Synthesis Processes, Photoluminescence Mechanism, and the Toxicity of Amorphous or Polymeric Carbon Dots

Abstract

Conspectus Fluorescence is the emission of light following photon absorption. This optical phenomenon has many applications in daily life, such as in LED lamps, forensics, and bioimaging. Traditionally, small-molecule fluorophores were most common, but the types of molecules and particles with compelling fluorescence properties have expanded. For example, green fluorescent protein (GFP) was isolated from jellyfish and won the Nobel prize in 2008 due to its significant utility as a fluorescent biomarker. Using the intrinsic fluorescence of GFP, many previously invisible biological processes and substances can now be observed and studied. Other fluorescent materials have also been developed, greatly expanding the potential applications. Semiconductor quantum dots (QDs), which have bright fluorescence and a narrow bandwidth, are a popular choice for display technologies. However, QDs are made of heavy metal elements such as Cd and Se, which pose potential safety concerns to the environment and human health. Thus, new fluorescent organic materials are being developed to mitigate the toxicological concerns while maintaining the QD advantages. One type of new material attracting great attention as an environmentally friendly substitute for semiconductor QDs is carbon dots (CDs). CDs have been developed with strong fluorescence, good photostability, and low toxicity using a variety of precursors, and some synthesis processes have good potential for scale-up. However, since they are made of a variety of materials and through different methods, the structure and properties of CDs can differ from preparation to preparation. There are three major types of CDs: graphene quantum dots (GQDs), carbon quantum dots (CQDs), and amorphous or polymeric carbon dots (PCDs). This Account focuses on PCDs and their unique properties by comparing it with other types of CDs. The synthesis processes, fluorescence properties, fluorescence mechanisms, and toxicity are discussed below with an emphasis on the distinct attributes of PCDs. PCDs can be synthesized from small molecules or polymers. They have an amorphous or cross-linked polymer structure with bright fluorescence. This fluorescence is possibly due to cross-link-enhanced emission or clusteroluminescence that arises from the through-space interactions of heteroatomic-rich functional groups. Other fluorescence mechanisms of CDs, including distinct contributions from the carbon core and surface states, may also contribute. The toxicological profiles of CDs are influenced by the chemical composition, surface functionalization, and light illumination. CDs are generally thought to be of low toxicity, and this can be further improved by removing toxic byproducts, functionalizing the surface, and reducing light exposure to minimize the generation of reactive oxygen species.

Supramolecular Chemistry of Carbon‐Based Dots Offers Widespread Opportunities

“…The surface charge had therefore an impact on the proteins bound to the CD, which in turn drove the cellular uptake and ultimately the toxicity. [ 52,99 ]…”

“…The surface charge had therefore an impact on the proteins bound to the CD, which in turn drove the cellular uptake and ultimately the toxicity. [52,99] Ionic bonds between carbon dots and charged inorganic nanomaterials is also an effective approach to prepare organicinorganic nanocomposites. For example, CDs could be combined with gold nanoclusters to achieve intracellular temperature detection, [100] with 2D transition metal carbides (MXenes) to prepare supercapacitors, [101] or with tungsten disulfide to prepare electrochemical sensors.…”

Shining Light on Carbon Dots: New Opportunities in Photocatalysis

“…[25][26] In addition, some organic solvents, such as formamide, can as well act as dopants, since during the synthesis is probable that they decompose and their by-products become incorporated into the CD structure. [27] The optical properties of these nanoparticles are primarily investigated by UV-Vis spectroscopy. In general, most CDs present a main absorption band below 300 nm, attributed to the π-π* transitions of C=C bonds, and another tail band ranges between 300-400 nm, which is linked to the n-π* transitions of C=O bonds.…”

“…[28] Moreover, the presence of a graphitic core leads to increased photostability and longer wavelength emission. [27] Consequently, the photoluminescence properties of CDs, namely fluorescence, phosphorescence, and up-converted photoluminescence, are attractive features for many applications, even though fluorescence is the most common and exploited light-emitting process. [29] Generally, CDs display excitation-dependent emission spectra and large Stokes shifts.…”

“…Metal‐doped CDs are also reported, mainly derived from organometallic complexes or metallic salts [25–26] . In addition, some organic solvents, such as formamide, can as well act as dopants, since during the synthesis is probable that they decompose and their by‐products become incorporated into the CD structure [27] …”