Multicolor carbon dots (CDs) have been developed recently and demonstrate great potential in bio-imaging, sensing, and LEDs. However, the fluorescence mechanism of their tunable colors is still under debate, and efficient separation methods are still challenging. Herein, we synthesized multicolor polymeric CDs through solvothermal treatment of citric acid and urea in formamide. Automated reversed-phase column separation was used to achieve fractions with distinct colors, including blue, cyan, green, yellow, orange and red. This work explores the physicochemical properties and fluorescence origins of the red, green, and blue fractions in depth with combined experimental and computational methods. Three dominant fluorescence mechanism hypotheses were evaluated by comparing time-dependent density functional theory and molecular dynamics calculation results to measured characteristics. We find that blue fluorescence likely comes from embedded small molecules trapped in carbonaceous cages, while pyrene analogs are the most likely origin for emission at other wavelengths, especially in the red. Also important, upon interaction with live cells, different CD color fractions are trafficked to different sub-cellular locations. Super-resolution imaging shows that the blue CDs were found in a variety of organelles, such as mitochondria and lysosomes, while the red CDs were primarily localized in lysosomes. These findings significantly advance our understanding of the photoluminescence mechanism of multicolor CDs and help to guide future design and applications of these promising nanomaterials.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation under the NSF Center for Sustainable Nanotechnology, CHE-2001611. The CSN is part of the Centers for Chemical Innovation Program. Part of this work was done at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE's office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The TD-DFTB computations were supported by the grant from the NIH to QC (R01 GM106443). Computational resources from the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF grant number ACI-1548562, are greatly appreciated. Part of the computational work was performed on the Shared Computing Cluster which is administered by Boston University's Research Computing Services. Part of this work utilized the CEM Discover SP microwave that was purchased with funding provided by the NSF Center for Sustainable Polymers, CHE-1901635. We thank Shuyi Xie in Prof. Timothy P. Lodge's group (Department of Chemistry, University of Minnesota) for his help in using the vacuum oven. We also thank Jiayi He from Christy L. Haynes' group for making the gold substrate used for Raman measurements.
© The Royal Society of Chemistry 2021.