Synthesis of Silver Nanoparticle Inside the Nonaqueous Ethylene

May 14, 2010 - Zia Ul Haq Khan , Amjad Khan , Yongmei Chen , Arif ullah Khan , Noor S. ... Madhurima Paul Chowdhury , Kaushik Kundu , Soumik Bardhan ...
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J. Phys. Chem. B 2010, 114, 7557–7564

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Synthesis of Silver Nanoparticle Inside the Nonaqueous Ethylene Glycol Reverse Micelle and a Comparative Study to Show the Effect of the Nanoparticle on the Reverse Micellar Aggregates through Solvation Dynamics and Rotational Relaxation Measurements Palash Setua,† Rajib Pramanik,† Souravi Sarkar,† Chiranjib Ghatak,† S. K. Das,‡ and Nilmoni Sarkar*,† Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB, India, and Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata-700 064, WB, India ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: April 28, 2010

In this work, we have reported the application of less familiar ethylene glycol-AOT reverse micelle for the synthesis of silver nanoparticle using glucose as mild reducing agent and isooctane as the continuous media. We have also studied the pure ethylene glycol-AOT reverse micelle and the perturbed reverse micelle (containing silver nanoparticle in its womb) through solvation dynamics measurement using steady-state and time-resolved fluorescence spectroscopy. Finally, we compared both of the results to get the valuable information about the perturbed reverse micellar system containing silver nanoparticle. Through the work, we found that in the pure reverse micellar system, with increasing ethylene glycol loading, solvation time was decreasing and anisotropy value became slower. In the perturbed reverse micellar system (containing silver nanoparticle) having the same environmental state, that is, at the same ethylene glycol content, solvent and rotational relaxation became slower and faster, respectively. 1. Introduction Surfactants are a special class of molecules having one polar group on one end and a long nonpolar hydrocarbon part on the other end. Here we used AOT (sodium bis-2-ethylhexyl sulfosuccinate) as surfactant for its good surfactant properties and well-known nature. It has been applied in many areas. It has been widely used for reverse micelle preparation.1-4 Structural concept of the reverse micelle is very simple. When a surfactant molecule is dissolved in a nonpolar medium, its nonpolar part interacts with the nonpolar solvent and tries to shade its polar part from the nonpolar dispersant. This results the formation of a special assembled structure where all hydrocarbon parts remain directed toward the nonpolar medium (outside) and polar end inside. These types of aggregates are known as reverse micelle.5-7 Reverse micellar (RM) aggregates are interesting because of this polar core, which can be used for different application. It has been used in nanomaterial synthesis,8,9 active catalyst preparation,10 hydrogel formation,11 nanogel creation,12 protein separation and refolding,13 enzyme stabilization and activity enhancement,14,15 preparative chemistry,16 and in supercritical liquid media.17 Recently it is being used in medicinal chemistry, blood research, and even in cancer targeted drug delivery.18-21 In all application, we have to incorporate some material inside the reverse micelle. Incorporated materials perturb the normal environment and perturbed state produces “feedback” effect. Such effect will play a significant role in RM nanoparticle preparation22 because here not only we incorporate some material, but they form some other type of rigid material, nanoparticles. This type of system (nanoparticle inside the reverse micelle) is known as the hybrid system because the applicability of two systems remains combining in one system. * Corresponding author. E-mail: [email protected]. Fax: 913222-255303. † Indian Institute of Technology. ‡ Variable Energy Cyclotron Centre.

The beauty of the hybrid system is that it increases the efficiency and feasibility of success many times. This new idea has recently been used to create the “hydrogel nanoparticle system”,12 where characteristics of hydrogel (hydrophilicity, high water content) have been combined with the nanoparticle (desired properties like small size, magnetism). Such feasibility is also present in this noble field. As we have already stated, reverse micelles can be used in protein folding, enzymatic and normal reaction, storage, and delivery of medicine, and nanoparticles are being used in catalysis in “target locked” approach. Therefore, it is possible to combine the “reaction” with the “catalysis” and “medicinal application” with “target specific” approach.

Figure 1. DLS size distribution of the reverse micelles at w ) 1 25 °C: (A) pure ethylene glycol-AOT reverse micelle having no precursor and (B) the same containing precursor, that is, silver ion 5.34 × 10-3 M and glucose 5.67 × 10-3 M.

10.1021/jp1008048  2010 American Chemical Society Published on Web 05/14/2010

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SCHEME 1: Structure of Coumarin 480

SCHEME 2: Structure of AOT Molecule

Several works have been done in this field, but no one is concerned about the fate of the RM aggregates after its application. We are working to explore this unknown field. One part of our work is related to aqueous reverse micelle that we have already published.23 Another part is related to nonaqueous reverse micelle where polar solvents other than water are used. This is the first work related to the nonaqueous category. Lastly, we chose ethylene glycol as the polar solvent because recently it is receiving increasing interest in nonaqueous category and has been applied in different application from photophysical studies to nanoreactor and enzymatic reaction.24-27 Novelty of this RM system and its structural feature have been studied by Falcone et al16b and Durantini et al16c in their recent work. Finally, we also compare our present results with previous results obtained in aqueous RM study.23 2. Experimental Section 2.1. Materials Used. Coumarin-480 (C-480) (laser grade from Exciton) was used as received. (For structure, see Scheme 1.) We purchased 4-tert-butyl phenol from Loba Chemie, Mumbai, India and used as received. AOT (dioctylsulfosuccinate, sodium salt, Aldrich) was purified by standard procedure.28 (For structure, see Scheme 2.) Cetylpyridinium chloride monohydrate (CPC) (Aldrich) and 8-anilino-1-naphthalenesulfonic acid hemimagnesium salt hydrate (ANS) (Sigma-Aldrich) were used as received. 4-tert-Butyl phenol, CPC, and ANS were used for the CMC and aggregation study, whose results are given in the Supporting Information. Isooctane of spectroscopic grade (spectrochem) was freshly distilled over calcium hydride (spectrochem) before use. Ethylene glycol, silver nitrate, and D-glucose (analytical grade) were purchased from Sisco Research Laboratories (India) and was used without further purification. Concentration of C-480 was maintained 5 × 10-5 M in all the measurements, and that of AOT was 0.089 M. 2.2. Reverse Micellar Solution Preparation. For the normal RM solution (reverse micelles having no nanoparticle inside its core) preparation, we followed the following procedure. Initially, we prepared two stock solutions, 0.089 M solution of AOT in isooctane and 2 × 10-3 M solution of C-480 in methanol, and using these two stock solutions, we prepared 0.089 M AOT RM solution in isooctane having probe (C-480) concentration 5 × 10-5 M. Using this probe (C-480), we carried out both the solvation dynamics and the anisotropy measurements. The detailed procedure is given in ref 23. For each normal RM system measurement, we took required volume of this mixture and added required volume of ethylene glycol to attain the desired w value. We calculated the required volume

Setua et al. of ethylene glycol to a specific volume of AOT-isooctane and probe mixture depending on the w value calculation procedure. The w value represents the molar ratio of polar solvent to surfactant, that is, here w ) ([ethyleneglycol])/([AOT]). 2.3. Preparation of Reverse Micellar Solution Containing Silver Nanoparticle. For the synthesis of nanoparticle inside the RM core, we prepared two stock solutions, one is 3.2 M silver nitrate in ethylene glycol and the other is 1.7 M glucose in ethylene glycol. Using these two and the previously mentioned C-480 [concentration 5 × 10-5 M], AOT, and isooctane solutions, we prepared two independent same w valued RM solutions. In one RM solution, we kept the silver ion, and in the other we kept glucose such that after mixing, the final concentration of silver ion and glucose became 5.34 × 10-3 and 5.67 × 10-3 M, respectively. The rest of the details are given in the Supporting Information. Another important point that is relevant to this work is that the synthesized nanomaterials were nonfluorescent. Because the nanoparticles were nonfluorescent, the possibility of slight quenching (which is expected from Figure 5) that normally occurs in the presence of nanoparticle will not affect the solvation process. For clarification, we are giving the fluorescence spectra (Supporting Information, Figure 10) recorded in pure RM solution containing only silver nanoparticle in the following Figure. Excitation wavelength was 408 nm. 2.4. Instruments Used. A Simadzu (model UV1601) UV-vis spectrophotometer and a Spex Fluorolog-3 (model FL3-11) spectrofluorimeter were used to collect absorption and emission spectra, respectively. Fluorescence spectra were corrected for the spectral sensitivity of the instrument. For the steady-state experiment, all samples were excited at 408 nm. The details of the time-resolved fluorescence setup were described in our previous publication.28 In brief, the samples were excited at 408 nm picosecond diode laser (IBH, nanoled), and the signals were collected at the magic angel (54.7°) using a Hamamatsu MCP PMT detector (3809U). The instrument response function of our setup was ∼90 ps. The same setup was also used for the anisotropy measurement. The analysis of both the time-resolved decay and the anisotropy data were carried out using the IBH DAS 6 decay analysis software. The temperature was kept at 25 °C for all measurements except dynamic light scattering (DLS). DLS measurements were performed using a Malvern Nano ZS instrument (model No-ZEN3600) equipped with a thermostated sample chamber. All experiments were carried out using a 4 mW He-Ne laser (λ ) 632.8 nm) at a scattering angle of 173°. DLS measurements were performed at two temperatures, 25 and 56 °C. TEM pictures were taken using a (JEOL) JEM-2100 transmission electron microscope operating at 200 keV. Samples of TEM were prepared by blotting a carbon-coated (50 nm carbon film) Cu grid (300 mesh, Electron Microscopy Science), with a drop of the Ag-micro emulsion and allowed to dry. We constructed histograms manually by measuring the individual particle diameter with the help of Imaje-J software. We have measured 240-250 particles for the construction of the size distribution histogram for each w value. 3. Results 3.1. DLS Measurement of the Reverse Micelles and TEM Picture Results. We carried out the DLS study on the pure ethylene glycol/AOT/isooctane system to get an overall idea about its structural feature and characteristics. All DLS measurements were done at two different temperatures, 25 and 56 °C. At 25 °C, in case of pure reverse micelle, we obtained mean diameter value of 5 and 11.3 nm at w ) 1 and 2, respectively.

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Figure 4. Size distribution histogram of silver nanoparticle from TEM image analysis prepared in AOT/ethylene glycol/isooctane reverse micellar system at w ) 2. Figure 2. DLS size distribution of the reverse micelles at w ) 2 at 25 °C: (A) pure ethylene glycol-AOT reverse micelle having no precursor and (B) the same containing precursor, that is, silver ion 5.34 × 10-3 M and glucose 5.67 × 10-3 M.

TABLE 1: Steady-State Absorption and Emission Spectra of C-480 in Pure Isooctane, Ethylene Glycol, and AOT/ Ethylene Glycol/Isooctane Reverse Micellar System at Different w Value and in Different Environment probe used C-480 excitation wavelength ) 408 nm

Figure 3. Size distribution histogram of silver nanoparticle from TEM image analysis prepared in AOT/ethylene glycol/isooctane reverse micellar system at w ) 1.

The respective value became 3.9 and 8.8 nm at 56 °C. The same experiments were done on the precursor-loaded reverse micelles (containing silver ion and glucose). At 25 °C, we observed a value of 4.1 and 8.7 nm at w ) 1 and 2, respectively, and the values were decreasing to 3.5 and 6.1 nm at 56 °C. The results are shown in Figures 1 and 2. RM size distribution obtained at 56 °C from the DLS measurement is given in Figure 6 of the Supporting Information. To get an idea about the shape, size, and morphology of the synthesized particle, we carried out the TEM picture analysis by direct measurement of the particle diameter and constructed the particle size distribution histogram. During this analysis, large agglomerates were avoided. Particle size distribution histogram (Figures 3 and 4) constructed at w ) 1 and 2, respectively, showed almost the same particle diameter, ∼3.6 nm. 3.2. Steady-State Absorption and Emission Results. The absorption and emission spectra of C-480 were taken in

system

λabs (max)/nm

λemi (max)/nm

isooctane ethylene glycole AOT/ethylene glycol/isooctane w)1 w)2 AOT/ethylene glycol/isooctane with silver nanoparticle w)1 w)2

363 393

421 478

363 363

455 465 481 500

isooctane, ethylene glycol, and in RM solution containing nanoparticle and without nanoparticle. The values are listed in Table 1. In pure isooctane solvent, absorption and emission maxima of C-480 were at 363 and 421 nm. The same values were shifted to 393 and 478 nm when we used ethylene glycol instead of isooctane. In the case of AOT-ethylene glycol RM solution, we observed an absorption maximum at 363 nm. But the emission maxima were situated at 455 nm at w ) 1 and were shifted to 465 nm at w ) 2. We did not observe the probe absorption peak in RM solution containing silver nanoparticle. A large intensity and broad absorption profile of silver nanoparticle were centered at 420 nm but clearly got emission peak. Emission peak maxima were centered at 481 and 500 nm at w ) 1 and w ) 2, respectively. Absorption and emission spectra of the RM solutions are shown in Figure 5. For isooctane and ethylene glycol absorption and emission spectra, see Figure 3 of the Supporting Information. 3.3. Time-Resolved Anisotropy Results. A qualitative idea already obtained from the steady-state analysis that the stable reverse micelles are forming and the probe molecules are successfully sensing the changed environment. A more accurate and precise result about the probe position can be obtained from the time-resolved anisotropy analysis. Time-resolved fluorescence anisotropy was calculated using the following equation

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r(t) )

I|(t) - GI⊥(t) I|(t) + 2GI⊥(t)

Setua et al.

(1)

where G represents the correction factor of the detector sensitivity to the polarization direction of the emission. In our instrument, the value of G is 0.6. I|(t) and I⊥(t) are the fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. The anisotropy decay parameters are listed in Table 2. The Table clearly shows that we were getting a anisotropy decay time of 89 ps in pure isooctane and the time was increasing to 804 ps when we used ethylene glycol as a solvent. In the case of dry reverse micelles, we observed a decay time of 1.37 ns. This value was increasing with increasing w value in the case of pure ethylene glycol reverse micelle and became 2.36 ns at w ) 1 and 2.72 ns at w ) 2. (See Figure 6.) In the case of silver-nanoparticle-containing reverse micelle, we got anisotropy decay times of 1.79 and 2.63 ns at w ) 1 and 2, respectively. For the comparison of the anisotropy decay in the presence and in the absence of silver nanoparticle at particular w value, see Figure 8A,B, respectively. 3.4. Dynamics of Solvent Relaxation. We carried out the solvation dynamics study in pure AOT-ethylene glycol reverse micelles at different w values by measuring the time-resolved decays at different wavelength. To get a closer look at the timeresolved decay spectra at different wavelength, we have added some representative decays at different wavelength in Figure 4 of the Supporting Information. The time-resolved emission spectra (TRES) were constructed following the procedure of Fleming and Maroncelli.32 The solvent relaxation dynamics was monitored by the solvent response function defined as

c(t) )

ν(t) - υ(∝) ν(0) - υ(∝)

(2)

where υ(t), υ(0), and υ(∞) are the peak frequencies at time t, time zero, and time infinity. The peak frequencies were calculated from the TRES. A representative TRES of the pure

Figure 5. Absoption and emission spectra of C-480 in AOT-ethylene glycol reverse micellar solution with and without silver nanoparticle. Significance of the symbol used: absorption at w ) 1 without nanoparticle (s0s), absorption at w ) 2 without nanoparticle (sOs), absorption at w ) 1 with nanoparticle (s9s), absorption at w ) 2 with nanoparticle (sbs), and dot lines with the same symbol represent the respective emission spectra.

AOT-ethylene glycol reverse micelles are shown in Figure 5 of the Supporting Information. Solvent relaxation time obtained from the solvent correlation function (C(t)) are summarized in Table 3. The representative decays of C-480 in pure AOT-ethylene glycol system at w ) 1 and 2 are shown in Figure 7. We were getting an average solvation time of 7.94 ns at w ) 1. With increasing ethylene glycol content, the average solvation time was decreasing, and we got a value of 4.27 ns at w ) 2. In the case of silver nanoparticle containing ethylene glycol reverse micelle, we observed an average solvation time of 8.91 ns at w ) 1 and 5.05 ns at w ) 2, respectively. Because our instrument response function is broad (∼90 ps), we calculate the υ(0) frequency according to the concept developed by Fee and Maroncelli in their work.39 Using that value, we calculated the percentage of the missing component observed in pure AOT/ ethylene glycol/isooctane system. The percentage of the missing component was ∼42% at w ) 1. The value was increasing with ethylene glycol loading and became ∼47% at w ) 2. This will certainly give a clear idea to the reader what percentage of the solvation dynamics we are missing and which components are being taken care of in this work. To get the details of the probe location, we carried out the partition coefficient study of C-480 in this RM system using the method applied by Novaira et al.40 (for details, see the Supporting Information), which was clearly showing that more and more probe molecules were going inside the RM core with the increase in w value. 4. Discussion DLS results showed us that AOT/ethylene glycol/isooctane system was creating a stable RM system. It was also following the conventional RM nature. The size of the RM was increasing with increasing ethylene glycol content (5 nm at w ) 1 and 11.3 nm at w ) 2 at 25 °C). Similar observation was also reported by Falcone et al. in their recent DLS study performed on nonaqueous RM system.41 At 56 °C, we also observed the presence of stable RM aggregates. (See Figure 6 of the Supporting Information.) Under this condition, the system was also following its normal behavior, that is, with increasing w value, size was also increasing (3.9 nm at w ) 1 and 8.8 nm at w ) 2). At this temperature, it was also supporting the formation of silver nanoparticle in its core, which was another direct evidence of its stable nature. The only effect that was produced by the temperature on the RM aggregates was a decreasing tendency in its mean diameter. Precursor loading (incorporation of silver ion and glucose) was also affecting the normal RM aggregates in the same way at both temperatures and at both w values. We were getting lower mean aggregates diameter value in precursor-loaded RM system than their respective counterpart having pure ethylene glycol. At 25 °C, the size was shrinking from 5 to 3.9 nm at w ) 1. The same tendency was observed at 56 °C temperature at w ) 1, the size was decreasing from 4.1 to 3.5 nm. Similar trend was also followed at w ) 2 at both temperature values. For detail of results, see Table 1 of the Supporting Information. The reason for this shrinking was probably due to the increase in the ionic strength inside the RM core after the addition of the precursor. Change of ionic strength can affect the RM size and was also reported by Simorellis et al.29 in their recent work on aqueous RM system. The image obtained from the TEM micrograph clearly showed that the synthesized nanoparticles were spherical in shape. Histogram analysis of the TEM micrograph (Figures 3 and 4) showed almost the same average particle diameter ∼3.6 nm. This observation signifies that synthesized material size was regulated by the templating effect of the stable ethylene glycol

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TABLE 2: Rotational Relaxation Time (τr) of C-480 in AOT/Ethylene Glycol/Isooctane Reverse Micelles at Different w Value in Different Environment system isooctane ethylene glycol AOT/isooctanec AOT/ethylene glycol/isooctane AOT/ethylene glycol/isooctane with silver nanoparticle a

w w w w w

w

r0

a1r

τ1r (ns)

) ) ) ) )

0.2 0.39 0.38 0.29 0.32 0.29 0.29

1 1 0.38 0.20 0.31 0.22 1

0.089 0.804 0.68 0.505 0.533 0.534 2.63

0 1 2 1 2

a2r

0.62 0.80 0.69 0.78

τ2r (ns)

1.80 2.360 3.703 2.145

(ns)a,b 0.089 0.804 1.37 2.36 2.72 1.79 2.63

) a1rτ1r + a2rτ2r b Error in measurement is (5%. c Values are taken from ref 31.

Figure 6. Decays of fluorescence anisotropy r(t) of C-480 in pure AOT/ethylene glycol/isooctane system: 9, w ) 1; O, w ) 2.

reverse micelles. The w-independent nature of the synthesized material is a beauty of the mild reduction condition and nonaqueous reverse micelles.30 Absorption and emission peaks of C-480 in isooctane were observed at 363 and 421 nm. They were shifted to 393 and 478 nm, respectively, in the case of ethylene glycol solvent. This clearly showed the solvent-sensitive behavior of the C-480 molecule. In the case of pure ethylene glycol RM solution, the absorption peak was situated at 363 nm, but we observed emission peak maxima at 455 nm at w ) 1, and the value was shifted to 465 nm at w ) 2. Also, we observed a clear rise in the red end shoulder of the absorption peak, which indicated that with increasing ethylene glycol loading, more and more probe molecules were entering into the RM core. In the case of nanoparticle-containing RM system, absorption peak of C-480 was masked by the broad absorption profile of the silver nanoparticle, but we were getting the same trend in the emission spectra (peak maxima at 481 nm at w ) 1 and at 500 nm at w ) 2). Steady-state analysis already showed us that the stable RM aggregates were forming and the probe molecules were successfully sensing the changed environment. For a more precise interpretation of the probe location, we carried out the timeresolved anisotropy analysis. We got an anisotropy value of 89 ps in pure isooctane, and the value was increasing to 804 ps when we used ethylene glycol. This result was a direct consequence of the higher viscosity of the ethylene glycol solvent. This also clearly indicated the highly sensitive nature of the time-resolved anisotropy value with respect to probe’s local environment. In isooctane AOT mixture, we got an anisotropy value of 1.37 ns. In the case of pure ethylene glycol

RM solution, we obtained anisotropy value of 2.36 ns at w ) 1 and 2.72 ns at w ) 2. High rotational relaxation time in the presence of ethylene glycol clearly indicates that probe molecules were experiencing a changed environment that was certainly different from pure isooctane solvent or the dry RM solution. Another important observation was that with increasing ethylene glycol loading, average anisotropy value was increasing (2.36 ns at w ) 1 and 2.72 ns at w ) 2) (See Table 2.) This was happening because of two reasons. First, with the addition of more and more ethylene glycol, viscosity was increasing because ethylene glycol is a viscous solvent. Second, it has very strong hydrogen bonding property, which was increasing the probability of hydrogen bond with the probe molecule. Both of these effects result in an increase in the average anisotropy value. For better understanding and to get a clear idea about the nature of the anisotropy decay, a graphical comparison of representative anisotropy decays of C-480 at w ) 1 and 2 in pure RM solution (without nanoparticle) is shown in Figure 6 where anisotropy decay at w ) 1 is running faster than that at w ) 2. In the case of the RM system containing silver nanoparticle, the anisotropy value was decreasing from 2.36 to 1.79 ns at w ) 1. (See Table 2.) In the case of w ) 2, anisotropy was also decreasing, but the decrease was much less in comparison to the first case (from 2.72 to 2.63 ns). This is because at w ) 1, the size of the reverse micelle is slightly larger than the dimension of the silver nanoparticle. This comparable size of the nanoparticle was highly influencing the AOT-ethylene glycol RM environment and most probably pushing the core solvent to the RM interface. The increase in ethylene glycol percentage in the interface was certainly influencing the interfacial nature and making it more flexible and fluidic. Therefore, probe molecules present in this environment were sensing a less rigid and less strained state, which was reflected in their faster rotational relaxation. This idea was further confirmed by the decrease in the value of slow component from 2.36 (without nanoparticle) to 2.14 ns (with nanoparticle) without changing its relative contribution. At w ) 2, the change was much less ∼100 ps. This is because at higher w value, RM size (∼9 nm) is much larger than the dimension the silver nanoparticle (∼3.5 nm) under the experimental condition at 25 °C. Therefore, in this situation, the presence of the nanoparticle was not influencing the normal RM aggregates so drastically. Moreover, the reverse micelles were encapsulating the nanoparticle inside their womb almost in an unaltered state. For better clarification, we have constructed Figure 8, where we have plotted the variation in the anisotropy decay at w ) 1 and 2 in the presence and in the absence of silver nanoparticle. A similar trend was also observed in the solvation dynamics study and their average solvation time variation. Results of the solvation dynamics study depend on the nature of the polar solvent used. In general, in the case of pure solvent like water23,33,36b,38 or some other nonaqueous solvents,34 we get

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TABLE 3: Decay Characteristics of C-480 in AOT/Ethylene Glycol/Isooctane Reverse Micellar System system AOT/ethylene glycol/isooctane AOT/ethylene glycol/isooctane with silver nanoparticle a

w value

∆ν (cm-1)a

a1

τ1 (ns)

a2

τ2 (ns)

(ns)b,c

w)1 w)2 w)1 w)2

1604 1562 1415 639

0.32 0.54 0.42 0.52

0.864 0.644 0.909 0.888

0.68 0.46 0.58 0.48

11.283 8.541 14.7 9.56

7.94 4.27 8.91 5.05

∆ν ) ν0 - ν∞. b ) a1τ1 + a2τ2. c Error percentage is (5%.

Figure 7. Decays of solvent correlation function C(t) of C-480 in pure AOT/ethylene glycol/isooctane reverse micelle at w ) 1 (0) and 2 (O).

a solvent relaxation time of femtoseconds time scale (except some highly viscous solvent like glycerol,31 ionic liquid, etc.). It is unanimously accepted that this very fast relaxation in pure solvent is mainly contributed by the intermolecular vibrational and librational motions because these processes require very small activation energy,36b but when we incorporated these solvents inside the RM core, their solvent responses were ∼1000 times retarded. This is due to the nanocage confinement of the solvent inside the RM core.3,35,36 This whole retarded solvent response phenomenon was explained by the dynamic exchange model. In this model,37 it was proposed that a dynamic exchange of bound and free solvent molecules always take place. The thermodynamics of this exchange process totally depends on the strength and number of hydrogen bonds between the solvent and surfactant molecules. With the increasing strength of the hydrogen bond, the relative population of bound solvent increases, which is consequently reflected by the higher contribution of the slow component in the overall solvation process. Perfectly, the same types of observations are reflected in our experiment. (See Table 3.) When we incorporated ethylene glycol inside the AOT RM core, its solvent response was very much retarded (7.94 ns at w ) 1). This was happening because large populations of ethylene glycol molecules were forming a strong hydrogen bond with the anionic sulfonate polar headgroup of the AOT molecules. With increasing ethylene glycol content, the average solvation time was decreasing, and we got a value of 4.27 ns at w ) 2, and the reason was with increasing ethylene glycol content, the relative population of the hydrogen bonded molecule was decreasing (as the concentration of AOT remained fixed). (See Figure 7.) These result in a decreased contribution of the slow component. Table 3 is clearly showing a large decrease in slow component (both in absolute value and relative percentage) from 11.28 to 8.54 ns and from 68 to 46% with increasing w value from 1 to 2. This

type of interesting observation was also reported by Chakraborty et al.31 in their unconventional, glycerol-AOT reverse micelles study. In the case of silver nanoparticle containing ethylene glycol reverse micelles, the average solvation time was increasing. At w ) 1, the value became 8.91 ns from the previous value of 7.94 ns (pure reverse micelle system). This result indicates and can also be explained by the changed RM structure, which was triggered by the silver nanoparticle. In the normal RM environment, probe partitioned in the reverse micelles can be present in two areas, one is the RM interface and the other is the RM core. Molecules that remain present in the RM core sense faster solvent response. Molecules in the interface sense slower solvent response because of its strained environment. In the changed or perturbed state, the population of the core solvent molecules was decreasing and was shifted to the RM interface region. In this situation, the contribution of the faster component was decreasing, making the average solvation time slower. For better comparison, we have plotted the solvent response function C(t) at w ) 1 in the presence and in the absence of silver nanoparticle in Figure 9A. At w ) 2, the average solvation time of the system was also increasing in the presence of silver nanoparticle. In the absence of silver nanoparticle, the value was 4.27 ns, and in the presence of silver nanoparticle, the value changed to 5.05 ns, but the change was less (∼750 ps) in comparison with the change observed at w ) 1 (∼1.0 ns). The reason lies in their structural morphology and their extent of perturbation. Because the size of the nanoparticle was much less than the size of the RM aggregates at w ) 2, it was perturbing the core solvent structure much less and thus influencing the interface very little. (For better clarification and comparison, see Figure 9B.) This ultimately shows little influence in the average solvation time. Finally, our present work on the nonaqueous RM system is in good agreement with our previous work on the aqueous RM system23 despite their apparently different nature. In the aqueous RM system, the nanoparticle diameter was comparable but slightly higher than the diameter of the RM aggregates. Therefore, in that system, perturbation was very strong, and we were getting a drastic change in solvation time and in anisotropy value, but here the diameter of the nanoparticle is always less than the diameter of the RM aggregates. Therefore, we are getting a less drastic change, and this concept was further clarified at higher ethylene glycol loading. 5. Conclusions This work certainly proves that nonconventional nonaqueous AOT-ethylene glycol RM media can be successfully applied as a nanoreactor for the nanoparticle synthesis. We observed fully satisfactory nonaqueous type of templating effect in this RM system using this synthesis procedure because with increasing w value, RM size was increasing largely but nanoparticle size remained almost constant. During the precursor addition in the RM media, a small amount of perturbation in the aggregate media was observed. Synthesized nanoparticle inside the RM core also changed the RM environment. This change

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Figure 8. (A) Decays of fluorescence anisotropy of C-480 at w ) 1 in AOT ethylene glycol reverse micellar system in the absence of silver nanoparticle (0) and in the presence of silver nanoparticle (9). (B) The same at w ) 2 in AOT ethylene glycol reverse micellar system in absence of silver nanoparticle (O) and in presence of silver nanoparticle (b).

Acknowledgment. N.S. thanks the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy (DAE), and Department of Science and Technology (DST), Government of India for generous research grants. R.P. and C.G. thank CSIR for the research fellowship. S.S. thanks BRNS for SRF. Supporting Information Available: Preparation procedure of nanoparticles, time-resolved decays, and spectra. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 9. Decays of solvent correlation function C(t) of C-480 in pure AOT/ethylene glycol/isooctane reverse micelle (A) at w ) 1 in the absence (0) and in the presence (9) of silver nanoparticle and (B) at w ) 2 in the absence (O) and in the presence (b) of silver nanoparticle.

is comparatively strong in low ethylene glycol loading (at low w value) and becomes weak with high ethylene glycol loading (at high w value). This is because with increasing ethylene glycol loading, a very large increase in the AOT-ethylene glycol RM size is observed, and smaller size nanoparticle could not perturb the larger RM aggregates strongly. This observation is successfully reflected in the solvation dynamics results. In the low ethylene glycol loading (at w ) 1), in the presence of silver nanoparticle, the solvation process became slower and anisotropy became faster in comparison with the pure ethylene glycol reverse micelle in the same environment. The same trend was also observed at high ethylene glycol loading (at w ) 2), but both of the effects were less prominent. Finally, this RM system retains its characteristics still after the formation of nanoparticle in its core, although in the perturbed state. Therefore, this modified hybrid type of system can be successfully applied for further application.

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