The Development of a Selective Colorimetric Sensor for Cu and Zn in Mineral Supplement with Application of a Smartphone Paper-Based Assay of Cu in Water Samples.

Herein, we developed a colorimetric method for the determination of Cu and Zn 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 Cu and Zn, its color changes selectively to red (510 nm) and orange (480 nm), respectively. NBD-G was used as a sensor for Cu and Zn, showing a high sensitivity down to 0.77 µM for Cu and 1.66 µM for Zn. 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 Cu was developed as, interestingly, the paper showed no change upon the addition of Zn. Next, Cu 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 Cu using this method was 3.9 µM. Finally, the NBD-G test paper method was able to satisfactorily quantify Cu spiked into the rainwater.