Paper-Assisted Ratiometric Fluorescent Sensors for On-Site Sensing of Sulfide Based on the Target-Induced Inner Filter Effect

Dissolved sulfide tends to species transformation and loss upon leaving the matrix, thus the development of a practical on-site determination of sulfide is crucial for environmental monitoring and human health. In this work, a novel paper-based ratiometric fluorescence sensor was developed for the field analysis of sulfide, which system was constructed by the inner filter effect of CdS quantum dots (QDs) toward carbon dots (C-dots). Instead of an aqueous phase system, the conversion of sulfide to its hydride would induce the in-situ formation of CdS QDs on the paper, which acted as an energy acceptor to quench the emission of C-dots, leading to a variation from blue to yellow with the increasing concentration of sulfide. Moreover, we proposed a smartphone-based fluorescence capture device integrated with a programmed Python program, accomplishing both color recognition and accurate detection of sulfide. This ratiometric fluorescence sensor allowed for the on-site analysis of sulfide with a limit of detection of 0.05 μM and the field analysis of environmental water samples. So far, a portable fluorescence device, especially for sulfide analysis was still limited, this developed system is label-free, low-cost, and easy to operate, showing great potential for the measurement of sulfide on-site and environmental monitoring.