Kinetic study of natural deep eutectic solvents by digital images: The importance of preparation time in sensing applications

Abstract

The development of green solvents is a trending topic in both the industrial and the scientific community. In recent years, there has been a considerable interest in Deep Eutectic Solvents (DES) and Natural Deep Eutectic Solvents (NADES) since they are in agreement with Green Chemistry principles, their low cost, and also because of they are easy to prepare. However, no studies have been reported so far on the importance of preparation temperature and storage time of this kind of solvents. In this work, the preparation temperature, storage time and optical properties of a natural deep eutectic solvent composed of fructose, citric acid and water (1:1:5) were assayed by means of a kinetic study through digital images. The fluorescence response was monitored for at least 20 days, at preparation temperatures from 61 to 100 °C. Results showed, in most cases, an abrupt ascent and a linear decay of the signal whose slope depended on preparation temperature. 1H NMR techniques showed that no new C–C bonds were formed in NADES preparation and thus no chemical reaction was involved. Besides, by using 2D-NOESY, it was confirmed that the variation in NADES fluorescence cannot be attributed to changes in the spatial interaction modes of the eutectic system molecules. In order to demonstrate the scope of this study in analytical applications, the behavior of the eutectic system was tested with curcumin and neomycin as fluorescent and non-fluorescent molecules, respectively. The former showed a linear trend regardless of the time since preparation, while the latter only showed a linear response at short storage times.