Carbon dots originated from methyl red with molecular state and surface state controlled emissions for sensing and imaging

In this article, carbon dots (CDs) were synthesized using methyl red (MR) as precursor in the absence and presence of ethanediamine (EDA) by facile hydrothermal pyrolysis, referring to as MR-CDs and MR-EDA-CDs, respectively. MR-CDs synthesized at different temperatures exhibited molecular state controlled photoluminescence (PL) property with the same characteristics (emission position and shape, no contribution from carbon core) as MR, except that their quantum yields (QYs) decreased with the reaction temperature increasing because of the consumption of MR. On the other hand, MR-EDA-CDs were fractionated into four fractions, which individually displayed unique surface state controlled PL property. Moreover, four fractions isolated from MR-EDA-CDs have specific surface groups that can selectively recognize pH, Fe³⁺, Ag⁺ and Hg²⁺, respectively, in aqueous solution, then were developed as the corresponding sensors. In addition, MR-EDA-CDs are potential fluorescent agents with excellent biocompatibility for bioimaging of fungal cells.