Abstract
Six fluorescence based probes for the optical detection of pH (Figure 1) were synthesized, purified, characterized, and tested. To make these fluorescent probes I used rhodamine 6G and rhodamine B. The probes contain a cycloalkyl-amine group, which reacts to different pH levels. The probes provide a sensitive measurement of pH by undergoing a colorimetric and fluorescent reaction as an aqueous environment changes from basic to acidic. At high pH, the methylamino group interacts with the fluorophore by forming a colorless, non-fluorescent, spiro-cyclic structure. As the pH is lowered, near the pKa of the methylamino group, the spiro-amine is protonated and the ring opens resulting in a brightly colored, highly fluorescent species. This series of rhodamine derivatives differ by the type of alkyl substitution on the methylamino nitrogen atom. Specifically the cyclic alkyl systems were compared and characterized the effect those systems have on the pKa of the spiro-amine ring opening process. It was found that the series of cyclic amines pH response correlated well with the Baeyer strain theory on how ring structure affects the basicity of amines by showing that an increase in the size of the ring structure directly increases the bascicity.