Lowering the detection limit towards nanomolar mercury ion detection via surface modification of N-doped carbon quantum dots

Lowering the detection limit of trace analysis is always a challenge in analytical chemistry and environmental analysis. Herein, we present a surface modification strategy to enhance photoluminescence, selectivity and lower the limit of detection towards mercury ions (Hg2+) via the solvothermal preparation of carbon quantum dots (Cdot) from three different components. Branched polyethyleneimine was used to modify Cdots through formation-selective functional groups on the surface of Cdots. The surface modified Cdots showed strong blue fluorescence in aqueous solution (ΦPL 45), and it was found that Hg2+ ions could efficiently quench the fluorescence of the Cdots, which was utilized as a simple, selective, and sensitive fluorescence probe for the detection of nanomolar Hg2+ in aqueous solution. The analytical performance of the method was demonstrated in a wide concentration range, from 20 nM to 800 nM, with a detection limit of 10.0 nM. The selectivity of the synthesized Cdots for the detection of Hg2+ was also studied. The Cdots were stable in a wide pH range and extreme ionic strength. The modified surface Cdots with excellent sensitivity and selectivity can be successfully used in environmental applications for monitoring Hg2+, and opens a door for improving the properties of other carbon quantum dots for various applications.