Improved mercury ions (Hg2+) detection by composite silver nanoparticles (AgNPs) and nitrogen - Sulfur co-doped carbon dots (N, S-CDs)

Abstract

Heteroatom doping has been demonstrated as an efficient method to increase the fluorescence quantum yield efficiency of carbon dots. Co-doping with heteroatoms leads to additional active sites on carbon dots, both extending their use in sensing systems and improving the sensitivity of detection. In this study, the synthesis of nitrogen and sulfur co-doped carbon dots (N, S-CDs) through a single step hydrothermal process was reported, utilizing citric acid and thiourea as sources of carbon, nitrogen, respectively. This N, S-CDs was composited with silver nanoparticles (AgNPs) to detect mercury ions (Hg²⁺) with reabsorption principle. The fluorescence intensity of the carbon dots was vanished by AgNPs due to the reabsorption effect due to overlap between emission spectrum of N, S-CDs and absorption spectrum of AgNPs, and it is recovered by the addition of Hg²⁺ due to formation Ag/Hg amalgam. As a result, composite of N, S-CDs-AgNPs can be used as a fluorescence "turn-on" probe for rapid and highly sensitive detection of Hg²⁺. The lowest limit of detection (LOD) of Hg²⁺ is 10 nM with linear detection in the range of 1 nM - 1.0 μM. This LOD is improved three-order of magnitude as compared to the N, S-CDs only with detection limit in 10 μM. This result proves that the composite of N, S–CDs-AgNPs is a very sensitive fluorescence probe for mercury ions detection. This work will also provide ideas for creating and building novel fluorescence probes for similar applications.