Browsing by Subject "Scanning electrochemical microscopy"
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Item Applications of scanning electrochemical microscopy in biological systems(2011-08) Koley, Dipankar; Bard, Allen J.; Crooks, Richard M.; Stevenson, Keith J.; Mullins, Charles B.; Humphrey, Simon M.The main theme in this dissertation is to develop Scanning Electrochemical Microscopy (SECM) based electroanalytical techniques to study quantitative biology in real time. The multidrug resistance (MDR) phenomenon in live cancer cells was studied using mimic drug molecules such as menadione with the aid of SECM. Real time quantitative detection of thiodione (menadione-conjugate) pumped out of the cells was determined to be 140 μM due to exposure of 500 μM menadione to the cells. Selective blocking of these MDR pumps in live intact cells was also achieved by small molecules such as MK571 as well as by the MDR specific antibody. An approximately 50% drop in thiodione flux was observed in both cases of MDR pumps inhibition. This SECM technique was also extended to measure the permeability of a highly charged hydrophilic molecule passing through the membrane of a single living cell. The permeability was measured to be 6.5 ± 2.0 × 10-6 m/s. Real time monitoring of morphological changes in a live HeLa cell due to addition of varying concentration of surfactant such as Triton X-100 was also demonstrated by SECM. This electroanalytical technique was also expanded to study quantitative microbiology. Real time quantitative detection of pyocyanin produced by Pseudomonas aeruginosa (PA14 strain) biofilm locally was determined to be 2.5 μM after 6 h. Pyocyanin (PYO) was also observed to be reduced by PA14 biofilm, thus maintaining a reduced atmosphere above the biofilm even in presence of oxygen. Spatial mapping of this reduced PYO showed that this reduced zone was only formed up to 500 μm above the biofilm. The cells are also able to modulate the height of the reduced PYO zone in accordance to the availability of Fe(III/II) in the solution to scavenge iron from the surrounding environment. Real time spatial mapping hydrogen peroxide across polymicrobial biofilm (Sg and Aa) was also achieved with the aid of SECM. The local peroxide concentration produced by Sg was measured to be 1 mM, which is significantly higher than the bulk peroxide concentration. This study also showed that the local concentration across the microbial film is more important than the bulk concentration since bacteria communicate locally in real world.Item The bromine/nitrobenzene redox flow battery : mechanism of the bromide/bromine reaction in nitrobenzene and characterization of supporting electrolytes(2015-12) Bennett, Brenton Edgar; Bard, Allen J.; Mullins, Charles B; Manthiram, Arumugam; Stevenson, Keith J; Meyers, Jeremy PThis work presents the first redox flow battery (RFB) to use the redox active liquids bromine (Br2) and nitrobenzene (NB) as both solvents and redox species. Because the supporting electrolyte (SE) is the only solute, instead of SE and an additional salt containing a redox species, the capacity of this RFB is limited only by the solubility of the SE and not by the solubility of the redox species coexisting in solution with SE. In addition to the increased capacity, this battery has a nominal cell voltage of 2.1 V, compared to 1.5 V or less for most aqueous systems. Therefore, this new all-liquid RFB system can reach energy densities more than twice that of traditional RFBs, over 100 Wh/L with 3 M SE and potentially close to 200 Wh/L with 5 M SE. In addition to the development of a prototype Br2/NB RFB, two specific aspects of this system were studied. First, the mechanism of the Br-/Br2 redox reaction in NB was characterized for the first time. As with the majority of halide/halogen reactions in nonaqueous solvents, this reaction proceeds in two distinct steps through a stable Br3- intermediate. An intermediate during Br- oxidation was identified using scanning electrochemical microscopy (SECM), and a mechanism for Br- oxidation was proposed to explain that observation. Then a full Br-/Br2 reaction mechanism, including the elementary reaction steps and relevant parameters, was developed using simulations of cyclic voltammograms in a variety of Br-, Br2, and Br3- solutions. Second, the solubility, conductivity, and viscosity of novel SEs containing the asymmetric cation dimethyldipropylammonium (DMDP+) in NB solutions were measured. This cation has the same molecular weight as the symmetric tetraethylammonium cation (TEA+), but its salts are roughly an order of magnitude more soluble in NB, sometimes exceeding 3 M. Compared to solutions with salts containing the larger tetrabutylammonium cation (TBA+), which are equally as soluble in NB, solutions with the smaller DMDP+ salts are up to seven times less viscous at 2 M concentrations. The conductivity of these solutions increases proportionally with the decrease in viscosity. Electrolysis of NB to NB-. was performed in solutions containing TBA+ and DMDP+ salts, and an increase in viscosity, accompanied by a decrease in conductivity, was observed as the electrolysis progressed. Finally, the performance of a Br2/NB RFB was evaluated using these same solutions.Item Detection of unstable intermediates and mechanistic studies in multisteps, two-electron transfer reactions by cyclic voltammetry and scanning electrochemical microscopy(2014-05) Chang, Jinho; Bard, Allen J.; Crooks, Richard M; Mullins, C.Buddie; Willets, Katherine; Rose, Michael JUnstable Sn(III) intermediates generated in the Sn(IV)/Sn(II) redox reaction in 2 M HBr + 4 M NaBr media were detected by scanning electrochemical microscopy (SECM) and cyclic voltammetry (CV). In CV, the underpotential deposition of Sn(0) and its stripping peaks severely perturbed the analysis of diffusional reactions. In SECM, however, the detection of diffusional Sn(III) bromide species was clearly observed due to the absence of the perturbation from the surface reactions. The ECEC-DISP mechanism in both the reduction and oxidation reactions was proposed via Sn(III) bromide intermediates. CVs at different concentrations of Sn(IV) and at various scan rates were fit by numerical simulations based on the proposed mechanism with good agreement. Enhanced electrochemical reversibility in the Sn(IV)/Sn(II) redox reaction was observed at the elevated temperature of 80 °C. We attributed such observation to changes in the rate of bromide loss from Sn(IV)Br₆²⁻ to Sn(IV)Br₅⁻ based on the CV simulation. In a similar approach, a short-lived intermediate, presumably bromine anion radical Br₂⁻·, was detected in the Br⁻ /Br₃⁻ electro-oxidation reaction in nitrobenzene solution by SECM and CV. The reaction mechanism was proposed based on a detected Br₂⁻· intermediate as follows: (1) the one electron transfer of Br⁻ to Br·, (2) the dimerization of 2Br· to Br₂, (3) the bromide addition reaction of Br₂ to Br₃⁻ , (4) the bromide addition reaction of Br· to Br₂⁻·, and (5) the Br· addition reaction of Br₂⁻· to Br₃⁻. The simulation based on the proposed mechanism fitted well with the experimental SECM and CV results. At last, the applicability of the Sn/Br system as electrolyte for electrochemical energy storage was tested. A redox flow battery was constructed, where the Sn(IV)/Sn(II) reduction was carried out on the negative electrode, while the Br· /Br₂ oxidation was carried out on the positive electrode during charging. Cyclability was tested up to 35 charge/discharge cycles, and 100 % coulombic efficiency was observed in all cycles. However, only 40 % of voltage efficiency was obtained, mainly due to the large irreversibility of the Sn(IV)/Sn(II) redox reaction in the bromide media.Item Scanning electrochemical microscopy studies applied to biological systems(2004) Mauzeroll, Janine; Bard, Allen J.This dissertation specifically deals with scanning electrochemical microscopy (SECM) studies of cellular transport processes that involve ion channels or activated transport proteins. To study biological systems, the substrate generation-tip collection (SG-TC) mode of SECM is used. Because of the inherent difficulty in quantifying such measurements, a two electrode system is first used to understand the general behavior of SG-TC transients. Numerical simulations confirm that the transient currents measured in SG-TC mode of SECM agree with the experimental behavior of simple electroactive compounds. The influence of the ultramicroelectrode (UME) geometry on recorded and simulated transient response is discussed. Finally, significant experimental and theoretical differences in the feedback mode of SECM for hemispherical and disk UMEs are presented. This knowledge is subsequently applied to a biomimmetic system where an ion channel is inserted in a self-assembled monolayer. The transport of thallium ions across gramicidin ion channels is detected at a nearby hemispherical mercury UME. This transport system is a model system for real biological systems. Nevertheless, important kinetic information about differences in the transport energy barrier between the two ends of the gramicidin half-channel can be obtained. The small level current measured in the above work confirmed that the SG-TC mode of SECM had the sensitivity to measure the efflux of electroactive biological material from cells. As such, the uptake of menadione and subsequent release of an electroactive biological metabolite from yeast cells is also reported. The synthesis, electrochemical characterization and detection of the menadione glutathione conjugate export from yeast cells are discussed. Kinetic treatment of the collection currents from yeast aggregates revealed that the uptake of menadione is the slow dominant step in the experiment. Finally, single human liver cells are studied using SECM measurements while exposed to cytotoxic concentrations of menadione. A determined cytotoxic dose of menadione imposes a chemical stress on hepatocytes and leads to the export of the menadione-glutathione conjugate via an ATP-dependent pump. The process is observed and imaged for both isolated and differentiated cells and has some interesting biological relevance.