Thiolate-protected gold cluster

Thiolate-protected gold clusters are a type of ligand-protected metal cluster, synthesized from gold ions and thin layer compounds that play a special role in cluster physics because of their unique stability and electronic properties. They are considered to be stable compounds.[1]

These clusters can range in size up to hundreds of gold atoms, above which they are classified as passivated gold nanoparticles.

Synthesis

Wet chemical synthesis

The wet chemical synthesis of thiolate-protected gold clusters is achieved by the reduction of gold(III) salt solutions, using a mild reducing agent in the presence of thiol compounds. This method starts with gold ions and synthesizes larger particles from them, therefore this type of synthesis can be regarded as a "bottom-up approach" in nanotechnology to the synthesis of nanoparticles.

The reduction process depends on the equilibrium between different oxidation states of the gold and the oxidized or reduced forms of the reducing agent, or thiols. Gold(I)-thiolate polymers have been identified as important in the initial steps of the reaction.[2] Several synthesis recipes exist that are similar to the Brust synthesis of colloidal gold, however the mechanism is not yet fully understood. The synthesis produces a mixture of dissolved, thiolate-protected gold clusters of different sizes. These particles can then be separated by gel electrophoresis (PAGE).[3] If the synthesis is performed in a kinetically controlled manner, particularly stable representatives can be obtained with particles of uniform size (monodispersely), avoiding further separation steps.[4][5]

Template-mediated synthesis

Rather than starting from "naked" gold ions in solution, template reactions can be used for directed synthesis of clusters. The high affinity of the gold ions to electronegative and (partially) charged atoms of functional groups yields potential seeds for cluster formation. The interface between the metal and the template can act as a stabilizer and steer the final size of the cluster. Some potential templates are dendrimers, oligonucleotides, proteins, polyelectrolytes and polymers.

Etching synthesis

Top-down synthesis of the clusters can be achieved by the "etching" of larger metallic nanoparticles with redox-active, thiol-containing biomolecules.[6] In this process, gold atoms on the nanoparticles' surface react with the thiol, dissolving as gold-thiolate complexes until the dissolution reaction stops; this leaves behind a residual species of thiolate-protected gold clusters that is particularly stable. This type of synthesis is also possible using other non thiol-based ligands.

Properties

Electronic and optical properties

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 existence of discrete electronic states was first indicated by the discrepancy between their optical absorption and the predictions of classical Mie scattering.[7] The discrete optical transitions and occurrence of photoluminescence in these species are areas where they behave like molecular, rather than metallic, substances. This molecular optical behavior sharply distinguishes thiolate-protected clusters from gold nanoparticles, whose optical characteristics are driven by Plasmon resonance. Some of thiolate-protected clusters' properties can be described using a model in which the clusters are treated like "superatoms".[8] According to this model they exhibit atomic-like electronic states, that are labeled S,P,D,F according to their respective angular momentum on the atomic level. Those clusters that have a "closed superatomic shell" configuration have indeed been identified as the most stable ones. This electronic shell closure and the resulting gain in stability is responsible for the discrete distribution of a few stable cluster sizes (magic numbers) observed in their synthesis, rather than a quasi-continuous distribution of sizes.

Magic numbers

Magic numbers are connected with the number of metal atoms in those thiolate-protected clusters which display an outstanding stability. Such clusters can be synthesized monodispersely and are end products of the etching procedure after an addition of excess thiols does not lead to further metal dissolution. Some important clusters with magic numbers are (SG:Glutathione): 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]

Au20(SCH2Ph)16 is also well-known.[9] It was greater than representatives Au102(p-MBA)44 with the para-mercaptobenzoice (para-mercapto-benzoic acid, p-MBA) produced ligand.[10]

Structure prediction

Worthy of note is that in 2013, a structural prediction of the Au130 (SCH3)50 cluster, based on Density Functional Theory (DFT) was confirmed in 2015.[11] This result represents the maturity of this field where calculations are able to guide the experimental work.[12] The following table features some sizes.

Composition database
CompositionMass Spec.Crystal StructureDFT modelsExp. UV-VisExp. powder XRD
Au10(SR)10JACS 2005JACS 2000-ExampleExample
Au15(SR)13JACS 2005Not knownJACS 2013, PCCP 2013JACS 2005
Au18(SR)14Angew. Chem Int. Ed. 2015, Angew. Chem Int. Ed. 2015PCCP 2012
Au24(SR)20JPCL 2010Nanoscale 2014JACS 2012JPCL 2010
Au40(SR)24JACS 2010 Nano Lett 2015Sci Adv 2015JACS 2012 Nanoscale 2013 Sci Adv 2015Anal. Chem. 2013 Nano Lett 2015
Au130(SR)50J. Phys. Chem. A 2013
Au187(SR)68not knownPCCP 2015

Applications

In bionanotechnology, intrinsic properties of the clusters (for example, fluorescence) can be made available for bionanotechnological applications by linking them with biomolecules through the process of bioconjugation.[13] The protected gold particles' stability and fluorescence makes them efficient emitters of electromagnetic radiation that can be tuned by varying the cluster size and the type of ligand used for protection. The protective shell can function (have functional groups added) in a way that selective binding (for example, as a complementary protein receptor of DNA-DNA-interaction) qualifies them for the use as biosensors.[14]

References
  1. Rongchao Jin: Quantum sized, thiolate-protected gold nanoclusters; Nanoscale, 2010, 2, 343–362l (doi:10.1039/B9NR00160C).
  2. 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. Y, Negishi (June 1994). "Magic-numbered Au(n) clusters protected by glutathione monolayers (n = 18, 21, 25, 28, 32, 39): isolation and spectroscopic characterization". J Am Chem Soc. 126 (21): 6518–6519. doi:10.1021/ja0483589. PMID 15161256.
  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-7-277-3479.48780458).
  6. Atomically monodispersed and fluorescent sub-nanometer gold clusters 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. Alfredo Tlahuice-Flores, Ulises Santiago,Daniel Bahena , Ekaterina Vinogradova , Cecil V Conroy , Tarushee Ahuja , Stephan B. H. Bach , Arturo Ponce , Gangli Wang , Miguel Jose-Yacaman , and Robert L. Whetten: On the Structure of the Thiolated Au130 Cluster, J. Phys. Chem. A. 2013, Volume 117, Number 40, pages 10470–10476 (doi:10.1021/jp406665m).
  12. Yuxiang Chen, Chenjie Zeng, Chong Liu, Kristin Kirschbaum, Chakicherla Gayathri, Roberto R. Gil, Nathaniel L. Rosi, and Rongchao Jin: Crystal Structure of Barrel-Shaped Chiral Au130(p-MBT)50 Nanocluster, Journal of the American Chemical Society 2015, Volume 137, Number 32, pages 10076–10079 (doi:10.1021/jacs.5b05378).
  13. Synthesis and Bioconjugation of 2 and 3 nm-diameter Gold Nanoparticles (doi:10.1021/bc900135d).
  14. 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 Archived 2015-06-10 at the Wayback Machine).
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