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Doctoral Dissertation Announcement
Candidate: Mary Sajini Devadas
Doctor of Philosophy
Title: Optical and Electrochemical Properties of Monolayer Protected Gold Clusters Modified with Fluorophores
Dr. Ekkehard Sinn, Chair
Dr. Ramakrishna Guda
Dr. Gellert Mezei
Dr. Pamela Hope
Date: Thursday, March 1, 2012 11:15 a.m. to 1:15 p.m.
Chemistry Building, Room 1260
This research focused on the synthesis, characterization and the investigation of the optical and electrochemical properties of monolayer-protected quantum-sized gold clusters. Significant research attention was focused on the solution phase optical and electrochemical properties of these clusters. Highly monodisperse Au clusters with different types of ligands were successfully synthesized with sizes varying from 1 nm to 13 nm. These preformed gold clusters were modified with fluorophores and pseudo-rotaxanes for developing better nonlinear optical materials for sensing and biological imaging purposes. Focus was on the interaction of the outer ligand shell, made up of –S-Au-S-Au-S- bonds, with the core gold states, which alludes to changes in the optical and electrochemical footprint of these AuMPCs.
Ultrafast luminescence investigations on Au25L18 clusters with two ligands glutathione (GS) and hexanethiol (C6S) have revealed the nature of the higher excited states in these clusters. The growth of luminescence is independent of the ligands and is ascribed to the luminescence from the Au13 core states and the decay dynamics is mainly governed by its relaxation to S-Au-S-Au-S semi ring states. Temperature-dependent electronic absorption properties of the Au25L18 and Au38L24 clusters have been studied for the first time and are explained on the basis of electron-phonon interactions.
Pyrene and coumarin analogs were used as chromophores labels. It is unambiguously shown that Au25 clusters can work as electron donors with electrochemical and time-resolved fluorescence upconversion and transient absorption measurements for the first time to understand the excited state dynamics. Coumarin labeled MPC was a selective turn-on fluorescent sensor for DiethylChloroPhosphate, a nevre gas mimic, in both one- and two-photon excited fluorescence. The research has shown that 7-mercapto-4-methylcoumarin was used as a passivating ligand successfully and this process can be followed by common spectroscopic techniques. Two-photon measurements prove its further use as nanoparticle beacons and in the development of non-linear optical material for biological imaging. Irrespective of the size of the AuMPC, or nature of the passivating layer, electron transfer seems to be taking place, hampering the threading of the electron-rich crown ether wheel component to form a pseudo-rotaxane.