On the myth of “red/near-IR carbon quantum dots” from thermal processing of specific colorless organic precursors

We report a systematic investigation on the mixtures of citric acid with urea or formamide or with their partially methylated or permethylated derivatives for the carbonization processing under conditions similar to and beyond those commonly used and reported in the literature

Weixiong Liang; Ping Wang; Mohammed J. Meziani; Lin Ge; Liju Yang; Amankumar K. Patel; Sabina O. Morgan; Ya-Ping Sun

2021

Scholarcy highlights

  • Carbon dots have emerged to represent a rapidly advancing and expanding research eld, as made evident by the large and ever increasing number of relevant publications in the literature. Originally, CDots were prepared by the chemical functionalization of pre-processed and selected small carbon4186 | Nanoscale Adv., 2021, 3, 4186–4195Paper uorescence emissions of CDots at different excitation wavelengths do cover the red region, extending into the near-IR, their associated quantum yields are generally low to very low, representing a major disadvantage of CDots in some biomedical imaging and related applications
  • On the samples from the thermal processing of the various colorless organic precursor mixtures in this work, our focus on optical absorptions at longer wavelengths in the visible spectrum is deliberate, even though these samples are emissive in the similar spectral region, as our purpose is to eliminate the need to deal with photoexcited states and related complications
  • The red/near-IR absorptions must be due to the intrinsic optical transitions of the nanoscale carbon domains or entities in the dots, not due to any organic dyes or similar molecular chromophores in the dot structures
  • Red dyes incorporated, doped, or modi ed dot-like carbon nanostructures are by no means red carbon dots, just like in the case of colloidal TiO2 that similar red dye modi cations obviously do not make “red TiO2 nanoparticles”
  • With respect to the repeated claims of “red/near-IR carbon dots” in the literature, there has been a surprisingly lack of considerations in the reported studies on why and how the simple and speci c organic precursor mixtures processed under relatively mild thermal conditions could produce the presumed “carbon nanomaterials” of so dramatically different optical transitions from those of the pre-existing carbon nanoparticles
  • A simple and much more logical explanation is that the observed red/near-IR absorption features are due to molecular chromophores in the thermally processed samples, namely that the chromophores are formed in the chemical reactions under processing conditions intended for carbonization

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