Phosphoric acid densified red emissive carbon dots with a well-defined structure and narrow band fluorescence for intracellular reactive oxygen species detection and scavenging

Abstract

Reactive oxygen species (ROS) play important roles in cell signaling, immunity, and tissue homeostasis. The overproduction of ROS can induce the pathogenesis of various diseases. Carbon dots (CDs) are promising fluorescence materials for sensing and imaging ROS in cells, and they have great therapeutic potential for scavenging ROS. Herein, a kind of red emissive carbon dots (R-CDs) was synthesized by the hydrothermal treatment of o-phenylenediamine (OPD) and catechol in the presence of phosphoric acid (H₃PO₄). With the strong traction effect of H₃PO₄ in the hydrothermal process, these precursors undergo orientated polymerization and carbonization to form R-CDs with a uniformly conjugated structure. More significantly, the resultant R-CDs have a high yield (87.0 wt%), narrow full width at half maxima (FWHM) emission band of 21 nm, and sensitive colorimetric and fluorescence response to hydroxyl radical (˙OH), superoxide radical anion (˙O₂⁻), and singlet oxygen (¹O₂) without interference from H₂O₂. Femtosecond transient absorption (fs-TA) spectra confirmed that the excited states of oxidized R-CDs (Ox-R-CDs) have a significantly increased absorbance in the range of 600–700 nm, and the absorbed photons are deactivated in a non-radiation form, which is responsible for the decrease in fluorescence upon reaction with ROS. Experimental characterizations and density functional theory (DFT) calculations revealed that R-CDs comprised two five-membered aromatic rings linked by pyrophosphate groups. The amino terminals of the aromatic ring are protected by phosphate groups, which should be responsible for the inactivity of R-CDs to H₂O₂. This study provides a new strategy for the synthesis of R-CDs with narrow FWHM and also provides a new metal-free nanoprobe with high sensitivity for ROS detection and imaging.