Browsing by Author "Chen, Xin"

Now showing 1 - 5 of 5
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    Development of Electrochemical Sensors for Analytical and Biomedical Applications
    (2019-08) Chen, Xin
    The focus of this dissertation is on two main topics: the development of chemical sensors with reduced biofouling for applications in biological samples (Chapter I–II), and the development of chemical sensors with improved biocompatibility (Chapter III–V). Conventional polymeric membrane-based ion-selective electrodes (ISEs) rely on plasticized poly(vinyl chloride) (PVC) as sensor membranes. The plasticizers that solubilize PVC backbone—a prerequisite for PVC-phase ISEs—leach out gradually, resulting in a limited sensor lifetime. Polar groups in the plasticizer may also lower the sensor selectivity. To improve selectivity and expand working ranges, fluorous-phase ISEs relying on nonpolar perfluorinated compounds as sensing membrane were developed. A novel fluorophilic ionophore was synthesized and used to make ionophore-doped fluorous-phase ISEs with Nernstian responses and an optimal working range centered around neutral pH—suitable for most biological samples. The reproducibility of fluorous-phase ISEs was enhanced by a new electrode body design. Importantly, fluorous-phase ISEs maintained their excellent selectivity after prolonged exposure in serum whereas PVC-phase ISEs lost selectivity considerably. Insights were also obtained on the optimal ionophore-to-ionic site ratio. To improve biocompatibility, silicone-based reference and ion-selective electrodes were developed to eliminate plasticizers. Reference electrodes doped with several ionic liquids showed sample-independent and long-term stable potentials in artificial blood electrolytes and serum samples. Potassium-selective silicone-based ISEs developed with two ionophores and two silicones showed Nernstian responses and good selectivities. In an attempt to prevent leaching of ionophores from ISE membrane into samples, a well-known potassium ionophore was covalently attached to silicone membranes. Miniaturized microelectrodes suitable for implantable devices were also developed based on this platform. In a similar effort, plasticizer-free polymethacrylate-based ISEs exhibited Nernstian responses to pH and selectivities comparable to PVC-phase ISEs. To further improve biocompatibility for applications in the pharmaceutical and food industries, either an ionophore or ionic site or both were covalently attached to sensor membranes. Sensors with either ionophore or ionic site attached provided similar good characteristics whereas when both were attached, Nernstian responses were not found consistently. Furthermore, heating experiments showed that sensors exposed to 90 ˚C heating maintained good selectivity.
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    Global nitrogen deposition (2°×2.5° grid resolution) simulated with GEOS-Chem for 1984-1986, 1994-1996, 2004-2006, and 2014-2016
    (2018-05-31) Ackerman, Daniel E; Chen, Xin; Millet, Dylan B; dackerma@umn.edu; Ackerman, Daniel E; Ecology, Evolution, and Behavior Department, University of Minnesota
    Atmospheric deposition of inorganic nitrogen is critical to the function of ecosystems and elemental cycles. During the industrial period, humans have doubled the amount of inorganic nitrogen in the biosphere and radically altered rates of atmospheric nitrogen deposition. Despite this rapid change, estimates of global nitrogen deposition patterns generally have low, centennial-scale temporal resolution. Lack of information on annual- to decadal-scale changes in global nitrogen deposition makes it difficult for scientists researching questions on these finer timescales to contextualize their work within the global nitrogen cycle. Here we use the GEOS-Chem Chemical Transport Model to estimate wet and dry deposition of inorganic nitrogen globally at a spatial resolution of 2°×2.5° for 12 individual years in the period from 1984 to 2016. During this time, we found an 8% increase in global inorganic nitrogen deposition from 86.6 TgN yr-1 to 93.6 TgN yr-1, a trend that comprised a balance of variable regional patterns. For example, inorganic nitrogen deposition increased in areas including east Asia and Southern Brazil, while inorganic nitrogen deposition declined in areas including Europe. Further, we found a global increase in the percentage of inorganic nitrogen deposited in chemically reduced forms from 30% to 35%, and this trend was largely driven by strong regional increases in the proportion of chemically reduced nitrogen deposited over the United States. This study provides spatially explicit estimates of inorganic nitrogen deposition over the last four decades and improves our understanding of short-term human impacts on the global nitrogen cycle. We provide all output from these GEOS-Chem simulations related to atmospheric deposition. We provide all output from these GEOS-Chem simulations related to atmospheric deposition.
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    Sufficient dimension reduction and variable selection.
    (2010-12) Chen, Xin
    Sufficient dimension reduction (SDR) in regression was first introduced by Cook (2004). It reduces the dimension of the predictor space without loss of information and it is very helpful when the number of predictors is large. It alleviates the “curse of dimensionality” for many statistical methods. In this thesis, we study the properties of a dimension reduction method named “continuum regression”; we propose a unified method – coordinate-independent sparse estimation (CISE) – that can simultaneously achieve sparse sufficient dimension reduction and screen out irrelevant and redundant variables efficiently; we also introduce a new dimension reduction method called “principal envelope models”.
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    "Twitter Archeology" of Learning Analytics and Knowledge Conferences
    (2015-03) Chen, Bodong; Chen, Xin; Xing, Wanli
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    Understanding the fuel of atmospheric chemistry: Comprehensive analyses of volatile organic compounds (VOC) measured from aircraft missions in the past decade (2010-19)
    (2021-09) Chen, Xin
    Volatile organic compounds (VOC) and their secondary products degrade human health, modulate the atmosphere’s self-cleaning capacity, and alter Earth’s climate on short timescales through formation of ozone and secondary aerosol. Current scientific understanding of atmospheric VOCs is limited in several important respects. A large number of well-instrumented aircraft missions have been conducted in the past decade, providing a wealth of new information for addressing these gaps. This dissertation research focuses on integrated analyses of these datasets using a chemical transport model (GEOS-Chem) to advance our knowledge of the sources and fate of VOCs.From an integrated analysis of aircraft campaigns conducted over North America between 2010 and 2014, biogenic emissions are identified as the predominant source of reactive organic carbon even over many major cities. Current state-of-the-art modeling persistently underestimates the abundance of VOCs in the upper troposphere due to inadequate treatment of vertical transport. However, the overall model-observation differences are dominated by a modest number of individual compounds that merit prioritization in future work. Next, the budget of formic acid (HCOOH) in the remote atmosphere is characterized for the first time using in-situ measurements, based on observations collected during the 2016-2018 ATom aircraft mission. Background HCOOH levels are well-captured by the model without the need to invoke a broad-scale missing source in the remote atmosphere, as had previously been suggested. The dataset shows clear evidence of HCOOH wet scavenging and ocean uptake, with the former underestimated and the latter well-captured by the current model treatment. Fire and other pollution sources are the main drivers of HCOOH variability in remote environments, with HCOOH found to be an unexpectedly large gas-phase reactive organic carbon reservoir in aged fire plumes. I apply data from the 2015-2017 NAAMES campaigns to perform the first direct, aircraft-based quantification of ocean-atmosphere VOC fluxes over the ocean. Eddy covariance analyses over the Western Subarctic Atlantic reveal net ocean emissions of dimethyl sulfide, primarily deposition of methanol and acetone, and bidirectional exchanges for acetaldehyde, acetonitrile, and methyl ethyl ketone. The largest fluxes are seen during a low-pressure system driven by extremely unstable weather conditions. Finally, methane fluxes in the eastern US are constrained through inverse analysis of airborne data from the 2016-2019 ACT-America study. A GEOS-Chem adjoint inversion framework indicates that current bottom-up inventories underestimate oil and natural gas emissions in the southcentral US and livestock emissions in the central US. Northern wetlands tend to release much lower methane than prior estimates, while seasonally varying biases are seen along the southeastern US coast.