Unconventional aliphatic fluorophores discovered as the luminescence origin in citric acid-urea carbon dots

Xiaoxiao Yao, Yinhan Wang, Fangjia Li, Joseph J. Dalluge, Galya Orr, Rigoberto Hernandez, Qiang Cui, Christy L. Haynes

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Abstract

Carbon dots (CDs) are emerging as the material of choice in a range of applications due to their excellent photoluminescence properties, ease of preparation from inexpensive precursors, and low toxicity. However, the precise nature of the mechanism for the fluorescence is still under debate, and several molecular fluorophores have been reported. In this work, a new blue fluorophore, 5-oxopyrrolidine-3-carboxylic acid, was discovered in carbon dots synthesized from the most commonly used precursors: citric acid and urea. The molecular product alone has demonstrated interesting aggregation-enhanced emission (AEE), making it unique compared to other fluorophores known to be generated in CDs. We propose that this molecular fluorophore is associated with a polymer backbone within the CDs, and its fluorescence behavior is largely dependent on intermolecular interactions with the polymers or other fluorophores. Thus, a new class of non-traditional fluorophores is now relevant to the consideration of the CD fluorescence mechanism, providing both an additional challenge to the community in resolving the mechanism and an opportunity for a greater range of CD design schemes and applications.

Original languageEnglish (US)
Pages (from-to)9516-9525
Number of pages10
JournalNanoscale
Volume14
Issue number26
DOIs
StatePublished - Jun 17 2022

Bibliographical note

Funding Information:
This work is supported by the National Science Foundation under Grant No. CHE-2001611, the NSF Center for Sustainable Nanotechnology (CSN). The CSN is part of the Centers for Chemical Innovation Program. We thank Yukun Cheng, Dr Yuan Sheng, Dr Thomas R. Hoye, Mengyuan Jin, and Dr Letitia Yao (all from the Department of Chemistry, University of Minnesota) for meaningful discussions in the spectral interpretation and proposing the structures of the fluorophores. The fluorescence macroscopic images were taken with the resources and help from Mary Brown and Guillermo Marques at the University of Minnesota University Imaging Centers (UIC). SCR_020997. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from the NSF through the MRSEC (Award Number DMR-2011401) and the NNCI (Award Number ECCS-2025124) programs. The computing resources necessary for this research were provided in part by the National Science Foundation through XSEDE resources under Grant No. CTS090079 and the Advanced Research Computing at Hopkins (ARCH) facilities supported by the NSF MRI Grant (OAC-1920103). Part of the research was performed in the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility sponsored by the Biological and Environmental Research program under Contract No. DE-AC05-76RL01830. Part of this work utilized the CEM Discover SP microwave from Jane Wissinger's group (University of Minnesota) that was purchased with funding provided by the NSF Center for Sustainable Polymers, CHE-1901635. The table of contents image was created with BioRender.com.

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© 2022 The Royal Society of Chemistry

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