A transition from metal-like double-layer capacitive charging to redox-like charging was observed in electrochemical ensemble Coulomb staircase experiments on solutions of gold nanoparticles of varied core size. The monodisperse gold nanoparticles are stabilized by short-chain alkanethiolate monolayers and have 8 to 38 kilodaltons core mass (1.1 to 1.9 nanometers in diameter). Larger cores display Coulomb staircase responses consistent with double-layer charging of metal-electrolyte interfaces, whereas smaller core nanoparticles exhibit redox chemical character, including a large central gap. The change in behavior is consistent with new near-infrared spectroscopic data showing an emerging gap between the highest occupied and lowest unoccupied orbitals of 0.4 to 0.9 electron volt.

We report the fabrication and characterization of an optical fiber biochemical sensing probe based on localized surface plasmon resonance (LSPR) and spectra reflection. Ordered array of gold nanodots was fabricated on the optical fiber end facet using electron-beam lithography (EBL). We experimentally demonstrated for the first time the blue shift of the LSPR scattering spectrum with respected to the LSPR extinction spectrum, which had been predicted theoretically. High sensitivity [195.72 nm/refractive index unit (RIU)] of this sensor for detecting changes in the bulk refractive indices has been demonstrated. The label-free affinity bio-molecules sensing capability has also been demonstrated using biotin and streptavidin as the receptor and the analyte.

Electronic conductivity, σEL, in solid-state films of alkanethiolate monolayer protected Au clusters (Au MPCs) occurs by a bimolecular, electron self-exchange reaction, whose rate constant is controlled by (a) the core-to-core tunneling of electronic charge along alkanethiolate chains and (b) the mixed valency of the MPC cores (e.g., a mixture of cores with different electronic charges). The tunneling mechanism is demonstrated by an exponential relation between the electronic conductivity of Au309(C n )92 MPCs (average composition) and n, the alkanethiolate chainlength, which varies from 4 to 16. The electron tunneling coefficient β n = 1.2/CH2 or, after accounting for alkanethiolate chain interdigitation, βdis = 0.8 Å-1. Quantized electrochemical double layer charging of low polydispersity Au140(C6)53 MPCs was used to prepare solutions containing well-defined mixtures of MPC core electronic charges (such as MPC0 mixed with MPC1+). Electronic conductivities of mixed-valent, solid-state Au140(C6)53 MPC films cast from such solutions are proportional to the concentration product [MPC0][MPC1+], and give a MPC0/1+ electron self-exchange rate constant of ca. 1010 M-1 s-1.

Rotated disk electrode voltammetry is described for CH2Cl2 solutions of cluster molecules with nanometer-sized gold cores and stabilizing ligand shells consisting of mixed monolayers of octanethiolate and ω-ferrocenyloctanethiolate ligands in molar ratios ranging from 2:1 to 24:1. Voltammograms for the cluster molecules exhibit a ferrocene oxidation wave with a limiting current that is under hydrodynamic mass transport control. The current−potential curves preceding (“prewave”) and following (“postwave”) the ferrocene wave, which are ideally flat, are decidedly sloped. The Δi/ΔE slopes are proportional to the square root of electrode rotation rate, i.e., are also under hydrodynamic control. The Δi/ΔE slopes are due to the charging of the electrical double layers of the cluster molecules, showing them to act as diffusing, molecule-sized “nanoelectrodes”. A theoretical analysis is presented of the transport control of the double layer charging. Possible reasons that the values of the cluster molecule capacitance (per unit surface area of cluster molecule, which entails use of models for the shape of the Au core of the cluster) are somewhat larger than the literature expectation for octanethiolate monolayers on flat gold surfaces are discussed. The tiny capacitances of the cluster molecules means that changing their charges by small potential increments can require an average of less than a single electron per cluster molecule.