Role of Phenol Derivatives in the Formation of Silver Nanoparticles

Dec 21, 2007 - Jasmine A. Jacob, Harbir S. Mahal, Nandita Biswas, Tulsi Mukherjee, and Sudhir Kapoor*. Radiation and Photochemistry DiVision, Bhabha ...
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Langmuir 2008, 24, 528-533

Role of Phenol Derivatives in the Formation of Silver Nanoparticles Jasmine A. Jacob, Harbir S. Mahal, Nandita Biswas, Tulsi Mukherjee, and Sudhir Kapoor* Radiation and Photochemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed July 12, 2007. In Final Form: October 8, 2007 We demonstrate that dihydroxy benzenes are excellent reducing agents and may be used to reduce silver ions to synthesize stable silver nanoparticles in air-saturated aqueous solutions. The formation of Ag nanoparticles in deaerated aqueous solution at high pH values suggests that the reduction of silver ions occurs due to oxidation of dihydroxy benzenes and probably on the surface of Ag2O. Pulse radiolysis studies show that the semi-quinone radical does not participate in the reduction of silver ions at short time scales. Nevertheless, results show that primary intermediates undergo slower transformation in the presence of dihydroxy benzenes than in their absence. This slow transformation eventually leads to the formation of silver nanoparticles. The Ag nanoparticles were characterized by UV-vis absorption spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). XRD and TEM techniques showed the presence of Ag nanoparticles with an average size of 30 nm.

Introduction Metal nanoparticles have become subject of intense interest in various fields of chemistry and physics during the past few decades.1-4 Interest in nanoparticles stems from their unique optical, electronic, and catalytic properties, which are different from their bulk counterparts.5-15 The size, shape, structure, and morphology of nanoparticles play important roles in modulating their electric and optical properties. Gold and silver nanoparticles synthesized by various techniques have received special attention because they have found potential application in many fields such as catalysis, sensors, SERS enhancement, etc.6-22 In most of the applications, nanoparticles are used as building blocks toward functional nanostructures. The coinage metal nanoparticles such as silver, gold, and copper have been exploited for the previously mentioned purposes as they show a surface plasmon resonance absorption in the UV-vis region.6,9,12,14,15 The surface plasmon band arises from the coherent existence of free electrons in the conduction band due to the small particle size effect, which is dependent on the particle sizes, chemical surroundings, adsorbed * Corresponding author. E-mail: [email protected]; fax: (+)-91-2225505151; tel.: (+)-91-22-25590298. (1) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Nature (London, U.K.) 2005, 437, 121. (2) Yin, Y.; Alivisatos, P. Nature (London, U.K.) 2005, 437, 664. (3) Capek, I. AdV. Colloid Interface Sci. 2004, 110, 49. (4) Goia, D. V.; Matijevic, E. New J. Chem. 1998, 22, 1203. (5) Mallik, K.; Witcomb, M. J.; Scurrel, M. S. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 797. (6) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (7) Fendler, J. H. Chem. ReV. 1987, 87, 877. (8) Schmid, G. Chem. ReV. 1992, 92, 1709. (9) Kamat, P. V. Chem. ReV. 1993, 93, 267. (10) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (11) Gates, B. C. Chem. ReV. 1995, 95, 511. (12) Henglein, A. Chem. ReV. 1989, 89, 1861. (13) Ozin, G. A. AdV. Mater. 1992, 4, 612. (14) Liz-Marzan, L. Langmuir 2006, 22, 32. (15) Mulvaney, P. Langmuir 1996, 12, 788. (16) Grunnert, W.; Bruckner, A.; Hofmeister, H.; Claus, P. J. J. Phys. Chem. B 2004, 108, 5709. (17) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (18) Cao, Y. W.; Jin, R.; Nam, J.; Thaxton, C. S.; Mirkin, C. A. J. Am. Chem. Soc. 2003, 125, 14676. (19) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Field, M. S. Chem. ReV. 1999, 99, 2957. (20) Biswas, N.; Thomas, S.; Kapoor, S.; Misra, A.; Wategaonkar, S.; Venkateswaran, S.; Mukherjee, T. J. Phys. Chem. A 2006, 110, 1805. (21) Thomas, S.; Biswas, N.; Venkateswaran, S.; Kapoor, S.; Naumov, S.; Mukherjee, T. J. Phys. Chem. A 2005, 109, 9928. (22) Nie, S.; Emory, S. R. Science (Washington, DC, U.S.) 1997, 275, 1102.

species on the surface, and dielectric constants.6,12,14,15 Another unique feature of these coinage metal nanoparticles is that a change in the absorbance or wavelength provides a measure of the particle size, shape, and inter-particle properties.14,15 In this capacity, one-dimensional nanostructures such as wires, rods, etc. have become the focus of intensive research owing to their unique applications.1,2 Thus, intense efforts have been put forth to search for methods that are capable of synthesizing and stabilizing nanoparticles.6,9,12,14,15 It has been observed that in most cases, nanoparticles will congregate because nanosized metal particles in the solution are active and prone to coalesce due to van der Waals forces and high surface energy. Therefore, to protect them, stabilizers or polymers are used.6-15,23-27 Since silver nanoparticles possess an excellent biocompatibility and low toxicity, they have important applications in the field of biology such as antibacterial agents, DNA sequencing, and so on.28-32 In addition, there is a considerable interest in phenolic acids due to their powerful antioxidant properties. Phenolic acids in particular have drawn much attention from various researchers due to their potential contribution to defend against oxidative stress and protect against cancer and cardiovascular diseases.33,34 It has been suggested that the great ability of phenolic acids to reduce Au(III) to Au nanoparticles correlates with the ease with which they are able to donate electrons.35 Thus, considerable efforts have been directed toward the preparation of metal nanoparticles in the presence of biologically important com(23) Ahmadi, T.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science (Washington, DC, U.S.) 1996, 272, 1924. (24) Longenberger, L.; Mills, G. J. Phys. Chem. 1995, 99, 475. (25) Sarkar, A.; Kapoor, S.; Mukherjee, T. J. Phys. Chem. B 2005, 109, 7698. (26) (a) Kapoor, S. Langmuir 1998, 14, 1021. (b) Kapoor, S. Langmuir 1999, 15, 4365. (27) Sarkar, A.; Kapoor, S.; Yashwant, G.; Salunke, H. G.; Mukherjee, T. J. Phys. Chem. B 2005, 109, 7203. (28) Zhang, L.; Yu, J. C.; Yip, Y. H.; Li, Q.; Kwong, K. W.; Xu, A.-W.; Wong, P. K. Langmuir 2003, 19, 10372. (29) Shi, Z.; Neoh, K. G.; Kang, E. T. Langmuir 2004, 20, 6847. (30) Cao, Y. W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961. (31) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128. (32) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042. (33) Antolovich, M.; Prenzler, E. D.; Patsalidas, E.; McDonald, S.; Roboards, K. Analyst 2002, 127, 183. (34) Robbins, R. J. J. Agric. Food Chem. 2003, 51, 2866. (35) Scampicchio, M.; Wang, J.; Biasco, A. J.; Arribas, A. S.; Mannino, S.; Escarpa, A. Anal. Chem. 2006, 78, 2060.

10.1021/la702073r CCC: $40.75 © 2008 American Chemical Society Published on Web 12/21/2007

Phenol DeriVatiVes in Ag Nanoparticle Formation

pounds. However, the details of the reduction process in many of these synthetic methods have not been elucidated.36,37 Similarly, catechols are bio-synthesized and used as ironsequestering agents by microorganisms. Catechols have low reduction potentials, and they can act as good ligands for forming chelates with metal ions such as Cu, Mn, etc. As mentioned earlier, the use of metal nanoparticles as supports for many biological applications is being actively explored.28-32,35,38,39 Because the redox reactions occur in vivo, there exists a possibility of forming various oxidation states of metal ions in the presence of antioxidants, especially those having phenolic groups.35 The aim of this work is to prepare Ag nanoparticles by dihydroxy benzenes in the absence of any stabilizer. Most of the work reported so far has dwelt on the synthesis of silver nanoparticles in the presence of amino acids and phenolic compounds; however, there are relatively few studies on the mechanistic aspects regarding the formation and stabilization of Ag nanoparticles.40 Experimental Procedures Reagents. Catechol, hydroquinone, resorcinol, AgClO4, and sodium azide were obtained from Aldrich. Na2HPO4 and KH2PO4 were obtained from BDH. MnO2 was obtained from Sarabhai Chemicals. All reagents were used as received without further purification. Millipore purified water was used in making the solutions. IOLAR grade N2 and N2O gas (purity g99.99%) used for purging solutions was obtained from Indian Oxygen Limited. All solutions were prepared just before the experiments and kept in the dark to avoid any photochemical reactions. Appropriate amounts of Ag+ were placed in a quartz cuvette and saturated with N2 or N2O, and to these, requisite quantities of dihydoxy benzenes were added during bubbling. As a precautionary check on dihydroxy benzene stability, its absorption spectrum under identical conditions was always verified prior to pulse radiolysis study. All experiments were carried out at ambient temperature close to 27 °C. Methods. The 7 MeV pulse radiolysis kinetic spectrophotometric detection setup used in this study and the data analysis protocols have been discussed in detail before.41 Briefly, samples were irradiated in a 1 cm × 1 cm suprasil quartz cuvette kept at a distance of approximately 12 cm from the electron beam window, where the beam diameter was approximately 1 cm. An optical detection system comprised of a 450 W xenon arc lamp, lenses, mirrors, and a monochromator monitored the transient changes in absorbance of the solution following the electron pulse. An aerated 10-2 mol dm-3 KSCN solution was used for dosimetry, and the (SCN)2•- radical was monitored at 475 nm. The absorbed dose per pulse was calculated42 assuming G [(SCN)2•-] ) 2.6 × 10-4 m2 J-1 at 475 nm. Absorbed doses per pulse were on the order of 16-18 Gy (1 Gy ) 1 J kg-1). For practical purposes, the G-unit rather than the SI-unit for radiation chemical yields is used. The G-unit denotes the number of species formed or converted per 100 eV of absorbed energy in aqueous solution; G ) 1 corresponds to 0.1036 µM per 1 J of absorbed energy in aqueous solution.43 Characterization. Absorption measurements were carried out on a Chemito UV-vis spectrophotometer (model UV-2600). X-ray diffraction patterns were taken using a Phillips Analytical automated powder diffractometer employing Cu KR radiation. Samples for transmission electron microscopy (TEM) were prepared by putting (36) Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 14014. (37) Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.; Plech, A. J. Phys. Chem. B 2006, 110, 15700. (38) Moridani, M. Y.; Pourahmed, J.; Bui, H.; Siraki, K.; O’Brein, P. J. Free Radical Biol. Med. 2003, 34, 243. (39) Letan, A. J. Food Sci. 1966, 31, 395. (40) Selvakannan, P. R.; Swarni, A.; Srisathiyanarayanan, D.; Shirude, P. S.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2004, 20, 7825. (41) Kapoor, S.; Varshney, L. J. Phys. Chem. A 1997, 101, 7778. (42) Buxton, G. V.; Stewart, C. R. J. Chem. Soc., Faraday Trans. 1995, 91, 279. (43) Von Sonntag, C. The Chemical Basis of Radiation Biology; Taylor and Francis: New York, 1987.

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Figure 1. Absorption spectrum of silver particles formed on addition of an aqueous solution of 5 × 10-4 mol dm-3 catechol to 5 × 10-4 mol dm-3 AgClO4 solution at pH 7. a drop of the colloidal solution on a copper grid coated with a thin amorphous carbon film. The excess solvent was removed using a filter paper and letting the solvent evaporate at room temperature. Dried samples were kept under vacuum in a desiccator before they were placed in a specimen holder. TEM characterization was carried out using a JEOL JEM-2000FX electron microscope. Particle sizes were measured from the TEM micrographs and calculated by taking the average of at least 100 particles.

Results and Discussion Reduction of Silver Ions by Dihydroxy Benzenes at Ambient and Neutral pH. (a) Under Aerated Conditions. It was observed that when an aqueous solution of 5 × 10-4 mol dm-3 silver perchlorate was added to Millipore water containing 5 × 10-4 mol dm-3 hydroquinone or catechol at ambient pH 5.4 or 7, the color of the solution changed to yellow. The absorption spectrum of the solution showed absorption with a maximum around 420 nm, indicating the formation of the silver nanoparticles. A typical case for catechol is shown in Figure 1. After an hour, the color of the solution turned to pink, which showed an absorption maximum at around 520 nm, indicating aggregation of particles. The particles so formed were not stable and precipitated in 24 h. The previous experiments show that for the synthesis of silver nanoparticles, hydroquinone or catechol serve as the reducing agent. It is pertinent to mention here that in the case of resorcinol, the formation of silver nanoparticles was not observed at pH 7. It is known that metal sols are stabilized in solution by the presence of a charged double layer surrounding the colloidal nanoparticle. This produces a coulomb barrier that inhibits aggregation. Adsorption of uncharged adsorbate or neutralization of the surface charge often lead to aggregation and show a pink color in the sol. An aqueous solution of dihydroxy benzene is feebly acidic. The pKa1 and pKa2 values for hydroquinone (9.9 and 11.5), resorcinol (9.3 and 11.2), and catechol (9.4 and 13.0) confirm that at pH 7, all of them exist in an undissociated form, that is, the O-H bond is not dissociated.44 It is well-known that catechol and hydroquinone oxidize readily to the corresponding quinones.45,46 However, in the case of resorcinol, no quinone is possible upon two-electron oxidation. This could be the reason for not observing silver particles in the presence of resorcinol. (b) Under Deaerated Conditions. To study the effect of O2 in the formation of Ag nanoparticles, an aqueous solution of AgClO4 at pH 7 was N2-bubbled for 10 min. This was followed by the (44) Huie, R. E.; Neta, P. J. Phys. Chem. 1985, 89, 3918. (45) Brede, O.; Kapoor, S.; Mukherjee, T.; Hermann, R.; Naumov, S. Phys. Chem. Chem. Phys. 2002, 4, 5096. (46) (a) Land, E. J. J. Chem. Soc., Faraday Trans. 1993, 89, 803. (b) Roginsky, V. A.; Pisarenko, L. M.; Bors, W.; Michael, C.; Saran, M. J. Chem. Soc., Faraday Trans. 1999, 94, 1835.

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Figure 2. Absorption spectrum of silver particles formed on mixing an aqueous solution of 5 × 10-3 mol dm-3 AgClO4 and 0.5 mol dm-3 resorcinol at pH 10.5 under N2-saturated conditions.

addition of hydroquinone or catechol while N2 gas was bubbled. It was observed that the color of the solution remained completely transparent with no visible tinge of yellow. This is in agreement with previous results obtained for Cu(II) complexes with phenolic derivatives.47 The results show that under these conditions, the reduction of silver ions, if at all, is insignificant. However, at pH values higher than 10.2, the absence of O2 had no affect on the formation of Ag nanoparticles. The formation of Ag nanoparticles in deaerated solutions at high pH values and in the presence of dihydroxy benzenes is probably due to the formation of Ag2O at high pH values and/or due to oxidation of dihydroxy benzenes (Figure 2). It is pertinent to mention here that Huang et al.48 have reported the formation of Ag colloids by the reduction of Ag+ ions in basic and air-saturated solutions of 2-propanol. It is proposed that the reduction of Ag+ ions takes place on Ag2O particles, which result from the interaction of Ag+ ions with OH-. Thus, as a priori, one can say that by inhibiting the formation of oxidation products of dihydroxy benzene and/ or the colloidal support Ag2O, the reduction of Ag+ ions can be avoided. Indeed, we have observed this phenomenon at pH 7 and in the absence of O2. Thus, it seems that the solution pH has a profound impact on the growth of the silver nanoparticles and that under inert conditions, increasing the solution pH accelerates the formation of Ag nanoparticles. It is known that MnO2 oxidizes catechol and hydroquinone.49 Hence, to further confirm the role of oxygen in the reduction of Ag+ ions, the following experiment was carried out. An aqueous solution at ambient pH (5.4) containing 0.01% (w/v) MnO2 and 1 × 10-3 mol dm-3 Ag+ was N2-bubbled for 10 min. This was followed by the addition of 1 × 10-3 mol dm-3 hydroquinone or catechol while N2 gas was bubbled. It was observed that upon the addition of hydroquinone or catechol, the color of the solution changed to yellow. The UV-vis spectrum showed a surface plasmon absorption band of Ag nanoparticles (see Figure S1, Supporting Information). The previous experiment shows that under deaerated alkaline conditions, the formation of Ag nanoparticles is due to the oxidation of dihdroxy benzenes. However, the role of Ag2O cannot be ruled out completely. Transmission Electron Micrographs and X-ray Diffraction Studies. It was observed that at high pH values, the addition of dihydroxy benzene either under inert conditions or under aerated conditions resulted in the formation of a black precipitate. The phase structure of the prepared particle was characterized by X-ray diffraction (XRD). A representative case for silver particles (47) Mahal, H. S.; Kapoor, S.; Satpati, A. K.; Mukherjee, T. J. Phys. Chem. B 2005, 109, 24197. (48) Huang, Z.-Y.; Mills, G.; Hajek, B. J. Phys. Chem. 1993, 97, 11542. (49) Laha, S.; Luthy, R. G. EnViron. Sci. Technol. 1990, 24, 363.

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Figure 3. X-ray diffraction pattern of silver nanoparticles obtained at pH 10.5. Other conditions are the same as in Figure 1.

formed in the presence of catechol is shown in Figure 3, with peaks at 37.8, 43.8, and at 64.2°, which agree well with the (111), (200), and (220) diffractions of face centered cubic (fcc) silver (JCPDS file no. 04-0783). The absence of peaks in any other planes indicates a high purity of the product. The size of the Ag particles was calculated using Scherrer’s method50 and was found to be 30 nm. Similar results were obtained from TEM. The precipitation of the particles could be due to the aggregation of Ag particles.51 Recent SERS studies have shown that Ag nanoparticles undergo aggregation due to interaction with hydroquinone and catechol derivatives.52-54 In a recent study, Tripathi55 has shown the adsorption of a semi-quinone radical at the surface of the Ag particles at alkaline pH. Mechanism of Formation of Ag Nanoparticles in Presence of Hydroquinone and Catechol at pH 7. One of the objectives of the present work was to address the importance of the role played by the formation of the phenoxyl radical and subsequently the formation of quinone in the preparation of Ag nanoparticles. For this purpose, a pulse radiolytic study was employed to probe the initial reduction of silver ions and primary growth steps involved in the formation of nanocyrstallites. The radiolysis of water produces free radicals according to the stochiometry56,57 shown in eq 1

H2O ' •OH(0.28) + •H(0.06) + eaq-(0.27) + H2(0.05) + H2O2(0.07) + H3O+(0.27) (1) where the numbers in parentheses represent the G-values, the number of species formed per 100 eV of energy at pH 7 in µmol J-1. Total radical concentrations in this study were ∼5-10 µM per pulse. Scavenging of either •OH radicals or eaq- can be used to control reductive or oxidative conditions. For studying oneelectron oxidation reactions, the solutions were purged with N2O, which quantitatively converted the hydrated electron, eaq-, to the •OH radical (50) Langford, J. L.; Wilson, A. J. C. J. Appl. Cyrstallogr. 1978, 11, 102. (51) Panigrahi, S.; Praharaj, S.; Basu, S.; Ghosh, S. K.; Jana, S.; Pande, S.; Vo-Dinh, T.; Jiang, H.; Pal, T. J. Phys. Chem. B 2006, 110, 13426. (52) Biswas, N.; Kapoor, S.; Mahal, H. S.; Mukherjee, T. Chem. Phys. Lett. 2007, 444, 338. (53) Sanches-Cortes, S.; Garcia-Ramos, J. V. Appl. Spectrosc. 2000, 54, 230. (54) Sanches-Cortes, S.; Garcia-Ramos, J. V. Spectrochim. Acta, Part A 1999, 55, 2935. (55) Tripathi, G. N. R. J. Am. Chem. Soc. 2003, 125, 1178. (56) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (57) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry; J. Wiley and Sons Inc.: New York, 1990.

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eaq- + N2O + H2O f N2 + •OH + OH-

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(2)

k2 ) 9.1 × 109 M-1 s-1 As the •OH radical can add or oxidize the organic compounds, a specific one-electron oxidant, N3•, was produced by the following reaction:56,58

N3- + •OH f N3• + OH-

(3)

For studying the reaction of only eaq-, conditions were achieved by presaturating the solutions with N2 in the presence of 1.0 mol dm-3 tert-butanol, which acts as a scavenger for the •OH radical. The hydrated electron shows an absorption maximum at ∼700 nm. The tert-butanol radical formed is quite innocuous and does not have any absorbance in the visible range. The reactivity of eaq- toward dihydroxy benzenes was studied by monitoring the decay of eaq- at 700 nm. It was observed that the reaction of eaqtoward dihydroxy benzenes is slow (k e 106 dm3 mol-1 s-1). However, the time-resolved spectrum showed a clear difference in the spectral profile at a longer time scale (1.8 ms). The transient absorption spectrum at 1.8 ms in the presence of hydroquinone is shown in Figure 4. The difference in the absorption spectrum might be due to the formation of higher Ag aggregates in the presence of hydroquinone. To further confirm the aforementioned results, a N2-bubbled aqueous solution at pH 7 containing 1 × 10-4 mol dm-3 Ag+, 1 mol dm-3 tert-butanol, and 1 × 10-4 mol dm-3 hydroquinone was irradiated by two consecutive electron pulses. We noticed that following the first electron pulse, a second pulse to the same solution resulted in the faster growth of transient absorption at all wavelengths of interest. A typical case at a wavelength of 410 nm is shown in Figure 5. During radiolysis of aqueous solutions, silver seeds are produced by the reduction of silver ions. Initially, small silver clusters or aggregates are formed from seeds that grow further into large clusters. Although there was no evidence of the formation of silver nanoparticles instantly after the first pulse, however, in the solution containing silver clusters prepared by irradiating them with a single electron pulse, the formation of silver nanocrystallites does occur with some induction time even in the absence of oxygen. With the progression of time, the appearance of the surface plasmon absorption band can be easily followed by a change in the optical density at the wavelength of the absorption maximum. Figure 6 shows the progressive evolution of the UV-vis spectra of silver nanoparticles. A plot of the absorbance at λmax versus reaction time yields a sigmoidal curve, which indicates that the formation of Ag nanoparticles has an induction period. Results are depicted in Figure 7. Similar results were obtained in the presence of catechol. The previous results clearly show that the formation of Ag nanoparticles in the presence of hydroquinone and catechol depends on the generation of initial Ag clusters. The development of yellow Ag sol after a certain induction time shows that the seeds generated were low in concentration and hence took time to aggregate to form Ag nanoparticles. We have tried to increase the concentration of seeds by using electron pulses of high dose. It was observed that with an increase in the absorbed dose, the induction time decreases (Figure 7A). However, an increase in the dose caused aggregation of the particles (Figure 7B). The color of the sol changed from yellow to pink. It is important to mention here that under identical conditions but without Ag+ ions in the solution, we have not observed any absorption in the visible region. It is (58) Neta, P.; Huie, R. E.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 1027.

Figure 4. Transient absorption spectrum obtained on pulse irradiation of N2-bubbled aqueous solution containing 1 × 10-4 mol dm-3 AgClO4 and 1 mol dm-3 tert-butanol at pH 7. (O) Without hydroquinone and (b) in the presence of 1 × 10-4 mol dm-3 hydroquinone.

Figure 5. Kinetic traces of the transient at 410 nm on pulse irradiation of a N2- bubbled aqueous solution containing 1 × 10-4 mol dm-3 AgClO4, 1 mol dm-3 tert-butanol, and 1 × 10-4 mol dm-3 hydroquinone at pH 7. (a) After one pulse and (b) after second pulse to the same solution.

Figure 6. Absorption spectrum of silver nanoparticles formed after irradiation of N2-bubbled aqueous solution by a 500 ns electron pulse containing 1 × 10-4 mol dm-3 AgClO4, 1 mol dm-3 tertbutanol, and 1 × 10-4 mol dm-3 hydroquinone at pH 7. Dose ) 16 Gy. (a) After 5 min, (b) after 6 min, (c) after 7 min, (d) after 47 min, (e) after 4 h, (f) after 20 h, and (g) after 191 h. Absorption spectra are shown only for specific times to make them more presentable.

pertinent to mention here that Platzer et al.59 have also shown a similar phenomenon for silver aggregates in a nafion membrane. In the previously mentioned pulse radiolysis experiments, most of the eaq- react with Ag+ ions as the bimolecular rate constant56 for the reaction of eaq- with Ag+ ions is 3.5 × 1010 dm3 mol-1 s-1. The concentration of eaq- in the present study was 4.5 µM. Thus, the observed increase in the absorbance in Figures 6 and (59) Platzer, O.; Amblard, J.; Marignier, J. L.; Belloni, J. J. Phys. Chem. 1992, 96, 2334.

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Figure 7. (A) Increase in absorbance of silver particles at λ ) 404 nm after pulse irradiation of N2-bubbled aqueous solution containing 1 × 10-4 mol dm-3 AgClO4, 1 mol dm-3 tert-butanol, and 1 × 10-4 mol dm-3 hydroquinone at pH 7. (a) Dose ) 16 Gy and (b) dose ) 44 Gy. Absorbance values are plotted only for initial time to show clearly the induction period. (B) Absorption spectrum of silver particles formed after irradiation of N2-bubbled aqueous solution by a 500 ns electron pulse containing 1 × 10-4 mol dm-3 AgClO4, 1 mol dm-3 tert-butanol, and 1 × 10-4 mol dm-3 hydroquinone at pH 7. Dose ) 44 Gy. (a) After 4 min, (b) after 5 min, (c) after 6 min, (d) after 1.30 h, (e) after 6 h, (f) after 27 h, and (g) after 72 h. Absorption spectra are shown only for specific times to make them more presentable.

7 after the radiolytic reduction is probably due to the reduction of unreduced silver ions at the surface of silver aggregates and/ or nanoparticles. This shows that catechol and hydroquinone participate as reductants in the redox transformation of the Ag+ ions. This is similar to a process well-known in photography.60-62 The morphology of silver nanoparticles formed in the presence of hydroquinone (Figure 6) was further established by recording TEM images of the silver colloid suspension prepared by electron pulse irradiation at pH 7 in the early (15 min) and final stages (24 h) (parts a and b, respectively, of Figure 8). As shown in Figure 8, no discernible changes of the silver nanoparticles are observed, indicating that silver nanoparticles are stable. The aforementioned results obtained in pulse radiolysis at pH 7 and under steady state inert conditions suggest that the high pH or silver clusters are necessary in forming and stabilizing the silver nanoparticles. It is well-known that the catalytic efficiency of a metal nanoparticle for an electron-transfer process is closely related to the size-dependent redox properties that control its role as an electron relay. As a general rule, the reduction should (60) Mostafavi, M.; Delcourt, M. O.; Belloni, J. J. Imaging Sci. Technol. 1994, 38, 54. (61) Belloni, J.; Mostafavi, M.; Marignier, J. L.; Amblard, J. J. Imaging Sci. Technol. 1991, 35, 68. (62) Belloni, J.; Mostafavi, M. In Metal Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.; J. Wiley and Sons Inc.: New York, 1999; Vol. III, p 1213.

Figure 8. TEM images of silver nanoparticles.

be determined by the difference in the potential between the reductant and the oxidant used. The redox potential of the silver cluster is size-dependent, and it increases with an increase in size. Thus, it appears that after a certain critical size, catechol and hydroquinone are able to donate electrons to Ag clusters, which finally lead to the formation of Ag nanoparticles. To rule out the possibility of any initiation of reduction of Ag+ ions by a phenoxyl radical, we selectively generated phenoxyl radicals in a N2O-saturated aqueous solution containing Ag+ and N3- at pH 7. It was observed that the phenoxyl radical decay rate does not change significantly in the presence of Ag+ ions. Kinetic traces at different wavelengths are shown in Figure 9. At this juncture, it would be worthwhile to compare somewhat similar results obtained by Belloni et al.60-62 in the case of napthazarin. Belloni and co-workers measured the redox potential of Ag clusters and electron-transfer reactions from semiquinone of napthazarin to Ag aggregates. They attributed the transfer of electrons from a semiquinone radical to Ag aggregates. Although the property that was monitored by us is different, the results are somewhat similar.

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of the potential implications in understanding the role of metal nanoparticles for their use in bio-related applications. The present investigation was undertaken because of its relevance and the important conclusions are as follows. In this article, we have shown that silver ions get reduced and form silver nanoparticles in the presence of dihydroxy benzenes. The reduction, if plausible, occurs only when the oxidation of dihydroxy benzenes to quinone is feasible. When the studies were repeated at high pH values, reproducible results confirmed that the position of the -OH group had no influence on the reduction mechanism. As a corollary, it was proposed that the nature of dihydroxy benzenes would not have any role at pH 10.5. Figure 9. Decay kinetics traces obtained on pulsing N2O-saturated solution of 5 × 10-2 mol dm-3 sodium azide and 1 × 10-3 mol dm-3 hydroquinone at pH 7. (a) In the absence of silver ions at 360 nm, (b) in the presence of 1 × 10-4 mol dm-3 AgClO4 at 360 nm, (c) in the absence of silver ions at 410 nm, and (d) in the presence of 1 × 10-4 mol dm-3 AgClO4 at 410 nm. Hydroquinone is added while purging the solution.

Conclusion Understanding the reaction mechanism of the formation of metal nanoparticles in the presence of biologically important compounds is of immense interest to many researchers because

Acknowledgment. The authors are grateful to Dr. S. K. Sarkar, Head, Radiation, and Photochemistry Division, for his encouragement during the course of this study. The authors are also thankful to the entire LINAC team for providing the facility. Supporting Information Available: UV-vis spectrum of the Ag nanoparticles obtained in N2-bubbled aqueous solution containing Ag+, MnO2, and catechol. This material is available free of charge via the Internet at http://pubs.acs.org. LA702073R