Highly nitrogen-doped carbon dots (N-CDs) are prepared by the pyrolysis of konjac flour under mild conditions followed with a simple extraction by ethanol and water. The N-CDs exhibit excellent pH-switched photoluminescence (PL), and their PL intensity can be facilitated by either mixing with NaOH and basic amino acids or by surface passivation with non-amine-terminated polyethylene glycols of different molecular weights. Further, the fluorescence of N-CDs can be quenched with Fe³⁺ and recovered with L-lysine, accompanied with a red-shift of emission wavelength. In addition, the low toxicity and strongly fluorescent N-CDs are applied for cell imaging, and the quenched fluorescence by Fe³⁺ can be recovered inside the living cells.

High specific capacity as well as good cycling stability are crucial for practical applications of supercapacitors, but actually the specific capacitances of transition metal oxide-based electrodes are still far below their theoretical predictions, and their cycling stabilities cannot satisfy the commercial standard due to intrinsically poor electronic conductivity and volume structural changes during charging/discharging processes. Here, we report the synthesis of a hybrid NiO–CuO mesoporous nanowire array with abundant oxygen vacancies which constructs a three-dimensional (3D) hollow architecture via a facile hydrothermal method and subsequent annealing. Compared to single-phase NiO or CuO, the increased O-vacancies on the NiO–CuO hybrid nanowires probably arising from a high number of lattice defects can promote their electronic conductivity, as revealed by XPS O 1s spectra, EPR characterization and HRTEM images. In addition, the 3D hollow structure together with mesoporous nanowires composed of alternately linked NiO and CuO nanocrystals can relax strain stress either in a hollow 3D structure or along a one-dimensional (1D)-direction nanowire, which not only provides a fast diffusion pathway for electrolyte ions and a high surface area with more active sites, but also helps to maintain the structural integrity during charge/discharge cycles. Therefore, the as-prepared NiO–CuO@Ni foam has a high areal capacitance of 4.35 F cm⁻² and also a very high specific capacitance of 1450.8 F g⁻¹ at a current density of 2 mA cm⁻². Further, it can be utilized directly as a binder-free positive electrode, and assembled with a negative electrode based on a 3D porous graphene hydrogel (PGH) to construct an asymmetric supercapacitor, which shows a wide potential window of 1.6 V and an outstanding cycling stability of 90.4% capacitance retention after 5000 charge/discharge cycles.