Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

Aggregation-Induced Emission: Together We Shine, United We Soar!

DLS and zeta potential - What they are and what they are not?

At present, the actual mechanism of the photoluminescence (PL) of fluorescent carbon dots (CDs) is still an open debate among researchers. Because of the variety of CDs, it is highly important to summarize the PL mechanism for these kinds of carbon materials; doing so can guide the development of effective synthesis routes and novel applications. This review will focus on the PL mechanism of CDs. Three types of fluorescent CDs were involved: graphene quantum dots (GQDs), carbon nanodots (CNDs), and polymer dots (PDs). Four reasonable PL mechanisms have been confirmed: the quantum confinement effect or conjugated π-domains, which are determined by the carbon core; the surface state, which is determined by hybridization of the carbon backbone and the connected chemical groups; the molecule state, which is determined solely by the fluorescent molecules connected on the surface or interior of the CDs; and the crosslink-enhanced emission (CEE) effect. To give a thorough summary, the category and synthesis routes, as well as the chemical/physical properties for the CDs, are briefly introduced in advance.

The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

Carbon dots (CDs) with tunable photoluminescence (PL) and a quantum yield of up to 35% in water were hydrothermally synthesized in one pot and separated via silica column chromatography. These separated CDs emitted bright and stable luminescence in gradient colors from blue to red under a single-wavelength UV light. They exhibited high optical uniformity; that is, every sample showed only one peak in the PL excitation spectrum, only one peak in the excitation-independent PL emission spectrum, and similar monoexponential fluorescence lifetimes. Although these samples had similar distributions of particle size and graphite structure in their carbon cores, the surface state gradually varied among the samples, especially the degree of oxidation. Therefore, the observed red shift in their emission peaks from 440 to 625 nm was ascribed to a gradual reduction in their band gaps with the increasing incorporation of oxygen species into their surface structures. These energy bands were found to depend on the surfac...

Full-Color Light-Emitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism.

Multiple-color-emissive carbon dots (CDots) have potential applications in various fields such as bioimaging, light-emitting devices, and photocatalysis. The majority of the current CDots to date exhibit excitation-wavelength-dependent emissions with their maximum emission limited at the blue-light region. Here, a synthesis of multiple-color-emission CDots by controlled graphitization and surface function is reported. The CDots are synthesized through controlled thermal pyrolysis of citric acid and urea. By regulating the thermal-pyrolysis temperature and ratio of reactants, the maximum emission of the resulting CDots gradually shifts from blue to red light, covering the entire light spectrum. Specifically, the emission position of the CDots can be tuned from 430 to 630 nm through controlling the extent of graphitization and the amount of surface functional groups, COOH. The relative photoluminescence quantum yields of the CDots with blue, green, and red emission reach up to 52.6%, 35.1%, and 12.9%, respectively. Furthermore, it is demonstrated that the CDots can be uniformly dispersed into epoxy resins and be fabricated as transparent CDots/epoxy composites for multiple-color- and white-light-emitting devices. This research opens a door for developing low-cost CDots as alternative phosphors for light-emitting devices.

Synthesis of Carbon Dots with Multiple Color Emission by Controlled Graphitization and Surface Functionalization.

Carbon dots (CDs) are one of the advancing fluorescent materials, which draw increasing attention in both theoretical research and practical applications. However, the clear chemical structure and photoluminescence (PL) mechanism of CDs is still an open debate, which limits the development of CDs. Because of the diversity of CDs, it is highly important to clarify this issue for specific CDs models. Herein, a type of CDs, made from citric acid with extremely high quantum yield, is investigated. Through the separation of the CDs, a type of bright blue fluorophore (IPCA) was discovered. IPCA and its derivatives were investigated and they were proven to contribute to the molecular state PL. Other components in the CDs were related to the carbon core state PL, which included polymer clusters and nanosized carbon cores. We conclude that this type of CDs contained complex components and multiple PL centers and that an independent fluorophore strongly affects the PL properties of the CDs. These two conclusions can potentially be true for similar CDs.

Investigation from chemical structure to photoluminescent mechanism: a type of carbon dots from the pyrolysis of citric acid and an amine

Carbon dots (CDs) are novel fluorescent materials with low toxicity and good biocompatibility. Herein, the collisional/dynamic and photoluminescence (PL) center destruction quenching behaviors of a novel type of CDs were investigated. Moreover, the quenching behaviors of the CDs were exploited in applications. Firstly, dynamic PL quenching was achieved by Fe3+ ions, which was proved by the Stern–Volmer equation, temperature dependent quenching and fluorescence lifetime measurements. Furthermore, a hemin sensor based on the Fe3+/CDs system was achieved. Secondly, quenching induced by PL center destruction was caused by hydroxyl radicals (˙OH), which were produced by high power UV light or the H2O2/Fe2+ system; thus an H2O2 sensor with a low detection limit (0.9 ppb) was realized. Finally, we assumed that the CDs are really composed of cross-linked molecular clusters, and that the PL centers of the as prepared CDs are certain molecular/chemical groups.

Investigation into the fluorescence quenching behaviors and applications of carbon dots.

A new type of fluorescent material is presented, which is called non-conjugated polymer dots (NCPDs). The NCPDs only possess sub-fluorophores (which are groups such as C=O, C=N, N=O) instead of typical conjugated fluorophore groups, and thus these materials should not have strong photoluminescence (PL) in the usual sense. Nevertheless, the PL of these sub-fluorophores can be enhanced by chemical crosslinking or physical immobilization of polymer chains, which is named the crosslink-enhanced emission (CEE) effect. The significant advances achieved by us and other groups on both experimental and theoretical aspects are discussed, and the covalent-bond CEE, rigidity-aggregated CEE, or supramolecular CEE in NCPDs is elaborated. Moreover, synthetic strategies, unique optical properties, and the promise of NCPDs in bio-related fields, such as bioimaging and drug delivery, are systematically discussed.

Non-Conjugated Polymer Dots with Crosslink-Enhanced Emission in the Absence of Fluorophore Units.

The crosslink enhanced emission (CEE) in non-conjugated polymer dots: from the photoluminescence mechanism to the cellular uptake mechanism and internalization.

Xiaohuan Zhao

Papers

The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): current state and future perspective

Investigation from chemical structure to photoluminescent mechanism: a type of carbon dots from the pyrolysis of citric acid and an amine

Investigation into the fluorescence quenching behaviors and applications of carbon dots.

Non-Conjugated Polymer Dots with Crosslink-Enhanced Emission in the Absence of Fluorophore Units.

The crosslink enhanced emission (CEE) in non-conjugated polymer dots: from the photoluminescence mechanism to the cellular uptake mechanism and internalization.