Electrochemistry and Optical Absorbance and Luminescence of Molecule-like Au₃₈ Nanoparticles

Abstract

This paper describes electrochemical and spectroscopic properties of a well-characterized, synthetically accessible, 1.1 nm diam Au nanoparticle, Au(38)(PhC(2)S)(24), where PhC(2)S is phenylethylthiolate. Properties of other Au(38) nanoparticles made by exchanging the monolayer ligands with different thiolate ligands are also described. Voltammetry of the Au(38) nanoparticles in CH(2)Cl(2) reveals a 1.62 V energy gap between the first one-electron oxidation and the first reduction. Based on a charging energy correction of ca. 0.29 V, the indicated HOMO-LUMO gap energy is ca. 1.33 eV. At low energies, the optical absorbance spectrum includes peaks at 675 nm (1.84 eV) and 770 nm (1.61 eV) and an absorbance edge at ca. 1.33 eV that gives an optical HOMO-LUMO gap energy that is consistent with the electrochemical estimate. The absorbance at lowest energy is bleached upon electrochemical depletion of the HOMO level. The complete voltammetry contains two separated doublets of oxidation waves, indicating two distinct molecular orbitals, and two reduction steps. The ligand-exchanged nanoparticle Au(38)(PEG(135)S)(13)(PhC(2)S)(11), where PEG(135)S is -SCH(2)CH(2)OCH(2)CH(2)OCH(3), exhibits a broad (1.77-0.89 eV) near-IR photoluminescence band resolvable into maxima at 902 nm (1.38 eV) and 1025 nm (1.2 eV). Much of the photoluminescence occurs at energies less than the HOMO-LUMO gap energy. A working model of the energy level structure of the Au(38) nanoparticle is presented.

Highly Fluorescent Noble-Metal Quantum Dots

“…In 1998, Wilcoxon et al reported blue emission at 440 nm from small gold nanoparticles (diameter < 2.5 nm) (67), however, due to the heterogeneity of the solution, the exact size of emissive species was undetermined. Near IR emission was also reported from glutathione encapsulated Au 28 clusters (9) and from tiopronin-capped ~1-nm Au species (68)(69)(70)(71)(72)(73)(74)(75), both in aqueous solution. Compared to well-studied alkali clusters (24,25), the size-dependent electronic structure of these noble metals remained poorly understood, yet a framework (9,23,76) for linking nanoparticle and cluster optical properties through free electron scalings was in place.…”

Optical, Electrical, and Catalytic Properties of Metal Nanoclusters Investigated by ab initio Molecular Dynamics Simulation: A Mini Review

“…They have attracted intense research interests recently due to their unique electronic, optical and catalytic properties (33)(34)(35). They are composed of very few Au atoms in the center core with the size less than 1 nm, and as a result they show "molecule-like" optical transitions in the absorption and emission spectra (36). The small-sized thiolate protected Au nanoclusters exhibit exceptional stability and size dependent HOMO-LUMO gap.…”

Microspectroscopic characterisation of gold nanorods for cancer detection

“…[180][181][182] The onset of this IR emission is consistently appearing around 1100 nm, which suggests that visible and IR luminescence are caused by different photophysical mechanisms. As discussed earlier (section 5.2 of this chapter), IR emission is attributed to intraband rather than interband transitions.…”

“…[179][180][181][182][183] The localized surface plasmons near rough metal nanostructures are presumed to perform a mediating role in the IR emission, 184,185 because these strongly confined fields possess resonances of electrons in the metal, and the associated field gradients give rise to higher-order multipolar transitions. 179 Independently, Varnavski et al also speculated upon a possible role of surface plasmons in intraband excitations.…”

“…191,192 More recent studies have shown, however, that the photoluminescence behaviour of gold nanoclusters is fundamentally different from that of bulk gold and other larger structures such as nanorods and nanospheres. 164,181,183 Absorption studies have shown that the electronic and optical properties of gold nanoclusters require new concepts. 183,204 Earlier, it has been proposed that nanoclusters smaller than 4 nm in radii would experience quantum 192 confinement.…”