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Bulk-state and single-particle imaging are central to understanding carbon dot photo-physics and elucidating the effects of precursor composition and reaction temperature

Fathi, Parinaz, Khamo, John S., Huang, Xuedong, Srivastava, Indrajit, Esch, Mandy B., Zhang, Kai, Pan, Dipanjan
Carbon 2019 v.145 pp. 572-585
absorbance, biocompatibility, carbon quantum dots, fluorescence, image analysis, particle size, photobleaching, photoluminescence, temperature
Carbon dots have garnered attention for their strong multi-color luminescence properties and unprecedented biocompatibility. Despite significant progress in the recent past, a fundamental understanding of their photoluminescence and structure-properties relationships, especially at the bulk vs. single-particle level, has not been well established. Here we present a comparative study of bulk- and single-particle properties as a function of precursor composition and reaction temperature. The synthesis and characterization of multicolored inherently functionalized carbon dots were achieved from a variety of carbon sources, and at synthesis temperatures of 150 °C and 200 °C. Solvothermal synthesis at 200 °C led to quantum yields as high as 86%, smaller particle sizes, and a narrowed fluorescence emission, while synthesis at 150 °C resulted in a greater UV–visible absorbance, increase in nanoparticle stability, red-shifted fluorescence, and a greater resistance to bulk photobleaching. These results suggest the potential for synthesis temperature to be utilized as a simple tool for modulating carbon dot photophysical properties. Single-particle imaging resolved that particle brightness was determined by both the instantaneous intensity and the on-time duty cycle. Increasing the synthesis temperature caused an enhancement in blinking frequency, which led to an increase in on-time duty cycle in three out of four precursors.