Deep eutectic solvent-coated paper-based device for colorimetric detection of chromium(VI)

Abstract

Hexavalent chromium, Cr(VI), is one of the heavy metal ions often found in the environment that is highly toxic and classified as a carcinogen. In this work, rapid screening and onsite monitoring of Cr(VI) using paper-based has been developed. Furthermore, a natural-based deep eutectic solvent, comprising choline chloride and thymol that makes it an environmentally friendly solvent, is applied as a coating medium on the test strips for qualitative and quantitative analysis of Cr(VI) for the first time. 1,5-Diphenylcarbazide was used as an indicator and complexing agent for Cr(VI). The detection was conducted by using a smartphone equipped with an application for color analyzing. The result shows that at pH 2, the DES-fabricated test strip can detect Cr(VI) within 10 min. The calibration curve displays good linearity in the range of 250–5000 µg/L with R2 of 0.9997. The LOD and LOQ of the method are 103 and 343 µg/L, respectively, and the RSDs are less than 7.2%. This developed method demonstrates high selectivity towards Cr(VI), however some cations may interfere in high concentration. Furthermore, the method has successfully applied for determination of Cr(VI) in real environmental water samples, exhibiting an acceptable and satisfactory recovery (82-104%).