Observing Single Nanoparticle Collisions by Electrogenerated Chemiluminescence Amplification

We demonstrate a novel method of observing single particle collision events with electrogenerated chemiluminescence (ECL). A single event is characterized by the enhancement of ECL intensity during the collision of an individual platinum nanoparticle (Pt NP) on an indium tin oxide electrode, which catalyzes the oxidation of Ru(bpy)32+ and a coreactant, for example, tri-n-propylamine (TPrA), present in the solution. Every collision produces a unique photon spike whose amplitude and frequency can be correlated with the size and concentration of the Pt NPs. A large amplification of ECL intensity can occur by choosing an appropriate measuring electrode and using high concentrations of Ru(bpy)32+ and the coreactant

Chemical, Electrochemical, Gravimetric, and Microscopic Studies on Antimicrobial Silver Films

Silver compounds are of interest because of their antimicrobial and other biological activity. Electrochemical and chemical (e.g., dissolution) properties of silver films of various origins, e.g., sputtered Acticoat antimicrobial silver samples, electrodeposited Ag metal and electrooxidized silver samples in various media have been studied with electrochemical techniques, quartz crystal microbalance (QCM) gravimetry, X-ray diffraction, atomic force microscopy (AFM), and scanning electrochemical microscopy (SECM). Examination of several sputtered antimicrobial silver samples with AFM reveals their nanometer grainy aggregate structures. The electrochemical results suggest that the sputtered antimicrobial films contain both Ag(0) and Ag(I) (in the form of Ag2O, AgOH, or a mixture of these). While the dissolution of metallic Ag or antimicrobial films that were completely reduced to the Ag(0) form is small in aqueous 1.0 M NaClO4 solution, films containing Ag(I) are soluble. The initial dissolution rate of an antimicrobial film in 1.0 M NaClO4 under open-circuit conditions was estimated to be about 3.6 (μg/h)/cm2 in an unstirred condition. The SECM/QCM results suggest that the dissolved silver species contains both Ag(I) and Ag(0), with diffusion coefficients in the range (3.5−4.0) × 10-6 cm2/s. Small clusters containing Ag(0) and Ag(I) (as well as O and H) are proposed for these dissolved silver species

Single Molecule Electrochemistry

By using specially constructed nanometer tips of sharpened Pt-Ir wire in a wax sheath, small numbers of molecules (1−10) can be trapped between the tip and a substrate. Repeated electron transfers of an electroactive molecule as it shuttles by diffusion between tip and substrate produce a current (∼0.6 pA/molecule) that can be used to detect the trapped molecules. The tip electrode size and shape can be found from the electrode approach curves (current vs tip-to-substrate distance) based on approximate equations and digital simulations. Analysis of the observed fluctuating currents by autocorrelation, spectral density, and probability density functions is also described

Observing Iridium Oxide (IrO_<i>x</i>) Single Nanoparticle Collisions at Ultramicroelectrodes

We describe the electrochemical detection of single iridium oxide nanoparticle (IrOx NP) collisions on a NaBH4-treated Pt ultramicroelectrode (UME). We observe single NP events through the enhanced current by electrocatalytic water oxidation, when IrOx contacts the electrode and transiently sticks to it. The overall current transient consists of repeated current spikes that return to the background level, superimposed on a current decay, rather than the staircase response seen where an NP sticks on the UME. Here each event produces a unique current spike (or “blip”). The frequency of the spikes was directly proportional to the particle concentration, and the peak current increased with the applied potential. The observed current is very sensitive to the material and surface state of the measuring electrode; a NaBH4-treated Pt UME was important in obtaining reproducible results

Combinatorial Biomimetics. Optimization of a Composition of Copper(II) Poly-l-Histidine Complex as an Electrocatalyst for O₂ Reduction by Scanning Electrochemical Microscopy

A simple approach to prepare and characterize biomaterial-based electrocatalysts for oxygen reduction was carried out. Poly-l-histidine was used as a matrix and ligand to complex Cu2+ to mimic the active sites of laccases. A modified glassy carbon (GC) electrode with Cu2+-poly-l-histidine complex decreases the oxygen reduction overpotential as compared with the bare GC electrode. An array of Cu2+-poly-l-histidine spots with different compositions was deposited on a GC substrate, and their catalytic activity for oxygen reduction was evaluated by a scanning electrochemical microscopy-based screening technique. The electrocatalytic activities of complexes for oxygen reduction strongly depended on the mole ratio of Cu2+ to poly-l-histidine and the applied potential of the substrate

Combinatorial Biomimetics. Optimization of a Composition of Copper(II) Poly-l-Histidine Complex as an Electrocatalyst for O₂ Reduction by Scanning Electrochemical Microscopy

A simple approach to prepare and characterize biomaterial-based electrocatalysts for oxygen reduction was carried out. Poly-l-histidine was used as a matrix and ligand to complex Cu2+ to mimic the active sites of laccases. A modified glassy carbon (GC) electrode with Cu2+-poly-l-histidine complex decreases the oxygen reduction overpotential as compared with the bare GC electrode. An array of Cu2+-poly-l-histidine spots with different compositions was deposited on a GC substrate, and their catalytic activity for oxygen reduction was evaluated by a scanning electrochemical microscopy-based screening technique. The electrocatalytic activities of complexes for oxygen reduction strongly depended on the mole ratio of Cu2+ to poly-l-histidine and the applied potential of the substrate

Observation of Discrete Au Nanoparticle Collisions by Electrocatalytic Amplification Using Pt Ultramicroelectrode Surface Modification

After growing a thin layer of oxide (PtO_<i>x</i>) by anodization of a Pt electrode, it changed from catalytically active for electrochemical NaBH₄ oxidation into an inactive electrode. When held at a potential where the oxide film was maintained, collisions of individual 14 nm diameter Au nanoparticles (NPs) that catalyzed NaBH₄ oxidation were successfully observed as discrete current pulses (spikes or blips) for each NP interaction with the modified Pt electrode via amplification from NaBH₄ oxidation. The current response is affected by NP concentration and the applied potential

Electrochemical Studies of Guanosine in DMF and Detection of Its Radical Cation in a Scanning Electrochemical Microscopy Nanogap Experiment

This communication reports the findings of the investigation of the electrochemical (EC) oxidation of the important bimolecular guanosine (Gs) by scanning electrochemical microscopy (SECM) using carbon fiber ultramicroelectrodes (CF-UMEs) as the probe and substrate. The first attempt is to try to gain a steady-state voltammogram for EC oxidation of Gs at the CF-UME probe in aqueous buffer solutions with three different pH values. Experimental results indicate that due to serious adsorption of Gs on the CF-UME surface, an “S-shaped” steady-state voltammetric curve, which is required for SECM studies, cannot be obtained in aqueous solutions. To solve this adsorption problem, a series of experiments for studying the EC behavior of Gs in DMF are carried out. A well-defined “S-shaped” steady-state cyclic voltammogram (CV) could be achieved at the CF-UME in DMF containing 0.1M TBAPF6 as the supporting electrolyte. By combining several EC techniques, including cyclic voltammetry at glassy carbon (GC) macroelectrode and CF-UMEs, and chronoamperometry, the general chemical characteristics and EC behavior of Gs in DMF solution are studied. Furthermore, SECM detection of Gs•+, the radical cation of Gs electrogenerated in its first oxidation, is carried out by using feedback and tip generation/substrate collection modes in a nanogap configuration. Gs•+ has been electrochemically detected for the first time, with an estimated lifetime of ≤40 μs and E° = 1.55 V versus NHE for the Gs/Gs•+ couple

Single-Molecule Spectroelectrochemistry (SMS-EC)

We introduce single-molecule spectroelectrochemistry (SMS-EC), a powerful new technique for studying electrochemical kinetics in highly heterogeneous systems. This technique uses fluorescence single-molecule spectroscopy to indirectly measure electrochemical kinetics one molecule at a time, offering for the first time the distribution of key electrochemical variables, such as the half-wave potential, E1/2, not just the ensemble averages. In SMS-EC, the potential of the working electrode of an electrochemical cell is linearly scanned while simultaneously measuring the florescence intensity, Ifl(t), of individual single molecules as a function of time in a wide-field microscope. SMS-EC is used herein to study the oxidation at an indium tin oxide (ITO) electrode of single molecules of the organic conjugated polymer F8BT. The results reveal both excited singlet state and ground state oxidation of F8BT. The latter process occurs over a narrow distribution of single-molecule half-wave potential values, indicating a relatively uniform electrochemical potential at the electrode

Electrochemical, Spectroscopic, and Mass Spectrometric Studies of the Interaction of Silver Species with Polyamidoamine Dendrimers

Electrochemical, spectroscopic, and mass spectrometric (MS) methods were used to probe the interaction (complexation) of silver ions and zerovalent silver species with polyamidoamine generation 1 amine-terminated (PAMAMG1NH2) and generation 2 hydroxy-terminated (PAMAMG2OH) dendrimers (DDMs). Stability constants ( ) and stoichiometries (q) (i.e., the number of silver ions complexed per DDM molecule) were determined from the voltammetric data, that is, shifts in potential and changes in peak or limiting current with addition of DDM. When the mole ratio of DDM to Ag+ is ≥1, Ag+ binds with PAMAMG2OH to form a dominant 1:1 complex with a value of 1.1 × 107 M-1. Under similar conditions, Ag+ binds with PAMAMG1NH2, yielding a 1:1 complex with = 4 × 109 M-1, which is consistent with the finding of the MS experiments. When the mole ratio is q ≥ 2. The E0‘ of the Ag−PAMAMG1NH2+/0 couple shifted to a more negative value than that of the Ag+/0 couple. The negative shift in the halfwave potential also suggests that DDM binds more strongly with Ag+ than with zerovalent silver species. Spectroscopic results suggest that hydroxyl-terminated PAMAMG2OH favors the formation of small zerovalent silver clusters after reduction while amine-terminated PAMAMG1NH2 allows for simultaneous formation of both clusters and larger nanoparticles at similar conditions. Other quantities, such as diffusion coefficients of the complexes and molar absorptivity of the Ag+ DDMs, are also reported