Molecular-scale ligand effects in small gold-thiolate nanoclusters

Transcription

1 Molecular-scale ligand effects in small gold-thiolate nanoclusters Daniel M. Chevrier, 1, Lluís Raich, 2, Carme Rovira, 2,3 Anindita Das, 4 Zhentao Luo, 5 Qiaofeng Yao, 5 Amares Chatt, 1 Jianping Xie, 5 Rongchao Jin, 4 Jaakko Akola, 6,7 Peng Zhang 1 * Addresses: 1 Department of Chemistry, Dalhousie University, Halifax, NS, B3H 4R2, Canada 2 Departament de Química Inorgànica i Orgànica (Secció Química Orgànica) & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1, Barcelona, Spain 3 Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys, 23, Barcelona, Spain 4 Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA 5 Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore, Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway 7 Laboratory of Physics, Tampere University of Technology, P.O. Box 692, FI Tampere, Finland S1

2 Materials and Methods Au 18 (SC 6 H 11 ) 14 clusters were synthesized according to a one-pot protocol by Das et al. 1 See the primary reference for further details on the characterization of Au 18 (SC 6 H 11 ) 14 clusters. Au 18 (SG) 14 clusters were synthesized as a mixture of clusters and then isolated using Native polyacrylamide gel-electrophoresis (PAGE). 2 In a typical synthesis, 1 ml of 10X Phosphate buffered saline (PBS) (from Bio-Rad) was added to 8.33 ml water, and aqueous solutions of GSH (30 µmol, 0.30 ml) and HAuCl 4 (10 µmol, 0.25 ml) were added consecutively into the PBS solution, followed by vigorous stirring at 25 C for 30 min. NaBH 4 (2.5 µmol in ml aqueous solution of 0.2 mm NaOH) was then added to the solution in a dropwise manner. The reaction mixture was then incubated with vigorous stirring for 30 min before stored at 4 C. The isolation of Au 18 (SG) 14 from the resultant mixture by native PAGE experiments were carried out according to a previous reported protocol. 3 Au L 3 -edge XAFS data was collected for all Au 18 (SR) 14 samples from the CLS@APS beamline (Sector 20-BM) at the APS. Solid- and solution-phase measurements were conducted at 300 K and atmospheric pressure in transmission mode. The Au 18 (SC 6 H 11 ) 14 solid-phase sample was prepared by dissolving clusters in a small amount of toluene (100 ul) and drop-casting them onto Kapton film. The clusters were left to dry under N 2 for at least one hour and then folded until an absorption of at least 0.2 was achieved. Au 18 (SG) 14 clusters (from freeze-dried sample) were ground lightly using a mortar and pestle. The powder was dispersed onto Kapton tape, which was then folded to achieve absorption of at least 0.2. Solutionphase samples were prepared for both clusters using a ~10 mm solution and utilizing a Teflon liquid cell with Kapton windows (designed and constructed at the CLS@APS beamline). The amplitude reduction factor (S 0 2 ) was fixed at 0.90 for Au L 3 -edge EXAFS fitting, as determined by fitting the Au-Au scattering of a Au foil reference with a fixed Au-Au CN of 12. For multi-shell EXAFS fitting, CNs were fixed according to the Au 18 (SR) 14 crystal structure. ΔE 0 values provide an indication of the phase difference between simulated and experimental waves for a particular scattering type. ΔE 0 parameters were correlated for each fit and range between -2 ev and 1 ev. A k-range of 3 to 12 Å 1 was used for the Fourier transformation of all k-space data to R-space. In the R-space range of 1.5 to 3.5 Å, four scattering shells were employed to account for core, surface and metal-ligand bonding environments in the Au 18 (SR) 14 NC. These were assigned with the assisted inspection of Au-Au bonding distribution from crystal structure information (Figure S1). There is a small amount of separation between the bonding distributions of Au- Au core and Au-Au surf shells; nevertheless, these shells are both related to Au-Au bonding within the Au 9 core. Wavelet transformed (WT)-EXAFS plots 4 were generated using Cauchy-type wavelets (order of 100) and a k-range of 3 to 12 Å 1. To give insights into the structural effect of the ligands and solvent on Au 18 (SR) 14 clusters, molecular dynamics (MD) simulations were performed at atomistic level. Four systems in total were considered, including two ligands (Au 18 (SC 6 H 11 ) 14 and Au 18 (SG) 14 ) in two environments (gas-phase and solvent, toluene for Au 18 (SC 6 H 11 ) 14 and water for Au 18 (SG) 14 ). The Au 18 (SC 6 H 11 ) 14 system was directly obtained from the X-ray structure. 1 The structure of Au 18 (SG) 14 was manually built taking the X-ray structure of Au 18 (SC 6 H 11 ) 14 and substituting the cyclohexane ligands by glutathione, one by one, conserving the orientation of the original S-C bonds. A total number of 5166 toluene molecules and 5949 water molecules were added to the Au 18 (SC 6 H 11 ) 14 and Au 18 (SG) 14 solvated systems, respectively. Glutathione was considered neutral in the gas-phase and in its zwitterionic form in solution. Given the charged nature of glutathione in solution, 14 sodium counterions were included in the system to render a neutral complex. S2

3 MD simulations at 300 K were performed for the solvated systems to equilibrate their structures. Additionally, for the Au 18 (SG) 14 system in the gas-phase, we also performed MD simulations for 20 ns to sample configurations of SG ligands. These simulations were considered converged according to the root mean square deviation (RMSD) of all the atoms of each systems. We used the AMBER software 5 together with the FF99SB, 6 GAFF 7 and TIP3P 8 force-fields. Due to the lack of suitable force-field parameters, gold atoms were constrained during the classical simulations. A representative snapshot of each equilibrated system was taken for a subsequent equilibration within a Quantum mechanics/molecular mechanics scheme (QM/MM) 9 using the Car-Parrinello method, 10 which is based on density functional theory (DFT). The quantum part was described with the Perdew-Burke-Ernzerhoff (PBE) exchange-correlation functional 11 which has been used previously in other studies on AuNCs. Gold atoms were represented by a Goedecker type pseudopotential (5d 10 6s 1 valence configuration), 12,13 carbon, sulphur and oxygen atoms by a Troullier-Martins type 14 and hydrogen atoms by a Car-von Barth type. 15 The interface bonds that fall between the QM and the MM parts were modelled by the use of monovalent pseudopotentials. 16 QM/MM simulations were performed at 300 K to relax the structures and see the evolution of different parameters of interest during the simulation. Afterwards, geometry optimizations were performed to the last snapshots, using a variety of annealing factors that reduce the kinetic energy of the ions with the aim to reach a minimum. The optimizations were considered converged when the largest nuclear gradient was lower than 5.0 x 10-4 atomic force units. Charge and orbital analyses were performed on these optimized structures. Absorption and photoluminescence spectra were measured using CaryBio 100 UV-Vis and PerkinElmer LS-55 fluorescence spectrometers, respectively. Solutions were measured in a standard quartz cuvette (path length of 1 cm) at room temperature. The concentration of each Au 18 (SR) 14 cluster was adjusted to yield the same peak absorption at the excitation wavelength used for photoluminescence measurements (365 nm). S3

4 Table S1. Au L 3 -edge EXAFS multi-shell fitting results for Au 18 (SC 6 H 11 ) 14 in solid-phase at 90 K. ΔE 0 values provide an indication of the phase difference between simulated and experimental waves for each scattering type. *Average bond distance for each shell from the Au 18 (SC 6 H 11 ) 14 crystal structure 24 Shell CN R (Å) σ 2 (Å 2 ) ΔE 0 (ev) Au-S 1.56 Au-Au core 1.33 Au-Au surf 1.78 Au-Au auro (7) (2.326*) 2.69(3) (2.743*) 3.07(4) (2.940*) 3.29(5) (3.330*) (2) -1(2) 0.009(3) -1(2) 0.008(3) -1(2) 0.010(3) -1(2) S4

5 Figure S1. Distribution of Au-Au bond lengths from the crystal structure of Au 18 (SC 6 H 11 ) 14. S5

6 Figure S2. Au L 3 -edge k-space spectra of Au 18 (SC 6 H 11 ) 14 and Au 18 (SG) 14 NCs in solid-phase and solution-phase. S6

7 Figure S3. Au L 3 -edge FT-EXAFS spectra of Au 18 (SC 6 H 11 ) 14 and Au 18 (SG) 14 in solid-phase (a and b, respectively) and in solution-phase (c and d, respectively) (black line) with simulated fit (red line). S7

8 Figure S4. Au L 3 -edge WT-EXAFS plots of (a) Au 18 (SC 6 H 11 ) 14 and (b) Au 18 (SG) 14 in solid-phase. S8

9 Ring edge Ring Ring Figure S5. Two types of CH- interactions interactions between the cyclohexane ligands and toluene molecules: ring-edge and ring-ring. S9

10 Figure S6. Density of states of the 5d-band of Au 18 (SC 6 H 11 ) 14 (left) and Au 18 (SG) 14 (right) clusters in the gas phase and solvents (optimized snapshots). The location of the 5d-band of Au in the density of states (DOS) displays a visible change depending on the ligand type (Figure S5 shows the projection of the total density of states (P-DOS) on the Au-5d components). We observe that the Au 18 (SC 6 H 11 ) 14 system (with or without toluene) exhibits a shift to lower binding energies. The corresponding d-band centers are at ev and ev for the gas phase and optimized snapshot in toluene, respectively. On the other hand, the Au 18 (SG) 14 systems have d-band centers at ev and ev for the gas-phase and water-phase, respectively. E 0 shifts at the Au L 3 -edge (relative to Au foil reference) for Au 18 (SG) 14 clusters (+0.8 ev) are also higher in energy compared to Au 18 (SC 6 H 11 ) 14 clusters (+0.7 ev), supporting the slightly higher binding energy of electrons in Au 18 (SG) S10

11 References (1) Das, A.; Liu, C.; Byun, H. Y.; Nobusada, K.; Zhao, S.; Rosi, N.; Jin, R. Angew. Chem. 2015, 54, (2) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, (3) Luo, Z.; Yuan, X.; Yu, Y.; Zhang, Q.; Leong, D. T.; Lee, J. Y.; Xie, J. J. Am. Chem. Soc. 2012, 134, (4) Funke, H.; Scheinost, A. C.; Chukalina, M. Phys. Rev. B 2005, 71, (5) Case, D. A.; Darden, T. A.; Cheatham, T. E.; Simmerling, C.; Wang, J.; Duke, R.; Luo, R.; Crowley, M. F.; Walker, R.; Zhang, W.; et al. AMBER 11. University of California, San Francisco (6) Hornak, V.; Abel, R.; Okur, A.; Strockbine, B.; Roitberg, A.; Simmerling, C. Proteins Struct. Funct. Bioinforma. 2006, 65, (7) Wang, J. M.; Wolf, R. M.; Caldwell, J. W.; Kollman, P. A.; Case, D. A. J. Comput. Chem. 2004, 25, (8) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L. J. Chem. Phys. 1983, 79, (9) Laio, A.; VandeVondele, J.; Rothlisberger, U. J. Chem. Phys. 2002, 116, (10) Car, R.; Parrinello, M. Phys. Rev. Lett. 1985, 55, (11) Perdew, J.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, (12) Hartwigsen, C.; Goedecker, S.; Hutter, J. Phys. Rev. B 1998, 58, (13) Goedecker, S.; Teter, M.; Hutter, J. Phys. Rev. B 1996, 54, (14) Troullier, N.; Martins, J. L. Phys. Rev. B 1991, 43, (15) Krüger, D.; Fuchs, H.; Rousseau, R.; Marx, D.; Parrinello, M. J. Chem. Phys. 2001, 115, (16) Zhang, Y. K.; Lee, T. S.; Yang, W. T. J. Chem. Phys. 1999, 110, (17) Chen, S.; Wang, S.; Zhong, J.; Song, Y.; Zhang, J.; Sheng, H.; Pei, Y. Angew. Chemie Int. Ed. 2015, 54, (18) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Grönbeck, H.; Häkkinen, H. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, (19) Akola, J.; Walter, M.; Whetten, R. L.; Häkkinen, H.; Grönbeck, H. J. Am. Chem. Soc. 2008, 130, (20) Goh, J.-Q.; Malola, S.; Häkkinen, H.; Akola, J. J. Phys. Chem. C 2013, 117, S11