Precise control of the ratiometric fluorescence of dual-emissive B/N-doped carbon dots using pH-dependent bonds

Precise control of the structure and bonds of doped carbon dots (CDs) is important, so that their fluorescence can be tuned as desired. Up until now, there has been a lack of effective ways to control the bonds of doped CDs. In this article, we show that the fluorescence of B/N-doped carbon dots (B/N-CDs) can be precisely tuned just by controlling their precursors' pH values. The prepared B/N-CDs exhibit two emission bands, including one emission peak at around 450 nm due to the defect state caused by low sp² hybridization of carbon atoms, as well as another emission peak at around 360 nm caused by the B-N bond. The results for the ratio of the maximum intensity of the two emission peaks above show a linear relationship. Meanwhile, the role of the pH value of the precursors on the luminescence of B/N-CDs is deeply studied. On the one hand, the pH value affects the bonding process of CDs. N-H bonds tend to form at a low pH value, while more competitive B-N bonds exist at much higher pH value, leading to the pH-dependent intensity of the 360 nm emission band. On the other hand, a high pH value causes higher crystallinity, thus suppressing defect-state fluorescence at 450 nm. The dual effects of pH lead to precisely controlled dual-emission intensities as well as ratiometric fluorescence.