Detection of single gold (Au) nanoparticle (NP) collisions on a carbon-fiber (C-fiber) ultramicroelectrode (UME) was carried out successfully using the electrocatalytic amplification method. The C-fiber UME shows a low and quite stable background current for the hydrazine oxidation over a wide potential range. The electrocatalytic current transient generated by Au NP collision on the C-fiber UME was also detected simultaneously without any pretreatment of the UME or deformation of the Au NP. The magnitude and frequency of the transient current from Au NP collisions agree well with the calculated values based on the size and concentration of the Au NP.

We have studied electron transport properties of benzenedithiol and benzenedimethanethiol covalently bonded to gold electrodes by repeatedly creating a large number of molecular junctions. For each molecule, conductance histogram shows peaks at integer multiples of a fundamental conductance value, which is used to identify the conductance of a single molecule. The conductance values of a benzenedithiol and benzenedimethanethiol are 0.011 G 0 and 0.0006 G 0 (G 0 ) 2e 2 /h), respectively. The conductance peaks are broad, which reflects variations in the microscopic details of different molecular junctions. We have also studied electrochemical gate effect.

Electrochemical hydrazine oxidation and proton reduction occur at a significantly higher rate at Pt than at Au or C electrodes. Thus, the collision and adhesion of a Pt particle on a less active Au or C electrode leads to a large current amplification by electrocatalysis at single nanoparticles (NPs). At low particle concentrations, the collision of Pt NPs was characterized by current transients composed of individual current profiles that rapidly attained a steady state, signaling single NP collisions. The characteristic steady-state current was used to estimate the particle size. The fluctuation in collision frequency with time indicates that the collision of NPs at the detector electrodes occurs in a statistically random manner, with the average frequency a function of particle concentration and diffusion coefficient. A longer term current decay in single current transients, as opposed to the expected steady-state behavior, was more pronounced for proton reduction than for hydrazine oxidation, revealing microscopic details of the nature of the particle interaction with the detector electrode and the kinetics of electrocatalysis at single NPs. The study of single NP collisions allows one to screen particle size distributions and estimate NP concentrations and diffusion coefficients.

The electromechanical properties of a single molecule covalently attached to two gold electrodes are studied by simultaneously measuring the conductance and the force during the stretching of the molecule. The conductance, the spring constant of the molecular junction, and the dependence of the conductance on the stretching force are determined. Like the conductance, the spring constant of a molecule depends also on the molecule-electrode contacts. The forces required to break the molecule-gold contacts are 1.5 nN for alkanedithiols and 0.8 nN for 4,4' bipyridine, indicating that the breakdowns take place at the Au-Au bond and at the N-Au bond, respectively.

We have studied electron transport properties of unsubstituted oligo(phenylene ethynylene) (OPE) (1) and nitro-substituted OPE (2) covalently bound to two gold electrodes. The conductance values of single 1 and 2 are approximately 13 and approximately 6 nS, respectively. In addition to a decrease in the conductance, the presence of the nitro moiety leads to asymmetric I-V characteristics and a negative differential resistance-like (NDR-like) behavior. We have altered the nitro-substituted OPE by electrochemically reducing the nitro group and by varying the pH of the electrolyte. The conductance decreases linearly with the electron-withdrawing capability (i.e., Hammett substituent values) of the corresponding reduced species. In contrast, the conductance of 1 is independent of the pH and the electrode potential.

We have demonstrated a single molecule field effect transistor (FET) which consists of a redox molecule (perylene tetracarboxylic diimide) covalently bonded to a source and drain electrode and an electrochemical gate. By adjusting the gate voltage, the energy levels of empty molecular states are shifted to the Fermi level of the source and drain electrodes. This results in a nearly 3 orders of magnitude increase in the source-drain current, in the fashion of an n-type FET. The large current increase is attributed to an electron transport mediated by the lowest empty molecular energy level when it lines up with the Fermi level.

We have measured the conductance of single peptides covalently bonded to two Au electrodes via S-Au bonds by repeatedly forming a large number of molecular junctions. The conductance decreases exponentially with the peptide length, with a decay constant of beta = 0.9 +/- 0.1 A-1, suggesting that tunneling is the mechanism of electron transport in the peptides. The conductance of the peptides is sensitive to the solution pH, due to the protonation/deprotonation of the amine and carboxyl groups of the peptides, which provides titration measurements based on single-molecule conductance.

The electrocatalytic properties of individual single Pt nanoparticles (NPs) can be studied electrochemically by measuring the current−time (i−t) responses during single NP collisions with a noncatalytic ultramicroelectrode (UME). The Pt NPs are capped with citrate ions or a self-assembled monolayer (SAM) of alkane thiols terminated with carboxylic acid that affect the observed i−t responses. By varying the length of the SAMs or the composition of a mixed monolayer, we have studied the effect of adsorbed molecules on the catalytic activity of Pt NPs at the single particle level through electrocatalytic amplification of single NP collisions. Collisions of single NPs were triggered and recorded as individual current steps whose amplitude represents the reactivity of single Pt NPs for the reaction of interest, here hydrazine oxidation, at a given electrode potential. The catalytic properties of Pt NPs are dependent not only on the nature of the interaction between the adsorbed monolayer and the catalytic NP surface, but also on the rate of electron transfer through the SAMs, governed by their length.