The commercially acquired aqueous solution of “carbon quantum dots” sample was evaluated by optical absorption and fluorescence emission methods; in reference to aqueous
dispersed small carbon nanoparticles and representative carbon dots prepared from chemical functionalization of the carbon nanoparticles. The results suggest a very low content of carbon that is associated with nanoscale carbon particles/domains in the as-supplied sample; and likely significant contamination by dye-like species/mixtures. In the absence of any information on the synthesis and history of the commercial sample, the possible cause of the contamination was illustrated by an example on similar dye formation in the one-pot carbonization synthesis of “red carbon dots” from citric acid–formamide precursor mixtures under too mild processing conditions that were insufficient for the intended carbonization. The negative impacts to the carbon dots research field by the apparent proliferation and now commercial availability of carbon-deficient or even largely carbon-less “carbon quantum dots”, which are more susceptible to dye contamination
or dominance, are discussed.
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On the myth of “red/near-IR carbon quantum dots” from thermal processing of specific colorless organic precursors
Carbon dots were originally found and reported as surface-passivated small carbon nanoparticles, where the effective surface passivation was the chemical functionalization of the carbon nanoparticles with organic molecules. Understandably, the very broad optical absorptions of carbon dots are largely the same as those intrinsic to the carbon nanoparticles, characterized by progressively decreasing absorptivities from shorter to longer wavelengths. Thus, carbon dots are generally weak absorbers in the red/near-IR and correspondingly weak emitters with low quantum yields. Much effort has been made on enhancing the optical performance of carbon dots in the red/near-IR, but without meaningful success due to the fact that optical absorptivities defined by Mother Nature are in general rather inert to any induced alterations. Nevertheless, there were shockingly casual claims in the literature on the major success in dramatically altering the optical absorption profiles of “carbon dots” by simply manipulating the dot synthesis to produce samples of some prominent optical absorption bands in the red/near-IR. Such claims have found warm receptions in the research field with a desperate need for carbon dots of the same optical performance in the red/near-IR as that in the green and blue. However, by looking closely at the initially reported synthesis and all its copies in subsequent investigations on the “red/near-IR carbon dots”, one would find that the “success” of the synthesis by thermal or hydrothermal carbonization processing requires specific precursor mixtures of citric acid with formamide or urea. In the study reported here, the systematic investigation included precursor mixtures of citric acid with not only formamide or urea but also their partially methylated or permethylated derivatives for the carbonization processing under conditions similar to and beyond those commonly used and reported in the literature. Collectively all of the results are consistent only with the conclusion that the origins of the observed red/near-IR optical absorptions in samples from some of the precursor mixtures must be molecular chromophores from thermally induced chemical reactions, nothing to do with any nanoscale carbon entities produced by carbonization.
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- NSF-PAR ID:
- 10284465
- Date Published:
- Journal Name:
- Nanoscale Advances
- Volume:
- 3
- Issue:
- 14
- ISSN:
- 2516-0230
- Page Range / eLocation ID:
- 4186 to 4195
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Carbon Dots: Classically Defined versus Organic Hybrids on Shared Properties, Divergences, and Mythsmore » « less
Abstract Carbon dots are defined as small carbon nanoparticles with effective surface passivation via organic functionalization. The definition is literally a description of what carbon dots are originally found for the functionalized carbon nanoparticles displaying bright and colorful fluorescence emissions, mirroring those from similarly functionalized defects in carbon nanotubes. In literature more popular than classical carbon dots are the diverse variety of dot samples from “one‐pot” carbonization of organic precursors. On the two different kinds of samples from the different synthetic approaches, namely, the classical carbon dots versus those from the carbonization method, highlighted in this article are their shared properties and apparent divergences, including also explorations of the relevant sample structural and mechanistic origins for the shared properties and divergences. Echoing the growing evidence and concerns in the carbon dots research community on the major presence of organic molecular dyes/chromophores in carbonization produced dot samples, demonstrated and discussed in this article are some representative cases of dominating spectroscopic interferences due to the organic dye contamination that have led to unfound claims and erroneous conclusions. Mitigation strategies to address the contamination issues, including especially the use of more vigorous processing conditions in the carbonization synthesis, are proposed and justified.
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Carbon dots (CDots) are generally defined as small-carbon nanoparticles with surface organic functionalization and their classical synthesis is literally the functionalization of preexisting carbon nanoparticles. Other than these “classically defined CDots”, however, the majority of the dot samples reported in the literature were prepared by thermal carbonization of organic precursors in mostly “one-pot” processing. In this work, thermal processing of the selected precursors intended for carbonization was performed with conditions of 200 °C for 3 h, 330 °C for 6 h, and heating by microwave irradiation, yielding samples denoted as CS200, CS330, and CSMT, respectively. These samples are structurally different from the classical CDots and should be considered as “nano-carbon/organic hybrids”. Their optical spectroscopic properties were found comparable to those of the classical CDots, but very different in the related photoinduced antibacterial activities. Mechanistic origins of the divergence were explored, with the results suggesting major factors associated with the structural and morphological characteristics of the hybrids.more » « less
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null (Ed.)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.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1NA00286D
