Supporting Information for: Electrochemical Characterization of Water-Soluble Au 25

Transcription

1 Supporting Information for: Electrochemical Characterization of Water-Soluble Au 25 Nanoclusters Enabled by Phase Transfer Reaction Kyuju Kwak and Dongil Lee * Department of Chemistry, Yonsei University, Seoul , Korea Chemicals. Hydrogen tetrachloroaurate trihydrate (HAuCl 4 3H 2 O, reagent grade), sodium borohydride (NaBH 4, 99%), sodium (3-mercaptopropyl)sulfonate (MPS, 90%), tetraoctylammonium bromide (TOABr, 98%), and tetrabutylammonim hexafluorophosphate (Bu 4 NPF 6, >99%) were purchased from Aldrich. Extrapure grade acetonitrile, toluene and dichloromethane were used. Water was purified using a Millipore Milli-Q system (18.2 MΩ cm). All the chemicals were used as received without further purification. Synthesis of MPS-Au 25 Clusters. Synthesis of MPS-Au 25 was carried out at room temperature following the Brust-Schiffrin method with some modifications. S1 In a typical reaction, g of sodium (3-mercaptopropyl)sulfonate (MPS, 3 mmol) was dissolved and stirred in 80 ml of water g of HAuCl 4 3H 2 O (1 mmol) in 40 ml of acetonitrile was slowly added to the above solution and stirring was continued for ~30 min until the whitish yellow solution turned colorless. To the above solution, g of NaBH 4 (3 mmol) in 10 ml of water was added. The solution turned brown 1

2 immediately, indicative of particle formation. After stirring of 1 h, g of NaBH 4 (2 mmol) in 2 ml of water was added additionally with rapid stirring. After additional stirring of 30 min, the solution was rotary evaporated to near dryness and washed thoroughly with copious amounts of acetonitrile. To isolate Au 25 clusters from the product, recrystallization was carried out using water acetonitrile mixtures. The product was dissolved in 10 ml of water and then 2 ml of acetonitrile was added to induce precipitation of larger particles. The precipitate was separated by centrifugation and more acetonitrile was added to the supernatant to induce additional precipitation. The precipitate was separated by centrifugation again. This recrystallization step was repeated until a clear supernatant was obtained. Precipitates containing monodisperse Au 25 clusters (MPS-Au 25 ) were obtained in the middle fractions of recrystallization and identified by using UV-visible absorption spectrometry and electrospray ionization (ESI) mass spectrometry, providing a total yield of ~150 mg. Preparation of Phase-Transferred Au 25 Clusters (PT-Au 25 ). Water-soluble MPS-Au 25 clusters were transferred to organic phase using an immiscible biphasic method. S2 In a typical procedure, 10 mg of MPS-Au 25 clusters were dissolved in 20 ml of water in a 30 ml scintillation vial. 3 ml of 10 mm TOABr in toluene was added to make an immiscible layer above the water phase. The mixture was then vigorously hand-shaken. Phase transfer of water-soluble MPS-Au 25 to toluene phase occurred within 30 s, as noticed by the color changes of the water and toluene phases. After the phase transfer reaction, the aqueous phase was replaced with fresh water and then the mixture was shaken again to remove watersoluble impurities. This step was repeated several times. The toluene solution was then rotary evaporated to near dryness and washed thoroughly with a mixture of ethanol and water. The purified product (PT-Au 25 ) was fully characterized using ESI mass spectrometry, UV-visible absorption spectrometry, and transmission electron microscopy (TEM). PT-Au 25 clusters are soluble in organic solvents such as toluene and CH 2 Cl 2 just as other organic solvent soluble Au 25 clusters and their voltammetric properties were investigated in CH 2 Cl 2 or mixtures of CH 2 Cl 2 and toluene. 2

3 Measurements. Negative-mode ESI mass spectra were acquired on a Micromass Q-TOF Acquity UPLC-Mass system (Waters). Cluster solutions (1 mg/ml) of MPS-Au 25 in 50 % (v/v) water/acetonitrile or PT-Au 25 in acetonitrile were infused at a flow rate of 2.0 L/min. The ion transfer time was set to 120 μs, and scans were averaged in the data presented. All spectra were acquired in the reflectron mode. TEM images of PT-Au 25 clusters were collected on a Jeol JEM 2011 microscope. TEM samples were prepared by dropcasting a CH 2 Cl 2 solution of clusters (1 mg/ml) on a 400 mesh Formvar/carbon-coated copper grid (01814-F, Ted Pella) and drying for 2 h at room temperature before imaging. UV-visible absorbance spectra of clusters dissolved in CH 2 Cl 2 were acquired using a Shimadzu UV-visible (UV-3600) spectrometer. Square wave voltammetry (SWV) was performed at room temperature with an electrochemical workstation (Model 660B, CH Instruments) using a Pt working electrode (diameter 0.4 mm), a Pt wire counter electrode, and Ag/Ag + reference electrode (-0.23 V vs. ferrocene couple) in CH 2 Cl 2 containing 0.1 M Bu 4 NPF 6. All potentials in this article are reported with respect to the Ag/Ag + reference electrode. SWV scan was conducted with a pulse amplitude of 50 mv and a potential increment of 4 mv at a frequency of 30 Hz. Cluster solutions (3 mg/ml) were degassed and blanketed with high-purity Ar atmosphere during measurement. Voltammetry was also performed in CH 2 Cl 2 /toluene mixtures containing 0.1 M Bu 4 NPF 6. The final CH 2 Cl 2 /toluene composition after Ar purging was quantified by NMR analysis. S1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R., Synthesis of thiol-derivatized gold nanoparticles in a two-phase liquid-liquid system. J. Chem. Soc., Chem. Commun. 1994, S2) Habeeb Muhammed, M. A.; Pradeep, T., Aqueous to Organic Phase Transfer of Au 25 Clusters. J. Clust. Sci. 2009, 20,

4 Table S1: Comparison of experimental and calculated values of peaks observed in Figure 1A. Number of MPS Na Cluster Charge Calculated Experimenta Relative error (%) Table S2: Comparison of experimental and calculated values of peaks observed in Figure 1B. Na Number of TOA Cluster Charge Calculated Experimenta Relative error (%)

5 Absorbance Wavelength / nm Figure S1: UV-visible absorption spectra of the PT-Au 25 in CH 2 Cl 2 (red) and in toluene (blue). The absorbance was normalized at 230 nm for comparison. 5