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Silver and Gold Nanocluster Catalyzed Reduction of Methylene Blue by Arsine in a Micellar Medium Sujit Kumar Ghosh, Subrata Kundu, Madhuri Mandal, and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India Received February 22, 2002. In Final Form: August 12, 2002 In anionic micelles, SDS has been found to remove the kinetic barrier for the reaction between methylene blue (MB) and arsine. The color bleaching (reduction) of SDS micelle bound MB by arsine gives a quantitative measure of arsenic concentration in the sub-parts-per-million levels for a test sample. Arsine is generated by reacting NaBH4 with arsenic-containing samples present along with the other reagents. The extent of the micelle-bound dye reduction is facilitated in the presence of Ag or Au nanoparticles. The micelles, in turn, help to increase the collision probability between the dye and arsine, whereas nanoparticles help the electron relay from AsH3 to the micelle-bound dye. This method depicts for the first time a simple, reproducible way to demonstrate nanoparticle-catalyzed dye reduction by arsine in an organized medium.
1. Introduction Surfactants, surface-active reagents, or detergents are increasingly being utilized as reaction media. Their rates, products, and in some cases stereochemistry are affected. In some respects, these systems also provide models for membrane-mediated processes. Many of these reactions have potential industrial application in various fields such as pharmacy, photography, extraction, polymerization, and so forth.1 Rate acceleration or deceleration of organic/inorganic reactions in micellar solutions arises from the different rates of reaction of the substrate in the micellar phase and in the bulk solution and the distribution of the substrate between these two phases. Basically, these rate effects can be attributed to electrostatic and hydrophobic interactions between the substrate and the surfactant aggregate and in some cases to alterations in the structure of the surrounding water. Generally, anionic micelles enhance the rate of the reactions involving nucleophilic cations with an uncharged substrate while cationic micelles retard them and catalyze reactions involving anions. Uncharged or zwitterionic micelles will have little or no effect on reaction rate.1-4 Nanoscale systems have become an interesting area of research during the past few decades because of their various applications in science and technology. These particles have the ability to bridge the gap between atomic and macroscopic scales.5-7 Any particle whose size falls in the range of 1-100 nm can be broadly called a nanoparticle whether it is dispersed in a gaseous, liquid, or solid medium. These particles comprise clusters of a small number of atoms. Atoms on their surfaces are * To whom correspondence should be addressed. E-mail: tpal@ chem.iitkgp.ernet.in. (1) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (2) Fendler, J. H. Membrane Mimetic Chemistry; New York, 1982. (3) Pal, T.; De, S.; Jana, N. R.; Pradhan, N.; Mandal, R.; Pal, A.; Beezer, A. E.; Mitchel J. C. Langmuir 1998, 14, 4724. (4) Mallick, K.; Jewrajka, S.; Pradhan, N.; Pal, T. Curr. Sci. 2001, 80, 1408. (5) (a) Spiro, M.; Freund, P. L. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1649. (b) Freund, P. L.; Spiro, M. J. Phys. Chem. 1985, 89, 1074. (6) (a) Davis, S. C.; Klabunde, K. J. Chem. Rev. 1982, 82, 153. (b) Morse, M. D. Chem. Rev. 1986, 86, 1049. (c) Henglein, A. Chem. Rev. 1989, 89, 1861. (d) Schmid, G. Chem. Rev. 1992, 92, 1709. (7) Lewis, L. N. Chem. Rev. 1993, 93, 2693.
inherently reactive since they are coordinatively unsaturated with their unoccupied orbital. Metallic nanoparticles exhibit properties that differ significantly from those of the bulk.6 This arises due to their finite size and large surface area to volume ratio and mostly due to sizedependent reactivity. The size-dependent reactivity and large surface area of these systems lead to their use as efficient catalysts. Here, the use of Ag and Au nanoparticles in the reduction of methylene blue (MB) by arsine in a micellar medium has been documented. The present report covers two aspects of modern chemistry mentioned above: first, the micelle-mediated reaction,3,4 and second, the electron relay via metal nanoparticles. Both of these effects might help for the development of a sensitive one-pot arsenic detection method. Keeping an eye to the grave situation of arsenic poisoning in this subcontinent, the idea of this paper could be of some help for the development of a simple spectrophotometric method of arsenic quantification at the subparts-per-million (ppm) level. The quantitative color bleaching of MB, a well-known nontoxic cationic dye, by arsine (AsH3) has been studied in micelles (here, it was aqueous SDS, sodium dodecyl sulfate, 10-2 mol dm-3). Arsine is generated from arsenic compounds with NaBH4 present in the reaction mixture containing the dye and micelles. The progress of reduction of the dye was followed spectrophotometrically at the dye λmax of 660 nm. The participation of silver or gold nanoparticles as catalysts8,9 made the method more efficient. Silver served the purpose better than gold. The reaction is simply fascinating (outlined in Scheme 1) and is reported for the first time. This report helps in understanding an effective collision between a dye-micelle aggregate and arsine for a fruitful reaction. Moreover, this investigation offers us a new site in catalysis where in one system both the organized medium and nanoparticles act hand in hand. 2. Experimental Section 2.1. Reagents. Methylene blue (Qualigens Fine Chemicals, India) was purified by repeated recrystallization from alcohol. SDS (SISCO, India), cetyl trimethylammonium bromide (CTAB, Aldrich), and poly(oxyethylene) iso-octyl phenyl ether (TX-100, Aldrich) were used as received. Sodium arsenate and NaBH4 (8) Jana, N. R.; Sau, T. K.; Pal, T. J. Phys. Chem. B 1999, 103, 115. (9) Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B 2001, 105, 9266.
10.1021/la0201974 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/15/2002
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Scheme 1. Schematic Representation of the Reduction of Methylene Blue by Arsine in Micelles
were obtained from BDH Chemical Co. HAuCl4 and AgNO3 were purchased from Aldrich and were used as received. Urea (Loba Chemie) and sodium chloride (AR, SISCO) were used as obtained from the manufacturers. Double-distilled water was used to prepare aqueous solutions. The NaBH4 (0.1 mol dm-3) solution was prepared freshly in ice-cold distilled water. Aqueous 10-2 mol dm-3 SDS, 0.1 mol dm-3 Na3AsO4, and 5 × 10-4 mol dm-3 MB were used as stock solutions. For the preparation of metal nanoparticles of Ag and Au, 10-2 mol dm-3 AgNO3 and 5 × 10-3 mol dm-3 HAuCl4 were used, respectively. 2.2. Instruments. All UV-visible absorption spectra were measured in a Shimadzu UV-160 digital spectrophotometer (Kyoto, Japan) with 1 cm quartz cuvette. The hydrodynamic radii of the samples were measured in a dynamic light scattering DLS 7000 instrument (Otsuka Electronic Corp., Japan). 2.3. Preparation of Silver and Gold Nanoparticles. An aliquot of 20 µL of 10-2 mol dm-3 AgNO3 was taken in 2 mL of distilled water. The solution was then purged with N2 and stirred by a magnetic stirrer. Then 5 µL of 0.1 mol dm-3 NaBH4 was added to the previous solution all at once. The stirring was continued for another 30 min. The yellow silver sol that was generated showed a λmax of 400 nm. Similarly, a 40 µL aliquot of 5 × 10-3 mol dm-3 HAuCl4 was taken in 2 mL of distilled water. The solution was then purged with N2 and stirred by a magnetic stirrer. Then 5 µL of 0.1 mol dm-3 NaBH4 was added to the previous solution all at once. The stirring was continued for another 30 min. The pink gold sol showed a λmax of 520 nm. The average particle size of both the particles was in the ∼7 nm region.10 2.4. Quantitative Determination of Arsenic in the Test Sample. An aliquot of 3 mL of anionic micelles of SDS (10-2 mol dm-3) was taken in a stoppered cuvette, and 50 µL of MB (5 × 10-4 mol dm-3) was added to the micellar solution. Then varying amounts (20-100 µL) of sodium arsenate (0.1 mol dm-3) were (10) Pradhan, N.; Jana, N. R.; Mallick, K.; Pal, T. J. Surf. Sci. Technol. 2000, 16, 188.
introduced into the cuvette, and 40 µL of silver nanoparticles (10-4 mol dm-3) was added. Finally, 150 µL of NaBH4 (0.1 mol dm-3) was introduced into the reaction mixture. After 3 min, the decrease in the absorbance value of the micelle-stabilized dye was noted at 660 nm. The dilution effect was considered as far as practicable. The color bleaching of the dye gives a quantitative measure of the amount of sodium arsenate (and hence arsenic) present in the solution. 2.5. Study on the Effect of the One-Pot (In Situ) and the Two-Pot Method of Reduction by AsH3. The color of methylene blue was bleached quantitatively by arsine in the SDS micellar system. To study the reaction, arsine (AsH3) was generated in situ from sodium arsenate by NaBH4 reduction in the micellar (SDS-bound MB) solution (one-pot method). In an alternative arrangement (two-pot method), when exactly the same amounts of SDS and MB were taken in a test tube and arsine was generated in another test tube with the same amount of sodium arsenate (as was taken for the one-pot reaction), keeping NaBH4 in excess, and finally the two test tubes were connected by a U-shaped tube, the extent of reduction of SDS-bound MB was found to be the same as in the previous method, that is, one-pot reduction. So, in situ arsine generation did not hamper the reduction. Hence, we always used the one-pot reduction method not only because of experimental simplicity but also to minimize the chance of arsine loss.
3. Results and Discussion 3.1. Micellar Catalysis. Methylene blue has the basic dye skeleton of the thiazine group, and it exists as aggregates in water.11 It is used as an oxidation-reduction indicator in chemistry and biology.12 The oxidized and reduced forms of methylene blue are shown in Figure 1. The color of MB (5 × 10-4 mol dm-3) can be bleached (11) Mukherjee, P.; Ghosh, A. K. J. Am. Chem. Soc. 1970, 92, 6403. (12) Gurr, E. Synthetic Dyes in Biology, Medicine and Chemistry; Academic Press: New York, 1971; p 77.
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Figure 1. Oxidized and reduced forms of methylene blue.
Figure 2. Plot of the absorbance of the dye (at 660 nm) vs the concentration of As (in ppm). The inset shows the UV-vis spectra of the arsine-dependent reduction of methylene blue in the SDS micellar system. Conditions: [SDS] ) ∼10-2 mol dm-3, [MB] ) 8.2 × 10-6 mol dm-3.
quantitatively by arsine in a micellar medium. Arsine was generated in situ (i.e., in the reaction medium) from arsenic-bearing samples (here, it was sodium arsenate, 0.1 mol dm-3) by NaBH4 reduction (0.1 mol dm-3). Though NaBH4 is a strong reducing agent, it cannot reduce MB in the experimental time scale in an aqueous and micellar medium. In aqueous solution, arsine reduces methylene blue to a negligible extent. This thermodynamically favorable reduction of the dye (E° for MBox/MBred ) -0.18 V, H3BO3/BH4- ) -1.33 V, and AsH3/As ) +0.60 V vs normal hydrogen electrode (NHE)) was difficult to observe in water and cationic and nonionic micellar solutions; that is, almost no reduction of the dye was observed under our experimental conditions and time scale. To study the quantitative progress of the reaction (i.e., the arsinedependent reduction of MB), an anionic surfactant (above its critical micellar concentration) was found to be suitable. It has been observed that the reduction becomes many times faster when methylene blue is present inside anionic micelles such as those of sodium dodecyl sulfate (∼10-2 mol dm-3). This one-pot reaction condition provides a good measure (correlation coefficient ) 0.9902) of the arsenic content of a sample even in the sub-ppm level as is shown in Figure 2. Arsine-dependent reduction of MB in the SDS micellar system is shown in the inset of Figure 2. From the kinetic data, the activation energy of this reaction was found to be 13.36 kJ mol-1, which indicates that the reaction rate is nearly diffusion controlled. Micelles are well-known membrane mimetic systems.13-15 They function in many ways. First, micelles can lead to environmental changes decreasing the energy (13) Fendler, J. H. Chem. Rev. 1987, 87, 877. (14) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481. (15) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262.
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difference between the initial and transition states, that is, they alter the reaction pathways. Second, in the micellar medium there is also an increase in the frequency of molecular collisions due to the close association of the reactants at the micellar interface. Third, they lead to the solubilization of organic compounds.3,4 NaBH4 is a strong reducing agent. But in aqueous solution BH4- could not reduce MB. Hence the color bleaching of the dye could not be observed. Addition of sodium arsenate into the mixture containing the dye and NaBH4 did not show any sign of MB reduction even though arsine is generated in the reaction medium. This observation clearly indicates that arsine could not collide effectively with methylene blue molecules under the said experimental condition. However, in an anionic micellar medium, the reduction becomes quantitative because of the increased encounter probability due to the binding of the dye at the micellar surface. Investigating substrate partitioning and orientation of the microenvironment taking the hydrophobicity of surfactants and substrates into consideration can explore the catalytic efficiency of micelles.2 The color bleaching of MB by arsine was tried in three different types of micelles: SDS (anionic), CTAB (cationic), and TX-100 (nonionic). The color bleaching was instantaneous in SDS systems, slower with TX-100, and slowest with CTAB micelles. The bleaching of MB color by arsine could be explained with the concept of encounter probability3,16 and the fractal nature of the micelle surface.17-19 Micellar surfaces bind many organic/inorganic compounds by electrostatic and/ or hydrophobic interactions.20 The hydrophobic effect is the natural tendency of a hydrocarbon-like molecule to form aggregates in aqueous solution so as to minimize the water-hydrocarbon interfacial area.21 In the case of the cationic MB, the dye is bound to SDS by both electrostatic and hydrophobic interactions and to TX-100 and CTAB only by hydrophobic forces. The stronger force of attraction between SDS and MB (i.e., immobilization of MB) explains qualitatively the faster rate of reduction of MB in SDS micelles. Anionic micelles indirectly help to increase the collision probability between the dye and arsine through a physical factor, that is, through their incorporation in the micellar Stern layer which is the compact region of the charged headgroups and the relatively small counterions of the ionic micelle (as shown in Figure 3).1,2 The reduction occurs in a diffusion space of the micelle of dimension d < 3. Complete reduction of MB occurs when the number ratio of micelles and methylene blue was 10: 1. The color bleaching of the cationic MB in a cationic or nonionic micellar solution was not that efficient, presumably because of the low collision probability between MB and arsine. To check the effect of increased hydrophobicity of the dye, an interesting observation was noted using dimethylmethylene blue (DMMB) in lieu of MB. The electronic environment (cationic nature) remained the same for DMMB, but the increased substitution in the dye skeleton inhibits its immobilization onto the micellar surface. Thus collision probability of the micelle-bound DMMB with the incoming AsH3 has been observed to be minimum. The (16) Keizer, J. Acc. Chem. Res. 1985, 18, 235. (17) Blumen, A.; Klaufter, J.; Zumofen, G. Fractals in Physics; Pietronero, L., Tossati, E., Eds.; North-Holland: Amsterdam, 1986; p 399. (18) Lianos, P. J. Phys. Chem. 1998, 89, 5237. (19) Evesque, P. J. Phys. (Paris) 1983, 44, 1217. (20) Pal, T.; Jana, N. R. Langmuir 1996, 12, 3114. (21) Tanford, C. The hydrophobic effect: formation of micelles and membranes, 2nd ed.; Wiley: New York, 1980.
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Table 1. Comparison of the Catalytic Activity of Metal Nanoparticlesa volume of SDS (mL)
volume of MB (µL)
volume of Na3AsO4 (µL)
volume of NaBH4 (µL)
3 3 3 3
70 70 70 70
100 100 100
150 150 150
volume of silver sol (µL)
volume of gold sol (µL)
absorbance (at 660 nm)
40
0.977 0.806 0.238 0.561
40
Initial concentration of reagents: [SDS] ) 10-2 mol dm-3; [MB] ) 5 × 10-4 mol dm-3; [Na3AsO4] ) 0.1 mol dm-3; [NaBH4] ) 0.1 mol dm-3; [Ag] ) 9.9 × 10-5 mol dm-3; [Au] ) 9.9 ×10-5 mol dm-3. a
Figure 3. Schematic representation of the incorporation of arsine and methylene blue in the SDS micellar system.
slower reduction rate in the case of DMMB could be visualized from another angle. The increased hydrophobicity of the dye might cause a deeper penetration in the micelle that would also diminish the encounter probability with AsH3. Anyway, whatever may be the case, the reduction of DMMB has been observed to be much slower than the MB reduction. 3.2. Nanoparticle Catalysis. The presence of anionic micelles was found to be essential for the effective reduction of MB (5 × 10-4 mol dm-3) by 0.1 mol dm-3 NaBH4 in the presence of sodium arsenate. In the absence of SDS micelles, the reaction does not occur. Even in SDS solution, the color bleaching is relatively slower if the arsine concentration is small (less than 1.0 ppm). To make the reaction faster between the donor (AsH3) and the acceptor (micelle-bound methylene blue), the presence of metal nanoparticles was thought of and tested successfully. Interestingly, it has been observed that both silver and gold nanoparticles (10-4 mol dm-3 prepared and added; final concentration, 10-7 mol dm-3) served the purpose. Nanoparticles of silver and gold help the effective electron relay (outlined in Scheme 1) for the reduction of the dye.8 Even the dye reduction can be catalyzed by the metal nanoparticles alone in aqueous solution (devoid of micelles). However, the involvement of both the micelles and the nanoparticles showed improved efficiency of the reaction. The redox reaction between MB and a reductant can be viewed as an electron-transfer reaction.3 The rate enhancement in the presence of metal particles can be explained as follows. Metal nanoparticles help the electron
relay (promote the extent of reaction) from the donor to the acceptor. The particles possess a large surface area which acts as a substrate for the electron-transfer reaction. Just before the reaction, both of the reactants are adsorbed on the metal particle and the micelle presumably brings them into close proximity. Subsequently, the reductant releases an electron to the surface of the metal particle from which MB gains an electron and is reduced. Thus, the metal particles act as an efficient catalyst by being involved in the electron-transfer process. Repetitive kinetic rate measurements (25 °C) revealed that both the nanoparticles catalyzed the redox reaction but the extent of reaction was different; that is, silver nanoparticles served better than gold (in terms of rate of reduction) as is shown in Table 1. Under the said experimental condition, gold particles might be capped with S-containing ligands.22 Presumably, because of the presence of the heteroatom S in the MB skeleton, gold particle surfaces are reserved for the preferential dye adsorption. Under these circumstances, dye-covered Au particles did not provide a reasonable avenue for AsH3 adsorption onto the Au surface. Hence, electron transfer was not observed to be as efficient in the case of gold as in the case of silver. Next, the rate of aerial oxidation of reduced MB in the presence of surfactant and metal particles was probed. As in the case of reduction, the rate of back oxidation in the presence of metal particles is observed to be much faster than in the absence of any metal nanoparticles. Here also, silver particles happen to be better-suited than gold nanoparticles. 3.3. Mechanism. We have tried to get an insight into the physical basis of the reduction of methylene blue by arsine in micelles. First, we have studied the effect of “salting-in” and “salting-out” agents to have an idea about the position of methylene blue in the reaction medium. Therefore, the effects of electrolytes on the micellecatalyzed reaction are discussed in terms of their competition with the reagents for the available binding sites.1 The water-hydrocarbon interactions are modified in the presence of such additives.22 Organic moieties such as urea and big (guanidium; Gdm+ClO4-) ions disrupt hydrophobic aggregation of the micelles. Since these compounds increase the solubility of organic molecules in an aqueous medium, they are called “salting-in agents”.23a Small ions (Li+, Na+, Cl-, etc.) reduce the solubility of organic compounds in water and hence are called “saltingout agents”.23b The latter facilitate hydrophobic binding of MB with micelles and thus facilitate the reduction of the micelle-bound dye with the incoming AsH3. Higher amounts of MB (more than the amount required to saturate SDS micelles) could be bleached by arsine when 1.0 mol dm-3 NaCl is added into the reaction mixture (micelle-bound dye solution). But when the same reaction was repeated in the presence of 1.0 mol dm-3 urea or (22) Jana, N. R.; Pal, T. Langmuir 1999, 15, 3458. (23) (a) Greico, P. A.; Garner, P.; He, Z. M. Tetrahedron Lett. 1983, 24, 1987. (b) Loupy, A.; Tchoubar, B.; Astruc, A. Chem. Rev. 1992, 92, 1141.
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guanidium hydrochloride, no meaningful alteration in the extent of dye reduction was observed. This is due to the fact that NaCl, being ionic, is dissociated in the aqueous phase and forces a higher amount of MB (unbound) molecules to be incorporated in the micellar Stern layer. Now, that micelle-bound MB is reduced by arsine. Organic moieties such as urea or guanidium hydrochloride when added to the micellar system get incorporated in the micellar core structure, but addition of them does not affect the extent of the reaction, clearly indicating that MB molecules are not incorporated in the core region of the micelle. So we conclude that MB is not located in the core region but remains on the surface of the micelle. Second, we have successfully determined the average hydrodynamic diameter of the micelle before and after the introduction of arsine in SDS micelles. Dynamic light scattering (DLS) experiments revealed a 13-fold increase in the size of the SDS micelle (schematically represented in Scheme 1) while AsH3 saturates SDS at 25 °C. The average diameters (hydrodynamic diameter) of the SDS micelle and arsine incorporated into the SDS micelle were measured at different angles. The average diameter was 6.3 nm and that for arsine incorporated into the SDS micelle was 79 nm for 90° measurement. Thus, from the above measurements it is clear that arsine is highly soluble in SDS and definitely incorporated into the micelle (the solubility of AsH3 in water is very low and it does not dissociate). Micelles are dynamic systems, and any AsH3 present inside the micellar aggregates will eventually reach the water-micelle interface because of the rapid surfactant-micelle equilibrium. Hence, AsH3 easily reduces micelle-bound MB. 3.4. Effect of Various Parameters. The arsinedependent reduction reaction could be followed even when MB was present at a trace level (∼10-7 mol dm-3). This is possible due to the high molar extinction coefficient ( ) 1 × 105 L mol-1 cm-1) of the dye at 660 nm. A change in the concentration of MB from 10-7 to 10-5 mol dm-3 was required for samples containing higher amounts of arsenic. To make the AsH3 generation smooth and quantitative, the arsenate/arsenite sample was treated in an aqueous medium with NaBH4. Reproducible results could not be achieved with other reducing agents such as Zn or Sn in an acid medium. They seem to be too drastic to be used under our experimental conditions. So NaBH4 has been found to be the best choice. While NaBH4 remains the best-suited reducing agent for AsH3 generation, we have studied the effect of NaBH4 concentration also. NaBH4 alone cannot reduce MB and even micelle-bound MB in the aqueous phase whatever the concentration of NaBH4. But NaBH4 produces AsH3 from the arsenite/arsenate reaction, and typically for 1.0 ppm of Na3AsO4, a variation of NaBH4 concentration (1.14.3 × 10-3 mol dm-3) did not affect the reaction. We have investigated the effect of varying the concentration of sodium arsenate (Na3AsO4). In 3 mL of a 10-2 mol dm-3 SDS micelle solution, 50 µL of 5 × 10-4 mol dm-3 methylene blue was added. Now addition of varying amounts (0-100 µL) of sodium arsenate (0.1 mol dm-3) followed by addition of 150 µL of 0.1 mol dm-3 NaBH4 showed a linear decrease in the absorbance at 660 nm, and that enabled us to set up a calibration curve for the
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quantitative determination of arsenic as is shown in Figure 2. Gradual increases of the arsine concentration (a direct measure of the amount of any arsenic compound even if taken at the sub-ppm level) proportionately decrease the color of the micelle-bound dye. For the reaction, the concentration of MB lies in the range of ∼10-6 mol dm-3 and the concentration of metal nanoparticles (Ag or Au) was in the range of 10-7 mol dm-3 whereas the concentration of As was ∼10-3 mol dm-3. Therefore, the concentrations of MB and the metal nanoparticles were ∼103 times less or even much lower than that of As. So, pseudo-first-order kinetics (k ) 0.12 min-1) with respect to As could be used in this case to evaluate the catalytic rate.9 The reaction retained the pseudo-first-order kinetics for a wide range (1-10 ppm) of arsenic and other reagent concentrations. The sequence of addition of substrates has been observed to be extremely important. It has been observed that no meaningful reduction of MB took place when SDS was presaturated with arsine prior to the addition of MB. It seems that binding of MB with the micelle is a prime condition and after the dye-micelle aggregate is formed, AsH3 can initiate dye reduction. Hence the correct sequence of addition of the substrates should be SDS first and then MB followed by AsH3 (arsenic-bearing sample and NaBH4). To catalyze the dye reduction, the nanoparticles could be introduced into the mixture containing dye and micelle. 4. Conclusion This article discusses a simple chemical reaction studied in organized systems. The organized assemblies provide geometric control of the reaction and solubilization sites for the reactant. However, they are not only passive hosts. The study reports the catalytic effect offered by the restricted geometric and atypical environment of various organized assemblies on the well-known redox reaction of methylene blue. Of the two factors affecting the rate of reaction (i.e., free energy of activation (∆G) and encounter probability), the latter has been found to exert a dominating influence in the case of catalysis by micelles. Thus, micelles provide a method of organizing the reactant on a molecular scale and enhancing the rate of reaction. Electrostatic and hydrophobic forces exert a profound influence on the encounter probability. A delicate balance of both gives rise to an optimum condition for catalysis. In addtion, nanoparticle-induced catalysis has been successfully demonstrated. The higher efficiency of silver over gold as a catalyst may provide a clue of the possible catalyst poisoning through “soft-soft” interaction. Thus, this study and other related studies may be important precursors to many aspects of catalysis. Moreover, the proposed method could be applied successfully for arsenic determination (one-pot method) in water samples without any serious interference. Acknowledgment. S. K. Ghosh and S. Kundu are grateful to the Department of Science and Technology (DST), New Delhi, for financial support. M. Mandal thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, for financial assistance. LA0201974
