Enhanced Optical Properties of Graphene Oxide–Au Nanocrystal

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Enhanced Optical Properties of Graphene Oxide Au Nanocrystal Composites Yih Hong Lee,† Lakshminaraya Polavarapu,†,‡ Nengyue Gao,† Peiyan Yuan,†,‡ and Qing-Hua Xu*,†,‡ † ‡

Department of Chemistry, National University of Singapore, Singapore 117543 NUSNNI-Nanocore, National University of Singapore, Singapore 117576

bS Supporting Information ABSTRACT:

A simple strategy based on electrostatic interactions was utilized to assemble Au nanocrystals of various morphologies onto graphene oxide (GO). This method allows deposition of metal nanocrystals of different shapes onto GO. The linear and nonlinear optical properties of GO Au nanocrystal composites have been examined. The extinction spectra of Au nanocrystals became broadened and red-shifted from the visible to the near IR upon formation of GO Au nanocrystal composites. A more than 4-fold increase in two-photon excitation emission intensity was observed from the GO Au nanocrystal composites compared to pure Au nanocrystals. The SERS signals of the composites were found to be strongly dependent on the morphology of Au nanocrystals, with SERS enhancement factors ranging from 9 to 20.

1. INTRODUCTION The graphene based hybrid composites have attracted a lot of attention because inclusion of graphene helps to improve device performance.1,2 Novel graphene composites have been developed in combination with polymers, semiconductors, and metal nanocrystals.3,4 Graphene polymer composites were found to display improved electrical conductivity, electrochemical capacity, and mechanical strength.5 8 Graphene semiconductor composite materials have been demonstrated to display enhanced performance in energy related applications such as solar cells, Li-ion batteries and supercapacitors.9 12 The inclusion of metal nanoparticles into graphene based materials resulted in surface-enhanced Raman scattering (SERS), better catalytic activity and increased sensitivity in electrochemical sensing,13 16 chemical sensing17,18 and DNA detection.19 There have been significant effort on growing metal nanocrystals such as Pt,20 Pd,16,20,21 Ni,22 Cu,21 Ag,23 26 and Au27 30 onto graphene oxide (GO) sheets. Thermal evaporation,31 electroless metallization,32 microwave assisted synthesis,21 photochemical synthesis,33 and chemical reduction methods13,20,34,35 have been utilized to attach noble metal nanoparticles onto GO. Most of the metal nanoparticles in the prepared nanocomposite by the above-mentioned methods are spherical in shape. Recently Au r 2011 American Chemical Society

nanorods and dendritic nanostructures have been successfully grown on GO.34 36 It is important to allow metal nanocrystals of different morphologies to be deposited onto GO to achieve optimum performance as the plasmon resonance of metal nanoparticles strongly depends on particle morphology. The ability to tune the shape of metal nanoparticles grown on GO would allow development of new applications of GO metal nanocomposites. Here we investigated the linear and nonlinear optical properties of GO Au nanocrystal composites. Au nanocrystals of various morphologies were assembled onto GO sheets based on electrostatic interactions between negatively charged GO and positively charged Au nanocrystals. The assembly of positively charged Au nanoparticles onto the negatively charged surface of GO was monitored by their extinction spectra. The surface plasmon resonances (SPR) bands of the GO Au nanocrystal composites were found to become broadened and red-shifted compared to those of pure Au nanocrystals. More than 4-fold increase in two-photon excitation emission intensity was observed from the GO Au nanocrystal composites compared to Received: October 15, 2011 Revised: November 27, 2011 Published: November 30, 2011 321

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pure Au nanocrystals. The SERS signals of the GO Au nanocomposites were also found to be enhanced by a factor of 9 20, depending on the shape of the Au nanocrystals.

2.5. Growth of Au Nanospheres (NSs). Two types of Au NSs were prepared: CTAB stabilized and citrate stabilized Au NSs.40 CTABcapped Au NSs were prepared through a seed-mediated method. The seed solution was prepared by addition of HAuCl4 (0.01 M, 0.25 mL) and citrate (0.01 M, 0.25 mL) into water (9.5 mL). After the solution was mixed by inversion, a freshly prepared ice-cold NaBH4 (0.1 M, 0.3 mL) was added followed by rapid inversion-mixing for 2 min. The resulting citrate stabilized seed solution was kept at room temperature for at least 3 h before use. The seed mediated growth procedure was divided into two steps. First, HAuCl4 (0.01 M, 0.225 mL) and water (1.575 mL) were added into a CTAB solution (0.1 M, 7.2 mL) in a plastic centrifuge tube, followed by inversion mixing. A freshly prepared ascorbic acid (0.1 M, 0.05 mL) solution was then added, followed by the addition of 1.0 mL of seed solution. The resulting solution was stirred for 30 min. In the second step, HAuCl4 (0.01 M, 0.225 mL), water (1.575 mL), and CTAB (0.1M, 7.2 mL) were mixed together followed by addition of ascorbic acid (0.1 M, 0.05 mL). A total of 1 mL of the solution from the first step was then added, and the resulting reaction mixture was left undisturbed overnight. The CTAB-capped Au NSs prepared were purified through centrifugation at 8500 rpm for 15 min and redispersed in an equivalent amount of water. Citrate-capped Au NSs was prepared by mixing HAuCl4 (2.5 mM, 20 mL) with water (30 mL) and boiling the solution at 100 °C.41 A citrate solution (0.04 M, 5 mL) was then added to the boiling solution and the reaction mixture was subsequently refluxed for 30 min. The citrate-capped Au NSs were then purified through centrifugation at 8,500 rpm for 15 min and redispersed in an equivalent amount of water. 2.6. Preparation of Graphene Oxide (GO) Sheets. GO was synthesized from graphite by using a modified Hummers and Offeman’s method.42 Graphite flakes (1.5 g) and NaNO3 (1.0 g) were placed in a flask. Concentrated H2SO4 (45 mL) was subsequently added into the flask, and the mixture was stirred overnight at room temperature. KMnO4 (6.0 g) was then slowly added into the mixture with an ice bath to avoid rapid heat evolution. After 4 h of stirring, the flask was shifted to an oil bath, and the reaction mixture was stirred at 35 °C for 2 h. The temperature was slowly increased to 60 °C and stirred for 4 h. Finally, water (40 mL) was added to the reaction mixture and stirred at 90 °C for 5 h. The reaction was stopped by addition of H2O2 (30 wt %, 10 mL). The warm solution was then filtered and washed with 5% HCl and water. The obtained solid was dissolved in water and sonicated to exfoliate oxidized graphene. The oxidized graphene was centrifuged at 1000 rpm for 2 min. After removing all visible graphite particles, it was centrifuged again at 15 000 rpm for 2 h. This washing procedure was repeated until the pH of the supernatant was in the range of 4 5. For complete oxidation, the above dried GO was further treated with 70% HNO3 (10 mL of HNO3/100 mg of GO). The mixture was sonicated for 8 h at 60 °C, and the sediment was dispersed in water. The obtained GO was purified by washing multiple times with ethanol and water and then completely dried by using a rotary evaporator. Finally 10 mg of dired GO was dispersed in 10 mL of water to obtain a concentration of 1.0 mg/mL. This solution was further diluted to a concentration of 10 μg/mL and used for all the measurements. 2.7. Assembly of Au Nanocrystals on GO Sheets. The concentrations of various Au nanocrystals were adjusted such that the extinctions of the nanocrystal solutions were in the range of 0.1 0.2 in a 1 cm path length cuvette. GO was then added into the Au nanocrystal solutions and the extinction spectra were monitored by using with a Shimadzu UV 2550 spectrometer.

2. EXPERIMENTAL METHODS 2.1. Materials. Cetyltrimethylammonium bromide (CTAB, 99%, Sigma), ascorbic acid (>99%, Sigma-Aldrich), poly(diallyldimethylammonium chloride) (PDDA, Mw 400 000 500 000, 20 wt % in H2O, Sigma), Trisodium citrate dihydrate (>99%, Aldrich), myristyltrimethylammonium bromide (MTAB, 99%, Aldrich), NaBH4 (98%, Sigma-Aldrich), HAuCl4 3 3H2O (>99.9%, metal basis, Sigma-Aldrich), AgNO3 (>99%, metals basis, Sigma-Aldrich), graphite flakes (Asbury Carbons Ltd.), ethylene glycol (EG, g99%, Sigma-Aldrich). 2.2. Growth of Au Nanorods (NRs). Au NRs were prepared by using a seed-mediated method.37 Specifically, the seed solution was prepared by injecting a freshly prepared ice-cold aqueous NaBH4 solution (0.01 M, 0.6 mL) into an aqueous mixture consisting of HAuCl4 (0.01 M, 0.25 mL) and CTAB (0.1 M, 9.75 mL), followed by rapid inversion mixing for 2 min. This seed solution was kept at room temperature for more than 2 h before use. The growth solution was prepared by sequential addition of aqueous HAuCl4 (0.01 M, 2 mL), AgNO3 (0.01 M, 0.6 mL), HCl (1.0 M, 0.8 mL), and ascorbic acid (0.1 M, 0.32 mL) solutions into an aqueous CTAB (0.1 M, 40 mL) solution. The resultant solution was mixed by swirling for 30 s, followed by addition of 0.3 mL of the seed solution. The reaction solution was gently mixed by inversion for 2 min and then left undisturbed overnight. Ten mL of the obtained Au NR solution was then used to react with H2O2 (30 wt %, 60 μL) to form Au NRs with extinction maximum at 600 nm. This oxidation reaction process was monitored through periodic measurements of their extinction spectra. The oxidation process was stopped by centrifugation at 8,500 rpm for 10 min to remove unreacted H2O2. The obtained Au NRs were then dispersed in water for use. 2.3. Growth of Au Nanooctahedra (NO). Two types of Au NO were prepared: CTAB-capped and PDDA-capped Au NO. CTAB-capped Au NO was prepared by first mixing HAuCl4 (0.01 M, 0.25 mL), CTAB (0.1 M, 1 mL) with water (19.75 mL).38 Ascorbic acid (0.1 M, 0.1 mL) was then added into the reaction mixture, followed by addition of NaOH (0.1 M, 0.1 mL). The solution was gently swirled for 10 s and left undisturbed for 2 h. The resulting CTAB-capped Au NO was then purified through centrifugation at 7000 rpm for 10 min and subsequently redispersed in an equivalent amount of water. PDDA-capped Au NO was synthesized using a polyol reduction method.39 PDDA (20 wt %, 0.4 mL) was mixed with EG solution (20 mL) in a flask, and the mixture was stirred vigorously at room temperature. HAuCl4 (0.1 M, 0.1 mL) was then added to the EG solution under stirring. The flask was then capped and heated at 195 °C in an oil bath for ∼45 min. At the end of the reaction, the mixture was cooled and collected by centrifugation. The PDDA-capped Au NO was purified through centrifugation at 7000 rpm for 20 min three times and subsequently redispersed in an equivalent amount of water. 2.4. Growth of Au Nanobranches (NBs). Gold NBs were prepared by using citrate-stabilized Au nanoparticles as seeds.40 The seeds were prepared by subsequently adding HAuCl4 (0.01 M, 0.125 mL), citrate (0.01 M, 0.25 mL) and freshly prepared ice-cold NaBH4 (0.01 M, 0.15 mL) into water (9.625 mL) under vigorous stirring. The resultant seed solution was kept at room temperature for at least 2 h before use. The solution for growing Au NBs was prepared by the sequential addition of HAuCl4 (0.01 M, 1.8 mL), AgNO3 (0.01 M, 0.27 mL), and ascorbic acid (0.1 M, 0.3 mL) solutions into an aqueous MTAB (0.1 M, 42.75 mL) solution. A total of 0.04 mL of the citrate-stabilized seed solution was then added. The entire reaction mixture was mixed by gentle inversion for 30 s and then left undisturbed overnight. The Au NBs were then purified through centrifugation at 7,000 rpm for 10 min and subsequently redispersed in an equivalent amount of water.

2.8. Physical Characterization of Au Nanocrystals, GO Sheets and GO Au Nanocrystal Composites. The transmission electron microscopy (TEM) images of Au nanocrystals, and GO Au nanocrystal composites were characterized by using a JEOL 2010 electron microscope. The GO sheets are characterized by atomic force microcopy (AFM, Veeco metrology Digital Instruments). The extinction spectra of various 322

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exfoliated to obtain individual GO sheets. The prepared GO sheets are rich in oxy-functional groups such as epoxy, hydroxyl (OH ) and carboxyl (COO ) functional groups.43,44 The presence of these functional groups enable GO to be easily dispersed in water. The AFM images (Figure 1g) show that the GO sheets are a mixture of single-layer and multilayer sheets. The topographic heights of the GO sheets were measured to be 1 2 nm. CTAB-capped Au nanospheres (NSs), nanooctahedra (NO), and nanorods (NRs); MTAB-capped Au nanobranches (NBs); PDDA stabilized Au NO; and citrate stabilized Au NSs were prepared, and their TEM images are shown in Figure 1. All Au nanocrystals exhibit a high number yields (above 85% on average) with good size monodispersity. The citrate-stabilized and CTAB-capped Au NSs have diameters of 16 ( 2 nm and 19 ( 1 nm respectively. CTAB-capped Au NO have an average edge length of 53 ( 4 nm. PDDA stabilized Au NO have slightly larger edge lengths of 61 ( 4 nm and better defined edges compared to CTAB-capped Au NO. The Au NRs have an average aspect ratio of 1.9, with widths of 20 ( 2 nm and lengths of 37 ( 4 nm. The extinction spectra of different Au nanocrystals dispersed in water are shown in Figure 1h. All the “isotropic” nanoparticles, such as NSs and NO, exhibit a single plasmon resonance band in their extinction spectra, while the “anisotropic” nanoparticles such as Au NRs and NBs display multiple peaks. Citratestabilized and CTAB-capped Au NSs support a single dipolar plasmon mode at 520 and 522 nm respectively. A single extinction peak was observed for CTAB-capped Au NO at 545 nm and PDDA-stabilized Au NO at 565 nm. Due to their small particle sizes, only the dipolar plasmon mode is supported in the Au NO.39 The extinction peaks at 518 and 600 nm of Au NRs correspond to their characteristic transverse and longitudinal plasmon resonances respectively. Au NBs were previously reported to display two extinction peaks.40 Here only the peak at 654 nm is seen because the second peak lies beyond the detection window (300 900 nm) of our spectrometer. The extinction peak of Au NBs is broader compared to other nanocrystals. The broadening is due to the polydispersity of the particles, as well as varying numbers of tips of individual NB and different distances between the centers of the NBs to the tips. 3.2. Assembly of Au Nanocrystals onto GO. Scheme 1 shows the strategy of assembling Au nanocrystals onto GO and the molecular structures of capping agents for various Au nanocrystals. Electrostatic interaction between oppositely charged Au nanocrystals and GO is the primary driving force for the facile assembly of Au nanocrystals onto GO. Different amounts of GO were added into a solution of CTAB-capped Au nanocrystals of different morphologies. The successful assembly of Au nanocrystals onto GO was confirmed by the TEM images of the GO Au nanocrystal composites (Figure 2). Au nanocrystals are randomly attached onto the surface of GO due to the arbitrary distribution of the oxy-functional groups on GO. The extinction spectra of the Au nanocrystals displayed significant changes upon the assembly onto the GO sheets (Figure 2). A broadening and red-shift of the plasmon band were observed upon the addition of GO, accompanied by an increase in extinction in the longer wavelength range (Figure 2a). Red-shifts of 5 and 19 nm were observed for the plasmon bands of Au NO and NSs upon assembly respectively. The observed red-shift of extinction peaks and increased extinction in the near IR range arise from the coupling of the plasmon resonances between closely spaced Au nanocrystals brought by the GO sheets.45,46

Figure 1. (a f) TEM images of the various prepared Au nanocrystals, including citrate-capped Au NSs (a), CTAB-capped Au NSs (b) and Au NOs (c), PDDA-capped Au NO (d), CTAB-capped Au NRs (e), and MTAB-capped Au NBs (f). (g) AFM image of GO. (h) Corresponding extinction spectra of the respective Au nanocrystals. Au nanocrystals, GO sheets and GO Au nanocrystal composites were measured using a Shimadzu UV 2550 spectrometer.

2.9. Two-Photon Excitation (TPE) Emission Measurements of GO Au Nanocrystal Composites. TPE emission measurements were performed by using a Spectra-Physics femtosecond Ti: sapphire oscillator as the excitation source. The output laser pulses have a central wavelength of 820 nm with a pulse duration of 80 fs and a repetition rate of 80 MHz. The laser beam was focused onto the samples contained in a cuvette with a path length of 1 cm. The emission from the samples was collected at an angle of 90° to the excitation beam by a pair of lenses and an optical fiber that was connected to a spectrometer consisting of a monochromator (Acton, Spectra Pro 2300i) and CCD (Princeton Instruments, Pixis 100B). A short pass filter with cutoff wavelength of 750 nm was placed before the monochromator to minimize the scattering from the excitation beam. The TPE emission of Au nanocrystal solutions with linear extinction of 0.1 0.2 in a 1 cm path length cuvette was first measured. The emission was measured while different amounts of GO (10 to 100 μL) was then sequentially added into the Au nanocrystal solutions (2 mL) to form the GO Au composites. The power was fixed at 100 mW for all the measurements.

2.10. Surface-Enhanced Raman Scattering (SERS) Experiments of GO Au Nanocrystal Composites. Different GO Au nanocrystal composites were prepared by mixing solutions of Au nanocrystals and GO together. A total of 10 μL of the mixtures was drop-cast onto glass slides for SERS measurements. Pure GO was used as the standard reference against which the respective SERS enhancement factor was determined. The SERS measurements were performed on a Renishaw micro-Raman spectrometer with an excitation wavelength of 514 nm and a power of 1 mW right before the sample.

3. RESULTS AND DISCUSSION 3.1. Preparation of GO and Au Nanocrystals. GO sheets were prepared by using a modified Hummer and Offeman’s method,42 in which graphite flakes were chemically oxidized and 323

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Scheme 1. Strategy for the Assembling of Au Nanocrystals onto GO Sheeta

a

The molecular structures of the various capping agents for different Au nanocrystals are also shown.

no red-shift or broadening of the extinction band was observed (Figure S1). To extend our strategy of assembling Au nanocrystals onto GO by electrostatic interactions, several other commonly used cationic capping agents were also tested. MTAB-capped Au NBs were successfully attached to the GO sheets (Figure S2a). The extinction spectra of GO Au NB composites only exhibited some subtle changes. As the amount of GO increases, the main extinction peak at 654 nm decreased faster than the decrease in the longer wavelength range. The extinction spectra change is similar to the previous studies on glutathione induced coupling of NBs.40 Upon the assembly of NBs, plasmon peaks of individual NB (both the 654 nm band and the longer wavelength region) will decrease while the new Plasmon peaks associated with the assembled NBs (in the near IR range with maximum at ∼1700 nm, beyond our spectrometer measurement window) will increase.40 The extinction spectra of PDDA-stabilized Au NO upon addition of GO resembled those of CTAB-capped Au NO assembly (Figure S2b). Successful assembly of PDDA-stabilized Au NO onto GO was further confirmed by the TEM image of GO Au NO composites (Figure S2d). The number densities of the Au nanocrystals on GO sheets are estimated to be ∼120 NO(CTAB)/μm2, 509 NS (CTAB)/μm2, 234 NR(CTAB)/μm2, 97 NB(MTAB)/μm2, and 115 NO(PDDA)/μm2. 3.3. Enhanced Two-Photon Excitation Emission of GO Au Nanocrystal Composites. Assembly of gold nanoparticles was known to significantly enhance the two-photon excitation (TPE) emission of metal nanoparticles.45,49 The TPE emission spectra of various GO Au nanocrystal composites (Figure 3) were measured by using femtosecond laser pulses at 820 nm as the excitation source. The broad spectra in the 450 700 nm range arise from TPE emission of Au nanocrystals. The sharp peak around 710 nm arises from the scattering of the excitation source. The TPE nature of the emission was confirmed by the dependence of emission intensity at 600 nm on the incident power, in which a gradient of 2 was obtained from the log log plot (Figure S3). The TPE emission intensity of the gold nanoparticles were observed to increase steadily upon the addition of GO. Compared to pure Au nanocrystals, the TPE emission enhancement factors were 5.3, 4.7, and 4.0-fold for the GO composites of Au NSs, NO, and NRs, respectively. The enhanced TPE emission could be explained as a result of increased extinction at the excitation wavelength when the Au nanocrystals are assembled

Figure 2. Extinction spectra of CTAB-capped Au NO (a), Au NSs (b), and Au NRs (c) upon addition of GO; (d f) corresponding TEM images of the GO Au nanocrystal composites.

The new extinction peak of the assembled Au NSs and NO resembles that of Au nanochains.46 The red-shift of the extinction spectra of the assembled Au NRs is similar to the previously reported results,47,48 suggesting that the dominant contribution is head-to-head assembly. In a control experiment, citratecapped, negatively charged Au NPs were mixed with GO and 324

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Figure 4. SERS signals from composites of GO with various CTABcapped Au nanocrystals.

GO Au nanocomposites relative to that of GO, suggesting that attachment of Au nanocrystals does not change the size of inplane sp2 domains.13 Figure 4 shows that both D and G bands of the GO Au nanocrystal composites are significantly higher than those of the pure GO. The SERS enhancement factor was calculated using (IGO‑Au/CGO‑Au)/(IGO/CGO), where IGO‑Au and IGO are the Raman scattering intensities of the D or G bands of the GO Au nanocrystal composites and GO respectively. CGO‑Au and CGO refer to the concentrations of the GO sheets for GO Au nanocrystal composites and GO respectively. The calculated enhancement factors for the D band of GO Au nanocrystal composites are 9, 15, and 18-fold for Au NSs, NO, and NRs, respectively. The SERS enhancement factors of the G band were determined to be 10, 16, and 20-fold for Au NSs, NO, and NRs, respectively. The obtained enhancement factors are consistent with the previously reported SERS studies on the GO materials (with enhancement factor of 2 50)24,35,36,51 It is noted that the SERS enhancement factors of GO (generally from 2 to 50) is much less than that of small dye molecules.24,35,36,46,51 The difference might be due to the 2-dimensional nature of the graphene.51 The detail mechanism of SERS effects of 2-D dimensional system has not been well understood so far. Both electromagnetic field enhancement and chemical effects have been previously proposed to explain the SERS effects in the GO/Au nanocomposites.15,24,35,36 It is difficult to distinguish the relative contributions of two mechanisms to the observed SERS effects here. The dependence of the enhancement factors on the morphologies of Au nanocrystals suggests that electromagnetic field enhancement plays an important role to the observed SERS since electromagnetic field enhancement is strongly dependent on the shape of the nanocrystals.52 The close proximity of Au nanocrystals on the GO sheets can create localized hot spots with strong local electric fields for enhanced SERS signal.

Figure 3. Enhancement of TPE emissions of GO Au nanocrystal composites of Au NO (a), Au NSs (b), and Au NRs (c) upon assembling onto GO sheets.

onto GO, which provide an intermediate state for two-photon excitation processes.45,49 On the other hand, the enhanced local electric field near the excitation wavelength due to plasmon coupling45 also contributes to the enhanced TPE emission of the GO Au nanocomposites. GO has negligible contribution to the observed TPE emission signal of the nanocomposites, which was confirmed by the negligible TPE emission of the pure GO solution of identical concentration under the same experimental conditions. 3.4. Surface Enhanced Raman Scattering (SERS) of GO Au Nanocrystal Composites. It is well-known that metal nanocrystals serve as excellent substrates for surface enhanced Raman scattering.50 The Raman signals of graphene oxide or graphene have also recently been shown to be significantly enhanced in the presence of metal nanocrystals, with enhancement factors ranging from a few times up to tens of times.15,24,35,36 So far there was no report on the morphological effects of metal nanoparticles on the SERS signal of graphene oxide or graphene. The current method allows attaching Au nanocrystals of various shapes onto the GO sheets, which allows us to investigate the morphology effect of Au nanocrystals on the SERS of GO. The SERS spectra of different GO Au nanocomposites are shown in Figure 4. Two characteristic bands were observed at 1350 and 1600 cm 1, corresponding to the D band and G band respectively.23 The D and G bands were assigned to defects in the curved graphene sheet and staging disorder and graphitic hexagon-pinch mode, respectively.13 There were no significant differences in the D/G intensity ratios of

4. CONCLUSIONS Assembly of Au nanocrystals onto GO was performed by using a simple strategy based on electrostatic interactions between oppositely charged Au nanocrystals and GO. This strategy allows Au nanocrystals of various morphologies to be easily attached onto GO. The linear and nonlinear optical properties of GO Au nanocrystal composites have been examined. Red-shift and broadening of the extinction spectra of Au nanocrystals from the visible to the near IR were observed in GO Au nanocrystal composites. The two-photon excitation emission of GO Au 325

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nanocrystal composites were found to be enhanced by >4-fold compared to pure Au nanocrystals. SERS enhancement factors of ranging from 9 to 20 were observed, depending on the morphology of Au nanocrystals. These studies on the enhanced optical properties of GO Au composites could be utilized to develop various applications such as chemical, biological sensing and imaging.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Extinction spectra of citratecapped Au NSs, MTAB-capped Au NBs, and PDDA-capped Au NO upon addition of different amounts of GO and incident power dependence of the two-photon excitation emission intensity of various Au nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We are thankful for the financial support from DSTA Singapore (DSTA-NUS-DIRP/9010100347) and Singapore National Research Foundation Singapore under its Competitive Research Program. ’ REFERENCES (1) Compton, O. C.; Nguyen, S. T. Small 2010, 6, 711. (2) Loh, K. P.; Bao, Q. L.; Eda, G.; Chhowalla, M. Nat. Chem. 2010, 2, 1015. (3) Huang, X.; Yin, Z.; Wu, S.; Qi, X.; He, Q.; Zhang, Q.; Yan, Q.; Boey, F.; Zhang, H. Small 2011, 7, 1876. (4) Jiang, H. Small 2011, 7, 2413. (5) Kulkarni, D. D.; Choi, I.; Singamaneni, S.; Tsukruk, V. V. ACS Nano 2010, 4, 4667. (6) Wu, Q.; Xu, Y. X.; Yao, Z. Y.; Liu, A. R.; Shi, G. Q. ACS Nano 2010, 4, 1963. (7) Zhang, K.; Zhang, L. L.; Zhao, X. S.; Wu, J. S. Chem. Mater. 2010, 22, 1392. (8) Zhou, X. S.; Wu, T. B.; Hu, B. J.; Yang, G. Y.; Han, B. X. Chem. Commun. 2010, 46, 3663. (9) Kamat, P. V. J. Phys. Chem. Lett. 2010, 1, 520. (10) Sun, S. R.; Gao, L.; Liu, Y. Q. Appl. Phys. Lett. 2010, 96, 083113. (11) Wu, Z. S.; Ren, W. C.; Wen, L.; Gao, L. B.; Zhao, J. P.; Chen, Z. P.; Zhou, G. M.; Li, F.; Cheng, H. M. ACS Nano 2010, 4, 3187. (12) Yang, S. B.; Feng, X. L.; Ivanovici, S.; Mullen, K. Angew. Chem., Int. Ed. 2010, 49, 8408. (13) Guo, S. J.; Dong, S. J.; Wang, E. W. ACS Nano 2010, 4, 547. (14) Guo, S. J.; Wen, D.; Zhai, Y. M.; Dong, S. J.; Wang, E. K. ACS Nano 2010, 4, 3959. (15) Lee, J.; Shim, S.; Kim, B.; Shin, H. S. Chem.—Eur. J. 2011, 17, 2381. (16) Scheuermann, G. M.; Rumi, L.; Steurer, P.; Bannwarth, W.; Mulhaupt, R. J. Am. Chem. Soc. 2009, 131, 8262. (17) Yang, M.; Choi, B. G.; Park, T. J.; Heo, N. S.; Hong, W. H.; Lee, S. Y. Nanoscale 2011, 3, 2950. (18) Liu, X. J.; Cao, L. Y.; Song, W.; Ai, K. L.; Lu, L. H. ACS Appl. Mater. Interfaces 2011, 3, 2944. (19) Du, Y.; Guo, S. J.; Dong, S. J.; Wang, E. K. Biomaterials 2011, 32, 8584. (20) Xu, C.; Wang, X.; Zhu, J. W. J. Phys. Chem. C 2008, 112, 19841. 326

dx.doi.org/10.1021/la204047a |Langmuir 2012, 28, 321–326