Assembly of Gold Nanoparticles Mediated by Multifunctional

Sep 6, 2007 - ... and The Graduate Center, City University of New York, Staten Island, New York 10314 ... These findings provide new insights into the...
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Langmuir 2007, 23, 10715-10724

10715

Assembly of Gold Nanoparticles Mediated by Multifunctional Fullerenes I-Im S. Lim,† Yi Pan,‡ Derrick Mott,† Jianying Ouyang,‡ Peter N. Njoki,† Jin Luo,† Shuiqin Zhou,*,‡ and Chuan-Jian Zhong*,† Department of Chemistry, State UniVersity of New York at Binghamton, Binghamton, New York 13902, and Department of Chemistry, College of Staten Island and The Graduate Center, City UniVersity of New York, Staten Island, New York 10314 ReceiVed June 22, 2007. In Final Form: July 16, 2007 The understanding of the interparticle interactions of nanocomposite structures assembled using molecularly capped metal nanoparticles and macromolecular mediators as building blocks is essential for exploring the fine-tunable interparticle spatial and macromolecular properties. This paper reports the results of an investigation of the chemically tunable multifunctional interactions between fullerenes (1-(4-methyl)-piperazinyl fullerene, MPF) and gold nanoparticles. The interparticle spatial properties are defined by the macromolecular and multifunctional electrostatic interactions between the negatively charged nanoparticles and the positively charged fullerenes. In addition to characterization of the morphological properties, the surface plasmon resonance band, dynamic light scattering, and surface-enhanced Raman scattering (SERS) properties of the MPF-mediated assembly and disassembly processes have been determined. The change of the optical properties depends on the pH and electrolyte concentrations. The detection of the Ramanactive vibration modes (Ag(2) and Hg(8)) of C60 and the determination of their particle size dependence have demonstrated that the adsorption of MPF on the nanoparticle surface in the MPF-Aunm assembly is responsible for the SERS effect. These findings provide new insights into the delineation between the interparticle interactions and the nanostructural properties for potential applications of the nanocomposite materials in spectroscopic and optical sensors and in controlled releases.

Introduction The assembly of nanoparticles using macromolecules as mediators into composite architectures provides opportunities for exploring the fine-tunable interparticle spatial and macromolecular properties in many areas of nanoparticle-based technologies.1 Some of the existing assembly strategies involved layer-by-layer,2,3 DNA,4 polymeric5 recognition, and mediator templating6 of molecularly capped nanoparticles. Macromolecules such as fullerenes have attracted increasing interest in many emerging assembly strategies. One of the main attractions of incorporating fullerenes7 into nanocomposite materials is the exploitation of the physical, chemical, photophysical, electronic,

* To whom correspondence should be addressed. E-mail: cjzhong@ binghamton.edu (C.J.Z.); [email protected] (S.Z.). † State University of New York at Binghamton. ‡ City University of New York. (1) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (b) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (c) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Wuelfing, W. P.; Zamborini, F. P.; Templeton, A. C.; Wen, X. G.; Yoon, H.; Murray, R. W. Chem. Mater. 2001, 13, 87. (3) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514. (4) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (b) Mirkin, C. A. Inorg. Chem. 2000, 39, 2258. (5) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (6) (a) Maye, M. M.; Lim, I-I. S.; Luo, J.; Rab, Z.; Rabinovich, D.; Liu, T.; Zhong, C. J. J. Am. Chem. Soc. 2005, 127, 1519. (b) Lim, I-I. S.; Maye, M. M.; Luo, J.; Zhong, C. J. J. Phys. Chem. B 2005, 109, 2578. (c) Lim, I-I. S.; Vaiana, C.; Zhang, Z.; Zhang, Y.; An, D. L.; Zhong, C. J. J. Am. Chem. Soc. 2007, 129, 5368. (d) Lim, I-I. S.; Ip, W.; Crew, E.; Njoki, P.; Mott, D.; Zhong, C. J. Langmuir 2007, 23, 826. (7) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162.

and electrochemical properties.8 Furthermore, while fullerene itself is highly hydrophobic, functionalized fullerenes can be hydrophilic, which have been synthesized by incorporation of polymer chains9 along with biological molecules.10 The study of fullerene (C60)-capped nanoparticles has recently begun to attract interest in materials research.11-19 Examples include van der Waals interaction-based assembly of unfunctionalized C60 and Au nanoparticles,11 covalent layer-by-layer assembly of C60-capped nanoparticles,12 and electrostatically linked C60 derivatives to CdTe nanoparticles.13,14 However, little is known about the ability to assemble C60 and nanoparticles into 3D architectures with tunable interparticle chemistry. Recently, we reported preliminary findings of a study of a novel strategy for the creation of such an ability by exploring the chemically tunable multifunctional interactions between C60 macromolecules and molecularly capped nanoparticles.20 In comparison with the (8) (a) Kroto, H. W.; Allaf, A. W.; Balm, S. P. Chem. ReV. 1991, 91, 1213. (b) Smalley, R. E. Acc. Chem. Res. 1992, 25, 98. (c) Guldi, D. M.; Zerbetto, F.; Georgakilas, V.; Prato, M. Acc. Chem. Res. 2005, 38, 38. (d) Wudl, F. Acc. Chem. Res. 1992, 25, 157. (e) Haddon, R. C. Pure Appl. Chem. 1993, 65, 11. (9) Giacalone, F.; Martin, N. Chem. ReV. 2006, 106, 5136. (10) (a) Nakamura, E.; Isobe, H. Acc. Chem. Res. 2003, 36, 807. (b) Pan, N.-Y.; Shih, J.-S. Sens. Actuators, B 2004, 98, 180. (11) Brust, M.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. J. Am. Chem. Soc. 1998, 120, 12367. (12) Shon, Y.-S.; Choo, H. Chem. Commun. 2002, 2560. (13) Guldi, D. M.; Zilbermann, I.; Anderson, G.; Kotov, N. A.; Tagmatarchis, N.; Prato, M. J. Am. Chem. Soc. 2004, 126, 14340. (14) Guldi, D. M.; Zilbermann, I.; Anderson, G.; Kotov, N. A.; Tagmatarchis, N.; Prato, M. J. Mater. Chem. 2005, 15, 114. (15) Shih, S.-M.; Su, W.-F.; Lin, Y.-J.; Wu, C.-S.; Chen, C.-D. Langmuir 2002, 18, 3332. (16) Zhang, P.; Li, J.; Liu, D.; Qin, Y.; Guo, Z.-X.; Zhu, D. Langmuir 2004, 20, 1466. (17) Guo, Z.-X.; Sun, N.; Li, J.; Dai, L.; Zhu, D. Langmuir 2002, 18, 9017. (18) Deng, F.; Yang, Y.; Hwang, S.; Shon, Y.-S.; Chen, S. Anal. Chem. 2004, 76, 6102. (19) Fujihara, H.; Nakai, H. Langmuir 2001, 17, 6393. (20) Lim, I-I. S.; Ouyang, J.; Luo, J.; Wang, L.; Zhou, S.; Zhong, C. J. Chem. Mater. 2005, 17, 6528.

10.1021/la701868b CCC: $37.00 © 2007 American Chemical Society Published on Web 09/06/2007

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Scheme 1. Schematic Illustration of Fullerene-Mediated Assembly of Nanoparticles via Electrostatic R+-X- Interactionsa

a

The structures drawn are not to scale.

Scheme 2. Schematic Illustration of the Synthesis of MPF

Scheme 3. Models Used for Estimating the Interparticle Edge-to-Edge Distances for Cit-Capped Au11nm Particles (A) and MPF-Cit/Au11nm Assemblies (B)a

design of functional nanomaterials (e.g., controlled drug delivery and optical sensing), an important driving force for the fullerenenanoparticle combination is to harvest the unique electron or energy transfer properties which cannot be obtained with individual gold nanoparticles or fullerenes. For example, the exploration of the photophysical properties in the self-organization of porphyrin (donor) and fullerene (acceptor) units by clusterization with gold nanoparticles has recently been demonstrated for organic solar cells.21 The self-assembly of gold nanoparticles as the central nanocore and appended fullerene moieties as the photoreceptive shell is also demonstrated for a photoactive antenna system.22 The exploration of these properties expands the welldocumented photochemical properties of porphyrin-fullerene dyads.23 To develop an in-depth understanding of the interparticle interactions and reactivities, this report describes the results of a detailed investigation of the MPF-Au nanoparticle assemblies. The investigation focuses on the characterization of the optical absorption, light scattering, and Raman scattering properties. The fundamental understanding of these properties has important implications for the design and exploitation of the functional nanocomposite materials derived from the combination of the two unique nanoscale building blocks. Experimental Section

a

The structures drawn are not to scale.

other mediator-template assemblies of nanoparticles in solutions,6 the interparticle linkages by design for this nanocomposite structure involve the interactions between negatively charged groups on gold nanoparticles (Aunm) and positively charged piperazinyl groups on 1-(4-methyl)-piperazinyl fullerene (MPF), as ideally illustrated in Scheme 1. This strategy is in contrast to other earlier work on the preparation of fullerene-capped nanoparticles11-19 because of the exploitation of the multifunctional interparticle electrostatic interactions. It exploits the electrostatic interaction between the multiple negative charges of the citrate-capped gold nanoparticles and the multiple positive charges of the piperazinyl-functionalized fullerenes. It is this interaction that leads to the macromoleculenanoparticle assembly in aqueous solutions. In addition to the

Chemicals. The chemicals used included C60 (99.9%, SES Research Company), 1-methylpiperazine (99%), chlorobenzene (99%), hydrogen tetrachloroaurate (HAuCl4, 99%), sodium citrate (Cit, 99%), dimethyl sulfoxide (DMSO, 99.9%), and sodium chloride (NaCl, 99%). All chemicals, except C60, were purchased from Aldrich unless otherwise stated and were used as received. Water was purified with a Millipore Milli-Q water system. Synthesis. The synthesis of 11 nm citrate-capped gold nanoparticles (Au11nm) followed the reported procedure.24 Briefly, aqueous AuCl4- (1 mM) was heated to boiling under vigorous stirring in a cleaned glass flask. At reflux, an excess (×3.8) of sodium citrate (38.8 mM stock solution) was quickly added into the solution. Citrate (21) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 2005, 127, 1216. (22) Sudeep, P. K.; Ipe, B. I.; Thomas, K. G.; George, M. V.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2002, 2, 29. (23) (a) Schuster, D. I. Carbon 2000, 38, 1607. (b) Schuster, D. I.; Cheng, P.; Jarowski, P. D.; Guldi, D. M.; Luo, C.; Echegoyen, L.; Pyo, S.; Holzwarth, A. R.; Braslavsky, S. E.; Williams, R. M.; Klihm, G. J. Am. Chem. Soc. 2004, 126, 7257. (c) Schuster, D. I.; Cheng, P.; Wilson, S. R.; Prokhorenko, V.; Katterle, M.; Holzwarth, A. R.; Braslavsky, S. E.; Klihm, G.; Williams, R. M.; Luo, C. J. Am. Chem. Soc. 1999, 121, 11599. (24) Lim, I-I. S.; Goroleski, F.; Mott, D.; Kariuki, N. N.; Ip, W.; Luo, J.; Zhong, C. J. J. Phys. Chem. B 2006, 110, 6673.

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Figure 1. TEM images of Au11nm nanoparticles (a) and MPF-Au11nm assemblies obtained at r ∼ 4.39 × 103 (b) and r ∼ 7.44 × 103 (c).

Figure 2. TEM images of MPF-Au11nm assemblies obtained at r ∼ 5.88 × 102 (a and b) and r ∼ 1.15 × 103 (c and d) in the absence (a and c) and presence (b and d) of NaCl (5 mM).

Figure 3. TEM images of MPF-Au30nm assemblies obtained at r ∼ 1.72 × 104 (a) and r ∼ 2.50 × 104 (b) in the presence of NaCl. ([NaCl] ) 16 mM.) acts as both a reducing and capping agent. The color of the solution turned from pale yellow to clear and then to light red. This solution was allowed to react while stirring under reflux for 30 min, after which the heating mantle was removed and the solution was stirred under room temperature for another 3 h. The particle size determined by transmission electron microscopy (TEM) was 11.3 ( 0.8 nm. The synthesis of 30 nm acrylate-capped gold nanoparticles (Au30nm) followed one of our previously reported procedures.25 The particle size determined by TEM was 27.3 ( 1.9 nm. The stock concentration (25) Njoki, P. N.; Jacob, A.; Khan, B.; Luo, J.; Zhong, C. J. J. Phys. Chem. B 2006, 110, 22503.

of Aunm was determined based on the mass, the molar absorptivity, and the average particle size.26 The synthesis of 1-(4-methyl)-piperazinyl fullerene (MPF) was carried out following a published method.27 As illustrated in Scheme 2, typically 15 mL of 1-methylpiperazine was added to a solution of 500 mg of C60 in 150 mL of chlorobenzene (C60 was not totally dissolved yet) in a three-neck round-bottomed flask. The mixture was then stirred continuously under purified nitrogen for 3 days (26) Maye, M. M.; Han, L.; Kariuki, N. N.; Ly, N. K.; Chan, W.-B.; Luo, J.; Zhong, C. J. Anal. Chim. Acta 2003, 496, 17. (27) Goh, S. H.; Lee, S. Y.; Lu, Z. H.; Huan, C. H. A. Macromol. Chem. Phys. 2000, 201, 1037.

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Figure 4. Spectral evolution of the SP band for the MPF-Au11nm assembly. The initial spectra are represented by the red curves. r ∼ 5.88 × 102 (A), 8.70 × 102 (B), and 1.15 × 103 (C) in the absence (top panel and line a in the bottom panel) and presence (middle panel and line b in the bottom panel) of NaCl (5 mM). The bottom panel shows plots of the absorbance at 700 nm versus time (s) corresponding to the three different r values, and the dotted lines are the fitted kinetic results using the first-order reaction model. The arrows indicate the direction of the spectral evolution. until C60 dissolved completely. Afterward, the crude product was washed with diethyl ether three times (the product was then centrifuged, and the diethyl ether washings were decanted) after excess 1-methylpiperazine and chlorobenzene were removed by rotary evaporation at 75 °C. At last, the purified product was dried in vacuo at 30 °C for 3 days to afford brown powdery MPF in a high yield. The obtained MPF was characterized by 1H NMR spectroscopy (in deuterated chloroform), infrared spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy (XPS). The results of the characterization confirmed that it has the structure with the secondary amino groups covalently attached to the C60 framework via nucleophilicmultiadditions.Anaveragestoichiometryof[C60H9(NC4H8NCH3)9] was determined by XPS. The number of piperazinyl groups on MPF is synthetically or chemically controllable, which we consider as an ideal building block for defining the interparticle spatial or chemical properties. Stock solutions of MPF were prepared by dissolving MPF in H2O with 1% DMSO and then filtering the solution using 0.45 µm filters. Measurements and Characterization. In a typical measurement, the concentration of the Aunm solution (11 and 30 nm) was adjusted to reach the desired absorbance (usually ∼0.5 Abs), which corresponds to the nanomolar range. A quantitative amount of MPF (in the micromolar range) was added to the Aunm solution and quickly mixed for a few seconds. For some samples, a quantitative amount of NaCl solution (typically 0.5 or 1 M stock solution) was first added to the Aunm solution and mixed for 15 min before a quantitative amount of MPF was added into the solution. The reaction was typically followed for a few hours before TEM images or surfaceenhanced Raman scattering (SERS) spectra were taken. Throughout this paper, r was used to define the [MPF]/[Aunm] ratio. For example, a Aunm concentration of 2.4 nM and an MPF concentration of 2.8 µM in the assembly reaction solution corresponds to r ∼ 1145.

Control experiments using 1% DMSO/H2O in the absence of MPF showed no indication of color change in the Aunm solution and no change in the UV-vis spectrum. Control experiments for the Aunm solution in the presence of NaCl in the experimental range also showed no changes or aggregation. We also note that large assemblies precipitated within 1 day and the precipitated assemblies could be redispersed back into the solution upon brief sonication. The morphology of the nanoparticle assemblies remained largely unchanged. UV-visible (UV-vis) spectra were acquired with a HP 8453 spectrophotometer. Spectra were collected over the range of 2001100 nm. A cuvette with a path length of 1.0 cm was utilized. Transmission electron microscopy (TEM) was performed on a Hitachi H-7000 electron microscope (100 kV). The gold nanoparticle assembly samples were dissolved in aqueous solution and were drop cast onto a carbon-coated copper grid sample holder followed by evaporation in air at room temperature. Dynamic light scattering (DLS) was performed on a standard laser light scattering spectrometer (BI-200SM) equipped with a BI9000 AT digital time correlator and a He-Ne laser (35 mW, 633 nm) to determine the evolution of the hydrodynamic diameters of the nanoparticle assemblies in solution. All solutions were filtered using 0.45 or 0.22 µm filters before they were mixed for the assembly. Surface-enhanced Raman scattering (SERS) spectra were collected using a Raman Instrument Advantage 200A spectrometer with a 5 mW laser at 632 nm.

Results and Discussion 1. Morphology of the Assembly. The MPF-mediated assemblies of Au11nm were first examined. The solution was sampled

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Figure 5. (A) Spectral evolution for the MPF-Au30nm assembly. r ∼ 1.72 × 104 (a), 2.50 × 104 (b), and 3.23 × 104 (c) in the presence of NaCl (16 mM). (B) Plots of Abs at 750 and 800 nm versus time (s). r ∼ 1.72 × 104 (black filled circle), 2.50 × 104 (blue filled square), and 3.23 × 104 (red filled triangle). The dotted lines represent the kinetic fitting for the corresponding curves using the first-order reaction model. The arrows indicate the direction of the spectral evolution. Table 1. Apparent Rate Constants [k (s-1)] for the Assembly of MPF-Aunm MPF-Au11nm r)

700 nm, [NaCl]added ) 0

700 nm, [NaCl]added ) 5 mM

5.88 × 102 8.70 × 102 1.15 × 103

1.89 × 10-3 2.35 × 10-3 4.11 × 10-3

8.65 × 10-3 1.02 × 10-2 3.76 × 10-2

MPF-Au30nm r)

750 nm, [NaCl]added ) 16 mM

800 nm, [NaCl]added ) 16 mM

1.72 × 104 2.50 × 104 3.23 × 104

3.13 × 10-3 5.39 × 10-3 7.28 × 10-3

2.21 × 10-3 3.57 × 10-3 4.31 × 10-3

after adding MPF into an aqueous solution of Au11nm and then examined using TEM (Figure 1). In contrast to the relatively scattered 2D fractal morphology for Au11nm (Figure 1a), the observation of highly clustered features with arrays or ensembles of Au11nm for the MPF-mediated assembly (Figure 1b and c) indicates 3D interparticle linking. The typical MPF-to-Au11nm molar ratio (r ) [MPF]/[Au11nm] ∼ 1000) translates to ∼10 piperazinyl groups anchored on fullerene per citrate group capped on Au11nm (MPF has 9 piperazinyl groups, whereas the 11.5 nm particle can accommodate ∼1250 citrates), implying that there are more positive charges from fullerenes than negative charges from Au particles. It is unlikely to be the simple surface neutralization effect that is responsible for the interparticle assembly. The size of the assembly was found to increase with the molar ratio r and assembling time. For r ) 7.44 × 103 (Figure 1c), the resulting highly clustered 3D ensemble of Au11nm exhibits a large spherical shape. The less-well-defined edge is likely due to the

Figure 6. Experimental data (black solid lines) and simulation results (red dashed lines) for the SP bands of the MPF-Au11nm assembly (∆RI ) 1.30) (top panel) and MPF-Au30nm assembly (∆RI ) 1.38) (bottom panel).

drying effect of water from the aqueous environment, in contrast to the well-defined spherical edges found for assemblies in an organic environment, for example, multidentate thioethermediated assembly of nanoparticles in toluene.6 If the assembly were due to simple surface neutralization, fused nanoparticles would have been formed because of the disruption of surface protection. In fact, separate experiments using other positively charged molecules (e.g., tetramethylammonium) did not show any assembly even at much higher concentrations, further ruling out the possibility of a simple surface neutralization effect. The fact that there were hardly any free nanoparticles or fractal morphologies spotted for the MPF-Au11nm samples (Figure 1b and c) was indicative of the interparticle linkages of particles by the multifunctional MPF, which was substantiated by the analysis

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of the interparticle distance. The edge-to-edge interparticle distance determined from the interconnected Au11nm particles in the TEM image (Figure 1a) yielded an average value of 1.1 ( 0.2 nm. For interparticle linking via two surface-adsorbed citrate molecules, a theoretical estimate of the edge-to-edge interparticle distance would be 1.3 nm (Scheme 3A), which could be slightly overestimated in view of the likelihood of interdigitated shell interactions. It is quite close to the observed interparticle distance. In contrast, the measured interparticle distance for MPF-Au11nm assemblies (Figure 1b) displays a larger value (2.5 ( 0.3 nm). Considering that the piperazinyl group on MPF is protonated at the nitrogen next to the methyl group, a theoretical estimate of the edge-to-edge interparticle distance for the MPF-Au11nm assembly yields a value of 3.2 nm (Scheme 3B). This value is slightly larger than the measured one, likely implying a certain degree of interpenetration between the protonated piperazinyl groups and the carboxylate shells. Figure 2 shows another set of TEM images for assemblies obtained in solution with different concentrations of MPF and in the absence and presence of added NaCl. Clearly, increasing the concentration of MPF leads to an increased packing density of the nanoparticles. Increasing the concentration of NaCl does not seem to further increase the local packing density, but it improves the apparent separation between the neighboring assemblies. Similar morphologies have also been observed with the MPFmediated assembly of gold nanoparticles of larger sizes. Figure 3 shows a representative TEM image for the assembly of 30 nm sized Au nanoparticles (Au30nm) obtained in solution with different concentrations of MPF and in the presence of added salt. Similar to the case of the smaller-sized particles, the increase of the concentration of MPF leads to an increased packing density of the nanoparticles. 2. Surface Plasmon Resonance Optical Properties. The solution of gold nanoparticles has been shown to display a gradual color change from red to purple as a result of the assembly. This change is indicative of the overall change in the interparticle dipole interaction and dielectric medium constant or the refractive index (RI) properties in the surrounding medium.6,28 In contrast to the appearance of a surface plasmon (SP) resonance band at a lower wavelength (520 nm) for Au11nm nanoparticles before assembly, a new SP band for MPF-Au11nm assemblies was found at ∼680-700 nm (Figure 4, top panel). Increasing the r value further increases the reactivity of the assembly, which is evidenced by the increase in the kinetics of the absorbance at 700 nm versus time plot (Figure 4, bottom panel). The assemblies in the solutions eventually precipitated after ∼1 day due to the formation of large-sized assemblies. The precipitated assemblies were redispersable by brief sonication. Importantly, the distinct isosbestic point observed at ∼580 nm indicates the presence of two lightabsorbing species at equilibrium, that is, Au11nm and MPFAu11nm assemblies. The effect of the electrolyte concentration on the SP band evolution was also examined (Figure 4, middle panel). In comparison with those without any addition of electrolytes, the addition of NaCl speeds up the overall reaction rate of the assembly. This is clearly evidenced by kinetic plots in Figure 4 (bottom panel). More specifically, the addition of NaCl causes a broadening of the longer wavelength region of the spectra (750-1100 nm), in contrast to the well-defined spectral evolution and absence of broadening in the longer wavelength region for the cases without the addition of NaCl. As a result of the overall broadening of the SP band in the longer wavelength region, the (28) Lim, I-I. S.; Zhong, C. J. Gold Bull. 2007, 40, 59.

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Figure 7. pH-tuning of MPF-Au11nm assembly and disassembly. UV-vis spectra for Au11nm (dashed line) and the MPF-Au11nm assembly at pH 5 (a) and upon changing the pH to 10 (b), pH 7 (c), and pH 2 (d). Each curve was taken after 30 min.

absorbance of the SP band (at 520 nm) is lower than those in the absence of NaCl. This observation is consistent with the TEM data, indicating the formation of more complete assemblies. The dependence of SP band evolution on particle size was also examined. Figure 5 shows a representative set of UV-vis spectra and kinetic data for the MPF-mediated assembly of Au30nm in the presence of NaCl. In the case of the MPF-Au30nm assembly, it was observed that the spectral evolution is insignificant in the absence of NaCl. As such, the addition of NaCl is a requirement for the assembly of the larger-sized nanoparticles (e.g., Au30nm). In comparison with the spectral evolution for MPF-Au11nm assembly, the evolution of the SP band shows a clear shift toward a longer wavelength for MPF-Au30nm assembly. This shift is indicative of the formation of larger sized assemblies. The solution changed from a red to bluish color within a 1 hr reaction time and showed precipitation in less than a day. By increasing the molar ratio r (Figure 5A, a-c), the reaction rate was observed to increase, as evidenced by the kinetic plots. The absorbance versus time plots in Figures 4 and 5 were further assessed by fitting the data based on a pseudo first-order reaction model (y ) y0 + a(1 - e-kt)). This assumption is justified by the high molar ratio of fullerenes to Au nanoparticles used in this study. The apparent rate constants (k) for the above assembly processes are summarized in Table 1. For the MPFAu11nm assembly process, the overall trend of the data shows that, as r increases, the apparent rate constant k increases. When comparing the k values for those between the absence and the presence of NaCl, the assembly process in the presence of NaCl was found to display a larger k value, indicating a faster assembly reaction. This trend is also observed for the MPF-Au30nm assembly process. It appears that the assembly of the largersized particles exhibits a somewhat slower rate in comparison with the assembly of the smaller sized particles. The characteristics of the SP band evolution in Figures 4 and 5 can be further understood based on Mie theory simulation.6,28-30 The change in size, shape, and dielectric medium or the change in the refractive index (RI) leads to a shift in the SP band.28-30 Figure 6 compares the experimental and simulated results. Since the red shift is known to increase with the RI change, a basic assumption for applying the theoretical simulation to this system is that the nanoparticles within the assembly environment have a higher refractive index than those in an aqueous environment.28 The simulation results showed that there was an increase in the (29) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409. (30) Sonnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P. Nat. Biotechnol. 2005, 23, 741.

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Figure 8. TEM micrographs for MPF-Au11nm assemblies sampled at pH 5 (a), pH 10 (b), and pH 2 (c), corresponding to the spectral data shown in Figure 7. Scheme 4. Schematic Illustration of the pH-Dependent Electrostatic Interactions between the Piperazinyl Groups on MPF and the Citrate Groups on the Nanoparticlesa

a (A) no electrostatic interaction, (B) electrostatic interaction, and (C) no electrostatic interaction and instability. The structures drawn are not to scale.

RI by 1.30 and 1.38 for the MPF-Au11nm and MPF-Au30nm assemblies, respectively. This is in qualitative agreement with the findings from both the optical data and the TEM data for the assembly of the different sized nanoparticles. It is observed that the SP band evolution is dependent on the pH of the solution (Figure 7), which is believed to suggest that the interparticle interaction for the assembly is pH-tunable. On the basis of pKa values for the citric acid (pKa) 3.1(a1), 4.8(a2), and 6.4(a3)31a) and piperazine (pKa) 4.2(a1) and 8.4(a2)31b) groups, the electrostatic interaction involved at least two of the deprotonated -CO2H groups in citrate and one protonated piperazinyl (-NC4H8N+HCH3) of C60 at pH ∼ 5. Upon increasing the pH from ∼5 (Figure 7a) to 10 (Figure 7b), the SP band shifted from ∼580 nm to the wavelength approaching the SP band for Au11nm (520 nm). Indeed, when the pH is increased to above 8, no apparent spectral evolution was observed. The control of the pH was found to tune the assembly and disassembly processes effectively. The pH-tuned nanoparticle assemblies were further examined by TEM. The comparison of the TEM data between samples taken from the solutions of the two pHs (Figure 8) clearly shows that the assembly at pH 5 was torn apart at pH 10 (Figure 8b). This observation is thus indicative of the disassembly of MPFAu11nm into individually isolated Au11nm particles. However, at much lower pH, for example, pH < 2 (Figure 8c), the nanoparticles became unstable, and the disassembly of the nanoparticles led to an irreversible fusion of the nanoparticles. The assembly-disassembly process was reversible because the SP band shifted back upon decreasing the pH to ∼7 (Figure (31) (a) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill: New York, 1972. (b) Santos, M. A.; Esteves, M. A.; Vaz, M. C.; Frausto da Silva, J. J. R.; Noszal, B.; Farkas, E. J. Chem. Soc., Perkin Trans. 2 1997, 2, 1977.

7c). At pH ∼ 2 (Figure 7d), irreversible precipitation and longwavelength broadening of the SP band were observed as a result of the instability of the citrate capping on the nanoparticles at low pH, which was supported by TEM observations of larger aggregates in which individual nanoparticles could hardly be identified (Figure 8c). Control experiments were performed, and the results showed that the pH-tuned changes in color or precipitation were different between samples with and without the fullerene mediator, demonstrating the importance of interparticle molecular interaction, which would be pH-independent if there were no binding of MPF to the gold particles by electrostatic interaction. The pH-tuned change in optical properties has implications for the controlled release of molecules from the nanoparticle assembly, an area of importance in controlled drug release. On the basis of the above analysis of the pH effect, pH-tuned disassembly is believed to reflect the change in the interparticle electrostatic interactions. Scheme 4 illustrates the pH-tuned change in terms of the charge on MPF and on the nanoparticles, and the corresponding variation in the interparticle electrostatic interaction. The electrostatic interaction is not operative between MPF and Aunm when MPF is neutral and citrate is negatively charged. 3. Dynamic Light Scattering Measurements. The dynamic light scattering (DLS) data for the assembly of MPF-Au11nm (Figure 9) evidenced the change of the apparent hydrodynamic radius (Rh) corresponding to the above spectral evolution. It is evident that the size of the assemblies grew with reaction time (up to Rh ∼ 700 nm). The Rh distributions are based on scattered intensity contributions. In the case of large assemblies with Rh ∼ 200 nm, the detection of the small peak with Rh ∼ 28 nm could be attributed to the early stage of assembly. The small-sized

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Figure 9. Apparent hydrodynamic radius (Rh) distributions by intensity for the MPF-mediated assembly of Au11nm (scattering angle: 30°) determined by DLS measurements. (The main peaks for the curves of 5, 13, 22, 32, and 50 min correspond to the rightmost five curves at 201, 246, 311, 355, and 627 nm, respectively.) ([Au11nm] ) 0.65 nM and [MPF] ) 2.63 µM.) (B) Plot of the average hydrodynamic radius (Rh) versus time (closed circles: citrate-capped Au11nm; open circles: MPF; and filled squares: MPF-Au11nm).

assemblies were dominant at the early stage of assembly. After ∼10 min, only highly monodispersed assemblies with Rh ∼ 246 nm were observed. The size of the monodispersed MPF-mediated assemblies was observed to continuously grow with reaction time until the occurrence of phase separation (∼40 min). The findings support that the size of the MPF-Au11nm assembly is controllable in terms of the control of assembly time, in addition to the other parameters. Further DLS measurements compared the growth of the assembly in the absence and presence of NaCl (Figure 10). It is evident that the hydrodynamic diameter of the nanoparticle assembly increases with assembly time. The data suggest that, at a low concentration of Au nanoparticles and MPF, the addition of NaCl to the assembly has little or no effect. This observation is apparently different from those shown in the UV-vis data where the addition of salt speeds up the evolution of the SP band. One intriguing phenomenon is the observation of the initial drop of the hydrodynamic diameter of the MPF (at ∼4 min, Figure 10A) before the nanoparticle assembly growth becomes dominant. While the exact origin is under detailed investigation, one possible scenario involves the adsorption of MPF on the surface of the nanoparticles which breaks the clustering force of MPFs in aqueous solution. Such a cluster involves possibly a number of loosely connected MPFs in the solution. Upon mixing of nanoparticles and MPFs in the solution, the adsorption of MPF on the Au nanoparticle surface breaks apart those loose connections and forms Au-MPF assemblies. This process thus leads to a drop in Rh (Figure 10A) as well as a slight decrease in the scattering intensity within the first 10 min (Figure 10B).

Figure 10. Plots of hydrodynamic diameters (Dh) (A) and scattering intensities (kHz) (B) versus time for MPF-Au11nm assemblies with (blue filled triangle) and without (black filled circle) the presence of NaCl. ([Au11nm] ) 0.81 nM; [MPF] ) 0.77 µM; and [NaCl] ) 3 mM.) The point at 0 min represents the measurement result for MPFs in the solution before their mixing with Au nanoparticles, whereas the point at 5 min represents the first measurement result of the solution after mixing.

4. Surface-Enhanced Raman Scattering. Surface-enhanced Raman scattering (SERS) has been used in many studies of surface or interfacial systems as a powerful tool to probe the adsorption of macromolecules on the surface of noble metals.32-36 In this work, SERS was used to assess the adsorption of MPF on the gold nanoparticle surface. The spectra were collected from two types of samples from the MPF-Aunm assembly with various NaCl and MPF concentrations including solution-phase samples and samples cast on a gold film substrate. The control experiment of samples taken from the MPF solution with concentrations identical to those of the MPF-Aunm assembly solution in the (32) Zhang, X.; Young, M. A.; Lyandres, O.; Van Duyne, R. P. J. Am. Chem. Soc. 2005, 127, 4484. (33) Cao, Y.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536. (34) Doering, W. E.; Nie, S. Anal. Chem. 2003, 75, 6171. (35) Kneipp, J.; Kneipp, H.; Rice, W. L.; Kneipp, K. Anal. Chem. 2005, 77, 2381. (36) (a) Luo, Z.; Fang, Y. Vib. Spectrosc. 2005, 39, 151. (b) Yang, X. C.; Fang, Y. J. Phys. Chem. B 2003, 107, 10100. (c) Luo, Z.; Fang, Y. J. Colloid Interface Sci. 2005, 283, 459.

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Figure 11. SERS spectra for MPF-Au11nm assemblies in aqueous solutions in the absence (A) and presence (B) of NaCl (5 mM) (at r ∼ 5.88 × 102 (a), 8.70 × 102 (b), and 1.15 × 103 (c)) and for MPF-Au30nm assemblies in the presence of NaCl (16 mM) (C) (at r ∼ 1.72 × 104 (a), 2.50 × 104 (b), and 3.23 × 104 (c)). (Integration time ) 50 s.)

absence of nanoparticles showed practically background-like spectra (not shown). In contrast, two clear bands are observed at 1456 and 1562 cm-1 for the MPF-Aunm assembly solution. These two bands can be assigned to the Ag(2) mode (1448 cm-1) and Hg(8) mode (1561 cm-1) for the Raman-active vibration modes of C60.37,38 The detection of SERS for these two bands is clearly due to the surface effect of the gold nanoparticles, a strong piece of evidence supporting the adsorption of MPF on the surfaces of gold nanoparticles which is responsible for the MPF-Aunm assembly. Figure 11 shows a representative set of SERS spectra for the solution-phase samples. The spectra were collected after the assembly reaction of Aunm and MPF had taken place for a few hours. The assemblies were then centrifuged (10 000 rpm for 15 min) and redispersed in deionized water. The overall features and peak positions remained identical before and after centrifugation. Two intense peaks were observed at 1456 cm-1 (Ag(2)) and 1562 cm-1 (Hg(8)) for the Raman-active vibration modes of MPF. The overall trend in Figure 11A indicates that there is an increase in the Raman intensity of the MPF-Au11nm assembly as the concentration of MPF increases, implying that the nanoparticles are not fully covered with MPF at relatively low concentrations of MPF. The addition of NaCl to the assembly solution (Figure 11B) does not seem to increase the peak intensity, the reason of which is under investigation. In addition, the intensity values for the assembly of the two different molar ratios (Figure 11B, b and c) appear similar. This observation suggests that the quantity of MPF is sufficient to form full monolayer coverage on Aunm and thus the SERS intensity shows no observable difference even though the addition of electrolytes speeds up the assembly process. These results further demonstrate that the SERS effect originates from the assembly of the nanoparticles via the adsorption of MPF onto the Aunm surface and is not due to the effect of electrolyte in the assembly solution. For the assembly of larger sized nanoparticles (Figure 11C), it appears that there is also no major difference observed for the different amounts of MPF added to the solution, again indicating that Au30nm nanoparticles are fully covered with MPF at relatively low concentrations of MPF. Importantly, the SERS intensity is much stronger for the Au30nm than that for the Au11nm particles, (37) Chase, S. J.; Bacsa, W. S.; Mitch, M. G.; Pilione, L. J.; Lannin, J. S. Phys. ReV. B 1992, 46, 7873. (38) Mene´ndez, J.; Page, J. B. Vibrational Spectroscopy of C60. In Light Scattering in Solids VIII; Cardona, M., Gu¨ntherodt, G., Eds.; Springer: Berlin, 2000.

Figure 12. SERS spectra comparing solution (black curve, a) and surface (blue curve, b) samples of MPF-Au11nm (A; r ) 1.15 × 103; NaCl ) 5 mM) and MPF-Au30nm assemblies (B; r ) 1.72 × 104; NaCl ) 16 mM). (Integration time ) 50 s.) Black curve: (A) ×4.5 and (B) ×7.5.

indicating that Au30nm has a greater enhancement than Au11nm. This observation is consistent with the size dependence of the SERS effect reported for gold nanoparticles of different sizes.39-43 Figure 12 shows a representative set of SERS spectra comparing samples of the assembled nanoparticles in solution and on a gold film surface. For the surface samples, the spectra were taken after the samples were dried. The SERS effect is clearly dependent on the sample form. The surface sample showed a much stronger intensity than that for the solution sample. By comparing the SERS spectra between the solution sample and the surface sample for the assemblies of the two different sized particles (Figure 12), it is evident that the peak intensity for Au30nm is much higher than that for Au11nm, though the spectral characteristics are very similar. These differences in the enhancement factor reflect in part the size and surface coupling effects. There is an additional surface plasmon resonance coupling between the sample surface (39) Freeman, R. G.; Bright, R. M.; Hommer, M. B.; Natan, M. J. J. Raman Spectrosc. 1999, 30, 733. (40) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Nano Lett. 2005, 5, 1569. (41) Driskell, J. D.; Lipert, R. J.; Porter, M. D. J. Phys. Chem. B 2006, 110, 17444. (42) Zhu, T.; Zhu, Z. H.; Wang, J.; Wang, Y. C.; Liu, Z. F. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1999, 337, 237. (43) Zou, X.; Ying, E.; Dong, S. Nanotechnology 2006, 17, 4758.

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and the nanoparticle assembly under the incident excitation beam.44 An important implication of these findings is that the MPF-Aunm combination provides an effective spectroscopic probe for high-sensitivity detection as a result of the SERS effect.

Conclusions In conclusion, the interparticle electrostatic interactions in the multifunctional fullerene-mediated assembly of gold nanoparticles produce 3D architectures with optical and spectroscopic tunabilities corresponding to the surface adsorption and the controllable assembly and disassembly processes. The optical and spectroscopic characteristics provided strong evidence for the surface binding of fullerene on the nanoparticles in the (44) Hao, E.; Schatz, G. C. J. Chem. Phys. 2004, 120, 357.

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assembly. The detection of the Raman-active vibration modes of C60 and the particle size effect demonstrated that the adsorption of MPF on the nanoparticle surface in the MPF-Aunm assembly is responsible for the SERS effect. With further correlation between the optical or spectroscopic properties and the nanoscale morphologies, these findings are expected to constitute the basis for refining the design parameters of novel nanocomposite materials for potential applications such as controlled drug delivery, optical sensors, and spectroscopic probes. Acknowledgment. This work is supported by the National Science Foundation (CHE 0349040 (for SUNY) and CHE 0316078 (for CUNY)). S.L. acknowledges the support of the National Science Foundation Graduate Research Fellowship. LA701868B