Emission Behavior of 1-Methylaminopyrene in Aqueous Solution of

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Langmuir 2004, 20, 5209-5213

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Emission Behavior of 1-Methylaminopyrene in Aqueous Solution of Anionic Surfactants Sujit Kumar Ghosh, Anjali Pal,† Subrata Kundu, Madhuri Mandal, Sudip Nath, and Tarasankar Pal* Department of Chemistry and Department of Civil Engineering, Indian Institute of Technology, Kharagpur-721302, India Received August 20, 2003. In Final Form: February 1, 2004 A new fluorescent probe, methylamino derivative of pyrene, has been considered to characterize the concentration dependent emission behavior of an aqueous solution of anionic surfactants, viz., SDS, DSS, and SDBS. It was found that the emission of the probe is uniquely sensitive to the changes in surfactant (anionic) concentration due to the functional group effect of the probe over the parent moiety, pyrene. Here, 1-methylaminopyrene (MAP) showed significant quenching of emission well below the critical micellar concentration (cmc) of the surfactant. Excimer emission of the probe due to the formation of premicellar aggregates of the surfactant solutions at a concentration close to but below the cmc and again an enhanced emission of the probe above the cmc were observed as a consequence of definite MAP-surfactant interactions. These observations assisted the possible quantification of surfactant concentrations and their chain length dependent premicellar aggregate formations. Significant monomer emission in relation to probe distribution in micelle was analytically authenticated. Dynamic light scattering (DLS) studies revealed the incorporation of the probe molecules in the micellar core. The fluorophore emission showed nonlinear behavior when the surfactant concentration was far above the cmc. Abrupt changes in the emission characteristics in relation to the micellar concentration led to the determination of the cmc of the surfactants.

1. Introduction During the past few years, biophysical study of biological macromolecules1-4 has become an important area of research. Since fluorescence spectroscopy is a very sensitive technique, suitable fluorescent probes5 have elegantly been employed to elucidate the structural problems in a variety of organized molecular assemblies i.e., micelles,3 polymers,6 biological membranes,4 vescicles,2 and microemulsions.7 The structure and function of such biological molecules have attracted considerable interest of researchers because of their ability to achieve specific chemical efficiency as a result of organization in the reaction media.8 Since surfactant molecules originate distinct microenvironments in different concentration ranges and represent models by which one might study the chemical behavior of a microenvironment analogous to a membrane system, significant efforts have been executed to study the static and dynamic properties of such moieties. Different techniques such as luminescence measurement,3,9 lifetime measurements,10 and fluores* Corresponding author. E-mail: [email protected]. † Department of Civil Engineering. (1) Novikov, E. G.; Visser, N. V.; Malevitskaia, V. G.; Borst, J. W.; van Hoek, A.; Visser, A. J. W. G. Langmuir 2000, 16, 8749. (2) Jung, M.; Hubert, D. H. W.; van Veldhoven, E.; Frederik, P. M.; Blandamer, M. J.; Briggs, B.; Visser, A. J. W. G.; van Herk, A. M.; German, A. L. Langmuir 2000, 16, 968. (3) Anghel, D. F.; Toca-Herrera, J. L.; Winnik, F. M.; Rettig, W.; Klitzing, R. V. Langmuir 2002, 18, 5600. (4) Panasu, R. B.; Yoshihara, K.; Arai, T.; Tokumaru, K. J. Phys. Chem. 1993, 97, 1125. (5) (a) Wang, D.; Wang, J.; Moses, D.; Bazan, G. C.; Heeger, A. J. Langmuir 2001, 17, 1262. (b) Rubio, D. A. R.; Zanette, D.; Nome, F.; Bunton, C. A. Langmuir 1994, 10, 1151. (c) Kawamoto, T.; Morishima, Y. Langmuir 1998, 14, 6669. (6) Gao, C.; Yan, D.; Zhang, B.; Chen, W. Langmuir 2002, 18, 3708. (7) Mishra, B. K.; Mukherjee, T.; Monohar, C. Colloids Surf. 1991, 56, 229. (8) Elworthy, P. H.; Florence, A. T.; MacFarlane, C. B. Solubilization of Surface Active Agents; Chapman and Hall, Ltd.: London, 1968; Chapter 1. (9) Schore, N. E.; Turro, N. J. J. Am. Chem. Soc. 1974, 96, 306.

cence depolarization studies11 have been adopted so far in the literature for the investigation of environmental changes in organized assemblies. Pyrene was introduced as a probe for micelle-forming surfactants by Fo¨rster and Selinger in 1964.12 There have been extensive studies on the photophysics of pyrene: its electronic spectrum and state assignments,13 kinetic details of excimer formation,14 spectral pressure effects,15 formation and kinetics of excited states,16 photoionization,17 delayed luminescence,18 and quasilinear spectra,19 etc. However, very few studies have been directed so far toward the functional group effect on the pyrene moiety.20,21 In the present study, we have employed the methylamino derivative of pyrene as a fluorescent molecule to probe the behavior of anionic surfactants in aqueous solution. Sodium dodecyl sulfate (SDS) is considered as the prototype anionic surfactant by many researchers and thus has been the focus of exhaustive (10) Patterson, L. K. J. Phys. Chem. 1973, 77, 1191. (11) Kubota, Y.; Kodama, M.; Miura, M. Bull. Chem. Soc. Jpn. 1973, 46, 100. (12) Fo¨rster, T.; Selinger, B. K. Z. Naturforsh. Teil 1964, A19, 38. (13) (a) Becker, R. S.; Singh, I. S.; Jackson, E. A. J. Chem. Phys. 1963, 38, 2144. (b) Geldof, P. A.; Rettschnick, R. P. H.; Hoytnik, G. J. Chem. Phys. Lett. 1969, 4, 59. (14) (a) Th. Fo¨rster, Angew. Chem., Int. Ed. Engl. 1969, 8, 33. (b) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970. (15) Offen, H. W. In Organic Molecular Photophysics; Birks, J. B., Eds.; Wiley-Interscience: New York, 1975; Vol. 1. (16) Richards, J. T.; West, A.; Thomas, J. K. J. Phys. Chem. 1970, 74, 4137. (17) (a) Gary, L. P.; de Groot, K.; Jarnagin, R. C. J. Chem. Phys. 1968, 49, 1577. (b) Gratezel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 2208. (18) Parker, C. A. Photoluminescence of Solutions; Elsevier: New York, 1968. (19) (a) Pesteil, L.; Troiplis, R.; Pesteil, P. J. Chim. Phys. 1963, 60, 1296 (b) Pellois, A.; Ripoche, J. Chem. Phys. Lett. 1969, 3, 280. (20) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (21) Bertolotti, S. G.; Zimmerman, O. E.; Cosa, J. J.; Previtali, C. M. J. Lumin. 1993, 55, 105.

10.1021/la035536n CCC: $27.50 © 2004 American Chemical Society Published on Web 05/18/2004

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Ghosh et al.

study.3,22 We have reported here the results on the fluorescence behavior of MAP in SDS solutions over a wide range of concentrations, and the study has also been extended to other anionic surfactants such as decyl sodium sulfate (DSS) and sodium dodecyl benzene sulfonate (SDBS). The changes in fluorescence behavior (quenching, enhancement, and excimer formation) of MAP in different surfactants helped to quantify their chain length, cmc, etc. and identification of premicellar aggregation (denoted by IE/IM) of the surfactants. 2. Experimental Section All the reagents were of analytical reagent grade. Three anionic surfactants, viz. SDS, DSS, and SDBS, were purchased from Sigma and were used without further purification. The fluorescent probe MAP was purchased from Aldrich and was used as received. Doubled distilled water was used throughout the experiment. All chemical reactions were carried out in well-stoppered quartz cuvettes. Fluorescence spectra were recorded with a Perkin-Elmer LS-50B spectrofluorimeter equipped with a 9.9 W xenon flash lamp and a photomultiplier tube with S-20 spectral response. The spectrofluorimeter was linked to a personal computer and utilized the Perkin-Elmer FLDM software package for data collection and processing. The experimental setup for the picosecond time correlated single photon counting technique was as follows. Briefly, a picosecond diode laser at 408 nm (IBH, UK, NanoLED-07, s/n 03931) was used as a light source. The fluorescence signal was detected by magic angle (54.7°) polarization using a Hamamatsu MCP PMT (3809U). The typical system response of this laser system is ∼75 ps. The decays were analyzed using IBH DAS-6 decay analysis software. Dynamic light scattering studies were carried out by a DLS 7000 instrument (Otsuka Electronic Corp.) using an argon ion laser of wavelength 488 nm and at an angle of 20°. The hydrodynamic radius was calculated using the Stokes-Einstein equation. The reproducibility of the determination was (20% for SDS micelle at ∼10-2 mol dm-3 concentration. The temperature was 298 ( 1 K for all measurements. In a typical set, an aqueous solution (3 mL) containing 3.3 µmol dm-3 of MAP was taken in the presence of different concentrations (10-6-10-1 mol dm-3) of SDS in a 1 cm quartz cuvette. After proper mixing of the probe, the emission spectrum of each set was recorded in the spectrofluorimeter.

3. Results and Discussion Figure 1 shows the total fluorescence intensity of MAP (3.3 µmol dm-3) as a function of SDS concentration. The aqueous solution of MAP possesses well-defined emission bands in the visible region with maxima at 376, 396, and 417 nm (trace a, Figure 1) due to the monomeric form of the fluorophore.23 In the presence of a low concentration of the surfactant (10-6-10-4 mol dm-3, i.e., well below the cmc of SDS), there was a considerable quenching of MAP emission compared to that of the simple aqueous solution (trace b-d, Figure 1). At only slightly higher concentration range (∼10-3 mol dm-3), the emission intensity of MAP decreased sharply and a broad featureless emission band appeared (trace e, Figure 1) with a maximum at 480 nm (bandwidth at half-maximum = 74 nm). Addition of SDS just above the cmc (10-2 mol dm-3) led to an enhancement in the intensity monomer emission versus that of in aqueous solution and no characteristic broad band was observed at 480 nm (trace f, Figure 1). When the surfactant concentration was far above the cmc (0.1 mol dm-3), the emission intensity was again found to diminish (trace g, Figure 1). At surfactant concentration greater than 0.1 (22) Sakagami, K.; Yoshimura, T.; Esumi, K. Langmuir 2002, 18, 6049. (23) George Thomas, K.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655.

Figure 1. Fluorescence emission of MAP (3.3 µmol dm-3) in the presence of (a) 0, (b) 10-6, (c) 10-5, (d) 10-4, (e) 10-3, (f) 10-2, and (g) 10-1 mol dm-3 SDS, respectively. The emission maxima at 376, 396, and 417 nm were assigned as peak I, II, and III, respectively. Table 1. Relative Emission Peak Intensities for the Three Vibronic Bands of MAP Solution at Different SDS Concentrationsa SDS concn (mol dm-3)

I

0 1.0 × 10-6 1.0 × 10-5 1.0 × 10-4 1.0 × 10-3 1.0 × 10-2 1.0 × 10-1

1.00 1.00 1.00 1.00 1.00 1.00 1.00

relative peak intensities II III 0.630 0.721 0.723 0.725 0.726 0.920 0.918

0.216 0.213 0.211 0.212 0.210 0.358 0.359

a The relative peak intensities were measured at an excitation wavelength of 278 nm. For all measurements, both the excitation and emission slits were 2.5 nm. All solutions contained 3.3 µmol dm-3 of MAP solution.

mol dm-3, nonlinear variation (i.e., the variation in intensity does not follow a regular trend) of the fluorescence intensity was noticed. When DSS or SDBS were used as the surfactant, similar trends in the emission spectra of MAP were observed. It is to be mentioned that when pyrene was used as the fluorescence probe, it could identify only two environments (i.e., above and below the cmc) and hence showed two types of emission behavior. As mentioned earlier, the monomeric form of MAP possesses characteristic emission bands with maxima at 376, 396, and 417 nm, which were assigned as peak I, II, and III, respectively. Table 1 shows the relative peak intensities (normalized with respect to peak I) for the three vibronic bands in aqueous SDS solutions of different concentrations. Below the cmc, with an increase in surfactant concentration over a wide range, the relative peak intensities were found to increase very slowly. But as the surfactant concentration reaches the critical micellar concentration, a large increment in the peak intensities were observed and again remained almost constant above the cmc. Therefore, the relative emission peak intensities of MAP could be utilized in realizing the environmental changes surrounding the probe molecules arising due to surfactant molecules.24 In this experiment, to have a general conclusion, we employed three anionic surfactants, viz., SDS, DSS, and (24) Nascimento, D. B.; Rapuano, R.; Lessa, M. M.; Carmona-Ribeiro, A. M. Langmuir 1998, 14, 7387.

Emission Behavior of 1-Methylaminopyrene

SDBS, to study their effects on the fluorescence behavior of the methylamino derivative of pyrene. It was found that the nature and intensity of MAP emission in an anionic surfactant varies substantially with its concentration. At a surfactant concentration well below the cmc (10-6-10-4 mol dm-3), the quenching of MAP emission could be explained by considering an ionic complex formation. Here, the complexation took place between the protonated form of the probe molecules and the negatively charged surfactant monomers in aqueous medium. The following observations were in agreement with this explanation. In water, the aminomethyl derivative of pyrene remained in its protonated form. On the other hand, when the fluorophore was taken in a cationic (cetyltrimethylammonium bromide, CTAB) or nonionic [poly(oxyethylene) isooctylphenyl ether, TX-100] surfactant solution well below the cmc, MAP showed an emission profile that is exactly similar to that of an aqueous solution. Therefore, the presence of anionic surfactant was responsible for the quenching of MAP fluorescence. This situation led us to doubt that such ionic complex formation might also be possible between the protonated form of an amine and any negatively charged ionic components. But when we added MAP in aqueous sodium sulfate (Na2SO4) solution (devoid of any surfactant), the emission spectrum of MAP showed similar nature as was observed in aqueous solution. It is, therefore, apparent that not only a negative charge but also a hydrophobic chain is required for an effective MAP-surfactant interaction.25 When the surfactant concentration approached close to but below the cmc (∼10-3 mol dm-3), the appearance of a new broad band at 480 nm speaks for the formation of an excimer. This could be ascribed to the formation of undefined aggregates between the surfactant molecules and the fluorophores and this has been documented as a premicellar effect.21,26 We have observed the excimer emission of the probe while the concentration of surfactant falls just below the cmc. Previtali et al.21 discussed the excimer formation of ionic pyrene derivatives in variable concentrations of ionic detergents. They reported that at low surfactant concentration, premicellar aggregates containing only two pyrene groups are responsible for the excimer emission. Therefore, in the present case, the assumption of excimer formation with two probe molecules per micelle was conceived. The premicellar effect was found to be prominent for a surfactant concentration in the proximity of the cmc and we also noted that MAP became quite ineffective at concentrations below ∼10-8 mol dm-3 to exhibit any detectable excimer emission. Several workers21,26 have explained this premicellar effect by a lowering of the cmc due to the presence of substrates (i.e., probe) with opposite charge to that of the surfactant. A comparative study has been made with other fluorescent probes containing an amine group, viz., 1-aminonaphthalene, 2-aminonaphthalene, benzylamine, and aniline. The appearance of the 480-nm band was not observed for any of these probe molecules. The hydrophobic part of these molecules is not large enough to form premicellar aggregates as was possible due to the presence of the large pyrene moiety in the case of MAP. One might rationalize that the broad band at 480 nm might be due to some kind of intra- or intermolecular exciplex emission from the probe molecules. The emission of the probe molecule in solvents of different dielectric constants (viz., cyclohexane, diethyl ether, ethyl acetate, tetrahydrofuran, and dimethylformamide) in lieu of surfactant was studied with an intention (25) Malik, W. U.; Pal Verma, S. J. Phys. Chem. 1966, 70, 26. (26) Atik, S. S.; Singer, L. A. J. Am. Chem. Soc. 1979, 101, 6759.

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Figure 2. Excimer (480 nm) to monomer (376 nm) fluorescence ratio (IE/IM) vs surfactant concentration for the (A) DSS-MAP, (B) SDS-MAP, (C) SDBS-MAP system. All solutions contained 3.3 µmol dm-3 of MAP solution.

to draw a Lippert plot. But the broad band at 480 nm did not appear in these cases, so there was no charge-transfer syndrome involving the probe molecules. Thus, the possibility of exciplex formation was ruled out. A critical examination has shown that the intensity of the 480-nm band was a maximum for an optimum surfactant concentration of 2.5 mmol dm-3 and a further increase in the concentration of surfactant (below cmc) again diminished the intensity of the 480-nm band. In Figure 2 B, the ratio of the intensity of the excimer and monomer fluorescence, IE/IM, is plotted against the SDS concentration. Values of IM and IE were noted at 376 and 480 nm, respectively, so that the two maxima at which IM and IE were measured are well-separated. Curve A and C in Figure 2 shows the plot of IE/IM as a function of DSS and SDBS concentration, respectively. The ratio of excimer to monomer increased with the surfactant concentration and reached a maximum at a particular concentration of the surfactant. In all cases, it was seen that the position of the maximum was attained below the cmc and was dependent on the nature of the surfactant. The shorter the chain length of the surfactant, the more pronounced was the shift of the maximum toward higher concentration. Therefore, it was concluded that formation of premicellar aggregates becomes more effective in case of surfactant with a longer chain length.

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Ghosh et al. Table 3. Results of Dynamic Light Scattering (DLS) Experiments concn (mol dm-3) SDS MAP 1.0 × 10-2 1.0 × 10-2 1.0 × 10-2

Figure 3. Time dependent decay of the excimer emission. The emission was measured after (a) 0, (b) 60, (c) 120, (d) 180, (e) 300, and (f) 420 min of mixing the solution. The inset shows the plot of the intensity of excimer emission (480 nm) as a function of time. Conditions: [SDS] ) 2.5 mmol dm-3, [MAP] ) 3.3 µmol dm-3. Table 2. Fluorescence Lifetimes of Excimer Emission SDS concn (mol dm-3)

lifetime of excimer (ns)

SDS concn (mol dm-3)

lifetime of excimer (ns)

0 1.0 × 10-6 1.0 × 10-5 1.0 × 10-4 1.0 × 10-3

2.03 2.67 2.91 2.94

2.5 × 10-3 5.0 × 10-3 7.5 × 10-3 1.0 × 10-2 1.0 × 10-1

2.98 2.96 2.95 2.75 2.71

All solutions contained 3.3 µmol dm-3 of MAP solution.

To elucidate the mechanism of excimer formation, we have analyzed the time dependent decay of excimer, as shown in Figure 3. The excimer was formed for a MAP concentration of 3.3 µmol dm-3 and surfactant concentration of 2.5 mmol dm-3. The inset shows the plot of excimer intensity as a function of time. As the time was increased, the rate of the excimer dissociation increased, but the intensity corresponding to monomer emission did not increase. This suggested that the dissociation of premicellar aggregates and the formation of free monomer were not reversible. The lifetime of the excimer was measured with the variation in SDS concentration. The excimer lifetime as a function of surfactant concentration for the SDS-MAP system is presented in Table 2. It is observed that the excimer lifetime shows no noticeable rise time in the premicellar concentration range (1.0-7.5 mmol dm-3), from which it can be concluded that the excimer formation occurred via a static mechanism. The change in the excitation profiles of the monomer and the excimer under the prescribed experimental condition suggests that the excimer emission originated from the direct excitation of mixed aggregates of methylaminopyrene and the surfactant.26 The red-shifted excimer excitation function corresponds to the more hydrophobic environment of the pyrene group, and therefore, the emission at 480 nm could not be ascribed to “pure excimer emission”. At SDS concentration above the cmc, considerable enhanced emission corresponding to the monomeric form of MAP relative to that in pure water was noticed. This spectral phenomenon has been observed by several workers in studies of the association of the dye with the

3.3 × 10-6 3.3 × 10-5

hydrodynamic radius (nm) 1.1 ( 0.2 2.0 ( 0.1 2.8 ( 0.1

micelle.27 This is due to the solubilization of the fluorophore in the highly viscous hydrocarbon core of the micelle as a monomeric form.28 The compartmentalization of the probe molecules in the micellar interior has been authenticated from dynamic light scattering (DLS) experiments. It was concluded from the specific increase in hydrodynamic radius upon incorporation of the probe molecules in micelle. The results of DLS studies with the reproducibilty have been presented in Table 3. It was found that the size of the micelles increases upon addition of MAP in the micellar solution. No broad band due to excimer was found to appear above the cmc of the surfactant. Predominant emission of MAP corresponding to its monomeric form from a micellar solution could be accounted for by considering a Poisson distribution29 of the probe molecules over the micellar aggregates. The probability P(n) of having n probe molecules solubilized in the same micelle is given by

P(n) ) µne-µ/n!

(1)

where µ denotes the average number of probe molecules per micelle. The concentration of micelle, [M], can be calculated using the expression

[M] ) (Cs - cmc)/N

(2)

where Cs is the total surfactant concentration and N is the aggregation number. The average number of probes per micelle, µ is related to the micelle concentration as

µ ) c/[M]

(3)

where c is the analytical concentration of methylaminopyrene. Assuming that the cmc of SDS ) 8.1 mmol dm-3, N ) 62 for 0.01 mol dm-3 SDS and taking c ) 3.3 µmol dm-3, we obtained [M] ) 3.06 × 10-5 mol dm-3 and m ) 0.107 84. Now, using eq 1, we get, P(0) ) 0.897 11, P(1) ) 0.096 81, P(2) ) 0.005 22, P(3) ) 0.000 19, and so on. This clearly showed that the number of micelles containing one probe molecule was manifold higher than the micelles containing two probe molecules. This is why we observed monomer fluorescence spectrum in the micellar solution. It is very interesting to note that, for this particular concentration of MAP, most of the micelles are left without any probe molecules. When the surfactant concentration was far above the cmc (0.1 mol dm-3), the emission intensity decreases more than that in 10-2 mol dm-3 SDS solution. A further increase in surfactant concentration (g0.1 mol dm-3) led to an enhanced but nonlinear emission from the fluorophore. This phenomenon may be attributed to a change in the micellar structure at the higher concentration of the surfactant. Kubota et al.11 noted that for SDS in addition (27) Mukherjee, P.; Mysels, K. J. J. Am. Chem. Soc. 1955, 77, 2937. (28) Pownall, H. J.; Smith, L. C. J. Am. Chem. Soc. 1973, 95, 3136. (29) Kim, J.-H.; Domach, M. M.; Tilton, R. D. Langmuir 2000, 16, 10037.

Emission Behavior of 1-Methylaminopyrene

Langmuir, Vol. 20, No. 13, 2004 5213 Table 4. Comparative Study between the Experimental Cmc and the Literature Values

Figure 4. Variation of fluorescence intensity of MAP (3.3 µmol dm-3) as a function of SDS concentration. In the curve, I, II, III, and IV indicate the different nature of emission of the probe well below the cmc, in the premicellar region, above the first cmc, and above the second cmc of SDS, respectively.

to the first critical micellar concentration there remained a second cmc (at about 0.07 mol dm-3 of SDS), where a change in the micelle structure took place. Such a modification in the micellar structure was responsible for making the fluorophore exhibit nonlinear emission properties. In these days, the critical micelle concentration is very often determined by spectral change techniques, as they are found to offer more accurate results than the conventional methods (i.e., conductance, viscosity, surface tension). The sensitivity of MAP fluorescence with the change in surfactant concentration gained a ground in determining the cmc of the host surfactant. A plot of the total fluorescence intensity at 376 nm with variation in SDS concentration is shown in Figure 4. The inflection point in Figure 4 occurs at 7.5 × 10-3 mol dm-3, whereas the cmc of SDS is reported as 8.1 × 10-3 mol dm-3. It was noted that the appearance of the break point indicating the cmc did not depend on the MAP concentration. Table

cmc (mol dm-3)

surfactant system

exptl

lit.

DSS SDS SDBS

3.0 × 10-2 7.5 × 10-3 2.0 × 10-3

3.3 × 10-2 8.1 × 10-3 1.6 × 10-3

4 shows a comparison between cmc values determined by this method and literature values30 for the three anionic surfactants. The excellent agreement between the cmc determined by this method and the literature values suggested that the incorporation of the probe in the surfactant aggregates did not interrupt considerably the micellization process. 4. Conclusion We have reported here a new fluorescent probe, methylaminopyrene, showing exceptional efficiency to characterize the concentration dependent phenomenon in organized media. The higher solubility of the probe (due to the presence of the aminomethyl functionality) over pyrene rendered it to sense uniquely each microenvironment originated from small variation in surfactant concentration. This study has explored several aspects of micellar chemistry in one hand and versatile behavior of a fluorescent probe on the other. The determination of the cmc appeared as a bonus feature in the present study. Acknowledgment. S.K.G. and S.K. are grateful to the Department of Science and Technology, New Delhi, for financial support. M.M. and S.N. thank the Council of Scientific and Industrial Research, New Delhi, for financial assistance. LA035536N (30) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975; pp 19-22.