Letter pubs.acs.org/JPCL
Electrochemical Characterization of Water-Soluble Au25 Nanoclusters Enabled by Phase-Transfer Reaction Kyuju Kwak and Dongil Lee* Department of Chemistry, Yonsei University, Seoul 120-749, Korea S Supporting Information *
ABSTRACT: We report the synthesis and electrochemical characterization of a new watersoluble Au25 cluster protected with (3-mercaptopropyl)sulfonate. The presence of sulfonate terminal groups on the surface of the cluster enabled facile phase transfer of the water-soluble cluster to organic phase by ion-pairing with hydrophobic counterions. The phase-transferred form of the water-soluble Au25 cluster was found to retain its integrity and allowed investigation of its electrochemical properties in organic media. The voltammetric investigation of the phase-transferred Au25 cluster in mixtures of CH2Cl2 and toluene has revealed that the cluster exhibits the characteristic Au25 peak pattern, but the electrochemical HOMO−LUMO energy gap of the cluster varies from 1.39 to 1.66 V depending on the solvent polarity. The origin of the solvent dependence is explained by the electrostatic field effect of the sulfonate anion on the redox potentials of the Au25 cluster. SECTION: Physical Processes in Nanomaterials and Nanostructures
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energy gap is considered to be a characteristic property of a gold cluster. However, the electrochemical method has not been widely utilized for water-soluble gold clusters. This is because of the narrow potential window allowed in aqueous media and the high dielectric constant of the water-swollen ligand shell that would make the voltammetric peak less resolved.8 In fact, our recent voltammetric investigations of glutathione-protected Au25 clusters in water have revealed that only a broad current peak was observed for the oxidation of Au25− clusters in water and the first and second oxidation peaks were not separated at all.7,28 Whether the water-soluble Au25 clusters adopt the same electronic structure and exhibit the same HOMO−LUMO energy gap still remains unaddressed. The precise estimation of the energy levels and the redox potentials of the nanoclusters is of crucial importance in their use in electrochemical sensing, photocatalysis, and photovoltaic applications.3−7,12 Herein we report the synthesis of a new water-soluble Au25 cluster protected with (3-mercaptopropyl)sulfonate (MPSAu25). The presence of sulfonate terminal groups on the surface of the cluster enables phase transfer of the cluster into organic phase by ion-pairing with hydrophobic counterions. The phase-transferred Au25 cluster (PT-Au25) is stable in organic solvent and allows subsequent investigation of its electrochemical properties. This work reports the first quantitative results demonstrating that the electrochemical
ecent advances in the synthesis of atomically precise, thiolate-protected gold clusters containing ∼10 to ∼100 core atoms have opened the avenue to utilize them in several technological applications.1−7 These clusters exhibit unique electrochemical, optical, and catalytic properties that differ substantially from the corresponding atoms and bulk materials.8−16 A number of analytical techniques have been employed to investigate the chemical and physical properties of these clusters. For example, crystallographic studies of Au25(SR)18 (SR = thiolate) revealed that the structure consists of Au13 surrounded by six Au2(SR)3 oligomeric units.17,18 Comparative optical studies of the Au25 clusters using experimental and computational methods further revealed the presence of a highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) energy gap and discrete electronic states.18−21 Voltammetry has proven to be a powerful means of investigating the electrochemical properties and HOMO− LUMO energy levels of gold clusters.8 The electrochemical HOMO−LUMO gap is typically determined by the difference between the first oxidation and reduction potentials of a cluster. The electrochemical HOMO−LUMO gaps of Au25(SR)18, Au38(SR)24, and Au75(SR)40 clusters have been voltammetrically determined to be ∼1.65, 1.2, and 0.74 V, respectively.22−25 Recently, we have found that Au 25 clusters adopting biicosahedral structure exhibit a different HOMO−LUMO gap (1.54 V).26 Whereas the HOMO−LUMO gaps of the gold clusters are critically dependent on the core size (core atom number) and core structure, the gaps have been found to be insensitive to the change in the ligand shell surrounding the gold core.22,27 Therefore, the electrochemical HOMO−LUMO © 2012 American Chemical Society
Received: July 30, 2012 Accepted: August 20, 2012 Published: August 20, 2012 2476
dx.doi.org/10.1021/jz301059w | J. Phys. Chem. Lett. 2012, 3, 2476−2481
The Journal of Physical Chemistry Letters
Letter
would make the MPS-Au25 cluster a useful building block for many functional nanoassemblies.30,31 The anionic MPS ligands are readily utilized to convert the water-soluble MPS-Au25 cluster to organic solvent soluble one. The conversion was performed by transferring the MPS-Au25 clusters into organic phase in the presence of a phase-transfer reagent, tetraoctylammonium bromide (TOABr).32 The electrostatic attraction between the hydrophilic sulfonate anion of the MPS ligand on the cluster surface in aqueous phase and the hydrophobic TOA cation in toluene phase enables the MPS-Au25 clusters to transfer to toluene phase. The terminal group of the MPS ligand is sulfonate anion in neutral pH, and thus the phase transfer occurs readily by shaking the two immiscible solvents (water and toluene) without requiring pH control. (See the Supporting Information for experimental details.) As can be seen in Figure 2A, the phase transfer occurred readily (within 30 s), and the phase-transferred clusters (PT-Au25) were found to be very stable in toluene phase. The PT-Au25 clusters were subsequently purified, dried, and stored in a freezer prior to use. Figure 1B shows the negative-mode ESI mass spectrum of the PT-Au25. There are peaks observed with m/z 1500−3000 that represent PT-Au25 ions with overall charge of 7−, 6−, and 5−, which are determined by the total number of counterions (TOA+ and Na+) paired with the sulfonate anions. As can be seen in Figure 1B, the cluster ions generated still contain Na+ counterions (
