Hydrogels for biomedical applications.

Despite the various synthesis methods to obtain carbon dots (CDs), the bottom-up methods are still the most widely administrated route to afford large-scale and low-cost synthesis. However, as CDs are developed with increasing reports involved in producing many CDs, the structure and property features have changed enormously compared with the first generation of CDs, raising classification concerns. To this end, a new classification of CDs, named carbonized polymer dots (CPDs), is summarized according to the analysis of structure and property features. Here, CPDs are revealed as an emerging class of CDs with distinctive polymer/carbon hybrid structures and properties. Furthermore, deep insights into the effects of synthesis on the structure/property features of CDs are provided. Herein, the synthesis methods of CDs are also summarized in detail, and the effects of synthesis conditions of the bottom-up methods in terms of the structures and properties of CPDs are discussed and analyzed comprehensively. Insights into formation process and nucleation mechanism of CPDs are also offered. Finally, a perspective of the future development of CDs is proposed with critical insights into facilitating their potential in various application fields.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201901316

Evolution and Synthesis of Carbon Dots: From Carbon Dots to Carbonized Polymer Dots

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

Nitrogen-doped carbon dots synthesized from citric acid as a carbon precursor have recently been considered to contain fluorescent derivatives of citrazinic acid, which contribute to their emission in the blue spectral range. To study the impact of such molecular fluorescent species on the optical properties of carbon dots, we synthesized three samples employing citric acid and three different nitrogen sources: ethylenediamine, hexamethylenetetramine, and triethanolamine. On the basis of the analysis of the nitrogen content and its coordination by X-ray photoelectron spectroscopy, FTIR spectra, and systematically comparing absorption, steady-state emission, and photoluminescence decays of each kind of carbon dot, we derive the influence of the molecular precursors and gain further understanding of the complex structure of carbon dots highlighting the strong impact of molecular fluorescence in the samples produced with ethylenediamine and hexamethylenetetramine.

Molecular Fluorescence in Citric Acid-Based Carbon Dots

Carbon dots (CDs) are potentially useful in many areas such as bioimaging, light-emitting diodes, and sensing because of their excellent optical properties, high biocompatibility, and low toxicity. Knowledge of their photoluminescence (PL) mechanisms, which have been widely studied, is of significance in guiding the synthesis and promoting applications of CDs with tunable PL emissions. However, the intrinsic mechanism of PL emission remains unclear, and a unified mechanism has not been found because of differences in particle structures. This review generalizes the categories of CDs, noting their structural diversity. Three types of PL mechanism pertaining to structural differences are outlined: internal factors dominated emission (including the conjugation effect, the surface state, and the synergistic effect), external factors dominated emission (including the molecular state and the environment effect), and crosslink-enhanced emission. Optical applications of CDs are also briefly mentioned. Finally, the prospects for research into PL mechanisms are discussed, noting the remaining challenges and directions for future work.

Insights into photoluminescence mechanisms of carbon dots: advances and perspectives

In this report, we present the results of our investigations into the elucidation of the chemical structure of moieties responsible for the blue and green luminescence of CDs derived from the microwave-assisted pyrolysis of citric acid in the presence of urea. The molecular fluorophore that forms during the synthesis of green fluorescing CDs is 4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione (HPPT).

Luminescence phenomena of carbon dots derived from citric acid and urea – a molecular insight

A series of novel fluorescent compounds was isolated from condensation mixtures of citric acid and specific β-amines. Their chemical structure was determined using 1H, 13C and HSQC experiments, elemental analysis data, high resolution and fragmentation spectra ESI-MS analyses. These compounds consisted of five-membered ring fused 2-pyridones. The fluorophores exhibited quantum yields as high as 79%.

Novel efficient fluorophores synthesized from citric acid

Luminescence phenomena of biodegradable photoluminescent poly(diol citrates)

We synthesized water-insoluble polymers, poly(β-cyclodextrin-co-citric acid)s, by heating a mixture of citric acid, cyclodextrin (CD), and Na2HPO4 as a catalyst with a 6 : 1 : 2 molar ratio at 160, 170, and 180°C for 10 and 20 min. The chemical composition of the polyesters was determined by high pressure liquid chromatography (HPLC) analysis of the polymer hydrolysates. The crosslinking mechanisms and thermal degradation of the polymers were also investigated. The polyesters contained 30–35% citric acid, 1–4% unsaturated carboxylic acids (i.e., itaconic, cis-aconitic, trans-aconitic, and mesaconic acids), and 60–70% CD, whereas about 40% of them were able to form inclusion complexes. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Chemical structure of poly(β‐cyclodextrin‐co‐citric acid)

https://pubs.rsc.org/en/content/articlepdf/2021/py/d1py00228g

New horizons for carbon dots: quantum nano-photoinitiating catalysts for cationic photopolymerization and three-dimensional (3D) printing under visible light

Wiktor Kasprzyk

Papers

Luminescence phenomena of carbon dots derived from citric acid and urea – a molecular insight

Novel efficient fluorophores synthesized from citric acid

Luminescence phenomena of biodegradable photoluminescent poly(diol citrates)

Chemical structure of poly(β‐cyclodextrin‐co‐citric acid)

New horizons for carbon dots: quantum nano-photoinitiating catalysts for cationic photopolymerization and three-dimensional (3D) printing under visible light