A Simple Ratiometric Probe System for the Determination of Water Content in Organic Solvents

Abstract

Determination of water content in organic solvents is very important in many of chemical processes and industrial applications. Chromatography and Karl Fischer titration are one of the most widely used techniques for this purpose. However, chemosensors are more convenient and simple to use in routine laboratory manipulations. Among the chemosensing systems, chromogenic or fluorogenic sensors are particularly attractive due to their easy of signaling and signal transducing method. Many interesting sensing systems have been reported including merocyanine dyes, chromone, flavone derivatives, membrane and opticalfiber sensor employing acridine dyes, and metal complex of Ru(bpy)(CN)4 , and are well reviewed in recent report. Fluorescein and its related compounds are known to have an interesting fluorescence signaling behavior in many biological systems. The spectral properties of fluorescein derivatives are strongly affected by the microenvironments and seems to be promising as molecular probes for the assessment of polarity of the biological systems. Particularly, it is well-known that the fluorescence of some fluorescein derivatives is strongly dependent on the media and the presence of a specific chemical stimulus. Based on this, we tried to elucidate the possibility of the fluorescein for the signaling of water content in common organic solvents. In preliminary studies, the UV-vis and fluorescence spectra of fluorescein 1 were found to be significantly affected in response to the increase in water content in common organic solvents. The significant fluorescence signaling behavior of the fluorescein was ratiometrically analyzed in reference to the relatively unaffected fluorescence of anthracene as an internal standard. In fact, the devised system exhibited a sensitive ratiometric fluorogenic behavior in response to the changes in water content of acetone and acetonitrile. First, UV-vis spectral behavior of the fluorescein 1 was studied in aqueous acetonitrile (Figure 1). In 100% acetonitrile solution, compound 1 exhibited no significant absorption band above 400 nm, which means that the fluorescein exists mainly in lactone form. As the water content increased, a strong absorption band around 452 and 488 nm emerged and steadily increased. That might be due to the shift in equilibrium of 1 toward ring-opened form over spectroscopically inactive lactone form. The changes in absorbance at 488 nm were not so pronounced up to 5% water then a significant enhancement was observed with further increasing water content in acetonitrile. Next, the changes in fluorescence of fluorescein in response to the water content in aqueous acetonitrile were measured. As discussed earlier, the fluorescence of 1 was strongly dependent on the water content in aqueous organic solvents. In 100% acetonitrile, compound 1 exhibited a very weak fluorescence around 541 nm, which is due to the fact that the fluorescein exists predominantly in lactone form in organic solvents. As the water content increased up to around 2%, the emission of 1 was significantly enhanced (20-fold) with some blue shift to 529 nm. In response to the further increase in water content from 2 to 50%, further blue shift in absorption maximum from 529 to 521 nm with another large (5.5-fold) fluorescence enhancement was observed. The chemosensing behavior of fluorescein was found to be efficient and a sensitive signaling of water content in aqueous acetonitrile seems to be realizable. However, the