Collision, Adhesion, and Oxidation of Single Ag Nanoparticles on a Polysulfide-Modified Microelectrode.

We report the collision, adhesion, and oxidation behavior of single silver nanoparticles (Ag NPs) on a polysulfide-modified gold microelectrode. Despite its remarkable success in volume analysis for smaller Ag NPs, the method of NP-collision electrochemistry has failed to analyze particles greater than 50 nm due to uncontrollable collision behavior and incomplete NP oxidation. Herein, we describe the unique capability of an ultrathin polysulfide layer in controlling the collision behavior of Ag NPs by drastically improving their sticking probability on the electrode. The ultrathin sulfurous layer is formed on gold by sodium thiosulfate electro-oxidation and serves both as an adhesive interface for colliding NPs and as a preconcentrated reactive medium to chemically oxidize Ag to form Ag2S. Rapid particle dissolution is further promoted by the presence of bulk sodium thiosulfate serving as a Lewis base, which drastically improves the solubility of generated Ag2S by a factor of 1013. The combined use of polysulfide and sodium thiosulfate allows us to observe a 25× increase in NP detection frequency, a 3× increase in peak amplitude, and more complete oxidation for larger Ag NPs. By recognizing how volumetric analysis using transmission electron microscopy (TEM) may overestimate quasi-spherical NPs, we believe we can have full NP oxidation for particles up to 100 nm. By focusing on the electrode/solution interface for more effective NP-electrode contact, we expect that the knowledge learned from this study will greatly benefit future NP collision systems for mechanistic studies in single-entity electrochemistry as well as designing ultrasensitive biochemical sensors.

Electrochemiluminescence (ECL)-Based Electrochemical Imaging Using a Massive Array of Bipolar Ultramicroelectrodes.

In this report, we describe the fabrication, characterization, and use of a massive array of closed bipolar ultramicroelectrodes (UMEs) in electrochemical imaging applications. The bipolar UME array is 1 cm2 in size and contains >146 000 carbon electrodes embedded in a 15 µm thick insulating and freestanding membrane of Parylene C. Structural characterization with optical and electron microscopies shows that the carbon UMEs are highly uniform in size, shape, and interelectrode spacing. The bipolar UME array was used in electrochemical imaging to probe highly dynamic redox processes in which the reduction of redox molecules on one side the array is electrically coupled to an oxidative electrochemilumescence (ECL) process on the opposite side. This allows one to simultaneously monitor electrochemical reactions on hundreds of thousands of individual electrodes with millisecond temporal resolution. Our results suggest that microfabricated closed bipolar UME arrays can be useful for imaging fast and transient electrochemical processes in which scanning probe methods are inapplicable due to their limited temporal resolution.

Temporally-Resolved Ultrafast Hydrogen Adsorption and Evolution on Single Platinum Nanoparticles.

The hydrogen evolution reaction (HER) was studied on single platinum nanoparticles (NPs) using a microjet NP collision system. By ejecting NPs onto a closely positioned ultramicroelectrode (UME), one can study single-particle collision electrochemistry in acid concentrations as high as 3 M or 750 mM on the electrode surface. This is nearly 2 orders of magnitude greater than previously reported in NP-collision studies. The use of high-bandwidth recording and higher acid concentration allows us to temporally resolve the ultrafast hydrogen adsorption and evolution processes in HER on single colliding Pt NPs. Moreover, the electroactive surface area (EASA) and roughness factor ( Rf) of single Pt NPs can be readily derived from the integrated hydrogen adsorption charge. The use of high acid concentrations is critical toward obtaining a full monolayer of adsorbed hydrogen before overlapping with the evolution process. This method allows one to study electrochemical and electrocatalytic behavior of metal NPs in a chemical environment that is close to that used in real applications, for example, fuel cells and electrolyzers.

Electrodeposited Gold on Carbon-Fiber Microelectrodes for Enhancing Amperometric Detection of Dopamine Release from Pheochromocytoma Cells.

Exocytosis is an ultrafast cellular process which facilitates neuron-neuron communication in the brain. Microelectrode electrochemistry has been an essential tool for measuring fast exocytosis events with high temporal resolution and high sensitivity. Due to carbon fiber's irreproducible and inhomogeneous surface conditions, however, it is often desirable to develop simple and reproducible modification schemes to enhance a microelectrode's analytical performance for single-cell analysis. Here we present carbon-fiber microelectrodes (CFEs) modified with a thin film of electrodeposited gold for the detection of exocytosis from rat pheochromocytoma cells (PC12), a model cell line for neurosecretion. These new probes are made by a novel voltage-pulsing deposition procedure and demonstrate improved electron-transfer characteristics for catecholamine oxidation, and their fabrication is tractable for many different probe designs. When we applied the probes to the detection of catecholamine release, we found that they outperformed unmodified CFEs. Further, the improved performance was conserved at cells incubated with L-DOPA (l-3,4-dihydroxyphenylalanine), a precursor to dopamine that increases the quantal size of the release events. Future use of this method may allow nanoelectrodes to be modified for highly sensitive detection of exocytosis from chemical synapses.