Controlled reduction for size selective synthesis of thiolate-protected gold nanoclusters Aun(n = 20, 24, 39, 40)
© Meng et al.; licensee Springer. 2012
Received: 5 February 2012
Accepted: 30 May 2012
Published: 30 May 2012
This work presents a controlled reduction method for the selective synthesis of different sized gold nanoclusters protected by thiolate (SR = SC2H4Ph). Starting with Au(III) salt, all the syntheses of Au n (SR) m nanoclusters with (n, m) = (20, 16), (24, 20), (39, 29), and (40, 30) necessitate experimental conditions of slow stirring and slow reduction of Au(I) intermediate species. By controlling the reaction kinetics for the reduction of Au(I) into clusters by NaBH4, different sized gold nanoclusters are selectively obtained. Two factors are identified to be important for the selective growth of Au20, Au24, and Au39/40 nanoclusters, including the stirring speed of the Au(I) solution and the NaBH4 addition speed during the step of Au(I) reduction to clusters. When comparing with the synthesis of Au25(SC2H4Ph)18 nanoclusters, we further identified that the reduction degree of Au(I) by NaBH4 also plays an important role in controlling cluster size. Overall, our results demonstrate the feasibility of attaining new sizes of gold nanoclusters via a controlled reduction route.
KeywordsGold nanoclusters Size selective synthesis Controlled reduction
Gold nanoclusters [1–6] have received extensive attention owing to their interesting optical properties [6–9], magnetism [10, 11], fluorescence [12–16], chirality [17–20], redox properties [21–27], as well as potential applications in many fields such as catalysis and biological labeling [28–33]. The new physicochemical properties of gold nanoclusters are largely imparted by the discrete electronic structure of the metal core due to quantum confinement effects. The surface of the cluster may also influence some of the material properties, such as chirality [18, 19].
Recently, major advances in wet chemical synthesis of nanoclusters have been achieved, and it has been possible to control nanoclusters at the atomic level. A number of well-defined nanoclusters have been reported; however, only a few can be obtained in bulk quantities and in high yields via facile synthetic methods . Among the various thiolate-protected gold nanoclusters, Au25(SR)18 has been extensively studied [21–27, 35–41]. Other well-defined nanoclusters have also been attained, such as Au36, Au38[43, 44], Au102, and Au144[46, 47].
We previously reported a kinetically controlled synthetic approach for the synthesis of highly pure Au25(SR)18 nanoclusters [48, 49]. The method involves a size focusing mechanism, that is, the initial cluster product of mixed sizes is converged to a specific size of highest stability under appropriate conditions . By controlling the size range of the initial nanoclusters, one can achieve atomic monidispersity of nanoclusters . This synthetic approach constitutes a versatile strategy for gold nanocluster synthesis  and has been demonstrated in the synthesis of quite a number of atomically precise Au n (SR) m nanoclusters, such as Au25, Au38, and Au144.
Herein, we demonstrate that a controlled reduction method can lead to different sizes of gold nanoclusters. By making a modification of the synthetic method of Au24 nanoclusters , we have obtained two new sizes, including Au39(SC2H4Ph)29 and Au40(SC2H4Ph)30. Our results explicitly show that the initial growth stage of nanoclusters is critical and can be largely influenced by experimental conditions. This method of controlled reduction has expanded the synthetic approaches for preparing nanoclusters with size control.
The following chemicals were used: tetrachloroauric(III) acid (HAuCl4·3H2O, >99.99 % metals basis, Sigma-Aldrich Corporation, St. Louis, MO, USA), tetraoctylammonium bromide (TOAB, ≥98%, Fluka Chemicals Limited, Gillingham, Medway, UK), phenylethanethiol (PhC2H4SH, 99%, Acros Organics, Thermo Fisher Scientific, NJ, USA), and sodium borohydride (99.99%, metals basis, Sigma-Aldrich). The solvents include toluene (HPLC grade, ≥99.9%, Sigma-Aldrich), ethanol (absolute, 200 proof, PHARMCO-AAPER, Shelbyville, KY, USA). Pure water was from Wahaha Co. LTD (Hangzhou, China). All glassware was thoroughly cleaned with aqua regia (HCl: HNO3 = 3:1 vol), rinsed with copious pure water, and then dried in an oven prior to use.
All UV-visible (vis) absorption spectra of Au nanoclusters in either toluene or methylene chloride were recorded using a Hewlett-Packard (HP, Palo Alto, CA, USA) 8453 diode array spectrophotometer. Electrospray ionization mass spectra were acquired using a Waters Q-TOF (Waters Corporation, Milford, MA, USA) mass spectrometer equipped with a Z-spray source. The sample solution (approximately 1 mg/mL) dissolved in toluene was diluted in dry methanol (50 mM cesium acetate CsAc, 1:2 vol). The sample was directly infused at 5 μL/min. The source temperature was kept at 70°C. The spray voltage was kept at 2.20 kV; the cone voltage, at 60 V.
Synthesis of Au n nanoclusters (n = 39 and 40)
HAuCl4·3H2O (0.1612 g, 0.41 mmol) was dissolved in 5 mL water, and tetraoctylammonium bromide (TOAB, 0.2541 g, 0.465 mmol) was dissolved in 10 mL toluene. These two solutions were combined in a 25-mL tri-neck, round-bottom flask. The solution was vigorously stirred (approximately 1,100 rpm) with a magnetic stir bar to facilitate phase transfer of Au(III) salt into the toluene phase. After approximately 15 min, phase transfer was completed; the clear aqueous phase was then removed. The toluene solution was cooled down to 0 °C in an ice bath over a period of approximately 30 min under constant magnetic stirring. After that, magnetic stirring was reduced to a slow speed (approximately 100 rpm), PhC2H4SH (0.20 mL, approximately threefold the moles of gold) was added, and the solution was kept under slow stirring. The solution color changed slowly from deep red to faint yellow and to colorless over approximately 1 h. After that, the speed of magnetic stirring was increased from approximately 100 to 400 rpm. At the same time, 1 mL aqueous solution of NaBH4 (0.44 mol/L, freshly made with ice-cold water) was dropwise added to the toluene solution over a 15-min period using a 1-mL syringe. The color of the solution turned black gradually. After the dropwise addition of NaBH4, the reaction was allowed to further proceed overnight. The optical absorption spectrum of the crude reaction product (diluted with toluene) shows a distinct absorption band at approximately 800 nm.
Post-synthetic treatment of the crude product
The aqueous layer at the bottom of the flask was removed using a syringe, and the toluene solution was concentrated by rotary evaporation at room temperature. Ethanol (approximately 50 mL) was added to precipitate the Au nanoclusters. The brown, turbid solution was allowed to stand on bench for several hours. The precipitate was collected and redissolved in toluene. This precipitation/dissolution process was repeated with ethanol. The crude mixture was extracted with methylene chloride/acetonitrile (1:9 vol) to remove a small amount of Au20(SC2H4Ph)16 (its optical absorption band at approximately 485 nm) . After Au20 was removed from the product, Au24(SC2H4Ph)20 nanoclusters were removed by a second extraction with methylene chloride/acetonitrile (1:2 vol) . The final remaining product was collected and characterized by mass spectrometry.
Results and discussion
Identification of Au39(SC2H4Ph)29 and Au40(SC2H4Ph)30
Starting with an Au(III) salt precursor, the synthesis of gold nanoclusters involves two primary stages: (a) reduction of Au(III) to Au(I) by HSR, during which the formed Au(I) intermediate species spontaneously aggregates into polymeric Au(I) species (unknown structure), and (b) reduction of Au(I) to Au n (SR) m nanoclusters by NaBH4.
In this work, we have identified several important factors for the synthesis of nanoclusters Au39 and Au40, including the stirring speed of the reaction mixture, the addition speed, and the amount of NaBH4 solution to reduce Au(I) into clusters. The synthetic conditions reported in this work differ from the previous syntheses of Au19(SC2H4Ph)13, Au20(SC2H4Ph)16 and Au24(SC2H4Ph)20 (see ‘Methods’ section) [50–52]. Specifically, in the present work, our major modification lies in the stirring speed of the Au(I) intermediate solution when reduced by NaBH4. In a previous work, Au20(SC2H4Ph)16 and Au24(SC2H4Ph)20 were synthesized by controlling the stirring speed for the reduction step of Au(I) by NaBH4; for example, approximately 50 rpm for Au20(SC2H4Ph)16 and approximately 100 rpm for Au24(SC2H4Ph)20.
We employ electrospray ionization mass spectrometry (ESI-MS) to determine the composition of the new gold cluster product. A solution of cesium acetate (CsAc, 50 mM, in dry methanol) was added to a toluene solution of gold clusters at 1:1 or 1:2 (vol). ESI-MS detects the cluster-Cs adducts that are positively charged due to Cs+ addition to the cluster surface.
List of candidate formulas for the two clusters
(n,m) Candidate formulas for m/z 11,660
(n,m) Candidate formulas for m/z 11,993
Deviation from 11,660 Da
Deviation from 11,993 Da
Insight into the size-controlled synthesis of Au39/40 nanoclusters
The attainment of two new nanoclusters, Au39(SC2H4Ph)29 and Au40(SC2H4Ph)30, demonstrates the feasibility of controlling cluster size via controlled reduction. Herein, we discuss some insights obtained regarding the synthetic processes of Au20, Au24, and Au39/40. These four cluster species belong to a new series, as Au20 and Au24 are formed concurrently, albeit in small amounts, in the synthesis of Au39/40 nanoclusters. However, by controlling the reaction process, one may selectively produce Au20, Au24, or Au39/40. When using Au(III) as the starting material for nanocluster synthesis, two primary stages include (a) the reduction of Au(III) to Au(I) by HSR and (b) the conversion of Au(I) to Au n (SR) m clusters by reduction with NaBH4. Factors that are important for Au n size control with n = 20, 24, 39 and 40 include (a) the stirring speed of the reaction mixture, (b) the addition speed of NaBH4, and (c) the amount of NaBH4 added (vide infra).
Stirring speed and effect on cluster size
The respective conditions for the syntheses of Au 20 , Au 24, and mixed Au 39/40
Amount of NaBH4(concentration, 0.44 M) per mole of gold and speed of addition
Stirring speed at the stage of Au(III) reduction to Au(I) by 3X HSC2H4Ph
Stirring speed at the stage of Au(I) reduction to clusters by NaBH4
Approximately 50 rpm
1 mL (1×), dropwise added over a 30-min period
Approximately 6 hrs
50 to 100 rpm
Approximately 100 rpm
1 mL (1×), dropwise added over a 15-min period
50 to 100 rpm
Approximately 400 rpm
1 mL (1×), dropwise added over a 15-min period
50 to 100 rpm
50 to 400 rpm
5 mL (5×), Dropwise added over a 50-min period
During stage II (the reduction of Au(I) by NaBH4), the stirring speed is even critical for the selective formation of Au20, Au24, or Au39/40. A slow stirring (approximately 50 rpm) during stage II favors the formation of Au20(SC2H4Ph)16, while a slightly higher speed of stirring (approximately 100 rpm) favored the formation of Au24(SC2H4Ph)20, and with further increased speed to approximately 400 rpm, we obtained new clusters of Au39(SC2H4Ph)29 and Au40(SC2H4Ph)30. This controlled reaction process for tuning cluster size is quite interesting. We believe that the aggregated Au(I)SR species are broken into small fragments upon reduction or partial reduction by NaBH4. Different stirring speed for stage II would influence the kinetics of the reduction reaction of Au(I)SR, and the different stirring speeds also give rise to different sheering forces that would break polymeric Au(I)SR into different sized fragments; such different sized fragments seem to subsequently grow into different sized clusters based upon our results.
Regarding the aggregated Au(I)SC2H4Ph species in the solution, structural characterization (e.g., by NMR or X-ray diffraction) is still difficult to carry out as the Au(I) intermediate species is poorly soluble in common solvents. Thermal gravimetric analysis determined the composition of Au(I):SR to be 1:1, but the aggregation degree (e.g., how many repeat units in [Au(I)SC2H4Ph] x ) and what structures [Au(I)SC2H4Ph] x may adopt are all unknown yet. Possible structures of [Au(I)SC2H4Ph] x include linear chains, ring [53–57], or lamellar structures. The characterization of Au(I)SR still need major efforts in future work.
Dropwise addition speed of NaBH4 and effect on cluster size
In addition to the stirring speed during stage II, the addition speed of NaBH4 (aqueous solution) to reduce Au(I) into clusters also plays an important role in controlling the final cluster size. We have tested that, in the synthesis of Au20(SC2H4Ph)16, if the initial drops of NaBH4 solution (0.44 mol/L, 1 mL) were added rapidly, the light yellow solution of Au(I) aggregates would rapidly become brown or deep black, and the product would contain more Au24 and Au39/40 clusters, instead of the predominant Au20 as the case of very slow addition of NaBH4 (over a period of approximately 30 min) (entry 1, Table 2). After optimization, we found that adding NaBH4 (0.44 mol/L, 1 mL, same concentration and amount as the Au20 synthesis) over a period of approximately 15 min gave rise to predominant Au24 (under approximately 100 rpm stirring condition) or Au39/40 (under approximately 400 rpm stirring condition); see entries 2 and 3 of Table 2. Thus, controlled reduction of Au(I) is very important for size selective formation of Au20, Au24, or Au39/40. The selective formation of Au39/40 over Au24 - which differ only in the stirring speed during stage II (i.e., 400 vs 100 rpm, Table 2) - should be due to the different reaction kinetics in the reduction process of Au(I) species into clusters. In a recent synthetical work on gold/phosphine nanocluster synthesis, Pettibone and Hudgens identified a growth-etching cyclic process that occurs around the most stable cluster species to form monodisperse product [4, 58, 59]. This size selective growth mechanism provides important information on the gold/phosphine system. The reaction kinetics of the gold/thiol system, however, still needs significant input in order to unravel the details of the kinetic process. Mass spectrometric monitoring of the reaction intermediates would provide valuable information and should be pursued in future work.
Degree of reduction of Au(I) and effect on cluster size
With respect to the growth of Au20, Au24, and Au39/40 nanoclusters, an interesting question arises naturally: why the ubiquitous Au25 nanocluster is not formed under these conditions (entries 1 to 3, Table 2). The synthesis of Au25(SC2H4Ph)18 is typically done under experimental conditions of fast stirring and rapid reduction of Au(I) with large excess of NaBH4 (approximately 10 equivalents (eq) of NaBH4 per mole of gold). An important difference lies in that the syntheses of Au20, Au24, and Au39/40 clusters all involve 1 eq of NaBH4 per gold, i.e., 1 mL of NaBH4 solution (0.44 mol/L), Table 2. This implies that the degree of reduction of Au(I) might affect the cluster size, and the formation of Au25 clusters might necessitate over reduction of Au(I) with a large excess of NaBH4.
To find out whether the reduction degree of Au(I) species affect the final cluster size, we adopt the same stirring conditions as the synthesis of Au24 and Au39/40 (see entry 4, Table 2), but we add more NaBH4 (e.g., 5 eq, rather than 1 eq for the synthesis of Au24 and Au39/40). The addition speed of NaBH4 solution is kept comparable to the syntheses of Au24 and Au39/40 clusters. Interestingly, dropwise addition of 5 eq of NaBH4 does result in selective formation of Au25, instead of Au24 or Au39/40, evidenced by its characteristic spectroscopic features (see Figure S2 in Additional file 1). Thus, the growth of Au25 nanoclusters does require a rich reductant (NaBH4), as opposed to lean NaBH4 for Au20, Au24, and Au39/40 synthesis. The fast stirring and rapid addition of NaBH4 seem not the key to the synthesis of Au25 nanoclusters.
This work has demonstrated the effectiveness of controlled reduction for synthesizing different sized gold nanoclusters. Specifically, slow stirring and slow addition of 1 eq NaBH4/mol of gold are critical to effect the preferential growth of the series of Au20, Au24, and Au39/40 nanoclusters. In addition to the reaction kinetics, controlling the degree of reduction also leads to different sized nanoclusters, as demonstrated in the selective formation of Au25 over Au n (n = 20, 24, 39/40). Future work is hoped to offer deeper mechanistic understanding of the Au(I) formation and the Au(I) reduction process by NaBH4. Mechanistic understanding of the cluster growth process will eventually permit high yielding synthesis of a full series of monodisperse gold nanoclusters.
XM is presently working at Anhui University (China). He received his PhD in Chemistry from the University of Science and Technology of China in 2007. His research interest focuses on chemosensors. ZL is a graduate student in the Zhu group at Anhui University (China). He obtained his BS in Chemistry (2010) from Anhui University. His research interest is noble metal nanoparticles. MZ presently works at Anhui University (China). He received his PhD in Chemistry from the University of Science and Technology of China in 2000. Before joining the Jin group as a postdoctoral researcher in February 2007, he worked at the University of Science and Technology of China. His research interests focus on photoinduced electron transfer, sensors, and nanomaterials. RJ received his BS in Chemical Physics from the University of Science and Technology of China (Hefei, China) in 1995, MS in Physical Chemistry/Catalysis from Dalian Institute of Chemical Physics (Dalian, China) in 1998, and PhD in Chemistry from Northwestern University (Evanston, Illinois) in 2003. After 3 years of postdoctoral research at the University of Chicago (Illinois), he joined the Chemistry faculty of Carnegie Mellon University in September 2006. His current research interests focus on atomically precise noble metal nanoparticles, evolution of their structure, electronic and optical properties, and utilizing these well-defined nanoparticles in catalysis, optics, sensing, and so forth.
Manzhou Zhu acknowledges financial support by NSFC (20871112, 21072001), 211 Project of Anhui University. Rongchao Jin acknowledges the support by the Air Force Office of Scientific Research under AFOSR award no. FA9550-11-1-9999 (FA9550-11-1-0147).
- Jin R, Zhu Y, Qian H: Quantum-sized gold nanoclusters: bridging the gap between organometallics and nanocrystals. Chem Eur J 2011, 17: 6584–6593. 10.1002/chem.201002390View Article
- Negishi Y, Takasugi Y, Sato S, Yao H, Kimura K, Tsukuda T: Magic-numbered Aunclusters protected by glutathione monolayers (n = 18, 21, 25, 28, 32, 39): isolation and spectroscopic characterization. J Am Chem Soc 2004, 126: 6518–6519. 10.1021/ja0483589View Article
- Wyrwas RB, Alvarez MM, Khoury JT, Price RC, Schaaff TG, Whetten RL: The colours of nanometric gold-optical response functions of selected gold-cluster thiolates. Eur Phys J D 2007, 43: 91–95. 10.1140/epjd/e2007-00117-6View Article
- Hudgens JW, Pettibone JM, Senftle TP, Bratton RN: Reaction mechanism governing formation of 1,3-Bis(diphenylphosphino)propane-protected gold nanoclusters. Inorg Chem 2011, 50: 10178–10189. 10.1021/ic2018506View Article
- Parker JF, Kacprzak KA, Lopez-Acevedo O, Häkkinen H, Murray RW: Experimental and density functional theory analysis of serial introductions of electron-withdrawing ligands into the ligand shell of a thiolate-protected Au25 nanoparticle. J Phys Chem C 2010, 114: 8276–8281. 10.1021/jp101265vView Article
- Maity P, Tsunoyama H, Yamauchi M, Xie S, Tsukuda T: Organogold clusters protected by phenylacetylene. J Am Chem Soc 2011, 133: 20123–20125. 10.1021/ja209236nView Article
- Zhu M, Aikens CM, Hollander FJ, Schatz GC, Jin R: Correlating the crystal structure of a thiol-protected Au25 cluster and optical properties. J Am Chem Soc 2008, 130: 5883–5885. 10.1021/ja801173rView Article
- Ramakrishna G, Varnavski O, Kim J, Lee D, Goodson T: Quantum-sized gold clusters as efficient two-photon absorbers. J Am Chem Soc 2008, 130: 5032–5033. 10.1021/ja800341vView Article
- Liao B, Chen JA, Huang HW, Li XF, He BQ: Gold nanocluster-based light-controlled fluorescence molecular switch. J Mater Chem 2011, 21: 5867–5869. 10.1039/c0jm04146gView Article
- Negishi Y, Tsunoyama H, Suzuki M, Kawamura N, Matsushita MM, Maruyama K, Sugawara T, Yokoyama T, Tsukuda T: X-ray magnetic circular dichroism of size-selected, thiolated gold clusters. J Am Chem Soc 2006, 128: 12034–12035. 10.1021/ja062815zView Article
- Zhu M, Aikens CM, Hendrich MP, Gupta R, Qian H, Schatz GC, Jin R: Reversible switching of magnetism in thiolate-protected Au25 superatoms. J Am Chem Soc 2009, 131: 2490–2492. 10.1021/ja809157fView Article
- Bigioni TP, Whetten RL: Near-infrared luminescence from small gold nanocrystals. J Phys Chem B 2000, 104: 6983–6986.View Article
- Wang G, Huang T, Murray RW, Menard L, Nuzzo RG: Near-IR luminescence of monolayer-protected metal clusters. J Am Chem Soc 2005, 127: 812–813. 10.1021/ja0452471View Article
- Chaudhari K, Xavier PL, Pradeep T: Understanding the evolution of luminescent gold quantum clusters in protein templates. ACS Nano 2011, 5: 8816–8827. 10.1021/nn202901aView Article
- Devadas MS, Kim J, Sinn E, Lee D, Goodson T, Ramakrishna G: Unique ultrafast visible luminescence in monolayer-protected Au25 clusters. J Phys Chem C 2010, 114: 22417–22423. 10.1021/jp107033nView Article
- Wu Z, Jin R: On the ligand’s role in the fluorescence of gold nanoclusters. Nano Lett 2010, 10: 2568–2573. 10.1021/nl101225fView Article
- Gautier C, Bürgi T: Chiral gold nanoparticles. Chemphyschem 2009, 10: 483–492. 10.1002/cphc.200800709View Article
- Zhu M, Qian H, Meng X, Jin S, Wu Z, Jin R: Chiral Au25 nanospheres and nanorods: synthesis and insight into the origin of chirality. Nano Lett 2011, 11: 3963–3969. 10.1021/nl202288jView Article
- Qian H, Zhu M, Gayathri C, Gil RR, Jin R: Chirality in gold nanoclusters probed by NMR spectroscopy. ACS Nano 2011, 5: 8935–8942. 10.1021/nn203113jView Article
- Garzón IL, Rovira C, Michaelian K, Beltrán MR, Junquera J, Ordejón P, Artacho E, Sánchez-Portal D, Soler JM: Do thiols merely passivate gold nanoclusters? Phys Rev Lett 2000, 85: 5250–5251. 10.1103/PhysRevLett.85.5250View Article
- Zhu MZ, Eckenhoff WT, Pintauer T, Jin R: Conversion of anionic [Au25(SCH2CH2Ph)18]− cluster to charge neutral cluster via air oxidation. J Phys Chem C 2008, 112: 14221–14224. 10.1021/jp805786pView Article
- Beasley CA, Sardar R, Barnes NM, Murray RW: Persistent multilayer electrode adsorption of polycationic Au nanoparticles. J Phys Chem C 2010, 114: 18384–18389. 10.1021/jp1065665View Article
- Negishi Y, Chaki NK, Shichibu Y, Whetten RL, Tsukuda T: Origin of magic stability of thiolated gold clusters: a case study on Au25(SC6H13)18. J Am Chem Soc 2007, 129: 11322–11323. 10.1021/ja073580+View Article
- Zhu MZ, Chan GR, Qian HF, Jin R: Unexpected reactivity of Au25(SCH2CH2Ph)18 nanoclusters with salts. Nanoscale 2011, 3: 1703–1707. 10.1039/c0nr00878hView Article
- Choi JP, Fields-Zinna CA, Stiles RL, Balasubramanian R, Douglas AD, Crowe MC, Murray RW: Reactivity of [Au25(SCH2CH2Ph)18]1− nanoparticles with metal ions. J Phys Chem C 2010, 114: 15890–15896. 10.1021/jp9101114View Article
- Venzo A, Antonello S, Gascón JA, Guryanov I, Leapman RD, Perera NV, Sousa A, Zamuner M, Zanella A, Maran F: Effect of the charge state (z = −1, 0, +1) on the nuclear magnetic resonance of monodisperse Au25[S(CH2)2Ph]18z clusters. Anal Chem 2011, 83: 6355–6362. 10.1021/ac2012653View Article
- Liu Z, Zhu MZ, Meng XM, Xu GY, Jin R: Electron transfer between [Au25(SC2H4Ph)18]−TOA+ and oxoammonium cations. J Phys Chem Lett 2011, 2: 2104–2109. 10.1021/jz200925hView Article
- Tsunoyama H, Sakurai H, Negishi Y, Tsukuda T: Size-specific catalytic activity of polymer-stabilized gold nanoclusters for aerobic alcohol oxidation in water. J Am Chem Soc 2005, 127: 9374–9375. 10.1021/ja052161eView Article
- Liu Y, Tsunoyama H, Akita T, Tsukuda T: Efficient and selective epoxidation of styrene with TBHP catalyzed by Au25 clusters on hydroxyapatite. Chem Commun 2010, 46: 550–552. 10.1039/b921082bView Article
- Zhu Y, Qian H, Zhu M, Jin R: Thiolate-protected Aun nanoclusters as catalysts for selective oxidation and hydrogenation processes. Adv Mater 2010, 22: 1915–1920. 10.1002/adma.200903934View Article
- Zhu Y, Wu Z, Gayathri GC, Qian H, Gil RR, Jin R: Exploring stereoselectivity of Au25 nanoparticle catalyst for hydrogenation of cyclic ketone. J Catal 2010, 271: 155–160. 10.1016/j.jcat.2010.02.027View Article
- Zhu Y, Qian H, Jin R: Catalysis opportunities of atomically precise gold nanoclusters. J Mater Chem 2011, 21: 6793–6799. 10.1039/c1jm10082cView Article
- Lin CAJ, Yang TY, Lee CH, Huang SH, Sperling RA, Zanella M, Li JK, Shen JL, Wang HH, Yeh HI, Parak WJ, Chang WH: Synthesis, characterization, and bioconjugation of fluorescent gold nanoclusters toward biological labeling applications. ACS Nano 2009, 3: 395–401. 10.1021/nn800632jView Article
- Jin R, Qian H, Wu Z, Zhu Y, Zhu M, Mohanty A, Garg N: Size focusing: a methodology for synthesizing atomically precise gold nanoclusters. J Phys Chem Lett 2010, 1: 2903–2910. 10.1021/jz100944kView Article
- Shichibu Y, Negishi Y, Tsunoyama H, Kanehara M, Teranishi T, Tsukuda T: Extremely high stability of glutathionate-protected Au25 clusters against core etching. Small 2007, 3: 835–389. 10.1002/smll.200600611View Article
- Parker JF, Fields-Zinna CA, Murray RW: The Story of a monodisperse gold nanoparticle: Au25L18. Acc Chem Res 2010, 43: 1289–1296. 10.1021/ar100048cView Article
- Negishi Y, Kurashige W, Niihori Y, Iwasa T, Nobusada K: Isolation, structure, and stability of a dodecanethiolate-protected Pd1Au24 cluster. Phys Chem Chem Phys 2010, 12: 6219–6225.View Article
- Jiang DE, Dai S: From superatomic Au25(SR)18− to superatomic M@Au24(SR)18q core-shell clusters. Inorg Chem 2009, 48: 2720–2722. 10.1021/ic8024588View Article
- Dharmaratne AC, Krick T, Dass A: Nanocluster size evolution studied by mass spectrometry in room temperature Au25(SR)18 synthesis. J Am Chem Soc 2009, 131: 13604–13605. 10.1021/ja906087aView Article
- Sfeir MY, Qian H, Nobusada K, Jin R: Ultrafast relaxation dynamics of rod-shaped 25-atom gold nanoclusters. J Phys Chem C 2011, 115: 6200–6207. 10.1021/jp110703eView Article
- MacDonald MA, Chevrier DM, Zhang P, Qian H, Jin R: The structure and bonding of Au25(SR)18 nanoclusters from EXAFS: the interplay of metallic and molecular behavior. J Phys Chem C 2011, 115: 15282–15287. 10.1021/jp204922mView Article
- Nimmala PR, Dass A: Au36(SPh)23 nanomolecules. J Am Chem Soc 2011, 133: 9175–9177. 10.1021/ja201685fView Article
- Qian H, Eckenhoff WT, Zhu Y, Pintauer T, Jin R: Total structure determination of thiolate-protected Au38 nanoparticles. J Am Chem Soc 2010, 132: 8280–8281. 10.1021/ja103592zView Article
- Qian H, Zhu Y, Jin R: Size-focusing synthesis, optical and electrochemical properties of monodisperse Au38(SC2H4Ph)24 nanoclusters. ACS Nano 2009, 3: 3795–3803. 10.1021/nn901137hView Article
- Hulkko E, Lopez-Acevedo O, Koivisto J, Levi-Kalisman Y, Kornberg RD, Pettersson M, Hakkinen H: Electronic and vibrational signatures of the Au102(p-MBA)44 cluster. J Am Chem Soc 2011, 133: 3752–3755. 10.1021/ja111077eView Article
- Qian H, Jin R: Controlling nanoparticles with atomic precision: the case of Au144(SCH2CH2Ph)60. Nano Lett 2009, 9: 4083–4087. 10.1021/nl902300yView Article
- Chaki NK, Negishi Y, Tsunoyama H, Shichibu Y, Tsukuda T: Ubiquitous 8 and 29 kDa gold: alkanethiolate cluster compounds: mass-spectrometric determination of molecular formulas and structural implications. J Am Chem Soc 2008, 130: 8608–8610. 10.1021/ja8005379View Article
- Zhu M, Lanni E, Garg N, Bier ME, Jin R: Kinetically controlled, high-yield synthesis of Au25 clusters. J Am Chem Soc 2008, 130: 1138–1139. 10.1021/ja0782448View Article
- Wu Z, Suhan J, Jin R: One-pot synthesis of atomically monodisperse, thiol-functionalized Au25 nanoclusters. J Mater Chem 2009, 19: 622–626. 10.1039/b815983aView Article
- Zhu MZ, Qian HF, Jin R: Thiolate-protected Au24(SC2H4Ph)20 nanoclusters: superatoms or not? J Phys Chem Lett 2010, 1: 1003–1007. 10.1021/jz100133nView Article
- Zhu M, Qian H, Jin R: Thiolate-protected Au20 clusters with a large energy gap of 2.1 eV. J Am Chem Soc 2009, 131: 7220–7221. 10.1021/ja902208hView Article
- Wu Z, MacDonald MA, Chen J, Zhang P, Jin R: Kinetic control and thermodynamic selection in the synthesis of atomically precise gold nanoclusters. J Am Chem Soc 2011, 133: 9670–9673. 10.1021/ja2028102View Article
- Gronbeck H, Walter M, Hakkinen H: Theoretical characterization of cyclic thiolated gold clusters. J Am Chem Soc 2006, 128: 10268–10275. 10.1021/ja062584wView Article
- Shao N, Pei Y, Gao Y, Zeng XC: Onset of double helical structure in small-sized homoleptic gold thiolate clusters. J Phys Chem A 2009, 113: 629–632. 10.1021/jp810447kView Article
- Kacprzak KA, Lopez-Acevedo O, Häkkinen H, Grönbeck H: Theoretical characterization of cyclic thiolated copper, silver, and gold clusters. J Phys Chem C 2010, 114: 13571–13576. 10.1021/jp1045436View Article
- Yu SY, Zhang ZX, Cheng EC, Li YZ, Yam VW, Huang HP, Zhang R: A chiral luminescent Au16 ring self-assembled from achiral components. J Am Chem Soc 2005, 127: 17994–17995. 10.1021/ja0565727View Article
- Simpson CA, Farrow CL, Tian P, Billinge SJL, Huffman BJ, Harkness KM, Cliffel DE: Tiopronin gold nanoparticle precursor forms aurophilic ring tetramer. Inorg Chem 2010, 49: 10858–10866. 10.1021/ic101146eView Article
- Pettibone JM, Hudgens JW: Synthetic approach for tunable, size-selective formation of monodisperse, diphosphine-protected gold nanoclusters. J Phys Chem Lett 2010, 1: 2536–2540. 10.1021/jz1009339View Article
- Pettibone JM, Hudgens JW: Gold cluster formation with phosphine ligands: etching as a size-selective synthetic pathway for small clusters? ACS Nano 2011, 5: 2989–3002. 10.1021/nn200053bView Article
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