Biological volatile organic compounds (Bio-VOCs) play crucial roles in living organisms such as plants, microbes, and the animals. Sub-ppm level of Bio-VOCs could work as indicators to provide information about metabolism or hormones to facilitate different stages of growth in an organism. For example, less than 25 ppb of ethylene can reduce flowering time, increase seed weight and promote ripening of plants. Thus, there is a need for sensitive detection to provide valuable information for in situ monitoring of biological ecology, as well as for environmental controlling and managing. In situ monitoring of Bio-VOCs requires highly sensitive detections (at mostly sub-ppm concentration level) using portable and accurate sensors, which is extremely challenging for most analytical methods currently available. This work examines the feasibility of an intensified capture and detection strategy for detection of trace amounts of Bio-VOCs, with the sensor unit suitable for miniaturized design for eventually remote and unmanned vehicle sensing applications. In the first part of the study, activated carbon fibers were developed using a reductive reduction procedure, and were examined for pre-concentrating of Bio-VOCs (from ppb-level raised to ppm-level). We expect the reductive carbonization can effectively remove the oxygenated groups from the cellulosic materials, producing fine-tuned electronic properties, which promote pi-pi interaction for intensified the adsorption of nonpolar VOCs (especially for multiple pi bonds compounds). The such produced carbon fibers were examined by XPS (X-ray Photoelectron Spectroscopy), which showed that 53% of the carboxylic and hydroxy groups have been successfully removed. The performance of reductive treated carbon fibers as an adsorbent was examined. Three nonpolar VOCs, methane, ethylene and benzene, were selected as typical biological and chemical VOCs. A sixteen-times increase of benzene (has multiple pi bonds) adsorption can be observed in comparison to carbon fibers without reductive treatment. The unique network structure of the reductive treated carbon fibers also provides a fine electrical conductivity (6.36 Ωcm), that makes it possible for electrothermal desorption for material regeneration. A full regeneration of VOCs was observed in repeated adsorption-desorption cycles, indicating excellent reusability and stability of reductive carbonized carbon fibers. When applied as a pre-concentration absorbent, the carbon fibers successfully increased the concentration of typical VOCs from 500 ppb to 3.5 ppm (700% increase) within 20 min. In order to develop a miniaturized sensor with ultra-high sensitivity and stability for in situ monitoring of Bio-VOCs, the electrochemical sensing system was employed because it is able to identify and quantify various VOCs with high accuracy and sensitivity. However, most of traditional electrochemical sensors have been developed for analysis of aqueous samples, they are easily impacted by the evaporation of water (changing the concentration of electrolyte) when applied to gaseous samples as concerned in the current work. To improve the stability of electrochemical sensing, we developed a unique thin film ionic liquid (IL)-gel coated sensor employing ionic liquids in poly(acrylamide) hydrogels as a solution-free electrolyte. We assumed the stability of the analysis can be improved since the solution of the electrochemical system can be locked in a gel phase, minimizing the evaporation. The ion liquids also have been selected as an electrolyte since the acidity of IL can facilitate the detection of ethylene (one critical Bio-VOCs), preventing the oxidization of the working electrode before ethylene oxidization. A series of experiments were conducted to confirm the performance of IL-gel coating sensor. The results showed the sensor has excellent sensitivity and linearity of our sensor with low detection limit to 650 ppb and 0.99 of R2 values within 0~15 ppm. Decent stability was obtained with a relative standard deviation below 1% for 1.5 months of storage. In addition, the strategy of using reductive fabricated carbon fibers as a pre-concentrating material and a thin film IL-gel coated sensor as a detection unit was also examined. Overall, our work successfully demonstrated the capture-detection strategy is suitable for stable detecting of an extremely low (sub-ppb level) concentration of VOCs. By integrating the preconcentration and senor units, we could eventually develop sensors that are capable of detecting VOC samples in the order of ppb. This approach is promising for building up miniatured Bio-VOCs sensors for in situ monitoring in future applications.
University of Minnesota M.S. thesis. December 2018. Major: Microbial Engineering. Advisor: Ping Wang. 1 computer file (PDF); x, 74 pages.
Activated Carbon Fibers from Cellulosic Biomass with Surface Reductive Treatment for Air Cleanup and VOCs Sensing.
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