Thiolate protected gold clusters

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Structure of Au 25 R 18 -, determined by X-ray single crystal diffractometry (R = SCH 2 Ph, white: H, gray: C, light yellow: S, yellow: Au). Upper left: complete structure, middle: core and Au-S shell, lower right: only Au 13 core

Thiolate- protected gold clusters are special ligand- protected metal clusters that play a special role in terms of their stability and electronic properties. Some particularly stable representatives of these clusters are monodisperse and easily accessible synthetically - in aqueous solution - and remain stable over a long period of time.

They range in size from a few to several hundred gold atoms. From this size on, their special properties can no longer be determined and they correspond to passivated gold nanoparticles.

presentation

Wet chemical process

The wet-chemical generation of thiolate-protected gold clusters is based on the reduction of gold (III) salt solutions by means of a gentle reducing agent in the presence of thiols. Since the starting point are individual gold ions, this type of generation is a " bottom-up " process. The reduction process includes equilibria between different gold oxidation states as well as the oxidized and reduced form of the reducing agent and the thiol and starts with gold (I) thiolate polymers. For this purpose, there are a number of synthesis instructions that are similar to the breast synthesis of colloidal gold , although the mechanism is not yet fully understood at this point in time. The product obtained is a mixture of dissolved thiolate-protected gold clusters of various sizes, which are separated by means of gel electrophoresis ( PAGE ). The synthesis can also be kinetically controlled in order to obtain particularly stable monodisperse clusters and thus avoid a separation process.

Template-directed synthesis

Instead of reducing “naked” gold ions in solution, templates can also be used for targeted synthesis. The affinity of the gold ions for electronegative and (partially) charged atoms of functional groups results in the provision of nuclei for the formation of the clusters. The interface between template and cluster can also have a stabilizing effect and control the final size of the cluster. As a template z. B. dendrimers, oligonucleotides, proteins, polyelectrolytes and polymers in question.

Etching process

Your synthetic " top-down " production is is (Engl. By so-called "etching" Etching ) by larger metallic nanoparticles with the aid of redox-active, thiol containing biomolecules possible. Gold atoms on the nanoparticle surface are dissolved as gold-thiolate complexes until the resulting gold-thiolate cluster is particularly stable and the dissolution reaction comes to a standstill. Gold clusters protected with other ligands that do not contain thiol (e.g. DNA ) are also accessible in this way.

properties

Electronic and optical properties

The electronic structure of the thiolate-protected gold cluster is characterized by quantum effects in terms of discrete energy states. This was first established based on the discrepancy between their optical absorption and the classical Mie theory . Discrete optical transitions and the appearance of photoluminescence in these species reflect properties of molecules rather than of metallic objects. They are therefore differentiated from the gold nanoparticles, the optical characteristics of which are largely caused by plasmon resonance . Some of their most important properties can be adequately described using a general model in which the clusters themselves are assumed to be atom-like ( superatoms ). According to this model, the clusters have atomic electronic states, which are identified with the letters S, P, D, F etc. based on analogous orbital angular momentum designations. Such clusters, which have a closed-shell electron configuration in this model, were also identified as particularly stable. This stability, which is based on the electronic structure, is held responsible for the occurrence of a discrete distribution of fewer individual cluster sizes (magic numbers) instead of a quasi-continuous cluster size distribution during their synthesis.

Magic Numbers

So-called magic numbers indicate the number of metal atoms in the clusters, which are particularly stable. An indicator of such stability is the possibility of making it monodisperse. Such clusters are end products in the etching process and are retained with the addition of thiol. Important representatives of clusters with magic numbers are e.g. B. (SG: Glutathione): Au 10 (SG) 10 , Au 15 (SG) 13 , Au 18 (SG) 14 , Au 22 (SG) 16 , Au 22 (SG) 17 , Au 25 (SG) 18 , Au 29 (SG) 20 , Au 33 (SG) 22 , and Au 39 (SG) 24 .

Au 20 (SCH 2 Ph) 16 is also known. As a larger representative, Au 102 (p-MBA) 44 was prepared with the para-mercapto-benzoic acid (p-MBA) ligand

application

Bionanotechnology

The intrinsic properties of the clusters (e.g. in some cases their fluorescence) can be made available for applications in bionanotechnology by functionalization with biomolecules (bioconjugation) .Thus, fluorescent representatives of these species are to be regarded as stable and efficient emitters whose properties are determined by their size the cluster and the type of protective ligand can be adjusted. The protective shell can be constructed in such a way that selective binding (e.g. via complementary protein-receptor or DNA-DNA interaction) qualifies the clusters for applications as biosensors.

Individual evidence

  1. ↑ Rongchao Jin: Quantum sized, thiolate-protected gold nanoclusters; Nanoscale , 2010 , 2, 343-362l ( doi : 10.1039 / B9NR00160C ).
  2. ↑ 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 ).
  3. ↑ Negishi, Y. et al. J. Am. Chem. Soc. 2004, 126, 6518.
  4. ↑ 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 ).
  5. ↑ 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 ).
  6. ↑ 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 ).
  7. ↑ 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 ).
  8. ↑ A unified view of ligand-protected gold clusters as superatom complexes ( doi : 10.1073 / pnas.0801001105 ).
  9. ↑ 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 ).
  10. ↑ 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 )
  11. ↑ Synthesis and Bioconjugation of 2 and 3 nm-diameter Gold Nanoparticles ( doi : 10.1021 / bc900135d ).
  12. ↑ 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 ( Memento of the original from June 10, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. ). @1@ 2Template: Webachiv / IABot / jmbe.bme.ncku.edu.tw