentrations of GNPs, the transition temperature again decreases (32.9 °C for 2.4 mg of GNP/2 mL of H1 phase and 31.9 °C for 12 mg of GNP/2 mL of H1 phase), which may be attributed to the beginning of the phase separation. For the 10CB-thiolated GNPs, the transition temperature is 33.6 °C at the concentration of 1 mg of GNP/2 mL of H1 phase, while for the lower concentration of 0.5 mg of GNP/2 mL of H1 phase, the transition temperature has shifted to 33.1 °C. The SAXS studies have been performed for the pure Triton X-100/water system and for the GNP-dispersed Triton X-100/ water systems. The diffraction patterns are shown in Figure 4,
Figure 3. (a) Optical micrographic texture showing the columnar hexagonal phase (H1 phase) of Triton X-100/water system, on cooling from the isotropic phase (29 °C). (b) Texture showing the columnar hexagonal phase of 1 mg of hexanethiol stabilized GNP/2 mL of H1 phase at 25 °C on cooling from the isotropic phase (32.9 °C). (c) Texture showing the H1 phase of 1 mg of 10CB-thiol stabilized GNP/2 mL of H1 phase at 25 °C on cooling from the isotropic phase (31.6 °C). Scale bar: 100 µm
where the intensity is plotted against the scattering vector q. Table 1 shows the d spacing corresponding to (1,0), (1,1), (2,0), and (2,1) reflections. It is clear that in the magnitudes of the scattering vectors q is in the ratio 1:x3:x4, showing a 2-D hexagonal lattice belonging to P6m space group with a lattice parameter a ) 60.66, 60.01, and 61.60 Å for the pure H1 phase, C6-GNP-dispersed H1 phase, and 10CB-GNP-dispersed H1 phase, respectively. This also supports the POM observation of a hexagonal columnar phase (H1) in these systems. The GNP-dispersed systems also show the H1 phase as seen from the figure. The extended array of ordering seen after the addition of GNPs with the appearance of an additional Bragg peak at the x7 position due to the (2,1) reflection shows that the H1 phase is more ordered within the core. A small decrease in the d spacing for the hexanethiolated nanoparticle dispersion, as shown in Table 1, can be explained in terms of our proposed model where the GNPs are distributed outside the column boundaries and in the inter domain spaces. Obviously, to minimize the hydrophilic-hydrophobic interactions between the GNPs and the polar boundaries of the columns, the individual columns will tend to shrink. On the other hand, we have observed a small increase in the d spacing for the 10CB-thiolated GNP dispersion. This can be attributed to the interaction between the terminal polar cyano group of the nanoparticles and the water molecules
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Figure 5. Optical polarographic textures of (a) pure AOT, (b) 1 mg of C6-GNP/2 g of AOT, and (c) 1 mg of 10CB-GNP/2 g of AOT systems at room temperature. Scale bar: 50 µm.
Figure 4. SAXS patterns for the systems under study showing the vector q vs intensity. (a) 42% Triton X-100/58% water system. (b) 1 mg of C6-GNP/2 mL of 42% Triton X-100/58% water system. (c) 1 mg of 10CB-GNP/2 mL of 42% Triton X-100/58% water system at room temperature. The Intensity is in an arbitrary scale. Table 1. d Values (in Angstroms) of Triton X-100/Water System and 1 mg of GNP/2 mL of Triton X-100/Water Systems at Room Temperature Measured by SAXS pure H1 phase C6-GNP/H1 phase 10CB-GNP/H1 phase
d1
d2
d3
d4
52.54 51.97 53.35
30.42 29.85 31.04
26.29 26.15 27.20
20.80 21.88
on the micelle, which leads to a small expansion of the micellar core. However, since the changes in the d values are quite small, the GNPs do not significantly alter the columnar arrangement of the H1 phase in both the dispersions. If, instead, the gold nanoparticles have to reside inside the hydrophobic core of Triton X-100, then the large size of the thiol stabilized Au nanoparticle (about 3.6 nm) will severely distort the hexagonal columns (about 5 nm). In that event, this distortion should have been reflected in a large shift in the d spacing as measured by the SAXS studies.
We believe that the gold nanoparticle will gain entropically in the inter-domain spaces instead of being confined inside the hydrophobic columns. The nanoparticles have adequate space between the domain planes where the hydrophobic alkanethiol stabilizers of the gold nanoparticles can also interact with the methylene chains of Triton X-100. We have calculated the molar mass of an individual GNP by assuming a spherical shape for the GNPs with an average diameter of about 4 nm obtained from the STM. The composition of GNP has been assumed to be Au140[S(CH2)5CH3]53 as reported earlier.7 We obtain a volume fraction of 1:3300 for the GNP:H1 phase. This means that the number of GNPs is not significantly high in the dispersion to perturb the d spacing of the columns in agreement with our small-angle X-ray scattering studies. Dispersion of GNPs in AOT. The dispersions of both hexanethiolated and 10CB-thiolated nanoparticles in the reverse hexagonal phase of AOT (H2 phase) have been prepared as described in the Experimental Procedures. The POM study of the pure and nanoparticle-dispersed AOT has revealed the liquid crystalline textures as shown in Figure 5. Since the nanoparticles may not be stable up to the transition temperature of AOT (about 180 °C), the textures have been obtained at room temperature without heating and subsequent cooling as is done in the case of H1 phase of Triton X-100/water system as discussed earlier. The SAXS studies have been performed at room temperature for the pure AOT as well as for the GNP-dispersed AOT systems. The diffraction patterns are shown in Figure 6, where the intensity
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Figure 6. SAXS pattern of (a) pure AOT, (b) 1 mg of C6-GNP/2 g of AOT, and (c) 1 mg of 10CB-GNP/2 g of AOT systems at room temperature. The intensity is in an arbitrary scale. Insets show the magnification of the main figure. Table 2. d Values (in Angstroms) for Pure AOT System and 1 mg of GNP/2 g of AOT Systems at Room Temperature Measured by SAXS pure AOT C6-GNP/AOT 10CB-GNP/AOT
d1
d2
d3
21.26 23.10 22.61
13.33 13.01
11.61 11.33
columnar phase (H2) even after the incorporation of the nanoparticles. The values of the scattering vectors q of the main and additional peaks, which are in the ratio of 1:x3:x4, point to the fact that the nanoparticles enhance the organization of the phase. It is obvious from the table that the addition of nanoparticles has increased the d spacing of the AOT system in both the dispersions. For pure AOT, the d value of 21.26 Å is in good agreement with the reported value in the literature.38-39 The addition of nanoparticles increases the d spacing for both C6-GNP and 10CBGNP as seen in Table 2, and the maximum increase is observed in the hexanethiolated nanoparticle dispersion where the d spacing is 23.10 Å. In this case, due to the presence of the hydrophobic terminal groups, the nanoparticles can be accommodated within the hydrophobic surface of the reverse hexagonal phase of AOT, which results in the increase of d spacing. In the dispersion of 10CB-thiolated nanoparticles, the d spacing has increased from 21.26 to 22.61 Å. Since in both cases the increment in d spacing is small, it is easy to rule out the possibility of the presence of nanoparticles inside the columns of the H2 phase. Moreover, for a reverse hexagonal phase, the outer surface of the domains is hydrophobic, while the inner core is hydrophilic. The thiol stabilized nanoparticles, being hydrophobic, will obviously prefer the outer hydrophobic surface of the hexagonal columns. In the case of 10CB-thiolated nanoparticles, due to the presence of a polar cyano group, the outer surface of these GNPs is less hydrophobic as compared to the hexanethiolated ones. This reduces the interaction between the terminal group of GNPs and the outer surface of the reverse hexagonal columns of AOT, thereby showing a smaller increment in the d spacing. It is clear from the SAXS studies that the nanoparticles are dispersed outside the columns made by the surfactant micelles, irrespective of the H1 or H2 phases being the dispersing medium. In the H1 phase of the Triton X-100/water system, the GNPs prefer the outer surface due to larger free space available in between the columns. In the H2 phase, the outer surface of the hexagonal domains is hydrophobic, and the hydrophobic nanoparticles naturally prefer this hydrophobic region. But in both the systems, viz. the Triton X-100/water system and AOT system, it has been observed that the addition of nanoparticles has enhanced the organization of the liquid crystalline phase within the core as evidenced by the additional peaks in the SAXS. Since GNPs have been considered as a potential candidate for targeted drug delivery applications, the lyotropic liquid crystalline medium provides an ideal matrix for such studies. The present method of dispersion of thiol stabilized nanoparticles can be extended to several other types of nanoparticles as well. This will offer a very convenient route to study several physical, electrical, magnetic, and rheological properties of GNPs.
Conclusion is plotted against the scattering vector q. The d spacings obtained for these systems have been presented in Table 2. In the case of C6-GNP and 10CB-GNP-dispersed in AOT, the magnitude of the scattering vectors, q, varies in the ratio 1:x3:x4 corresponding to the (1,0), (1,1), and (2,0) reflections from a 2-D hexagonal lattice. The lattice parameter values were calculated to be a ) 24.55, 26.67, and 26.11 Å for the pure H2 phase, C6-GNP-dispersed H2 phase, and 10CB-GNP-dispersed H2 phase, respectively, which belong to the P6m space group. This fact along with the POM textures discussed previously supports our conclusion that this system exhibits a reverse hexagonal (38) Hyde, S. T. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley and Sons: New York, 2001; Ch. 16. (39) Ruggirello, A.; Turco Liveri, V. Chem. Phys. 2003, 288, 187.
In conclusion, we have demonstrated a novel method of formation of a dispersion of thiol functionalized GNPs in a lyotropic liquid crystalline phase. We have measured the dimensions of the hexagonally ordered phases both with and without nanoparticles using SAXS. The results show that the GNPs are dispersed in between the columns, essentially retaining the column dimensions. The proposed method provides an effective tool to study physicochemical properties of alkanethiol stabilized gold nanoparticles in an aqueous micellar/liquid crystalline medium. Acknowledgment. We thank M. Jayadevaiah for the STM studies and D. Samanta and Mrs. Vasudha for the SAXS studies. LA063318Z
