This work proposes a highly specific method of Cr6+ determination based on the double reactions of two metals, Co2+ with dithizone to form a (DTZ)-Co2+ complex, and the replacement of Co2+ in the formed complex with Cr6+. The fast degradation of DTZ in solution in wet analysis was overcome by preparing dithizone functionalized polyurethane nanofibers that were electrospun into a membrane (DTZ/PU-NF) and a microwell plate film (DTZ/PU-MPF). For comparison, the performance of diphenylcarbazide (DPC), a currently used complexing agent for Cr6+, was also investigated. Colour changes were detected as red-green-blue values. The DTZ/PU-NF was smooth, with an average diameter of 384.09 nm and no bead appeared. A dense network structure was formed. The best formulation of DTZ, PU and Co2+ was also applied as a microwell plate film. In the presence of Cr6+, the colour of DTZ-Co2+ changed from red to magenta. Among the three studied methods, the colorimetric DTZ-Co2+/PU-NF presented the best results. Its linearity range was 0.001-1.0 mg L-1, with a regression equation of Cr6+ = -0.189 + (0.0056 × red) + (0.0086 × green) - (0.0129 × blue), R 2 of 0.990. The limit of detection was 0.001 mg L-1 and the precision was 1.7%. The applicability of DTZ/PU-NF was validated for Cr6+ in vegetable oils with recoveries of 89.5-116.8%. The sensitivity of DTZ/PU-NF was ten times higher than that of DTZ/PU-MPF. The methods based on DTZ-Co2+/PU-NF and DTZ-Co2+/PU-MPF proved to be highly selective, rapid, user-friendly, simple and reliable.
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