Dan-Dan Liu,Hua Su,Qian Cao,Xue-Yi Le,Zong-Wan Mao
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.
Herein, carboxyl-rich cellulose nanocrystals (CNC) were obtained through a designed TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) mediated oxidation process without any subsequent mechanical treatment, which was proved to be widely suitable for commonly used cellulosic sources, including fibril-like fibers and lignocellulose biomasses. CNC prepared from the acid-free process showed better dispersion stability, less aggregation, higher aqueous UV transmittance and crystallinity compared with that from sulfuric acid hydrolysis process. Furthermore, the residuals in the insoluble precipitate were further conducted ultrasonic treatment and extra CNC was obtained from the oxidized cotton and ramie cellulose while cellulose nanofibril (CNF) was extracted from rice straw and pine powder, attributing to their different native structures. The total yields of nanocelluloses (NCs) were up to 63.2–68.3 %, improving the utilization ratio of the raw materials. This work provided a widely applicable acid- and mechanic-free route in fabricating CNC with inherent carboxyl groups and described an understanding of the relationship between the native structure of raw materials and their products, which is beneficial for improving the application of NCs.
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