Carbon Dots: Classically Defined versus Organic Hybrids on Shared Properties, Divergences, and Myths

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.

Synthetic strategies, properties and sensing application of multicolor carbon dots: recent advances and future challenges

“…At present, there are many raw materials for synthesizing M-CD, with the three most commonly used types being organic small molecules (i.e., citric acid (CA), EDA, urea, thiourea, etc. ), 72 aromatic organic molecules (i.e., phenylenediamine (PD), dihydroxybenzene, etc. ), 58,73,74 polymers (i.e., polythiophene derivative, polyvinyl alcohol (PVA), and polyethylene glycol (PEG), etc.)…”

“…The emission color of M-CDs could be tuned by excitationdependent FL features, which are attributed to quantum size effect, carbon core state and surface state. 19,72 As the excitation wavelength increases, the FL emission peak redshifts. However, most of the emissions by the excitation-dependent FL approach are focused on the short wavelength range from blue to green, and their FL intensity is extremely low within the long wavelength range.…”

“…, polyethylene glycol (PEG) 6 and heteroatom doping with elements such as B, N, P, and S, and metal elements are required to alter the structural defects and surface functional groups, 34 which improves QYs, and tunes the emission wavelength throughout the entire visible light region. 35–38 For instance, Zhu et al 39 used an environmentally-friendly and scalable method for the fabrication of multicolor N-doped CDs with a high QY (89–62%), which is the utilization of red-near-infrared NGQDs in bio-imaging. Sato et al 40 proposed multicolor CDs with solvatochromism in the visible region through surface modification using decanoic acid and perfluorodecanoic acid as passivation agents, which opens a new path to tune the emission wavelengths.…”

Size Isn’t Everything: Geometric Tuning in Polycyclic Aromatic Hydrocarbons and Its Implications for Carbon Nanodots

“…Due to this complexity, it is extremely difficult to diagnose or predict their optical properties based on simple rules such as size-confinement. Select GQDs have been shown to exhibit semiconductor nanocrystal-like properties. − Yet, in all other CNDs, a mixture of cooperative, inter- and intraheterogeneous effects influence their emissions. − This means that experimental measurements of optical properties often cannot be directly correlated with structural and chemical properties.…”