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Solvent-Adaptable Silver Nanoparticles B. L. V. Prasad,* Sujatha K. Arumugam, Tanushree Bala, and Murali Sastry* Nanoscience Group, Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received September 14, 2004. In Final Form: December 16, 2004 A simple and efficient way of obtaining silver nanoparticles that are dispersible both in organic and in aqueous solvents using a single capping agent is described. The silver nanoparticles are initially prepared in water in the presence of aerosol OT [sodium bis(2-ethylhexyl)-sulfosuccinate, AOT]. Thereafter, transfer of the AOT-capped silver nanoparticles to an organic phase is induced by the addition of a small amount of orthophosphoric acid during shaking of the biphasic mixture. The AOT-stabilized silver nanoparticles could be separated out from the organic phase in the form of a powder. The hydrophobic nanoparticles thus prepared are stable and are readily resuspended in a variety of other polar (including water) and nonpolar solvents without further surface treatment. The amphiphatic nature of the silver surface is brought about by a small orientational change in the AOT monolayer on the silver surface in response to the polarity of the solvent.
Introduction The exciting application potential of nanomaterials,1-5 especially metal nanoparticles, has resulted in a plethora of experimental recipes for their synthesis either in water6 or in nonpolar organic solvents.7 Often the stabilization of nanoparticles in different physicochemical environments (in different solvents) is of paramount importance for their utilization as building blocks from both fundamental and applied considerations.8 Because the asproduced nanoparticles are often soluble in either the aqueous phase or the organic phase but not in both, a great amount of work is also being done toward the phase transfer of nanoparticles from an aqueous phase into an organic phase9 and vice versa.10 (A summary of the efforts currently in vogue can be found in the reviews written by Sastry.)11 While many ingenious ways have been developed * To whom correspondence should be addressed. Phone: 9120-25893400, ext. 2260 (B.L.V.P.); 2013 (M.S.). Fax: 91-2025893044 (B.L.V.P.); -25893952 (M.S.). E-mail: blvprasad@ dalton.ncl.res.in (B.L.V.P.);
[email protected] (M.S.). (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56. (3) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (4) Esumi, K.; Hosoyo, T.; Suzuki, A.; Torigoe, K. Langmuir 2000, 16, 2978. (5) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (6) (a) Turkevich, J.; Garton, G.; Stevenson, P. C. J. Colloid Sci. 1954, 9, 26. (b) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301. (c) Henglein, A. Langmuir 1999, 15, 6738. (d) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. New J. Chem. 1998, 1257. (e) Watson, K. J.; Zhu, J.; Nguyen, S. B. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462. (f) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075. (g) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (7) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (c) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J. J. Nanopart. Res. 2000, 2, 154. (d) Lin, X. M.; Wang, G. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 1999, 103, 5488. (e) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515. (f) Stoeva, S. I.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. J. Am. Chem. Soc. 2002, 124, 2305. (g) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (h) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (8) Science 2000, 290, 1523-1558 (Special Issue on Nanotechnology).
to affect the phase transfer from aqueous to organic12-18 or the more difficult organic to aqueous10,19-21 phases, most of the methods are laden with their own disadvantages and often require elaborate procedures to achieve the same.11 In this regard a glaring lacuna has been the development of one simple nanoparticle surface capping procedure that leads to the stabilization of nanoparticles in both polar and nonpolar environments, and in this paper we address this issue. Our strategy involves the usage of a water-soluble bifunctional surfactant, aerosol OT [sodium bis(2-ethylhexyl)-sulfosuccinate, AOT] during the preparation of (9) (a) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (b) Copanik, N.; Talapin, D. V.; Rogach, A. L.; Eychmuller, A.; Weller, H. Nano Lett. 2002, 2, 803. (c) Lala, N.; Lalbegi, S. P.; Adyanthaya, S. D.; Sastry, M. Langmuir 2001, 17, 3766. (d) Mayya, K. S.; Caruso, F. Langmuir 2003, 19, 6987. (e) Yao, H.; Momozawa, O.; Hamatani, T.; Kimura, K. Chem. Mater. 2001, 13, 4692. (f) Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2003, 125, 4046. (g) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (10) (a) Templeton, A. C.; Hosteler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (b) Simard, J.; Briggs, C.; Boal, A. K.; Rotello, V. M. Chem. Commun. 2000, 1943. (c) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001. (11) (a) Sastry, M. Curr. Sci. 2003, 85, 1735. (b) Sastry, M. In The Chemistry of Nanomaterials; Rao, C. N. R., Muller, A., Cheetham, A. K., Eds.; Wiley-VCH: Weinheim, 2004; p 31. (12) Sarathy K. V.; Kulkarni, G. U.; Rao, C. N. R. Chem. Commun. 1997, 537. (13) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (14) Fu, X.; Wang, Y.; Wu, N.; Gui, L.; Tang, Y. J. Colloid Interface Sci. 2001, 243, 326. (15) Zhao, S.-Y.; Chen, S.-H.; Wang, S.-Y.; Li, D.-G.; Ma, H.-Y. Langmuir 2002, 18, 3315. (16) (a) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277. (b) Kumar, A.; Mukherjee, P.; Guha, A.; Adyantaya, S. D.; Mandale, A. B.; Kumar, R.; Sastry, M. Langmuir 2000, 16, 9775. (17) Kumar, A.; Joshi, H.; Pasricha, R.; Mandale, A. B.; Sastry, M. J. Colloid Interface Sci. 2003, 396. (18) (a) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (b) Wang, W.; Chen, X.; Efrima, S. J. Phys. Chem. B 1999, 103, 7238. (19) (a) Swami, A.; Kumar, A.; Sastry, M. Langmuir 2003, 19, 1168. (b) Swami, A.; Jadhav, A.; Kumar, A.; Adyanthaya, S. D.; Sastry, M. Proc. Indian Acad. Sci., Chem. Sci. 2003, 115, 679. (20) Wang, Y.; Wong, J. F.; Teng, X.; Lin, X. Z.; Yang, H. Nano Lett. 2003, 3, 1555. (21) Pellegrino, T.; Manna, L.; Kudera, S.; Liedl, T.; Koktysh, D.; Rogach, A. L.; Keller, S.; Radler, J.; Natile, G.; Parak, W. J. Nano Lett. 2004, 4, 703.
10.1021/la047707+ CCC: $30.25 © 2005 American Chemical Society Published on Web 01/06/2005
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nanoparticles, and AOT is one of the most widely used and inexpensive surfactants. It has two ester functional groups and a sulfonate group, which renders it highly water-soluble, and has been widely used as a surfactant in the synthesis of nanoparticles especially by the reverse micelle methods discussed by Pileni and co-workers.7g,h,22 Here, we found that silver nanoparticles prepared in an aqueous medium and capped with AOT may be rendered hydrophobic by the addition of phosphoric acid. This hydrophobic silver colloid, where the sulfonate and ester groups cap the nanoparticles, is very stable, and the particles retain their integrity even after the solvent is evaporated. We also found that the dried deposit may be readily resuspended in a variety of other nonpolar solvents as well as water. Presented below are the details of the investigation. Experimental Section Silver sulfate (Ag2SO4), sodium borohydride (NaBH4), AOT, phosphoric acid (H3PO4), and other solvents were purchased from Sigma Aldrich and used as received. Silver Hydrosol Preparation. In a typical experiment, 1 × 10-4 M of Ag2SO4 in water was reduced with 0.1% NaBH4 in the presence of AOT at different concentrations (1 × 10-4, 5 × 10-3, and 1 × 10-2 M). A yellowish-brown colloidal solution of silver was obtained almost immediately. However, the reaction was continued for 2 h to ensure completion. To remove excess AOT used during the preparation, the samples were subjected to dialysis in a dialysis bag (12 kDa) for 24 h with regular change of water every 6 h. Silver Organosol. To a biphasic mixture of 25 mL each of the silver hydrosol and cyclohexane was added 0.2 mL of 0.1 M H3PO4 under vigorous stirring. The color of the organic phase changes to bright yellow immediately when H3PO4 is added to the silver nanoparticle solution. Redispersion of Silver Nanoparticles. The silver organosol thus prepared was extremely stable. The silver solution was evaporated in a vacuum, and the silver nanoparticle powder remaining could be readily redispersed in both water and in different nonpolar organic solvents. The different samples prepared by the above procedures were probed by UV-vis spectroscopic studies, powder X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM), and the details are given below. The optical properties of the Ag hydrosol, Ag organosol in cyclohexane, and Ag nanoparticles dispersed in various nonpolar solvents were monitored on a Jasco UV-vis spectrophotometer (V570 UV-vis-NIR) operated at a resolution of 2 nm. FTIR spectra were recorded from drop-coated films of the different nanoparticle samples deposited on a Si(111) substrate on a Perkin-Elmer Spectrum-One spectrometer operated in the diffuse reflectance mode at a resolution of 4 cm-1. The spectrum of pure AOT was also recorded for comparison. TEM measurements were performed on a JEOL model 1200EX instrument operated at an accelerating voltage at 120 kV. Samples of Ag nanoparticles from aqueous and chloroform solutions for TEM studies were prepared by placing a drop of the solutions on carbon-coated copper grids. The films on the TEM grids were allowed to dry for 2 min, following which the extra solution was removed using a blotting paper. XRD measurements of the Ag nanoparticles were performed by casting the respective nanoparticle solutions in the form of films on glass substrates by simple solvent evaporation. The XRD measurements were carried out on a Philips PW 1830 instrument operating at 40 kV and a current of 30 mA with Cu KR radiation.
Results and Discussion The phase transfer of the aqueous silver nanoparticles prepared with various concentrations of AOT into the organic phase is illustrated in Figure 1A, which shows (22) (a) Bagwe, R. P.; Khilar, K. C. Langmuir 2000, 16, 905. (b) Kitchens, C. L.; McLeod, M. C.; Roberts, C. B. J. Phys. Chem. B. 2003, 107, 11331.
Figure 1. (A) UV-vis spectra recorded from the as-prepared silver hydrosol (with an AOT concentration of 5 × 10-3 M; curve 1) and silver organosols after phase transfer into cyclohexane (curves 2-4); curves 2-4 correspond to the spectra recorded from silver nanoparticles capped with 5 × 10-3, 1 × 10-2, and 1 × 10-4 M AOT, respectively. The curves have been displaced vertically for clarity. The inset shows pictures of test tubes of silver nanoparticle solutions of the samples in curves 1-4, respectively. (B) UV-vis spectra recorded from the silver nanoparticle powder after redispersion in aqueous and nonpolar organic solvents (curves 1-4). Silver nanoparticles in water (curve 1), chloroform (curve 2), toluene (curve 3), cyclohexane (curve 4), and benzene (curve 5). These spectra have been displaced vertically for clarity.
pictures of a test tube before (test tube 1 on the left) and after (test tubes 2-4 on the right) phase transfer of the colloidal Ag particles into cyclohexane (see Experimental Section). The UV-vis spectra reveal only small shifts in the absorbance maximum from the aqueous to the organic phase, where the phase transfer is achieved by shaking a biphasic mixture of the silver hydrosol and cyclohexane following the addition of phosphoric acid. Curve 1 corresponds to the UV-vis spectrum recorded from the aqueous phase and shows a sharp resonance at about 406 nm. This band arises because of excitation of surface plasmon vibrations in the silver nanoparticles and is responsible for the striking yellow color of the silver nanoparticle solutions. After the phase transfer, the peak is marginally shifted to around 410 nm. The fact that very little shifts are observed in the peak positions clearly reveals that the particles are practically phase transferred as-is without undergoing any agglomeration. The resulting organosol was then rotary evaporated and resulted in a brownish solid powder that could be readily redispersed in water and different solvents such as toluene, benzene, chloroform, and cyclohexane. We would like to emphasize that we did not see perceptible differences in the time for the redispersibility of this powder in water or cyclohexane, two solvents differing vastly in their polarities, indicating that there is no particular polarity preference that these AOT-capped silver particles display. We also note that there does not seem to be any particular size separation upon phase transferring into different solvents apart from the general size selection that otherwise seems to occur.23 Apart from the visual observation to check the phase transferability, (23) Organic dispersions of nanoparticles obtained after phase transfer from an aqueous medium in general seem to be more monodisperse (see ref 18, for example). This could be due to the fact that the bigger particles present in the aqueous dispersions are not phase transferred or that evaporation of organic dispersions on the TEM grids lead to better size segregation and good hexagonal arrangement of the same sized particles on the grid.
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Figure 2. (A) Representative TEM micrograph from a drop-cast film of the as-prepared silver hydrosol. The inset shows the SAED pattern of the particles indexed to fcc silver. (B) Particle size distribution histogram of the silver nanoparticles. The solid line is a Gaussian fit to the data. The AOT concentration for the preparation of the sample used in this image is 5 × 10-3 M.
Figure 3. (A-C) Representative TEM micrographs recorded from drop-cast films of silver organosol in cyclohexane obtained from the hydrosol prepared with different concentrations of AOT: A, 1 × 10-2 M; B, 5 × 10-3 M; and C, 1 × 10-4 M. The respective particle size distributions are plotted in the insets. (D) Powder XRD pattern of a film prepared from the sample depicted in Figure 3B.
we have recorded the UV-vis spectra of the solutions that we obtained after the brownish powder has been redispersed in water and different solvents such as toluene, benzene, chloroform, and cyclohexane. Figure 1B, curve 1, shows the spectrum of the AOT-capped silver nanoparticle powder now redispersed in water. Curves 2-5 correspond to the spectra recorded from the brownish silver nanoparticle powder after redispersion in chloroform, toluene, cyclohexane, and benzene, respectively. The small shifts observed in the peak positions approximately follow a trend as expected from the refractive index changes in the solvent. This exercise clearly proves that our simple method is very effective in getting silver nanoparticles dispersed in a variety of solvents, including redispersibility in water. Figure 2A shows a representative TEM picture of the as-prepared AOT-capped silver nanoparticles (AOT concentration ) 5 × 10-3 M) in water. Different concentrations of AOT were used to probe its effect on the resulting nanoparticle sizes. These changes are reflected better in the TEM images obtained from the organosols and are given in Figure 3. The inset of Figure 2A shows the selected area electron diffraction (SAED) pattern recorded from the AOT-capped silver hydrosol. In the figure the diffraction rings have been indexed and show that crystal-
linity of the particles is consistent with the face-centered cubic (fcc) structure of silver. The particle size distribution analysis (Figure 2B) reveals that the particles are polydisperse with an average size of 12 ( 3 nm. When the initial concentration of AOT in water is varied systematically the average particle sizes varied as 6.1 ( 0.9, 12.3 ( 3, and 10.6 ( 1.8 nm for the AOT concentrations 1 × 10-2, 5 × 10-3, and 1 × 10-4 M, respectively (Figure 3A-C). The critical micelle concentration (cmc) of AOT is reported to be 2.5 × 10-3 M. Around the cmc or below the cmc the particles tend to be bigger. Micelles are considered equilibrium structures where the surfactant molecules are always coming out and joining the micellar moiety. This could explain the larger sizes of particles observed when we have lower concentrations of the surfactant where the micelles may not possess very compact structures. However, when we take an excess quantity of the surfactant compared to the cmc the micelles are packed probably in a more compact manner leading to the smaller sizes of particles observed as in the case of the 1 × 10-2 M concentration. The XRD pattern recorded from a dropcast film of the silver hydrosol prepared in the presence of 5 × 10-3 M AOT shows a number of strong Bragg reflections corresponding to the (111), (200), (220), and (311) reflections of fcc silver (Figure 3D). The XRD results,
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Figure 4. FTIR spectra in the spectral range 1000-3100 cm-1 recorded from the drop-coated film on the Si(111) substrate. For clarity the spectra are split in the regions 950-1800 cm-1 (A) and 2600-4000 cm-1 (B). The details of the spectra are as follows: pure AOT (curve 1); as-prepared AOT-capped silver hydrosol (curve 2); the silver hydrosol after removal of excess AOT by dialysis (curve 3); the AOT-capped silver nanoparticle powder after redispersion in water (curve 4); and the AOTcapped silver nanoparticle powder after redispersion in chloroform (curve 5). The peaks from the carbonyl stretches almost disappear after transfer to the organic solvent, and the weak transmittance in this region is shown in the inset of part A after a magnification of 100 times.
thus, show that the as-prepared silver nanoparticles are crystalline. A systematic FTIR investigation of the as-prepared, organic dispersed, and aqueous redispersed samples was carried out to understand the mechanism of phase transfer. Figure 4 provides the FTIR spectra of the silver nanoparticles in the aqueous as well as organic environment (curve 1, spectrum of pure AOT; curve 2, spectrum of the as-prepared hydrosol in the presence of 5 × 10-3 M AOT; curve 3, spectrum recorded from the dialyzed silver hydrosol; curve 4, spectrum of the AOT-capped silver nanoparticle powder after redispersion in water; and curve 5, spectrum of the AOT-capped silver nanoparticle powder after the redispersion in chloroform). For this discussion the major interest is in the regions 950-1800 cm-1 and 2500-4000 cm-1, and these are plotted separately in Figure 4A,B, respectively. The CsH symmetric and antisymmetric stretching vibration frequencies of the s CH2 groups occur at 2877 and 2934 cm-1 for pure AOT, and those in as-prepared hydrosol are at approximately the same frequencies (at 2882 and 2944 cm-1). This might be due to the large excess of AOT present in the as-prepared hydrosol. After dialysis the excess AOT is removed and only the molecules bound to the silver nanoparticle surface remain. In curve 3 the FTIR spectrum from the hydrosol after dialysis and removal of excess AOT is displayed. The stretching frequencies now occur at 2866 and 2945 cm-1 for the dialyzed silver hydrosol indicating that the hydrocarbon chain now acquires some kind of order because it is adsorbed to the silver surface. However, in the hydrosol the functional (ester and sulfonate) groups of the AOT molecule are directed toward the solvent and the molecule is only weakly adsorbed to the silver surface as a result of some weak van der Waals interactions (Scheme 1). In the redispersed organosol, on the other hand, the functional groups are now directed toward the silver surface and the molecules are arranged like a self-assembled monolayer with a very good packing arrangement (Scheme 1). We would like to mention here that the packing of AOT on the surface may be different for particles of different sizes because of vast differences in their curvature. However, with current data we are
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unable to make specific comments on the size-dependent phase transferability aspect of the work. Further work is in progress to address this interesting point. Here, the better packing of AOT molecules on the silver surface in the organic media is known24 to result in stronger shifts of the sCsH symmetric and antisymmetric stretches toward lower frequencies, and indeed we observe peaks at 2852 and 2933 cm-1 (curve 5). The IR spectrum of hydrosol obtained from the redispersion of the organic solvent evaporated powder is an exact replica of that of the dialyzed hydrosol. This clearly indicates that once the powder is redispersed in water it is undergoing an orientational change where the functional groups are again directed toward the solvent environment and that the hydrocarbon chains are attached to the silver surface, leading to a poorer packing order. The absorption band at 1466 cm-1 is ascribed to the methylene (CH2) scissoring mode in pure AOT (curve 1). In the as-prepared case this undergoes almost no difference and occurs at 1471 cm-1 (curve 2). For the dialyzed and redispersed hydrosols where we probe only the AOT adsorbed on the silver surface the band is shifted to a lower frequency at 1460 cm-1 as expected for a poorly ordered molecule on the surface (curves 3 and 4). Finally, in the silver organosol (curve 5) this peak is shifted to a lower frequency and comes at 1448 cm-1 as expected for a packed and ordered monolayer of hydrocarbons on the silver surface.23 Coming to the peaks from functional groups we first consider the sulfonate group. The peaks at 1059 and 1160 cm-1 in the case of pure AOT (curve 1) are assigned to the symmetric and asymmetric -SdO stretching vibration of the sulfonate group present in the AOT molecules and are shifted to 1049 and 1154 cm-1 in as-prepared silver hydrosol (curve 2). In curves 3 and 4 (dialyzed hydrosol and redispersed hydrosol, respectively), these occur at exactly the same frequencies at 1049 and 1154 cm-1, but in curve 5, that is, silver organosol, they are shifted to lower frequencies and come at 1033 and 1100 cm-1. This is because in the organosol the SO3- group is bound to the silver surface whereas in the hydrosol it is free and is directed toward the solvent medium. The most dramatic changes are depicted in the carbonyl stretches. First, the band at 1736 cm-1 due to carbonyl stretch vibrations in the AOT molecules (curve 1) is shifted to 1749 cm-1 in silver hydrosol (curve 2). At this moment it is not clear why we are observing a small upward shift in this peak position in the as-prepared hydrosol. However, once the excess AOT is removed by dialysis the peak comes at 1729 cm-1, a small shift probably arising because of the poor ordered state of the molecules on the silver surface. The scenario changes drastically once the phosphoric acid is added to the silver hydrosol when the AOT molecule undergoes a major orientational change and the functional groups are strongly attached to the silver surface forming a self-assembled monolayer. Accordingly, in the silver organosol the peaks due to sCdO almost disappear and occur at 1736 and as a broad peak at 1667 cm-1 (curve 5). These peaks are very weak and are displayed in the inset of Figure 4A after a magnification of 100 times. If the disappearance of these peaks in the organic medium is due to the binding and close proximity of these functional groups to the silver nanoparticle surface, redispersion of the powder in water should regenerate these peaks. Indeed, as displayed in curve 4 redispersion of the organic solvent evaporated powder in water regenerates the peak (24) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.
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Scheme 1. Schematic Representation of the AOT Orientations on the Silver Nanoparticle Surface in Different Environments and the Molecular Structure of AOT
at 1729 cm-1 in very good agreement of our hypothesis of orientational change in AOT molecules brought about by the solvent environment. To get a more distinct picture of the role of phosphoric acid in the phase transfer process, we have tried other acids to effect the phase transfer of AOT-capped silver nanoparticles from water to the organic phase. Our results indicate that the extent of phase transfer varies as HClO4 g H3PO4 > H2SO4 and that addition of HCl destabilizes the Ag colloid in the aqueous phase itself. While we do not yet have a clear explanation for the role of the phase transfer agents, the trends are exactly the same as those observed by Efrima and co-workers for the oleic acid capped silver nanoparticle phase transfer procedures. Hence, we tentatively assume that the hydrogen bonding mechanism proposed by them is operational in our case as well.18 As in the Efrima study, it is possible that the capping layer composed of the phosphoric acid or its anion and functional moieties of the AOT molecule form a net of hydrogen bonds and force the functional group of the AOT to turn over toward the nanoparticle surface in the organic solvent. In the aqueous phase, on the other hand, the functional
moieties are better stabilized if they are directed toward the solvent resulting in the amphiphatic nature observed here. Conclusion A very simple and efficient way where the same molecule AOT is used for the synthesis, stabilization, and phase transfer of silver nanoparticles is described. The phase transfer is actually the most versatile of the reported methods so far in that the powder obtained after the organic solvent is evaporated can be redispersed in a variety of solvents including water. Acknowledgment. S.K.A. and T.B. thank the Department of Science and Technology (DST) and Council for Scientific and Industrial Research (CSIR), New Delhi, respectively, for financial assistance. This work was partially funded by an internal grant of the National Chemical Laboratory, Pune, to B.L.V.P., and it is gratefully acknowledged. LA047707+
