This investigation was carried out with the aim of verifying whether transmittance detection modes on soaked paper, which is a simple and attractive determination approach for microfluidic paper-based analytical devices, can be improved by using deep eutectic solvents (DESs) as suitable media for minimizing reflection and refraction phenomena affecting these measurements. With this purpose, we tested two DESs such as ethaline and glyceline by checking that paper soaked with both two DESs displayed a quite higher transparency than that soaked with water (the absorbance decreased of about 25%). This result is particularly important in that DESs offer the appreciable advantage of being characterized by a negligible vapor pressure, unlike water or the majority of organic media, thus allowing long-term detections or analysis of very small samples to be conducted avoiding any problem caused by the solvent evaporation. As a proof of concept, we have then compared the detection of the patent blue V dye (E131) in ethaline solutions with that of the same dye dissolved in water. These determinations were performed on paper discs defined by a circular hydrophobic barrier which were installed into a plastic holder, which was constructed on purpose by the 3D technology to fit the cell housing of a Varian Cary 50 bio benchtop spectrophotometer adopted in our spectrophotometric measurements. The results found on paper discs soaked with ethaline were totally satisfactory in that they were characterized by a good repeatability and allowed a calibration plot to be constructed in a quite wide concentration range which allowed quite low concentrations to be detected (in the μM range) and was characterized by a good enough correlation coefficient. On the contrary, the results found on paper discs soaked with water were totally unsatisfactory since they were strongly affected by both the solvent evaporation and a more marked ripple present in the recorded spectra.
