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Abstract
Arsenic compounds are classified as group 1 carcinogens and the maximum contamination level in drinking water is set at 10 ppb by the United States Environmental Protection Agency. Methods currently approved for monitoring environmental arsenic are classified as colorimetric and instrumental methods. However, toxic byproducts (arsine gas and mercury) are formed in colorimetric methods, while instrumental methods require high maintenance costs. Therefore developing a sensitive, safe, and less expensive detection technique for arsenic is needed. One strategy involves the development of new imaging tools using small molecule fluorescent sensors, which will offer a sensitive and inexpensive method of arsenic detection in environmental samples. Emphasis will be on As(III) compounds because they are prevalent in the environment. To accomplish this goal, the coordination chemistry of As(III) was studied to discover new As(III) chemistry/preference for thiols. One finding from this work is the As(III)-promoted redox rearrangement of a benzothiazoline-containing compound to afford a four-coordinate As(III) complex and the benzothiazole analog. The knowledge gained was used to design two fluorescent chemodosimeters for As(III). The first generation sensors, named ArsenoFluors (AFs), were designed to contain a benzothiazoline functional group appended to a coumarin fluorescent reporter and were prepared in high yield by multi-step organic synthesis. The sensors react with As(III) to afford a highly fluorescent coumarin-6 dye (benzothiazole analog), which results in a 20 25 fold increase in fluorescence intensity and 0.14 0.23 ppb detection limit for As(III) in THF at 298 K. In addition, the reaction is complete within 30 min and is selective for As(III) over other toxic ions such as Hg(II) and Pb(II). The sensors also react with a common environmental species of As(III), namely sodium arsenite, in a THF/CHES (1:1, pH 9) buffer mixture. However, the reaction is slower (time > 5 h) and the enhancement of fluorescence is modest (1.5- to 3- fold) due to the quenching of the coumarin dye in a high polarity solvent. Finally, the mechanism for the As(III)-promoted formation of the fluorescent benzothiazole compounds from the benzothiazolines is proposed.