A smartphone-based ratiometric fluorescent device for field analysis of soluble copper in river water using carbon quantum dots as luminophore.

Sensitive detection of copper is valuable for environment and human healthy. Herein, we reported a smartphone-based ratiometric fluorescent device for field analysis of soluble copper in river water samples, which is based on the fluorescence quenching effect of Cu to N-doped carbon quantum dots. The fluorescent images in the sample and reference areas in a twin cuvette were captured by the camera in a smartphone. The non-linearity response of the image brightness to the exposure was corrected by (B-B)/(255 -B), where B and B are the brightness with and without luminophore, respectively. It was shown that corrected brightness ratio is insensitive the variation in light source intensity, ambient temperature, shooting parameters and focus of the camera, which is helpful to improve the stability and reliability of the cheap portable detection device. By using a balanced calibration method, the accuracy for concentration estimation is improved further. Under optimized conditions, the proposed method was applied to detect Cu in the range from 1 to 200 nM with the detection limit of 0.43 nM. This simple fluorescence device offers the advantages of portability, reliability and accuracy for field analysis.