Photoinduced Charge Separation in a Fluorophore−Gold Nanoassembly

Abstract

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

Beyond superquenching: Hyper-efficient energy transfer from conjugated polymers to gold nanoparticles

“…A recent report suggests that both charge transfer and energy transfer play a role in quenching in fluorophore-gold particle nanoassemblies (30). We have investigated the roles that energy transfer and electron transfer play in the extremely efficient quenching reported here and find that, for gold nanoparticles of Ͼ2 nm, resonance energy transfer dominates the quenching mechanism.…”

Optical investigation of the electron transfer protein azurin–gold nanoparticle system

“…The lack of a preferential orientation of Az molecules with respect to the nanoparticle surface suggests that this orientation-dependent effect cannot fully explain the outlined fluorescence quenching of bound Az molecules. The characteristics of the investigated system suggest that an important role in fluorescence quenching is played by non radiative decay rate increase, which is thought to be induced by the opening of an additional non radiative decay path connected to electron and/or energy transfer processes [47,49,51,52] (in such cases, the total non radiative rate would be k nr '+k T , where k T is the transfer decay rate, hence, the denominator in Eq.4b becoming k r '+k nr '+k T ). In fact, on one hand, the injection of a charge from the fluorophore excited state to the metal surface provides the additional non radiative decay route that, in specific conditions, can compete with the naturally available de-excitation processes [49,52].…”

Spectroscopic properties of a two-level atom interacting with a complex spherical nanoshell

“…10 only shows the upper theoretical limits on the fluorescence yields. Indeed, the Ohmic loss is only one of many other nonradiative mechanisms, such as, for instance, multiphoton relaxation, coupling to defects, direct electron-transfer processes, and concentration quenching, which all contribute to the nonradiative decay rate W nrad [39,40,41,42,43]. It turns out that even in a purely dielectric case, in the absence of any Ohmic losses and for small fluorescence atom concentrations, the nonradiative decay W nrad can be higher than the radiative decay W rad , resulting in the fluorescence yield smaller than 0.5 [3,52,53].…”