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We report the first spectroscopic demonstration of direct electron transfer between a gold nanoparticle and a surface-bound fluorophore induced by pulsed laser irradiation. Binding of pyrene thiol directly to the gold nanoparticle results in quenching of its singlet excited state. The suppression of S 1 -T 1 intersystem crossing process as well as the formation of pyrene radical cation confirm the excited-state interaction between the metal nanoparticle and the surface-bound fluorophore. The charge separation is sustained for several microseconds before undergoing recombination.Unique electronic and chemical properties of metal nanoparticles have drawn the attention of chemists, physicists, biologists, and engineers who wish to use them for a new generation of nanodevices. 1-10 Electrochemical studies have established the electron-storing properties of gold nanoparticles and their ability to act as an electric relay on a given nanotemplate structure. [11][12][13][14] Modification of the gold nanoparticles with fluorophores is important for the development of biological tracers as well as optoelectronic devices. 12,15,16 Gold nanoparticles themselves show limited photoactivity under UV-visible irradiation, although photoinduced fusion and fragmentation have been observed under laser irradiation. [17][18][19][20][21][22] Binding of a photoactive fluorophore such as pyrene to a gold nanoparticle renders the organic-inorganic hybrid nanoassemblies suitable for light-harvesting and optoelectronic applications. 23,24 Direct binding of a fluorophore to the metal surface often results in the quenching of excited states. 25-28 Both energytransfer and electron-transfer processes are considered to be major deactivation pathways for excited fluoroprobes on metal surface. Most of these studies are limited to bulk gold surfaces modified with self-assembled monolayers. Indirect evidence for electron transfer between the chromophore and the gold surface has been obtained from photocurrent measurements. 29,30 Obtaining insight into such processes using spectroscopic measurements is important to improve the charge separation efficiencies in gold-fluorophore nanoassemblies. We now report transient absorption studies that relate the excited-state quenching of the surface-bound fluorophore to an electron-transfer process in the pyrene thiol-bound gold (Au-SR-Py) nanoassemblies (Scheme

A one-step process of solubilization of single wall carbon nanotubes (SWCNT) in an organic solvent has enabled us to polarize them asymmetrically in a dc electric field. Quaternary ammonium ion-capped SWCNTs readily suspend in organic solvents; under the influence of a dc electric field, they assemble as stretched bundles anchored on the positive electrode. At low dc applied field (approximately 40 V), all of the SWCNTs from the suspension are deposited on the electrode, thus providing a simple methodology to design robust SWCNT films. At higher applied voltages (>100 V), the SWCNT bundles stretch out into the solution and orient themselves perpendicular to the electrode surface. The alignment of these bundles is responsive to the ON-OFF cycles of the applied electric field. The possibility of modulating the alignment of SWCNT in an electric field opens new ways to achieve electrical contacts in nano- to micro-devices.