Light-Induced Aggregation of Colloidal Gold Nanoparticles Capped by

Mar 7, 2008 - Ian Wark Research Institute, University of South Australia, Mawson Lakes, Adelaide, South Australia 5095. Langmuir , 2008, 24 (9), pp 45...
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Langmuir 2008, 24, 4506-4511

Light-Induced Aggregation of Colloidal Gold Nanoparticles Capped by Thymine Derivatives Jingfang Zhou, Rossen Sedev, David Beattie, and John Ralston* Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes, Adelaide, South Australia 5095 ReceiVed NoVember 29, 2007. In Final Form: January 24, 2008 The colloid stability of thymine-coated gold nanoparticles under light irradiation as a function of particle size, surface charge, and exposure time was investigated in alkaline, aqueous solutions as well as in a 0.5 vol % of DMF in H2O mixture. With increasing exposure to light irradiation at 280 nm, more and more particles coagulated. Lightinduced aggregation of colloidal gold nanoparticles was attributed to reorientation of thymine terminal groups tethered on gold particle surfaces. A smaller particle size and negatively charged surface reduced the rate of photodimerization or even inhibited the photoreaction. UV-vis and FTIR spectroscopy confirmed the photodimerization of terminal thymine molecules under 280 nm light irradiation. The reaction kinetics of thymine photodimerization appears to be a combination of first-order reactions, each having different rates, reflecting the inhomogeneity and high curvature of the gold nanoparticle surfaces.

I Introduction The stability of dispersions of colloidal gold nanoparticles is of great importance for their practical applications in many fields such as nanooptoelectronics, catalysis, and bioassays.1-3 Colloid stability can be controlled by various methods,4,5 for example, increasing electrolyte concentration, adjusting solution pH, adding different inorganic salts, and changing the capping agent or dispersion medium. However, the composition of the colloid system itself is substantially altered if these methods are used. In recent years, external stimuli have been used to control the stability of colloidal dispersions. For example, heat can control the stability of thermosensitive nanoparticle dispersions,6-8 while the aggregation and dispersion of magnetic particles can be controlled by magnetic forces.9,10 Light is another commonly encountered stimulus which can be applied to trigger instability in photoresponsive particle dispersions. Chromophores such as azobenzene or spiropyrene have been attached to oxides or metal particle surfaces as terminal groups.11-19 These particles can be dispersed in some organic * Corresponding author. E-mail: [email protected]. (1) Watanabe, K.; Menzel, D.; Nilius, N.; Freund, H.-J. Chem. ReV. 2006, 106, 4301-4320. (2) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (3) Bonnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 2455-2480. (4) Hiemenz, P. C. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1997. (5) Lyklema, J. H. Fundamentals of Interface and Colloid Science; Academic Press: San Diego, 2005, Vol. 4. (6) Suzuki, D.; McGrath, J.; Kawaguchi, H.; Lyon, L. J. Phys. Chem. C 2007, 111, 5667-5672. (7) Ballauff, M.; Lu, Y. Polymer 2007, 48, 1815-1823. (8) Li, D.; He, Q.; Cui, Y.; Wang, K.; Zhang, X.; Li, J. Chem.sEur. J. 2007, 13, 2224-2229. (9) Zhang, J.; Srivastava, R.; Misra, R. Langmuir 2007, 23, 6342-6351. (10) Racuciu, M.; Creanga, D.; Badescu, V.; Airinei, A. J. Optoelectron. AdV. Mater. 2007, 9, 1530-1533. (11) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436-6440. (12) Bell, N. S.; Piech, M. Langmuir 2006, 22, 1420-1427. (13) Ueda, M.; Fukushima, N.; Kudo Ichimura, K. J. Mater. Chem. 1997, 7, 641-645. (14) Ueda, M.; Kim, H.-B.; Ichimura, K. J. Mater. Chem. 1994, 4, 883-889. (15) Zhang, J.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2001, 13, 23232331. (16) Hu, J.; Zhang, J.; Liu, F.; Kittredge, k.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc 2001, 123, 1464-1470. (17) Ipe, B. I.; Mahima, S.; Thomas, K. G. J. Am. Chem. Soc 2003, 125, 7174-7175.

solvents and stabilized mainly by solvation forces. Upon UV irradiation, the chromophores display a dramatic polarity change due to photoisomerization, which can alter the solvation forces, resulting in the aggregation of particle dispersions. However, the coagulated particles can be redispersed not only by light irradiation but also by thermal back reaction or even by simply letting them sit for a long time. Thymine is one of the bases in DNA. When compared with photoisomerization of azobenzene or spiropyrene, thymine molecules undergo dimerization and cleavage upon exposure to UV light irradiation of different wavelengths, producing two photoswitchable and stable states without thermal reversal reactions.20 However, the photodimerization of thymine is a wellknown [2 + 2] cycloaddition, which is sensitive to the distance and orientation of the thymine molecules. Based on these properties, thymine molecules can be used as a sensitive molecular probe to study the ordering and structure of thymine-terminated SAMs formed on the surfaces, which, in turn, reflects the properties of surface roughness or curvature of the particles. The photoreaction of thymine has been investigated in solutions21,22 as well as in solid states, for example, in cast films,23,24 LB films,25 polymer matrices,26,27 and single crystals.28,29 Itoh30 et al. have prepared thymine-functionalized mixed-SAMs protected gold nanoparticles recently. The photochemical assembly of gold (18) Manna, A.; Chen, P.-L.; Akiyama, H.; Wei, T.-X.; Tamada, K.; Knoll, W. Chem. Mater. 2003, 15, 20-28. (19) Sidhaye, D. S.; Kashyap, S.; Sastry, M.; Hotha, S.; Prasad, B. L. V. Langmuir 2005, 21, 7979-7984. (20) Inaki, Y. Senryo to Yakuhin 1991, 36, 342-55. (21) Lamola, A.; Mittal, J. Science 1966, 154, 1560-1561. (22) Greenstock, C. L.; Brown, I. H.; Hunt, W.; Johns, H. E. Biochem. Biophys. Res. Commun 1967, 27, 431-436. (23) Inaki, Y.; Wang, Y.; Kubo, M.; Takemoto, K. Chem. Funct. Dyes, Proc. Int. Symp. 1993, 365-70. (24) Tohnai, N.; Sugiki, T.; Mochizuki, E.; Wada, T.; Inaki, Y. J. Photopolym. Sci. Technol. 1994, 7, 91-2. (25) Li, C.; huang, J.; Liang, Y. Langmuir 2001, 17, 2228-2234. (26) Kita, Y.; Inaki, Y.; Takemoto, K. J. Polym. Sci., Part A: Polym. Chem. 1980, 18, 427-439. (27) Kita, Y.; Uno, T.; Inaki, Y.; Takemoto, K. J. Polym. Sci., Part A: Polym. Chem. 1981, 19, 477-485. (28) Inaki, Y.; Mochizuki, E.; Yasui, N.; Miyata, M.; Kai, Y. J. Photopolym. Sci. Technol. 2000, 13, 177-182. (29) Mochizuki, E.; Yasui, N.; Kai, Y.; Inaki, Y.; Yuhua, W.; Saito, T.; Tohnai, N.; Miyata, M. Bull. Chem. Soc. Jpn. 2001, 74, 193-200. (30) Itoh, H.; Tahara, A.; Naka, K.; Chujo, Y. Langmuir 2004, 20, 19721976.

10.1021/la703746w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

Light-Induced Aggregation of Gold Nanoparticles

nanoparticles was observed via interparticle photoreaction of thymine molecules attached to different gold particles. We31,32 have studied thymine-functionalized, single component SAMs on smooth gold surfaces and observed photocontrolled reversible contact angle changes as a consequence of the photoreaction of thymine occurring on the surfaces. Lake et al.32 observed that the photoreaction is accompanied by a substantial decrease in wettability as well as a molecular reorientation when the dimer is formed. We further assembled thyminethiol derivatives onto curved gold nanoparticle surfaces and found that a well-ordered monolayer of thyminethiol derivatives with a long hydrocarbon chain was formed on the particle surfaces.33 We have also identified that hydration forces largely dictate the colloid stability of these nanoparticle dispersions at high electrolyte concentrations and pH.34 In this present study, however, we wanted to know whether or not the intraparticle photoreaction of thymine terminal groups attached on gold particle surfaces could occur. The curvature of the gold particles, the nature of the dispersion medium, the sign and magnitude of the surface charge, as well as the influence of light were therefore examined in this study of the colloid stability of gold nanoparticles capped by thymine derivatives. II Experimental Section 1. Reagents. The thyminethiol derivative, 1-(10-mercaptodecyl)5-methylpyrimidine-2,4-dione (TSH), was synthesized in our laboratory.33 All solvents and reagents, such as hydrogen tetrachloroaurate (HAuCl4‚3H2O, 99.999%), sodium borohydride (NaBH4, AR grade), tetraoctylammonium bromide (98%), sodium hydroxide (NaOH, semiconductor grade 99.99%), AR grade toluene, chloroform, and methanol, were purchased from Aldrich or Chem-supply and used as received. DMF (CHROMASOLV plus, HPLC grade, g99.9%, Aldrich) was used as a solvent for photoreaction. Water was purified by a Millipore Ultrapure water system and had a resistivity of 18.2 MΩ·cm at 25 °C. 2. Techniques. The TEM samples were prepared by placing one drop of an aqueous particle dispersion at pH 12.5 onto a standard copper grid (with 200 mesh). TEM images were obtained with a Philips CM100 electron microscope operating at 100 kV. UVvisible absorption spectra were performed on a Varian Cary 5 UVvis-NIR spectrophotometer, with dispersions contained in quartz curvettes. Absorbance spectra are shown for gold nanoparticle surface, described as surface plasmon band (SPB) in relevant diagrams. The FTIR samples were prepared as follows: after light irradiation, the deposited gold particles were collected and dried under vacuum. The gold powder was then dispersed in DMF. One drop of gold particle dispersion in DMF was placed onto a silicon wafer and dried under vacuum. Then, transmittance infrared absorption spectra of the particles attached to the silicon wafer were acquired using a Nicolet Magna-IR 750 spectrometer at 2 cm-1 resolution. 3. Photoirradiation. The gold particle dispersions at a concentration of 0.1 mg/mL were irradiated at room temperature using an MM3 diffraction grating type illuminator equipped with a 300 W xenon lamp. The wavelengths of the irradiated light were selected at 280 and 240 nm , with a bandwidth of 20 nm. Prior to irradiation, the dispersions were contacted with high purity argon for approximately 15 min to remove dissolved oxygen from the solution, and then the quartz cuvettes were sealed during irradiation. The aggregation behavior of the gold particles in the dispersion as well as the photoreaction of thymine molecules were monitored by UV absorption spectroscopy. (31) Lake, N.; Ralston, J.; Reynolds, G. Lamgmuir 2005, 21, 11922-11931. (32) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923-8928. (33) Zhou, J.; Beattie, D.; Sedev, R.; Ralston, J. Langmuir 2007, 23, 91709177. (34) Zhou, J.; Beattie, D.; Ralston, J.; Sedev, R. Langmuir 2007, 1209612103.

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Figure 1. UV-vis absorption spectra of a 7.0 nm gold particle dispersion in pH 12.5 water under 280 nm light irradiation at different exposure times. 4. Synthesis and Dispersibility of Thymine-Coated Au Nanoparticles. Thymine functionalized gold nanoparticles were prepared using a modified two-phase transfer method. The small and large gold particles were obtained using in situ and ligand replacement methods, respectively. The purification of gold particles followed a standard procedure typically used for this type of synthesis. The detailed procedure has been described elsewhere.33 The particles could be dispersed in highly polar organic solvents, such as DMF, DMSO, and DMF-H2O mixture, as well as in high pH (>12) aqueous solution, due to deprotonation of hydrogen attached to thymine at N3 position. The smaller gold particles formed a brown, transparent dispersion, while the larger ones formed a purplish-red, transparent dispersion.34 However, the gold particle dispersions in 0.5 vol % of DMF in H2O mixture were less stable than those in DMF, DMSO, and pH 12.5 aqueous solution, where the particles were stable for more than 1 year. In DMF-H2O mixtures, the smaller gold particle dispersions are stable for more than 2 months, whereas in contrast, the larger gold particle dispersions retained their stability for only around 10 h. TEM images33 showed that the average diameters of the small and large particles were 2.2 ( 0.3 and 7.0 ( 1.0 nm, respectively, with a reasonably narrow size distribution. The characteristic absorption peak of thymine is found near 270 nm in the UV spectra, and the appearance of the gold surface plasmon band absorption near 520 nm confirmed the gold formation. Both FTIR and TGA results indicated that well-ordered thymine-terminated SAMs were formed on the smaller gold particle surfaces, whereas more ordered SAMs were formed on the larger gold particles.33

III Results The colloid stability of gold nanoparticles34,35 and Carey Lea silver nanoparticles36,37 has been investigated thoroughly using UV-vis absorption. Peak broadening and absorbance changes with time reflect aggregation of these nanoparticle dispersions, as we and others34-37 have shown. The aggregation behavior of thymine-coated 7.0 nm gold particles after irradiation with 280 nm light was investigated in two different solvents. These small particles are Rayleigh scatterers, and their absorption is determined by their size and concentration.4 The UV-vis absorption spectra of the dispersions were recorded as a function of irradiation time to monitor the aggregation process. The results are shown in Figures 1 and 3 for the dispersion in high-pH (12.5) water and in a DMF-H2O mixture, respectively. The corresponding gold and thymine (35) Rouhana, L. L.; Jaber, J. J.; Schlenoff, J. B. Langmuir 2007, 23, 1279912801. (36) Fornasiero, D.; Grieser, F. J. Colloid Interface Sci. 1991, 141, 168-179. (37) Karpov, S. V.; Slabko, V. V.; Chiganova, G. A. Colloid J. 2002, 64, 425-441.

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Figure 2. Thymine and gold SPB absorbance as a function of irradiation time for a 7.0 nm gold particle dispersion (pH 12.5) under 280 nm light irradiation.

Figure 3. UV-vis absorption spectra of a 7.0 nm gold particle dispersion in a 0.5 vol % of DMF in H2O mixture under 280 nm light irradiation at differentexposure times (irradiation time interval ) 39 s; total irradiation time ) 156 s). Inset: thymine absorbance versus gold SPB absorbance.

absorbance versus irradiation time in high-pH water are plotted in Figure 2. In high-pH water, the thymine peak decreased to zero at a changing rate; at the same time, the gold peak also decreased slowly for the first 8 h and then more rapidly (Figures 1 and 2). The gold peak reflects the behavior of the nanoparticles; initially, the dispersion was stable but later coagulation occurred (increase in absorbance), and finally the settling of the dispersion is clearly indicated by the total disappearance of both the gold and thymine peaks. The behavior was very different when the nanoparticles were dispersed in the mixed DMF-water solvent (Figure 3). Upon light irradiation, both the thymine and the gold peaks decreased sharply. The gold particles coagulated and settled fully at very short exposure times (39 s), adhering to the walls at the base of the cuvettes. The thymine and gold peaks decreased in parallel in the DMF-H2O mixture (inset, Figure 3); in highpH water, the thymine peak decreased much faster than the gold one (Figures 1 and 2). The thymine peak at 270 nm is due to the conjugation of 5,6-double bond and carbonyl group (C5dC6sC4dO) in the thymine ring.38-40 During photodimerization a cyclobutane ring is formed between adjacent double bonds and disrupts the (38) Beukers, R.; Berends, W. Biochim. Biophys. Acta 1960, 41, 550-551. (39) Herbert, M. A.; LeBlanc, J. C.; Weinblum, D.; Johns, H. E. Photochem. Photobiol. 1969, 9, 33-43. (40) Wulff, D. L.; Fraenkel, G. Biochim. Biophys. Acta 1961, 51, 332-339.

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Figure 4. FTIR spectra of 7.0 nm gold particles: (a) before light irradiation; (b) after 280 nm light irradiation in high pH water; (c) after 280 nm light irradiation in a 0.5 vol % of DMF in H2O mixture. Inset: enlarged CdO vibration spectra.

conjugation.31,32 Therefore the decrease of thymine peak has been always taken as solid evidence for the photodimerization of thymine. In our dispersions, however, the thymine peak decrease can be caused by either photodimerization or settling of the particles. In order to confirm whether or not dimerization occurred, we used FTIR spectroscopy as a diagnostic tool. The transmittance FTIR spectra of thymine-coated gold nanoparticles obtained before and after light irradiation are shown in Figure 4. The spectrum recorded before irradiation (Figure 4a) showed the antisymmetric and symmetric CH2 stretching vibrations in the hydrocarbon chain located at 2920 and 2850 cm-1, respectively. The corresponding CH2 and CH3 bending vibrations were found at 1468 and 1432 cm-1. The C-N vibration was situated at 1352 cm-1. The N-H and aromatic C-H stretching vibrations appeared at 3185 and 3052 cm-1, respectively. A broad band, peaking at approximately 1678 cm-1, was attributed to the stretching vibrations of the two secondary amide carbonyl CdO groups in the thymine ring.41-43 This confirmed that the thyminethiol derivatives were grafted on the surface of the gold particles. The spectra obtained after exposure to 280 nm light are shown in Figure 4b (high-pH water) and Figure 4c (DMFwater mixture). The C-H, N-H, and C-N vibrations were unchanged. The carbonyl CdO vibrations shifted to a higher wavenumber in high-pH water but remained unchanged in the DMF-H2O mixture (Figure 4, inset). The carbonyl stretching vibrations in a thymine monomer are situated at a lower wavenumber due to the conjugation between the 5,6-double bond and the carbonyl group in the C4 position. The saturation of the 5,6-double bond during photodimerization will therefore shift the CdO vibrations to a higher wavenumber.43-45 Thus, FTIR spectra confirm that photodimerization of thymine molecules occurred on the gold particles dispersed in high-pH water. The situation is less clear for particles dispersed in the DMF-H2O mixture. The thymine peak decrease coincided with the settling of the dispersion. As the 7.0 nm gold particles settled very quickly after light irradiation, it was not possible to determine whether or not photodimerization occurred in this case. The aggregation behavior of smaller (2.2 nm) gold nanoparticles dispersed in the same two solvents was distinctively (41) Briggs, D.; Riviere, J. C. Spectral Interpretation; Wiley: Chichester, 1990. (42) Rastogi, V. K.; Singh, C.; Jain, V.; Palafox, M. A. J. Raman Spectrosc. 2000, 31, 1005-1012. (43) Silverstein, R. M.; Webster, F. X. Spectrometric identification of organic compounds, 6th ed.; John Wiley & Sons: New York, 1998. (44) Bergmann, F.; Dickstein, S. J. Am. Chem. Soc. 1955, 77, 691-696. (45) Shim, S. C.; Lee, S. H. Photochem. Photobiol. 1979, 29, 1035-1038.

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from the gold particle surface. Furthermore, the lack of any significant change in the FTIR spectra after light irradiation means that decomposition of thyminethiol derivatives did not occur. The photodimerization of thymine is a pseudo-first-order reaction, according to the known reaction mechanism,46,47 where one excited thymine monomer reacts with another ground state thymine monomer to form a thymine dimer. Since the thymine monomer is excited immediately by light and the concentration is much higher than that of the ground state thymine monomer, the concentration of excited thymine monomer can be considered to be constant during photoreaction. The kinetic rate equation is written as:

-d[M]/dt ) k′[M][M*] ) k[M] Figure 5. UV-vis absorption spectra of a 2.2 nm gold particle dispersion in pH 12.5 water under 280 nm light irradiation at different exposure times. Inset: thymine absorbance versus gold SPB absorbance.

(1)

Upon integration, where k is the pseudo-first-order rate constant and noting that absorbance is proportional to concentration, then

ln(At/A0) ) -kt

(2)

where A0 and At are the UV absorbances at 270 nm at time zero and t, respectively. The dependence is shown in Figure 3a,b of Supporting Information for 7.0 nm gold particles dispersed in high-pH water, before settling (t e 9 h), and for 2.2 nm gold particles dispersed in DMF-H2O mixture, respectively. The kinetics of the photoreaction in both cases is certainly not simple first order.

IV Discussion

Figure 6. FTIR spectra of 2.2 nm gold particles: (a) before light irradiation; (b) after 280 nm light irradiation in high pH water; (c) after 280 nm light irradiation in a 0.5 vol % of DMF in H2O mixture. Inset: enlarged CdO vibration spectra.

different. The UV-vis spectra for 2.2 nm particles dispersed in high-pH water are shown in Figure 5. In this case, the thymine and gold peaks decreased during UV exposure in parallel (Figure 5, inset), suggesting that no dimerization occurred. On the contrary, in the DMF-H2O mixture, dimerization apparently occurred (Figure 1 of Supporting Information). The thymine peak was not correlated with the gold peak, with the latter remaining practically constant (Figure 2 of Supporting Information). Thus dimerization occurred without affecting the stability of the suspension. The FTIR spectra of thymine-coated 2.2 nm gold particles, obtained before and after UV irradiation, are shown in Figure 6. The spectrum in high-pH water (Figure 6b) was unchanged, confirming that photodimerization did not occur. In the DMFH2O mixture, a shift of the carbonyl vibrations to higher wavenumbers was observed after light irradiation (Figure 6c). Thus both UV-vis and FTIR spectra demonstrate that thymine dimerization occurred in the DMF-H2O mixture. After irradiation with 280 nm light, the coagulated/aggregated dispersion was then subjected to irradiation with 240 nm light. No redispersion of the gold particles, however, was observed in any of the cases. Both UV-vis and FTIR spectra after exposure to 240 nm light did not show any change, which indicate that the cleavage of the dimer on the gold particles did not occur, as observed by Itoh.30 After repeated exposure to light irradiation, UV-vis spectra did not show any detectable products in the solution, implying that thyminethiol derivatives did not detach

1. Photoinduced Aggregation of Thymine-Coated Gold Nanoparticles. Before UV irradiation, the aqueous dispersions (pH ) 12.5) of thymine-coated gold particles are stable. At this high ionic strength and pH, electrostatic repulsion is insignificant and the particles are mainly stabilized by hydration forces, as we have shown previously.34 In a neutral DMF-H2O mixed solvent, we also propose that the hydration force plays a major role in stabilizing the gold particles, as there is little or no charge existing on the gold surface.34,48 Hydration forces arise from water layers, adjacent to the surface of the particles, and are more structured than the bulk solution.48 The magnitude of the hydration repulsion is strongly related to the hydrophobicity/hydrophilicity of the gold particle surface. During photodimerization, the orientation of thymine molecules on the gold surface changes, as we have shown earlier in our study of flat gold surfaces.31 As a consequence, the gold surface becomes more hydrophobic when the dimer is present. We propose that similar changes occur on the surface of the gold nanoparticles and that this change in hydrophobicity is sufficient to trigger the aggregation of the 7.0 nm gold particles in high-pH water and ultimately lead to the coagulation of the suspension. We have certainly observed a change of this nature occurring in our previous study on the stability of these gold nanoparticles in the presence of salt, where a pH decrease resulted in surface protonation, decrease in the hydration force, and increase in hydrophobicity, and as a result particle coagulation occurred.34 Dimerization was also observed for the smaller (2.2 nm) gold particles dispersed in the DMF-H2O mixture, however, the dispersion remained stable and there was no change in the state of particle aggregation. This different behavior for the large and small gold nanoparticles shows that dimerization is an intraparticle surface process, which is in contrast with the study of Itoh et al.30 (46) Mochizuki, E.; Tonai, K.; Inaki, Y. Kobunshi Kako 1999, 48, 489-497. (47) Wagner, P. J.; Bucheck, D. J. J. Am. Chem. Soc 1970, 92:1, 181-185. (48) Israelachvili, J. Intermolecular & surface forces, 2nd ed.; Academic Press: London, 1991.

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We have observed that thymine-coated particles can be dispersed in pure DMF but not in neutral pure water, indicating that DMF has a higher affinity for thymine than does water. DMF is polar and can clearly hydrogen bond. How the interaction of DMFwater mixture occurs with the small gold particle surfaces is unclear. Steric repulsion may be involved.5 We recall from our previous study34 that the van der Waals attraction is reduced for the 2.2 nm particles compared with the larger ones. Coagulation was observed for the 7.0 nm particles dispersed in DMF-water and also with the 2.2 nm particles dispersed in high-pH water. Interestingly, photodimerization occurred in the first case only. The structural components of the thymine molecule have very different wettabilities as we have shown in our previous studies.31,32 The carbonyl and amine groups are polar and hydrophilic, while the phenyl ring edge and the methyl group are rather hydrophobic. The two sides of the thymine ring have very different wettabilities and one might expect the molecule to reorient itself, depending on the polarity of the dispersion phase it contacts.48 When thymine molecules attached to flat gold surfaces are subjected to light irradiation, the molecules reorient,31,32 as they also do in thin films.49,50 We propose that UV light can provide enough energy so that the orientation of the thymine molecules alters when dimerization occurs, as noted above, and also when it does not. Thus hydration forces are diminished significantly and coagulation ensues for both 2.2 and 7.0 nm gold particles in high-pH water. In our previous study,33 we have shown that the 2.2 nm particles have a higher packing density than the 7.0 nm particles. However, when either the 2.2 or the 7.0 nm thymine-coated gold particles come into contact with each other, the adhesion force between the gold particles is very strong because of strong interactions between the thymine end-groups, irrespective of the size, as we have noted.33 Irradiation with 240 nm light is not sufficient to weaken the interactions between the aggregated gold particles, and redispersion of the coagulated gold nanoparticles does not occur. Thus orientation changes due to photodimerization (as with 7.0 nm particles in high-pH water) and excitation (as with 2.2 nm particles in highpH water), on their own, can cause the coagulation of thyminecoated gold particle dispersions. 2. Factors Affecting the Photodimerization of ThymineCoated Gold Nanoparticles. Photodimerization of thymine, which is a [2 + 2] cycloaddition reaction, is very sensitive to the distance and orientation of the two dimerizing molecules. These need to be aligned face-to-face and the distance between them should be 3.7 to 4.1 Å for the photoreaction to occur.29,49,51,52 The fact that photodimerization occurred in high-pH water with the larger (7.0 nm) gold particles, but not when the particles were smaller (2.2 nm), correlates well with our previous results showing that thymine terminated SAMs are more ordered on the larger particles.33 The reduced curvature of the larger gold particles, resulting in a closer distance between adjacent thymine molecules, may play a part in the photodimerization as well.53 In neutral DMF-water, however, photodimerization occurred even on 2.2 nm particles. Thymine molecules are negatively charged in pH 12.5 water, while they are neutral in DMFwater.31,33,34 As noted already, there are favorable interactions (49) Inaki, Y.; Mochizuki, E.; Yasui, N.; Miyata, M.; Kai, Y. J. Photopolym. Sci. Tech. 2000, 13, 177-182. (50) Inaki, Y. CRC Handbook of Organic Photochemistry and Photobiology, 2nd ed; Boca Raton, FL, 2004; pp 104/1-104/34. (51) Tohnai, N.; Inaki, Y.; Miyata, M.; Yasui, N.; Mochizuki, E.; Kai, Y. Bull. Chem. Soc. Jpn. 1999, 72, 851-858. (52) Tohnai, N.; Inkai, Y.; Miyata, M.; Yasui, N.; Mochizuki, E.; Kai, Y. Bull. Chem. Soc. Jpn. 1999, 72, 1143-1151. (53) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911.

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between the neutral thymine molecules, e.g., dipole-dipole interactions, π-stacking, and hydrogen bonding.49 These interactions appear to lead to a sufficient degree of closeness and preferential orientation so that dimerization can occur during UV irradiation. Further, we have also observed that a higher molecular packing density of thymine derivatives occurs on 2.2 nm gold particles,33 which is expected to facilitate the photoreaction and overcome the influence of higher curvature of the smaller gold particles on the photodimerization.54 Unfortunately, before light irradiation in DMF-H2O mixture, the dispersion was stable kinetically for a long time for the 2.2 nm gold particles but only for a brief period for the larger 7.0 nm ones, due to the stronger influence of van der Waals forces between the larger gold particles.34 Upon light irradiation, the 7.0 nm gold dispersions settled far too quickly for any possible photoreaction to be examined. In comparing the photodimerization of 7.0 nm gold particles in high-pH water and 2.2 nm gold particles in the neutral DMF-H2O mixture, the negatively charged surface, higher curvature, and less ordered SAMs together prevented photodimerization of thymine-capped 2.2 nm gold particles in highpH water. The strongly favorable influence of the mixed solvent is further exemplified by comparing the rate of dimerization in high-pH water (7.0 nm particles) and neutral DMF-water (2.2 nm particles). A significantly longer irradiation was needed to achieve the same level of conversion in high-pH water (Figure 2 and Figure 2 of Supporting Information). The initial reaction rate was 0.004 min-1 in high-pH water but 0.12 min-1 in the mixed solvent. We conclude that the quality of the solvent and the amount of surface charge determine the behavior of the dispersed particles under UV irradiation. The reaction kinetics in both high-pH water and DMF-H2O mixture were certainly not simple first order. A similar situation was described by Inaki and co-workers in their investigations of the photodimerization of thymine derivatives in cast films.46,55 In single crystals, where the distance and orientation between monomers were optimal, the rate of dimerization followed a first-order reaction.28,51,52,56 In cast films, where orientation was random and distances varied significantly, the thymine molecules dimerized rapidly or slowly or not at all. The amorphous structure of the films caused this spread, and the kinetic data showed a nonlinear behavior. It has been shown for chromophore-capped gold nanoparticles that the presence of the gold core does not affect the reaction order of the photoreaction.15,16,18,57,58 The results obtained for this present system are probably caused by the inhomogeneity and high curvature of the gold nanoparticle surfaces, where thymine molecules dimerize according to a firstorder reaction but with different reaction rates, reflecting the surface character. Such a combination of first-order reaction with different rates would of course produce the nonlinear behavior observed in Figure 3 of Supporting Information.

V Conclusion The aggregation of gold particles, dispersed in both pH 12.5 water and DMF-H2O mixtures, was observed upon exposure to (54) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; 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, 17-30. (55) Mochizuki, E.; Tohnai, N.; Wang, Y.; Saito, T.; Inaki, Y.; Miyata, M.; Yasui, N.; Kai, Y. Polym. J. (Tokyo) 2000, 32, 492-500. (56) Tohnai, N.; Sugiki, T.; Mochizuki, E.; Wada, T.; Inaki, Y. J. Photopolym. Sci. Technol. 1994, 7, 91-92. (57) Thomas, K. G.; Ipe, B. I.; Sudeep, P. K. Pure Appl. Chem. 2002, 74, 1731-1738. (58) Zhang, J.; Whitesell, J. K.; Fox, M. A. J. Phys. Chem. B 2003, 107, 6051-6055.

Light-Induced Aggregation of Gold Nanoparticles

280 nm light irradiation. Photoinduced colloid instability was attributed to reorientation of thymine terminal groups attached to the gold particles. The reorientation occurred following dimerization31 or light excitation. Photodimerization of thyminecapped gold particles was influenced by particle size, surface charge, and solvent type. The inhomogeneity and high curvature of the gold nanoparticle surfaces accounted for the photodimerization of thymine molecules exhibiting different reaction rates. Redispersion of gold aggregates irradiated at 240 nm did not occur. Thymine-capped gold particles provide an interesting route for the control of colloid stability using light irradiation, with potential for applications in biosensing.

Langmuir, Vol. 24, No. 9, 2008 4511

Acknowledgment. Financial support through The Australian Research Council Special Research Centre Scheme is gratefully acknowledged. Discussions with Nicky Lake and Anuttam Patra are warmly acknowledged. Supporting Information Available: UV-vis absorption spectra of 2.2 nm gold particle dispersion in 0.5 vol% of DMF in water, thymine and gold SPB absorbance as a function of irradiation time for 2.2 nm gold particle dispersion in 0.5 vol % of DMF in water, and reaction kinetics of thymine-coated gold nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. LA703746W