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Microelectrodes are typically used for neurotransmitter detection, but nanoelectrodes are not because there is a trade-off between spatial resolution and sensitivity, which is dependent on surface area. Cavity carbon nanopipette electrodes (CNPEs), with tip diameters of a few hundred nanometers, have been developed for nano-scale electrochemistry. Here, we characterize the electrochemical performance of CNPEs with fast-scan cyclic voltammetry (FSCV) for the first time. Dopamine detection is compared at cavity CNPEs, with a depth equivalent to a few radii, and open-tube CNPEs, an essentially infinite geometry. Open-tube CNPEs have very slow temporal response that changes over time as the liquid rises in the pipette. However, the cavity CNPEs have a fast temporal response to a bolus of dopamine that is not different than traditional carbon-fiber microelectrodes. Cavity CNPEs, with a tip diameter of 200-400 nm, have high currents because the small cavity traps and increases the local dopamine concentration. The trapping also leads to a FSCV frequency independent response and the appearance of cyclization peaks that are normally observed only with large concentrations of dopamine. CNPEs have high dopamine selectivity over ascorbic acid (AA) due to the repulsion of AA by the negative electric field at the holding potential and the irreversible redox reaction. In mouse brain slices, cavity CNPEs detected exogenously-applied dopamine, showing they do not clog in tissue. Thus, cavity CNPEs are promising neurochemical sensors that provide spatial resolution on the scale of hundreds of nanometers, useful for small model organisms or locating near specific cells.

Fast and cost-efficient detection and identification of bacteria in food and water samples and biological fluids is an important challenge in bioanalytical chemistry. It was shown recently that bacteria can be detected by measuring the decrease in the diffusion current to the ultramicroelectrode caused by cell collisions with its surface. To add selectivity to the bacteria detection, herein we show the possibility of collision experiments with the signal produced by electrochemical activity of bacterial cells reducing (or oxidizing) redox species. The mediator oxidation/reduction rate can be used to identify different types of bacteria based on their specific redox activities. Here we report the analysis of electrochemical collision transients produced by two kinds of bacteria, Escherichia coli and Stenotrophomonas maltophilia. The effects of the charge and redox activity of bacterial cells on collision events are discussed. The current transients due to live cell collisions were compared to those produced by bacteria killed either by heavy metal ions (cobalt) or by an antibiotic (colistin). This approach is potentially useful for evaluating the effectiveness of antimicrobial agents. Finite-element simulations were carried out to model collision transients.

Electrochemical experiments at individual nanoparticles (NPs) can provide new insights into their structure-activity relationships. By using small nanoelectrodes as tips in a scanning electrochemical microscope (SECM), we recently imaged individual surface-bound 10-50 nm metal NPs. Herein, we introduce a new mode of SECM operation based on tunneling between the tip and a nanoparticle immobilized on the insulating surface. The obtained current vs. distance curves show the transition from the conventional feedback response to electron tunneling between the tip and the NP at separation distances of less than about 3 nm. In addition to high-resolution imaging of the NP topography, the tunneling mode enables measurement of the heterogeneous kinetics at a single NP without making an ohmic contact with it. The developed method should be useful for studying the effects of nanoparticle size and geometry on electrocatalytic activity in real-world applications.

Ion transport controlled by electrostatic interactions is an important phenomenon in biological and artificial membranes, channels, and nanopores. Here, we employ carbon-coated nanopipets (CNPs) for studying permselective electrochemistry in a conductive nanopore. A significant accumulation (up to 2000-fold) of cationic redox species and anion depletion inside a CNP by diffuse-layer and surface-charge effects in a solution of low ionic strength were observed as well as the shift of the voltammetric midpeak potential. Finite-element simulations of electrostatic effects on CNP voltammograms show permselective ion transport in a single conducting nanopore and semiquantitatively explain our experimental data. The reported results are potentially useful for improving sensitivity and selectivity of CNP sensors for ionic analytes.

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Conductive nanopipettes have been extensively used as powerful multifunctional probes for electrochemical and ion transport measurements, while the involving charge transfer processes have not been fully explored. In this paper,...

In a typical intracellular electroanalytical measurement, a nanoelectrode is located inside a living cell and a reference electrode outside the cell. This setup faces a problem to drop a certain potential across the cellular plasma membrane that might interrupt the cellular activity. To solve this problem, a self-referenced nanopipette is assembled by incorporating a reference electrode inside the nanocapillary, with a Pt ring at the tip as the electrochemical surface. The potential applied between the Pt ring and the reference electrode is restricted inside the capillary and thus has a negligible effect on the surrounding cellular environment. Using this new setup, the nanopipette pierces into the nucleus of a single living cell for the measurement of hydrogen peroxide under oxidative stress. It is found that a lesser amount of hydrogen peroxide is measured in the nucleus compared with the cytoplasm, revealing uneven oxidative stress inside the cell. The result will not only greatly improve the current setup for intracellular electrochemical analysis but also provide biological information of the compartment inside the living cell.