Carbon dots versus nano-carbon/organic hybrids – dramatically different behaviors in fluorescence sensing of metal cations with structural and mechanistic implications

Abstract

Carbon dots (CDots) are defined as surface-passivated small carbon nanoparticles, with the effective passivation generally achieved by organic functionalization. Photoexcited CDots are both potent electron donors and acceptors, and their...

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

“…Logically, the dependence on the thermal processing conditions for the mixtures containing the unsubstituted amide groups is such that at lower processing temperatures, probably up to around 200 C, the dominating reactions in the mixtures must be chemical in nature to produce organic chromophores of absorptions in the visible spectrum, including the red/near-IR for some products. The intended carbonization is secondary at best under the relatively mild processing conditions, [37][38][39][40][41] but does play a role in complicating the product mixtures and hindering the separation and isolation of the organic chromophores in the product mixtures, as discussed in more detail below. With more aggressive processing conditions at higher temperatures, the carbonization apparently becomes more signicant, carbonizing not only the precursor mixtures, but also the initially produced organic chromophores (thus eliminating their corresponding absorptions).…”

“…On the preparation of dot samples in general, overwhelming majority of the syntheses reported in the literature have been based on the carbonization of organic precursors, oen in "onepot" thermal or hydrothermal processing, mostly under conditions such as 200 C or lower in temperature for a few hours or less. [30][31][32][33][34][35][36][37] With colorless organic molecules as precursors, such as commonly used mixtures of citric acid and amino molecules or oligomers (oligomeric polyethylenimine or PEI, as an example), the carbonization processing typically yields colored samples of uorescence emissions resembling those of the classically dened CDots (Fig. 1).…”

“…36,37 The spectroscopic evidence has generally been considered as sufficient for labeling the carbonized samples as carbon dots (or carbon nano-dots or various other naming variations), though the validity of such practice has raised serious concerns in recent studies, including the experimental evidence on molecular chromophores in the samples being primarily or majorly responsible for the spectroscopic results. [38][39][40][41][42] Since by denition the purpose of carbonization is to carbonize the organic precursors, namely to produce carbon nanoparticle-like entities (or denoted as nanoscale carbon domains in some publications 3,37 ), the optical absorptions of the samples thus prepared should in the best case be the same as or similar to those of the pre-existing carbon nanoparticles, which are again characterized by progressively decreasing absorptivities toward longer wavelengths in the visible. 36 While uorescence emissions in the red (extending into the near-IR) could be obtained with excitations at nearby wavelengths, the corresponding uorescence quantum yields are always low, similar to what have been observed with the classically dened CDots.…”

Chemical Reactions in Thermal Carbonization Processing of Citric Acid-Urea Mixtures

“…Such a comparability in nanoscale configuration illustrated in Figure 1 must be the assumption implicitly or explicitly behind the popular "one-pot" carbonization approach for the synthesis of samples that exhibit some of the characteristic properties of CDots, such as the bright and colorful fluorescence emissions. [1,[22][23][24] While convenient, the thermal carbonization synthesis is understandably difficult to control, generally yielding complicated mixtures at both the sample and sample structure levels. In fact, the "dot" samples from the thermal carbonization synthesis should be considered more appropriately as carbon/organic hybrids that are randomly structured at the nanoscale, dubbed "nano-hybrids" (Figure 1).…”

“…Such passivation of defects serves as the basis for CDots, in which the surface defects of small carbon nanoparticles are passivated effectively by chemical functionalization with selected organic molecules/species (Figure 1). [1,20,21] In principle the same configuration of nanoscale carbon with surface passivation by organic moieties could also be found in a comparable or even equivalent fashion in the nano-carbon -organic moiety "mixtures" (Figure 1), [3,22,23] in which the nanoscale carbon entities or domains could be Organic Functionalization Species…”