Sustainable route for synthesis of nitrogen-doped carbon dots with high efficiency for iron(III) and copper(II) ions detection

The environmentally friendly luminescent sensing materials are desired for detection of heavy metal ions. Herein, two kinds of fluorescent nanomaterials, carbon dots (CDs-1) and N-doped carbon dots (CDs-2) were designed and synthesized as effectively sensors for simultaneous detection of Fe 3+ and Cu 2+ ions. A simple one-step hydrothermal synthesis was adopted by using cost-effective precursor humic acid (HA) as a carbon source and ammonium hydroxide as a nitrogen source. Photoluminescence (PL) spectra and TEM images reveal the distinct structure and property of these two CDs, in which CDs-1 display blue fluorescence with the average diameter of about 1.73 nm, while CDs-2 possess larger size (av. 2.41 nm) because of the larger conjugated structures and exhibit green fluorescence. The successful doping of N on the surface of CDs-2 was confirmed by characterizations of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR), which endows CDs-2 improved detection performance than that of CDs-1. CDs-2 could selectively and sensitively detect iron(III) and copper(II) ions, and the limit of detection (LOD) can reach 73 nM and 61 nM with a liner range from 0 to 20 μM and 0 to 50 μM, respectively. The fluorescence static quenching effect and the formation of the ground-state complex may explain the quenching mechanism of CDs-2 in this process. This work demonstrates the more possibilities of applying the low-cost waste and developing high-efficiency CDs-based luminescent sensors.