Browsing by Subject "SECM"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
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 effects of polymicrobial metabolism on pathogenesis and survival in Aggregatibacter actinomycetemcomitans(2011-08) Ramsey, Matthew M.; Whiteley, Marvin; Marcotte, Edward; Meyer, Richard; Trent, Stephen M.; Walker, JamesIn this dissertation I describe a model system to characterize the response of an oral bacterial pathogen, Aggregatibacter actinomycetemcomitans to the metabolic byproducts of a representative member of the oral flora, Streptococcus gordonii. A. actinomycetemcomitans is a causative agent of periodontal infections in humans. To cause infection, A. actinomycetemcomitans must overcome numerous challenges, including the host immune system and toxic metabolite production from other microbes. The most numerically dominant flora in the oral cavity are oral streptococci, which are well known for their ability to produce copious amounts of lactic acid and H₂O₂. By studying the response to H₂O₂ and lactic acid in pure and co-cultures, I have demonstrated that A. actinomycetemcomitans responds to these metabolites by several novel mechanisms that both enhance its survival in the presence of the host immune system and in the presence of the model oral streptococci S. gordonii. These studies have demonstrated that metabolites produced by normal flora can impact the survival of a single species in vivo as much as previously known virulence factors have done. In addition, I present a new method for measuring metabolite production in an attached cell population. This method is a novel application of scanning electrochemical microscopy (SECM) and I used this technique to study H₂O₂ production in the three dimensional space surrounding a multispecies biofilm in real time. In a related study I present the use of SECM to discover a novel redox chemistry phenomenon in the opportunistic pathogen Pseudomonas aeruginosa.Item The electrogenerated chemiluminescence of novel organic donor-acceptor emitters as well as study heterogeneous electron transfer kinetics using scanning electrochemical microscopy(2011-08) Shen, Mei; Bard, Allen J.; Crooks, Richard M.; Stevenson, Keith J.; Vandenbout, David A.; Manthiram, ArumugamNew modalities and novel emitters were investigated for the production of electrogenerated chemiluminescence (ECL). In annihilation ECL, a light-emitting excited state is formed upon reaction of two electrochemically generated species, typically a radical anion and a radical cation. Donor-acceptor (DA) molecules provide a means of generating these two reactive species within the same molecule but where the oxidized and reduced centers are separated; furthermore, they allow one to explore the ECL properties of multiply charged radical ions. Three new efficient ECL-emitting donor-acceptor molecules were investigated. The effects of conjugation in the electrochemistry of diphenylaminospirobifluorenylfumaronitrile (FPhSPFN), which has the structure of D-X-A-A-X-D, where X is a linker, as well as the effects of the stability of its (multiply charged) radical ions on its red ECL emission (λmax= 708 nm) were studied; the molecule shows solvatochromism and different emission yields on both photoluminescence and ECL in benzene:acetonitrle mixtures. The possibility of generating ECL through multiply charged radical ions was further tested with the very efficient 1b emitter (4,7-bis(4-(4-sec-butoxyphenyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole). Two reversible oxidations and one reduction were observed. The more sluggish reduction is proposed to be a consequence of a long distance electron transfer to the buried acceptor center; further confirmation of this effect was pursued by application of the scanning electrochemical microscope (SECM) to model systems. 1b emits intense ECL with λmax= 635 nm and with an efficiency 330% of the ECL standard 9, 10-diphenylanthracene and similar intensity to the red emitting standard tris(2,2′-bipyridine)ruthenium(II) perchlorate (Rubpy). The generation of asymmetric chronoamperometric ECL pulses upon generation of radical anion-radical dication annihilation events was explained by the use of digital simulation, and proven to be a consequence of asymmetry in the amount of generated charges rather than instability of the electrogenerated species. ECL was also produced from a film of a red fluorophore 1a (4,7-bis(4-(n-hexyl)-5-(3,5-di(1-naphthyl)phenyl)thiophen-2-yl)-2,1,3-benzothiadiazole) with a coreactant in PBS buffer solution. The electrochemical synthesis of carbon quantum Dots (C QDs) in inert atmosphere was explored using highly oriented pyrolytic graphite as the starting material, for its later use in the production of ECL in the radical annihilation mode. FT-IR (ATR), mass spectrometry (desorption chemical ionization), Raman and TEM analysis were used to characterize the C QDs.Item Platinum catalysts degradation by oxide-mediated platinum dissolution in PEMFCs (Proton Exchange Membrane Fuel Cells)(2011-12) Kim, Seok koo 1973-; Meyers, Jeremy P.Proton exchange membrane fuel cells (PEMFCs) have attracted great attention due to their high power density, low-temperature operation and high energy conversion efficiency. However, the high cost of Pt catalysts and durability problems hinder their commercialization. So their cost must be lowered drastically and their durability must be extended. In an effort to overcome these problems, there have been intensive efforts to enhance the activity, durability and to lower the price of catalysts by alloying with other less expensive metals. In particular, the sluggish kinetics of ORR caused by Pt oxide at cathode and Pt catalyst degradation by electrochemical surface area (ECSA) loss have been a huge research area where a lot of researchers have paid lots of attention to solve. In this regard, the objective of this dissertation is to evaluate a series of Pt catalyst electrode surface electrochemical reactions on PEMFC electrode in order to help searching new catalysts and enhancing system design, assist in the search for new catalysts and improved system design by suggesting the developed mechanism of electrocatalyst activity and stability (durability). We have been focused on understanding the oxide-mediated dissolution of Pt by using electrochemical experiment methods such as RRDE, EQCN, SECM with a combination of ICP-MS and computational simulation with COMSOL Multiphysics. Firstly, in chapter 3, we showed the oxide-mediated Pt dissolution rate and the influence of hydrogen and cation underpotential deposition on Pt dissolution. In chapter 4, we revealed oxygen reduction reaction (ORR) plays a significant role in Pt oxide formation and reduction that influences the Pt catalyst dissolution, resulting in accelerated Pt dissolution rate at specific potential range. Finally, we found out the nature of mobile species generated during PtO₂ reduction process which have been disputed as Pt ion or other mobile species and fulfilled computational simulation for evaluation of SECM experiment in chapter 5. Based on these experiments and simulation, we were able to explain some mechanism of literature results that already were reported but have not been clearly explained so far. In terms of the purpose of this dissertation, the mechanism of oxide-mediated Pt dissolution, influence of ORR to Pt oxide formation/reduction and Pt dissolution, the nature of mobile species generated during PtO₂ reduction process, are sure to be very helpful in developing new catalysts and enhancing system design and suggested operating conditions.Item The use of scanning electrochemical microscopy for the detection and quantification of adsorbed intermediates at electrodes(2010-08) Rodriguez Lopez, Joaquin, 1983-; Bard, Allen J.; Crooks, Richard M.; Stevenson, Keith J.; Mullins, C. B.; Henkelman, GraemeScanning electrochemical microscopy (SECM) was used for the study and characterization of catalytic and electrocatalytic processes occurring at electrodes. The Surface Interrogation mode (SI-SECM) was introduced for the detection and quantification of adsorbed intermediates and products of catalyzed chemical and electrochemical reactions at noble metals (Pt, Au). In SI-SECM two micro electrodes (i.e. an SECM tip and a substrate of the desired material) are aligned concentrically at a micrometric distance where SECM feedback effects operate. A contrast mechanism based on feedback effects allows for the detection of reactive adsorbed intermediates at the substrate: the SECM tip generates a reactive homogeneous species that “micro-titrates” the substrate adsorbates to yield an electrochemical signal that contains information about the amount of intermediate and about its kinetics of reaction with the redox mediator. The technique was used for the study of the reactivity of three model small adsorbates: 1) the reactivity of adsorbed oxygen on Au and Pt with a reducing mediator was explored and suggested the detection of “incipient oxides” at these surfaces; kinetic parameters of the reactivity of Pt oxides with mediators were obtained, fit to theory and used to explain observations about the electrocatalytic behavior of Pt under anodizing conditions; 2) the reactivity of oxidizing mediators with adsorbed hydrogen on Pt was studied and showed the cation of N,N,N,N-tetramethyl-p-phenylenediamine (TMPD) to be a successful interrogation agent, the detection of hydrogen generated by the decomposition of formic acid on Pt at open circuit was investigated; 3) electrogenerated bromine was used to catalytically interrogate carbon monoxide at Pt, this reaction was previously unreported. The mentioned applications of SECM were validated through the use of digital simulations of diffusion in the complex SECM geometry through flexible commercial finite element method software.