A rational synthesis strategy of multicolor carbon nanodots for efficient visual detection of NaNO2

Carbon nanodots (CDs) have seen unprecedented growth in recent years. Here, we report a facile synthesis strategy of tunable luminescent CDs via sulfur (S)-nitrogen (N) co-doping for visually detecting sodium nitrite (NaNO). The prepared multicolor CDs have outstanding stability and good emission properties, and the emitted fluorescence of the CDs can be redshifted from 520 nm to 555 nm to 625 nm by introducing S and N elemental doping, reaching a redshift of wavelength greater than 100 nm. The narrowing of the energy gap caused by the increase in graphitic N and C = O levels might be the reason behind the adjustable luminescence mechanism of the CDs. This also suggests an increase in sp conjugated domains, which shifts the emission wavelength towards the red color. Besides, yellow CDs (Y-CDs) and red CDs (R-CDs) synthesized by S and N co-doping are extremely sensitive and specific in detecting NaNO. Importantly, the detection of NaNO can be visualized by the color recognition software in the smart phone. We expect that this synthesis strategy will provide favorable technical support for the synthesis of emission redshift of high-performance CDs with special applications.