Biomass-Based Carbon Dots: Current Development and Future Perspectives.

ZnCl2 Enabled Synthesis of Highly Crystalline and Emissive Carbon Dots with Exceptional Capability to Generate O2⋅–

SummaryProtons are highly related to cell viability during physiological and pathological processes. Developing new probes to monitor the pH variation could be extremely helpful to understand the viability of cells and the cell death study. Carbonized polymer dots (CPDs) are superior biocompatible and have been widely applied in bioimaging field. Herein, a new type of extreme-pH suitable CPDs was prepared from citric acid and o-phenylenediamine (CA/oPD-CPDs). Due to the co-existence of hydrophilic and hydrophobic groups, CA/oPD-CPDs tend to aggregate in neutral condition with a dramatic decrease of fluorescence, but disperse well in both acidic and alkaline conditions with brighter emission. This specialty enables them to selectively illuminate lysosomes in cells. Moreover, CA/oPD-CPDs in the cytoplasm could serve as a sustained probe to record intracellular pH variation during apoptosis. Furthermore, CA/oPD-CPDs present a continuous fluorescence increase upon 2-h laser irradiation in living cells, underscoring this imaging system for long-term biological recording.

Functionalized carbon nanoparticles: Syntheses and applications in optical bioimaging and energy conversion

Light-emitting carbon dots extracted from naturally grown torreya grandis seeds

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.

Semiconductor quantum dots (SQDs) are extensively used nanomaterial for sensing, electronics, drug delivery, and bioimaging. However, their poor aqueous solubility, lack of uniformity in synthesis, toxicity, and challenges in scalable fabrication frequently limit their applications. Carbon quantum dots (CDs) emerged as new safer alternatives to SQDs with comparable optical properties and diverse applications. Herein, fluorescent, biocompatible, and water‐soluble, carbon dots were synthesized via cost‐effective hydrothermal method using ascorbic acid (AA) as the sole precursor. The as‐synthesized spherical 3–4 nm ascorbic acid‐derived carbon dots (AA‐CDs) exhibited maximum excitation at 340 nm, emission at 400 nm and displayed fluorescence quantum yield of ∼29.89% with average fluorescence lifetime decay of ∼1.12 ns. For technological domain, this study successfully formulated AA‐CDs into anti‐counterfeiting fluorescent ink that resembled conventional ink under visible light but exhibited bright blue fluorescence under UV excitation. Furthermore, AA‐CDs displayed negligible toxicity toward both mycobacterial (e.g., Mycobacterium marinum ) and mammalian (phorbol myristate acetate‐treated human THP‐1 macrophages) cells, supporting their safe applications in biomedicine. Rapid and efficient internalization of AA‐CDs into M. marinum and THP‐1 macrophage cells was evidenced by presence of bright blue fluorescence inside the cells, which highlighted their potential as nano‐probe for real‐time bioimaging and tracking cellular processes. Collectively, this study demonstrated AA‐CDs as a proof‐of‐concept dual‐function nanomaterial for applications across fluorescent ink‐based security technology and nanomedicine.

Luminescence of carbon quantum dots and their application in biochemistry

Due to the different emission mechanism between fluorescent carbon dots and semiconductor quantum dots (QDs), it is of interest to explore the potential emission in hetero-structured carbon dots/semiconducting QDs. Herein, we design carbon dots coated CdTe QDs (CDQDs) and investigate their inherent emission. We demonstrate switchable emission for the hetero-interactions of the CDQDs. Optical analyses indicate electron transfer between the carbon dots and the CdTe QDs. A heterojunction electron process is proposed as the driving mechanism based on N atom protonation of the carbon dots. This work advances our understanding of the interaction mechanism of the heterostructured CDQDs and benefits the future development of optoelectronic nanodevices with new functionalities.

A single molecular-beam epitaxy process is used to produce GaAs-based heterostructures containing two-dimensional arrays of InAs semiconductor quantum dots and AsSb metal quantum dots. The twodimensional array of AsSb metal quantum dots is formed by low-temperature epitaxy which provides a large excess of arsenic in the epitaxial GaAs layer. During the growth of subsequent layers at a higher temperature, excess arsenic forms nanoinclusions, i.e., metal quantum dots in the GaAs matrix. The two-dimensional array of such metal quantum dots is created by the δ doping of a low-temperature GaAs layer with antimony which serves as a precursor for the heterogeneous nucleation of metal quantum dots and accumulates in them with the formation of AsSb metal alloy. The two-dimensional array of InAs semiconductor quantum dots is formed via the Stranski–Krastanov mechanism at the GaAs surface. Between the arrays of metal and semiconductor quantum dots, a 3-nm-thick AlAs barrier layer is grown. The total spacing between the arrays of metal and semiconductor quantum dots is 10 nm. Electron microscopy of the structure shows that the arrangement of metal quantum dots and semiconductor quantum dots in the two-dimensional arrays is spatially correlated. The spatial correlation is apparently caused by elastic strain and stress fields produced by both AsSb metal and InAs semiconductor quantum dots in the GaAs matrix.

Optical Bioimaging and Therapy

Chapter 15 - Future aspects of carbon and graphene quantum dots for biomedical applications

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

The light of carbon dots: From mechanism to applications

Luminescent carbon dots (CDs) are emerging as a potential eco‐friendly alternative to standard metal‐based semiconductor quantum dots. A solvatothermal method to prepare crystalline, nitrogen‐rich, surface‐passivated, and rare excitation wavelength‐independent green‐emitting (λmax ≈ 522 nm in solution and 536 nm in solid) CDs from a pyridine diamine precursor in ethanol solvent is reported here. In comparison to a neat thin film of CDs, a film of CD‐doped polystyrene microspheres (PS‐CDs) displays a remarkable enhancement of the photoluminescence emission intensity driven by optical cavity effect of the composite microspheres. Apart from downconversion one‐photon luminescence, upon excitation with 800 nm femtosecond pulse laser, pure CDs and PS‐CDs exhibit enhanced upconversion two‐photon luminescence in the range of about 410–580 nm.

Chapter 23 - Multicolor carbon dots for imaging applications