A novel double metal-dithizone functionalized polyurethane electrospun nanofiber and film for colorimetric determination of hexavalent chromium

Abstract

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), R² 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.