Amino Acid

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Br--Induced Facile Fabrication of Spongelike Gold/Amino Acid Nanocomposites and Their Applications in Surface-Enhanced Raman Scattering Yan Liu, Lili Liu, and Rong Guo* College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, P. R. China Received March 28, 2010. Revised Manuscript Received July 11, 2010 We report a facile method for the fabrication of spongelike gold/amino acid nanocomposites by the addition of NaBr to glutamic acid-stabilized gold nanoparticles (GNPs) at room temperature. The gold/glutamic acid (Glu) nanocomposite is characterized by TEM, SEM, UV-vis spectroscopy, and XRD measurements. The results suggest that the three-dimensional spongelike gold/Glu nanocomposites with mean diameter of 50 nm are formed via the nanospheres fusing into one another. The driving force for the fabrication of spongelike gold/amino acid nanocomposites is the van der Waals attractive forces of Br- partially coated GNPs. Furthermore, the obtained spongelike gold nanocomposites can be used as surface-enhanced Raman scattering (SERS) substrates with high SERS activity and stability for detecting Rhodamine 6G (R6G) molecules. Hence, NaBr-mediated preparation of SERS substrates described in this work has potential applications in chemical and biological analysis as well as medical detection.

Introduction Fabricating gold with three-dimensionally networked porous architectures has received much attention in the recent year.1-4 Such nanomaterial has a wide range of applications including surface-enhanced Raman spectroscopy, efficient catalysts for chemical reactions, sensing, electrochemistry, and so on.2,4-6 The popular approaches for the generation of porous gold include dealloying of the Au-Ag alloy1,3,6,7 and directing synthesis using soft and hard templates.8-10 Generally, these methods involve some procedures for the removal of the sacrificial metal or organic template. Therefore, some intriguing approaches have been explored for the design and fabrication of such gold porous nanostructures.2,11,12 Gold nanoparticles (GNPs) are promising building blocks for assembling into nanostructured functional materials, and some *To whom correspondence should be addressed: Tel þ86-514-87971858, Fax þ86-514-87311374, e-mail [email protected].

(1) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450. (2) Gao, S.; Zhang, H.; Wang, X.; Yang, J.; Zhou, L.; Peng, C.; Sun, D.; Li, M. Nanotechnology 2005, 16, 2530. (3) Ding, Y.; Erlebacher, J. J. Am. Chem. Soc. 2003, 125, 7772. (4) Zhang, J.; Liu, P.; Ma, H.; Ding, Y. J. Phys. Chem. C 2007, 111, 10382. (5) Bonroy, K.; Friedt, J.-M.; Frederix, F.; Laureyn, W.; Langerock, S.; Campitelli, A.; Sara, M.; Borghs, G.; Goddeeris, B.; Declerck, P. Anal. Chem. 2004, 76, 4299. (6) Shulga, O. V.; Jefferson, K.; Khan, A. R.; D’Souza, V. T.; Liu, J.; Demchenko, A. V.; Stine, K. J. Chem. Mater. 2007, 19, 3902. (7) Ding, Y.; Kim, Y. J.; Erlebacher, J. Adv. Mater. 2004, 16, 1897. (8) Wang, D.; Luo, H.; Kou, R.; Gil, M. P.; Xiao, S.; Golub, V. O.; Yang, Z.; Brinker, C. J.; Lu, Y. Angew. Chem., Int. Ed. 2004, 43, 6169. (9) Zhang, H.; Hussain, I.; Brust, M.; Cooper, A. I. Adv. Mater. 2004, 16, 27. (10) Bartlett, P. N.; Baumberg, J. J.; Birkin, P. R.; Ghanem, M. A.; Netti, M. C. Chem. Mater. 2002, 14, 2199. (11) Zhang, Y. X.; Zeng, H. C. J. Phys. Chem. C 2007, 111, 6970. (12) Zhang, Y. X.; Zeng, H. C. Langmuir 2008, 24, 3740. (13) Polavarapu, L.; Xu, Q. Nanotechnology 2008, 19, 75601. (14) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. Adv. Mater. 2005, 17, 2553. (15) Dai, Q.; Worden, J G.; Trullinger, J.; Huo, Q. J. Am. Chem. Soc. 2005, 127, 8008. (16) Aldaye, F. A.; Sleiman, H. F. J. Am. Chem. Soc. 2007, 129, 4130. (17) Huang, Y.; Chiang, C.; Lee, S. K.; Gao, Y.; Hu, E. L.; Yoreo, J. D.; Belcher, A. M. Nano Lett. 2005, 5, 1429.

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new properties often emerge from the assemblies that are distinctly different from those of the corresponding isolated nanoparticles.13-19 In the past few decades, extensive researches have been carried out on the self-assembly of GNPs into one-, two-, and three-dimensional nanostructured materials using various interactions. For example, electrostatic action,20,21 hydrogen bonding,22-24 and van der Waals forces23,24 have mostly been utilized to prepare nanostructured materials with various architectures. Recently, Zen et al. fabricated gold sponges via hydrothermally activated self-assembly of GNPs.11,12 It is found that the hydrothermal condition is essential to detach surfactants and thus trigger the self-assembly of GNPs. Although a variety of gold nanoparticles assemblies are now available,25-29 yet it still remains as a challenge to fabricate porous gold sponges through a one-step process under moderate conditions. Many earlier studies have confirmed that halide ions have different affinity to gold surface. I-, Br-, and Cl- can chemisorb on the gold surface with the Au-X binding strength varying as I>Br>Cl.30-33 Recently, there is a growing interest in investigating the effect of halide ion addition on the shape and (18) Maxwell, D. J.; Taylor, J. R.; Nie, S. J. Am. Chem. Soc. 2002, 124, 9606. (19) Nam, J.; Won, N.; Jin, H.; Chung, H.; Kim, S. J. Am. Chem. Soc. 2009, 131, 13639. (20) Adachi, E. Langmuir 2000, 16, 6460. (21) Kolny, J.; Kornowski, A.; Weller, H. Nano Lett. 2002, 2, 361. (22) Si, S.; Mandal, T. K. Langmuir 2007, 23, 190. (23) Lim, I. S.; Mott, D.; Ip, W.; Njoki, P. N.; Pan, Y.; Zhou, S.; Zhong, C. Langmuir 2008, 24, 8857. (24) Han, L.; Luo, J.; Kariuki, N. N.; Maye, M. M.; Jones, V. W.; Zhong, C. Chem. Mater. 2003, 15, 29. (25) Ramanath, G.; D’Arcy-Gall, J.; Maddanimath, T.; Ellis, A. V.; Ganesan, P. G.; Goswami, R.; Kumar, A.; Vijayamohanan, K. Langmuir 2004, 20, 5583. (26) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955. (27) Guan, J.; Jiang, L.; Li, J.; Yang, W. J. Phys. Chem. C 2008, 112, 3267. (28) Zhong, Z.; Luo, J.; Ang, T. P.; Highfield, J.; Lin, J.; Gedanken, A. J. Phys. Chem. B 2004, 108, 18119. (29) Cheng, W.; Dong, S.; Wan, E. Angew. Chem., Int. Ed. 2003, 42, 449. (30) Magnussen, O. M.; Ocko, B. M.; Wang, J. X.; Adzic, R. R. J. Phys. Chem. 1996, 100, 5500. (31) Ocko, B. M.; Watson, G. M.; Wang, J. J. Phys. Chem. 1994, 98, 897. (32) Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2002, 106, 4782. (33) Singh, S.; Pasricha, R.; Bhatta, U. M.; Satyam, P. V.; Sastry, M.; Prasad, B. L. V. J. Mater. Chem. 2007, 17, 1614.

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size of preformed gold nanoparticles as well as their pronounced effect as additives during the synthesis of nanoparticles. Many investigations show that the addition of I- or Br- will lead to the aggregation and the fusion of GNPs and the formation of anisometric nanocrystals in some cases.29,33-37 The proposed aggregation and fusion process is considered to be the chemisorption of iodine or bromine on the gold nanoparticle surface by displacing the stabilizer. For example, Dong et al. reported the aggregation and fusion/fragmentation of gold nanoparticles by the addition of KI, and they considered that the replacing effect of KI played an important role.29 Herein, for the first time, we demonstrated a facile self-assembled procedure to prepare threedimensionally spongelike gold/Glu nanocomposites with gold nanoparticles serving as building blocks by adding NaBr at room temperature. Furthermore, the obtained gold spongelike nanocomposites could be used as surface-enhanced Raman scattering (SERS) substrates with high SERS activity and stability for detecting Rhodamine 6G (R6G) molecules. Hence, NaBrmediated preparation of SERS substrates described in this work has potential applications in chemical and biological analysis as well as medical detection.

Experimental Section Materials. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O), L-glutamic acid, and L-histidine were purchased from Aldrich Chemicals and Fluka Chemicals. Sodium borohydride (NaBH4), NaF, NaCl, NaBr, and NaI used were analytical reagent grade (Shanghai Chemical Co., China). All chemicals were used as received without further purification. All glassware were cleaned in aqua regia (three parts of concentrated HCl and one part of concentrated HNO3 by volume) and rinsed with deionized water thoroughly. Ultrapure water (18 MΩ from an Elga LabWater purification system) was used as the solvent throughout. Synthesis of Glu-Stabilized GNPs. In a typical experiment, 25.0 mL of 0.25 mM HAuCl4 was reduced by quickly adding 0.4 mL of 30 mM freshly prepared ice cold solution of NaBH4 in the presence of 6.8 mM glutamic acid. The HAuCl4 was immediately reduced to gold nanoparticles upon addition of NaBH4, manifested by an immediate solution color change from colorless to ruby red. Synthesis and Purification of Three-Dimensional Spongelike Gold/Glu Nanocomposites. The gold/Glu nanocomposite was prepared by the addition of NaBr to GNPs solution. After being left 24 h, sedimentation of the nanocomposites was completed when the supernatant solution became colorless. Then the supernatant solution was removed, and the sample was washed with water and ethanol three times and finally was dried in air. All experimental procedures for the nanocomposites preparation were carried out at room temperature. Instruments. UV-vis spectra of the nanoparticles were measured using a Shimadzu UV 2450 spectrometer. The morphologies and microstructures of the samples were observed using a Tecnai-12 transmission electron microscope (TEM) and a Hitachi S4800 field emission scanning electron microscope (FE-SEM). The particle composition was analyzed by an energy-dispersive X-ray spectrum (EDX) analyzer attached to the microscope. X-ray diffraction (XRD) was carried out by using an X-ray diffractometer model D8 advance (Bruker) with Cu KR radiation (λ = 1.5418 A˚). All Raman spectra were recorded by a Renishaw inVia Raman microscope using a 532 nm laser excitation source (34) (35) (36) (37) 2683.

Rai, A.; Singh, A.; Ahmad, A.; Sastry, M. Langmuir 2006, 22, 736. Pang, S.; Kondo, T.; Kawai, T. Chem. Mater. 2005, 17, 3636. Ha, T. H.; Koo, H. J.; Chung, B. H. J. Phys. Chem. C 2007, 111, 1123. Kawasaki, H.; Nishimura, K.; Arakawa, R. J. Phys. Chem. C 2007, 111,

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Figure 1. (A) TEM image and (B) UV-vis absorption spectrum of GNPs obtained in Glu solution at pH 10.0. The inset in (A) shows the selected area electron diffraction pattern of GNP.

with 2 μm diameter spot size and 1 mW power using a single 60 s accumulation unless otherwise stated.

SERS Measurements of R6G on GNP Assemblies. Washed GNP assemblies were transferred into R6G solution with various concentrations ranging from 1  10-7 to 1  10-10 M at room temperature. Twenty minutes later, GNP assemblies with adsorbed R6G were transferred onto quartz coverslip, and the remaining solvent was allowed to evaporate under ambient conditions. Finally, a Raman spectrometer was used to measure the SERS activities of these GNP assemblies directly. In this experiment, more than three SERS-active substrates of each GNP assemblies were prepared, and at least 10 different points on each substrate were selected to detect the R6G probes, to verify the stability and reproducibility of these SERS-active substrates.

Results and Discussion Fabrication of Spongelike Gold/Glu Nanocomposites by Addition of NaBr. Glu solutions containing HAuCl4 with an initial pH 10.0 result in the formation of stable ruby red GNP dispersions after the addition of NaBH4. Figure 1A,B shows the typical TEM image and UV-vis extinction spectra of these nanoparticles. The obtained nanoparticles are nanospheres with the mean diameter of 6.8 nm. The extinction spectrum of the sample shows a sharp peak at 518 nm, which is a typical plasmon resonance band of gold nanospheres. The diffraction rings of the gold nanoparticles ED pattern (inset in Figure 1A) correspond well to the crystalline planes of the face-centered-cubic (fcc) structured gold, suggesting the crystalline nature of these gold nanoparticles. The GNPs are found to be stable over 6 months and do not show any signs of aggregation. After the addition of 0.1 M NaBr, the solution color turns from ruby red to purple and dark blue immediately. By further extending the aging time to 20 h, the solution became colorless accompanied by the occurrence of blue precipitate that is phaseseparated from the aqueous media. The color changes are a direct consequence of appearance and assembly of gold nanoparticles. The SEM images of the product at different magnifications are shown in Figure 2. As shown, the addition of 0.1 M NaBr results in a highly spongelike structure, which consists of interconnected ligaments with the mean diameter being 50 nm. The chemical composition of the product is determined by elemental analysis. The peaks of Au, C, O, and Br are noticed, as shown in Figure 2C. Obviously, the spongelike nanocomposite is of high-purity gold with only traces of organics from Glu molecules as well as Br. Thus, we can conclude that the spongelike structure is gold/Glu hybrid. Furthermore, the XRD study further verifies that the gold sponge is a face-centered-cubic (FCC) structure (Figure 2D). FTIR analysis further confirms the formation of gold/Glu hybrid. Figure 2E shows the FTIR spectra for pure glutamic acid and Langmuir 2010, 26(16), 13479–13485

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Figure 2. SEM observation of spongelike gold/Glu nanocomposite with low (A) and high (B) magnification. (C) EDX analysis and (D) XRD pattern of the spongelike nanocomposite. (E) FTIR spectra of pure glutamic acid (a) and gold/Glu hybrid (b).

gold/Glu hybrid. The symmetrical amino bending mode at 1516 cm-1 and the asymmetric stretching mode of NH3þ with a maximum at 3079 cm-1 both decrease in intensity in gold/Glu hybrid compared to that of pure glutamic acid. This is due to the binding of NH3þ to the gold surface.13 Transmission electron microscope (TEM) images of the dispersions at two typical times (Figure 3) support the sequentially fusion of the GNPs. Upon the addition of NaBr, i.e., the process time is very short (e.g., less than 5 min), a three-dimensional network starts to take place (Figure 3A). It is clearly indicated that the assembled GNPs are concentrated in the junctures of the 3D network, and many primary GNPs fuse into (the inset in Figure 3A). High-resolution TEM images (Figure 3B) recorded at the joint regions of the small Au nanochains provide solid evidence that the nanoparticles have fused into, rather than physically adsorbed onto, each other. Furthermore, the measured lattice fringe spacing is 0.230 nm at the joint regions, corresponding to (111) plane of cubic Au. With time, the extension degree of the three-dimensional networks becomes higher and the magnified TEM image reveals a long pearl-chain-like 3D superstructure (Figure 3C,D). Obviously, with time, in the network, the primary GNPs fuse themselves into larger particles in the diameter range 40-50 nm; meanwhile, the resultant particulates fuse one another into 3D networks, giving rise to a spongelike structure. The magnified TEM image (Figure 3D) also provides solid evidence that nanoparticles have fused into, rather than physically adsorbed onto, each other. The color changes of the media and the appearance of the blue precipitate can also be traced by the UV-vis absorption method (Figure 3E). After the addition of salt, the peak position red-shifts from 518 to 537 nm. Also mentioned is that the peak shape becomes broad during the reaction, which can be attributed to ineffectual stabilization of Langmuir 2010, 26(16), 13479–13485

Figure 3. TEM images of the purple (A) and blue (C) gold solution. The insets in (A) and (D) show the TEM images with high magnification. (B) HRTEM image of the small gold chain shown in (A). (E) UV-vis absorption spectra of gold solution at different times after the addition of 0.1 M NaBr. Insets show photographs of the samples at four typical times: (a) before adding NaBr, (b) 1 min, (c) 30 min, and (d) 6 h after adding NaBr.

Glu for gold nanoparticles resulting in multiple size domains.38,39 Meanwhile, a decrease in the intensity of the surface plasmon absorption peak of the individual GNPs at about 518 nm is accompanied by an increased absorption in the near-IR region with time. The appearance of the plasmon band at longer wavelength results from the formation of networks.40 It has also been found that NaBr with relative high concentration range can form the spongelike structure. Figure 4 shows three representative products after the addition of NaBr with different concentration. As shown, the addition of NaBr with concentration of 0.08, 0.15, and 0.2 M all lead to the formation of spongelike superstructure of GNPs, and the magnified SEM images all reveal a long pearl-chain-like 3D nanostructure (Figure 4 and Figure S1). It can also be found from Figure 4 that the size of the mean diameter of ligaments is very similar to different NaBr (38) Wang, T.; Zheng, R.; Hu, X.; Zhang, L.; Dong, S. J. Phys. Chem. B 2006, 110, 14179. (39) Slocik, J.; Wright, D. Biomacromolecules 2003, 4, 1135. (40) Pei, L.; Mori, K.; Adachi, M. Langmuir 2004, 20, 7837.

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Figure 4. SEM images of gold/Glu nanocomposites formed by adding 0.08 (A), 0.2 (B), 0.05 (C), and 0.4 M (D) NaBr. Insets in (A) and (B) show the SEM images with high magnification.

Figure 5. Time for appearance of blue precipitate and the mean ligament diameter of the spongelike nanocomposites at several typical NaBr concentrations.

concentration. However, as shown in Figure 5, the time for the appearance of the blue precipitate depends much on the NaBr concentration. For the system with 0.08 NaBr, 15 h is needed to form 3D structure, and only 2 h is needed for the system with 0.20 M NaBr. This means that the mean diameter of ligaments is determined by NaBr with low concentration, and the kinetics of the superstructure formation is faster with higher NaBr concentration. However, the addition of NaBr with too low and high concentration cannot lead to the formation of the spongelike structure. For example, the addition of 0.05 M NaBr only leads to the partially aggregation of GNPs and a small increase in the GNP size (Figure 4C), and the addition of 0.4 M NaBr leads to the formation of irregular aggregates (Figure 4D). Effect of Other NaX Salt on the Assembling Behavior of Glu-Stabilized GNPs. To understand the effect of NaBr better and to find out the real species responsible for the spongelike nanocomposites, we further investigate the effect of other NaX on the assembling behavior of Glu-stabilized GNPs. Similarly, the addition of other NaX also leads to the blue precipitate, but the time for the appearance of the precipitate differs much on the content of NaX. Figure 6G-I shows the UV-vis absorption spectra of gold solution after the addition of other NaX with different time. Evidently, the change tendency of the spectra after the addition of other NaX is almost the same as that of NaBr. That is, a decrease in the intensity of the surface plasmon absorption peak 13482 DOI: 10.1021/la101219r

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of the individual GNPs at about 518 nm is accompanied by an increased absorption in the near-IR region with time. However, compared with the case of NaBr, the change tendency is much faster for NaI and is much slower for NaCl and NaF. Furthermore, the increase in the absorption in the near-IR region follow sequence is low for NaF and NaCl, indicating that different assembling behavior of GNPs occurs compared with the case of NaBr. High-resolution SEM observation shows the microstructure of the precipitated gold/Glu nanocomposite after the addition of 0.1 M NaF, NaCl, and NaI, respectively (Figure 6A-C). As expected, different from NaBr, the three-dimensional spongelike structure cannot be obtained. From the SEM images, the morphologies of the gold assembly are considerably different between other NaX (Figure 6A-C) and NaBr (Figure 2). The addition of other NaX (NaF, NaCl, and NaI) all result in the formation of irregular aggregates. In the case of NaI, more gold nanoparticles with larger size are detected (Figure 6C). TEM images of the blue dispersions after the addition of NaF, NaCl, and NaI (Figure 6D-F) also confirm the irregular aggregation of the GNPs. As shown, the blue dispersions all exhibit large aggregates of GNPs, and no threedimensional spongelike structure is obtained. For NaF and NaCl, GNPs form densely packed aggregates, but many of them retain their individual character, without fused into each other. And the addition of NaI leads to the obvious increase in the size of gold nanoparticles. Evidently, changing the content of NaX can control the assembling behavior of GNPs. Thus, the anionic ions must predominantly influence the degree of coalescence and hence the different morphologies of GNP assemblies. Proposed Mechanism for the Spongelike Gold/Glu Nanocomposites Formation. As stated in the literature,13 the amino group in Glu has a strong interaction with gold surfaces. Evidently, at pH 10.0, the carboxyl groups almost are negatively charged due to deprotonation, so the electrostatic repulsion between the carboxyl groups leads to the high stability of the GNPs. As is well-known, the stability of charged particles in aqueous solutions depends on the exquisite balance of the electrostatic repulsion and van der Waals attractive forces between the charged particles. In general, the addition of salts leads to the screening of the electrostatic repulsion between the charged GNPs and hence the assembly. As expected, the addition of all 0.1 M NaX results in the assemblies of Glu-stabilized GNPs. Surprisingly, just as discussed above, the assembling behavior differs much according to the content of NaX. Typically, that is the time for the appearance of and the morphologies of the assemblies. Thus, the screening effect of NaX is not the unique factor in determining the assembling behavior of GNPs, since the added salt has the same concentration in all the studied systems. Here, some other factor plays an important role in controlling the assembling behavior of GNPs. Some earlier studies have shown that halide ions have different affinity to gold surface. I-, Br-, and Cl- all chemisorb on the gold surface with the Au-X binding strength varying as I > Br > Cl.32,34,36 Therefore, the chemisorption of iodine or bromine on the gold nanoparticle surface by displacing the stabilizer will lead to the aggregation and fusion of GNPs and the formation of anisometric nanocrystals in some cases.29,33-37 Similarly, I- and Br- may replace Glu from GNPs surfaces, which plays an important role in determining the assembling behavior of Glu-stabilized GNPs here. In the case of Br-, on the one hand, the addition of salt screens the electrostatic repulsion between the charged GNPs. On the other hand, the adsorption of Br- on GNP surfaces by displacing some Glu molecules could also reduce surface charge of GNPs, increasing the van der Waals attractive forces between the gold surfaces and resulting in the attractive interaction. Furthermore, Biggs et al. found that the gold surfaces jumped toward contact with an Langmuir 2010, 26(16), 13479–13485

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Figure 6. SEM (A-C) and TEM images (D-F) of GNP assemblies formed by adding 0.1 M salt. The samples were taken for TEM imaging after the solution color turned blue. UV-vis absorption spectra (G-I) of gold solution at different times after the addition of 0.1 M salt. (A, D, G) NaF, (B, E, H) NaCl, and (C, F, I) NaI.

interparticle distance of 10 nm, driven by the large van der Waals attraction forces.40,41 Thus, because of the increased attraction between nanoparticles and the jump in model,40 gold nanoparticles hit and stick together very rapidly, and then nanoparticles fuse into each other followed by Ostwald ripening. This leads to the first formation of 3-D network with low extension and coalescence degree and finally a pearl-chain-like 3D superstructure with nanoparticles fully fused into each other (Figure 7). Yonezawa et al.42 also reported that their unstable gold particles displayed automatic fusion into larger particles in the assemblies, where bromide ions surrounding gold nanoparticles were considered to accelerate the fusion of the particles to form the wirelike structures. Notably, the binding ability between the amine group of Glu and Au surface is different for different facets. The binding of amino acid molecules to the (111) facet might be relatively weaker compared to other facets, but Br- has a strong affinity to gold (111) surface.30,33,43 Thus, Glu bound on (111) fact is easy to be replaced by Br-, so fusion of nanopaticles occurs first along the (111) facet first as shown in Figure 3B. Furthermore, a proper concentration of NaBr is essential to the formation of the threedimensional spongelike structure. The key factor for formation of such assemblies is believed to be a subtle balance between the van der Waals attractive forces of Br- partially coated GNPs, (41) Biggs, S.; Mulvaney, P.; Zukoski, C. F.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150. (42) Yonezawa, T.; Onoue, S.; Kimizuka, N. Chem. Lett. 2002, 12, 1172. (43) Pong, B. K.; Lee, J. Y.; Trout, B. L. Langmuir 2005, 21, 11599.

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Figure 7. Schematic illustration of the formation process of the pearl-chain-like 3D superstructure by the addition of NaBr.

working against the electrostatic repulsion among gold colloid surfaces bearing negative charges. The addition of F- and Cl- cannot replace Glu on gold surfaces, so only the screening effect of the two salts plays a role. However, the electrostatic repulsion between GNPs only decreases to some extent, and this cannot lead to the rapid hit and stick of GNPs and hence the slow aggregation of GNPs. In the system, only irregular aggregates are formed, and many GNPs retain their individual character, without fused into each other (Figure 6D,E). Compared with the case of Br-, I- has a stronger affinity to gold surfaces, and thus more Glu molecules are replaced from GNP surfaces. Thus, the van der Waals interactions among I--coated GNPs is stronger, and gold aggregates are formed more quickly after the addition of salt. During the fast assembling process, some larger spherical-like GNPs with wide size distribution are formed, so those GNPs cannot assemble orderly and only are stacked randomly (Figure 6C,F). DOI: 10.1021/la101219r

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Figure 8. UV-vis absorption spectra of histidine-stabilized gold solution before (a) and after the addition of 0.1 M NaBr (b) and NaI (c).

Further evidence regarding the replacing effect of Br- is supported by adding NaBr to histidine-stabilized GNPs. The addition of Br- could not lead to any shifts in the SPR of histidine-stabilized GNPs (Figure 8). The unshifted SPR bands suggest that the surface modification can hinder the injected Brions adsorbing onto the gold surface and hence reduce the assembling ability of Br-. Some earlier studies show that GNPs are coated and stabilized by the histidine ligands with both carboxyl and imidazole side group binding to their surface.27 Thus, the stronger complexing interaction between GNP and histidine hinder the replacement of amino acid molecules by Brand hence the loss of the assembling ability of Br-. However, the addition of NaI leads to the red shift of the absorption peak from 510 to 524 nm, indicating larger nanoparticles are formed. This is caused by a stronger affinity of I- to gold surfaces compared with the case of Br-. SERS Measurements of R6G on Spongelike Gold/Glu Nanocomposites. The spongelike gold/Glu nanostructure can be readily used as surface-enhanced Raman scattering (SERS) substrates for molecular sensing. The target molecule, R6G, is used to investigate the SERS sensitivity of these substrates. Figure 9 shows the SERS spectrum of R6G with different concentration (10-7, 10-8, and 10-9 M) on spongelike gold/Glu nanostructure. A detailed assignment of the spectral features of R6G has been reported previously and will not be repeated here.44 As shown, on the SERS-active substrates, even 1  10-9 M R6G produces a clear enhanced effect at 1650 cm-1, one of the main characteristic bands. As the concentration of R6G increases, the SERS intensities progressively increase. It is found that the detection limit is 0.5 nM when the intensity of C-S stretching vibration band at 1650 cm-1 is selected. Therefore, the spongelike gold may be an appropriate Raman-active substrate. As for the enhancement factor (EF) for R6G, it is calculated by the equation45 EF ¼ ðISERS =IRaman ÞðNneat =Nads Þ where ISERS is the band intensity of the selected band at 1650 cm-1 obtained by SERS, IRaman is the corresponding band intensity of the concentrated aqueous solution sample, and Nneat and Nads are the numbers of molecules in the cross section of the laser beam from the concentrated sample and that of adsorbed molecules in (44) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935. (45) Huang, G. G.; Han, X. X.; Hossain, M. K.; Yukihiro, O. Anal. Chem. 2009, 81, 5881.

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Figure 9. Typical SERS spectra of R6G with different concentration on spongelike gold/amino acid nanocomposites substrate.

Figure 10. SERRS spectra of R6G (1  10-8 M) on the active spongelike substrate of initially prepared (a) and after 1 month of storage (b).

the cross section of laser beam, respectively. The cross sections of the laser beam are identical for the Raman and SERS measurements because we used the same Raman spectrometer. Thus, Nneat and Nads may be replaced by the concentrations of the neat and SERS samples, respectively. The calculated enhancement factor is 6.0  106 for the SERS detection of R6G under the experimental conditions. Notably, gold substrates have historically been less sensitive to analytes such as R6G, so they have been used less frequently.46 R6G readily adsorbs to silver and is detectable at nanomolar concentrations and below.47,48 However, gold substrates have many advantages over silver. That is, gold is resistant to oxidation and is more tunable in the selection of excitation wavelengths. The stability of the spongelike nanocomposite-modified substrate is investigated by measuring the Raman activity with time. Figure 10 shows the aging of the SERRS spectrum for R6G (1  10-8 M) on spongelike nanocomposites. There is only very a small reduction in the SERRS intensity over a period of 1 month. Thus, the spongelike nanocomposite-modified substrate displays (46) Schwartzberg, A. M.; Grant, C. D.; Wolcott, A.; Talley, C. E.; Huser, T. R.; Bogomolni, R.; Zhang, J. Z. J. Phys. Chem. B 2004, 108, 19191. (47) Polavarapu, L.; Xu, Q. Langmuir 2008, 24, 10608. (48) Sun, L.; Song, Y.; Wang, L.; Guo, C.; Sun, Y.; Liu, Z.; Li, Z. J. Phys. Chem. C 2008, 112, 1415.

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Liu et al.

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nanocomposite-modified substrates are much stronger than those for GNPs (Figure 11). Thus, SERS enhancement is mainly due to the local electromagnetic field enhancement. The as-prepared gold sponges, unlike spherical nanoparticle SERS substrates that rely on interparticle fusion, have a great number of particle junctions, which can act as “hot sites” for surface plasma and cause “rough surface” EM enhancement. The plasmon coupling of gold nanospheres at these hot spots will produce a very intense local electromagnetic field and consequently strong SERS signals. Obviously, some of the GNPs will also self-assemble to form hot spots for SERS enhancement, though they will be much weaker when NaF and NaCl are added. Gold substrate with fewer junctions has smaller enhancement ability and GNP assembly substrates with considerable coalescence of nanoparticles have the largest SERS signal. Figure 11. SERS spectrum of 1  10-8 M R6G on GNP and GNPs assemblies formed by adding 0.1 M NaX.

high SERS activity and stability for detecting Rhodamine 6G (R6G) molecules. Figure 11 displays the SERS spectra of 1  10-8 M R6G collected from GNPs and GNP aggregates formed by adding different NaX, respectively. As shown, no SERS signal is observed when the concentration of R6G is 1  10-8 M on GNPs substrate. As for gold/Glu nanocomposite formed by the addition of salt, the SERS signals obtain obvious improvement. The most intense SERS spectrum is shown on the spongelike nanocomposite by the addition of NaBr, accompanied by little background noise. The R6G on GNP aggregates (adding NaF and NaCl) substrate gives a weak signal, and R6G on the GNP aggregates (adding NaI) substrate gives a moderate SERS signal. In the spongelike nanocomposites, the ligaments are welded by many gold nanoparticles, resulting in an abundance of grain boundaries, which provides a larger number of high activity sites for SERS sensors. The SERS mechanism is believed to be due to a combination of an electromagnetic field enhancement and a chemical effect. In the present study, the SERS signals on the spongelike

Langmuir 2010, 26(16), 13479–13485

Conclusions In summary, we have demonstrated a simple, template-free, and environmentally friendly route for the synthesis of spongelike gold/amino acid nanocomposites by NaBr-assisted fusion GNPs at room temperature. Apart from the screening function, the replacing function of NaBr plays an important role in controlling the GNPs assembly. The driving force for the formation of spongelike gold/amino acid nanocomposites should be van der Waals attractive force between the partially Br--coated colloidal particles. This spongelike gold/amino acid nanocomposite has been shown to be an active SERS substrate. The SERS activity exhibited is closely related to the abundant junction sites originating from the spongelike structure, which makes the surface unique in comparison to other SERS substrates currently in use. Acknowledgment. This work was supported by the National Nature Science Foundations of China (20633010, 20773106, and 20803061). Supporting Information Available: Figure S1. This material is available free of charge via the Internet at http://pubs. acs.org.

DOI: 10.1021/la101219r

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