Photochemical Assembly of Gold Nanoparticles Utilizing the

Feb 4, 2004 - performed using a TG/DTA 6200, SEIKO Instruments,. Inc., with a heating rate of 10 °C min-1 in air. Colloid Synthesis. Gold nanoparticl...
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Langmuir 2004, 20, 1972-1976

Photochemical Assembly of Gold Nanoparticles Utilizing the Photodimerization of Thymine Hideaki Itoh, Akitomo Tahara, Kensuke Naka,* and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Received October 23, 2003. In Final Form: December 25, 2003 Irradiation of UV light to the solution of gold nanoparticles modified with thymine units resulted in the formation of aggregates comprising chemical cross-linking gold nanoparticles through the photodimerization of the thymine units. Transmission electron microscopy and UV-visible absorption measurement showed the aggregates consisting of the gold nanoparticles. The effect of thymine unit density on the nanoparticle surface and the concentration of the gold nanoparticles in solution to the aggregation process were studied by UV-visible absorption measurement.

Introduction Size-quantized noble metal nanoparticles have drawn considerable interest in various fields of science and engineering because of their unique physical and chemical properties leading to potential applications in electronics, for optical and magnetic devices.1 Of particular interest is the possibility of tailoring the metal nanoparticles surface with a molecular arrangement consisting of organic molecules that possess responsive properties to a certain stimulation. Several papers have been described for the metal nanoparticles with various kinds of stimuliresponsive property, such as light,2 heat,3 pH,4 and so on.5 For example, Fox and co-workers have demonstrated the ability of gold nanoparticles to preserve the photoreactivity of the trans-stilbene and o-nitrobenzyl ether moieties similar to the one observed in the solution phase.2c Such photoactive metal nanoparticles are important for designing light-energy-harvesting devices of nanometric dimension and photocatalysts. The ability to functionalize gold nanoparticles with a stimuli-responsive property has opened new avenues to utilizing these nanomaterials in optical and electronic applications. Apart from the studies of discrete nanoparticles, it is also very important to fabricate nanoparticles into one-, two-, and three-dimensional structures because the collective properties of the resulting structures are expected to be different from those of the corresponding isolated nanoparticles.6 Development of simple and easy methods * Authors to whom correspondence should be addressed. Email: [email protected]. (1) (a) Schmid, G. Chem. Rev. 1992, 92, 1709. (b) Andres, R. P.; Bein, T.; Dorogi, M.; Fend, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (c) Alivisatos, A. P. Science 1996, 271, 933. (d) Schmid, G.; Ba¨umle, M.; Geerkens, M.; Heim, I.; Osemann, C.; Sawitowski, T. Chem. Soc. Rev. 1999, 28, 179. (2) (a) Manna, A.; Chen, P.-L.; Akiyama, H.; Wei, T.-X.; Tamada, K.; Knoll, W. Chem. Mater. 2003, 15, 20. (b) Ipe, B. I.; Mahima, S.; Thomas, K. G. J. Am. Chem. Soc. 2003, 125, 7174. (c) Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464. (d) Zhang, J.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2001, 13, 2323. (e) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (3) (a) Nath, N.; Chilkoti, A. J. Am. Chem. Soc. 2001, 123, 8197. (b) Miyazaki, A.; Nakano, Y. Langmuir 2000, 16, 7109. (c) Raula, J.; Shan, J.; Nuopponen, M.; Niskanen, A.; Jiang, H.; Kauppinen, E. I.; Tenhu, H. Langmuir 2003, 19, 3499. (4) (a) Simard, J.; Briggs, C.; Boal, A. K.; Rotello, V. M. Chem. Commun, 2000, 1943. (b) Sastry, M.; Mayya, K. S.; Bandyopadhyay, K. Colloids Surf., A 1997, 127, 221. (c) Shiraishi, Y.; Arakawa, D.; Toshima, N. Eur. Phys. J. E 2002, 8, 377. (5) Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 1999, 121, 4914.

for the fabrication of controlled organized structures is indispensable for preparing new nanodevices. Various strategies (solvent evaporation,7 electrostatic attraction,8 hydrogen bonding,9 DNA-driven assembly,10 and crosslinking induced by organic molecules)11 have been developed to form nanoparticle assemblies. The organization of metal nanoparticles in superstructures of desired shape and morphology is a challenging research area. Recently, we have reported the self-assembly of colloidal gold nanoparticles into macroscopic aggregates by chargetransfer interaction between a pyrenyl unit as an electron donor immobilized on the surface of the gold nanoparticles and a bivalent linker containing two dinitrophenyl units as electron acceptors.12 This methodology allows the gold nanoparticles to self-assemble reversibly into aggregates by controlling the temperature. That is, this system is a thermally responsive system. Here, we describe a new concept for the fabrication of metal nanoparticle assemblies by light irradiation (Figure 1). Our strategy is based on the photodimerization of thymine units immobilized on the surface of the gold nanoparticles. It is well-known that thymine bases photodimerize upon the irradiation above 270 nm and revert back to thymine again upon the irradiation below 270 nm.13 Irradiation of UV light to a solution of the gold nanoparticles modified with the thymine units resulted in the formation of aggregates comprising chemical cross(6) (a) Xia, Y.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (b) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. Rev. 2000, 29, 27. (c) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (7) (a) Stowell, C.; Korgel, B. A. Nano Lett. 2001, 1, 595. (b) Ohara, P. C.; Gelbart, W. M. Langmuir 1998, 14, 3418. (8) Sastry, M.; Rao, M.; Ganesh, K. N. Acc. Chem. Res. 2002, 35, 847. (9) (a) Boal, A. K.; Llhan, 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 (10) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (11) (a) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (b) Naka, K.; Itoh, H.; Chujo, Y. Nano Lett. 2002, 2, 1183. (12) Naka, K.; Itoh, H.; Chujo, Y. Langmuir 2003, 19, 5496. (13) (a) Overberger, C. G.; Inaki, Y. J. Polym. Sci.: Polym. Chem. Ed. 1979, 17, 1739. (b) Moghaddam, M. J.; Hozumi, S.; Inaki, Y.; Takemoto, K. Polym. J. 1989, 21, 1739. (c) Moghaddam, M. J.; Kanbara, K.; Hozumi, S.; Inaki, Y.; Takemoto, K. Polym. J. 1990, 22, 369. (d) Moghaddam, M. J.; Hozumi, S.; Inaki, Y.; Takemoto, K. Polym. J. 1989, 21, 203.

10.1021/la0359777 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/04/2004

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reaction was performed at room temperature and was followed by the absorption spectroscopy. Results and Discussion

Figure 1. Schematic illustration for the photochemical assembly of metal nanoparticles by the photodimerization of thymine units.

linking gold nanoparticles through the photodimerization of the thymine units. That is, this methodology is a photoresponsive system. Several authors reported a thermal- and pH-responsive assembly system of metal nanoparticles.4,5 Several papers have reported the photochemical control of metal nanoparticle morphologies.14 To the best of our knowledge, however, this is the first report for a photochemical assembly system without a morphological change of the metal nanoparticles. Materials and Methods Materials. All solvents and reagents were obtained from a commercial source and used as supplied except the following. Dimethylformamide (DMF) was distilled and stored under nitrogen. Measurement. 1H NMR spectra were obtained with a JEOL JNM-EX270 spectrometer (270 MHz for 1H NMR) in chloroform-d. UV-visible spectra were measured on a Jasco V-530 spectrometer. Transmission electron microscopy (TEM) was performed using a JEOL JEM-100SX operated at 100 kV. Thermogravimetric analysis was performed using a TG/DTA 6200, SEIKO Instruments, Inc., with a heating rate of 10 °C min-1 in air. Colloid Synthesis. Gold nanoparticles were prepared via a modified Brust’s method.2b HAuCl4 (100 mg, 0.24 mmol) was dissolved in 30 mL of H2O. Tetraoctylammonium bromide (130 mg, 0.24 mmol) was then added as a solution in 30 mL of chloroform, and the reaction mixture was stirred until the yellow aqueous layer was clear and the organic layer was red. 10,10′-Dithiobis(undecanoic acid 2-(thymine-1-yl)ethyl ester) (1) and 1-dodecanethiol in different feed ratios were then added, followed by the dropwise addition of NaBH4 (26.1 mg, 0.70 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 methanol (100 mL). The precipitates were isolated by centrifugation. The gold nanoparticles stabilized by 1 and 1-dodecanethiol were obtained as a black powder. The product molar ratios of 1/1-dodecanethiol on the surface of the gold nanoparticles were determined on the basis of the corresponding 1H NMR spectra. Photodimerization. Photodimerization of the thymine units was induced by the irradiation of light from a 175-W Xe lamp filtered with a Toshiba UV-D33S glass filter. The (14) (a) Dawson, A.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 960. (b) Murakoshi, K.; Tanaka, H.; Sawai, Y.; Nakato, Y. J. Phys. Chem. B 2002, 106, 3041. (c) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 6152. (d) Chandrasekharan, N.; Kamat, P. V.; Hu, J.; Jones, G., II. J. Phys. Chem. B 2000, 104, 11103.

1. Synthesis of Gold Nanoparticles. The gold nanoparticles were prepared by reduction of HAuCl4 (100 mg, 0.24 mmol) with NaBH4 in the presence of 1 (18.0 mg, 0.024 mmol) and 1-dodecanethiol (48 µL, 0.24 mmol). The mixtures of 1 and 1-dodecanethiol were used to suppress intramolecular photoreaction of the thymine units on the nanoparticle surface. The UV-vis absorption spectrum of a red solution after chemical reduction with NaBH4 indicated the formation of gold nanoparticles with a surface plasmon absorption band at 496 nm. The average diameter of the 1-modified gold nanoparticles (gold nanoparticles (I)) was 3.2 ( 0.1 nm as measured by a TEM (Figure 2). Figure 3 shows the 1H NMR spectra of 1 and gold nanoparticles (I) in CDCl3. In the 1H NMR of the gold nanoparticles (I), signals appeared corresponding to those of the proton signals of the parent 1 (Figure 3B). The signals of the methylene protons in gold nanoparticles (I) are broadened compared to those of 1. The signals of the methylene protons closest to the thiolate/Au interface are hardly recognized because the motion of the methylene groups close to the nanoparticle surface was constrained. This type of signal broadening has been observed in bipyridyl or alkanethiolate-modified metal nanoparticles.15 These results indicate clear evidence for the attachment of the thiolated thymine to the surface of the gold nanoparticles. Comparing the integrated areas for signal at 0.92 ppm (due to the CH3 group of 1-dodecanethiol) and the signal at 3.95 ppm (due to the CH2 group next to ester group of 1) revealed that the product molar ratio of 1/1-dodecanethiol on the nanoparticle surface was 1:20. Thermogravimetric analysis established that the content of 1 and 1-dodecanethiol in the gold nanoparticles was 25 wt %. On the basis of the fact that the average particle size was 3.2 nm, the number of 1 and 1-dodecanethiol adsorbed on the surface of each gold nanoparticle was about 200. According to the previous paper,16 the CdO stretch and CdC stretch of the thymine ring were observed around 1731 and 1678 cm-1. The Fourier transform infrared measurement of gold nanoparticles (I) showed these two peaks, indicating the presence of thymine units. 2. Assembly of Nanoparticles by Photoirradiation. Photoirradiation on the solution of gold nanoparticles (I) was carried out to induce the photodimerization of thymine. The concentration of gold nanoparticles (I) in chloroform solution was 0.16 mg/mL. The solution color gradually changed from red to purple with increasing the reaction time. The photoreaction of gold nanoparticles (I) was followed as a function of time through optical changes in the surface plasmon band in the UV-vis absorption spectrum. The UV-vis absorption spectrum is a strong method to investigate the formation of particle aggregates. It is well-known that the aggregation can be conveniently monitored by the change of the surface plasmon band. (15) (a) Hostetler, M. J.; Wingate, J. E.; 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. (16) Zhang, S. L.; Michaelian, K. H.; Loppnow, G. R. J. Phys. Chem. A 1998, 102, 461.

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Figure 2. (A) TEM image of gold nanoparticles (I) and (B) size distribution histogram of gold nanoparticles (I).

Figure 3.

1H

NMR spectra of (A) 1 and (B) gold nanoparticles (I).

Figure 4 shows UV-vis absorption spectra recorded at different times after photoirradiation. A shift and decrease in the surface plasmon band was observed, indicating the formation of nanoparticle aggregates. The surface plasmon band changed from 496 to 525, 538, and 544 nm after photoirradiation for 22, 32, and 46 h, respectively. The precipitation of black powders was observed after 72 h, indicating that the nanoparticle aggregates eventually became too large to remain in the solution. One control experiment was carried out. When gold nanoparticles prepared with only 1-dodecanethiol were used under the same condition, the colorimetric change

was not detectable, indicating that no assembly of the gold nanoparticles proceeded. The difference between these nanoparticles is the absence of the thymine unit. These results suggest that gold nanoparticles (I) aggregate through cross-linking by the photodimerization of the thymine units on the nanoparticle surface. 3. TEM Image of the 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 photoirradiation showed

Photochemical Assembly of Gold Nanoparticles

Figure 4. UV-vis absorption spectra of the solution with gold nanoparticles (I) after photoirradiation for (a) 0, (b) 22, (c) 32, and (d) 46 h.

the well-separated gold nanoparticles with a uniform size distribution (Figure 2A), the TEM image after photoirradiation to the solution containing gold nanoparticles (I) showed the formation of aggregates (Figure 5). The aggregates consisted of individual gold nanoparticles. The diameter of the obtained aggregates became larger with increasing photoirradiation time. The formation of the aggregates with average diameters of 0.15, 0.25, and 1 µm was observed after 6, 22, and 72 h, respectively. Thus, the degree of colloidal association would be controlled by adjusting the photoirradiation time. In contrast, a TEM image after photoirradiation on the solution of the gold nanoparticles prepared with only 1-dodecanethiol showed no assembly of the gold nanoparticles. 4. Conversion of the Photodimerization Reaction. Conversion of the photodimerization reaction of the thymine units immobilized on the surface of the gold nanoparticles after photoirradition was studied. According to the literature,13 the photodimerization can be monitored by a UV-vis absorption measurement. The decline of the absorption at 270 nm indicates the progress of the photodimerization. However, the UV-vis absorption spectra gave no clear decline of the photodimerization because of the high extinction coefficients of the gold nanoparticles. To confirm the photodimerization in the present system, 1H NMR measurement was carried out. To make the obtained photodimers available for a 1H NMR measurement, degrafting of the obtained photodimers from the gold nanoparticles is required. After photoirradiation for 72 h at room temperature, a solution of

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excess of I2 was added to reoxidize the gold nanoparticles. The 1H NMR measurement of the resulting products showed the decline of the signal at 7.00 ppm (due to the CH group of thymine ring) in comparison with that of the parent 1, indicating the progress of the photodimerization of the thymine units. Conversion of the photodimerization reaction of thymine units was calculated from the 1H NMR measurement to be 36%. The conversion of the photodimerization reaction in the present system is low. The reason for this low conversion is as follows. Suppression of the photodimerization may arise from the steric hindrance associated with a tight monolayer packing and from quenching of the excited states by the metal core. Several papers investigated the photoefficiency of photoactive molecules on the metal surface.17 These studies also showed that the efficiencies of the photoreaction of the photoactive molecules immobilized on the metal surface are significantly lower than those of the free photoactive molecules as a result of the same reasons as just described. Alternatively, the concentration of the thymine units is too low to photodimerize. 5. Effect of the Thymine Unit Density of the Gold Nanoparticles on the Aggregation Process. The effect of the thymine unit density of the 1-modified gold nanoparticles on the aggregation process was studied. The gold nanopaticles (gold nanoparticles (II)) were prepared with 1 (12.0 mg, 0.016 mmol) and 1-dodecanethiol (48 µL, 0.24 mmol) by the same method as previously described. The product molar ratio of 1/1-dodecanethiol calculated from the 1H NMR spectrum is 1:40. The thymine unit density of this gold nanoparticles (II) is lower than that of the gold nanoparticles (I), as previously described. Because of the lower concentration of the thymine unit, that the the lesser number of thymine units is close enough for photodimerization is expected. That is, the slower aggregation process will be expected by the decrease of cross-linking points in the case of gold nanoparticles (II). Photoirradiation on the solution of gold nanoparticles (II) under the same conditions also led to the red shift of the surface plasmon band in the UV-vis absorption spectra. Figure 6A showed the UV-vis absorption spectra recorded at different times after photoirradiation. Figure 6B shows the time course plots of the wavelength maximum of the gold nanoparticles (I) and gold nanoparticles (II) solutions. Comparison between these nanoparticles revealed that the shift of the gold nanoparticles (I) solution was larger than that of gold nanoparticles (II) solution. That is, the aggregation of gold nanoparticles (I) proceeded faster than that of gold nanoparticles (II). This experimental result is in accordance with the described expectation, demon-

Figure 5. TEM images of the obtained aggregates consisting of gold nanoparticles (I) after photoirradiation for (A) 6 and (B) 72 h.

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Figure 7. Absorbance versus time for solutions a and b.

Conclusion In conclusion, we have demonstrated the fabrication of the aggregates consisting of the chemical cross-linking gold nanoparticles via photodimerization of the thymine unit. The present results suggest that the assembly process of the gold nanoparticles is controlled by the thymine unit density on the nanoparticle surface and the concentration of the gold nanoparticles in solution. Investigation on the photocleavage of thymine dimer on the nanoparticle surface to realize the photoreversible assembly system is in progress. We expect that this concept represents a powerful and general strategy for the creation of highly structured multifunctional materials. Figure 6. (A) UV-vis absorption spectra of the solution with gold nanoparticles (II) after photoirradiation for (a) 0, (b) 25, (c) 31, and (d) 47 h. (B) Absorbance versus time for the solution with gold nanoparticles (I) and gold nanoparticles (II), respectively.

strating the control of the aggregation process by turning the thymine unit density on the nanoparticle surface. 6. Effect of the Concentration of the Gold Nanoparticles on the Aggregation Processs. The effect of the concentration of gold nanoparticles (I) in a solution on the aggregation process was studied. We prepared two kinds of solutions with a gold nanoparticle concentration of 0.16 mg/mL (solution a) and 0.08 mg/mL (solution b). The aggregation process via photoirradiation was monitored by optical changes in the surface plasmon band in the UV-vis absorption spectrum. The solution color changed in the cases of both solutions. Figure 7 shows the time course plots of the wavelength maximum of solutions a and b. Comparison between these results showed the slight difference in the optical changes of the surface plasmon band. The high concentration in the case of solution a showed a larger red shift in the surface plasmon band, demonstrating the fast aggregation process. This is because gold nanoparticles (I) tend to meet each other in a higher concentration and photodimerization of the thymine unit proceeds easily. (17) (a) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955. (b) Fox, M. A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14, 816. (18) Gi, H. J.; Xiang, Y.; Schinazi, R. F.; Zhao, K. J. Org. Chem. 1997, 62, 88. (19) Niemz, A.; Jeoung, E.; Boal, A. K.; Deans, R.; Rotello, V. M. Langmuir 2000, 16, 1460.

Experimental Section (a) 1-(2-Hydroxyethyl)thymine. 1-(2-Hydroxyethyl)thymine was prepared according to the previous literature.18 (b) 11,11′-Dithiobis(undecanoic acid). 11,11′-Dithiobis(undecanoic acid) was prepared via a modified method reported in the previous literature.19 To a solution of 11-mercaptoundecanoic acid (200 mg, 0.92 mmol) in H2O (100 mL) was added sodium hydroxide (500 mg, 12.5 mmol). Hydrogen peroxide solution (2 mL) was added to a well-stirred resulting mixture. After being stirred for 30 min, concentrated HCl (2 mL) was added and the mixture was extracted with EtOAc several times. The organic portion was washed with saturated aqueous NaCl and dried over MgSO4. After the removal of the solution, disulfide was obtained as a colorless solid (110 mg, 55% yield). 1H NMR (DMSO): δ 1.23-1.35 (12H, m), 1.46 (2H, t), 1.57 (2H, t), 2.26 (2H, t), 2.69 (2H, t). (c) 1. To a solution of 1-(2-hydroxyethyl)thymine (1.5 g, 8.88 mmol) in DMF were added 11,11′-dithiobis(undecanoic acid) (1.7 g, 4.06 mmol), dicyclohexyl carbodiimide (3.4 g, 16.5 mmol), and 4-(dimethylamino)pyridine (0.2 g, 1.67 mmol). The mixture was stirred at room temperature for 72 h. After the removal of the precipitate by filtration, the solution was evaporated under a reduced pressure. The remaining white solid was subjected to column chromatography. The second fraction containing product was concentrated by evaporation and dried under a reduced pressure. 1 was obtained (19.5%). 1H NMR (CDCl3): δ 1.21-1.62 (38H, m), 2.31 (4H, t), 2.67 (4H, t), 3.96 (4H, t), 4.31 (4H, t), 7.00 (2H, s), 8.72 (2H, s). Anal. Calcd for C36H58O8S2N4 (739.00): C, 58.51; H, 7.91; O, 17.32; S, 8.68; N, 7.58. Found: C, 58.64; H, 7.78; O, 17.34; S, 8.69; N, 7.55.

Acknowledgment. We thank Professor T. Fukuda, Dr. M. Tsujii, and Dr. S. Yamamoto (Institute of Chemical Research, Kyoto University) for the TEM micrographs. LA0359777