Development of an electrochemical sensor for detection of 2,4-dinitrotoluene.

University of Minnesota Ph.D. dissertation. July 2012. Major: Chemistry. Advisor: Philippe Bühlmann. 1 computer file (PDF); xxi, 185 pages.The main goal of this work is to develop a sensing device that is capable of selectively binding and detecting DNT. Because fluorous sensing matrixes have been previously shown to greatly enhance the stability of host-guest complexes, the ultimate goal of this work is to incorporate DNT-selective receptors into a fluorous matrix for enhanced sensitivity and selective for DNT. Therefore, in Chapter 2, an electrolyte for performing electrochemistry with fluorous matrixes is introduced. As a proof-of-concept, cyclic voltammetry of ferrocene was performed. Quantitative fitting of the resulting CVs shows that the transfer coefficient for ferrocene in the fluorous phase is well within the range previously reported in the literature. Conversely, the ko determined by this fitting is orders of magnitude lower than in solvents typically used for electrochemistry. Using well-established Marcus theory, this discrepancy is explained. Because the electrolyte used for voltammetry is not commercially available at this time, the use of large sample volumes is undesirable. Thus, a sample cell that allows for performing cyclic voltammetry with sample volumes of 200 μL is presented in Chapter 3. This sample cell also has the potential of greatly reducing the size of samples typically used for electrochemistry experiments in undergraduate teaching laboratory experiments; thus, this chapter is presented in terms of reducing sample sizes for such experiments. Chapter 4 begins to explore the development of receptors for the detection of DNT by attempting to determine the formation constants for the interaction of DNT with aliphatic amines in so-called "Meisenheimer complexes." However, we definitively show that DNT does not form an appreciable concentration of Meisenheimer complexes in the presence of amines in dimethylsulfoxide; rather, DNT is deprotonated. While developing the theory for interpretation of the Job's plot data collected for the interaction of DNT with these amines, a new method for quantitatively interpreting these plots was developed and is presented in Chapter 5. This method allows for quantitatively interpreting the type of interaction observed with a Job's plot by comparing the area under the curve to the reactant stoichiometry. In Chapter 6, the electrochemistry of DNT is further explored using cyclic voltammetry. We observe that DNT is reduced in two well-resolved electron transfers in aprotic media. Quantitative interpretation of these CVs is immensely complicated, as the radical anion formed by reduction of DNT is sufficiently basic to deprotonate neutral, uncharged DNT. Upon addition of an acidic species, the two reduction waves coalesce into a one-step, six-electron irreversible transfer. This is explained as the reduction of the two nitro groups to N-hydroxylamino groups. Chapter 7 brings together the knowledge gained in Chapters 4 and 6 to construct a sensing device for DNT. In collaboration with Melissa Fierke from the Stein research group, a sensing electrode was constructed from 3-dimensionally ordered macroporous (3DOM) carbon. The surface of the 3DOM carbon monolith was modified by covalent attachment of a receptor molecule for DNT. Using cyclic and square wave voltammetry, the response of the electrode was characterized for DNT. These electrodes also showed selectivity over interferents commonly tested for DNT sensors. Chapter 8 explores increasing the electrochemical window for solvent/electrolyte systems. This research has a large bearing on increasing the bias voltage window accessible for electrochemical capacitor devices. While this chapter is a slight departure from the sensing theme of previous chapters, it does provide an increase in knowledge for the fundamental electrochemistry upon which electrochemical capacitors operate. Lastly, Chapter 9 summarizes the results that have been presented in this thesis and discusses the directions for possible continuation of this work