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2009, 113, 5677–5680 Published on Web 01/14/2009
Direct Observation of Solvation Dynamics in an Aqueous Reverse Micellar System Containing Silver Nanoparticles in the Reverse Micellar Core Palash Setua, Rajib Pramanik, Souravi Sarkar, Debabrata Seth,† and Nilmoni Sarkar* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India ReceiVed: NoVember 21, 2008; ReVised Manuscript ReceiVed: December 19, 2008
In this work we have reported the synthesis of silver nanoparticles in aqueous AOT (dioctylsulfosuccinate, sodium salt) n-heptane reverse micelles and then carried out the study of solvation dynamics keeping the system unaltered, i.e., inside the reverse micelles containing silver nanoparticles. Solvation dynamics and anisotropy studies showed that they were highly affected and became slower in reverse micelles containing silver nanoparticle in comparison to the pure reverse micellar system. Introduction The research related to reverse micelles or the microemulsion field is neither a new nor an emerging area of research.1 But still it is blooming and astonishing us by giving birth to newer areas of research and application. Today it is being applied in medicinal chemistry,2 preparative chemistry,3 catalysis,4 nanoparticle synthesis, and compartmentalization of biochemical reaction,5a,b and even solving the world’s biggest problem, global warming, by producing a more efficient and more environmentally friendly fossil fuel.6 Among these, the most interesting and growing field is nanoparticle synthesis. The first application of a reverse micelle as a template in nanomaterial synthesis was done by Boutonnet et al.7 26 years ago. Within this small period, different researchers have published thousands of works. But to the best of our knowledge all the studies follow the same trend: first they gather the information about the pure reverse micellar system, then they apply these to synthesize the nanomaterial inside it, and finally they conclude that the reverse micellar media is acting as a “passive nanocage” and thus “templating” the structure and morphology of the synthesized materials. Because reverse micelles do not break during and after synthesis, they act as a “templating media” and protect the nanoparticle inside its core, but at the same time, because it is inevitable for micellar accommodation, nanoparticles will exert some force on the aggregates. So we might describe the situation as “when a reverse micelle is not a regular reverse micelle”. Moreover, the reverse micellar radii are very sensitive to the small amount of solute present inside the reverse micellar core.8 This also implies that from the initial moment, reverse micelles have been modified (as we incorporated the precursor for the nanoparticle synthesis) and the particle synthesis is taking place in an environment that has already been changed. As a response to this, numerous cases of experimental deviation from the proposed correlation were reported.9,10 The most questionable things in this respect are why the nanoparticle size is larger than the reverse micellar size11 and how one gets different shaped particles when the reverse micelle itself in most of the † Present address: Indian Institute of Technology Patna, Navin Government Polytechnic Campus Patliputra Colony, Patna 800 013, Bihar, India. E-mail:
[email protected]. Fax: 91-3222-255303.
10.1021/jp810229m CCC: $40.75
SCHEME 1: Structure of Coumarin-153
cases is spherical or sometimes elliptical.12 Effects of some macroscopic variables like composition, pH, concentration, ionic strength, and heat content have been studied.13 We think that this feedback interaction, which is triggered by the nanoparticle formation inside the reverse micellar core, affects the structure of the parent microemulsion and therefore is of deep concern. Bernickel et al. first synthesized the silver nanoparticle in microemulsion.14 Castner, Maroncelli, and Flemming developed a technique known as dynamics of polar solvation.15a-c This technique has successfully been applied to study the dynamics of a water molecule inside the reverse micelle and other constrained environments.16a-f In this Letter, we have applied both techniques: we have synthesized silver nanoparticles in a water/aerosol-OT/n-heptane reverse micelle, then investigated the solvation dynamics, and finally compared the result with the pure aqueous AOT reverse micelle in n-heptane. Experimental Section Coumarin-153 (C-153) (laser grade from Exciton) was used as received (structure in Scheme 1). AOT (dioctylsulfosuccinate, sodium salt, Aldrich) was purified by a standard procedure.17 n-Heptane of spectroscopic grade (spectrochem) was freshly distilled over calcium hydride (spectrochem) before use. DGlucose was purchased from Sisco Research Laboratories, Mumbai and was used without further purification. The concentration of C-153 maintained in all the measurements was 5 × 10-5 M and that of AOT was 0.09 M. Throughout the work, we used triple distilled water where it was required. For the reverse micelles preparation, we prepared a 0.09 M solution of AOT in n-heptane by direct weighing and then made the probe concentration 5 × 10-5 M in this solution. To reach 2009 American Chemical Society
5678 J. Phys. Chem. B, Vol. 113, No. 17, 2009 the above mentioned probe concentration, first we prepared a stock solution of the probe in methanol having a concentration of 2 × 10-3 M by direct and accurate weighing. Then we calculated the volume requirement of this stock solution to reach the concentration of 5 × 10-5 M for a specific volume of n-heptane and AOT solution. Next we poured that volume into a volumetric flax using a microliter syringe. After that, we completely removed the methanol by proper drying (in a dry atmosphere under mild hot air blow). Finally, we added that specific volume of n-heptane and AOT mixture into the flax. Each time we took the required amount of this solution and added the required volume of water to reach the specific w0 value (w0 ) molar ratio of polar solvent to surfactant) for normal reverse micellar solution preparation. For nanoparticle synthesis we prepared two stock solutions of silver nitrate (having concentration 1 M) and glucose (having concentration 1.1 M). Using these, the previously mentioned probe [C-153, concentration 5 × 10-5 M], and the AOT and n-heptane solution, we prepared two independent reverse micellar solutions with the same w0 value (w0 ) 4). In one solution we kept the silver ion, and in the other solution, we kept glucose, such that the final concentrations of Ag+ and glucose were 1 × 10-3 and 1.1 × 10-3 M, respectively, after mixing. The volume requirement of the Ag+ and glucose stock solution was less than the total volume of water required to reach the w0 value of 4. In each case, the remaining portion of the water requirement was fulfilled by direct addition of triple distilled water. Finally, we mixed these two solutions. For nanoparticle synthesis we kept this final reverse micellar solution in a temperature bath for 2 days (∼52 h), keeping the temperature fixed at 56 ( 2 °C. During this period we took necessary precautions and tightly sealed the mouth of the pot such that no liquid can escape by vaporization. In the final state, the colorless solution turned a beautiful yellow color due to the formation of silver nanoparticles. Finally, we removed the solutions from the temperature bath and kept them at normal temperature for about 8 h. Then we used this solution for solvation dynamics study. TEM analysis was also done from this solution. To get an idea how many reverse micelles contained nanoparticles and how many of them remained in the normal state (without nanoparticle), we carried out an approximate calculation. A previous study by Monoj et al.19 on the isooctane/AOT/water reverse micellar system showed that the CMC (critical micellar concentration) value increases with increasing w0, from 1.7 mM at w0 ) 1.6 to 2.4 mM at w0 ) 2.9, and the aggregation number in the pure aqueous AOT reverse micellar system was ∼120 ( 20. In our system we used n-heptane instead of isooctane. So it is reasonable to assume the same aggregation number for an approximate calculation. As evidence, we measured the CMC value of the pure n-heptane/AOT/water system at w0 ) 4 using pyrene as a probe and constructed the derivative spectra [∆I/ ∆C ) change in pyrene peak one intensity (373 nm, I1)] with a change in surfactant (AOT) concentration. For spectra see Supporting Information (Figure 5). This figure shows a CMC value of ∼3.5 mM. The value obtained is slightly higher because we measure it at a comparatively higher w0 value (w0 ) 4). For approximate calculation we have taken the higher limit of this aggregation number, 140 (we have taken a slightly higher value because in our case we are getting comparatively higher CMC values and carried out the experiment at w0 ) 4). The number of reverse micelles from the calculation was on the order of 1020/liter. The number of nanoparticles, which was calculated from the average radius of the particle, was on the order of 1018/liter. A slight difference was observed. We think that the
Letters
Figure 1. Size distribution histogram (A) of pure water/AOT/n-heptane reverse micelle at w0 ) 4. (B) Same but having Ag+ ion and glucose as precursor.
Figure 2. Inset: silver nanoparticle synthesized in water/AOT/nheptane system at w0 ) 4. The histogram represents the particle distribution profile.
source of this slight deviation lies in our assumption that the aggregation number of AOT is the same and is equal to the aggregation number of the pure isooctane/AOT/water system. The reasons are as follows a. When we added the precursor, the average diameter of the reverse micellar system was increased around 3.5 times (see Figure 1). So, certainly, to give the system stability, the aggregation number will also increase. b. Moreover, after completion of the nanoparticle formation process, the aggregation number will be changed and from common intuition it will be increased to give stability to the nanoparticle containing reverse micelles because nanoparticles have an inherent tendency for agglomeration. From the above consideration, we are thinking that the actual aggregation number will be of higher value than we have taken for approximate calculation. The higher aggregation number will decrease the number of reverse micelles, and in that situation, the difference will be much less and almost ideal where both values will be of the same order considering experimental error. Unfortunately, we cannot apply those processes19 in our system, because they are used to calculate the aggregation
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J. Phys. Chem. B, Vol. 113, No. 17, 2009 5679
TABLE 1: Rotational Relaxation parameter of C-153 in Normal Water Reverse Micelle and in Water Reverse Micelle Containing Silver Nanoparticle system
w0
r0
a1r
a2r
τ1r (ns)
τ2r (ns)
〈τr〉a (ns)
C-153 in water/AOT/n-heptane reverse micelle C-153 in water/AOT/n-heptane reverse micelle with silver nanoparticle
4 4
0.24 0.14
0.73 0.56
0.27 0.44
0.100 0.140
0.840 1.020
0.300((0.01) 0.530((0.01)
a
〈τr〉 ) a1rτ1r + a2rτ2r.
number of pure reverse micellar media. Our system is much more complicated, and such an attempt would make the system perturbed and unstable. The details of TEM measurements, anisotropy, and timeresolved studies are described in the Supporting Information. Results and Discussion First, we mention the reverse micelle and nanomaterial that we prepared. They are not directly related to the solvation dynamics study but play a decisive role in creating the system. Moreover, they can be used in other systems where reverse micellar methods were applied or will be used for nanoparticle synthesis. Figure 1 shows the size distribution histogram of the reverse micelles. The average diameter of the normal water/ AOT reverse micelle was ∼1 nm, but it increased to ∼3.5 nm as we incorporated the precursor, Ag+ ion and glucose (for detail see Supporting Information). The size distribution and the average size of precursor containing reverse micelles were almost equal to the size distribution of the silver nanoparticle synthesized (see Figure 2). The average diameter of the particle was ∼3.7 nm (deviation is well within experimental error and matching is excellent). The details of the steady state absorption spectra in different environments are given in the Supporting Information (Figure 1). Before going to the direct discussion of the solvation dynamics data, we shed some light on the environment of the probe molecule through time-resolved fluorescence anisotropy (see Supporting Information for details). The anisotropy data are given in Table 1. In the case of pure water/AOT reverse micelles, we were getting two components; the faster one was ∼100 ps, very similar to the component of pure n-heptane.17 The first component arises from the probe molecule residing in the pool of the reverse micelle, and the slower one was around 840 ps, which was from the probe molecule present at the reverse micellar interface near the AOT headgroup. In the reverse micellar system containing silver nanoparticle, both components became slow. The faster component became 140 ps, and the slower component became 1.02 ns. Moreover, the contribution of the faster component decreased from 73% to 56% and the slower component increased to 44% from 27%. These certainly signify that in reverse micelles containing silver nanoparticles, probe molecules are experiencing a further restricted environment and shifting in position to the more restricted interfacial region. This is due to the strain exerted by the nanoparticle residing in the core. The anisotropy decay curves also show a slower decay profile in reverse micellar system containing silver nanoparticle (Figure 3). Figure 4 shows the decay profile of solvent correlation function, C(t), and the values are summarized in Table 2. From the values and the nature of the curve, it is clear that both follow the biexponential decay path. When the system was changed from normal reverse micelles to reverse micelles containing silver nanoparticle, both values (from 11.9 to 15.9 ns) and the contribution of the slow component increased (55% to 89%) largely. Though the value of the faster component decreases slightly (from 3.9 ns to 3.0
Figure 3. (i) Decay profile of anisotropy in water/AOT/n-heptane reverse micellar system at w0 ) 4. (ii) Same condition but in the presence of silver nanoparticle.
Figure 4. (i) Decay of solvent correlation function C(t) in water/AOT/ n-heptane reverse micellar media at w0 ) 4. (ii) Same as (i) but in the presence of silver nanoparticle.
ns), the value can be considered as constant and unchanged with respect to the other. More importantly, the contribution of the faster component decreased significantly (from 45% to 11%). All these things simultaneously produced a dramatic change in solvation time and finally the average solvation time increased to a value of 14.45 ns, which is surprisingly large and almost twice the value of solvation time (8.36 ns) in a normal reverse micelle. Details of the solvation dynamics study are given in the Supporting Information. To the best of our knowledge, still there is no reporting of a solvation dynamics study in reverse micelles containing nanoparticles. So we cannot compare our result and make further assumptions on the system. During our literature survey, we
5680 J. Phys. Chem. B, Vol. 113, No. 17, 2009
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TABLE 2: Decay Parameter of C(t) for C-153 in Normal Water Reverse Micelle and in Water Reverse Micelle Containing Silver Nanoparticle system
w0
∆νa (cm-1)
a1
a2
τ1 (ns)
τ2 (ns)
〈τ〉 (ns)
C-153 in water/AOT/n-heptane reverse micelle C-153 in water/AOT/n-heptane reverse micelle with silver nanoparticle
4 4
1615 1820
0.45 0.11
0.55 0.89
3.970 3.000
11.950 15.870
8.36((0.03) 14.45((0.03)
a
∆ν ) ν0 - ν∞.
found two references of Levinger et al.18a,b where they attached the probe molecule at the surface of the ZrO2 nanoparticle and carried out the study by dispersing them in the bulk solvent. Because they attached the probe molecule on the surface, they got lower solvation times and higher anisotropy values. But there is no similarity between those works and the present work because here the probe molecule is inert and has no such functional group that can be used for surface attachment, the system is different and the aim is different, and finally, we did not tamper with any component of the system including nanoparticles and kept them in their natural habitation. It is wellknown that reverse micelles have widespread use in extraction, synthesis, purification, and medicinal chemistry and can act as a nanoreactor and that, similarly, nanoparticles have excellent application in catalysis and drug delivery and have paramount importance for their utilization as a basic unit from both fundamental and applied consideration. So such a study can seal the power in a single system. It will certainly be helpful for the individual system and will also increase their efficiency. Currently, further work is going on to improve and expand its area to different conditions and in different systems. Acknowledgment. N.S. is indebted to the Department of Science and Technology (DST) and Council of Scientific and Industrial Research (CSIR) for generous research grants. R.P. and S.S. are thankful to CSIR for research fellowships. Supporting Information Available: The experimental procedure for nanoparticle synthesis, time-resolved fluorescence and anisotropy decays are given. The absorption spectra, timeresolved fluorescence decays, time-resolved emission spectra (TRES) are also shown. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Mayers, D. Surfactant Science and Technology; VCH: New York, 2006. (b) Fendler, J. H.; Ed.; Nanoparticle and Nanostructured films: preparation characterization and application; Wiley-VCH: Weinheim, 1998.
(2) Carlile, K.; Rees, G. D.; Robinson, B. H.; Steer, T. D.; Sevensson, M. J. Chem. Soc., Faraday Trans. 1996, 92, 4701. (3) (a) Lopez-Quintela, M. A.; Tojo, C.; Blanco, M. C.; Garcia, R. L.; Leis, J. R. Curr. Opin. Colloid Interface Sci. 2004, 9, 264. (b) Klyachko, N. L.; Levashov, A. V. Curr. Opin. Colloid Interface Sci. 2003, 8, 179. (4) Biasutti, M. A.; Abuin, E. B.; Silber, J. J.; Correa, N. M.; Lissi, E. A. AdV. Colloid Interface Sci. 2008, 136, 1. (5) (a) Correa, M. N.; Durantini, E. N.; Silber, J. J. J. Phys. Org. Chem. 2006, 19, 805. (b) Falcone, R. D.; Correa, N. M.; Biasutti, M. A.; Silber, J. J. Langmuir 2000, 16, 3070. (6) Lin, Y. C.; Lee, W. J.; Chao, H. R.; Wang, S. L.; Tsou, T. C.; Guo-Ping, C. C.; Tsai, P. J. EnViron. Sci. Technol. 2008, 42, 3849. (7) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (8) (a) Pileni, M. P.; Zemb, T.; Petit, C. Chem. Phys. Lett. 1985, 118, 414. (b) Pileni, M. P. J. Phys. Chem. C 2007, 111, 9019. (c) Silber, J. J.; Biasutti, A.; Abuin, E.; Lissi, E. AdV. Colloid Interface Sci. 1999, 82, 189. (9) Barnickel, P.; Wokaun, A.; Sager, W.; Eickel, H. F. J. Colloid Interface Sci. 1992, 148, 80. (10) Dresco, P. A.; Zaitsev, V. S.; Gambino, R. J.; Chu, B. Langmuir 1999, 15, 1945. (11) Hua, R.; Zang, C.; Shao, C.; Xie, D.; Shi, C. Nanotechnology 2003, 14, 588. (12) Holmberg, K. J. Colloid Interface Sci. 2004, 274, 355. (13) Uskokovic, V.; Drofenik, M. AdV. Colloid Interface Sci. 2007, 133, 23. (14) Barnickel, P.; Wokaun, A. Mol. Phys. 1990, 69, 1. (15) (a) Castner, E. W., Jr; Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1986, 90, 5623. (b) Maroncelli, M.; MacInnis, J.; Fleming, G. R. Science 1989, 243, 1674. (c) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471. (16) (a) Sarkar, N.; Das, K.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem. 1996, 100, 10523. (b) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. ReV. 2000, 100, 2013. (c) Bhattacharyya, K. Acc. Chem. Res. 2003, 36, 95. (d) Bhattacharyya, K.; Bagchi, B J. Phys. Chem. A 2000, 104, 10603. (e) Moilanen, D. E.; Levinger, N. E.; Spry, D. B.; Fayer, M. D. J. Am. Chem. Soc. 2007, 129, 14311. (f) Bagchi, B. Chem. ReV. 2005, 105, 3197. (17) Hazra, P.; Chakrabarty, D.; Sarkar, N. Chem. Phys. Lett. 2003, 371, 553. (18) (a) Pant, D.; Levinger, N. E. Chem. Phys. Lett. 1998, 292, 200. (b) Pant, D.; Levinger, N. E. J. Phys. Chem. 1999, 103, 7846. (19) Manoj, K. M.; Jayakumar, R.; Rakshit, R. Langmuir 1996, 12, 4068.
JP810229M
