11:00-11:45

Luminescent Clusters of Noble Metals

In this talk I shall present our most recent work on monolayer protected sub-nanometer clusters of gold and silver. All the clusters are molecule-like with well-defined absorption and emission features, spanning across the visible-near infrared window. In a few clusters, the emission is intense. In general, several clusters are prepared simultaneously by reduction of metal ions in presence of appropriate ligands and subsequently size separated. However, we have also developed new methods for fast and efficient synthesis of specific clusters. All of these clusters are stable, highly water-soluble and can be made in the solid state. The clusters have been characterized by electrospray ionization mass spectrometry which gives multiply charged species. Isotope patterns and theoretical mass spectra have been used to characterise the molecular composition. We have shown that fluorescence resonance energy transfer is possible in these clusters by linking appropriate fluorescent molecules to the cluster core. We have developed interfacial routes for the direct synthesis of silver clusters. More recent excitement in this category of materials is the creation of clusters in protein templates. Bio-mineralization of Au³⁺ by native lactoferrin (NLf) and bovine serum albumin (BSA) resulting in near infrared (NIR) luminescent gold QCs will be discussed. We show that it occurs through a protein bound Au¹⁺ intermediate and subsequent release of free protein. The evolution was probed by diverse tools, principally, using matrix assisted laser desorption ionization mass spectrometry (MALDI MS), x-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). We have also shown that it is possible to make clusters starting from colloids of noble metals. The synthesis of a diverse variety of clusters, their chemical stability and intense luminescence offer numerous applications in areas such as energy transfer, sensors, bio-labeling and metal ion scavenging. Several of these applications are described, especially in the context of biological imaging and sensing.

1. Two luminescent silver clusters by interfacial synthesis, T. Udaya Bhaskara Rao and T. Pradeep, Angew. Chem. Int. Ed., 49 (2010) 3525-3529.
2. Luminescent quantum clusters of gold in bulk by BSA-induced core etching of nanoparticles: Metal ion sensing, metal enhanced luminescence and biolabeling, Madathumpady Abubaker Habeeb Muhammed, Pramod Kumar Verma, Samir Kumar Pal, Archana Retnakumari, Manzoor Koyakutty, Shantikumar Nair, and T. Pradeep, Chem. Eur. J., 16 (2010) 10103-10113.
3. Ag9 quantum cluster through a solid state route, T. U. B. Rao, B. Nataraju and T. Pradeep, J. Am. Chem. Soc., 132 (2010) 16304-16307.
4. Understanding the evolution of luminescent gold quantum clusters in protein templates, K. Chaudhari, P.L. Xavier, T. Pradeep, ACS Nano, 5 (2011) 8816-8827.
5. Ag₇Au₆: A 13 atom alloy quantum cluster, T. U. B. Rao, Y. Sun, N. Goswami, S. K. Pal, K. Balasubramanian, and T. Pradeep, Angew. Chem. Int. Ed., 51 (2012) 2155-2159.
6. Selective visual detection of TNT at the sub-zeptomole level, Ammu Mathew, P. R. Sajanlal, and T. Pradeep, Angew. Chem. Int. Ed., 51 (2012) 9596-9600.

Speaker

Dr. T. Pradeep, Department of Chemistry, Indian Institute of Technology Madras, India

Chair

Dr. Ryutaro Souda, MANA Scientist, MANA, NIMS