Strategies for Enhancing the Performance of Chemical Sensors Based on Microcantilever Sensors

Microcantilever (MC) based chemical sensors have become more widely used during the past 10 years due to the advantages they possess over other chemical sensors. One of the most significant characteristics is their extremely high surface to volume ratio. This key facet allows surface forces that can be ignored on a macroscale to become a significant sensing transduction mechanism. MC based sensors also exhibit a higher mass sensitivity to adsorbates than do many other chemical sensor platforms. Under many conditions, MC based sensors directly translate changes in Gibbs free energies due to analyte-surface interactions into mechanical responses. However, the widespread application of MCs in the field of sensors has yet to be fully realized. This is primarily due to the lack of a unifying methodology and instrumentation that would allow various research groups to benefit from a combined wealth of knowledge on the subject. The underlying goal of this research is to broaden the depth and scope of knowledge of MC based chemical sensors. By working on several areas in a coherent order, the limitations of MC based sensors have been determined and largely overcome. The information gathered in all aspects of this project will be useful to present and future researchers in this field. The initial research was focused on the application of various chemical films to MC sensors to be able to measure a wide range of chemical species. In one case, thin films of polymeric gas chromatography (GC) phases were deposited onto V-shaped MCs. A main strength to using GC phases was that the responses of the analytes could be predicted before hand by using the McReynolds constants of the phases used. This allowed for the detection and quantification of various chemical species using these moderately selective phases. vi During this phase of research it was discovered that methods for enhancing MC response were needed to overcome some of the traditional problems facing MC based sensors. By employing a new type of underlying nanostructured metallic film, MC response was greatly enhanced. This resulted in a better limit of detection and wider dynamic range relative to previous results with smooth surface MCs. In addition to advances resulting from nanostructuring, important advances were made in MC coating strategies. The widely used and well-characterized process of physical vapor deposition was used to deposit both organic and polymeric materials onto the MC surface. This process allowed for uniform films to be deposited with tailored thicknesses and for individual MCs on a single chip to be coated selectively. Another approach involving the immersion of MCs into fused silica capillaries containing solutions of thiolated materials was also developed. This method also allowed for individual MCs in an array to be selectively coated. Finally, out of these results and a developing trend of using sensor arrays came the need to increase the robustness and selectivity of MC based systems. Two different systems for achieving these goals were developed. First, a simple differential system based upon dual diode lasers was constructed in order to eliminate common sources of noise and non-specific interactions that decrease the dynamic range of these sensors. This system was also applied to the quantification of individual components in a binary mixture. While this system has met only limited success, it has been a beneficial first step towards MC systems of higher order. Towards that goal, a system designed to measure multiple MCs simultaneously using an array of vertical cavity surface emitting lasers was also used. This system measures the responses of multiple MCs exposed to an vii analyte in a single run and provides unique response patterns for that analyte. This allowed for the qualitative analysis of a simple mixture to be performed

Exploiting the use of a liquid waveguide capillary cell for spectrophotometric determinations in flow-based systems

In this thesis, the use of a liquid waveguide capillary cell (LWCC) was exploited for the spectrophotometric determination of several analytes in different types of water. With the purpose of in-line sample handling, different flow approaches were used for the development of simple, robust, cheap and automated analytical procedures. The first procedure was based on a sequential injection analysis (SIA) system for the determination of iron in coastal waters. With the goal of reaching low levels of iron, a LWCC was coupled to the system. This procedure used a doubled-line SIA system to improve mixing conditions between sample and reagents. The detection was based on a colorimetric reaction and two different reagents (ferrozine and 1,10-phenanthroline) were tested. The absorbance measurements were carried out at the wavelengths of 512 and 562 nm for the detection of iron-1,10-phenathroline and iron-ferrozine complex, respectively. An interference study was performed for both reagents. The developed method was also applied to natural waters (river, well, ground, potable and sea waters) and then compared with the reference procedure. One certified reference water sample was used to test the accuracy of the developed method. The objective of the second work was to determine iron at lower levels than the previous work and, as a consequence, to measure the levels of iron in ocean waters. With this in mind, a LWCC and a pre-concentration resin were coupled to a multisyringe flow injection analysis (MSFIA) system. Two different pre-concentration resins (Chelex 100 and NTA Superflow) were tested and evaluated. The determination of iron was also based on a colorimetric reaction and two reagents were tested, ferrozine and ammonium thiocyanate. The reactions were monitored at the wavelengths of 480 and 562 nm for the detection of iron-ammonium thiocyanate and iron-ferrozine complex, respectively. The accuracy was assessed using a certified reference water sample. A multi-parametric system for the spectrophotometric determination of zinc and copper at low levels in waters was the third work of this thesis. To attain this objective, a LWCC was coupled to a MSFIA system. The developed procedure for both analytes was based on a colorimetric reaction with zincon reagent at different pH values and monitored at 620 nm. Zincon reagent reacts only with copper at pH 5 and with copper and zinc at pH 9. An interference study for both determinations was carried out. The developed work was also applied to natural waters and three certified reference water samples. Subsequently, a multi-pumping flow system (MPFS) coupled with a LWCC was developed for the determination of titanium. This determination was based on the colorimetric reaction of titanium with chromotropic acid and the absorbance measurements were carried out at 425 nm. An interference study was performed in order to evaluate possible interferences. The developed procedure was applied to natural waters as well as to sunscreen formulations (the results were compared with the reference procedure). The accuracy was assessed with one certified lake sediment. The development of a spectrophotmetric method for bromate determination in waters at trace levels was the last work of this thesis. With this objective, a LWCC was coupled to a MPFS. The proposed methodology was based on a colorimetric reaction and two different colour reagents were tested, chlorpromazine and trifluoperazine. The lack of repeatability detected in this approach led to the development of a FIA approach in order to find out the reasons of this occurrence.Nesta tese, utilizou-se uma célula de fluxo de percurso óptico longo na determinação espectrofotométrica de vários analitos em diferentes tipos de água. Com o propósito de manusear/transportar as amostras, vários sistemas de fluxo foram utilizados de forma a desenvolver procedimentos analíticos mais simples, robustos, automatizados e de baixo custo. O primeiro procedimento baseou-se no uso de um sistema de injecção sequencial para a determinação de ferro em águas estuarinas. Com o objectivo de determinar os níveis baixos de ferro presente neste tipo de amostra, acoplou-se ao sistema uma célula de fluxo de percurso óptico longo. Este procedimento utilizou um sistema de injecção sequencial de duplo canal de forma a melhorar a mistura entre amostra e reagentes. A detecção baseou-se numa reacção colorimétrica e dois reagentes foram testados (ferrozina e 1,10-fenantrolina). As medidas espectrofotométricas foram realizadas aos comprimentos de onde de 512 e 562 nm para a detecção dos complexos formados de ferro-1,10-fenantrolina e ferro-ferrozina, respectivamente. Realizou-se um estudo de possíveis interferentes para ambos os reagentes. O método desenvolvido foi igualmente aplicado a diferentes tipos de água (águas de rio, poço, mina, mar e amostras de água potáveis) e os resultados obtidos foram comparados com o procedimento de referência. Usou-se uma amostra de água certificada de forma a comprovar a exactidão do método desenvolvido. O segundo método desenvolvido teve como principal objectivo determinar ferro em concentrações mais baixas do que no trabalho anterior e assim atingir os valores de ferro presentes em águas do mar. De forma a poder atingir este objectivo, uma célula de percurso óptico longo e uma coluna de pré-concentração foram acopladas a um sistema de fluxo baseado numa multi-seringa. Dois tipos de colunas de pré-concentração, Chelex 100 e NTA Superflow, foram testadas e avaliadas. A determinação de ferro baseou-se numa reacção colorimétrica e foram testados dois reagents (ferrozina e tiocianato de amónio). As medidas espectrofotométricas foram realizadas aos comprimentos de onda de 480 e 562 nm para a detecção dos complexos formados de ferro-tiocianato de amónio e ferro-ferrozina, respectivamente. A exactidão deste procedimento foi avaliada através de uma amostra de água certificada. O terceiro procedimento realizado nesta tese envolveu um sistema multiparamétrico para a determinação espectrofotométrica de zinco e cobre em águas a concentrações baixas foi. Com este intuito, acoplou-se uma célula de fluxo de percurso óptico longo a um sistema de fluxo baseado numa multi-seringa. O procedimento desenvolvido para ambos os analitos baseou-se numa reacção colorimétrica com o reagente zincon a diferentes valores de pH e monitorizada a 620 nm. A pH 5 o reagente zincon reage apenas com o cobre e a pH 9 com ambos. Foi efectuado para ambos os analitos um estudo de possíveis interferentes. O procedimento desenvolvido foi igualmente aplicado a diferentes tipos de amostras de água e a três amostras de águas certificadas. O quarto método desenvolvido visou determinar titânio a concentrações muito baixas. Neste procedimento utilizou-se um sistema de fluxo baseado em micro-bombas com uma célula de fluxo de percurso óptico longo acoplada. A determinação baseou-se novamente numa reacção colorimétrica entre o titânio e o ácido cromotrópico e a detecção foi realizada a 425 nm. Foi efectuado um estudo de interferentes com o intuito de avaliar possíveis interferências. O procedimento desenvolvido foi aplicado a diferentes tipos de água e a amostras de cremes solares (os resultados obtidos para os cremes solares foram comparados com o procedimento de referência). A exactidão deste método foi avaliada através de uma amostra certificada de um sedimento de lago O desenvolvimento de um método espectrofotométrico para a determinação de bromato em águas a concentrações baixas foi o último trabalho desta tese. Com este objectivo utilizou-se uma célula de fluxo de percurso óptico longo acoplada a um sistema de fluxo baseado em multi-bombas. A metodologia proposta baseou-se igualmente numa reacção colorimétrica e foram testados dois reagentes (cloropromazina e trifluoperazina). A falta de repetibilidade detectada neste sistema levou ao desenvolvimento de um sistema de análise por injecção em fluxo com o objectivo de determinar as causas deste acontecimento

Development of a Separation-Based Sensor using Microdialysis Coupled to Microchip Electrophoresis with Electrochemical Detection for Monitoring Catecholamines

Microdialysis is a powerful separation technique capable of simultaneously monitoring multiple analytes in the extracellular fluid of the brain. This technique generates small sample volumes in a continuous flow stream. Traditional methods used for sample analysis forfeit temporal information regarding dynamic neurochemical processes due to the larger volumes necessary for analysis. Additionally, sample acquisition methods traditionally involve some form of tethering or anesthetizing the animal under study, greatly reducing the available behavioral information. In order to preserve both temporal resolution and behavioral information, the ideal analysis system is one that can be employed on-line, has fast analysis times of small sample volumes, and can be placed on a freely-roaming animal. Microdialysis sampling coupled on-line to microchip electrophoresis with electrochemical detection creates a separation-based sensor that fulfills these constraints. The ability to place the device directly on-animal, without tethering, allows for the neurochemical information to be correlated with the animal’s behavior, allowing for further understanding of the neurochemical basis behind each behavior. Additionally, neuroactive drug metabolism can be monitored alongside behavior when employing an on-animal separation-based sensor, potentially aiding in drug development. The goal of this thesis is therefore to develop a separation-based sensor that is capable of monitoring neurochemicals in vivo. Towards this aim, the separation and detection of analytes in the dopamine metabolic pathway was accomplished using microchip electrophoresis with electrochemical detection at a carbon electrode. The substrate material in this separation was also optimized. In order to integrate this separation and detection with microdialysis sampling, a novel fabrication procedure was developed. This procedure creates a PDMS/glass hybrid device capable of integrating hydrodynamic microdialysis flow with electrophoretic flow and detection at a carbon electrode using a flow-gated interface. Lastly, the developed method was used to monitor the dopamine metabolic pathway in vivo in rat after the administration of L-DOPA. In the future, the complete device and associated instrumentation can be used remotely and on-animal, for near-real time in vivo monitoring

Development of In-Situ sensors for Nutrients in Marine Waters

In particular, I focussed on the development, improvement, and deployment of autonomous analysers for on-site monitoring of nutrients that have shown promise for further application in field monitoring either on shore or in the water column (in-situ).Macronutrients include nitrate, nitrite, phosphate, and silicic acid. These macronutrients are important for primary production, and silicic acid is essential for the hard tissue of siliceous phytoplankton (e.g., diatoms). The importance of macronutrients for photosynthesis means that their supply rates have major implications for the functioning of phytoplankton communities, which are considered an carbon dioxide sink in the ocean. The analysis of macronutrients is still more difficult than physical parameters such as salinity and temperature or dissolved oxygen. In addition, analysis of macronutrients in a complex matrix such as seawater can be challenging because their concentrations are typically low, in the range from nanomolar to micromolar. This thesis reports on the development of an electrochemical approach for the determination of orthophosphate in estuarine and seawater, reporting the initial investigations of our technique with a further application in an automated analyser which uses a bipotentiostat approach to enhance data quality. In addition, the improvement of on-site determination of macronutrients with a long-term application in estuarine and coastal water is reported

Light-emitting diodes and photodiodes in the deep ultra-violet range for absorption photometry in liquid chromatography, capillary electrophoresis and gas sensing

Absorbance measurement in the deep ultra-violet range (below 300 nm) has been one of the most widely used detection methods for analytical techniques as a large number of organic compounds have strong absorption bands in the deep UV region. The use of incandescent or discharge lamps coupled to a monochromator for the wavelength selection in a conventional UV detector makes it complex and costly. Light-emitting diodes (LEDs) for the deep UV range commercially available in recent years have become potential alternatives to thermal light sources. LEDs with their relatively narrow emission bandwidths (typically 20 nm) are well suited for absorption photometry in which a monochromator is not required. This dissertation, therefore, concerns the utilization LEDs and photodiodes (PDs) in the deep UV range as radiation sources and light detectors, respectively for absorption photometry in high-performance liquid chromatography (HPLC), capillary electrophoresis (CE) and gas sensing. LEDs were known to perform as light detectors. In measuring systems based on LEDs as light sources, PDs have been normally employed for detection devices. The practical reasons for the use of LEDs as alternatives to PDs, however, have not been demonstrated. Only an advantage of cost-saving was pointed out. In the first project, the performance of LEDs in the light intensity measurement was investigated and compared to that of standard silicon PDs in three different measuring configurations: current follower mode to measure to photocurrents, photovoltaic mode to determine the voltage developed across the diode on irradiation without load and discharge time mode to measure the rate to discharge the junction capacitance of diodes. LEDs as detectors were generally found to be adequate for the analytical work but PDs offered higher sensitivity and linearity as well as provided stable readings with faster settling times. Absorbance detectors for narrow-column HPLC (250 μm inner diameter) and CE (50 μm inner diameter) based on deep UV-LEDs and PDs selective for emission wavelengths were developed and evaluated in the quantification of model compounds at 255 and 280 nm. Absorbance measurements were directly obtained by the use of a beam splitter and PDs for reference signals and a logarithmic ratio amplifier-based circuitry to emulate the Lambert-Beer’s law. Narrow-column HPLC is useful for the applications in which the reduction in eluent consumption is desired or only limited amount of samples is available when utmost sensitivity is not required. In CE, the use of a capillary as the separation channel to minimize the peak broadening downscales the detection window to micrometer range which is even much narrower than that of a narrow-bore HPLC. This makes the design and construction of these LED-based detectors for narrow detection channels more challenging than for a standard HPLC as the higher efficiency for light coupling and stray light avoidance is essentially required. Additionally, high mechanical stability is needed to minimize the noise resulted from mechanical fluctuations. The performance of these optical devices at two measured wavelengths was excellent in terms of the baseline noise (low μAU range), linearity between absorbance values and concentrations (correlation coefficients > 0.999) and reproducibility of peak areas (about 1%). Not only was the potential of a deep UV-LED as a radiation source for absorption spectroscopy investigated for separation techniques but also for the detection of benzene, toluene, ethylbenzene and the xylenes compounds in the gas phase at 260 nm. In the first part of this work, its performance in the acoustic waves excitation was preliminarily investigated with some different measuring systems for the detection of the toluene vapor. It was found that the intensity of a deep UV-LED was insufficient to produce detectable acoustic signals. This was followed by the construction of an absorbance detector for the determination of these target compounds based on the combination of a deep UV-LED and PDs. This optical device was designed to use optical fibers for the light coupling from the LED to a measuring cell and a reference PD, that allows removing a beam splitter previously required for detectors of a narrow column HPLC and CE. Its performance with regard to linearity and reproducibility was satisfactory. Detection limits of about 1 ppm were determined. It could be concluded that viable absorbance detectors for narrow-column HPLC, CE and gas sensing based on deep UV-LEDs and PDs as light sources and light detectors, respectively can be constructed. The performance of these inexpensive LED-based optical devices with regard to linearity, reproducibility and baseline noise was satisfactory and found to be comparable to that of more complex and expensive commercial detectors. These detectors with features of low power consumption and small size are useful for portable battery-powered devices

Enhancing the limit of detection of biomarkers in serum using a SPRi nano-aptasensor

Surface Plasmon Resonance imaging (SPRi) is a label-free, ultrasensitive detection method for monitoring biomolecular interactions in real-time with high throughput. Diagnostic biomarkers for cancer, cardiovascular disease, and Alzheimer’s disease are often in low abundance in serum, presenting many challenges for their detection. SPRi has great potential as a diagnostic tool because its limit of detection (LOD) for many biomarkers falls in the nanogram per milliliter range, but in order to further enhance its usefulness, its LOD must be reduced to even lower concentrations. We have developed a detection scheme that improves SPRi sensitivity by several orders of magnitude. This increase in sensitivity relies upon the integration of SPRi with nanomaterials and microwave-assisted surface functionalization. This approach makes it possible for the SPRi biosensor to detect C-reactive protein in spiked human serum at concentrations of 5 fg/ml or 45 zeptomole. This scheme was then compared to commercial ELISA kits for the detection of human Growth Hormone, which has a LOD of 1 ng/ml. In order to directly compare the two platforms the antibody sandwich assay was copied in the SPRi scheme and with nanomaterial enhancement, an LOD of 9.2 pg/ml was achieved

Analysis of relevant technical issues and deficiencies of the existing sensors and related initiatives currently set and working in marine environment. New generation technologies for cost-effective sensors

The last decade has seen significant growth in the field of sensor networks, which are currently collecting large amounts of environmental data. This data needs to be collected, processed, stored and made available for analysis and interpretation in a manner which is meaningful and accessible to end users and stakeholders with a range of requirements, including government agencies, environmental agencies, the research community, industry users and the public. The COMMONSENSE project aims to develop and provide cost-effective, multi-functional innovative sensors to perform reliable in-situ measurements in the marine environment. The sensors will be easily usable across several platforms, and will focus on key parameters including eutrophication, heavy metal contaminants, marine litter (microplastics) and underwater noise descriptors of the MSFD. The aims of Tasks 2.1 and 2.2 which comprise the work of this deliverable are: • To obtain a comprehensive understanding and an up-to-date state of the art of existing sensors. • To provide a working basis on “new generation” technologies in order to develop cost-effective sensors suitable for large-scale production. This deliverable will consist of an analysis of state-of-the-art solutions for the different sensors and data platforms related with COMMONSENSE project. An analysis of relevant technical issues and deficiencies of existing sensors and related initiatives currently set and working in marine environment will be performed. Existing solutions will be studied to determine the main limitations to be considered during novel sensor developments in further WP’s. Objectives & Rationale The objectives of deliverable 2.1 are: • To create a solid and robust basis for finding cheaper and innovative ways of gathering data. This is preparatory for the activities in other WPs: for WP4 (Transversal Sensor development and Sensor Integration), for WP(5-8) (Novel Sensors) to develop cost-effective sensors suitable for large-scale production, reducing costs of data collection (compared to commercially available sensors), increasing data access availability for WP9 (Field testing) when the deployment of new sensors will be drawn and then realized

Biosensor system with an integrated CMOS microelectrode array for high spatio-temporal electrochemical imaging, A

2019 Fall.Includes bibliographical references.The ability to view biological events in real time has contributed significantly to research in life sciences. While optical microscopy is important to observe anatomical and morphological changes, it is equally important to capture real-time two-dimensional (2D) chemical activities that drive the bio-sample behaviors. The existing chemical sensing methods (i.e. optical photoluminescence, magnetic resonance, and scanning electrochemical), are well-established and optimized for existing ex vivo or in vitro analyses. However, such methods also present various limitations in resolution, real-time performance, and costs. Electrochemical method has been advantageous to life sciences by supporting studies and discoveries in neurotransmitter signaling and metabolic activities in biological samples. In the meantime, the integration of Microelectrode Array (MEA) and Complementary-Metal-Oxide-Semiconductor (CMOS) technology to the electrochemical method provides biosensing capabilities with high spatial and temporal resolutions. This work discusses three related subtopics in this specific order: improvements to an electrochemical imaging system with 8,192 sensing points for neurotransmitter sensing; comprehensive design processes of an electrochemical imaging system with 16,064 sensing points based on the previous system; and the application of the system for imaging oxygen concentration gradients in metabolizing bovine oocytes. The first attempt of high spatial electrochemical imaging was based on an integrated CMOS microchip with 8,192 configurable Pt surface electrodes, on-chip potentiostat, on-chip control logic, and a microfluidic device designed to support ex vivo tissue experimentation. Using norepinephrine as a target analyte for proof of concept, the system is capable of differentiating concentrations of norepinephrine as low as 8µM and up to 1,024 µM with a linear response and a spatial resolution of 25.5×30.4μm. Electrochemical imaging was performed using murine adrenal tissue as a biological model and successfully showed caffeine-stimulated release of catecholamines from live slices of adrenal tissue with desired spatial and temporal resolutions. This system demonstrates the capability of an electrochemical imaging system capable of capturing changes in chemical gradients in live tissue slices. An enhanced system was designed and implemented in a CMOS microchip based on the previous generation. The enhanced CMOS microchip has an expanded sensing area of 3.6×3.6mm containing 16,064 Pt electrodes and the associated 16,064 integrated read channels. The novel three-electrode electrochemical sensor system designed at 27.5×27.5µm pitch enables spatially dense cellular level chemical gradient imaging. The noise level of the on-chip read channels allow amperometric linear detection of neurotransmitter (norepinephrine) concentrations from 4µM to 512µM with 4.7pA/µM sensitivity (R=0.98). Electrochemical response to dissolved oxygen concentration or oxygen partial pressure (pO2) was also characterized with deoxygenated deionized water containing 10µM to 165 µM pO2 with 8.21pA/µM sensitivity (R=0.89). The enhanced biosensor system also demonstrates selectivity to different target analytes using cyclic voltammetry to simultaneously detect NE and uric acid. In addition, a custom-designed indium tin oxide and Au glass electrode is integrated into the microfluidic support system to enable pH measurement, ensuring viability of bio-samples in ex vivo experiments. Electrochemical images confirm the spatiotemporal performance at four frames per second while maintaining the sensitivity to target analytes. The overall system is controlled and continuously monitored by a custom-designed user interface, which is optimized for real-time high spatiotemporal resolution chemical bioimaging. It is well known that physiological events related to oxygen concentration gradients provide valuable information to determine the state of metabolizing biological cells. Utilizing the CMOS microchip with 16,064 Pt MEA and an improved three-electrode system configuration, the system is capable of imaging low oxygen concentration with limit of detection of 18.3µM, 0.58mg/L, or 13.8mmHg. A modified microfluidic support system allows convenient bio-sample handling and delivery to the MEA surface for sensing. In vitro oxygen imaging experiments were performed using bovine cumulus-oocytes-complexes cells with custom software algorithms to analyze its flux density and oxygen consumption rate. The imaging results are processed and presented as 2D heatmaps, representing the dissolved oxygen concentration in the immediate proximity of the cell. The 2D images and analysis of oxygen consumption provide a unique insight into the spatial and temporal dynamics of cell metabolism

A Micro-Analytical System for Complex Vapor Mixtures - Development and Application to Indoor Air Contaminants.

This dissertation concerns the development of two fully integrated, automatically controlled, field-deployable Si-microfabricated gas chromatograph (µGC) prototypes, and their application to indoor-air monitoring of trace-level trichloroethylene (TCE) vapor concentrations. Each µGC prototype has a pre-trap and a partially selective high-volume sampler of conventional design, a micromachined-Si focuser for injection, dual micromachined-Si columns for separation, and an integrated array of four microscale chemiresistors with functionalized gold-nanoparticle interface films for detection. Scrubbed ambient air is used as the carrier gas. Operating conditions and control settings are user-defined through a laptop computer, providing real-time data display and continuous unattended operation. A meso-scale GC employing the same detector technology as in the µGC prototypes was adapted for the same application, and the laboratory results obtained were used to guide the design and operating conditions of the µGCs. Application of a multivariate curve resolution method for deconvoluting microsensor array responses from partially overlapping interferences was also demonstrated. The µGC prototypes were characterized in the laboratory and then field tested in Utah in a house with active TCE vapor intrusion. In the laboratory, the separation of TCE from 45 other VOCs in < 60 sec, unique sensor-array response patterns, and accurate quantification of as little as 0.12 parts per billion (ppb) of TCE were demonstrated. In the field, the projected single-microsensor detection limit was 0.052 ppb for an 8-L air sample collected and analyzed in 20 min. Above the mitigation action level (MAL) of 2.3 ppb for the field-test site, accurate TCE determinations were achieved in the presence of up to 52 documented background VOCs. Below the MAL, positive biases were observed, which are attributable to background VOCs that were unresolvable chromatographically or by analysis of the sensor-array response patterns. Spatial and temporal variations in TCE concentrations, ranging from 0.23 to 56 ppb, provided by the prototypes were in good agreement with reference method values. This is the first study to validate the performance of a µGC in the field. Results demonstrate that µGC technology could provide selective, trace-level, on-site determinations of VOCs in numerous applications relevant to occupational and environmental exposure assessment.Ph.D.Environmental Health SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91391/1/airbuff_1.pd

Theoretical and Experimental Development of Bipolar Based Fluorescence Detection for Microchip Electrophoresis

Abstract Methods for the separation and detection of reactive oxygen and nitrogen species (RNOS) at the cellular level can be useful tools for the study of the biochemical mechanisms of neurodegenerative diseases. Microchip electrophoresis (ME) is a promising analytical separation technique that can be used to separate these short-lived RNOS since it offers sub-minute analysis times, low sample volumes, and the ability for single cell analysis. Amperometric detection is one of the most popular detection methods for ME and has been used for the detection of RNOS and related antioxidants. In this thesis, a dual-channel/dual-parallel electrode system is developed to identify electroactive species based on their redox properties without the need for complicated data correction procedures. This new strategy was applied to distinguish nitrite from azide in a cell sample. Azide is a contaminant that is introduced by the filters used to remove cell debris. Microchip electrophoresis can also be coupled to fluorescence detection (FL) for the investigation of RNOS production in macrophage cells using different fluorescent dyes for specific RNOS that exhibit similar excitation and emission wavelengths. Using ME-FL, the effect of engineered carbon nanoparticles on ROS production by microglia and lung epithelial cells was investigated. In this dissertation, a novel detection method for ME was developed that takes advantage of both electrochemical and fluorescence detection. This method involves transforming an electrochemical signal to a fluorescence signal using a bipolar electrode. The new method was evaluated with two model reducible analytes using 2,7-dichlorodihydrofluorescein (DCFH2) as the fluorescence reporter. In addition, modeling of the ME-bipolar electrochemistry/ fluorescence experimental setup was performed using COMSOL Multiphysics. Programs were developed to generate bipolar cell voltammograms and to model the effect of the flow rate on the size of the fluorescence plug formed at the detector electrode. As a result of these studies, a bipolar fluorescence detection method was developed that was able to obtain low micromolar detection limits for reductive analytes. The method was further developed to obtain the bipolar fluorescence response without a potentiostat and with a simplified experimental setup. This development will be extended in the future to detect oxidizable analytes such as RNOS in cells. Additionally, chemiluminescence reporting can be used instead of fluorescence reporting to obtain better detection limits. Lastly, this system could be coupled to a miniaturized optical detection system to develop a portable microchip device capable of detecting electroactive species on-site