Identifying the existence of highly-fluorescent carboxylic group-rich carbon nanodots during a one-pot synthesis of branched polyethylenimine-passivated amine group-rich carbon nanodots

Abstract

Carbon nanodots (C-Dots) have recently been proposed as promising alternatives to traditional semiconductor quantum dots and organic fluorophores. However, C-Dots were often prepared and studied as a mixture, and the ignorance of the real composition significantly limits the accurate understanding and control of physical/chemical properties of C-Dots and further practical applications. Herein, we demonstrated and identified for the first time the co-existence of highly-fluorescent carboxylic group-rich C-Dots (CR-C-Dots) and amine group-rich C-Dots (AR-C-Dots) during the one-pot pyrolysis of citric acid and branched polyethylenimine precursors. Transmission electron microscopy demonstrated that the average size of CR-C-Dots was slightly smaller than that of amine group-rich C-Dots (AR-C-Dots), 1.5 nm vs. 1.9 nm. Photophysical characterization indicated that CR-C-Dots had a longer fluorescence lifetime and higher quantum yields (QYs) than the AR-C-Dots, despite the same excitation-independent emission at 445 nm, unlike most previous reports, which stated that amine group passivation enhanced the QYs of C-Dots. Both the C-Dots showed excellent biocompatibility and great potential for cellular imaging. A fluorescence- and electron spin resonance-responsive dual-model probe for ascorbic acid detection was also constructed, further proving the existence of CR-C-Dots. This study facilitates the understanding of the nucleation/surface passivation process during C-Dots formation and the development of a new synthetic and purification strategy for C-Dots.