Temperature-Dependent Reversible Self-Assembly of Gold

Colloidal gold nanoparticles self-assembled into macroscopic aggregates by charge-transfer interaction between pyrenyl units as an electron donor immo...
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Temperature-Dependent Reversible Self-Assembly of Gold Nanoparticles into Spherical Aggregates by Molecular Recognition between Pyrenyl and Dinitrophenyl Units Kensuke Naka,* Hideaki Itoh, and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Received November 25, 2002. In Final Form: April 4, 2003 Colloidal gold nanoparticles self-assembled into macroscopic aggregates by charge-transfer interaction between pyrenyl units as an electron donor immobilized on the surface of the gold nanoparticles and a bivalent linker containing two dinitrophenyl units as electron acceptors. Transmission electron microscopy and scanning electron microscopy images showed the spherical aggregates consisting of the gold nanoparticles. The UV-vis absorption spectrum showed a thermally reversible self-assembly process.

Introduction Metal nanoparticles have potentially useful optical, optoelectronic, and material properties derived from the quantum size effect.1 These properties would lead to applications including chemical sensors and quantum dots. In particular, the building and patterning of the nanoparticles into two- and three-dimensional organized structures by manipulation of individual units is of interest to realize nanodevices with these unique properties. While a number of different approaches have been developed to self-assemble the metal nanoparticles in hexagonal closepacked monolayers,2 the controlled assembly of the nanoparticles in solutions is an area that is relatively unexplored.3 One of the more efficient methodologies for organization of the metal nanoparticles involves the selfassembly based on selective control of noncovalent interactions. Several approaches (hydrogen bonding interaction,4 π-π interaction,5 host-guest interaction,6 van der Waals forces,7 electrostatic forces,8 and antibodyantigen recognition9) have been described for the formation of arrays of the metal nanoparticles. These results provided a powerful method for employing preprogrammed materials with the potential for multidimensional ordering for the creation of well-defined structures at a molecular level. However, the ability to assemble them with controllable dimensions in solutions has met with limited success.4,5 (1) (a) Schmid, G. Clusters and Colloids from Theory to Application; VCH: Weiheim, 1992. (b) Simon, U.; Scho¨n, G.; Schmid, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 250. (2) (a) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (b) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 11, 1007. (c) Motte, L.; Billoudet, F.; Lacaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 138. (d) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (3) Adachi, E. Langmuir 2000, 16, 6460. (4) (a) Boal, A. K.; Ilhan, F.; Derouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (b) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2000, 122, 734. (c) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (5) Jin. J.; Iyoda, T.; Cao, C.; Song, Y.; Jiang, L.; Li, T. J.; Zhu, D. B. Angew. Chem., Int. Ed. 2001, 40, 2135. (6) (a) Liu, J.; Mendoza, S.; Roman, E.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (b) Lin, S.-Y.; Liu, S.-W.; Lin, C.-M.; Chen, C.-H. Anal. Chem. 2002, 74, 330. (7) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281. (8) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (9) Shenton, W.; Davis, S. A.; Mann, S. Adv. Mater. 1999, 11, 449.

Figure 1. Schematic illustration for thermally reversible selfassembly of metal nanoparticles by charge-transfer interaction.

The programmed organization of the nanoparticles in superstructures of desired shape and morphology is a challenging research area. Here we describe a new method for self-assembling colloidal gold nanoparticles reversibly into macroscopic aggregates (Figure 1). Our strategy is based on the charge transfer (CT) interaction between pyrenyl units as an electron donor immobilized on the surface of the gold nanoparticles and a bivalent linker containing two dinitrophenyl units as electron acceptors. Addition of the bivalent linker to a solution of individual gold nanoparticles results in the formation of macroscopic aggregates comprising noncovalently linked gold nanoparticles through the CT interaction between the pyrenyl units and the dinitrophenyl units. The advantages of this method are as follows. First, the stability of CT complexes depends on temperature. This methodology allows the modified gold nanoparticles to self-assembly into aggregates, in which the sizes might be thermally controlled. Second, the stability of CT complexes also depends on the structure of components of the CT complexes. That is, the binding strength would be easily tailored by the modification of the structure of the electron donor and electron acceptor moieties. Unlike the previous methods, the self-assembly process would be controlled easily through tuning the binding strength between the gold nanoparticles by choosing the structure of the bivalent linker. To the best of our knowledge, the use of CT interaction to realize nanoparticle assemblies has not been reported so far.

10.1021/la026898i CCC: $25.00 © 2003 American Chemical Society Published on Web 05/24/2003

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Figure 2. (a) TEM image of the 1-modified gold nanoparticles. (b) Size distribution histogram of the 1-modified gold nanoparticles. Chart 1. Structures of 1, 2, 3, and 4

Materials and Methods Materials. All solvents and reagents were obtained from commercial sources and used as supplied except for the following. Tetrahydrofuran (THF) was distilled under nitrogen. Methanol was distilled with sodium under nitrogen. Measurement. 1H NMR spectra were obtained with a JEOL JNM-EX270 spectrometer (270 MHz for 1H NMR) in chloroformd. UV-visible spectra were measured on a Jasco V-530 spectrometer. Transmission electron microscopy was performed using a JEOL JEM-100SX operated at 100 kV. Scanning electron microscopy was performed using a JEOL JNM-5310/LV system. Fluorescence emission spectra were recorded on a Perkin-Elmer LS 50B luminescence spectrometer. Thermogravimetric analysis was performed using a TG/DTA 6200, SEIKO Instruments, Inc., with heating rate of 10 °C min-1 in air. Colloid Synthesis. Gold nanoparticles were prepared according to the procedure described by Brust.10 HAuCl4 (20.0 mg, 0.048 mmol) was dissolved in 30 mL of H2O. Tetraoctylammonium bromide (26.5 mg, 0.048 mmol) was then added as a solution in 30 mL of toluene, and the reaction mixture was stirred until the yellow aqueous layer was clear and the organic layer was red. 1 (6 mg, 6.3 × 10-3 mmol) was then added, followed by the dropwise addition of NaBH4 (5.23 mg, 0.14 mmol) as a solution in 5 mL of H2O. This caused an immediate color change to dark black. The reaction mixture was stirred for 10 min, and the organic phase was collected and added to MeOH (100 mL). The precipitates were isolated by centrifugation. The gold nanoparticles stabilized by 1 were obtained as a black powder.

Results 1. Synthesis of Gold Nanoparticles. The gold nanoparticles were prepared by reduction of HAuCl4 (20.0 mg, 0.048 mmol) with NaBH4 in the presence of 10,10′dithiobis(decanoic acid 4-(1-pyrenyl)butyl ester) (1) (6 mg, 6.3 × 10-3 mmol) (Chart 1). The UV-vis absorption (10) Brust, M.; Walker, M.; Bethell, D.; Schiffrin. D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801.

spectrum of a red solution after chemical reduction with NaBH4 indicated the formation of gold nanoparticles with a surface plasomon absorption band at 500 nm. The average diameter of the 1-modified gold nanoparticles was 2.35 ( 1.00 nm as measured by transmission electron microscopy (TEM) (Figure 2). Figure 3 shows the 1H NMR spectra of 1 and the 1-modified gold nanoparticles in CDCl3. In the 1H NMR of the 1-modified gold nanoparticles, signals appeared corresponding to those of the proton signals of the parent 1 (Figure 3b). The signals of the methylene protons in the 1-modified gold nanoparticles are broadened and shifted compared to those of 1. The signals of methylene protons closest to the thiolate/Au interface are hardly recognized, since the motion of the methylene groups close to the surface of the gold nanoparticles was constrained. This type of signal broadening has been observed in bipyridyl or alkanethiolate-modified metal nanoparticles.11 These results indicate clear evidence for the attachment of the thiolated pyrene to the surface of the gold nanoparticles. On the basis of the fact that the proton signals at 3.4 and 4.1 ppm derived from the free parent 1 were not observed in the 1-modified gold nanoparticles, the free ligand 1, which did not bind to the gold nanoparticles, was not involved. Thermogravimetric analysis established that the content of 1 in the gold nanoparticles was 34 wt %. On the basis of the fact that the average particle size was 2.35 nm, the number of 1 adsorbed on the surface of each gold nanoparticle was 86. (11) (a) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-Z.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (b) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (c) Naka, K.; Yaguchi, M.; Chujo, Y. Chem. Mater. 1999, 11, 849. (d) Itoh, H.; Naka, K.; Chujo. Y. Polym. Bull. 2001, 46, 357.

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Figure 3. 1H NMR spectra of (a) 1 and (b) the 1-modified gold nanoparticles.

Figure 4. UV-vis absorption spectra by changing the weight ratio of 2 against the 1-modified gold nanoparticles. 2/1-modified gold nanoparticles ) (a) 0, (b) 1, (c) 5, (d) 10.

2. Assembly of Nanoparticles in the Presence of Linkers. The bivalent dinitrophenyl linker (2) was added to a toluene solution of the 1-modified gold nanoparticles at room temperature. The solution color gradually faded with increasing the incubation time. The reaction of the 1-modified gold nanoparticles with 2 was followed as a function of time through optical changes in the surface plasmon band in the UV-vis absorption spectrum. After addition of 2, a decrease in the surface plasmon resonance was observed. The absorption of the surface plasmon band was almost saturated at 24 h after addition of 2. The UVvis absorption spectrum showed no change for several days. Figure 4 shows a UV-vis absorption spectrum recorded at 24 h after addition of 2 with varying concentration to colloidal solutions of the 1-modified gold nanoparticles. Decreasing in the surface plasmon band became more significant with increasing the concentra-

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tions of 2, indicating the formation of the particles aggregates.6 Two control experiments were carried out. First, to confirm the recognition property between the pyrenyl units and the dinitrophenyl units, we used 1,10-decanediol dibenzoate (3) with two benzoate groups. In contrast to the results obtained with 2, addition of 3 to the solution of the 1-modified gold nanoparticles caused no color change and no change in the UV-vis absorption spectrum. Second, the colorimetric change was not detectable using 1,3dinitrobenzene instead of 2 for the solution of the 1-modified gold nanoparticles. The stability of the CT complexes depends on the structure of the dinitrophenyl unit. To confirm the effect of the acceptor strength, 1,10-decanediol di(3,4-dinitrobenzoate) (4) with o-dinitrophenyl units at each end was used. In contrast to the results obtained with 2, addition of 4 to the solution of the 1-modified gold nanoparticles showed no change in the UV-vis absorption spectrum, indicating no aggregate formation of the gold nanoparticles by 4. 3. Fluorescence Quenching. According to the literature,12 the CT band between pyrene and dinitrobenzene is observed around 430 nm, which shows a complete overlap with the spectra of the surface plasmon band of the gold nanoparticles. Thus, the UV-vis absorption spectrum gave no clear proof of the presence of the CT complex. To confirm the presence of the CT interaction in the present system, fluorescence measurement was carried out. Figure 5a shows the quenching of the pyrenyl units on the gold nanoparticles by 5 as a model compound of 2 in toluene at 25 °C to eliminate the effect of the particle aggregates. As the concentration of 5 increased, the emission intensity decreased without change in the spectral pattern. This indicates that the quenching process occurred via the charge transfer from the pyrenyl units to the dinitrophenyl units. No exciplex emission was detectable in the spectral region under these experimental conditions. The Stern-Volmer plots of the pyrenyl units of the fluorescence quenching using 5 are shown in Figure 5b. The plots are not linear, indicating a static-type quenching mechanism via a ground-state complex formation. The static quenching mechanism was substantiated by studying the temperature effect on the fluorescence quenching efficiency. The effect of temperature on the fluorescence quenching by 5 in toluene in the presence of the 1-modified gold nanoparticles was studied at 25 and 0 °C. As the temperature increased, the fluorescence efficiency of the pyrenyl units on the surface of the gold nanoparticles decreased (Figure 5b), and this is due to the effect of thermal energy in destabilizing the groundstate CT complex.13 4. TEM and Scanning Electron Microscopy (SEM) Image of Assembly of Gold Nanoparticles. TEM was used to characterize the aggregates. One drop of the solution containing the obtained product was placed on a copper grid and allowed to evaporate the solvent under atmospheric pressure at room temperature. Although the TEM image before addition of 2 showed the well-separated gold nanoparticles with uniform size distribution (Figure 2a), the TEM image after addition of 2 (1 mg) to the toluene solution containing the 1-modified gold nanoparticles (1 mg) showed the formation of large, spherical aggregates with diameter of 1 ( 0.7 µm (Figure 6a). The microscopic aggregates consisted of individual gold nanoparticles. The SEM observation also indicated that the obtained product (12) Ayad, M. M. Z. Phys. Chem. Bd. 1994, 187, 123. (13) Ebeid, E. M.; Gaber, M.; Habib, A. M.; Issa, R. M.; El-Azim, S. A. J. Chim. Phys. 1989, 86, 2015.

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Figure 5. (a) Fluorescence quenching of pyrenyl units on the surface of the gold nanoparticles by 5 in toluene (λex ) 330 nm). The concentrations of 5 at decreasing emission intensities are (1) 0.0 M, (2) 8.8 × 10-3 M, (3) 1.76 × 10-3 M, and (4) 3.53 × 10-3 M. (b) Stern-Volmer plots of the fluorescence quenching by 5 in toluene in the presence of the 1-modified gold nanoparticles at (9) 0 °C and (2) 25 °C. I0 and I represent the emission intensities in the absence and presence of 5, respectively.

Figure 6. (a) TEM image, (b) magnified TEM image, and (c) SEM image of the spherical aggregates of the 1-modified gold nanoparticles.

was spherical in shape (Figure 6b). The diameter of the obtained spherical aggregates became larger with increasing the concentrations of 2. The addition of 5 and 10 mg of 2 resulted in the formation of spherical aggregates with diameters of 5.2 ( 3.2 and 7.2 ( 4.7 µm, respectively (see Supporting Information). Thus, the degree of colloidal association would be controlled by adjusting the concentration of 2 in the medium. Temperature strongly affects the morphology of the resulting aggregates. A SEM investigation showed at

0 °C that the formation of larger aggregates with a diameter of 5 ( 1.9 µm was observed compared to that with a diameter of 1 ( 0.7 µm at room temperature. These results indicate that a stronger recognition process at lower temperatures resulted in the larger aggregate structures. 5. Assembly and Disassembly of Aggregates Depending on Temperature. The reversibility of the binding between 2 and the 1-modified gold nanoparticles was confirmed by monitoring an optical absorption de-

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Figure 7. (a) UV-vis absorption spectra before and after heating at 50 °C. (b) UV-vis absorption spectra before and after incubation for 24 h. (c) Absorbance versus temperature for the solution containing the 1-modified gold nanoparticles (1 mg) and 2 (1 mg).

Figure 8. (a) UV-vis absorption spectra of the CT complexes between 5 and 7. The concentrations of 7 (solid line) are (1) 0 M, (2) 3.70 × 10-3 M, (3) 7.67 × 10-3 M, and (4) 20.0 × 10-3 M. The concentration of 5 is 3.20 × 10-3 M. (b) UV-vis absorption spectra of the CT complexes between 6 and 7. The concentrations of 7 (solid line) are (1) 0 M, (2) 3.32 × 10-3 M, (3) 10.2 × 10-3 M, and (4) 20.0 × 10-3 M. The concentration of 6 is 3.20 × 10-3 M. Chart 2. Structures of 5, 6, and 7

pendent on the aggregation. Heating the solution at 50 °C for 10 min resulted in the complete recovery of the surface plasmon band prior to addition of 2 (Figure 7a), indicating complete aggregate dissociation of the gold nanoparticles cross-linked with 2 (see Supporting Information). When the solution was left at room temperature after heating, the absorbances of the surface plasmon band returned the same values as those obtained before heating (Figure 7b), indicating the formation of microspheres consisting of the gold nanoparticles again. The reversibility of the surface plasmon band is shown in Figure 7c, as the temperature was cycled between 25 and 50 °C. This reversible process was confirmed to be repeatable through several cycles. No difference of the UV-vis absorption spectra for the 1-modified gold nanoparticles was observed in the absence of 2 by heating at 50 °C, indicating that the particle size and shape of the 1-modified gold nanoparticles were maintained by heating. 6. Equilibrium Constants. To determine the equilibrium constants (Kc) of the CT complex, 5, 6, and 7 (Chart 2) were employed as model compounds for 2, 4, and 1, respectively. The absorption spectra of mixed solutions

with a fixed concentration of 7 and varying concentrations of 5 and 6 in toluene at room temperature are shown in Figure 8. The absorbance of the broad absorption band, which is seen up to 500 nm in Figure 8, increased with an increase in the concentration of 7 as the donor compound. This band is ascribed to the CT complex of 5 or 6 with 7. Comparison of parts a and b of Figure 8 revealed that the increase of the absorption band in Figure 8a was larger than that in Figure 8b. These results suggest that the Kc values of the CT complexes with pyrenyl group decreased in the order m-dinitrophenyl group > odinitrophenyl group. The Rose and Drago equation14 was used to calculate the Kc value of the CT complex from the spectrophotometric data. The order of Kc values of the CT complexes of different acceptors with 7 appeared to be 5 (Kc ) 11.2) > 6 (Kc ) 2.2). Discussion 1. Molecular Recognition. The UV-vis absorption spectrum is a strong method to investigate the formation (14) Rose, N. J.; Drago, R. S. J. Am. Chem. Soc. 1959, 81, 6138.

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of the particle aggregates. It is well-known that the aggregation can be conveniently monitored by the change of surface plasmon band. As described above, the broadening of the surface plasmon band in the presence of 2 indicated the assembly of the 1-modified gold nanoparticles. The two control experiments provided additional insights into the correlation of the spectral evolution. The addition of 3 instead of 2 to the solution resulted in no change in the UV-vis absorption spectrum, indicating that no assembly of the gold nanoparticles proceeded. The difference between 2 and 3 is terminal units. The benzoate units of 3 did not show CT interaction between pyrenyl units. This result indicates that the self-assembly of the 1-modified gold nanoparticles proceeded through specific CT interaction. The second control experiment by using 1,3-dinitrobenzene also showed no change in the UV-vis absorption spectrum. Based on the fact that the fluorescence quenching experiment showed the formation of the ground-state CT complex between the pyrenyl units on the surface of the gold nanoparticles and the dinitrophenyl units, the bivalent linker was required for the aggregation. Both TEM and SEM results combined with the UV-vis results indicate clearly that the 1-modified gold nanoparticles aggregated through cross-linking by 2 which recognizes the pyrenyl residues on the surface of the gold nanoparticles. The molecular structures of the acceptors on the CT complex formation have an important role for the assembly of the gold nanoparticles. Larger aggregates would be expected by a more efficient recognition process. The addition of 4 with o-dinitrophenyl units at each end showed no change in the UV-vis absorption spectrum. The difference between 2 and 4 was the structure of the dinitrophenyl units. Investigation of the equilibrium constants (Kc) of the CT complex by employing a model compound indicated that the order of Kc values of the CT complexes of different acceptors with 7 appeared to be 5 (Kc ) 11.2) > 6 (Kc ) 2.2). That is, the aggregation of the 1-modified gold nanoparticles via CT interaction with 2 is efficient in comparison with 4. The described experimental results were in accordance with the expectation based on the equilibrium constants of the CT complex, demonstrating control of the aggregate process by turning the structure of the acceptor compounds. 2. Reversible Self-Assembly. The addition of the linker to solution of the 1-modified gold nanoparticles led

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to the aggregation. We expected that the role of thermal energy in destabilizing the CT complexes led to the dissociation of the aggregates. The reversibility of this process was demonstrated by the temperature-dependent changes of the UV-vis absorption spectrum. Heating the solution showed that complete aggregate dissociation occurs as evidenced by a return to the original extinction values associated with the dispersed particles. This result also indicated that the 1-modified gold nanoparticles was stable against fusion of the nanoparticles. The stability of CT complexes depends on temperature. Conclusion In conclusion, we have demonstrated the self-assembly of the nanoparticles into microscopic aggregates by the CT interaction. The shape of the microscopic aggregates formed using this strategy is regular and spherical. The ability to tailor binding strength and assembly reversibility by this methodology would be highly desirable in many applications. The present results suggest that the selfassembly process of the gold nanoparticles is controlled by turning of the structure of the acceptor compounds. Synthesis of a series of the ligands with various CT complexes (for example, carbazolyl groups and dinitrophenyl groups15) and preparation of gold nanoparticles by using these ligands are in progress to control the sizes and shapes of nanoparticle assembly and to develop the highly selective recognition process. This strategy would be easily extended to fabricate two-dimensional gold nanoparticle monolayers. We expect that this concept represents a powerful and general strategy for the creation of highly structured multifunctional materials. Acknowledgment. We thank Professor T. Fukuda, Dr. Y. Tsujii, and Dr. S. Yamamoto (Institute of Chemical Research, Kyoto University) for the TEM micrographs. Supporting Information Available: Experimental details for the preparation of compounds 1-7 and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. LA026898I (15) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768.