This one-step synthesis of N-CQDs provides a feasible method for preparing portable paper-based ECL sensors for future applications. The sensor shows overall excellent ECL stability, reproducibility, and selectivity, with a wide linear relationship ranging from 0.01 to 1000 μM in the detection of Cu 2+ ions. The binding energy is calculated from the difference in the energy of the x-ray source and the kinetic energy of the photoelectron being detected. This axis is traditionally displayed as Binding Energy in electron volts (eV). The ECL strength of the N-CQDs-K 2S 2O 8 system increases with increasing Cu 2+ ions concentration. In XPS analysis, the position of a peak on the x-axis indicates the elemental and chemical composition. A paper-based ECL sensor is developed with N-CQDs, based on the easy-scalable screen printing technology, and K 2S 2O 8 is used as a co-reactant to detect Cu 2+ ions. An up- shift in the positions of C1s XPS peak and a down-shift in the positions of the N1s XPS peak, has been observed along with an up-shift in the G-peak position in the Raman spectra, which suggest the occurrence of inter-molecular charge transfer from carbon atoms in MWCNTs to N + centres in PDDA. Compared with carbon quantum dots (CQDs) synthesized using glucose in a similar manner, N-CQDs show a much clearer green fluorescence at the long wavelength of ultraviolet light (365 nm) and a more stable and improved electrochemiluminescence (ECL) activity. A one-step hydrothermal method using glucosamine as the carbon and nitrogen source, mixed with dipotassium hydrogen phosphate (K 2HPO 4) in an aqueous solution, is used to prepare nitrogen-doped carbon quantum dots (N-CQDs).
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