Synthesis of Aqueous Au Coreâ Ag Shell Nanoparticles Using

Langmuir 2004, 20, 7825-7836

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Synthesis of Aqueous Au Core-Ag Shell Nanoparticles Using Tyrosine as a pH-Dependent Reducing Agent and Assembling Phase-Transferred Silver Nanoparticles at the Air-Water Interface PR. Selvakannan,† Anita Swami,† D. Srisathiyanarayanan,† Pravin S. Shirude,‡ Renu Pasricha,† Anandrao B. Mandale,† and Murali Sastry*,† Nanoscience Group, Materials Chemistry Division, and Organic Chemistry (Synthesis) Division, National Chemical Laboratory, Pune - 411 008, India Received March 22, 2004. In Final Form: June 3, 2004 We demonstrate that the amino acid tyrosine is an excellent reducing agent under alkaline conditions and may be used to reduce Ag+ ions to synthesize stable silver nanoparticles in water. The tyrosine-reduced silver nanoparticles may be separated out as a powder that is readily redispersible in water. The silver ion reduction at high pH occurs due to ionization of the phenolic group in tyrosine that is then capable of reducing Ag+ ions and is in turn converted to a semi-quinone structure. These silver nanoparticles can easily be transferred to chloroform containing the cationic surfactant octadecylamine by an electrostatic complexation process. The now hydrophobic silver nanoparticles may be spread on the surface of water and assembled into highly ordered, linear superstructures that could be transferred as multilayers onto suitable supports by the versatile Langmuir-Blodgett technique. Further, tyrosine molecules bound to the surface of Au nanoparticles through amine groups in the amino acid may be used to selectively reduce silver ions at high pH on the surface of the Au nanoparticles, thus leading to a simple strategy for realizing phase-pure Au core-Ag shell nanostructures.

Introduction There is much current interest in metal nanoparticles due to their fascinating electronic and optical properties and potential application in the areas of catalysis,1a-f single electron tunneling devices,2 nonlinear optical devices,3 electron microscopy markers,4 DNA sequencing,5a-d and the emerging area of plasmonics.6 Consequently, development of new synthesis procedures for metal nanoparticles is an essential aspect of nanoscience. There are a number of experimental recipes available in the literature for synthesizing gold nanoparticles of variable size and shape in both polar solvents7 and nonpolar organic solvents,8a-d the gold nanoparticles prepared by these methods showing excellent stability over long periods of time. * To whom correspondence should be addressed. E-mail: [email protected]. † Nanoscience Group, Materials Chemistry Division. ‡ Organic Chemistry (Synthesis) Division. (1) (a) Schmid, G. M. Chem. Rev. 1992, 92, 1709. (b) Le Bars, J.; Specht, U.; Bradley, J. S.; Blackmond, D. G. Langmuir 1999, 15, 7621. (c) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385. (d) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938. (e) Dai, J.; Bruening, M. L. Nano Lett. 2002, 2, 497. (f) Yoo, J. W.; Hathcock, D.; El-Sayed, M. A. J. Phys. Chem. A 2002, 106, 2049. (2) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. J.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (3) Galletto, P.; Brevet, P. F.; Girault, H. H.; Antoine, R.; Broyer, M. J. Phys. Chem. B 1999, 103, 8706. (4) Baschong, W.; Wrigley, N. G. J. Electron Microsc. Tech. 1990, 14, 313. (5) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (b) Reynolds, R. A.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 2000, 122, 3795. (c) Zanchet, D.; Micheel, C. M.; Parak, W. J.; Gerion, D.; Williams, S. C.; Alivisatos, A. P. J. Phys. Chem. B 2002, 106, 11758. (d) Nam, J.-M.; Park, S.-J.; Mirkin, C. A. J. Am. Chem. Soc. 2002, 124, 3820. (e) Parak, W. J.; Pellegrino, T.; Micheel, C. M.; Gerion, D.; Williams, S. C.; Alivisatos, A. P. Nano Lett. 2003, 3, 33. (6) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. Adv. Mater. 2001, 19, 1501.

Recognizing the need to develop metal nanoparticle synthesis processes that are eco-friendly, we have recently studied the possibility of using microorganisms such as fungi7b-d and biomolecules such as amino acids as reducing agents for metal ions and have shown that the amino acids tryptophan9a and aspartic acid9b may be used as versatile reducing agents in the synthesis of stable aqueous gold nanoparticles. However, these amino acids were found to be incapable of reducing silver ions to generate silver nanoparticles, which are known to be excellent candidates for application in surface-enhanced Raman spectroscopy (SERS).10,11 We address this issue and show in this paper that the amino acid tyrosine is capable of reducing silver ions under alkaline conditions to yield highly stable silver nanoparticles but not under neutral and acidic solution (7) (a) Handley, D. A. Colloidal Gold: Principles, Methods and Applications; Hayat, M. A., Ed.: Academic Press: San Diego, 1989; Vol. 1, Chapter 2. This comprehensive review by Hayat lists at least one dozen protocols for the syntheses of gold hydrosols with particle sizes in the range of 10-640 Å. (b) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Sainkar, S. R.; Khan, M. I.; Ramani, R.; Pasricha, R.; Kumar, P. V. A.; Alam, M.; Sastry, M.; Kumar, R. Angew. Chem., Int. Ed. 2001, 40, 3585. (c) Mukherjee, P.; Ahmad, A.; Mandal, D.; Senapati, S.; Sainkar, S. R.; Khan, M. I.; Ramani, R.; Pasricha, R.; Kumar, P. V. A.; Alam, M.; Sastry, M.; Kumar, R. Nano Lett. 2001, 1, 515. (d) Ahmad, A.; Senapati, S.; Khan, M. I.; Kumar, R.; Sastry, M. Langmuir 2003, 19, 3550. (8) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (c) Porter, L. A.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (d) Chen, S.; Murray, R. W. Langmuir 1999, 15, 682. (9) (a) Selvakannan, PR.; Mandal, S.; Phadtare, S.; Gole, A.; Pasricha, R.; Adyanthaya, S. D.; Sastry, M. J. Colloid Interface Sci. 2004, 269, 97. (b) Mandal, S.; Selvakannan, PR.; Phadtare, S.; Pasricha, R.; Sastry, M. Proc. Indian Acad. Sci. (Chem. Sci.) 2002, 114, 513. (10) Xiong, Y.; Xie, Y.; Wu, C. Z.; Yang, J.; Li, Z.; Xu, F. Adv. Mater. 2003, 15, 405. (11) (a) Nie, S.; Emory, S. R. Science 1997, 275, 1103. (b) Emory, S. R.; Haskins, W. E.; Nie, S. J. Am. Chem. Soc. 1998, 120, 8009. (c) Tripathi, G. N. R. J. Am. Chem. Soc. 2003, 125, 1178.

10.1021/la049258j CCC: $27.50 © 2004 American Chemical Society Published on Web 07/16/2004

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conditions. This pH-dependent reducing capability of tyrosine is shown to arise due to ionization of the phenolic group of tyrosine at high pH which by electron transfer to silver ions is then transformed to quinone (Scheme 1). The silver nanoparticles thus obtained are highly stable and may be separated as a powder and readily redispersed in water. The redispersibility of the silver nanoparticles in water indicates that they are capped by the oxidized tyrosine molecules. Hitherto, thiol-coordinated functional groups such as tiopronin,12,13 glutathione,14 mercaptosuccinic acid,15 sulfonic acid,16 and ammonium ions17,18 bound to nanoparticles have been shown to result in metal nanoparticles that are readily redispersible in water. A recent report describes the visible luminescence from such water-dispersible gold nanoparticles.19 The construction of two-dimensional organized assemblies of nanoparticles and predefined topologies such as linear structures is important for application as nanowires and in the newly emerging area of plasmonics.6 Highly monodisperse nanoparticles synthesized in nonpolar organic solvents have been shown to self-assemble into ordered structures during solvent evaporation on suitable substrates.20,21 However, these nanoparticle assemblies generally lack long-range order and good reproducibility. Other methods for nanoparticle assembly rely on immobilizing them on the surface of self-assembled monolayers21a,22 or surface-modified polymers21a,23 by covalent attachment, electrophoretic assembly onto suitable substrates,24,25 or immobilizing nanoparticles at the air-water interface by using electrostatic interactions between nanoparticles and oppositely charged Langmuir (12) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (13) Templeton, A. C.; Chen, S.; Gross, S. M.; Murray, R. W. Langmuir 1999, 15, 66. (14) Schaff, T. G.; Knight, G.; Shaffigulin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643. (15) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075. (16) Shon, Y. S.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 17, 1255. (17) Cliffel, D. E.; Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2000, 16, 9699. (18) Yonezawa, T.; Onoue, S.; Kimizuka, N. Langmuir 2000, 16, 5218. (19) Huang, T.; Murray, R. W. J. Phys. Chem. B 2001, 105, 12498. (20) Wang, Z. L. Adv. Mater. 1998, 10, 13. (21) (a) Sastry, M. Handbook of Surfaces and Interfaces of Materials. Volume 3: Nanostructured Materials, Micelles, and Colloids; Nalwa, H., Ed.; Academic Press: San Diego, 2001; Chapter 2, p 87 and references therein. (b) Brown, L. O.; Hutchison, J. E. J. Phys. Chem. B 2001, 105, 8911. (c) Pileni, M. P. J. Phys. Chem. B 2001, 105, 3358. (d) He, S.; Yao, J.; Jiang, P.; Shi, D.; Zhang, H.; Xie, S.; Pang, S.; Gao, H. Langmuir 2001, 17, 1571. (e) Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3353. (22) (a) Hutter, E.; Fendler, J. H.; Roy, D. J. Phys. Chem. B 2001, 105, 11159. (b) Nakanishi, T.; Ohtani, B.; Uosaki, K. J. Phys. Chem. B 1998, 102, 1571. (c) Chan, E. W. L.; Yu, L. Langmuir 2002, 18, 311. (d) Jiang, P.; Liu, Z. F.; Cai, S. M. Langmuir 2002, 18, 4495. (e) Okamoto, T.; Yamaguchi, I. J. Phys. Chem. B 2003, 107, 10321. (f) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (23) (a) Hao, E.; Lian, T. Chem. Mater. 2000, 12, 3392. (b) Malynych, S.; Luzinov, I.; Chumanov, G. J. Phys. Chem. B 2002, 106, 1280. (c) Sun, S.; Anders, S.; Hamann, H. F.; Thiele, J.-U.; Baglin, J. E. E.; Thomson, T.; Fullerton, E. E.; Murray, C. B.; Terris, B. D. J. Am. Chem. Soc. 2002, 124, 2884. (d) Tanaka, H.; Mitsuishi, M.; Miyashita, T. Langmuir 2003, 19, 3103. (e) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234. (24) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706.

Selvakannan et al.

monolayers.21a,26 Slightly different approaches involve the spontaneous growth of thin films of nanoparticles at the air-organic solvent interface27 and the diffusion of charged nanoparticles into thin, ionizable fatty lipid films.21a,26a Insofar as using the air-water interface for nanoparticle assembly is concerned, perhaps the oldest variant was based on the assembly of hydrophobic (water-insoluble) nanoparticles and formation of Langmuir-Blodgett (LB) films thereafter. This was demonstrated for the first time by Fendler and co-workers28 wherein compact, ordered arrays of monolayer protected clusters (MPCs) of different chemical compositions were formed by spreading the nanoparticle organic solution at air-water interface and then transferring the nanoparticle monolayers onto suitable substrates using the LB technique.28 Later, this approach was extensively used by other groups to form superlattices of gold29 and magnetic colloidal nanoparticles stabilized with different passivating agents.30 Chen and co-workers have shown that two-dimensional gold nanoparticle networks cross-linked by bifunctional bridging linkers such as TBBT (4,4′-thiobisbenzenethiol) may be achieved at the air-water interface.31 In an interesting application-related report, Brust and co-workers have demonstrated the fabrication of two-dimensional nanowires by the assembly of dodecanethiol-capped gold nanoparticles in the presence of the surfactant dipalmitoylphosphatidylcholine (DPPC) at the air-water interface.32 The unidirectional sintering of particles accompanied by packing into a mazelike structure was shown to be due to a templating effect of the surfactant at the molecular level. Recently, Xia and co-workers33 have demonstrated the use of the LB technique to assemble monolayers of aligned silver nanowires, which serve as excellent substrates for surface-enhanced Raman spectroscopy.33 The assembly of oleic acid stabilized silver nanoparticles on the surface of water and formation of multiparticulate films on different substrates have been reported.34a,b Heath and co-workers have demonstrated the spontaneous assembly of dodecanethiol-capped silver nanoparticles at the air-water interface into arrays of high aspect ratio (25) (a) Li, H. X.; Lin, M. Z.; Hou, J. G. J. Cryst. Growth 2000, 212, 222. (b) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (c) Chandrasekharan, N.; Kamat, P. V. Nano Lett. 2001, 1, 67. (26) (a) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35, 847. (b) Sastry, M. In Colloids and Colloid Assemblies; Caruso, F., Ed.; Wiley-VCH: Weinheim, 2004; Chapter 12, p 369. (27) (a) Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902. (b) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Agrawal, V. V.; Saravanan, P. J. Phys. Chem. B 2003, 107, 7391. (28) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607 and references therein. (29) (a) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (b) Bourgoin, J. F.; Kergueris, C.; Lefevre, E.; Palacin, S. Thin Solid Films 1998, 327-329, 515. (c) Swami, A.; Kumar, A.; Selvakannan, PR.; Mandal, S.; Sastry, M. J. Colloid Interface Sci. 2003, 260, 367. (d) Swami, A.; Kumar, A.; Sastry, M. Proc. Indian Acad. (Chem. Sci.) 2003, 115, 185. (e) Huang, S.; Minami, K.; Sakaue, H.; Shingubara, S.; Takahagi, T. Langmuir 2004, 20, 2274. (f) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955. (g) Brust, M.; Stuhr-Hansen, N.; Nørgaard, K.; Christensen, J. B.; Nielsen, L. K.; Bjørnholm, T. Nano Lett. 2001, 1, 189. (h) Santhanam, V.; Andres, R. P. Nano Lett. 2004, 4, 41. (i) Brown, J. J.; Porter, J. A.; Daghlian, C. P.; Gibson, U. J. Langmuir 2001, 17, 7966. (30) Lefebure, S.; Me´nager, C.; Cabuil, V.; Assenheimer, M.; Gallet, F.; Flament, C. J. Phys. Chem. B 1998, 102, 2733. (31) (a) Chen, S. Adv. Mater. 2000, 12, 186. (b) Chen, S. Langmuir 2001, 17, 2878. (32) Hassenkam, T.; Nørgaard, K.; Iversen, L.; Christopher, J. K.; Brust, M.; Bjørnholm, T. Adv. Mater. 2002, 14, 1126. (33) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang P. Nano Lett. 2003, 3, 1229. (34) (a) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (b) Wang, W.; Chen, X.; Efrima, S. J. Phys. Chem. B 1999, 103, 7238. (c) Chung, S.-W.; Markovich, G.; Heath, J. R. J. Phys. Chem. B 1998, 102, 6685.

Synthesis of Au Core-Ag Shell Nanoparticles

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Scheme 2. Phase Transfer of Ag Nanoparticles and Formation of Their LB Films

Scheme 3. Formation of Au Core-Ag Shell Nanoparticles

wires whose width was found to be a function of the solvent and particle size.34c In the present study, we report on the hydrophobization of tyrosine-reduced silver nanoparticles, their assembly on the surface of water, and formation of ordered LB films on different substrates (Scheme 2). The hydrophobization of the silver nanoparticles was accomplished by vigorous shaking of a biphasic mixture of aqueous tyrosine-reduced silver nanoparticles and chloroform containing octadecylamine (ODA). Under suitable pH conditions, electrostatic interaction between positively charged protonated amine groups of ODA and negatively charged tyrosinereduced silver nanoparticles results in the formation of a hydrophobic sheath around the silver nanoparticles that are thereby rendered hydrophobic and soluble in chloroform. The hydrophobic tyrosine-capped silver nanoparticles may be spread on the surface of water, compressed, and transferred onto suitable supports by the LB method, yielding close-packed assemblies of silver nanoparticles. An interesting observation is the formation of quasi-linear assemblies of the silver nanoparticles on the surface of water that in many respects are similar to those observed by Heath and co-workers.34a Further, by capping gold nanoparticles with tyrosine, we show that the pH-dependent reducing capability of the tyrosine molecules may be used to reduce silver ions under alkaline conditions to yield a gold core-silver shell bimetallic structure (Scheme 3). It may be immediately recognized that since the reducing agent is bound to the surface of the gold nanoparticles and is not present in solution, silver ion reduction takes place only on the gold surface, thus avoiding nucleation and growth of silver nanoparticles in solution. We have recently synthesized gold core-silver shell nanoparticles using Keggin ions as UV-switchable reducing agents,35 and elucidation of tyrosine as a pH-switchable reducing agent that may be bound to nanoparticles represents an important advance in this line of research. Interest in such bimetallic nanoparticles of the core-shell structure partially stems (35) Mandal, S.; Selvakannan, PR.; Pasricha, R.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 8440.

from the fact that they have important applications in catalysis36 and in biological applications such as DNA sequencing.37 Presented below are details of the investigation. Experimental Section Chemicals. Chloroauric acid (HAuCl4), silver sulfate (Ag2SO4), sodium borohydride (NaBH4), potassium hydroxide (KOH), tyrosine (C9H11NO3), and octadecylamine (C18H39N) were obtained from Aldrich Chemicals and used as received. Synthesis of Tyrosine-Reduced Silver Nanoparticles. In a typical experiment, 10 mL of 10-3 M aqueous silver sulfate solution was taken along with 10 mL of 10-3 M aqueous solution of tyrosine and this solution was diluted to 100 mL with deionized water. To this solution, 1 mL of 10-1 M solution of KOH was added, and this solution (solution pH ∼ 10) was allowed to boil until the colorless solution changed into a yellow solution, indicating the formation of silver nanoparticles. To prove that the reducing ability of tyrosine occurred only at alkaline pH, a control experiment was carried out in which the above-mentioned reaction was carried out without the addition of KOH. It was observed that there was no formation of silver nanoparticles under neutral and acidic pH conditions. In a control experiment, the silver sulfate solution was boiled with KOH in the absence of tyrosine. Silver nanoparticles were not observed to form even after prolonged boiling, clearly underlining the crucial role of tyrosine as a pH-dependent reducing agent. Phase Transfer of Silver Nanoparticles from Aqueous to Chloroform Phase. Twenty-five milliliters of an aqueous dispersion of silver nanoparticles was taken, and its pH was adjusted to 5 using dilute hydrochloric acid (HCl). pH adjustment did not result in visible destabilization of the nanoparticles. This solution was taken along with 25 mL of 10-3 M solution of ODA in chloroform. Vigorous shaking of the mixture results in the (36) (a) Zhong, C.-J.; Maye, M. M. Adv. Mater. 2001, 13, 1507. (b) Wu, M.-L.; Chen, D.-H.; Huang, T.-C. Langmuir 2001, 17, 3877. (c) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (d) He, J.; Ichinose, I.; Kunitake, T.; Nakao, A.; Shiraishi, Y.; Toshima, N. J. Am. Chem. Soc. 2003, 125, 11034. (e) Doudna, C. M.; Bertino, M. F.; Blum, F. D.; Tokuhiro, A. T.; Lahiri-Dey, D.; Chattopadhyay, S.; Terry, J. J. Phys. Chem. B 2003, 107, 2966. (f) Lu, P.; Teranishi, T.; Asakura, K.; Miyake, M.; Toshima, N. J. Phys. Chem. B 1999, 103, 9673. (37) Cao, Y. W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961.

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transfer of silver nanoparticles from the aqueous to chloroform phase and is seen as a transfer of yellow color from water to chloroform. The colorless aqueous phase was then separated, and the process of phase transfer was continued with fresh tyrosine-reduced silver nanoparticle solution until the organic phase was fully saturated with silver nanoparticles (i.e., until almost all ODA molecules are consumed in complexing with the silver nanoparticles). After completion of the phase transfer, the organic phase was separated from the aqueous phase, rotavapped, and washed with ethanol to remove excess uncoordinated ODA molecules (if any). Removal of excess ODA was checked by monitoring the intensity of the signal at 3330 cm-1 which arises due to N-H stretching and bending vibrations of free amine groups in ODA. Moderate washing of the ODA-capped silver nanoparticles resulted in complete loss of the 3330 cm-1 band and indicated removal of uncoordinated ODA molecules (within detection limits of the Fourier transform infrared (FTIR) instrument). The purified ODA-capped silver nanoparticle powder (ODA-Tyr-Ag) could be readily redispersed in different organic solvents such as chloroform, toluene, hexane, and so forth. Organization of ODA-Tyr-Ag Nanoparticles at the AirWater Interface and Formation of LB Films. Thin multilayered films of the ODA-Tyr-Ag nanoparticles were formed in a Nima model 611 LB trough equipped with a Wilhelmy plate as the surface-pressure sensor and pure double-distilled water as the subphase. In a typical experiment, the ODA-Tyr-Ag nanoparticle solution was prepared in chloroform at a concentration of 1 mg/mL. An aliquot of 110 µL was then spread slowly onto the water surface in a dropwise fashion using a Hamilton microliter syringe. At least 15 min was allowed for solvent evaporation prior to pressure-area (π-A) isotherm measurements. π-A isotherms were recorded at different times after spreading of the ODA-Tyr-Ag nanoparticle monolayer at a compression rate of 20 cm2/min. After stabilization of the π-A isotherms, the Ag nanoparticle monolayers were deposited onto carbon-coated copper grids, quartz slides, and Si(111) wafers by the LB method at a controlled surface pressure of 40 mN/m at a substrate immersion/withdrawal speed of 10 mm/min. The quartz and Si(111) substrates were made hydrophobic prior to transfer of the Ag nanoparticles by deposition of three monolayers of lead arachidate onto the substrates. Metal salts of fatty acids (cadmium arachidate, lead arachidate, etc.) form stable monolayers strongly bound to substrates with oxide layers, and hydrophobization of the support resulted in significantly better transfer ratios of the nanoparticle monolayers. For the LB films grown on different substrates, monolayer transfer was observed during both upward and downward strokes of the substrate at close to unity transfer ratio. Synthesis of Au Core-Ag Shell Nanoparticles. A 10-4 M aqueous solution (100 mL) of chloroauric acid (HAuCl4) was reduced by 0.01 g of sodium borohydride (NaBH4) at room temperature. This procedure results in a ruby-red solution containing gold nanoparticles of dimensions 35 ( 7 Å (solution pH ∼ 9.5).38 The gold nanoparticles were capped with tyrosine by addition of 10 mL of an aqueous solution of 10-3 M tyrosine to 90 mL of the gold hydrosol. The tyrosine-functionalized gold nanoparticle solution was heated to remove excess sodium borohydride ions from the solution, following which this solution was allowed to age for 1 day. This solution was dialyzed using a semipermeable membrane and copious amounts of doubledistilled water to remove excess uncoordinated tyrosine molecules present in solution. To 90 mL of the dialyzed tyrosine-capped gold nanoparticle solution, 10 mL of 10-3 M Ag2SO4 and 1 mL of 10-1 M KOH solution were added, and the solution was allowed to boil until its color changed from purple to brownish yellow (Au/Ag core shell 1). To vary the thickness of the silver shell over the tyrosine-capped gold nanoparticles, 5 mL of 10-3 M silver sulfate solution was added to 95 mL of dialyzed tyrosine-capped gold nanoparticle solution and allowed to boil till completion of the reaction (Au/Ag core shell 2). Isothermal Titration Calorimetry (ITC). The binding of tyrosine to borohydride-reduced gold nanoparticles after dialysis was studied by ITC. These measurements were carried out in a (38) Patil, V.; Malvankar, R. B.; Sastry, M. Langmuir 1999, 15, 8197.

Selvakannan et al. MicroCal VP-ITC instrument in which 10 µL of 5 × 10-4 M aqueous tyrosine was repeatedly injected into 1.47 mL of the dialyzed gold nanoparticle solution and the calorimetric response was monitored. Conductivity Measurements. The real-time formation of the silver shell on tyrosine-capped gold nanoparticles was followed by monitoring the conductivity of the tyrosine-capped gold nanoparticle solution after pH adjustment using KOH and addition of Ag2SO4 solution. The conductivity of this solution as a function of time of reaction at 27 °C was measured using a LabIndia Pico conductivity meter with a conductivity electrode of cell constant 1. UV-Vis Spectroscopy Studies. The optical properties of the tyrosine-reduced silver nanoparticles, the ODA-Tyr-Ag nanoparticle solution in chloroform, the LB films of ODA-Tyr-Ag nanoparticles, and the Au core-Ag shell bimetallic nanoparticles were monitored on a Jasco V-570 UV/Vis/NIR spectrophotometer operated at a resolution of 1 nm. Transmission Electron Microscopy (TEM) Measurements. Samples for TEM analysis were prepared by placing drops from solutions of tyrosine-reduced silver nanoparticles, ODA-Tyr-Ag nanoparticles, and Au core-Ag shell bimetallic nanoparticles on carbon-coated copper grids. The films on the TEM grids were allowed to stand for 2 min, following which the extra solution was removed using a blotting paper, and the grid was allowed to dry prior to measurement. A monolayer of ODA-Tyr-Ag nanoparticles was deposited from the LB trough. TEM measurements were performed on a JEOL model 1200EX instrument operated at an accelerating voltage of 120 kV. FTIR Measurements. FTIR measurements of purified powders of tyrosine, tyrosine-reduced silver nanoparticles, pure ODA and ODA-Tyr-Ag nanoparticles, tyrosine-capped gold nanoparticles after dialysis, and the Au core-Ag shell bimetallic nanoparticles were carried out on a Perkin-Elmer FTIR Spectrum One spectrophotometer in the diffuse reflectance mode operating at a resolution of 4 cm-1. X-ray Photoemission Spectroscopy (XPS) Measurements. XPS measurements of ODA-Tyr-Ag nanoparticles, tyrosine-capped gold nanoparticles, and Au core-Ag shell bimetallic gold nanoparticles cast in the form of films onto Si(111) substrates were carried out on a VG MicroTech ESCA 3000 instrument at a pressure better than 10-9 Torr. The general scan and C 1s, O1s, Ag 3d, and Au 4f core level spectra were recorded with unmonochromatized Mg KR radiation (photon energy ) 1253.6 eV) at a pass energy of 50 eV and electron takeoff angle (the angle between the electron emission direction and the surface plane) of 60°. The overall resolution of measurements is thus 1 eV for the XPS measurements. The core level spectra were background corrected using the Shirley algorithm, and the chemically distinct species were resolved using a nonlinear leastsquares procedure. The core level binding energies (BEs) were aligned with respect to the Au 4f7/2 BE of 84 eV.

Results and Discussion Figure 1A shows the UV-vis spectra recorded from aqueous solutions of tyrosine (curve 1), tyrosine solution to which KOH was added (curve 2), a mixture of tyrosine and silver sulfate solution (curve 3), tyrosine-reduced silver nanoparticles (curve 4), and the tyrosine-reduced silver nanoparticles after drying in the form of a powder and redispersion in water (curve 5). The strong absorption at ca. 415 nm in curve 3 clearly indicates formation of silver nanoparticles, this absorption arising due to excitation of surface plasmons in the nanoparticles. It is clear from the controls that both pure tyrosine (curve 1), tyrosine solution after addition of KOH (curve 2), and the mixture of tyrosine and silver sulfate solution without the addition of KOH (curve 3, to simulate the conditions employed for silver nanoparticle synthesis) do not absorb in the visible region and therefore the absorption band at 410 nm in the case of the Ag2SO4 solution after reaction with tyrosine at pH 10 (by addition of KOH, Experimental Section; curve 4) is due to the silver nanoparticles alone. Tyrosine has the ability to reduce silver ions only under alkaline conditions,


Figure 1. (A) UV-visible absorption spectra of aqueous solutions of tyrosine before (curve 1) and after addition of KOH (curve 2), a mixture of tyrosine and silver sulfate solution (curve 3), a tyrosine-reduced silver nanoparticle solution (curve 4), and the tyrosine-reduced silver nanoparticles after drying as a powder and redispersion in water (curve 5). (B) FTIR spectra of pure tyrosine (curve 1) and tyrosine-reduced silver nanoparticles (curve 2).

while at other pH values, it is unable to reduce silver ions to yield silver nanoparticles. It is known that phenolic protons are weakly acidic. Therefore, under alkaline conditions phenols undergo deprotonation to give phenolate anions. The phenolate anions transfer electrons to the silver ions (one-electron reduction) to form metallic silver and are simultaneously transformed into semiquinone. Tyrosine contains a phenolic group which can be “switched on” as a reducing agent under alkaline conditions. The ionization of the phenolic group (Scheme 1) enables facile electron transfer from the phenolate ion to the silver cations, resulting in the formation of silver nanoparticles, the phenolate ions being converted into a semi-quinone type of structure concomitantly (Scheme 1). The pH at which the reduction takes place (pH 10) is well above the isoelectric point of tyrosine (pI ∼ 5.3), and therefore it is likely that the carboxylate groups of the oxidized tyrosine molecules complex with Ag+ ions bound to the silver nanoparticle core. The formation of a semiquinone type of structure which results from the oxidation of tyrosine can be confirmed by the red shift of the π-π* absorption of the phenyl ring of tyrosine at 270 nm (curve 1) to 285 nm (curve 2), which is characteristic of π-π* absorption due to the interconversion of phenol to quinone under alkaline conditions (Scheme 1). The tyrosinereduced silver nanoparticles display excellent stability over time both in solution and as a powder obtained after solvent evaporation. The powder obtained after complete removal of water is readily redispersible in water as can be seen from the optical absorption spectrum recorded from the aqueous dispersion (curve 5). A small peak centered at ca. 525 nm is observed in the redispersed nanoparticle solution that is attributed to some aggregation of the particles during solvent removal. Representative TEM images of the tyrosine-reduced silver nanoparticles (Figure 2A,B) reveal that the particles are spherical in shape and reasonably uniform in size. The selected area electron diffraction (SAED) pattern of these nanoparticles shown in the inset of Figure 2A reveals that the particles are polycrystalline in nature and the rings could be indexed based on the face-centered cubic (fcc) structure of silver. From the TEM images, the particle size distribution was measured, and the histogram obtained is shown in Figure 2C. The mean size of the particles was measured to be 22 nm with a standard deviation of 3.6 nm. Along with these larger particles, a

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Figure 2. Representative TEM images (A,B) of silver nanoparticles synthesized by the reduction of silver ions by tyrosine at pH 10. The selected area electron diffraction pattern of the silver nanoparticles in image A is shown as the inset. (C) Particle size distribution histogram of the silver nanoparticles measured from images A and B. The solid line is a Gaussian fit to the data.

small percentage of spherical nanoparticles in the size range of 1-10 nm are seen (Figure 2B). It is clear that under the present experimental conditions, the silver nanoparticles are not monodisperse and further work is required to improve the polydispersity and remove the bimodal nature of the distribution. The tyrosine-reduced silver nanoparticles are stable in solution and in the form of a powder over a period of several months. Identifying capping molecules to render powders of nanoparticles that are readily redispersible in water is considerably more difficult than obtaining organically soluble nanoparticles,8 and our discovery that tyrosine promotes water-dispersion of silver nanoparticles is an important advance in this direction. FTIR studies of pure tyrosine (Figure 1B, curve 1) and tyrosine-reduced silver nanoparticles (Figure 1B, curve 2) show that the carbonyl stretching vibration from the carboxylate ion in tyrosine occurs at 1610 cm-1 in the case of pure tyrosine but shifts to 1674 cm-1 after oxidation of tyrosine (curve 2). This shift may be attributed to formation of a quinone type structure due to oxidation of the phenolic group in tyrosine (Scheme 1).39 We believe that the Ag nanoparticles in this case are stabilized by oxidized tyrosine molecules bound to the Ag surface through amine groups. We tried to transfer these nanoparticles to chloroform as mentioned in the Experimental Section using the surfactant ODA as the phase transfer molecule. It was observed that the percentage of phase transfer of the tyrosine-capped silver nanoparticles from water to chloroform was strongly dependent on the pH of the aqueous nanoparticle solution and that phase transfer at pH 5 was most efficient. This clearly indicates that the nanoparticle transfer is governed by the electrostatic interactions between ODA and nanoparticle surface bound tyrosine molecules. The phase transfer of silver nanoparticles from aqueous to chloroform phase was monitored by UV-vis spectroscopy measurements. Curves 1 and 2 in Figure 3 correspond to the spectra of the aqueous silver nanoparticle solution before and after phase transfer while curves 3 and 4 are the spectra of the chloroform solution containing ODA before and after phase (39) Silverstein, R. B.; Bassler, G. C. Spectrometric Identification of Organic Compounds, 2nd ed.; John Wiley & Sons: New York, 1967; Chapter 3, p 88.

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Figure 3. UV-vis spectra of the silver sols before and after phase transfer. Curves 1 and 2: spectra of the aqueous tyrosinereduced silver solution before and after phase transfer, respectively. Curves 3 and 4: chloroform phase containing 10-3 M ODA before and after phase transfer of silver nanoparticles, respectively.

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Figure 5. (A) Ag 3d, (B) C 1s, and (C) N 1s XPS core level spectra recorded from the ODA-Tyr-Ag nanoparticle film deposited on a Si(111) substrate by drop-coating. The solid lines are nonlinear least-squares fits to the data.

transfer, respectively. The sharp absorption peak at 410 nm in the as-prepared silver nanoparticle solution (curve 1) is due to excitation of surface plasmons in the silver nanoparticles. The complete disappearance of this peak after vigorous shaking with the chloroform solution of ODA (curve 2) clearly indicates almost total phase transfer of the silver nanoparticles to the chloroform phase. This is also evident from the appearance of the sharp surface plasmon resonance at 417 nm observed in the spectrum of the chloroform phase after phase transfer (curve 4). The small damping and red shift of the surface plasmon band observed after the phase transfer are attributed to surface complexation of ODA and to a change in the refractive index (refractive indices: chloroform, 1.446; water, 1.33) of the medium in which nanoparticles are suspended.40 To understand the exact nature of interaction between ODA molecules and tyrosine-reduced silver nanoparticles, FTIR and XPS measurements were carried out. Figure 4A,B shows the FTIR spectra recorded from powders of pure ODA (curve 1) and ODA-Tyr-Ag nanoparticles (curve 2) taken in a KBr pellet in different spectral windows. The peaks obtained at 3330 cm-1 (feature a) and 1565 cm-1 (feature b) in the case of pure ODA (curve 1) can be

attributed to N-H stretching and bending vibrations, respectively. These features are completely broadened in the case of the ODA-Tyr-Ag nanoparticle powder (curve 2). The appearance of a new peak at 1601 cm-1 (feature c) in curve 2 is due to the carbonyl stretching frequency from the carboxylate groups in the silver nanoparticle surface bound tyrosine molecules. The carbonyl stretch for carboxylic acids is known to occur at close to 1700 cm-1.39 The shift to lower wavenumbers clearly implies that the carboxylic acid group is ionized and that it is most likely complexed electrostatically with the protonated amine groups of ODA. The features d and e centered at 2920 and 2850 cm-1 are due to methylene antisymmetric and symmetric vibrations, respectively, which are common in both cases. Thus, the disappearance of N-H stretching and bending vibrations and the appearance of carboxylate stretching vibrations clearly indicates that the amine group of ODA interacts with the carboxylic acid group of tyrosine molecules bound to the surface of the silver nanoparticles (Scheme 2). This supports the experimental observation that efficient phase transfer of the silver nanoparticles requires suitable adjustment of pH (pH 5) of the aqueous nanoparticle solution. The ODA-Tyr-Ag nanoparticles in chloroform were dropcoated in the form of a film onto a Si(111) substrate and analyzed by XPS. The general scan spectrum of the film showed the presence of strong C 1s, N 1s, and Ag 3d core levels with no evidence of impurities. The film was sufficiently thick, and therefore no signal was measured from the substrate (Si 2p core level). The Ag 3d, C 1s, and N 1s core levels recorded from the film are shown in Figure 5A-C, respectively. The spectra have been background corrected using the Shirley algorithm41 prior to curve resolution, and the core levels were aligned with respect to the adventitious C 1s BE of 285 eV. The Ag 3d core level spectrum (Figure 5A) could be stripped into two chemically distinct spin-orbit pairs. Two chemically distinct Ag 3d5/2 components are observed at 368 and 370 eV BEs (labeled 1 and 2 in Figure 5A). The low BE component is attributed to electron emission from the silver metal nanocore42 while the high BE component arises from silver ions, indicating that a small fraction of unreduced Ag+ ions remain bound to the surface of the nanoparticles. The C 1s core level spectrum recorded from the nanoparticle film could be stripped into three main chemically distinct components at 285, 286.4, and 288.1 eV (Figure 5B). The high BE component observed at 288.1 eV can be assigned to the carboxylate carbon from the silver-bound tyrosine mol-

(40) (a) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (b) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668.

(41) Shirley, D. A. Phys. Rev. B 1972, 5, 4709. (42) Fadley, C. S.; Shirley, D. A. J. Res. Nat. Bur. Stand. (U.S.) 1970, 74A, 543.

Figure 4. FTIR spectra of powders of pure ODA (curve 1) and ODA-Tyr-Ag nanoparticles (curve 2) in the spectral windows (A) 3500-2800 cm-1 and (B) 1800-1500 cm-1.


Figure 6. π-A isotherms during one compression and expansion cycle (indicated by arrows), recorded at different time intervals after spreading the ODA-Tyr-Ag nanoparticle solution in chloroform on the surface of pure water. Curve 1, t ) 15 min; curve 2, t ) 1 h; curve 3, t ) 2 h; curve 4, t ) 3 h; curve 5, t ) 4 h.

ecules. The 286.5 eV BE peak is attributed to the carbon adjacent to the carboxylic acid group in tyrosine as well as the carbon bound to the amine groups in ODA. Two chemically distinct components are observed at 400 and 401.5 eV in the case of the N 1s core level spectrum (Figure 5C). The presence of two N 1s signals is indicative of protonated amine groups in ODA and surface-bound amine groups from tyrosine (Scheme 2). Formation of LB Films of Hydrophobic Silver Nanoparticles. The assembly of ODA-Tyr-Ag nanoparticles into monolayers on the surface of water was followed by π-A isotherm measurements. Figure 6 shows the π-A isotherms recorded as a function of time after spreading the ODA-Tyr-Ag nanoparticle Langmuir monolayer on double-distilled water. Curve 1 is the isotherm recorded 15 min after spreading the monolayer, while curves 2-5 are the isotherms recorded 1, 2, 3, and 4 h after spreading the monolayer, respectively. The arrows in the isotherms indicate the compression and expansion cycles of the monolayer. During compression, the surface pressure builds up to reach a limiting value of ∼40 mN/m (curve 1). However, it is observed that the monolayer expands gradually as a function of time, which is seen clearly by an increase in takeoff area from 300 cm2 at time t ) 15 min (curve 1) to 420 cm2 at t ) 3 h (curve 4). Minimal difference is seen in the isotherms recorded at time t ) 3 h (curve 4) and t ) 4 h (curve 5), indicating that the silver nanoparticle monolayer has stabilized 3 h after spreading the monolayer. From the π-A isotherm measurements (Figure 6, curve 4), a region of reasonably large incompressibility is seen to occur up to a surface pressure of ca. 50 mN/m, and therefore multilayer films of ODATyr-Ag nanoparticles of different thickness were transferred onto different substrates at a surface pressure of 40 mN/m by the LB technique 3 h after spreading the monolayer for further studies. During expansion of the monolayer, some hysteresis is observed, but this is not surprising given that the hydrophobic Ag nanoparticles cannot be considered to be truly amphiphilic. The hysteresis observed may also arise due to rearrangement of the Ag nanoparticles within domains (and reorganization of the domains themselves) on release of surface pressure with a time scale larger than the experimentally controlled expansion rate of the monolayer. The advantage of organizing the nanoparticles on the surface of water is that ordered, large-scale assemblies of the nanoparticles

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can be achieved which can subsequently be transferred to different substrates for further characterization. Figure 7A-D shows representative TEM images recorded at different magnifications from one monolayer of ODA-Tyr-Ag nanoparticles transferred by the LB method from the air-water interface onto a carbon-coated TEM grid. The low-magnification images (A,B) reveal that the silver nanoparticles assemble into linear, nanowire-like superstructures extending up to 3-4 µm in length. Each individual linear assembly is observed to consist of highly ordered domains of silver nanoparticles in a close-packed configuration (Figure 7C). The hexagonally close-packed structure of the silver nanoparticles is clearly seen in the Fourier transform of the region bounded by the square in Figure 7C. The hexagonal arrangement of the spots in the Fourier transform (inset, Figure 7C) attests to the existence of reasonably long-range ordering of the silver nanoparticles within the linear domains. From the periodicity of the spots in the Fourier transform, the average silver nanoparticle core-core separation was determined to be ca. 24 nm. We recollect that the average diameter of the silver particles was determined to be 22 ( 3.6 nm.22 The center-center distance of 24 nm implies that the hydrophobic sheath surrounding the particles contributes 2 nm to the particle separations. Thus the bilayer thickness of the stabilizer on the surface of the silver nanoparticles is ca. 2 nm, which is less than twice the length of the stabilizer ODA calculated from the empirical formula of Bain et al. (l ) 0.25 nm + 0.127n nm, where n is the number of CH2 groups, and 0.25 nm takes into account the amine and methyl terminal functional groups; l ) 2.41 nm for ODA).43 Thus, there is considerable interdigitation of the hydrocarbon chains of ODA in the close-packed silver nanoparticle assemblies. The highmagnification image of the silver nanoparticles in the linear superstructures shows that the particles are faceted with significant contrast within individual particles, indicating that they are multiply twinned nanocrystals (Figure 7D). For comparison, the TEM image recorded from a drop-coated film of ODA-Tyr-Ag nanoparticles is shown in Figure 7E. The average particle diameter determined for the silver nanoparticles in the drop-coated film is ca. 22 nm, which is the same as the particle size of the nanoparticles in the LB film (Figure 7D). This clearly indicates there is no sintering of the silver nanoparticles on the surface of water as observed previously in the case of ODA-capped gold nanoparticles organized at the airwater interface.29c The nanoparticles in the drop-coated film do not show any particular ordered packing or longrange order suggesting that compression of the silver nanoparticle monolayer on the surface of water and hydrophobic association of the interdigitating ODA chains are responsible for the ordered superstructures formed. At this stage, it is not clear why the particles assemble into linear superstructures and why so little of the totally available film area is covered with the particles. However, the rather small surface coverage obtained in the present case is consistent with previous reports for the twodimensional assembly of gold nanoparticles44 and may be rationalized in terms of strong interparticle repulsive interactions which we believe will be operative in the phase-transferred silver nanoparticles. The charge on the surface of the nanoparticles could arise either from (43) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (44) (a) Grabar, K. C.; Allison, K. J.; Baker, B. E.; Bright, R. M.; Brown, K. R.; Freeman, R. G.; Fox, A. P.; Keating, C. D.; Musick, M. D.; Natan, M. J. Langmuir 1996, 12, 2353. (b) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575.

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Figure 7. (A-D) Representative TEM images recorded at different magnifications from one monolayer of ODA-Tyr-Ag nanoparticles transferred onto a carbon-coated TEM grid by the LB method. The inset of image C shows the Fourier transform of the section outlined by the box. (E) TEM image of a drop-coated film of ODA-Tyr-Ag nanoparticles deposited on a carbon-coated TEM grid. (F) Electron diffraction pattern recorded from the nanoparticles shown in image D.

incomplete or over-charge-compensation of the negative charge by the ODA molecules (illustrated in Scheme 2 for incomplete charge-compensation by ODA of the negatively charged oxidized product of tyrosine). It is clear from the electron diffraction pattern recorded from the silver nanoparticles shown in Figure 7C that they are crystalline in nature. The characteristic rings in the diffraction pattern could be indexed as the (111), (200), (220), and (311) allowed reflections from fcc silver. LB films of the silver nanoparticles of different thickness were deposited on quartz substrates to study their optical properties (Scheme 2). Figure 8A shows the UV-vis spectra of the different LB films of silver nanoparticles (the number of monolayers in the film is indicated next to the respective curves). A broad absorption band centered at ca. 540 nm is observed in the LB films whose intensity increases with increasing number of monolayers in the LB film. The absorption band at 540 nm is due to excitation of surface plasmon vibrations in silver nanoparticles in the film. Nanosized metal particles exhibit a unique surface plasmon absorption band which is superimposed onto the exponential-decay Mie scattering profiles.40 Welldispersed, spherical silver nanoparticles in solution in general show a sharp surface plasmon resonance in the range of 400-420 nm (Figure 3, curve 4). The surface plasmon band of the silver nanoparticles in the LB films is considerably red-shifted and broadened, indicating considerable interaction between the nanoparticles due to the close-packed nature of organization of the nanoparticles (Figure 7C). A similar observation was made by Efrima’s group in the case of LB films of oleic acid capped silver nanoparticles.34b The position of the absorption band in the UV-vis spectra of the silver nanoparticle LB films is very close to that normally observed for dispersed gold nanoparticles,27 and indeed, direct viewing of the films showed that they possessed a purple color normally

associated with gold nanoparticles (picture shown in the inset of Figure 8B to the left). The LB films of the silver nanoparticles when viewed slightly off-normal showed that they had a golden hue (inset of Figure 8B, photo to the right). One of the problems associated with preparing thick LB films of pure, undiluted rigid cluster monolayers from the air-water interface is the poor monolayer transfer manifested by continuous breaking of the meniscus in both dipping directions (except for the first dipping cycle). However, in the present case, the absorption at 540 nm increases linearly with film thickness up to 24 monolayers (Figure 8B), clearly indicating close to unity monolayer transfer ratios up to this thickness. This is an interesting result given that the ODA-capped silver nanoparticles are hydrophobic and in that sense would not be expected to behave like a classical amphiphile. Earlier studies from this laboratory have shown that amino acids bind to gold nanoparticles through their amine groups, stabilize them against aggregation, and in certain cases render the surface of the nanoparticles sufficiently hydrophilic to make them water redispersible.45 It is clear from the above experiments that reduction of Ag+ ions by tyrosine involves the oxidation of the phenolic group while the rest of the molecule is relatively unperturbed. It should be possible, in principle, to bind tyrosine to the surface of gold nanoparticles though the amine group and then use the surface-bound phenolic groups of the amino acid to selectively reduce silver ions on the gold core under alkaline conditions to form Au core-Ag shell nanostructures (Scheme 3). We demonstrate the efficacy of this strategy below. ITC is an extremely powerful and sensitive thermodynamic technique to probe interactions of reacting species (45) Selvakannan, PR.; Mandal, S.; Phadtare, S.; Pasricha, R.; Sastry, M. Langmuir 2003, 19, 3545.


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Figure 9. (A) ITC data obtained during successive injections of 10 µL of 5 × 10-4 M aqueous tyrosine into 1.47 mL of aqueous borohydride-reduced gold nanoparticles. (B) The binding isotherm, obtained from the raw data shown in panel A where the total heat per injection (cal per mole of tyrosine solution injected) is plotted against the volume of the tyrosine added to the gold nanoparticle solution.

Figure 8. (A) UV-vis spectra recorded from ODA-Tyr-Ag nanoparticle LB films of different thicknesses transferred onto hydrophobized quartz substrates. The number of monolayers in the LB films is indicated next to the respective curves. (B) A plot of intensity of the surface plasmon resonance (absorbance at 540 nm) as a function of number of ODA-Tyr-Ag nanoparticle LB films grown on a quartz substrate. The inset shows photographs of a 40 monolayer LB film of the silver nanoparticles on a quartz substrate recorded normal to (a) and slightly off normal to the surface of the film (b).

in solution.46 ITC has been used with considerable success in studying biomolecular recognition processes in solution such as enzyme-substrate47 and protein-protein reactions,48 DNA hybridization,49 and metal ion binding to oligopeptides and proteins50 and processes occurring during processing of crude oil (interaction of surfactants with asphaltene).51 The basic principle behind the technique is fairly simple and involves measuring the heat of reaction of two interacting species in solution during (46) Jelesarov, I.; Bosshard, H. R. J. Mol. Recognit. 1999, 12, 3. (47) Cai, L.; Cao, A.; Lai, L. Anal. Biochem. 2001, 299, 19. (48) Pierce, M. M.; Raman, C. S.; Nall, B. T. Methods 1999, 19, 213. (49) (a) Kamiya, M.; Torigoe, H.; Shindo, H.; Sarai, A. J. Am. Chem. Soc. 1996, 118, 4532. (b) Holbrook, J. A.; Capp, M. W.; Saecker, R. M.; Record, M. T., Jr. Biochemistry 1999, 38, 8409. (50) Zhang, Y.; Akilesh, S.; Wilcox, D. E. Inorg. Chem. 2000, 39, 3057. (51) Merino-Garcia, D.; Anderson, S. I. Langmuir 2004, 20, 1473.

titration. Recognizing that ITC could provide valuable information on the binding of ligands to gold nanoparticles, we have attempted to apply it (to our knowledge for the first time) to understanding the strength and nature of the interaction of tyrosine with aqueous borohydridereduced gold nanoparticles. Figure 9A shows the ITC data obtained in an experiment wherein the heat released/ absorbed was measured during injection of 10 µL injections of aqueous tyrosine (5 × 10-4 M) into 1.47 mL of aqueous borohydride-reduced gold nanoparticles after thorough dialysis contained in the sample cell. As tyrosine is introduced into the reaction cell, it is seen that the interaction of the amino acid with the gold nanoparticles is highly exothermic, indicating strong binding of tyrosine with the gold surface. As the number of injections increases and the concentration of tyrosine in solution builds up, the exothermicity of the peaks is monotonically reduced, indicating that the free gold nanoparticle surface available for complexation of tyrosine is depleted. After ca. 15 injections, the surface of the gold nanoparticles is fully saturated with a monolayer of tyrosine and there is no further change in the exothermicity of the reaction. Figure 9B shows the binding isotherm determined from the raw ITC data of Figure 9A, where the total heat per injection (kcal per mole of tyrosine injected; obtained by integrating the heat evolved/absorbed during each injection) is plotted against the volume of the tyrosine solution added to the gold nanoparticle solution in the reaction vessel. The exothermic nature of the isotherm observed in this case clearly shows that tyrosine molecules bind to the gold nanoparticle surface (at pH 10, conditions of the ITC experiment). At pH 10, the amine groups in tyrosine are expected to be unprotonated (pI of tyrosine ∼ 5.6) and are therefore available for binding with the gold surface. Figure 10A shows the UV-vis absorption spectra recorded from the tyrosine-capped gold nanoparticles before (Figure 10A, curve 1) and after addition of KOH (curve 2) and after reaction with aqueous silver ions whose concentration is 10-4 M (curve 3) and 5 × 10-5 M (curve 4). While the optical properties of the tyrosine-capped gold nanoparticles do not change significantly after addition of KOH (curves 1 and 2), reaction of the amino acid capped gold nanoparticles with Ag+ ions does lead to large changes in the absorption spectra (compare curve 1 with curves 3 and 4). The common feature in the spectra of curves 3 and 4 after reaction of the tyrosine-capped gold nanoparticles with silver ions is the appearance of an absorption band

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Figure 10. (A) UV-visible spectra recorded from tyrosinecapped gold nanoparticles (curve 1); the tyrosine-capped gold nanoparticle solution after addition of KOH (curve 2); Au coreAg shell bimetallic nanoparticles synthesized as described in the Experimental Section wherein the silver ion concentration in solution is 5 × 10-5 M (curve 3) and 10-4 M (curve 4). (B) The absorbance values at 405 nm from the UV-vis absorption spectra plotted against time during the silver shell formation on the surface of tyrosine-capped gold nanoparticles (circles, right axis) and the solution conductivity data (stars, left axis).

centered at ca. 405 nm in addition to the plasmon vibration band of gold nanoparticles at 520 nm. The resonance at 405 nm is due to excitation of surface plasmon vibrations in silver nanoparticles52 that are formed by the reduction of Ag+ ions by the ionized tyrosine molecules present on the gold nanoparticle surface. For smaller amounts of Ag+ ions in the reaction medium (curve 3, 5 × 10-5 M silver ions), the damping of the gold plasmon band and intensity of the silver plasmon band are less than those observed for the 10-4 M Ag+ ion reaction case (curve 4). Since the tyrosine is bound to the surface of the gold nanoparticles (note that uncoordinated tyrosine was removed by dialysis of the gold nanoparticle solution), reduction of the silver ions is expected to occur only on the surface of the gold particles, leading to an Au core-Ag shell bimetallic structure (Scheme 3). The possibility of alloy formation may be ruled out since in the case of alloy formation, a single surface plasmon band is expected, the position of which would depend on the relative concentration of gold/ silver in the alloy.53 The kinetics of formation of the silver shell around the gold core in the 10-4 M Ag+ ion reaction was followed by UV-vis absorption spectroscopy (spectra not shown for brevity), and the absorption at 405 nm (due to the silver shell) was plotted against time (Figure 10B, circles, right axis). During this reaction, the conductivity of the reaction solution was monitored and is plotted in Figure 10B (stars, left axis). Immediately after the addition of silver ions to the tyrosine-capped gold nanoparticle solution at pH 10, it is seen that the plasmon resonance intensity increases rapidly and is accompanied by a large fall in solution conductivity, both of which indicate reduction of the metal ions to metallic silver. After ca. 24 h of reaction, the conductivity and plasmon absortion intensity achieve saturation indicating completion of the reduction of silver ions. Thus, the surface reduction of the Ag+ ions by tyrosine is fairly slow. It is interesting to note the faithful tracking of the solution conductivity with increase in the silver nanoshell plasmon intensity (Figure 10B). (52) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (b) Kumar, A.; Joshi, H.; Pasricha, R.; Mandale, A. B.; Sastry, M. J. Colloid Interface Sci. 2003, 264, 396. (53) (a) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (b) Sandhyarani, N.; Pradeep, T. Chem. Mater. 2000, 12, 1755.

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Figure 11A,B shows representative TEM images recorded from the tyrosine-capped gold nanoparticles after dialysis of the solution for 1 day. The gold particles are fairly spherical (6 nm average size) and are assembled into quasi-linear superstructures, presumably by hydrogen bonding between the tyrosine molecules bound to the gold nanoparticles.54 After the addition of silver ions to the solution, it was observed that the solution color turned rapidly to light blue, suggesting Ag+ ion induced aggregation of the tyrosine-capped gold nanoparticles. During heating of the Ag+-tyrosine-capped gold nanoparticle solution, it was observed that the blue color disappeared and that the solution attained a pale yellow color indicative of formation of nanoparticles of metallic silver. Representative images of the tyrosine-capped gold nanoparticles after reaction with aqueous solutions of 10-4 and 5 × 10-5 M Ag+ ions are shown in Figure 11C,D, respectively. The formation of a silver shell around the gold core is seen in many of the nanoparticles as a “halo” around a dark core. One of the particles showing this effect very clearly is identified by arrows in Figure 11C. While the halo could be observed in the case of the 10-4 M experiment, it was not so evident in the case of core-shell structures formed in the 5 × 10-5 M case (Figure 11D). In this experiment, we see that the gold nanoparticles are aligned in a chainlike superstructure. In the above experiments, care was taken to remove uncoordinated tyrosine in solution by dialysis so that silver ion reduction occurs only on the surface of the gold nanoparticles. We also carried out the reduction of aqueous 10-4 M Ag+ ions with tyrosine-capped gold nanoparticles without subjecting the solution to dialysis and obtained interesting nanostructures as shown in Figure 12A,B. In these TEM images, it is observed that the gold nanoparticles are assembled into a tenuous fibrous structure. A number of wires and rodlike structures together with extremely flat nanotriangles are observed (Figure 12B). While free tyrosine molecules in solution would be expected to reduce Ag+ ions to silver nanoparticles that are expected to be reasonably spherical (Figure 2A,B), we are currently unable to explain the presence of wires, rods, and triangular silver particles in the reaction medium. It does appear from the higher magnification TEM image that the wirelike and triangular silver structures grow outward from gold nanoparticles that presumably act as seeds. Figure 12C shows the FTIR spectra recorded from pure tyrosine (curve 1) and tyrosine-capped gold nanoparticles (curve 2). The symmetrical NH3+ bending vibration band observed in the case of pure tyrosine at 1510 cm-1 (curve 1) disappears in the case of tyrosine bound to the surface of the gold nanoparticles (curve 2), indicating strong binding of the amine group in tyrosine to the gold nanoparticles. The FTIR spectrum recorded from the tyrosine-capped gold nanoparticle solution after reaction with 10-4 M aqueous Ag+ ions is shown as curve 3 in Figure 12C. After formation of the silver shell around the gold core, a new band appears at 1680 cm-1 which is assigned to the carbonyl stretching frequency of the semi-quinone formed by the oxidation of the phenolic group in tyrosine. Usually the carbonyl stretch band is intense; we believe the reduction in intensity of this resonance is due to the fact that the tyrosine molecules in the gold core-silver shell structure are trapped beneath the silver shell. A chemical analysis of the tyrosine-capped gold nanoparticles and the gold core-silver shell nanostructures (54) Mandal, S.; Gole, A.; Lala, N.; Sastry, M. Langmuir 2001, 17, 6262.


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Figure 11. Representative TEM images (A,B) of tyrosine-capped gold nanoparticles after dialysis. (C,D) Representative TEM images of Au core-Ag shell nanoparticles synthesized under conditions where the silver ion concentration in solution was 10-4 and 5 × 10-5 M, respectively.

Figure 12. Representative TEM images (A,B) of anisotropic silver nanostructures originating from spherical gold nanoparticles capped by multilayers of tyrosine. (C) FTIR spectra of pure tyrosine (curve 1), tyrosine-capped gold nanoparticles (curve 2), and Au core-Ag shell bimetallic nanoparticles (curve 3).

was carried out by XPS. Figure 13A,B shows the Au 4f core level spectra recorded from the tyrosine-capped gold and gold core-silver shell nanoparticles, respectively. In both cases, the Au 4f signal could be decomposed into two spin-orbit pairs. The low BE Au 4f7/2 appears at 84 eV and is attributed to fully reduced metallic gold, while the presence of a higher BE component (BE ∼ 85.6 eV) indicates the presence of a small percentage of unreduced AuCl4- ions (Au3+) on the surface of the metallic gold core.

This result is in agreement with our earlier findings on the electrostatic complexation of alkylamines with aqueous gold nanoparticles.55 Figure 14A shows the Ag 3d core level spectrum recorded from the Au core-Ag shell nanoparticles. The Ag 3d core level could be decomposed into two chemically distinct (55) Kumar, A.; Mandal, S.; Selvakannan, PR.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277.

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Figure 13. Au 4f core level spectra recorded from tyrosinecapped gold nanoparticles (A) and Au core-Ag shell bimetallic nanoparticles (B). The spectra have been split into two chemically distinct spin-orbit pairs.

Figure 14. (A) Ag 3d core level spectrum recorded from the Au core-Ag shell nanoparticles. The spectrum has been decomposed into two chemically distinct spin-orbit pairs. O 1s core level spectra recorded from tyrosine-capped gold nanoparticles (B) and Au core-Ag shell nanoparticles (C). Two chemically distinct species are observed in each case (see the text for details).

species with Ag 3d5/2 BEs of 368 and 370.2 eV that are assigned to metallic silver and unreduced silver ions, respectively. The broad peak appearing at 378 eV is possibly the Auger electron signal from nitrogen atoms present in the gold nanoparticle surface bound tyrosine. As briefly discussed earlier, the phenolic part of tyrosine

Selvakannan et al.

is responsible for reduction of the Ag+ ions, in the process getting converted into a semi-quinone type of structure (Scheme 1). The O 1s core level binding energy of tyrosinecapped gold nanoparticles appears at 532.4 eV (Figure 14B), while it is shifted to 531.8 eV in the case of Au core-Ag shell bimetallic nanoparticles (Figure 14C). We believe this shift in O 1s BE is indicative of semi-quinone formation, a result in agreement with the FTIR and UV-vis spectroscopy results. In conclusion, it has been shown that stable and waterredispersible silver nanoparticles can be synthesized by the reduction of silver ions by tyrosine at alkaline pH. The fact that reduction of the silver ions does not occur under neutral and acidic pH conditions indicates that ionization of the phenolic group of tyrosine at high pH and reduction of the silver ions accompanied by formation of a semi-quinone is the likely mechanism for formation of the silver nanoparticles. Phase transfer of the tyrosinereduced silver nanoparticles to an organic phase was achieved by electrostatic complexation with fatty amines in the organic phase. The hydrophobic ODA-capped silver nanoparticles could then be organized on the surface of water, yielding close-packed nanoparticle Langmuir monolayers. The silver nanoparticles assembled into quasilinear superstructures with long-range hexagonal ordering upon compression of the nanoparticle monolayer on the surface of water. Excellent quality LB films of the silver nanoparticles could be grown on different substrates without significant variation in the nanoparticle density in the individual layers. Binding tyrosine to the surface of gold nanoparticles through the amine groups in the amino acid provides a versatile means of selectively reducing silver ions on the surface of the gold nanoparticles at alkaline pH to yield phase-pure Au core-Ag shell nanostructures. Acknowledgment. PR.S., A.S., and P.S.S. thank the Council of Scientific and Industrial Research, Government of India, for research fellowships. This work was partially sponsored by a grant from the Council of Scientific and Industrial Research (P23CMM0022), Government of India, which is gratefully acknowledged. LA049258J

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