Pattern-Based Discrimination of Carboxylate Anions in Aqueous Media and on Solid Supports
Carboxylate-containing species play important roles in biosynthesis, energy generation and biological signaling; they are also common functional groups in pharmaceuticals, and potential pollutants in aqueous media. Sensing and discrimination of carboxylates was achieved using non-covalent interactions mediated by poly(amidoamine) (PAMAM) dendrimers, a family of macromolecular polyelectrolytes.Our focus was on fast, sensitive, and inexpensive detection methods based on optical spectroscopy. However, neither the dendrimers nor the analytes have spectroscopic signals in the visible region, so we first designed and tested an indicator displacement assay (IDA), built from PAMAM dendrimers and common organic dyes, whose response was monitored through absorbance, fluorescence emission and fluorescence anisotropy (Chapter 2). We achieved qualitative and quantitative discrimination among structurally similar carboxylates using pattern-based clustering methods applied to the output of the IDA sensing system (Chapter 3). It is noteworthy that we obtained not only qualitative discrimination (where pattern recognition commonly excels) but also quantitative measurements, a much rarer accomplishment. In Chapter 4, the scope was extended to the discrimination of β-lactam antibiotics, a family of environmentally and biologically relevant analytes that contain carboxylate groups and comprise more than half of the world market for antibiotics. A novel partial hydrolysis method, introduced here, significantly increased the affinity of these compounds for the PAMAM hosts; this in turn allowed us to successfully differentiate samples drawn from an analyte panel comprising two penicillins, five cephalosporins, and two references. In Chapter 5 we move these carboxylate sensing methods to solid supports with lower cost and higher stability, on a path towards the construction of a portable, deployable device. Studied support media included cellulose acetate, SiO2 dispersions, regular printing paper, filter paper, and chromatography paper. Chromatography paper, the best performer, yielded excellent differentiation and repeatability, and improved long-term stability over the same system in water. Finally, Chapter 6 describes our efforts to improve the determination of the concentration of chloroform, a halogenated byproduct of water chlorination and a dangerous water pollutant. We attempted to improve upon the common colorimetric Fujiwara test, using the intensely colored Fujiwara product as an optical filter to modulate the fluorescence emission of organic fluorophores.