Toward the Detection and Identification of Single Bacteria by Electrochemical Collision Technique.

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.

Dissolution of Pt during Oxygen Reduction Reaction Produces Pt Nanoparticles.

The loss of Pt during the oxygen reduction reaction (ORR) affects the performance and economic viability of fuel cells and sensors. Our group previously observed the dissolution of Pt nanoelectrodes at moderately negative potentials during the ORR. Here we report a more detailed study of this process and identify its product. The nanoporous Pt surface formed during the ORR was visualized by AFM and high-resolution SEM, which also showed ∼5 nm sized Pt particles on the glass surface surrounding the electrode. The release of these nanoparticles into the solution was confirmed by monitoring their catalytically amplified collisions with a Hg-coated microelectrode used as the tip in the scanning electrochemical microscope (SECM).

Assessments of the Effect of Increasingly Severe Cathodic Pretreatments on the Electrochemical Activity of Polycrystalline Boron-Doped Diamond Electrodes.

The electrochemical response of many redox species on boron-doped diamond (BDD) electrodes can be strongly dependent on the type of chemical termination on their surface, hydrogen (HT-BDD) or oxygen (OT-BDD). For instance, on an HT-BDD electrode the [Fe(CN)6](3-/4-) redox system presents a reversible voltammetric behavior, whereas the oxidation overpotential of ascorbic acid (AA) is significantly decreased. Moreover, the electrochemical activity of BDD electrodes can be significantly affected by electrochemical pretreatments, with cathodic pretreatments (CPTs) leading to redox behaviors associated with HT-BDD. Here we report on the effect of increasingly severe CPTs on the electrochemical activity of a highly doped BDD electrode, assessed with the [Fe(CN)6](3-/4-) and AA redox probes, and on the atomic bonding structure on the BDD surface, assessed by XPS. The hydrogenation level of the BDD surface was increased by CPTs, leading to decreases of the total relative level of oxidation of the BDD surface of up to 36%. Contrary to what is commonly assumed, we show that BDD surfaces do not need to be highly hydrogenated to ensure that a reversible voltammetric behavior is obtained for Fe(CN)6](3-/4-); after a CPT, this was attained even when the total relative level of oxidation on the BDD surface was about 15%. At the same time, the overpotential for AA oxidation was confirmed as being very sensitive to the level of oxidation of the BDD surface, a behavior that might allow the use of AA as a secondary indicator of the relative atomic bonding structure on the BDD surface.

The analysis of estrogenic compounds by flow injection analysis with amperometric detection using a boron-doped diamond electrode.

We report on the use of flow injection analysis with amperometric detection (FIA-EC) to evaluate the potential of using diamond electrodes for the analysis of three estrogenic compounds: estrone, 17-ß-estradiol, and estriol. Amperometric detection was performed using a cathodically pretreated boron-doped diamond electrode that offered low background current, relatively low limits of detection, and good response reproducibility and stability. For all three compounds, response linearity was observed over the concentration range tested, 0.10 to 3.0µmol L(-1), the sensitivity was ca. 10mA L mol(-1), and the minimum concentration detection (S/N≥3) was 0.10µmol L(-1) (~27µg L(-1)). The response variability with multiple injections was ca. 10% (RSD) over 20 injections. For estrone, the oxidation reaction on diamond does not proceed through an adsorbed state like it does on glassy carbon. After an initial current attenuation, the diamond electrode exhibited a stable response (oxidation current) for 3 days of continuous use, indicative of minimal surface contamination or fouling by reaction intermediates and products. The method for estrone was assessed using spiked city tap and local river water. Estrone recoveries in spiked city and river water samples presented standard deviations of less than 10%. In summary, the FIA-EC method with a diamond electrode enables sensitive, reproducible, stable, quick, and inexpensive determination of estrogenic compounds in water samples.

Electrochemical activation of diamond microelectrodes: implications for the in vitro measurement of serotonin in the bowel.

The electrochemical pretreatment of diamond microelectrodes was investigated for the purpose of learning how an anodic, cathodic or a combined anodic + cathodic polarization affects the charge-transfer kinetics for two surface-sensitive redox systems: ferri/ferrocyanide and serotonin (5-hydroxytryptamine, 5-HT). The pretreatments were performed in 0.5 mol L(-1) H2SO4. The anodic pretreatment was performed galvanically for 30 s at 250 mA cm(-2). The cathodic pretreatment was performed for 180 s at -250 mA cm(-2). The combined pretreatment involved application of the anodic step first followed by the cathodic step. The results clearly demonstrate that the best performance for both redox systems is obtained after the cathodic polarization, which presumably activates the electrode by cleaning the surface and removing site-blocking surface carbon-oxygen functionalities. The cathodic pretreatment was found to be effective at activating a fouled microelectrode in situ. This observation has important implication for the measurement of 5-HT in the bowel.