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 5   h 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 800   nm, two-photon fluorescence (TPF) of the nanocomposites was observed, with TPF peaks of CDs at 415   nm and NB at 675   nm. The addition of Cu 2+ could lead to TPF quenching of CDs via inner filter effect, but hardly any impacted on TPF of NB. Afterward, the added S 2- combined with Cu 2+ to form stable species that caused the separation of Cu 2+ 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 S 2- was developed and showed a good linear relationship ( R 2 =0.9933) between ratiometric TPF intensity ( I 415 / I 675 ) and S 2- concentration (0.1–8.0   μM), with a low detection limit of 0.06   μM. This probe was highly selective and sensitive toward S 2- over potential interferences in real biological fluids, with high detection recoveries.