Carrot-derived carbon dots modified with polyethyleneimine and nile blue for ratiometric two-photon fluorescence turn-on sensing of sulfide anion in biological fluids.

In this article, a facile and green synthesis of carbon dots (CDs) was developed by using natural carrot as new carbon source. After direct hydrothermal carbonization for 5h at 180°C, CDs were prepared facilely. Then, CDs were conjugated with polyethyleneimine (PEI) and Nile Blue (NB) chloride to produce CDs/PEI/NB nanocomposites under electrostatic interactions. Upon excitation at 800nm, two-photon fluorescence (TPF) of the nanocomposites was observed, with TPF peaks of CDs at 415nm and NB at 675nm. The addition of Cu2+ could lead to TPF quenching of CDs via inner filter effect, but hardly any impacted on TPF of NB. Afterward, the added S2- combined with Cu2+ to form stable species that caused the separation of Cu2+ from CDs surface and the TPF recovery of CDs, with negligible effects on TPF of NB. Herein, a new CDs-based ratiometric TPF turn-on probe of S2- was developed and showed a good linear relationship (R2 =0.9933) between ratiometric TPF intensity (I415 /I675 ) and S2- concentration (0.1-8.0μM), with a low detection limit of 0.06μM. This probe was highly selective and sensitive toward S2- over potential interferences in real biological fluids, with high detection recoveries.