Browsing by Subject "Electrochemical analysis"
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Item Amperostatic-potentiometric detection for Micro LC(Texas Tech University, 1996-05) Siddiqui, Aftab A.The current trend of separation science is towards miniaturization. Micro separations such as microcolumn liquid chromatography (Micro LC) and capillary electrophoresis have gained considerable importance over the past decade. The early developments of Micro LC were mainly due to the efforts of Horvath and coworkers [1,2], Ishii et al. [3, 4], Scott and Kucera [5, 6], and Novotny et al. [7, 8]. Since then, the field of Micro LC has been steadily growing. Higher column efficiencies, lower mobile phase flow rates, less consumption of mobile and stationary phases, and increased mass sensitivity due to smaller peak volumes are some of the appealing advantages of Micro LC [9, 10]. Also, Micro LC is ideal for the analysis of complex sample mixtures such as exotic physiological fluids and the contents of a single cell [11,12]. In order to maintain the separation efficiency obtained from Micro LC, extra-column volume in injection, detection and connection systems should be minimized [10]. While injectors are commercially available to inject appropriately small amounts of sample (60 - 100 nL), connecting tubing should be minimized wherever possible in order to reduce the extra-column broadening. The maximum permissible detector volume for a 250 |im I.D. column packed with 5 |im particles is 400 nL [10]. Smaller capillaries demand even lower detector volumes.Item Conductometry and admittance spectroscopy of micellar solutions(Texas Tech University, 1998-05) Houlne, Michael PatrickNot availableItem Development of combined scanning electrochemical optical microscopy with shear force feedback using a tuning fork and current feedback(2001-12) Lee, Young Mi; Bard, Allen J.A technique that combines scanning electrochemical microscopy (SECM) and optical microscopy (OM) was developed. To accomplish SECM/OM, the most important aspect is the conception, design and fabrication of a special probe tip, which can serve as a light source and microelectrode. Once fabricated, the tip must then be characterized to validate all future experimental measurements. One particular probe tip that was investigated for SECM/OM contained a ring ultramicroelectrode. Theoretical SECM tip current–distance (approach) curves for all ring electrodes studied were calculated by numerical (finite element) analysis. The SECM curves obtained were a function of the geometry of the tips including the thickness of the ring and the insulating sheath. Comparison of experimental and theoretical SECM curves provided a good method of evaluating the size and shape of ring electrodes. Out of the numerous tips designed and fabricated, the most reliable tip for SECM/OM was constructed by electrochemically depositing electrophoretic paint onto a gold metal film instead of an aluminum film used for a typical NSOM tip. The development of valiadation techniques for the optical and electrochemical characterization of such tips is an important part of this work. The reliable probe tip exhibited stable steady-state current and well-defined SECM approach curves for both conductive and insulating substrates. We consistently fabricated quite durable tips whose geometry was a ring with < 1 µm as an inner ring diameter. Simultaneous electrochemical/optical images of an interdigitated array (IDA) electrode were obtained with a resolution on the micrometer scale, demonstrating good performance of the tip as both an optical and electrochemical probe for imaging microstructures. Another key point of SECM/OM is that the tip must be positioned within nanometers above the substrate. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force provided a feedback to regulate tip-substrate distance as well as simultaneous topography with electrochemical and optical images. The capacity of this technique was confirmed by obtaining simultaneous topographic, electrochemical, and optical images of an IDA electrode in a constant distance mode. Imaging in this mode based on a tuning fork allowed a closer proximity between a tip and a substrate than in a constant height mode. Thus, a better spatial resolution was obtained in terms of both electrochemical and optical imaging. Application of SECM/OM to the imaging of soft biological samples was accomplished with SECM tip current rather than shear force as a feedback signal to control tip-sample proximity. Imaging in a constant current mode was an excellent imaging tool for soft materials because it preserves the benefits of the constant distance mode but eliminates the strong interaction between a tip and samples, which may damage the samples.Item Electrochemical evaluation of nanocarbons for biogenic analyte detection(2007-12) Lyon, Jennifer Lee, 1980-; Stevenson, Keith J.This dissertation explores the use of nanocarbons both as conductive supports for redox enzyme electrochemistry and as electrocatalytic components for the nonmediated detection of biogenic analytes. More specifically, the influence of nitrogen doping of these nanocarbons (referred to herein as nitrogen-doped carbon nanotubes, or N-CNTs) on their bioelectrocatalytic performance is studied through direct enzyme adsorption and exploitation of the N-CNTs' inherent reactivity toward H₂O₂ to create H₂O₂-based sensing strategies. Both nondoped CNTs and N-CNTs may be effectively incorporated into biogenic sensing assemblies, as demonstrated herein using a variety of electrochemical techniques. Chapter 1 gives a general overview of the scope of this research and describes previous studies conducted within our laboratories that demonstrate our CNTs' promise as biogenic electrode materials. Chapter 2 describes the chemical vapor deposition (CVD) method used to prepare both CNTs and N-CNTs and establishes their suitability for use in the detection schemes outlined in later chapters through long-term stability studies. Additionally, the redox activity of Fe nanoparticles entrapped in the CNTs as a result of this CVD growth process is examined using a host of electrochemical experiments. Importantly, the data presented in this chapter show that these Fe particles do not explain the observed electrocatalytic response of the CNTs. Chapter 3 explores the direct adsorption of horseradish peroxidase (HRP) at both nondoped and N-CNTs. Spectroscopic and electrochemical assays are used to compare the extent of HRP enzymatic activity upon immobilization at both types of CNTs. Both types of HRP/CNT composites are then utilized in a quantitative H₂O₂ sensing strategy. Chapter 4 discusses the intrinsic reactivity of N-CNTs toward H₂O₂. Koutecky-Levich plots are used to demonstrate differences in H₂O₂ consumption mechanisms between NCNTs and traditional peroxidases. By replacing HRP with N-CNTs in an amperometric glucose detection scheme, the versatility of N-CNTs as a peroxidase substitute for biogenic analyte detection is demonstrated. Chapter 5 outlines future directions for this research, including possible strategies for improving electron transfer between HRP and both types of CNTs. This chapter also presents a newly developed, mediated oxidase-substrate electrochemical detection method that can easily be modified to incorporate CNTs.Item Electrochemical Reactions and Mechanisms of Organic and Organometallic Compounds in Aprotic Solvents(Texas Tech University, 1980-08) Root, Duane KeithNot Available.Item Investigations of diffuse intermolecular electronic systems(Texas Tech University, 1992-05) Muguet, Francis F.Diffuse intermolecular electronic systems, such as the hydrated electron or the immonia and water dimers, present both a theoretical and a practical computational challenge. The hydrated electron was discovered more than 25 years ago, yet there is still no consensus on an explanation of this phenomenon. A novel model is presented here whereby the hydrated electron consists in an itinerant dihydronium radical structure. Although electrostatically neutral, the itinerant radical is shown to behave as a negative charge carrier under the influence of an electric field. Within this perspective, the hydrated electron may be considered a quasiparticle. Contrary of the absence of agreement between many experiments and the old but still popular cavity model description, the energetics in the new model, are shown to be consistent with photophysical experimental data. In order to understand negatively charged water clusters, it is also proposed that a metastable bifurcated water dimer structure is able to bind an extra electron. Prior to our studies, no ab initio computations had been able to reproduce the experimental geometry of the ammonia dimer nor to predict a water dimer anion with Franck-Conden factors agreeing with those recently found in molecular beam experiments. In both cases the potential energy surface is determined by attractors corresponding to nonlinear and linear hydrogen bonded geometries, respectively. One attracter receives an unfair advantage in the computational procedure mainly because of the basis set superposition error (ESSE). There is still no agreement on a scheme for correcting the ESSE. A widely employed error estimation method is the counterpoise correction. A completely different new method is proposed using reerthonermalizatien of purified localized molecular orbitals. In terms of a ESSE corrected potential energy surface of the water dimer, a multi-attractor model of water is very briefly discussed. For further water molecular dynamics studies, we offer a new algorithm which we have developed specifically for a massively parallel computer.