A paper-based ratiometric sensor for mercury (II) detection using fluorescent carbon dots

Mercury is a highly toxic environmental contaminant known for its significant bioaccumulation, making the rapid and sensitive detection of mercury ions (Hg²⁺) essential for environmental and public health. In this study, we developed a multicolor nanosensor utilizing dual-emissive fluorescent carbon dots (CDs) for the swift detection of Hg²⁺. The sensor comprises yellow-emissive o-phenylenediamine-synthesized CDs (OPD-CDs) and cyan-emissive m-phenylenediamine-synthesized CDs (MPD-CDs), synthesized via a one-step solvothermal method. Comprehensive structural and physicochemical characterization of OPD-CDs and MPD-CDs was performed utilizing transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR). Upon exposure to Hg²⁺, the fluorescence of OPD-CDs is significantly quenched due to electron transfer, while MPD-CDs maintain their emission, resulting in a color change from yellow to cyan. We thoroughly investigated the luminescence and sensing mechanisms of OPD-CDs and developed a ratiometric detection method that exhibited excellent selectivity, a low limit of detection at 26.8 nM, and high accuracy in real sample analyses, with apparent recovery rates between 99.5 % and 107.5 %. Additionally, we fabricated a portable paper-based device that employs smartphone technology for on-site visual detection of Hg²⁺, featuring a control zone for signal calibration to mitigate interference from external conditions, demonstrating its potential for accurate on-site detection of Hg²⁺ in real samples.