Carbon dots with tunable dual emissions: from the mechanism to the specific imaging of endoplasmic reticulum polarity

Regulating the fluorescence of carbon dots (CDs) is important but highly challenging. Here, carbon dots with tunable dual emissions were facilely fabricated via modulating the polymerization and carbonization processes of o-phenylenediamine (OPD) with lysine (Lys) as the co-precursor and modulator, respectively. The self-polymerization/carbonization of the OPD molecules contributed to the blue/green emission of the OPD-derived CDs. The introduction of Lys in the CD fabrication process efficiently suppressed the carbonization of the OPD polymer chains and enhanced the self-polymerization of the OPD molecules. Meanwhile, the formed OPD-Lys co-polymer chains endowed the final CD product with a new green emission center. The dual-emissive CDs were distinctly sensitive to polarity fluctuations, providing a ratiometric fluorescence response towards solution polarity. Due to their specific distribution in the endoplasmic reticulum (ER), the as-prepared dual-emissive CDs successfully distinguished the polarity variations in ER under stress, which offers a new approach for the early diagnosis of cell injury.