Fig. 2. (A) The emission spectra of CuNCs under 300 nm excitation with the increasing concentration of the Cu²⁺. Inset: the calibration plot of (F − F₀) versus Cu²⁺ concentration, where F and F₀ respectively represented the fluorescence intensity with and without Cu²⁺. (B) Selectivity of the “turn on” sensor for Cu²⁺. The concentration of each metal ion were 1.0 μM. The error bars are the standard deviation of three measurements.