Early and accessible diagnostics are important elements to reducing the negative side-effects of untreated disease. One key advancement in diagnostic monitoring is through the development of highly sensitive sensors that have the capability to detect lower concentrations, while still remaining accessible for point-of-care use. This dissertation characterizes electrochemical sensing platforms that are enhanced by convection and by 3D electrodes made from high surface area, vertically aligned carbon nanotubes (VACNTs). Free-standing VACNTs were patterned into microchannel arrays for flow-through amperometric sensing. Convective mass transfer enhancement was shown to improve sensor performance in amperometric sensing through the use of high surface area to fluid volume structures and concentration boundary layer confinement. Through-flow sensing of hydrogen peroxide produced drastically higher signals than stirred sensing, with over 90% of the hydrogen peroxide being oxidized as it passed through the channels. Non-enzymatic sensing of glucose was achieved by chemical reaction of glucose with methyl viologen to produce on average 3.4 electrons per glucose molecule, significantly higher than that obtained with enzymatic sensing with glucose oxidase. A scaled down sensor enabled detection from 200 μL of glucose by flow injection analysis with a limit of detection of 360 nM and a linear sensing range up to at least 150 μM glucose. Such sensing range offers the potential to measure glucose levels found in saliva. This work demonstrates the utility of high aspect ratio electrodes made of VACNTs. Convection and surface area are shown to enhance the sensitivity of flow-through VACNT amperometric sensors by effectively utilizing the available analyte to increase the measured current density. Advances in nanomaterials, combined with electrochemical impedance spectroscopy, have allowed impedimetric biosensors to have high sensitivity while remaining label-free, pushing towards enabling portable diagnosis at the point-of-care. Porous, 3D VACNT electrodes for impedance-based biosensing were fabricated with different electrode height, gap width, and configuration. Sensitivity was characterized by functionalizing the representative protein streptavidin onto VACNT electrodes for detection of biotin. Tall, closely-spaced VACNT interdigitated electrodes are shown to have the highest electroactive surface area (15x the 2D geometric area) and the highest sensitivity, allowing for a 1 ng/mL limit of detection. Aspect ratio and surface area are shown to be important factors in determining the sensitivity of 3D VACNT interdigitated electrodes for impedimetric sensing of biomolecules bound to electrode surfaces. Although this biosensing platform is shown with streptavidin and biotin, it could be extended to other proteins, antibodies, viruses, and bacteria.

This study used electrospinning to fabricate a polyvinylidene fluoride (PVDF) piezoelectric nanofiber harvesting device with interdigitated electrode to capture ambient energy. According to d33 mechanical-electric energy conversion mode, the energy harvesting device can be applied on the low frequency ambient vibration and impact abilities for the transformation mechanical energy into electrical energy effectively. First, the PVDF powder was mixed in acetone solution uniformly and the dimethyl sulfoxide (DMSO) was mixed with multi-walled carbon nanotube (MWCNT) to prepare PVDF macromolecular solution. The mixed solution was filled in a metals needle injector and contacted hundreds of voltage. After the PVDF drop in the needle was subjected to high electric field, the drop overcame surface tension of the solution itself, then extremely fine PVDF fiber was formed and spun out. The electrospun was collected orderly using X-Y digital control stage and the linear diameter of electrospun can be controlled easily by adjusting the travelling speed of the stage. In the spinning process, as affected by stretching strain and electric field at the same time, the PVDF piezoelectric fiber resulted in electric polarization and transformed £] piezoelectric crystal phase, in which the dipoles are oriented in the same direction. Furthermore, MWCNT was added to improve the mechanical properties of fiber and increase £] phase, to enhance the tensile strength and piezoelectric property of PVDF fiber effectively. Finally, the photolithography was used to fabricate interdigitated electrodes with 100£gm gap on the flexible PI substrate. The PVDF fibers, with a length and diameter of approximately 1cm and 700-1000nm, were aligned on interdigitated electrodes and packaged with the PI film. In order to increase the conversion efficiency of piezoelectric fiber in d33 mode, the PVDF fibers were repolarized in a high electric field. The results showed that the PVDF fiber energy harvesting device can generate 15mV open-circuit voltage under low frequency vibration of 4Hz and generate above 30mV open-circuit voltage under 6Hz vibrations. As compared with the piezoelectric fiber not repolarized by interdigitated electrode, its output voltage was increased by1- 2 times.

Methods for integrating enzyme systems with electrochemical reactions having applications to diagnostic sensing are described. Diagnostic tests that include biological molecules can be classified as biosensors. Existing testing methods often require trained technicians to perform, and laboratory settings with complex infrastructure. The theme of this dissertation is the development of methods that are faster, easier to use, and more applicable for non-laboratory environments. These goals are accomplished in systems using enzymatic catalysis and galvanic processes. Two biosensors with specific model pathologies have been designed and demonstrated in this study. The first assay senses a DNA fragment representing the Epstein Barr virus and uses enzyme-mediated Ag deposition over a v microfabricated chip. The chip contains a specially designed pair of electrodes in an interdigitated array (IDA). Detection is signaled by a change in the resistance between the two electrodes. The second biosensor discussed in this study is targeted towards the digestive enzyme trypsin. It is selfpowered due to its construction within an open-circuit galvanic cell. In this system, a small volume of blood serum is introduced onto the device over barriers made of protein and Al that block the anode from solution. In the presence of trypsin, the protein gel is rendered more permeable to sodium hydroxide. Adding hydroxide initiates the dissolution of the Al layer, closing the cell circuit and illuminating a light-emitting diode (LED). A relationship was observed between LED illumination time and trypsin concentration. Biosensors that utilize enzymes to generate or amplify a detectable signal are widely used, and the final project of this study uses a nanoparticle based approach to protect the catalytic activity of alkaline phosphatase (AlkP) from hostile chemicals. By incubating Au colloid with AlkP overnight and adding Ag+, core@shell nanoparticles of Au@Ag2O can be isolated that show AlkP activity. The resulting enzyme-metal composite material was analytically characterized and demonstrated greater activity in the presence of organic inhibitors relative to either wild type vi or Au colloid-associated AlkP without the Ag2O shell. The stabilization procedure is complete in one day using a onepot synthesis. This method may provide opportunities to carry out biosensing chemistry in previously incompatible chemical environments. / text