Nitrogen-doped carbon dots as fluorescence ON–OFF–ON sensor for parallel detection of copper(ii) and mercury(ii) ions in solutions as well as in filter paper-based microfluidic device

Due to improper garbage disposal and rapid industrialization, concentrations of different metal ions are rising to toxic levels in natural water sources. Development of novel, selective and sensitive sensors for different metal ions is in high demand for rapid detection and remediation. Herein, we report nitrogen-doped carbon dots (NCDs) with high blue fluorescence, synthesized by a new one-step pyrolytic method using urea and ethylenediaminetetraacetic (EDTA) acid as precursors. The NCDs were used for parallel detection of Hg(2+) and Cu(2+) ions in aqueous medium through a fluorescence ON–OFF–ON process. The minimum detection limit for Hg(2+) and Cu(2+) were 6.2 nM and 2.304 nM, respectively, in aqueous medium, which is close to or below the allowed levels of Hg(2+) and Cu(2+) ions, i.e., 6 ppb and 2 ppm, respectively, in drinking water as per World Health Organisation (WHO). Hg(2+) and Cu(2+) ions were discriminated with vitamin C (ascorbic acid) and trisodium citrate by a fluorescence turn on process. A filter paper based microfluidic device loaded with NCDs, vitamin C and trisodium citrate was developed using candle wax channels on a filter paper as a proof of principle, projecting NCDs as a promising material for parallel detection of multiple metal ions. The device demonstrated herein is capable of detecting Hg(2+) and Cu(2+) ions up to 0.1 μM. This simple, low cost, disposable paper-based device will be very useful for rapid onsite analysis.