The Development of a Selective Colorimetric Sensor for Cu²⁺ and Zn²⁺ in Mineral Supplement with Application of a...

Herein, we developed a colorimetric method for the determination of Cu²⁺ and Zn²⁺ using NBD-G as a novel selective metal sensor. NBD-G was easily synthesized by a nucleophilic substitution reaction between 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl) and Girard's Reagent P. The NBD-G solution is yellow, but when it reacts with Cu²⁺ and Zn²⁺, its color changes selectively to red (510 nm) and orange (480 nm), respectively. NBD-G was used as a sensor for Cu²⁺ and Zn²⁺, showing a high sensitivity down to 0.77 µM for Cu²⁺ and 1.66 µM for Zn²⁺. NBD-G could determine both metals simultaneously; thus, it was applied to determine them in multimineral supplements, which showed excellent recoveries. Next, a filter paper impregnated with NBD-G was prepared as a test paper, and a simple, selective, and rapid onsite method for quantifying Cu²⁺ was developed as, interestingly, the paper showed no change upon the addition of Zn²⁺. Next, Cu²⁺ could be quantified with high selectivity and accuracy by photographing the color change with a smartphone camera and processing the image with Image J. The detection limit for Cu²⁺ using this method was 3.9 µM. Finally, the NBD-G test paper method was able to satisfactorily quantify Cu²⁺ spiked into the rainwater.