Synthesis of Gold Nanoparticles Supported by Aggregated ...

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J. Surface Sci. Technol., Vol 24, No. 3-4, pp. 163-177, 2008 © 2008 Indian Society for Surface Science and Technology, India.

Synthesis of Gold Nanoparticles Supported by Aggregated Assemblies of Triblock Copolymers in Aqueous Phase : Effect of Temperature POONAM BHANDARI1, POONAM SHARMA2, GURINDER KAUR3, MANDEEP SINGH BAKSHI1, and TARLOK SINGH BANIPAL2* 1 Department of Chemistry, 2Department of Applied Chemistry, Guru Nanak Dev University, Amritsar-143 005, Punjab, India. 3College of North Atlantic, Labrador City, Canada A2V 2K7 NF

Abstract — The effect of temperature on the self-assembled behavior of polymers P103 and P84, and their subsequent use as soft templates for the synthesis of gold (Au) nanoparticles (NP) have been studied with the help of SEM, TEM, and UV-vis spectral measurements. Both the triblock copolymers (TBP) exist in the form of liquid crystalline thread like assemblies. P103 being more hydrophobic shows a structural transition from liquid crystal (LC) threads to sheets at 50ºC and bear uniformly distributed Au NP, the size of which increases with the increase in temperature. P84 being more hydrophilic shows only LC threads and no sheets, but the LC threads bearing running groove at 50ºC, act as wonderful nucleation sites for the growth of large cubic Au NP. The presence of surface cavities constituted by polyethylene oxide (PEO) and polypropylene oxide (PPO) blocks on LC phase of both TBPs are considered to be the nucleation sites for Au NP. The greater hydrophobicity of P103 in comparison to P84 favors the uniform distribution of NP throughout the LC phase while an increase in the temperature facilitates this process. Keywords : Gold nanoparticles, triblock copolymers, liquid crystalline structures, temperature effect, hydrophobicity.

INTRODUCTION

Synthesis of gold nanoparticles (Au NP) is a subject of great importance as these particles have potential applications in a variety of areas, e.g., catalysis, electronics, optics, biosensors, biomedical, etc. [1]. Although the covalent strategy provide a stable *Author for correspondence.

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linkage between the ligand and the particles but it may also alter the properties of the particles. Therefore to maintain the properties of the nanomaterial, the physical adsorption of ligands over the surface of the nanoparticles may be a useful method. The ligands used to achieve an efficient physical stabilization are usually surfactants or macromolecules, such as linear and hyperbranched polymers [2]. Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPOPEO) triblock copolymers (TBPs) belong to an important category of water soluble polymers with numerous industrial applications[3-5]. Vast literature on their selfassembling behavior in aqueous phase has suggested the formation of various aggregated assemblies from spherical to thread like micelles under the effect of concentration and temperature variations [6-11]. Increase in the concentration leads to an increase in the polydisperse behavior which is mainly responsible for the anisotropic structural transitions [9]. Increase in the temperature has drastic effect on the hydration of TBP micelles and a subsequent dehydration process produces predominantly hydrophobic micelles with significant structural transitions[12-15]. Recently, TBPs have been used as capping agents for the synthesis of gold (Au) nanoparticles (NP) [16-21]. The work by Sakai and Alexandridis [18-20] requires special mention. The use of TBPs both as reducing as well as capping agents in the synthesis of Au NP has been explained. They have studied [18-20] the effects of copolymer characteristics such as molecular weight, PEO and PPO block lengths, and critical micelle concentration by examining several PEO-PPO-PEO block copolymers. From these studies, they have suggested that the formation of gold nanoparticles from AuCl4- comprises three main steps : (1) reduction of metal ions by block copolymer in solution, (2) absorption of block copolymer on gold clusters, and (3) growth of metal particles stabilized by block copolymers. Like other neutral polymers such as poly(ethylene glycol), poly(vinyl alcohol), poly(vinyl pyrrolidone) etc., TBPs wrap themselves around the metal nanoparticles and prevent them from coagulating in aqueous phase. In this way, a stable metal colloidal suspension is achieved in which hydrophobically predominant PPO groups are mainly in contact with metal surface while hydrophilic PEO groups interact with dipolar water molecules through hydrogen bonding. This kind of association is mainly possible in the premicellar concentration range of the micelle forming TBP. However, as the concentration of TBP exceeds the critical micelle concentration (cmc), the nucleation of Auo atoms essentially takes place on the surface cavities of TBP micelles[18-21]. A typical micelle of TBP consists of hydrophobic core made up of PPO units while PEO units occupy the corona. The PEO units basically form surface cavities which act as wonderful templates for the nucleation of Auo atoms due to their reducing properties [18-21], and therefore, varying the number of PEO groups has significant effect on the NP synthesis.

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Apart from this, due to the presence of oxyethylene and oxypropylene groups in PEO and PPO blocks, these blocks attain some degree of hydration even in the aggregated state, which is quite significant for the PEO blocks rather than the PPO ones [22-24]. Thus a change in the hydration of these groups with respect to temperature drastically influences the hydrophobicity of a TBP micelle and its stability as a whole. An increase in the temperature leads to an enormous dehydration of the TBP macromolecules which drives them towards micellization. The temperature at which this phenomenon occurs is known as the critical micelle temperature (cmt). In this way, by varying both concentration and temperature, one can achieve the micellization of a TBP. We have tried to evaluate the effect of temperature on the Au NP synthesis in the presence of P103 and P84, where P103 ((EO)17(PO)60(EO)17) is much more hydrophobic than P84 ((EO)19(PO)43(EO)19), by first studying the critical micelle temperature behaviour using fluorescence spectroscopy. The difference in their hydrophobicity and its effect on the synthesis of Au NP has been evaluated. EXPERIMENTAL

Materials : Triblock copolymers designated by their commercial names, i.e., Pluronic P103 and P84, were received from BASF, Germany. Tetrachloroauric acid (HAuCl4) and sodium borohydride (NaBH4) were obtained from Acros and Aldrich, respectively. All reagents were used as received. Water used was doubly distilled. Method : Preparation of simultaneous TBP self assembled threads and TBP-capped gold nanoparticles — First of all, 20 ml of HAuCl4 aqueous solution ([HAuCl4] = 2.5 × 10–4 mol dm–3) was prepared in a double walled glass flask to circulate thermostated water at a fixed temperature. Under constant stirring, a required amount of P103 or P84 was added to this solution so as to make [TBP] = 0.5 × 10–3 mol dm–3. After the complete dissolution of P103 or P84, 0.6 ml of cold freshly prepared NaBH4 aqueous solution ([NaBH4] = 0.1 mol dm–3) was added and the solution was kept under stirring condition for at least 12 hours at 30ºC. The temperature was precisely maintained by circulating thermostated water (using Julabo F25 thermostat) within the uncertainties of ±0.01ºC. After 12 hours of stirring at 30ºC, the solution was transferred to 10 ml tightly capped sample tubes and kept for aging in dark for at least 1 month. Similar reactions were carried out at 40, 50, and 60ºC. In each case, a light pink color appeared initially with different color intensities at different temperatures. The color slowly disappeared on aging within 4-5 days. This simultaneously leads to the appearance of black threads floating at the bottom of the sample tube. After this, the nature of the solution remained essentially the same for several months.

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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, 200-900 nm. The formation of Au nanoparticles was monitored in the visible absorption range, around 540 nm. The formation of TBP threads and self aggregated assemblies has been determined with the help of Field Emission Scanning Electron Microscope (FESEM). A drop of the sample was dried on silicon wafer and imaged by Hitachi S-4500 SEM. 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 both FESEM as well as TEM observations were carried out for each sample after aging for at least 2 months to allow sufficient time for stabilisation. RESULTS AND DISCUSSION

It is known that TBPs undergo micelle formation with increase in temperature at cmt, however, more increase in the temperature even further dehydrates the TBP micelles leading to the cloud point where typical phase separation occurs[25-26]. Thus, the purpose of carrying out the Au NP synthesis at different temperatures was simply to study the effect of such TBP micellar transitions on Au NP formation. Pyrene fluorescence is the most appropriate method to study the cmt of a TBP by monitoring a change in the polarity of pyrene emission spectrum. It is given by the ratio, I1/I3 of the intensities of the first and the third vibronic bands with respect to a change in temperature[12]. Fig. 1 shows clear cmt of both P103 and P84 ([TBP] = 0.5 × 10–3 mol dm–3) at 22.8 and 32.6ºC, respectively, and there is no appreciable change in the I1/I3 plot beyond this point up to 55ºC suggesting the absence of cloud point (Cp) in both the cases in the temperature range studied. The Cp determined separately by visual observation [25-26] for P103 and P84 are 82.3 and 88.0ºC, respectively, at [TBP] = 0.5 × 10–3 mol dm–3. This suggests that the Au NP synthesis performed in the presence of P84 below 32.6ºC, i.e., at 30ºC, was especially in the premicellar region (although the presence of premicellar aggregates cannot be avoided even at 30ºC due to the polydispersed nature of P84). On the other hand, the synthesis of Au NP at 40, 50, and 60ºC was essentially taking place in the post micellar regions. In the presence of P103, the synthesis was always in the post micellar region of P103 (cmt = 22.8 ºC). The UV-visible spectra were taken immediately after each reaction. They showed clear absorption bands for each NP solution around 540 nm at all temperatures


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Temperature / ºC Fig. 1. Plot of the ratio of intensities of the first to third vibronic emission bands (I1/I3) of pyrene versus temperature at fixed TBP concentration = 0.5 × 10–3 mol dm–3, showing critical micelle temperature (cmt). Oex = 335 nm.

(not shown). Fig. 2 shows the UV-visible spectra of various Au-P103 and Au-P84 NP solutions prepared at different temperatures after giving sufficient time for stabilization. At 30 and 40ºC, the Au NP solution in the presence of both P103 and P84 shows absorbance around 540 nm though strong peaks are observed in the former case. At 50 and 60ºC, the absorbance bands are not clear which indicate insignificant number of NP in the solution. Since the concentrations of all the reagents are always constant in each reaction performed at different temperatures, insignificant absorbance (at 540 nm) at 50 and 60ºC is directly related to the nature of TBP present. Generally increase in the temperature increases the reaction kinetics and is expected to facilitate the nucleation process at higher temperature. Before evaluating how P103 or P84 assemblies affect the synthesis of Au NP, it is important to know what kind of P103 or P84 self assembled structures are present in the solution. This has been observed by obtaining the FESEM micrographs of aqueous P103/P84+Au NP at all temperatures (Fig. 3 and 4).

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Fig. 2. Plots of absorbance of gold nanoparticle solution versus wavelength in the presence of (a) P103 and (b) P84, at different temperatures.

Some recent studies [6-9] on the aggregation behavior of block copolymers have indicated that their self aggregation depends on the degree of hydration of the hydrophilic blocks. Greater hydration keeps hydrophilic blocks in contact with water


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Fig. 3. Scanning electron micrographs (SEM) of liquid crystalline (LC) assemblies of P103 bearing gold nanoparticles at 30, 40, 50, and 60 ºC. All left hand side panels show a change in the LC texture with the increase in temperature. The right hand side panels show the presence of gold nanoparticles as bright spots indicated in white circles.

and thus shields the hydrophobic blocks from any contact with water. This leads to self-arrangement of the polymer in small spherical aggregates with minimum surface

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to volume ratio. Any factor which reduces the amount of water available for the hydration of hydrophilic blocks leads to the fusion of such drops in long thread like structures. Fig. 3 shows the SEM images at all temperatures of this study. At 30ºC (Fig. 3a), P103 is arranged in the form of three-dimensional thread like liquid crystalline (LC) structures which are interconnected with each other (the lamellar blocks leading to the LC formation are quite clear for P84, Fig. 4e). The PEO groups present at the surface of LC phase create small cavities which act as nucleation sites for NP (Fig. 5). A close inspection of such LC structures (Fig. 3b) shows the presence of Au NP as bright dots (shown in a white circle). At 40ºC also similar thread like LC structures can be seen which are relatively more interconnected with each other (Fig. 3c). Again, a close look clearly indicates the presence of Au NP as bright small dots (Fig. 3d). Further increase in the temperature at 50ºC almost converts the thread like structures into interwoven sheets (Fig. 3e) which still bear NP in small pockets (Fig. 3f). At 60ºC, such structures even look more porous (Fig. 3g) and carrying large number of Au NP on them (Fig. 3h). On the other hand, very interesting arrangement was observed for P84 especially at 50 and 60ºC (Fig. 4). Fig. 4a shows clear thick smooth thread like structure with running groove in the center. This groove contains small pockets containing groups of large Au NP (Fig. 4b). Further increase in the temperature to 60ºC makes these threads more fibrous (Fig. 4c) which still bear NP on them. However, at 30 and 40ºC, only large vesicles bearing NP could be observed (not shown) without the presence of any thread like LC arrangement. The results are further complemented from the TEM studies. Fig. 6a and 6b show the TEM images of Au NP synthesized in the presence of P103 at 30ºC. Several small NP can be seen entangled with each other in many network structures, which might arise due to the cohesive interactions among various PEO groups of P103 capping the adjoining NP [27]. It is expected that P103 wrap the NP via hydrophobic interactions and PPO groups being predominantly hydrophobic in nature play major role in such interactions leaving PEO groups predominantly facing the aqueous phase. This arrangement brings various P103 capped NP together through cohesive interactions provided by the adjoining PEO groups while entrapped NP further act as nucleation sites for more Auº atoms. Such assemblies are also evident from SEM images (Fig. 3b, indicated by white circle) which are available on the surface of LC assemblies. In our previous studies, we have reported that the surface cavities made up of PEO groups on the surface of TBP micelles act as nucleation sites for the synthesis of organized morphologies of NP[21,28]. We expect similar surface cavities


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Fig. 4. Scanning electron micrographs of liquid crystalline (LC) assemblies of P84 bearing gold nanoparticles at 50 and 60 ºC. The left hand side panels show a change in the LC texture with the increase in temperature. The right hand side panels show the presence of group of cube shaped gold nanoparticles indicated in white circles. Fig 4e shows the linings of lamellar blocks forming LC phase.

in the present case which further activate the nucleation leading to large Au NP. Fig. 6b clearly demonstrates the presence of several hexagonal shaped NP with size around 150 nm especially arranged on P103 lamellar phase. Further increase in the temperature at 40ºC simply increases their number significantly (Fig. 6c), while some plate like (two dimensional, 2D) large hexagonal nanosheets with size greater than 250 nm have also appeared. Similar much larger (>1 mm) nanosheets have also been reported by Kim et al. [29] for HAuCl4/ PEO20PPO70PEO20 system when this mixture

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Fig. 5. A schematic representation of liquid crystalline phase showing a typical arrangement of TBP. The surface is in line with the cavities formed by the PEO groups which accommodate the gold nanoparticles.

was heated at 70 ºC for 24 hours. A closer look of 2D hexagonal plate (Fig. 6d) indicates that it is capped by about 20 nm thick sheet of P103. The SEM images (Fig. 3d) further support the presence of even more dense network structures of these assemblies with several bright spots showing the presence of large NP. Unfortunately, we could not detect any such assemblies at 50ºC in aqueous phase, though thread like structures were still present. It seems that due to the porous nature of P103 threads, the NP are entrapped in the threads (Fig. 4f) and thus completely eliminating their availability in the aqueous phase. Due to the extraordinary large size (few to several microns) of LC threads, it was really hard to observe them through TEM at nanoscale. However, increase in the temperature at 60 ºC, further leads to the presence of small sized Au NP (10 nm) adsorbed on P103 lamellar phase (Fig. 6e) and some large size NP with somewhat irregular shapes (50 nm) scattered around (Fig. 6f). The SEM images (Fig. 4h) suggest a greater smoothness of the surface in comparison to the presence of porous surface at 50 ºC (Fig. 4f), which might release some of the entrapped NP and made them available in the aqueous bulk phase.


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Fig. 6. Transmission electron micrographs of gold nanoparticles in the presence of P103 at 30, 40 and 60 ºC. All scale bars are equal to 100 nm or otherwise specified.

Shifting our attention on the Au NP synthesized in the presence of P84, Fig. 7a shows the presence of some spherical large vesicles with size around 500 nm bearing several small NP. Apart from this, small NP arranged in numerous wire-

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like structures, can also be seen. Interestingly, increase in the temperature to 40 and 50 ºC completely removes the presence of Au NP from aqueous bulk phase, though SEM images show the presence of groups of large cubic NP (700 nm) in the grooves of P84 threads (Fig. 4b). Increasing the reaction temperature to 60 ºC brings back a few NP in the aqueous phase (Fig. 7b), which might arise from the facilitation in NP formation at elevated temperatures. However, it is to be mentioned that we do not observe such large cubic NP in the solution from TEM observations, and it seems that such large NP are the result of facilitated nucleation at the LC phase.

Fig. 7. Transmission electron micrographs of gold nanoparticles in the presence of P84 at 30 and 60 ºC.


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All the above results, can be explained on the basis of hydrophobic/hydrophilic (PPO/PEO) ratio (based on the molecular weights) of the present TBPs (P103PPO/PEO = 2.32, P84PPO/PEO = 1.49). The PPO/PEO is much higher for P103 than for P84, and clearly suggests that former is more hydrophobic than the latter. That is the reason why the cmt of P103 is lower than that of P84. If we compare the absorbance spectra of Au NP synthesized in the presence of P103 with that in the presence of P84 (Fig. 2), it is very clear that at least at 30 and 40ºC, the NP formation is much significant in the presence of P103 than in the presence of P84. Certainly, the stronger hydrophobicity of P103 is playing significant role in achieving this, in comparison to a cmt effect because at all temperatures the concentration of P103 is always greater than its cmt. At 50 ºC, a further dehydration of P103 LC threads makes them more hydrophobic which is evident from an instant fall in I1/I3 value around 50 ºC after the cmt (Fig. 1). This change is clearly shown by the SEM images of P103 in Fig. 3, where the P103 threads available at 30 and 40ºC are converted into fibrous sheets at 50ºC. Such sheets act as wonderful templates with large surface area and thereby facilitate the adsorption of NP from aqueous phase to lamellar or LC phase. Exactly similar mechanism is observed for P84 immediately after the cmt. UV spectrum at 30ºC (Fig. 2b) shows the presence of NP in aqueous phase which is also evident from Fig. 7a. But as we cross the cmt, i.e., 40ºC, no aqueous phase NP were observed, and P84 start self aggregating into large vesicles and then into long threads bearing NP at 50 and 60ºC. One can see a clear difference between the capability of P103 and P84 LC phases to accommodate Au NP. P103 LC phase accommodates relatively small sized NP equally distributed throughout the whole LC phase contrary to the selective presence of large Au NP on P84 LC phase. Here too the stronger hydrophobicity of P103 LC phase proves to be more favorable for the nucleation of Au NP, since both PEO and PPO blocks participate in the nucleation of Au NP. The greater molecular weight of P103 is considered to be more favorable for the equal distribution of small NP on its LC phase in comparison to selective distribution over P84 LC phase. This study is a step forward in synthesizing Au NP on templated microstructures of TBPs. The liquid crystalline templates can be easily dissolved in nonaqueous solvents such as alcohols and hence Au NP can be recovered to use for different applications. CONCLUSIONS

It has been found that LC phase of TBPs act as wonderful templates for the nucleation of Au NP. This is achieved due to the presence of surface cavities made up of PEO

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and PPO groups on the micellar phase of TBP aggregated assemblies which act as reducing sites and thus facilitate the nucleation. Greater hydrophobicity achieved by P103 with the increase in temperature helps in uniform distribution of Au NP throughout the LC phase while this is not observed in the case of P84 due to its predominant hydrophilic nature. Thus greater hydrophobicity of self-assembled aggregates of a TBP is necessary to use it as an appropriate template for the synthesis of Au NP. ACKNOWLEDGEMENT

The authors thank CSIR, New Delhi, India, for research grants (Ref. No. 01(2102)/ 07/EMR-II). M S Bakshi also thanks UGC for financial assistance (Ref. No. 33-291/ 2007(SR)). REFERENCES 1. T. Azzam, L. Bronstein and A. Eisenberg, Langmuir, 24, 6521 (2008). 2. K. Rahme, F. Gauffre, J-D. Marty, B. Payre and C. Mingotaud, J. Phys. Chem. C, 111, 7273 (2007). 3. J. M. Harris, S. Zalipsky, Poly(ethylene glycol) : Chemistry and Biological Applications; American Chemical Society ; Washington DC, (1997). 4. D. W. Tedder, F. G. Pohland, Emerging Technologies in Hazardous Waste Management; American Chemical Society; Washington DC, (1990). 5. P. Alexandridis, T. A. Hatton, Colloids Surf. A, 96, 1 (1995). 6. H. W. Shen, L. F. Zhang, A, Eisenberg, J. Am. Chem. Soc., 121, 2728 (1999). 7. L. F. Zhang, K. Yu, A. Eisenberg, Science, 272, 1777 (1996). 8. L. F. Zhang, A. Eisenberg, J. Am. Chem. Soc., 118, 3168 (1966). 9. N. S. Cameron, M. K. Corbierre, A. Eisenberg, Can. J. Chem., 77, 1311 (1999). 10. M. Almgren, J. Van Stam, C. Lindblad, P. Li, P. Stilbs, P. Bahadur, J. Phys. Chem., 95, 5677 (1991). 11. M. Almgren, P. Bahadur, M. Jansson, P. Li, W. Brown, A. Bahadur, J. Colloid Interface Sci., 151, 157 (1992). 12. P. Alexandridis, J. F. Holzwarth, T. A. Hatton, Macromolecules, 27, 2414 (1994). 13. P. Alexandridis, T. Nivaggioli, T. A. Hatton, Langmuir, 11,1468 (1995). 14. T. Nivaggioli, B. Tsao, P. Alexandridis, T. A. Hatton, Langmuir, 11,119 (1995). 15. T. Nivaggioli, P. Alexandridis, T. A. Hatton, A. Yekta, M. A. Winnik, Langmuir, 11, 730 (1995).


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16. J. P. Spatz, S. Mössmer and M. Moller, Chemistry A European Journal, 2, 1552 (1996). 17. A. B. R. Mayer and J. E. Mark, Polymer Preprints; New Orleans, Spring, 74 (1996). 18. T. Sakai and P. Alexandridis, Langmuir, 20, 8426 (2004). 19. T. Sakai and P. Alexandridis, J. Phys Chem B, 109, 7766 (2005). 20. T. Sakai and P. Alexandridis, Nanotechnology, 16, 344 (2005). 21. M. S. Bakshi, A. Kaura, P. Bhandari, G. Kaur, K. Torigoe, and K. Esumi, J. Nanoscience and Nanotechnology, 6, 1405 (2006). 22. M. S. Bakshi, S. Sachar, K. Singh, A. Shaheen, J. Colloid Interface Sci., 286, 369 (2005). 23. M. S. Bakshi, J. Singh, G. Kaur, J. Colloid Interface Sci., 285, 403 (2005). 24. M. S. Bakshi, G. Kaur, I. Ahmad, Colloids Surf. A, 253, 1 (2005). 25. M. S. Bakshi, J. Singh, and J. Kaur, J. Colloid Interface Sci., 287, 704 (2005). 26. M. S. Bakshi, N. Kaur, R. K. Mahajan, J. Singh, and N. Singh, Coll. Polym. Sci., 284, 8792 (2006). 27. K Niesz, M. Grass, G. A. Somorjai, Nano Lett., 5, 238 (2005). 28. M. S. Bakshi, A. Kaura, G. Kaur, K. Torigoe, and K. Esumi, J. Nanoscience and Nanotechnology, 6, 644 (2006). 29. J-U Kim, S-H Cha, K. Shin, J. Y. Jho, J-C Lee, J. Adv. Materials, 16, 459 (2004).