Stability of the DMF-Protected Au Nanoclusters - American Chemical

Dec 11, 2009 - Physics, Faculty of Engineering, Science Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680,. Japan. Received October 13 ...
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Stability of the DMF-Protected Au Nanoclusters: Photochemical, Dispersion, and Thermal Properties Hideya Kawasaki,*,† Hiroko Yamamoto,† Hiroaki Fujimori,† Ryuichi Arakawa,† Yasuhiko Iwasaki,† and Mitsuru Inada‡ † Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan and ‡Department of Pure and Applied Physics, Faculty of Engineering, Science Kansai University, 3-3-35 Yamate-cho, Suita-shi, Osaka 564-8680, Japan

Received October 13, 2009. Revised Manuscript Received November 25, 2009 We have reported the synthesis of dimethylformamide (DMF)-protected gold nanoclusters using a surfactant-free DMF reduction method. DMF-protected gold nanoclusters (Au NCs) are obtained without the formation of gold nanoparticles and bulk metals as byproducts using a hot injection process for the homogeneous reduction. The asprepared DMF-protected Au NCs were a mixture of various-sized Au NCs with a cluster number of less than 20 including at least Au8 and Au13. The photoluminescence emission from Au8 and Au13 was confirmed in the photoluminescence spectra. The Au NCs are stabilized with DMF molecules through the interaction of amide groups of DMF with Au NCs. DMF-protected Au NCs in solution were found to have high thermal stability, high dispersion stability in various solvents, and high photochemical stability. The DMF-protected Au NCs dispersed well for at least a month in various solvents such as water, acid (pH 2), alkali (pH 12) and 0.5 M NaCl aqueous solution, and methanol without further surface modification. The thermal stability of DMF-protected Au NCs was ∼150 °C, which was comparable to that of thiolate-protected Au NCs. The photobleaching of Au NCs in water gradually occurred under UV light irradiation (356 nm, 1.3 mW/cm2) because of the photoinduced oxidation of Au NCs. After 8 h irradiation, the fluorescence intensity slowly decreased to ∼50% of the maximum and to ∼20% after 96 h under the present condition, compared to the photobleaching of CdSe semiconductor quantum dots. We also found that the fluorescence intensity remained to be about 30% of the maximum even in the presence of concentrated 30% H2O2. These findings demonstrate that the photobleaching process under the UV irradiation is effectively suppressed for DMF-protected Au NCs.

1. Introduction Gold metal clusters have recently attracted much attention in many areas of physics, chemistry, materials science, and biosciences.1-6 The size-dependent effects of metal clusters are only observed when the free electrons are confined relative to the Fermi wavelength (∼1 nm) in the cluster conduction band.7-9 As a result, subnanometer-sized metal clusters consisting of only several tens of atoms are likely to exhibit molecule-like behavior, including having discrete electronic states and size-dependent fluorescence.1-9 Among various metal clusters, much attention has been given to gold (Au) clusters because of the relative ease of their synthesis by the chemical reduction of their salts in solution *Corresponding author. E-mail: [email protected]. (1) Michael, D.; Mingos, P. Gold Bull. 1984, 17, 5. (2) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (3) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science 2001, 291, 103. (4) Haruta, M. Chem. Rec. 2003, 3, 75. (5) Lee, T.-H.; Gonzalez, J. I.; Zheng, J.; Dickson, R. M. Acc. Chem. Res. 2005, 38, 534. (6) Zheng, J.; Nicovich, P. R.; Dickson, R. M. Annu. Rev. Phys. Chem. 2007, 58, 409. (7) Vollmer, M.; Kreibig, U. Optical Properties of Metal Clusters; Springer Series in Material Sciences Vol. 25; Springer-Verlag: Berlin, 1994.Haberland, H. Clusters of Atoms and Molecules; Springer-Verlag: Berlin, 1994.Meiwes-Broer, K.-M. Metal Clusters at Surfaces: Structure, Quantum Properties, Physical Chemistry; Springer Series in Cluster Physics; Springer: Berlin, 2000. (8) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutierrez-Wing, C.; Ascensio, J.; Jose-Yacaman, M. J. J. Phys. Chem. B 1997, 101, 7885. (9) Aikens, C. M. J. Phys. Chem. C 2008, 112, 19797.

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using strong stabilizing agents such as thiol and phosphine compounds.10-29 However, the quantum yields (QYs) of these Au clusters are relatively low, ranging from 10-4 to 10-5.10,15 On (10) Link, S.; Beeby, A.; FitzGerald, S.; El-Sayed, M. A.; Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2002, 106, 3410. (11) Zheng, J.; Petty, J. T.; Dickson, R. M. J. Am. Chem. Soc. 2003, 125, 7780. (12) Zheng, J.; Zhang, C.; Dickson, R. M. Phys. Rev. Lett. 2004, 93, 077402. (13) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. J. Am. Chem. Soc. 2004, 126, 6518. (14) Balasubramanian, R.; Guo, R.; Mills, A. J.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 8126. (15) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261. (16) Hussain, I.; Graham, S.; Wang, Z.; Tan, B.; Sherrington, D. C.; Rannard, S. P.; Cooper, A. I.; Brust, M. J. Am. Chem. Soc. 2005, 127, 16398. (17) Duan, H.; Nie, S. J. Am. Chem. Soc. 2007, 129, 2412. (18) Bao, Y.; Zhong, C.; Vu, D. M.; Temirov, J. P.; Dyer, R. B.; Martinz, J. S. J. Phys. Chem. C 2007, 111, 12194. (19) Tsunoyama, H.; Nickut, P.; Negishi, Y.; Al-Shamery, K.; Matsumoto, Y.; Tsukuda, T. J. Phys. Chem. C 2007, 111, 4153. (20) Gies, A. P.; Hercules, D. M.; Gerdon, A. E.; Cliffel, D. E. J. Am. Chem. Soc. 2007, 129, 1095. (21) Shichibu, Y.; Negishi, Y.; Tsunoyama, H.; Kanehara, M.; Teranishi, T.; Tsukuda, T. Small 2007, 3, 835. (22) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754. (23) Zhu, M.; Lanni, E.; Garg, N.; Bier, Mark, E.; Jin, R. J. Am. Chem. Soc. 2008, 130, 1138. (24) Liu, X.; Li, C.; Xu, J.; Lv, J.; Zhu, M.; Guo, Y.; Cui, S.; Liu., H.; Wang, S.; Li, Y. J. Phys. Chem. C 2008, 112, 10778. (25) Xie, J.; Zheng, Y.; Ying, J. Y. J. Am. Chem. Soc. 2009, 131, 888. (26) Qian, H.; Zhu, M.; Andersen, U. N.; Jin, R. J. Phys. Chem. A 2009, 113, 4281. (27) Wu, Z.; Jin, R. ACS Nano 2009, 3, 2036. (28) Sakamoto, M.; Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Am. Chem. Soc. 2009, 131, 6. (29) Shafai, G.; Hong, S.; Bertino, M.; Rahman, T. J. Phys. Chem. C 2009, 113, 12072.

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the other hand, Dickson and colleagues reported highly fluorescent Au nanoclusters (Au NCs) with more than 10% QYs for ultraviolet (UV) (Au5), blue (Au8), green (Au13), red (Au23), and near-infrared (Au31) emitting species in the presence of stabilizing ligands of poly(amidoamine)dendrimers and much increased QYs of Au NCs (QYs of 0.7 for Au5, 0.42 for Au8, and 0.25 for Au13).5,11,12 Highly fluorescent Au NCs in solutions have also since been reported by other research groups.17,18,25 Recently, photoluminescent Au NCs of Au11 were prepared in dimethylformamide (DMF) solvent without any other reducing agents or thiol compounds by Liu et al.,24 who obtained Au11 atomic clusters with QYs of 4-15% by removing larger Au nanoparticles as a byproduct via centrifugation and passing through a silica gel column. The Au NCs are of interest for their further functionalization with various thiolate ligands. Although there have been extensive studies on thiolate- and/or phosphineprotected Au NCs, there have been few reports on surfactant-free Au NCs,24 and it is not clear how the surfactant-free Au clusters are stabilized in DMF solution. In this paper, we investigated fluorescent gold nanoclusters synthesized using a surfactant-free DMF reduction method. In using hot injection for the homogeneous reduction, it was found that gold nanoclusters (Au NCs) can be obtained without the formation of gold nanoparticles and bulk metals as byproducts. As a result, we do not require centrifugation to remove larger gold nanoparticles or the passing through a silica gel column to obtain pure Au NCs. Fourier transform infrared spectroscopy and thermogravimetric analysis indicate that DMF molecules bind to Au clusters through the amide group. As for DMF-protected Au NCs, we are interested in the stability of Au NCs. The main objectives of the present study are (1) thermal stability, (2) dispersion stability in various solvents, and (3) photochemical stability (i.e., photobleaching) in water of the DMF-protected Au NCs. The results reported here on the stability of Au NCs will be important in understanding the physicochemical stabilities of Au NCs as well as in practical applications of such nanoclusters in optics and catalysis.

2. Experimental Section 2.1. Materials. HAuCl4 3 4H2O as a source of gold atoms was

obtained from Wako Chemical Co. All solvents used in this study were reagent grade from Wako Chemical Co. and were used without further purification. 2-Mercaptobenzothiazole (MBA) and NaCl were obtained from Wako Chemical Co. The ultrapure water used throughout all experiments was purified with an Advantec RFD 250 NB system. 2.2. Preparation of Au NCs. Au NCs were prepared according to the DFM reduction method of Liu et al.;24 however, the preparation method was partly modified in terms of the heating process. A solution of 150 μL of 0.1 M aqueous HAuCl4 was added to 15 mL of preheated DMF at 140 °C, and the DMF solution was refluxed by simply heating with an oil bath (140 °C for 6 h) with vigorous stirring under an atmosphere. After evaporating the excess solvent under a vacuum of less than 10 mHg at 80 °C for 3 h, the residue was redissolved in various solvents such as water, methanol, hexane, and 0.5 M NaCl aqueous solution. 2.3. Photophysical Properties of Metal Clusters. UV-vis absorption spectra were measured by a JASCO V-670 spectrometer. Fluorescence excitation and emission spectra were obtained on a JASCO FP-6300 fluorimeter. Photochemical stability (i.e., photobleaching) of Au clusters were examined using a UV light (SLUV-6, AS ONE Co., Japan) with 1.3 mW/cm2 at a distance of 5 cm from the light. Langmuir 2010, 26(8), 5926–5933

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2.4. Thermogravimetry/Differential Thermal Analysis (TG-DTA). Thermogravimetric analysis was performed with a Rigaku Thermo plus EVO TG-DTA. Approximately 1 mg of dried Au NCs, which were prepared under a vacuum of less than 10 mHg at 80 °C for 3 h, was put into an aluminum pan, and the temperature was ramped from room temperature to 480 °C at a heating rate of 10 °C/min.

2.5. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectroscopy was performed on a Jasco FT-IR 4200 spectrometer with an attenuated total reflection (ATR) attachment (ATR8100H). Approximately 1 mg of dried Au NCs, which were prepared under a vacuum of less than 10 mHg at 80 °C for 3 h, was put onto the ATR ZnSe crystal for the FT-IR-ATR measurements. 2.6. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra were recorded with Quantera SXM spectrometer (Physical Electronics, Inc.) using monochromatic Al KR line (1486.7 eV). The base pressure was ∼2  10-8 Torr. The binding energies were referenced to C 1s at 284.7 eV from hydrocarbon to compensate charging effect.

2.7. Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS). MALDI-MS was conducted with an AXIMA CFR MALDI-TOF mass spectrometer. After evaporating excess DMF solvent under a vacuum, the resultant Au NCs were dispersed in 15 mL of methanol, and solid powder (1.5 μmol, which gave a concentration equal to that of Au atoms) of MBA was added to the solution, which was stirred for 24 h. We thus obtained MBA-modified Au NCs. MBA adsorbs the UV laser light (337 nm) of MALDI, and hence MBA works as the MALDI matrix. 1.0 μL of gold clusters in methanol was deposited on a MALDI target plate and air-dried. The sample was irradiated by a 337 nm N2 pulse laser. In general, the results from 100 laser pulses were averaged to obtain the spectrum.

3. Results and Discussion 3.1. Synthesis of DMF-Protected Au NCs and Spectroscopic Measurements. Using hot injection for the homogeneous reduction, it was found that gold clusters can be obtained without the formation of gold nanoparticles and bulk metals as byproducts. Prior to the reduction, a DMF solution containing HAuCl4 was nonfluorescent. During refluxing via oil-bath heating at 140 °C, the solution changed from being light yellow to being clear. The colorless solution then gradually turned yellow over a period of 2-4 h. The reduction reaction was almost complete after 6 h. After 8 h reaction, the photoluminescence spectrum had almost no change, suggesting completion of the reduction, which was also confirmed by further reduction by adding sodium borohydride from no observation of precipitates of Au NPs. The resultant yellow solution exhibited photoluminescence from Au NCs in DMF, as shown in the photograph of Figure 1a. It has been reported that the surfactant-free DMF reduction method produces a brown solution of Au clusters for as-prepared product,24 while our DMF reduction method produced a yellow solution of Au clusters. This different appearance of the Au cluster solution may be attributed to the formation of smaller Au clusters in our synthesis.21 In the UV-vis absorption spectrum of Au NCs in water, the absorption increases strongly toward shorter wavelengths from around 400 nm. The UV-vis spectrum has a distinct peak at 220 nm and a shoulder peak around 300-350 nm (Figure 1a). The former may correspond to the absorption of DMF molecules on Au clusters because it is consistent with the absorption peak of DMF in water at 216 nm in the spectrum. The latter may originate from the multipleabsorption band spectrum of Au clusters, which is similar to those DOI: 10.1021/la9038842

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Figure 1. (a) UV-vis spectrum of an aqueous solution of DMF-protected Au clusters. The photographs of DMF-protected Au clusters in DMF under ambient light (left) and UV light of 365 nm (right) are shown in the figure. (b) Photoluminescence spectra of DMF-protected Au clusters in DMF for different excitation wavelengths of 329, 358, and 433 nm.

of Au13 NCs protected with N-acetyl-L-cystein.30 The UV-vis spectrum show no surface plasmon resonance (SPR) band from gold nanoparticles (∼520 nm), suggesting that all Au NCs have core diameters of less than 2 nm. In the reaction, an important step was that HAuCl4 was injected into hot DMF after oil-bath heating. When the HAuCl4 was instead injected into DMF at room temperature and then heated slowly, a precipitate of bulk Au or large Au nanoparticles formed in addition to the formation of Au clusters. DMF is thermally decomposed into dimethylamine and carbon monoxide above ∼100 °C through the formation of unstable carbonic acid.31 The pyrolytically generated carbon monoxide may work as the reduction agent for AuCl4ions. It has been suggested that photoluminescence from gold clusters is attributed to transitions between the filled 5d10 band and 6sp1 conduction band of the gold atom.10 In general, the decrease in metal cluster size leads to a blue shift in fluorescence relative to that from larger clusters, and thus the mixture of various-sized metal clusters has different color emissions when excited at an appropriate wavelength.18 Figure 1b shows photoluminescence spectra of as-prepared Au NCs, exhibiting the multiple emission maxima when excited at different wavelengths. The visible excitation at Eex = 433 nm (2.76 eV) resulted in an emission wavelength maxima of Eem = 496 nm (2.5 eV) (Figure 1b), which is close to that of Au13 clusters at Eem = 2.4 eV.12 The UV excitation at Eex = 385 nm (3.2 eV) produced an emission wavelength maxima of Eem = 476 nm (2.6 eV) (Figure 1b), which is close to that of Au8 clusters at Eem = 2.7 eV.12 These results suggest the presence of Au8 and Au13 clusters in the mixture at least. On the other hand, the UV excitation at Eex = 329 nm (3.8 eV) produced an emission peak with shoulder peaks, suggesting the emission from Au clusters other than Au13 and Au8. Because the red emission of Au23 was not observed, it is likely that Au clusters do not have cluster numbers of more than 20. MALDI-MS of the light-emitting species supported the presence of Au cluster peaks less than Au15 (Figure 2a), although there was accompanying fragmentation of the clusters. This is consistent with the result of the photoluminescence spectra that Au clusters do not have the cluster numbers above 20. Here, we used MBA as a new MALDI matrix in MALDI-MS of the An (30) Zhang, Y.; Shuang, S.; Dong, C.; Lo., C.-K.; Paau, C.; Choi, M. M. F. Anal. Chem. 2009, 81, 1685. (31) Dickmeis, M.; Ritter, H. Macromol. Chem. Phys. 2009, 19, 1–10.

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NCs because we could not detect gold clusters with more than cluster’s number of 8 without additional MALDI matrix and the fragment ions were observed in the mass range less than 800 in the absence of MALDI matrix. MBA can adsorb the UV laser light (337 nm) of MALDI, and it seems that the use of the additional MALDI matrix allows relatively soft ionization of the Au clusters. Thus, MBA-modified Au NCs were detected in the MALDI-MS as (Au)nSm ions, as shown in the expanded mass spectrum of Figure 2b. The sum of peak intensities of (Au)nSm ions against the number of gold atoms is shown in Figure 2c, indicating the maxima of Au13 in the distribution, which has been reported to be a stable structure.29 Laser desorption/ionization is apparently very complex,32 and we do not intend to interpret the detailed gas phase fragmentation. However, the combination of the analysis of photoluminescence spectra with MALDI-MS indicates that the as-prepared product is a mixture of different sized clusters with cluster numbers less than 20, including at least Au8 and Au13. We attempted to observe Au NCs in TEM images; however, the subnanometer atomic clusters smaller than 1 nm were too small to be visible under TEM observations. It has been reported that such DMF-protected Au11 clusters with subnanometer sizes are not visible under TEM.24 3.2. Oxidation State of DMF-Protected Au NCs Determined by XPS. The oxidation state of DMF-protected Au NCs was determined by XPS. The XPS Au(4f7/2) spectrum obtained from the dried Au cluster is shown in Figure 3. The main peak at 85.4 eV is of higher binding energy than that of bulk Au (84 eV), and it is of lower binding energy than that of tetrachloraurate ion (87.3 eV). The main peak (85.4 eV) might be due to the binding energy of the surface of the Au cluster. It is known that the binding energy of the metal cluster increases with a decrease in the cluster size.33,34 Bulk Au metal has a binding energy of 84 eV, while the Au(4f7/2) binding energy of Au clusters could be 1.0-2.0 eV higher.15,21,34 These features are qualitatively interpreted on the basis of recent XPS studies on thiolate-protected Au clusters and nanoparticles.15,35 Tanaka and co-workers found from (32) Schaaff, T. G. Anal. Chem. 2004, 76, 6187. (33) Wertheim, G. K.; DiCenzo, S. B. Phys. Rev. B 1988, 37, 844. (34) Turner, M.; Golovoko, V. B.; Vaughan, O. P. H.; Abdulkin, P.; Murcia, A. B.; Tikhov, M. S.; Johnson, B. F. G.; Lambet, R. M. Nature 2008, 454, 981. (35) Tanaka, A.; Takeda, Y.; Imamura, M.; Sato, S. Phys. Rev. B 2003, 68, 195415.

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Figure 2. (a) MALDI mass spectrum of MBA-protected Au clusters in the negative ion mode, which were obtained from DMF-protected Au clusters by the ligand exchange reaction. Here, we used MBA as a new MALDI matrix in the MALDI-MS. The assignment of the cluster numbers for the peaks are shown in the mass spectra. (b) The expanded mass spectrum of (a) for MBA-modified Au NCs detected as (Au)nSm ions. (c) The sum of peak intensities of (Au)nSm ions against the number of gold atoms.

Figure 3. XPS Au(4f 2/7) spectrum obtained from DMF-protected Au clusters.

detailed peak shape analysis that the Au(4f7/2) peaks can be deconvoluted into two components associated with the inner and surface atoms of gold.35 The Au(4f7/2) peak positions for the inner Au atoms were found to monotonically shift from 84.0 to 84.3 eV with a reduction of the core size. The Au(4f7/2) peak positions of the surface Au components were at higher energy (84.3-84.7 eV) than those of the corresponding inner components. Tsukuda and coworkers observed that the Au(4f7/2) binding energies of thiolateprotected Au clusters are around 85 eV,15 which is consistent with the binding energy of Au(4f2/7) observed for DFM-protected Au clusters (85.4 eV) in this study. Thus, we consider that AuCl4- ions are reduced to Au0 atoms and grow to form Au clusters; however, there is no further growth into Au nanoparticles. 3.3. FT-IR Analysis of DMF-Protected Au NCs. In the FT-IR spectrum of DMF-protected Au NCs, the spectral region Langmuir 2010, 26(8), 5926–5933

of 2000-600 cm-1 was taken into consideration to examine the interaction of amide groups of DMF (i.e., CdO and C-N vibration modes) with Au NCs. We found that the CdO and C-N vibration modes of DMF in DMF-protected Au NCs were dramatically different to those in DMF solvent. Figure 4a shows FT-IR spectra of (i) DMF solvent and (ii) DMF-protected Au NCs in a dried state. The dried Au NCs were prepared under a vacuum of less than 10 mHg at 80 °C for 3 h in order to remove free DMF solvents. The main bands of DMF in the spectrum of Au clusters are clearly seen. The similarity of the features in the two FT-IR spectra confirms that the DMF is an essential component of the dried Au NCs. All the peaks for DMFprotected Au NCs are significantly broader than those of free DMF (i.e., only DMF solvent), which may reflect on the interaction of the gold clusters with DMF molecules. A very strong absorption at ∼1660 cm-1, mainly due to ν(CdO), is the most sensitive among the DMF bands with respect to the interaction. The carbonyl band has additional contributions from C-N stretching (∼23%) and C-H bending (∼12%).36 Indeed, the band shifts toward lower wave numbers (from 1660 to 1653 cm-1) and broadens for DMF-protected Au NCs, suggesting the interaction of the CdO group in DMF with Au NCs. A new band appearing at 1713 cm-1 for the Au NCs might be assigned to the ν(CdO) vibration mode of the carboxylic acid group because a broad band around 3500-2500 cm-1 due to the ν(OH) vibration mode of the carboxylic acid group was also observed (not shown). The synthesis of DMF-protected Au NCs seems to be accompanied by the byproduct of the oxidation of DMF. The (36) Durgaprasad, G.; Sathyanarayana, D. N.; Patel, C. C. Bull. Chem. Soc. Jpn. 1971, 44, 316.

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Figure 4. (a) FT-IR spectra of (i) DMF solvent and (ii) DMF-protected Au clusters. (b) TG-DTA thermogram of DMF-protected Au clusters with a heating rate of 10 °C/min.

1387 cm-1 band for DMF solvent is a coupled vibration with major contributions from CH3 deformation (∼35%) and C-N stretching (∼24%) modes, and this band has been reported to be sensitive for metal complexes.36 In this study, this 1387 cm-1 band for DMF solvent was found to shift to 1403 cm-1 for DMFprotected Au NCs, suggesting the metal complexes with DMF. The two absorption bands at 1090 and 1063 cm-1 for DMF solvent are assigned to methyl rocking vibrations coupled with a C-N characteristic (10%), and this band is also sensitive for metal complexes.36,37 These methyl rocking vibration bands for DMF solvent also shift to 1043 and 1023 cm-1 for DMFprotected Au NCs. The band at 660 cm-1 for DMF solvent is assigned to the major O-C-N deformation contribution (∼36%) coupled with C-N stretching (∼34%) and C-N-C rocking (∼13%).36 The O-C-N deformation band shifts from 660 to 703 cm-1 for DMF-protected Au NCs. These large changes in CdO and O-C-N related bands observed for DMF-protected Au NCs (denoted by dotted lines) strongly suggest the interaction of amide groups of DMF with Au NCs. Owing to the contribution of the two possible resonance structures of the amide group in a DMF molecule, the bond order of the carbonyl CdO bond is reduced, while that of the C-N bond is increased. Because of the partial double bond characteristic, the amide group of DMF has a large dipole moment of 3.82 D. The interaction of the large dipole with Au NCs is expected to play an important role in the stabilization of Au NCs. 3.4. Thermal Stability of DMF-Protected Au NCs. The thermal desorption of DMF in the Au NCs and the thermal stability of Au NCs were examined by TG-DTA at a heating rate of 10 °C/min in a N2 atmosphere. The dried Au NCs for TG-DTA were prepared in an aluminum pan under a vacuum of less than 10 mHg at 80 °C for 3 h. Figure 4b shows the TG-DTA thermogram for the dried Au clusters. In the thermogram, there is a large mass loss of 67% in the range of 150-300 °C around the boiling temperature of DMF (∼153 °C) corresponding to the loss of DMF molecules adsorbed on Au NCs. The negligible change in the total TG weight and the broad exothermic curve in the temperature range above 200 °C are probably due to the formation of metallic gold through the fusion of Au NCs. Observation of a color change for the dried Au NCs from burnt orange to black, as shown in Figure 4b, indicated that the formation of metallic gold began at a temperature of 160-200 °C. Thus, (37) Biliskov, N.; Baranovic, G. J. Mol. Liq. 2009, 144, 155.

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TG/DTA results indicate that the DMF-protected Au NCs are stable up to ∼150 °C (the clusters were kept at this temperature for 1 h) and that the thermal stability of Au NCs is determined by the stabilization of DMF molecules. The thermal stability of DMF-protected Au NCs is similar to that of thiolate-protected Au25(SG)18 NCs (where SG represents glutathionate).27 Assuming that the mass loss of 67% above 150 °C can be assigned to the DMF-protected Au13 clusters, there are ∼15 DMF molecules per Au13 cluster, suggesting that the surface atoms of Au NCs are mostly capped by DMF molecules. 3.5. Dispersion Stability of DMF-Protected Au NCs in Various Solvents. For a number of applications, it is necessary to use different solvents. We found that DMF-protected Au NCs can disperse in various polar solutions such as water, broad pH range (pH 2-12) solutions, solutions with a high salt concentration (e.g., 0.5 M NaCl), and alcohols (e.g., methanol) without needing additional stabilizing agents because of the high polarity and wide solubility range of DMF for polar organic solvents. Almost no decrease in fluorescence intensity was observed for a month, as indicated by the fluorescence intensities after 20 and 30 days (Figure 5). When using nonpolar hexane with low solubility for DMF, on the other hand, Au NCs precipitated in hexane just after the injection of Au NCs into hexane, and the colorless supernatant showed no photoluminescence, as shown in Figure 5. Thus, it can be concluded that the DMF-protected Au NCs disperse well in various solvents such as water, acid, alkali and NaCl aqueous solutions, and methanol without surface modification. 3.6. Photochemical Stability of DMF-Protected Au NCs in Water. It has been reported that photobleaching irreversibly destroys photoluminescence molecules stimulated by radiation within the excitation spectrum, thus eliminating potentially useful photoluminescence properties. In contrast to organic fluorescent probes, semiconductor quantum dots (QDs) have much better photostability; however, photochemical instability (i.e., photobleaching) has still been observed even for QDs because of photooxidation.38-41 Generally, bulk Au metal is known to have high tolerance of oxidation, and it is interesting to examine the (38) Ma, J.; Chen, J.-Y.; Zhang, Y.; Wang, P.-N.; Guo, J.; Yang, W.-L.; Wang, C.-C. J. Phys. Chem. B 2007, 111, 12012. (39) Stouwdam, J. W.; Shan, J.; van Veggel, F. C. J. M.; Pattantyus-Abraham, A. G.; Young, J. F.; Raudsepp, M. J. Phys. Chem. C 2007, 111, 1086. (40) Chen, H.; Gai, H.; Yeungac, E. S. Chem. Commun. 2009, 1676. (41) Cooper, D. R.; Suffern, D.; Carlini, L.; Clarke, S. J.; Parbhoo, R.; Bradforth, S. E.; Nadeau, J. L. Phys. Chem. Chem. Phys. 2009, 11, 4298.

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Figure 5. Photoluminescence spectra of DMF-protected Au clusters at an excitation wavelength of 350 nm in different solvents: water, acid (pH 2) and alkali (pH 12) aqueous solutions, 0.5 M NaCl aqueous solution, and methanol. The spectra recorded 20 and 30 days after preparation are shown. In the case of hexane, the spectra immediately after preparation and 1 day later are shown.

Figure 6. (a) Photoluminescence spectra of DMF-protected Au clusters in waterat an excitation wavelength of 350 nm as a function of the UV irradiation time (356 nm, 1.3 mW/cm2) for up to 96 h. (b) UV-vis spectrum of an aqueous solution of DMF-protected Au clusters. Normalized photoluminescence intensities of DMF-protected Au clusters in water as a function of the UV irradiation time (356 nm, 2 mW/cm2) for up to 96 h.

photochemical stability of Au NCs. However, there has been little study on the photochemical stability of Au NCs in solution. In this study, therefore, the photobleaching properties of the DMF-protected Au NCs in water were examined. Figure 6 shows photoluminescence spectra of Au NCs in water with a quartz cell as functions of the UV light continuous irradiation time (356 nm, 1.3 mW/cm2). It was found that the photobleaching of Au NCs gradually occurred under the UV irradiation. The fluorescence intensity decreased to ∼50% of its maximum after 8 h and to ∼20% 96 h later (Figure 6a). Thus, even for Au NCs, we observed photobleaching behavior; however, it appears that the photobleaching of Au NCs proceeds slowly over several days under the present condition. The photobleaching of Au NCs is also clear from the UV-vis Langmuir 2010, 26(8), 5926–5933

spectrum and leads to a depression of the UV-vis absorption and/or a blue shift of the onset of absorption (Figure 6b). This may be because transformation of the original clusters into small clusters has occurred, which can be attributed to core etching through the photo-oxidation of Au clusters. The core etching of Au NCs has been observed for glutathionateprotected (Au)n clusters (n < 25) by adding a large excess of free glutathione.21 It should be noted that the decrease in the peak intensity around 415 nm from smaller clusters in the photoluminescence spectrum was more dominant under the UV irradiation of 96 h compared to the decrease in the shoulder peak at 450 nm from larger clusters (as shown in dotted lines of Figure 6a). This implies that the photo-oxidation of Au NCs is more enhanced for smaller Au NCs. DOI: 10.1021/la9038842

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Figure 7. (a) Photoluminescence spectra of DMF-protected Au clusters in water at an excitation wavelength of 350 nm (i) initially, (ii) after UV irradiation (356 nm, 1.3 mW/cm2) for 96 h, and (iii) after the addition of 0.2 M NaBH4. (b) Photoluminescence spectra of DMF-protected Au clusters at an excitation wavelength of 350 nm in (i) water and (ii) 30% H2O2 aqueous solution.

Figure 8. Photoluminescence spectra of DMF-protected (a) Au clusters and (b) CdSe QDs in toluene at an excitation wavelength of 350 nm as a function of the UV irradiation time (356 nm, 1.3 mW/cm2) for up to 10 h.

It has been reported that DMF-protected Au NCs are reduced with the addition of NaBH4, resulting in higher QYs of Au NCs after the NaBH4 reduction.24 In this study, we attempted to perform the treatment with 0.5 M NaBH4 as a strong reducing agent for the DMF-protected Au NCs in the photobleaching state. This resulted in partial recovery of the fluorescence intensity of Au NCs in the photobleaching state (Figure 7a). These results suggest that the DMF-protected Au NCs can be oxidized by UV irradiation, while the Au NCs in the oxidized state can be partially reduced by NaBH4 although this is not absolute proof of the clusters’ oxidation and reduction states at present. To induce the oxidation of the Au NCs, the DMF-protected Au NCs were also dispersed in 30% H2O2 aqueous solution for 24 h, indicating a decrease in fluorescence intensity in the presence of the strong oxidization agent (Figure 7b). Irrespective of the presence of the concentrated H2O2, however, the fluorescence intensity remained at ∼30% of the initial maximum. This suggests that DMFprotected Au NCs are highly tolerant to oxidation. For comparison, we also compared the photochemical stability of Au NCs and CdSe QDs. Here, CdSe QDs were synthesized

from CdO and elemental Se using a kinetic method on the reaction time of 10 s.42 Figure 8 shows the variations in the fluorescence intensities of Au NCs and CdSe QDs in toluene with a quartz cell as functions of the UV light continuous irradiation time (356 nm, 1.3 mW/cm2). The fluorescence intensity of Au NCs stays above about 70% of the original one over the duration of 2 h, while that of CdSe QDs decreases to 15% of the original one (Figure 9a). This is represented in the photographs of Au NCs and CdSe QDs under a UV light of 365 nm (a) in an original one and (b) after the UV irradiation for 2 h (Figure 9b,c). After the UV irradiation of 10 h, there was no observation of the photoluminescence from CdSe QDs, in contrast to the high photoluminescence from Au NCs. Recently, Guo et al. reported that the reduction of HAuCl4 by aniline in aqueous HCl gave irregularly layered gold nanoplates, where the rate of reduction was slowed because of oxidative etching of gold atoms by Cl- ions.43 As the possible gold photooxidation products for the Au NCs, we consider that a combination of Cl- ions and the photo-oxidation can result in the oxidative etching of Au NCs to dissolve into the solution, which

(42) Boatman, E. M.; Lisensky, G. C.; Nordel, K. J. J. Chem. Educ. 2005, 82, 1697.

(43) Guo, Z.; Zhang, Y.; Xu, A.; Wang, M.; Huang, L.; Xu, K.; Gu, N. J. Phys. Chem. C 2008, 112, 12638.

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Figure 9. (a) Normalized photoluminescence intensities at 445 nm for DMF-protected Au clusters and those at 505 nm for CdSe QDs as a function of the UV irradiation time (356 nm, 1.3 mW/cm2). Photographs of DMF-protected Au clusters and CdSe QDs in toluene (b) before and (c) after the UV irradiation for 2 h.

might be the AuCl2- complex. The Cl- ions are originated from Au Cl4- precursor ions, and the Cl 2p XPS spectrum of Au NCs indicated the existence of chloride compounds (not shown). Further study is underway to clarify the gold photo-oxidation products and the oxidation state of Au NCs.

4. Summary In this study, we showed that fluorescent DMF-protected Au NCs can be synthesized using a surfactant-free DMF reduction method without the formation of gold nanoparticles and bulk metals as byproducts and hot injection for the homogeneous reduction. As-prepared DMF-protected Au NCs are a mixture of various-sized Au NCs of less than 20 atoms including at least Au8 and Au13. Development of the surfactant-free DMF reduction

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method could result in attaining truly atomic-monodisperse (Au)n NCs (where n is a given value) in the future. The Au NCs dried under a vacuum were stable and hence dispersed again in various solvents. The DMF-protected Au NCs dispersed well in various solvents such as water, acid (pH 2), alkali (pH 12) and 0.5 M NaCl aqueous solutions, and methanol without further surface modification. The thermal stability of the DMF-protected Au NCs was ∼150 °C, which is comparable to that of thiolate-protected Au NCs. The DMF-protected Au NCs can be oxidized gradually by UV irradiation (356 nm, 2 mW/cm2) over several days, while the Au NCs in the oxidized state can be partially reduced using NaBH4. Irrespective of the presence of concentrated H2O2, however, the fluorescence intensity remained at ∼30% of the initial maximum. This suggests that DMF-protected Au NCs are highly tolerant to photo-oxidation. We also found that the photobleaching process under the UV irradiation is suppressed for Au NCs compared to that of CdSe QDs. Even in the presence of Cl- ions, the DMF-protected Au NCs are very stable under the UV photoirradiation even for several hours, and there was almost no change in the fluorescence intensity the UV photoirradiation for 2 h, irrespective of the case in the presence of Cl- ions. These findings demonstrate that the photobleaching process under the UV irradiation may be more effectively suppressed for DMF-protected Au NCs in the absence of Cl- ions for the metal ion. In spite of the absence of ligands such as thiolate compounds, therefore, the high stability of DMF-protected Au NCs in dispersion, photochemical, and thermal treatments will contribute to practical applications of such nanoclusters in the fields of optics and catalysis. Acknowledgment. This work was supported by the Kansai University Special Research Fund (#120135).

DOI: 10.1021/la9038842

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