This award by Chemical Measurement and Imaging (CMI) program supports work by Professor Isiah Warner and his collaborators at Louisiana State University & Agricultural and Mechanical College to explore the theoretical foundation of using quartz crystal microbalance (QCM) for both mass and molecular weight determination of volatile organic compounds at the presence of a group of uniform materials based on organic salts (GUMBOS) and cellulose acetate (or other polymers). This research is based upon the researcher's recent discovery that in the presence of aforementioned composite sensing materials, the ratio of two parameters, which are simultaneously acquired during QCM measurements, can provide molecular weight estimates of absorbed molecules. In the proposed studies, the research groups will (1) conduct a detailed examination of sensing composites to achieve a full understanding of their characteristics to provide an appropriate ratio of df to dR; (2) develop computational methods and theoretical models to better understand how these composites function at a fundamental level; and (3) evaluate overall morphology and other properties of composites that contribute to optimal use in QCM measurements and analytical applications. Professor Warner will continue to actively engage undergraduate and graduate students of diverse backgrounds in science, technology, engineering, and mathematics (STEM) education.

Project Report

Volatile organic compounds (VOCs) are emitted from a variety of natural and anthropogenic sources. There has been an ever-expanding need to develop reliable sensors for detection and discrimination of these chemical species. VOC sensors are used in extensive applications ranging from medical diagnosis to environmental monitoring. Among a number of currently used sensors, the quartz crystal microbalance (QCM) is an attractive option because it is simple, compact, and suitable for creation of multiple sensor arrays. A typical QCM comprises a quartz wafer inserted between two metallic electrodes, and this quartz crystal is coated with a suitable sensing material to generate a QCM sensor. Our project was focused on evaluating the VOC sensing characteristics of composites of ionic liquids or GUMBOS coated QCM sensors. Ionic liquids (ILs) are most often defined as organic salts that melt below 100 °C. A group of uniform materials based on organic salts (GUMBOS) are solid phase analogues of ionic liquids with melting points between 25 to 250 °C. ILs and GUMBOS have been demonstrated to be excellent sensing materials since they are easy to synthesize, exhibit high chemical and thermal stability, and their physicochemical properties can be easily modulated. In our studies, binary blends of ILs (or GUMBOS) and polymers were immobilized onto the surface of a quartz crystal, and the resulting sensor was exposed to a wide range of organic vapors. Upon exposure to these vapors, the QCM sensor exhibited changes in two parameters. One of these parameters involves a change in frequency (Δf) and the other parameter a change in motional resistance (ΔR) or change in dissipation factor (ΔD). It is interesting to note that most previous studies are based on measurement of only Δf, and therefore, such QCM sensors are almost exclusively used for mass measurements Due to the viscous/viscoelastic characteristics of ionic liquids/ GUMBOS and the binary blends obtained from them, two parameters can be obtained in our QCM studies. We have utilized these two QCM parameters to obtain qualitative, as well as quantitative information about an analyte. Interestingly, we have also demonstrated a direct relationship of the ratio of these two QCM parameters to the molecular weight of vapor absorbed by the coating materials. We have determined that this relationship can be best expressed as follows: *See attachment In this equation, k' and k are constants that are dependent on the coating. Similar relationships can be obtained if ΔD is replaces ΔR because these two quantities are directly proportional. Our in-depth analyses have revealed that Δf is dependent on the mass of vapor absorbed, while ΔR (or ΔD) was found to depend on the viscosity change of the coating materials. In fact, previous studies have shown that the viscosity of ILs depend on the mole fraction of the absorbed vapors. Hence, it follows that ΔR (or ΔD) depends on the moles of vapors absorbed irrespective of their characteristics. We have also synthesized different novel GUMBOS and their vapor sensing characteristics were analyzed using a QCM sensor which shows that these materials exhibit excellent sensitivities to a wide range of organic vapors. Our finding is very useful for detection, discrimination, and molecular weight determination of organic vapors. In addition, we have developed a QCM virtual sensor array in which a single QCM sensor is used for discrimination of different closely related organic vapors. Our findings should assist in easy detection and differentiation of different real-word samples. We are currently assessing the quality of food samples by analyzing the volatile organic compounds emitted from these sources. In addition, we are analyzing different gasoline samples in order to assess gasoline quality.

National Science Foundation (NSF)
Division of Chemistry (CHE)
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Michelle Bushey
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Louisiana State University
Baton Rouge
United States
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