Benutzer:CoolEsza/Thiolate protected goldclusters
Thiolate protected gold clusters are special representatives of ligand-protected metal clusters, that play a special role in cluster physics thanks to unique stability and electronic properties. Some of these clusters can be synthesized, comparably simply even monodispersely, and in aqueous solution. These clusters as stable compounds[1]
They belong to a size regime of up to some hundreds of gold atoms. The special properties are not observed for clusters bigger than this size, which can be regarded as passivated gold-nanoparticles rather than ligand protected clusters.
Synthesis[Bearbeiten | Quelltext bearbeiten]
Wet-chemical synthesis[Bearbeiten | Quelltext bearbeiten]
The wet-chemical synthesis of thiolate-protected goldclusters is achieved via the reduction of gold(III)-salt solutions using a mild reducing agent in presence of thiol compounds. The initial species of the chemical transformation are the gold ions. Hence, this type of synthesis can be regarded as a "bottom-up" approach. The reduction process depends on equilibria between different oxidation states of the gold as well as the oxidized or reduced forms of the reducing agent or the thiols. Gold(I)-thiolate polymers have been identified as important in the initial steps of the reaction.[2] Several synthesis recipes exist, that are similiar to the Brust-synthesis of colloidal gold, although the mechanism is not yet fully understood. As product, a mixture of dissolved, thiolate-protected gold clusters of different sizes is obtained. This mixture can be separated by gel-electrophoresis (PAGE).[3] If the synthesis is performed in a kinetically controlled manner, particularly stable representatives can be obtained monodispersely, avoiding further separation steps.[4][5]
Template-mediated synthesis[Bearbeiten | Quelltext bearbeiten]
Rather than starting from "naked" gold-ions in solution, Templates can serve for the directed synthesis. The high affinity of the gold-ions to electronegative and (partially) charged atoms of functional groups yields potential seeds for the cluster formation. The interface between the metal and the template can act as a stabilizer and steer the final size of the cluster. Potential templates are e.g. dendrimers, oligonucleotides, proteins, polyelektrolytes und polymers.
Etching-procedure[Bearbeiten | Quelltext bearbeiten]
The protected gold clusters' „top-down“-synthesis can be achieved by the so-called „etching“ of bigger metallic Nanoparticles with redox-active, thiolcontaining biomolecules.[6] In this process, gold atoms dissolve from the particles' surface as gold-thiolate complexes until the residual species is particularly stable. At this point the dissolution reaction stops. Also with other, non thiol-based ligands, this type of synthesis is possible.
Properties[Bearbeiten | Quelltext bearbeiten]
Electronic und optical Properties[Bearbeiten | Quelltext bearbeiten]
The electronic structure of the thiolate-protected gold clusters is characterized by strongly pronounced quantum-effects. These result in discrete electronic states and a nonzero homo-lumo gap. This discretization was first evidenced by the discrepancy between their optical absorption and classical Mie-Theory.[7] Discrete optical transitions and the occurrence of photoluminescence in these species reflect a molecular-like rather than metallic behaviour. By this difference the thiolate-protected clusters sharply distinguish from the gold-Nanoparticles, whose optical characterisics are driven by Plasmon resonance. Some of the thiolate-protected clusters' properties can be described invoking a model in which clusters are treated like "superatoms" .[8] According to this model they exhibit atomic-like electronic states, that are labelled S,P,D,F acoording to the respective angular momentum in the atomic picture. Those clusters, that are assigned to a "closed superatomic shell" configuration, have indeed been identified as the most stable ones. The electronic shell closure and the resulting gain in stability is made responsible for the discrete distribution of few cluster sizes (magic numbers) observed in the symtheses rather than the quasi-continous one.
Magic Numbers[Bearbeiten | Quelltext bearbeiten]
Magic numbers are connected with the number of metal atoms in the thiolate-protected clusters, which display an outstanding stability. Such clusters can be synthesized monodispersely and are end-products of the etching-procedure, where an addition of exess thiols does not lead to further metal dissolution. Important representatives of clusters with magic numbers are e. g. (SG:Glutathion): Au10(SG)10, Au15(SG)13, Au18(SG)14, Au22(SG)16, Au22(SG)17, Au25(SG)18, Au29(SG)20, Au33(SG)22, and Au39(SG)24.[2]
Auch Au20(SCH2Ph)16 ist bekannt.[9] Als größerer Vertreter wurde Au102(p-MBA)44 mit dem para-mercaptobenzoesäure (para-mercapto-benzoic acid, p-MBA) Liganden hergestellt[10]
Applications[Bearbeiten | Quelltext bearbeiten]
Bionanotechnology[Bearbeiten | Quelltext bearbeiten]
The intrinsic properties of the clusters (e. g. fluorescence in several cases) can be made available for bionanotechnological applicathions through their functionalization with biomolecules (bioconjugation)[11] The stable and fluorescent protected gold clusters are recognized as efficient emitters, that can be tuned by varying the cluster size and the type of ligand used for protection. The protective shell can be functionalized in a way that selective binding(e. g. via complementary protein receptor of DNA-DNA-interaction) qualifies them for the use as biosensors. [12]
References[Bearbeiten | Quelltext bearbeiten]
- ↑ Rongchao Jin: Quantum sized, thiolate-protected gold nanoclusters; Nanoscale, 2010, 2, 343–362l (doi:10.1039/B9NR00160C).
- ↑ a b Yuichi Negishi, Katsuyuki Nobusada, Tatsuya Tsukuda: "Glutathione-Protected Gold Clusters Revisited: Bridging the Gap between Gold(I)−Thiolate Complexes and Thiolate-Protected Gold Nanocrystals", J. Am. Chem. Soc., 2005, 127 (14), 5261–5270 (doi:10.1021/ja042218h).
- ↑ Negishi, Y. et al. J. Am. Chem. Soc. 2004, 126, 6518.
- ↑ Manzhou Zhu, Eric Lanni, Niti Garg, Mark E. Bier, and Rongchao Jin: Kinetically Controlled, High-Yield Synthesis of Au25 Clusters, J. Am. Chem. Soc., 2008, 130 (4), 1138–1139 (doi:10.1021/ja0782448).
- ↑ Xiangming Meng, Zhao Liu, Manzhou Zhu and Rongchao Jin: Controlled reduction for size selective synthesis of thiolate-protected gold nanoclusters Aun (n = 20, 24, 39, 40), Nanoscale Research Letters, 2012, 7, 277 (doi:10.1186/1556-276X-7-277-3479.48780458).
- ↑ Atomically monodispersed and fluorescent sub-nanometer gold clusters created by biomolecule-assisted etching of nanometer-sized gold particles and rods (doi:10.1002/chem.200802743).
- ↑ Marcos M. Alvarez, Joseph T. Khoury, T. Gregory Schaaff, Marat N. Shafigullin, Igor Vezmar, and Robert L. Whetten: Optical Absorption Spectra of Nanocrystal Gold Molecules, J. Phys. Chem. B, 1997, 101 (19), 3706–3712 (doi:10.1021/jp962922n).
- ↑ A unified view of ligand-protected gold clusters as superatom complexes (doi:10.1073/pnas.0801001105).
- ↑ Manzhou Zhu, Huifeng Qian and Rongchao Jin: Thiolate-Protected Au20 Clusters with a Large Energy Gap of 2.1 eV, Journal of the American Chemical Society 2009, Volume 131, Number 21, pages 7220-7221 (doi:10.1021/ja902208h).
- ↑ Yael Levi-Kalisman, Pablo D. Jadzinsky, Nir Kalisman, Hironori Tsunoyama, Tatsuya Tsukuda, David A. Bushnell, and Roger D. Kornberg: Synthesis and Characterization of Au102(p-MBA)44 Nanoparticles, Journal of the American Chemical Society 2011, Volume 133, Number 9, pages 2976–2982 doi:10.1021/ja109131w)
- ↑ Synthesis and Bioconjugation of 2 and 3 nm-diameter Gold Nanoparticles (doi:10.1021/bc900135d).
- ↑ Cheng-An J. Lin, Chih-Hsien Lee, Jyun-Tai Hsieh, Hsueh-Hsiao Wang, Jimmy K. Li, Ji-Lin Shen, Wen-Hsiung Chan, Hung-I Yeh, Walter H. Chang: Synthesis of Fluorescent Metallic Nanoclusters toward Biomedical Application: Recent Progress and Present Challenges, Journal of Medical and Biological Engineering, (2009) Vol 29, No 6, (Abstract).